Development of a selective dissolution protocol for pyrrhotite quantification in sulfide-bearing concrete aggregates

Concrete and its basic components are essential resources in modern society, with their usage steadily increasing over the years. Ensuring the production of high-quality concrete mixtures is critical to the long-term economic sustainability and infrastructure progress of any developed nation. Coarse rock aggregates make up most of the total volume in any concrete structure. Therefore, their suitability and performance must be ensured through proper evaluation and selection. One of the most significant concrete issues associated with the use of unsuitable aggregates is Internal Sulfate Attack (ISA). ISA can occur when sulfide-containing lithotypes, namely pyrrhotite, become reactive after the concrete has set. Through a series of complex chemical reactions, pyrrhotite releases sulfur compounds into the cement paste, where they react with the primary hydrated phases to form expansive by-products. These secondary expansive mineral phases cause internal swelling and cracking, leading to a significant reduction in the structural integrity of the concrete. Currently, a procedure that measures the total sulfur content (TS%) of aggregates is widely used and serves as an effective screening method to quickly identify and select aggregates with low to no sulfide content. However, this analysis only reports the TS% of the entire sample and does not account for the presence of non-reactive sulfide minerals, such as chalcopyrite, pentlandite, and most types of pyrite. The objective of this study is to improve this widely used analysis by developing a selective dissolution protocol that can attribute the TS% of an aggregate sample to its specific pyrrhotite content. This will allow the accurate quantification of sulfur associated with pyrrhotite, the most reactive sulfide mineral. The methodology includes a primary analysis to determine the initial TS% of the sample. This is followed by two selective dissolution cycles to separate the sulfide phases present. The first step uses an HCl solution designed to selectively dissolve the pyrrhotite content. In the second step, the remaining sample is dissolved in an aqua regia solution to digest the other sulfide phases. The solid residue from the first dissolution is analyzed for TS%, representing the sulfur fraction associated with the other sulfide phases in the sample that are non-reactive in the context of ISA. The difference between the TS% value of the solid residue from the first dissolution and the initial TS% corresponds to the sulfur specifically associated with pyrrhotite. The final solid residue of the second dissolution is also analyzed for TS%, which should be negligible and close to zero. Preliminary results from aggregate samples with medium to high sulfur content have shown promising findings. Samples with up to 2.7% TS% showed negligible amounts of pyrrhotite content after the first dissolution step, with very low TS%, averaging 0.3% after the second dissolution. Samples with an initial TS% of 0.6% yielded an average TS% of 0.03% after the second dissolution phase. These results indicate that the parameters established for the method (acid strength, dissolution temperature, sample amount) perform as expected for the range of sulfur values currently under evaluation in the industry.