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SHRINKAGE MITIGATION STRATEGIES FOR INTERNALLY CURED ALKALI-ACTIVATED SLAG MORTARS
ABSTRACT
Ground granulated blast furnace slag (GGBFS) is a by-product of iron manufacturing. When GGBFS is activated by an alkaline solution, calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H) and calcium aluminate silicate hydrate (C-A-S-H) are formed, which provide similar compressive strength to hardened ordinary portland cement. Since the alkaliactivated slag (AAS) can provide the binder phase in concrete, it can be used to replace up to 100% of portland cement in concrete. However, AAS concretes undergo significant shrinkage, which may cause a high risk of cracking and impede its industrial application.
Internal curing, which provides internal water reservoirs and distributes the reservoirs inside the concrete, is proved to be an efficient curing method for concrete with low permeability. Porous lightweight aggregate with high absorption can be used as an internal curing agent. The application of lightweight aggregate to the portland cement concrete shows that it has a significant effect on reducing autogenous shrinkage and delaying drying shrinkage. However, there is little research about its application to AAS concrete.
The main objective of this research was to investigate the effect of internal curing with lightweight aggregate on AAS binders. The properties of AAS investigated included the autogenous shrinkage, drying shrinkage, setting time and compressive strength. Three lightweight aggregate contents (0%, 10% and 20% volumetric replacement of normal weight aggregate) and two types of slag (Grade 100 and Grade 120 slag) were studied. The activating solutions included two different concentrations of sodium hydroxide (2 molar sodium hydroxide and 4 molar sodium hydroxide) and the combination of sodium hydroxide and sodium silicate with different concentrations (lower silica concentration and higher silica concentration).
The effect of lightweight aggregate on the setting time of AAS was found to be negligible, while the setting time increased with the decreasing content of Na2O and SiO2 and pH value.
Lightweight aggregate significantly reduced the autogenous shrinkage of AAS, regardless of the type of slag and the type and concentration of the activators. The AAS mixtures internally cured by lightweight aggregate showed lower drying shrinkage and lower shrinkage rates in the early age, but in the long term, lightweight aggregate showed a negligible effect on drying shrinkage and shrinkage rate. Weight change of AAS mixtures in drying shrinkage experiment was higher with the higher content of lightweight aggregate, likely due to the fact that more water absorbed in the lightweight aggregate can be lost when the mixtures with higher content of lightweight aggregate are subject to drying environment. The effect of lightweight aggregate on the compressive strength of AAS depends on the type and concentration of activators and the grade of slag. The water absorbed in the lightweight aggregate may be released into the paste and affect the concentration of the activating solution, which ultimately affects the compressive strength of
AAS.
TABLE OF CONTENTS
List of Figures ………………………………………………………………………………………………………….. v
List of Tables …………………………………………………………………………………………………………… vi
Acknowledgements …………………………………………………………………………………………………… vii
Chapter 1 Objectives and Organization ……………………………………………………………………….. 1
1.1 Introduction …………………………………………………………………………………………………. 1
1.2 Research Objectives ……………………………………………………………………………………… 3
1.3 Organization ………………………………………………………………………………………………… 4
Chapter 2 Literature Review ………………………………………………………………………………………. 5
2.1 Introduction …………………………………………………………………………………………………. 5
2.2 Alkali-activated slag systems …………………………………………………………………………. 5
2.2.1 Properties of AAS ………………………………………………………………………………. 10
2.3 Internal Curing …………………………………………………………………………………………….. 18
2.3.1 Historical Development of Internal Curing …………………………………………….. 18
2.3.2 Mechanism of Internal Curing and the Effect of Internal Curing on
Properties of High Performance Concrete ……………………………………………….. 24
2.4 Literature Review Summary ………………………………………………………………………….. 32
Chapter 3 Materials and Experimental Methods ……………………………………………………………. 33
3.1 Materials……………………………………………………………………………………………………… 33
3.2 Experimental Methods ………………………………………………………………………………….. 36
3.2.1 Mixing ………………………………………………………………………………………………. 38
3.2.2 Setting Time ………………………………………………………………………………………. 38
3.2.3 Autogenous Shrinkage ………………………………………………………………………… 39
3.2.4 Drying Shrinkage ……………………………………………………………………………….. 40
3.2.5 Restrained Shrinkage …………………………………………………………………………… 41
3.2.6 Compressive Strength …………………………………………………………………………. 42
Chapter 4 Results and Discussion ……………………………………………………………………………….. 43
4.1 Setting Time of AAS Mortars ………………………………………………………………………… 43
4.2 Autogenous Shrinkage of AAS Mortars ………………………………………………………….. 474.3 Drying Shrinkage of AAS Mortars …………………………………………………………………. 524.4 Restrained Shrinkage of AAS Mortars…………………………………………………………….. 59
4.5 Compressive Strength of AAS Mortars …………………………………………………………… 63
Chapter 5 Conclusions and Future Research ………………………………………………………………… 69
5.1 Conclusions …………………………………………………………………………………………………. 69References ……………………………………………………………………………………………………………….. 71
Chapter 1
Objectives and Organization
1.1 Introduction
The annual production of concrete exceeds 1010 tones and is the highest among the manmade engineering materials produced globally (Figure 1-1). This large-scale production results in high-energy consumption and greenhouse gas emissions, which are harmful to the environment and negatively affect human health. As shown in Figure 1-2, the annual carbon dioxide emission caused by the production of concrete exceeds 109tonnes/year, placing concrete second only to iron and steel [1] .
