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Capillary water absorption characteristics of some Cappadocian ignimbrites and the role of capillarity on their deterioration

  • İsmail DinçerEmail author
  • Meliha Bostancı
Original Article
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Abstract

Cappadocia stands out as one of the most important regions in Turkey for its natural, historical, and cultural heritages. The region is under the influence of wetting and drying and freezing and thawing weathering processes, which are largely controlled by the water content. Water migrates through rock pores via different processes, such as capillary water absorption, which is one of the most common ways observed in the Cappadocia region. In this study, a research program consisting mainly of laboratory studies was carried out to investigate the capillary water absorption characteristics of ignimbrites, which are the host rocks of many natural and historical heritage structures in Cappadocia. Accordingly, XRD analyses, mercury porosimeter, and capillary water absorption tests were performed. The pore size distribution, which is a function of welding degree, controls the capillary water absorption process of ignimbrites. Particularly, ignimbrites that have a uniform pore size distribution of around 5–10 µm reveal higher capillary water absorption potential. Ignimbrites have a considerable and variable potential in terms of capillarity, and the capillarity plays a major role on the deterioration and decomposition of many of the historical and natural heritage structures in the Cappadocia region.

Keywords

Ignimbrite Capillarity Deterioration Anisotropy Cappadocia 

Introduction

It is known that rocks also deteriorate due to the effect of moisture, which may vary in different ways of water infiltration inside the rocks. Capillary water absorption is one of the most common ways. Once porous media touches liquid, the media absorbs the liquid because of its capillarity characteristics. The water rises from the lower to the upper part depending on the capillarity characteristics. The water evaporates and causes deterioration in rocks by its solute property contained in the absorbed water. Numerous studies have reported on the kinetics and overall theory of capillary water absorption and the resulting rock deterioration that occurs as a result of the capillarity (Benavente et al. 2001, 2002; Özdemir 2002; Franzen and Mirwald 2004; Cueto et al. 2009; Yıldız et al. 2010; Vázquez et al. 2010; Özvan et al. 2015). Benavente et al. (2001) carried out salt weathering tests on porous stones using the partial immersion method. They concluded that the weight loss due to salt crystallization is localized in the capillary zone. The salt crystallization pressure and solution flow within the rock are controlled by porosity. A similar result was also reported by Benavente et al. (2002), on the fact that the capillary process is a function of rock petrography and pore structure. Cueto et al. (2009) suggested a practical and simple linear model to estimate the permeability using the capillary imbibition test, vacuum saturation porosity test, and fissure density quantification. They also indicated that the fissure density strongly influences the water transport properties of rocks. Vázquez et al. (2010) assessed the relationship between some crack network parameters and properties such as capillary absorption and P-wave velocity of granites from Galicia, Spain. Subsequently, the physical properties were found to be closely correlated to the crack network parameters. Török et al. (2016) studied eight different types of acidic tuffs of the Eger Castle (Hungary) and a couple of tuffs from nearby quarries. They stated that pore size distribution rather than effective porosity controls the weathering susceptibility of tuffs and frequent larger micropores are the main causes of freezing and thawing-related weathering. There are also many studies on the effect of pore size and its effects on the expansion potential of tuff. For example, Stück et al. (2008) consolidated the different volcanic tuffs of Hungary and Germany under laboratory conditions. They concluded that the water uptake of the consolidated tuff is influenced by the shift in pore size distribution (Wedekind et al. 2013; López-Doncel et al. 2013; Siegesmund and Snethlage 2014; Pötzl et al. 2018a, b; Sousa et al. 2018). Wedekind et al. (2013) identified a correlation between microporosity, average pore radius, and moisture expansion. López-Doncel et al. (2013) indicated that the pore radii distribution is decisive for the effectiveness of porosity and the water transport into the rock. Similarly, porosity and distribution of micropores directly related to the water transport and retention properties of the tuff rock (Pötzl et al. 2018a). In addition to these, the pore radii size and distribution are the main factors controlling the behavior of the granitoids under salt action (Sousa et al. 2018). The location of the clay minerals in the tuf rock is important. Even small amounts of swellable clay minerals can cause significant expansion of the material if they are located in critical spots in the rock fabric. The disjoining pressure plays an important role for the hydric expansion of two varieties, which are free of swellable clay minerals but show high microporosity and pores smaller than 2 nm (Pötzl et al. 2018b).

