OPTIMISED MICROSTRUCTURE OF CALCIUM SULPHATE BASED MORTARS FOR THE RESTORATION OF HISTORIC MASONRY

Tesch, V., Middendorf, B. University Kassel, Faculty of Civil Engineering, Department of Structural Materials, Moenchebergstr. 7, D-34109 Kassel, Germany Abstract: The use of gypsum based mortar has a very long tradition. In Germany, especially in the neighbourhood of natural gypsum outcrops, gypsum mortars, gypsum lime and gypsum anhy-drite lime mortars were used for construction of sacred buildings. Despite of the high water solubility (2.6 g/l) and of the low wet compressive strength of gypsum mortars they were used for exterior masonry. Today even joints of the masonry exposed to weathering are still in rea-sonable good condition. However, some of the cultural historical important buildings are due to restoration. Nowadays it is well known that restoration should consider the original build-ing materials. So it is necessary to develop a compatible calcium sulphate based restoration mortar. Depending on the original substance restoration mortars have to accomplish certain physico-mechanical properties (e.g., comp. strength, porosity). 1. Introduction In spite of their low water resistance CaSO4 based mortars were used in numerous cultural and historical buildings even in building zones exposed to atmospheric conditions. Despite of this long time exposition these mortars are still well preserved. Physical- mechanical and chemical- mineralogical investigations of these historical calcium sulfate based mortars have shown that their structure has consolidated in the course of time by processes of crystalline transformation and recristallisation, which causes an improved weathering resistance [1]. The choice of a compatible restoration material is of main importance for the restoration of ma-sonry built with gypsum mortars. For the prevention of damages due to mechanical tension or to the establishing of secondary phases by moisture penetration the restoration mortars need to have physical and mechanical properties adjusted to the original substance and they have to be chemically and mineralogically compatible to the original substance. Recent field studies have shown that modern gypsum based mortars are not suitable for outside application. In contrast to historic gypsum based mortars modern gypsum based mortars deteriorate after short time of weathering. A thorough investigation of the historic material was the first step to develop suitable CaSO4 based restoration mortars. The development of these mortars happens on the base of the results of phase and structural analyses of historical CaSO4 based mortars. The main focus lies on structural optimisation while ensuring material authencity. 2. Properties and microstructure of historical CaSO4 based mortars Information of raw materials, burning processes, additives and processing techniques for the production of historical gypsum mortars was lost with the appearance of hydraulic binders at the beginning of the 19th century. So nowadays for the production of restoration mortars in-dustrially available CaSO4 binders are used because they are quality controlled. Mineralogical investigations of historical mortars from various different buildings in northern Germany and Italy have shown that gypsum and gypsum anhydrite lime mortars with low lime contents [3-5] were used and despite their high water solubility they have shown technical feasibility for interior as well as for exterior applications. According to [1, 2] the reason for the high resis-

tance against environmental influences is based in the mineralogical composition and in the pore structure; binder and aggregate consist of the same material. The rather high density of approx. 2.0 g cm-3 implies that a very low water/binder ratio (<0.4) was used for the produc-tion of the mortars. A good workability of mortars with such a low water/binder ratio can only be established by using additives such as retarders [6]. Strength investigations of approx. 100 different mortars from buildings in northern Germany and the Netherlands have shown [1] that the investigated mortars can be divided into 3 groups: lime mortars: compressive strength 4 – 10 N/mm² gypsum lime mortars: compressive strength 4 – 14 N/mm² gypsum mortars: compressive strength 11 – 31 N/mm². In general it can be said that the strength of mortars increases with enhanced gypsum content. Before the use of hydraulic binders, CaSO4 based mortars were favourised to build churches with complex structures like arches or columns because of their strength development and set-ting behaviour which is controllable by using chemical additives [1, 3]. Furthermore it could be observed that these gypsum mortars were almost completely free of aggregates. The low contents of silicate and carbonate aggregates that could be found can be interpreted as impuri-ties [1]. Phase analysis and microscopical investigations of historical gypsum mortars have also shown that they often possess a low content of anhydrite which indicates the use of high fired gypsum as binders [13]. MIP (Mercury intrusion porosimetry) and SEM (scanning electron microscopy) investigations have also demonstrated that the porosity of historic gypsum mor-tars is noticeably lower than that of commercial available CaSO4 based restoration mortars [7-9].So crystalline transformation and recristallisation must have taken place during the time of exposition. Figure 1 shows the mortar structure of a section of masonry of Hohnstein castle, the granular cristalline structure is exact identifiable. In figure 2 microstructures of a historic mortar and of a mortar made with commercially produced hemihydrates are shown in com-parison. The commercial produced CaSO4 mortar consists of idiomorphical longprismatic gypsum crystals. This microstructure has a significant higher content of porosity, especially capillary pores. From the comparrison of the microstructures shown in figure 2, it can be con-cluded that the water resistance of a gypsum based mortar depends on the morphology and the habit of the gypsum crystals as well as of the porosity [10].

