EXAMINATION OF HYDRAULIC LIME MORTARS OF MEDIEVAL CHURCHES IN DENMARK.

Transcription

1 EXAMINATION OF HYDRAULIC LIME MORTARS OF MEDIEVAL CHURCHES IN DENMARK. Helle Dam Andersen 1, Hans Dieter Zimmermann 1, Henrik Friis 1, Ulrich Schnell 2 1.Geologisk Institut, C. F. Møllers Allé, DK-8000 Århus C. 2. Nationalmuseets bevaringsafdeling, Brede, DK-2800 Lyngby. Abstract We have studied hydraulic lime mortars of two different types: a) Mortars from four Danish village churches built between 1100 and 1450, and - for comparison - b) three modern mortars. Mortar mineralogy and texture were examined macroscopically and under the polarizing microscope. Phase composition and chemistry were determined by x-ray diffraction (XRD), x-ray fluorescence (XRF) and microprobe analysis. Scanning electron microscopy (SEM) was used to establish the microstructure and morphology of the different phases. This study has three objectives: We want 1) to clarify how the church mortars carbonatised and 2) to determine the causes for their disintegration. Furthermore, we are trying 3) to compose a mortar suitable as replacement mortar for the restauration of these churches. The old church mortars typically show a low aggregate/binder ratio of approximately 1:3. The mineralogy of the aggregates is varied. Besides quartz, it contains up to five volume percent of strongly seriticised feldspar and accessory amounts of biotite, hornblende and iron-oxide. Also, clasts of limestone and fossil fragments are seen in the samples. Among the possible replacement mortars we produced, one was made using bryozoic limestone from a Danien locality in Jutland, Denmark. The limestone for the original mortars was probably from this deposit. 37

2 1. Introduction Damage of medieval mortars manifest as peeling off of fragments, and is caused by alternating crystallisation and dissolution of salts. These form in response to humidity and temperature variations. The replacement of wall mortars damaged in this way requires a mortar type which - in terms of surface structure and properties - ressembles the original ones as much as possible, but at the same time is more resistent to decay. With earlier church restaurations, cement mortar was the preferred mortar type. This, however, proved disadvantageous, as various salts were found to precipitate on it. The pertaining damage makes it necessary to remove this mortar again and to replace it by an appropriate hydraulic lime mortar. The same goes for mortars containing unburnt clay particles. They too, tend to come off, and have to be substituted. 2. Materials Specimens of the old church mortars were provided by The National Museum of Denmark, and had been collected from the interior of four medieval churches on Zealand, Denmark. These were all intact mortar samples. The new mortars were prepared from products commercially available. One mortar was made from a ready-to-use mortar mix from the company Falkenløwe in Denmark. The other two mortars differ by their lime component: For the one we used bryozoic limestone from a Danien locality in Jutland, Denmark and for the other one Jurassic limestone from the company Skandinavisk Jurakalk A/S. The sand grading of the aggregate is from 0,2-0,5 mm. The mixing ratio was 1:3 binder to aggregate by volume. 3. Methods Macroscopic inspection. All mortar samples were examined with respect to size, shape, amount, colour and type of aggregates, colour of binder, presence of cracks and cavities, damages and precipitates. Microscopic examination. Mineralogy and texture were obtained by light microscopy of thin sections. X-ray diffraction. Phase composition was determined by XRD powder technique using a Philips PW X-ray fluorescence. The chemical bulk composition was analysed using Philips PW

3 Microprobe analysis. The chemical identification of the different mineral phases and determination of their composition were carried out on a JEOL JXA-8600 superprobe. Scanning electron microscope. Microstructure and morphology of the different mineral phases were investigated using a Camscan Maxim. 4. Results and discussion X-ray fluorescence was carried out on all the mortars but one (insufficient amount of sample). Figure 1 shows the large difference in the chemical bulk composition of the old and the new mortars. The SiO 2 concentration of the old mortars range from 38 to 70 weight percent, compared to 88 to 91 weight percent in the new ones. Concomitantly, the CaO content of the old mortars lies between 13 and 30 weight percent, whereas it is between four and five weight percent in the new ones. The volatile concentration differ accordingly: Old 11 to 24, new two to three weight percent. Also the Al 2 O 3 content differ. While the old mortars contain three weight percent, the new mortars contain only one weight percent Al 2 O 3. All the remaining oxides have weight percentages < 1 in all mortar types. Concentrations of TiO 2, Fe 2 O 3, Mn 3 O 4 and MgO are about the same in all samples % 20% 40% 60% 80% 100% SiO2 % CaO % Al2O3 % Na2O+K2O % Other oxides % Volatiles % Figure 1. Chemical composition of old church mortars (1-3) and new mortars (4-6). The term Other oxides include TiO 2, Fe 2 O 3, Mn 3 O 4, MgO and P 2 O 5. The concentrations are given in weight percent. X-ray diffraction patterns permit the identification of the main crystalline phases in the mortars. By this method we determined the phases present in the samples to be, for the 39

