silica
Silica is available abudantly in many forms. Some the forms of silica forms are:
clay, marl, sand, flyash, slag, quartz, flint,felspar, mica, sandstone, Illite, diatomite
chemical name: silicon di oxide (SiO2)
Melting point : 2072 deg celcius
No need to add silica seperately as it was present in mixed condition along with limestone and with iron & alumina. The basic thumb rule is:
A high lime material will have less silica and low lime material will have high silica.
Two types of silica is availabe:
1. Reactive silica : This silica will react, burn easily with rawmix (limestone with iron and alumina ) and without causing burning, operational or cement manufacturing problems.
2. Free silica : This silica will not react or burn easily with rawmix (limestone with iron and alumina). It requires very high temperature compared to reactive silica.It is also difficult to grind. So ensure care while selecting raw material which contains silica in the form of free silica.
CLAYS :
These are aluminim silicates which contain about 50 % to 70 % of silicon di oxide. They are fine grained in appearance and size under 2 microns in diameter. The fusion temperature of clays varies between 1150 deg celcius to 1750 deg celcius. The following table provides you about general clay types:
KAOLIN GROUP
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MONTMORILLONITE GROUP
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Kaolinite
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Montmorillonite
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Dickite
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Beidellite
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Nacrite
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Nontronite
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Halloysite
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Saponite
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Saponite is magnesium silicate based and remaining clay types are aluminium silicate types. Clay mica including illite are alkali with magnesium based clays.
Clays with no impurities will be white in colour, but it contains iron hydroxide which is the principle colouring matter in clay.
(clay image, courtesy : www.learnnc.org)
What to do if silica content is low in raw mix ?
Just add sand to raw mix as it contains more silica.
How to select a raw material containing silica for cement manufacturing ?
1. Ensure care while selecting alkali and magnesium based clays (as they create operational and cement quality problems like cement expansion and cracking) by keeping them under permissible limits.
2. Again and always try to keep auxillary components under limits as they also create cement expansion with cracking and operational problems. (clays are major source of alkalies in cement)
3.If high silica, then it means material having low lime. So add high grade limestone or any material of high grade lime so that the lime increases and silica decreases.
What to do if you have more free silica ?
Under such condition,grind the raw mix less than 45 microns. This makes material with free silica reactive and easy burning type.
