Evaluation of alkali–silica reaction potential of quartz-rich rocks by alkaline etching of polished rock sections

Rock sections mainly composed of quartz (coming from orthoquartzite, quartz metagreywacke, pegmatite quartz, vein quartz, chert, and flint) were subjected to experimental etching in 1 M NaOH solution at 80 °C. After 4 h–14 days, the rock sections were analysed using a scanning electron microscopy (SEM) combined with energy-dispersive spectrometer. Representative areas were documented in backscattering mode and analysed using fully automatic petrographic image analysis (PIA). Specific features connected to alkaline etching were observed on the surface of the rock sections: dissolution gaps, accentuation of pre-existing microcracks, etched out parts of aggregates, and the formation of altered silica-rich layer, and alkali–silica gels, all influencing the greyscale spectrum of the BSE microphotographs. Using SEM-PIA, it was possible to quantify the area affected by alkaline etching (etched area) and to express it as a percentage of the total area analysed. A very good correlation was found between the etched area and alkali–silica reaction potential of investigated rocks measured according to an accelerated mortar bar test.