International Journal of Science, Engineering and TechnologyAuthors: Engr. Ezea Boniface, Olorunshola Banjo, Fadiran David, Engr. Dr. Ugwuoke Malachy Okonkwo, Dr. J O Labiran
Abstract: Fire exposure significantly alters the mechanical and durability properties of concrete, often leading to substantial reductions in structural capacity. This study investigates the improvement of residual mechanical properties of concrete exposed to elevated temperatures through partial replacement of ordinary Portland cement with calcined kaolin (metakaolin, MK). Concrete specimens were produced with 0%, 2.5%, 5%, 7.5%, and 10% metakaolin replacement by weight of cement. The water-cement ratio was varied appropriately to maintain workability as MK content increased. Specimens were cured for 28 and 56 days and subsequently exposed to heat loads of 200°C, 400°C, 600°C, and 800°C. Residual compressive strength, mass loss, and water absorption were determined to evaluate post-fire performance. Results indicate that compressive strength improved at ambient conditions with increasing MK content, with optimum performance observed between 7.5% and 10% replacement. At elevated temperatures, MK-modified concrete demonstrated superior residual strength retention compared to control specimens, particularly at 600°C and 800°C. Mass loss and water absorption increased with temperature across all mixes; however, mixes containing higher MK percentages exhibited comparatively reduced deterioration, attributed to pore refinement and reduced calcium hydroxide content. Specimens cured for 56 days consistently showed better residual performance than 28-day specimens, highlighting the importance of hydration maturity in thermal resistance. Correlation analysis revealed an inverse relationship between water absorption and residual compressive strength, confirming that microstructural densification significantly enhances fire resistance. The findings support the use of metakaolin as an effective supplementary cementitious material for improving the thermal stability and post-fire structural integrity of concrete. The study contributes to performance-based fire design by emphasizing material performance enhancement rather than reliance solely on prescriptive measures such as concrete cover thickness.
