152 Materials 1521 Portland Cement

HPCs have been produced successfully using Types I, II, and III Portland cements that meet the ASTM Standard Specification C 150. Unfortunately, ASTM C 150 is very imprecise in its chemical and physical requirements, and so cements that meet these rather loose specifications can vary quite widely in their fineness and chemical composition. Consequently, the same type of cements have quite different rheological and mechanical characteristics, particularly when used in combination with chemical admixtures and mineral admixtures. Therefore, when choosing Portland cements for producing HPC three requirements should be satisfied: (1) it can develop the appropriate strength at given ages; (2) it exhibits good rheological behavior, particularly when chemical admixtures are incorporated; and (3) it should not generate too much hydration heat.

The strength of the cement has a strong influence on the properties of HPC. If the cement cannot develop reasonable strength at given ages, it is impossible to achieve the mechanical and durability requirements of HPC.

TABLE 15.3 Properties of Cement Compounds

Component

Rate of reaction

Heat liberated

Ultimate cementing value

Tricalcium silicate, C3S Dicalcium silicate, C2S Tricalcium aluminate, C3A Tetracalcium aluminoferrate, C4AF

Medium Slow Fast Slow

Medium Small Large Small

Good Good Poor Poor

The strength of cement paste is the result of the process of hydration. The hydration process results in the formation of crystal phases such as Ca(OH)2, AFt/AFm, and calcium silicate hydrate (C-S-H) gel phase. The crystal phases interlock each other and construct the bone of the harden paste. For C-S-H gels, it fills in the cavities among the bone. Table 15.3 shows the relative contribution of each component of the cement toward the rate of gain in strength. The early strength of portland cement is mainly controlled by the percentages of C3S. C3S contributes most to the strength developed during the first day after placing the concrete.

In general, no special compressive strength criterion is specified for cement used for the manufacture of HPC; however, Portland cement is preferable with 52.5 MPa or greater compressive strength at 28 days.

Increasing the fineness of the Portland cement will, on the one hand, increase the early strength of the concrete. On the other hand, higher fineness will lead to a more rapid release of hydration heat and a big rheological problem, which are attributed to the rapid reaction and the great amount of ettringite formation at early ages. Early work by Perenchio [7] indicated that fine cements produced higher early strengths, though at later ages the strength differences of different fineness cement concrete were not significant. Most cements now used to produce HPC have Blaine finenesses that are in the range of 300 to 400 m2/kg.

The work of Perenchio [7] indicates that cements with higher C3A contents lead to higher strengths. However, subsequent work [8] has shown that high C3A contents generally leads to rapid loss of flow for fresh concrete. Therefore, cements with high C3A contents should be avoided for the production of HPC. A'itcin [9] has shown that C3A should be primarily in its cubic, rather than its orthorhombic, form. Further, A'itcin suggests that attention must also be paid not only to the total amount of SO3 in the cement but also to the amount of soluble sulfates. Thus, the degree of sulfurization of the clinker is an important parameter. In addition to commercially available cements conforming to ASTM Types I, II, and III, a number of cements have been formulated specifically for HPC.

Generally, HPC typically contains higher cement content than normal concrete in order to develop good workability, high strength, low permeability, and excellent durability. Thus, much attention should be paid on hydration heat for HPC, particularly for massive HPC structures.

Since different types of cement generate different amounts of hydration heat at different rates, the type of structure governs the type of cement to be used. The bulkier and the heavier in cross-section the structure is, the less generation of heat of hydration is desired. In massive HPC structures such as transferring floor, type IV cement is a good candidate to use. In addition, the reduction in hydration heat can also be accomplished in the presence of mineral admixture, ASTM type II cements, or moderate-heat blended cements. From this discussion it is seen that the type of structure, the weather, and other conditions under which the structure is built and that will exist during its life span are the governing factors in the choice of the type of cement that should be used.

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