Precipitates in a Cr(VI)-contaminated concrete

Examination of Cr(VI)-contaminated concrete from a former hard-chrome plating shop revealed the presence of long, thin crystals varying from white to bright yellow. Many of the crystals examined by scanning electron microscopy (SEM) were acicular and 25-100 mu m in length and 1-15 mu m wide. The composition, determined by energy-dispersive X-ray spectroscopy, morphology, and d spacings, measured by electron diffraction, identifies these crystals as chromate enriched ettringite (Ca6Al2((S,- CT)O-4)(2)(OH)(12). 26H(2)O) With the mole fraction of CrO42- in the SO42- position being 0.41 and 0.72. A nearly pure CrO42- - hydrocalumite (3CaO . Al2O3. CaCrO4. nH(2)O) is also observed. Some of these crystals appear to be pseudomorphs of ettringite. The CrO42--hydrocalumite crystals are coated with smaller acicular crystals that are most likely solid solutions between Si-ettringite (Ca6Al2(SiO3)(3)(OH)(12). 26H(2)O) and CrO42--ettringite. These crystals are bound together by an amorphous appearing SiO2 phase. Ca-silicate-hydrate gel (CSH) has precipitated around ettringite-group crystals containing chromium indicating that some CSH formation occurred after contamination of the concrete.