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Composition of Portland Cement
·
The principal raw materials used
in the manufacture of cement are
·
Argillaceous of alumina in the
form of clay and shales
·
Calcareous or calcium carbonate,
in the form of lime stone, chalk and marl which is a mixture of clay and
calcium carbonate,
·
The ingredients are mixed in the
proportion of about two parts of calcareous materials to one part of
argillaceous
· Portland cement is prepared by burning slurry in a rotary Kiln at a temperature between 1400 to 1500°C.
Constituents of Portland Cement are
Lime (CaO) - 60 to 67%
Silica (SiO2) - 17 to 25%
Alumina (Al2O3) - 3 to 8%
Ironoxide (Fe2O3) - 0.5
to 6%
Magnesia (MCO) - 0.1 to 4%
Sulphur Trioxide (SO3) - 1 to 3%
Soda and Potash - 0.5 to 1.3%
Above raw
material undergoes chemical reactions to form the following compounds on
fusion.
1. Tricalcium
Silicate (C3S) → 3CaO.SiO2
2. Dicalcium
Silicate (C2S) → 2CaO.SiO2
3. Tricalcium
Aluminate (C3A) → 3CaO.Al2O3
4.
Tetracalcium Alumino Ferrite (CAF) → 4CaO.Al2O3. Fe2O3
·
C3S and C2S
contribute most to the eventual strength.
·
Initial setting is due to
tricalcium Aluminate.
·
C3S hydrates quickly
and contributes more to the early strength.
·
C2S strengthen the concrete
from 7 days to 1 year.
·
C4AF is comparatively
inactive.
·
C3A hydrates quickly,
generates much of heat and makes only a small contribution to the strength
within the first 24 hrs. C3A generates maximum heat and is responsible
for most of the undesirable properties of concrete. Cement having less C3A
will have higher ultimate strength, less generation of heat and less cracking.
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