Performance of Concrete Containing a Variety of Black Shale

Abstract

The combined mechanisms of oxidation of iron sulfides followed by sulfatation of both the surrounding calcite and the hydration products of Portland cement are well known. Nevertheless, cases where important breakdown of concrete structures have occurred, with less than 2% shale in the coarse aggregate fraction seem to be very rare.Based mainly on petrographic examination, the expansive mineral phases were identified in the black shale and in the concrete paste. It is believed that sulfuric acid, formed within the shale by oxidation of the pyrrhotite, has transformed the calcite into gypsum. In the concrete, this newly formed gypsum or that formed by sulfatation of the portlandite Ca(OH)2 has reacted in turn with the tricalcium aluminate of the cement paste to form ettringite. Both reactions, within the shale or with the cement paste, occur with an important volume increase.