Arab J Geosci DOI 10.1007/s12517-015-1910-8

ORIGINAL PAPER

Experimental study to evaluate the effect of travertine structure on the physical and mechanical properties of the material M. Chentout 1 & B. Alloul 1 & A. Rezouk 1 & D. Belhai 1

Received: 14 October 2014 / Accepted: 1 April 2015 # Saudi Society for Geosciences 2015

Abstract Geology, hydrophysical properties, and mechanical behavior of two main varieties of ornamental stones in Algeria have been studied in detail. Travertines and onyx in western Algeria are summarized to two large layers, the Bouhanifia and the Ain Takbalat deposits. Their formation is related to the hydrothermalism following the great fault of Tafna. The travertine formed in water overflow while onyx is set up along the lines of outputs of the sewage of dissolved CaCo3. The results obtained show a good correlation between petrography and the mechanical behavior; the bedding planes and laminations of the travertines give a petrophysical anisotropy across the sample. Two types of mechanical testing: compressive and flexural strength were carried out either parallel or perpendicular to the veins. This test showed that mechanical behavior is guided by the nature, thickness, composition, and orientation of the veins. Keywords Travertine . Petrography . Structure . Petrophysical anisotropy . Mechanical behavior

Introduction The mineralogical composition and structure are closely related; they determine the mechanical properties of rocks. To

* M. Chentout [email protected]; [email protected] 1

Laboratoire de Géodynamique, Géologie de l’Ingénieur et de Planétologie (LGGIP), Faculté des Sciences de la Terre, Géographie et Aménagement du Territoire (FSTGAT), Université des Sciences et de la Technologie Houari Boumediene (USTHB), B.P. 32 El Alia, Dar el Beida, Alger 16111, Algérie

understand and define this relationship, two travertines and two onyxes were investigated in this study. These stones have been chosen for several raisons. They are the principal varieties of ornamental stone in the western of Algeria. They are widely used in construction as well as imposed for restoration of many historical monuments. In addition, travertine is a common building and dimensional stone that has been explored and used in many countries (Benavente et al. 2008; Molina et al. 2013; Vazquez et al. 2013). The main Algerian travertine production originated in western Algeria, with two main quarries: Ain Takbalat and Bouhanifia (Tawab 1970). These two quarries are thought to have been used in ancient times. So, they have been heavily exploited in the Middle Ages (Playfair 1895), and again at 19 century, known then as the “Algerian Onyx”. The term travertine is generally used to refer to carbonate rocks formed on the continent (Sanders and Friedman 1967) and consisting of calcite or aragonite. Then, the definition of travertine as sedimentary rock formed by precipitation of CaCO3 from waters saturated with Ca (HCO3)2 is not unequivocal without taking into account the origin of these waters. Depending the training process and the origins of the water, two groups are distinguished (Pentecost and Viles 1994). (i) Meteogene Travertine: these travertine types are normally deposited in cold water springs in areas underlain by carbonates. In some cases, they are formed by evaporation, through the degassing of CO2, so their formation is closely linked to climatic factors. (ii) Thermogene Travertine: this stone type forms at high temperatures, from rich sources of dissolved Ca (HCO3)2, that emerge along active faults in active tectonic setting. Travertine deposition is induced by the loss of carbon dioxide (CO2) through the reaction: H2O+ CO2+CaCO3 ↔Ca (HCO3)2. Another way to form is decarbonation, with heating of carbonates includes clay and other minerals. Several authors reserve the term travertine to the

