Recycled aggregate mortar enhanced by microbial calcite precipitation

Abstract

The reuse of construction and demolition waste (CDW) is hampered by its high water absorption and weak bonds with new cement paste. This problem is more severe for recycled fine aggregates (RFAs) that are involuntarily generated during the production of recycled coarse aggregates. To explore the feasibility of reusing RFAs in construction, microbial carbonate precipitation (MCP) is presented in this study to consolidate the RFAs derived from CDW. The effects of calcium sources on bacterially induced MCP were investigated, and the efficiency of MCP for improving RFAs was evaluated in terms of physical and mechanical properties. Results show calcium chloride is an optimal calcium supplement that is useful for generating calcium carbonate, as compared with the effect of calcium nitrate. MCP treatment has improved the quality of RFAs, with decreases in water absorption, crushing value and the powder content. Furthermore, mortars containing 100% treated RFAs exhibited better workability and improved compressive strength, with a maximum increment of 32% after 28 d of curing. The pore structures of mortars containing modified RFAs were densified, with decreased porosities detected by the mercury intrusion porosity method. X-ray diffraction and scanning electron microscopy revealed the precipitation of calcium carbonate on the RFAs.