Coupled influences of carbon dioxide (CO₂) and Bacillus mucilaginosus on the strength of recycled aggregate concrete

Curing recycled aggregates (RAs) and recycled aggregate concrete (RAC) with carbon dioxide (CO₂) has been proposed to enhance their performance. However, the carbonation is usually ineffective. Recently, Bacillus mucilaginosus, which produces carbonic anhydrase to potentially facilitate carbonation, has been explored to strengthen RAs but its applications to RAC are rarely considered. This study examines the coupled influences of CO₂ and B. mucilaginosus on RAC. Untreated RAs, carbonated RAs and bacterially carbonated RAs were adopted to prepare concrete specimens at two water-cement (w/c) ratios with air and CO₂ curing. Unconfined compressive strength of specimens and water absorption of RAs were measured. Chemical properties and microstructure of RAC were determined by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Results reveal that the combined method of carbonated RAs and B. mucilaginosus was the most effective at a high w/c ratio in improving the strength of RAC after curing. Bacterial specimens exhibited the greatest strength which was at least 22 % higher than untreated specimens. The improvement was contributed by substantial formation of calcium carbonate (CaCO₃), as indicated by FTIR and TGA, and lower water absorption of treated RAs. Microstructural analysis also shows that B. mucilaginosus can improve the strength by reducing and filling specimen pores. At a low w/c ratio, bacterial application became ineffective since CaCO₃ likely covered the specimen surface to hinder its further formation. Overall, this study is the first to provide guidance on developing RAC with better performance through CO₂ curing and carbonic anhydrase-producing bacteria, thereby promoting sustainability and carbon neutrality.