Experimental study on mechanical properties of silica fume modified steel fibre reinforced geopolymer recycled aggregate concrete

Steel fiber reinforced geopolymer recycled concrete (SFRGRAC) presents several benefits, including reduced carbon emissions, conservation of natural minerals, and enhanced ductility and toughness, rendering it a promising material for broad applications. This study aims to augment the mechanical characteristics of SFRGRAC by incorporating silica fume as a reinforcing agent. It examines the influence of silica fume content, steel fiber volume fraction, and recycled aggregate substitution ratio on mechanical performance of SFRGRAC through cube compression, split tensile strength, flexural strength, and modulus of elasticity test. Additionally, the study elucidates the modification mechanism of silica fume via SEM and low field NMR test. The findings indicate that silica fume addition extends the setting time of SFRGRAC, with 15% substitution rate of silica fume leading to a 29.68% increase in initial setting time and a 22.98% increase in final setting time. The accelerated hydration reaction, prompted by the prompt pozzolanic reaction between silica fume and the alkali activator, results in the SFRGRAC achieving over 85% of its 28d compressive and tensile strengths within 3d. The compressive strength improves by 17.44% as steel fiber volume fraction increases from 0% to 1.5%, whereas a 50% substitution of recycled aggregate causes a 9.79% reduction in strength than 0% substitution rate. A 10% inclusion of silica fume diminishes total porosity by 37.38%, substantially enhancing compressive, tensile, and flexural strengths. When the silica fume substitution rate reaches 15%, the excessive silica fume reduces the alkalinity of matrix, resulting in an incomplete hydration reaction of the geopolymer. Consequently, this causes a decline in its mechanical properties. These findings offer valuable insights for the revision and enhancement of specifications related to recycled concrete.