036 Lime slaking in baskets

Transcription

1 036 Lime slaking in baskets Robert Wacha 1 and Farkas Pintér 2 1 Information and Training Centre of Cultural Heritage Preservation, Federal Monuments Authority Austria, Kartäuserplatz 2, A-3001 Mauerbach, Austria, robert.wacha@bda.gv.at 2 Scientific Laboratory, Federal Monuments Authority Austria, Arsenal 15/4, A-1030 Vienna, Austria, farkas.pinter@bda.gv.at Abstract: The method of slaking lime using baskets can be mostly found in manuscripts from the 19 th century and describes mainly an easy procedure for producing calcium hydroxide powder [1]. For this, small pieces of quicklime were put in wooden or iron baskets and immersed in water for a few seconds. To test this rather unconventional technique under field conditions a locally burned limestone was slaked in wicker and iron baskets according to the description of the historical manuals. Although the approach of using lime with very little water contradicts most modern conventions for producing durable and stable coatings, the basket lime showed surprisingly good workability in its practical application such as a thin lime coat. When the samples of the test fields were analysed, the microscopic and SEM images showed signs of a specific structure of shrinkagecracks, which seem to have some, probably, positive effects on the material. The paper shows the first results of practical tests of producing and working with basket slaked lime and the experiences with this special technique. 1 Slaking by immersion in historical manuscripts Aside Vitruvius de architectura, where lime was slaked by intinctus in aqua [2] the unusual technique of lime slaking by immersion or basket slaking is mainly reported in different German manuscripts and manuals of the 19 th Century. [1,3]. The process obviously was used as an easy way to get hydrated lime powder as agricultural mineral fertilizer and soil compacter, but also to produce material for special coatings and plaster. The easiest way to break up big quicklime lumps was simply to immerge them in water for a few seconds. So normally big baskets of iron or wicker were filled with quicklime lumps (Figure 1) and dunked into water basins. After a few seconds the exothermic reaction of hydration started and as soon as the water showed first signs of bubbles, the baskets were taken out with long bars and set aside. Here the quicklime lumps began to crumble to a smooth paste and dried up quickly to a homogenous powder, which was then filled up in air tight barrels. This powder was then used mainly for agricultural purposes, but sometimes also for mixing mortars, for plastering and lime coatings. This approach of using relatively poorly

2 2 hydrated quicklime was often considered useless and dangerous [3], as it was supposed to produce active swellings in the mortar, which could break up the layer or destroy the surface. Figure 1: Slaking by immersion in 1925, [3] 2 Reconstruction of basket lime slaking An occasion to test this technique came up during some restoration works in Upper Austria, where basket lime was made according to a historical manual by August Scheffer released in 1865 [1]. The quicklime used was produced from a local limestone with small fractions of dolomite and iron oxides typical for this area, which were burned in a field kiln by a historic society (Museumsverein Molln) and so probably very close to the material used in rural areas in the past. For slaking, larger pieces were first hammered to the size of an egg and then filled

3 3 in an iron and a wicker basket. These baskets were then immersed in a water basin, where the reaction started immediately (Figures 2 and 3). Figure 2: Immersion of quicklime Figure 3: Slaking reaction Figure 4: Partly hydrated lime paste at the end of the reaction

4 4 After the basket was taken out and set aside (Figure 4), the reaction slowed down after a few minutes and the quicklime crumbled to a fine and homogeneous paste with nearly no splinters or grains in it. However, the use of the iron basket was fairly dangerous. As the wicker was still able to hold some water, the iron basket leads to a very short immersion and triggers a fierce reaction, where the quicklime burst with small explosions (Figure 5). Small pieces of hot lime were tossed a few meters and made gloves and goggles essential for safety. Maybe iron baskets were not really suitable for the field slaking of highly reactive lime. Figure 5: Exploding lime lumps After the experimental slaking, one part of the lime paste was used to form mortar samples while another part was stored in an air tight box with some wet cloth in it (historically a sealed barrel with wet linen). The stored lime was later worked up as a thin lime coat for testing its workability. Therefore, no aggregates were used and polishing the surface with a mason s trowel was astoundingly easy (Figure 6). 3 Laboratory analyses The test samples consisted of three volume parts natural carbonate sand from a near river and one volume part basket lime. Later a lime coat free of aggregates was also applied on a 2 cm thick plaster in the pedestal of a historical stone wall on the outside of the monastery of Mauerbach in Austria. After carbonating two years under fairly harsh weather conditions the mortar samples and the coating showed no significant signs of damage.

