LIME MORTAR FOR RENDERS: IS THERE A RELATIONSHIP BETWEEN FINISHING TECHNIQUE AND PROPERTIES? PRELIMINARY RESULTS

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1 LIME MORTAR FOR RENDERS: IS THERE A RELATIONSHIP BETWEEN FINISHING TECHNIQUE AND PROPERTIES? PRELIMINARY RESULTS A. Jornet 1), M. Somaini 2) and A. Romer 3) 1) Laboratorio Tecnico Sperimentale, DACD-SUPSI, Switzerland 2) Conservazione e Restauro, DACD-SUPSI, Switzerland 3) Bausystem- & Baustofftechnik, Switzerland 1. Introduction Lime mortars have traditionally been used both in wall construction, and as protective and decorative treatment, in harling, thrown coating, and in rendering, smoother finish. Experience gathered in investigations conducted on historic buildings shows that once, skilled craftsmen were able to obtain a large variety of surface textures producing many different aesthetic effects (Emmenegger, 1994, Historic Scotland, 1996). Although the lime based construction materials have been displaced by modern materials for more than a century, in the last two decades there has been a quite wide revival in the use of lime, specially in conservation and restoration work. Thus, although not without difficulties, lime has been successfully re-introduced in Scotland (Historic Scotland TAN 1, 1995) and Sweden (Lime News, 1995), and the interest for a better knowledge about lime technology and lime use has considerably increased in many other countries (Historic Scotland, 2000). Different working groups dealing with lime production and lime technology have been created (Eurolime, Cost Action C5), and a basic understanding on lime technology and how the lime mortars behave, has been newly developed. However, additional research is still required in order to ensure a successful application and in order to produce lime mortars with a good durability. The preliminary results reported here are part of an on going project intended as a contribution aiming at the assessment of basic knowledge related to lime mortar as such and to lime mortar surface coatings. The research project is composed of three main parts: the first one is focused on the investigation of the relationship between mortar composition and properties, the second one concerns the investigation of the relationship between finishing technique and properties, and the third one deals with the investigation of the relationship between historic wall painting technique and properties. Although the testing programme of this second part contains the determination of porosity parameters, water absorption coefficient and rate of water vapour diffusion, along with the examination of the microscopic features related to the different finishing techniques used, only the results concerning fresh mortar and mechanical properties, and the microscopic observations will be presented in this paper albert.jornet@supsi.ch Pagina 1 di 9

2 2. Materials and samples Insulating wood panels of 600x300x30 mm were used as substrate for the application of eleven different mortar finishes. On top of the panels, a first, thin mortar coat prepared by mixing cement type CEM II B-LL 32.5 R and a commercially available, well graded sand (Figure 1) with a void ratio of 35 % and a maximum grain size of 4 mm, was applied by throwing. The components were mixed in dosages corresponding to 600 kg/m 3 and 1600 kg/m 3 respectively, and to a w/c ratio of One week later, a single mortar mix was prepared mixing 1 part by volume of fat lime putty and 3 parts of the previously mentioned sand. A small amount of water was added to the mix in order to obtain the right workability. With this mortar, a second coat about 10 mm thick, was applied on top of the first one. From both mixes, bulk density (UNI EN , 2000), porosity (UNI EN , 2000), and consistency (UNI EN , 2000), were measured on fresh mortar. In addition, 9 standard prisms 40x40x160 mm were prepared Passing [%] Metric size [mm] Figure 1: Grain size distribution curve Using a wide variety of techniques, the following eleven different surface textures were obtained: left as thrown (I), thrown and levelled with the trowels edge (II), thrown, levelled and flattened (III), applied with the back of the steel trowel and flattened (IV), thrown, levelled, flattened and smoothed (V), like V and rubbed off with the lime brush (VI), like V and rubbed off with the jute sack (VII), like V and rubbed off with the sponge (VIII), like V and rubbed off with the wood float (IX), like V and rubbed off with the felt float (X), and finally, like V and coated with a lime-fine sand wash (XI). The prepared panels were stored at 20 C and 80 % r.h. for 24 hours and they remained for additional 4 to 6 days at the laboratory. They were then stored outdoors, in a rain protected area for at least 56 days, while the prisms were stored at the laboratory at about 20 C and 50 % r.h. Small prismatic samples were cut from the panels with a diamond blade saw and after resin pre-impregnation, thin sections were prepared and examined albert.jornet@supsi.ch Pagina 2 di 9

3 using optical and fluorescence microscopy. Flexural and compressive strength, and unit weight were determined on the standard prisms of both mixes (UNI EN ). 3. Results Fresh mortar properties In Table 1 it can be observed that the first coat mortar is characterized by a fluid consistency with 222 mm spread, and by a normal unit weight, for a Portland cement mortar, while the lime mortar shows a lower, although normal, unit weight value and a plastic consistency with 180 mm spread. The porosity is higher in the lime putty mix. Table 1: Fresh mortar properties Property 1 st coat 2 nd coat Unit Weight [kg/dm 3 ] Porosity [vol. %] Spread [mm] Mechanical properties Indicatively, flexural and compressive strength and unit weight of the Portland cement mix have been reported along with the values of the lime mix in Table 2. The values summarized there were obtained on 28 days old prisms, for the Portland cement mortar, and on 90 days old ones, for the lime putty mortar. The differences observed are related to the different nature of both mixes and correspond to what it could be expected. Table 2: Mechanical properties Properties 1 st coat 2 nd coat Flexural strength [N/mm 2 ] 5.3± ±0.02 Compressive strength [N/mm 2 ] 20.5± ±0.05 Unit Weight [kg/dm 3 ] 1.99± ±0.00 Microstructure The observation of the thin sections shows that a well graded sand, composed mostly by silicatic, angular and sub-angular grains, is uniformly distributed in a carbonated matrix. In addition to fragments of different metamorphic rocks, crystals of quartz, feldspars, and mica can also be observed. Carbonatic grains are rare, while mica particles can frequently be seen. Some amorphous, slag-like grains are present as well. The finegrained matrix is composed by calcite crystals of micrometric size. The structure is characterized by a high capillary porosity and by a relatively high amount of irregularly shaped pores or cavities, with sizes comprised between 0.2 and 0.8 mm. Very few round pores and shrinkage cracks have been observed (Figure 2) albert.jornet@supsi.ch Pagina 3 di 9

