Andesite is a very common type of extrusive volcanic rock (second most common after basalt). Its intrusive equivalent, diorite, can often be found as volcanic intrusions in limestone deposits.
As well as being much harder than limestone, both andesite & diorite are invariably quite high in alkalis (Na & K) and magnesium. These chemical and physical characteristics make most volcanic rocks unsuitable as a raw materials for cement manufacture and they are usually avoided or discarded when mining limestone. However, small amounts (<5%) can be tolerated chemically in the raw mix if the alkali content of the limestone and other raw materials is low.
Being harder to grind, as well as containing a moderate amount of coarse silica, too much andesite in the raw mix can cause an increase in the percentage of silica particles >45um in the raw meal, resulting in poor kiln feed burnability and high clinker free lime contents.
Too much andesite can also interfere with the operation of the raw mill resulting in a decrease in mill throughput and increased wear of the mill internals.
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