High-Temperature Properties and Microstructure of a High-Performance Concrete With Recycled Aggregates Modified by Steel Fibers and Nano-Silica

To promote sustainability, this study incorporates coarse recycled aggregate (RA) from crushed concrete blocks into high-performance concrete (HPC) to reduce the cost of expensive mineral components and reduce surface flaking and thermal degradation at elevated temperatures. Steel fibers (SF) and nano-silica (NS) were introduced as co-modifying agents to enhance HPC's mechanical and microstructural properties. In total, 36 mixtures with varying proportions were designed and subjected to compressive, splitting tensile, and modulus of elasticity tests, considering different RA replacement rates, SF contents, and NS contents. Microstructural analyses, including SEM, XRD, pore distribution, and thermal conductivity tests, were also conducted. Results revealed that SF and NS significantly improved the residual compressive and splitting tensile strengths of HPC with RA (HPC-RA) at elevated temperatures. As temperature increased, residual compressive strength initially rose but then declined, while splitting tensile strength showed a continuous decrease. SEM and XRD analyses confirmed that NS enhanced C-S-H gel formation, improving heat resistance. However, at 600°C, dehydration and C-S-H decomposition led to strength reduction. Pore analysis indicated that higher RA replacement rates introduced more detrimental pores, impacting thermal conductivity. A linear relationship between compressive and splitting tensile strengths was established, along with a temperature-dependent fitting equation to predict residual properties.