The addition of fly ash in the mix design results in reduction in: (i) permeability (ii) chemical attack (iii) heat of hydration (iv) initial setting time
Correct Answer: D. Only (i), (ii) and (iii)
📚 Detailed Explanation: Fly Ash Reduces (i) Permeability, (ii) Chemical Attack, (iii) Heat — NOT Initial Setting Time
Why D (Only i, ii and iii) is correct: Fly ash is a pozzolanic supplementary cementitious material (SCM). Its effects on concrete are significant, but it is critical to know that fly ash does NOT reduce initial setting time — in fact, it extends it (retarding effect due to slower pozzolanic reaction). Therefore statement (iv) is incorrect.
Effects of Fly Ash on Concrete — Detailed
| Statement | Effect | Mechanism | Correct? |
|---|---|---|---|
| (i) Reduces permeability | YES | Pozzolanic reaction: SiO2 in fly ash + Ca(OH)2 → secondary C-S-H gel; fills capillary pores; denser microstructure | ✓ Correct |
| (ii) Reduces chemical attack | YES | Consumes Ca(OH)2 (the vulnerable compound to acid/sulfate attack); lower Ca(OH)2 = higher resistance to sulfate and acid attack | ✓ Correct |
| (iii) Reduces heat of hydration | YES | Fly ash reacts slowly (secondary/pozzolanic); replaces faster-reacting OPC clinker compounds (C3S, C3A) that generate most heat; 30% fly ash reduces heat by 50–60% | ✓ Correct |
| (iv) Reduces initial setting time | NO | Fly ash EXTENDS initial setting time (retarding effect); slower pozzolanic reaction | ✗ Wrong |
Fly Ash: Benefits vs. Limitations
| Benefits | Limitations |
|---|---|
| Reduced permeability (long-term denser matrix) | Slower early strength gain; lower 7-day strength |
| Improved workability (spherical particles) | Extended initial setting time |
| Reduced heat of hydration (mass concrete applications) | May increase requirements for wet curing duration |
| Higher ultimate strength (28d+) | Not suitable where early strength is critical (formwork removal, prestress transfer) |
| Reduced material cost; uses industrial waste | Variable quality depending on coal source and combustion temperature |
Typical Fly Ash Dosage:
15–35% replacement of OPC by mass (IS 456 and IS 1489)
30% replacement → 50–60% reduction in heat of hydration
Pozzolanic reaction: SiO2 + Ca(OH)2 + H2O → C-S-H (gel)
This secondary C-S-H fills pores → permeability reduction after 28 days
15–35% replacement of OPC by mass (IS 456 and IS 1489)
30% replacement → 50–60% reduction in heat of hydration
Pozzolanic reaction: SiO2 + Ca(OH)2 + H2O → C-S-H (gel)
This secondary C-S-H fills pores → permeability reduction after 28 days
- Fly ash reduces: (i) permeability, (ii) chemical attack susceptibility, (iii) heat of hydration.
- Fly ash does NOT reduce initial setting time — it actually extends it.
- IS 1489 governs Portland-Pozzolana Cement (PPC) where fly ash is blended with OPC.