Cement

Portland cement consists of the following major compounds : tricalcium alumniate, tricalcium silicate (alite), dicalcium silicate (belite), tetracalcium aluminoferrite and gypsum.

Reactions

When water is added to cement, the following series of reactions occur

• The tricalcium aluminate reacts with the gypsum in the presence of water to produce ettringite and heat (▲H = 207 cal/g). Ettringite consists of long crystals that are only stable in a solution with gypsum.
• The alite is hydrated to produce calcium silicate hydrates (CSH), lime and heat (▲H = 120 cal/g). The CSH has a short-networked fiber structure which contributes greatly to the initial strength of the cement glue.
• Once all the gypsum is used up in the first reaction, the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals.
  • The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of sulfates, the crystals resort back into ettringite, whose crystals are bigger than the monosulfates (two-and-a-half-times). It is this increase in size that causes cracking when cement is subjected to sulfate attack.
• The belite also hydrates to form calcium silicate hydrates and heat (▲H = 62 cal/g).
  • Like the second reaction, the calcium silicate hydrates contribute to the strength of the cement paste. This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of belite to the strength of the cement paste will be slow initially. This compound is however responsible for the long term strength of portland cement concrete.
• The ferrite undergoes two progressive reactions with the gypsum. In the first of the reactions, the ferrite reacts with the gypsum and water to form ettringite, lime and alumina hydroxides (i.e. ferric aluminum hydroxide). The ferrite further reacts with the ettringite formed above to produce garnets.
  • The garnets only take up space and do not in any way contribute to the strength of the cement paste.

In the end, the hardened cement paste consist of the following :

1. Ettringite : 15 to 20%
2. Calcium silicate hydrates, CSH : 50 to 60%
3. Calcium hydroxide (lime) : 20 to 25%
4. Voids : 5 to 6% (in the form of capillary voids and entrapped and entrained air)

References

1. The Pennsylvania State University Department of Civil and Environmental Engineering and Instructional Systems "Concrete Clinic - Hydration of Portland Cement"