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Open AccessArticle

Raman Microspectroscopic Imaging of Binder Remnants in Historical Mortars Reveals Processing Conditions

by Thomas Schmid 1,2,*,† and Petra Dariz 2,3,†
1
BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Str. 11, 12489 Berlin, Germany
2
School of Analytical Sciences Adlershof (SALSA), Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
3
Bern University of Applied Sciences, Bern University of the Arts, Conservation-Restoration, Fellerstr. 11, 3027 Bern, Switzerland
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Heritage 2019, 2(2), 1662-1683; https://doi.org/10.3390/heritage2020102
Received: 26 April 2019 / Revised: 27 May 2019 / Accepted: 12 June 2019 / Published: 14 June 2019
(This article belongs to the Special Issue Advances of Analytical Chemistry in Cultural Heritage)
Binder remnants in historical mortars represent a record of the connection between the raw materials that enter the kiln, the process parameters, and the end product of the calcination. Raman microspectroscopy combines high structural sensitivity with micrometre to sub-micrometre spatial resolution and compatibility with conventional thin-sectional samples in an almost unique fashion, making it an interesting complementary extension of the existing methodological arsenal for mortar analysis. Raman spectra are vibrational fingerprints of crystalline and amorphous compounds, and contain marker bands that are specific for minerals and their polymorphic forms. Relative intensities of bands that are related to the same crystalline species change according to crystal orientations, and band shifts can be caused by the incorporation of foreign ions into crystal lattices, as well as stoichiometric changes within solid solution series. Finally, variations in crystallinity affect band widths. These effects are demonstrated based on the analysis of three historical mortar samples: micrometric distribution maps of phases and polymorphs, crystal orientations, and compositional variations of solid solution series of unreacted clinker grains in the Portland cement mortars of two 19th century castings, and the crystallinities of thermal anhydrite clusters in a high-fired medieval gypsum mortar as a measure for the applied burning temperature were successfully acquired. View Full-Text
Keywords: cement clinker remnants; polymorphic transitions; striations; remelted belite; high-fired gypsum; thermal anhydrite; pyrometamorphism; spectroscopic imaging; Raman microscopy cement clinker remnants; polymorphic transitions; striations; remelted belite; high-fired gypsum; thermal anhydrite; pyrometamorphism; spectroscopic imaging; Raman microscopy
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Schmid, T.; Dariz, P. Raman Microspectroscopic Imaging of Binder Remnants in Historical Mortars Reveals Processing Conditions. Heritage 2019, 2, 1662-1683.

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