Which of the following compounds of cement is responsible for early strength?
Different Properties of cement compound
These compounds contribute to the properties of cement in different ways.
Tricalcium silicate (C3S):Tricalcium silicate compound is largely responsible for the cement's rapid hardening and early strength gain. The increase in the percentage of this compound causes early strength in cement and is responsible for generating more heat from hydration reactions. High heat leads to rapid strength gain and drastically reduces workability. Tricalcium Silicate is one of the key clinker compounds, which form C-S-H, heat, and lime when reacts with water.
Tricalcium aluminate (C3A): Tricalcium aluminate is the compound of cement in which hydration reaction takes place first, thus its contribution in the development of strength is highest in the first week of concrete, cement, and mortar. It emits large quality of heat and attributes to rapid gain in strength. A high concentration of Tricalcium aluminate is accountable for low resistance to sulfate and escalation in shrinkage in drying mass. Therefore less percentage of Tricalcium aluminate in cement will result in low heat of hydration, good sulfate resistance, and good strength gain. Tricalcium aluminate liberates heat during the early stages of hydration and reacts with the gypsum when mixed with water to produce ettringite and heat.
Tricalcium aluminate + gypsum + water — ettringite + heat
Dicalcium silicate (C2S): It is the important compound of the cement responsible for the development of slow strength but responsible for long term strength. Dicalcium silicate hydrates along with water form calcium silicate hydrates and lime:
Dicalcium silicates + water — calcium silicate hydrate + lime
Tetracalcium aluminoferrite (C4AF): The major role of Tetracalcium Aluminoferrite is to lower down the temperature of clinker. In C4AF hydration occurs at a very fast rate and it functions as filler, but its contribution to the strength gain process of cement is less. The grey color of cement is due to Tetracalcium Aluminoferrite, when it reacts with water and gypsum, it forms alumina hydroxides, lime and ettringite.
Gypsum (CaSO.2H20): It is a very important constituent of cement and it is hydrated calcium sulfate in chemical form. During the manufacturing process of the cement after the cooling of the clinker, a small amount of gypsum (3%) is added during the final grinding process. Gypsum is added to control the “setting of cement”, thus it plays a very important role in controlling the rate of hardening of the cement. Without gypsum, cement will be set immediately after mixing of water and its workability will considerably reduce as the time for placing concrete will reduce.