IS 455 fixes the permitted slag content range in the blended cement and the chemical limits (SO₃, MgO, chloride, sulphide). The GGBS itself must conform to IS 12089 (glass content, chemical moduli). The chemistry — low effective C₃A and free lime in the slag system — is the basis of PSC's sulphate/chloride resistance and low heat.
Key Requirements
•Slag content within the IS 455 permitted range; chemical limits (SO₃, MgO, chloride, sulphide) met
•GGBS conforms to IS 12089 (glass content / chemical moduli) — reactivity depends on it
•Low effective C₃A/free lime in the slag system → sulphate/chloride resistance & low heat
•Chloride contributes to the IS 456 total-chloride budget for RCC
•Verify by chemical analysis on a representative sample (IS 3535)
Practical Notes
✓PSC's chemical resistance comes from the slag chemistry (little of the C₃A/free lime sulphates attack) — but only if the slag is glassy/reactive (IS 12089).
✓Higher slag proportion increases chemical resistance and lowers heat, at the cost of slower early strength.
Common Mistakes
⚠Ignoring IS 12089 slag conformity (glass content) — non-reactive slag.
⚠Accepting chemistry on the bag/mill tag without representative testing.
⚠Overlooking the chloride contribution to the IS 456 budget.