Influence of Carbonated Calcium Generated from Waste Concrete Sludge on the Strength and Microstructural Properties of Portland Cement Mortar

Abstract. It is imperative to identify solutions to lower the carbon footprint of construction materials. One of the ways is to recycle and reuse waste concrete materials through indirect CO2 mineral carbonation. This study replaces Portland cement (PC) in mortar composites with up to 15% carbonated calcium (CC) generated from waste concrete at 5% intervals (i.e, 0%, 5%, 10%, 15%) by mass of PC. The slump and flow of the mortar composites (MC) developed (MC0, MC5, MC10, and MC15) were determined using the mini-slump cone and flow table tests. 50 × 50 ×50 mm cubes and 160 mm × 40 mm × 40 mm beam specimens were cured in a climate-controlled room at 23 ± 2 ℃ and a relative humidity of 65 ± 5% for 24 hours before being water cured for 7 and 28 days, respectively. The bulk density (BD), dry density (DD), compressive strength (CS), and flexural strength (FS) were determined. Additionally, the morphology of the hardened mortar was examined using scanning electron microscopy (SEM). The slump decreases from 8 mm in MC0 to 4 mm in MC15, and the BD is 2272–2462 kg/m3. The DD of the hardened mortar increases from 2104–2290 kg/m3. At 28 days, MC0, MC5, MC10, and MC15 exhibit CS of 16.8, 19.2, 22.8, and 24.9 MPa, respectively, and FS of 3.4, 3.7, 3.9, and 4.3 MPa. The microstructural analysis shows that the rhombohedral-shaped CC matrix is formed around the cement particles, depicting a whitish hydration product termed calcium silicate hydrate, with minimal visible porosity as CC increases in the SEM images. Conclusively, 15% CC inclusion in MC exhibited a 50% decrease in slump. Also, BD, DD, CS, and FS increased by 7.7%, 8.1%, 33%, and 2.1%, respectively, at optimum CC content due to the dense microstructure revealed in the SEM.