Weathering of natural stone used as building material

Physical, Chemical and Physico-Chemical Introduction


Research Paper (postgraduate), 1992

67 Pages


Free online reading

TABLE OF CONTENTS

1. INTRODUCTION

2. WEATHERING, DETERIORATION, DECAY
2.1 NATURAL WEATHERING
2.1.1 STONE
2.1.2 PHYSICAL FORCES
2.1.3 TEMPERATURE
2.1.4 AIR AND WIND
2.1.5 WATER
2.2 ANTHROPOGENIC DETERIORATION
2.2.1 DIRECT DECAY INDUCED BY HUMAN INADEQUACIES
2.2.2 INDIRECT DECAY CAUSED BY ENVIRONMENTAL POLLUTION
2.2.2.1 SOLUBLE SALTS
2.2.2.2 ACIDIC AIR POLLUTANTS
2.3 BIOGENIC DECAY
2.3.1 BIO-DETERIORATION OF NATURAL BUILDING STONE
2.3.2 TYPES OF ORGANISMS INVOLVED IN BIO-DETERIORATION
2.3.2.1 MACRO-ORGANISMS : i. HIGHER PLANTS Ü. ANIMALS
2.3.2.2 MICRO-ORGANISMS: i. LICHEN AND MOSSES Ü. ALGAE AND FUNGI

iii. BACTERIA

3. BIBLIOGRAPHICAL REFERENCES

1- INTRODUCTION

In the last one hundred years buildings have deteriorated faster and more dramatically than in the five or more preceding centuries.

In this paper the most important causes of decay to natural stones, used as building material, are reviewed:

i. natural weathering by air, water and characteristics of the material itself;

ii. man-made decay caused by incompetence, neglect, vandalism, and environmental pollution;

iii. biogenic damage due to animals, plants and micro-organisms.

Weathering can be broadly defined as the chemical alteration and the physical breakdown of rock material in response to environmental conditions (Trudgill, S.T. 1983). It is a process involving the interaction of many different factors and the resulting products are complex (fig.l).

Ose factors: Design of system

Installation and maintenance procedures Normal wear and tear Abuse by the user

Degradation factors affecting the service life of building components and materials (Clifton, J.B. & Frohnsdorff, G.J.C. 1982)

Weathering is classified in most qeology textbooks as: (i) physical weathering: frost action, insolation, salt weathering such as crystallization and hydration pressures, plant root action and fire; and (ii) chemical weathering : dissolution of carbonates and sulphates, solubilisation by leaching of elements from silicates and sulfides, followed by oxidation and hydration.

In other words, if a solid rock is broken down into a mass of rock fragments by entirely mechanical methods, the process is known as physical weathering and little change in the minerals of the rock take place. In chemical weathering the chemical elements in the minerals are redistributed to a greater or lesser extent.

On both natural outcrops and urban buildings, the process turns out to be more complex (Winkler, E.M. 1987). Recent research shows that most processes are physico-chemical in nature and that the line between the different types of processes is not easy to draw (fig.2). Furthermore, the study of the problem is complicated by the fact that it is difficult to separate the effects of the various agents of decay. No weathering agent acts alone. The relative importance of each one is influenced by the concurrent effect of other agents. Indeed exposure to the action of one may render the material more susceptible to the subsequent action of another. Some agents are predominant in relation to the decay of certain materials, but have no significant action on others. In most cases, it is clear that more than one factor is necessary to bring about an observed effect (Schaffer, R.J. 1972). In other cases, the absence of one factor entails another. For example when a building is less strained by frost and rain, it is more exposed to thermal stress and particulate abrasion. Stress takes different forms in different climates.

2. WEATHERING, DETERIORATION, DECAУ

In the scope of this paper the weathering behaviour of natural stone will be classified by cause and not by effect. The three main actors involved in stone decay are identified as (i) the natural environment, (ii) man and (iii) other living organisms.

2.1. Natural weathering:

2.1.1. Stone:

Besides the obvious physical features of the building stone such as volume, size and weight there are a number of additional important factors that will influence the suitability ot a certain type of stone as a building material.

Based on their origin rocks are classified in three major groups: igneous, sedimentary and metamorphic reck. Sandstone, lor example, is one type of sedimentary rock (fig-3) formed by clastic sediments (residual rock fragments originating from the disintegration of primary igneous rocks or of pre-existing sediments). It is composed of the more resistant constituents of igneous rocks, and consist essentially of fragments of quartz (Silicon dioxide Si02) with subsidiary amounts of other minerals such as feldspar and mica. The essential feature of the sandstones is that the consistent grains of quartz are relatively indestructible, therefore the weathering susceptibility is largely determined by the chemical stability and cementing properties of the material forming the intergranular cement.

Pig.3:

Although most sandstones are composed mainly ot quartz which is highly resistant to weathering, the grains are bonded together by a cement or a matrix which varies in its tenacity. The main forms of decay are as follows:

i. crumbling: loss of cohesion between the grains with the progressive loss of body, in individual grains or aggregates, resulting from frost action or salt attack.
ii. exfoliation: (contour-spalling), loss of the surface in one or more layers. Generally the result of the failure of a case-hardened surface. A single layer may spall of, or there may be a progression of thin layers.
iii. cracking: development of penetrating fractures through blocks; appears to be associated with the presence of swelling clays.
iv. splitting: cracking parallel to bedding planes, usually associated with clay-rich partings or flakes of mica.
v. honeycomb: (alvéolisation): development of a pattern of cavities leaving raised ridges with a vague honeycomb pattern. Associated with coastal exposures and air-borne salt spray attack.

The degree of decay depends on the durability of the sandstone itself, on the degree of aggressiveness of the environment, on the aspect of exposure, and on architectural detail.

The use of dimensional stone in building requires a number of compromises. In early simple building, stone was utilised because it was available locally, was cheap and was a simple constructional material, however, it is now an expensive, prestige material which may be transported long distances (Spry, A.H.).

The inhomogeneity of some types ol sandstone can create a broad spectrum of variations in properties, even in one type of stone. The tensile strength is strongly influenced by the rock matrix, while the compressive strength depends largely on the porosity (Schuh, H. 1987). Natural defects inherent in the material are an important factor in its susceptibility to weathering and become apparent when the material is exposed in the building.

As stone is not a homogeneous substance and furthermore characterised by wide ranges of mineral composition, texture and structure, the durability can be extremely variable.

Uneven weathering may be the result of soft beds. Some types of sandstone, for example, contain beds of different structure with a lower content of intergranular cement. They supply a sandy type of stone which is certainly not suitable for exterior building work (Schaffer, R.J. 1972; Joway, H.F. 1985; Mamillan, M. 1985).

Another type of natural defect which may have serious consequences is the presence of minute fissures in the stone. They are a source of weakness and will ultimately cause cracking and/or spalling.

Because sedimentary rocks are formed by the weathering of igneous and metamorphic rocks, they contain clay minerals. These are sub-microscopic (grain-size 0.004 mm) particulates thus having a high surface/volume ratio. Therefore, they have large unsatisfied charges at their surfaces. This provides them with a capacity for
the adsorption and exchange of ions from circulating waters. The resulting expansion of these minerals makes them a potential source of stone decay (Cauri, K.L. 1980; Snethlage, R. 1981; Felix, C. 1983; Knofel, D.K. et al. 1987; Schuh, H. 1987; Yatsu, E. 1988). Some sandstones, for example, may also contain a small amount of clay. They are therefore marginally susceptible to deterioration caused by swelling.

Other characteristics of natural stone which will influence its resistance to weathering are: residual stresses and constraint stresses, bulk specific gravity, hardness, compressive strength, tensile strength, modulus of elasticity, rock creep, modulus of rupture, heat and temperature conductivity, and water sorption as well as porosity, pore size distribution and capillarity. In other words, the durability of natural stone mainly depends on the quality and quantity of the cementitious matter bonding the grains.

The pore structure of stone is determined by:

i. porosity: this is the volume of pores within a stone, expressed as a percentage of the total volume of the stone, values around 10 to 20 % are typical, although porosity may be as low as 1 % and as high as 40 %;
ii. pore size distribution: this gives an indication of the way in which the pore space is distributed within a stone: whether there are many fine pores, for example, or a smaller number of coarse pores. This affects the durability and some predictions can be made concerning frost resistance and susceptibility of salt crystallization (Knöfel, D.K. et al. 1987).

Pores with r > 0.03 μτη are empty, thus, unless special conditions prevail, the presence of such pores does not enhance the danger of frost damage. On the contrary, by acting as reservoirs, they can accommodate the water expelled from the smaller capillaries as it cools to low temperatures. Pores with very small diameters are also harmless because water contained in them becomes unstable only at very low temperatures which seldom, if ever, occur in nature. From the point of view of frost action, pores with radii in the range of 0.03 to 0.004 μιη most often cause frost damage (Litvan, G.G. 1973);

iii. capillarity : is the property of materials to absorb liquids in their pores by capillary suction. Capillarity is conditioned by porosity and pore size distribution (Mamillan, M. 1985).

The natural decay rate (Anon. 1987) can be estimated at:

- 0.3 to 1 cm per century for sandstone;
- 0.1 to 0.3 cm per century for less porous material such as limestone ;
- 0.02 to 0.05 cm per century for more compact stone such as granite and marble.

NOTE:

According to W.E. Krumbein et al. (1987) there is one more natural cause of stone degradation: the biophysically produced damage. It will be mentioned here only for the sake of
completeness. On the quantum-level, a series of forces can induce changes in mineral building-materials, e.g. proton transfer, electron transfer, hydrophily, hydrophoby and Van der Waals forces.

2.1.2. Physical forces:

The three most important natural physical forces acting upon structures are: gravity, wind load and seismic activity (Delrue, J. 1988).

i. gravity, a purely vertical force, has to be taken into account in the choice of building materials and construction methods.

ii. wind load has a purely horizontal effect with a variable orientâtion.

iii. seismic forces are a very complex phenomenon of accelerations of the ground on which the structure rests, as a consequence of the inertia of a building, these accelerations release enormous destructive forces. These forces consist of:

- a horizontal component. This causes a dynamic and rhythmic alteration of heavy horizontal loads;
- a torsion around a vertical axis. This axis is located in the centre of rigidity which is defined by the spatial arrangement of the stiffening walls and plates;
- the mode or frequency of the actual construction itself. Hereby the construction is simply excited and consequently produces a very important distortion response.

As can be seen from the above, these three physical forces have hardly any influence on the building materials as such, but are very important to the stability of the entire construction.

2.1.3. Temperature:

i. Depending on its type, the durability of a material may be affected by air temperature and solar irradiation (thermal and ultraviolet). The temperature of a given material, or component of a structure, is a continuously variable factor determined by the diurnal atmospheric temperature change, solar radiation, thermal characteristics of the material, wind speed and the heat gain or loss from the ground and the occupied space (Ashton, H.E. & Sereda, P.J. 1982).

Temperature can affect the performance of materials in at least two ways :

- causing movements of building elements or components that result in stress and deformation;
- causing internal stresses in composite materials when components of the system have different coefficients of thermal expansion.

It is evident that it is necessary to measure the temperature of a material in its micro environment in order to determine the effect on performance and durability.

Rock is a poor conductor of heat, so a thermal gradient is set up between the surface and the interior of a rock when it is heated. The surface of the rock, therefore, expands more than the interior, setting up a stress that can eventually lead to fracture (Ollier, С. 198ч).

The rapid changes in strains and stresses associated with temperature fluctuations highlight the dynamic nature of these processes and suggest that they can accelerate decay, particularly when the stone is already weathered. Expansion and contraction of the clay minerals, caused by temperature and moisture changes, can result in micro-cracking and a breakdown of the microstructure. Salt attack, associated with urban pollution or other forms of contamination, can also be accelerated by frequent temperature and moisture changes (Herman, J.L. 1987). Diurnal heat cycles cause differential thermal expansion. Quartz, for instance, can produce an expansion pressure of 545 kg/cm- if heated to 40°C. This difference of temperature is not exceptional in urban environments (Winkler, E.M. 1975).

