Wet grinding of mixtures of calcium hydroxide and silica gel with different molar ratios (Ca/Si) ranging from 0.5 to 3.0 was conducted in a planetary ball mill. Three hours of wet grinding enables us to synthesize tobermorite mechanochemically when the molar ratio of the mixture is controlled at about unity, while calcium silicate hydrate-(B) (CSH(B)) is mechanochemically synthesized within about 1.5 h. The presence of excess water, by about four times the mixture’s weight in the mill, leads effectively to the mechanochemical synthesis of tobermorite by grinding. Tobermorite synthesized mechanochemically shows similar thermal behavior to that synthesized hydrothermally. However, crystallinity of the former compound is slightly inferior to that of the latter one. The ground mixture containing tobermorite is thermally decomposed at about 1120 K, forming wollastonite via an amorphous phase, while the mixture with synthesized CSH(B) is decomposed at about 1100 K, forming epsilon-wollastonite via a transitional crystalline phase.