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Microscopic and macroscopic characterization of the porosity of marble as a function of temperature and impregnation
Construction and Building
MATERIALS
Construction and Building Materials 20 (2006) 939–947
www.elsevier.com/locate/conbuildmat
Microscopic and macroscopic characterization of the porosity of marble as a function of temperature and impregnation K. Malaga-Starzec b
˚ kesson , U. A
a,b,*
b,c
, J.E. Lindqvist b, B. Schouenborg
b
a Department of Inorganic Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden Swedish National Testing and Research Institute, Building Technology, Building Materials, Brinellgatan 4, Box 857, SE-501 15 Bora˚s, Sweden c Department of Geology, Earth Sciences Centre, Go¨teborg University, P.O. Box 460, SE-405 30 Go¨teborg, Sweden
Received 23 April 2004; received in revised form 3 April 2005; accepted 30 June 2005 Available online 27 October 2005
Abstract Various methods were used in order to study how temperature cycling initiates changes in the porosity of fresh and impregnated marbles. The results indicated that intergranular decohesion was more pronounced in calcitic marble than dolomitic marble. The impregnation agents had a mitigating effect on the intergranular decohesion. Use of fluorescence microscopy, among the other methods, appears to give inexpensive and reliable information about the internal structure of marble. A better understanding of the effect that temperature has on the porosity of marble could be used as a guide for selection of suitable stone material for exterior use as well as an indication for appropriate conditioning of the samples before physical property testing. 2005 Elsevier Ltd. All rights reserved. Keywords: Thermal expansion; Marble; Porosity
1. Introduction Anisotropy of the calcite crystals contributes to predominantly high thermal stresses in calcitic marbles. Elevated temperatures lead to expansion in the crystallographic c-direction and contraction in perpendicular directions, whereas thermal expansion in dolomite is positive in all crystallographic directions [1]. The early stages of thermal weathering of marble are characterized by the progressive granular decohesion that results in increased porosity and subsequently, through expansion, to loss of strength. Many studies [1–5] describe the thermal expansion of marble. It has been demonstrated that rate of expansion increases with the temperature and is accelerated in the presence of humidity or water [6]. Sug* Corresponding author. Tel.: +46 31 772 2888; fax: +46 31 772 2853. E-mail addresses: [email protected], [email protected] (K. Malaga-Starzec).
0950-0618/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.06.016
aring of the surface leads to increased roughness. A rough surface can easily accumulate dirt and particles and form a crust, which has a negative influence on humidity and water transport out of the stone. This problem has gained increasing attention since the marble claddings on the Amoco Building in Chicago and Finlandia House in Helsinki started to bow and had to be replaced [7,8]. This type of failure is sometimes visible some 10–15 years after the completion of the building. Thermal stress resulting from changes in the temperature of ambient air in temperate climates can be sufficient to cause micro-fractures between the mineral grains of a rock [9]. The analysis of porosity in combination with studies of grain size is not commonly used for characterization of marble. The strength of a material decreases with increasing porosity but is also related to pore size, pore shape and spatial distribution. Pores may occur within the grains or in the grain boundaries. The latter are related to grain shape, size and distribution. Pore
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K. Malaga-Starzec et al. / Construction and Building Materials 20 (2006) 939–947
characteristics influence the internal surface area per unit material volume and this, in turn, determines the transport of gaseous and liquid phases, the freeze thaw properties as well as salt crystallization and durability of the material. A comprehensive study of porosity can provide valuable information in order to determine whether a given type of marble is susceptible to thermal stresses or not. In order to determine how temperature cycling initiates changes in the porosity of untreated and impregnated stones, two mineralogically different marble types were tested for water absorption and ultrasonic velocity propagation, and were analysed by fluorescence microscopy and nitrogen adsorption. The influence of the protective agents on porosity was also evaluated. Additionally, a separate long-term study of thermal expansion was performed on fresh untreated samples.
2. Materials
stones. However, some of these methods (e.g., measuring percentage weight loss) do not reflect the initial stages of weathering on the microscopic scale. This study compares results from different methods for evaluating weathering of the stone at the microscopic and macroscopic scales. The samples were submitted to 50 temperature cycles in a climatic chamber. The temperatures chosen for the experiment ranged from 15 C (12 h) to +80 C (6 h). One cycle lasted for 24 h, including the time necessary to go from minimum to maximum and back to minimum temperature. The choice of the temperatures between 15 and +80 C was intended to imitate the climatic conditions that could be found on natural stone claddings outdoors. The samples were dried at 40 C for two weeks in order to exclude the influence of freezing and thawing, and the temperature cycling started with heating them up to 80 C. Table 1 gives a short description of the samples used in the study.
2.1. Stone materials
3.1. Quantitative microscopy
Two types of marble were used in the study: one, nearly pure calcitic (>99%) from Italy and another, dolomitic (93%) with some phlogopite (4%) and tremolite (3%) from Sweden. The average grain sizes in the calcitic and dolomitic samples ranged between 0.13– 0.21 and 0.09–0.2 mm, respectively, and had varying grain size distributions (Fig. 1). The marbles were characterized by very low total porosity of about 0.5%. The samples were fresh, each of them sawn from a large block directly from the quarry.
3.1.1. Sample preparation After temperature cycling, the samples were cut and vacuum-impregnated with epoxy resin containing a fluorescent dye. One thin section was made from each sample. Two reference thin sections from untreated marbles (CR0 and DR0) were also prepared in order to compare the pore structure before temperature cycling.
2.2. Impregnation agents Two impregnation agents: GypStop P17 and GypStop P22 were used in this study. GypStop (GS) is a protective treatment product for stone materials, for pre-consolidation, consolidation, desalination and gap filling. GS products consist of a colloidal dispersion of dense, amorphous particles of SiO2. In order to ensure the maximum penetration depth, the GS products have various distributions of SiO2 particle size (4–150 nm) giving different surface area (P17 gives 170 m2/g and P22 gives 220 m2/g). The marble samples were immersed in the GS and water solution (1:4) for 2 h. Several laboratory studies have shown that GS significantly reduces decay of the calcareous stone due to acid rain and salt action and could also be used as a desalination product [10,11].
