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Conservation products inside building stones V. Cnudde1, P. Dubruel2, K. De Winne2, I. De Witte3, B. Masschaele1, P. Jacobs1 and E. Schacht2

1 Ghent University, Department of Geology and Soil Science Krijgslaan 281/S8 B-9000 Gent, Belgium 2 Ghent University, Polymer Materials Research Group Krijgslaan 281/S4 B-9000 Gent, Belgium

3 FTB Restoration Bouwelven 19 B-2280 Grobbendonk, Belgium Corresponding author: Veerle Cnudde

Veerle.Cnudde@ugent.be Tel: 00-32-9-2644580 Fax: 00-32-9-2644943

Abstract

To reduce the weathering rate of natural building stones, a wide variety of water repellents and

consolidants are commercially available. Although a lot of research is performed on these products, it

remains difficult to determine which product is appropriate to use for a certain type of building stone.

Each type of building stone has its own physical and technical properties which influence its rate of

decay. The localisation of the products inside a stone type is not only depending on the properties of

the products themselves, but also linked to the texture and structure of the stone. The impregnation

depth of the products strongly influences their efficiency and is therefore a key issue in the

determination if a product is functional for a certain type of stone. Here non-destructive X-ray

computed micro-tomography (micro-CT) turns out to be a powerful tool as it can visualise the presence

of water repellents and consolidants inside the stone and can help to detect the influence that these

treatments exert on porosity and pore-size distribution.

X-ray micro-CT has recently been introduced as a non-destructive material evaluation technique for

engineering and geology purposes. The fact that micro-CT can provide information about the internal

structure and properties of natural building stones, is a major advantage in the study of their

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conservation. Firmly linked with more classical research techniques, this non-destructive technique

offers an extra dimension to the cultural heritage research.

Keywords: water repellent, consolidant, natural building stones, CT, micro-CT, SEM, MIP, NMR,

viscosity, 3-bromopropyltrimethoxysilane

Introduction Currently consolidants and water repellents are often applied in restoration and conservation of cultural

heritage. But the use of consolidants can not be taken for granted. Some types of stone show an

important colour change after treatment with a consolidant. Ferruginous sandstone becomes darker and

has a “wet” look. Some materials that contain glauconite exhibit stains. Unpleasant surprises can be

avoided by putting up a test patch. During the gel formation of a consolidant, shrinking occurs that can

not be neglected. This means that the grain size of the materials exerts a big influence on the efficiency

of the treatment. This suggests that materials that become coarser grained during erosion like the

Euville sandstone, can hardly or not be consolidated. Meanwhile finer grained materials like some

chalks exhibit cracks caused by shrink ing. These few examples demonstrate the importance of

integrated research on conservation products combined with knowledge about the physical and

chemical properties of natural building stones. The products used in restoration are not only very

important for their constructive properties but also the esthetic part of the restoration is influenced by

their properties. The impregnation depth and localisation of the se products strongly influences their

efficiency and is therefore a key issue in the determination of the efficiency of a product for a certain

type of stone. Different techniques can be applied to determine this impregnation depth.

The drilling force measurement system (DFMS) is one of the tests that can be used to determine the

presence of a consolidant. This technique directly determines mechanical features like the hardness of

natural building stones by measuring its drilling resistance. During this experiment a rotating drill

progresses with a constant velocity, while the exerted force is measured with an estensimetric

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transducer which translates the gauge deformation into a electrical signal. The load cell deformation is

correlated with the “resistance of penetration”, hence to the “stone cohesion” (Tiano, P., 2003). This

procedure can assist in the evaluation of the consolidating performance of the applied conservation

treatments. However, in the case of heterogeneous structures, the results of DFMS can be difficult to

interpret.

A method frequently used to study hydrophilic/hydrophobic properties of material surfaces, is static

contact angle measurements. During these tests a video camera records an amount of ultra pure water

positioned on the surface of non-treated and treated samples. This movie is then used to determine the

impregnation speed of the water droplet and its contact angle. Measurements of the contact angle can

be performed by using an optical sessile drop method in combination with imaging software. The

contact angle (?), which is the angle between the droplet baseline and the tangent to the circle (fig. 1),

results from the balance among the three surface tensions (s) acting at the interface between liquid,

solid and gas. Thus the contact angle is a function of the characteristics of the liquid, gas and solid

material.

s sg = s sl + s lg cos? ? cos? = (s sg – s sl)/ s lg

where sg, sl and lg denote respectively the solid/gas, solid/liquid and liquid/gas interfaces.

In general it is assumed that when ?<90° the surface is hydrophilic (s sl < s sg ) and when ?>90° the

surface is hydrophobic.

Fig. 1: Visual photo image used to determine the contact angle of the water droplet.

