Chemistry

Non-hydraulic cement, such as slaked lime (calcium hydroxide mixed with water), harden by carbonation in presence of the carbon dioxide naturally present in the air. First calcium oxide is produced by lime calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure:

CaCO₃ → CaO + CO₂

The calcium oxide is then spent (slaked) mixing it to water to make slaked lime:

CaO + H₂O → Ca(OH)₂

Once the water in excess from the slaked lime is completely evaporated (this process is technically called setting), the carbonation starts:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is small. The reaction of carbonation requires the air be in contact with the dry cement, hence, for this reason the slaked lime is a non-hydraulic cement and cannot be used under water. This whole process is called the lime cycle.
Conversely, the chemistry ruling the action of the hydraulic cement is hydration. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being:

rotary Kiln

Belite (2CaO·SiO₂);

Alite (3CaO·SiO₂);

Celite (3CaO·Al₂O₃);

Brownmillerite (4CaO·Al₂O₃·Fe₂O₃).

The silicates are responsible of the mechanical properties of the cement, the celite and the browmillerite are essential to allow the formation of the liquid phase during the cooking. The chemistry of the above listed reactions is not completely clear and is still the object of research.