"PPC has ash in it, so it must be cheap cement" is one of the most common myths on Indian building sites. The chemistry says otherwise.
What Fly Ash Does
When cement hydrates, it forms calcium silicate hydrate (the main binder) and also releases calcium hydroxide, which contributes little strength and can leach out. Fly ash is a pozzolan: in the presence of water it reacts with that calcium hydroxide to form more calcium silicate hydrate. The result, over weeks and months:
- Denser concrete, with finer pores.
- Lower permeability, so water and chemicals penetrate less.
- Better resistance to sulphates and alkali-aggregate reaction.
- Lower heat of hydration, which reduces thermal cracking.
The Trade-Off: Time
| Age | PPC vs OPC (typical) |
|---|---|
| 3 – 7 days | PPC lower |
| 28 days | Close |
| 90 days and beyond | PPC continues gaining |
This is why PPC needs longer curing and patience before formwork comes off, and why OPC 53 is preferred where very early strength is essential.
Myths and Facts
| Myth | Fact |
|---|---|
| "Ash makes the cement weak" | PPC meets IS 1489 strength requirements and gains strength longer |
| "PPC is only for plaster" | PPC is used for foundations, masonry, plaster and, where specified, RCC |
| "Cheaper means lower quality" | It is cheaper because less clinker is burned |
| "PPC cracks more" | Its lower heat generally reduces thermal cracking |
Using PPC Well
- Cure for at least 10 days; up to 14 in Hyderabad's hot months.
- Keep props longer than for OPC.
- Do not mix PPC and OPC within one pour.
- Check the packing date — fresh cement matters for any type.
The Short Version
Fly ash makes PPC slower at first and better over time. Use it confidently for most of a house, cure it longer, and use OPC 53 only where your engineer needs early strength.
