The effect of boron oxide(B₂O₃) and colemanite(2CaO·3B2O3·5 H2O) additives on the aerating, mechanical, post-fire, and microstructural behavior of perlite-based geopolymer lightweight concrete

In this study, the aerating effect and contribution to mechanical performance of boron oxide (B₂O₃) and colemanite (2CaO·3B₂O₃·5 H₂O) in ground raw perlite (GRP)-based lightweight geopolymer concrete (GPLC) were investigated. Recent studies on lightweight geopolymer concrete have shown that, despite the advantage of low density, there are significant limitations in simultaneously achieving environmental sustainability, diverse production methods, adequate mechanical strength, and high-temperature performance. Instead of GRP, boron compounds were used at ratios of 5%, 10%, and 15%; a fixed amount of GGBFS and silica fume (SF) was added to improve bonding. Expanded perlite (EP) aggregate of 2–4 mm was used for lightness, and carbon fiber (CF) at ratios of 0%, 1%, and 2% was used for strength. Additionally, a hyperplasticizer was added at a rate of 1.93% of the total binder and activators to improve workability. The gel structure and phase formation were examined using FE-SEM, EDX, and F-TIR analyses; regression and Taguchi methods determined the optimized parameters. The findings showed that 2CaO·3B₂O₃·5 H₂O formed a more homogeneous, but less ductile N-A-S-H network, while B₂O₃ provided a denser matrix and higher compressive strength with hybrid N-A-S-H/C-A-S-H gels. In high-temperature tests, both boron additions increased secondary phase formation and thermal stability. The best performance was achieved with a 10% boron content and a 2% CF mixture, offering a balance of low density, high strength, and high post-fire resistance.