Figure 1: Mortar structure taken from a long time exposed part of the historic masonry of

Hohnstein castle in the Harz mountains. Width 145 µm (right figure).

Figure 2: Microstructure of CaSO4 based mortars, SEM figures; width: 145 µm

Left side: restoration mortar, right side: historic mortar 3. Requirements for restoration mortars Masonry mortars are designed to establish a homogenous and friction locked joint between the bricks. The state of preservation mainly depends on the properties of the mortar and the building stones. This demands a detailled pre-analysis of the used materials for every restora-tion of historic buildings. Investigations of historic CaSO4 based mortars by [1, 4, 5, 11-16] and own investigations of historic mortars of Hohnstein Castle in the Harz mountains and the church in Bosau in Schleswig-Holstein (Germany) form the basis for the establishment of re-quirements for the development of restoration mortars. As the composition of the restoration mortars should be close to the historic ones a binder content of 70-80 wt.-% of CaSO4 components should be aspired. Fresh mortar properties of historic mortars are not documented in literature so the properties of modern mortars were adopted to guarantee a good workability. A low expansion is advantageous for a good adhe-sion at the stones. If a high shrinkage appears later on, this causes a cracking fissuration

mostly combined with a separation from the flanks of the stone. Furthermore the mortars should have an adapted water retention. The water retention should be higher than 85% [1]. To guarantee a workability retarders are necessary. These kinds of chemical additives effect a change of shape and habit of the growing dihydrate crystals [7, 17, 18]. The properties of the hardened mortars have to be adopted to those of the masonry stones. The strength and elasticity modulus of the mortars must be lower than those of the masonry stones to prevent damage resulting from external loads. Historic mortars show a low porosity due to the numerous processes of crystalline transforma-tion and recristallisation which in turn is responsible for their good resistance against weather-ing. The aim is to adjust the restoration mortars regarding porosity to the historical ones which show an average total porosity of approx. 25 vol.-%. Based on the knowledge about historic mortars the content of capillary pores of the restoration mortars should be in a range between 11 and 15 vol.-%. The gypsum crystals should have a low aspect ratio to build up a dense microstructure, which can be controlled by using chemical additives. Knowing the ef-fectiveness of the chemical additives in detail a tailored microstructure can be designed. Humidification and subsequent dehydration is frequently observed in historic buildings and supports efflorescence. The mortar’s content of salts and agents subserving efflorescence should be as low as possible. 4. Development and application 4.1 Raw materials For the development of compatible restoration mortars commercial available raw materials (binder, aggregate, additives) were used. Due to quality control this approach ensures constant and reliable product quality. As binding materials α-hemihydrate, β-hemihydrate, anhydrite and slaked lime (CL 80 acc. EN 459 [21]) were applied. The chosen aggregate components were lime stone and quartz with grain sizes of 0.125-2 mm as well as fine grained lime stone powder. Hydroxy carboxyl acids were used as retarder for the hemihydrate samples and K2SO4 as activator for the anhydrite samples. 4.2 Mixtures 4.2.1 Binding agents The mortars were produced with a CaSO4 amount of >70 wt.-% to fulfil the requirements listed in chapter 3. Additionally slaked lime (CL 80) was used as binding agent. By variation of the binding agents in type and quantity an optimum of resistance against weathering should be achieved. The compositions of the investigated mixtures are listed in table 1. Table 1: Composition of mixtures sample α-HH

