Construction and Building Materials 24 (2010) 686–694

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Construction and Building Materials

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‘‘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, Portugalb Centro de Investigação em Ciência e Engenharia Geológica, Departamento de Ciências da Terra, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugalc Department of Materials Engineering, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

a r t i c l e i n f o

Article history:Received 24 July 2008Accepted 28 October 2009Available online 30 November 2009

Keywords:DegradationAlterationLimestonePavementCapillarityEfflorecences

0950-0618/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2009.10.040

* Corresponding author. Tel.: +351 938 436 402.E-mail address: vera.pires@frontwave.pt (V. Pires)

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 appli-cation signs of degradation were detected (efflorescences) on the tile surface. The limestone containsirregular patterns due to rock cutting across stylolites. These decorative features represent ideal roadsfor fluid circulation through the rock where reactions between minerals and fluids (essentially waterfrom the mortar) occurred. Rock analyses obtained from microscopic petrography, SEM, EDS and XRDemphasize the high calcite content of the rock and the variable composition and porosity of the styloliticzones. Results show the importance to acknowledge the rock structure before setting these natural prod-ucts as construction materials. In the present work, the need for surface treatments was found importantto avoid water arising by capillarity from the pavement substrate, and hence contribute to avoid the rockweakness.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Limestone has been used as a construction material since his-torical times. It has been applied as facing stone in many types ofbuildings e.g. churches, monuments, palaces, etc., due to its pleas-ant light colours and ability to cut and shape in various forms anddesigns [1].

Limestone is the most abundant of the non-clastic sedimentaryrocks on the Earth’s crust. A large proportion of limestones areformed by the mineral calcite – calcium carbonate (CaCO3) andclasts (including tiny shells and micro-skeletons) deposited onthe sea bed. Other types of limestone are formed by precipitationof calcite, after dissolution of previous limestone, like travertine,which is also a common rock used as building stone. They are tex-turally and structurally extremely diverse, as a result of differentenvironment depositions and origins [2].

The principal aspect which unifies carbonate rocks is their highreactivity to acids. Wine, carbonated drinks, fruits and fruit juices,vinegar, and even some natural waters, all will react with carbon-ate rocks.

Another essential characteristic of limestones is their softnessrelative to other rocks such as silicate rocks. The softness is mainlya function of the mineral composition, calcite, whose hardness is 3in Moh́s scale (1–10) [3].

A physical feature of limestones which is technically importantis their porosity. Many limestones, particularly the biogenic ones,

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have a medium to high porosity. Nonetheless, although being tech-nically weak and very absorbent, certain construction techniques(impermeability techniques) allow them to be used successfullyand effectively. In cold climates, however, a porous limestone cansuffer rapid degradation due to freeze–thaw alternate cycles andsome protection might be required in order to assure its long life[3].

One important structural aspect inherent to many limestones isthe presence of very fine lines, usually brownish in colour, whichare pressure-solution features formed during the compaction andlithification. These structural aspects are denominated stylolites.The brownish colour is mostly due to hydrated iron oxide, but itcan be also caused by concentrations of clays and/or sulphide min-erals. Due to the fact that these stylolites are natural planes ofweakness and can easily act as pathway for fluid circulation sincethey usually are not fully closed, any expanding clays present inthe rock can react with fluids and physically weaken the limestone[3].

Rock decay is a dynamic process where the entropy of the sys-tem (the rock) increases with the increasing disorder of theirphases (the minerals) [4]. This process produces mineralogicalbreakdown by means of crystalline lattice disruption enabling io-nic migration to build up new materials. These new materials arenow in thermodynamic equilibrium with the new environmentalconditions.

In order to have a comprehensive idea of the decay process in-volved in this case under study and to find solution for minimisingthe degradation effect, it is very important to acquire specific under-standing 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 impor-tant characteristic to consider when selecting materials, whose useimpose constrains whenever vulnerability to alteration is effective.The durability of a material is the property that has to stand the ef-fects caused by time once installed, that is to say, it is the capacityto resist the action of the degradation agents, either from chemical,organic, physical or mechanical origin [5].

The main degradation agents which easily may affect limestoneapplied in interior floors can be summarised as: soluble salts andwater [4].

