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“Bianco di Asiago” limestone pavement – Degradation and alteration study
Construction and Building Materials 24 (2010) 686–694
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
‘‘Bianco di Asiago” limestone pavement – Degradation and alteration study Vera Pires a,*, Z.S.G. Silva b, J.A.R. Simão b, C. Galhano b, P.M. Amaral c a
FrontWave – Materials Engineering, Taguspark, Núcleo Central 389, 2740-122 Porto Salvo, Portugal Centro de Investigação em Ciência e Engenharia Geológica, Departamento de Ciências da Terra, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c Department of Materials Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b
a r t i c l e
i n f o
Article history: Received 24 July 2008 Accepted 28 October 2009 Available online 30 November 2009 Keywords: Degradation Alteration Limestone Pavement Capillarity Efflorecences
a b s t r a c t Pavement tiles made of a micritic limestone were set on a residence building and shortly after its application signs of degradation were detected (efflorescences) on the tile surface. The limestone contains irregular patterns due to rock cutting across stylolites. These decorative features represent ideal roads for fluid circulation through the rock where reactions between minerals and fluids (essentially water from the mortar) occurred. Rock analyses obtained from microscopic petrography, SEM, EDS and XRD emphasize the high calcite content of the rock and the variable composition and porosity of the stylolitic zones. Results show the importance to acknowledge the rock structure before setting these natural products as construction materials. In the present work, the need for surface treatments was found important to avoid water arising by capillarity from the pavement substrate, and hence contribute to avoid the rock weakness. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Limestone has been used as a construction material since historical times. It has been applied as facing stone in many types of buildings e.g. churches, monuments, palaces, etc., due to its pleasant light colours and ability to cut and shape in various forms and designs [1]. Limestone is the most abundant of the non-clastic sedimentary rocks on the Earth’s crust. A large proportion of limestones are formed by the mineral calcite – calcium carbonate (CaCO3) and clasts (including tiny shells and micro-skeletons) deposited on the sea bed. Other types of limestone are formed by precipitation of calcite, after dissolution of previous limestone, like travertine, which is also a common rock used as building stone. They are texturally and structurally extremely diverse, as a result of different environment depositions and origins [2]. The principal aspect which unifies carbonate rocks is their high reactivity to acids. Wine, carbonated drinks, fruits and fruit juices, vinegar, and even some natural waters, all will react with carbonate rocks. Another essential characteristic of limestones is their softness relative to other rocks such as silicate rocks. The softness is mainly a function of the mineral composition, calcite, whose hardness is 3 in Moh´s scale (1–10) [3]. A physical feature of limestones which is technically important is their porosity. Many limestones, particularly the biogenic ones, * Corresponding author. Tel.: +351 938 436 402. E-mail address: [email protected] (V. Pires). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.10.040
have a medium to high porosity. Nonetheless, although being technically weak and very absorbent, certain construction techniques (impermeability techniques) allow them to be used successfully and effectively. In cold climates, however, a porous limestone can suffer rapid degradation due to freeze–thaw alternate cycles and some protection might be required in order to assure its long life [3]. One important structural aspect inherent to many limestones is the presence of very fine lines, usually brownish in colour, which are pressure-solution features formed during the compaction and lithification. These structural aspects are denominated stylolites. The brownish colour is mostly due to hydrated iron oxide, but it can be also caused by concentrations of clays and/or sulphide minerals. Due to the fact that these stylolites are natural planes of weakness and can easily act as pathway for fluid circulation since they usually are not fully closed, any expanding clays present in the rock can react with fluids and physically weaken the limestone [3]. Rock decay is a dynamic process where the entropy of the system (the rock) increases with the increasing disorder of their phases (the minerals) [4]. This process produces mineralogical breakdown by means of crystalline lattice disruption enabling ionic migration to build up new materials. These new materials are now in thermodynamic equilibrium with the new environmental conditions. In order to have a comprehensive idea of the decay process involved in this case under study and to find solution for minimising the degradation effect, it is very important to acquire specific understanding of the types of damage and the appropriate mechanisms set
V. Pires et al. / Construction and Building Materials 24 (2010) 686–694
up in order to slow down the process. Durability is the most important characteristic to consider when selecting materials, whose use impose constrains whenever vulnerability to alteration is effective. The durability of a material is the property that has to stand the effects caused by time once installed, that is to say, it is the capacity to resist the action of the degradation agents, either from chemical, organic, physical or mechanical origin [5]. The main degradation agents which easily may affect limestone applied in interior floors can be summarised as: soluble salts and water [4]. 1.1. Soluble salts Salt crystallization is one of the most common decay processes that affect materials such as rocks, ceramics and concrete. It generates pressure variation, depending on the size and type of the rock pore. Soluble salts (hydrous and anhydrous forms of carbonates, chlorides and sulphates of Ca, Na and Mg) form due to reactions between the rock components and fluids which circulate through the rock. These salts have strong effect on the degradation of rocks, their presence is often detected by a sugary white coating (efflorescences) on the surfaces and once they crystallize, salt crystals within the rock (subefflorescences) can promote further decay. Spontaneous crystallization of a soluble salt is a function of concentration in solution and temperature [4]. Theoretical calculations of growth pressure have been studied for crystal growth in single pores and fissures [6]. Crystal growth within pores and fissures creates maximum pressure when a large crystal grows in a pore with small entries as it cannot penetrate to the surrounding small pores until high saturation is achieved, producing great stress which in turn damages the material [6].
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the stylolites surfaces). Along these ‘‘lines”, white efflorescences developed as the tiles were set. Several samples were collected for analyses of both types: sound and altered tiles (see Figs. 1 and 2). 2.1. Floor structure The floor structure on which the tiles were set is schematically shown in Fig. 3. One should emphasize that: No waterproofing product was applied on the inside face of the ‘‘Bianco di Asiago” (BA) tile.
Fig. 1. Example of a disaggregated BA tile collected form the bathroom floor.