A method to determine the efficiency of a water repellent is based on water absorption measurements

with the help of a Karsten pipe (Commission 25-PEM, 1980). This water pipe that consists of a vertical

tube with a gradation of 0 to 4 ml and with a surface area of 5 cm², is applied to the stone surface with

special mastic that does not leave stains. After filling the pipe with water to the highest gauge line, the

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absorbed amount of water is read after 5, 10 and 15 minutes. The amount of water absorbed between

the 5th and 10th minutes is considered as a measure for the water absorbing behaviour of the material

and is called the water absorption coefficient (WAC). The hydrostatic pressure on the surface is

determined by the height of the water level in the Karsten pipe. When the water level is 98 mm, this

corresponds to a wind speed of 40 m/s, perpendicular to the test surface (WTCB, 2002). Measurements

are generally performed before and after application of the water repellent, preferably after 168 hours

(or 1 week) of age.

Treatment with water repellents should normally not change the colour of the natural building stones

drastically, but often small colour changes do appear due to treatment. These colour changes can be

measured primarily in order to determine if there exists a colour deviation between non-treated and

treated samples and secondly to obtain the colour deviation size. Because visual perception is very

subjective, instrumental colour measurements were introduced to provide accurate colour

identification. Today, the most commonly used instruments to measure colour are spectrophotometers,

that measure the reflected light at various points in the visual spectrum, resulting in a curve. This curve

is unique for each colour which makes it an excellent tool for identifying, specifying and matching

colours.

Other frequently used techniques to detect products inside stones are optical microscopy and scanning

electron microscopy (SEM). For consolidated stones these microscopical destructive techniques are

very practical and thus widely used, but water repellents turn out to be much more difficult to detect.

Because all these classical research techniques are mostly performed at the stone surface, X-ray micro-

computed tomography (micro-CT) was used as a new research tool to visualize the products inside

natural building stones.

Micro-CT finds its origin in 1895 when Wilhelm Conrad Röntgen discovered the X-rays. The first

commercial application was in the late 1960s and early 1970s, when Hounsfield developed the first

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computed tomography (CT) scanner, also known as the Computed Axial Tomography (CAT) scanner

(Losano et al., 1999). These early developments were mainly used for medical diagnostic imaging.

Although medical X-ray CT was in the beginning mainly used to image bone structures, it soon became

clear that X-ray CT could prove to be of high value for other applications. By the end of the 1970s

special efforts were made towards the application of computed tomography in the industrial

environment. CT was rapidly applied in paleontology (Fourie, 1974; Conroy & Vannier, 1984; Haubitz

et al., 1988), soil research (Petrovic et al., 1982, Hainsworth & Aylmore, 1983; Warner et al., 1989;

Anderson et al., 1990; Pierret et al., 2002) and sedimentology (Kenter, 1989; Peyton et al., 1992; Zeng

et al., 1996). Rocks and natural building stones were examined in the 1990’s with CT (Carlson and

Denison, 1992; Keller, 1997; Jacobs, P. et al, 1995; Jacobs, P. et al, 1997; Klobes et al., 1997; Philips

& Lannutti, 1997). At the end of the 1990’s micro-CT, possessing a higher resolution, was introduced

for the analysis of rocks (Rosenberg et al., 1999; Carlson et al., 1999; Landis et al. 2000; Van Geet et

al., 2001). Although micro-CT proved to be a very promising non-destructive technique in different

domains of geological and material research, the obtained data needs to be firmly linked with

information obtained from more classical non-destructive and destructive techniques. Therefore,

information of different techniques, including SEM, MIP, NMR and viscosity measurements, was

linked with the µCT data providing a detailed analysis of the concerned material.

Materials

Water repellents

Hydro 10, an oligomer siloxane (10 volume % in white spirit) was selected as a water repellent for

stone materials. This oligomer siloxane contains a small quantity of trifunctional monomers. Upon

applying the water repellent on the stone, it reacts with the surrounding atmospheric water molecules,

resulting in the release of an alcohol (ethanol or methanol) and the formation of silanol groups. These

groups can further react resulting in the formation of three dimensional crosslinked polysiloxanes (fig.

2).

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Fig. 2: Reaction scheme of Hydro 10. Consolidants

For the consolidants, SH75 and SH100 were used (both based on ethylorthosilicate). While SH75 is

diluted in methylethylketon (25%), SH100 is solvent free. The product is a combination of pure ethyl

silicate and some oligomers. These are low molecular prepolymers that are formed by controlled

precondensation of ethylorthosilicate. A catalyst (dibutyltinlaurate) is added to control the reaction

speed in such a way that the increase in viscosity is not too fast (the product has to penetrate the

substrate) and not too slow (the product could migrate back to the surface). In optimal conditions, an

amorphous silicium dioxide is formed in the weak zone of the stone as presented in figure 3. This

amorphous silicium dioxide is the result of the reaction between the ethylorthosilicate and the

atmospheric humidity.

Fig.3: Reaction scheme for the consolidants Although this reaction requires water, it does not imply that the product can be applied on a wet

surface. On the contrary, the surface has to be as dry as possible. In the capillaries of each building

material, enough moisture is present to start the reaction. Half the amount of water is recuperated at the

end of the reaction. Depending on the petrographic composition of the material, the weathered zone

needs to be more or less coarse grained, because fine grained zones treated with consolidant will

develop shrinking cracks. Mercury porosimetry demonstrated that the silicium dioxide will mainly

concentrate in the capillaries with a diameter of 10 ? m (De Witte, 1999).

