Geotechnical and geological properties of Mokattam limestones: Implications for conservation strategies for ancient Egyptian stone monuments

Engineering Geology 104 (2009) 190–199

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Engineering Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e n g g e o

Geotechnical and geological properties of Mokattam limestones: Implications for conservation strategies for ancient Egyptian stone monuments H.D. Park a,⁎, G.H. Shin b a b

Department of Energy Systems Engineering, Seoul National University, Korea Environmental Planning Team, Korea Water Resources Corporation, Korea

a r t i c l e

i n f o

Article history: Received 27 November 2007 Received in revised form 9 September 2008 Accepted 23 October 2008 Available online 6 November 2008 Keywords: Stone monuments Conservation Chemical consolidant Limestone Egypt

a b s t r a c t Geotechnical and geological properties of limestone samples from the Mokattam Quarry in Cairo, Egypt, were determined in order to provide prior information for the selection of suitable methods for the conservation of stone monuments around Cairo. A commercial chemical consolidant (Wacker OH 100) was applied to fill the pore spaces and to strengthen the weathered rock. Filling of pore spaces was confirmed by the decrease of both porosity and permeability of rock samples after the application of the consolidant. Analysis by mercury porosimeter showed most effective consolidation results for pore spaces from 0.75 to 1.0 µm in diameter, which were those mainly observed in the samples. Ultrasonic velocity did not show any significant evidence but an increase in strength, observed as an increase in the point load index after the consolidation process was completed, confirmed that the filling and consolidation process worked effectively. Point load testing can thus be used in preference when the number of samples available for laboratory testing is limited. From the color analysis, it was shown that there was no noticeable color change after the application of consolidant Wacker OH 100. The combinations of laboratory tests adopted in this study can be applicable to the planning of conservation of other stone monuments. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Conservation strategies for stone monuments require information on the geotechnical and geological properties of their rock material. Laboratory tests of rock samples from the monument itself can provide the necessary information (e.g., Wüst and McLane, 2000) but sampling ancient stone monuments for laboratory tests may not be possible due to their sensitivity and cultural value. This is also true for the Sphinx and the pyramids at Giza, Egypt, which have been severely weathered (Fitzner et al., 2002). In order to prepare long-term conservation programs, detailed information on the geotechnical and geological properties of the rocks used in the construction of these monuments is of vital importance. Previous studies on samples of the Sphinx showed that (i) the uneven distribution of salts throughout the rocks has caused differential weathering (Hughes, 1988; Dixon and Reader, 2001) and (ii) the variable pore structure has also caused differential weathering (Gauri et al., 1988). Although many stone blocks of the Giza pyramids exist around the pyramid structures, it is difficult to obtain material for destructive laboratory tests. Some of the stone of the pyramids at Giza were quarried from the Giza Plateau. Other parts, such as casing stones, were quarried from the Tura area (Romer, 2007).

⁎ Corresponding author. E-mail address: [email protected] (H.D. Park). 0013-7952/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2008.10.009

However, there are several other stone structures in the city of Cairo that exhibit similar weathering problems to the Ancient Egyptian structures. These structures have obtained their building blocks from a nearby quarry. Therefore, our study focused on material from the Mokattam Quarry in order to be able to conduct destructive tests. A series of laboratory tests was conducted to assess (i) the geotechnical and geological properties before and after the application of consolidant for conservation, and (ii) the effectiveness of the penetration of

Fig. 1. Mokattam Quarry where samples for this study were taken.

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Fig. 2. Procedure of experimental work.

consolidant into rock samples and its positive contribution to the increase in strength of the rock for conservation purposes. 2. Methods for experimental work 2.1. Sampling and mineralogy The Mokattam Quarry at Cairo is believed to be one of the main sources of rock used in the Ancient Egyptian constructions around Cairo. The quarry is located in the south-eastern part of Cairo and is not in use anymore (Fig.1). The regional significance of Mokattam limestone is that it belongs to the lower to middle part of the Mokattam formation of the middle Eocene (Said, 1990; Tawadros, 2001), which extends from the

Giza Plateau to Mokattam with a gentle dip of 5–10°SE. Several blocks of limestone (700 mm each side) were carefully selected based on their similar appearance. The blocks were then cut into smaller blocks (160 mm each side) at a local stone factory and then shipped to Seoul National University. In the laboratory, smaller cubes (20 mm each side) were cut from one block and cylindrical cores (diameter 38.1 mm with lengths of 50–70 mm) were cut from another block for further experiments (Fig. 2).

