Title: Mechanisms of Alkali-Aggregate Reaction
Author(s): Robert G. Pike, Donald Hubbard, and Herbert Insley
Publication: Journal Proceedings
Appears on pages(s): 13-34
Keywords: no keywords
An interferometer procedure was used to determine the attack on character-istic active and nonreactive aggregates over an extended pH range, in solutions of NaOH, Ca (OH),, NaOH + Ca (OH), and in aqueous extracts of high-and low-alkali cements. Curves are given showing the hygroscopicity of high- and low-alkali cement pastes and powders and the same cements with added reactive and nonreactive aggregates. Curves are given showing ex-pansion of the mortar bars containing various percentages of Pyrex glass of different grain sizes. Microscopic studies of reactive aggregates in high-alkali cements are described, and photographs show instances where the major reaction takes place inside the opal grain rather than at the outside of the particle. This effect is rationalized in terms of the uneven distribution of the migratable ions. An expansion of 300 percent is demonstrated when opal reacts with soda to form a sodium silicate hydrate complex. It is believed that this expansion is the major cause of alkali-aggregate distress in concrete.