High Alumina Cement (IS: 6452)

High Alumina Cement (IS: 6452)

High Alumina Cement (IS: 6452): This is not a type of Portland cement and is manufactured by fusing 40 per cent bauxite, 40 per cent lime, 15 per iron oxide with a little of ferric oxide and silica, magnesia, etc. (Table 5.5) at a very high temperature. The alumina content should not be less than 32%. The resultant product is ground finely. The main cement ingredient is monocalcium aluminate CA which interacts with water and forms dicalcium octahydrate hydroaluminate and aluminium oxide hydrate.

2(CaO.AL₂O₃.10H₂O) + H₂O = 2CaO.Al₂O₃.8H₂O + 2Al(OH)₂

The dicalcium hydroaluminate gel consolidates and the hydration products crystallise. The rate of consolidation and crystallisation is high leading to a rapid gain of strength. Since C₃A is not present, the cement has good sulphate resistance.

Table 8 Composition of a Typical High Alumina Cement

Composition                 Percentage

Al₂O₃, TiO₂ 43.5

Fe₂O₃, FeO, Fe₃O₄          13.1

CaO  37.5

SiO₂  3.8

MgO  0.3

SO₃   0.4

Insoluble material 1.2

Loss on ignition   0.2

Properties: It is not quick setting: initial setting time (minimum) is 30 minutes, even up to 2 hours. The final setting time should not exceed 600 minutes. It attains strength in 24 hours, high early strength, high heat of hydration and resistance to chemical attack. Compressive strength after one day is 30.0 N/mm² and after 3 days it is 35.0 N/mm². After setting and hardening, there is no free hydrated lime as in the case of ordinary Portland cement. The fineness of the cement should not be less than 225 m²/kg. The cement should not have expansion more than 5 mm.

Uses: It is resistant to the action of fire, sea water, acidic water and sulphates and is used as refractory concrete, in industries and is used widely for precasting. It should not be used in places where temperature exceeds 18 o C.