A thermodynamic and kinetic model for paste–aggregate interactions and the alkali–silica reaction

Abstract

a b s t r a c tA new conceptual model is developed for ASR formation based on geochemical principles tied to aqueous speciation, silica solubility, kinetically controlled mineral dissolution, and diffusion. ASR development is driven largely by pH and silica gradients that establish geochemical microenvironments between paste and aggregate, with gradients the strongest within the aggregate adjacent to the paste boundary (i.e., where ASR initially forms). Super-saturation of magadiite and okenite (crystalline ASR surrogates) occurs in the zone defined by gradients in pH, dissolved silica, Na + , and Ca 2+ . This model provides a thermodynamic rather than kinetic explanation of why quartz generally behaves differently from amorphous silica: quartz solubility does not produce sufficiently high concentrations of H 4 SiO 4 to super-saturate magadiite, whereas amorphous silica does. The model also explains why pozzolans do not generate ASR: their fine-grained character precludes formation of chemical gradients. Finally, these gradients have interesting implications beyond the development of ASR, creating unique biogeochemical environments.

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

“…These phenomena are both in accordance with decreasing Na + ion concentration. These results further demonstrated that the ASR gel can only be generated under high silicate ion condition after the consumption of portlandite by comparing the Figures and , which is in accordance with the findings in reference that the high silicate ion concentration gradient is essential for ASR gel formation. The change in the CSH gel amount obtained from the thermodynamic simulation is shown in Figure .…”

“…The thermodynamic simulation has demonstrated the ASR gel can only be formed after the silica anion reached the specific threshold. During the 30 day age reaction, only the samples at 80°C condition have reached that step …”

“…Furthermore, the kinetic analysis for alkali‐silica reaction in cementitious material has also been conducted. Guthrie and Carey studied the alkali‐silica reaction kinetics by considering the influence of the specific surface area and saturation index. The simulation results demonstrated the formation of a geochemical gradient is essential to the formation of ASR gel.…”

“…First of all, calcium content plays an essential role in the solution chemistry. The ASR gel cannot be formed with the existence of Ca(OH) 2 and the high silicate ion concentration are needed for the formation of ASR gel . Furthermore, the generated ASR gel will react with the calcium hydroxide content in concrete.…”

“…The simulated ion concentration changes are in accordance with the measured experimental data. Guthrie and Carey analyzed the influence of pH and silica gradient on the ASR gel formation using the thermodynamic model and PFLOTRAN database.…”