Investigations carried out by the Research Institute revealed that the lower values were not attributable to the omission of the oxidation step, but rather to slight temporal differences in the analysis. As a consequence of these differences, not all the pretreated cement extracts exhibited the typical red-violet coloration of the DPC-Cr(III) complex. In some cases, the test solution was red instead of red-violet (Fig. 1). This observation, which had been made by visual inspection, was confirmed by the UV/VIS spectra of the solutions.

In the chromate solution, the DPC-Cr(III) complex absorbs visible light. The absorption maximum ranges around a wavelength of about 540 nm. In the processed cement extract, by contrast, a solution component absorbs with an absorption maximum at a wavelength below 350 nm additionally. This generates a slightly yellow coloration and the yellowish portions in the solution cause the analysis solution as a whole to become red instead of red-violet. Obviously, another cation reacts with the diphenyl carbazone to form a complex generating yellow coloration. As a consequence, the concentration of the DPC-Cr(III) complex in the solution is reduced, which results in a deficiency in the findings of hexavalent chromium. Competing reactions did not occur during chromate analyses in which diphenyl carbazide was added immediately after acidification or which had a pH value of less than 2. A reduction of the pH value below 2, however, requires a prior oxidation step.

The reduction of bagged cements has been primarily accomplished by adding ferrous sulphate compounds in the form of granulate so far. The crystal water contained in this granulate may react with the cement to form a capsule of cement hydration products. Although this capsule reduces the solubility of the compounds, it also conserves the core of the ferrous sulphate particles. This conservation enhances the durability of reducing efficacy. When the cement sample is suspended or when the cement is processed, the encapsuled particles are comminuted mechanically, which accelerates their dissolution.

If, however, the chromate reducing agent in the cement is fine-grained, a hydrate layer is formed around the particles of the chromate reducing agent as well, but the mechanically aided dissolution of the reducing agent is possible to a limited extent only. Such a fine-grained structure of the reducing agent is obtained when it is ground together with the cement or the cement components. For that reason, the cement industry has increasingly used tin(II) sulphate free of crystal water when the chromate reducing agent is metered in this way. The costs incurred for the use of tin(II) compounds are, however, significantly higher than those for reducing agents based on ferrous sulphate. The Research Institute is therefore conducting trials on ferrous sulphates low in crystal water. The trials are to clarify whether these compounds can be of any help in reaching a compromise between technically convenient metering and adequately durable reducing efficacy.