Performance evaluation of concrete containing fly ash, silica fume and m-sand under high temperatures: mechanical and microstructural approach

Abstract Fire is one of the most serious threats that building structures may face during their lifespan. Experiencing a fire that spreads over the whole building might lead to unforeseen damage to the structural components. The structural element and concrete are affected by the mechanical and thermal characteristics of the concrete. The current study aims to investigate the thermal properties of concrete using silica fume (SF), flyash (FA), with natural sand (NS) and M-Sand (100%) in concrete at 27, 250, 500, and 750 °C at a steady state of 2.5 h. Six mixes were generally considered in two series: with NS as M1 mix and with M-Sand mixes (M2-M5). The SF (0%, 6%, 12%, 18% & 24%), FA (10%), and M-Sand (100%) at the substitution rates. Each mixtures are assessed for residual compressive strength, mass loss, density, and ultrasonic pulse velocity, and statistical and microstructural analysis, like using scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), and energy dispersive X-ray spectroscopy (EDS), were performed at high temperatures. The results found that substituting SCMs for cement with FA, SF with M-Sand in concrete shows the improvement in the residual compressive strength (R-CMS) at individual high temperatures of the mixes compared with mixes (M1 and M2). The combination M4 (10% FA and 12% SF, substituting with cement and 100% M-Sand) shows good performance, having 47.65 MPa, 51.25 MPa, 44.5 MPa, and 38.63 MPa at high temperatures, respectively. Microstructural analysis revealed that a compact dense nano C-S–H structure at individual temperature levels of the mixes via pozzolanic reactions. Overall this study emphasizes the role of FA, SF & M-Sand performance in concrete subjected to fire, making a valuable contribution to the development of green and sustainable infrastructure to improve the life span of the structure under the spread of fire.