The production of Portland cement clinker has a share of about 6% to 8% on the global CO2-emissions . Approximately 60% of those emissions are attributable to the decarbonation of limestone. A widespread approach for the reduction of the CO2-emissions is to
replace the clinker in the cement by pozzolanic waste materials, e.g. fly ash. The reaction of fly ash is generally slow. Due to this slow reaction cement pastes mixed of Portland cement and fly ash have a slower strength development than pure Portland cement, at least at early ages up to 7 days. The goal of this study was to increase early strength properties of Portland cement/fly ash blends by increasing the early ettringite formation in order to decrease the porosity. Therefore various amounts of anhydrite and laboratory synthesised C3A were added to Portland cement/fly ash systems that contained 30% fly ash. The behavior of these systems in terms of kinetics, phase development and microstructure was studied by means of strength tests, isothermal calorimetry, thermogravimetric analysis, chemical shrinkage, X-ray diffraction, backscattered electron image analysis, mercury intrusion porosimetry and thermodynamic modeling. In addition the activation by Na2SO4 was investigated on certain systems.
It was found that the amount of ettringite formed increases gradually with increasing amounts of anhydrite and C3A. The formation of ettringite was completed at approximately 1 day. Further the activation with Na2SO4 increased the amount of ettringite
and shifted the completion of its formation to approximately 2 days. Due to the addition of anhydrite and C3A the porosity decreased corresponding to the amount of ettringite formed. The effects on the development of early strength were most distinct for very high
additions and for systems activated by Na2SO4. All systems showed equal strengths at 7 d and 28 d. For later ages up to 90 days, the benefits at early ages were found to be inversed, leading to lower strength values compared to systems without or with low additions
of anhydrite, C3A or Na2SO4. Eventually the increase in early strength was either too low to be significant or only relevant at very high additions of anhydrite and C3A with simultaneous system activation by Na2SO4. In terms of kinetics a remarkable impact of
the added C3A was observed as this seemed to significantly accelerate the silicate reaction of the Portland cement.
Table of Contents
1 Introduction
1.1 Outline of the Development of Ordinary Portland Cement
1.2 Research Objectives
2 Fundamentals
2.1 Ordinary Portland Cement
2.1.1 Mechanisms of Portland Cement Hydration
2.1.2 Development of Microstructure
2.1.3 The System C3A-CaSO4-H2O
2.2 Fly Ash
2.2.1 The Pozzolanic Reaction and the Hydration of Fly Ash
2.2.2 Interactions between Ordinary Portland Cement and Fly Ash
2.2.3 Activation of Fly Ash
3 Materials
3.1 Portland Cement
3.2 Fly Ash
3.3 Anhydrite
3.4 Laboratory synthesised C3A
4 Sample Preparation and Methods
4.1 Mix Design
4.2 Procedures of Sample Preparation
4.2.1 Paste Samples
4.2.2 Mortar Samples
4.3 Investigation of Flexural and Compressive Strength
4.4 Isothermal Calorimetry
4.5 Thermogravimetric Analysis
4.6 Chemical Shrinkage
4.7 X-Ray Diffraction
4.8 Scanning Electron Microscopy
4.9 Mercury Intrusion Porosimetry
4.10 Thermodynamic Modelling
5 Experimental Results and Discussion
5.1 Effect of Fly Ash, Anhydrite and C3A on the development of compressive strength
5.1.1 Compressive strength
5.1.2 Isothermal Calorimetry
5.1.3 Thermogravimetric Analysis
5.1.4 Chemical Shrinkage
5.1.5 X-ray Diffraction
5.1.6 SEM Image Analysis
5.1.7 Mercury intrusion Porosimetry
5.2 Effect of elevated Anhydrite and C3A contents upon presence of Fly Ash
5.2.1 Compressive strength
5.2.2 Isothermal Calorimetry
5.2.3 Thermogravimetric Analysis
5.2.4 Chemical Shrinkage
5.2.5 X-ray Diffraction
5.2.6 SEM Image Analysis
5.2.7 Mercury Intrusion Porosimetry
5.3 Effect of System Activation via Na2SO4
5.3.1 Mechanical Properties
5.3.2 Isothermal Calorimetry
5.3.3 Thermogravimetric Analysis
5.3.4 Chemical Shrinkage
5.3.5 X-ray Diffraction
5.3.6 SEM Image Analysis
5.3.7 Mercury intrusion Porosimetry
5.4 Thermodynamic Modelling
5.4.1 Calculated volume of the phases as a function of time
5.4.2 Calculated volume of the phases at complete reaction
6 Conclusions
Research Objectives and Key Themes
This thesis investigates methods to improve the early strength development of Portland cement/fly ash blends, which typically show slower strength gain than pure Portland cement due to the slow pozzolanic reaction of fly ash. The research aims to decrease early-age porosity by increasing early ettringite formation through the addition of anhydrite and synthetically produced tricalcium aluminate (C3A), as well as by investigating chemical activation using sodium sulfate (Na2SO4).
- Optimization of early-age compressive strength in blended cement systems.
- Impact of anhydrite and C3A additions on hydration kinetics and ettringite formation.
- Chemical activation of fly ash systems using Na2SO4.
