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Discuss the setting and hardening of portland cement as well as the function of gypsum with balanced equations.
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SETTING AND HARDENING OF PORTLAND CEMENT:

When cement is mixed with water to a plastic mass, called "cement paste", hydration reaction begin, 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-like material.

The process of solidification comprises of:

  • Setting and then
  • 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 progress of crystallization in the interior of the mass.

The strength developed by cement paste at any time, depends upon the amount of gel fromed and the extent of crystallization.

Initial setting of cement-paste is mainly due to the hydration of tricalcium aluminate $\text{(C}_3\text{A)}$ and gel formation of tetracalcium aluminoferric.

$3\text{CaO}\cdot\text{Al}_2\text{O}_3+6\text{H}_2\text{O}\rightarrow3\text{CaO}\cdot\text{Al}_2\text{O}_3\cdot6\text{H}_2\text{O}+880kJ/kg$

or

$\text{C}_3\text{A} + 6\text{H}_2\text{O} \rightarrow \text{C}_3\text{A}\cdot 6\text{H}_2\text{O} + 880kJ/kg$

Where,

$\text{C}_3\text{A} $-Tricalcium Aluminate

$\text{C}_3\text{A}\cdot 6\text{H}_2\text{O}$-Hydrated Tricalcium aluminate (crystalline)

$\text{4CaO}\cdot \text{Al}_2\text{O}_3\cdot \text{Fe}_2\text{O}_3 + 7\text{H}_2\text{O} \rightarrow \text{3CaO}\cdot\text{Al}_2\text{O}_3\cdot 6\text{H}_2\text{O} \\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ + \text{CaO}\cdot\text{Fe}_2\text{O}_3\cdot\text{H}_2\text{O} + 420kJ/kg$

Where,

$\text{4CaO}\cdot \text{Al}_2\text{O}_3\cdot \text{Fe}_2\text{O}_3$-Tetracalcium aluminoferrite

$\text{3CaO}\cdot\text{Al}_2\text{O}_3\cdot 6\text{H}_2\text{O}$-Crystalline

$\text{CaO}\cdot\text{Fe}_2\text{O}_3\cdot\text{H}_2\text{O}$-Gel

$\text{CuAF} + 7\text{H}_2\text{O} \rightarrow \text{C}_3\text{A}\cdot 6\text{H}_2\text{O} + \text{CF}\cdot\text{H}_2\text{O} + 420kJ/kg$

Where,

$\text{C}_3\text{A}\cdot 6\text{H}_2\text{O}$-Crystalline

$\text{CF}\cdot\text{H}_2\text{O} $-Gel

Also dicalcium silicate starts hydrolising to tobermonite gel, which also contributes to initial setting.

$2[2\text{CaO}\cdot\text{SiO}_2]+4\text{H}_2\text{O}\rightarrow3\text{CaO}\cdot 2\text{SiO}_2\cdot 6\text{H}_2\text{O}+\text{Ca(OH)}_2+250kJ/kg$

or

$2\text{C}_2\text{S} + 4\text{H}_2\text{O} \rightarrow \text{C}_3\text{S}_2\cdot 6\text{H}_2\text{O} + \text{Ca(OH)}_2 + 250kJ/kg$

Where,

$2\text{C}_2\text{S} $-Dicalcium Silicate

$\text{C}_3\text{S}_2\cdot 6\text{H}_2\text{O}$-Tobermonite gel

$\text{Ca(OH)}_2$-Crystalline calcium hydroxide

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

$2[2\text{CaO}\cdot\text{SiO}_2]+6\text{H}_2\text{O}\rightarrow3\text{CaO}\cdot 2\text{SiO}_3\cdot 3\text{H}_2\text{O}+3\text{Ca(OH)}_2+500kJ/kg$

or

$2\text{C}_3\text{S} + 6\text{H}_2\text{O} \rightarrow \text{C}_3\text{S}_2\cdot 3\text{H}_2\text{O} + 3\text{Ca(OH)}_2 + 500kJ/kg$

Function of Gypsum in cement:

Tricalcium aluminateĀ $\text{(C}_3\text{A)}$ combines with water very rapidly with the evolution of a large amount of heat.

$\text{C}_3\text{A}+6\text{H}_2\text{O}\rightarrow\text{C}_3\text{A}\cdot 6\text{H}_2\text{O}+Heat$

After the initial-set, the paste becomes somewhat stiff.

However, the added gypsum retards the dissolution of $\text{(C}_3\text{A)}$ by forming insoluble calcium sulpho-aluminate:

$3\text{CaO}\cdot \text{Al}_2\text{O}_3\cdot x\text{CaSO}_4\cdot 7\text{H}_2\text{O}$

This reaction prevents a high concentration of alumina in the cement solution, thereby retarding the early initial-set of the cement.

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