INTRODUCTION: Technical OPC normally contains mixed sulfate carriers in varying amounts. Gypsum (calcium sulfate dihydrate, CaSO₄ ·2 H₂O) is added to the clinker before the milling process and dehydrates partially to bassanite (calcium sulfate hemihydrate, CaSO₄ ·0.5 H₂O) and anhydrite (CaSO₄). The dehydration temperature affects the resulting calcium sulfates [1]. Anhydrite exists in different polymorphes (anhydrite I-III) of which anhydrite III shows the highest reactivity with H₂O. Due to different kinetics of the calcium sulfate phases, which strongly influence hydration, it is crucial to be able to characterize the sulfate carrier composition in a cement system. Since bassanite and high reactive anhydrite III have very similar structures, a discrimination via X-ray methods is not easy. In this investigation a focus was put on the discrimination between the bassanite and anhydrite III structure as well as on transformation processes by rehydration of ambient humidity.

EXPERIMENTAL METHODS: Syntheses of different calicum sulfates were carried out by dehydration in chamber furnaces at 85 °C to 800°C ± 5 °C in air. The initial high purity gypsum (CaSO₄ ·2 H₂O, 99.9 %, Fluka) – was weighed in corundum crucibles of 60 ml size and dehydrated for 16 to 120 hours (deydration time is dependant on dehydration temperature). Preparation of the dehydrated gypsum samples for X-ray investigations was carried out under different humidity conditions (47 % relative humidity and 7 % relative humidity). The sample was covered by a capton film to prevent further reactions. Phase composition of the samples was examined at room temperature by quantitative X-ray powder diffraction (XRPD) with a Siemens D5000 X-ray diffractometer in combination with Rietveld refinements. Rietveld refinement was performed using the structural models (ICSD) of all occurring calcium sulfate phases. Refined parameters were: scale factor, zero displacement, background as Chebychev polynomial of 5th grade, crystallite size and lattice parameters for anhydrite III and anhydrite II. Lattice parameters of bassanite were kept fix in order to achieve stable refinement results.

RESULTS: All investigated powders consisted of at least two different calcium sulfates. Less reactive anhydrite II was found in all samples besides highly reactive anhydrite III and bassanite in different ratios. With increased dehydration temperature bassanite contents are decreasing in the samples. Anhydrite III is formed at the expense of bassanite and transformed to anhydrite II at higher temperatures. A determination of bassanite and anhydrite III in the same powder via Rietveld refinement can be accomplished with fixed lattice parameters of bassanite. Coevally, the investigations showed the hygroscopic character of dehydrated calcium sulfates. High humidity causes an instant transformation of anhydrite III to bassanite. Refinements of long time investigations showed a clearly detectable and continuous transformation of anhydrite III to bassanite over time. Rietveld refinement could be utilized to differentiate calcium sulfates with very similar crystalline structure and to measure transformation processes by rehydration in pure calcium sulfates.

REFERENCES:

[1] W. Abriel, K. Reisdorf, Dehydration Reactions of Gypsum: A neutron and X-Ray Diffraction Study, J. Solid State Chem. 85 (1990) 23-30

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