3. Experimental There is a range of different methods commonly used for quantifying the degree of breakdown of natural
3.1.2. Image analysis procedure Images used for the porosity analyses were taken with an optical microscope, using fluorescent and polarized light. The area covered by each image was 1.0 · 1.3 mm2. In order to analyse a larger area the thin section was fixed on a motorized stage that was programmed for edge-by-edge photographing, creating an image mosaic containing six images, covering an area of 2.0 · 4.0 mm2. Four mosaic images, giving an area of 32 mm2 were analysed from each thin section. Greyscale binary images were created from the fluorescent mosaic, using a threshold technique. The same threshold values were used for all images. The binary and polarized images were overlaid in order to evaluate where the pores were formed. 3.1.3. Pore measurements The pores and voids are smaller than the thickness of the thin section (25 lm). A consequence of this is that it is not possible to recalculate a measured area percentage in the fluorescence images to give the volume percentage in the sample. A large number of the pores are finer than the resolution of the microscope but the light from them can be observed. All the open porosity is therefore treated in the same way and no distinction based on shape is made. The porosity is
K. Malaga-Starzec et al. / Construction and Building Materials 20 (2006) 939–947
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Fig. 1. Grain size distribution for the calcitic (a) and dolomitic (b) marbles determined by the linear traverse method. A number of lines were drawn at random on the polarized images, intersecting several mineral grains. The chord length of each mineral grain was measured by measuring the distance between the points intersecting the grain boundaries along the line transects and the median grain size and the grain size distribution were determined.
concentrated to the grain boundaries and is measured as the length of porous grain boundaries. Even if the whole boundary emits fluorescence, it may still be only partly open as the individual pores and voids
in the grain boundary cannot be resolved in the image. The binary images were printed in A4 format in order to measure the length of cracks and grain boundaries
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Table 1 Description of the marble samples used in this study Analyses method/samples
Calcitic
Dolomitic
Water absorption (100 · 50 · 50 mm prisms)
Temp. uncycled (reference): – Untreated (CR0) – Impregnated with GS P17 – Impregnated with GS P22 Temp. cycled: – Untreated (C0) – Impregnated with GS P17 – Impregnated with GS P22 CR0, C0, C1, C2 CR0, C0, C1, C2 CR0, C0, C1, C2 Fresh untreated samples
Temp. uncycled (reference): – Untreated (DR0) – Impregnated with GS P17 – Impregnated with GS P22 Temp. cycled: – Untreated (D0) – Impregnated with GS P17 – Impregnated with GS P22 DR0, D0, D1, D2 DR0, D0, D1, D2 DR0, D0, D1, D2 Fresh untreated samples
Ultrasonic (100 · 50 · 50 mm prisms) Microscopy (thin sections) Gas adsorption (crashed to 4–5 mm fraction) Thermal expansion (250 · 25 · 25 mm prisms)
with continuous porosity, referred to as pore length in this document, and pores and cracks were traced manually by using transparent paper. The line drawings were scanned into the computer and the width was reduced to one pixel (=1.72 lm) before the length was measured. However, the pores are strongly correlated with grain boundaries and thus the grain size influences the pore structure. A linear-traverse method was applied to quantify the amount of pores related to grain boundaries, in order to compare the different samples. The numbers of porous and non-porous grain boundaries intersecting a 1 mm linear transect were counted by using the combined fluorescent/polarized mosaic images. A total of 215 linear transects were counted on each sample. 3.2. Gas adsorption The nitrogen adsorption technique was used to investigate the surface area, average pore size and the pore size distribution, (TriStar 3000 Analyser by Micromeritics). A compact degassing unit was used for sample drying (SmartPrep by Micromeritics). Both units were computer controlled and all calculations were performed using Win3000 software. The TriStar 3000 Analyser uses multi-point analysis for each sample. The temperature-cycled marble samples were crushed and analysis of the BET (Brunauer, Emmett, and Teller) surface area, average pore diameter (4 V/A byBET) and pore size distribution were performed by applying the BJH (Barrett, Joyner and Halenda) method with Halsey equation [12]. The marble samples were crushed and sieved to form 4–5 mm test specimens. About 7 g were used for one sample and analysis. Six samples were used for each type of marble.
(CR1) (CR2)
(C1) (C2)
(DR1) (DR2)
(D1) (D2)
ence samples, dried for one week at 40 C. The second set comprised samples subjected to temperature cycling. The reference samples were included in the test in order to determine the influence of temperature cycling on capillary absorption. The absorption was determined by mass gain after placing one side of the sample in a water bath. The readings of mass gain were taken at the following intervals (in minutes): 0, 30, 90, 240, 3000, 37,600, 47,600. 3.4. Ultrasonic velocity measurements The technique is non-destructive and has the potential of detecting deterioration causing reduction in the strength of the material, even where there are no visible signs of deterioration. Three measurements in three directions were performed for each sample in order to find signs of deterioration and anisotropy of the marbles. The measurements were performed on dry samples using an AU 2000 Ultrasonic Tester (CEBTP). 3.5. Thermal expansion The thermal expansion was measured by the NT BUILD 479 [13] method on 250 · 25 · 25 mm prisms, using a removable mechanical strain gauge. The temperature intervals were 20, 40 and 60 C and at each interval the relative humidity was adjusted to 40%, 70%, 90% and 99%. After the experiment, all samples were cooled down to 20 C in order to measure permanent expansion.
4. Results and discussion 4.1. Quantitative microscopy
3.3. Capillary water absorption The water absorption by capillary action was determined, using two sets of the calcitic marble and two sets of the dolomitic marble. The first set comprised refer-
Results from the microscopic analyses are presented in Table 2 and in Figs. 2 and 3. The results of the analyses indicate that an increase in porosity has occurred for all samples subjected to temperature cycling. The
K. Malaga-Starzec et al. / Construction and Building Materials 20 (2006) 939–947 Table 2 Pore- and grain boundaries/mm traverse length and the measured pore length for the investigated samples Sample
Pores (mm)
Grain boundaries (mm)
Pores/grain boundaries
Pore length (pore area) (mm)
CR0 CR C1 C2 DR0 D0 D1 D2
3 6 6 6 1 5 5 3
4 6 6 6 4 6 7 6
0.75 1.0 1.0 1.0 0.25 0.83 0.71 0.5
41.3 77.7 85.1 96.0 17.7 76.9 64.8 44.1
greatest difference between the reference samples and the temperature-cycled samples was observed in the dolomitic samples. The pore system has mainly developed
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along the grain boundaries as indicated in Fig. 2 and most pores were observed in the untreated dolomitic sample, where the pore length became four times longer than in the reference sample. The dolomitic sample impregnated with GS P22 showed the strongest resistance to temperature cycling (Pores/Grain boundary in Table 2). However, this sample showed more microcracks within the mineral grains compared to the other cycled samples. When the existing pores are filled with the SiO2 particles precipitated from GS, the boundaries may become less elastic and during the expansion some of the crack propagation might occur within the mineral grains. The relationship between open porous grain boundaries and closed non-porous grain boundaries showed that none of the dolomitic samples had all grain boundaries partly open (all values are below 1; value 1 indicates that all grain boundaries are open).