Fig. 1. Example of a disaggregated BA tile collected form the bathroom floor.

1.1. Soluble salts

Salt crystallization is one of the most common decay processesthat affect materials such as rocks, ceramics and concrete. It gener-ates pressure variation, depending on the size and type of the rockpore.

Soluble salts (hydrous and anhydrous forms of carbonates, chlo-rides and sulphates of Ca, Na and Mg) form due to reactions be-tween the rock components and fluids which circulate throughthe rock. These salts have strong effect on the degradation of rocks,their presence is often detected by a sugary white coating (efflores-cences) on the surfaces and once they crystallize, salt crystals with-in the rock (subefflorescences) can promote further decay.Spontaneous crystallization of a soluble salt is a function of con-centration in solution and temperature [4].

Theoretical calculations of growth pressure have been studiedfor crystal growth in single pores and fissures [6]. Crystal growthwithin pores and fissures creates maximum pressure when a largecrystal grows in a pore with small entries as it cannot penetrate tothe surrounding small pores until high saturation is achieved, pro-ducing great stress which in turn damages the material [6].

1.2. Water

Frozen water within the pore-spaces promotes expansion thatcan impart a force which is often superior to the tensile strengthof the rock. This effect will originate a localised stress concentra-tion field that will probably originate the occurrence of criticalflaws, which will accelerate rock disruption under any of loadingcondition.

Water may also disaggregate rock through its ability to dilate,by osmotic absorption, water-imbibing minerals and/or throughhydration of expandable clays or anhydrous salts. Water not onlydissolves hydrates and hydrolyzes minerals, but also can transportreagents capable of oxidizing and reducing atoms in the minerals.Water may also introduce soluble minerals in solution which, onprecipitation, disrupt the fabric [4].

Depending on the type of construction and on the particularenvironment in which it is inserted, the rates of thermal expansionand contraction of limestone tile (typically in a range of thick-nesses between 2.5 and 3.0 cm), vary substantially from those ex-pected in thicker specimens; this parameter should be taken intoaccount. Special attention should be given to the drying rates ofthe other construction materials such as mortars, glues andconcrete.

Fig. 2. Non-altered (sound) BA limestone tiles: (a) cross-section; and (b) top view.

2. Case study

Limestone tiles were set on a bathroom floor in a residencebuilding and in a very short time after the application (approxi-mately 1 month) several degradation signs were observed on thetiles surfaces. The material itself is a fine limestone having thinirregular dark patterns due to the way the rock was cut (crossing

the stylolites surfaces). Along these ‘‘lines”, white efflorescencesdeveloped 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 schematicallyshown in Fig. 3. One should emphasize that:

� No waterproofing product was applied on the inside face of the‘‘Bianco di Asiago” (BA) tile.

Fig. 3. Illustration of the floor constructive solution.

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� 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) ofthe BA tiles.

� There are no references concerning the mortar drying times. It isassumed regular drying times as usually done in similar casesfor home construction.

� Concrete was placed on the top of the pipe system in order toflatten the pavement surface.

� A waterproof liquid product layer was applied above theconcrete.

� The mortar was placed above in order to protect the waterprooflayer;

� The BA stone tiles were set with white cement–glue on the topof the mortar layer.

3. Sample description and analyses

‘‘Bianco di Asiago” is a lower cretaceous light coloured micriticlimestone from pelagic environment, containing fossils, nodulesand ‘‘selce” (mixture of silicon and clay of various types) layers,with typical conchoidal surface fracture. Fig. 4 illustrates a com-mon tile showing the ‘‘decorative” irregular pattern due to the styl-olite configuration.

Several samples were observed under the stereoscope and pet-rographic microscopes. In order to compare different textures andfeatures exposed on the rock tiles the samples were selected insuch a way so that all parts of the tile structure were represented:limestone, mortar and the stylolites area, of both sound and alteredrock.

3.1. Petrography

Petrography is considered a powerful tool for the characteriza-tion and classification of rocks, allowing a way for interpretation of

Fig. 4. Example of a BA tile.

alteration aspects and microscopic features developed within min-eral components.

Thin sections from different parts of the tile, cut parallel andacross the tile surface were analysed under a petrographic micro-scope (Olympus AH-3 at DCT, UNL). The contrast between thelimestone and the mortar ended up being a difficult task for thesample preparation as well as the preservation of the thin flakesalong the stylolites.

