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Which impurities must be limited in raw materials for 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.
To ensure the quality and performance of portland cement, specific impurities in the raw materials must be strictly limited.
The content of magnesia (magnesium oxide, MgO) must be kept low, as the permissible limit in the final product is between 4 and 5 percent. Other impurities that require strict limitation include:1
- Fluorine compounds1
- Phosphates1
- Metal oxides and sulfides1
- Excessive alkalies1
Additionally, while alkalies are naturally present, their total content is sometimes restricted to 0.6 percent or less in "low-alkali cements" to prevent harmful reactions with certain types of aggregates that can cause concrete expansion and disruption.2