Fig. 2. Photomicrographs of the calcitic samples, where the processed fluorescent and polarised images are combined in order to evaluate where the pores are situated.
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Fig. 3. Photomicrographs of the dolomitic samples, where the processed fluorescent and polarised images are combined in order to evaluate where the pores are situated.
The differences between the temperature-cycled calcitic samples did not show the same variation as for the cycled dolomitic marble. The reference sample (CR0) shows that there was already well-defined grain matrix porosity in the calcitic marble compared to the dolomitic marble. The pore length (equivalent of the pore area of the single pixel line) is approximately twice as long for all temperature-cycled samples, compared with the reference sample. All grain boundaries in the temperaturecycled samples are assumed to have open porosity. It should be borne in mind that this method does not measure the width of the cracks and pores, it only measures existing and newly generated cracks and porosity. 4.2. BET surface area and porosity The results of BET surface area, mean pore size and pore size distribution are presented in a series of graphs
(Figs. 4–7) to highlight not only changes within the same type of marble but also between the types of marble. The BET surface area showed slightly higher values for dolomitic marble than for calcitic marble (Fig. 4), which was also confirmed by previous studies done by MalagaStarzec et al. [8]. Temperature cycling increased the BET surface area for both marble types. The increase in the BET surface area for the impregnated samples might be additionally explained by the contribution of the precipitated SiO2 GS particles, which is also confirmed by a reduction in mean pore size (Fig. 5). The difference between the marbles impregnated with GS P17 and GS P22 is very small. The dolomitic marble had the highest mean pore size as well as BET surface area, which is assumed to be determined by the original porosity of the marble. The pore size distribution of the analyzed pores (2– 200 nm) shows that the untreated calcitic sample (C0)
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pore size distribution, compared to the impregnated samples (long tail-off towards smaller pore diameter), may reflect rather extensive micro-cracking and widespread intergranular decohesion for untreated sample whereas the GS treatment seems to mitigate these processes. 4.3. Capillary water absorption
Fig. 4. BET surface area for calcitic and dolomitic marble samples.
Fig. 5. Mean pore size for calcitic and dolomitic marble samples.
For each diagram, two sets of samples are presented: reference samples and temperature-cycled samples. A comparison of the two marbles shows that the capillary absorption of calcitic marble was about twice the absorption of dolomitic marble (Figs. 8 and 9). The differences between reference and temperature-cycled samples are apparent for the calcitic samples. Capillary absorption increases from about 55 g/m2 for the reference samples to 100 g/m2 for the temperature-cycled samples. The differences between untreated and impregnated samples are relatively small; however the untreated samples show the highest capillary absorption for both calcitic and dolomitic marble. The capillary absorption results for the dolomitic samples display an opposite trend compared to the calcitic samples. The reference samples show higher capillary absorption than the temperature-cycled samples. In this case, the influence of temperature cycling is not
Fig. 6. Pore size distribution versus volume % for calcitic marble. Fig. 8. Absorbed water evolution during capillary absorption test as a function of the square root of the time for the calcitic reference samples and temperature-cycled samples.
Fig. 7. Pore size distribution versus volume % for dolomitic marble.
is the marble that is most sensitive to temperature cycling of all the samples (Figs. 6 and 7). The reverse pattern (long tail-off towards larger pore diameter) of the
Fig. 9. Absorbed water evolution during capillary absorption test as a function of the square root of the time for the dolomitic reference samples and temperature-cycled samples.
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reflected in the measured water absorption. Despite the same origin of the samples, the influence of the natural heterogeneity of the material on the results is higher than the temperature cycling. The untreated sample shows the highest capillary absorption of all dolomitic samples. The difference in capillary absorption between samples impregnated with GS P17 and GS P22 is difficult to interpret. The influence of the impregnation material on capillary absorption is smaller than the effect of heterogeneity of the stones. The capillary absorption for the samples is relatively small, which means that the presence of existing cracks in the reference material might have a significant influence on the results.
Fig. 10. Thermal expansion of the calcitic and dolomitic marbles. The residual expansion after 294 days is 0.33 and 0,05 mm/m, respectively.