5 5 Figure 6: Applying the thin coat with basket lime Although the lack of reaction water was significant, it obviously did not damage the surface or the coating itself. Thin section analyses and SEM observations on broken surface samples clearly showed the nature of the samples and the coating. While the samples exhibited a porous structure and large shrinkage cracks (Figures 7 and 8), the finishing coat was characterized by a fairly dense microstructure and almost free of shrinkage cracks, probably because of the polishing with the trowel.(figures 9 and 10). Although the present experiment and research has not dealt with the investigation of mechanical parameters, we assume that this rather uncommon structure of lime bridges and high open porosity might have lead to lower modulus of elasticity and thus a better performance of the mortar (as reported by Jan Valek at the HMC 2010) [5]. Furthermore, due to the place of application in the pedestal zone we assume that the zone contained a considerable amount of damaging salts. Nevertheless, two years after the application test the thin coat was in a surprisingly good condition. If this phenomenon could be used to characterize the material as a slightly salt resistant cannot be answered yet, as there are no data about the exact salt content. We assume however, that the high capillary and macro porosity which naturally promote the extraction and deposition of masonry salts may also support their resistance which allows using them as a sacrificing render on historical masonries. The unusual characteristics could also be the reason for its good workability and very smooth and lightweight consistency.

6 6 Figure 7: Microstructure of the test mortar sample containing large shrinkage cracks (plane polarized light) Figure 8: Large shrinkage cracks in the matrix of the test mortar sample (SEM-BSE)

7 7 Figure 9: Thin section image of the dense finishing coat on top of the lime render (plane polarized light) Figure 10: Microstructure of the lime coat showing a dense and homogenous microstructure of micritic CaCO 3 (SEM-SE) 4 Conclusions Even in the past the technique of slaking by immersion or basket slaking was a rare und unconventional way to use lime for constructional purpose. Perhaps it was mainly applied when barrels of lime fertilizer were easy available and normal lime putty was not an issue. Although it has a lot of interesting qualities, especially when applied as a thin layer (such as the famous lime polish in the Roman times) it was and is definitely an exotic technique. With the exception of the above mentioned Roman examples, historical example of its usage as a mortar

8 8 or plaster has not been clearly identified yet, but this maybe just because the material itself is known so little by now. As the field test and practical exercise showed that the method itself worked out as historically described, the next step of the project will be to replicate the experiment under laboratory conditions. It is also planned to take various samples of historical thin coatings from different ages, analyse and compare them with the observed characteristics of slaked lime produced by immersion. Also further testing with different sorts of quick lime, mortar samples with varying b/a-ratios and the investigation of their main structural and mechanical properties as well as salt resistance is planed. 5 References 1. Scheffers, A, (1865) Handbuch des bürgerlichen und ländlichen Hochbauwesens in besonderer Berücksichtigung der Bau-Constructionslehre(!) zum Gebrauch für Bauhandwerker, Verlag von E. Seemann, Leipzig 2. Vitruvius, M, (33-22 BC) De architectura, libri decem, Rom 3. Lebrun, F.M.,(1837) Der Steinmörtel, Praktische Anweisung den Steinmörtel im Allgemeinen besonders aber bei Bauten an und unter dem Wasser bei Gewölben etc. statt jeder anderen Art von Maurerarbeit mit Vortheil(!) zu benützen, Verlag Hübling, Ulm 4. Eisemann, F, (1925) Kalklöschen. Anleitung zum Löschen aller Kalkarten mit Berücksichtigung der Kalklagerung und der Mörtelbereitung., Kalkverlag, Berlin 5. Valek J, Matas T (2012) Experimental study of hot mixed mortars in comparison with lime putty and hydrate mortars. In: Valek J, Hughes J, Groot C (eds) Historc Mortars, Cahracterisation, Assessment and Repair, Springer, London