[wt.-%] β-HH

[wt.-%] anhydrite [wt.-%]

lime [wt.-%]

binder / aggre-gate-ratio

chemical addi-tives

KS 1 20 50 / 30 2 / 1 tartaric acid KS 2 40 30 / 30 2 / 1 tartaric acid KS 3 100 / / / 2 / 1 tartaric acid KS 4 / / 80 20 2 / 1 K2SO4

4.2.2. Aggregates As aggregates lime stone and quartz (grain size 0.125-2 mm) and lime stone powder were used. The binder/aggregate ratio was 2:1. The grain size distribution of the aggregates was chosen in order to support a rather dense structure. Furthermore the humidity transport from the inside to the outside should be possible to inhibit surface damage of the mortar and stone. 4.3 Investigation of the weathering behaviour The weathering behaviour of the mortar samples was investigated in fast motion with an spe-cial developed experimental setup which rains the samples in defined time intervals and under defined climatic conditions. For this prisms (acc. DIN 1164, Part 7 [19]) with the dimensions 4cm x 4cm x 16cm were chosen. Before the weathering procedure the samples were stored 28 days at a climate of 20 °C / 65 % relative humidity. The raining was realised by a constantly spraying system. The volumetric flow rate was 2,0 l/min*m². After 30 minutes when the hu-midity penetration of the samples was reached they were stored for 3 hours at 40°C / 40% relative humidity and after that dried again for 2.5 hours at 20°C / 40% relative humidity. This cycle was repeated several times to simulate a repeated humidifying and drying. 4.4. Analysis of the structure For structural analysis samples which had previously passed through the weathering process as described above and samples stored in an climatic room at 20°C / 65% relative humidity for the same time were compared. The pore volume of the samples was measured by using MIP. For the analysis of the surface topography and the structure of the weathered and the unweathered samples SEM was used. 5. Results and discussion 5.1. Strength and porosimetry The compressive strength of the samples was measured acc. DIN 18555, Part 3 [20] after 7 and 28 days. The results have shown that the compressive strength of hemihydrate containing mixtures can be controlled by changing the ratio of α- / β-hemihydrates. An increase of the amount of α-hemihydrates in the binder component effects a gain of compressive strength. In this way any required compressive strength can be achieved. The compressive strength corre-lates with the porosity in a way that a higher amount of α-hemihydrates results not only in an increase of compressive strength but also in a decrease of porosity (figure 3, mixtures table 1).

0

10

20

30

40

KS 1KS 1

*KS 2

KS 2 *

KS 3KS 3

*KS 4

KS 4 *

* after weathering

tota

l por

osity

[Vol

.-%]

air voids capillary pores gel pores

9 N/mm² 11 N/mm² 46 N/mm² 27 N/mm²

Compressive strength of the unweathered samples after 28 days

Figure 3: Porosity and compressive strength values of differently effected samples There is a conspicuous difference in porosity between the weathered and the unweathered samples. After weathering the samples show a decrease of capillary pores and an increase of gel pores. This implies that processes of crystalline transformation and recristallisation must have taken place within the microstructure during weathering. These products of crystalline transformation and recristallisation could be traced using SEM (figure 3). Samples based on pure α-hemihydrate mixtures have the lowest porosity. These samples are very dense and are not resistant against weathering. In contrary to samples combined of α-hemihydrate / β-hemihydrate and slaked lime showed a high weathering resistance despite their high total porosity. Using slaked lime with hydraulic components results a high weather-ing resistance because of formation CSH-phases into the microstructure. 5.2 Results of microscopical analysis In the surface region of the mortar (< 1 cm) the gypsum crystals of the weathered samples showed etching effects whereas in deeper regions (> 1 cm) no significant differences in the crystalline morphology could be observed. Sporadic recrystallisations could be traced in the surface regions (see figure 4).

