Influence of inclusion on the volume changes in sealed conditions of slag activated by sodium hydroxide: a multi-technique investigation

The production of ordinary Portland cement (OPC) contributes to around 8 % of global CO2 emissions caused by human activity. Reducing this impact is crucial, and exploring alternatives, like alkali-activated materials, can help reach this goal. Despite having promising mechanical properties to replace OPC, alkali-activated materials unfortunately exhibit significant volume changes at early-age. These include autogenous and thermal strains. This research investigates how the change of scale (paste to mortar) impacts the autogenous and thermal strains of blast-furnace slag activated with sodium hydroxide. The AutoShrink device used to determine the autogenous strain has been adapted to study the coefficient of thermal expansion by means of repeated thermal variations. A higher solution-to-binder ratio results in increased autogenous swelling of the paste and a decrease of the autogenous shrinkage. The increase of the coefficient of thermal expansion rises proportionally to the increase of the solution-to-binder ratio. Compared to the paste scale, both thermal and autogenous strains are reduced in mortar due to the restraining nature and the change in the thermal behaviour of the sand. The whole process was monitored through acoustic emission and ultrasonic pulse velocity to evaluate the evolution of micro-cracking and mechanical properties.