Table 2 Classification of chemical consolidants (based on De Witte, 2001) Material (Consolidant)

Characteristics

Polymer

–Acrylics (Paraloid B72) –PVA(Mowilith) –PEG –Polyesters –Epoxys

–Easy to deal with –Penetration depth would be influenced by the concentration and by solvent –Reacted by water and X-rays –Insoluble after consolidation –Viscosity can be decreased by adding other materials. –Polyesters: styrene, MMA –Epoxy: acetone, MEK –Deep penetration depth is possible.

Table 1 Characteristics of Wacker OH 100 (Data from technical reports of Wacker Chemie®, 2002) Property

Wacker OH 100

Remark

Prepolymer

Maker Ingredient Color Density Viscosity Flash point Ignition point

Wacker Chemie® Ethyl silicate Colorless to yellowish Approximately 1000 (kg/m3) Approximately 1.6 × 10− 3 (N s/m2) 40 °C 230 °C

Germany Ethyl silicate content: 95% – At 25 °C At 25 °C – –

Monomer –Acrylics –Silanes: ethyl silicate or mixture of ethyl silicate and methyl silicate

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Table 3 Summary of different applications of chemical consolidants Case no.

Consolidant (manufacturer)

Sample

Application of consolidant

Measurement properties

Reference

1

Wacker OH 100 (Wacker-Chemie)

Ganghwaae-stonestone

Immersion for 3 days

Salt crystallization test, water absorption

Won (2001)

2

Wacker OH 100 (Wacker-Chemie) Funcosil OH (Remmers) Wacker OH 100 (Wacker-Chemie) Brethane (Colebrand Pty. Ltd.) Mowilith 35/73 (Hoecht) Minersil SH & Minerxan (Prolab) H-224 (Rhone Poulenc) Wacker OH (Wacker-Chemie) Tegovakan V (Goldschmidt) Paraloid B72 (Röhm & Hass) Paraloid B72 (Röhm & Hass) DF104 (General Electric) Wacker OH (Wacker-Chemie) Rhodorsil RC70 (Rhone Poulenc) Merck (Riedel-De Haen) Silicic acid ester solution

Capillary suction for 10 min Flooding using squeeze bottle

Color difference on the surface of stone

Lukaszewicz and Kwiatkowski (1995) Caselli and Kagi (1995)

Sandstone Limestone

No description

Porosity, mercury porosity, water absorption

Pascua et al. (1995)

Calcitic sandstone Limestone Dolomitic limestone

No description

Mercury porosity

Perez et al. (1995)

Limestone

Applied with brush Repeated

Water absorption

Manaresi et al. (1995)

Fresco sample

No description

Color change

Marble, Sandstone Chalk, Limestone

Capillary absorption

Water absorption, compressive strength, ultrasonic velocity

Provinciali and Iazurlo (1995) Leroux et al. (2000)

3

4

5

6

7 8

Geochang-stone Gotland sandstone Bacchus Marsh sandstone

Both XRD (X-ray diffraction) and XRF (X-ray fluorescence) were selected for the identification of minerals and chemical composition. In addition, spectroradiometer analysis was carried out to compare mineral composition between samples from Mokattam Quarry and the Ancient Egyptian monuments in the Giza area. Spectroradiometry is a non-destructive method that measures reflectance of an object using very high spectral (i.e., hyperspectral) resolution. Each mineral has its own characteristic reflectance pattern, which can be identified (Clark, 1999). The portable spectroradiometer (FieldSpec3 by ASD Inc) used in this study has a spectral range of 350 nm to 2500 nm, with dual spectral resolutions of 3 nm at 700 nm, and 10 nm at 1400/2100 nm. The same

Strength, water absorption, permeability, consolidant absorption, artificial weathering

lighting conditions were used for the measurements of samples no.1 and no.2 from Mokattam Quarry and no.3 from a limestone block of the Giza pyramids. 2.2. Consolidation of samples using a commercial product Epoxy resin was one of the main chemical consolidants used for conservation but is no longer widely used due to yellowing problems. Reviews of the different chemical consolidants show that the ethylsilicate series is one of the best alternatives for conservation due to its lack of side effects and ease of penetration because of its low viscosity

Table 4 Preparation of samples Sample no.

