Thermodynamic calculation of CaCO3 polymorphs from aqueous carbonation of Portland cement with the addition of organic additives

The carbonation curing of cement-based materials has been widely recognized as one of the most promising technologies for CO₂ storage and utilization. Calcium carbonate is the main carbonation product of cement-based materials, which includes three anhydrous polymorphs: cubic calcite, needle-like aragonite, and amorphous vaterite. Products composed of aragonite with a large aspect ratio are inclined to develop whisker-like structures, which confer enhanced flexural strength and toughness. In this paper, a thermodynamic model for the formation of different CaCO₃ polymorphs during the aqueous carbonation with organic additives that selectively promote aragonite formation is proposed. The effects of four organic additives including polyacrylic acid (PAA), polyacrylamide (PAM), polyvinyl alcohol (PVA) and monoethanolamine (MEA) on the proportion of different CaCO₃ polymorphs produced during carbonation were quantified. By comparing the literature data and experimental results with the modelling output, the average error of the model for the four different organic additives (PAA, PAM, PVA, MEA) is 2.70%, 4.61%, 3.05% and 3.71% respectively. Through calculation, the thermodynamic mechanism of the selective adsorption of organic additives on the surface of aragonite has been revealed. The carbonation parameters, including temperature, CO₂ input and additives concentration have been found to specifically affect the polymorphs of CaCO₃ in three aspects: 1) adjusting the effective concentration of organic additives adsorbed on the surface of calcium carbonate; 2) altering the difference in surface energy and critical nucleation Gibbs free energy between aragonite and calcite; 3) regulating the reduction in surface energy attributed to per mole organic additive.