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

摘要

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.