Effect of silica fume on the microstructural and mechanical properties of concrete made with 100% recycled aggregates

Recycled concrete aggregate can be utilized in structural concrete in order to reduce the use of natural resources and the harmful impacts of waste concrete on the environment. This present research aimed to assess the effectiveness of using recycled aggregate concrete with the partial replacement of cement by silica fume (SF) to analyze the microstructural and mechanical properties of recycled aggregate concrete (RAC). In this study, recycled stone was used as coarse aggregate. The main variables of the study included the dosage of silica fume that was employed as a partial replacement of ordinary Portland cement (OPC) with four different percentages: 4%, 8%, 12%, and 16% by weight. Five different mixes were prepared, with four mixes created by varying amounts of silica fume, which were designated as RSACSF4, RSACSF8, RSACSF12, and RSACSF16. The other mix was created as a reference mix without silica fume and designated RSACSF0. Slump test was conducted to investigate the workability of concrete mixes. From the test result, a decreasing trend was found after adding more percentage of SF. Compressive and splitting tensile tests were conducted to analyze the mechanical properties of RSAC at 7 and 28 days. The results showed that the addition of SF improved the performance of RSAC at early and later curing ages, and a 12% addition of SF showed the best result. Scanning electron microscopy and X-ray diffraction analysis were performed to explore SF's microstructural performance and effect on RSAC. The results showed that silica fume showed a positive pozzolanic impact, and when combined with calcium hydroxide, it underwent a secondary hydration reaction that boosted the generation of calcium silicate hydrate and improved the parameters of the interface transition zone. X-ray diffraction analysis showed that silica fume and silica fume have similar pattern intensities. Finally, 12% SF is recommended as a partial replacement for cement in RSAC.