<p>Calcium sources play an essential role in the microbially induced calcium carbonate precipitation (MICP) process, and their production and extraction have a significant impact on both carbon emissions and costs. This study introduced a novel bio-mineralization technique for sand stabilization using ammonium chloride (NH₄Cl) to leach calcium from recycled concrete fines (RCF) as an economical and sustainable calcium source. The mechanical properties were assessed through permeability and unconfined compression tests on bio-cemented samples. The underlying mechanism was explored through microstructural characterization and process monitoring, which included ion concentration analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation. Test results showed that NH₄Cl demonstrated efficient and rapid leaching, achieving suitable calcium ion concentrations and a moderately alkaline environment, which is ideal for the subsequent precipitation processes. RCF-NH₄Cl leachate can achieve comparable or even better mechanical improvements, compared with RCF-HCl leachate or chemical CaCl₂. The presence of NH₄⁺ ions and the lower pH conditions facilitated the stable formation of vaterite that has large specific surface area and high solubility, facilitating enhanced particle adhesion and short-term mechanical strength. Finally, a systematic life cycle assessment indicated that the new method based on RCF-NH₄Cl leachate offered lower energy consumption, reduced carbon emissions, and obviously higher cost-effectiveness.</p>