Sample KS 1; width of picture: 290 µm

Sample KS 2; width of picture: 290 µm

Figure 4: recrystallised gypsum on the mortar surface after weathering

5.3 Weathering resistance Mixtures of α-hemihydrate, β-hemihydrate and slaked lime (KS 1 and KS 2) and mixtures of anhydrite and slaked lime show the best resistance against weathering. There is no remarkable ablation of material recognisable, neither optical nor gravimetrical. In contrast the samples of mixture KS 3 showed a reproducible mass loss of approx. 30 wt.-%. Slaked lime seems to play an important role for the weathering resistance for the mixtures. The slaked lime builds up a stabilising framework within the mortar which protects the CaSO4 binder from ablations by dissolving attacks. This is documented by SEM analyses. The details of these mechanisms are not completely discovered. Figures 5 compares mortars with lime (right prism) and without (left prism) after weathering process. The difference in material ablation is significant.

Figure 5: Samples after weathering

Left prism: KS 3 (α-hemihydrate without lime); Right prism: KS 2 (α-hemihydrate with lime).

6. Summary and future prospects The suitability of gypsum mortars for the restoration of historic buildings is defined by the composition (anhydrite, α- or β-hemihydrate), the binder content, the type and composition of the aggregates and the microstructure (morphology of the gypsum crystals and porosity). Be-sides the strength which should be adjusted to the stone material (natural stone, brick) the weathering resistance of the mortars is of utter importance. The investigations have shown that the values of strength and elasticity moduli of mortars can be affected by variation of the content of α− and β-hemihydrate. A variation of the binder/aggregate ratio and therefore of the mortar’s strength can only be achieved in a very small range because the workability has to be guaranteed. Present results show that a lime content of 30 wt.-% considerably improves the durability of gypsum based mortar.

In comparison to modern gypsum mortars historic gypsum mortars show a lower porosity caused by numerous delaminations– and crystallisation processes. Gypsum mortars with comparatively great and short prismatic crystals have a higher weathering resistance than those with longprismatic gypsum crystals. In a next step the influence of the binder/aggregate ratio and the aggregate/filler ratio will be investigated. The amount of aggregate and filler material will be optimised to ensure a high packing density. Subsequently, the influence of the grain size of the gypsum crystals on the packing density will be considered. Since a retarding agent is required to ensure the workabil-ity of the mortars a more careful selection of this compound may optimise the shape and habit of the gypsum crystals. 7. Acknowledgment The authors thank the Deutschen Bundesstiftung Umwelt (DBU) for financial support. 8. References [1] Middendorf, B.: Charakterisierung historischer Mörtel aus Ziegelmauerwerk und Ent-

wicklung von wasserresistenten Fugenmörteln auf Gipsbasis. University Siegen, Dis-sertation, 1994.

[2] Steinbrecher, M.: Gipsestrich und –mörtel: Alte Techniken wiederbeleben. Bausub-stanz, 10, 1992, 59-61.

[3] Cioni, P.: Small thickness Brick Vaults in Tuscany: Theirs Characteristics and Con-solidation. Proceedings of the 9th International Brick/Block Masonry Conference, Ber-lin, Germany, 1991, Vol. 3, 1523-1530.

[4] Middendorf, B.; Knöfel, D.: Gypsum and Lime Mortars of Historic German Brick Buildings: Analytical Results as well as Requirements for Adapted Restoration Mate-rial. Conservation of Historic Brick Structures: Case Studies and Reports of Research. Editors: N.S. Baer et al.; Donhead Publishing Ltd, England, 1998, ISBN 1 873394 34 9, 197-208.

