A comparative investigation of the degradation of pyrite and pyrrhotite under simulated laboratory conditions

The effect of the oxidation of iron sulfides contained in aggregates on concrete durability is well known. The mechanisms of formation of gypsum or ettringite have been well documented for a long time and also, for about 15 years, the formation of thaumasite although there are some unanswered questions concerning this compound, such as whether the ettringite is always needed as a precursor to thaumasite.However, there are few studies on behavioral differences of different oxidizable sulfides in the process of oxidation with more or less harming effects on concrete durability. As a result there is some confusion in the various standards of aggregates for concrete. The American Standard ASTM C294-05 (Section No. 13) does not make any distinction between the different sulfide types: marcasite, pyrite or pyrrhotite. In contrast, the European standard EN 12620:2008 specifically in paragraph 6.3.2 says that it is necessary to take special precautions if it detects the presence of pyrrhotite (an unstable form of iron sulfide FeS). Once confirmed of their presence the maximum total sulfur content is less than 0.1%. This amount is 10 times lower than would be permitted for aggregates containing other iron sulfides. This fact is exaggerated from the point of view of many engineers and could result that in practice, suitable aggregates have been rejected unnecessarily on some sites.This technical note presents the findings of a first experimental set-up that serves to analyze the products of an aqueous solution with constant oxygen supply in two pyrite and pyrrhotite samples as well as the physicochemical pH and electrical potential values of the solutions. The results show the existence of significant differences regarding the nature of the alteration products proving that the pyrrhotite is degraded more than the pyrite. In a second experimental set-up, the oxidation of both iron sulfides has been analyzed but three aggregate types coming from three different rocks were added: a marlstone, a limestone and a shale. In all three cases reveal that pyrrhotite is degraded more than pyrite.Despite the limitations in the representativity of this study in such a huge context, the results of this study permit some first reflections to be made about the normative that rules the characterization and quantification of sulfur compounds in concrete aggregates.

► The paper describes a methodology for measuring potential reactivity of iron sulfide.
► Pyrrhotite provides a larger quantity of SO42 − than pyrite to the solution.
► Contribution of host rock cannot be neglected and the normative should consider this.