The most common alternative material used in place of cement in cement concrete is
Correct Answer: B. fly ash
📚 Detailed Explanation: Fly Ash as the Most Common Cement Alternative
Supplementary cementitious materials (SCMs) are used to partially replace cement in concrete to reduce cost, reduce CO&sub2; emissions, and often improve specific properties. Among all SCMs, fly ash is the most widely used globally and in India.
Why B (fly ash) is correct: Fly ash is available in enormous quantities as a by-product of coal-fired thermal power plants — India produces over 220 million tonnes per year. It is classified as a pozzolana: it reacts with calcium hydroxide (Ca(OH)&sub2;) released during cement hydration to produce additional C-S-H gel, improving long-term strength and durability. IS 3812 governs fly ash use in India; IS 456 permits up to 35% replacement. While lime (A) and natural pozzolana (C) are also used, fly ash is by far the most common due to its availability and proven performance.
Common Supplementary Cementitious Materials (SCMs)
| SCM | Source | Max Replacement | Key Benefit |
|---|---|---|---|
| Fly Ash (Class F/C) | Coal power plants | Up to 35% (IS 456) | Most common; improves workability, durability |
| GGBS (Slag) | Steel plant slag | Up to 70% | High sulphate resistance |
| Silica Fume | Silicon/ferrosilicon production | Up to 10% | High strength, low permeability |
| Natural Pozzolana | Volcanic ash | Varies | Historical use (Roman concrete) |
Key Concepts for Students
- Fly ash reduces the water demand of concrete and decreases heat of hydration — beneficial in large mass pours.
- Class F fly ash (low calcium, from bituminous coal) is used in most structural applications in India.
- Fly ash is a pozzolan: it does not react with water alone; it needs the Ca(OH)&sub2; from cement hydration to develop its cementitious properties.