Ultra-high-performance geopolymer concrete in fire: mix design, phase evolution, and residual performance

Ultra-high-performance geopolymer concrete (UHPGC) has emerged as a promising low-carbon alternative to ultra-high-performance concrete, developed through the alkali activation of aluminosilicate precursors. It combines high compressive strength, a highly dense microstructure, and improved durability, making it an attractive candidate for sustainable and fire-resilient infrastructure. Despite these advantages, its performance under elevated temperatures and fire exposure remains insufficiently understood, with published findings often inconsistent due to differences in precursor chemistry, activator composition, curing conditions, and testing protocols. This review critically evaluates recent advances in the thermal and fire response of UHPGC. The influence of key mix design parameters is systematically discussed in relation to high-temperature behavior. The underlying physicochemical processes during heating, such as continued geopolymerization, dehydration, phase crystallization, and sintering, are examined in detail. Particular attention is given to the coupling between phase evolution, gel chemistry, pore structure development, microcracking, and residual mechanical performance. The role of fiber reinforcement and waste-derived aggregates in mitigating explosive spalling, crack propagation, and strength degradation is also assessed. Thermo-physical responses, including mass loss, thermal shrinkage, and strength retention across different temperature regimes, are synthesized. Microstructural evidence is integrated to clarify degradation and thermally induced densification mechanisms. Current shortcomings in mix design standardization and fire-resistance evaluation approaches are identified. Finally, key research gaps and future directions are outlined to support the reliable engineering application of UHPGC in fire-exposed structural systems.