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Clinker compounds and their functions

Short answer

Clinker compounds: C₃S gives early strength (7 days) and high heat; C₂S provides later strength and low heat; C₃A causes flash set (controlled by gypsum), high heat, poor sulphate resistance; C₄AF acts as flux, gives colour, little strength. C-S-H gel from C₃S and C₂S is the main strength source.

What are the compounds present in cement clinker? Explain their functions (role in setting and hardening).

Cement clinker is made by burning limestone and clay at about 1450 °C. It contains four main compounds (Bogue's compounds).

CompoundFormulaShortApprox. %
Tricalcium silicate3CaO·SiO₂C₃S40-50
Dicalcium silicate2CaO·SiO₂C₂S25-30
Tricalcium aluminate3CaO·Al₂O₃C₃A8-12
Tetracalcium alumino-ferrite4CaO·Al₂O₃·Fe₂O₃C₄AF8-12

Functions

  1. C₃S (alite): hydrates quickly and gives early strength (first 7 days); produces high heat of hydration. More C₃S gives rapid hardening cement.
  2. C₂S (belite): hydrates slowly and is responsible for the strength after 7 days, up to a year. Gives low heat and good resistance to chemicals.
  3. C₃A: reacts immediately with water and causes the initial flash set, controlled by adding 2 to 3% gypsum. It evolves large heat, adds some early strength, but makes cement weak to sulphate attack.
  4. C₄AF: acts as a flux that lowers the burning temperature; gives the grey colour, adds little strength and hydrates moderately fast.

Hydration of C₃S and C₂S produces calcium silicate hydrate (C-S-H gel), which gives the strength, and Ca(OH)₂.

✓ Answer checked by Claude Opus 5.5 (high effort)

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