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second category, while meteogene travertines are called calcareous tufa or crusting. Regarding to the physical properties, thermogene travertines are more durable and long lasting compared to meteogene travertine porous that is friable and contains fossil rests and plants. Various authors gave very different descriptions of some travertine properties, i.e., some authors wrote about travertine as a porous stone (Isık and Ozkahraman 2010; Erdoğan 2011), while others that it is compact with massive structure (Angeles Garcia-del-Cura et al. 2012). Compressive strength is the most important property to characterize the mechanical behavior of building stones (Murat and Hurriyet 2013). The mechanical behavior of rocks is closely related to lithology (porosity, microcracks). Over the last few years, several authors (Barruol and Kern 1996; Karpuz and Paşamehmetoğlu 1997; Kahraman 2001, 2007; Goudie 2006; Del Rio et al. 2006; Kahraman and Yeken 2008; Çobanoglu and Çelik 2008; Török and Vásárhelyi 2010) studied the relationship between lithology and the physical and mechanical properties. The effect of grain size and shape (Haney and Shakoor 1994; Wong et al. 1996; Přikryl 2001), porosity (Hatzor and Palchik 1997; Sabatakakis et al. 2008), and mineral composition (Tsiambaos and Sabatakakis 2004; Sabatakakis et al. 2008) on strength and crack propagation were analyzed for many lithologies, but few data available for travertine and onyx. Despite their intense use in several areas such as construction, decoration, and restoration, few studies have been conducted to evaluate the influence of bedding planes and veined structure on the mechanical behavior. This study has two main objectives: (i) to characterize four varieties of travertines, from a geological point of view so many geological perspective and the determinations of their engineering properties. (ii) Focusing on the mechanical properties, this paper tries to identify the relationship between the

structure of these travertine and their mechanical behavior. These veins rock give a unique characteristic pattern but also to ensure a uniform appearance across anisotropy block. This study contains the investigation of petrographic properties, physical; density, porosity, water absorption, and undestructive tests; ultrasonic pulse velocity, Schmidt hammer hardness, mechanical properties including compressive and flexural strength.

Fig. 1 a Column of Ain Takbalat Travertine, a laminated massive variety, late to time of Zianide (1235–1556) (Musée d’art et d’ Histoire de Tlemcen). b Column shaft of polished alabastro a pecorella, with

clouded structure, 1st–3rd century (Musei Capitolini, Rome). c Column of alabastro a pecorella flanking niche, triumphal arch, Timgad, late 2nd century

Historical background Travertine is a common building and dimension stone that has been explored and used in many countries. The use of travertine as building, constructive, and decorative element dates back to ancient times. There are evidences that the quarrying activity in the Ain Takbalat and Bouhanifia areas had begun during the reign of the Roman Empire, and that these stones were extensively used. Several remains have been spotted in various monuments around the Algerian territory, including the Museum of Djemila, Cherchell, and Tlemcen (Fig. 1a) attesting the extensive use of these two varieties. These varieties are extensively used especially in Italy known by other names. An alabastro pecorella, an ornamental stone, with vivid red and white marks with subordinate brown and yellow areas, and characterized by clouded structure, has been extensively used in ancient Roman cities. This stone was highly prized for decorative panels in the Renaissance and Baroque periods (Gnoli 1988; Lazzarini 2009). A spectacular example is an ancient column shaft in the Museo Capitolino, Rome (Fig. 1b) (Bertoletti 2002). In 1971, Raniero Gnoli recognized that the stone came from Algeria and ascribed it to the quarries of Bouhanifia in western Algeria. His views have been followed by scholars

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ever since (Gnoli 1997; Lazzarini 2002). In the effort to understand Algeria’s place in the international marble trade, these authors made trips by to Algeria in 2005, 2006, and 2008. The quarries visited were primarily those managed by Enamarbre (Entrerprise Nationale du Marbre, Spa). A study of stable isotopes of carbon and oxygen confirmed the origin of this ornamental stone. Gsell (1895) reports, according to Playfair (1895), the use of these travertines in the ruins of the Capitol Timgad. Also, fragments of alabastro a pecorella veneering were incorporated in the floor of the Timgad Museum (Fig. 1c). In addition to this, Dubois (1908) mentioned in his book that at Louvre Museum, there is an inscription made of translucent onyx quarries that the Vésinet Company operates at Ain Takbalet. This inscription was carefully described by the same author «This marble, coarsely veined with red, cracked in several places, unsuitable for fine sculpture, is a piece of waste that would have thrown in a ravine without registration that covered one of its sides. On this side, the block was polite

if not, at least somewhat worked, so as to eliminate roughness and obtain an almost flat surface. An outer bead was alone maintained. It forms a kind of frame that contains the text. The entire stone is 55 cm long and 30 cm wide; the covered part of writing 33 cm×20 cm Registration, writing mixed with uncial and cursive is arranged into 3 slices on vertical sections, we have not yet been able to decipher».