Natural building stones

The products were applied on two different types of building stones: a heterogeneous quartz arenite

from Bray (Belgium) with a porosity ranging from 5 % to 23 % and a more homogeneous highly-

porous bioclastic limestone from Maastricht (the Netherlands), with an average porosity of 51 %. The

quartz arenite (Upper-Landenian age, Lower Eocene) was mostly used as a local building stone in the

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Belgian cities of Bray, Binche and Mons, while the limestone (Maastrichtian, Upper Cretaceous) was a

typical building stone for the Romanesque and Gothic monuments in the area around Maastricht (Nijs,

1985).

Methods

CT

A highly new CT-scanner LightSpeed16 from GE Medical System was used to scan larger stone

samples, delivering 16 slices per CT revolution with a pixel size of 0,625 mm or 1,25 mm. Scans were

performed at 140 kV and 310 mA. The 21888 detector elements are composed of high density poly-

crystalline ceramic material. The absorption efficiency is better than 99 % allowing to scan stone

samples as large as 8 cm in diameter.

Micro-CT

An X-ray desktop micro-tomograph Skyscan 1072 was used to scan sample cores with 8 mm diameter.

The X-ray source is a Hamamatsu micro-focus tube with a focal spot size of 10 µm. The spot size

limits the spatial resolution of the reconstructed slices to 10 µm in the X, Y and Z directions. The

samples were scanned at a voltage of 130 kV and a current at 76 µA. Random movement and multiple-

frame averaging was used to minimise the Poisson noise in the images.

Due to the low X-ray tube current compared to the medical CT-scanner, the measuring time for 400

projections is about 4 hours. The true spatial resolution of the micro-CT scanner is about 100 times

better in comparison to the medical scanner.

SEM

Scanning electron microscopy (JEOL JSM 6400 and 5900) was used to examine surface details of the

stone material. Because both systems are equipped with an energy dispersive spectrometer, qualitative

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and quantitative compositional analysis could be obtained and precise elemental composition of

materials with high spatial resolution was accomplished. Secondary electron imaging was used to

analyse the morphology and the surface topography of the samples, while backscattered electron

imaging visualized compositional contrast in detail.

MIP

For the mercury intrusion porosimetry Autopore III (Micrometrics) was used to determine the pore-size

distribution and the porosity of the limestone of Maastricht and the sandstone of Bray.

NMR

The apparatus used for 1H-NMR spectra acquisition of the products was a Bruker 300 MHz, with

tetramethylsilane as internal reference.

Viscosity

For the viscosity measurements, the flow-through times were measured at room temperature using an

Ubbelohde viscosimeter (Schott Geräte) with a capillary diameter of 0,36 mm attached to a AVS/S

support (Schott Geräte). The processor controls pumping cycles and measures the flow-through time

with an electro-optical sensor.

Results

Products

The experimental dry weight of all the products was determined, in order to evaluate the amount of

weight loss after polymerization. Based on these data in combination with the original porosity results,

information about porosity changes due to treatment can be theoretically derived. The experimental dry

weight of Hydro 10, SH75 and SH100 was respectively 8,03 %, 31,99 % and 52,13 % (table 1). The

density of Hydro 10 before polymerization was 0,77 g/cm³ at 20°C, while after polymerization, when

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the polymer is composed out of [(R)2-Si-O-] as repeating units, it increased to 1,07 g/cm³. The density

of SH75 and SH100 before polymerization was respectively 0,93 g/cm³ and 0,95 g/cm³ at 20°C, while

after polymerization it attained 1,67 g/cm³ for both products. The basic elements used for SH100 has a

different composition (amount of ethylorthosilicate oligomers) compared to those used to prepare the

SH75. This explains why the density of the solvent free product is lower than the diluted product.

Table 1: Summary of product data (BR= 3-bromopropyltrimethoxysilane; * catalyst: n-butyltinmercaptide: di-n-butyltindilaurate).

The minimum molecule size of the ethylorthosilicate, which is the main component of bo th SH75 and

SH100 is 1,06 nm, taking into count the bond lengths and bond angles (Streitwieser et al., 1992) of the

constituting atoms. The size of polymers is typically in the range of a few tenths of nm.

MIP

The pore-size distribution of the limestone and the sandstone is a very important aspect in conservation

research. Porosity and pore-size distribution have an impact on different characteristic properties of the

samples, not only on capillarity, water vapour transport, contact angle and drying rate but also on the

impregnation of the products applied on the stone. When the polymer size is larger than the pore-size,

pores will remain untreated. In large cases the transition of untreated to treated pores will be gradual

over a broad pore-size range due to the statistical character of the polymerization process (Carmeliet et

al, 2002). In coarse pores the polymer network can fully develop, but these polymer structures can also

clog the pores, leading to a reduction of the connectivity within the pore structure. The reduction of the

pore section due to the development of the polymer network in the pore space will depend on the pore-

size. When the polymer size equals or exceeds the pore-size of the stone material, pore clodding will

occur. In pores with a large diameter, the polymer will result in a smaller reduction of the pore-size

radius (Carmeliet et al., 2002). Clodding and pore-size reduction will exert an influence on some very

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important rock characteristics, including water vapour permeability and capillarity. From MIP results

the limestone of Maastricht turned out to consist of 0,03 % of pore diameter of 1 µm, while 4,5 nm

seemed to be the smallest pore diameter to be detected (Table 2). 20 % of the pores, smaller than 10

µm diameter, will not be detected by micro-CT because of its spatial resolution. 76 % of the pores had

a diameter between 10 µm and 96 µm, while the largest detected pore had a diameter of 356 µm.

