High-temperature performance of clay-enhanced alkali-activated binders: A sustainable alternative to Portland cement

The cement industry's commitment to reducing CO2 emissions has led to exploring alternative processes and supplementary cementitious materials (SCMs). These include alkali-activated binders or geopolymers, promising cement alternatives due to their pozzolanic properties and superior strength and durability, especially at high temperatures. Using abundant raw materials such as clay further enhances the sustainability of geopolymers by reducing emissions associated with their production. Factors like raw material composition and activator type can affect binder properties, so careful material design is essential. This paper discusses experimental results on the mechanical behaviour of alkaline cements at high temperatures, testing two binders (FA=100 % fly ash and FB=70 % fly ash with 30 % clay) and two activators (WN and WK), selected from previous studies where residual strength after temperature exposure was excellent. Their performance was compared to two Portland types of cement (CEM I 52.5 N/SR and CEM I 42.5 R). Unlike cement pastes, the alkaline cements FA and FB retain and can even improve mechanical strength up to 600 degrees C. At this temperature, adding clay to the FB alkali cement promotes the formation of a viscous gel capable of filling cracks and voids in the geopolymer, thus increasing its high-temperature strength by 80 % compared to cement pastes.