Cement paste is the binder in concrete or mortar that holdsthe fine aggregate, coarse aggregate, or other constituents to-gether in a hardened mass. The term hydraulic is associatedwith the word cement in this document to point out to theconsumer that the basic mechanism by which the hardeningof the concrete or mortar takes place is the reaction of the ce-ment material with water.
The word hydraulic also differen-tiates this type of cement from binder systems that are basedon other hardening mechanisms.The properties of concrete depend on the quantities andqualities of its constituents. Because cement is the most activecomponent of concrete and usually has the greatest unit cost,its selection and proper use are important in obtaining mosteconomically the balance of properties desired for a particularconcrete mixture.
Most cements will provide adequate levelsof strength and durability for general use. Some providehigher levels of certain properties than are needed in specificapplications. For some applications, such as those requiringincreased resistance to sulfate attack, reduced heat evolution,or use with aggregates susceptible to alkali-aggregate reac-tion, special requirements should be imposed in the purchasespecifications.
While failure to impose these requirements may have serious consequences, imposing these requirementsunnecessarily is not only uneconomical but may degrade oth-er more important performance characteristics. For example,moderate sulfate resistance may be specified for certain plant-manufactured structural elements that require strength gain inthe production process. Because the compositional variationsthat impart sulfate resistance tend to reduce the rate ofstrength gain, some compromise must be made.
The goal of the specifier is to provide specifications thatwill ensure that the proper amounts and types of cement areobtained to meet the structural and durability require-mentsno more, no less. Due to gaps in our knowledge, thisgoal is seldom, if ever, fully achieved; economies, however,can often be obtained with little or no decrease in perfor-mance in service, if specifications are aimed at this goal.