Mineralogical and microstructural changes in alkali-activated and hybrid materials exposed to accelerated leaching

Alkali-activated materials (AAM) and hybrid cements (HC) have become sustainable alternatives to Portland cement (PC) due to their low carbon footprint. The main differences between AAM and HC lie in their content of clinker (none in the AAM and usually lower than 30% for HCs) and the type of activator used (strong alkaline solutions for AAM and small amounts of solid alkalis for HC). Durability problems related with microstructural changes due to decalcification and leaching of the cementitious paste have been well researched for PC pastes, but it is still not well known for AAM and HC. The present work aims to study the leaching process for cement pastes of both types of sustainable pastes. Blast furnace slag (BFS) was selected as a precursor to manufacture hybrid slag (HS) pastes and alkali-activated slag (AAS) pastes. A commercial CEM IV was selected as reference material. A 6 M NH4NO3 solution was used to accelerate leaching kinetics. After 28 days of immersion, the mineralogical and microstructural changes were evaluated. Results show that AAS pastes exhibited the highest leaching resistance of all the pastes under study, due to the absence of portlandite and the high level of polymerization of silicate chains. In HS pastes, the presence of portlandite (due to PC in the material) and gypsum (due to the activa-tor) explains their intermediate performance, in between CEM IV and AAS.