What sort of change takes place when cement powder is mixed with water?

Answer 1

Short Answer:

When adding water to cement a chemical reaction called hydration takes place, transforming the calcium and silicates into new mineral forms through an exothermic reaction. It takes minutes to days or longer to complete depending on the particular recipe being used and if it is combined with other materials such as those used to make concrete.

More:

Contrary to popular belief, concrete does not "dry," it cures. When properly mixed, water does not leave the mixture but gets chemically incorporated into the resulting minerals formed by chemical reaction. That is the reason newly poured concrete is often protected from evaporation since evaporation changes the chemical mix near the surface and the deficiency of water changes the final type of material and its properties.

We say cement sets or cures when mixed with water to describe a series of hydration chemical reactions. The constituents harden and the interlocking of the crystals gives cement its strength.

Using cement or making concrete is a lot like making bread. There is a huge knowledge base based on experience of what ingredients go into it and the process of putting them together and cooking the mixture, but the detailed chemistry of what takes place in the hardening phase is not easy and not completely understood and changes somewhat for different recipes.

Making cement is better understood. It takes limestone and silica. The first step is called the calcining of limestone (calcium carbonate) into lime (calcium oxide) which give off carbon dioxide. It takes place in a kiln heated to 1650F (900C). The second reaction is the bonding of calcium oxide and silicates to form dicalcium and tricalcium silicates. This is done at higher temperature. Other minor ingredients may be added to adjust final properties.