4.4. Ultrasonic velocity measurements The ultrasonic velocity was measured in three directions. The maximum and minimum velocities and the anisotropy are presented in Table 3. Open micro-cracks are assumed to reduce ultrasonic velocities. This is clearly visible for the calcitic marble where the difference between the reference samples and temperature-cycled samples is about 65%, whereas the dolomitic marble shows insignificant variation in velocities between the samples. The dolomitic samples exhibit pronounced anisotropy: AV p ¼ ðV pmax V pmin Þ=V pmax 100; [14], in contrast to the calcitic marble. Therefore, the detected differences for the dolomitic samples are results of the anisotropy of the stone and not an effect of microcracking. 4.5. Thermal expansion The results from the thermal expansion measurements performed on the separate set of fresh samples show that both stones are sensitive to heating. However, no influence of humidity on expansion was observed (Fig. 10). Expansion of the material only occurred when the temperature increased. The diagram shows that the calcitic marble is more sensitive to heat than the dolomitic. The greatest residual expansion was also mea-
Table 3 Maximum and minimum ultrasonic velocities and anisotropy data for the calcitic and dolomitic marbles measured at dry conditions Sample
Vpmax (km/s)
Vpmin (km/s)
AV p (%)
CR0 CR C1 C2 DR0 D0 D1 D2
4.17 2.59 2.54 2.67 4.51 4.76 4.85 4.67
3.98 2.56 2.47 2.53 3.56 4.40 4.05 4.05
5.7 1.0 2.8 5.4 21.0 7.6 12.9 10.8
sured in the calcitic sample (0.33 mm/m) whereas almost none of the expansion remained in the dolomitic sample. 4.6. Summary of the results The role of porosity in marble is important in terms of moisture transport, mechanical properties and durability. There is a variety of methods for determining porosity, however, no individual test is sufficient to properly classify the marble relative to its expected behaviour in use and when exposed to weathering. Different techniques for porosity determination may give different values for the same sample. The results from this study show good agreement between different methods in some cases. This is especially noticeable for calcitic marble. The increase in water absorption in temperature-cycled samples in relation to reference samples is confirmed by increase in BET surface area, increase in the area measured by fluorescence microscopy, noticeable thermal expansion and reduction in ultrasonic velocity. For cases in which there is good agreement between methods, it can be assumed that breakdown processes are operating and that the quantification of the intergranular decohesion is independent of the method used. The problem arises when there is poor agreement between methods and unclear results are obtained. This might be the case with dolomitic marble. The breakdown is of much narrower range than the calcitic marbles, which means that the results are dependent on the method used. The image analyses of the dolomitic marble show evidence of initial decohesion, which is not obtained by the other methods. A combination of measurement methods is normally used to determine the properties of rock. However, the classification of the rocks is still based on the ability to pass each of the tests. The validity of the results based on a single test should therefore be interpreted cautiously.
K. Malaga-Starzec et al. / Construction and Building Materials 20 (2006) 939–947
5. Conclusions This study has shown that the calcitic and dolomitic marbles were susceptible to thermal fluctuations causing local mechanical stresses between grains and resulting in intergranular decohesion. The results from all tests indicated that intergranular decohesion was more pronounced for the calcitic marble than the dolomitic marble. The differential thermal expansion/contraction associated with different mineralogical and textural properties of the materials is primarily responsible for the increase in numbers of pore spaces and their magnitude. Another important parameter that influences thermal expansion is the microstructure. Increasing complexity of the microstructure from equigranular (granoblastic) to seriate interlobate (xenoblastic) will probably make it more difficult for a stone material to expand. The results showed that most of the expansion had occurred along the grain boundaries. This indicates that the durability of a fine-grained sample is more sensitive to temperature changes than the coarse-grained ones since the total volume of grain boundaries increases with falling grain size. Grain boundary separation will then promote water ingress through capillary transport, including transport of dissolved salts, and subsequently the freeze/thaw and salt crystallization processes. Thermal expansion was apparent for both marbles and was independent of humidity; however, permanent thermal expansion was only significant in calcitic marble. Impregnation reduced the mean pore size and had a mitigating effect on water absorption and intergranular decohesion. Ultrasonic velocity measurements were not able to detect the micro-cracks in the dolomitic marble while in the calcitic marble this process was fully detectable. The use of fluorescence microscopy, among other methods used, gives relatively inexpensive and reliable information about the internal structure of the marbles. The first signs of thermal weathering are best analyzed by gas adsorption. However, the combination of gas adsorption and fluorescence microscopy gives thorough information about initial cracking and porosity. A better understanding of the effect that temperature has on the porosity of marble could be used as a guide for selection of suitable stone material for exterior use. In addition, the findings provide essential information that should be used when conditioning marble stones intended for mechanical testing. If intergranular decohesion in calcareous stones occurs at temperatures lower than the recommended temperatures for standardized
947
tests (110 C), the results may give misleading values. This has to be taken into account during interpretation of data and as an indication of appropriate conditioning temperatures for the samples before physical property testing.
Acknowledgements This work was financially supported by the KK Foundation, the Swedish National Testing and Research Institute and the Geological Survey of Sweden. Eka Chemicals is acknowledged for providing GypStop. References [1] Kessler DW. Physical and chemical tests on the commercial marbles of the United States. Technology Papers of the Bureau of Standards. Washington: Government Printing Office; 1919. p. 123. [2] Sage JD. Thermal microfracturing of marble engineering geology of ancient work monuments and historical sites. Rotterdam: Balkerna; 1988. [3] Tschegg KE, Widhalm C, Eppensteiner W. Ursachen mangelnder Formbesta¨ndigkeit von Marmorplatten. Zeitschrift der Deutscher Geologischer Gesellschaft 1999;150/2:283–97. [4] Royer-Carfagni GF. On the thermal degradation of marble. Int J Rock Mech Mining Sci 1999;36:119–26. [5] Siegesmund S, Weiss T, Tschegg EK. Control of marble weathering by thermal expansion and rock fabrics. In: Proceedings of the 9th International Congress on Deterioration and Conservation of Stone, Venice, Italy; 2000. p. 205–13. [6] Hudec PP, Sitar N. Effect of water sorption on carbonate rock expansivity. Can Geotech J 1975;12(2):179–86. [7] Jornet A, Ruck P. Bowing of calcitic marble slabs: A case study. Quarry – Laboratory – Monument. International Congress, Pavia; 2000. [8] Royer-Carfagni GF. Some considerations on the warping of marble facades: the example of Alvar AltoÕs Finlandia Hall in Helsinki. Construct Build Mater 1999;13:449–57. [9] Malaga-Starzec K, Lindqvist JE, Schouenborg B. Experimental study on the variation in porosity of marble as a function of temperature. Geol Soc, Special Edition No. 205; 2002. [10] Kozlowski R, Tokarz M, Persson M. GypStop – a novel protective treatment. In: Proceedings of the 7th International Congress on Deterioration and Conservation of Stone, Lisbon, Portugal; 1992. [11] Lind A-M. Gypstop. Institute of Conservation, Go¨teborg University. University Report, 5; 2000. [12] Webb PA, Orr C. Analytical methods in fine particle technology. USA: Micrometitics Instrument Corporation; 1997. [13] NORDTEST, NT BUILD 479. Coefficient of thermal expansion, NORDTEST, Espoo, Finland; 1997. [14] Weiss T, Siegesmund S, Rasolofosaon PNJ. The relationship between deterioration, fabric, velocity and porosity constraint. In: Proceedings of the 9th International Congress on Deterioration and Conservation of Stone, Venice, Italy; 2000, p. 215–23.
