Alite

Alite is an impure form of tricalcium silicate, Ca3SiO5, sometimes formulated as 3CaO·SiO2 (C3S in cement chemist notation), typically with 3-4% of substituent oxides.

During clinker grinding, first step of partial dissolution of C3S involves hydration of superficial oxide ions and leads to a hydroxylated C3S surface.

%) (see the hereabove table) may help to understand why in particular conditions, if a sufficiently high temperature is reached in a lime kiln during enough time, alite can also be directly formed by pyrolizing only siliceous limestone (containing amorphous SiO2 impurities up to 25 – 30 wt. %).

[6] In the area of Tournai (Belgium), the Tournaisian limestones are particularly rich in amorphous silica and exploited as building stone and for making lime mortar since very ancient ages.

It is unknown if the cathedral builders of this area were aware of the hydraulic properties of their lime mortar or intentionally developed its use after their fortuitous finding.