Tuffs and sandstones are formed by sedimentary processes. Pyroclastic sedimentation resulting from volcanic eruptions gives rise to tuffstones, whereas sandstones develop from the sedimentation of rock fragments generated by weathering processes. The duration of the sedimentation process however occurs under different time periods. For tuffstones the sedimentation history only needs a few weeks or months, while the sedimentation history of sandstones occurs over a time span of mil-lions of years. The distinguishing characteristics of tuffstones are primarily compaction processes and flow structures that occur immediately after the volcanic eruption and accumulation. In con-trast, the sedimentary conditions of sandstones can change considerably and are directly connected to changes in climatic and topographic conditions. These factors require the flow velocity of bodies of water and the possible influx and sedimentation of clay minerals.
Figure 20: Different schemes for categorizing tuffs and sandstones in comparison to the types of tuffs and sandstones investigated in this thesis. a) Tuff classification based on type of material, b) tuff classification based on the size of the
material and c) sandstone classification based on the composition of framework grains.
1. 5. 1 Classification of tuffs and sandstones
To categorize tuffs and sandstones different systems were developed. The most commonly used scheme for classifying tuffs is a system based on the grain size and type of materials or fragments present (Fig. 20 a and Fig. 20 b). McBride (1963) developed a descriptive classification for sands-tones based on the composition of framework grains (Fig 20 c).
1. 5. 2 Structural aspects
Anisotropies describe the structural differences of a material. These differences are caused by mine-ralogical and microstructural differences in the fabric of marbles (Zezza 1992; Siegesmund et al., 2000; Leiss, Weiss 2000) or in regards to sedimentary processes the differences are created by laye-ring, bedding, flow structures or the presence of clasts (Meng et al., 1991; Wedekind et al., 2011).
In this thesis a wide range of different tuff and sandstone varieties were analyzed with regard to their anisotropy. The results obtained were acquired by investigations on specimens oriented paral-lel and perpendicular to bedding, flow structures or different types of layering. The anisotropies of different properties related to the microstructure was used to define the predisposition to weathe-ring.
Different physical properties are compared for both stones types, by contrasting the various structu-ral features evident in both rock types. These properties include the porosity and bulk density, the pore radii class, capillary water absorption, hydric expansion, splitting tensile strength and the ultra-sonic velocity.
1. 5. 1 Porosity and bulk density
Porosity and density are important factors that control the properties of compressive strength and water transport. Both rock types show different but comparable properties. For example, both rock types show a comparable bulk density. Mosch (2008) statistically evaluated thousands of published values from the stone industry for various stone types. In his study he also considered sandstones and volcanites. The results from Mosch (2008) not only include different tuffstones, but also other volcanic rock types such as basalts, which as a rule have a high density and low porosity. The box-plot calculations and values determined by Mosch (2008) are used as a comparative basis for the 20 tuffstones and 20 sandstones investigated in this study (Fig. 21 b). The mean value (mean median) of the bulk density for the sandstones and volcanites varies by 2.5 gm/cm3 with a comparable aniso-tropy of around 8 % for both stone types. The value of the bulk density for the tuffstones investiga-ted in this study has an average value of 1.81 gm/cm3, the sandstones value is 2.25 gm/cm3, and thus a comparable anisotropy of 20 % is attained. Furthermore, the majority of the values for the tuffstones as well as for the sandstones do not fall into the tolerance range (quantile) of the specified stone industry values. In the case of the tuffstones only seven attain a value that is located in the
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quantile and in the sandstones it is only one variety.
Figure 21: a) Legend, b) bulk density and c) porosity of volcanites and sandstones after Mosch (2008) completed with the values of the samples investigated in this study.
Similar holds true for the porosity of both rock types. According to Mosch (2008), the porosity atta-ins on average a value less than 5 % for sandstones and around 18 % for volcanites (Fig. 21 c). The investigated stones in this study also show considerable variation. For the tuffs the average porosity is 27 % and for the sandstones 15 %. Both values lie significantly above those known in the stone industry, whereby the median of the investigated tuffstones are located in the upper quarter of the specified quantile given in Mosch (2008).
1. 5. 2 Pore radii classes
Pore radii distribution is divided into five different classes. The two classes with the smallest pores are defined as microporosity (Fig. 22 a). Water transport in this pore class takes place by vapor. The class with the smallest pores ranges from 0.001 - 0.01 µm and the second smallest one from 0.01 - 0.1 µm. The three classes with the larger pores consist of capillary pores. They have pores ranging 0.1 ≤ 10 µm (Fig. 21 a).
