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Why must the magnesia content of portland cement be kept low?
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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.
The magnesia (magnesium oxide, MgO) content in portland cement must be kept low—typically within a limit of 4 to 5 percent—because of its impact on the long-term stability of the material.1
If the concentration of free magnesia is too high, it can lead to a condition known as unsoundness. A sound cement must not undergo significant expansion after it has set, as such movement can cause internal stress and disrupt the integrity of mortar or concrete. Because excessive magnesia or hard-burned free lime can cause this delayed expansion, manufacturers strictly limit these components to ensure the durability and structural reliability of the finished product.21