- Microstructural and phase development characterization using advanced analytical techniques.
- Thermodynamic modeling to predict hydrate phase evolution.
Excerpt from the Book
2.1.2 Development of Microstructure
The study on microstructures of cement pastes via scanning electron microscope (SEM) or transmission electron microscope (TEM) supplies visual insight on the above described reactions. Figure 2.2 describes schematically the sequence of changes undergone by a typical, polymineralic cement grain [90].
Early Stage: Soon after mixing an amorphous and colloidal layer or membrane containing alumina, silica and also calcium and sulfate forms on the surface of the grains. The composition depends on that of the underlying surface. After about 10 minutes stubby AFt rods (2nd drawing) can be seen. They seem to be abundant near the surface of the aluminate phase and appear to be nucleated both, in the solution and on the outside surface of the formed gel layer.
Middle Stage: Within this stage which begins at about 3 hours and ends at about 24 hours approximately 30% of the cement react and a strong heat evolution occurs due to the rapid formation of C-S-H and CH (3rd drawing at approximately 10 hours). Undried C-S-H has a filmy and foil-like morphology while drying leads to give fibres9, 2 μm in diameter, as soon as space is available or reticular networks when it is more restricted. CH forms massive crystals in the former water-filled space and nucleation sites are relatively rare which is why the growing crystals may engulf some of the smaller cement grains. C-S-H forms outwards in a thickening layer around cement grains and possibly also on AFt rods. After about 12 hours the C-S-H layer will make contact with layers growing on adjacent grains as it is by now 0.5-1.0 μm thick. At this point a structure of interconnected shells is present which will play an important role in determining the mechanical properties that depends on the particle size distribution of the cement. A space of 0.5 μm develops between the shells and the anhydrous grains which is likely to be filled with highly concentrated colloidal solution. This space proves the reaction process to be based on dissolution and precipitation.
Summary of Chapters
1 Introduction: Provides an overview of the development of Portland cement, its industrial significance, and the research objectives regarding the acceleration of fly ash-blended cements.
2 Fundamentals: Covers the basic chemistry of Portland cement hydration, the role of supplementary cementitious materials like fly ash, and the relevant theories behind the hydration and pozzolanic reactions.
3 Materials: Details the chemical and physical properties of the materials used in the study, including Portland cement, fly ash, anhydrite, and the synthesised tricalcium aluminate.
4 Sample Preparation and Methods: Describes the experimental design, the mixing procedures, and the analytical methods employed, such as calorimetry, TGA, XRD, SEM, MIP, and thermodynamic modeling.
5 Experimental Results and Discussion: Presents and discusses the findings from the experimental matrix, analyzing the influence of fly ash, anhydrite, C3A, and Na2SO4 on hydration kinetics, phase development, and mechanical properties.
6 Conclusions: Summarizes the key findings of the research, noting that while the additions and activation affected hydration products like ettringite and porosity, they did not lead to significant long-term strength improvements.
Keywords
Portland cement, fly ash, hydration, ettringite, anhydrite, tricalcium aluminate, pozzolanic reaction, compressive strength, porosity, thermogravimetric analysis, mercury intrusion porosimetry, thermodynamic modeling, sodium sulfate, cement chemistry, microstructure.
Frequently Asked Questions
What is the primary goal of this research?
The primary goal is to improve the early-age compressive strength of Portland cement blended with fly ash, as these blends typically exhibit slower strength development compared to pure Portland cement.
What are the central themes of the work?
The central themes include cement hydration mechanisms, the influence of supplementary cementitious materials (fly ash), the role of ettringite in strength development, and the effectiveness of chemical activation in modifying the cement paste structure.
Which scientific methods are used in this study?
The study employs a variety of analytical techniques, including strength testing, isothermal calorimetry, thermogravimetric analysis (TGA), chemical shrinkage measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and thermodynamic modeling via GEMS.
What is the role of ettringite in these cement systems?
Ettringite formation is essential for set regulation and contributes to strength development by reducing the porosity of the hydrating system through the growth of solid phases in air voids.
What does the main part of the thesis cover?
The main part focuses on the experimental results divided into three groups: the effect of basic additions (fly ash, anhydrite, C3A), the effect of elevated levels of anhydrite and C3A, and the impact of chemical activation using sodium sulfate (Na2SO4).
What are the key technical terms characterizing this research?
Key terms include Ordinary Portland cement (OPC), pozzolanic reaction, hydration kinetics, microstructure, ettringite, C-S-H (calcium silicate hydrate), and phase development.
Does the addition of anhydrite and C3A consistently improve strength?
The study indicates that while these additions influence phase development and porosity, they do not lead to significant, consistent increases in compressive strength compared to the base blends across all time intervals.
What specific effect does sodium sulfate (Na2SO4) have on the activation of fly ash?
Na2SO4 acts as a chemical activator that generally accelerates the hydration processes in the short term, though this beneficial effect on early strength is often lost or inverted at later stages of hydration (e.g., from 7 days onwards).
- Quote paper
- Axel Schöler (Author), 2012, Study of hydration processes of Portland cements blended with supplementary cementitious materials, Munich, GRIN Verlag, https://www.grin.com/document/199418