MATERIALS
Construction and Building Materials 20 (2006) 939–947
www.elsevier.com/locate/conbuildmat
Microscopic and macroscopic characterization of the porosity of marble as a function of temperature and impregnation K. Malaga-Starzec b
˚ kesson , U. A
a,b,*
b,c
, J.E. Lindqvist b, B. Schouenborg
b
a Department of Inorganic Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden Swedish National Testing and Research Institute, Building Technology, Building Materials, Brinellgatan 4, Box 857, SE-501 15 Bora˚s, Sweden c Department of Geology, Earth Sciences Centre, Go¨teborg University, P.O. Box 460, SE-405 30 Go¨teborg, Sweden
Received 23 April 2004; received in revised form 3 April 2005; accepted 30 June 2005 Available online 27 October 2005
Abstract Various methods were used in order to study how temperature cycling initiates changes in the porosity of fresh and impregnated marbles. The results indicated that intergranular decohesion was more pronounced in calcitic marble than dolomitic marble. The impregnation agents had a mitigating effect on the intergranular decohesion. Use of fluorescence microscopy, among the other methods, appears to give inexpensive and reliable information about the internal structure of marble. A better understanding of the effect that temperature has on the porosity of marble could be used as a guide for selection of suitable stone material for exterior use as well as an indication for appropriate conditioning of the samples before physical property testing. 2005 Elsevier Ltd. All rights reserved. Keywords: Thermal expansion; Marble; Porosity
1. Introduction Anisotropy of the calcite crystals contributes to predominantly high thermal stresses in calcitic marbles. Elevated temperatures lead to expansion in the crystallographic c-direction and contraction in perpendicular directions, whereas thermal expansion in dolomite is positive in all crystallographic directions [1]. The early stages of thermal weathering of marble are characterized by the progressive granular decohesion that results in increased porosity and subsequently, through expansion, to loss of strength. Many studies [1–5] describe the thermal expansion of marble. It has been demonstrated that rate of expansion increases with the temperature and is accelerated in the presence of humidity or water [6]. Sug* Corresponding author. Tel.: +46 31 772 2888; fax: +46 31 772 2853. E-mail addresses: [email protected], [email protected] (K. Malaga-Starzec).
0950-0618/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2005.06.016
aring of the surface leads to increased roughness. A rough surface can easily accumulate dirt and particles and form a crust, which has a negative influence on humidity and water transport out of the stone. This problem has gained increasing attention since the marble claddings on the Amoco Building in Chicago and Finlandia House in Helsinki started to bow and had to be replaced [7,8]. This type of failure is sometimes visible some 10–15 years after the completion of the building. Thermal stress resulting from changes in the temperature of ambient air in temperate climates can be sufficient to cause micro-fractures between the mineral grains of a rock [9]. The analysis of porosity in combination with studies of grain size is not commonly used for characterization of marble. The strength of a material decreases with increasing porosity but is also related to pore size, pore shape and spatial distribution. Pores may occur within the grains or in the grain boundaries. The latter are related to grain shape, size and distribution. Pore
940
K. Malaga-Starzec et al. / Construction and Building Materials 20 (2006) 939–947
characteristics influence the internal surface area per unit material volume and this, in turn, determines the transport of gaseous and liquid phases, the freeze thaw properties as well as salt crystallization and durability of the material. A comprehensive study of porosity can provide valuable information in order to determine whether a given type of marble is susceptible to thermal stresses or not. In order to determine how temperature cycling initiates changes in the porosity of untreated and impregnated stones, two mineralogically different marble types were tested for water absorption and ultrasonic velocity propagation, and were analysed by fluorescence microscopy and nitrogen adsorption. The influence of the protective agents on porosity was also evaluated. Additionally, a separate long-term study of thermal expansion was performed on fresh untreated samples.
2. Materials
stones. However, some of these methods (e.g., measuring percentage weight loss) do not reflect the initial stages of weathering on the microscopic scale. This study compares results from different methods for evaluating weathering of the stone at the microscopic and macroscopic scales. The samples were submitted to 50 temperature cycles in a climatic chamber. The temperatures chosen for the experiment ranged from 15 C (12 h) to +80 C (6 h). One cycle lasted for 24 h, including the time necessary to go from minimum to maximum and back to minimum temperature. The choice of the temperatures between 15 and +80 C was intended to imitate the climatic conditions that could be found on natural stone claddings outdoors. The samples were dried at 40 C for two weeks in order to exclude the influence of freezing and thawing, and the temperature cycling started with heating them up to 80 C. Table 1 gives a short description of the samples used in the study.
2.1. Stone materials
3.1. Quantitative microscopy
Two types of marble were used in the study: one, nearly pure calcitic (>99%) from Italy and another, dolomitic (93%) with some phlogopite (4%) and tremolite (3%) from Sweden. The average grain sizes in the calcitic and dolomitic samples ranged between 0.13– 0.21 and 0.09–0.2 mm, respectively, and had varying grain size distributions (Fig. 1). The marbles were characterized by very low total porosity of about 0.5%. The samples were fresh, each of them sawn from a large block directly from the quarry.
3.1.1. Sample preparation After temperature cycling, the samples were cut and vacuum-impregnated with epoxy resin containing a fluorescent dye. One thin section was made from each sample. Two reference thin sections from untreated marbles (CR0 and DR0) were also prepared in order to compare the pore structure before temperature cycling.
2.2. Impregnation agents Two impregnation agents: GypStop P17 and GypStop P22 were used in this study. GypStop (GS) is a protective treatment product for stone materials, for pre-consolidation, consolidation, desalination and gap filling. GS products consist of a colloidal dispersion of dense, amorphous particles of SiO2. In order to ensure the maximum penetration depth, the GS products have various distributions of SiO2 particle size (4–150 nm) giving different surface area (P17 gives 170 m2/g and P22 gives 220 m2/g). The marble samples were immersed in the GS and water solution (1:4) for 2 h. Several laboratory studies have shown that GS significantly reduces decay of the calcareous stone due to acid rain and salt action and could also be used as a desalination product [10,11].
