Question

Explain with the help of chemical reactions “ Setting and Hardening “ of cement .

Answer 1

When cement is mixed with water to a plastic mass ,called “cement paste “ ,hydration reaction begins ,resulting in the formation of gel and crystalline products.The interlocking of the crystals ,finally bind the inert particles of the aggregates into a compact rock-line material .The process of solidification comprises of :

(i)setting and then , (ii) hardening

“ Setting “is defined as stiffening of the original plastic mass ,due to initial gel formation . “Hardening “is development of strength ,due to crystallization .

After setting ,hardening starts ,due to the gradual process of crystallization in the interior of the mass .The strength developed by cement paste at any time ,depends upon the amount of gel formed and the extend of crystallization .

Initial setting of cement-paste is mainly due to the hydration of tricalcium aluminate (C₃A) and gel formation of tetracalcium 

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

or  C₃A + 6H₂O → C₃A . 6H₂O  + 880 KJ/Kg

Tricalcium aluminate Hydrated tricalcium aluminate

4CaO . Al₂O_3. Fe₂O₃ + 7H₂O→ 3CaO . Al₂O₃ . 6H₂O + CaO . Fe₂O₃ . H₂O  + 420 KJ/Kg
Or C₄AF + 7H₂O → C₃A . 6H₂O (crystalline) + CF . H₂O (gel) + 420 KJ/Kg 

Dicalcium silicate starts hydrolysing to tabermonite gel (which possesses a very high surface area and thus ,very high adhesive property) ,which also contributes to initial setting .

2[2CaO . SiO₂] + 4H₂O → 3CaO . 2SiO₂ . 6H₂O + Ca(OH)₂ + 250 KJ/Kg
Or 2C₂S + 4H₂O → C₃S₂ . 6H2O + Ca(OH)₂ + 250 KJ/Kg 

Dicalcium silicate Tabermonite gel Crystalled calcium hydroxide
Final setting and hardening of Portland cement-paste is due to the formation of tabermonite gel (formed above ) plus crystallization of calcium hydroxide and hydrated tricalcium aluminate .
2[2CaO . SiO₂] + 6H₂O → 3CaO . 2SiO₂ . 3H₂O + Ca(OH)₂ + 500 KJ/Kg
Tricalcium silicate Tabermonite gel Calcium hydroxide
Or 2C₃S + 6H₂O → C₃S₂ . 3H2O + 3Ca(OH)2 + 500 KJ/Kg
In short ,

1) When water is added to cement ,at first hydraton of C₃A and C₄AF takes place within day .
2) C₃S begins to hydrate within 24 hrs and gets completed in 7 days .
3) Gel of aluminate begins to crystallise and at the same time C₂S begins to hydrate in 7-28 days . 
4) The development of early strength between 1-7 days is due to the hydration of C₃S and further hydration of aluminate 
5) The increase of strength between 7-28 days ,is due to hydration of C₂S and continued hydration of C₃s .