Relationship Between the Reduction of Mechanical Properties and Calcined Bauxite Content Under Thermal Conditions

Ultra-high-performance concrete (UHPC) has gained recognition as a cement-based material renowned for its outstanding strength, toughness, and durability. This study focuses on exploring the relationship between calcined bauxite (CB) content and the residual compressive strength of UHPC under various thermal conditions. To achieve this, a comprehensive series of experiments was conducted, including uniaxial compressive tests, spalling tests, and scanning electron microscope (SEM) analysis. Additionally, the influences of superplasticizer types and curing methods were characterized. The experimental findings revealed that UHPC specimens incorporating ISO standard sand exhibited higher compressive strength, while Silkroad 5000P proved to be the most effective superplasticizer, outperforming Sikament R7 and Silkroad 1000 in enhancing compressive strength. Among the curing methods, steam curing yielded the highest strength in UHPC specimens compared to ambient, hot air, and hot water curing. The study also investigated the thermal-microstructural evolution of UHPC. The results indicated that as the temperature increased from 20° C to 200° C, the C-S-H gel became denser, while the C-H phase diminished in both quantity and quality. The inclusion of calcined bauxite in the mixture enhanced the fire resistance of UHPC. At 400° C, the C-S-H gel remained compact, but the C-H phase disappeared entirely. At higher temperatures, partial decomposition of the C-S-H gel occurred, leading to the formation of a needle-like C-S-H phase, which contributed to a reduction in the residual compressive strength of UHPC. These findings underscore the importance of material composition and curing methods in optimizing the performance of UHPC under thermal conditions.