Influence of mineral admixtures on the strength, durability and microstructure of recycled sand concrete

To mitigate the durability deterioration of recycled sand concrete (RSC) induced by complex interfacial transition zones (ITZs) and high porosity, the effects of mineral admixtures on the compressive strength, durability and microstructure of RSC were systematically examined. The mineral admixtures studied were fly ash (FA), a binary combination of FA and ground granulated blast-furnace slag (SL) and a ternary blend of FA, SL and silica fume (SF) or metakaolin. The results show a threshold effect for single FA incorporation: the optimal comprehensive performance was achieved at 10% FA (FA10). Compared with the reference mix (FA0), the 28-day compressive strength of FA10 was increased by 0.9%; the carbonation depth, water absorption and electrical flux were reduced by 15.3%, 11.8% and 30.6%, respectively, and the 180-day corrosion resistance coefficient was improved by 27.9%. Excessive FA (>10%) degraded performance. The composite mineral admixtures provided superior enhancement owing to multi-component synergy, with the ternary system FA15SL15SF8 (15% FA, 15% SL, 8% SF) showing the best performance. Compared with FA0, the 28-day compressive strength of FA15SL15SF8 was increased by 4.7%; the carbonation depth, water absorption and electrical flux were reduced by 51%, 31%, and 64%, respectively, and the 180-day corrosion resistance coefficient was improved by 37%. Microstructural analysis revealed that composite admixtures refined the pore structure and densified ITZs through reactive component gradient matching and multi-scale particle packing, forming a robust barrier against the ingress of deleterious agents.