The micritic limestone portion of the tile did not show specialaspect due to the size of the crystals; however, along the styloliticzone, coarser calcite crystals are present, justifying the strong reac-tion to the acid observed previously. No clayish material was de-tected, but a few sparse, non-identified red flakes.

3.2. Stereoscopic microscopy

Observations with the stereoscopic microscope allowed theidentification, with a low magnification, of the principal featuresthat characterizes the stone alteration.

Hand specimens examination was performed under the stereo-scope (Olympus SZ 51 at DCT, UNL). Fig. 5 is an illustration of thegeneral aspect of the non-altered BA limestone, a soft cream rockcontaining very thin grey and green veining. This most striking fea-ture is shown in the picture as well as its stratified aspect.

The BA tiles disaggregations generally take place along thestratification surfaces. As a result, tiles placed on the bathroomfloor started to show effloresences (white coating) and severe dam-age on the floor pavement. (see Fig. 6a–c).

It is possible to visualise the generalized appearance of whiterecrystallized material (new material, neo – appearance material)at the BA tiles surface, namely on the stratified areas (thin veinsof clay), as illustrated in Fig. 7. It should be emphasized that whenthe tiles were tested with a dilute HCl solution (10%), the greenishareas where the clay material was dominant showed a strongereffervescence and reaction to the acid.

Fig. 5. Example of BA tiles stratified structure.

Fig. 6. (a–c) – Examples of white efflorescences on a BA tile.

Fig. 7. Example of the BA stratified structure.

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3.3. Scanning electron analysis (SEM) and energy dispersive X-rayspectroscopy (EDS)

BA limestone specimens were analysed in a high vacuum scan-ning electron microscope in secondary electron mode. Jeol 330ASEM, 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 aSEM coating unit (Polaron equipment – E5000).

The first images were collected from a free green vein zonespecimen with much superficial efflorescence (Fig. 8).

In this zone, it is possible to identify the presence of 8.89% ofAluminium (Al), 13.08% of Silicon (Si) and 58.22% of Calcium (Ca)(weight percentage). It was also detected the lower presence of So-dium (Na), Magnesium (Mg), Manganese (Mn) and Iron (Fe).

With this EDS elementary analysis it was clear that the efflores-cences are formed by calcium still, the global specimen calciumpercentage (%) is lower than the observed for similar limestone’smajority. This fact leads to a lower than expected percentage ofthe CaCO3 (calcium oxide) phase, what indicates other phasespresence.

The efflorescence elementary composition was determined byEDS analysis and it was possible to identify the presence of (weightpercentage): 9.43% of Magnesium (Mg), 10.32% of Silicon (Si),57.54% of Calcium (Ca). This analysis revealed that this efflores-cence was mainly constituted by calcium. Calcium may have differ-ent origins: cement-glue, mortar (mixture of sand, cement,adjutants and water) – see Fig. 9.

The second group of images was collected from two distinctspecimens, with green vein areas. In both areas, it was possibleto identify foliated structures with different textures from the cal-cium carbonated matrix (Fig. 10).

In the first green vein area the elementary compositiondetermined by EDS analysis detected the presence of (weightpercentage): 43.02% of Aluminium (Al), 8.13% of Silicon (Si),7.82% of Iron (Fe). It was also detected the lower presence ofSodium (Na) and Magnesium (Mg). These results are shown inFig. 10.

In the second green vein area the elementary compositiondetermined by EDS analysis detected the presence of (weight per-centage): 20.11% of Aluminium, 68.19% of Silicon (Si) and 3.45% ofMagnesium (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 alumin-ium – silicates which give a porous foliated structure [7]. Thesezones will act as sinks to absorb water and form large crystalsand that will be vital to the overall rock strength as high stressintensity factor values are reached and the tips of large fissures[6].

Green vein zones increment the stone chemical potential repre-senting preferential places for oxidation reactions due to theirchemical composition (rich in metals like iron).

