Performance of some commercial consolidating agents on porous limestones from Egypt

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1 Performance of some commercial consolidating agents on porous limestones from Egypt H. Ahmed Helwan University, Academy of Applied Arts, Dept. of Sculpture and Monument Conservation, Orman Giza, Egypt Á. Török Budapest University of Technology and Economics, Budapest, Hungary, J. Lőcsei H-2013 Pomáz, Széchenyi u. 21, Hungary ABSTRACT: Two types of porous Egyptian limestones from Mokattam and Tura quarry were used to test the performance of four different consolidants. The stones were intensively used in the monuments of Ancient Egypt (Pyramids of Sakkara Plateau, Old Cairo City). The quarry were consolidated under laboratory conditions by silica acid ester, aliphatic uretan resin, acrylate resin and polymethyl methacrylate. Before and after the consolidation physical parameters such as density, Duroscope rebound value and ultrasonic sound velocity was measured. The absorption rate of different consolidants varies and it reflects the differences in porosity of limetones, and very probably it also marks differences in pore size distribution and viscosity of consolidants. Tura limestone has en effective porosity of 6.5% and it absorbs 3.7% of acrylate resin, the most. Mokattam limestone with its greater porosity (9.2%) absorbs more consolidant. A maximum of 7% of polymethyl methacrylate absorption was recorded. Duroscope tests have shown that after the consolidation there is an increase in surface strength by 2-13% and 12-67% for Tura and Mokattam limestones, respectively. For Tura limestone silica acid ester while for the more porous Mokattam limestone aliphatic uretan resin was the most effective consolidant. 1 INTRODUCTION In Egypt limestone was one of the most important raw material used from Pharaonic, Greco-Roman and Arab times. This raw material was exploited in the Nile valley, mostly on the eastern side of the river (Klemm & Klemm 2001). The limestones vary from dense compact types with minor porosity to chalky very porous lithologies (Punru et al. 1990). The monuments that have been constructed from various limestone types now show various signs of decay under the Egyptian climate including granular disintegration, pulverization, weathering crust formation, scaling and flaking (Fitzner et al. 2002). To avoid these weathering related changes several stone conservation methods were applied in the past to protect Egyptian limestone monuments (Helmi & Attia 1996, Gauri & Bandyopadhyay 1999). Among the different procedures, consolidation is certainly one of the most studied problems, which has been also applied for Pharaonic limestone sculptures (Thicket et al. 2000). Although several consolidants were applied in the past detailed laboratory analyses are needed to exclude the adverse effect of such treatments. The key issues are to choose an appropriate chemical and to select the proper technique of stone consolidation. To address the problem limestone from two Egyptian quarries were tested under laboratory conditions by using various stone consolidants. The physical properties of treated and non-treated are compared to analyse the performance of silica-acid ester, polymethyl methacrylate, uretanand acrylate resins. 2 METHODOLOGY Cubic specimens (5 cm) were used for the consolidant treatments. From sound blocks of Mokattam and Tura quarry, (Cairo, Egypt) more than 120 specimens were cut by using diamond saw. The textural properties of limestones were described by using petrographic microscope. Mineralogical composition was determined by X-Ray Diffraction (XRD) with a Phillips Diffractometer (PW 1130 generator, PW 1050 goniometer, Cu anode and monochromator, 40kV, 20mA, angle 5-70, step size 0.02, time per step 1.0 second). The physical parameters of each test specimen were analysed before the treatment procedure. Bulk density, apparent porosity was determined according to the test procedures described in the European standard EN Duroscope rebound values and ultrasonic

