Which of the following statements with regard to Portland cement are correct? 1. Silica imparts strength to cement. 2. Alumina makes the cement quick setting. 3. Excess of lime increases the strength of cement. 4. Calcium sulphate decreases the initial setting time of cement. Select the correct answer using the code given below.

Portland cement's properties are determined by its chemical composition. Silica (SiO2) is a primary ingredient that reacts with lime to form dicalcium and tricalcium silicates, which are responsible for imparting strength and durability to the cement [1]. Alumina (Al2O3) acts as a flux and imparts quick-setting properties by facilitating the formation of tricalcium aluminate. While lime (CaO) is essential for strength, an excess of lime is detrimental as it causes the cement to expand, crack, and become unsound rather than increasing its strength [1]. Calcium sulphate, typically added in the form of gypsum, acts as a retarder; it increases the initial setting time to prevent 'flash set,' allowing for better workability. Therefore, only statements 1 and 2 are correct.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > 13.5. Some Rock-Forming Minerals > p. 175