Chemical Requirements (LSF, C₃A, MgO, SO₃, IR, Alkali, Chloride)
IS 269 caps the chemical composition that governs cement behaviour: lime saturation factor (LSF) and the alumina/silica moduli, C₃A (sulphate-attack and heat driver), MgO (soundness/periclase), SO₃, insoluble residue (IR), loss on ignition (LOI), and alkali and chloride content. The chemistry explains and bounds heat of hydration, sulphate resistance, soundness and durability — it is verified by IS 4032 chemical analysis.
Key Requirements
•LSF and alumina/silica moduli within the specified range (clinker phase balance)
•C₃A limited — high C₃A → higher heat and lower sulphate resistance (cf. SRPC IS 12330 caps it tighter)
•MgO, SO₃, insoluble residue, loss on ignition within limits (MgO → soundness/periclase risk)
•Alkali content relevant to alkali-aggregate reaction (with reactive aggregate — IS 2386 Part 7)
•Chemistry verified by IS 4032 analysis on a representative sample (IS 3535)
Practical Notes
✓Chemistry should explain the physical behaviour: high C₃A/fineness → more heat; high MgO → soundness risk. A contradiction usually means a test/calibration fault, not a paradoxical cement.
✓Where reactive aggregate is possible, the cement alkali figure (IS 12813 AAS) drives the alkali-aggregate-reaction decision.
Common Mistakes
⚠Treating chemistry as academic — it bounds heat, sulphate resistance and soundness.
⚠Ignoring alkali content where alkali-reactive aggregate is in play.
⚠Accepting chemistry on a mill tag without representative sampling/IS 4032 testing.