Characterization and optimization of a two-step carbonation process for valorization of recycled cement paste fine powder

In a previous study, a novel two-step carbonation technique was developed to convert finely crushed hardened cement paste into a calcium-rich and a silica-rich residue. This paper reports a further work on optimizing the two-step carbonation process by studying the influences of a series of experimental variables including particle size, Na₂CO₃ concentration, reaction time and temperature, CO₂ flow-rate, and CO₂ concentration on the carbonation processes. The characteristics of the chemical and physical properties of calcium-rich (mainly calcite) and silica-rich (silica and alumina-silica gel) residues formed were analyzed by using X-ray fluorescence (XRF), powdered X-ray diffraction (XRD), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR). It was found that the changes in the experimental variables of both Steps 1 and 2 mainly affected the amounts of new products formed, but only slightly changed the chemical structures of the new products. Through a series of comparative tests, the most efficient conditions for the two-step carbonation process was reported. Comparing to the previous exploratory reaction conditions of 24 h stirring and 7 h flow-through carbonation using pure CO₂ gas, the optimum conditions would be: i) in Step 1: a 5–10 wt% Na₂CO₃ solution, a reaction duration of 5–7 h under ambient temperature, ii) in Step 2: a CO₂ gas with a concentration of>50%, and the carbonation ending with a final pH ≤ 9.8 in less than 90 mins. Meanwhile, the estimated CO₂ uptake in Step 2 was 0.136 g per gram of RCP. In addition, the supernatant of Step 2 after centrifugation can be recycled and reused as the Na₂CO₃ solution in Step 1 for another reaction cycle after pH adjustment by NaOH.