Improving Recycled Concrete Aggregate Performance via Microbial-Induced Calcium Carbonate Precipitation: Effects of Bacterial Strains and Mineralization Conditions

The use of recycled coarse aggregates (RCA) in concrete production offers significant environmental and economic benefits. However, the high water absorption and low mechanical strength of RCA, caused by residual mortar and internal cracks, severely limit its application. This study employed microbial-induced calcium carbonate precipitation (MICP) technology to improve RCA performance, systematically investigating the effects of key parameters such as bacterial strains, bacterial concentration, modification duration, and urea addition sequence. This study employed microbial-induced calcium carbonate precipitation (MICP) technology to enhance the performance of RCA. The investigation systematically examined the effects of key parameters, including bacterial strains (Bacillus subtilis, urease mixed bacteria, and Bacillus pasteurii), bacterial concentrations (0, 2.4 × 107 cells/mL, 9.3 × 107 cells/mL, 2.49 × 108 cells/mL, and 2.36 × 109 cells/mL), modification durations (0 d, 3 d, 7 d and 14 d), and urea addition sequences (urea added to the calcium source, urea added to the culture medium, and added to the bacterial solution followed by 2 h of incubation). The impact of MICP treatment on RCA’s water absorption, apparent density and resistance to ultrasonic impact was analyzed. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) were used to characterize the microstructure and composition of calcium carbonate deposits, revealing the mechanisms by which MICP enhances RCA performance. The results showed that optimized MICP treatment reduced RCA water absorption by 32.5%, with the optimal conditions being a bacterial concentration of 2.4 × 107 cells/mL, a modification duration of 7 days, and a two-hour urea resting period. It is primarily due to calcium carbonate filling pores and sealing cracks, which significantly improves the structural integrity of RCA. This study demonstrates that MICP is an effective and sustainable method for RCA modification, providing theoretical support and practical insights for the recycling of construction waste and the promotion of green building materials.