Mineral carbonation, which is CO 2 fixation through a carbonation reaction using alkaline earth metals, is being investigated as a carbon capture and utilization method to reduce CO 2 atmospheric emissions. Concrete sludge is an alkali waste material from the concrete industry and contains abundant calcium components. We investigated the applicability of concrete sludge for mineral carbonation. In this study, gas containing CO 2 was bubbled through the model concrete sludge solution and the effects of the solid−liquid ratio, bubbling time, gas flow rate, and the partial pressure of CO 2 on the CO 2 fixation ratio and fixation rate were investigated. The CO 2 fixation ratio decreased with increasing CO 2 bubbling time, CO 2 flow rate, and CO 2 partial pressure. The CO 2 fixation rate increased with increasing CO 2 flow rate and CO 2 partial pressure. The formation of calcite, a form of calcium carbonate, was confirmed.
A new type of mineral carbonation process for concrete sludge, a waste of fresh concrete under hydration, was developed, and the carbonation performances of the process were examined by laboratory-scale experiments. The process is composed of two steps; filtration of concrete sludge and bubbling of CO 2 into the filtrate to form calcium carbonate. Model concrete sludge, a mixture of cement and water, was filtered through a cellulose filter after hydration for 24 h to obtain a solution containing dissolved calcium ions. Then, the model flue gas containing CO 2 (10%) was bubbled through the filtrate solution, and calcium carbonate was precipitated by the carbonation reaction. About 3% of calcium in the concrete sludge could be extracted into the filtrate in a single filtration step, and more than 95% of dissolved calcium was recovered as calcium carbonate by the bubbling of CO 2 . The obtained calcium carbonate was calcite with a high purity (>95%) and 5–10 μm. The solid residue (concrete sludge) after filtration was mixed with fresh water and filtered through a cellulose filter. Then, the model flue gas was bubbled into the filtrate solution for carbonation. This filtration–bubbling step was repeated 5 times, and 10.8% of calcium in the feed cement was extracted into the filtrates in total. More than 95% of the extracted calcium could be recovered as calcium carbonate with high purity (>97%), and the overall conversion of calcium in the feed cement to calcium carbonate was 10.1%. The purity of calcium carbonate and the calcium conversion were much higher than those for the direct bubbling method, where the model flue gas is bubbled into concrete sludge.
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