1.2. Water Frozen water within the pore-spaces promotes expansion that can impart a force which is often superior to the tensile strength of the rock. This effect will originate a localised stress concentration field that will probably originate the occurrence of critical flaws, which will accelerate rock disruption under any of loading condition. Water may also disaggregate rock through its ability to dilate, by osmotic absorption, water-imbibing minerals and/or through hydration of expandable clays or anhydrous salts. Water not only dissolves hydrates and hydrolyzes minerals, but also can transport reagents capable of oxidizing and reducing atoms in the minerals. Water may also introduce soluble minerals in solution which, on precipitation, disrupt the fabric [4]. Depending on the type of construction and on the particular environment in which it is inserted, the rates of thermal expansion and contraction of limestone tile (typically in a range of thicknesses between 2.5 and 3.0 cm), vary substantially from those expected in thicker specimens; this parameter should be taken into account. Special attention should be given to the drying rates of the other construction materials such as mortars, glues and concrete.
2. Case study Limestone tiles were set on a bathroom floor in a residence building and in a very short time after the application (approximately 1 month) several degradation signs were observed on the tiles surfaces. The material itself is a fine limestone having thin irregular dark patterns due to the way the rock was cut (crossing
Fig. 2. Non-altered (sound) BA limestone tiles: (a) cross-section; and (b) top view.
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Fig. 3. Illustration of the floor constructive solution.
A white cement–glue was used to set the tiles on the pavement. After the setting, no finishing treatment or product was applied on the visible face (place where the alterations start to occur) of the BA tiles. There are no references concerning the mortar drying times. It is assumed regular drying times as usually done in similar cases for home construction. Concrete was placed on the top of the pipe system in order to flatten the pavement surface. A waterproof liquid product layer was applied above the concrete. The mortar was placed above in order to protect the waterproof layer; The BA stone tiles were set with white cement–glue on the top of the mortar layer.
3. Sample description and analyses ‘‘Bianco di Asiago” is a lower cretaceous light coloured micritic limestone from pelagic environment, containing fossils, nodules and ‘‘selce” (mixture of silicon and clay of various types) layers, with typical conchoidal surface fracture. Fig. 4 illustrates a common tile showing the ‘‘decorative” irregular pattern due to the stylolite configuration. Several samples were observed under the stereoscope and petrographic microscopes. In order to compare different textures and features exposed on the rock tiles the samples were selected in such a way so that all parts of the tile structure were represented: limestone, mortar and the stylolites area, of both sound and altered rock. 3.1. Petrography Petrography is considered a powerful tool for the characterization and classification of rocks, allowing a way for interpretation of
Fig. 4. Example of a BA tile.
alteration aspects and microscopic features developed within mineral components. Thin sections from different parts of the tile, cut parallel and across the tile surface were analysed under a petrographic microscope (Olympus AH-3 at DCT, UNL). The contrast between the limestone and the mortar ended up being a difficult task for the sample preparation as well as the preservation of the thin flakes along the stylolites. The micritic limestone portion of the tile did not show special aspect due to the size of the crystals; however, along the stylolitic zone, coarser calcite crystals are present, justifying the strong reaction to the acid observed previously. No clayish material was detected, but a few sparse, non-identified red flakes. 3.2. Stereoscopic microscopy Observations with the stereoscopic microscope allowed the identification, with a low magnification, of the principal features that characterizes the stone alteration. Hand specimens examination was performed under the stereoscope (Olympus SZ 51 at DCT, UNL). Fig. 5 is an illustration of the general aspect of the non-altered BA limestone, a soft cream rock containing very thin grey and green veining. This most striking feature is shown in the picture as well as its stratified aspect. The BA tiles disaggregations generally take place along the stratification surfaces. As a result, tiles placed on the bathroom floor started to show effloresences (white coating) and severe damage on the floor pavement. (see Fig. 6a–c). It is possible to visualise the generalized appearance of white recrystallized material (new material, neo – appearance material) at the BA tiles surface, namely on the stratified areas (thin veins of clay), as illustrated in Fig. 7. It should be emphasized that when the tiles were tested with a dilute HCl solution (10%), the greenish areas where the clay material was dominant showed a stronger effervescence and reaction to the acid.
Fig. 5. Example of BA tiles stratified structure.
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Fig. 7. Example of the BA stratified structure.
Fig. 6. (a–c) – Examples of white efflorescences on a BA tile.
3.3. Scanning electron analysis (SEM) and energy dispersive X-ray spectroscopy (EDS) BA limestone specimens were analysed in a high vacuum scanning electron microscope in secondary electron mode. Jeol 330A SEM, with 20 kV accelerating voltage was used (DCT/FCT/UNL). An elemental EDS analysis was also performed. The samples were coated previously with a thin gold film on a SEM coating unit (Polaron equipment – E5000). The first images were collected from a free green vein zone specimen with much superficial efflorescence (Fig. 8). In this zone, it is possible to identify the presence of 8.89% of Aluminium (Al), 13.08% of Silicon (Si) and 58.22% of Calcium (Ca) (weight percentage). It was also detected the lower presence of Sodium (Na), Magnesium (Mg), Manganese (Mn) and Iron (Fe).
