Deleterious cement-aggregate reaction is a reaction between soluble silica in aggregate and the alkali-hydroxides derived from the portland cement, which produces abnormal expansion and cracking of mortar and concrete. The alkalies, sodium and potassium, are normally present in small amounts in portland cement. The reactive siliceous aggregate constituents include widely distributed normal constituents of aggregates.The definitely established reactive constituents of natural aggregates are opal, acid and intermediate volcanic glass, cristobalite, tridymite, chalcedony. Synthetic glasses and silica gel are also reactive. All these substances can be described as "highly siliceous materials which are thermodynamically metastable at ordinary temperatures"–silica not tied up in a crystalline structure that is stable under normal ambient conditions.In 1942 Stanton published an account of some failures of concrete pavements and other concrete structures in coastal southern California from Monterey County south to Los Angeles County. This kind of failure or distress had developed in bridges, pavements, and buildings dating from 1914; it had first been noticed but not identified in 1923. Stanton and the State Division of Highways were aroused by the failure in 1938 of a 2-year-old pavement that buckled at the expansion joints and cracked throughout the length of some of the slabs.After a great deal of investigation in the field and laboratory, Stanton and his colleagues had established some common characteristics of the distressed structures, had some ideas about the causes of the trouble, and had a test method that appeared to pick out undesirable constituents. The common characteristics of the distressed structures were: