| Summary: | This study examines the effect of silica fume (SF) as a partial replacement for cement to prevent strength retrogression in hardened cement paste (HCP) and mortar at elevated temperatures. An experimental procedure was conducted on 320 specimens, replacing 0, 10, 20, and 30% of the cement by weight with SF. The residual compressive strength of the specimens was evaluated at room temperature (25 °C) and at 100, 200, 300, and 400 °C for 7, 28, and 56 days. The results indicate that the addition of SF to the cement paste and mortar improves the compressive strength both at 25 °C and at temperatures up to 400 °C. That is attributed to the formation of C-S-H phases, such as tobermorite and xonotlite. Additionally, the optimal residual compressive strength was achieved by adding 30% of SF. Therefore, XRD, SEM, and EDS techniques were employed to evaluate the microstructure of HCP specimens with 30% of SF. The results show that adding SF leads to a denser microstructure and lower porosity, resulting in more durable cement paste and mortar at ambient and elevated temperatures. In conclusion, using SF as a partial replacement for cement can be an effective way of developing sustainable fire-resistant construction materials.
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