The precipitation of calcium carbonate was studied by adding the sodium carbonate solution to a mixed system such as Still effluent (i.e., CaCl2 + NaCl solution) of the soda ash manufacturing (Solvay) process, and the effects of controlling factors on calcium carbonate polymorphs were investigated. The products were characterized with scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and powder X-ray diffraction analysis. The particle size distribution of products was determined as dry powder using laser diffraction technique. It was found that the dilute solutions of the reactants favor the aragonite formation without affecting much the particle size. The formation of calcite is favored with the increase in sodium carbonate concentration. The reaction temperature has no significant effect on polymorphism as well as particle size of calcium carbonate precipitated from solutions having concentration 1.25 M Ca2+ and 1.25 M CO32- and reaction time of 30 min. However, the mode of mixing the reactants has a remarkable effect on the type of polymorph formed and its morphology. When sodium carbonate solution is added to Still effluent maintained at 95 degrees C, mostly calcite is formed. When the mode of mixing the reactants is reversed, a mixture of calcite and aragonite is formed under identical experimental conditions. In the case of simultaneous addition of both the reactants, aragonite formation is favored. These results indicated that a selective type of polymorph can be produced simply by changing the mode of mixing the reactants. Various morphologies of calcium carbonate crystals such as rhombohedral and needlelike were produced, depending on the preparation conditions.