Effect of Internal and External Factors on the Volume Changes of Slag Binder Activated by Sodium Hydroxide at Early-Age

As concrete is the second most used material after water, representing 5% to 7% of global anthropogenic CO2 emissions, it is crucial to decrease its CO2 production. One way to reach this goal is to use alkali-activated materials. It is known that they have adequate mechanical properties for the construction sector. However, alkali-activated materials suffer from significant early-age volume changes such as autogenous and thermal strains. Factors such as the amount of solution, activator type, molarity, curing conditions and internal relative humidity have an important influence on how the early-age volume develops. The objective of this research is to study the impact of internal (solution-to-binder ratio) and external (curing temperature) parameters on the autogenous strain of slag activated by sodium hydroxide. A revisited version of the AutoShrink device, developed at ULB, has been used to determine the autogenous strain as well as the coefficient of thermal expansion (CTE) through temperature cycles applied to the samples since casting. In a general way, increasing the solution-to-binder ratio has a magnifying effect on the cumulative heat while the autogenous shrinkage decreases. Moreover, increasing the solution-to-binder ratio and the curing temperature leads to a higher autogenous swelling of the paste after setting. Decreasing the curing temperature results in a lower heat flow while the autogenous strain increases in magnitude. The increase in the CTE is proportional to the increase in the solution-to-binder ratio. Additionally, lowering the temperature induces a decrease in the CTE.