We have previously precipitated highly pure calcium carbonate (CaCO3) crystals from a filtered solution extracted in the course of the direct mineral carbonation of flue gas desulfurization gypsum. However, the maximum amount of pure CaCO3 obtained by the process was less than 5% of the gypsum. We thus attempted to manipulate the induction period by adding a polyacrylic acid (PAA), with the expectation of dissolving more Ca2+ in the solution. This was because it is known to inhibit the formation of CaCO3. The PM was actually found to prolong the induction period to over 1.5 h, with the addition of 2.7 g/L PAA specifically increasing the amount of dissolved Ca2+ to 60% of the gypsum. It was also found that the addition of PM lowered the overall carbonation yield via CO2(g) emission, thus reducing the CO2 dissolution. In addition, ammonium carbonate ((NH4)(2)CO3) was discovered to be usable for producing pure CaCO3 in the presence of PM. An in-situ liquid-phase X-ray diffraction of the filtered solution revealed the sequential formation of calcite crystals from the amorphous phase on exposure to air. The morphology of the obtained calcite comprised twinned pseudo-spherical or dumbbell-shaped crystals, unlike the typical rhombohedral calcite phase obtained without PAA. It is clearly demonstrated that the precipitation of clean CaCO3 in direct mineral carbonation of FGD gypsum can be enhanced by additives as PM for a better economic viability. (C) 2016 Elsevier B.V. All rights reserved.