Thermodynamic modelling of CaCO3 polymorphs during CO2 sequestration by cement slurry with the addition of MgCl2

Abstract

As the largest consumed man-made materials, the production of cement releases a huge amount of CO2 into the atmosphere and dramatically accelerates the global warming. However, the main mineral component of cement can solidify CO2 and convert it into stable calcium carbonate, thus producing low-carbon building materials. In this paper, a thermodynamic model was proposed to simulate the formation of different calcium carbonate polymorphs during the aqueous carbonation of Portland cement with the addition of the most common ionic crystal stabilizer MgCl2. Based on the variations of the calculated Gibbs free energy due to the substitution of magnesium ions for calcium ions, the model can be applied to quantify the proportion of different calcium carbonate polymorphs under different CO2 input amount, MgCl2 concentrations, and temperatures. By comparing the experimental results with the modelling results, the developed model was proved to be able to predict the proportion of calcium carbonate polymorphs with an average relative error of 5.51%. Through experimental and simulation studies, it has been found that 80–90 °C is the most favorable temperature range for the formation of aragonite in carbonation systems. When the concentration of MgCl2 exceeds 0.18 mol/L, calcium carbonate can exist as pure aragonite at near room temperature (25 °C). Considering both the maximum CO2 sequestration rate and the proportion of aragonite, the appropriate CO2 input range is M(CO2)/M(PC) = 0.45–0.5.