Electron probe microanalysis data indicates that sodium, potassium, and calcium ions migrate into opal aggregate from the cement paste in experimental alkali-silica reactive concretes. The reaction also modifies the development of cement phases at the interface with the opal and these modifications are reflected in modifications of the tensile strength of the opal/cement bond. It is suggested that the calcium silicate hydrates which develop in the cement at the interface with opal may control the migration of alkali ions into the opal. With high alkali concentration in the cement pore fluids this hydrate layer at the interface may not develop completely allowing high concentrations of alkali ions to reach the reacting opal causing a change in the composition and properties of the alkali-silica gel reaction product.

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