Effect of calcium on dissolution and precipitation reactions of amorphous silica at high alkalinity

Abstract

A better understanding of silica dissolution-precipitation reactions at high pH aqueous solutions allows for promotion of favorable (e.g., pozzolanic) reactions and mitigation of deleterious (e.g., alkali-silica) reactions in concrete. In this paper, the kinetics and products of silica glass dissolution are studied as a function of solution pH, temperature, and availability of calcium. It was observed that dissolution rate versus time increases linearly with pH and reaches a maximum at pH = 14, with slower dissolution at higher alkalinities. In solutions with similarly high pH, but saturated with portlandite, glass dissolution is significantly slower. This is due to formation of a dense, low porosity, and strongly bonded C-S-H layer on the surface of glass, which serves as a barrier against diffusion of OH − and alkali ions towards the substrate glass. This protective layer forms only when Ca is abundant and portlandite saturation can be maintained on a local scale.

Topological controls on the dissolution kinetics of glassy aluminosilicates

“…As expected, the dissolution rate increases with pH because greater hydroxyl concentrations (activities) facilitate hydrolysis of silicate networks. 37 Surface speciation induced by the presence of sodium (i.e., from NaOH which exchanges with the protons of terminal silanol groups 24 ) may somewhat enhance fly ash dissolution rates, 24 but this effect should be relatively constant across all fly ash compositions because it is related solely to the concentration of added sodium. In the high pH range used in this study, the dominant driving force for dissolution arises from the elevated activity of [OH] À ions, which induces nucleophilic attack of tetrahedral [SiO 4 ] 4À or [AlO 4 ] 5À units present in the glassy compounds in fly ashes.…”

Kinetic analysis and thermodynamic simulation of alkali‐silica reaction in cementitious materials

“…The influence of hydroxyl ion activity on the dissolution rate can be represented by the Equation , which has also been adopted by the references . Particularly, the partial order β θ in Equation used for the silica dissolution in 298.15 K and 343.15 K are 0.411 and 0.5, respectively.…”

Mechanisms of Alkali-Silica Reaction in Alkali-Activated High-Volume Fly Ash Mortars

“…As such, it is of concern that if alkali leaching from alkali-activated specimens occurs during testing, it could lower the ASR development. However, the dissolution rate of amorphous silica is at the highest at an intermediate concentration of NaOH solution (at around 1.0 M) (Maraghechi et al 2016;Tarnopol and Junge 1946). In that context, the alkali leaching, if occurring to some extent in alkaliactivated mortars, does not seem to considerably alter ASR development.…”