9-9 PORTLAND CEMENT COMPOSITION

9-9 PORTLAND CEMENT COMPOSITION

Portland cement is the most important hydraulic cement used extensively in various types of construction, as in mortars, plasters, grouting, and concrete. Portland cement is obtained by burning an intimate mixture, composed mainly of calcareous and argillaceous materials, or other silica-, alumina-, and iron oxide-bearing materials, at a clinkering temperature of about 1400°C (2552°F).

The partially sintered material, called clinker, is then ground to a very fine powder. A small amount of gypsum, from 2% to 4%, is usually added to the clinker before grinding. The chemical analysis of Portland cement reveals its composition of calcium oxide, silica, alumina, iron oxides, magnesium oxide, and sulfur trioxide, but this does not indicate its complex chemical character. Microscopic investigations have proved that these oxide constituents exist in Portland cement mainly as calcium silicates and

aluminates. They are mainly tricalcium silicate (3CaO.SiO 2 ), dicalcium silicate (2CaO.SiO 2 ), tricalcium aluminate (3CaO.Al 2 O 3 ), and tetracalcium aluminoferrite (4CaO.Al 2 O 3 .Fe 2 O 3 ). In the nomenclature of the cement industry these compounds are usually written as C3S, C2S, C3A, and C4AF, respectively, where C stands for CaO, S for SiO 2 , A for Al 2 O 3 , and F for Fe 2 O 3 . Small quantities of pentacalcium trialuminate (5CaO.3A1 2 O 3 ), free magnesium oxide and calcium oxide, calcium sulfate, and even smaller quantities of titanium dioxide and potassium and sodium oxide may also be present.

FIGURE 9-5 Comparison of compressive strengths of cement compounds. (From R. H. Bogue and W. Lerch Industrial and Eigineering Chemistry, 26 837, 1934.)

The properties of the four main cement compounds are illustrated by Fig.9-5, indicating that the most desirable constituent is the tricalcium silicate (C 3 S) because it hardens The properties of the four main cement compounds are illustrated by Fig.9-5, indicating that the most desirable constituent is the tricalcium silicate (C 3 S) because it hardens

possess cementing properties. If the amount of dicalcium silicate (C 2 S) formed during the burning of the mixture is not too high, a rapid cooling of the clinkerwill inhibit the transformation of the a and forms to the y form.Both tricalcium aluminate (C 3 A) and tetracalcium aluminoferrjte (C 4 AF)give, on hardening, a product of low strength, which would tend to make them undesirable constituents. The presence, however, of some alumina andiron oxides in the raw mixture is necessary because they function as fluxes to lower the fusion temperature, thereby facilitating the recrystallization of the desirable tricalcium silicate from the liquid phase.

There are five major types of Portland cement covered by ASTM and federal specifications. The compositions of these cements in terms of their compounds are shown in Table 9-2.

Type I is the most universally used cement in concrete construction when the special properties specified for the other types are not required.

Types II, IV, and V are characterized by lower contents of tricalcium silicate and tricalcium aluminate. This accounts for their moderate or low heat evolution and the fact that smaller volume changes occur during hydration than in the Type I cement. Type IV is used for massive concrete work in which a low evolution of heat is required, whereas Type Vis used when high resistance to sulfate attack is essential. Type II also shows improved resistance to moderate sulfate action.

Type III contains a high proportion of tricalcium silicate and is known as high-early- strength cement, which hardens rapidly and shows high heat evolution. It is made by increasing the lime content of the cement and by finer grinding.