Concrete Technology
Why Cement is grey in colour

Upon addition of water Ordinary cement undergo ย Hydration and gains strength.
The reactions involved in this Hydration are :
Hydration of ย C4AF ย gives ย hydration ย products ย that ย are ย similar in ย many ย respects ย to ย those ย formed ย from ย ย ย C3A ย under ย comparable ย conditions, though ย typically ย they contain ย Fe3+ย as well as ย Al3+. An ย iron (III) hydroxide gel ย and calcium ferrite gel ย are also ย possible ย products ย of ย C4AF ย hydration. The ย reactivity ย of ย the ย pure ย C4AF ย is, in general, ย much ย slower ย than ย that ย of ย the ย C3A.
ย
- Ferrite + gypsum + waterย ยฎย ettringite + ferric aluminum hydroxide + lime
- C4AF + 3CSH2ย + 3Hย ยฎย C6(A,F)S3H32ย + (A,F)H3ย + CH
C4AF ย is responsible for the grey colour of cement.
The other reactions involved in the hydration of cement are :
Tricalcium silicate (3CaOโขSiO2ย = C3S)
C3S on reaction with water produces C-S-H and calcium hydroxide, CH, (also known as Portlandite). The hyphens used in the C-S-H formula are to depict its variable composition: CSH would imply a fixed composition of CaO.SiO2.H2O. C/S ratios in C-S-H vary from 1.2 to 2.0, and H/S ratios vary between 1.3 and 2.1.
Tricalcium silicate + waterย ยฎย calcium silicate hydrate + lime + heat
2C3S + 6Hย ยฎย C3S2H3ย + 3CH,ย Dย H = 120 cal/g
Tricalcium silicate + waterย ยฎย calcium silicate hydrate + lime + heat
2C3S + 6Hย ยฎย C3S2H3ย + 3CH,ย Dย H = 120 cal/g
ย
Dicalcium silicate (2CaOโขSiO2ย = C2S)
The kinetics and hydration mechanism for C2S are similar to those of C3S, except that the rate of reaction is much slower. The hydration products are the same except that the proportion of CH produced is about one-third of that obtained on hydration of C3S.
Dicalcium silicates + waterย ยฎย calcium silicate hydrate + lime
C2S + 4Hย ยฎย C3S2H3ย + CH,ย Dย H = 62 cal/g
Dicalcium silicates + waterย ยฎย calcium silicate hydrate + lime
C2S + 4Hย ยฎย C3S2H3ย + CH,ย Dย H = 62 cal/g
ย
Tricalcium aluminate (3CaO.Al2O3ย = C3A)
The initial reaction of C3A with water in the absence of gypsum is vigorous, and can lead to โflash setโ caused by the rapid production of the hexagonal crystal phases, C2AH8 (H = H2O) and C4AH19. Sufficient strength is developed to prevent continued mixing. The C2AH8ย and C4AH19ย subsequently convert to cubic C3AH6ย (hydrogarnet), which is the thermodynamically stable phase at ambient temperature. Typically, gypsum is added to retard this reaction, though other chemical additives can be used.
Tricalcium aluminate + gypsum + waterย ยฎย ettringite + heat
C3A + 3CSH2ย + 26Hย ยฎย C6AS3H32,ย Dย H = 207 cal/g
Tricalcium aluminate + gypsum + waterย ยฎย ettringite + heat
C3A + 3CSH2ย + 26Hย ยฎย C6AS3H32,ย Dย H = 207 cal/g
The reaction products formed on reaction of C3A in the presence of gypsum depend primarily on the supply of sulfate ions available from the dissolution of gypsum. The primary phase formed is ettringite (C6AS3H32) (S = SO3). Ettringite is the stable phase only as long as there is an adequate supply of soluble sulfate. A second reaction takes place if all of the soluble sulfate is consumed before the C3A has completely reacted. In this reaction, the ettringite formed initially reacts with the remaining C3A to form a tetracalcium aluminate monosulfate-12- hydrate known as monosulfate or monosulfoaluminate (C4A SH12).
Tricalcium aluminate + ettringite + water ย ย monosulfate aluminate hydrate
2C3A + 3 C6AS3H32ย + 22Hย ยฎย 3C4ASH18,