Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1992-11-12
1994-11-08
Bell, Mark L.
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106736, 106766, 106769, C04B 2208
Patent
active
053623215
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a cement composition with reduced content of water-soluble chromate, a method for reducing the amount of water-soluble chromate in a cement composition, and to methods for producing a cement composition with reduced content of water-soluble chromate.
BACKGROUND OF THE INVENTION
It is well known that water-soluble chromate in cement gives rise to allergic reactions in workers subjected to contact with cement-containing products. In particular, construction workers who are subjected to regular skin contact with wet cement such as wet cement paste, wet mortar, and concrete mixes, have a risk of contracting chromate-related eczema.
It is known to reduce the content of water-soluble chromate in a cement composition by addition of ferrous sulphate. The ferrous sulphate may be added, e.g., during the preparation of cement-containing mixes or during manufacture of the cement, vide e.g. WO 82/02040. The iron(II) sulfate reduces Cr.sup.+6 to Cr.sup.+3, the latter having low solubility in cement-water mixes. The reaction between Fe.sup.+2 and Cr.sup.+6 takes place in aqueous solution, e.g. when water is added to a cement containing iron(II) sulfate.
SUMMARY OF THE INVENTION
It has now been found that manganese(II) compounds are effective for reducing the content of water-soluble chromate in cement compositions. Manganese(II) compounds, even in technical grade, have the advantage that they can be easily milled or ground to a small particle size well suited for addition to cement or cement compositions to obtain a satisfactorily homogeneous distribution. Some manganese(II) compounds such as manganese(II) sulfate have the advantage that they are oxidation stable in dry cement compositions even at high temperature, are available as dry free-flowing powders in technical grades and have high chromate-reduction efficiency when they are interground with cement.
DETAILED DISCLOSURE OF THE INVENTION
Thus, in one aspect, the invention relates to a cement composition comprising 1) a cement containing water-soluble chromate and 2) at least one manganese(II) compound in an amount sufficient to reduce the amount of water-soluble chromate to at the most 2 mg of Cr(VI) per kg of the cement, the determination of the chromate content being carried out by elution of the composition in water for 15 minutes at a water/cement ratio of 1, filtration, and determination of the chromate content in the filtrate.
The cement is one which, if no measures are taken, contains water-soluble chromate in amounts which are objectionable from a health hazard point of view, typically amounts of above 2 mg of Cr(VI) per kilogram of the cement, determined as described above, this determination preferably being performed in accordance with Danish Standard DS 1020. In this standard method, an agent which forms a coloured complex with chromate, i.e. s-diphenyl carbazide, is added to the filtrate obtained following elution of the cement, whereupon the intensity of the coloured complex is measured spectrophotometrically, and converted into content of water-soluble chromate by means of a calibration curve. As will be understood, the content of water-soluble chromate in the filtrate may also be measured in other suitable manner.
It is normally preferred that the manganese(II) compound is present in the composition in an amount sufficient to reduce the amount of water-soluble chromate to at the most 1 mg, preferably at the most 0.5 mg, in particular at the most 0.1 mg, especially at the most 0.01 mg of Cr(VI) per kg of the cement component.
The amount of manganese(II) compound sufficient to obtain the above reduction in the content of water-soluble chromate will typically be an amount representing a molar excess, calculated on the chromate, and thus, the minimum amount of manganese(II) compound is dependent on the natural content of water-soluble chromate in the cement in question. However, as a guideline, the manganese(II) compound may be present in an amount corresponding to from 1 to 1000 mmol man
REFERENCES:
patent: 1865021 (1932-06-01), Larmour
patent: 4572739 (1986-02-01), Rasmussen
patent: 4784691 (1988-11-01), Rasmussen
Chemical Abstract-"Change in the Electrode Potential of Zinc in Hardening Cement Paste" Berdov et al. (1989), vol. 12 pp. 52-55 (Russian).
Aalborg Portland A/S
Bell Mark L.
Marcantoni Paul
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