2 sound velocities (Controls Ultrasonic tester E-46) were determined under laboratory conditions before and after the conserving trials. For each test specimen five measurements were made and average values and standard deviations were calculated. Water absorption was also determined by capillary rise method (procedure is described in the European standard EN 1925). This method is aimed to evaluate the kinetics of water absorption and to provide an estimation of the penetration depth of consolidants into the porous stone. The postconservation tests were performed on fully saturated cubic 5 months after the treatment trials. The products, which have been applied, are selected according to their frequent use in the field of stone conservation. Table 1 shows the most important parameters of the applied stone consolidants. Table 1. Stone consolidants and their properties (PMMA: polymethyl Consolidant Diluting Density Viscosity agent [g/cm 3 ] [mpas] Silica acid ester ready to use Aliphatic uretan resin white spirit Acrylate resin white spirit nitro-thinner more resistant variety of limestone is found which was used for casing of the pyramids from the time of King Snofru ( BC) onwards. Today, the quarries of the region supply the extensive lime and cement industries of Tura and Helwan. 3.2 Texture and mineralogy The white limestone from Tura quarry is very fine grained. Under the microscope it has a peloidal microbiocalstic wackestone texture. Thus it prevailingly consists of very fine micrometer-size calcite micrite. Very small detrital quartz grains are seldomly visible. Entire microfossils are also present having a size of few tens of micrometers. No iron staining and iron minerals were observed in thinsections. XRD analyses have shown that calcite is the prevailing mineral, but minor amount of quartz and cristobalite was also found. 3 MATERIALS 3.1 Provenance of limestones Limestone is available along the entire length of the Nile Valley from Cairo far beyond Aswan, and easily accessible along the Nile river valley, as well as in the Eastern Desert (Figure 1). The most important limestone quarries supplied ancient dynasties with varying quantities of buildings and monumental stone, mainly for funereal and sacral purposes such as pyramids, temples, sculptures, decorative elements and tombs (Klemm & Klemm 2001). In the area of Cairo and Giza Eocene limestone is exposed in Mokkatam plateau (Eastern riverside of the Nile), the Helwan plateau (Eastern riverside of the Nile) and Giza plateau (few km west from the Nile) (Tawadros 2001). The test were taken from two quarries, which are located on the eastern side of the Nile in Cairo region. Mokattam quarry is in Gebel El Mokattam, while Tura quarry is not far from Helwan (Figure 1). Both quarries expose Middle Eocene limestones of Mokkatam Group (Said 1990). The simple structural setting of Gebel El Mokattam, the thick-bedding of the limestone, the presence of perfect horizontal bedding-planes, and the comparatively soft nature of the rock make the quarrying and subsequent shaping of the limestone quite easy. In Tura quarry a slightly Figure 1. Limestone quarries in Egypt (after Klemm & Klemm 2001) and the location of Mokatam and Tura quarries near Cairo. The limestone from Mokattam quarry is very similar to the one of Tura quarry, although it has a

3 slightly creamy colour. It is very fine grained and contains minor amount of silt. The microfabric is characterised by the dominance of micrite (very fine calcite) and sparse fragments of micro fossils. Weak bioturbation is also visible in the microbioclastic wackestone. According to XRD analyses this calcitic limestone consists of dolomite and traces of hematite. Quartz, tridymite, Ca-sulphates such as gypsum and bassanite, as well as clay minerals, such as montmorillonite were also detected in the powdered. Compared to Tura limestone Mokattam limestone is enriched in sulphates and swelling clay minerals. penetration varies between 1.8 and 3.7 %, and 2.8 to 7 % for Tura and for Mokattam limestone, respectively. The graphs also illustrate that in general most consolidants have a higher penetration rate for Mokattam limestone than for Tura limestone. The only exception is acrylate resin, from which Tura limestone absorbs more than Mokattam limestone (Figure 5). 3.3 Physical properties Despite their similarities in visual appearance the limestones differ in their physical properties. Mokattam limestone is more porous, while Tura limestone is denser and has an increased ultrasonic sound velocity (Table 2). Table 2. Physical properties of limestones (average values) Density Ultrasonic s. vel. Effective porosity Limestone [g/cm 3 ] [km/s] [%] Tura Mokattam Figure 3. Absorption of silica-acid-ester consolidant in Tura and Mokattam limestones compared to absorption of nontreated The capillary-rise tests have shown that Mokattam limestone is not only more porous than Tura limestone but it also absorbs water more rapidly (Figure 2). Figure 4. Absorption of aliphatic uretan resin consolidant in Mokatam and Tura limestones compared to absorption of nontreated Figure 2. Capillary-rise water absorption of untreated Mokattam and Tura limestones 4 CONSOLIDATION TESTS 4.1 Absorption rate Figures 3-6 show the absorption rate of various consolidants in Tura and Mokattam limestones compared to water absorption of non-treated at room temperature. The average

