Utilization of Citrus Peel Waste for Regulating Enzyme-Induced Carbonate Precipitation in Cement-Based Materials: Mechanical Performance and Freeze–Thaw Resistance

This study investigates citrus peel powder (CP) as an environmentally friendly admixture to regulate plant-derived urease (with soybean powder (SP) as the urease source) and to promote bio-mediated CaCO3 mineralization, thereby improving the mechanical and freeze–thaw (FT) resistance properties of cement-based materials. When CP is combined with urea and soybean urease, it exhibits a regulatory effect on urease activity. For the CPUD (CP-encapsulated urea combined with soy powder)-modified material with SP dosage in cement content of 0.2 wt%, the CP–urea modification treatment can effectively improve their mechanical properties and FT durability. The flexural and compressive strengths at 28 days are increased by 10.53% and 11.19%, respectively, compared to the blank group. After freeze–thaw cycles, the strengths are still 27.08% and 26.67% higher than those of the blank group, and their respective strength loss rates are 7.58% and −5.77% (negative indicating a net strength increase), compared with 21.31% and 9.48% for the blank group. X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy analyses reveal that CP–urea promotes the formation and effective packing of calcium carbonate. Mechanistically, CP establishes a stable hydrogen-bonding network with both urea and urease, exerting a dual regulatory effect: it enhances the electrophilicity of urea while also creating a physical mass transfer barrier to precisely control biomineralization. Notably, CP can be directly used without pretreatment, offering a sustainable strategy for citrus peel waste valorization.