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Publication

Early-Age Structural Development and Autogenous Shrinkage of Alkali-Activated Slag/Fly-Ash Cements

Book - Dissertation

Alkali-activated cement is considered to be a promising eco-friendly alternative to Ordinary Portland Cement (OPC). In addition to its eco-friendly performance, the alkali-activated cements synthesized with blast furnace slag and fly ash offers several potential advantages as compared to OPC, including higher rate of strength development and good resistance to chemical and fire attacks. Despite these advantages, the acceptance and application of the alkali-activated slag/fly ash (AASF) cements in the construction industry is hampered by concerns related to their rapid hardening behaviour resulting in very short setting times and large autogenous shrinkage. However, the scientific reasoning and the mechanism behind quick setting process and large autogenous shrinkage remain poorly understood in the literature.Activator type is one of the most dominant factors that influences setting and autogenous shrinkage behavior in AASF cements. Sodium silicate is the typically used activator in AASF cements due the superior or comparable strength properties to Portland Cement. However, AASF cements activated with sodium silicate have critical shortcomings such as quick setting and large shrinkage deformations which cannot secure their performant functions in concrete applications. Moreover, sodium silicate is not naturally available, and its production process is expensive with a high environmental impact. On the other hand, AASF cements activated with naturally occurring minerals like sodium carbonate and sodium sulfate generates much longer setting time and lower autogenous shrinkage than sodium silicate activation. The fundamental elements behind the setting and shrinkage mechanisms are not fully understood. The main objective of this doctoral research is to explore the setting and autogenous shrinkage behavior in AASF cements with sodium silicate, sodium carbonate and sodium sulfate solutions through an interlinked assessment of reaction kinetics, phase formations, gel structure, stiffness development, mechanical properties, and pore structure. The evolution of early-age structure development is continuously monitored using ultrasonic P-wave velocity in order to understand the setting and hardening behaviour. The pore pressure caused by self-desiccation is calculated and related to the measured autogenous shrinkage towards a better understanding of complex autogenous shrinkage behavior in AASF cements. The cracking tendency under restrained autogenous shrinkage is investigated experimentally using circular ring test. Moreover, the influence of curing temperature at 40°C is investigated as a mitigation strategy to control the autogenous shrinkage.Regardless of the activator type, Al substituted C-S-H gel and N-A-S-H gel are observed as the main reaction products. The partial substitution of sodium silicate with sodium carbonate and sodium sulfate significantly reduces the rate of reaction due to decrease in alkalinity (OH- ions) of the activating solution. At equal replacement of the sodium silicate by sodium carbonate and sodium sulfate, the samples with sodium carbonate exhibits higher rate of reactivity due to the formation of calcite as a nucleation seed to the precipitation of C-A-S-H gels.Comparative assessment of ultrasonic measurements with Vicat setting times and calorimetry heat flow curves shows that condensation reaction between the aluminosilicate units formed near the particle surface is most likely responsible for setting process in AASF cements. Research findings confirm that self-desiccation is one of the main driving forces for autogenous shrinkage in AASF cements. Besides self-desiccation, the Si-polycondensation reaction at solid-liquid interface or between adjacent aluminosilicate units during induction period of heat flow curves is also responsible for the autogenous shrinkage.The autogenous shrinkage of sodium silicate-activated samples significantly reduces during early-ages if sodium carbonate or sodium sulfate is incorporated in the solution. The reduced autogenous shrinkage is attributed to a decrease in self-desiccation and capillary stresses in the pore fluid. The decrease in viscoelastic deformation associated to the rearrangement of C-A-S-H gels under internal stresses, on the other hand, reduces the autogenous shrinkage. Besides the self-desiccation process, reduction in the Si polycondensation reaction during induction period of the heat flow curves is also responsible for a lower autogenous shrinkage in samples activated with sodium carbonate and sodium sulfate. As a result of considerably reduced autogenous shrinkage and rarely affected tensile strength, the time of cracking under restrained condition is prolonged in case of sodium carbonate and sodium sulfate activation.The increase in curing temperature to 40°C can effectively reduce the autogenous shrinkage in AASF cements. This is the result of suppression of the self-desiccation process due to decrease in rate of reaction, formation of a matrix with higher elastic modulus at early ages, formation of more 3D-structured N-A-S-H type gels, and considerable increase in crystallinity of C-A-S-H type gels.
Publication year:2021
Accessibility:Embargoed