A statistical evaluation of the pore classes of all sandstones and tuffstones investigated in this study shows the differences of the two types of stone according to their pore radii distribution (Fig. 22).
Sandstone shows a unimodal pore radii distribution dominated by capillary pores, whereas volcani-tes exhibit a bimodal pore system (Fig. 22 a). The largest anisotropy in the pore classes are found in the pores >10 µm with more than 60 % (Fig. 22 b). The lowest anisotropy occurs in the next smaller pore size class ranging between 1 µm and 10 µm with 17%. The anisotropy related to the micropo-res is around 40 % (Fig. 22 b).
Figure 22: a) The pore size classes of the sandstones and tuffs investigated in this study. B) The anisotropy of the single classes comparing sandstones and tuffs.
1. 5. 3 Capillary water absorption
A value that is closely linked with the porosity is the water uptake coefficient, which also describes the structural character of a rock. This value is calculated by the amount of water in liters that a sto-ne can absorb by capillary uptake during a specific amount of time (see also capillary water uptake, section 3.2.1) On average the tuffs in this investigation are able to absorb 4.6 kg/m3 of water within one hour. The sandstones with 6.3 kg/m3 absorb much more (Fig. 23 a).
The anisotropy of the water uptake in dependence of the different directions describes the effect of a possible layering or other structural features. The average anisotropy of the water uptake coefficient is 30 % for the investigated tuffs, whereas the sandstones only reach a value of 18 % and is thus lo-wer by almost a half. Even the spread of the measured values differs considerably. In the case of the tuffstones the anisotropies can attain a value of 98 %, whereas the highest anisotropy in the sands-tones only reaches 40 % (Fig. 23 b). To summarize, tuffssands-tones show considerable structural diffe-rences with respect to the water uptake. These are indicated by a factor of 40 % in comparison to sandstones.
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Figure 23: a) Water absorption in the sandstones and tuffs and b) anisotropy of water absorption in the tuffs and sandstones.
1. 5. 4 Hygric expansion
Deterioration on buildings is often concentrated in areas affected by temporary moisture infiltration and leads to different damage phenomena (see Fig. 17 a, d, e and 18 c and d)). Moisture expansion is clearly recognized as one of the most important factors contributing to the weathering and dete-rioration of building stones (Ruedrich et al., 2011 (a); Weiss et al., 2004), and this is mostly asso-ciated with the swelling and shrinking of clay minerals (Schuh 1987; De la Calle, Suquet 1988;
Snethlage et al., 1995; Jimenez Gonzalez, Scherer 2004; Dixon, Weed 1989; Moore and Reynolds 1997; Graf v. Reichenbach, Beyer 1995). The intensity of moisture expansion varies markedly de-pending on the type of stone and can have a significant anisotropy in regards to the bedding. In ge-neral, volcanic stones, and primarily tuffs, have a wide range of moisture expansion, which, as mea-sured in this study, can reach a dilatation of up to 6 mm/m (Fig. 24).
Besides clay minerals the disjoining pressure can lead to moisture expansion, especially if the rock material contains a pore space dominated by micropores. The disjoining pressure (Πd) arises from an interaction between two flat and parallel surfaces. The value of the disjoining pressure can be calculated as the derivative of the Gibbs energy of interaction per unit area with respect to distance.
This energy, also known as free energy, is defined as a thermodynamic potential that measures the process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure (Greiner et al., 1995). The disjoining pressure will be affected when the distance between them is less than two times the thickness of adsorbed moisture on a free surface, where the force
needed to keep the forced distance is determined by the so-called “disjoining pressure equation” (Nielsen 1994).
Figure 24: Swelling and moisture expansion of building stones (data from different authors and from this study). Modi-fied from Kocher (2005): Granite, marble und sandstone I from Hockmann and Kessler (1950), from Snethlage (1984), limestone from Lukas (1990), sandstones II from Schuh (1987), sandstones III from Snethlage and Wendler (1997),
concretes from Wesche (1977), mudstones from Madson (1976) and Madson and Nueesch (1990).
Ruedrich et al., (2011 a) presented a detailed overview of the different types and causes of moisture expansion. Whatever the cause of the moisture expansion may be, the water uptake and distribution into the rock is the principal mechanism allowing water (moisture and humidity) to interact with the clay minerals present and is only realizable through the porosity. The porosity, therefore, is one of the most important parameters that must be known and determined.
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Figure 25: a) Hydric dilatation in the Z-direction of the tuffs and sandstones and b) the anisotropy of hydric dilatation of both rock types.