Heating-cooling cycles may cause (Bernardi, A. 1990):

- mechanical stresses due to thermal expansion and contraction ;
- evaporation-condensation cycles within micro-pores;
- migration of hygroscopic salts from and to the súriace;
- deposition of pollutants due to thermophoresis;
- deposition of pollutants due to diffusophoresis;
- chemical reactions;
- increase in biological attack due to enhanced growth conditions.

ii. Fire causes severe damage to buildings made of stone. This may vary from unsightly discolourations, over extensive spalling and exfoliation to the complete disintegration oí the structure.

Exposure of rock to intense heating results in differential expansion of the minerals. Breakdown of rocks may occur at the boundary of heated and unheated rock, especially in quartz- containing stones (Winkler, E.M. 1975). Quartz expands about four times more than the feldspars and twice as much as hornblende. It is, therefore, considered as the most critical mineral under conditions of intense heating (fig.4).

Quartz has a volume expansion of 3.76% when heated from 0°C to 570 C. This produces pressures of about 1.000 - 2.500 atm. At 573°C, the phase boundary from low quartz to high-quartz, another volume increase of 0.7% causes further disruption of the stone. At 870°C recrystailisation occurs with further volume increase. When expanding during heating quartz exerts pressure against its surroundings which unavoidably causes spalling, cracking and rock disintegration, especially in along the boundary between the heated and unheated substance. The stresses reach their maximum along this line where the thermal gradient is greatest. As a consequence, the characteristic physical damage caused to stone by fire consists of the exfoliation of a thick crust parallel to the original stone face (Kieslinger, A. 1954). Uneven quenching with cold water from fire hoses can also cause surface spalling by creating great internal differential stresses. Spalling and cracking can then occur well below 573°C.

When over-heating occurs, chemical deterioration of sandstone, for example, is caused by the expulsion of capillary and chemically bound water. Dehydration of hydroxides induces important colour changes, especially limonite containing cement which turns red at about 25СГС. Red discolouration also occurs as a result of the oxidation of ferrous to ferric iron caused by over-heating.

Fig.4: Volume expansion of some common rock-fomimj minerdis (Winkler, E.M. 1- 5)

iii. Frost resistance of rock or stone, as a problem, implies a coalescence of material properties and environmental factors (Fagerlund, G. 1974). Frost mechanisms are, in any case, induced by temperature effects. The decay caused by frost can not, however, simply be assigned to volumetric expansion of H20 during freezing in sealed cracks or pores, or to the frequency of freeze-thaw cycles alone (Wälder, J. & Hallet, R. 1985). Freezing-induced damage in porous building materials is, on the contrary, strongly related to water movement. The attraction of water to freezing sites, due to chemical potential gradients, is a central part of the physics of frost cracking (Wälder, J. & Hallet, В. 1986). Slow, steady growth of cracks occurs as water migrates towards ice bodies within rocks. Negative pore water pressures at or near the freezing front will result in a constant movement of water through porous rock from the water table and will cause a continuous increase in the amount of segregated ice in pores (fig.5). Such an excess of water migration in porous rocks during freezing is found to be similar to the frost heaving processes in soils (Fukuda, M. 1983).

Measured weight increases are observed prior to visible breakdown which are related to (a) continual absorption of moisture and (b) modification of pore structure by freezing water which, by enlarging pores, permits absorption and retention of greater amounts of moisture (McCreevy, J.P. & Whalley, w.B. 1985).

Crack growth depends on the magnitude and the maintenance of pressure inside cracks. Therefore, it will be controlled both by their pressure and the rate of water flow necessary to maintain pressure in an expanding crack. At very low temperatures, high pressure can be generated in cracks, but crack expansion is limited because water mobility decreases sharply with decreasing temperature. In contrast, at negative temperatures closer to 0nC, water is considerably more mobile, but pressures inside cracks become lower. Por most rocks the most rapid breakdown will occur at temperatures which range from -5°C to -15UC (Hallet, В. 1981) (lig.5-6). So frost-induced cracking will occur only below a critical temperature. Below this temperature, the rapi i decrease in permeability causes the crack growth rate to diminish with temperature because water flux becomes severely limiting (fig.6).

5 .and U ИРй sre ^.rerally appropriate values f or sedinentary rocks an*:: tor plulcnk rocks, respectively.

Pig.6: Rate of crack propagation as a function of temperature and ice pressure in cracks (Ballet, B. 1983).

The breakdown of rock due to freezing depends on the properties of rock, thermal regime, moisture conditions and solute effects:

- four rock properties will influence crack growth: elastic moduli, fracture mechanical properties, grain size and shape, crack size and pore size (Wälder, J. & Hallet, В. 1985). Among the pores of different sizes within a porous material the following interactions can occur (fig.7). In very small pores (micro-pores) or very close to the pore/mineral surlaces the water molecules are attracted by electrostatic Van dec' Waals forces or hydrogen bonds. This type of water contained in the pores does not freeze under normal frost conditions. The frost action is strongly influenced by the cooling rate. Under slow cooling rates the ice pressure may be reduced by the plastic flow of ice along these water films (Knöfel, D.K. et al. 1987);

- thermal regime has many aspects favouring frost wedging. Apart from a critical temperature (fig.6) the frequency of freeze-thaw cycles, rate of cooling, minimum temperature and duration of cold periods are also important parameters;

- moisture content of rocks can strongly affect i rost wedging rates primarily by controlling the pore water pressure. High initial moisture content and ample water supply also favour frost damage;

- solutes affect frost wedging mainly because the freezing temperature is lowered thereby increasing the unfrozen water content and water mobility at sub-freezing temperatures. This can either increase or decrease frost wedging rates, depending on the effective shift of the curves in fig.6, This effect is important for low soluble concentrations, the greatest
frost-induced damage is associated with the most dilute solutions used (McGreevy, 1982).

(a) filled nacropores;
(b) filled nacropores in connection with meso]-ores;
(c) nacropores connected with mesopores, with freezing occuring in the mesopores and nacropores;
(d) inicropores connected with mesopores, with freezing occuring in the aesopores;
(e) Mero-, neso-, and aicropores connected with each other, with freezing in the aesopores.

Classification of the pore size ; ■acropores ffy>50nn aesopores 2ni<RH<50nn aicropores RH<2nn

Explanation of symbols : pore type not existing - filled pore I partly filled pore empty pores (j

Abbildung in dieser Leseprobe nicht enthalten

Fig.7: Mechanical effect of ice formation in different porous systems (Knofel, U.K. 1987).

2.1.4. Air and wind :

In addition to air temperature which was discussed above, air is a factor because it contains a number of products which iniluence the condition of stone buildings.

The most active natural factor in this case Is water vapour. Depending on temperature and level of saturation, such phenomena as condensation (inside or on the surface of the stone), mist, rain or snow, can occur with water in a liquid state dissolving the gaseous contaminant or of air (СОг, S03, etc) and becoming very active chemically (Jedrzejewska, H. 1931).

Erosion due to wind-borne dust or sand sometimes causes damage to building stone. The relative susceptibility of building stones to this type of erosion largely depends on their hardness and homogeneity. The effect of wind will increase according to the height of the building, the degree of exposure (obstructions, topography, trees, etc.) and the proximity of the sea (Meisel, U. 1988) .

Wind can also deposit dust, soot, salts, spores, etc. on the face of the building. Dust particles and gaseous pollutants can combine to form a black crust on the building stone. This soiling is enhanced by stronger wind speeds. On high buildings, for example, the accumulation of detrimental substances on the facade increases with height.

Natural atmospheric pollutants of importance in material deterioration are carbon dioxide, chloride ion, small amounts of naturally occurring other ions, and dust from natural sources (Hollander, J.C. & Lanting, R.W. 1986):

- the main effect of carbon dioxide on stone is the formation of calcium carbonate (carbonation), with simultaneous reduction of the alkalinity of the stone matrix. It also reacts with metal hydroxides to form metal carbonates, which may be leached out by highly acidic rain. Although a natural constituent of the atmosphere, significant anthropogenic emissions of carbon dioxide occur in urban and industrial areas;
- the most important natural source of chlorides is sea spray and is therefore one of the dominant factors in atmospheric corrosion of coastal areas;
- the effect of dust from natural sources is mainly soi Ling of surfaces. Interaction with gaseous air pollutants may Lead to the formation of aggressive substances causing pit corrosion.

2.1.5. Water:

Most processes of deterioration and corrosion involve moisture as a direct agent in the process, either as a medium of reaction or as a constituent of the micro-environment fostering the process.

Hardly any chemical weathering processes would occur without water. In fact, in the absence of water (i) there would be no chemical reaction of stone constituents; (ii) soluble salts would not be transported and would not migrate, crystallize and recrystallize; (iii) airborne atmospheric pollutants would not dissolve in water droplets and would thus not remain in contact with the stone constituents for a long time, therefore reducing the attack on the stone (Amoroso, G.G. & Fassina, V. 1983).

Other than causing surface effects such as run-off staining, water induces or exerts a mechanical action under certain conditions. The most important physical action is ascribed to its increase in volume during freezing. Physico-chemical processes occur mostly during the transportation of water through the capillaries. The distribution of the pore size rather than the integral porosity, plays the most important role here.

The chemical reactions which can cause weathering as well are:

- solution, which is usually the first stage of chemical weathering. It may take place in running water or in a thin film of water around a solid particle. The amount of solution depends on the amount of water passing over the surface of the
particle and the solubility of the solid being dissolved. Solutions may precipitate chemicals, which can lead to changes in volume, thus enhancing physical weathering. Even quartz, one of the most resistant minerals, shows a rapid increase in solubility following ferrous-ferric iron reactions (Morris, L.C. & Fletcher, A.В. 1987);
- migration or surface movement of water containing soluble constituents of rock minerals. This is caused by three important forces: diffusion, filtration and capillarity (Stegena, L. 1983):

(i) diffusion is the force that tries to eliminate concentration differences in the medium. The medium in which the diffusion takes place does not move. Only the minerals dissolved unequally are redistributed;
(ii) filtration is the process by which liquids move in the pores of rocks by the force of hydraulic pressure difference. This hydraulic pressure means the difference between existing and hydrostatic pressure;
(iii) capillarity can also cause water movements, even directed upwards. This process originates in the surface tension of the liquid and acts between water and rock;

- oxidation means a reaction with oxygen to form oxides, or if water is also incorporated, hydroxides;
- reduction is the opposite and takes place usually in waterlogged anaerobic sites, mostly through the action of bacteria;
- carbonation is the reaction of carbonate or bicarbonate ions with minerals, carbon dioxide leads to rapid carbonation;
- hydration is the addition of water to a mineral. Iron oxides, for instance, may absorb water and turn into hydrated iron oxides or iron hydroxides. Hydration is an exothermic reaction and involves a considerable volume change which may be important in physical weathering such as exfoliation and granular disintegration (Ollier, C. 1984);
- chelation (or complexing) involves the holding of an ion, usually a metal, within a ring structure of organic origin. Chelating agents can extract ions from otherwise insoluble solids, and enable the transfer of ions in chemical environments where they would normally be precipitated (Ollier, C. 1984);
- hydrolysis is a chemical reaction between minerals and water, that is between the H or OH ions of water, and the ions of the mineral.

As a result of one or a combination of these chemical reactions the most important types of mineral deterioration which can occur are (Cawsey, D.C. & Mellon, P. 1983):

i. pseudomorphing of primary minerals by alteration products;
ii. etching of minerals and formation of residual alteration products ;
iii. etching of minerals and removal of alteration products.

Experimental investigations have confirmed that water is indeed the most important agent of chemical weathering, acting mainly through the processes of dissolution, hydrolysis and hydration, and that the progress of chemical weathering in a given rock body is influenced by three principal factors: oxidation, pH and drainage (fig.8-9):

i. Oxidizing conditions convert Fe*+ to Fe+++ affecting Fe"

within the minerals as well as leached-out Fe" ±n solution. The insoluble Fe"' hydroxides v/hich tend to form remain in the zone of weathering, leading to a residual concentration of iron in the weathering profile, i.e. crust formation. Under reducing conditions, the opposite effect occurs, enabling iron to be mobilized as soluble Fel% and leached from the profile, leading to loss of iron cement (fig.9).

