15411Cement

The choice of Portland cement for HPC is extremely important. Different brands of cements will have different strength development characteristics because of the variations in chemical composition and fineness that are permitted by ASTM C 150.

Initially, silo test certificates should be obtained from potential suppliers for the previous 6 to 12 months. Not only will this give an indication of strength characteristics from the ASTM C 109 mortar cube test, but also, more importantly, it will provide an indication of cement uniformity. The cement supplier should be required to report uniformity in accordance with ASTM C 917. If the tricalcium silicate content varies by more than 4%, the ignition loss by more than 0.5%, or the fineness by more than 375 cm2/g (Blaine), then problems in maintaining a uniform high strength may result.

Special concern should be given to the contents of C3A and SO3 in the cement, because slump loss in concrete is usually associated with the formation of ettringite caused by the two components. When early setting and hardening properties are desired, it is preferable to use more finely ground Type II Portland cement than Type I and III cements, which generally contain more than 8% C3A. On the other hand, cements with a very low or negligible C3A content, such as ASTM Type V, are generally high in C4AF and their setting and hardening rates may be unusually slower than desired. Portland pozzolan (ASTM Type IP) and portland-slag (ASTM Type IS) cements should also be seriously considered for use in HPC mixtures because of certain desirable physical-chemical effects associated with fine particles of a pozzolan or slag. The same effects can generally be achieved by using a high-quality pozzolan or slag as a mineral admixture in Portland cement concrete mixtures.

In addition, alkali content should also be strictly controlled below the limitation level of ASTM standard specifications in order to prevent the potential alkali-aggregate reaction.

A further consideration is the optimization of the cement-admixture system. The exact effect of a water-reducing agent on water requirement will depend on the cement characteristics. Mechanical and durability performance developments will depend on both cement characteristics and cement content.

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