Current concern with alkali silica reactions is due to rising alkali contents of cements, changed concrete technology, and necessity of employing marginal aggregates in many areas, as well as to new reports of field damage coming to light.
A brief overall view of the physicochemical basis for alkali-silica reactions is given.
The reaction is primarily one of hydroxide ions rather than of alkali cations;
nevertheless the latter are of critical importance. Alkalies in cement are found primarily as alkali sulfates or as solid solution substituents in calcium aluminate or in belite.
The rates at which these reach the pore solution phase are discussed, and data indicating that concentrations as high as 0.7 molar may be attained in a month or so and maintained indefinitely are discussed.
The relationship between alkali cation concentration and hydroxide ion concentrations are explored, and after a few days of hydration published data indicate that the two are substantially equivalent.
Thus reacting pore solutions may have hydroxyl ion concentrations of the order of 0.7 molar, more than 15 times that of pure saturated calcium hydroxide solutions.