Using boron as a tracer reveals how surface precipitates inhibit silicate dissolution in hyperalkaline solutions during alkali–silica reaction

Silicate dissolution under hyperalkaline conditions is a key process that affects the evolution of alkali–silica reaction (ASR) in concrete. In these environments, dissolution may involve multiple overlapping processes—including ion adsorption on reactive sites, transport of dissolved species, and the formation of alteration layers. In addition to complex, and often incongruent dissolution processes, coupled precipitation reactions can make it difficult to isolate influential mechanisms precisely. Therefore, we propose a new approach to use boron as a tracer to indirectly track silicon dissolution rates and their alterations, for example, in situations wherein dissolution may be altered by ionic additives in solution. We emphasize the use of glass microspheres—such as those produced by in-flame spheroidization—that exhibit congruent dissolution of boron and silicon. The validity of using boron as a tracer is based on the absence of borate salt precipitation during dissolution, and negligible boron uptake by any coprecipitated solids (i.e., hydroxides and C–S–H). We demonstrate that the total amount of Si dissolved into alkaline solutions rich in Ca, Mg, and Al can be monitored over time by tracking boron concentrations. This approach allows us to distinguish Si uptake into surficial alteration layers and secondary coprecipitates, and to thereby unambiguously assess the underlying origins of dissolution inhibition. The findings offer a mechanistic basis to identify and rank chemical additives to mitigate ASR via silicate-dissolution suppression in concrete.