Tests

Consolidation

Specific gravity

Core #1–6

–Consolidant absorption –Ultrasonic velocity –Water absorption –Porosity –Permeability –RGB value (Color) –Consolidant absorption –Ultrasonic velocity –Water absorption –Porosity –Permeability –RGB value (Color) –Consolidant absorption –Ultrasonic velocity –Water absorption –Porosity –Permeability –RGB value (Color) –Point load test –Ultrasonic velocity –Water absorption –Permeability –Point load test –Ultrasonic velocity –Water absorption –Permeability

Treated twice

2.19–2.34 (mean 2.25)

Treated

2.05–2.40 (mean 2.23)

Untreated

2.10–2.39 (mean 2.23)

Treated

2.25–2.31 (mean 2.27)

Untreated

2.23–2.31 (mean 2.27)

Core #7 ~ 21

Core #22–35

Cube #1–8

Cube #9–15

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(Gauri et al.,1988; Caselli and Kagi, 1995; Lukaszewicz and Kwiatkowski, 1995) (Tables 1 and 2). Ethyl-silicate reacts with water and produces both ethanol and gel type Si–O, which will consolidate within the pores and cracks in rock. Wacker OH 100 is one of the commercial products of the ethyl-silicate series and has been widely used for recent conservation work around the world (Table 3) and was chosen for this study because it has been proven to be a successful consolidant (De Witte, 2001). All samples were oven-dried at 60 °C for 48 h to eliminate water and to increase the penetration of consolidant (De Witte, 2001). Samples for consolidation (Table 4) were then immersed in consolidant for 72 h with the upper surface exposed to the air for easy escape of air bubbles from the sample. Although such an immersion method cannot be directly

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applied to large stone columns in the field, this experiment may provide the basic information on the process of consolidation. For better consolidation, all immersed samples were then put under controlled temperature and humidity conditions (i.e.,10 °C with 40% RH for the first three weeks and then 20 °C with 60% RH for deep penetration). 2.3. Absorption, porosity, and permeability Water absorption was calculated from the sample immersed in water for 48 h at a temperature of 25 °C. Consolidant absorption was also calculated employing the same principle as water absorption. Again, these measurements were based on the recommendation from ICCROM (Falcone, 2003). High porosity is one characteristic that allows severe

Fig. 3. Spectral responses of (a) samples from Mokattam quarry (No. 1 and No. 2) and stone block at Giza pyramid (No. 3) and (b) standard library (based on Clark, 1999).

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weathering (Benavente et al., 2001) and it is very important to obtain the value by measurement. Porosity changes from before to after the application of consolidant were measured using both a helium porosimeter and a mercury porosimeter. Because the mercury porosimeter is based on the measurement of pressure and surface tension with regard to the radius of a pore, it is possible to get information on the pore size distribution using the following equation. Permeability is one of the most important factors controlling the penetration of a consolidant. A total of 13 cylindrical core samples were used for the measurement using helium gas. For the cubic samples, a mini permeameter was used. P = ð2σcosθÞ=r

ð1Þ

where P r σ θ

absolute pressure exerted (bar or atm; 1 bar = 0.9678 atm) pore radius (μm) surface tension of mercury (dyn/cm2) contact angle (radian)

2.4. Geotechnical rock properties The velocity of ultrasonic waves through rock depends on the density of the structure and has been used as a good indicator of the current weathering state (Uchida et al., 1999; Esaki and Jiang, 1999). If the pore spaces are filled with consolidant and are solidified, the ultrasonic velocity will increase. Measurements were conducted with 500 kHz and 100 kHz for P-waves and S-waves, respectively, using the Sonic Viewer 170 by Oyo. It is desirable that the consolidant works to increase the strength of the weathered stone monument. The change of strength values of rock samples after consolidation can be detected using a point load test. Two groups of cubic samples were prepared; one group (with six samples) representing natural stone before any consolidation, and the other representing stone (with five samples) treated with consolidant. Tests were conducted according to the ISRM Suggested Method (ISRM, 1985).

monuments and has also been used for the assessment of the durability of weathered rock (Aboushook et al., 2006).

