Effect of granite waste and steel fibers on the mechanical properties of geopolymer concrete before and after fire exposure

This study investigates the post-fire performance and mechanical properties of geopolymer concrete incorporating granite industry waste as a full replacement for natural fine aggregates, reinforced with steel fibers. High-calcium fly ash was used to produce high-strength concrete cured at room temperature. The results indicated that incorporating granite waste and steel fibers reduced workability due to increased internal friction. The lowest slump flow (37 cm), observed in the mix with 2 % steel fibers, represented a 38 % reduction compared to the control mixture. Despite the reduced workability, the concrete achieved a 28-day compressive strength of 55 MPa and a 45 % increase in flexural strength with 2 % steel fibers. Toughness improved by up to 13 times compared to concrete without fibers. Post-fire testing showed that granite-waste concrete retained mechanical performance comparable to that made with natural sand. After 30 min of fire exposure, the residual compressive strength of concrete without fibers dropped to 39 % of its original value, while flexural strength decreased by approximately 70 %. Incorporating 1 % steel fibers increased toughness by 1.7–2.2 times, whereas 2 % fibers enhanced flexural strength retention by up to 10 % after 60 min of exposure. Microstructural analysis confirmed that steel fibers delayed crack propagation and influenced the thermal transformation of gel phases by increasing the silicon-to-aluminum and calcium-to-silicon ratios. These findings highlight the potential of granite waste and steel fibers as sustainable alternatives to natural aggregates and conventional reinforcement for enhancing both fire resistance and mechanical performance in geopolymer concrete.