A value-added method for cement mortar preparation: Utilizing pre-carbonated concrete wastewater

Concrete wastewater (CWw), a byproduct of ready-mixed concrete plants, poses significant disposal challenges due to its high alkalinity and residual chemical admixtures. Reusing CWw directly as mixing water in new concrete can adversely affect workability and durability. To address this issue, this study proposes a novel pre-carbonation method using low-concentration CO 2 to treat CWw before its reuse in mortar production. CO 2 gas mixtures with varying concentrations (0%, 25%, 50%, 75%, and 100% by volume) were bubbled through synthetic CWw. The treatment effectively mitigated the rapid fluidity loss and accelerated setting typically induced by untreated CWw, restoring workability as the CO 2 concentration increased. The key mechanism involves the reaction of CO 2 with Ca 2+ and OH ⁻ ions in CWw to precipitate CaCO 3 . This in-situ generated CaCO 3 acted as both a filler and a nucleation site, promoting cement hydration, refining the pore structure, and enhancing the compressive strength of the mortar. An optimal CO 2 concentration of 75% was identified, which increased the 28-day compressive strength by approximately 3.5% compared to mortar made with untreated CWw. Furthermore, the pre-carbonation process significantly reduced the autogenous shrinkage of the mortar, thereby lowering its cracking risk. The proposed method not only improves the performance of CWw-incorporated mortar but also offers a promising pathway for the synergistic recycling of industrial wastewater and low-concentration CO 2 , presenting notable economic and environmental benefits.