When the basement floor of a church in the New Edinburgh area of Ottawa heaved, it was discovered that the concrete under the floor had been reduced to a mushy consistency due to attack by sulfate solution formed by the oxidation of pyrite in the underlying shale. Gypsum was the main product in the weathered zone of the shale. It is shown that heaving resulted from the growth of gypsum crystals between the lamellae in the shale. Jarosite, which is formed by a series of oxidation reactions from pyrite, was also found in the weathered shale. Some oxidation reactions occur only in the presence of sulfur bacteria. The cementitious portion of the concrete was leached out by acid, leaving a weak porous material. The importance of safeguarding concrete in contact with potentially expansive shales from sulfate attack is emphasized.