Portland cement is a hydraulic cement, hence it derives its strength from chemical reactions between the cement and water. The process is known as hydration.
Cement consists of the following major (see compounds composition of cement):
- Tricalcium silicate, C3S
- Dicalcium silicate, C2S
- Tricalcium aluminate, C3A
- Tetracalcium aluminoferrite, C4AF
- Gypsum, CSH2
Chemical reactions during hydration
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:
Tricalcium aluminate + gypsum + water ® ettringite + heat
C3A + 3CSH2 + 26H ® C6AS3H32, D H = 207 cal/g
Ettringite consists of long crystals that are only stable in a solution with gypsum. The compound does not contribute to the strength of the cement glue.
- The tricalcium silicate (alite) is hydrated to produce calcium silicate hydrates, lime and heat:
Tricalcium silicate + water ® calcium silicate hydrate + lime + heat
2C3S + 6H ® C3S2H3 + 3CH, D 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 as per reaction (i), the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals:
Tricalcium aluminate + ettringite + water ® monosulfate aluminate hydrate
2C3A + 3 C6AS3H32 + 22H ® 3C4ASH18,
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 two-and-a-half times the size of the monosulfate. It is this increase in size that causes cracking when cement is subjected to sulfate attack.
- The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat:
Dicalcium silicates + water ® calcium silicate hydrate + lime
C2S + 4H ® C3S2H3 + CH, D H = 62 cal/g
Like in reaction (ii), 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 C
S 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 ettringite reacts with the gypsum and water to form ettringite, lime and alumina hydroxides, i.e.
- Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime
- C4AF + 3CSH2 + 3H ® C6(A,F)S3H32 + (A,F)H3 + CH
- the ferrite further reacts with the ettringite formed above to produce garnets, i.e.
- Ferrite + ettringite + lime + water ® garnets
- C4AF + C6(A,F)S3H32 + 2CH +23H ® 3C4(A,F)SH18 + (A,F)H3
The garnets only take up space and do not in any way contribute to the strength of the cement paste.
The hardened cement paste
Hardened paste consists of the following:
Ettringite – 15 to 20%
Calcium silicate hydrates, CSH – 50 to 60%
Calcium hydroxide (lime) – 20 to 25%
Voids – 5 to 6% (in the form of capillary voids and entrapped and entrained air)
It can therefore be seen that each of the compounds in cement has a role to play in the hydration process. By changing the proportion of each of the constituent compounds in the cement (and other factors such as grain size), it is possible to make different types of cement to suit several construction needs and environment.