Answer: Your pictures show examples of staining due to the oxidation of the pyrite or marcasite. The process is identified as "other reactions" in section 2.1.8 of ACI 221 "Normal Weight and Heavyweight Aggregates." It might appear that the aggregate dissolves to leave the popout, but actually the popout occurs first, and then the mineral weathers to leave the telltale stain.
The popout forms because a chemical reaction creates a substantial volume increase in the area immediately surrounding the aggregate piece. According to Mielenz (Ref. 1), this is a three-step chemical reaction.
After the popout occurs, iron sulfide material becomes exposed to water and air. In his research on the staining caused by Thames River pyrite, British researcher H. G. Midgley identified the brown stain that often appears to be flowing from the surface defect as goethite, an iron oxide. The only effective way to avoid these types of popout reactions and stains is to make sure these aggregates aren't found in the coarse aggregate used for the concrete. Pyrite and marcasite are close enough in specific gravity that mechanical or hydraulic separation during crushing, screening, or washing is economically infeasible. Producers should purchase aggregates from sources free of ferrous sulfides when supplying concrete used for decorative flatwork. This might be difficult, as testing coarse aggregates for ferrous sulfides is not a requirement for meeting ASTM C33, "Standard Specifications for Aggregates." Nor is this particular reaction identified in ACI 221's Table 1.1, which shows properties of concrete influenced by aggregate properties.
Producers need to work with their aggregate suppliers to prevent the problem by first identifying those ferrous oxides in sources that are potentially deleterious. Once identified, geologists can check potential aggregate sources for their presence and concentrations using a petrographic examination, as outlined in ASTM C 295, "Standard Guide for Petrographic Examination of Aggregates for Concrete."
Mielenz suggests three procedures that can be used to detect potentially deleterious types and proportions of ferrous sulfides.
The Midgley procedure requires the immersion of suspected particles in saturated lime water. Technicians then observe the formation of blue-green ferrous sulfides that turn brown when exposed to air. Midgley reports that lack of any reaction within 30 minutes indicates that the ferrous sulfides are absent or are not potentially deleteriously reactive in concrete.
The Seaton procedure is now incorporated in ASTM Specifications C 330 and C 331 for lightweight aggregate. The method involves exposure of a portion of the aggregate to a steam bath for 16 hours, after which the degree of stain resulting from the liberation of iron compounds is noted and the proportion of such compounds is determined by partial chemical analysis.
In a third test described by Hagerman and Roosaar, the aggregate is mixed in concrete cast into beams fitted with terminal gauge studs, and the specimens are subjected to laboratory or outside storage for several months. During this period, the beams are measured for length change.
- Richard Mielenz, "Reactions of aggregates involving solubility, oxidation, sulfates, or sulfides," Highway Research Record No. 43, Highway Research Board, 1963, pp. 8-18.
- H. G. Midgley, "The staining of concrete by pyrite," Magazine of Concrete Research: August 1958,Vol. 10, No. 28, pp. 75-78 in Section 2.1.8.