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The chemical interaction of certain siliceous mineral constituents of aggregates with the alkalies in portland cement is known to be a cause of abnormal expansion and pattern cracking in some concrete structures. The most practical and most certain method of preventing this type of expansion and pattern cracking is to select a non-reactive aggregate. When the use of a reactive aggregate is unavoidable, control of the alkali content of the cement may be considered after thorough tests justify it, or the use qf certain proved pozzolanic materials may provide protection. This paper describes progress made toward the development of various methods for gaging the presence of reactive minerals in the aggregates, and discusses the procedure for limiting the alkali content of the cement and the use of proved pozzolanic admixtures as alternate methods of minimizing the expansion and pattern cracking associated with the alkali-aggregate reaction, as shown by field experience and results of laboratory tests available to date.
Manager, Portland Cement Association, Chicago, Ill.