Figure 1-1. Annual world production of 23 materials on which industrialized society
depends [1]
Figure 1-2. Annual carbon dioxide emissions to the atmosphere from material production [1]
Common methods to reduce the energy consumption and the carbon dioxide emission of concrete industry include reducing the consumption of concrete, reducing the paste content and reducing the clinker content [2] . The production of clinker requires high temperatures, as combusting fuels consume significant amounts of energy and high volumes of carbon dioxide are emitted to the atmosphere. Additionally, substantial quantity of carbon dioxide is released due to calcining process, i.e., decomposition of CaCO3 to CaO and carbon dioxide. Grinding clinker and adding small amounts of gypsum makes the most widely used construction material, ordinary Portland cement (OPC). To reduce the energy consumption and the carbon dioxide emission in the concrete industry, research on using an environmentally friendly alternative for binder production has been initiated.
Blast furnace slag is a byproduct of steel manufacturing. When slag particles come in contact with alkaline solution of high pH value, low Ca/Si ratio calcium silicate hydrate (C-S-H), calcium aluminate hydrate (C-A-H) and calcium aluminate silicate hydrate (C-A-S-H) are formed that provide the binder phase in the cement-free concrete, which has similar compressive strength to the hardened OPC [5] . However, AAS undergoes a much higher autogenous shrinkage and drying shrinkage than OPC and the volumetric instability of AAS is an obstacle for its wider application in the industry [4] [14] .
One way to mitigate large shrinkage deformations is to implement internal curing. Internal curing has been rapidly emerging as an effective curing method for high performance concrete (HPC) over the last decade. Since the permeability of HPC is low due to its low water to cement ratio, it is hard for the outer water to enter the paste to provide curing water for the HPC. In this case, internal curing provides internal water resources and distributes them inside the paste to enable efficient curing water. It is also proved to be an efficient method of mitigating autogenous shrinkage, as well as delaying drying shrinkage of HPC [36] .
Even though internal curing has a significant effect on shrinkage behavior of HPC, and is the focus of multiple researchers [27-37] , there is little work available documenting application of internal curing to AAS. This research aims to apply internal curing to AAS, and to properly measure the effect of internal curing on shrinkage behavior of AAS.
1.2 Research Objectives
AAS is a potentially environmentally friendly construction material, which can replace ordinary portland cement in concrete to produce entirely portland cement-free binder. However, questions have been raised with respect to the volumetric instability of the AAS systems [14] [23] . This volumetric instability may also result in a high potential of cracking. The objective of this study is to investigate shrinkage mitigation strategies for AAS, namely internal curing with saturated lightweight aggregate. The effect of internal curing on the properties of AAS, including autogenous shrinkage, drying shrinkage, setting time and compressive strength, are investigated.
Additionally, the effects of the type and concentration of the activator and the type of slag on the shrinkage behavior of AAS system and the internal curing efficiency are studied.
1.3 Organization
This thesis is organized into five chapters:
Chapter 1 – Introduction. The objectives of the study and organization of the thesis are described.
Chapter 2 – Overview of literature related to the development and mechanism of internal curing, and the effect of internal curing on high performance concrete. An introduction of the AAS system is also provided with the properties of AAS system including compressive strength, shrinkage and cracking susceptibility.
Chapter 3 – Experimental approach of the thesis describing. Materials used and the experimental methods are described, including restrained shrinkage, setting time, autogenous shrinkage, drying shrinkage and compressive strength.
Chapter 4 – Presentation of the experimental results and discussion
Chapter 5– Research conclusions and future work.
References are provided at the end of the thesis.