Table 2: MIP data Maastricht limestone and Bray sandstone .

For the sandstone of Bray a similar pore-size distribution was found as for the limestone of Maastricht,

but the percentage of pores smaller than 10 µm diameter was only 10 % (Table 2). The smallest

detected pores in the sandstone had a diameter of 4,5 µm (0,35 % of the total porosity), while the

largest reached 356 µm in diameter. 0,3 % of the pores had a diameter smaller than 1 µm. Although

these data give a good indication of the pore-size distribution, it is very important to keep in mind that

MIP has the tendency to overestimate the smaller pores, which might lead to erroneous conclusions on

their number. The ethylorthosilicate molecules and the polymers of the Hydro 10 can thus impregnate

the smallest pores of both the sandstone of Bray and the limestone of Maastricht. SH75 contains beside

ethylorthosilicate molecule s also a small fraction of oligomers. It can thus be assumed that these

oligomers will not impregnate the smallest pores of both types of natural building stones.

Medical CT-scan

The sandstone and limestone samples were first scanned with a medical CT-scanner to obtain 3D

information about the texture inside the samples. Depending on the direction of the layering,

conservation products will penetrate into the stone in a different way. In a first series of tests the

medical CT-scanner was used to visualize denser areas inside stones, which could act as natural

boundaries and obstacles preventing the products to penetrate inside the stone. The sandstone often

showed very heterogeneous areas with fine iron oxide in the matrix. This heterogeneity was even

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detectable on hand pieces (Fig. 4a) but could also be clearly identified with the CT-scanner. Because of

the attenuation differences between iron and silicium, the presence of iron inside the sandstone could

be easily demonstrated on the 2D cross-sections derived from the CT-radiograms (fig. 4b and fig. 5).

Fig. 4: a) Photo of a heterogeneous piece of the Bray sandstone; b) 2D reconstructed slice of the heterogeneous piece of the Bray sandstone derived from the CT -radiograms .

Fig. 5: Stack of 2D images derived from the CT-scan, showing the position of the denser iron layer inside the sandstone .

CT images of the Maastricht limestone confirm the presence of regular layering and of shells (fig. 6).

Although CT can be used to obtain a rapid and general overview of the sample, the current resolution

limitation still renders it impossible to visualize pores or to determine any pore-size distribution.

Fig. 6: 2D cross-section of the limestone of Maastricht derived from the CT-scan. Because CT- images can visualize denser areas inside larger rock samples, some samples were treated

with a consolidant. Treatment with consolidant should cause a higher density inside certain parts of the

treated stone samples, because of their structure and their high percentage of dry weight. CT-images of

sandstone and limestone samples treated with SH100 and SH75 revealed regions with a higher density,

probably due to their consolidation. But because these denser areas could also be present due to the

heterogeneity of the stone and because of the resolution limitations of the medical CT-scanner, a

correct interpretation remains complicate. Only when the scans would be performed before and after

treatment, subtraction of the corresponding images could provide detailed information on the induced

changes.

Although the acquisition time for the CT-scans is very fast, detailed data on the structures inside the

samples cannot be obtained due to the resolution limitation of the CT-technique. Therefore micro-CT

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was used in order to obtain more detailed information at a higher resolution, although its limitation lies

in the fact that small samples need to be used.

Micro-CT and SEM

The micro-CT images reveal inside the sandstone samples not only the pores but also some very bright

spots (fig. 7).

Fig. 7: 2D cross-section of Bray sandstone derived from micro-CT data. To identify these bright spots, SEM in combination with BSE- images and EDS analysis was used.

They often turned out to be rutile (TiO 2) and sometimes zircon (ZrSiO4). Both heavy minerals are very

resistant to chemical weathering and mechanical abrasion. The y are accessory grains present in a

concentration less than 1 %. On the BSE- image (fig. 8) rutile (60 µm length) and zircon (23 µm length)

are both visible as well as on the 2D cross-section derived from the micro-CT images due to their high

attenuation for X-rays. Both minerals are also larger than the spatial resolution of the micro-CT.

Fig. 8: BSE image of quartz grains from Bray sandstone with intragranular zircon and rutile. In the 2D reconstructions of the limestone samples the observed brighter areas are derived from denser

parts like shells (fig. 9).

Fig. 9: 2D-cross-section of Maastricht limestone derived from micro-CT data. Samples treated with consolidant and water repellent were also investigated with SEM and micro-CT.

