I.12 Lime Mortar with Natural Hydraulic Components Characterization of Reaction Rims with FTIR-Imaging in ATR-Mode

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1 I.12 Lime Mortar with Natural Hydraulic Components Characterization of Reaction Rims with FTIR-Imaging in ATR-Mode Anja Diekamp 1, Roland Stalder 1, Jürgen Konzett 1 and Peter W. Mirwald 1 1 Institute of Mineralogy and Petrography, University of Innsbruck, Austria, anja.diekamp@uibk.ac.at, roland.stalder@uibk.ac.at, juergen.konzett@uibk.ac.at, peter.mirwald@uibk.ac.at Abstract Reaction rims of natural hydraulic relicts in historic mortars were investigated using a novel technology, a FTIR-spectrometer equipped with a focal plane array detector enabling in ATR-mode IR-imaging with a spatial resolution of 1.0μm. IR spectra show two regions with main absorption bands at cm -1 and cm -1. Bands at 1450cm -1 and 1396cm -1 correspond to the asymmetric stretching of CO 3 2-, indicating two different forms of CaCO 3 ; the cm -1 group of bands is assigned to Si-O stretching vibrations indicating C S H phases. The ratio of the integral absorbance of these two main regions of absorption bands shows an inhomogeneous spatial distribution in the reaction rim. Our investigation shows that the reaction rims consist of CaCO 3, C S H phases and SiO 2 gel in variable proportions. In areas with low carbonate-to-c S H phase ratios, carbonate is mainly present as metastable aragonite. The associated C S H phases show a comparatively low Ca/Si ratio and a high degree of polymerization. In areas with high carbonate-to-c S H phase ratios, both, aragonite and calcite are present. The C S H phases show a comparatively higher Ca/Si ratio combined with a lower degree of polymerization. The presence of SiO 2 gel and aragonite indicate carbonation of C S H phases. The matrix outside of the cloudy reaction rim consists mainly of calcite. FTIR imaging measured in ATR-mode is a promising method to gain information on the hydraulicity of natural hydraulic mortars in its spatial distribution on a micrometer scale and to characterize the hydration and carbonation products as well. 111

2 1 Introduction The major mineral phase of limestone that is used for the production of lime mortar is calcite [CaCO 3 ]. If silicate phases, in particular clay minerals and quartz, are present in addition, then hydraulic components may form during firing of the limestone, provided that the firing temperatures are sufficiently high. In contact with water these hydraulic components (activated by calcium hydroxide) will hydrate to form calcium silicate hydrate [C S H] phases. The degree of hydraulicity of the binder has an important effect on the properties of the mortar. C S H phases are the main hydration products in Portland cement and are mainly responsible for its physical, chemical, and mechanical properties. Although the amount of hydraulic components may be very small in natural hydraulic lime binder (so-called sub-hydraulic binders), their effect on final strength and porosity cannot be excluded [1]. In Portland cement C S H appear as extremely variable, poorly crystalline or amorphous phases [2]. The detection of hydraulicity of a natural hydraulic lime binder, especially in sub-hydraulic binders, with common methods such as wet chemical analyses, differential thermal analysis (DTA) or X-ray diffraction (XRD) is difficult. Virtually nothing is known about the nature of the hydration products in sub-hydraulic lime binders. The degree of hydraulicity of any binder can be quantified with wet chemical methods by determination of the amount of acid-soluble silica [3, 4]. This is considered a proxy for the hydraulic components. Nevertheless, there are limitations imposed on the determination of the exact amount of the acid-soluble silica-content and its interpretation. This is the case (1) when carbonate-bound aggregate is used [4], (2) when very fine grain size fractions of the aggregates (e.g. quartz sand) act as pozzolanic components [5] or (3) dolomitic lime is part of the binder [6]. Alternatively the degree of hydraulicity can be evaluated by differential thermal analysis (DTA), where the weight loss of the binder fraction in the temperature range C (loss of water chemically bound to C S H or calcium aluminate hydrates) is set in relation to the weight loss at temperatures > 600 C (loss of CO 2 due to decarbonation) [7, 8]. However, to make use of this method, the presence of organic substances, clay minerals, hydrated salts, hydrous magnesium phases (e.g. in dolomitic lime mortars [9]) and relicts of carbonatebound aggregates whose decomposition would interfere with the release pattern of H 2 O or CO 2 must be excluded. In many cases XRD is not an option for the determination of hydraulicity due to the low modal amount and/or poorly crystalline character of the hydraulic phases. Even more difficult is the characterization of hydraulic products in their spatial context. SEM investigations on fracture surfaces only reveal the textures along an internal zone of weakness. In polished thin sections C S H phases are not 112

