Hydration and microstructure of ternary slag cements incorporating recycled concrete fines

This study investigates the hydration mechanisms and performance of ternary cements formulated with slag and recycled concrete fines (RCF), a secondary fine powder waste derived from high-quality recycled concrete aggregate production. The cement blends were formulated using 50% CEM I 52.2 R cement with 50% RCF-slag, with RCF content varying from 10% to 30%. A binary slag cement and a conventional ternary limestone-slag cement at equivalent cement replacement levels were set as benchmark groups. The results indicate that RCF is composed of calcite, quartz, vaterite, feldspar, gypsum and around 39% amorphous phases. RCF-slag cements exhibit significantly higher rates of clinker hydration and slag reaction compared to ternary limestone-slag and binary slag cements. Formation of AFm-CO 3 phases is favored over monosulfoaluminate, and ettringite is stabilized in RCF blends. Also, RCF alters C-S-H composition by increasing Ca/Si and reducing Al/Si ratios. Setting and hardening of composite cement is also affected by RCF, with initial setting threshold time advancing as the RCF content increases. A faster strength gain speed and higher strength at late age is also found in RCF-slag cement mortar. This research demonstrates the potential of RCF to enhance the performance of slag-based cements and contribute to sustainable construction practices.