Fly ash reduces the quantity of water requirement by
Correct Answer: C. 3 to 5 per cent
📚 Detailed Explanation: Fly Ash and Water Demand Reduction
When fly ash replaces a portion of Portland cement in a concrete mix, it changes the particle shape distribution of the cementitious materials. Unlike cement particles (which are irregular angular particles), fly ash consists predominantly of smooth, spherical glassy particles. This spherical shape produces the “ball-bearing effect” that reduces friction in the paste and thereby reduces water demand.
Why C (3 to 5 per cent) is correct: Standard concrete technology references and IS 3812 guidelines indicate that fly ash reduces water demand by approximately 3–5% compared to a reference OPC mix at the same workability. This reduction comes from: (1) The spherical morphology of fly ash particles reducing internal friction; (2) Fly ash particles filling micro-voids in the paste, reducing the water needed for lubrication. Option A (1–3%) is too conservative; option D (5–10%) is an overstatement for typical Class F fly ash; 3–5% is the well-established standard figure.
Fly Ash Benefits in Concrete
| Property | Effect of Fly Ash |
|---|---|
| Water demand | Reduces by 3–5% (ball-bearing effect) |
| Workability | Improves for same water content |
| Heat of hydration | Reduces significantly (pozzolanic reaction is slow) |
| Early strength | Slightly lower than OPC |
| Long-term strength | Equal or higher than OPC (pozzolanic contribution) |
| Permeability | Lower (denser microstructure from C-S-H formation) |
Key Concepts for Students
- The 3–5% water reduction is the primary cited figure in IS 10262 and standard concrete mix design guides for fly ash concrete.
- IS 456 permits up to 35% cement replacement by fly ash; IS 1489 Part 1 specifies Portland Pozzolana Cement (PPC) made with 15–35% fly ash.
- Fly ash concrete gains strength more slowly but achieves better long-term durability (lower permeability = higher resistance to sulphate, chloride, and carbonation).