With this EDS elementary analysis it was clear that the efflorescences are formed by calcium still, the global specimen calcium percentage (%) is lower than the observed for similar limestone’s majority. This fact leads to a lower than expected percentage of the CaCO3 (calcium oxide) phase, what indicates other phases presence. The efflorescence elementary composition was determined by EDS analysis and it was possible to identify the presence of (weight percentage): 9.43% of Magnesium (Mg), 10.32% of Silicon (Si), 57.54% of Calcium (Ca). This analysis revealed that this efflorescence was mainly constituted by calcium. Calcium may have different origins: cement-glue, mortar (mixture of sand, cement, adjutants and water) – see Fig. 9. The second group of images was collected from two distinct specimens, with green vein areas. In both areas, it was possible to identify foliated structures with different textures from the calcium carbonated matrix (Fig. 10). In the first green vein area the elementary composition determined by EDS analysis detected the presence of (weight percentage): 43.02% of Aluminium (Al), 8.13% of Silicon (Si), 7.82% of Iron (Fe). It was also detected the lower presence of Sodium (Na) and Magnesium (Mg). These results are shown in Fig. 10. In the second green vein area the elementary composition determined by EDS analysis detected the presence of (weight percentage): 20.11% of Aluminium, 68.19% of Silicon (Si) and 3.45% of Magnesium (Mg). It was also detected the lower presence of Iron (Fe) – see Fig. 11. The same elements were found in different green vein zones. Mainly, these areas are formed by magnesium and iron aluminium – silicates which give a porous foliated structure [7]. These zones will act as sinks to absorb water and form large crystals and that will be vital to the overall rock strength as high stress intensity factor values are reached and the tips of large fissures [6]. Green vein zones increment the stone chemical potential representing preferential places for oxidation reactions due to their chemical composition (rich in metals like iron). The third group of images was collected from cement–glue specimen. In the analysed areas it was possible to identify a highly porous structure that can easily promote the water transport by capillarity (Fig. 12). If no waterproof treatment has been applied on the inside face of the BA tiles, the water resulting from the drying of all constructive materials will ascend by capillarity and will remain on the tile green vein zones (with higher porosity than the cream light zones) leading to premature detachment and fragmentation.
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Fig. 8. SEM image of the BA specimen surface showing recrystallized material and EDS emission spectrum.
Fig. 9. SEM image of efflorescence.
The elementary composition of a cement–glue specimen, determined by EDS analysis, detected the presence of (weight percentage): 11.58% of Aluminium, 67.33% of Silicon, 9.54% of Calcium. It was also detected the presence of Magnesium (Mg) and Sodium (Na). In Fig. 13 it is possible observe a SEM image showing a detail of a quartz grain inside a cement–glue specimen. The analysed mortar specimen exhibits a highly porous structure (Fig. 14). The mortar EDS analysis determined the presence of 10% of calcium, due to the mortar manufacturing process that incorporates lime and limestone aggregates. During the mortar drying process, which still occurs after the pavement setting, all the water will evaporate, and depending on the degree of acidity, may react with calcium present in it. Typically, Calcium is a very reactive element and if occurs a pH alteration on the water (used to produce the mortar) a violent reaction may happen and normally will affect all the existing calcium on the system (mortar, cement–glue and limestone). The elementary composition of a mortar specimen, determined by EDS analysis, detected the presence of (weight percentage): 6.38% of aluminium, 78.42% of Silicon, 10.45% of Calcium. In Fig. 15 it is also possible to observe a SEM image showing a detail of a quartz grain inside a mortar specimen.
Fig. 10. SEM image of the BA green veined zone and EDS emission spectrum.
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Fig. 11. SEM image of the BA green veined zone and EDS emission spectrum.
Fig. 12. SEM image of a cement–glue specimen and EDS emission spectrum.
Fig. 13. SEM image of a cement–glue specimen – quartz grain.
3.4. X-ray diffraction analysis X-ray diffraction (XRD) is also an important tool for analysing rock samples through crystalline structures. Experiments such as
those reported in this work should be undertaken in a systematic way always using the same equipment, experimental procedure, experimental parameters, and using a known software database for the treatment of the data [8]. XRD analysis is currently used to verify the existence of a certain mineral phase in specific stones, as a measurement that, besides identifying mineral contents with economical importance, allows the interpretation of possible phase transitions that took place in the past and observe the occurrence of other solid-state reactions, as part of the geological research [8]. The experimental procedure for the mineral identifications using XRD was: The powder samples were provided from three tile portions collected from the damaged bathroom floor. Three materials were separated and grounded: limestone cream parts, limestone green clay parts and cement–glue parts. For experimental reasons, only a small amount of material (approximately 3 g) was needed for accurate intensity measurements. Grinding should not affect the crystallite size or the coherent domain. However, as micro absorption is probably a major factor affecting accuracy in the acquisition of the intensity data, the most suitable way to eliminate this effect is powder size reduction. A reasonable and attainable upper grain size limit was determined to be around 30 lm [8]. The equipment used to perform the experimental work was a RIGAKU, Model D/MAX 3 at INETI. All test parameters were kept constant for all the XRD determinations during this work (see Table 1).
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Fig. 14. SEM image of a mortar specimen and EDS emission spectrum.
Fig. 15. SEM image of a mortar specimen – quartz grain.
aluminium silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage of muscovite [9]. It should be pointed out that these minerals were not identified through the petrologycal microscopy, but only after XRD as it can be observed in Fig. 16. Qualitative X-ray diffraction analysis of beige/light zones with efflorescences revealed the presence of: Calcite (CaCO3) and Quartz (SiO2). This indicates that the efflorescences on BA surface have their origins on Calcium (Ca) that with high probability came by capillarity from the mortar and cement–glue phases through the clay green areas of BA (see Fig. 17). Qualitative X-ray diffraction analysis of cement–glue specimens revealed the presence of: Quartz (SiO2), Rutile (TiO2), Calcite (CaCO3), Portlandite or Portland cement (Ca(OH)2), Halloysite (Al2Si2O5(OH)4) – see Fig. 18. Rutile is often used in industry as a white pigment for plastics, paints, papers and its presence is registered in the cement–glue technical specification [9]. Portlandite is formed during the curing of concrete which in turn, is one of the cement–glue components. Halloysite, quartz and calcite are also common components of white cement-glue. 3.5. Water absorption at atmospheric pressure, apparent density and open porosity
Table 1 Test parameters used in all XRD determinations. Parameters
Value
Sample amount (approx.) Powder grain size 2h (range) Scanning rate (2h) Slew Anode
3g Max 30 lm 5–104.996 (°) 0.012 (°/s) 1.2 (d/m) Cu (45 kV, 20 mA)
Qualitative X-ray diffraction analysis of green veined areas revealed the presence of: Quartz (SiO2), Calcite (CaCO3), Nacrite (Al2Si2O5(OH)6) and Potassium Aluminium Silicates (KAl3Si3O11). This indicates that these zones are visibly weakness areas formed by clay minerals like Nacrite, a hydrous aluminium phyllosilicate similar to Kaolinite. Nacrite is also identified as a low temperature hydrothermal alteration product which indicates that these areas can absorb water [9]. It was also revealed the presence of Potassium Aluminium silicate (KAl3Si3O11) – muscovite. This is a common rock forming mineral that has a layered structure of
Density and porosity are useful parameters always required when studying natural stones properties. The principle for determining the properties is very simple. After drying to constant mass, the apparent density and the open porosity are determined by vacuum assisted water absorption and submerged weighing of specimens. Open porosity (ratio of void space in sample accessible from its outer surface to its total bulk volume, expressed as a percentage or a volume fraction) and apparent density (ratio of the mass of a sample to its geometrical volume including closed pores but not open pores) – probably the most assessed properties of stones, were determined according to the standard EN 1936:1999. Water absorption at atmospheric pressure was determined according to the standard EN 13755:2001 [8,10,11]. Apparent density can be defined as the ratio between the mass of the dry specimen and its apparent volume (volume limited by the external surface of the specimen, including any voids). Open porosity can be described as the ratio (as a percentage) between the volume of the open pores and the apparent volume of the specimen [10]. Experimental tests revealed that the medium value for
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Fig. 16. Qualitative X-ray diffraction analysis of the green veined zones on BA limestone.
