Physicochemical evaluation of oxidation of diabase aggregates with sulfide minerals: influence of particle size, pH of the medium and pyrite concentration

Abstract

Large constructions, such as concrete dams, commonly use aggregates extracted from rock masses near the implantation sites, which may contain sulfide minerals in its composition. These aggregates, inside the concrete, may oxidize, forming sulfate ions (SO42-), which can cause the internal sulfate attack. Therefore, in order to study aggregates for concrete dosages of these structures, aimed to evaluate on this paper the oxidation process of aggregate containing sulfide minerals in amounts of up to 5.0 %, by way of studies into the effects of particle size and pH of the medium (Study 1) and the pyrite content (Study 2). In Study 1, diabase aggregates containing pyrite (FeS 2) were separated into three particle sizes (sand, coarse aggregate 0 and coarse aggregate 2). Samples were analyzed in two groups and immersed in solutions of distilled water and calcium hydroxide, with pH and electric potential measurements. In Study 2, diabase sand from the same origin of Study 1, but in smaller particle size, and natural aggregates (sand) with the addition of pyrite in four levels (0, 0.5, 1.0 and 5.0 %) were immersed in distilled water, with measurements of pH, electric conductivity and total dissolved solids. The results of the tests, under constant oxygenation, showed that the reduced grain size and alkaline medium accelerate the oxidation reactions, in contrast to aggregates immersed in neutral solutions. Comparing the results by ANOVA test indicates the presence of around 0.5 % of sulfide mineral in the diabase aggregates, with better results from monitoring of electric conductivity and dissolved solids in contrast to pH readings.