High temperature resistance of binary and ternary geopolymer-based ultra-high-performance concrete

The development of building materials with excellent high-temperature resistance is crucial for fire safety. As a low-carbon alternative to cement, geopolymer has attracted extensive attention, and geopolymer concrete has demonstrated promising performance at elevated temperatures. This study evaluated the high-temperature performance of two ambient-cured, fibre-free G-UHPC systems, namely a fly ash-slag-silica fume (FA-S-SF) ternary system and a slag-silica fume (S-SF) binary system, with OPC-UHPC used as a reference. The investigation was carried out through room-temperature compressive strength testing, elevated-temperature exposure under a controlled heating regime, residual mass and residual compressive strength measurements, and microstructural characterization using SEM, XRD, and TGA. The results show that raw material composition significantly influenced the strength development of G-UHPC, and the compressive strength reached a maximum at 20% silica fume, followed by a slight reduction at 30%. Compared with OPC-UHPC, both G-UHPC systems exhibited superior residual performance after high-temperature exposure and maintained structural integrity without explosive spalling even at 800 ℃, with all 60 G-UHPC specimens remaining macroscopically intact. At 800 ℃, the ternary G-UHPC exhibited more pronounced expansion and cracking than the binary system, while the binary G-UHPC retained a residual compressive strength of more than 30 MPa. The observed deterioration at high temperatures is interpreted as the result of multiple coupled mechanisms. In addition to pore-pressure-related effects during heating, crystallization-related changes, thermal incompatibility, and thermally induced microcracking may also have contributed to the deterioration behavior. Overall, this study provides a direct comparison between ambient-cured, fibre-free binary and ternary G-UHPC systems under elevated temperatures, expands the experimental database on the high-temperature performance of G-UHPC, and offers useful comparative evidence for the design of high-temperature-resistant geopolymer cementitious materials.