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.
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