Study on the Mineralization Modification of Recycled Coarse Aggregate by Bacillus pasteurii and the Performance of Recycled Concrete

Abstract

Recycled aggregate concrete is an important approach for realizing the resource utilization of construction waste and alleviating the shortage of natural aggregates [1,2,4]. However, the engineering application of recycled coarse aggregate is severely limited by adhered old mortar, high porosity, and high water absorption [3-6]. In this study, microbially induced calcium carbonate precipitation (MICP) using Bacillus pasteurii was adopted to modify recycled coarse aggregate [7,13]. The optimal mineralization process was determined through single-factor tests, and the mechanical properties of modified recycled concrete with different replacement ratios were investigated at a fixed water-cement ratio of 0.45. In addition, the deformation and crack propagation behavior during the whole compression process was analyzed by digital image correlation (DIC). The results showed that the bacterial solution cultured for 24 h exhibited the best mineralization activity [9]. The optimum modification effect was achieved at a Ca2+ concentration of 0.50 mol/L, a urea concentration of 0.50 mol/L, and a mineralization duration of 7 d [16,19,20]. Under this condition, the water absorption of recycled coarse aggregate decreased by 46.72%, while the apparent density and bulk density increased significantly. Compared with unmodified recycled concrete, both the compressive strength and splitting tensile strength of the modified concrete were obviously improved at all replacement ratios [15,17-19]. The best comprehensive performance was obtained at a recycled aggregate replacement ratio of 50%, at which the 28 d compressive strength reached 37.62 MPa. The DIC results indicated that microbial mineralization modification effectively delayed local strain concentration, inhibited the early initiation and rapid propagation of interface cracks, and transformed the failure mode of concrete from brittle failure to quasi-ductile failure [10,12-15]. The results can provide theoretical support and technical reference for the green modification and engineering application of recycled concrete.