High Temperature Resistance of Fly Ash-Enhanced Alkali Activated Portland Cement Mortar: Microstructural Evolution and Strength Retention

The limited high-temperature resistance of Ordinary Portland Cement (OPC) remains a critical challenge for fire-exposed and industrial concrete structures. Its performance deterioration above 500 °C is associated with dehydration and recrystallization of hydration products, leading to structural degradation of the cement matrix. To address this limitation, partial clinker replacement with fly ash combined with sodium water glass activation was proposed to enhance thermal stability. Physico-chemical analysis revealed the absence of portlandite and the formation of C-A-S-H and zeolite-like N–C–A–S–H phases in the fly ash-containing alkali-activated Portland cement. Upon heating, C-A-S-H phases sintered into stable high-temperature calcium aluminosilicate phases and zeolite-like phases underwent topotactic recrystallization into feldspathoid-type structures, preserving matrix integrity at high temperatures. The optimized composition region of cement system (fly ash—12.0–16.5 wt. %, density of water glass—1220–1240 kg/m3) was characterized by residual strength ≥ 50%, while compressive strength at 28 days was ≥80 MPa, exceeding the residual performance typically reported for conventional OPC systems under similar conditions (5–35%). The study was devoted to revealing the potential of low-emission Portland cements in high-temperature-resistant concretes through the utilization of fly ash. The mechanism that controls the compressive strength and temperature resistance of such cements has been demonstrated.