Abbildung in dieser Leseprobe nicht enthalten

Fiq.8: Solubility in relation to pH for some components released by chemical weathering (Loughnan, F.C. 1969).

The way liquid water controls both the rate and morphology of the deterioration crust formed on a stone surface indicates that local rainfall is a critical factor in determining the type of deterioration. Different weathering patterns depend on the; way the interaction o rain with the stone surface occurs.

Wetting, resulting from moisture condensation, appears to play a minor role compared to rainfall (Camuffo, D. et al. 1982).

ii. Low pH promotes hydrolysis of silicate minerals by providing additional H* ions which enter the crystal lattice, displacing metal cations and disrupting the silicate framework. Zi When the pH is higher than 9 the solubility of silica increases appreciably; aluminium is soluble at a pH less then 4 and iron hydroxides at a pH lower than 2 (Cawsey, D.C. & Mellon, P. 1983) (fig.8).

iii. Drainage of water passing through the zone of weathering influences the nature of secondary minerals which form. Even poorly soluble products of hydrolysis may be leached out leaving an insoluble resin largely of Fe1" and Al hydroxides.

Where drainage is poor, clays will be the stable secondary minerals.

The penetration of water into porous stone can be caused by different factors:

- air currents and air pressure due to wind will cause rain penetration ;

- capillary absorption and rising damp being a function of the porosity, size and distribution of the pores and permeability of the stone;

- surface condensation will cause the formation of a film o! liquid water on the face of the stone in the presence of two simultaneous conditions: iow surface temperature and high relative humidity of the nearby air (Amoroso, G.G. & Fass ina, V. 1983).

Abbildung in dieser Leseprobe nicht enthalten

Fig.9: Eh - pH relations. Iron mobilised at low pH (high acidity) ind when reduced (low Eh) {Tnidgill, S.T. 1983).

It is important to note that deterioration processes do not take place during the absorption of water, but when water evaporates, that is, during the so-called drying phase. Evaporation of water takes place at the surface and depends on the moisture supply from the interior, the temperature of the ambient air, the relative humidity and the ventilation on the surface.

As water evaporates at the surface, florescences tend to accumulate in surface regions of the stone. Their migration is facilitated by increased ionic concentration resulting from atmospheric pollution. Repeated dissolution and crystallization of florescences (wetting-drying cycles) in subsurface regions generate stresses which disintegrate the stone (Gauri, K.L. 1980). Even pure water may cause the breakdown of stone as it undergoes alternate wetting and drying cycles, without any change of temperature. Furthermore, the effects of wetting and drying on rocks containing expanding clay minerals cannot be underestimated.

« Fig.io:

Proa archive texts it к known that the building stone for this church was kept in a pool next to th? river. Possibly this is tl· ï·■■ ¡son for the bad conditio:, οι the stone.

I Fig.11:

The ferrous iron precipitates near the suit : horning hard crust. In a short tine flaking will occur exposing a sandy enabling area.

2.2. Anthropogenic deterioration

2.2.1. Direct decay induced by human inadequacies:

This type of stone deterioration brings about purely physical damage patterns. These may be induced by one or a combination of the following man-made causes:

(i) Faulty craftsmanship is one important cause affecting the condition of buildings. Defects may occur as a result of lack of skill in manipulation. For example all sedimentary rocks have a laminated structure. From the point of view of weathering it is advisable that all stone be laid on its natural bed, because stone usually shows a higher compressive strength at right angles to the bedding plane (Schaffer, R.J. 1972). Face bedding is sometimes adopted because the bed in the quarry is not thick enough to supply stone of sufficient size for mullions, pillars, etc. Spalling and exfoliation in building stones is often due to face bedding.

(ii) Seasoning is also an important factor affecting the quality of ashlar. As far as ease of working is concerned, there is no doubt that it is preferable to work the stone immediately after its removal from the quarry and before it has had time to harden. Therefore, it is common practice to keep freshly quarried limestone and marl in pools of lime water in order to avoid evaporation of the quarry sap. After dressing the stones, drying out of the surface causes the quarry sap to move to the outer layer and precipitate there, forming a hard crust which is said to be a protective covering against weathering.

In other cases, e.g. ferruginous sandstone, however, this practice induces rapid exfoliation and considerable damage. The ferrous iron contained in the quarry sap has to be given time to precipitate in situ and form the hard iron cement which is one of the principal qualities of iron-sandstone. If the stone is dressed and used in a building without proper seasoning the soluble ferrous iron will move to the exposed face and precipitate there forming a hard crust. As a result a sandy area will form behind this crust containing hardly any cementitious matter. In a relatively short time blisterina and flaking will occur (fig.10+11). The sandy area which is then exposed is an ideal substratum for bacteria, fungi, lichens and mosses. These, in turn, accelerate further decay.

(iii) Improper methods of quarrying and dressing may weaken the stone. Inadequate selection of stone in the quarry and blasting operations, for example, may influence the ultimate life expectancy of a building (Mamillan, M. 1979).

(iv) The wrong choice ol materials for a particular purpose is another important reason for stone decay (Joway, H.F. 1985). For example, materials unsuited to the design, a particular purpose, structural movement and settlement, neglect or faulty restoration attempts can cause various categories of damage (Ashurst, J. & Ashurst, N. 1989):

- Cracking of stones and joints due to structural movement and settlement of large areas of a building, or unequal settlement of elements tied to each other;
- Cracking due to poor detailing and construction, such as inadequate bearings for lintels, thin stone facing to poor quality core-filling and sills cracking over hard spots in

I Fig.12:

Hur,an induced decay: lack of naintenance to gutters and rainwater pipes.

- Fig.13:

Run-off water from the linestone of the rose window has caused danage to the sandstone.

bedding ;

- Spalling, splitting and lifting due to the volume increase of rusting, embedded iron cramps, straps, window and door ferramenta ;

- Staining and decay of open joints with lime runs, due to neglect of the joint's condition, giving water freer access to the interior of the wall;

- Water staining and scouring sometimes associated with frost damage due to inadequate provision for rainwater disposal or failure of rainwater drainage systems (fig.12);

- Cracking and advanced decay around mortar which is too dense and impervious for the stone;

- Spalling and other damage around joints caused by careless cutting out when repointing.

- Pitting and dishing from careless (air) abrasive or disc cleaning;

- Staining and efflorescence associated with inexpert or inappropriate chemical cleaning;

- Surface discolouration, flaking and pitting due to shallow 'preservative' surface treatments. Decay due to poor selection or misuse of stone can be added to this list. The classic example of the latter is the placing of limestone over sandstone in a building, resulting in the accelerated decay of the sandstone. The carbonates washed out of the limestone by rain are absorbed in the sandstone due to capillary suction. When drying, these salts will crystallize and expand causing cracking and exfoliation (fig.13).

(v) Vandalism can without doubt be considered as the largest and most damaging among destructive factors affecting our cultural heritage. Wilful and malicious destruction of art objects can be considered as one of the main sources of anthropogenic "pollution". Whether this tendency to inflict damage originates from motives of gain, craving for political power, lack of proper planning or sheer ignorance is often a matter of debate or even a matter of "fashion". The root of vandalism is in all cases foolish spitefulness (Gibbs, N.R. 1987).

Acts of vandalism can take different forms;

- The first one is war: the wilful destruction and looting caused by the abuse of power by politicians;
- The second one is fanaticism and zealotry: the destruction of "cult objects" (images, objects, buildings, etc.) by religious or anti-religious fanatics;
- The third-one is unbridled pursuit of gain by so-called "developers" who in the name of modernization purposely neglect and destroy old cities to replace them with nondescript constructions (Benaire, M. 1988);
- The fourth-one is individual senselessness, mostly enhanced by mass-tourism. Souvenir hunting, graffiti and deliberate damaging of cultural property has entailed huge repair-bills in all major cities of the world.

(vi) Vibrations caused by traffic, trains, machinery or sonic boom induce rapidly alternating tensile and compressive stresses in building structures (Torraca, G. 1982).

The stresses caused by the most frequent types of vibration (e.g. traffic) are not sufficient to cause damage to a building if considered alone. When superimposed, however, on other types of
stress which act on structures and materials (load and environmental factors) it must be expected that vibrations will cause an increase in the rate of deterioration.

2.2.2. Indirect decay caused by environmental pollution:

2.2.2.1. Soluble salts

i. Freshly quarried stone seldom contains any significant quantities of soluble salts but it can become contaminated before or after positioning in a building. If there is no damp-proof course or the damp-proof course is defective, salts can be absorbed in solution from the soil. They can originate from the stone itself, nortar, in brick or concrete backing, air pollution, biological agents, by exposure to salt spray (sea, de-icing) or by the use of ill-advised methods of cleaning and maintenance. Some salts are more harmful than others and their effects are more pronounced on some materials than on others (Schaffer, R.J. 1967).

Salt action can cause great destructive effects. The visual evidence of salt corrosion or salt fretting occurs along the upper fringe of the capillary water transport (Arnold, A. 1981). Salt weathering produces rapid splitting and granular disintegration of the rock even within a relatively limited temperature range.

Besides calcium and magnesium sulphates derived from calcareous or dolomitic materials, soluble salts such as sodium sulphate, magnesium sulphate, sodium chloride or magnesium chloride, derived from other sources, commonly cause deterioration in porous building materials.

ii. Four types of soluble salts are found in the efflorescences of stone walls (fig.14): carbonates, chlorides, nitrates and sulphates:

- carbonates mostly originate from constituents of the stone itself. Na2C0, and its hydrates can be supplied by ground moisture and proves to be very effective in stone deterioration (Goudie, A.S. 1986). Many carbonate minerals are produced by modern alkaline building materials such as portland cement, water glass products and alkaline cleaning materials (Zehnder, K. & Arnold, A. 1984);
- chlorides are quite common in coastal regions (NaCl) and when found inland they mainly originate from de-icing salts. They only seem to occur in small amounts but their frequency may be underestimated by the fact that most of them are more or less hygroscopic. Some of them can not crystallize under normal conditions on walls because of their very high hygroscopicity even if they are present in considerable amounts (Arnold, A. 1981);
- nitrates are quite common and are only found in the neighbourhood of decomposing organic matter. The most commonly occurring nitrate is KN03 (niter);
- sulphates are the most common soluble salts. Sodium sulphate (Na^SO,) is the principal constituent in most of the efflorescences encountered (de Quervain, F. 1945 & 1951). Sodium sulphate and magnesium sulphate are known to be the worst cause of stone deterioration (Goudie, A.S. 1986). Their effect is due to the increase in volume when getting higher
degrees of hydration. The best known action is that of sodium sulphate :

Abbildung in dieser Leseprobe nicht enthalten

Pig.14: Saline Binerais found in walls (Arnold, A. 1981), including carbonate rainerais originating frora the building materials thenselves.

The weathering eliiciency of the sodium sulphate system results from a combination of the following factors :

a. the large increase in volume (313.67%) at the point of the hydration of thenardite to mirabilite.

b. the solubility characteristics of Na .SO*. 1QH ,0 change very rapidly with temperature, so that on a marked temperature reduction, concentration of salt solution in the rock pores leading to supersaturation will quickly take place and crystal growth ensue. For Na,S04 there is an inverse relation between temperature and solubility with the result that high daytime ground surface temperatures will also cause the precipitation of sodium sulphate crystals out of solution ;

c. the crystallography: the pressures caused by crystal growth are effective in promoting disintegration of the host rock, especially when the growth pressure is concentrated along one axis (Sperling, C.H.B. & Cooke, R.U. 1985). This crystallization pressure is more effective in rock disintegration than the hydration of sodium sulphate.

iii. The most important potential causes of disruption by salt actions are crystallization pressures, hydration pressure and hygroscopic water retention -

- crystallization pressure depends on the temperature and the degree of supersaturation of the solution, C/Cs. Depending on the ambient temperature and relative humidity some salts may recrystallize to different hydrates, which occupy a larger space, being less dense, and exert additional pressure. Resistance to crystallization damage is strongly dependant on the internal structure of the stone and decreases as the proportion of fine pores increases. The pressures produced in small pores by crystallization are appreciable. Gypsum (CaS0,.nH?0) for Instance exerts a pressure of up to 100 MN/nr; anhydride (CaSOJ, 120MN/&2; kieserite (MgSO, ,пНйО) , 100MN/m ; halite (NaGl), 200MN/m all are sufficient to cause disruption (Bell, F,G. & Dearman, W.R. 1988);

- hydration pressure: when for example Na,S0« becomes Na,S04.10H50 the crystal volume of the salt increases by over 300% (see above). Some salts attract moisture through hygroscopicity without hydrating. NaCl present in de-icing salt, nitrates from fertilizers and excrements retains a considerable amount of moisture when trapped in stone, keeping the area of salt concentrations continuously moist; hygroscopicity is also determined by relative humidity. For example in the case of NaCl the critical limit is a relative humidity in the ambient air of 75% (Vos, B.H. 1988) (fig.15).