3 detectable by microscopy. Nevertheless, diffuse cloudy structures [5, 10] and reaction rims around hydraulic particles [6, 11] are taken as indication for the presence of hydraulic phases. While elemental mapping of Si with electron microprobe (EMPA) is able to detect the spatial distribution of C S H phases in reaction rims, the detection of C S H phases in binder areas is very difficult or even impossible because of the additional presence of very fine grained siliceous aggregates [12]. An alternative method to observe the spatial distribution and the structural character of solids is high resolution Fourier transform infrared (FTIR) microspectroscopy mapping or raster scanning [13]. In this study we characterized the distribution and nature of C S H phases in reaction rims with FTIR imaging using the attenuated total reflection (ATR) mode combined with a focal plane array (FPA) detector consisting of 64 x 64 elements. This combination enables a spatial pixel resolution of less than 1 µm. 2 Analytical methods 2.1 Microscopy and electron microprobe analysis The mineralogy and textures of a representative selection of historic mortar and plaster samples from Western Austria (Tyrol) and Northern Italy (South Tyrol) were studied using polished thin sections. Identification and quantitative analysis of the mineral phases were carried out by electron microprobe analysis (EMPA, JEOL JXA 8100) in both energy and wave length dispersive analytical modes. Analytical conditions of 15 kv accelerating voltage and 10 or 5 na beam current were used with spot sizes between 3 and 50 µm to minimize beam damage of the analyzed phases. To study element distributions wavelength dispersive X-ray mapping was performed. In addition to quantitative and mapping analysis, the backscattered electron (BSE) imaging mode was used to study textures and assemblages on a micrometer scale. 2.2 FTIR spectroscopy IR absorption spectra were recorded at room temperature using a Bruker Vertex 70 FTIR spectrometer combined with a Hyperion 3000 microscope equipped with a Ge-ATR-objective, a focal plane array (FPA) detector, a globar light source and a KBr beamsplitter. The FPA detector consists of 64 x 64 mercury-cadmiumtelluride (MCT) detectors, enabling in combination with the ATR-objective FTIR imaging of an area measuring 32 x 32 μm with a spatial pixel resolution of 0.5 μm. Using a FPA-detector, the locus of the measured point is determined by 113

4 the position of the detector itself and no optical aperture is necessary. Thus, the spatial resolution of the mid-ir-signal is only limited by the wavelength of the IRradiation (i.e., approximately 1 μm at 2000 cm -1 when an ATR-crystal with an index of refraction of 5 is applied). For each spectrum, 64 scans in the cm -1 range were performed with a spectral resolution of 8 cm -1. Thus, an area of 32 x 32 μm could be imaged within less than one minute, and by sequential analysis larger areas could be mapped in a few minutes. Data reduction was performed by defining the high and low wave number margin of an absorption band. In between these two points a linear background was subtracted and the area of the absorption band was calculated. The values of the most characteristic absorption bands were set into relation, colour-coded and graphically displayed as map. 3 Results and discussion 3.1 Microscopy and electron microprobe analyses Fig. 1 Thin section photomicrograph with (a) plane polars, (b) crossed polars of an area in a lime lump (Gothic plaster from Finstermünz-Fortress/Austria). A glassy, hydraulic relict (located at the left image edge) is in contact with a cloudy reaction rim and surrounded by a fine grained matrix. Parts of a pore structure are visible on the right hand side. (Field of view 0.18 mm, the frame in Fig. 1 (a) marks the region for the FTIR-imaging). Optical microscopic images of the investigated mortars often show diffuse cloudy structures in the binder and in binder related particles (lime lumps sensu stricto [10]). In some instances transparent glass patches of up to several hundred µm in size (Fig. 1) surrounded by these cloudy structures may also be present. In a Gothic plaster (~1500 AD) from the Finstermünz-Fortress in the upper Inn-Valley, Austria, the glasses are rich in K and Si (average composition: 67.3 wt% SiO 2 ; 15.1 wt% K 2 O; 7.4 wt% CaO; 6.7 wt% Na 2 O; 1.1 wt% MgO and 0.9 wt% Al 2 O 3 ) and frequently contain idiomorphic needles of wollastonite [CaSiO 3 ]. 114

5 They formed during partial melting of silicates in the firing process of impure limestone. The most likely source of K is mica (biotite or muscovite), which is a very common constituent of siliceous carbonates (marls). Even though individual phases within the cloudy rims cannot be identified due to their extremely small grain size, BSE-imaging clearly shows a mixture of phases. Elemental mapping reveals that these cloudy rims are Ca- and Si-rich. Large area EMPA with a rastered beam of the Ca- and Si- rich reaction rims yield a wide range in Ca/Si-ratios [12]. Fig. 2 FTIR-ATR image of the reaction rim (marked area in Fig. 1 (a)): colours (scale bar on the right hand side) represent the ratio of the integral absorbances of the absorption bands in the region cm -1 to the absorption bands in the region cm -1. Fig. 3 ATR spectra obtained from areas (a), (b) and (c) in Fig. 2: (a) area consisting of aragonite, C S H phases and SiO 2 gel; (b) area consisting of calcite, aragonite, C S H phases and SiO 2 gel; (c) area next to the reaction rim (within the white area in Fig.2), dominated by calcite. 115

