From the view point of chemistry, the function of water in different types of cementitious materials is analyzed. In some books about cement technology and cement chemistry, authors say:“It should be noticed, that the term, hydration, can not be used to describe the reaction between water and cementitious materials, even Portland cement.” In the fact that the reaction of water with cementitious materials may results in hydration and hydrolysis, water sometimes may only play the role of solvent to dissolve the reactants. Moreover the water is not the reactant, but is even the reaction product.