Other processes in which salt crystallization plays an important role in the deterioration of porous stone are :

- the expansion and contraction causeo by changes in temperature and relative humidity or vapour pressure (Pühringen, J. 1983);
- alveolar erosion caused by strong winds which accelerate the evaporation of water circulation in the pores to such an extend that no liquid film can be formed on the external surface. Evaporation is actually taking place immediately below the surface, in the pores, and this is where the disruptive effect of crystallization takes place. The process undergoes a progressive acceleration when a cavity is formed because wind speeds increase inside the air eddies. This further enhances evaporation in that specific area and alveoli form (Torraca, G. 1982);
- efflorescences are salt crystals which form on the surface of porous materials when water evaporates there, because the water supply is constant and the wind speed low. In this case the salt crystals are formed mainly out of the pores and the disruptive effect is smaller;
- frost weathering in combination with salt-contaminated pore-water has been investigated several times with quite divergent results (Goudie, A. 1974; Hochstetter, R. 1974; Williams, R.B.G. & Robinson, D.A. 1981; Mc Greevy, J.P. 1982; Bos, K. 1990; De Witte, E. & Bos, K. 1992).

Rock breakdown induced by a combination of frost shattering and salt weathering appears to be a much more complex phenomenon than either of these two processes operating independently. The question as to whether or not the presence of salts enhances or inhibits frost shattering will depend upon the attainment of a particular set of environmental conditions. For example, the experiments suggest that the amounts of weathering are inversely related to solution concentrations. There probably exists a threshold degree of salinity of a given solution below which the freezing behaviour of the rock would not be significantly affected regardless of solution concentration. Above this threshold frost damage would progressively diminish. Temperature, hydration, solution, type and concentration are also important factors affecting the breakdown (fig.18). For each type of salt, for example, the greatest damage is associated with the weakest solution. The ability of solutions to penetrate and saturate available pore space seems to affect the weathering rates. For example, NaCl solutions are absorbed more rapidly than Na?S0, solutions and resulted in a greater degree of saturation. The breakdown caused by "frost-salt" weathering is greater in the case of NaCl solutions . With more concentrated solutions the presence of salts in pores may alter the physical properties of the rock. A large amount of salts in the pores causes a significant increase in elastic modulus. Saturated salt solutions can also cause chemical surface etching on quartz grains in a short period of time (Magee, A.G. et al. 1988 a+b). Sodium chloride, sodium carbonate and sodium sulphate are particularly effective chemical weathering agents of fresh quartz, unrelated to any natural temperature fluctuations! This process may slightly increase the pore volume of rock.

These factors can increase the ability to withstand freezing stresses and hence less frost damage occurs. Another way in which the various salts dissolved in the pore water may influence the amount of frost damage is exfoliation due to the following mechanism. As the concentration of the salts in solution is greatest in the vicinity of the surface of the stone, and as a consequence of this the freezing point of the solution there is considerably lowered, the moisture held deep inside the stone freezes first. When the temperature outside, however, decreases sufficiently the moisture in the uppermost layer of the stone freezes too. Yet under this ice there is still an interstitial layer of fluid sandwiched between the already formed lower and upper ice-lenses. This is the last to freeze, and because enclosed, the expansion caused by its conversion to solid ice would force the surface layer of the stone to flake off (Stambolov, T. & Van Asperen De Boer, J.R.J. 1984).

iv. Soluble salts are subject to outward migration due to moisture travelling through the complex pore and capillary system and ultimately to crystallization near the surface during the evaporation cycle (fig.16). As a result, the distribution of soluble salts decreases exponentially from the surface toward the inside of the rock. Besides the effect due to crystallization, differences in the thermal expansion of salts trapped in the pores of the surface layer of the rock, may favour the disruption of the face of the monuments where thermal shock is maximal. This effect becomes dramatic for example in the case of gypsum wedges in calcite (Camuffo, D. et al. 1983).

Transport of salts to the surface from the inside of porous stone may give rise to the formation of a case hardened outer crust. The density of this surface skin increases by the precipitation of salts, by as much as 20% in soft friable stones, 5 to 10% for semi dense stones and only 1 to 5% for dense stones. Such crusts may act as a protective coating if composed on secondary calcite or calcium sulphite. Superficial induration is temporary if the disintegrating stone substance is the sole supplier of salts for the outer crust. At the same time the interior structure of the stone is weakened (Bell F.G. & Dearman W.R. 1988). This is the case, for example, with ferruginous sandstone where the sandy layer behind the surface crust is deprived of its internal cement.

v. The effectiveness of salt action depends on the kind of salts present, on the size and shape of the capillary system, on the moisture content and on the exposure to solar radiation (Winkler, E.M. 1987).

Factors affecting the resistance of rock to breakdown are surface area and shape. A significant inverse relationship has been found between the resistance of a block to weathering and the initial surface area of the block. The total edge length, exposed to weathering depending on the shape of the block, may be even more significant than relative surface area. The orientation of the bedding within the sandstone has no significant effect on the weathering rate (Robinson, D.A. & Williams, R.B.G. 1982).

Under conditions of low relative humidity the disintegration rate for most rock types is more rapid. The saturation coefficient is not a satisfactory measure for discriminating between rock types with different physical properties and, therefore, cannot be used to predict rock durability (Sperling, C.H.B. & Cooke, R.U. 1985).

According to the calculations of Fitzner & Snethlage (1982 a+b) the vulnerability to deterioration due to salt crystallization increases with the progressing of smaller pores. Evidently the presence of two pore radius maxima sufficiently removed from each other signifies a special risk factor. The relation of coarse pores to fine pores has an enormous influence on the degree of pore filling and hence on the deterioration by hydration processes. The coarse pores are the site of salt crystal growth and the content of the fine pores acts as a solution supply to the coarse pores. It can therefore be concluded that deterioration due to hydration will increase the higher the proportion of fine pores to large pores.

Salts can only crystallize when the ambient relative humidity is lower than the equilibrium relative humidity of the saturated salt solution (fig.15). If that is the case at the surface of a stone or mortar joint the salt will crystallize producing decay to some degree.

If the ambient relative humidity exceeds the equilibrium relative humidity crystallized salts dissolve and the solutions

Abbildung in dieser Leseprobe nicht enthalten

î Fig.16: Salt efflorescences caused by external contamination.

*- Fig.17:

Dissolution of a porous solid. In this schematic representation, acid diffuses from left to right and is consumed by cherei cal reaction with the solid. The metal ions produced by this reaction, can under some conditions, diffuse into the pores and precipitate as more solid (Cussiet, E.L & Featherstone, J.D.B. 1981).

Abbildung in dieser Leseprobe nicht enthalten

Fig.15: Crystallization fron solution as a function of concentration and teiperature. Supersaturation may

be obtained in three ways. A to B: by decrease in te»perature at constant concentration; possible in daily temperature cycles. ? to C: by increase in concentration through evaporation of solvent at constant teiperature. A to Û: a combination of A to B, and A to c; nost common in nature (Winkler, E.H. & Singer, P.C. 1972).

dilute until equilibrium is reached. Efflorescences then disappear and humidity spots become visible, this is the hygroscopic humidity of soluble salts.

vi. The presence of salts in building stone has a negative

influence on the effectiveness of protective silicon products and consolidating silicic esters. The influence of soluble salts has repercussions for the colour of the stone, the contact angle, the penetration depth of the product, the capillary water absorption and the strengthening effect of the consolidation products (Biscontin, G. et al. 1988; Fritsch, H. & Schamberg, E. 1988; Vanden Broeck, P. 1989). The presence of salts in building stones treated with water repellent products (methyl alkoxy silane) will cause further deterioration of the stone (fig.19+20). Because the resin-impregnated hydrophobic surface layer impedes the migration of water to the outside, the water evaporates underneath this layer. This evaporation causes the crystallization of soluble salts underneath the hydrophobic layer. If this layer is thin, it is easily exfoliated by the growing stress of the crystal formation. If the impregnated layer is thick exfoliation will take longer, but eventually the layer will be detached. Condition for this type of deterioration is the moisture supply from behind the treated surface layer and the continuous evaporation of this moisture through the same (Nishiura, T. 1985).

2.2.2.2. Acidic air pollutants:

Man's activity influences the quality of the atmosphere, particularly in dense urban areas and near large emission sources. The total mass of pollutants emitted by nature exceeds those emitted by man. For sulphur dioxide, however, pollution sources in

Fig.19: Damage will increase dramatically when walls containing soluble salts are treated with water- repellent products (Bos, K. 1990).

Fig-20:

the Northern Hemisphere are already greater than natural sources.

Recent estimations have in fact attributed about 60% of atmospheric sulphur to man-made sources (Fassina, V. 1988). In urban areas the situation is even more serious than in rural areas because anthropogenic emissions of sulphur are several orders of magnitude higher than natural ones, and therefore the deterioration processes of building stones are rapidly increasing.

Air pollution is made up of a multitude of individual constituents. Some of the most important groups are :

- Inorganic gases: oxygenous sulphur compounds, nitrous oxides, hydrochloric acid, hydrogen fluoride, chlorine compounds, fluor compounds, ammonia and hydrogen sulphide;
- Organic gases and vapours: hydrocarbons, aromates, other organic compounds (alcohols, esters, hetones, aldehydes and phenols);
- Dusts: these may be of very complex composition (inorganic and organic compounds).

The weathering of building stones has become a major political and scientific issue within the debate on acid rain. It is widely assumed that the pollution of urban atmospheres by the combustion of fossil fuel leads to the acceleration of stone decay.

Since last century, phenomena of dramatic deterioration of building stones have been observed. Growing industrialization has been indicated as the cause of this runaway disintegration of our cultural heritage.

Anthropogenic air pollution, however, is only partly responsible for this atmospheric corrosion. Factors such as natural weathering (see 2.1) and biodeterioration (see 2.3) should not be underestimated.

The natural and artificial pollutants relevant to stone decay are: sulphur compounds (sulphur dioxide, hydrogen sulphide and sulphate aerosols and mist), nitrogen oxides, ozone, hydrogen chloride, hydrogen fluoride and carbon dioxide.

These acidic air pollutants accelerate the natural weathering processes (Hollander, J.C.T. & Lanting, R.W. 1986). Damage from their dry or wet deposition, however, is hardly distinguishable from damage caused by natural phenomena.

Pollutants may be subdivided into:

- primary pollutants: emitted directly from identifiable sources; and
- secondary pollutants: produced in the air by the interaction of two or more primary pollutants or by reaction with normal atmospheric constituents.

Their effectiveness depends on: the available moisture (rain, fog, humidity); the temperature of the air; the cooling and heating of surfaces (wind and solar radiation) and the evaporation and condensation of moisture on them; the presence of air constituents and contaminants (gaseous and aerosol); seasonal and meteorological phenomena and human activities.

Most building materials react with different constituents of air and rain water and undergo changes in the presence of ultraviolet light.

The changes in appearance of the building facades are caused by the deposition of these constituents:

- dry deposition of reactive gases (HN03, HC1 r S02f NO.,, CO,, H2S. 02), of water-soluble solid material such as sea salt and of insoluble matter such as dust and soot;
- wet deposition of rainwater and its constituents, solubilizing the dry deposited products. This can cause direct deterioration of the building stone or provides the wet condition necessary for corrosion. It can also transport the deposited element over the facade or into the stone itself (Verhoef, L.G.W. 1988).