The tuffs and sandstones investigated in this study show different values of hydric dilatation. Mois-ture dilatation in the Z-direction of the different sandstones can show very high expansion but even shrinking by reaching an average value of 0.48 mm/m (Fig. 25 a). By eliminating the extreme va-lues the average value of 0.56 mm/m becomes higher. Comparably the hydric expansion of all tuff samples is smaller and only attains a value of 0.37 mm/m (Fig. 25 a). In the case of the anisotropy the opposite relation can be seen. The directional dependence of the anisotropy in the sandstones attains a value of 34 % and 38 % for the tuffs. The spread of the values for the tuffs is much larger than for the sandstones (Fig. 25 b). By eliminating the extreme value in the case of the sandstones, the anisotropy only reaches a value of 30 %.
1. 5. 5 Splitting tensile strength
Splitting tensile strength tests are done on slices of drill core material. To understand weathering processes as well as conservation treatments in relation to the bedding, the splitting tensile strength can give more information in general than the compressive strength tests because less strength is needed to split the sample. The strength and anisotropies according to textual properties like the bedding is sometimes small as well as the increase of strength due to consolidation. Therefore a smaller force to the sample can give a clearer result.
Sandstone samples show a significant directional dependence of the tensile strength to the loading direction (Siegesmund, Duerrast 2011). The same situation was determined for the tuffstones in this study. To perform the test under water-saturated conditions, information about the presence and
bedding of clay minerals can be determined because this leads to a decrease in the strength.
Figure 26: a) Splitting tensile strength of the tuffs and sandstones and b) anisotropy in splitting tensile strength.
The other reason why splitting tensile strength tests should be preferred is that less material is requi-red to obtain significant results. Therefore, the intervention on historical materials studied can be limited.
The tuffs measured in this thesis reach an average value of 3.43 MPa, whereas the sandstones show a greater strength with around 5 MPa (Fig 26 a). The anisotropic behavior of the directional depen-dence of splitting tensile strength of the sandstone is 23 % (Fig. 26 b). By ignoring the outliers the value is only 19 %. In contrast the tuffs have an average value of 29 %, ten percent more than the sandstones (Fig. 26 b).
1. 5. 6 Ultrasonic velocity
Ultrasonic velocity measurements are an important non-destructive tool for the evaluation of struc-tural damage on historical architecstruc-tural elements and artwork created from stone (Siegesmund, Du-errast 2011). The P-wave velocity (Vp) is determined by a transducer and measured by a receiver.
The ultrasonic velocity (V) is calculated with respect to the distance between transducer and recei-ver, respectively. The material thickness is determined by an ultrasonic testing machine. The V-va-lue is given in units of distance over time (km/sec).
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Figure 27: a) Ultrasonic velocity of the tuffs and sandstones and b) their anisotropy.
The average values for the tuffs and sandstones seem to be quite similar. The tuffs attain a velocity of 2.72 km/sec and the sandstones a value of 2.68 km/sec (Fig. 27 a). The anisotropic behavior of the directional dependence of the sandstone is 9.73 % (Fig. 27 b) and without the outliers 7.78 %. In the tuffs the anisotropy reaches a value of 12.78 % (Fig. 27 b) and without the outliers 10.94 %.
A clear correlation in the increase of porosity due to fabric damages is evident. Furthermore there is a correlation to salt resistance, probably controlled by the density and strength of the rock (see sec-tion 1.5.7).
1. 5. 7 Salt resistance
Crystallization tests on stone samples for determining salt resistance is a common method. In this test a sample is soaked in a salt solution and afterwards dried in an oven. The index used to charac-terize salt resistance in this study is the number of cycles until a 30 % material loss has taken place.
The tuffs show a wide range of resistance against salt bursting with an average of 24 cycles until a 30 % material loss is reached (Fig. 28 a). One half of the tuffs show a low salt resistance with 5 to 20 cycles. Other samples with 20 to nearly 70 cycles show a high resistance to salt bursting. The investigated sandstones have a lower salt resistance with an average value of 19 cycles until a 30 % material loss is attained (Fig. 28 b). Similar to the tuffs the sandstones show two groups: a larger
group that is markedly below 20 cycles and a smaller one that shows resistance up to 44 cycles (Fig.
28 a).
Salt resistance in general is related to the presence of a special pore size distribution (Benavante 2011). Stone material with a high amount of micropores is classified as less resistant towards salt crystallization (Wellmann, Willson 1965; Siegesmund et al., 2010). On average the investigated tuffs attain a value of 38 % microporosity, the sandstones 22 % (Fig. 28 b). Most sandstones only show a microporosity of less than 10 %, whereas the spread of the values in the case of the tuffs is highly diversified (Fig. 28 b).
Figure 28: a) Salt resistance in the tuffs and sandstones and b) the microporosity.