Fig. 17. Qualitative X-ray diffraction analysis of a cream/light zone with efflorescences on BA limestone.
Fig. 18. Qualitative X-ray diffraction analysis of a cement–glue specimen.
BA apparent density was 2560 ± 12 (kg/m3) and also that the medium value for BA open porosity was 2.4 ± 0.3 (%). Water absorption at atmospheric pressure results are listed in Table 2. Based on the results, BA cannot be considered as a high porosity limestone; however, it is a heterogeneous rock having two distinct phases: light/beige phase and a green veined phase [12]. As such studies with heterogeneous rocks, rock durability depends not only on properties such as open porosity and apparent density, but is also controlled by the size and spatial distribution of the textural elements (e.g. green vein zones) within the rock [6]. The particular deterioration of BA is more closely associated with the absorbed water from the green veined zones and fissures than with the any other mechanical property [6].
Table 2 Water absorption at atmospheric pressure for BA limestone. Mass of the samples (g) Dry
Saturated
Water absorption at atmospheric pressure (%)
295.9 284.9 288.9 292.5 292.8
298.7 287.2 291.9 295.1 295.9
0.94 0.81 1.04 0.91 1.06
Average Standard deviation Variance
0.95 0.10 0.20
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4. Comments and conclusions ‘‘Bianco di Asiago” is a sedimentary chemical limestone showing stratification surfaces, more visible in some areas. These surfaces reflect the limestone sedimentary origin and are usually associated to the weakest tile zones, not only with low mechanical resistance but also with an expected lower durability. Its chemical composition is normally formed by calcium carbonate and by some oxides of iron and aluminium. It is recognised that the green veined zones showed on BA give a favourable aesthetical effect of antique; however, this weakest stratified zones work like fluid transportation paths and ways in which the water and mineral salts can ascend by capillarity until reaching the tile surface. Hydrochloric acid (HCl) tests on altered specimens of BA showed cold effervescence on calcium carbonate (light cream) and green veined zones, although the effervescences were stronger on the green zones. This test was also performed on the cement– glue used to set the stone pavement. In line with the limestone, this glue material reacts positively to the HCl test. All these data lead to the following interpretation and main conclusions: The alterations on BA tiles are caused by capillarity phenomena. During the mortar drying process, which still occurs after the pavement setting, all the water will evaporate, and reach the rock tiles. This type of rock presents stratified areas (denominated stylolites) that contribute to the water absorption and thus to the premature damage of the pavement. The absorbed water that comes from the mortar and cement– glue will bring dissolved salts that will remain on the stratified areas. These zones are visibly weakness points formed by clay minerals like Nacrite, which is a hydrous aluminium phyllosilicate with a structure similar to the micas and therefore forms flat hexagonal sheets. Nacrite is also identified as a low temperature hydrothermal alteration product which indicates that these areas can absorb water.
The reaction of the minerals in the stratified areas along with the water will cause the appearance of a white coated layer on the tiles surface and the consequent premature mechanical damage of the rock. Results show that if no waterproof treatments are performed on BA tiles, water will arise by capillarity (until it reaches the equilibrium between evaporation and capillarity) from the pavement substrate and will reduce the rock strength. The general conclusion of this work is that all fabric and petrophysical rock properties must be considered in order to evaluate, predict and understand the vulnerability to water action and salt weathering in carbonate natural stones. References [1] Sharma PK, Singh TN, Khandelwal M. Effect of the pH on the physical– mechanical properties of marble. Bull Eng Geol Environ 2007;66:81–7. [2] Press F, Siever R. Understanding earth. 2nd ed. New York: WH Freeman and Company; 1998. p. 162. [3] Cruz A, Assunção A, Casal Moura A, Esteves Henriques, Carvalho A, Ramos C, et al. Manual da pedra natural para arquitectura. DGGE; 2006. p. 97–114. [4] Monte MD. Stone monument decay and air pollution. In: Corso I, editor. Weathering and air pollution, Communità delle Università Mediterranee, Scuola Universitaria C.U.M. Conservacione dei Monumenti; 1991. p. 101–10. [5] Aires-Barros L. The decay of stonework: mechanism, methodology of study. In: Corso I, editor. Weathering and air pollution, Communità delle Università Mediterranee, Scuola Universitaria C.U.M. Conservacione dei Monumenti; 1991. p. 111–8. [6] Benavente D, Cueto N, Martinez-Martinez J, Garcia del Cura MA. Salt weathering in dual-porosity building dolostones. Eng Geol 2007;94:215–26. [7]. [8] Amaral PM, Fernandes JC, Rosa LG. A comparison between X-ray diffraction and petrography techniques used to determine the mineralogical composition of granite and comparable hard rocks. Mater Sci Fórum 2006;514– 516:1628–32. [9] Fardon J. The complete guide to rocks and minerals. London: Hermes House; 2006. p. 136. [10] EN 1936. Natural stone test methods – determination of real density and apparent density, and of total and open porosity. Brussels: CEN – European Committee for Standardization; 1999. [11] EN 13755. Natural stone test methods – determination of water absorption at atmospheric pressure. Brussels: CEN – European Committee for Standardization; 2001. [12] Carvalho J, Manuppella G, Moura C. Calcários Ornamentais Portugueses. Estudos, notas e trabalhados de IGM. Porto: IGM; 2001. p. 30.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
‘‘Bianco di Asiago” limestone pavement – Degradation and alteration study Vera Pires a,*, Z.S.G. Silva b, J.A.R. Simão b, C. Galhano b, P.M. Amaral c a
FrontWave – Materials Engineering, Taguspark, Núcleo Central 389, 2740-122 Porto Salvo, Portugal Centro de Investigação em Ciência e Engenharia Geológica, Departamento de Ciências da Terra, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c Department of Materials Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b
a r t i c l e
i n f o
Article history: Received 24 July 2008 Accepted 28 October 2009 Available online 30 November 2009 Keywords: Degradation Alteration Limestone Pavement Capillarity Efflorecences