3. Results 3.1. Mineral composition Both XRD (X-ray diffraction) and XRF (X-ray fluorescence) results on prepared samples show that Mokattam limestone is mainly composed of calcite with minor amounts of dolomite, quartz, and gypsum. Mokattam limestone contains a fossil nummulite, which is usually observed in the limestones of the Sphinx and pyramids of Giza (Purton and Brasier, 1999). Results from spectroradiometer analysis show that strong absorption peaks (i.e., convex downward) at 2.30 to 2.35 µm and at 2.50 to 2.55 µm are typical responses of calcite (i.e., combinations of CO3 fundamentals). Another absorption peak at 1.9 µm is for gypsum (i.e., combinations of the H–O–H bend with the OH stretches) (Fig. 3a,b). Among the three samples, there is no difference in the absorption peak pattern. This result shows that although a spectroradiometer is a tool for simple assessment of the general similarity of mineral composition due to its non-destructive characteristic, the details of the mineral composition should always be determined by other methods such as XRD or XRF analyses, as long as direct sampling is allowed.

2.5. Change of color value Any consolidant should not change the appearance of the original stone monument, especially in terms of color (Aboushook et al., 2006). In practice, a certain amount of color change after the application of consolidant is allowable if it is not noticeable by visual observation. Color values before and after the application of consolidant were measured both by a commercial colormeter (DR LANGE Micro color) and ImageCraft, a computer software developed by Chang and Park (2001). Commercial colormeters measure color components such as CIE L⁎, a⁎, b⁎, which are based on black-white, red-green, and yellow-blue color space. Thus a⁎, b⁎ are related to hue and chroma, and L⁎ represents brightness. The difference (ΔE) is calculated using the following equation.

ΔE =

 2  2  2 1=2 ΔL⁎ + Δa⁎ + Δb⁎

ð2Þ

where ΔE ΔL⁎ Δa⁎ Δb⁎

Color difference Difference of color component L⁎ Difference of color component a⁎ Difference of color component b⁎

ImageCraft is software that calculates the R (Red), G (Green), and B (Blue) values by calibrating the image taken by a digital camera using a standard color chart as reference. ImageCraft has been successfully applied to analyze the color change of several Egyptian stone

Fig. 4. Water absorption of (a) core samples and (b) cubic samples between natural samples and samples treated with consolidant.

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Fig. 5. (a) Comparison of porosities measured by Helium porosimeter and by Mercury porosimeter. (b) Pore size distribution of treated and untreated samples measured by Mercury porosimeter.

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3.2. Water absorption vs consolidant absorption

Table 5 Color analysis before and after consolidation by colormeter

The different values of water absorption in cubes and cores are believed to be a result of the variations between different sample blocks. However, both core and cube samples show that water absorption decreased by 50% after treatment by consolidant (Fig. 4a,b). Consolidant absorption has a good linear correlation with water absorption (R2 = 0.989) and with pore volume (R2 = 0.951). Consolidant absorption values changed due to the further reaction with water to produce ethanol to be evaporated over up to 28 days for cores and 17 days for cubes. The initial absorption of consolidant (12.2% on average) is larger than the maximum water absorption (10.8% on average) but eventually less than the value (7.3% on average) after the full curing process. Larger values of consolidant absorption were achieved because the method was different from that of water absorption, i.e., trapped air in the rock sample could freely escape when the sample was immersed in consolidant, whereas no escape was allowed during the full immersion of the sample in water. Consolidant absorption after the second immersion (3.1% on average) is lower than that after the first (7.3% on average) due to the decrease of available pore space for accommodation of consolidant. This is also proven from the observation that water absorption decreased from 5.3% to 0.8% after the second consolidation. Among the samples, there is good linear correlation between first and second consolidant absorption values (R2 = 0.859). Thus, it is shown that there could be a further chance to apply consolidant again to increase the amount of total penetrated consolidant, as long as the first process of consolidation absorption is effective.