Cappadocia is one of the most important touristic areas in Turkey and was added to the World Heritage List by UNESCO in 1985 thanks to its natural, historical, and cultural heritages. These heritage sites, however, are at risk of deterioration because of environmental and atmospheric effects (Topal and Doyuran 1997, 1998; Ulusay et al. 2006; Aydan et al. 2007, 2008a, b). The structural stability of the fairy chimneys is controlled by strength reduction due to moisture, poor to very poor durability, and the adverse effects of joints (Topal and Doyuran 1997). The durability of the Cappadocian tuff was assessed through wetting and drying, freezing and thawing, and salt crystallization by Topal and Doyuran (1998) and the durability of tuffs was classified as poor to very poor. Ulusay et al. (2006) concluded that discontinuities and rock weathering are the main factors controlling the stability of rock-hewn structures in the region. Similarly, deterioration occurred as a result of capillary water absorption in tuffs has been reported by Aydan et al. (2007, 2008a, b). Cappadocia is located in a region with a cold continental climate, which has a considerable effect on the deterioration processes of rock masses. In this region, the winters are cold and snowy, the springs are rainy, and the summers feature hot and dry weather conditions. There are two processes causing weathering whose climatic regimes impact the region’s heritages, namely, the wetting and drying as well as the freezing and thawing. Both processes are mainly controlled by the water content. Water migrates into rock through different mechanisms, such as capillary water absorption, which is one of the most common ways in the Cappadocia region. Ergüler (2009) reported that capillary action likely accelerates the weathering rate in the region based on field observations. Kaşmer and Ulusay (2013) and Kaşmer et al. (2013) were also noted that the Zelve tuff may suffer from deterioration particularly at the toe of natural slopes during freezing and thawing cycles as the rock mass is likely to be saturated at those locations due to capillarity. Nevertheless, the capillarity performance and its role on deterioration were not discussed in detail. Therefore, the determination of the capillary absorption behavior of ignimbrite and its effect on heritage structures are highly important for conservation projects.

As indicated in the brief summary of literature given above, capillary water absorption capacity of rocks is highly controlled by petrophysical properties and pore geometry structure. Although a significant part of previous studies were focused on this point, the relationship between pore structure and capillarity in rocks was not fully understood depending on clear descriptions of pore structure. Because of this reason and the role of capillarity in deterioration of natural and historical structures in Cappadocia, a research was conducted to investigate the capillary water absorption phenomenon of ignimbrites. For this purpose, a number of block samples were employed from various locations around the Cappadocia that are covered by ignimbrite outcrops (Fig. 1). The block samples were previously used in a research project which was performed by Orhan and Dinçer (2015) to evaluate the usability of the rock material as natural building stone. Consequently, a series of physico-mechanical tests were carried out in the aforementioned project. Then, within the scope of this current research, new core and cubic samples were extracted from the same blocks to perform the laboratory tests including XRD, pore size distribution, capillary water absorption, and mercury porosimetry. Capillary water absorption tests were carried out under different conditions and different directions to evaluate the capillary behavior of ignimbrites. The data obtained from the laboratory studies are presented in the following sections.

Fig. 1

Geological map of the study area, including sampling locations, a geological map, b generalized stratigraphic vertical column of the Cappadocia region (from Aydar et al. 2012)

Materials

Geology

The Cappadocia region is located within the Cappadocian Volcanic Province (CVP), which extends in a belt in a NE–SW direction for a length of more than 250 km and a width of 40–60 km (Toprak et al. 1994). This region is surrounded by the Taurus Mountain range in the south, Kırşehir massive in the north, and two prominent Quaternary stratovolcanoes (Hasan Dag and Erciyes Dag) in the west and east, respectively. Structurally, the western and eastern boundaries of the plateau are defined by the Tuzgölü and Ecemis faults. In the Central Anatolian Volcanic Province (CAVP), Miocene–Pliocene ignimbrites collectively cover an area of 20,000 km2 (Le Pennec et al. 1994). These pyroclastic deposits, intercalated with terrestrial sediments and local lava flows, reveal a unique landscape (Aydar et al. 2012). The Cappadocia region mainly consists of Pre-Neogene basement rocks (Cretaceous granitic and gabbroic rocks) Neogene sedimentary rocks (red mudstones, sandstones, and conglomerates), Neogene volcano–sedimentary units (tuffs and ignimbrites), and volcanic rocks (ignimbrites, andesites, and basalts) from the Quaternary period. The stratigraphy of ignimbrites of the Miocene–Holocene age is recently modified by Aydar et al. (2012) based on the geochronological data in the Central Anatolian Volcanic Province (CAVP). They follow the terminology outlined in Le Pennec et al. (1994), identifying ten ignimbrite members (in stratigraphic order from oldest to youngest), namely, Kavak, Zelve, Sarımadentepe, Sofular, Cemilköy, Tahar, Gördeles, Kızılkaya, Valibabatepe, and Kumtepe (Fig. 1). A brief summary of the stratigraphy of ignimbrites is presented with respect to the recent study of Aydar et al. (2012) carried out in the Cappadocia region.

