Studies Relating To the Mechanism by Which the Alkali-Aggregate Reaction Produces
EXPANSION IN CONCRETE . . . . . .
W. C. Hanson
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The expansion and cracking of * certain concrete structures have been attributed to a reaction between the aggregate and the alkali hydroxides produced by the hydration of the cement minerals. An hy- pothesis of the mechanism by which this reaction produces expansions in concrete is given. This hy- pothesis is based on the reaction of alkali hvdroxides with opaline silica in the aggregate producing alkali silicates which because of the semipermeable nature of the hardened cement paste give rise to osmotic pressures. Several lines of evidence support this hypothesis, and the hypothesis appears to explain satisfactorily why opaline silica does not produce abnormal expansions through reaction with calcium hydroxide.