Depositions of these gases and particles are governed by different mechanisms according to the relevant atmospheric factors.

Abbildung in dieser Leseprobe nicht enthalten

Fig.21: Classification of nechaniSES of deposition of gases and particles (Tombach, L 1982)

So the main weathering processes are due to the combined action of rainwater and various atmospheric pollutants. In our urban environment the dry deposit on the surface of a monument may be greater than a factor of ten in comparison to pollutants scavenged by rain from the atmosphere. The way in which the monument surface is wetted is therefore very important.

For example dew has the ability to increase the deposition rate of acids and acid precursors on some surfaces. The presence of dew enhances both the retention of dry-deposited particles and ♦’he absorption of water soluble gases. (Mulawa, P.A. et. al. 1986).

In the case of ashlar, damage as a result of emission often occurs as reaction products, conversions and the formation of new phases. The degree of damage inflicted on stone depends, therefore, on its chemical composition (reactive substances), mineralogical composition (mineral components susceptible to weathering), mineralogical behaviour (particle bonding, grain size distribution) and physical properties such as porosity, pore volume, surface characteristics, etc...(fig.17). The damage caused is not so much produced by the concentration of a pollutant in the air, but rather by the amount of pollutant absorbed by a unit of area in a unit of time.

Abbildung in dieser Leseprobe nicht enthalten

Fig.2?: Airborne pollutants: eiission to deposition cycle (Schroeder, W.B. & Lane, D.A. 1988)

Annual surface recession and material losses have been calculated for several buildings in different countries. For example the mean annual surface recession of the St. Rombouts Cathedral has been calculated to be around 20 дт. The yearly material loss through rain water was estimated to be roughly 3.650kg (Roekens, Б. & Van Grieken, R. 1989).

In summary, the following components are assumed to have significant corrosive effects on building materials :

- gas phase: S02, SO,, N02/ HN03, HC1, organic acids, 03;
- aerosols: H3S04, NH4HS04, (NH4) 3H( S04) 2/ (NH4)S04r NaCl ;
- rainwater ions:H‘, NH?, HS03, SO?', NO,, NO;, HCO,*.

Emissions of SO, and NOK are directly or indirectly responsible for most of the pollutants above, via atmospheric reactions such as :

N0a + 02 + hv -*· NO + 0*

H2S04 + NaCl -+ NaHSO* + HCl etc.., (Rosvall, I. 1988).

The corrosion caused by acidic air pollutants is especially effective on limestones, marbles and sandstones with calcium carbonate cements (Jayves, S.Μ, & Cooke, R.U. 1987).

Possible weathering reactions of stones containing calcite are:- natural weathering: CaCOs + C02.H20 « Ca(HC03)2 - possible deterioration due to air pollution:

CaC03 + SCI, + 2H20 - CaS03.2H20 + C02 CaS04.2H20 (gypsum)

CaC03 + H2S04 + H20 -* CaS04.2H20 (gypsum) + C02 CaC03 + 2HN03 - Ca(N03)2 + H20 + C02 CaC03 + 2HC1 + 5H30 -* CaCl2.6H20 + C02 2HF + CaO. ( CaC03 ) - CaF2 + H20

Silicate-based rocks are typically far more resistant to weathering in urban climates. Acid rain does not or hardly react with sandstone buildings. Dry deposition still occurs, however, and adheres to the stone causing a uniform black colour to develop (black soiling) (fig.44+45)(see also 2.3.2.2).

Dissenting opinions have also been voiced asserting that the contribution of air pollution to stone decay is unimportant.

According to Riederer, for example, (1973 & 1982) the decay of building stones is hardly related to air pollution. The rapid decay of monuments is first of all due to the idea of the preservation of monuments. He asserts that considerable damage to cultural property is caused in the first place by (i) a lack of or inadequate maintenance and (ii) 19th century interventions.

Another alternative hypothesis for the accelerated decay in the last one hundred years is the existence of "latent damage". This type of deterioration happens during the initial period of the monument's existence, called the "resistance phase". During this period no visible damage occurs but factors, depending on climate and environment in relation to individual characteristics of the stone, will cause internal structural changes. In time (100, 200, ... years ?) this largely invisible degradation will become apparent at the surface. At that moment the "weathering phase", characterized by a runaway deterioration of the material, starts. Environmental pollution can accelerate this process but is not to be considered as the primary cause (Künzel, H. 1987).

- Carbon dioxide (C02) is a normal constituent of the atmosphere and is not generally considered as an air pollutant. It is also by far the most abundant of all atmospheric gases generated by man's activity being one of the end products of forest fires and waste incineration as well as combustion of organic materials such as coal, petroleum and gas.

Rain is naturally slightly acidic because of the presence of CO, in the air which dissolves in water to form carbonic acid. In a simplified way the reaction can be written as:

h2o + co2 « h2co3 «* H* + hco;

"Natural" rain has a pH of about 5.65.

The weak acid solution formed by dissolution of C02 in rainwater dissolves the calcium carbonates in limestone, marble, lime mortars and plasters and it forms the much more soluble calcium bicarbonates. The attack of C02 proceeds more rapidly in urban areas than in rural ones because both the C02 concentration may be increased and the pH decreased.

Calcite (CaC03) and dolomite CaMg(C03)2 are the common carbonate minerals found in limestone and marble which make these rocks highly susceptible to attack by acid precipitation.

H.O, CO, ,

CaC03 — Ц> Ca(IIC03)2 - Ca3 + HCO;

The reaction only occurs when C02 is dissolved in water. Immediately on drying, CaC03 is precipitated from the solution. The effectiveness of the C02 reaction is, therefore, confined to the period that the structure is wet.

- Sulphur dioxide (SO„) and its transformation products are the main man-made agents in the atmospheric corrosion of building stones. S02 can be oxidized to S03 by the oxygen or ozone in the air under the influence of LTV light and through catalytic action of dust particles, building surfaces and other agents. S03 then dissolves in water to produce sulphuric acid. The reaction can be written as: H20 + S03 « H2SO, «* H' + HSO;

The pH of this rain depends on the concentration of S03 (Charola, A.E. 1987).

The sulphurous compounds (including aerosols) which affect the stone are transported mainly by air. Rainwater, on the other hand, is only a secondary vector playing a decisive part in the distribution of the sulphurous compounds and in the processes of alteration (Furlan, V. & Girardet, F. 1983).

The overall process whereby atmospheric pollutants are delivered to a stone surface, form reactive species, attack the surface and ultimately are responsible for loss of surface material can be separated into individual steps governed by distinct physical and chemical parameters. Aerodynamic factors control the delivery of SO, to the stone surface; S02 is then oxidized to sulphate and subsequently reacts with the carbonates of the stone. Mechanical stresses are introduced by the reaction products disrupting the stone structure. Finally material is lost as stone and alteration products are removed (Livingston, R.A. et al. 1983).

There are great uncertainties associated with these critical parameters so that at present only qualitative interpretation of the decay process is possible.

The S02 reaction with calcite can be written as:

caso3.2H.o + co2

Camuffo (1982;1986) classifies the deterioration of carbonatic rocks in urban areas into three kinds of visible feature patterns which relate to the way in which the wetting of the stone occurs:

a. black areas consisting of a black crust, characterized by gypsum crystals and calcite with carbonaceous particles, produced by fuel combustion, embedded in the crust. This area of the wall is only wetted by wind-borne droplets or by percolation at the edges of the descending streams. In these protected areas S02 continues to attack almost perpetually forming black crusts which have incorporated soot in the process of crystallization (Gauri, K.L. et al. 1981 & 1982).

b. grey areas originating from dry deposits of dust on the stone. They are mainly observed in areas where both beating rain and washing out are prevented even if condensation is still possible,*

c* white areas are the surfaces which are well-exposed to wetting and are subject to a mechanism which regularly removes by rain washout the deterioration products. The partial dissolution washout and partial recrystallization of the rock results in a thinning of the stone.The degree of severity depends on the intensity of the chemical attack and the amount of water flowing down the surface.

Abbildung in dieser Leseprobe nicht enthalten

According to Amoroso & Fassina (1983) (see fig. 23-26) there are three possible ways in which the stone and sulphur dioxide interact transforming calcite into gypsum in limestone monuments: i. wet deposition of S02 involving (a) absorption in water by precipitation; (b) aqueous-phase oxidation of dissolved S03, and (c) deposition on the surface.

Sulphuric acid formed according to this mechanism can reach the stone surface as droplets or can be formed on the surface in the presence of a liquid film. The main stages in this process may be outlined as follows:

SO- -*· SO . H30 -* H2S04 ( droplets ) -* CaSO„ (Fig. 23+24) ii . wet deposition of S03 involving (a) conversion into sulphate in the gas phase; (b) assimilation with existing sulphate aerosol by precipitation; and (c) deposition on the surface.

Part of the sulphate in the droplets can be present as sulphuric acid, which reacts very aggressively with limestone. The main stages in this process may be outlined as follows:

S02(gas) -* SO, (aerosol) -» H,S04 ( droplets ) CaS04 (fig. 25).

iii. Dry deposition of SO, which is a slow but continuous mechanism. In the presence of water calcium sulphite is formed and can be oxidized to sulphate by oxygen catalyzed by carbon particles and metal oxides. At first gypsum crystals are formed which together with carbonaceous particles are major constituents of black scabs (Fassina, V. 1988) (fig.24).

- Nitrogen oxides (NO*) as a form of anthropogenic air pollution mainly originate from industry and automobiles. Nitrous monoxide and nitrous dioxide are lumped together as NO.. They interconvert rapidly depending on the available sunlight. They are deposited and penetrate the stone as nitrous (HN03) and nitric acid (HN03) (Wolters, В. et al. 1988).

Considering its chemical characteristics it can be accepted that N0a plays an (intermediate) role in the decay of natural building st)ne. The direct chemical reaction of dry deposition SOa and N02 will cause decay in materials such as lime (Caco ). At the same concentration level, the deterioration will be caused 80% by S02 and 20% by NO,. In any case the direct role of NO, in the deterioration process is very much less important than that of SO,. The contribution of the reactive products related to NO* to the total dry deposition of N-oxides on material surfaces is:

Besides direct chemical attack by N02 and its reactive products there are a number of interactions with existing corrosion products. These are enhanced by the fact that the surfaces remain moist over a longer period due to the hygroscopic character of nitrate salts (Lanting, R.W. 1983). Investigations show that N02 drastically increases the corrosion rate of calcareous stones in humid so2-containing atmospheres (RH > 50%).

- Ozone (O,) is known to attack organic materials, it is also a strong and active agent in S02 oxidation. It certainly contributes to the formation of secondary air pollutants, such as sulphuric acid and nitric acid. Ozone also has a direct effect on building materials by reacting with the sulphur dioxide and nitrogen dioxide absorbed by the surface layers of building materials.

- Hydrogen chloride (HC1) is a common air pollutant emitted from industrial sources. Its role in the deterioration of stone has never been clarified. In quantitative studies, however, chlorides have been shown to constitute an average 1%, with peaks of up to 5-6%, of the scab samples taken from Venetian buildings (Fassina, V. 1988).

- Hydrogen fluoride (HF) acts as a gaseous pollutant and originates from specific industrial operations. The role played by fluorides in stone deterioration has been pointed out by Cortès and Martin (1982). They created some accelerated alteration cycles by exposing siliceous and calcareous stone to an artificial hydrogen fluoride atmosphere. Two types of deterioration were found:

a. nucléation and growth of new crystalline phases.
b. formation of fissures, micro-cracks and orifices.

- Particulate matter or aerosols comprise all solid or liquid substances that can be airborne, for however short a time, in the gaseous medium. Aerosols are defined as mixtures of sufficiently finely dispersed particles in the air. The particles can be solid or liquid droplets and can be produced either by disintegration of solid or liquid matter (e.g. dust and spray) or by condensation from the gas phase (e.g. all smokes produced by industry) (Fassina, V. 1988).

