On an architectural concrete job where we used plastic-coated plywood to get an especially smooth finish, we were shocked to find random bluish stains marring what should have been a beautiful, smooth surface. On areas near form joints and where plain plywood was used, the concrete color was mostly normal. What could have gone wrong?
From time to time bluish-green or greenish-blue discoloration is reported. Usually it is linked to iron sulfides and ferrous oxides found in blast furnace slag used either as aggregate or in the manufacture of slag cement. This question is quite similar to one reported nearly 20 years ago (Concrete Construction, March 1968, page 80), even to the irregular, free-form shapes of the discolorations. The stains seem to be more severe where the forms are more airtight. The unwanted color generally will fade with age where the concrete is exposed to indoor drying or outdoor weathering conditions. If it is necessary to speed up the decolorizing action, wash the surface with a 3 percent solution of hydrogen peroxide as early as possible after removing the forms.