Fresh and mechanical properties on brittleness and fracture characteristics of kaolin limestone blend self-compacting concrete

This study investigates the effect of a novel supplementary cementitious material called kaolin-limestone blend (KLB) on the mechanical and fracture performance of high-strength self-compacting concrete of M60 grade. The KLB, composed of 66% calcined kaolin, 33% limestone powder, and 1% gypsum, was incorporated as a partial replacement of cement at 0%, 10%, 20%, 30%, 40%, and 50%. The experimental program evaluated impact energy, flexural strength, fracture energy, and critical stress intensity factor (KIC), along with brittleness through characteristic length. Microstructural investigations were conducted using a scanning electron microscope and X-ray diffraction analyses. The results showed that incorporating KLB effectively reduced the clinker factor, with KLB.5 exhibiting a 15-20% higher impact resistance than conventional SCC (CONV) at 28 days, 13.63-13.04% at 56 days, and 17.39-12.5% at 90 days. Flexural strength improved by 14.03% at 28 days, 12% at 56 days, and 10.24% at 90 days. Fracture energy peaked at KLB.4, surpassing CONV by 6.33%, 2.105%, and 2.41% at 28, 56, and 90 days, respectively. Whereas KLB.5 fell below CONV. This suggests that while the reactivity of the materials stabilizes at higher replacement levels, the filler and packing effects contribute to a dense and cohesive microstructure. At higher replacement levels, a reduction in KIC and characteristic length indicates increased brittleness, attributed to rapid crack propagation after peak load. However, prolonged curing partially mitigates this effect due to continued pozzolanic reactions. Overall, the KLB system demonstrated optimal performance at 50% replacement and shows strong potential for application in high-performance and sustainable concrete structures, particularly in high-rise buildings and precast elements where enhanced workability and strength are required.