The Kavak ignimbrites represent the oldest pyroclastic deposits in the CAVP (Fig. 1b). They are interbedded with fluvio-lacustrine sediments indicating multiple eruptive episodes. The ignimbrite was subdivided into four different sub-units such as Kavak-1, Kavak-2, Kavak-3, and Kavak-4 (Fig. 1b). The Kavak-1 sub-unit is characterized by reverse graded pumice-rich flow and is separated by lacustrine sediments from the overlying Kavak-2 unit which consists of ash-rich flow deposits with lithic materials. The Kavak-3 sub-member is consolidated with several pumice-rich horizons in an ash matrix and overlain by the Kavak-4. The ignimbrite is signified by pale pinkish pyroclastic flow deposits with lithic and pumice clasts in an ash-rich matrix (Aydar et al. 2012). The Zelve ignimbrite is represented by white basal pyroclastic fallout and pink ignimbrite and is the host rock of numerous natural heritages in the Cappadocia region together with the Kavak ignimbrite. The Zelve unit approximately covers an area of 4200 km2 around Ürgüp, Avanos, and Nevşehir districts with total volume of 120 km3 (Le Pennec et al. 1994). The Sarımadentepe ignimbrite overlies the paleosol developed above the Zelve ignimbrite and is localized in the Mustafapaşa and Ayvalı villages (Fig. 1). The unit mainly consists of two dissimilar levels. The lower part is composed of a basal fallout deposit with reverse grading. The upper section is strongly welded and displays columnar jointing with color variations from pale yellow to brownish-scarlet (Aydar et al. 2012). The Sofular ignimbrite is dominated by a 25-m-thick single flow unit with an ash-supported lithic content. The Cemilköy ignimbrite covers an area of 8600 km2 with a volume of 300 km3 (Le Pennec et al. 1994). The unit is unwelded and represented by smooth surfaces. This ignimbrite involves pale-gray pumice fragments with a prismatic shape (Aydar et al. 2012). The Tahar ignimbrite is generally characterized by pale-pink to scarlet-brown color and has very rich lithic content specifically at the lower part of the unit. The main body of unit comprises with glassy, beige to pinkish pumice with incipiently flatted vesicles. Aydar et al. (2012) stated that the Gördeles ignimbrite may be confused with the Kızılkaya or Sarımadentepe ignimbrites. The ignimbrites are incipiently to moderately welded and pale gray to light brownish in color as in Gördeles ignimbrite. Gördeles was divided into two different sub-units as lower and upper which are separated by a paleosol (Aydar et al. 2012). The Kızılkaya ignimbrite is the most widespread member in the CAVP where it covers an area of 8500–10,600 km2 with a volume of 180 km3 (Le Pennec et al. 1994; Schumacher and Mues-Schumacher 1996). The Kızılkaya ignimbrite consists of a basal pre-ignimbrite Plinian fallout deposit and two main flow units. The thickness of the unit is indicated as > 40–50 m (Derinkuyu underground city) and a maximum of 80 m (Ihlara valley) by Aydar et al. (2012). The Valibabatepe ignimbrite is dark, strongly welded, and displays eutaxitic textures with well-developed fiammes. It widely crops out in the eastern parts of the CAVP. The youngest ignimbrite in the CAVP is the Kumtepe member which was erupted from Acıgöl Volcanic Complex in Quaternary (Druitt et al. 1995; Schmitt at al. 2011). It is divided into two different layers which consist of lapilli-sized and basal lapilli fallout deposits. A majority of the above-mentioned ignimbrite units are also included in the Ürgüp formation by Pasquarè (1968) which is accepted to be the main lithostratigraphic unit of the region.

Sampling locations

The sampling area is generally underlain by thick and extensive pyroclastic deposits of the Upper Miocene-Quaternary age in the Cappadocian Volcanic Province (Fig. 1). The volcano-sedimentary sequence is mainly represented by the Ürgüp formation, from which emerged the unique geological structures known popularly worldwide as “fairy chimneys.” As these rock units can be processed easily, they have been preferred for the construction of rock-hewn structures in the region (Topal and Doyuran 1995; Aydan and Ulusay 2003; Ulusay et al. 2006; Korkanç 2007). The deposits of the Ürgüp formation are fine to coarse grained containing large and small pumice and obsidian fragments. At some locations, these deposits are interbedded with ignimbrites and tuffs, while in other places they are interbedded with clay and marly clay beds (Aydan and Ulusay 2003). In this study, only the Kavak, Zelve, Sarımaden, Tahar, and Kızılkaya ignimbrites were focused on, since these units are the host rocks of the most important and typical structures of Cappadocia, like the fairy chimneys and underground cities (Fig. 1a). In this study, a total of 14 block samples were obtained from six different locations (Fig. 2) (Başdere, Demirtaş, Ortahisar, Böltaş, Nevbitaş, and Kavak) and detailed descriptions are given in Table 1.

Fig. 2

General view of sampling ignimbrite quarries a Başdere (BD01), b Demirtaş (DT01 and DT02), c, d Ortahisar (OH01 and OH02), e Böltaş (BT01, BT02, and BT03), f Nevbitaş, g Kavak (KV01 and KV02), h Kavak (KV03)

Table 1

General characteristics of ignimbrite samples

Sample no.

