MICP using calcium-rich waste solution from acetic acid treatment as a calcium source to improve the properties of recycled coarse aggregate
Wiracha Thaue, Mitsuyasu Iwanami, Kazuhide Nakayama, Nobuhiro Chijiwa
Abstract
Due to the porous adhered mortar on the surface of recycled coarse aggregate (RCA), the mechanical properties and durability performance of recycled aggregate concrete (RAC) are inferior to natural aggregate concrete (NAC). Microbially induced calcium carbonate precipitation (MICP) is an innovative and environmentally sustainable approach to improve the quality of RCA. However, MICP requires a large amount of calcium source, in which CaCl 2 is the most commonly employed calcium source. The presence of chloride ions can cause steel corrosion, thereby compromising the durability performance of reinforced concrete. In addition, acetic acid treatment effectively enhances RCA quality while generating calcium-rich waste solution as a by-product. In this study, the feasibility of employing the generated calcium-rich waste solution as a sustainable alternative calcium source for MICP to improve the quality of low-quality RCA was evaluated. The effects of calcium concentrations and sources on the physical properties and microstructural characteristics of RCA, including morphology and polymorph of bacterial-induced calcium carbonate (CaCO 3 ) crystals and pore structure of RCA were investigated. The results show that treating RCA with MICP using the calcium-rich waste solution with the calcium concentration of 0.2 M as the calcium source had the greatest modification performance. The reduction in the water absorption of the RCA reached 17.9 %. Moreover, due to a large amount of rhombohedral calcite crystals precipitated within the pores, the cumulative pore volume of RCA decreased by 62.4 %. As a result of the enhanced pore structure of MICP-modified RCA and increased micro-hardness of the interfacial transition zone (ITZ) between the adhered mortar and new cement paste, the compressive strength and chloride ion penetration resistance of RAC were significantly improved. The findings of this study highlight its significant potential of calcium-rich waste solutions to promote broader acceptance in biotechnical engineering. This approach offers a sustainable and environmentally friendly method to reduce the use of chemical calcium sources and promote the broader use of low-quality RCA in durable concrete construction.