Sulfide oxidation in concrete aggregates: behavior under NaOH, NaCl, and bleach exposure and implications for accelerated mortar bar testing

This study investigates the oxidation of iron sulfides in concrete aggregates under conditions simulating accelerated mortar bar test (AMBT) protocols. An aggregate bearing both pyrrhotite and pyrite was exposed to NaOH (80 °C), NaCl (50 °C), and bleach (23 °C) solutions for 30 days. The chemical-mineralogical evolution was tracked using in situ techniques: micro X-ray fluorescence (μXRF), micro X-ray diffraction (μXRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA); and by bulk analyses such as X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), and total sulfur analysis. The results indicate an apparent sulfide reactivity order of pyrrhotite > framboidal pyrite > cubic pyrite. μXRF showed local sulfur depletion of 98% at the sample surface after bleach exposure, against 46% and 38% after NaOH and NaCl, respectively. Over the same period, the bulk sulfur content of the powdered aggregate decreased by approximately 27% in bleach, while remaining unchanged in NaOH and NaCl. The two measurements therefore describe different quantities: the oxidation attained at an exposed sulfide surface and the sulfur effectively removed from the aggregate. The alkaline solutions were neutralized from pH 12.5 to 7, consistent with acid generation by sulfide oxidation, which may in turn have limited the alkali-silica reaction (ASR) during the test period. These results indicate that the oxidizing capacity of the immersion solution, rather than its pH alone, governs sulfide oxidation, and identify solution stability as parameters worth considering in the optimization of accelerated test protocols.