Thesis for degree of Doctor of Technology Traditional Lime Mortar and Plaster – Reconstruction with emphasis on durability kristin balksten Traditional Lime Mortar and Plaster Reconstruction with Emphasis on Durability Kristin Balksten Department of Chemical and Biological Engineering chalmers university of technology Göteborg, Sweden 2007 Research School NMK – Natural Materials within Environmental and Conservation techniques Department of Chemical and Biological Engineering chalmers university of technology Göteborg, Sverige 2007 Abstract Lime mortar and plaster have been investigated with the aim to improve the knowledge on how to make them as durable as before the cement technology was developed. The background was the durability problems experienced for newly produced lime plaster on the medieval churches on the island of Got- Traditional lime mortar and plaster land, Sweden. In some cases the new lime plaster façades showed severe frost Reconstruction with emphasis on durability damages after only one winter. Although the lime was burnt and produced according to old local traditions, the lime mortar was still mixed and worked on according to methods developed for lime-cement mortar. This often led to a Kristin Balksten very porous lime plaster with a lime shell in the surface and such a plaster has been shown to be sensitive to frost expansion. Isbn 978-91-7291-990-7 Field studies were combined with laboratory studies of thin section specimens. Optical microscopy and scanning electron microscopy have been important analytical methods showing the porosity and the structure of the binder and aggregate materials. The investigations have been carried out on both historic and on newly made reference mortar and plaster. Key words: lime plaster, lime mortar, masonry, craftsmanship, pore The field studies were carried out mainly on Gotland, using local mate- structure, frost resistance, historic mortar rials. The influence of the raw materials, i.e. lime, aggregate and blending ratio was investigated. The focus has been on the workability of the fresh mortars as Cover from Othem church of Gotland well as the pore structure of the carbonated plaster. The craftsmanship, mean- All photos and drawings are made by Kristin Balksten. ing mixing and application of mortar and working the plaster surface, was stu- All thin sections were prepared by Minoprep. died in order to clarify its final pore structure. The pore structure in a material determines many of its technical properties, such as moisture transportation, compressive strength, permeability and frost resistance. All these properties are closely connected to the durability of the mortar and plaster. The permeability © Kristin Balksten, 2007 of the plaster has an impact also on the durability of the covered construction materials. Behind low-permeable plasters made with hydraulic binder, exam- Doktorsavhandlingar vid Chalmers Tekniska Högskola ples of extensive damages of rotten wood and leached lime have been shown Ny serie nr: 2671 The investigations have shown the importance of choosing a mortar ad- Issn 0346-718X justed to the building construction. They also showed the importance of choo- sing a blending ratio adjusted to the specific binder and sand used in order to Department of Chemical and Biological Engineering get a mortar with a suitable pore structure and good durability. It has also Chalmers University of Technology shown the importance of knowing when and how to work on the plaster surface Se-412 96 Göteborg in order to obtain a homogenous material that is well receptive for lime wash Sweden and has a good frost resistance. The combination of all the investigations has Phone +46 31 772 10 00 led to a method for reconstructing historic mortar and plaster with good dura- bility. [email protected] Printed on Munken Lynx 100 g Arkitektkopia Uddevalla, Sverige 2007 Traditional lime mortar and plaster – reconstruction with emphasis on durability a I Preface List of publications This doctoral thesis is primarily based on the work presented in the following papers: This research project was carried out within the NMK En- terprising Research School at the Department of Chemical and Paper I Biological Engineering, Chalmers University of Technology, Gö- Balksten, K., Magnusson, S. (2004) The pore structure in lime plaster as a key teborg, Sweden in co-operation with the Department of Building to understanding moisture transportation properties and frost damages. Conservation at Gotland University, Visby, Sweden. The project Proceedings of the 10th International Congress on the Deterioration and was funded by the KK-foundation, Samfälligheten Gotlands kyr- kor, Byggnadshyttan på Gotland, Svenska kyrkans församlings- Conservation of Stone. Stockholm, Sweden 2004. förbund, Nordkalk, Cementa Research AB, Mur- och putsinfor- Paper II mation MPI. Balksten, K., Broström, T. (2004) Permeability in lime plaster in relation to The research project aimed to study traditional lime plaster durability of covered materials. Proceedings of the 32nd IAHS World 92 and its problems. Some of the medieval churches on the island Congress on Housing – Sustainability of the Housing Projects. Trento, Italy of Gotland were in focus for the field studies. 