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CARBONATION OF ALKALI-ACTIVATED SLAG MORTAR
ABSTRACT
Sustainability is an important global issue nowadays, and it is known that production of portland cement is highly energy-intensive (embodied energy of OPC production is 5.3 MJ/kg OPC [1] ) with a significant contribution to greenhouse gas emissions (0.97 ton CO2/ton OPC [1] ). While the absolute value of the embodied energy of OPC appears to be small, considering the large global consumption and production of OPC (annual world production of cement is 109tons [2] ) the negative environmental impact of OPC becomes evident. Therefore, many researchers have focused on development of Portland cement substitutes to produce a binder with a lower environmental impact. Alkali-activated binders, such as alkali-activated slag or fly ash (AAS or AAF) have shown to be promising alternative materials in fitting this purpose and can provide similar mechanical and fresh properties to portland cement. Slag and fly ash are industrial byproducts, and their utilization in concrete, besides the added environmental values, will result in a more durable concrete in most aggressive environments compared to ordinary portland cement (OPC) [3] . Use of alkali activated slag binder and its potential to replace portland cement requires improvement in the knowledge of the durability aspects of this material. Carbonation of alkali-activated slag concrete is known to be higher than ordinary portland cement and can influence the long-term performance (durability) of this binder. Nevertheless, there are limited numbers of research focused on this issue.
Carbonation is an important consideration in concrete durability, since it can promote corrosion of embedded reinforcement and disintegration of concrete matrix. The goal of this study is to investigate the mechanism of AAS carbonation in order to provide the means needed to develop a high performance and durable AAS concrete.
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It was found that the effect of carbonation depends to a large extent on the type of the activator. Carbonation rate of sodium silicate-activated slag was almost twice the carbonation rate of sodium hydroxide-activated slag (sodium silicate and sodium hydroxide are two commonly used alkaline activators). Additionally, while carbonated sodium silicate activated slag lost half of its strength during carbonation, the compressive strength of sodium hydroxide activated slag did not decrease after accelerated carbonation. The results were consistent with the observations in microanalysis techniques. When sodium hydroxide was used as the activator, carbonation products (calcite crystals) formed densely in the binder. For sodium silicate activated slag, deposition of crystalline calcium carbonate due to carbonation was not significant. The results also showed that natural carbonation in air softens AAS binder and leads to a higher shrinkage for AAS paste.
Utilizing an NDT method to monitor carbonation progress in OPC and AAS, it was observed that microcracks develop during carbonation of AAS binder caused by C-A-S-H decalcification. The results showed that nonlinearity of sodium hydroxide activated slag uniformly increased by 90% during carbonation. For sodium silicate activated slag nonlinearity increased monotonically by 85% until carbonation reached its half, after which, the internal damage was beyond micro- damage measurement range of the implemented NDT method. In OPC, carbonation decreased the nonlinearity by 38% overall, due to transformation of portlandite to calcium carbonate.
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Table of Contents
List of Figures………………………………………………………………………………………………………………. viii
List of Tables…………………………………………………………………………………………………………………. xi
Acknowledgements……………………………………………………………………………………………………….. xii
1.1 Introduction…………………………………………………………………………………………………………….. 1
1.2 Research objectives…………………………………………………………………………………………………… 1
1.3 Outline…………………………………………………………………………………………………………………… 2
2.1 Hydration of ordinary portland cement and alkali-activated slag binders……………………………….. 3
2.2 Carbonation process in concrete materials………………………………………………………………………. 5
2.2.1 Carbonation of OPC…………………………………………………………………………………………….. 6
2.2.2 Carbonation of AAS…………………………………………………………………………………………….. 8
2.2.3 Different performance of OPC and AAS carbonation…………………………………………………… 9
2.2.4 Effect of activator on carbonation of AAS……………………………………………………………….. 13
2.2.5 Effect of slag type on carbonation of AAS………………………………………………………………. 14
2.2.6 A summary of literature on carbonation of AAS vs OPC…………………………………………….. 16
2.2.7 Carbonation depth measurement……………………………………………………………………………. 17
2.3 Alkali-activated slag concrete research needs………………………………………………………………… 18
2.4 References…………………………………………………………………………………………………………….. 20
3.1 Introduction……………………………………………………………………………………………………………. 24
3.2 Theoretical background…………………………………………………………………………………………….. 26
3.3 Materials and experimental method……………………………………………………………………………… 27
3.3.1 Specimens preparation………………………………………………………………………………………… 27
3.3.2 Carbonation depth……………………………………………………………………………………………… 29
3.3.3 Compressive strength…………………………………………………………………………………………. 29
3.3.4 SEM……………………………………………………………………………………………………………….. 30
3.3.5 XRD……………………………………………………………………………………………………………….. 30
3.3.6 TGA……………………………………………………………………………………………………………….. 30
3.4 Test Results and Discussion………………………………………………………………………………………. 31
3.4.1 Carbonation depth……………………………………………………………………………………………… 31
3.4.2 Compressive strength of mortar specimens………………………………………………………………. 32
3.4.3 SEM……………………………………………………………………………………………………………….. 34
3.4.4 X-day Diffraction Analysis………………………………………………………………………………….. 36
3.4.5 TGA Analysis…………………………………………………………………………………………………… 39
3.5 Conclusion…………………………………………………………………………………………………………….. 42
3.6 References…………………………………………………………………………………………………………….. 44