Carbonaceous particles deposited on stone strongly accelerate its deterioration mainly through catalyzing S02 oxidation to H2S04 and consequent gypsum formation (Del Monte, M. & Vittori, О. 1985). Along with carbonaceous particles, glassy spherical particles are also frequently encountered on stone monuments and natural outcrops. Opposed to the general belief that carbonaceous particles are the main aerosol species it is in fact the fly ash (the glassy spherical particles produced by coal combustion which makes up the major part of anthropogenic atmospheric aerosol). They also contribute to the transformation of CaC03 to CaS0,.2H20, whether starting from the stone on which they are deposited or subsequently contributing independently to the growth of sulphation crusts (Del Monte, M. 1987).

2.3. Biogenic decay

2.3.1. Bio-deterioration of natural building stone:

The term bio-deterioration refers to any undesired or detrimental change in material properties due to the activity of micro- and macro-organisms (Caneva f G, & Salvador!; 0. 1988).

Once the surface of a stone has been exposed to the environment, considerable numbers of different types of organisms can be observed. Stone and mineral materials have always been good substrates for a large number of different micro- and macro-organisms (Eckhard, F.E.W. 1985; Krumbein, W.E. 1988 a+b).

The biological alterations which they cause differ according to the substrate, the nature of the organisms and the characteristics of the environment. Bio-deterioration can never be considered as an isolated phenomenon, it always occurs along with other physical, chemical or physico-chemical deterioration processes, with which it is intimately related.

This biological weathering of silicate rocks and minerals consists of both biophysical and biochemical weathering processes (Silverman, M.R. 1979).

- Biogeophysical weathering processes are those by which life forms cause mechanical fracturing and breakdown of rocks and minerals to produce particles smaller than the original material. For example the rhizines of lichens penetrate rock and, upon wetting and drying, tear loose rock fragments of the substratum. The roots of higher plants penetrate rocks and split them into smaller fragments (fig.30).

The loosened substrate is then vulnerable to attack by animals. Not only mites, fly larvae and boring wasps, but also birds, bats, molluscs, etc. add to the deterioration of building stone.

- Biochemical weathering refers to all other processes, direct or indirect, by which living organisms and their metabolic processes and products affect the chemical stability and composition ol silicate rocks and minerals.

Various bacteria, cyano-bacteria, algae, fungi, lichens, mosses, higher plants, have been found to be involved in both the degradation and the formation of minerals and rocks, and in the recycling of mineral-forming elements. In addition different mechanisms have been recognized in the mobilization, transport and deposition of mineral cations by micro-organisms (Wilson, M.J. & Jones, D. & McHardy, W.J. 1981; Krumbein, W.E. & Werner, D. 1983).

The chemical processes involved in these mechanisms develop in two different ways:

(l) the production of organic and/or inorganic acids? and

(ii) the production of chelating substances.

Acids may vary in strength and react directly with minerals of the substrate. Carbon dioxide (СОг)г one acid forming compound, is produced by all aerobic organisms tnrough cellular respiration. In an aqueous environment it changes into carbonic acid which then dissolves the calcium and magnesium carbonates in limestone,

marbles, lime mortars, plasters, etc. (Amoroso, G.G. & Fassina, V. 1983 )

* Fig.27:

Extraction of cations fron biotite by citric and oxalic acids (Boyle, J.R. et al. 1974).

I Fig.28:

delation reactions lay solubilise iron which is redeposited in a hardened surface crust.

Chelation is a phenomenon by which one atom of hydrogen or oí a metal is shared by two atoms of the same molecule. As virtually all organisms require iron as an essential trace nutrient m iny acid and basic organic compounds, such as 2-ketogluconic, citric and oxalic acids, produced by microorganisms and organisms involved in bio-deterioration, possess the property of complexing or chelating metal ions of a substrate. The removal from the mineral surface leads to mineral dissociation (fig.27).

In natural environments, such chelation reactions may stabilize iron (either in the ferrous or ferric state) as soluble chelates, thereby increasing the amount of iron in the solution to levels far above those expected from thermodynamic calculations (Nealson, K.A. 1983). In other words chelation contributes to the weathering of rock, solubilizing or maintaining in solution metals which are not easily soluble (fig.29).

Abbildung in dieser Leseprobe nicht enthalten

Fig.29: Iron-binding and transport compounds (Nealson, K.B. 1983)

Usually, the micro-flora transfers iron and/or manganese to, or close to, the surface oi the substrate and then the iron an·:'/or manganese is redeposited in a surface or near-surtace crust (Krumbein, W.E. 1988 a+b) .

It is evident that these iron and manganese encrustations are in many cases associated with cement losses from deeper within the rock. This leads to a hardened surlace Layer underneath which, water, ice and efflorescences can produce physical stress. This in turn leads to exfoliation of large chips of the rock after which a new cycle of crust formation and destruction immediately begins (Krumbein, W.E. & Grote, G. & Peterson, К. 1987) (fig.28).

The rates of biological and organic chemical weathering of silicates are however very difficult to assess. Although it happens at a remarkably rapid rate in the laboratory it would be premature to extrapolate the results to the natural environment. Such variables as the number and different kinds of living organisms present, their interaction with one another and the organic matter and metabolic products present, the availability of water and fluctuations in temperature, pH, Eh, etc, all acting in different combinations, make predictions of natural events uncertain, if not impossible (Silvermann, M.P. 1979).

Plant roots accelerate the weathering of rocks by exchanging H-ions from the roots for metal ions in the rock (Keller, W. et al. 1975).

2.3.2. Types of organisms involved in bio-deterioration :

2.3.2.1. Macroorganisms: (i) higher plants and (ii) animals.

i. In the case of higher plants, roots can cause damage of a mechanical nature since they may exert considerable pressure which contributes to the widening of crevices thus facilitating water penetration. The process of deterioration becomes more threatening to the stability of the monument if the plants are perennial (Tiano, P. 1987).

The roots can radically alter the physical conditions of the surface on which they grow. Apart from mechanical disturbance and gradual fouling with organic material, there is an increase in water content (Fisher, G.G. 1982). For example, creepers and ivy are detrimental to the fabric of the stone (fig.30). They tend to maintain the walls in a permanently moist condition, and the secretion of acid substances by the suckers and tendrils is also a contributory cause of decay.

In addition to the mechanical force exerted by root growth, an other important factor in stone deterioration is the chemical action of the exudates and acidity of rootlets.

Root systems are able to alter the grains and cause the release of potassium and other ions from mineral particles (Boyle, J.R. & Voigt, G.K. 1973) (fig.31). This happens through the action of root exudates which are substances released into the surrounding medium by the roots. The composition of the exudates includes sugars, amino-acids, organic acids, nucleotides, flavonones and enzymes (Caneva, G. & Altieri, A. 1988). Some of these compounds are toxio and some are fungal stimulators or inhibitors while others have chelating abilities. The most effective chelating agents for the organic acids secreted by roots are 2-ketogluconate, oxalate and citrate (Duff, R.B.& Webley, D.M. & Scott, R.O. 1963). The role of carbonic acids, produced by cellular respiration processes, must also be taken into account.

Modification of the mineral composition of the substrate is hereby induced as well as mineral etching due to the contact exchange mechanisms of ions in solution. Control of such processes falls, however, within the normal routine of building maintenance. So these processes become important only in the case of semi-abandoned structures (Torraca, G. 1982).

ii. In many cases animals contribute to the direct or indirect damage of building stone. Insects, spiders, larvae, etc, damage softer stone by boring holes in the surface (fig.32). Mammals such as goats, sheep and cattle may, in rural areas, contribute to the deterioration of masonry by trampling, pushing and rubbing on surfaces.

The direct destructive action of birds, especially in urban areas, is of both a mechanical and a chemical nature. Mechanical action caused by trampling and pushing may be harmful to components with poor cohesion (Caneva, G. & Salvadori, O. 1988). Chemical action is caused by excrement containing organic or inorganic nitrogen, phosphates, sodium, potassium and calcium. Some of these acids etch stone and chemically react with carbonates to form phosphates and nitrates which cause dangerous corrosive action » Fig. 32:

Borinq wasps penetrate the softer parts of sandstone.

1 Fig,33:

JLj-Manni tol forms large cristal:, in the presence of moisture, and is excreted by some of the most согаиоп lichens (e.g. Candelariella vitellina).

(Winkler, E.Μ. 1975). This viewpoint is not corroborated by the experiments of Bassi & Chi atante (1976) who did not find any surface alterations when covering their samples with sterilized pigeon excrement.

However indirect damage is caused by the contribution of organic substances present in pigeon and bat excrement. These constitute a highly favourable medium for microbial growth. A suitable relative humidity of the environment is all that is needed to promote the development of the fungal spores which are either endemic or mapped in the excrement. Some of the fungal species which grow on pigeon excrement secrete acidic products which contribute to the chemical erosion of the stone surface. In the case of marble this takes place within a relatively short period of time (~20 days) (Bassi, M. & Chiatante, D. 1976).

2.3.2.2. Micro-organisms: (i) lichen and mosses; (ii) algae and fungi; (iii) bacteria.

The atmosphere naturally contains viable particles or micro-organisms such as bacteria, algae and fungi in the form of fragments or spores. It also contains large numbers of pollen, grains and animal life, ranging from microscopic creatures such as protozoa to larger forms of insect life. The different micro-organisms can be grouped together in larger particles or attached to other elements such as dust or pollen grains (Verhoef, L.G.W. 1988). For most building materials, the porosity and, therefore, the water retention capacity is sufficient to support the attachment, existence and growth of the micro-organisms found in and on their surface (fig.36).

The development of these micro-organisms depends on:

- the pH of the substrate: micro-organisms can be classified as acidiphiles, neutrophiles or basiphiles (Dommergues Y. & Mangenot, F. 1970), but some normally develop in a wide range of pH.
- water is undoubtedly essential to the life and growth of micro-organisms, this may be obtained from rain, dew, condensation, rising damp or even from the relative humidity in the air.
- temperature is also an important factor as most micro­organisms have an optimal temperature for growth and a range outside which development is impossible. Some organisms are very tolerant and are resistant to high and low temperatures, showing a remarkable ability to adapt to thermal and hydric cycles.
- light and carbon dioxide is needed by photosynthetic organisms such as algae and certain bacteria for their normal development.
- the substratum plays a dominant role in the nutritive conditions necessary for the micro-organisms' metabolism.

Although the organisms may obtain some of their nutrients from rainwater and dew they show a marked preference for substrata with specific chemical characteristics. Investigations into this relationship between rock type and species have shown that it is the silicon content of the rock, rather than the calcium content, which determines their distribution (Haynes, F.N. 1964). Indeed, living micro-organisms draw the minerals necessary for their nutrition from the substratum and excrete waste products back into their surroundings, causing bio-deterioration in the process.

i. Lichens are unique among living organisms in that an individual lichen thallus is a combination of two distinct types of plant, a cyanobacterium or an alga and a fungus, which together form a structure that behaves as one plant that can be classified on a similar basis to other living organisms (Haynes, F.N. 1964). This association is referred to as "symbiosis". In other words, lichens constitute a stable self-supporting association of mycobiont (fungi) and photobiont (alga or cyanobacterium). About one fifth of all fungi are lichenized (Hawksworth, D.L. & Hill, D.J. 1984).

The lichens dealt with here are saxicolous (rock encrusting) species and may be divided into several distinct groups. The largest group is described as crustose. They grow directly on the immediate substructure and are firmly attached to it by a general mass of hyphae. These crustose lichens, which comprise over 60% of the species, can be subdivided into endolithic (immersed in rock) and epilithic (growing on rock) types. Saxicolous lichens may also be foliose, where they bear leaf-like lobes, or fruticose, which are shrubby or beard species.

Foliose species are attached more loosely to the substratum by distinct clusters of hyphae, termed rhizinae, whereas fruticose species may be unattached or may arise from a cluster of rhizoids termed the holdfast. Two further types of lichens are described as squamulose and filamentous. Squamulose lichens are characterized by the presence of small scales, whereas filamentous lichens are characterized by the dominance of their algal partner (Jones, D. & Wilson, M.J. 1985).

Of the lichens which grow on rocks, some varieties prefer calcareous rocks, other siliceous rocks: they are known as calcicolous and silicolous lichens respectively (Schaffer, R. 1972) .