Fig. 2 a Simplified geological map of western Algeria showing the tectonic sketch of the Western Tell Atlas. b Extract the Beniskrane map 1/50.000, showing the most important outcrops of travertine. c Geology

of the region of Bouhanifia, drawn from the map of Mascara 1/50,000 and field observations

Geology of the deposits The studied travertines come from the Tafna basin which is rich in thermal spring water. These sources, six in number, may be cited among the curiosities of Algeria. All travertine listed in western Algeria (Fig. 2a) are formed by extensive hydrothermal activity, whose sources are currently dried up and deposited in relatively long trails on all surfaces at least flat, and most likely in valleys or depressions, is now formed on top of hills due to ablation of the surfaces that had limited

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their gully and the more or less deep. Travertine deposition from these sources is still present on many points. In this study, we focused on two main deposits, the Bouhanifia and Ain Takbalat deposit; the geology of these main deposits is as described below. a. Ain Takbalat deposit: these travertine outcrop widely in the region between the Traras and Tlemcen mountains (Fig. 2b). This travertine characterized by its compactness and calcium veins of different colors (white, pink, brownferrous). Its peculiarity is the presence of geodes filled with calcite. This deposit is represented by Quaternary lacustrine limestone deposited on the Neogene—Quaternary in Tafna basin. The geological setting and travertines deposition are shown in (Fig. 2b). They are thermogenes generated by hot water springs associated with hydrothermal activity along the Tafna fault. It is represented by thick deposits of travertine in an extremely varied range with brighter colors (gray, cream, and yellow), onyx, and dolomite. The quarry rock face shows some variety in appearance with a clear grain, dark or less marked. It has by a simple subhorizontal lode in conformity with the enclosing rocks; visible thickness is 6–7 m. One of the features of this layer is the typical horizontal banding and alternation in some places of travertine and onyx, offering a decorative appearance in which colors vary dramatically. Cavities of some millimeters in length and depth are observed on the rock face, corresponding either to the calcite dissolution or clay leaching. Rarely found separated, Onyx is often confined to the travertine layers as veins, varying in thickness from a few centimeters to sometimes even 1 m. The deposit is still active by the National Company of granules (ENG). The extraction is done by a series of modern tools (Fig. 3a). b. Bouhanifia deposit: it is Eocene and vein type, consisting of deposits of travertine and onyx, which are sometimes difficult to separate during machining. The travertine deposits that occupy large areas in Bouhanifia depression (Fig. 2c) are the result of the bicarbonate hot springs deposits. Their thickness exceeds 30 m. The deposit is confined to onyx travertine surrounding the Fig. 3 a Quarry of Ain Takbalat still active by the ENG. b A sector of the quarries of Bouhanifia showing ancient abandoned blocs

Bouhanifia hydrothermal station. Their location at the terraces notes that they likely were formed simultaneously with the terraces. The deposition of CaCO3 in these forms is due, firstly, to chemical and physical characteristics of the warm waters rich in bicarbonate (ion concentration of CaCO3, Mg, Fe, temperature, and pressure) and, secondly, to the state of the deposit environment (liquid medium, turbulent, quiet, deposition surface…) and defining multiple structures; massive, recrystallized, colorful, and clouded. The deposit is confined to onyx travertine surrounding the hydrothermal station of Bouhanifia. They were deposited in approximately parallel veins with NE–SW orientation, corresponding, probably, to bicarbonate water output lines to the surface. Apart from these veins, some onyx veins were identified, which are difficult in most cases to separate from the travertine. The Bouhanifia area has two travertine quarries, Sidi Sliman and Douar Krerma (Fig. 2c), oriented NE–SW, respectively. Both deposits correspond to small hills separated by the Hammam valley. The Bouhanifia quarry was active until recent times, managed by Enamarbre (Entrerprise Nationale du Marbre, Spa). But nowadays, it has been abandoned due to the urban growth of Bouhanifia (Fig. 3b). The field work revealed anisotropy in both Ain Takblat and Bouhanifia quarries, where color and structural transitions in both vertical and horizontal directions are obvious. Different colors, especially white, grayish white, beige, yellow, yellow brownish, and red were found. This means that several colors and textures can be found within the same block. Considering the described differences in structure and color, there are many travertine varieties such as vuggy travertine, massive travertine, and red travertine commonly called “Marbre rouge”.