3. Experimental There is a range of different methods commonly used for quantifying the degree of breakdown of natural
3.1.2. Image analysis procedure Images used for the porosity analyses were taken with an optical microscope, using fluorescent and polarized light. The area covered by each image was 1.0 · 1.3 mm2. In order to analyse a larger area the thin section was fixed on a motorized stage that was programmed for edge-by-edge photographing, creating an image mosaic containing six images, covering an area of 2.0 · 4.0 mm2. Four mosaic images, giving an area of 32 mm2 were analysed from each thin section. Greyscale binary images were created from the fluorescent mosaic, using a threshold technique. The same threshold values were used for all images. The binary and polarized images were overlaid in order to evaluate where the pores were formed. 3.1.3. Pore measurements The pores and voids are smaller than the thickness of the thin section (25 lm). A consequence of this is that it is not possible to recalculate a measured area percentage in the fluorescence images to give the volume percentage in the sample. A large number of the pores are finer than the resolution of the microscope but the light from them can be observed. All the open porosity is therefore treated in the same way and no distinction based on shape is made. The porosity is
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Fig. 1. Grain size distribution for the calcitic (a) and dolomitic (b) marbles determined by the linear traverse method. A number of lines were drawn at random on the polarized images, intersecting several mineral grains. The chord length of each mineral grain was measured by measuring the distance between the points intersecting the grain boundaries along the line transects and the median grain size and the grain size distribution were determined.
concentrated to the grain boundaries and is measured as the length of porous grain boundaries. Even if the whole boundary emits fluorescence, it may still be only partly open as the individual pores and voids
in the grain boundary cannot be resolved in the image. The binary images were printed in A4 format in order to measure the length of cracks and grain boundaries
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Table 1 Description of the marble samples used in this study Analyses method/samples
Calcitic
Dolomitic
Water absorption (100 · 50 · 50 mm prisms)
Temp. uncycled (reference): – Untreated (CR0) – Impregnated with GS P17 – Impregnated with GS P22 Temp. cycled: – Untreated (C0) – Impregnated with GS P17 – Impregnated with GS P22 CR0, C0, C1, C2 CR0, C0, C1, C2 CR0, C0, C1, C2 Fresh untreated samples
Temp. uncycled (reference): – Untreated (DR0) – Impregnated with GS P17 – Impregnated with GS P22 Temp. cycled: – Untreated (D0) – Impregnated with GS P17 – Impregnated with GS P22 DR0, D0, D1, D2 DR0, D0, D1, D2 DR0, D0, D1, D2 Fresh untreated samples
Ultrasonic (100 · 50 · 50 mm prisms) Microscopy (thin sections) Gas adsorption (crashed to 4–5 mm fraction) Thermal expansion (250 · 25 · 25 mm prisms)
with continuous porosity, referred to as pore length in this document, and pores and cracks were traced manually by using transparent paper. The line drawings were scanned into the computer and the width was reduced to one pixel (=1.72 lm) before the length was measured. However, the pores are strongly correlated with grain boundaries and thus the grain size influences the pore structure. A linear-traverse method was applied to quantify the amount of pores related to grain boundaries, in order to compare the different samples. The numbers of porous and non-porous grain boundaries intersecting a 1 mm linear transect were counted by using the combined fluorescent/polarized mosaic images. A total of 215 linear transects were counted on each sample. 3.2. Gas adsorption The nitrogen adsorption technique was used to investigate the surface area, average pore size and the pore size distribution, (TriStar 3000 Analyser by Micromeritics). A compact degassing unit was used for sample drying (SmartPrep by Micromeritics). Both units were computer controlled and all calculations were performed using Win3000 software. The TriStar 3000 Analyser uses multi-point analysis for each sample. The temperature-cycled marble samples were crushed and analysis of the BET (Brunauer, Emmett, and Teller) surface area, average pore diameter (4 V/A byBET) and pore size distribution were performed by applying the BJH (Barrett, Joyner and Halenda) method with Halsey equation [12]. The marble samples were crushed and sieved to form 4–5 mm test specimens. About 7 g were used for one sample and analysis. Six samples were used for each type of marble.
(CR1) (CR2)
(C1) (C2)
(DR1) (DR2)
(D1) (D2)
ence samples, dried for one week at 40 C. The second set comprised samples subjected to temperature cycling. The reference samples were included in the test in order to determine the influence of temperature cycling on capillary absorption. The absorption was determined by mass gain after placing one side of the sample in a water bath. The readings of mass gain were taken at the following intervals (in minutes): 0, 30, 90, 240, 3000, 37,600, 47,600. 3.4. Ultrasonic velocity measurements The technique is non-destructive and has the potential of detecting deterioration causing reduction in the strength of the material, even where there are no visible signs of deterioration. Three measurements in three directions were performed for each sample in order to find signs of deterioration and anisotropy of the marbles. The measurements were performed on dry samples using an AU 2000 Ultrasonic Tester (CEBTP). 3.5. Thermal expansion The thermal expansion was measured by the NT BUILD 479 [13] method on 250 · 25 · 25 mm prisms, using a removable mechanical strain gauge. The temperature intervals were 20, 40 and 60 C and at each interval the relative humidity was adjusted to 40%, 70%, 90% and 99%. After the experiment, all samples were cooled down to 20 C in order to measure permanent expansion.
4. Results and discussion 4.1. Quantitative microscopy
3.3. Capillary water absorption The water absorption by capillary action was determined, using two sets of the calcitic marble and two sets of the dolomitic marble. The first set comprised refer-
Results from the microscopic analyses are presented in Table 2 and in Figs. 2 and 3. The results of the analyses indicate that an increase in porosity has occurred for all samples subjected to temperature cycling. The
K. Malaga-Starzec et al. / Construction and Building Materials 20 (2006) 939–947 Table 2 Pore- and grain boundaries/mm traverse length and the measured pore length for the investigated samples Sample
Pores (mm)
Grain boundaries (mm)
Pores/grain boundaries
Pore length (pore area) (mm)
CR0 CR C1 C2 DR0 D0 D1 D2
3 6 6 6 1 5 5 3
4 6 6 6 4 6 7 6
0.75 1.0 1.0 1.0 0.25 0.83 0.71 0.5
41.3 77.7 85.1 96.0 17.7 76.9 64.8 44.1
greatest difference between the reference samples and the temperature-cycled samples was observed in the dolomitic samples. The pore system has mainly developed
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along the grain boundaries as indicated in Fig. 2 and most pores were observed in the untreated dolomitic sample, where the pore length became four times longer than in the reference sample. The dolomitic sample impregnated with GS P22 showed the strongest resistance to temperature cycling (Pores/Grain boundary in Table 2). However, this sample showed more microcracks within the mineral grains compared to the other cycled samples. When the existing pores are filled with the SiO2 particles precipitated from GS, the boundaries may become less elastic and during the expansion some of the crack propagation might occur within the mineral grains. The relationship between open porous grain boundaries and closed non-porous grain boundaries showed that none of the dolomitic samples had all grain boundaries partly open (all values are below 1; value 1 indicates that all grain boundaries are open).
