Synergistic multi-scale reinforcement of recycled aggregate concrete with basalt fibers and calcium carbonate whiskers: From micro-filling to macro-bridging

This study develops a synergistic multi-scale reinforcement strategy for recycled aggregate concrete (RAC) by combining macro-scale chopped basalt fibers (BF) with micro-scale calcium carbonate whiskers (CW) in concrete containing 25% recycled coarse aggregate that was pretreated using a two-step nano-silica treatment followed by cement-slurry coating. An experimental program of 144 specimens was conducted to quantify fresh workability and hardened performance, complemented by scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and X-ray diffraction (XRD), to elucidate reinforcement mechanisms from pore refinement to crack bridging. Compared with plain RAC, the hybrid system delivered a distinct dosage-dependent synergy. The mix with 0.3% whiskers showed strong positive synergy in compression (synergy index = 1.63). Microstructural results indicate that CW primarily densifies the cementitious matrix and refines the recycled aggregate-related interfacial regions, including the old mortar-associated weak zones and the newly formed aggregate–paste ITZ, through micro-filling and hydration promotion, while BF primarily enhances crack bridging and stress transfer at the fiber–matrix interface. Their coupled action improves fiber anchorage, enables hierarchical crack control from microcrack suppression to macrocrack restraint, and shifts failure from brittle fracture toward pseudo-ductile multi-cracking behavior. The main practical trade-off is reduced workability, indicating that admixture optimization and mixing procedures are essential for field implementation. This study validates that the proposed multi-scale reinforcement strategy effectively transforms waste concrete into a robust, high-performance material for sustainable structural applications. • Multi-scale reinforcement of RAC using basalt fibers and CaCO 3 whiskers is proposed. • Hybrid system shows strong synergy with a 31.5% increase in tensile strength. • Whiskers densify the ITZ and refine pores, enhancing fiber bridging efficiency. • Reinforcement transforms brittle RAC failure into pseudo-ductile behavior. • Optimal synergy is achieved at 0.3% whisker and 0.1% basalt fiber content.