Mineral thermometry of anthropogenic pyrometamorphism products in high-fired gypsum mortars

• Applicant: Dr. Thomas Schmid
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Analytical Chemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 458702855

The project aims to retrieve information on ancient manufacturing processes of mortar binders and corresponding technological advancements of past societies. Scientific analyses often provide the only access to this lost knowledge, enabling to re-enact historical building materials for both, restoration and modern use due to advantageous properties (e.g., less CO2 emission during calcination, or higher strength than modern counterparts), but systematic studies on indicators for process parameters of man-made materials are missing so far. High-fired medieval gypsum mortars and early industrial Keene’s marble cement will be studied. In addition to the determination of calcination temperatures, the unambiguous identification of marble cement in historical samples is aimed to be achieved. Mineral thermometry is the determination of the temperature a rock or material was previously exposed to, by studying mineral compositions and mineral-related properties resulting from pyrometamorphic conversions. While the established geothermometry is largely based on pressure-temperature diagrams of equilibrium compositions, the project aims to develop a mineral thermometry of mortars, which needs to consider the influence of specific production and processing workflows, e.g., by calcining and hydrating minerals under defined laboratory conditions. Main analytical innovation is the use of Raman microspectroscopic imaging, which combines chemical-structural information with micrometre-scale spatial resolutions in a unique fashion. This is of paramount importance for the detection of indicative mineral microparticles within complex mortar mixtures (e.g., silicates in gypsum mortars). Due to overlapping compositions and the generally applicable strategies for the application of mineral thermometers aimed to be developed, the project results will be relevant for a range of mortar components including cement, lime and slag. Beyond, mineral thermometry is expected to have impact in additive manufacturing (3D printing) involving ceramic processes. The limestone blocks at Notre Dame, Paris, classified only according to heat-induced colour changes so far, demonstrate the need for a quantitative approach to stability assessments of building stones after fire events. A Raman spectroscopic database library planned to be developed – connecting Raman spectra with mineral types and elemental compositions – is expected to have impact in soil, geological and planetary studies. An interdisciplinary collaboration between the analytical sciences and fields including art technology, conservation-restoration, and the re-enactment of historical mortars is needed to reach the project goals. Project partners from the latter fields provide samples of high-fired medieval and early industrial gypsum mortars from the alpine region, Keene’s marble cement from 19th-century building Neues Museum Berlin, and high-fired gypsum-based mortar binders re-enacted for modern use.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Cooperation Partner: Dr. Petra Dariz