4 old mortars quartz, calcite, plagioclase and alkali feldspar, and for the new mortars quartz, calcite and alkali feldspar. In the mortar from the company Falkenløwe additionally portlandite was present. In all the mortars, the calcareous mass of the binder is cream coloured. In the old mortars, the aggregates show various colours from colourless over pink to red and black. In the new mortars the aggregates are beige to greyish. There is a good adherence between the aggregate and the binder in the old mortars compared to the new mortars, which easily crumble. This is probably due to the fact, that the proportion of binder to aggregate is approximately 3:1 and 1:3 in the old and new mortars, respectively. (Figure 2 and 3). Figure 2. Photomicrograph of a thin section of an old mortar, seen under crossed polars. The area of the photo is 1,0 mm x 1,3 mm. It shows quartz grains of different sizes and shapes in a calcite binder. In the upper left corner is a partly seriticised feldspar. The two black areas SE and NW of the center are pores. 40

5 Figure 3. Photomicrograph of a thin section of a new mortar, seen under crossed polars. The area of the photo is 1,0 mm x 1,3 mm. It shows round isometric quartz grains of almost uniform size. The calcite binder forms a rim around the quartz grains. A large pore (black) is seen to the right of the center. Also five smaller pores are seen in the photo. In both mortar types the aggregates mainly consist of quartz. In the old mortars the quartz grains vary between 0,1-1,0 mm, typically with angular to round shapes. In comparison the quartz grains in the new mortars vary between 0,1-0,5 mm, and mainly show round contours. In all the mortar samples, quartz is generally found as single grains, although minor amounts of multi-granular quartz aggregates are present, too. Also very fine grained flint particles are found. Apart from quartz the samples contain both plagioclase and alkali feldspar. These minerals are accessory minerals in the new mortars, but exist in concentrations of up to five volume percent in the old mortars. In these, half of the feldspars are more or less altered to sericite. Biotite, hornblende and iron-oxide are present as accessory minerals in all of the old mortars and in the new mortar from the Falkenløwe company. The binder consists in all of the samples of very fine-grained calcite. Because of the binder to aggregate ratio of 1:3 in the new mortars, the calcite is only seen as a thin rim around the quartz grains glueing the samples together. In the new mortars, pore spaces are interconnected. They form more or less spherical, millimeter wide, volumes. Also 41

6 cracks in the calcite rim are found. These, however, seem to be caused by drying of the mortars. A very different texture is displayed by the old mortars. This is largely due to their very different binder to aggregate ratio of 3:1. The aggregates are homogenously distributed in the binder. Two types of voids are seen: One is round and up to 0,3 mm in diameter, the other has the form of contraction cracks caused upon drying. The latter are up to five mm long and 0,05 mm wide. Hardly any of the pores in the old mortars are interconnected. In the old mortar samples relict bryozoans and limestone aggregates are seen in the binder. 5. Further investigations Our investigation of the mortars is still ongoing, conclusions cannot be made yet. It is obvious however, that a large difference exists between the mortars used for restauration today and the mortars used for construction work in medieval times. Further work will comprise the examination of another seven medieval mortars, together with six examples of the new mortar type. In addition, it is intended to use image analysis in order to determine the distribution of the sizes and shapes of the pores and of the aggregate grains. This will be carried out on thin sections, where the epoxy is mixed with fluorescent yellow dye. Also, cathodoluminescence will be performed to reveal the growth of different calcite phases in the samples. Furthermore, we will try to prepare a new mortar with an aggregate to binder ratio similar to that of the old mortars. Finally, we plan to do experiments to clarify how the mortars carbonatised. 6. References Although not cited explicitely in the text, the following references were used. 1. Schäfer, J.and Hilsdorf, H. K., Ancient and new lime mortars - the correlation between their composition, structure and properties, Conservation of Stone and Other Materials 2 (1993) Suenson, E., Dansk hydraulisk kalk og hydratkalk, København (1945) 3. Gülec, A. and Tulun, T., Physico-chemical and petrographical studies of old mortars and plasters of Anatolia, Cement and Concrete Research 27 (2) (1997) Luxán, M. P. and Dorrego, F., Ancient XVI century mortar from the Dominican Republic: Its characteristics, microstructure and additives, Cement and Concrete Research 26 (6) (1996) Sarkar, S. L., Microstructural investigation of renaissance mortar from Montreal, Québec, Canada, Cement and Concrete Research 22 (1992)