JNTU B.Tech III CONCRETE TECHNOLOGY Previous Question Paper (August - 2022 - R18)

Ordinary Portland Cement (OPC) is manufactured primarily from calcareous and argillaceous materials. The key raw materials are limestone (calcium carbonate) and clay or shale (silica, alumina, and iron oxide).

The functions of these materials are determined by their chemical oxides:

• Lime (CaO): Derived from limestone, it is the most critical component. It forms the strength-giving compounds like Tricalcium Silicate and Dicalcium Silicate. Excess lime causes expansion and unsoundness, while deficiency reduces strength.
• Silica (SiO2): Primarily from clay, it provides strength through the formation of calcium silicates. It contributes to the slow development of strength.
• Alumina (Al2O3): Acts as a flux to lower the clinkering temperature and is responsible for the initial setting time of cement.
• Iron Oxide (Fe2O3): Imparts the characteristic grey color to cement and assists in the fusion of raw materials during the burning process.
• Magnesia (MgO): Provides color and hardness but must be limited to prevent long-term expansion and cracking.

Mineral admixtures are finely divided siliceous or aluminous materials added to concrete to improve its properties, enhance durability, or replace a portion of Portland cement to reduce costs and environmental impact. They react chemically with calcium hydroxide, a byproduct of cement hydration, to form additional cementitious compounds (pozzolanic reaction).

Two common mineral admixtures are:

• Fly Ash: A byproduct of coal-fired power plants, fly ash is a pozzolanic material that improves the workability of fresh concrete and reduces water demand. It refines the pore structure, leading to lower permeability and increased resistance to sulfate attack and chloride penetration. Its use also reduces the heat of hydration, which is beneficial for mass concrete structures.
• Ground Granulated Blast Furnace Slag (GGBS): A byproduct of the iron manufacturing industry, GGBS is latent hydraulic. When mixed with cement, it reacts to form calcium silicate hydrate (C-S-H) gel. It significantly improves the long-term strength of concrete, enhances resistance to chemical attack, and produces a lighter-colored finish. It is widely used in marine environments due to its superior resistance to chloride-induced corrosion.