Wt% of aragonite, a CaCO3 polymorph, increased with higher hydraulic activity (C) of limestone in precipitated calcium carbonate (PCC) from the lime-soda process (Ca(OH)(2)-NaOH-Na2CO3). Only calcite, the most stable polymorph, was crystallized at hydraulic activity under 10 degrees C, whereas aragonite also started to crystallize over 10 degrees C. The crystallization of PCC is more dependent on the hydraulic activity of limestone than CaO content, a factor commonly used to classify limestone ores according to quality. The results could be effectively applied to the determination of polymorphs in synthetic PCC for eco-friendly paper manufacture.