Location

Geological unit

Welding degree

Components

Descriptions

Quarry type

BD01

Başdere

Kızılkaya ignimbrite

Incipient to partial

Pumice, biotite, quartz, feldspar, and rock fragments

Dark gray to black in color and present a massive structure

Ancient quarry

DT01

Demirtaş

Kızılkaya ignimbrite

Moderate to well

Quartz, plagioclase, pyroxene and biotite, shards, and lithic fragments

Pinkish brown color, significant fiamme structures, lithic fragments

Abandoned quarry

DT02

Demirtaş

Kızılkaya ignimbrite

Moderate

Quartz, plagioclase, and pumice

High porosity and high number of lithic fragments

Abandoned quarry

OH01

Ortahisar

Kavak member

Poorly to moderate

Volcanic shards, low ratio of crystals

Massive structure and white to cream in color

Abandoned quarries

OH02

Ortahisar

Kavak member

Partially to moderate

Pumice, quartz, plagioclase, and lithic fragments

Yellowish white in color, high pumice ratio

Abandoned quarries

BT01

Böltaş

Zelve ignimbrite

Partial

Pumice, biotite, quartz, feldspar, and rock fragments

High porosity and a high number of pumice fragments

Active quarry

BT02

Böltaş

Kavak member

Moderate

Quartz, plagioclase, pyroxene and biotite, shards, and lithic fragments

Pinkish red

Active quarry

BT03

Böltaş

Kavak member

Moderate

Quartz, plagioclase, pyroxene and biotite, shards, and lithic fragments

Yellowish cream

Active quarry

NBT01

Nevbitaş

Tahar ignimbrite

Partially to moderate

Pumice, lava, basalt fragments, and olivine

Yellowish white in color, pumice fragments is fairly high

Abandoned quarries

NBT02

Nevbitaş

Tahar ignimbrite

Partially to moderate

Pumice, lava, basalt fragments, and olivine

Cream in color, low pumice ratio

Abandoned quarries

NBT03

Nevbitaş

Kavak ignimbrite

Partially to moderate

Quartz, plagioclase, pyroxene and biotite, shards, and lithic fragments

Light white in color, pumice fragments is fairly high

Abandoned quarries

KV01

Kavak

Kavak ignimbrite

Poor

Phenocrysts, pumice fragments, shards, and lithic fragments

Brownish cream in color

Abandoned quarry

KV02

Kavak

Kavak ignimbrite

Partial to moderate

Phenocrysts, pumice fragments, shards, and lithic fragments

Dark gray and black in color

Abandoned quarry

KV03

Kavak

Sarımaden ignimbrite

High

Hyaline groundmass, biotite, pumice and rock fragments

Dark gray in color

Outcrop

Methods

The ignimbrites used in this study were evaluated in terms of their physical, mechanical (ISRM 2007), and petrographical properties in the project by Orhan and Dinçer (2015). Below is the methodology applied for the additional laboratory and field works performed in this current study.

X-ray diffraction (XRD) analyses of the ignimbrite samples were performed at the Central Laboratory of Nevşehir Hacı Bektaş Veli University using a Rigaku diffractometer system with Cu&Kα radiation. Samples were run from 5ο to 70ο 2θ with a step increment of 0.02ο and a counting time of 2 s/step; the relevant data were stored in a digital form. The analyses of major oxides of the ignimbrite samples were performed at the ACMELab Analytical Laboratories in Vancouver (Canada), using ICP-MS (Inductively Coupled Plasma Mass Spectrometry).

As a part of the study, the capillary water absorption capacity of the ignimbrites was determined and the role of pore size distribution on capillary water absorption was discussed. Accordingly, capillary water absorption tests were carried out in accordance with the standards of UNE-EN 1925 (1999). Before the tests, the ignimbrite samples were dried at 105 °C. Pore size distribution was determined by applying the mercury intrusion technique, as suggested by the ASTM method D4404 (1984). Mercury porosimetry tests were performed on oven-dried samples using a Quantachrome Poremaster 60 porosimeter, which is capable of delivering a maximum pressure of 60 psi and measuring pore diameters ranging between 0.006 and 950 µm at the Central Laboratory of Nevşehir Hacı Bektaş Veli University.

Characteristics of Cappadocian rocks

Mineralogical, petrographical, and geochemical properties

Some of the collected ignimbrites are composed of hyaline groundmass with a hypohyaline texture. The rest of the samples (OH02, BT01, BT02, BT03, NBT01, NBT02, and NBT03) present a hypocrystalline texture, and their groundmass consist of volcanic shard material. The ignimbrites are composed of varying proportions of quartz phenocrysts, plagioclase, lithic fragments, biotite, pyroxene, opaque minerals, and pumice (Fig. 3). In addition, alteration products such as quartzolite, iddingsite, and chlorite are also traced. XRD analyses were performed on 14 samples to confirm the results of the optical microscopy and all mineral content ratios were obtained according to Rietveld (1969). In order of abundance, the following minerals were identified through XRD analyses of the Cappadocian ignimbrites: quartz, plagioclase, feldspar, calcite, and pyroxene, with small amounts of amphibolite, biotite, and clay minerals. Pyroxene and calcite are present only in the locations of Ortahisar, Böltaş, and Başdere. Additionally, cristobalite was observed at a very low rate in a few ignimbrite samples. The ratios of the main minerals obtained from XRD analysis are given in Table 2. Topal and Doyuran (1997) performed XRD analyses on both lichen-covered and iron-stained samples to assess the abundance of all minerals and clay mineral types within the tuff. They proposed similar XRD diffraction patterns to this study and their samples contain dominantly feldspar, some quartz, small amounts of clay (smectite), and mica minerals. The ratio of SiO2 is higher than 65% for all ignimbrite samples, with the highest value being obtained from the sample locations of Böltaş and Nevbitaş (Orhan and Dinçer 2015). On the total alkali (Na2O + K2O) vs. silica (SiO2) classification diagram (Total Alkali Silica) (Le Bas et al. 1986; Middlemost 1994), all ignimbrite samples are plotted in trachydacite, dacite, and rhyolite fields (Fig. 4).

Fig. 3

General macroscopic views of KV01 (a, b), KV02 (c, d), KV03 (e, f) (KP rock fragments, P pumice, M mica, LM lithic materials, K quartz, F feldspar, B void)

Table 2

Mineral ratio of investigated ignimbrites obtained from XRD analyses

Sample no.

