3CaOSiO2

3CaOSiO2, commonly written as 3CaO·SiO2 or Ca3SiO5, is the chemical composition of tricalcium silicate, one of the principal phases in Portland cement clinker. In cement chemistry it is abbreviated as C3S and is sometimes referred to by the mineral name alite.

In clinker, C3S is formed during high-temperature fusion of calcium oxide (CaO) and silicon dioxide (SiO2). It is a reactive calcium silicate that hydrates rapidly when mixed with water. The hydration reaction produces calcium silicate hydrate (C-S-H) gel and calcium hydroxide (Ca(OH)2), with the C-S-H phase largely responsible for the early strength development of cementitious systems. C3S contributes most of the early strength within the first days after setting, while other phases such as C2S contribute to later strength.

C3S content is a key parameter in cement performance. Higher proportions of C3S lead to faster strength gain and higher early-age heat of hydration, which can affect setting time and temperature rise in large pours. The proportion of C3S in Portland cement is controlled by clinker production conditions and the presence of other phases such as C2S, C3A, and C4AF. While 3CaOSiO2 itself is not typically used as a standalone material, its presence and proportion in clinker determine important properties of the final cementitious material.

See also: Portland cement clinker, calcium silicate hydrates, hydration of cement.