Color component

Treated sample Average

SDa)

Average

SDa)

L⁎ a⁎ b⁎ Difference (ΔE)

77.66 4.99 20.84

1.47 0.67 1.92

76.42 5.42 21.46 1.45

1.21 0.58 2.01

3.3. Porosity and permeability The measurements by both the gas porosimeter and mercury porosimeter show a similar trend, i.e., a decrease of total porosity by 1/3 after the consolidant treatment (Fig. 5a). This means that not all the pores were occupied by cured consolidant. The results of the pore size distribution from the mercury porosimeter (Fig. 5b) are as follows: (i) an increase in the relative proportion of smaller pores (i.e., those less than 0.5 µm in diameter), and (ii) a decrease in the relative proportion of pores of 0.75 to 1.0 µm in diameter. This means that the consolidant (Wacker OH100 in this study) filled the pores from 0.75 to 1.0 µm in diameter. Such an effective range of pore size depends on the viscosity of

a)

Untreated sample

SD: Standard deviation.

the consolidant. Further decrease of porosity after the second consolidation confirms that more consolidant can be accommodated in the pore spaces by further application of consolidant. Permeability averaged from 13 cylindrical samples decreased from 3.1 md before consolidation to 2.2 md after the first consolidation (i.e., decreased by 30%) and then decreased further to 0.3 md after the second consolidation. This shows the consolidation worked effectively to fill the pore spaces. Results from the cubic samples also show that permeability decreased due to consolidation: 0.9 md before consolidation and 0.5 md after consolidation. Smaller values than those of the cylindrical samples are observed because the mini permeameter provides an approximation of permeability around a specific point and can vary from point to point within the sample. However, it also shows that the consolidation worked to fill the pore spaces. 3.4. Changes of geotechnical rock properties After the application of consolidant both P-wave and S-wave velocities seemed to be slightly increased when the average values are considered. However, the ranges of velocity values do not show any significant differences. Thus ultrasonic velocity does not provide enough evidence for effective consolidation in this study. Point load test results are expressed as Is(50) (i.e., point load index value corrected to a diameter of 50 mm) and showed that the average value of Is(50) after consolidation was increased by 18% from 3.4 MPa to 4.0 MPa and by 42% from 2.6 MPa to 3.7 MPa for the weakest sample (Fig. 6). This proves that rock samples were consolidated, which positively contributed to the increase in strength of weak samples (or weak parts of the rock structure), leading to higher stability of the stone monument. 3.5. Change of color value Results from the colormeter show ΔE = 1.45 (Table 5). According to the guidelines provided by DR LANGE Micro color, the color change in two neighboring surfaces cannot be noticed by visual observation if ΔE is less than 2. Color analysis from ImageCraft also shows some differences where the green component changed more than the other two components (Fig. 7). The average RGB values initially lowered soon after the immersion (i.e., darkening of the surface) and then increased (i.e., brightening of the surface) with the continuing consolidation process, up to 2.1%, 11.4%, and 12.6% for R, G, and B, respectively. This shows that ImageCraft is sensitive to the color changes and can continuously monitor color changes before and after immersion in consolidant. 4. Discussion

Fig. 6. Point load index value of cubic samples before and after consolidation.

Considering that the consolidant should penetrate and bind the stone to increase the durability of the stone, the overall results in this study showed that the commercial consolidant Wacker OH100 will help in the conservation of stone monuments made with Mokattam limestone. As the samples are porous enough (i.e., 24% total porosity), the penetration of consolidant was effective without using any special treatment such as the application of pressurized consolidant or the use of a vacuum chamber to take out the air bubbles in the stone. After

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Fig. 7. Color analysis of Mokattam limestone surface before and after consolidation by ImageCraft.