Experimental program Samples We selected four stone types that represent the principal varieties used in the ancient monument in the western of Algeria.

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The samples were selected and monitoring during their extraction and preparation in the quarry in order to maintain the same orientation during our studies, specially, the mechanical behavior. The geographical and geological situation of the samples is as shown in Fig. 2. Table 1 summarizes the mineralogical composition of each sample in percentage. Ain Takabalat Travertine “TA” It presents a heterogeneous aspect with a multitude of colors due the numerous veins. The geotechnical behavior of these rocks is commonly related to its stratigraphical position and controlled by their petrography (Fig. 4a), textural and diagenetic features. This travertine is largely used in this region. It comes in a many variety of color. These colors range from a very light white to a deeper beige color. Ain Takbalat Onyx “OA” It is quarried in the same quarry and marketed as small slabs with a thickness ranging from 2 to 10 cm (Fig. 4c), highly recommended for indoors decoration. This stone is widely used all over the inner coating of the mosques of the region. Bouhanifia Travertine “TB” It comes from the ancient quarry of Bouhanifia and it is located in the north of the Bouhanifia depression. This stone dates from the Eocene age and has been quarried and used especially for the touristic purpose, i.e., in the restoration of monuments, which are today in a dilapidated state. It has a concretionary aspect, at least vacuolar coarsely bedded, characterized by its unique clouded structure (Fig. 4e) and it appears weathered in outcrop.

Methods The petrological examination of the samples was done by means of polarized light microscope. In order to quantify the porosity, the slabs were impregnated with resin, then the pore system will be filled by the resin. The physical and hydric tests were carried out on six cubes of 5×5×5 cm for each stone type. Apparent density and open porosity were obtained. For mechanical characterization, both destructive (uniaxial compression and flexural strength) and non-destructive (ultrasound and Schmidt hammer hardness) tests were carried out. After all the tests, the average values for each sample are recorded along with standard deviations. Apparent density “ρa” (NF B 10–503) Apparent density “ρa” (NF B 10–503) of a rock is defined as the ratio of its mass to its volume, including the volume of voids and grains. It was calculated using geometrical methods based on the volume of the specimens.
ρa ¼ mo =ðms −mh Þ*pa t=m3 where mo is the dry weight of the specimen, ms is the saturation weight, mh is the hydrostatic weight, and pa is the water density. Open porosity “po” (UNE-EN 1936) The porosity is mathematically derived from the test for the determination of the apparent density and expressed in volume %. It represents the percentage of open spaces in the stone. It was calculated using the hydrostatic method. po ¼ ððms −mo Þ=ðms −mh ÞÞ*100 ð%Þ

Bouhanifia Onyx “OB” It is often found associated with mass travertine with a solid, compact appearance, and dirty white color (Fig. 4g). It is considered as an ornamental stones known as “Marbre blanc” because of being affected by red veins. The natural veining of this variety can form different patterns that give an interesting look to the stone.

Schmidt hammer hardness Schmidt hammer rebound tests were applied on the rock blocks having varying dimensions between 2 and 6 m3. The tests were performed with an N-type hammer with impact energy of 2.207 J. ISRM suggested that 20 rebound values from single impacts separated by at least a plunger diameter should be recorded, and the ten highest values were averaged.

