Removal of free lime from thermally activated recycled cement powder using phosphoric acid modification

Abstract

Thermal activation of waste concrete or cement fines is a potential approach for recycling these into reactive cementitious materials. However, the presence of residual free lime (f-CaO) causes a substantial heat release upon contact with water. Moreover, its rapid setting significantly limits the practical application of thermally activated recycled cement (T-RC). To address these challenges, this study proposes the chemical modification of T-RC using phosphoric acid (PA) with isopropanol (IPA) as the solvent. The physicochemical transformations in the T-RC particles, including the variations in the free lime content, particle size distribution, specific surface area, phase assemblage, and morphology, were characterized under varying PA concentrations in the solvent. In addition, the fluidity, setting time, isothermal hydration heat, compressive strength, hydration products, and pore structure of T-RC paste with and without PA modification were compared systematically. The results show that treating T-RC with 3 wt% PA for 2 h effectively reduced f-CaO content from 14.10 % to 4.35 %, without significantly altering the contents of other reactive mineral phases. Compared to the unmodified T-RC paste, the mini-slump and the final setting time of the modified T-RC paste have extended from 54.22 mm to 187.11 mm, and 20 mins to 160 mins, respectively. Most importantly, PA modification helps densify the pore structure in the matrix of T-RC paste, and its compressive strength is greatly increased from 3.74 MPa to 13.30 MPa (a 255 % increase) at 1 day and from 16 MPa to 31.32 MPa (a 196 % increase) at 28 days.