Enhancing mechanical and thermal performance of ultra-high-performance calcium aluminate cement concrete (UHP-CACC) using hybrid steel-basalt fibers

This study investigates the thermal and mechanical performance of ultra-high-performance calcium aluminate cement concrete (UHP-CACC) reinforced with hybrid steel and basalt fibers, at temperatures up to 1000 °C. Three mix proportions include (i) control UHP-CACC without fibers, (ii) 0.5 % basalt fibers (BF) and 1.5 % steel fibers (STF) and (iii) 1.0 % BF and 1.0 % STF. Compressive strength results demonstrated that UC-0.5BF+ 1.5STF exhibited superior thermal resistance, retaining the highest residual strength at 1000 °C. The use of hybrid fibers enhanced the crack resistance and matrix integrity at high temperatures and led to strong interfacial bonding, as confirmed by scanning electron microscopy (SEM) and surface analysis. Although porosity and cracking increased significantly beyond 500 °C in control UHPCACC without fibers, but better crack resistance was observed in UC-0.5BF+ 1.5STF at high temperatures. However, theporosity was reduced at 250 °C due to matrix densification. The heat of hydration analysis for UC-0.5BF showed delayed reaction peaks and reduced thermal sensitivity in hybrid fiber-reinforced mixes, contributing to better thermal stability compared to that of UC. Thermogravimetric analysis (TGA) identified three distinct stages of thermal decomposition in UC-0.5BF, with hybrid fibers enhancing thermal resistance by having stable phases at elevated temperatures. Fourier-transform infrared spectroscopy (FTIR) analysis for UC-0.5BF confirmed the dissociation of hydration products and structural transformations in the matrix. These findings conclude that the incorporation of 1.5 % steel fibers and 0.5 % basalt fibers optimally enhances the thermal and mechanical performance of UHP-CACC.