Quartz

Plagioclase

Feldspar

Calcite

Pyroxene

Biotite

Amphibolite

Clay minerals

BD01

14.4

19.0

8.0

12.0

37.0

9.6

DT01

22.4

30.5

44.0

3.1

 

DT02

21.3

9.1

65.0

3.84

< 1.0

OH01

21.3

48.0

28.0

2.7

  

OH02

78.0

17.1

4.9

  

BT01

93.0

< 1.0

1.6

5.1

 

< 1.0

BT02

81.0

12.3

3.4

3.0

 

< 1.0

BT03

> 95.0

 

< 1.0

NBT01

82.0

< 1.0

 

17.6

NBT02

> 95.0

2.5

 

NBT03

79.0

 

21.0

KV01

9.2

89.0

1.8

 

KV02

85.0

10.0

5.0

 

KV03

14.8

70.2

12.8

2.2

 

Fig. 4

Classification of ignimbrites based on the total alkali (Na2O + K2O) vs. silica (SiO2) classification diagram (Middlemost 1994)

Physico-mechanical properties

Short- and long-term material properties of Cappadocian ignimbrites are recently defined by Ulusay and Aydan (2018) in which a detailed comparison of the physical, index, and mechanical properties of different Cappadocian ignimbrites (Kavak, Zelve, Gördeles, Cemilköy, Kızılkaya, and fallout product of Kavak tuffs) are presented. Additionally, the physico-mechanical properties of the Sarımaden and Tahar ignimbrites are determined in this study. The physico-mechanical properties of various ignimbrite units in the region are summarized in Table 3 with respect to previous researchers and this study. The dry unit weight of ignimbrites considered in this study varies between 11.43 and 18.68 kN/m3, which is almost similar to the range reported by Ulusay et al. (2006, 2013), Tuncay (2009), Kaşmer and Ulusay (2013) and Aydan and Ulusay (2013). All ignimbrite samples can be classified as high to very high-porosity rock based on NBG (1985). The P-wave velocity ranges between 1237.28 and 3158.73 m/s, while water absorption by weight is in the range of 10.39 and 28.03%. Dry uniaxial compressive strength of investigated ignimbrites is between 5.91 and 32.56 MPa, and the rocks are regarded as having medium–low strength according to Deere and Miller (1966). Nonetheless, the average uniaxial compressive strength of the KV03 sample is as high as 78.65 MPa due to its high degree of welding, texture, and composition (Fig. 3e, f). The ignimbrite samples are classified as slightly weathered according to ISRM (1981).
Table 3

Physico-mechanical properties of Cappadocian ignimbrites

Material properties

Ulusay et al. (2006)

Tuncay (2009)

Kaşmer and Ulusay (2013)

Aydan and Ulusay (2013)

Ulusay et al. (2013)

This Study

KI

KI

KZI

CI

ZI

GI

KTa

KI

ZI

SI

TI

KZI

Unit weight (kNm− 3)

12.4–14.4

13.3–14.3

12.2–14.4

11.2–14.6

11.9–14.6 (12.9)

13.7–15.9 (14.9)

8.5–10.6 (9.12)

15.98–11.43

14.70

18.20

17.26–18.68

13.79–14.44

Porosity (%)

21–27

27–37

28–35

19.52–41.10 (29.24)

 

18.28–35.14

26.50

20.23

17.18–18.40

31.49–35.12

P-wave velocity (ms− 1)

1236–2299 (1672)

1500–2200 (1800)

900–1340 (1240)

1237–2789

1425

3158

1902–2689

1358–1864

Water absorption by Weight (%)

13.70–33.21 (22.29)

11.52–28.03

17.76

10.78

8.84–10.39

18.68–24.11

Dry uniaxial compressive strength (MPa)

2.3–9.1 (4.6)

3.60–5.0

3.4–6.3

1.2–2.2

3.85–6.32 (4.95)

7.9–10.3 (9.0)

1.97–2.45 (2.26)

5.91–32.56

13.36

78.65

18.91–28.99

8.76–14.41

Saturated uniaxial compressive strength (MPa)

1.10–1.56

3.27–3.88

0.44–0.52

0.67–0.92 (0.80)

3.1–4.9 (6.8)

1.12–1.45 (1.22)

2.86–31.74

9.54

74.29

12.85–23.36

6.33–13.25

KI Kavak ignimbrite, KTa fallout product of Kavak ignimbrite, ZI Zelve ignimbrite, SI Sarımadentepe ignimbrite, TI Tahar ignimbrite, GI Gördeles ignimbrite, KZI Kızılkaya ignimbrite, CI Cemilköy ignimbrite

Pore size distribution

Ignimbrites with different welding degrees may present different pore structures due to vertical stresses and temperatures being exposed during the formation process. The physical and mechanical properties of rocks are highly affected by their pore size and structure (Blows et al. 2003). Therefore, the determination of pore characteristics is extremely important for assessing the deterioration mechanism of dimension stones. In this study, mercury porosimetry tests were carried out to evaluate the pore size and its distribution. The pore size distribution results obtained from mercury porosimeter tests for each sample and the typical porosimetry curves of plotted cumulative pore volume with respect to pore diameter are given in Fig. 5.