On the SEM images of sandstone and limestone samples treated with SH75 or SH100, the typical

structure of the consolidant wrapped around the grains is visible (fig. 10a). This consolidation will

change the porosity of the sample and its pore-size distribution. By analysing different positions inside

the consolidated samples, the presence of the consolidant could be identified over a depth of more than

1 cm. The BSE- images obtained from polished thin sections of limestone treated with SH100 revealed

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that the consolidant tends to concentrate in small blocks (fig. 10b), which is due to the tetra-

functionality of the consolidant, that sets out four bonds to a variety of other atoms.

Fig. 10: a) SE-image of lime stone treated with SH100 b) BSE-image of a polished surface of limestone treated with SH100. The water repellent has the tendency to create a thin film. The identification of the water repellent

Hydro 10 inside the natural building stones turned out to be very difficult with microscopy. In thin

sections investigated with optical microscopy, the water repellent seemed to be removed while

polishing the thin section. On freshly broken pieces of treated stones, SEM-research in combination

with EDS revealed the presence of silicium on the limestone, probably derived from the polysiloxanes,

but unambiguous pieces of polymerized product were nevertheless difficult to find.

Visualization of the consolidants and water repellents with micro-CT

When the consolidant polymerizes into pieces larger than 10 µm, it should theoretically be possible to

visualize this with micro-CT. The visualization of the consolidant inside the stone will depend on the

density of the consolidant and on the contrast in X-ray absorption between the mineral constituents of

the stone material and the consolidant. When this contrast is low, it remains very difficult to determine

their exact localization inside the stone. Only when the consolidant would completely block the pores

and would remain concentrated in a small area only, clear visualization would be possible with micro-

CT. The consolidant will cause reduction of the pore-size and of the total porosity, but because it tends

to spread inside the stones in a more homogeneous way, precise localization of the delicate pieces of

amorphous silicium dioxide remains very difficult to obtain due to resolution limitations. When

analyzing the micro-CT images of samples scanned before and after treatment in 3D with

“µCTanalySIS” (Cnudde & Jacobs, 2004), small porosity changes could be detected in the apparently

slightly denser images of the consolidated samples. Without the information provided by the images of

the samples scanned before consolidation, it would remain difficult to determine the precise

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localization of the consolidant. Obviously, for the visualization of Hydro 10, which polymerizes in

even thinner layers, micro-CT turned out not to be a suitable technique. Clear visualization of the water

repellents should be accomplished by doping these products with a material that causes a higher

attenuation for X-rays.

Doping of the consolidants and water repellents

The doping product

In this study the doping product was 3-bromopropyltrimethoxysilane (fig. 11).

Fig. 11: Chemical structure of 3-bromopropyltrimethoxysilane .

The presence of the bromine atom will cause a higher attenuation of X-rays. Also, the molecule will

react with the siloxanes (consolidants or water repellents) resulting in a cross-linked structure. The

chemical structure of 3-bromopropyltrimethoxysilane is quite similar to the structure of the used

consolidants and therefore does not significantly influence the consolidation process and the water

repellent properties of the stone material.

Chemical properties of doped products

To find out if the doping product influences the chemical properties of the original products,

measurements of viscosity, NMR and determination of both dry weight and volume (table 1) were

performed. 1H- NMR spec troscopy revealed that mixing of 3-bromopropyltrimethoxysilane with SH75,

SH100 and Hydro 10 in a free-of-water environment (deuterated chloroform, CDCl3) does not

influence the chemical shift of the products, indicating absence of reaction.

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After complete drying, the resulting products did not dissolve any more due to the cross- linking

reaction during polymerisation. Due to the addition of the high amount of 3-

bromopropyltrimethoxysilane, the percentage dry weight of SH75 and Hydro 10 will change.

Since 3-bromopropyltrimethoxysilane is required as a doping agent for visualising the impregnation of

stones with consolidants and water repellents, the possible effects of the doping agent on the viscosity

of the products needed to be investigated. MEK, the solvent used for the SH75 formulation has the

lowest viscosity in terms of flow-through times (461 seconds, see table 1) followed by SH100 (672

seconds). SH75, which contains MEK, has a higher flow-through time than SH100. This is most likely

due to the presence of low molecular-weight oligomers in the SH75 formulation, which renders the

SH75 composition more viscous. Addition of 33 % 3-bromopropyltrimethoxysilane to the Hydro 10,

reduces the flow-through times with 9 %. Water turns out to be more viscous than the consolidants.

Based on the viscosity data it should be taken into count that doping with 3-

bromopropyltrimethoxysilane has the largest influence on the viscosity of the product SH100.

Micro-CT of doped consolidants and water repellent

A composition of different materials was scanned in order to determine the attenua tion differences

between the various objects. The samples were composed of calcite, polymerized SH100 and

polymerized SH100 and Hydro 10 both doped with 33 % 3-bromopropyltrimethoxysilane. The

reconstruction images after acquisition show a visible difference between the different objects (fig. 12).

