The preparation of CaCO3 from a supersaturated solution of Ca(HCO3)(2) in a microwave oven was studied at 25 degrees C and pH of similar to 7.1 and 7.3, with and without colloidal SiO2 as an additive. SiO2 particles controlled the nucleation, size, and shape of crystals and were incorporated into the CaCO3/SiO2 nanocomposite. Incorporation of SiO2 increased with increased SiO2 addition into the solution and with increased rate of precipitation. Heating the above nanocomposite precipitate led to crystallization of calcium silicate at a low temperature of 450 degrees C, due to intimate contacts of CaCO3 and SiO2 in the composite.