The word cement is used to define adhesive substances. It is derived from the Latin caementum, which meant stone chippings such as were used in Roman mortar, not the binding material itself. In its general sense it covers a wide variety of binding materials used for many different purposes, but more commonly it implies the binding materials used in building and civil engineering construction. These constructional cements, which will set and harden under water, are often called hydraulic cements. The most important of these is Portland cement.
Cements are finely ground powders that, when mixed with water, set to a hard mass.
Small amounts of uncombined lime and magnesia also are present, along with alkalies and minor amounts of other elements. Titanium oxide is usually present to the extent of 0.1 to 0.4 percent. The strength developed by Portland cement depends on its composition and the fineness to which it is ground. The C 3 S is mainly responsible for the strength developed in the first week of hardening, and the C 2 S for the subsequent increase in strength. The alumina and iron compounds that are present only in lesser amounts make little direct contribution to strength.