The third group of images was collected from cement–gluespecimen. In the analysed areas it was possible to identify a highlyporous structure that can easily promote the water transport bycapillarity (Fig. 12). If no waterproof treatment has been appliedon the inside face of the BA tiles, the water resulting from the dry-ing of all constructive materials will ascend by capillarity and willremain on the tile green vein zones (with higher porosity than thecream light zones) leading to premature detachment andfragmentation.

Fig. 8. SEM image of the BA specimen surface showing recrystallized material and EDS emission spectrum.

Fig. 9. SEM image of efflorescence.

Fig. 10. SEM image of the BA green veine

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The elementary composition of a cement–glue specimen, deter-mined by EDS analysis, detected the presence of (weight percent-age): 11.58% of Aluminium, 67.33% of Silicon, 9.54% of Calcium. Itwas also detected the presence of Magnesium (Mg) and Sodium(Na). In Fig. 13 it is possible observe a SEM image showing a detailof a quartz grain inside a cement–glue specimen.

The analysed mortar specimen exhibits a highly porous struc-ture (Fig. 14). The mortar EDS analysis determined the presenceof 10% of calcium, due to the mortar manufacturing process thatincorporates lime and limestone aggregates.

During the mortar drying process, which still occurs after thepavement setting, all the water will evaporate, and depending onthe degree of acidity, may react with calcium present in it.

Typically, Calcium is a very reactive element and if occurs a pHalteration on the water (used to produce the mortar) a violent reac-tion may happen and normally will affect all the existing calciumon the system (mortar, cement–glue and limestone).

The elementary composition of a mortar specimen, determinedby EDS analysis, detected the presence of (weight percentage):6.38% of aluminium, 78.42% of Silicon, 10.45% of Calcium. InFig. 15 it is also possible to observe a SEM image showing a detailof a quartz grain inside a mortar specimen.

d zone and EDS emission spectrum.

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.

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3.4. X-ray diffraction analysis

X-ray diffraction (XRD) is also an important tool for analysingrock samples through crystalline structures. Experiments such as

those reported in this work should be undertaken in a systematicway always using the same equipment, experimental procedure,experimental parameters, and using a known software databasefor the treatment of the data [8].

XRD analysis is currently used to verify the existence of a cer-tain mineral phase in specific stones, as a measurement that, be-sides identifying mineral contents with economical importance,allows the interpretation of possible phase transitions that tookplace in the past and observe the occurrence of other solid-statereactions, as part of the geological research [8].

The experimental procedure for the mineral identificationsusing XRD was:

The powder samples were provided from three tile portions col-lected from the damaged bathroom floor. Three materials wereseparated and grounded: limestone cream parts, limestone greenclay parts and cement–glue parts.

For experimental reasons, only a small amount of material(approximately 3 g) was needed for accurate intensity measure-ments. Grinding should not affect the crystallite size or the coher-ent domain. However, as micro absorption is probably a majorfactor affecting accuracy in the acquisition of the intensity data,the most suitable way to eliminate this effect is powder size reduc-tion. A reasonable and attainable upper grain size limit was deter-mined to be around 30 lm [8].

The equipment used to perform the experimental work was aRIGAKU, Model D/MAX 3 at INETI. All test parameters were keptconstant for all the XRD determinations during this work (seeTable 1).

Fig. 14. SEM image of a mortar specimen and EDS emission spectrum.

Fig. 15. SEM image of a mortar specimen – quartz grain.

Table 1Test parameters used in all XRD determinations.

Parameters Value

Sample amount (approx.) 3 gPowder grain size Max 30 lm2h (range) 5–104.996 (�)Scanning rate (2h) 0.012 (�/s)Slew 1.2 (d/m)Anode Cu (45 kV, 20 mA)

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Qualitative X-ray diffraction analysis of green veined areas re-vealed the presence of: Quartz (SiO2), Calcite (CaCO3), Nacrite (Al2-

Si2O5(OH)6) and Potassium Aluminium Silicates (KAl3Si3O11). Thisindicates that these zones are visibly weakness areas formed byclay minerals like Nacrite, a hydrous aluminium phyllosilicate sim-ilar to Kaolinite. Nacrite is also identified as a low temperaturehydrothermal alteration product which indicates that these areascan absorb water [9]. It was also revealed the presence of Potas-sium Aluminium silicate (KAl3Si3O11) – muscovite. This is a com-mon rock forming mineral that has a layered structure of

aluminium silicate sheets weakly bonded together by layers ofpotassium ions. These potassium ion layers produce the perfectcleavage of muscovite [9].

