Ceramics-Silikáty 50, (3) 167 - 177 (2006)

This study aimed to investigate the effect of heat treatment on microstructure and phase composition of pastes containing fly ash (FA) and limestone (LS). Two groups of cement pastes were prepared. In the first group, Portland cement (OPC) has been partially replaced by 10, 20 and 30 wt.% FA (M.0, M.10, M.20, M.30). The second group of specimens is constituted by samples derived from M.20, in which, FA has been replaced by 5, 10, 15 and 20 wt.% LS (L.5, L.10, L.15 and L.20). After 28 and 366 days of water curing, the hardened cement pastes were dried at 105°C for 24 hours, subjected to thermal treatment for 3 hours at 250, 450, 650 and 800°C, then cooled to room temperature in the furnace switched off. The compressive strength and weight loss were determined. Scanning electron microscopy, differential scanning calorimetry and X-ray diffraction were used for identification of the changes occurring in the microstructure of the formed or decomposed phases. The results show that additional hydration of unhydrated cement grains, recrystallization, deformation and transformation of CSH phases were occurred. The changes in physical state of cement pastes were studied by measuring deterioration in the compressive strength with temperature.