Novel dual-strain biomineralization method for performance enhancement of recycled coarse aggregate

The utilization of recycled coarse aggregate (RCA) conserves natural resources and minimizes construction waste. However, its inferior properties remain a major concern. Ureolytic bacterial treatment effectively enhances RCA; nevertheless, its associated ammonia emissions critically constrain practical applications, particularly in indoor environments. This study explored a dual-strain biomineralization method, which not only significantly enhanced the properties of RCA but also sharply reduced the ammonia emission. A specific strain with nitrifying and denitrifying ability was supplied to the ureolytic bacterial solution, composing a dual-strain bacterial solution. The dual-strain bacterial solution was adopted to treat RCA. The results demonstrated that the 14-day treatment significantly improved the RCA microstructure, yielding a 21.5 % reduction in water absorption, a 20.8 % decrease in crushing value, and a 4.1 % increase in apparent density. Notably, compared with ureolytic bacterial treatment, the ammonia release of this method decreased by 93.4 %. Compared to concrete with 50 % of untreated RCA, the compressive strength and tensile splitting strength of concrete with 50 % of treated RCA improved by 22.1 % and 6.9 %, respectively, and the relative dynamic modulus of elastic after 100 freeze-thaw cycles decreased by 78.0 %. The SEM, EDS, and XRD analysis showed that the dual-strain biomineralization modified the RCA by the bio-induced calcium carbonate crystals in calcite formation. This method effectively enhanced the performance of RCA with negligible release of ammonia, thereby overcoming one of the major long-term constraints that has restricted the practical implementation of biomineralization technology in industrial production.