Fig. 2. Photomicrographs of the calcitic samples, where the processed fluorescent and polarised images are combined in order to evaluate where the pores are situated.
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Fig. 3. Photomicrographs of the dolomitic samples, where the processed fluorescent and polarised images are combined in order to evaluate where the pores are situated.
The differences between the temperature-cycled calcitic samples did not show the same variation as for the cycled dolomitic marble. The reference sample (CR0) shows that there was already well-defined grain matrix porosity in the calcitic marble compared to the dolomitic marble. The pore length (equivalent of the pore area of the single pixel line) is approximately twice as long for all temperature-cycled samples, compared with the reference sample. All grain boundaries in the temperaturecycled samples are assumed to have open porosity. It should be borne in mind that this method does not measure the width of the cracks and pores, it only measures existing and newly generated cracks and porosity. 4.2. BET surface area and porosity The results of BET surface area, mean pore size and pore size distribution are presented in a series of graphs
(Figs. 4–7) to highlight not only changes within the same type of marble but also between the types of marble. The BET surface area showed slightly higher values for dolomitic marble than for calcitic marble (Fig. 4), which was also confirmed by previous studies done by MalagaStarzec et al. [8]. Temperature cycling increased the BET surface area for both marble types. The increase in the BET surface area for the impregnated samples might be additionally explained by the contribution of the precipitated SiO2 GS particles, which is also confirmed by a reduction in mean pore size (Fig. 5). The difference between the marbles impregnated with GS P17 and GS P22 is very small. The dolomitic marble had the highest mean pore size as well as BET surface area, which is assumed to be determined by the original porosity of the marble. The pore size distribution of the analyzed pores (2– 200 nm) shows that the untreated calcitic sample (C0)
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pore size distribution, compared to the impregnated samples (long tail-off towards smaller pore diameter), may reflect rather extensive micro-cracking and widespread intergranular decohesion for untreated sample whereas the GS treatment seems to mitigate these processes. 4.3. Capillary water absorption
Fig. 4. BET surface area for calcitic and dolomitic marble samples.
Fig. 5. Mean pore size for calcitic and dolomitic marble samples.
For each diagram, two sets of samples are presented: reference samples and temperature-cycled samples. A comparison of the two marbles shows that the capillary absorption of calcitic marble was about twice the absorption of dolomitic marble (Figs. 8 and 9). The differences between reference and temperature-cycled samples are apparent for the calcitic samples. Capillary absorption increases from about 55 g/m2 for the reference samples to 100 g/m2 for the temperature-cycled samples. The differences between untreated and impregnated samples are relatively small; however the untreated samples show the highest capillary absorption for both calcitic and dolomitic marble. The capillary absorption results for the dolomitic samples display an opposite trend compared to the calcitic samples. The reference samples show higher capillary absorption than the temperature-cycled samples. In this case, the influence of temperature cycling is not
Fig. 6. Pore size distribution versus volume % for calcitic marble. Fig. 8. Absorbed water evolution during capillary absorption test as a function of the square root of the time for the calcitic reference samples and temperature-cycled samples.
Fig. 7. Pore size distribution versus volume % for dolomitic marble.
is the marble that is most sensitive to temperature cycling of all the samples (Figs. 6 and 7). The reverse pattern (long tail-off towards larger pore diameter) of the
Fig. 9. Absorbed water evolution during capillary absorption test as a function of the square root of the time for the dolomitic reference samples and temperature-cycled samples.
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reflected in the measured water absorption. Despite the same origin of the samples, the influence of the natural heterogeneity of the material on the results is higher than the temperature cycling. The untreated sample shows the highest capillary absorption of all dolomitic samples. The difference in capillary absorption between samples impregnated with GS P17 and GS P22 is difficult to interpret. The influence of the impregnation material on capillary absorption is smaller than the effect of heterogeneity of the stones. The capillary absorption for the samples is relatively small, which means that the presence of existing cracks in the reference material might have a significant influence on the results.
Fig. 10. Thermal expansion of the calcitic and dolomitic marbles. The residual expansion after 294 days is 0.33 and 0,05 mm/m, respectively.