[5] Middendorf, B.; Knöfel, D.: Characterisation of Historic Mortars from Secular and Re-ligious Buildings in Germany and the Netherlands. In: Conservation of Historic Brick Structures: Case Studies and Reports of Research. Editors: N.S. Baer et al.; Donhead Publishing Ltd, England, 1998, ISBN 1 873394 34 9, 180-196.

[6] Middendorf, B.: Physico-mechanical and microstructural characteristics of historic and restoration mortars based on gypsum: current knowledge and perspective, Geo-logical Society London, Special Publications, 205, 2002, 165-176.

[7] Middendorf, B.; Budelmann, H.: Effects of different Additives on Microstructural De-velopments in Gypsum based Materials. Proceedings of the Fifth Euroseminar on Mi-croscopy Applied to Building Materials, Leuven-Belgium, 1995, 40-49.

[8] Middendorf, B.; Budelmann, H.: Evaluation and Optimisation of Calciumsulfate Based Flooring Plaster with Regards to Water Resistance. Proceedings of the 20th In-ternational Conference on Cement Microscopy, USA, 1998, 246-258.

[9] Singh, M.; Garg, M.: Relationship between mechanical properties and porosity of wa-ter-resistant gypsum binder. Cement and Concrete Research, 26, 1996, 449-456.

[10] Middendorf, B.; Vellmer, C.; Schmidt, M.: Take a Closer Look: Calcium Sulphate Based Building Materials in Interaction With Chemical Additives, 1st International Symposium on Nanotechnology in Construction, Paisley 2003, im Druck.

[11] Rüth, G.: Schäden, Schutz und Sicherungsmaßnahmen bei Bauten mit Gipsmörtel. Der Bautenschutz, 1 & 3, 1932.

[12] Lucas, H.G.: Gipsstein und Gipsmörtel als Baustoffe im alten Windsheim. Der Stucka-teur, 8, 1986.27-32.

[13] Lucas, H.G.: 1992. Gips als historischer Außenbaustoff in der Windsheimer Bucht –Verbreitung, Gewinnung und Beständigkeit im Vergleich zu anderen Natursteinwer-ken. Dissertation, 1992, RWTH Aachen.

[14] Werner, A.: Sanierung von Kirchenbauten an der Elbe. Bausubstanz, 5, 1986, 36-40.

[15] Arens, P.: Untersuchung und Entwicklung von Gipsmörteln für den Aussenbereich un-ter besonderer Berücksichtigung der Wasserresistenz, Dissertation Universität-GH-Siegen, 2002.

[16] Weichmann, M.J.: Historische Gipsmörtel in Deutschland: Mineralogische, chemische und physikalische Eigenschaften, Ableitung der Brennprozesse und Rezepturen, Wechselwirkung mit Werksteinen. Clausthaler Geowissenschaftliche Dissertationen, 1998.

[17] Matyszweski, T.; Burdzinska, T.; Saladajczyk, A.: Modifizierung der Eigenschaften des Chemiegipses mit Hilfe verschiedener Zusatzmittel. TIZ-Fachberichte Rohstoff-Engineering, 2, 1980, 89 – 91.

[18] Koslowski, Th.: Zitronensäure - Ein Verzögerer für Gips. Dissertation, RWTH Aa-chen, Germany, 1983.

[19] DIN 1164, T. 7: Ausgabe 1978-11 Portland-, Eisenportland-, Hochofen- und Trasszement.

[20] DIN18555 T.3: Ausgabe:1982-09 Prüfung von Mörteln mit mineralischen Bindemitteln; Festmörtel; Bestimmung der Biegezugfestigkeit, Druckfestigkeit und Rohdichte

[21] DIN EN 459-1, Ausgabe: 2002-02 Baukalk - Teil 1: Definitionen, Anforderungen und Konformitätskriterien; Deutsche Fassung EN 459-1:2001

Authors Dipl.-Ing. Viola Tesch vtesch@uni-kassel.de Dr. Bernhard Middendorf midden@uni-kassel.de