2004. I would like to thank all who has shown great interest in the project, you have been important for my motivation and inspi- Paper III ration Balksten, K., Klasén, K. (2005) The influence of craftsmanship on the inner structures of lime plaster. Proceedings of the International RILEM Workshop Repair Mortar for Historic Masonry. Delft, Holland 2005. Munkedal 2007, Paper IV Kristin Balksten Myrin, M., Balksten, K. (2006) Lime-based repair mortar – Influence by surface working methods on behaviour and durability of mortar. Proceedings of the International Conference on heritage, Weathering and Conservation, HWC-2006, 21-24 June 2006, Madrid, Spain. pp 763-770. Paper V Balksten, K., Steenari, B-M. (2007) The influence of particle size and structure in hydrated lime on the properties of the lime putty and lime mortar. Submitted to Journal of Cultural Heritage, in April 2007. Paper VI Balksten, K., Steenari, B-M. (2007). A method to recreate historic mortar applied at Norrlanda church on the island of Gotland, Sweden. Submitted to Journal of Cultural Heritage, in April 2007. II a© Kristin Balksten Traditional lime mortar and plaster – reconstruction with emphasis on durability a III 6 Reconstruction of durable lime mortar 63 Content 6.1 Understanding historic mortar 64 6.2 A method to reconstruct historic mortar 69 7 Concluding remarks 79 8 Suggestions for further research 83 1 Introduction 1 1.1 Reading instructions 4 9 Acknowledgements 85 2 Materials in lime mortar 5 10 References 87 2.1 Binder 6 2.2 Aggregate materials 15 2.3 The underlying and covered materials 16 2.4 Methods for material analyses 17 2.5 Materials used in experimental work 18 2.6 The crystal structure of traditionally produced lime 20 3 Mixing of mortar 27 3.1 Blending ratio 27 3.2 Mixing techniques 32 4 The influence of craftsmanship 37 4.1 Application 38 4.2 The drying time of mortar after application 39 4.3 Working the surface 41 4.4 Lime wash 43 5 The influence of pore structure 47 5.1 Damages connected to mortar 47 5.2 Moisture properties connected to pore structure 51 5.3 Strength connected to pore structure 56 5.4 Experimental studies of the pore structure of lime based mortar and its effect on durability 58 IV a© Kristin Balksten Traditional lime mortar and plaster – reconstruction with emphasis on durability a V 1 Introduction The general background for this research project was the frequent occurrence of frost damages on recent made lime plaster in Scan- dinavia. The present study was initiated due to many problems with newly made lime plaster surfaces on the medieval churches of Gotland In some cases there were frost damages already during the first winter. The raw materials for the lime mortar were sim- ilar to the historic ones but the different types of mortar were often mixed and worked on as if they were lime-cement mortar rather than traditional lime mortar. The result was a lime plaster with poor durability. Figure 1:1: Frost dam- ages on the tower of Ot- hem church on Gotland. VI a© Kristin Balksten Introduction a 1 To make research about lime plaster and lime mortar is more than To understand the influence of pore structure on moisture just to study a material. It is about studying a process from choo- transportation properties and durability. sing materials and mixing them to applying and working the mor- To find a method for reconstructing traditional lime mortar tar surface. Each step involves a number of variables that deter- and plaster in order to increase the durability of the plaster as mine the crucial properties of the mortar such as pore structure well as of the historical buildings. and durability. The goal was to define how to make traditional lime mortar and K N O W L E D G E plaster from choosing and mixing materials via working techni- ques to a pore structure providing good durability. The goal was MATERIALS MIXING CRAFTSMANSHIP PORE STRUCTURE DURABILITY also to find a method to make restoration mortar as close as possible to the original mortar of a historical building. Figure 1.2: An illustration of the different parameters in the research problem The scientific approach has been multidisciplinary and the of lime mortar. problem has been investigated with perspectives from environ- Lime plaster and mortar were first described in literature by mental science, building material science, inorganic chemistry and Vitruvius in Rome 2000 years ago. The principles are still the building conservation. The inclusion of craftsmanship in the in- same. In literature written before the 1940’s lime was described as vestigation has helped to bring theory and practice together. a material in its own right, to be used as an alternative to hydrau- The horizons of understanding can be found in the backgro- lic lime mortar.