Table. 1 Petrographic synthesis of the four ornamental stones expressed in percentage

Ultrasonic pulse velocity (NF P94-411 2002) The transmission method was used, which consists of two piezoelectric sensors coupled to the sample at constant pressure. One of the transducers is stimulated using an ultrasonic pulser, and the other is used as a receptor sensor. Therefore, this method measures the propagation wave characteristics induced by microstructural factors. The pulse velocity is given by the formula:

Carbonates

Silicates

Clay minerals

Stone

Cal

Dol

Q

Mica

K

TA OA TB OB

80 75 90 >85

T 10 5 T

1 2

T

5

–

T

C

5 5

Io

T

6 6 5 7

Cal calcite, Dol dolomite, Q quartz, K kaolinite, C chlorite, Io iron oxide, T traces

Vp ¼

L T

ðm=sÞ

where Vp is pulse velocity (km/s), L is path length (cm), and T is transit time (μs).

Arab J Geosci Fig. 4 Petrography of the four ornamental stone studied. a Macroscopic aspect, sample of Ain takbalat travertine with vuggy and massive structure. b Microscopic aspect showing a recrystallized massive banded structure. c Recrystallized banded Onyx of Ain Takbalat. d Microscopic aspect of BOA^ showing calcite pigmented by the iron oxide. e Macroscopic aspect, red travertine with clouded texture of the BTB^. f Microscopic observation reveals microcrystals of calcite. g Macroscopic aspect, massive translucent Onyx of Bouhanifia BOB^. h Microscopic observation of OB

Compressive strength (UNE-EN 1926) (Anon 1999) The uniaxial compressive strength test is used to determine the maximum value of stress attained before failure. For this experiment, 10 cubic samples were cut with stone saw machine. The load applied was 0.6 Mpa/s. An Ibertest MEH-2000 H/ FIB-50 machine was used. Two types of tests were carried out either perpendicular or parallel to the veins. The faces subjected to compression were prior corrected so as to make planar and parallel. The compressive strength Cs is expressed as:

Rc ¼

F S

ðMpaÞ

where Rc is the uniaxial compressive strength (MPa), F is maximum failure load (N), S is the section area of specimen (mm2). Flexural strength (UNE-EN 13161) (Anon 2002) The flexural strength is the ability of a rock to withstand bending or flexural (Winkler 1997); it is considered as an indirect

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OB: This onyx sample consists of 98 % calcite with angular grains, fibrous by well oriented place; it has microcrystalline texture (Fig. 4h). Fine cracks were observed corresponding to the crystal boundaries; they are filled by secondary calcite. The porosity observed on this sample is of the dissolution, corresponds to the dissolution of calcite.

measurement of tension stress (Jaeger and Cook 1976). The sizes of test specimens were 40×40×160 mm. The load applied was 0.12 Mpa/s. The length between two supports was 100 mm. Failure load was recorded as F. The flexural strength of stones was calculated by the following equation: 3 FL Rt f ¼ ðMpaÞ 2bh2 where Rtf is the flexural strength of stone (MPa), F is the failure load (N), L is the length between two supports (mm), b is the width of the sample (mm), and h is the height of the sample (mm).

Results and discussion Petrographic characterization Table 1 shows the mineralogical composition of the stones in percentage. From the petrographic point of view, they consist mainly of calcite, clay minerals, and iron oxides. The examination of the samples using a polarized light microscope showed the different textures of the rocks (Fig. 4): TA: The microscopic studies revealed that is recrystallized limestone in which the sparite size calcite crystals well packed; these calcite crystals are well packed similar to granoblastic texture in marbles (Fig. 4b). It contains 1 % of insoluble residue composed mainly of quartz. Is has an anisotropic crystalline texture with bands of different colors and composition. There are also areas with dark crystals due to plant precursors and Mg and Fe oxides/hydroxides. A porosity of dissolution has also been observed at the edges of the bands. OA: The microscopic observation of this stone shows that is consists of calcite in which the crystals are well packed together (Fig. 4d); it contains few rhombohedral crystals of dolomite surrounding by the calcite thus forming acute angles. We note also the existence of several generations of iron oxide; few fibrous crystallization with heterogeneous sizes which are perpendicular to iron oxides are observed. TB: This travertine has a fairly high degree of compactness, a microcrystalline structure in general (more widely crystallized bands 0, 5 to 1 m); there is predominantly calcite pigmented by an iron oxide (Fig. 4f); there are also areas with dark crystals due to Mg and Fe oxides/hydroxides. It is natural for some clay minerals to be present in this type of rock. But it is not possible to define these clay minerals as modal due to their small sizes under microscope; they occupied the fractures. Polishing can give good results for this type of well-packed recrystallized limestone.

