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

Abstract

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.

Insights into the CaSO4–H2O System: A Raman-Spectroscopic Study

“…Paul Rohland mentions the temperature range of 525-600 • C to be ideal for synthesis of reactive anhydrite II, which coincides with the reactive and pure (bassanite-free) anhydrite II of the present study [57]. Our previous studies of several examples of high-fired medieval gypsum mortars yielded anhydrite II grains assigned to burning temperatures ranging from approximately 650 • C up to 900 • C (uncertainty of measurement: ± 50 K), [21][22][23][24] confirming previous light microscopic observations reported in References [58][59][60]. Even though heterogeneous heat distribution in medieval kilns is expected, including lower burning temperatures as well, anhydrite grains generated at the latter were completely converted into gypsum over the centuries, and only the much less reactive grains survived.…”

“…It was operated in the fully opened position (diameter of 1000 µm) in most experiments and tuned to 100 µm in depth-resolved experiments (surface vs. sub-surface of gypsum stones). That way, depth resolution is estimated to improve from 40 µm to 10 µm [24]. Typical acquisition time per spectrum was 60 s in ex situ experiments (see Figures 2,5,12,13,[17][18][19], usually split into several accumulations (depending on the intensity with the aim of avoiding saturation effects).…”

“…Spectroscopic data was plotted and analysed by using the software Origin 2019 (OriginLab). Exact band positions and band widths (expressed as full widths at half maximum or FWHMs, respectively) were determined by fitting Lorentzian peak profiles to the experimental data, according to References [21][22][23][24]. A detailed description of the choice of appropriate fit functions for Raman bands can be found in Reference [23].…”

“…Whereas predecessor studies of the CaSO 4 -H 2 O system employed Raman micro-spectroscopic imaging of high-fired medieval gypsum mortars for determining burning temperatures [21][22][23][24], the present study focuses on the structural sensitivity and in situ measurement capabilities of Raman spectroscopy while spatial resolution only plays a role in surface vs. sub-surface measurements within kinetic investigations and in heterogeneity tests of samples. The whole system of phases stable at room temperature ranging from gypsum to anhydrite II is studied, including their Raman spectra, temperature-dependence of spectroscopic data, phase transformations, and their significant process parameters as well as analytical approaches and structural insights to the α-form and β-form of bassanite and the sub-phases of anhydrite II.…”

Spectroelectrochemistry, the future of visualizing electrode processes by hyphenating electrochemistry with spectroscopic techniques

“…9, as reported by Schmid and Dariz. 119 It consists of a light source, most often a laser, focused on the sample using a microscope objective. The scattered light is guided back into the instrument and a filter removes the Rayleigh scattered light.…”