Ingress of NaCl in concrete with alkali reactive aggregate: effect on silicon solubility

Abstract

Enhanced damage due to the alkali-silica reaction (ASR) in concrete exposed to deicing salt (NaCl) is usually attributed to binding of chloride ions in the hydration products of cement. To balance charge, OH -ions are released into the concrete pore solution which increases alkalinity. However, during NaCl ingress a decrease in the OH -concentration of the concrete pore solution due to potassium leaching would reduce SiO 2 solubility and therefore ASR damage. The present work combines expansion measurements with pore solution analysis by ICP-OES and XRD measurements on concretes and hydrated cement pastes. Solubility equilibria calculations were performed with the hydrogeochemical simulation program PHREEQC. The investigations show that the OH -concentration of the pore solution is mainly lowered by potassium leaching during NaCl ingress. The OH -concentration also decreases owing to the formation of Friedel's salt from ettringite which is associated with the release of sulphate. Although the OH -concentration with NaCl is lower, ASR damage is intensified and the silicon concentration in the pore solution is higher. Higher silicon solubility is explained by the higher total alkali concentration which increases surface silicate solubility, the formation of an aqueous complex NaHSiO 3 0 and a higher ionic strength. These effects promote the sensitivity of silicate minerals to ASR, the formation of alkali silica gel and finally ASR damage.

Alkali-silica reaction and microstructure of concrete subjected to combined chemical and physical exposure conditions

“…Microstructural changes for concrete specimens were observed in terms of formation of Friedel's salt. The presence of Friedel's salt in the matrix resulting to the NaCl application is in accordance with other researchers findings, [16][17][18][19]. Sutter et al [20] confirmed the formation of Friedel's salt in voids in analyzed concrete specimens exposed to NaCl solution and examined by petrographic methods.…”

“…Bérubé et al [16], showed that the high sodium concentration in the near-surface layer of the concrete might not cause severe ASR owing to a decreased OHconcentration in this area. Heising et al [17] stated that the chloride binding on hydrated cement phases and the corresponding release of OHions does not significantly promote ASR. The OHconcentration, and therefore pH, of the concrete pore solution is mainly lowered by potassium leaching, but the OHconcentration is also lowered by the formation of Friedel's salt.…”

“…The OHconcentration, and therefore pH, of the concrete pore solution is mainly lowered by potassium leaching, but the OHconcentration is also lowered by the formation of Friedel's salt. Both studies [17] and [16] concerned long-term soaking of concrete specimens in NaCl solutions at various concentrations. The effect of cyclic wetting and drying, which significantly affects the durability of the concrete, has not been considered.…”

“…The gel consisted of both, sodium and potassium ions. According to [17], the Si concentration in the pore solution increases in the presence of dissolved NaCl. So the ASR damage should be intensified.…”

Alkali–silica reaction in plain and fibre concretes in field conditions

“…Recent literature shows great interest in the study of ASR. Different subjects related to products and kinetic of the reaction considering the influence of salts [5], powders to promote [6] or inhibit ASR [7], the effects of polymer action [8] were studied. There are also many works that use new NDT methodologies for following ASR [9][10][11][12][13], establish criteria for the characterization and assessment of potentially reactive aggregates [14], or improve accelerated tests, particularly the impact of alkali leaching on scale effects affecting expansion tests [15].…”