Effect of Additives and pH on the Formation of Carbonate Mineral by CO2 Sequestration of Cement Paste

Abstract:

CO₂ in the atmosphere causes a global warming that is a big issue nowadays. Many studies of CO₂ capture and storage (CCS) technologies have been studied all over the world. Waste cement is a good source for aqueous carbonation because it is rich in calcium. Therefore, this study was performed to develop the aqueous carbonation method for waste cement powder. Cement paste was made with water/cement ratio of 6:4 and cured for 28 days in water bath. The cement paste was pulverized into a fine powder sizing less than 0.15 mm. To study effect of additives and pH on the formation of carbonate minerals, aqueous carbonation experiments were conducted. The mineral compositions and morphology of carbonate mineral were identified by XRD and SEM/EDS analysis.

1.0 M NaCl and 0.25 M MgCl₂ were applied as additives. Aqueous carbonation experiment was conducted with injecting pure CO₂ gas (99.9%) to a reactor containing 200 ㎖ of reacting solution. The pH of reacting solution was controled to determine formational condition of carbonate minerals. In 0.25 M MgCl₂ solution, calcite was dominant mineral at high pH. More aragonite, however, formed as decreasing pH of solution with injection of CO₂. The presence of Mg²⁺ in solution makes aragonite more dominant than calcite. Aragonite was mainly formed at the high pH of solution with 1.0 M NaCl additive, whereas calcite was more preponderant mineral than aragonite as falling pH. It show that unstable aragonite transformed to calcite as decreasing pH. In no additive solution, vaterite was dominantly formed at the initial stage of experiement, but unstable vaterite transformed to well crystallized calcite with further carbonation.