a b s t r a c t Pavement tiles made of a micritic limestone were set on a residence building and shortly after its application signs of degradation were detected (efflorescences) on the tile surface. The limestone contains irregular patterns due to rock cutting across stylolites. These decorative features represent ideal roads for fluid circulation through the rock where reactions between minerals and fluids (essentially water from the mortar) occurred. Rock analyses obtained from microscopic petrography, SEM, EDS and XRD emphasize the high calcite content of the rock and the variable composition and porosity of the stylolitic zones. Results show the importance to acknowledge the rock structure before setting these natural products as construction materials. In the present work, the need for surface treatments was found important to avoid water arising by capillarity from the pavement substrate, and hence contribute to avoid the rock weakness. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Limestone has been used as a construction material since historical times. It has been applied as facing stone in many types of buildings e.g. churches, monuments, palaces, etc., due to its pleasant light colours and ability to cut and shape in various forms and designs [1]. Limestone is the most abundant of the non-clastic sedimentary rocks on the Earth’s crust. A large proportion of limestones are formed by the mineral calcite – calcium carbonate (CaCO3) and clasts (including tiny shells and micro-skeletons) deposited on the sea bed. Other types of limestone are formed by precipitation of calcite, after dissolution of previous limestone, like travertine, which is also a common rock used as building stone. They are texturally and structurally extremely diverse, as a result of different environment depositions and origins [2]. The principal aspect which unifies carbonate rocks is their high reactivity to acids. Wine, carbonated drinks, fruits and fruit juices, vinegar, and even some natural waters, all will react with carbonate rocks. Another essential characteristic of limestones is their softness relative to other rocks such as silicate rocks. The softness is mainly a function of the mineral composition, calcite, whose hardness is 3 in Moh´s scale (1–10) [3]. A physical feature of limestones which is technically important is their porosity. Many limestones, particularly the biogenic ones, * Corresponding author. Tel.: +351 938 436 402. E-mail address: [email protected] (V. Pires). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.10.040
have a medium to high porosity. Nonetheless, although being technically weak and very absorbent, certain construction techniques (impermeability techniques) allow them to be used successfully and effectively. In cold climates, however, a porous limestone can suffer rapid degradation due to freeze–thaw alternate cycles and some protection might be required in order to assure its long life [3]. One important structural aspect inherent to many limestones is the presence of very fine lines, usually brownish in colour, which are pressure-solution features formed during the compaction and lithification. These structural aspects are denominated stylolites. The brownish colour is mostly due to hydrated iron oxide, but it can be also caused by concentrations of clays and/or sulphide minerals. Due to the fact that these stylolites are natural planes of weakness and can easily act as pathway for fluid circulation since they usually are not fully closed, any expanding clays present in the rock can react with fluids and physically weaken the limestone [3]. Rock decay is a dynamic process where the entropy of the system (the rock) increases with the increasing disorder of their phases (the minerals) [4]. This process produces mineralogical breakdown by means of crystalline lattice disruption enabling ionic migration to build up new materials. These new materials are now in thermodynamic equilibrium with the new environmental conditions. In order to have a comprehensive idea of the decay process involved in this case under study and to find solution for minimising the degradation effect, it is very important to acquire specific understanding of the types of damage and the appropriate mechanisms set
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up in order to slow down the process. Durability is the most important characteristic to consider when selecting materials, whose use impose constrains whenever vulnerability to alteration is effective. The durability of a material is the property that has to stand the effects caused by time once installed, that is to say, it is the capacity to resist the action of the degradation agents, either from chemical, organic, physical or mechanical origin [5]. The main degradation agents which easily may affect limestone applied in interior floors can be summarised as: soluble salts and water [4]. 1.1. Soluble salts Salt crystallization is one of the most common decay processes that affect materials such as rocks, ceramics and concrete. It generates pressure variation, depending on the size and type of the rock pore. Soluble salts (hydrous and anhydrous forms of carbonates, chlorides and sulphates of Ca, Na and Mg) form due to reactions between the rock components and fluids which circulate through the rock. These salts have strong effect on the degradation of rocks, their presence is often detected by a sugary white coating (efflorescences) on the surfaces and once they crystallize, salt crystals within the rock (subefflorescences) can promote further decay. Spontaneous crystallization of a soluble salt is a function of concentration in solution and temperature [4]. Theoretical calculations of growth pressure have been studied for crystal growth in single pores and fissures [6]. Crystal growth within pores and fissures creates maximum pressure when a large crystal grows in a pore with small entries as it cannot penetrate to the surrounding small pores until high saturation is achieved, producing great stress which in turn damages the material [6].
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the stylolites surfaces). Along these ‘‘lines”, white efflorescences developed as the tiles were set. Several samples were collected for analyses of both types: sound and altered tiles (see Figs. 1 and 2). 2.1. Floor structure The floor structure on which the tiles were set is schematically shown in Fig. 3. One should emphasize that: No waterproofing product was applied on the inside face of the ‘‘Bianco di Asiago” (BA) tile.
