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Why is kaolin used to make white portland cement?
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Cement
The cement-making process, from crushing and grinding of raw materials, through roasting of the ground and mixed ingredients, to final cooling and storing of the finished product.
Kaolin is used as the argillaceous (clayey) component in the production of white portland cement primarily because it contains very little iron oxide.1
Standard portland cement typically derives its characteristic gray color from the presence of iron compounds. Because white portland cement requires a specific aesthetic appearance, manufacturers must use raw materials that are extremely low in iron and other metal oxides that would otherwise discolor the final product. As a white clay, kaolin provides the necessary alumina and silica required for the cement's chemical composition without introducing the iron pigments that would darken the cement.213