Cement and Admixtures

Cement and Admixtures

Portland cement is the most common hydraulic binder used in construction. It is a finely ground powder that reacts with water to form a hard, durable mass.

Portland Cement Types (ASTM C150)

• Type I: General purpose. Used when no special properties are required.
• Type II: Moderate sulfate resistance. Used in structures exposed to soil or water with moderate sulfate concentrations.
• Type III: High early strength. Used for rapid construction or in cold weather.
• Type IV: Low heat of hydration. Used in massive concrete structures (dams) to control temperature rise.
• Type V: High sulfate resistance. Used in structures exposed to severe sulfate action.

Heat of Hydration

Hydration is the exothermic chemical reaction between cement and water.

• $C_3S$: Responsible for early strength (first 7 days) and generates significant heat.
• $C_2S$: Contributes to long-term strength and generates less heat.
• $C_3A$: Reacts very quickly, generating high heat. Requires gypsum to control setting time.
• $C_4AF$: Reacts moderately, contributes little to strength.

Chemical Admixtures (ASTM C494)

Admixtures are materials other than water, aggregates, and hydraulic cement used as an ingredient of concrete or mortar to modify its properties.

• Type A: Water-reducing admixtures. Reduce water requirement for a given consistency.
• Type B: Retarding admixtures. Delay setting time (hot weather concreting).
• Type C: Accelerating admixtures. Accelerate setting and early strength development (cold weather).
• Type D: Water-reducing and retarding admixtures.
• Type E: Water-reducing and accelerating admixtures.
• Type F: High-range water-reducing admixtures (Superplasticizers). Allow significant water reduction (>12%).
• Type G: High-range water-reducing and retarding admixtures.

Mineral Admixtures (Supplementary Cementitious Materials)

• Fly Ash (ASTM C618): By-product of coal combustion. improves workability and durability.
• Slag Cement (ASTM C989): By-product of iron blast furnaces.
• Silica Fume (ASTM C1240): By-product of silicon metal production. Extremely fine, increases strength and impermeability.