Fig. 5

Typical porosimetry curves of investigated ignimbrites

The cumulative mercury intrusion per gram of sample (cm3/g) and pore diameter (mm) are shown on Y and X axes, respectively. As seen in Fig. 5, total pore volume varies between 0.075 and 0.398 cm3/g. In all samples, the diameter of pores ranges between 0.004 and 275 µm. Tuğrul (2004) noted that the smoothness of curves generally indicates an equal distribution of different pore diameters. Accordingly, it can be concluded that the regions wherein a sharp increase is observed specify the dominant pore diameter. Thus, the main pore diameter values for each sample are different from each other. In this study, the main pore diameter, dominant pore diameter range, secondary pore diameter range, and pore size distribution descriptions are defined based on the porosimetry curves of plotted cumulative pore volume versus pore diameter (Fig. 5). As seen in Fig. 6, typical pore size distribution is classified as uniform, semi-uniform, and non-uniform (Table 4). While the uniform type is represented by a very sharp increase, the non-uniform type is characterized by a smooth curve, as shown in Fig. 6. Moreover, the semi-uniform type is described by a curve showing a slightly sharp increase. As Tuğrul (2004) revealed, the smoothness of curves generally indicates an equal distribution of different pore diameters, whereas a uniform pore diameter distribution is represented by a sharp increase in curves. On the other hand, the pore diameter observed in the region of sharp increases, particularly in the middle of this region, is accepted as the main pore diameter for each ignimbrite in this study. In addition, the zone between the starting and ending points of the sharp increment is defined as the dominant pore diameter range (Fig. 6). The main pore diameter of the samples fluctuates between 12.00 and 0.74 µm (Table 4).

Fig. 6

Typical pore size distribution observed in the ignimbrites a the sample of uniform pore size distribution, b the sample of semi-uniform pore size distribution, c the sample non-uniform pore size distribution

Table 4

Pore size distribution parameters of the investigated ignimbrites

Sample no.

Pore diameter (µm)

Total intruded pore volume (cm3/gr)

Main pore diameter (µm)

Dominant pore diameter range (µm)

Secondary main pore diameter range (µm)

Pore size distribution description

BD01

276–0.004

0.264

7.64

10.00–5.00

Uniform

DT01

209–0.004

0.254

5.56

9.00–1.00

Uniform

DT02

227–0.004

0.118

12.00

20.00–8.00

0.005–0.004

Uniform

OH01

221–0.004

0.398

5.29

30.00–0.01

Non-uniform

OH02

213–0.004

0.204

3.46

5.00–0.008

Non-uniform

BT01

231–0.004

0.266

2.10

3.00–0.09

Semi-uniform

BT02

204–0.004

0.236

3.20

4.00–0.08

Semi-uniform

BT03

275–0.004

0.303

4.89

6.00–0.20

0.006–0.004

Semi-uniform

NBT-01

178–0.004

0.116

1.14

1.50–0.20

Uniform

NBT-02

197–0.004

0.181

0.74

1.50–0.02

0.006–0.004

Semi-uniform

NBT-03

178–0.004

0.178

1.94

3.00–0.20

0.05,0.03,0.005

Semi-uniform

KV01

205–0.004

0.227

0.20

1.00–0.08

Semi-uniform

KV02

253–0.004

0.242

8.92

12.00–0.06

0.06–0.01

Non-uniform

KV03

238–0.004

0.075

1.05

1.50–0.20

0.09–0.003

Non-uniform

Capillary water absorption characteristics

Ignimbrites expose high capillary water absorption potential (Özvan et al. 2015) due to their micropore structure. Average capillary water absorption of ignimbrites in gr/m2 in terms of the square root of the time (seconds) is shown in graph form in Fig. 7, and the capillary water absorption coefficient values are shown in Table 5, where it can be seen that the average capillary water absorption coefficients vary widely, having a range of 624.43–35.20 g/m2 s0.5. In this study, using classification system (Snethlage 2005), all ignimbrite samples are classified as “highly absorbing rocks” based on their water absorption values (w). The lowest capillary water absorption coefficients are obtained in NBT01, NBT02, and KV03, while the highest is obtained in DT01. The KV03 sample consists of small pores, ranging in size between 1.50 and 0.20 µm, its total pore volume is 0.075 cm3/gr, and its pore size distribution is defined as non-uniform. Conversely, the DT01 specimen contains uniform pores, with pore sizes varying between 9.00 and 1.00 µm. Thus, it can be concluded that the capillary water absorption behavior of ignimbrites is completely governed by the pore structure, as mentioned in several previous studies, such as the one by Benavente et al. (2001). The relation between the pore structure and capillary water absorption behavior of ignimbrite will be discussed in the following sections.

Fig. 7

Capillary water absorption curves of investigated ignimbrites in gr/m2 as a function of the square root of the time

Table 5

Capillary water absorption coefficient of investigated ignimbrite samples

Sample no.