4.1 Introduction……………………………………………………………………………………………………………. 47
4.2 Materials and Methods……………………………………………………………………………………………… 50
4.2.1 Preparation and curing of mortar and paste specimens……………………………………………….. 51
4.2.2 Shrinkage measurement………………………………………………………………………………………. 52
4.2.3 Vickers hardness……………………………………………………………………………………………….. 54
4.2.4 X-ray diffraction……………………………………………………………………………………………….. 56
4.2.5 Mercury intrusion porosimetry……………………………………………………………………………… 56
4.3 Results………………………………………………………………………………………………………………….. 57
4.3.1 Shrinkage…………………………………………………………………………………………………………. 57
4.3.2 Vickers hardness……………………………………………………………………………………………….. 64
4.3.3 MIP………………………………………………………………………………………………………………… 65
4.4 Discussion……………………………………………………………………………………………………………… 67
4.4.1 Key parameters in shrinkage………………………………………………………………………………… 67
4.4.2 Carbonation shrinkage of C−S−H………………………………………………………………………….. 70
4.5 Conclusion…………………………………………………………………………………………………………….. 75
4.6 References…………………………………………………………………………………………………………….. 76
5.1 Introduction……………………………………………………………………………………………………………. 81
5.2 Theoretical background…………………………………………………………………………………………….. 84
5.2.1 Carbonation of OPC……………………………………………………………………………………………. 84
5.2.2 Carbonation of AAS …………………………………………………………………………………………… 85
5.2.3 Nonlinear Resonant Ultrasound Spectroscopy (NRAS)………………………………………………. 86
5.3 Materials and methods……………………………………………………………………………………………… 87
5.3.1 Nonlinear Resonant Spectroscopy (NRAS)……………………………………………………………… 90
5.3.2 Compressive strength…………………………………………………………………………………………. 92
5.3.3 Carbonation depth……………………………………………………………………………………………… 92
5.3.4 Scanning Electron Microscopy (SEM)……………………………………………………………………. 93
5.3.5 X-ray diffraction (XRD)……………………………………………………………………………………… 94
5.3.6 Mercury intrusion porosimetry (MIP)…………………………………………………………………….. 94
5.4 Results………………………………………………………………………………………………………………….. 95
5.4.1 Evolution of the nonlinearity parameter α………………………………………………………………… 95
5.4.2 Changes in linear resonant frequency f0………………………………………………………………….. 97
5.4.3 Advance of carbonation front with time………………………………………………………………….. 98
5.4.4 Compressive strength development………………………………………………………………………… 99
5.5 Discussion: correspondence between elastic nonlinearity and microstructure………………………. 100
5.5.2 Evolution of alpha with time for OPC…………………………………………………………………… 107
5.6 Conclusions………………………………………………………………………………………………………….. 111
5.7 Acknowledgment…………………………………………………………………………………………………… 112
5.8 References……………………………………………………………………………………………………………. 113
6.1 Conclusions………………………………………………………………………………………………………….. 116
6.1.1 Summary of AAS carbonation mechanism…………………………………………………………….. 116
6.1.2 Main contributions……………………………………………………………………………………………. 119
6.2 Future research and potential mitigation strategies………………………………………………………… 121
1. Objectives and Organization
1.1 Introduction
In the field of construction materials, developing a strong and dense binder is desired to manufacture mortars and concretes. Some alternative binders for portland cement have been introduced that are seemingly able to provide similar mechanical and fresh properties as Portland cement. However, there are still limitations and concerns, such as lack of sufficient information with respect to their long-term performance and durability characteristics, which restricts widespread and confident application of these cement substitute materials. A full characterization of their behavior under different physical and chemical conditions is needed to confirm they will perform as expected in service life.
Carbonation is an important aspect of concrete durability considering the corrosion of reinforcing steel and impacts on mechanical properties of concrete. Carbonation reactions in ordinary portland cement (OPC) paste and concrete have been the subjects of multiple studies, while carbonation mechanism of alkali-activated slag (AAS) paste and concretes has not yet been fully evaluated and is the focus of this research.
1.2 Research objectives
The goal of this study is to investigate the mechanism of AAS carbonation in order to provide the means needed to develop a high performance and durable AAS concrete. Based on this goal, two main objectives are defined as: 1) Understanding the mechanism and microstructural
characteristics of carbonation in AAS, 2) Investigating the effect of carbonation on physical and mechanical properties of AAS concrete. To achieve these objectives, research approach with multiple experimental tasks are defined, discussed and presented in this document.
Bridging such knowledge gaps will then result in better understanding of the carbonation reactions in alkali-activated slag concrete, and potentially lead to development of effective carbonation mitigation techniques for alkali-activated slag concretes as a viable alternative for OPC concrete.
1.3 Outline
This thesis is presented in five (5) remaining chapters. Chapter 2 provides background knowledge and motivation of the research. In this chapter different aspects of OPC and AAS carbonation are introduced. Next, the outcome of this study is presented in three chapters.
Chapter 3 is named “Comparative evaluation of carbonation in alkali-activated slag concrete with various activators”. In this chapter the focus of investigation is the mechanism of carbonation in different AAS systems.
Chapter 4 “Evaluation of carbonation shrinkage in alkali-activated slag (AAS) concrete”, investigates carbonation shrinkage of AAS paste. Chapter 5 is “Carbonation-induced microstructural evolution of alkali-activated slag (AAS) and ordinary portland cement (OPC) revealed by nonlinear resonant acoustic spectroscopy (NRAS)”. Chapter 5 includes the results of applying an NDT method for evaluation of carbonation in AAS and OPC. Finally, Chapter 6 summarizes the findings of this thesis, with research conclusions and suggestions for future work.
CARBONATION OF ALKALI-ACTIVATED SLAG MORTAR
