Tri calcium silicate

C₃S constitutes about 45 percent of the cement & is very important constituent from the consideration of strength giving properties.

C₃S hydrates and hardens rapidly and is largely responsible for initial set and early strength. In general, the early strength of Portland cement concrete is higher with increased percentages of C₃S. A convenient approximate rule assumes that C₃S contributes most to the strength development during the first 4 weeks and C₂S influences the gain in strength from four weeks onwards.

At the end of one year the two compounds viz. C₃S & C₂S, weight for weight, contribute approximately equally to the ultimate strength.

The influence of the other major compounds on the strength development of cement has been established less clearly.

A cement with higher C₃S content is better for cold weather concreting. 

Di calcium silicate

C₂S constitutes about 25 percent of the cement & is very important constituent from the consideration of strength giving properties.  

C₂S hydrates and hardens slowly and contributes largely to strength increase at ages beyond one week.

A convenient approximate rule assumes that C₃S contributes most to the strength development during the first 4 weeks and C₂S influences the gain in strength from four weeks onwards. At the end of one year the two compounds, weight for weight, contribute approximately equally to the ultimate strength.

The influence of the other major compounds on the strength development of cement has been established less clearly.

Tricalcium aluminate

Tricalcium Aluminate, C₃A liberates a large amount of heat during the first few days of hydration and hardening.

The influence of C₃A on strength development of cement has been established less clearly. C₃A contributes to the strength of cement paste at one or three days ,and possibly longer, but causes retrogression at an advanced stage, particularly in cements with high C₃A or (C₃A+ C₄AF) content. The role of C₃A is still controversial.

Cements with low percentages of C₃A are more resistant to soils and waters containing sulphates. The amount of tricalcium aluminate present may well be limited as in the case of sulphate resisting Portland cement, to prevent adverse reactions between the hydrate and sulphates from the environment which can result in swelling and cracking of the cement matrix.

The great advantage of tricalcium aluminate is its ability to combine with chlorides, so removing them from the liquid phase of the cement. Chloride ions, as we know, are one of the major causes of corrosion of embedded steel.

The presence of C₃A in cement is considered by and large undesirable: it continues little or nothing to the strength of the cement except at early ages, and when hardened cement paste is attacked by sulphates, expansion due to the formation of calcium sulphoaluminate from C₃A may result in disruption of the hardened paste.

However, C₃A acts as a flux and thus reduces the temperature of burning of clinker and facilitates the combination of lime & silica. For this reason C₃A is useful in manufacture of cement.  

Tetracalcium Aluminoferrite

Tetracalcium Aluminoferrite, C4AF, is the product resulting from the use of iron and aluminum raw materials to reduce the clinkering temperature during cement manufacture.

The role of C₄AF in the strength development is also debatable, but it can be said with certainty that there is no appreciable positive contribution.

Most color effects that make cement gray are due to C₄AF and its hydrates.

C₄AF also acts as a flux.

Graph showing Compressive Strength development in pastes of pure cement compounds (Mindess et al, 2003)

Graph showing Degree of hydration of different compounds of cement? 

Comparative table showing Characteristics of hydration of the cement components (Mindess et al. 2003):

Heat of hydration of different compounds of cement

Different compounds hydrate at different rate & liberate different quantities of heat. Table below illustrates the same:

Since, retarders are added to control flash setting properties of C₃A, the early heat of hydration is mainly contributed by hydration of C₃S (the qty of C₄AF being much lesser).

The total heat of hydration is very nearly equal to the sum of heats of hydration of individual compounds of cement. Thus, knowing the chemical compound composition of cement, its heat of hydration can be calculated with a fair degree of accuracy.