The use of waste concrete sludge as alkali source to recover reactive MgO from reject brine

This study investigated the use of waste concrete sludge as an alkali source for MgO recovery from reject brine, aiming to reduce energy consumption and carbon footprint of synthesized MgO. Through simple soaking and filtration procedures, alkaline water was extracted from waste concrete sludge with a pH of 12.2. Brucite can be successful precipitated from the reaction between alkaline water and reject brine with a high Mg²⁺ extraction ratio of 94 % from the reject brine. After calcination at 500 °C, the reactive MgO can be obtained with high purity (> 80 %) and significant specific surface area (> 75 m²/g). The production of MgO proposed in the current study has a much lower carbon footprint (0.20–0.41 tCO₂/tMgO) compared to that of MgO derived from chemical-based alkali (1.94–27.5 tCO₂/tMgO) in literature. Even when compared with Portland cement, the production energy (2.96–5.40 GJ/tMgO) and carbon footprint of MgO in the current study are lower than that of Portland cement (PC, 9.5 GJ/tPC and 1.2 tCO₂/tPC). Overall, this study has demonstrated the feasibility of using waste concrete sludge as alkali for MgO recovery with minimum energy consumption and CO₂ emission, which brought new potentials to produce alternative cementitious binders with enhanced sustainability and reduced cost.