4.4. Ultrasonic velocity measurements The ultrasonic velocity was measured in three directions. The maximum and minimum velocities and the anisotropy are presented in Table 3. Open micro-cracks are assumed to reduce ultrasonic velocities. This is clearly visible for the calcitic marble where the difference between the reference samples and temperature-cycled samples is about 65%, whereas the dolomitic marble shows insignificant variation in velocities between the samples. The dolomitic samples exhibit pronounced anisotropy: AV p ¼ ðV pmax V pmin Þ=V pmax 100; [14], in contrast to the calcitic marble. Therefore, the detected differences for the dolomitic samples are results of the anisotropy of the stone and not an effect of microcracking. 4.5. Thermal expansion The results from the thermal expansion measurements performed on the separate set of fresh samples show that both stones are sensitive to heating. However, no influence of humidity on expansion was observed (Fig. 10). Expansion of the material only occurred when the temperature increased. The diagram shows that the calcitic marble is more sensitive to heat than the dolomitic. The greatest residual expansion was also mea-
Table 3 Maximum and minimum ultrasonic velocities and anisotropy data for the calcitic and dolomitic marbles measured at dry conditions Sample
Vpmax (km/s)
Vpmin (km/s)
AV p (%)
CR0 CR C1 C2 DR0 D0 D1 D2
4.17 2.59 2.54 2.67 4.51 4.76 4.85 4.67
3.98 2.56 2.47 2.53 3.56 4.40 4.05 4.05
5.7 1.0 2.8 5.4 21.0 7.6 12.9 10.8
sured in the calcitic sample (0.33 mm/m) whereas almost none of the expansion remained in the dolomitic sample. 4.6. Summary of the results The role of porosity in marble is important in terms of moisture transport, mechanical properties and durability. There is a variety of methods for determining porosity, however, no individual test is sufficient to properly classify the marble relative to its expected behaviour in use and when exposed to weathering. Different techniques for porosity determination may give different values for the same sample. The results from this study show good agreement between different methods in some cases. This is especially noticeable for calcitic marble. The increase in water absorption in temperature-cycled samples in relation to reference samples is confirmed by increase in BET surface area, increase in the area measured by fluorescence microscopy, noticeable thermal expansion and reduction in ultrasonic velocity. For cases in which there is good agreement between methods, it can be assumed that breakdown processes are operating and that the quantification of the intergranular decohesion is independent of the method used. The problem arises when there is poor agreement between methods and unclear results are obtained. This might be the case with dolomitic marble. The breakdown is of much narrower range than the calcitic marbles, which means that the results are dependent on the method used. The image analyses of the dolomitic marble show evidence of initial decohesion, which is not obtained by the other methods. A combination of measurement methods is normally used to determine the properties of rock. However, the classification of the rocks is still based on the ability to pass each of the tests. The validity of the results based on a single test should therefore be interpreted cautiously.
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5. Conclusions This study has shown that the calcitic and dolomitic marbles were susceptible to thermal fluctuations causing local mechanical stresses between grains and resulting in intergranular decohesion. The results from all tests indicated that intergranular decohesion was more pronounced for the calcitic marble than the dolomitic marble. The differential thermal expansion/contraction associated with different mineralogical and textural properties of the materials is primarily responsible for the increase in numbers of pore spaces and their magnitude. Another important parameter that influences thermal expansion is the microstructure. Increasing complexity of the microstructure from equigranular (granoblastic) to seriate interlobate (xenoblastic) will probably make it more difficult for a stone material to expand. The results showed that most of the expansion had occurred along the grain boundaries. This indicates that the durability of a fine-grained sample is more sensitive to temperature changes than the coarse-grained ones since the total volume of grain boundaries increases with falling grain size. Grain boundary separation will then promote water ingress through capillary transport, including transport of dissolved salts, and subsequently the freeze/thaw and salt crystallization processes. Thermal expansion was apparent for both marbles and was independent of humidity; however, permanent thermal expansion was only significant in calcitic marble. Impregnation reduced the mean pore size and had a mitigating effect on water absorption and intergranular decohesion. Ultrasonic velocity measurements were not able to detect the micro-cracks in the dolomitic marble while in the calcitic marble this process was fully detectable. The use of fluorescence microscopy, among other methods used, gives relatively inexpensive and reliable information about the internal structure of the marbles. The first signs of thermal weathering are best analyzed by gas adsorption. However, the combination of gas adsorption and fluorescence microscopy gives thorough information about initial cracking and porosity. A better understanding of the effect that temperature has on the porosity of marble could be used as a guide for selection of suitable stone material for exterior use. In addition, the findings provide essential information that should be used when conditioning marble stones intended for mechanical testing. If intergranular decohesion in calcareous stones occurs at temperatures lower than the recommended temperatures for standardized
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tests (110 C), the results may give misleading values. This has to be taken into account during interpretation of data and as an indication of appropriate conditioning temperatures for the samples before physical property testing.
Acknowledgements This work was financially supported by the KK Foundation, the Swedish National Testing and Research Institute and the Geological Survey of Sweden. Eka Chemicals is acknowledged for providing GypStop. References [1] Kessler DW. Physical and chemical tests on the commercial marbles of the United States. Technology Papers of the Bureau of Standards. Washington: Government Printing Office; 1919. p. 123. [2] Sage JD. Thermal microfracturing of marble engineering geology of ancient work monuments and historical sites. Rotterdam: Balkerna; 1988. [3] Tschegg KE, Widhalm C, Eppensteiner W. Ursachen mangelnder Formbesta¨ndigkeit von Marmorplatten. Zeitschrift der Deutscher Geologischer Gesellschaft 1999;150/2:283–97. [4] Royer-Carfagni GF. On the thermal degradation of marble. Int J Rock Mech Mining Sci 1999;36:119–26. [5] Siegesmund S, Weiss T, Tschegg EK. Control of marble weathering by thermal expansion and rock fabrics. In: Proceedings of the 9th International Congress on Deterioration and Conservation of Stone, Venice, Italy; 2000. p. 205–13. [6] Hudec PP, Sitar N. Effect of water sorption on carbonate rock expansivity. Can Geotech J 1975;12(2):179–86. [7] Jornet A, Ruck P. Bowing of calcitic marble slabs: A case study. Quarry – Laboratory – Monument. International Congress, Pavia; 2000. [8] Royer-Carfagni GF. Some considerations on the warping of marble facades: the example of Alvar AltoÕs Finlandia Hall in Helsinki. Construct Build Mater 1999;13:449–57. [9] Malaga-Starzec K, Lindqvist JE, Schouenborg B. Experimental study on the variation in porosity of marble as a function of temperature. Geol Soc, Special Edition No. 205; 2002. [10] Kozlowski R, Tokarz M, Persson M. GypStop – a novel protective treatment. In: Proceedings of the 7th International Congress on Deterioration and Conservation of Stone, Lisbon, Portugal; 1992. [11] Lind A-M. Gypstop. Institute of Conservation, Go¨teborg University. University Report, 5; 2000. [12] Webb PA, Orr C. Analytical methods in fine particle technology. USA: Micrometitics Instrument Corporation; 1997. [13] NORDTEST, NT BUILD 479. Coefficient of thermal expansion, NORDTEST, Espoo, Finland; 1997. [14] Weiss T, Siegesmund S, Rasolofosaon PNJ. The relationship between deterioration, fabric, velocity and porosity constraint. In: Proceedings of the 9th International Congress on Deterioration and Conservation of Stone, Venice, Italy; 2000, p. 215–23.