Minimum (g/m2 s0.5)

Maximum (g/m2 s0.5)

Mean (g/m2 s0.5)

BD01

514.93

614.51

564.72

DT01

650.03

598.84

624.435

DT02

405.79

752.6

579.195

OH01

68.29

96.74

82.51

OH02

136.7

148.18

142.44

BT01

86.00

97.28

91.64

BT02

106.16

93.84

100

BT03

327.92

351.58

339.75

NBT-01

24.16

46.25

35.20

NBT-02

45.97

52.60

49.29

NBT-03

186.64

211.14

198.89

KV01

87.22

113.49

100.35

KV02

118.22

162.17

140.19

KV03

75.2

68.33

71.15

Role of direction (anisotropy) on the capillarity

Three directions (X, Y, and Z) were defined to evaluate the effect of anisotropy on the capillary water absorption behavior of ignimbrites. For this aim, only the BT-designated ignimbrite samples were able to be used due to difficulties involving sample preparation. The X direction represents the surface perpendicular to horizontal plane, while Y and Z represent the surfaces parallel to horizontal plane (Fig. 8). As seen in Fig. 8a, the BT02 and BT03 samples have similar water absorption curves for all directions. The BT01 sample, however, shows dissimilar capillary water absorption behavior; that is, the capillary water absorption behavior observed along the Y and Z direction greatly differs from that seen in the X direction. In the BT01 samples, a lower capillary water absorption coefficient is obtained in the direction parallel to the overburden pressure. Therefore, the anisotropy effect is valid for the BT01, considering the capillary water absorption variation. On the contrary, there is no distinct variation in water absorption for the BT02 and BT03 samples, indicating that these samples are not anisotropic. P-wave velocity tests were also carried out for the same samples used in capillary water absorption tests, and the average values are given in Fig. 8b. While the highest values are obtained in the Y and Z directions, the lowest values are assigned to the X direction. For the BT02 and BT03, P-wave velocities in the X direction are very close to those seen in other directions. However, the P-wave velocity of the BT01 in X direction is half of those in the other directions. Anisotropy also has a significant effect on the P-wave velocity, as on the capillary water absorption behavior. The BT02 and BT03 specimens do not show any anisotropy, while the BT01 presents anisotropy in a direction parallel to overburden pressure. As aforementioned, the BT01 sample is stratigraphically located at the top of BT02 and BT03. Thus, it was slightly influenced by the overburden pressure and it exhibits higher porosity than that of lower units (BT02 and BT02). Dinçer et al. (2016) presented a classification of welding of Cappadocian rocks based on Quane and Russell (2005). The BT01 specimen is classified as partially welded, while BT02 and BT03 are defined as moderately welded with respect to the classification of Quane and Russell (2005). Considering the results of capillary water absorption and P-wave velocity measurements, it can be concluded that less welded or unwelded ignimbrites behave in a more anisotropic manner. The interpretation of the anisotropy of ignimbrites is performed with a very limited number of samples. So, the sample number limitation should be considered during the engineering geological assessments of ignimbrites and additional research should be carried out for precise outcomes.

Fig. 8

Anisotropy of selected ignimbrites samples a variation of capillary water absorption curves, b P-wave velocity

Effect of pore size distribution on capillarity

In porous rock material, capillarity is not only related to pore size and pore shape, but also to the connectivity and the topology of the porous structure (Beck et al. 2003). Besides, the pore size is the most important parameter in terms of stone deterioration (Winkler 1997; Benavente 2011; and Török et al. 2016). Benavente et al. (2001) suggest three different porosity classes based on the pore size and forces producing the movement of the fluid in porous rocks. These are macroporosity (pore size is higher than 2500 µm), mesoporosity (pore size ranges between 0.1 and 2500 µm), and microporosity (pore size is less than 0.1 µm). While fluid mobility is mainly driven by the force of gravity in macroporosity, fluid movement is chiefly dominated by capillary forces in mesoporosity. In microporosity, the fluid motion is only controlled by adsorption forces. The pore size of the ignimbrites used in this study commonly varies between 276.0 and 0.004 µm and is classified as mesoporosity and microporosity in accordance with the mercury intrusion tests. The classification ranges suggested by Benavente et al. (2001) are very wide, and it is not enough to simply classify ignimbrites in terms of capillary water absorption. Therefore, new range and threshold values of pore size should be defined for the ignimbrites. For this aim, a total of 14 different ignimbrite samples with varying capillarity are considered in this study. As mentioned in the previous sections, pore mercury tests were performed and new parameters representing the pore size distribution of ignimbrites are defined to evaluate the relationship between the capillary water absorption and pore size distribution. The pore size distribution of samples is categorized as uniform, semi-uniform, or non-uniform, and the capillary water absorption coefficients are correlated with pore size distribution description and the range of dominant pore diameter (Fig. 9a). Accordingly, it is determined that the ignimbrites with uniform pore sizes have higher capillary water absorption potential than that of semi-uniform and non-uniform samples. The graph plotted for the dominant pore diameter range with respect to average capillary water absorption is given in Fig. 9b. As clearly seen from this figure, capillary water absorption coefficients of ignimbrites that have a dominant pore diameter between the narrow range of 10 and 5 µm (uniform) are higher than those of semi-uniform and non-uniform specimens. The highest capillary water absorption coefficients are assigned to DT01, DT02, and BD01, all of which have dominant pore diameters within the relatively narrow range of 1–20 µm. On the contrary, the dominant pore diameter of OH01 and OH02 specimens varies between the wide range of 0.01 and 30 µm, and thus these samples are accepted to have lower average capillary water absorption coefficients. Although the NBT01 specimen presents a uniform pore size distribution, the lowest capillary water absorption coefficient is obtained for this sample. This exception can be attributed to the pore size distribution of this sample being dominantly smaller than 1.50 µm. If porous rocks have pore sizes smaller than 1.0 µm, when they are exposed to water adsorption and capillarity, condensation occurs instead of capillarity imbibition (Benavente 2011). Pore size distribution, which is a function of welding degree in pyroclastic rocks, plays a significant role on the capillary water absorption capacity of ignimbrites. The samples with a uniform pore size distribution of around 5–10 µm have higher capillary water absorption potential. In contrast, the samples with dominant pore diameters smaller than 1 µm are represented by lower water absorption coefficients (Fig. 9b).

