Effect of MCM-41 on high-temperature transformations in cement paste

The aim of the study was to investigate the effect of mesoporous silica MCM-41 (at 0–2% by cement mass) on the thermal degradation of cement pastes subjected to exposure up to 700 °C. The experimental programme included, among others, thermogravimetry (TG/DTG), isothermal calorimetry, SEM-EDS, ultrasonic pulse velocity (UPV) measurement, and compressive strength testing. The results indicated that MCM-41 accelerates and intensifies the cement hydration process while simultaneously reducing portlandite content after thermal exposure (500 °C: 17.44% → 13.07%). At 300 °C, the content of bound water increased compared to plain cement paste (from 3.77% to 5.03–5.20%), with comparable mass loss at 100 °C (17.33–17.60%). A higher susceptibility to secondary carbonation was also observed in pastes containing MCM-41. Despite the increase in microporosity, matrix continuity was better preserved (UPV: +16.9% at 500 °C and + 25.3% at 700 °C for 0.5% MCM-41). Moderate dosages enhanced the initial compressive strength by 26.9% for paste with 0.75% MCM-41 and mitigated strength losses after heating (700 °C: +7.1% for 1.0% MCM-41 and + 4.9% for 1.5% MCM-41). Excessive dosage (> 2.0%) diminished the benefits. It was demonstrated that MCM-41, when added in moderate amounts (up to 1% by cement mass), improves the thermal resistance of cement pastes by reducing portlandite, promoting secondary carbonation, and maintaining favourable mechanical performance after heating. Accordingly, the carefully selected dosage of MCM-41 constitutes one of the effective strategies for enhancing the thermal resistance of cement pastes.