Cement is a material that is composed of several compounds, including tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium alumino ferrite. These compounds not only determine the properties of cement but also react with water to form new materials through a process called cement hydration. This chemical reaction is responsible for the strength of concrete.
Among the compounds found in cement, tricalcium silicate is known to harden rapidly and generate a significant amount of heat during hydration. On the other hand, the hydration of dicalcium silicate, tricalcium aluminate, and tetracalcium alumino ferrite is slower, resulting in a lower heat of hydration.
Studies have shown that tricalcium silicate and dicalcium silicate contribute the most to the strength of concrete, while the contribution of tricalcium aluminate and tetracalcium alumino ferrite to both early and ultimate strength is relatively low. Additionally, tricalcium silicate is the only compound that provides high early strength to concrete.
Compounds in Cement and Their Influence on Concrete Strength
Tricalcium silicate and Dicalcium silicate
Tricalcium silicate (C3S) and dicalcium silicate (C2S) are two important compounds in cement, which play a significant role in determining its strength properties. The chemical formula for C3S is 3CaO.SiO2, while the formula for C2S is 2CaO.SiO2.
When these silicates undergo hydration, they both produce calcium silicate hydrate (C3S2H3) and calcium hydroxide. However, C3S exhibits a faster rate of reaction and generates more heat, resulting in the development of early strength in cement. On the other hand, C2S hydrates and hardens more slowly, but it contributes significantly to the ultimate strength of cement.
Both C3S and C2S phases are crucial in determining the overall strength of cement, as shown in Figure 1. While C3S provides early strength, C2S contributes to the long-term and ultimate strength of cement.
Fig. 1: Contribution of Cement Compounds to the Strength of Concrete
C3S (tricalcium silicate) and C2S (dicalcium silicate) are two main compounds in cement that require water for their chemical reaction. C3S needs approximately 24 percent water by weight, while C2S requires about 21 percent. However, C3S releases nearly three times as much calcium hydroxide during the chemical reaction as C2S. Despite this, C2S provides better resistance to chemical attack.
A higher percentage of C3S in cement results in rapid hardening with early strength gain and higher heat of hydration. On the other hand, a higher percentage of C2S leads to slower hardening, less heat of hydration, and greater resistance to chemical attack. C2S is responsible for concrete strength beyond 7 days of age.
Tricalcium aluminate (C3A) and tetracalcium alumino ferrite (C4AF) are other compounds in cement with chemical formulas 3CaO.Al2O3 and 4CaO.Al2O3Fe2O3, respectively. C3A is known for its fast reaction with water, which can cause immediate stiffening of the cement paste, termed flash set. Gypsum is added in the manufacture of cement to prevent such fast reaction. C3A requires 40 percent of water by mass for its hydration, which is more than what is needed for silicates. However, since the amount of C3A in cement is relatively small, the overall water requirement for cement hydration is not significantly affected.
C3A provides weak resistance against sulphate attack and its contribution to the development of cement strength is less significant. On the other hand, C4AF hydrates rapidly but contributes little to concrete strength. The majority of Portland cement strength is due to C4AF.
Fig. 2: Rate of Hydration of Pure Cement Compound
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 |