From Waste to Crystal: Synthesis of Aragonite Whiskers from Recycled Concrete Fine Powder via a Novel Carbonation Method

Recycled concrete fine (RCF) powder is a byproduct generated during the crushing of waste concrete, with a significant portion typically ending up in landfills. In this research, a novel leaching-carbonation process was proposed to synthesize aragonite, aiming to achieve effective CO2 sequestration and high-value utilization of RCF. The effect of carbonation conditions on the morphologies and mineral compositions of calcium carbonate (CC) was investigated, and the mechanisms governing the aragonite formation and growth were explored. Aragonite whiskers with a high aspect ratio of 10:1 were successfully obtained at 80 °C with the addition of 25 mmol of Mg, a CO2 flow rate of 500 mL/min, and a stirring speed of 200 rpm. Based on the results, the formation of aragonite could be divided into four stages: (1) CaCl2–Mg(OH)2-NH4OH system, (2) MgCl2–Ca(OH)2-NH4OH system, (3) CaSO4–MgCl2–NH4OH system, and (4) CaCO3–Mg(OH)2 system. Throughout these stages, various transformations occurred in CaCO3 particles: amorphous calcium carbonate (ACC)–ACC agglomerate–small particle–elongated aragonite–stout aragonite. When applied to real RCF, 1 kg of RCF was capable of sequestering 106.5 g of CO2 while producing 24.2 g of high-purity aragonite whiskers.