Hydration of Portland Cement
Introduction
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 compounds (see 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 C2S 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)
Conclusion
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.
References:
Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall,
Inc., Englewood Cliffs, NJ, pp. 671.
Steve Kosmatka & William Panarese (1988): Design and Control of
Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.
Michael Mamlouk & John Zaniewski (1999): Materials for Civil and
Construction Engineers, Addison Wesley Longman, Inc.,