Recycled coarse aggregate from waste concrete strengthened by microbially induced calcium carbonate precipitation

To realize the recycling of resources and reduce environmental pollution, waste sand was used as the inner core, waste powder and binding material were used as outer core, and the recycled coarse aggregate was prepared by using cold bonding ball forming technology. The comprehensive qualities were further improved by microbially induced calcium carbonate precipitation (MICP) that strengthening the inner core and outer core, respectively. This paper has studied the effects of the Ca^2 + concentration and biomineralized time on the mass gain and water absorption of the waste sand. The results show that the optimal experimental conditions for strengthening waste sand were Ca²⁺ concentration of 0.8 mol /L and biomineralized time of 3 days. Moreover, the surface morphology, phase composition and special functional groups of the recycled coarse aggregate were determined by Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The results show that spherical particles with a diameter of about 20 µm were filled in the cracks and pores, forming a layer of crystal protection layer with a dense structure. The comprehensive qualities of the recycled coarse aggregate were improved by analyzing the conglomeration rate, crushing index, apparent density, and water absorption rate. The results of the thermogravimetric analyses show that recycled coarse aggregate had high thermal stability and crystalline degree. Therefore, the qualities of recycled coarse aggregate strengthened by MICP can effectively alleviate the problem of insufficient aggregate supply. It is conducive to solve the problem of construction waste and realize sustainable development.