Compounds in cement mainly are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium alumino ferrite. Not only do these compounds control most of cement properties but also reacts with water to produce new materials (cement hydration) and consequently responsible for concrete strength.
For instance, tricalcium silicate hydrates and hardens rapidly, hence generates heat greatly whereas hydration of the other three compounds are slow and consequently heat of hydration would be much lower.
It is demonstrated that tricalcium silicate and dicalcium silicate provide most of concrete strength, but the contribution of tricalcium aluminate and tetracalcium alumino ferrite to the concrete strength are considerably low both at early strength and at ultimate strength. It is worth mentioning that tricalcium silicate is the only compound that provide high early strength to concrete.
Compounds in Cement and Their Influence on Concrete Strength
Tricalcium silicate and Dicalcium silicate
The chemical formula for Tricalcium silicate (C3S) and Tricalcium silicate (C3S) are 3CaO.SiO2 and 2CaO.SiO2, respectively. Two silicates, namely C3S and C2S, control most of the strength giving properties.
Upon hydration, both C3S and C2S give the same product called calcium silicate hydrate (C3S2H3) and calcium hydroxide. Tricalcium silicate (C3S) giving a faster rate of reaction accompanied by greater heat evolution develops early strength.
On the other hand, dicalcium silicate (C2S) hydrates and hardens slowly and provides much of the ultimate strength. It is likely that both C3S and C2S phases contribute equally to the ultimate strength of cement as can be seen in Fig. 1.
C3S and C2S need approximately 24 and 21 percent water by weight, respectively for chemical reaction, but C3S liberates nearly 3 times as much calcium hydroxide chemical reaction as C2S. However, C2S provides more resistance to chemical attack. That is why higher percentage of C3S results in rapid hardening with an early gain in strength at a higher heat of hydration.
On the other hand, a higher percentage of C2S results in slow hardening, less heat of hydration and greater resistance to chemical attack. The C2S is responsible for concrete strength beyond 7 days of age.
Tricalcium aluminate and Tetracalcium alumino ferrite
The chemical formula of tricalcium aluminate (C3A) and tetracalcium alumino ferrite (C4AF) are 3CaO.Al2O3 and 4CaO.Al2O3Fe2O3, respectively. The compound tricalcium aluminate (C3A) is characteristically fast reacting with water and may lead to an immediate stiffening of paste and this process is termed flash set.
The role of gypsum added in the manufacture of cement is to prevent such fast reaction. C3A reacts with 40 percent of water by mass, and this is more than that required for silicates. However, since the amount of C3A in cement is comparatively small, the net water required for the hydration of cement is not substantially affected.
It provides weak resistance against sulphate attack and its contribution to the development of strength of cement is perhaps less significant. Tetracalcium alumino ferrite hydrates rapidly but contribute little to concrete strength. The majority of Portland cement is because of C4AF.
Table 1 Percentage by Mass of Each Compound in Cement
| Compound | Percentage by mass in cement |
| C3S | 30 to 50 |
| C2S | 20 to 45 |
| C3A | 8 to 12 |
| C4AF | 6 to 10 |
Table 2 Contribution of Different Compounds in Cement to Concrete Strength to Early Strength and Ultimate Strength
| Cementing Value | C3S | C2S | C3A | C4AF |
| Early | Good | Low | Low | Low |
| Ultimate | Good | Good | Low | Low |