Development of the fire resistance and mechanical characteristics of silica fume-blended cement pastes using some chemical admixtures

[Display omitted] •The used admixtures enhancing both fire resistance and mechanical properties of silica-fume blended pastes.•The water reduction% of the prepared admixtures reaches 52.1% and 54.36%.•The compressive strength enhanced by (31.6% for TG, 50.3% for SI and 53.9 for SII).•Fast cooling in water causes the higher losses of strength than slow cooling in air for all temperatures. In this study, the fire resistance and mechanical properties of blended cement pastes were improved using three different admixtures namely; phenol formaldehyde sulfonilate (SI), and low phenol content phenol formaldehyde sulfonilate (SII) and sikament-TG (TG). The fire resistance tests were studied by measuring the weight loss percentages for some selected pastes (optimum dose) at each firing temperature. The phase composition and microstructure of the formed hydrates were examined using scanning electron microscopy, X-ray diffraction and differential scanning calorimetry analysis. The results show that the doses of 0.25% TG, 0.50% SI and 1.00% SII are the optimum doses. Moreover, in case of air cooling, the compressive strength of the hardened pastes admixed with the optimum doses increases by firing the specimens up to 500 °C, then decreases by firing the specimens at temperatures up to 700 °C, but still higher than the compressive strength at room temperature.