4 Figure 2: Photomicrograph showing the mortar structure taken under crossed polars. The long axis of the picture corresponds to 2.25 mm. Figure 3: Photographs taken with sidelight and showing the surface textures I and II (left side), and the corresponding photomicrographs taken under crossed polars of the same textures (right side). The long axis of the right side pictures = 1.0 mm albert.jornet@supsi.ch Pagina 4 di 9

5 Figure 4: Photographs taken with sidelight and showing the surface textures III, IV and V (left side), and the corresponding photomicrographs taken under crossed polars of the same textures (right side). The one corresponding to texture V shows a clearly visible upper surface layer. The long axis of the right side pictures = 1.0 mm albert.jornet@supsi.ch Pagina 5 di 9

6 Figure 5: Photographs taken with sidelight and showing the surface textures VI, VII and VIII (left side), and the corresponding photomicrographs taken under crossed polars of the same textures (right side). The one corresponding to texture VI shows clearly visible brush stroke tracks. The long axis of the right side pictures = 1.0 mm albert.jornet@supsi.ch Pagina 6 di 9

7 Figure 6: Photographs taken with sidelight and showing the surface textures IX, X and XI (left side), and the corresponding photomicrographs taken under crossed polars of the same textures (right side). The long axis of the right side pictures = 1.0 mm. Independently of the surface finishing technique used, the surfaces show a thin layer with calcite crystals slightly coarser than those present in the matrix (Figures 3 to 6). In some cases, this layer is particularly clearly visible like for the textures V and VI albert.jornet@supsi.ch Pagina 7 di 9

8 Probably due to the application fresh on fresh, the texture XI does not clearly show the applied lime-fine sand wash. The main difference seems to be either the presence of a rough surface, with partially uncovered sand grains, as it is the case for the textures I, II, VII, VIII, IX and X, or the presence of a flat surface, without uncovered sand grains, as it is the case for the textures III, IV, V, VI and XI. 4. Discussion Taking into consideration the quite important differences existing between the possible surface finishing treatments once used on harling and rendering, and knowing about the adhesion problems frequently occurring between render layers, it was assumed that some surface treatments could be able to produce not only different kinds of textures but also relevant changes within the mortar layer. It was also assumed that these changes would result in different mortar layer properties. Although some interesting surface texture features are nicely visible looking at the thin sections under the microscope, the results obtained so far do not show any existing relationship between surface texture and relevant changes within the structure of the mortar layer. It is however surprising that textures I, II, VI, VII and VIII, achieved with surface treatments producing a rather coarse texture, do not show stronger differences when compared to the textures III, IV, V, IX, X and XI, where it would be expected that the smoothing and compacting work would produce a surface layer with lower porosity and higher concentration of fine particles. Hopefully the future results related to the physical properties will contribute to better understand these somehow unexpected results, considering the observations made so far. In this regard, two factors could have played an important role. On the one side, the maximum grain size of the sand, and on the other side, the consistency of the fresh mortar. Both, a smaller maximum grain size and a much fluid consistency would probably promote the formation of a much pronounced upper layer. Finally, if we consider the possible contribution to durability of this upper layer, best results would probably be obtained applying a layer of limewash finish or, better several layers, although this would mask the aesthetic results initially wanted. 5. References 1. Emmenegger, O., Putztechniken Sonderdruck aus Die Burgenforschung und ihre Probleme, (1994), Historic Scotland, Case studies of Traditional Lime Harling Historic Scotland Case Study Series, (1996), Edinburgh. 3. Historic Scotland TAN 1, Preparation and Use of Lime Mortars An Introduction to the principles of using lime mortars, (1995), Edinburgh. 4. Lime News, Proceedings of the Historic Scotland International Lime Conference, Lime News Vol. 4 No. 1, (1996). 5. Historic Scotland, Cost Action C5 Lime Technology Workshop Workshop Proceedings, (2000), Edinburgh albert.jornet@supsi.ch Pagina 8 di 9

9 6. UNI EN , Metodi di prova per malte per opere murarie Determinazione della massa volumica apparente della malta fresca, (2000), Milano. 7. UNI EN , Metodi di prova per malte per opere murarie Determinazione del contenuto d aria della malta fresca, (2000), Milano. 8. UNI EN , Metodi di prova per malte per opere murarie Determinazione della consistenza della malta fresca (mediante tavola a scosse), (2000), Milano. 9. UNI EN , Metodi di prova per malte per opere murarie Determinazione della resistenza a flessione e a compressione della malta indurita, (2001), Milano albert.jornet@supsi.ch Pagina 9 di 9