Skip to main content

Harmful ingredients of cement

Short answer

Excess free lime, magnesia (>6%), and SO3 (>2.5%) cause unsoundness by late expansion. High alkalis (>0.6% Na2O eq) cause efflorescence and alkali-aggregate reaction. Insoluble residue and loss on ignition beyond limits indicate poor burning or adulteration, reducing strength.

What are the harmful constituents (ingredients) of OPC?

Harmful constituents are the compounds in cement that, if present in excess, reduce strength, cause cracking or damage the concrete in the long run.

  1. Excess free lime (CaO): left unburnt, it hydrates slowly after setting and expands, causing unsoundness and cracks.
  2. Excess magnesia (MgO): above 6% (IS 269) it also hydrates late and causes expansion and cracking.
  3. Excess sulphur trioxide (SO3): more than 2.5% (3% when C3A is over 5%) forms excess ettringite, causing unsoundness and expansion.
  4. Alkalis (Na2O and K2O): limited to 0.6% (as Na2O equivalent); they cause efflorescence and alkali-aggregate reaction.
  5. Insoluble residue and loss on ignition: kept within limits (about 5% each for OPC in IS 269:2015); high values indicate poor burning, adulteration or stale (partly hydrated) cement.
  6. Excess alumina/iron oxide and uncombined silica: upset the compound balance and lower strength.

Their limits are fixed in IS 269 and checked by chemical analysis.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Civil Engineering Materials chapter 5 questions →