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portland cement

updated tue 2 nov 04


Ivor and Olive Lewis on mon 1 nov 04

All Hydraulic Cements have a degree of water reactivity. Portland
Cement is no exception.
They are mixtures of compounds derived from Clay, Limestone and other
incombustible materials, including power station fly ash, that are
calcined at a very high temperature. During firing they become
mixtures of Calcium Aluminate and Calcium Silicate. Pulverised
material is partially soluble. Mixed with water and allowed to cure
these chemicals recrystallise as Hydrates (as does potters plaster)
then set into a solid hard substance.
It should be possible to calculate from the recipe which has been
given the proportions of Calcium Aluminate and Calcium Silicate in an
average Portland Cement.
I have used Portland Cement as a "setting agent" when making home made
rammed refractories. Because its strength comes from recrystallisation
bonding this is destroyed when it dehydrates. Sintering (Thermal
cementation) replaces the adhesive properties of the original cement
and makes the refractory durable.

<Silica 21.8%
Alumina 7.1%
Ferric Oxide(Fe2O3) 3.1%
Lime(CaO) 62.3%
Magnesia(MgO) 1.9%
Sulphur 1.3%
Alkalis 1.1%>>

Best regards,
Ivor Lewis.
S. Australia.