Evaluating the efficacy of concrete washout water utilization in wet carbonation of hydrated cement waste considering temperature effects

Wet carbonation is a promising method for recycling hydrated cement waste (HCW) while permanently sequestering CO 2 . However, the large amount of fresh water required in wet carbonation challenges its practical implementation. Concrete washout water (CWW) generated in ready-mixed concrete plants is high-alkalinity wastewater with adverse impacts on the environment that must be neutralized before disposal. To address these challenges, this study explores the feasibility of using CWW as a liquid medium in the wet carbonation of HCW. The effect of reaction temperature (20 °C, 45 °C, and 60 °C) on carbonation efficiency, growth of calcium carbonate phases (CaCO 3 ), crystallinity, and silicate polymerization is investigated. The results reveal that the main carbonation products are calcite, decalcified tobermorite-like C-S-H gel (with a low Ca/Si ratio), and amorphous silica gel. During the first 10 min of carbonation, the dissolution rates of crystalline and amorphous calcium silicate phases of HCW in CWW are slightly slower than those in distilled water, resulting in ⁓3 % lower CaCO 3 precipitation. Using elevated temperatures promotes the formation, crystallization, and growth of calcite crystals, owing to a significant impact on the carbonation of C 2 S and C-S-H gel at longer durations (> 10 min). However, the increased stress forces induced by structural mismatch within the precipitated CaCO 3 at 60 °C leads to a ⁓5 nm reduction in crystal size as the carbonation reaction progresses from 10 min to 60 min. • The carbonation of hydrated cement waste (HCW) in concrete washout water (CWW) is investigated. • The carbonation rate of C-S-H gel is slower in CWW than in distilled water (DW). • The morphology of calcite develops from irregular clusters to a rhombohedral structure at higher temperatures. • The crystallite size of calcite decreases with prolonged reaction duration at 60 °C. • A carbonation degree comparable to that in DW is achieved after 60 min.