Lichen growth and colonization is affected by the geomorphology of the rock substratum and by such factors as the aspect, weathering of the rock surface, texture, water-seepage, degree of exposure, slope, orientation and water-holding capacity of the rock (Lambinon, J. 1969; Brightman, F. A. & Seaward, M.R. 1977; Armstrong, R. A. 1977; Pentecost, A. 1979; Moxham, J.H. 1981; Hawksworth, D.L. & Hill, D.J. 1984; Wessels, D.C. & Büdel, В. 1989). Saxicolous lichens have significant preferences as far as the exposure, radiation and water content of the substratum are concerned. Lichen thalli are able to withstand repeated wetting and drying, and their tolerance to drying is much greater than that of most other fungi.

It was recently discovered that the ice nucléation activity (I.N.A.) of lichens may actively be involved in moisture uptake and frost protection (Kieft, T.L. 1988; Kieft, T.L. & Ahmadjian, V. 1989) .

The presence of lichens on stonework is invariably considered in different ways by specialists in different disciplines. Their attitudes are inevitably coloured by different aesthetic and practical considerations. The lichenologist for example, looks at lichen as a natural feature of ancient monuments, taxonomically and ecologically interesting. Indeed the assistance of specialists in lichenometry has been of great help in dating the exposure of natural rock surfaces by glaciologists, and of buildings and monuments by archaeologists. Furthermore, lichens are exceedingly sensitive to environmental changes and are reliable indicators of the level of atmospheric pollution (Hawksworth, D.L. & Rose, F. 1976). Those concerned with the conservation of our cultural heritage, however, look at the encroachment of lichens not only as a cause of disfigurement but also of accelerated deterioration of the stone (Giacobeni, G. et al. 1984). It remains however a point of discussion as to whether lichens should be systematically removed from building stone or not. It is a fact that lichens do cause stone decay through physical and chemical processes. Nevertheless, they may also act as a bio-protective layer hampering or decelerating extensive rock weathering caused by air pollution. One must always be wary of the risk of aggravating stone decay by cleaning surfaces and thus exposing them to other aggressive processes. In turn, air pollution may kill both protective as well as detrimental lichen covers (Fry, M. 1985b; Gehrrnann, C.K. et al. 1988; Krumbein, W.E. 1988a).

Damage attributed to the action of lichen can be both biochemical and biophysical. Some lichen species contribute to the bio-deterioration of a wide range of building materials. Electron microscopy and chemical techniques show that lichens are involved in both these processes. Many lichens create microclimates at the thallus/substratum interface, particularly in terms of water retention, which undoubtedly leads to mechanical damage to stonework. Their effect being enhanced by man-made environmental conditions. The climatic wetting and drying of the lichen thallus causes it to expand and contract, mechanically breaking down the substratum. Combined with the chemical breakdown of substrata by the metabolic products of lichens it is clear that all these bio­geophysical processes are of great importance to rock weathering (Seaward, M.R. et al. 1989) (fig.36).

Mechanisms involved in the biophysical weathering of the rock on which the lichens grow are principally hyphae penetration and thallus expansion and contraction.

Lichen growth causes physical damage to various sandstones by increasing the porosity of the stone. In this way the stone is made much more susceptible to decay by other agencies such as frost action (Bech-Anderson, J. & Christensen, P. 1983). The penetrating hyphae occupy the spaces among the fragments, sometimes replacing the cementing material and altering the physical structure of the stone (Pallecchi, R. & Pinna, D. 1988).

Foliose lichens (Xanthoria parietina, Parmelia saxatilis) form a compact and superficial covering of the rock surface but without penetrating the substratum. On the other hand, fruticose lichens form almost an integral part of the stone. They use existing micro­cracks in the material or create new ones by producing organic acids (Orial, G. & Brunet, A. 1988).

Even if only crustose lichens which have a deteriorating physical effect on the stone due to the anchoring of their rhizinae, all species shelter bacterial metabolisms which may be Pig.34: Diagrams of endoiithic and epilithic lichens in sandstone. Exfoliation of a surface crust due to biological activity (Friedaan, E.I. 1982).

detrimental to the condition of the stone (Orial, G. & Brunet, A 1988). Rhizines penetrate substrata and have a role in the breaking up of rock surfaces. Crustose species penetrate over the whole lower surface, tending to spread horizontally and so inducing a tendency for the surface to flake (fig.35). Penetration is usually not more than a few millimetres but has been recorded to a depth of 16 mm. Endolithic lichens have their hyphae and photobiont cells entirely immersed in channels or locules in the rock which to a large extent they produce themselves. The activities of these rock inhabiting lichens have obvious implications for weathering and paedogenesis (Hawksworth, D.L. & Hill, D.J. 1984) (fig.34). The erosive action of lichens is less destructive in humid than in dryer environments, although differences in lithology and species present may complicate the picture (Viles, H. 1987).

In addition to mechanical effects, crustose lichens affect the chemistry of the rocks on which they grow in a series of processes collectively termed biochemical weathering. In short bio­geochemical weathering is caused by the following three processes (Syers, J.K. & Iskandar, I.K. 1973; Wessels, D.C.J. & Schoeman, P. 1988):

a. The production of respiratory C02, a part of which may be dissolved in water and can cause a localised reduction in pH. This carbon dioxide in an aqueous environment changes into carbonic acid which dissolves the calcium and magnesium carbonates in limestones, marbles, lime mortars, plasters, etc. (Caneva, G. & Salvadori, 0. 1988):

C02 + H20 « Н2С03

CaC03 + Н2С0э « Ca(HC03)2

MgCO-, + HjCOj «■ Mg ( HC03 ) 2

b. The production of biochemical compounds or extra-cellular lichen substances. These substances are soluble in water and capable of forming complexes with cations such as calcium, magnesium, iron and aluminium (Hale, M.E. 1973). This can lead to modification of minerals in rocks through chelation (Schatz, A. 1963 a+b; Ascaso, C. et al. 1976; Ascaso, C. & Galván, J. 1976; Hallbauer, D.K. & Jahns, H.M. 1977);

c. The production and excretion of oxalic acid. This is of great interest in weathering processes, particularly in the case of lichen species living on limestones. The products resulting from the action of oxalic acid depend on the cations present in the rock, mainly Ca, Fe, Mg, and also on the hydration state of the rock (Ascaso, C. et al. 1982) (fig.37).

The importance of the lichens in the chemical weathering of their substrate is indicated by the following evidence:

a. The weathering crust of lichen-covered rock is thicker than the crust on bare rock;
b. The lichen-covered weathering crust is considerably enriched in Fe and impoverished in Ti, Si and Ca in comparison with the crust on unaltered rock;
c. The lichen covered weathering crust is an assemblage of residual oxides apparently devoid of clay minerals;
d. the iron oxide in the lichen covered weathering crust is mineralogically different from the iron oxide in the lichen free weathering crust (Jackson, T.A. & Keller, W.D. 1970).

Lichens are considered more effective weathering agents than other plants. The reasons for this are understandable. Lichens excrete greater amounts and a greater variety of chelating agents. The nutritional poverty of plain rock surfaces makes the large-scale production of such substances a luxury that saxicolous lichens can ill afford unless the acids are of some use to them. The only function which can be attributed to lichen acids is that of supplying the symbiont with needed minerals. Since these have literally to be extracted from the rocks, saxicolous lichens have to be efficient weathering agents, and lichen acids are the means by which they accomplish this action (Schatz, A. 1963b).

Lichens excrete a variety of chelating organic compounds which are unique for these organisms. The weathering capacity of lichenic substances is probably more related to their chemical structure than to the water solubility of the compounds. Polar groups such as -OH, -CHO and -COOH allow to function as metal-complexing agents and thus promote the chemical weathering of rocks (Gehrmann, C.K. et al. 1988).

Recorded values for lichen-substance contents of lichen thalli usually range from 1 to 10% of the dry weight (Hale, M.E. 1973) but may be as high as 25.6% (Brown, D.H. 1976). These so-called lichen acids, of universal occurrence in lichen species, are able to form metal complexes with substrate silicate minerals. The iron-chelating ability of lichen compounds was determined but found to be very small (williams, M.E. & Rudolph, E.D. 1974). The weathering potential of lichens as measured by iron chelation, was then compared directly with that of fungi isolated from the same rock surface. Most of the fungi isolates were found to chelate iron up to six times as much as does squamatic acid, the lichen acid isolated from Cladonia squamosa.

Endolithic lichens are able to grow between the crystals of porous rocks. Their activity results in the mobilisation of the iron compound and in rock weathering with a characteristic pattern of exfoliation (Friedmann, E.I. 1982) (fig.34). Soluble metal complexes are formed when lichen compounds react with minerals in rocks. The fact that they are soluble means that these products of the biochemical weathering process can be removed from the site of weathering (Syers, J.K. & Iskandar I.K. 1973). The cementing substance between the grains of sandstone is solubilized at the level of the lichens and the upper rock crust peels off. Hyphae then penetrate deeper and a new lichen zone is formed at an appropriate depth, while on the surface a new rock crust is formed. The mucus produced by the mycobiont is also important. In its dry state, it can produce high strengths of adhesion leading to a reduction in the cohesion and adhesion between the structural components of the stones. Moreover, many of the mucilaginous substances are aggressive and act on the surface (Gehrmann, C.K. et al. 1988) .

The ability of crustose lichens to decompose their immediate rock and mineral substrates is related to the fungal partner's (mycobiont) excretion of various chelating, organic acids (Jones, D. & Wilson, M.J. 1985).

The lichens' production of oxalic acid could explain their ability to etch surfaces, thus allowing colonization to occur. It is possible that the toxic effect of oxalic acid is neutralized by

“ Fig.36:

The nechanisis of biogenic decay caused by micro­organista.

I Fig.37:

A clear deterioration of the iron coating of grains in lerruginous Mudstone, caused by the action of oxalic acid (Bos, K. 1990). the production of calcium oxalate or by the formation of heavy metal complexes with iron (Bech-Anderson, J. 1987). X-ray powder diffraction and transmission electron microscopy have demonstrated the presence of the products at the interface between rocks and various lichens. Many lichens known to contain calcium oxalate, cause extensive corrosion of a range of rock substrata (fig.36-37).

Abbildung in dieser Leseprobe nicht enthalten

Fig.38: Biо лineralis ation products identified in lichens (Jones, D. £ Wilson, M.J. 1986)

Oxalic acid secreted by the mycobiont is soluble in water and acts as a chelator of metal ions. The oxalates produced are closely related to the chemical composition of the rock. It is known to cause extensive corrosion of primary minerals and the complete decomposition of ferruginous clay minerals. Iron oxides, amorphous gels and calcium oxalates are precipitated. There is evidence to implicate lichen acids and oxalic acid in the weathering process but it is possible that other organic acids are also involved. Microscopic examination of lichen/rock interfaces and of lichen thalli reveal the presence of a variety of minerals resulting from the activity of the mycobiont. Among these bio-minerals are a number of insoluble crystalline oxalates. Their composition depends directly on the composition of the substrate rock (Ornella, S. & Adreina, Z. 1981; Jones, D. & Wilson, M.J, 1986). The most common forms are calcium oxalates which are particularly abundant where lichens have incrusted limestones and other calcareous rocks. Magnesium oxalate (Wilson, M.J. et al. 1981), manganese oxalate, copper oxalate and others have also been reported (fig.38).

Oxalic acid has now also been established as the cause of surface etching, such as pitting and honeycombing, as seen in the minerals beneath crustose thalli (Hawksworth, D.L. & Hill, D.J. 1984). It is also responsible for the conversion of poorly ordered weathering products and the crystallization of oxalates at the rock/lichen interface and within the lichen thallus itself (Jones, D. et al. 1981; Wilson, M.J. & Jones, D. 1983; Wilson, M.J. et al. 1981) .

Lichen activity can also lead to the formation of iron oxide minerals (These oxides may be obvious from the colour of the lichen thallus). Two mineral species have been reported: ferrihydrate (Jackson, T. A. & Keller, W.D. 1970) and goethite (Ascosa, C. & Galván, J. 1976). The lichen iron oxides probably originate in the formation of soluble iron compounds following the decomposition of ferruginous silicates. These compounds are oxidised, possibly microbially, resulting in precipitation on rock surfaces or within lichen thalli (Jones, D. et al. 1981; Jones, D. & Wilson, M.J. 1986) .

