Why Cement is grey in colour

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Upon addition of water Ordinary cement undergo Hydration and gains strength.

The reactions involved in this Hydration are :

Hydration of C4AF gives hydration products that are similar in many respects to those formed from C3A under comparable conditions, though typically they contain Fe3+ as well as Al3+. An iron (III) hydroxide gel and calcium ferrite gel are also possible products of C4AF hydration. The reactivity of the pure C4AF is, in general, much slower than that of the C3A.

Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime
C₄AF + 3CSH₂ + 3H ® C₆(A,F)S₃H₃₂ + (A,F)H₃ + CH

C4AF is responsible for the grey colour of cement.

The other reactions involved in the hydration of cement are :

Tricalcium silicate (3CaO•SiO2 = 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 + water ® calcium silicate hydrate + lime + heat
2C₃S + 6H ® C₃S₂H₃ + 3CH, D H = 120 cal/g

Dicalcium silicate (2CaO•SiO2 = 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 + water ® calcium silicate hydrate + lime
C₂S + 4H ® C₃S₂H₃ + CH, D H = 62 cal/g

Tricalcium aluminate (3CaO.Al2O3 = C3A)

The initial reaction of C3A with water in the absence of gypsum is vigorous, and can lead to “flash set” caused by the rapid production of the hexagonal crystal phases, C2AH8 (H = H2O) and C4AH19. Sufficient strength is developed to prevent continued mixing. The C2AH8 and C4AH19 subsequently convert to cubic C3AH6 (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 + water ® ettringite + heat
C₃A + 3CSH₂ + 26H ® C₆AS₃H₃₂, D 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 monosulfate aluminate hydrate
2C₃A + 3 C₆AS₃H₃₂ + 22H ® 3C₄ASH₁₈,