Fig. 9

Variation in capillary water absorption values a based on the pore diameter size distribution types, b based on the dominant range of pore diameter

Results and discussion

In the Cappadocia region, many historical, cultural, and natural heritage structures are under the influence of deterioration due to the moisture effects derived from capillarity. To simulate this phenomenon and to investigate the role of capillary water absorption on deterioration, a series of laboratory tests were carried out, and the test results were correlated with field observations. The obtained experimental data, along with the significant results of capillarity, are discussed in the following paragraphs.

Mineralogical and geochemical characteristics of ignimbrites may have an effect on the capillary water absorption, especially in the case of high clay content, which can cause an increase in the capillary water absorption capacity (Aydan and Ulusay 2003; Ulusay et al. 2006; Yıldız et al. 2010). Moreover, the capillary water absorption capacity is not significantly affected by the mineralogical and geochemical features of ignimbrites with low clay content. Texture and micro-structure, which are the functions of welding degree, are more effective on capillary water absorption.

Topal and Doyuran (1997) suggested that the tuffs that crop out in the Cappadocia region do not show a distinct anisotropy based on the sonic velocity, elastic constants, and linear strain values. They also pointed out that a slight change in the strength of the tuffs is present in vertical (6.53 MPa) and horizontal (4.87 MPa) directions. In this study, the capillarity potential of ignimbrites and their P-wave velocity are evaluated in terms of anisotropy. Partially welded or unwelded ignimbrites behave in a more anisotropic manner in terms of capillary water absorption and P-wave velocity. The vertical values of the capillary water absorption coefficient and P-wave velocity are lower than those observed in the horizontal direction for partially or unwelded ignimbrites. In contrast, the welded ignimbrites do not present anisotropy, as also stated by Topal and Doyuran (1997).

Capillarity is directly governed by the pore structure, such as pore size and pore shape, in porous rock material (Beck et al. 2003; Benavente 2011). Benavente et al. (2001) suggested that three different porosity classes exist, in terms of the pore size and the forces producing the movement of the fluid in porous rocks. The classification of ignimbrites in terms of capillary water absorption capacity lacks accuracy and consistency. In this study, the pores of ignimbrites are categorized as uniform, semi-uniform, and non-uniform based on porosimetry curves of cumulative pore volume versus pore diameter. The ignimbrites with a uniform pore size distribution reveal higher capillary water absorption potential than that of the semi-uniform and non-uniform samples. The capillary water absorption coefficients of ignimbrites with a dominant pore diameter between 10 and 5 µm in a narrow range are higher than those of rocks with a wide pore diameter range. Pore size distribution, which is also a function of welding degree, plays an important role in the capillary water absorption process of ignimbrites. Besides, the samples that have dominant pore diameters smaller than 1 µm present lower capillary water absorption potential.

The ignimbrites in the region have variable capacity in terms of capillarity, and high capillary water absorption potential may cause the deterioration and decomposition of many historical and natural heritage structures in Cappadocia (Ergüler 2009; Kaşmer and Ulusay 2013; Kaşmer et al. 2013). Examples of deterioration triggered by capillary action are shown in Fig. 10a–d. An approximately 1.00-m-high deterioration zone is measured at the base of the fairy chimneys’ bodies. Ignimbrites, which are host rocks for all the heritage structures in the Cappadocia, have been used as dimension stone, both in historical and recent buildings. Similar deterioration types to nature can also be observed in these structures (Fig. 10e–g). Although the campus gate of Nevşehir Hacı Bektaş Veli University was constructed with ignimbrites 10 years ago, the gate is highly defected as a result of capillary action recently (Fig. 10e). The degree of deterioration in capillary zones is higher than in immersion zones, as indicated by Benavente et al. (2001). As aforementioned, in the Cappadocia region, deterioration is mainly triggered by capillarity.

Fig. 10

General views of deterioration triggered by capillary water absorption in natural heritage structures; fairy chimneys (ad), deterioration in newer buildings (ef), capillary effect on a historical mosque built of ignimbrite (g)

Notes

Acknowledgements

We are grateful to Dr. Mutluhan AKIN and Dr. Ahmet ORHAN for their constructive scientific discussion and excellent suggestions. We would also like to thank the Tufan KOÇYİĞİT for his help in laboratory studies. Constructive review by two anonymous reviewers is gratefully acknowledged. This study was financially supported by the Scientific Research Projects Office of Nevşehir Hacı Bektaş Veli University (Project Number NEÜBAP162F14).

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Geology Engineering, Engineering-Architecture FacultyNevşehir Hacı Bektaş Veli UniversityNevşehirTurkey

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