Chemie und Prozesstechnik
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Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars
(2019)
When used as a mineral binder, gypsum is thermally dehydrated and mixed with water, resulting in a paste hardening in the backreaction to calcium sulphate dihydrate (CaSO4 · 2 H2O). Although nowadays mainly hemihydratebased (CaSO4 · ½ H2O) binders are employed, higher firing temperatures in medieval kilns yielded anhydrite II (CaSO4). Except for the discrimination of the metastable phases anhydrite III and I due to different crystal structures, variations within the production temperature range of anhydrite II (approximately 300 to 1180°C) were not analytically accessible until recently. This study describes the development of an analytical technique, which is based on steady changes of band widths in room‐temperature Raman spectra of anhydrite II as a function of burning temperature. Raman microspectroscopic mapping experiments enable to pinpoint individual unreacted grains of thermal anhydrite in mortars and to discriminate them from natural anhydrites originating from the raw gypsum. The determination of band full widths at half maximum of down to 3 cm−1 and differences between them of a few tenths of wavenumbers is not a trivial task. Thus, a focus of this work is on peak fitting and strategies for correction of instrument‐dependent band broadening, which is often neglected also beyond the field of mortar analysis. Including other potential influences on band widths, burning temperatures of 400 to 900°C can be retraced in high‐fired medieval gypsum mortars with an uncertainty of approximately ± 50 K, as demonstrated with sample material of a stucco sculpture dated around 1400.
Draft standards for the determination of organic pollutants in the solid matter of environmental matrices such as sludge, treated biowaste and soil have been basically developed in the framework of the European standardization project HORIZONTAL. A research project financed by the German Federal Environment Agency was initiated to finalize some of these CEN standard drafts, since fully validated standard procedures are crucial for the evaluation of their reliability in the context of implementation in legislation on environmental health.
Approach: Appropriate test materials (< 2mm particle size) were prepared and homogenized from contaminated soils, sludge and treated biowaste containing polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), dioxins, furans and dioxin-like-PCB and served, along with reference solutions, as the basis for international interlaboratory comparisons. Performance data of three analytical standard procedures were obtained by the statistical evaluation of results received from 11 to 29 participants per test material.
Results: The overall variation coefficients of reproducibility (between-lab standard deviations) for the sum parameters were roughly between 10 and 35 %. The variation coefficients of repeatability (within-lab standard deviations) range between 3 % and 8 % and show no trend considering the substance groups or matrices.
The highest coefficients of reproducibility were found for the analysis of PAHs, which were between 26 and 35 %, depending on the matrix, whereas 7-17 % reproducibility was observed for toxicity equivalents (TEQ) comprising dioxins, furans and dl-PCB.
Conclusions: Overall, the results confirm that the procedures described in the Technical Specifications are fit for purpose for all three matrices and that the feasibility of the HORIZONTAL approach, to cover several matrices with one standard per analyte, was thereby proven.