The correct answer is D. All of these.
Cement is a binding agent that is used to hold together aggregates (sand, gravel, crushed stone, etc.) to form concrete. It is made by heating limestone and clay to a high temperature, which produces a clinker. The clinker is then ground into a powder and mixed with gypsum to form cement.
The quality of cement is affected by the presence of impurities, such as sulfur, magnesia, and lime. Sulfur can cause the cement to become unsound by forming ettringite, a mineral that expands and can cause cracks in the concrete. Magnesia can also cause the cement to become unsound by forming periclase, a mineral that is very hard and can make the concrete difficult to work with. Lime can cause the cement to become unsound by forming calcium carbonate, a mineral that is not as strong as the other minerals in the cement.
The presence of any of these impurities can reduce the strength and durability of concrete. Therefore, it is important to use cement that is free of impurities to ensure that the concrete will be of the highest quality.