Fig. 1. Example of a disaggregated BA tile collected form the bathroom floor.
1.2. Water Frozen water within the pore-spaces promotes expansion that can impart a force which is often superior to the tensile strength of the rock. This effect will originate a localised stress concentration field that will probably originate the occurrence of critical flaws, which will accelerate rock disruption under any of loading condition. Water may also disaggregate rock through its ability to dilate, by osmotic absorption, water-imbibing minerals and/or through hydration of expandable clays or anhydrous salts. Water not only dissolves hydrates and hydrolyzes minerals, but also can transport reagents capable of oxidizing and reducing atoms in the minerals. Water may also introduce soluble minerals in solution which, on precipitation, disrupt the fabric [4]. Depending on the type of construction and on the particular environment in which it is inserted, the rates of thermal expansion and contraction of limestone tile (typically in a range of thicknesses between 2.5 and 3.0 cm), vary substantially from those expected in thicker specimens; this parameter should be taken into account. Special attention should be given to the drying rates of the other construction materials such as mortars, glues and concrete.
2. Case study Limestone tiles were set on a bathroom floor in a residence building and in a very short time after the application (approximately 1 month) several degradation signs were observed on the tiles surfaces. The material itself is a fine limestone having thin irregular dark patterns due to the way the rock was cut (crossing
Fig. 2. Non-altered (sound) BA limestone tiles: (a) cross-section; and (b) top view.
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Fig. 3. Illustration of the floor constructive solution.
A white cement–glue was used to set the tiles on the pavement. After the setting, no finishing treatment or product was applied on the visible face (place where the alterations start to occur) of the BA tiles. There are no references concerning the mortar drying times. It is assumed regular drying times as usually done in similar cases for home construction. Concrete was placed on the top of the pipe system in order to flatten the pavement surface. A waterproof liquid product layer was applied above the concrete. The mortar was placed above in order to protect the waterproof layer; The BA stone tiles were set with white cement–glue on the top of the mortar layer.
3. Sample description and analyses ‘‘Bianco di Asiago” is a lower cretaceous light coloured micritic limestone from pelagic environment, containing fossils, nodules and ‘‘selce” (mixture of silicon and clay of various types) layers, with typical conchoidal surface fracture. Fig. 4 illustrates a common tile showing the ‘‘decorative” irregular pattern due to the stylolite configuration. Several samples were observed under the stereoscope and petrographic microscopes. In order to compare different textures and features exposed on the rock tiles the samples were selected in such a way so that all parts of the tile structure were represented: limestone, mortar and the stylolites area, of both sound and altered rock. 3.1. Petrography Petrography is considered a powerful tool for the characterization and classification of rocks, allowing a way for interpretation of
Fig. 4. Example of a BA tile.
alteration aspects and microscopic features developed within mineral components. Thin sections from different parts of the tile, cut parallel and across the tile surface were analysed under a petrographic microscope (Olympus AH-3 at DCT, UNL). The contrast between the limestone and the mortar ended up being a difficult task for the sample preparation as well as the preservation of the thin flakes along the stylolites. The micritic limestone portion of the tile did not show special aspect due to the size of the crystals; however, along the stylolitic zone, coarser calcite crystals are present, justifying the strong reaction to the acid observed previously. No clayish material was detected, but a few sparse, non-identified red flakes. 3.2. Stereoscopic microscopy Observations with the stereoscopic microscope allowed the identification, with a low magnification, of the principal features that characterizes the stone alteration. Hand specimens examination was performed under the stereoscope (Olympus SZ 51 at DCT, UNL). Fig. 5 is an illustration of the general aspect of the non-altered BA limestone, a soft cream rock containing very thin grey and green veining. This most striking feature is shown in the picture as well as its stratified aspect. The BA tiles disaggregations generally take place along the stratification surfaces. As a result, tiles placed on the bathroom floor started to show effloresences (white coating) and severe damage on the floor pavement. (see Fig. 6a–c). It is possible to visualise the generalized appearance of white recrystallized material (new material, neo – appearance material) at the BA tiles surface, namely on the stratified areas (thin veins of clay), as illustrated in Fig. 7. It should be emphasized that when the tiles were tested with a dilute HCl solution (10%), the greenish areas where the clay material was dominant showed a stronger effervescence and reaction to the acid.
Fig. 5. Example of BA tiles stratified structure.
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Fig. 7. Example of the BA stratified structure.
Fig. 6. (a–c) – Examples of white efflorescences on a BA tile.
3.3. Scanning electron analysis (SEM) and energy dispersive X-ray spectroscopy (EDS) BA limestone specimens were analysed in a high vacuum scanning electron microscope in secondary electron mode. Jeol 330A SEM, with 20 kV accelerating voltage was used (DCT/FCT/UNL). An elemental EDS analysis was also performed. The samples were coated previously with a thin gold film on a SEM coating unit (Polaron equipment – E5000). The first images were collected from a free green vein zone specimen with much superficial efflorescence (Fig. 8). In this zone, it is possible to identify the presence of 8.89% of Aluminium (Al), 13.08% of Silicon (Si) and 58.22% of Calcium (Ca) (weight percentage). It was also detected the lower presence of Sodium (Na), Magnesium (Mg), Manganese (Mn) and Iron (Fe).