Fig. 12: Different polymerized products and calcite scanned with micro-CT. The average grey value of each sample demonstrates that the various objects can be distinguished

based on their grey values (table 2).

Table 2: Grey values of different objects scanned with micro-CT.

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The images derived from the micro-CT illustrate that the doped samples possess a much higher

attenuation for X-rays than calcite and non-doped samples.

2/3 Hydro 10 mixed with 1/3 3-bromopropyltrimethoxysilane was applied by absorption on a sandstone

sample with 13,6 % porosity. In this set-up, the sample is incubated during 5 seconds into a dish filled

with the product that ascended to 2 ? 1 mm above the points of support. After absorption and

polymerization of the mixture, cylindrical samples were drilled out of the sample and scanned with

micro-CT. From the reconstructed cross-sections the presence of the product can be clearly observed

(fig. 13).

Fig. 13 : Cross-section of Bray sandstone sample with visualization of doped Hydro 10 (Cnudde et al., 2004). The same product was applied on cylindrical rock samples with 17 % porosity, but this time by using

the spray-flow technique to better simulate reality. After complete polymerization the samples were

scanned with micro-CT, but this time the presence of the product could not be visualized. Based on the

amount of product applied during the spray flow, an estimated dry weight can be calculated. This

amount can be linked to the corresponding volume and can therefore provide an idea about the amount

of pores filled with product after complete polymerization. When analyzing the images, no pore

blocking could be detected and it seemed that the amount of product was too low to be clearly detected.

Different factors are responsible to visualize the product. The amount of absorbed product, which will

depend on the porosity and the pore-size distribution of the stone, will influence its detectib ility.

Concentration in a certain area or dilution over the entire sample will result in a better or a reduced

visibility of the product respectively. To obtain a better visualization, the original products can be

doped with a higher amount of 3-bromopropyltrimethoxysilane or with a substance that causes even

more attenuation of the X-rays than the 3-bromopropyltrimethoxysilane. Care should be taken that the

doping product should not significantly influence the properties of the original products.

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Discussion

Doping of water repellents and of consolidants is an important research subject and can be used in

laboratory studies when treatment of natural building stones is planned. It can help to determine which

product will be suitable for a certain type of stone. Consequently, when buildings are treated with a

certain product, it would be very interesting to evaluate the presence of the product inside the building

stones. This however would be very expensive due to the high cost of the doping material currently

available.

If the products could be doped in advance before being applied to a monument, this could be a very

important controlling tool. At present the cost of these products would increase to a level that they

would be no longer competitive. A possible solution to reduce the cost of doping agents might be the

development of agents that contain more than one halogen atom (e.g. Br), resulting in doping materials

with a higher attenuation effect. These materials would reduce the total amounts required for doping

and thus the overall cost for treating stone materials.

Conclusion

The structure of natural building stones can be visualized in a detailed way with micro-CT. The link

between the pore-size distribution of the natural building stone and the molecule size of the

consolidants or water repellents is of importance when stones need to be treated with a certain type of

product. The visualization of the products inside natural building stones is possible with micro-CT. For

the visualization of the product its atomic number and density and the amount of product inside the

natural building stone is crucial. Besides the contrast in attenuation, the resolution of the micro-CT also

needs to be taken into account to obtain a good contrast between stone and product. By doping with 3-

bromopropyltrimethoxysilane, more contrast will occur between the stone material and the

conservation product. When the amount of product inside the samples is very low and the product is

homogeneously spread inside the sample, the doped product is difficult to distinguish from the stone

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material due to the resolution of the micro-CT. When the amount of doped product inside the stone

material reaches a certain threshold value, the dope will appear in the 2D cross-sections derived from

micro-CT, creating a good visualization of the products inside the stone samples. To enhance the

contrast even more, 3-bromopropyltrimethoxysilane or the use of doping materials with a higher

attenuation could be considered. However these products should not induce significant differences on

the properties of the original product.

Acknowledgements

The authors wish to thank Dr. D. Fonck and operator F. De Weer from the “Clinique Soeurs de la

Miséricorde” from Ronse, Belgium for the use of their medical CT-scanner. This study is supported by

the Institute for the Promotion of Innovation by Science and Technology in Flanders, Belgium through

a PhD grant for Veerle Cnudde.