6 3.2 FTIR spectroscopy For FTIR analyses two groups of bands were selected: group I in the range of cm -1 is assigned to carbonate phases, group II in the range of cm -1 is assigned to C S H phases. To detect the relative proportion of carbonates and C S H phases in the cloudy reaction rim (marked area in Fig. 1), the modal amount of these phases is then deduced from the area ratios of these groups of bands and are graphically displayed as map in Fig. 2. The map shows an inhomogeneous spatial distribution of the areas intensity ratios, whereby cold colours represent high II/I-ratios and warm colours (including pink and white) represent high I/II-ratios, respectively. Representative ATR spectra from the reaction rim and from the surrounding matrix were extracted and are shown in Fig. 3 (a), (b) and (c). All ATR spectra show bands at 1450 cm -1 and/or 1396 cm -1. These bands can be attributed to the asymmetric stretching (ν 3 ) of CO 3 2- in carbonates: the band at 1396 cm -1 to calcite; the bands at 1450 cm -1 and 1080 cm -1 to aragonite. This assignment is consistent with results from X-ray diffraction, where both polymorphs of CaCO 3 were detected in corresponding powder probes. The main bands at 1450 cm -1 and 1396 cm -1 are shifted towards lower wave numbers compared to literature data obtained by KBr-technique [14, 15], but compare well to ATR spectra from the RRUFF database [16]. Both spectra Fig. 3 (a) and (b) in the cloudy reaction rim show bands in the region of cm -1 and a shoulder at ~1200 cm -1. These bands are related to the presence of silica compounds. The bands at cm -1 correspond to the asymmetric and symmetric stretching vibrations of Si O bonds. C S H phases contain a characteristic set of bands centered at ~970 cm -1 which are assigned to Si O stretching vibrations of the Q 2 tetrahedra [15]. Yu et al. [15] show that this group of bands is shifted systematically towards higher wave numbers with decreasing Ca/Si ratios. They further show that in a mixture of C S H phases and SiO 2 gel the Si O band broadens, and the main position is shifted towards higher wave numbers. The shift of the Si-O band reflects a higher Si content and a higher degree of polymerization due to the presence of Q 3 and Q 4 sites in the SiO 2 gel. The occurrence of Q 3 sites is also specifically related to a broad shoulder at 1200 cm -1 [15]. Based to BSE imaging and elemental mapping [12], which reveal that the rim consists of newly formed Ca- and Si-phases, the 1041 cm -1 band in spectrum (a) and the bands at 964 cm -1 and 1026 cm -1, respectively, in spectrum (b) are attributed to C S H phases. This interpretation is in agreement with Yu et al. (op. cit.). In spectrum Fig. 3 (a) the main band in the cm -1 region is centered at 1041 cm -1 and is very broad. This indicates a high Si content of the C S H phases and, in addition, the occurrence of polymerized SiO 2 gel. The presence of SiO 2 gel is also consistent with the broad shoulder at 1200 cm -1. In spectrum Fig. 3 (b) bands at 910, 964, and 1026 cm -1 are assigned to Si O stretching vibrations at Q 2 sites of the C S H phase. The band at 1080 cm -1 is 116

7 attributed to aragonite. Again, the shoulder at 1200 cm -1 is assigned to Si O stretching vibrations in Q 3 sites of SiO 2 gel. The group of bands at cm -1 is shifted towards lower wave numbers, which might be due to a lower Si content of the C S H phases and a lower degree of polymerisation. In both areas (a) and (b) of the reaction rim, the presence of SiO 2 gel and aragonite indicate carbonation of C S H phases as observed in carbonated cement pastes [17]. Spectrum Fig. 3 (c) obtained from matrix (Fig. 1 (b)) is largely dominated by calcite. 4 Summary and conclusion K- and Si- rich glasses are present in the binder of ~500 year old mortars and plasters. The glasses formed during partial melting of silicates in the firing process of impure limestone. The reaction rims around the glass patches indicate that they act as hydraulic components, forming C S H phases. With FTIR-ATR imaging the spatial distribution of different phases in a reaction rim was determined on a micrometer scale. Our investigation shows that the reaction rims consist of CaCO 3, C S H phases and SiO 2 gel in variable proportions. In areas with low carbonate-to-c S H phase ratios, carbonate is mainly present as metastable aragonite. The associated C S H phases show a comparatively low Ca/Si ratio and a high degree of polymerization. In areas with high carbonate-to-c S H phase ratios, both, aragonite and calcite are present. The C S H phases show a comparatively higher Ca/Si ratio combined with a lower degree of polymerization. The presence of SiO 2 gel and aragonite indicate carbonation of C S H phases. The matrix outside of the cloudy reaction rim consists mainly of calcite. FTIR imaging measured in ATR-mode is a promising new method to gain information on the hydraulicity of natural hydraulic mortars in its spatial distribution on a micrometer scale and to characterize the hydration and carbonation products as well. 5 Acknowledgement This study was carried out within the framework an EU-Interreg project entitled Interdisciplinary investigations of selected monuments as key examples for optimized planning of conservation measures and a national project entitled Historic building materials in Tyrol. The funding for these projects, provided by the EU, the Austrian Federal Ministry for Education, Arts and Culture, the state of Tyrol and the Autonomous Province of Bolzano - South Tyrol is gratefully acknowledged. 117

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