the consolidation, the water absorption was observed as only 53% of the original water absorption value for the sample. The result is quite comparable to previous studies, e.g., other types of limestone with a 24% to 31% drop (Lukaszewicz, 2004), and marble with a 15% drop (Rohatsch et al., 2000), although a drop in water absorption depends on the specific rock type, consolidant, and method of consolidant application. A decrease in porosity and permeability due to the penetration of consolidant can contribute to the retarding of further weathering by reducing the water penetration in the manner that water repellent does (Alvarez De Buergo and Fort Gonzalez, 2002), although the main mechanism is different. Another role of consolidant for conservation is binding the loose mineral grains and this will help to increase mechanical durability. Although half of the total pores were initially filled with consolidant in liquid form, about one third of the total pores were finally left in

Fig. 8. Optimum time period for application of consolidant in terms of porosity and strength against time as weathering continues.

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a hardened form after curing. The hardening effect after consolidation has been frequently revealed by ultrasonic velocity measurement due to its non-destructive characteristic; (i) the ultrasonic velocity was measured in steps of 5 mm parallel to the treated surface with a point-shaped transducer (Leroux et al., 2000), (ii) the ultrasonic velocity of marble was doubled after consolidation with hydrophobe mixture of alkyl-alkoxysiloxane (Wacker 290) and KSE (ethylsilicate Wacker OH100) in ethanol (6:4:5) (Rohatsch et al., 2000). Although the average values of P-wave and S-wave values in this study were increased by 10% and 11%, respectively, it should be noted that there are no significant changes both in P-wave and Swave velocities because there are too many overlaps in the results. It is not clear whether ultrasonic measurement would be useful to assess the effectiveness of consolidation on Mokattam limestone samples in this study. If samples taken from the monument itself, or analogous samples from other sources such as the quarry, are available for destructive testing, this will provide good information on the binding effect of consolidation. The decrease in bi-axial flexural strength of weathered sandstone by 50% could be recovered after consolidation (Snelthage and Meinhardt-Degen, 2004). Effective consolidation of limestone was confirmed by the increased drill resistance (Leroux et al., 2000; Steinhäuβer and Wendler, 2004). The change of Brazilian tensile strength of core discs was observed on porous limestone after consolidation (Maravelaki-Kalaitzaki et al., 2004). About 20% to 49% of an increase in compressive strength on cylindrical samples was observed after consolidation (Rohatsch et al., 2000). Although drill resistance can be easily measured in the destructive tests, this value cannot be used as direct input of strength for numerical modeling because it represents the hardness of stone rather than strength. The advantage of acquiring strength values is also its direct input to structural analysis (e.g. Wüst and McLane, 2000). Most strength tests used in previous studies required significant volumes of samples either as disc cores (50 mm diameter in general) or cylindrical cores (50 mm diameter and lengths of up to 200 mm), whereas the samples for the point load test used in this study could be as small as a cube of 20 mm each side. This can be an advantage when the number of available samples is limited for testing. In this study strength values were obtained by point load testing and simple statistical tests such as the t-test showed that there is a significant increase in strength (18%) after consolidation. The result supports the view that the consolidant worked to increase the mechanical durability of the stone. As explained in Fig. 8 (see curve b), the porosity, permeability, and microcrack occurrence (including propagation) will generally increase as the weathering continues with time. The effective penetration of consolidant will be possible after passing a minimum weathering period (i.e., after th,min) with the development of certain flow passages such as pores and microcracks. Thus, the more weathered the rock is, the easier is the penetration of consolidant. However, as weathering continues, the structural stability (or strength) of stone monuments decreases (see curve a in Fig. 8). Thus, the timing of the application of consolidant should not be too late to avoid structural damage (i.e., tf). In addition, the result from the analysis of pore size distribution before and after the application of consolidant shows that there is a certain effective range of pore size, i.e., minimum pore size, dh,min for penetration of consolidant, for the applied consolidant with a specific viscosity. If the pore size is bigger than this critical value (i.e., dh,max) due to further weathering, then the effective consolidation cannot be made with a single application of consolidant, although the penetration is very effective. Thus, the maximum healable pore size by consolidant could be dh,max. There could be an optimum time window (i.e., Toptimum) for effective consolidation, depending on the specific consolidant and on the specific rock,

