The alkali–silica reaction: The effect of monovalent and bivalent cations on the surface charge of opal

The degree of expansion of alkali–silica reaction (ASR) gel is dependent on the valence and relative concentrations of cations present in the ASR gel. The electrical double-layer (EDL) model can predict which gel will be more expansive. If double-layer models are applied in a complex system, as in concrete, it is necessary to characterize the surface charge density of amorphous silica as a function of pH and electrolytes. The effects of Li +, Na +, K +, Mg +2, Ca +2, and Ba +2 on the surface charge density of opal have been measured as a function of pH. The results indicate that bivalent ions produce much lower surface charge density than monovalent ions, resulting in less expansive gels.