The presence of mosses which form thick green cushions is a sure sign of a damp environment and of the accumulation of soil particles on the area of the building where they are found. These organisms prefer a thin layer of soil to anchor their roots and to provide a constant level of humidity which allows them to complete their biological cycle (Tiano, P. 1987).

Mosses and liverworts, like lichens, have the ability to take up mineral nutrients from their substratum. Their mechanical action is small in comparison to higher plants, but they do produce small holes in sandstone. In this way they are able to improve the microclimate but at the same time cause deterioration of the building materials (Bech-Anderson, J. 1984).

In interpreting the deterioration of various types of materials by mosses and lichens, the specific urban climate and associated atmospheric pollutants should be taken into consideration, particularly as the latter dramatically affect the lichen flora. It was noted for example that several of the more pollutant-tolerant species {poleophiles) which have an aggressive behaviour pattern, were actively colonizing stonework, resulting in the most alarming short-term damage.

Lecanora muralis appears to be a highly successful lichen in urban environments, where its spread has been dramatic in the last few years (fig.39).

Recent environmental changes have been conducive to the detrimental increase in colonisation by certain species of lichen. This helps to explain, in addition to the known problems resulting from air pollution, why monuments undamaged for many centuries should now appear to be vulnerable to lichen attack (Seaward, M.R.D. 1988).

ii. Similar to lichens, algal and fungal growth on rocks and stone causes damage which can be ascribed to chemical and mechanical action. The sheets of algal growth, or patinas, trap water and delay subsequent drying increasing the water related damage to the substrate. These patinas trap dust particles, organic residues, spores, salts and pollution particles. This forms eventually a rich organic substrate favouring the growth of other organisms such as bacteria, lichens, mosses and ferns.

The parts of a building most prone to algal and fungal colonization are the ones not exposed to direct sunlight but which are shaded and damp (fig. 40). Algae on rocky substrate can be subdivided into epilithic (growing on the surface), chasmolithic (living inside existing pores and cavities) and endolithic (actively penetrating into the substrate) (Caneva, G. & Salvadori, 0. 1988).

Epilithic and chasmolithic algae contribute to the weathering of stone by retaining water which then expands and contracts in freeze-thaw cycles and causes loosening of the mineral grains.

Abbildung in dieser Leseprobe nicht enthalten

I Pig.39:

Lecanora muralis on ferruginous sandstone.

* Fig. 40:

Algal growth on humid side of buttress.

They have also been found to produce a variety of metabolic products, predominantly organic acids (lactic, oxalic, succinic, acetic, etc.) but also proteins, sugars, antibiotics, etc. Many of these compounds are able to complex or chelate organic and inorganic ions (Strzelczyk, A.B. 1981). The damage which they cause is similar to that described above in relation to licnens.

Endolithic algae actively dissolve carbonates, penetrating into the substrate and forming micro-cavities. As algae need light to photosynthesize, the depth of penetration remains limited. Therefore algae develop parallel to the surface of the substrate in order to gain the maximum amount of available light.

Fungi are heterotrophic organisms that largely depend on organic matter for their nutrition. Organic residues of various origin are always present on stones. The chemical action of fungi appears to be responsible for their most deteriorative effect. They produce powerful enzymes (Verhoef, L.G.W. 1988), carbonic-, nitric- and sulphuric acid and many other organic acids. Some of them form chelating complexes with metal cations of the substrate and thereby dissolve limestones, silicate minerals, iron- and manganese bearing minerals and different phosphates. Some of them produce large amounts of oxalic acid which is a well-known weathering agent, especially in relation to iron-bearing minerals (sre above) (Silvermann, M.P. & Munoz, E.F. 1970; Mehta, A. et al. 1978).

TYPE OF DAMAGE:

AESTHETIC

CHEMICAL

Abbildung in dieser Leseprobe nicht enthalten

PHYSICAL

Fig.41: Diagram of the relative inportance of the different types of damage caused by various groups of organisms according to Tiano, P. (1986)

iii. In addition to numerous fungi and algae, large numbers of different bacteria are present within the layers of stones exposed to weathering (Eckhard, F.E.W. 1988). Organisms which have been connected with the weathering process include:

- chemo-organotrophic bacteria (actinomycètes etc.);
- chemo-lithoautotrophic bacteria (nitric and sulphuric acid producing; iron and manganese oxidising);
- photo-autotrophic bacteria (cyanobacteria etc.);
- chemo-lithoheterotrophic bacteria;
- heterotrophic bacteria

(Krumbein, W.E. 1973; Mentler, A. & Müller, H.W. 1984; Eckhard,

F.E.W. 1985; Lewis, F.J. et al. 1987; Krumbein, W.E. 1987; May,

E. & Lewis, F.J. 1988; Krumbein, W.E. 1988a; Caneva, G. &

Salvador!, O. 1988).

The activity of these organisms in promoting stone deterioration depends on the production of corrosive metabolites which can solubilize stone minerals in a manner similar to chemical agents. Microbial solubilization processes are always coupled with an acidification of the cultural medium. In other words it is the relative acid-secreting potential of the bacteria rather than their presence that will influence the course of stone decay (Lewis, F.J. et al. 1987). This leads to difficulties in distinguishing between the chemical and biological causes of decay and often between the symptoms and causes of decay as well (Mentler, A. & Müller, H.W. 1984).

Bacterial bio-transfer includes the production of sulphides, carbonates, oxides, hydroxides, phosphates, silicates and sulphates and has been documented at least for the following elements: H, C, N, O, Mg, Al, si, P, S, K, Ca, Ti, V, Mn, Fe, Ca, Ni, Cu, Zn, As, Mo, Ag, Au, Pb, LJ (Lowenstam, H.A, 1981; Krumbein, W.E. 1986).

The types of bacteria which play the most important role in the weathering of rocks and minerals are autotrophic bacteria which use inorganic substrates, and heterotrophic bacteria, which use organic compounds (fig.42).

Abbildung in dieser Leseprobe nicht enthalten

Fig.42: Bacteria and their possible involvement in stone decay (May, Б. S Leáis, F.J. 1958)

Autotrophic bacteria consist of sulphur-oxidizing, nitrifying and iron bacteria, among others. The sulphur oxidizing bacteria utilize various sulphur compound: to produce sulphate ions which, when reacting with calcium ions of stone, form gypsum (CaSO,. 2H20 ) .

Therefore, limestones and sandstones with calcareous binders, are most sensitive to this type of microbial deterioration (Pochon, J. & Jaton, C. 1968). The problem, however, is explaining the origin of the reduced sulphur on which the oxidizing organisms depend. As there is no sulphur in freshly quarried stone there must be a continuous supply from external source. There is always a certain amount of sulphur in soils. So the hypothesis is that the reduction of sulphates to sulphides by bacteria occurs at the foot of wet walls. These sulphides are then absorbed into the wall by capillarity and come to the surface of the wall through superficial evaporation, There they are oxidised by the thiobacilli which produce sulphuric acid, thus causing scaling (Pochon, J. & Jaton, C. 1967).

Abbildung in dieser Leseprobe nicht enthalten

As far as deterioration is concerned, higher up on a building it is necessary to consider a sulphate contribution from pollutants or from previous biological colonizations, dust, soot or bird droppings. This possibility was tested experimentally and it was demonstrated that even at very high concentrations of SO,, the reaction may be considered negligible. So the actual importance of this phenomenon in the field is far from clear (Caneva, G. & Salvador!, 0. 1988).

According to Perrichet (1987) "black soiling" is due to the presence of sulphur-oxidizinq bacteria rather than to atmospheric pollution. This type of soiling is typical for rough textured surfaces, along tops of walls and at gable ends which are predominantly on or near horizontal water retaining areas (fig.43- 44-45) .

Nitrifying bacteria attack mainly limestone, using it as a source of carbon to produce nitric and nitrous acids from ammonia (NH3) in polluted air. The formation of calcium nitrate causes the surface of the stone to be transformed into a powdery alteration product which can be washed away by rain. It can also cause deterioration through crystallization (Strzelczyk, A.В. 1981; May,

E. & Lewis, F.J. 1988) .

Piq.44: SEU micrograph of "black soiling" on sandstone, possibly due to sulphur oxidising bacteria.

TN-5500 K I.K. - I R PR FRI 17-RUG-90 13¡30

Cursor: 0.000k©V = 0

EDX of fig.í4: К and Ca reading typical for glauconite containing ferruginous sandstone.

Abbildung in dieser Leseprobe nicht enthalten

ŕ'iq.W: Bacteria which precipitate or solubilise iron and manqanese compounds (Silverman, H.P. & Ehrlich, H.L, 1964)

The activities of iron and manganese-oxidizing bacteria on inorganic substrates may also lead to mineral formation or breakdown. For instance, through the indirect (accumulation already oxidised Fe or Mn) or direct (enzymatic oxidation) interaction of these micro-organisms with iron and manganese compounds, these metals can be precipitated or dissolved (fig.46+47).

There are many examples of organisms which catalyse iron oxidation and/or accumulation at rates significantly faster than occurs in the absence of such organisms (Nealson, K.H. 1983). For example thiobacillus ferrooxidans accelerates the oxidation of Fe¿* by up to 200.000 times compared with chemical oxidation by oxygen or ferric iron (Fea*) (Karavaiko, G.l. 1982).

Biological processes (such as the bio-transfer, bio-transport and bio-deposition of minerals in the context of destructive and reorganizational processes) are responsible for the formation of crusts, especially in the case of iron and manganese. These processes can also occur chemically, but are certainly enhanced by the presence of microbial mats and films (Krumbein, W.E. & Grote, G. & Petersen, К. 1987; Krumbein, W.E. 1988a). These biogenic metal crusts may form on and in rocks and contribute considerably to the deterioration of building stone (fig.47). The enrichment of iron and manganese oxides in these weathering crusts occurs more frequently in silicate rocks than in carbonate rocks. An equilibrium between ingoing and outgoing humidity, at different porosities and cementing minerals, influences the speed and importance of biological transfer of mineral oxides and the place of redeposition in interior and exterior crusts*

It is evident that iron and manganese encrustations are in most cases associated with cement losses deeper within the rock. This will eventually lead to exfoliation and accelerated decay (fig.11).

The activity of heterotrophic bacteria on stone is governed by the availability of suitable organic matter. the principal mechanism by which heterotrophic bacteria degrade minerals and stones is by excretion of organic acids, sometimes combined with an oxidation or reduction of mineral-forming cations (Eckhard, F. E.W. 1988). They are capable of causing severe damage when organic nutrients are available. Damage is related to the calcium-based mineral content of the stone and/or its porosity (Lewis, F.J. et al. 1987).

However, the relative contribution of bio-corrosion and chemo- corrosion to stone decay will ultimately depend on the geological nature of the stone and its in-service location. Bacterial activity is not always detrimental to stone conservation. Recent experiments have shown that certain types of sulphate reducing bacteria regenerate calcite from gypsum crusts on deteriorated marble monuments (Atlas, R.M. et al. 1988).

Abbildung in dieser Leseprobe nicht enthalten

Pig.47: Model of biogenic restai transfer with crust formation. Micro-organisms inoculate the rock surface, Biocorosion transfers metals towards the surface. Dependant on micro-cliiate and humidity the redeposition of minerals leads to internal crusts which alternate with zones empoverished in cement. Hereby exfoliation and desquamation is initiated. In other places, however, a consolidating crust may stabilise the rock (Krumbein, W.E. et al. 1987).

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66 of 67 pages

Details

Title
Weathering of natural stone used as building material
Subtitle
Physical, Chemical and Physico-Chemical Introduction
Author
Year
1992
Pages
67
Catalog Number
V160093
File size
15195 KB
Language
English
Tags
Weathering, Physical, Chemical, Physico-Chemical
Quote paper
Karel Bos (Author), 1992, Weathering of natural stone used as building material, Munich, GRIN Verlag, https://www.grin.com/document/160093

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