which can be assessed with the engineering geological properties described in this study. 5. Conclusion A series of laboratory experiments showed that analogous rock samples can be used as alternatives for the assessment of geotechnical and geological properties of stone monuments for conservation purposes, as long as careful selection is carried out. XRF analysis showed that the Mokattam limestone used in this study is mainly composed of calcite with minor amounts of dolomite, quartz, and gypsum. Although the use of a portable spectrometer is very convenient and advantageous as a nondestructive method, it cannot provide better information on mineral composition than XRD or XRF as long as direct sampling is allowed. From the laboratory tests on Mokattam limestone, it was clearly shown that a consolidant (Wacker OH 100 in this study) worked effectively on a range of pore sizes from 0.75 to 1.0 µm in diameter. Decreases of both porosity and permeability of 50% and 30%, respectively, after the application of consolidant were observed as proof of effective penetration of consolidant. A final increase of strength of 18% also supports the conclusion that the consolidation process worked effectively. However, no meaningful result was observed from ultrasonic velocity measurements. Thus, Mokattam limestone in its present state falls within the optimum time window for conservation work, based on the stability-timeweathering relationships. Additional analysis of the color change of the surface after consolidation showed that the consolidant did not cause any noticeable change in color and therefore is good for conservation work with Mokattam limestone. Acknowledgements This study was supported by KOSEF (Project No. 20016-312-01-2) and the Brain Korea21 Project in 2008, and the Research Institute of Engineering Sciences, Seoul National University, Korea. The authors wish to express their sincere thanks to Prof. Aboushook and EASRT (Egyptian Academy of Scientific Research and Technology). References Aboushook, M., Park, H.D., Gouda, M., Mazen, O., El-Sohby, M., 2006. Determination of durability of some Egyptian monument stones using the digital images. Proceedings of the 10th IAEG Congress – Engineering Geology for Tomorrow's Cities, Nottingham, UK. Alvarez De Buergo, M., Fort Gonzalez, R., 2002. Characterizing the construction materials of a historic building and evaluating possible preservation treatments for restoration purposes. In: Siegesmund, S., Weiss, T., Vollbrecht, A. (Eds.), Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies. Geological Society, London, Special Publications, vol. 205, pp. 241–254. Benavente, D., García del Cura, M.A., Bernabéu, A., Ordóñez, S., 2001. Quantification of salt weathering in porous stones using an experimental continuous partial immersion method. Engineering Geology 59 (3–4), 313–325. Caselli, A., Kagi, D., 1995. Methods used to evaluate the efficacy of consolidants on an Austrian sandstone. Preprints of International Colloquim on Methods of Evaluating Products for the Conservation of Porous Building Materials in Monuments, ICCROM, pp. 121–130. Chang, Y.S., Park, H.D., 2001. Quantitative classification of building stone using image processing technique. Proceedings of the International Conference on Aggregate 2001 – Environment and Economy, vol. 1, pp. 75–79. Clark, R.N., 1999. Spectroscopy of rocks and minerals and principles of spectroscopy. In: Rencz, A.N. (Ed.), Manual of Remote Sensing. John Wiley and Sons, New York, pp. 3–58. De Witte, E., 2001. Synthetic Polymers. Lecture Note of the 14th International Course on the Technology of Stone Conservation SC01, ICCROM, p. 18. Dixon, J., Reader, C., 2001. The riddle of the Sphinx – one geological solution, Geoscientist. The Geological Society of London 11 (7), 4–6. Esaki, T., Jiang, K., 1999. Comprehensive study of the weathered condition of welded tuff from a historic stone bridge in Kagoshima, Japan. Engineering Geology 55 (1–2), 121–130. Falcone, R., 2003. Lecture Note of the 15th International Course on the Technology of stone conservation SC03, ICCROM. Fitzner, B., Heinrichs, K., La Bouchardiere, D., 2002. Limestone weathering of historical monuments in Cairo, Egypt. In: Siegesmund, S., Weiss, T., Vollbrecht, A. (Eds.), Natural Stone, Weathering Phenomena, Conservation Strategies and Case Studies. Geological Society, London, Special Publications, vol. 205, pp. 217–239.

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