The crystals and the relative orientation of the veins of these varieties allows to define two main textures, a veined structure with horizontal to subhorizontal bands (TA), and a multicolored clouded texture, very characteristic of the TB. The mineralogical analyses have shown that there is not a significant difference in the composition of the two travertines. The major mineral of both travertines is calcite with a proportion of 85–98 %. Accessory minerals are represented by quartz (2 %) that were found in some of the samples and by iron oxides observed in TB. Physical properties As already mentioned, the physical and mechanical properties of natural stone narrow its use as building material. Table 2 shows the physical characteristics. The apparent density is one of the most important physical properties of the rocks; it is controlled by texture mineral composition and mineral density. The four stones showed few variations in density. Values ranged from 2.46 to 2.66 t/m3. OA has the highest apparent density, as might be expected due to the high density of dolomite (2.86 t/m3). TB showed the good value explained by the presence of iron oxides. The travertines and onyx have low porosity with values ranging from 1.5 to 4.28 %. The petrographic investigation revealed two types of porosity: a moldic porosity produced by the dissolution of some allochems and with irregular microchannels of stylolites, and a non-localized porosity in which pore size varies considerably. Of the four stones studied, TA gives the highest value (4.28 %) due essentially to dissolution of calcite. Water transport in travertines and onyx was quantified by the water absorption, Ab. The values are varying from 0.91 to 1.75 %. Water flow is conditioned by rock structure, pore size, Table. 2

Hydrophysical characteristics with standard deviation

Stone

ρa (T/m 3)

po (%)

Ab (%)

Sa (%)

TA OA TB OB

2.46±0.06 2.66±0.05 2.61±0.05 2.63±0.06

4.28±2.6 1.5±1.15 3.53±0.32 2.05±4.17

1.75±1.14 0.98±1.16 0.91±0.08 1.01±1.04

0.62±0.75 0.74±0.15 0.68±0.08 0.81±0.80

ρa apparent density, po open porosity, Ab absorption, Sa saturation coefficient

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and pore connectivity of vuggy porosity. As would be expected, the crystallized facies of TA show the highest value of Ab (1.75 %) due to the fact that it has the highest porosity and the separation lines between the different veins correspond to channels thus promoting the circulation of the water. Samples of TB recorded the lowest values of Ab, which can be explained by the non-connection between the porous system. The result of saturation coefficient “Sa” are very similar ranging from 0.62 to 0.81 %. Mechanical behavior All the mechanical results obtained are in the Table 3. Rebound index RI expresses the ability of thin slab to withstand a brutal shock on the surface. Tests conducted on this group of sedimentary rocks (travertine and onyx) revealed an average RI (<40), which implies a strong absorption of the impact energy of the stone surface. Indeed, the RI values were strongly reduced when the impact direction was normal to such planes as they absorb impact energy. The repetition of the tests at the same location on the onyx surface caused a spray of mineral matter, showing a rapid decrease of RI after each test. The values of P wave velocity “Vp” are largely influenced by the test direction parallel or perpendicular to veins (Fig. 5); for all the samples tested, six per stone, the Vp is bigger when the test is performed parallel to the veins; in this case the Vp values are varying between 4450 and 5975 m/s; the highest value obtained is attributed to OB (5975 m/s), which can be explained by the fact that it is constituted by calcite well packed by the clay cement and iron oxide. The TA gives the lowest value (4450 m/s) mainly due to its porosity. The value of the Vp realized perpendicular to the veins are ranging from 3280 to 5423 m/s. The analysis of Tables 2 and 3 shows a very clear proportional relationship between the P wave velocity Vp, rebound index (RI), apparent density ρa and open porosity po. The values of Vp and RI increase significantly with decreasing Table. 3 The average mechanical properties of the four ornamental stones studied with standard deviations Stone