4 Figure 5. Absorption of acrylate resin consolidant in Mokattam and Tura limestones compared to absorption of non-treated The maximum absorption rate for Mokattam limestone (nearly 7 %) and the minimum absorption rate for Tura limestone (1.8%) was observed when polymethyl methacrylate was used (Figure 6). Figure 6. Absorption of polymethyl methacrylate consolidant in Tura and Mokattam limestones compared to absorption of non-treated 4.2 Changes in physical properties The increase of density after consolidation shows variations for the two limestones. The density increase is less than 0.5 % for each consolidant for Tura limestone, except silica acid ester where 1.7% of increment in density was recorded (Table 3). The consolidant saturated Mokattam limestone cubes are denser (at least by 1%) than the pure. 3% of density augmentation was only measured on silica acid ester treated (Table 4). Ultrasonic sound velocities reflect better the consolidating effect of various consolidants. All values show a distinct increase after the treatment, but the most dramatic difference in velocities of untreated and treated was recorded on silica acid ester treated Tura and on polymethyl methacrylate saturated of Mokattam limestone. The Duroscope rebound values (surface strength) of consolidated Tura limestone are increased by 2 to 13% compared to the non-consolidated (Table 5). Whilst, 12 to 67% increase of strength was measured on consolidation saturated Mokattam limestone (Table 6). Silica acid ester was the most effective for Tura limestone (Table 5), whereas for Mokattam limestone aliphatic uretan resin brought about the most dramatic increase in surface strength (Table 6) Table 3. Density and ultrasonic sound velocity (USSV) of Tura limestone before and after treatment (PMMA: polymethyl Before treatment After treatment Consolidant density USSV density USSV [g/cm 3 ] [km/s] [g/cm 3 ] [km/s] Silica acid ester Aliphatic uretan resin Acrylate resin PMMA Table 4. Density and ultrasonic sound velocity (USSV) of Mokattam limestone before and after treatment (PMMA: polymethyl Before treatment After treatment Consolidant density USSV density USSV [g/cm 3 ] [km/s] [g/cm 3 ] [km/s] Silica acid ester Aliphatic uretan resin Acrylate resin PMMA Table 5. Duroscope rebound of Tura limestone before and after treatment (PMMA: polymethyl Consolidant Before After difference treatment treatment [%] Silica acid ester Aliphatic uretan resin Acrylate resin Table 6. Duroscope rebound of Mokattam limestone before and after treatment (PMMA: polymethyl Consolidant Before After difference treatment treatment [%] Silica acid ester Aliphatic uretan resin Acrylate resin DISCUSSIONS AND CONCLUSIONS The texture and mineralogy of the two studied limestones is very similar, although Mokattam limestone contains traces of swelling clay minerals and Ca-sulphates (gypsum and bassanite). The laboratory tests of non-treated cubic test specimens also have proven that the Tura limestone is denser and has a higher surface strength (Table 2, 5 and 6). No pore-size distribution measurements were made, albeit water absorption test by capillary-rise method have shown that Mokattam limestone rapidly absorbs water, while water penetration into Tura limestone is slower. It indicates that more small pores are found in Tura than in Mokattam limestone (Fig. 2), although the total porosity of the latter one is higher. The difference in porosity is reflected in the different rates of consolidation. According to Duroscope rebound test all consolidants strengthen the tested limestones. For the micro-porous and less porous Tura limestone silica acid ester (13% increase of Duroscope rebound) while for porous Mokattam limestone

