Examination of Cr(VI)-contaminated concrete from a former hard-chrome plati
ng shop revealed the presence of long, thin crystals varying from white to
bright yellow. Many of the crystals examined by scanning electron microscop
y (SEM) were acicular and 25-100 mu m in length and 1-15 mu m wide. The com
position, determined by energy-dispersive X-ray spectroscopy, morphology, a
nd d spacings, measured by electron diffraction, identifies these crystals
as chromate enriched ettringite (Ca6Al2((S,- CT)O-4)(2)(OH)(12). 26H(2)O) W
ith 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 togethe
r by an amorphous appearing SiO2 phase. Ca-silicate-hydrate gel (CSH) has p
recipitated around ettringite-group crystals containing chromium indicating
that some CSH formation occurred after contamination of the concrete.