Accelerating Precuring of Autoclaved Aerated Concrete (AAC) via Hybrid Alkali-Activated Pisha Sandstone: Suppression of Montmorillonite Swelling and Pore Structure Optimization

Pisha Sandstone (PS), characterized by its high silica content (76.94% SiO2), presents a promising alternative to traditional quartz sand in construction applications due to its abundant availability and low cost. However, its montmorillonite clay minerals induce expansion during the precuring stage of autoclaved aerated concrete. When the PS content accounts for 42% of the total siliceous materials, extending the precuring time to approximately 5.1 h and hindering gas evolution within the green body, thereby severely compromising production efficiency. This study employed a NaOH and Na2SiO3 hybrid alkali activation system to modify PS. Experimental results reveal that adding 1 wt % each of NaOH and Na2SiO3 (N1S1), dynamic equilibrium between slurry thickening and gas evolution rates was achieved, yielding homogeneous pore structures and reducing precuring time by 37.3%, from 5.1 to 3.2 h. Mechanistic analysis indicates that NaOH effectively disrupts montmorillonite's layered structure, while it synergizes with Na2SiO3 to promote cogeneration of C-(A)-S-H gels and ettringite (AFt), thereby enhancing early stage hardening.