TA OA TB OB

IR

23±4 30±3 38±7 42±3

Vp

CSS

Rtf

//

┴

//

┴

4450±60 5055±395 5426±1 5975±4

3280±2 4940±13 5234±4 5423±6

39±17 46±2 61±20 75±77

22±10 45±2 58±10 74±13

5.0±2.3 4.4±4.0 7.9±1.1 9.6±1.3

RI rebound index, Vp P wave velocity (m/s), CSS compressive strength resistance (Mpa), Rtf flexural strength (Mpa) Test done // parallel to veins, ┴ perpendicular to the bedding planes and veins

porosity, and they increase with increasing density; this allows to conclude that these physical parameters are directly related to the nature of the material. The evaluation of the physical properties of rocks can be a simple way to assess their quality and can assist with the interpretation of the results achieved by mechanical characterization. Previous studies have shown that mechanical properties such as compressive strength and elastic modulus are dependent on porosity and density. The dependence of the compressive mechanical properties on the physical properties of rocks has been reported by several authors. This trend is confirmed by the data published in the bibliography. Roman lowporous travertine studied by Jackson et al. (2005) presents porosity values of 0.8 % and strength values of 105 MPa. Yagiz (2006) studies travertines with porosities of 4 %, with a measured strength of approximately 70 MPa. Török (2006) reports Hungarian travertines with porosities between 2.11 and 13.13 %. The strength values registered at these extremes are 103.17 and 30.29 MPa, respectively. Travertine samples studied by Demirdag (2009) show a porosity value of 8 % and a strength of 52.3 MPa. Angeles Garcia-del-Cura et al. (2012) studied three varieties of Spanish travertine characterized by a porosity of 11.82, 6.69, and 4.48 %; the value of compressive strength is 36.82, 41.89, and 49.48, respectively. These data fully coincide with the values obtained in this study and confirm the trend observed. In general, increasing porosity is associated with decreasing compressive and tensile strength and a lower modulus of elasticity. This behavior is to great extent related to the higher heterogeneity and presence of weak bonds such as pores, voids, and microcracks in very porous rocks. In the field of materials, especially the rocks, the mechanical behavior and the porosity are closely related (i.e., references). This relationship shows an exponential trend, i.e., minimal changes in the porosity generate remarkable changes in mechanical behavior and particularly in compressive strength. Porosity is the most important factor controlling the mechanical behavior of rocks, indeed, fracture tends to propagate using the pore space rather than creating new fractures (Chentout et al. 2014). In general, in mechanical properties, the differences in composition are less important than those in texture (Vazquez et al. 2013). The most accepted criterion for anisotropic rocks, namely travertines, is that the highest strength is registered when the sample is loaded perpendicular to veins. However, the results obtained for these two varieties of travertine led to three major remarks. (i) The CSS conditioned by the nature, thickness, and orientation of the veins and the loading direction (┴ or //). This is due to the fact that discontinuity surfaces between bands work as weakness planes if they are orientated parallel to the load. (ii) Travertine predominantly micritic is less resistant; it is the case of TA. (iii) The TB of clouded structure containing a percentage of iron oxides

Arab J Geosci Fig. 5 Verification of anisotropy on onyx (OA). a test done // to veins; b test done ┴ to veins

The Young’s moduli were determined from the compressive strength test. Characteristic axial stress—axial strain curves are plotted in Fig. 6. It can be seen for all the ornamental stones studied a concave curve portion before becoming linear. This phase, called clamping, reflects the closure of microfissures and vugs oriented orthogonally to the loading. The shape of the graphs turns into a curvilinear shape just before failure; this means that the deformations are not linear and the stresses are not proportional to the deformation. The moduli are estimated approximately to 10 % of stress. Studies carried out on travertines showed that the travertines with low porosity offer the more rigid behavior. This relationship was not observed in the two varieties tested; the OA have the lowest porosity but also the lowest young’s modulus. The shape of the graph demonstrates, once again, the heterogeneity of these stones, due to the veins composition.

