Cements Users’ Expectations with Regard to Blended Cements


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Title: Cements Users’ Expectations with Regard to Blended Cements

Author(s): Bryant Mather

Publication: Special Publication

Volume: 79


Appears on pages(s): 255-266

Keywords: blast furnace slag; blended cements; costs; pozzolans; -- properties.

Date: 5/1/1983

In current practice in the U.S.A., the term "blended cements" is applied only if the addition to portland cement is blast-furnace slag or pozzolan. Blended cements are covered by ASTM specification C 595 and the most generally used types (IS and IP) are required to give a level of "performance" as good as Type cement. The user, if he pays the same price per unit mass pur-chased, pays a lower price per solid volume. A few people believe that a greater degree of sulfate resistance of concrete could be attained by the addition of "a suitable" pozzolan to Type V portland cement. Some pozzolans with high silica content can improve the sulfate resistance of an otherwise non-sulfate resistant portland cement, other pozzolans can make sulfate-resistance worse. Merits or demerits in the performance of cements may be quite unrelated to whether or not the cement is "blended." As ASTM specifications require that IS and IP cements meet the same 3-day strength limits as Type I portland cements, such blended cements are seldom competitive in mass concrete construction. Batching of pozzolan or slag at the ready-mixed concrete plant has the advantage that a wider range of admixture materials may be made available and the quantity of blending material can be varied at will. The disadvantages are that there is another ingredient to procure, test, store, and control and that it is possible to make a concrete with all slag or pozzolan and no portland cement. U.S.A. specifications for blended cements and for pozzolans still show serious deficiencies. For instance, the present classification of fly-ashes into Type F and C is misleading. Also, no basis exists for measuring for specification purposes the benefits of blended cement as compared with a straight portland cement with regard to sulfate resistance or reduction in tendency to concrete expansion due to alkali-silica reaction. The view is expressed that prices should be fixed according to performance requirements. If the customer wants more early strength or lower heat, the extra costs should be added on to the basic price. Producers of hydraulic cements in general and of blended cement in particular will have great opportunities in the future if they are aware that cement production is part of the chemical industry and concrete making is not simply putting mud in a hole. Quality control can save costs and permit the use of a variey of cements of a variety of levels of performance and prices. To make progress, efforts in research and development must be improved. Better methods of characterizing slags and pozzolans for better specifications must be developed.