Experimental Study on Mechanical Properties of Silica Fume Foam Concrete after Exposure to High Temperatures

To investigate how the content of silica fume (SF) influences the performance of foam concrete (FC) after high-temperature exposure and the underlying mechanisms, this study prepared standard FC cube specimens with SF contents of 0%, 0.15%, 0.2%, 0.25%, and 0.3%. The working properties of the material at room temperature were systematically tested, and the mass loss, residual compressive strength, failure mode, microstructure and acoustic emission (AE) data at different temperatures (100°C, 200°C, 300°C and 400°C) were analyzed. The test results indicate that increasing the SF content reduces the fluidity of the fresh paste yet significantly enhances the compressive strength and lowers the water absorption of FC at room temperature. After high-temperature exposure, the effect of SF exhibits a dual character: at 200 °C and below, SF effectively mitigates the performance degradation of FC. However, when the temperature reaches 300–400 °C, specimens with an excessively high SF content (e.g., 0.3%) experience rapidly built-up internal steam pressure that cannot escape in time, which triggers the formation and propagation of a microcrack network and leads to a sharp drop in strength. Based on AE detection and scanning electron microscopy (SEM) image analysis, the failure process of silica fume foam concrete (SFFC) proceeds through three stages: free water evaporation at low temperatures, dehydration shrinkage of the C-S-H gel at medium temperatures, and finally, structural failure marked by the collapse of the C-S-H gel network at high temperatures. This study indicates that an SF content of 0.25% allows FC to achieve an optimal balance between mechanical properties and high-temperature stability. The findings provide a theoretical basis for optimizing FC mix proportions and enhancing fire prevention design.