With this EDS elementary analysis it was clear that the efflorescences are formed by calcium still, the global specimen calcium percentage (%) is lower than the observed for similar limestone’s majority. This fact leads to a lower than expected percentage of the CaCO3 (calcium oxide) phase, what indicates other phases presence. The efflorescence elementary composition was determined by EDS analysis and it was possible to identify the presence of (weight percentage): 9.43% of Magnesium (Mg), 10.32% of Silicon (Si), 57.54% of Calcium (Ca). This analysis revealed that this efflorescence was mainly constituted by calcium. Calcium may have different origins: cement-glue, mortar (mixture of sand, cement, adjutants and water) – see Fig. 9. The second group of images was collected from two distinct specimens, with green vein areas. In both areas, it was possible to identify foliated structures with different textures from the calcium carbonated matrix (Fig. 10). In the first green vein area the elementary composition determined by EDS analysis detected the presence of (weight percentage): 43.02% of Aluminium (Al), 8.13% of Silicon (Si), 7.82% of Iron (Fe). It was also detected the lower presence of Sodium (Na) and Magnesium (Mg). These results are shown in Fig. 10. In the second green vein area the elementary composition determined by EDS analysis detected the presence of (weight percentage): 20.11% of Aluminium, 68.19% of Silicon (Si) and 3.45% of Magnesium (Mg). It was also detected the lower presence of Iron (Fe) – see Fig. 11. The same elements were found in different green vein zones. Mainly, these areas are formed by magnesium and iron aluminium – silicates which give a porous foliated structure [7]. These zones will act as sinks to absorb water and form large crystals and that will be vital to the overall rock strength as high stress intensity factor values are reached and the tips of large fissures [6]. Green vein zones increment the stone chemical potential representing preferential places for oxidation reactions due to their chemical composition (rich in metals like iron). The third group of images was collected from cement–glue specimen. In the analysed areas it was possible to identify a highly porous structure that can easily promote the water transport by capillarity (Fig. 12). If no waterproof treatment has been applied on the inside face of the BA tiles, the water resulting from the drying of all constructive materials will ascend by capillarity and will remain on the tile green vein zones (with higher porosity than the cream light zones) leading to premature detachment and fragmentation.
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Fig. 8. SEM image of the BA specimen surface showing recrystallized material and EDS emission spectrum.
Fig. 9. SEM image of efflorescence.
The elementary composition of a cement–glue specimen, determined by EDS analysis, detected the presence of (weight percentage): 11.58% of Aluminium, 67.33% of Silicon, 9.54% of Calcium. It was also detected the presence of Magnesium (Mg) and Sodium (Na). In Fig. 13 it is possible observe a SEM image showing a detail of a quartz grain inside a cement–glue specimen. The analysed mortar specimen exhibits a highly porous structure (Fig. 14). The mortar EDS analysis determined the presence of 10% of calcium, due to the mortar manufacturing process that incorporates lime and limestone aggregates. During the mortar drying process, which still occurs after the pavement setting, all the water will evaporate, and depending on the degree of acidity, may react with calcium present in it. Typically, Calcium is a very reactive element and if occurs a pH alteration on the water (used to produce the mortar) a violent reaction may happen and normally will affect all the existing calcium on the system (mortar, cement–glue and limestone). The elementary composition of a mortar specimen, determined by EDS analysis, detected the presence of (weight percentage): 6.38% of aluminium, 78.42% of Silicon, 10.45% of Calcium. In Fig. 15 it is also possible to observe a SEM image showing a detail of a quartz grain inside a mortar specimen.
Fig. 10. SEM image of the BA green veined zone and EDS emission spectrum.
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Fig. 11. SEM image of the BA green veined zone and EDS emission spectrum.
Fig. 12. SEM image of a cement–glue specimen and EDS emission spectrum.
Fig. 13. SEM image of a cement–glue specimen – quartz grain.
3.4. X-ray diffraction analysis X-ray diffraction (XRD) is also an important tool for analysing rock samples through crystalline structures. Experiments such as
those reported in this work should be undertaken in a systematic way always using the same equipment, experimental procedure, experimental parameters, and using a known software database for the treatment of the data [8]. XRD analysis is currently used to verify the existence of a certain mineral phase in specific stones, as a measurement that, besides identifying mineral contents with economical importance, allows the interpretation of possible phase transitions that took place in the past and observe the occurrence of other solid-state reactions, as part of the geological research [8]. The experimental procedure for the mineral identifications using XRD was: The powder samples were provided from three tile portions collected from the damaged bathroom floor. Three materials were separated and grounded: limestone cream parts, limestone green clay parts and cement–glue parts. For experimental reasons, only a small amount of material (approximately 3 g) was needed for accurate intensity measurements. Grinding should not affect the crystallite size or the coherent domain. However, as micro absorption is probably a major factor affecting accuracy in the acquisition of the intensity data, the most suitable way to eliminate this effect is powder size reduction. A reasonable and attainable upper grain size limit was determined to be around 30 lm [8]. The equipment used to perform the experimental work was a RIGAKU, Model D/MAX 3 at INETI. All test parameters were kept constant for all the XRD determinations during this work (see Table 1).
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Fig. 14. SEM image of a mortar specimen and EDS emission spectrum.
Fig. 15. SEM image of a mortar specimen – quartz grain.
