SSC JE Civil Engineering 23 Mar 2021 Shift 1 Paper

Concept: Sulphate attack:● Waterborne sulfates react with hydration products of the tri-calcium aluminate $(\mathrm{C}_3\mathrm{A})$ phase of portland cement, and with calcium hydroxide $(\mathrm{Ca}(\mathrm{OH})_2)$ to form an expansive crystalline product called ettringite(calcium sulphoaluminate hydrate). ● Expansion due to ettringite formation causes tensile stresses to develop in the concrete. ● When these stresses become greater than the concrete's tensile capacity, the concrete begins to crack. ● These cracks allow easy ingress for more sulfates into the concrete and the deterioration accelerates. ● Sulfates also cause the chemical disintegration of some of the cement hydration products. Principal factors that affect the rate and severity of sulfate attack are: 1. Permeability of the concrete. 2. The concentration of sulfates in the waterborne solution. 3. $\mathrm{C}_3\mathrm{A}$ content. 4. $\mathrm{Ca}(\mathrm{OH})_2$ content. Explanation ● $\mathrm{C}_3\mathrm{S}$ readily reacts with water and produces more heat of hydration. It is responsible for the early strength of concrete. ● The quality and density of calcium silicate hydrate formed out of $\mathrm{C}_3\mathrm{S}$ is slightly inferior to that formed by $\mathrm{C}_2\mathrm{S}$. ● A cement with $\mathrm{C}_3\mathrm{S}$ gets attacked by sulfate easily whereas $\mathrm{C}_2\mathrm{S}$ is offering better resistance against chemical attacks. $\mathrm{C}_2\mathrm{S}$ hydrates rather slowly. It is responsible for the later strength of concrete and produces less heat of hydration. ● The calcium silicate hydrate formed is rather dense and its specific surface is higher and the quality of the product of hydration of $\mathrm{C}_2\mathrm{S}$ is better than that produced in the hydration of $\mathrm{C}_3\mathrm{S}$. ● The hydrated aluminates do not contribute anything to the strength of concrete. Their presence is harmful to the durability of concrete, particularly where the concrete is likely to be attacked by sulphates. ● The calcium hydroxide also reacts with sulphates present in soil or water to form calcium sulphate which further reacts with $\mathrm{C}_3\mathrm{A}$ and cause deterioration of concrete. This is known as sulphate attack. ● To remedy the sulphate attack, the use of cement with low $\mathrm{C}_3\mathrm{A}$ content is found to be effective.