References Anderson, S.H. , Peyton, R.L., Gantzer, C.J., 1990. Evaluation of constructed and natural soil macropores using X-ray computed tomography. Geoderma, 46, pp. 13-29. Carlson, W.D., Denison, C., 1992. Mechanisms of porphyroblast crystallisation: results from high-resolution computed X-ray tomography. Science 257, pp. 1236-1239. Carlson, W.D., Denison, C., Ketcham, R.A., 1999. High-Resolution X-ray computed tomography as a tool for visualization and quantitative analysis of igneous textures in three dimensions. Electronic Geoscience 4:3. Carmeliet, J., Houvenaghel, G., Van Schijndel, J. & Roels, S., 2002. Moisture phenomena in hydrofobic porous building material Part1: Measurements and physical interpretations. Internationale Zeitschrift für Bauinstandsetzen und Baudenkmalpflege, Jahrgang 8, Heft 2/3, pp. 165-183. Commission 25-PEM Protéction et erosion des Monuments, 1980. Recommended tests to measure the deterioration of stone and to asses the effectiveness of treatment methods. Test No. II.4, Water absorption under low pressure (Pipe method). Materials and structures, vol. 13 (75), pp. 201-205. Conroy, G.C., Vannier, M.W., 1984. Noninvasive three-dimensional computer imaging of matrix- filled fossil skulls by high-resolution computed tomography. Science 226, pp. 456-458. Cnudde, V., Cnudde, J.P., Dupuis, C., Jacobs P.J.S., 2004. X-ray micro-CT used for the localization of water repellents and consolidants inside natural building stones. Materials Characterization 53, pp. 259-271.

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Cnudde V, Jacobs P., 2004 Preliminary results of X-ray micro-tomography applied in conservation and restoration of natural building stones. X-ray CT for Geomaterials, Soils, Concrete, rocks. Proceedings of the International Workshop on X-ray CT for Geomaterials, GeoX2003, 6-7 November 2003, Kumamoto, Japan, pp. 363-371. Otani & Obara (eds.). De Witte E. , 1999. Oppervlakteconsolidering. Steenachtige materialen. Kluwer Editorial, pp. 249-280. Fourie, S., 1974. The cranial morphologhy of Thrinaxondon liohinus Seeley. Annals of the South African Museum 65, pp. 337-400. Hainsworth, J.M., Aylmore, L.A.G., 1983. The use of computer-assisted tomography to determine spatial distribution of soil water content. Australian Journal of Soil Research, 21, pp. 435-443. Haubitz, B., Prokop, M., Dohring, W., Ostrom, J.H., Wellnhofer, P., 1988. Computed tomography of Archaeopteryx. Paleobiology 14 (2), pp. 206-213. Jacobs, P., Sevens, E., Kunnen, M., 1995. Principles of computerised X-ray tomography and applications to building materials. The Science of the Total Environment 167, pp. 161-170. Jacobs, P., Sevens, E., Vossaert, P. & Kunnen, M., 1997. Non-destructive monitoring of interactive physical and biological deterioration of building stones by computerized X-ray tomography. In: Marinos, P., Koukis, G., Tsiambaos, G. & Stournaras, G. (eds), Engineering Geology and the Environment. Balkema, Rotterdam, pp. 3163-3168. Keller, A.A., 1997. High resolution CAT imaging of fractures in consolidated materials. Int. J. Rock Mech. Min. Sci. vol. 34 (3-4), pp. 358. Kenter, J.A.M., 1989. Applications of computerized tomography in sedimentology. Marine Geotechnology 8, pp. 201-211. Klobes, P., Riesemeier, H., Meyer, K., Goebbels, J., Hellmuth, K.-H., 1997. Rock porosity determination by combination of X-ray computerized tomography with mercury porosimet ry. Fresenius J. Anal. Chem 357 (5), pp. 543-547. Landis, E.N., Petrell, A.L., Lu, S., Nagy, E.N., 2000. Examination of pore structure using three-dimensional image analysis of microtomographic data. Conrete Science and Engineering, 2, pp. 162-169. Losano, F., Marinsek, G., Merlo, A.M., Ricci, M., 1999. Computed tomography in the automotive field. Development of a new engine head case study. Computerized Tomography for Industrial Applications and Image Processing in Radiology, March 15-17, 1999, Berlin, Germany. DGZfP Proceedings BB67-CD pp. 65-73. Nijs R., 1985. Petrographical characterization of calcareous building stones in Northern Belgium. Vth International Congress on Deterioration and Conservation of Stone, Lausanne, 25-27.9.1985, pp. 13-21. Petrovic, A.M., Siebert, J.E., Rieke, P.E., 1982. Soil bulk density analysis in three dimensions by computed tomographic scanning. Soil Science Society of America Journal 46, pp. 445-450.

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Peyton, R.L., Haeffner, B.A., Anderson, S.H., Gantzer, C.J., 1992. Applying X-ray CT to measure macropore diameters in undisturbed soil cores. Geoderma 53, pp. 329-340. Philips, D.H., Lannutti, J.J., 1997. Measuring physical density with X-ray computed tomography. NDT&E International, Vol. 30, No. 6, pp. 339-350. Pierret, A., Capowiez, Y., Belzunces, L., Moran, C.J., 2002. 3D reconstruction and quantification of macropores using X-ray computed tomography and image analysis. Geoderma 106 (3-4), pp. 247-271. Rosenberg, E., Ferreira De Pavia, R., Guéroult, P. & Lynch, J., 1999. Microtomography applications in rock analysis and related fields. Computerized Tomography for Industrial Applications and Image Processing in Radiology. March 15-17, 1999, Berlin, Germany. DGZfP -Proceedings BB 67-CD. Streitwieser, A., Heathcock, C.H., Kosower, E.M., 1992. Introduction to Organic Chemistry (Fourth Edition). Macmillan Publishing Company (New York, 1992). Tiano, P., 2003. The use of microdrilling techniques for the characterization of stone materials. RILEM TC 177, Workshop 2003. Van Geet, M., Swennen, R. & Wevers, M., 2001. Towards 3-D petrography: application of microfocus computer tomography in geological science. Computers & Geoscience 27, pp. 1091-1099. Warner, G.S., Nieber, J.L., Moore, I.D. & Geise, R.A.,1989. Characterizing macropores in soil by computed tomography. Soil Science Society of America Journal. 53, pp. 653-660. WTCB Technische Voorlichting 224: Waterwerende oppervlaktebehandeling. Juni 2002. Zeng, Y., Gantzer, C.J., Peyton, R.L., Anderson, S.H.,1996. Fractal dimension and lacunarity of bulk density determined with X-ray computed tomography. Soil Science Society of America Journal 60, pp. 1718-1724.