aluminium silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage of muscovite [9]. It should be pointed out that these minerals were not identified through the petrologycal microscopy, but only after XRD as it can be observed in Fig. 16. Qualitative X-ray diffraction analysis of beige/light zones with efflorescences revealed the presence of: Calcite (CaCO3) and Quartz (SiO2). This indicates that the efflorescences on BA surface have their origins on Calcium (Ca) that with high probability came by capillarity from the mortar and cement–glue phases through the clay green areas of BA (see Fig. 17). Qualitative X-ray diffraction analysis of cement–glue specimens revealed the presence of: Quartz (SiO2), Rutile (TiO2), Calcite (CaCO3), Portlandite or Portland cement (Ca(OH)2), Halloysite (Al2Si2O5(OH)4) – see Fig. 18. Rutile is often used in industry as a white pigment for plastics, paints, papers and its presence is registered in the cement–glue technical specification [9]. Portlandite is formed during the curing of concrete which in turn, is one of the cement–glue components. Halloysite, quartz and calcite are also common components of white cement-glue. 3.5. Water absorption at atmospheric pressure, apparent density and open porosity
Table 1 Test parameters used in all XRD determinations. Parameters
Value
Sample amount (approx.) Powder grain size 2h (range) Scanning rate (2h) Slew Anode
3g Max 30 lm 5–104.996 (°) 0.012 (°/s) 1.2 (d/m) Cu (45 kV, 20 mA)
Qualitative X-ray diffraction analysis of green veined areas revealed the presence of: Quartz (SiO2), Calcite (CaCO3), Nacrite (Al2Si2O5(OH)6) and Potassium Aluminium Silicates (KAl3Si3O11). This indicates that these zones are visibly weakness areas formed by clay minerals like Nacrite, a hydrous aluminium phyllosilicate similar to Kaolinite. Nacrite is also identified as a low temperature hydrothermal alteration product which indicates that these areas can absorb water [9]. It was also revealed the presence of Potassium Aluminium silicate (KAl3Si3O11) – muscovite. This is a common rock forming mineral that has a layered structure of
Density and porosity are useful parameters always required when studying natural stones properties. The principle for determining the properties is very simple. After drying to constant mass, the apparent density and the open porosity are determined by vacuum assisted water absorption and submerged weighing of specimens. Open porosity (ratio of void space in sample accessible from its outer surface to its total bulk volume, expressed as a percentage or a volume fraction) and apparent density (ratio of the mass of a sample to its geometrical volume including closed pores but not open pores) – probably the most assessed properties of stones, were determined according to the standard EN 1936:1999. Water absorption at atmospheric pressure was determined according to the standard EN 13755:2001 [8,10,11]. Apparent density can be defined as the ratio between the mass of the dry specimen and its apparent volume (volume limited by the external surface of the specimen, including any voids). Open porosity can be described as the ratio (as a percentage) between the volume of the open pores and the apparent volume of the specimen [10]. Experimental tests revealed that the medium value for
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Fig. 16. Qualitative X-ray diffraction analysis of the green veined zones on BA limestone.
Fig. 17. Qualitative X-ray diffraction analysis of a cream/light zone with efflorescences on BA limestone.
Fig. 18. Qualitative X-ray diffraction analysis of a cement–glue specimen.
BA apparent density was 2560 ± 12 (kg/m3) and also that the medium value for BA open porosity was 2.4 ± 0.3 (%). Water absorption at atmospheric pressure results are listed in Table 2. Based on the results, BA cannot be considered as a high porosity limestone; however, it is a heterogeneous rock having two distinct phases: light/beige phase and a green veined phase [12]. As such studies with heterogeneous rocks, rock durability depends not only on properties such as open porosity and apparent density, but is also controlled by the size and spatial distribution of the textural elements (e.g. green vein zones) within the rock [6]. The particular deterioration of BA is more closely associated with the absorbed water from the green veined zones and fissures than with the any other mechanical property [6].
Table 2 Water absorption at atmospheric pressure for BA limestone. Mass of the samples (g) Dry
Saturated
Water absorption at atmospheric pressure (%)
295.9 284.9 288.9 292.5 292.8
298.7 287.2 291.9 295.1 295.9
0.94 0.81 1.04 0.91 1.06
Average Standard deviation Variance
0.95 0.10 0.20
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4. Comments and conclusions ‘‘Bianco di Asiago” is a sedimentary chemical limestone showing stratification surfaces, more visible in some areas. These surfaces reflect the limestone sedimentary origin and are usually associated to the weakest tile zones, not only with low mechanical resistance but also with an expected lower durability. Its chemical composition is normally formed by calcium carbonate and by some oxides of iron and aluminium. It is recognised that the green veined zones showed on BA give a favourable aesthetical effect of antique; however, this weakest stratified zones work like fluid transportation paths and ways in which the water and mineral salts can ascend by capillarity until reaching the tile surface. Hydrochloric acid (HCl) tests on altered specimens of BA showed cold effervescence on calcium carbonate (light cream) and green veined zones, although the effervescences were stronger on the green zones. This test was also performed on the cement– glue used to set the stone pavement. In line with the limestone, this glue material reacts positively to the HCl test. All these data lead to the following interpretation and main conclusions: The alterations on BA tiles are caused by capillarity phenomena. During the mortar drying process, which still occurs after the pavement setting, all the water will evaporate, and reach the rock tiles. This type of rock presents stratified areas (denominated stylolites) that contribute to the water absorption and thus to the premature damage of the pavement. The absorbed water that comes from the mortar and cement– glue will bring dissolved salts that will remain on the stratified areas. These zones are visibly weakness points formed by clay minerals like Nacrite, which is a hydrous aluminium phyllosilicate with a structure similar to the micas and therefore forms flat hexagonal sheets. Nacrite is also identified as a low temperature hydrothermal alteration product which indicates that these areas can absorb water.
The reaction of the minerals in the stratified areas along with the water will cause the appearance of a white coated layer on the tiles surface and the consequent premature mechanical damage of the rock. Results show that if no waterproof treatments are performed on BA tiles, water will arise by capillarity (until it reaches the equilibrium between evaporation and capillarity) from the pavement substrate and will reduce the rock strength. The general conclusion of this work is that all fabric and petrophysical rock properties must be considered in order to evaluate, predict and understand the vulnerability to water action and salt weathering in carbonate natural stones. References [1] Sharma PK, Singh TN, Khandelwal M. Effect of the pH on the physical– mechanical properties of marble. Bull Eng Geol Environ 2007;66:81–7. [2] Press F, Siever R. Understanding earth. 2nd ed. New York: WH Freeman and Company; 1998. p. 162. [3] Cruz A, Assunção A, Casal Moura A, Esteves Henriques, Carvalho A, Ramos C, et al. Manual da pedra natural para arquitectura. DGGE; 2006. p. 97–114. [4] Monte MD. Stone monument decay and air pollution. In: Corso I, editor. Weathering and air pollution, Communità delle Università Mediterranee, Scuola Universitaria C.U.M. Conservacione dei Monumenti; 1991. p. 101–10. [5] Aires-Barros L. The decay of stonework: mechanism, methodology of study. In: Corso I, editor. Weathering and air pollution, Communità delle Università Mediterranee, Scuola Universitaria C.U.M. Conservacione dei Monumenti; 1991. p. 111–8. [6] Benavente D, Cueto N, Martinez-Martinez J, Garcia del Cura MA. Salt weathering in dual-porosity building dolostones. Eng Geol 2007;94:215–26. [7]