Evaluation of CO2 supply method on CaCO3 formation in sludge water and effects of carbonated sludge powder on mortar strength

Sludge water generated from ready-mixed concrete plants is rich in calcium ions, and hydrated and unhydrated cement materials, making it promising for CO 2 capture. By introducing CO 2 into sludge water, a carbonated powdery material containing calcium carbonate (CC) can be produced and reused as a CO 2 -fixing cementitious material, thereby reducing the CO 2 emissions associated with the Portland cement production. This study aims to investigate the effective CO 2 supply method on CC formation and CO 2 fixation but also the effect of carbonated sludge powder (CSP) on the strength of mortar. Two CO 2 supply methods were explored: continuous and intermittent (stop-and-go) supply. The calcium hydroxide (CH) and CC contents, as well as CO 2 fixation were analyzed by thermogravimetry-differential thermal analysis (TG-DTA). Cement mortar samples were prepared by replacing 12.5 % and 25 % of the cement mass with CSP, and the compressive strength and activity index (SAI) of the mortars were evaluated. For continuous supply method, the lowest CO 2 flow rate (0.25 L/min) was the most efficient for generating the high CC content within a specific time frame. In stop-and-go supply method, the highest CC content was obtained in pH-controlled range of 10–12 and 270 kg-CO 2 /t of CSP was fixed with a lower cumulative CO 2 supply. The compressive strength of mortars decreased with increasing CSP replacement. The mortars with 12.5 % CSP exhibited SAI of > 85 % after 91 days of curing, which is similar to fly ash quality. Therefore, utilizing CSP as a CO 2 -fixing cementitious material can contribute to reducing CO 2 emissions. • Up to 30 L of cumulative CO 2 supply can be enough for achieving great CC content. • Continuous CO 2 supply had an advantage of time efficiency to reach high CC content. • Stop-and-go CO 2 supply required a lower cumulative CO 2 value to fix a higher CO 2 . • Compressive strength of mortars decreased as the replacement CSP ratio increased. • CSP did not negatively affect SAI, but it also had the benefit in CO 2 fixation.