Residual Mechanical and Structural Properties of Non-Calcined Hwangto Concrete After Exposure to High Temperatures

This study evaluated the residual mechanical properties of concrete in which Ordinary Portland Cement (OPC) was partially replaced with non-calcined Hwangto (NHT). Specimens were prepared with two water-to-binder (W/B) ratios (0.41 and 0.33) and three NHT replacement levels (0%, 15%, and 30%). The specimens were exposed to elevated temperatures of 20, 100, 200, 300, 500, and 700 °C at a heating rate of 1 °C/min. The results indicated that while the initial compressive strength at room temperature decreased with increasing NHT content, the residual mechanical performance at high temperatures significantly improved. Notably, temporary strength recovery was observed in the 200-300 °C range due to the internal autoclaving effect. At 700 °C, the NHTC (non-calcined Hwangto concrete)-30 series exhibited the highest thermal stability, retaining 28.2% of its initial compressive strength, whereas the Plain (OPC Concrete) and NHTC-15 series retained only 23.6% and 22.4%, respectively. Regarding energy absorption, the dissipated energy varied with the W/B ratio. In the W/B 41 series, the NHTC-30 specimen demonstrated superior ductility and energy dissipation capacity at 700 °C, outperforming the Plain specimen. This enhanced post-peak performance is attributed to the thermal activation of kaolinite into metakaolin, which preserves microstructural integrity by mitigating the severe degradation of hydration products and inhibiting crack propagation. These findings suggest that incorporating NHT effectively enhances the fire resistance and residual structural integrity of concrete, particularly in normal-strength matrices.