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Figure captions

Fig. 1: Visual photo image used to determine the contact angle of the water droplet.

Fig. 2: Reaction scheme of Hydro 10.

EtO Si OEt

OEt

OEt

+ H2O4 HO Si OH

OH

OH

+ EtOH4

Catalyst

HO Si OH

OH

OH

HO Si OH

OH

OH

Si O Si + H2O

Fig.3: Reaction scheme for the consolidants.

EtO Si OEt

R

R

+ H2O Si OH

R

R

+ EtOH

Si OH

R

RSi

R

R

O Si

R

R

Si

R

R

O Si

R

R

O Si

R

R

O Si

R

R

H2O+

22

a b fig. 4: a) Photo of a heterogeneous piece of the Bray sandstone; b) 2D reconstructed slice of the same heterogeneous piece of the Bray sandstone derived from the CT-radiograms.

Fig. 5: Stack of 2D images derived from the CT-scan, showing the position of the denser iron layer inside the sandstone.

Fig. 6: 2D cross-section of the limestone of Maastricht derived from the CT-scan.

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Fig. 7: 2D cross-section of Bray sandstone derived from micro-CT data.

Fig.8: BSE image of quartz grains from Bray sandstone with intragranular zircon and rutile.

Fig. 9: 2D cross-section of Maastricht limestone derived from micro-CT data.

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a) b) Fig. 10: a) SE-image of limestone treated with SH100; b) BSE-image of polished surface of limestone treated with SH100.

MeO Si (CH2)

OMe

OMe

Br3 + 4 H2O HO Si (CH2)

OH

OH

Br + 4 MeOH

Fig. 11: Chemical structure of 3-bromopropyltrimethoxysilane.(fig!!!!!!, 3 ontbreekt!)

Fig. 12: Different polymerized products and calcite scanned with micro-CT.

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Fig. 13 : Cross-section of Bray sandstone sample with visualization of doped Hydro 10 (Cnudde & Jacobs, 2004).

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Tables Name components Density (g/cm³

at 20°C) Experimental dry weight (%)

Density dry weight (g/cm³)

Time (s) to flow out tube viscosity (error 0,7 %)

SH100 ethylorthosilicate bases* 0,93 52,13 1,67 672 SH100 + BR 2/3 ethylorthosilicate bases*

1/3 3-bromopropyltrimethoxysilane 1,02 47,60 1,62 781

SH75 ethylorthosilicate bases * (75%) MEK (25%)

0,95 35,78 1,67 1049

SH75 + BR 2/3 [ethylorthosilicate bases* (75%) + MEK (25%)] 1/3 3-bromopropyltrimethoxysilane

1,05 51,83 1,38 1083

Hydro 10 10% Methylethoxypolysiloxane (mixture of silicon resin/organosiloxane/modified silane) 90% D40

0,77 8,025 1,07 1252

Hydro 10 + BR 2/3 [10% Methylethoxypolysiloxane (mixture of silicon resin/organosiloxane/modified silane) 90% D40] 1/3 3-bromopropyltrimethoxysilane

0,93 33,41 1,48 1145

D40 White spirit 0,76 - - 1140 MEK methylethylketon 0,81 - - 461 water 1 0 0 873

Table 1: Summary of product data (BR= 3-bromopropyltrimethoxysilane ; * catalyst: n-butyltinmercaptide: di-n-butyltindilaurate). MAASTRICHT LIMESTONE BRAY SANDSTONE Smallest pore diameter (µm) 0,0045 0,0045 Largest pore diameter (µm) 356 356 Average pore diameter (µm) 39,0 19,3 Pore diameter 1 µm (%) 0,03 0,3 Pore diameter = 10 µm (%) 20 10 10µm <Pore diameter = 96 µm (%) 76 84 Table 2: MIP data Maastricht limestone and Bray sandstone.

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MATERIAL MEAN GREY VALUE STD. DEV. GREY VALUE

calcite 178 5

SH100 + 33 % 3-bromopropyltrimethoxysilane 147 7

SH100 217 5

Hydro 10 + 33 % 3-bromopropyltrimethoxysilane 96 7

Table 2: Grey values of different objects scanned with micro-CT.