The indirect CO₂ mineralization by using Ca/Mg-containing industrial alkaline by-products or wastes is a promising way for mitigation of CO₂ emissions and valorization of wastes, although plagued by the utilization of precipitated calcium carbonate (PCC) if low-value product is obtained. It is difficult to acquire specific PCC with targeted morphology for commercial use since many parameters change in the gas–liquid carbonation process. In this work, the phase transition and morphology evolution behavior of PCC precipitated during carbonation of 0.1 mol·L⁻¹ NH₄Cl Ca-rich solution were studied. Results show that the polymorph transformation from rhombohedral calcite to spherical vaterite was found during the gas–liquid carbonation process, which is different from the traditional carbonation process (aqueous carbonation of lime). A promising result is that pure spherical vaterite of industrial interest was obtained under the studied conditions. In addition, to achieve conversion of CO₂(g) into desired morphology of PCC, the feasibility of introducing additives (ethylene glycol and citric acid) into the gas–liquid carbonation system was also evaluated in terms of CO₂(g) adsorption and PCC polymorphs control. Finally, the mechanism of additives on the growth and morphology of crystalline CaCO₃ was investigated by FTIR and interpreted at the atomic level.