gives very good results (CSS=75 MPa). To sum up, the influence of veined structure and nature is very important; the presence of veins weakens the material; the first cracks appear follow this weakened lines. In accordance with (Coca and Rosique 1992), the value of the flexural strength is ten times much lower than the compressive strength, and the failure load appears to be strongly influenced by the test direction and the mineralogical composition. Only one type of test is performed, perpendicular to the veins. Flexural strength test was impossible to carry out parallel to the bands; the sample broke with the only contact of the arm and without applying a load. The values obtained are ranging from 4.4 to 9.9 Mpa. The OA gives the lowest value, probably due to low resistance to flexural strength of calcite. The TB gives the highest value; the iron oxide offers to the stone a good compactness and a good mechanical characteristics. Fig. 6 Graph stress—strain, test done // to veins, asterisk indicates abrupt rupture during loading

50

(OA) * E= 1,92 Gpa

40

(TA) E= 9,06 Gpa

15

Stress (Mpa)

Stress (Mpa)

20

10 5

30 20 10 0

0

0

0.002

0.004

0

0.006

0.004

0.006

0.008

Strain ( )

Strain ( ) 51

101

(OB) * E= 2,61 Gpa

41

81

Stress (Mpa)

Stress (Mpa)

0.002

(TB) E=4,39 Gpa

61 41

31 21 11

21

1

1

0

0.005

0.01

Strain ( )

0.015

0

0.005

0.01

Strain ( )

0.015

Arab J Geosci

Conclusion This paper studied the geology, petrography, physical, and mechanical behavior of four main ornamental stone in Algeria. From a geological point of view, they are thermogene travertines associated with onyx. The deposition of CaCO3 in these petrographic forms is due to chemical and physical characteristics of bicarbonate warm (ion concentration of CaCO3, Mg, Fe) and to the state of depositional environment, giving and defining multiple structures: laminated, crystallized, and clouded. These travertines were taken as ornamental stones because they have a nice coloristic appearance, and can be extracted by blocks with large sizes, especially in shaping as substitution stones. These travertines are widely used in the domestic construction industry for internal and external use and also exported abroad in huge quantities as blocks, slabs, and tiles. They have varying colors from white, yellow, red, and brown. These travertines have a fairly degrees of compactness, a microcrystalline structure generally more widely crystallized bands. From the petrographic point of view, they consist mainly of calcite and iron oxide with percentages can vary significantly across the same sample. Both TA and TB travertine are affected by numerous fractures that are either open or sealed by iron oxides and clay minerals. Travertines are classified as heterogeneous rocks; a single block can contain several different textures. We carried out a petrophysical and mechanical characterization of travertines. This experience has allowed us to set the relation between their petrographical and physico-mechanical properties. The various results obtained for travertines and onyx led us to arise some remarks and especially of the relations between the petrography of the samples and their characteristics and thus what will influence their field of application: –

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The apparent density is related to the porosity which controlled by grain size and the type of cement between them. For instance, the OA gives a high apparent density due to percentage of dolomite. The influence of massive banded and clouded structures on Vp is very important. The ultrasonic response of travertines is also highly sensitive to porosity. To sum up, a simple analysis of the results brings out two main conclusions. Vp increases with decreasing porosity. Vp values are related to the sample orientation, the highest values were reached when the test was performed parallel to the veins. The study of the mechanical behavior conducted on this category of rocks has clarified the relationship between lithology and results, the mechanical properties of travertine depend on the relative orientation of the veins relative to the loading direction. However, the results obtained in this study, in which two main varieties of travertine and two onyx associated with them were analyzed, show that

resistance is much greater when the test is performed perpendicular to bedding planes and veins. This is due to the fact that the discontinuity surfaces between bands work as weakness planes if they are orientated parallel to the load.

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