What is the effect of fly ash on concrete?
Correct Answer: D. Reduces the heat of hydration
📚 Detailed Explanation: Fly Ash Reduces Heat of Hydration
Why D (Reduces the heat of hydration) is correct: Fly ash is a slow-reacting pozzolanic material. It does not hydrate directly; instead it reacts with Ca(OH)2 (a by-product of cement hydration) to form additional C-S-H gel. This secondary reaction is slow and generates far less heat than the primary hydration of OPC compounds. Replacing OPC with fly ash therefore significantly reduces the total heat generated.
Effects of Fly Ash on Concrete Properties
| Property | Effect of Fly Ash | Reason |
|---|---|---|
| Heat of hydration | Decreases (50–60% at 30% replacement) | Slow pozzolanic reaction vs. rapid C3S/C3A hydration |
| Workability | Improves | Spherical glassy particles act as ball bearings; reduce inter-particle friction |
| Permeability | Decreases (long-term) | Secondary C-S-H gel fills capillary pores |
| Early strength (7 day) | Slightly lower than OPC | Slower pozzolanic reaction; less early C-S-H |
| Ultimate strength (90d+) | Increases | Continued pozzolanic reaction produces more C-S-H over time |
| Cost | Reduces | Fly ash is cheaper than OPC; industrial waste utilisation |
Pozzolanic Reaction of Fly Ash
Primary OPC hydration (fast, high heat):
C3S + H2O → C-S-H (gel) + Ca(OH)2 [rapid; high heat]
C3S + H2O → C-S-H (gel) + Ca(OH)2 [rapid; high heat]
Pozzolanic reaction of fly ash (slow, low heat):
SiO2 (from fly ash) + Ca(OH)2 + H2O → C-S-H (gel) [slow; low heat]
Net effect: fly ash consumes Ca(OH)2 (a weak link in concrete)
and produces more C-S-H, improving both strength and impermeability long-term.
Fly ash heat reduction:
30% fly ash replacement: 50–60% reduction in heat of hydration
Why Other Options Are Wrong
| Option | Why Incorrect |
|---|---|
| A. Reduces secondary hydration | Fly ash IS the secondary/pozzolanic hydration; it participates in it, does not reduce it |
| B. Increases heat of hydration | Opposite; fly ash reduces heat by replacing fast-reacting OPC clinker |
| C. Increases air entrainment | Fly ash is not an air-entraining agent; it does not significantly increase air content |
| D. Reduces heat of hydration | Correct; the primary and most significant thermal effect of fly ash |
- Fly ash reduces heat of hydration — primary benefit in mass concrete (dams, raft foundations).
- Also improves workability, reduces permeability, increases ultimate strength, and lowers cost.
- IS 1489 covers Portland-Pozzolana Cement (PPC) blended with fly ash (15–35% replacement).