heritage Article Raman Microspectroscopic Imaging of Binder Remnants in Historical Mortars Reveals Processing Conditions 1,2, , 2,3, Thomas Schmid * y and Petra Dariz y 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; [email protected] 3 Bern University of Applied Sciences, Bern University of the Arts, Conservation-Restoration, Fellerstr. 11, 3027 Bern, Switzerland; [email protected] * Correspondence: [email protected] or [email protected] These authors contributed equally to this work. y Received: 26 April 2019; Accepted: 12 June 2019; Published: 14 June 2019 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. Keywords: cement clinker remnants; polymorphic transitions; striations; remelted belite; high-fired gypsum; thermal anhydrite; pyrometamorphism; spectroscopic imaging; Raman microscopy 1. Introduction Morphology and phase assemblage of unreacted binder grains in the hydrated and/or carbonated binder matrix of historical mortars reflect the composition, comminution, as well as the firing and cooling history (temperature of firing, residence time, kiln atmosphere, etc.) of the kiln raw feed. Thus, detailed analyses of historical mortars are particularly essential in developing a better understanding of the parameters of the historical production of mineral binders, which is a prerequisite for tailoring compatible restoration materials. Microscopically resolved measurements with high structural and chemical specificity are needed to pinpoint and analyse such indicative mineral clusters within the mortar samples. Raman measurements on polished thin sections in the imaging mode, complemented by scanning electron and (polarised) light microscopies, have demonstrated fulfilling the demands for analysis of architectural heritage; the utility of Raman microscopy particularly with regard to the Heritage 2019, 2, 1662–1683; doi:10.3390/heritage2020102 www.mdpi.com/journal/heritage Heritage 2019, 2 1663 Heritage 2019, 2 FOR PEER REVIEW 2 assessment of the original composition of aged mortars and the differentiation of the alteration phases 45 ismicroscopy discussed inparticularly Refs. [1–9 ].with regard to the assessment of the original composition of aged mortars 46 and theRaman differentiation spectroscopy of the yields alteration structural phases fingerprints is discussed that in Refs. enable [1–9]. the identification of organic 47 moleculesRaman and spectroscopy inorganic phases. yields Based structural on inelastic fingerprin light scattering—alsots that enable termedthe identification Raman scattering—the of organic 48 analyticalmolecules and technique inorganic provides phases. peaks Based (termedon inelastic bands) light thatscattering—also represent Raman-active termed Raman molecular scattering— or 1 49 crystal-latticethe analytical vibrations, technique whichprovides appear peaks at specific(termed vibrational bands) that frequencies represent or Rama wavenumbersn-active molecular (unit cm− or), 50 respectivelycrystal-lattice [10 vibrations,]. The samples which are appear excited byat anspecific intense vibrational and monochromatic frequencies source or wavenumbers of electromagnetic (unit 51 radiation,cm−1), respectively typically a[10]. continuous The samples visible laserare excited beam. Theby lightan intense that is scatteredand monochromatic by the sample source contains of 52 photonselectromagnetic with frequencies radiation, di typicallyffering from a continuous the excitation. visible This laser diff beam.erence The is called light that the Ramanis scattered shift by and the it 53 equalssample the contains frequency photons of a molecularwith frequencies or crystal-lattice differing from vibration, the excitation. which is This excited difference by this processis called (see the 54 FigureRaman1 ).shift Being and dependent it equals the on symmetry,frequency atomicof a molecular masses, or bond crystal-lattice strengths, lengths,vibration, and which angles, is theexcited set 55 ofby its this vibrational process (see frequencies Figure 1). (termed Being dependent the vibrational on symmetry, spectrum) atomic is highly masses, specific bond for strengths, a certain molecule lengths, 56 orand crystal. angles, Thus, the set Raman of its spectroscopyvibrational frequencies is related to (t infraredermed the (IR) vibrational spectroscopy spectrum) in terms is ofhighly information specific 57 contentfor a certain [11]. molecule or crystal. Thus, Raman spectroscopy is related to infrared (IR) spectroscopy 58 59 60 FigureFigure 1.1. Basic concept of elastic (Rayleigh) and inelastic (Raman) scattering of lightlight byby moleculesmolecules oror ν 61 crystals.crystals. AA monochromatic monochromatic laser laser beam beam with with frequency frequencyν0 excites 0 excites a characteristic a characteristic vibration vibration of a chemical of a ν 62 compoundchemical compound having the having frequency the frequencyνvib. The lightvib. The can light be either can be Rayleigh either Rayleigh scattered scattered resulting resulting in light ofin ν ν ν 63 thelight same of the frequency same frequencyν0 or Raman 0 or scatteredRaman scattered yielding yielding either Stokes either (ν Stokes0 νvib ()0 or − Anti-Stokesvib) or Anti-Stokes shifted ν ν − 64 (shiftedν0 + νvib ( )0 photons.+ vib) photons. 65 TheThe useuse of of visible visible light light instead instead of IRof radiation IR radiation is advantageous is advantageous for several for several reasons. reasons. 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The for latter the is focusing a measure angle, for the with focusing a high NAangle, or 70 angle,with a respectively,high NA or relatingangle, respectively, to a small focus relating spot, to and a small thus highfocus resolution spot, and (forthus a high detailed resolution explanation, (for a 71 seedetailed the Supplementary explanation, see Information the Supplementary for Ref. [13 Informatio]). The focusn for spot Ref. also [13]). shrinks The focus if a shorter spot also wavelength shrinks if 72 ofa shorter light is employed.wavelength Therefore, of light is theemployed. lateral resolution Therefore, of the IR microspectroscopylateral resolution of is IR in themicrospectroscopy order of several 73 tois in 10s the of orderµm, whileof several sub-micrometre to 10s of µm, resolution while sub- canmicrometre be achieved resolution with Ramancan be achieved microscopes with that Raman are 74 operatedmicroscopes in the that range are operated of visible in lightthe range [11]. of Due visible to this light spectral [11]. Due range, to this both spectral optics range, and samples both optics that 75 areand optimised samples that for classical are optimised light microscopy for classical are light typically microscopy compatible are withtypically this microscopic compatible technique.with this 76 Thus,microscopic polished technique. cross sections Thus, polished and thin cross sections sections of materials and thin sections on glass of slides materials can on be glass analysed slides with can 77 Ramanbe analysed microspectroscopy. with Raman microspectroscopy. 78 ThisThis last point point we we have have to to discuss discuss in inlittle little more more detail detail with with respect respect to typical to typical limitations limitations and