A contractor called the cement manufacturer when he noted a green coloration in bleed water when placing and consolidating concrete. The manufacturer conducted laboratory tests at the cement plant in an effort to reproduce the coloration, then retained CTLGroup to investigate and provide an external opinion on possible causes of the discoloration.
Cement and clinker samples were sent to CTLGroup for analysis. The firm used chemical analysis by X-ray fluorescence (XRF) and crystalline phase analysis by X-ray diffraction (XRD) to characterize the materials.
XRF analysis provided the first clue to the coloration problem: the suspect cement and its companion clinker both contained significantly higher amounts of manganese and chromium than does normal cement. This difference, however, seemed to cause no significant change in the two cements' clinker phase composition, according to XRD results.
To further investigate the role of these metals in causing the coloration, CTLGroup chemically analyzed extracts by atomic absorption spectroscopy. Extracts were obtained by adding extra water to the samples, shaking the mix, and filtering the water. The analysis revealed elevated levels of water-soluble chromium in the extracts of the suspect cement and its companion clinker, as compared to the normal cement. Further investigation, using a combination of techniques including selective dissolution, XRF, and XRD, suggested that the water-soluble chromium which caused the coloration was present in the form of a salt, probably potassium or sodium chromate.
CTLGroup recommended that the client assess the raw feed components, including slag, to determine what was causing high chromium levels in the cement and then dissolving upon contact with water during concrete mixing.