5 aliphatic uretan resin (67% increase of strength) was the most effective consolidant. For three consolidants; silica acid ester, aliphatic uretan resin and polymethyl methacrylate the more porous Mokattam limestone absorbs more than the less porous Tura limestone. Surprisingly, the opposite trend was observed for acrylate resin. The explanation, why the less porous Tura limestone absorbs more of this consolidant is given by the density and viscosity of acrylate resin. It is suggested that very light and viscous consolidant could penetrate into the smaller pores of Tura limestone gradually, while the penetration to the larger pores of the more porous Mokattam limestone was less effective. The swelling clay mineral content and possibly the Ca-sulphate content of Mokattam limestone also might have hampered the penetration of acrylate resin. It has been previously reported that salts present in Egyptian limestones might decrease the effectiveness of consolidants (Thicket et al. 2000). Steinhäuser & Wendler (2004) have demonstrated that modified ethylsilicates can be used to strengthen gypsum-rich weathered limestones. The gypsum content of the studied Mokattam limestone is far less than the one which was used by Steinhäuser & Wendler (2004), thus it is supposed that traces is of gypsum does not modify significantly the consolidation absorption. This study has demonstrated that a single product may show different effects on slightly different limestones. Minor differences in micro-texture, mineralogy and more obviously in pore size distribution and porosity appear to be the key control factors in the strengthening effect of the consolidant. These results are in good agreement with the previous limestone consolidation tests since Snethlage (1997), Alvarez de Buergo & Fort (2002), Steinhäuser & Wendler (2004.) have also emphasised the role of pore-size distribution and micro-pores in the strengthening effect of various consolidants. It has been also documented in this paper that for selecting the appropriate stone consolidant the recording of penetration rate or density changes is not appropriate and might be misleading. Conversely, Duroscope rebound values provide better information on the effectiveness of consolidation. ACKNOWLEDGEMENTS Financial support of Bolyai János Research Grant of the Hungarian Academy of Sciences (BO/233/04; ÁT) and the Hungarian Science Found (OTKA K63399, ÁT) is gratefully acknowledged. REFERENCES Alvarez de Buergo, M. & Fort, R Characterizing the construction materilas of a historic building and evaluating possible preservation treatments for restoration purposes. In: Siegesmund, S., Weiss, T., S., Vollbrecht, A (Eds.), Natural Stones, Weathering Phenomena, Conservation Strategies and Case Studies. Geological Society, London, Special Publications 205: Fitzner B., Heinrichs, K. & La Bouchardiere, D Limestone weathering of historical monuments in Cairo, Egypt. In: Siegesmund, S., Weiss, T., S., Vollbrecht, A (Eds.), Natural Stones, Weathering Phenomena, Conservation Strategies and Case Studies. Geological Society, London, Special Publications 205: Gauri K.L. & Bandyopadhyay, J.K Carbonate stone: chemical behavior, durability, and conservation. New York: Wiley. Helmi, F.M & Attia, H.R Characterization and conservation of Seti-I temple stone, Abydous, Upper Egypt. In: In: J. Riederer, (ed) Proceedings of the 8 th International Congress on Deterioration and Conservation of Stone, - Berlin 1996, Vol. 2: Berlin: Möller Druck Verlag. Klemm, D.D. & Klemm R The building stones of ancient Egypt a gift of its geology. Journal of African Earth Sciences, 33: Punuru, A.R., Chowdhury, A.N., Kulshreshtha, N.P & Gauri, K.L., Control of porosity on durability of limestone at the Great Sphinx, Egypt. Environmental Geology and Water Science, 15 (3): Said, R The Geology of Egypt. Rotterdam: Balkema. Snethlage, R Leitfaden Steinkonservierung. Stuttgart: Fraunhofer IRB Verlag. Steinhäuser U. & Wendler E Conservation of limestone by surfactants and modified ethylsilicates. In D. Kwiatkowski & R. Löfvendahl (Eds.), 10th International Congress on Deterioration and Conservation of Stone - Stockholm 2004: Stockholm: ICOMOS Sweden. Tawadros, E. E Geology of Egypt and Libya. Rotterdam: Balkema. Thicket, D, Lee, N.J & Bradley, S.M Assessment of the performance of silane treatments applied to Egyptian limestone sculptures displayed in a museum environment. In: V. Fassina (ed) Proceedings of the 9 th International Congress on Deterioration and Conservation of Stone 2000: , Venice: ICOMOS.