Question

Explain with the help of chemical reactions 'setting and hardening' process of cement.

Answer 1

When cement is mixed with water, it forms a cement paste. It undergoes several hydration reactions to from gel and crystalline products. The compounds thus formed are responsible for solidification and gradual hardening. The process comprises of i) Setting is the initial stiffening of the cement pate due to gel formation. ii) hardening is the gain of strength due to crystallisation.

The process of solidification consists of three steps:-

• Initial setting is due to the hydration of tricalcium aluminate(C₃A) and gel formation of
tetra calcium alumino ferrite {C₄AF)

3CaCOAl₂O₃ + 6H₂0 →3 CaOAl₂O₃6H₂O + 880 kL/kg

4CaOAl₂O₃Fe₂O₃ + 7H₂O → 3CaOAl₂O₃6H₂O + CaOFe₂O₃H₂O + 420kJ/kg

• Dicalcium silicate (C₂S) and Tricalcium silicate (C₃S) hydrolyses to from tobermonite gel

3[2CaOSiO]+ 4H₂0 → 3CaO.2SiO₂6H₂0 + Ca(OH)₂ + 250 kJ/kg

2[3CaOSiO₂] + 6H₂O→3Ca0.2Si0₂3H₂0 + 3Ca(OH)₂ + 500 kJ/kg

• Final setting and hardening of cement paste is due to the formation of tobermonite gel
and crystallisation of calcium hydroxide and hydrated tricalcium aluminate.

SEQUENCE OF CHEMICAL REACTIONS FOR SETTING AND HARDENING OF CEMENT.

• At first hydration of tricalcium aluminate (C₃A) and tetracalcium alumino ferrite (C₄AF)
  takes place. This leads to the initial setting of the cement.
• Next the hydration oftricalcium silicate (C₃S) begins within 24 hours and gets completed
  within 7 days. This provides the initial strength.
• Dicalcium silicate (C₂S) begins to hydrate after 7 days and gets completed in 28 days.
  This is responsible for increased strength of cement.
  CEMENT PASTE →HYDRATION OF C₃A AND C₄AF → GELATION OF  C₃S→HYDRATION OF C₃S AND C₂S.