It should be pointed out that these minerals were not identifiedthrough the petrologycal microscopy, but only after XRD as it canbe observed in Fig. 16.

Qualitative X-ray diffraction analysis of beige/light zones withefflorescences revealed the presence of: Calcite (CaCO3) and Quartz(SiO2). This indicates that the efflorescences on BA surface havetheir origins on Calcium (Ca) that with high probability came bycapillarity from the mortar and cement–glue phases through theclay green areas of BA (see Fig. 17).

Qualitative X-ray diffraction analysis of cement–glue specimensrevealed the presence of: Quartz (SiO2), Rutile (TiO2), Calcite(CaCO3), Portlandite or Portland cement (Ca(OH)2), Halloysite (Al2-

Si2O5(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–gluetechnical specification [9]. Portlandite is formed during the curingof concrete which in turn, is one of the cement–glue components.Halloysite, quartz and calcite are also common components ofwhite cement-glue.

3.5. Water absorption at atmospheric pressure, apparent density andopen porosity

Density and porosity are useful parameters always requiredwhen studying natural stones properties. The principle for deter-mining the properties is very simple. After drying to constant mass,the apparent density and the open porosity are determined by vac-uum assisted water absorption and submerged weighing ofspecimens.

Open porosity (ratio of void space in sample accessible from itsouter surface to its total bulk volume, expressed as a percentage ora volume fraction) and apparent density (ratio of the mass of asample to its geometrical volume including closed pores but notopen pores) – probably the most assessed properties of stones,were determined according to the standard EN 1936:1999. Waterabsorption at atmospheric pressure was determined according tothe standard EN 13755:2001 [8,10,11].

Apparent density can be defined as the ratio between the massof the dry specimen and its apparent volume (volume limited bythe external surface of the specimen, including any voids). Openporosity can be described as the ratio (as a percentage) betweenthe volume of the open pores and the apparent volume of the spec-imen [10]. Experimental tests revealed that the medium value for

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.

Table 2Water absorption at atmospheric pressure for BA limestone.

Mass of the samples (g)

Dry Saturated Water absorption at atmospheric pressure (%)

295.9 298.7 0.94284.9 287.2 0.81288.9 291.9 1.04292.5 295.1 0.91292.8 295.9 1.06

Average 0.95Standard deviation 0.10Variance 0.20

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BA apparent density was 2560 ± 12 (kg/m3) and also that the med-ium value for BA open porosity was 2.4 ± 0.3 (%).

Water absorption at atmospheric pressure results are listed inTable 2.

Based on the results, BA cannot be considered as a high porositylimestone; however, it is a heterogeneous rock having two distinctphases: light/beige phase and a green veined phase [12].

As such studies with heterogeneous rocks, rock durability de-pends not only on properties such as open porosity and apparentdensity, but is also controlled by the size and spatial distributionof the textural elements (e.g. green vein zones) within the rock [6].

The particular deterioration of BA is more closely associatedwith the absorbed water from the green veined zones and fissuresthan with the any other mechanical property [6].

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4. Comments and conclusions

‘‘Bianco di Asiago” is a sedimentary chemical limestone show-ing stratification surfaces, more visible in some areas. These sur-faces reflect the limestone sedimentary origin and are usuallyassociated to the weakest tile zones, not only with low mechanicalresistance but also with an expected lower durability. Its chemicalcomposition is normally formed by calcium carbonate and by someoxides of iron and aluminium.

It is recognised that the green veined zones showed on BA give afavourable aesthetical effect of antique; however, this weakeststratified zones work like fluid transportation paths and ways inwhich the water and mineral salts can ascend by capillarity untilreaching the tile surface.

Hydrochloric acid (HCl) tests on altered specimens of BAshowed cold effervescence on calcium carbonate (light cream)and green veined zones, although the effervescences were strongeron 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 mainconclusions:

� The alterations on BA tiles are caused by capillarity phenom-ena. During the mortar drying process, which still occursafter the pavement setting, all the water will evaporate,and reach the rock tiles. This type of rock presents stratifiedareas (denominated stylolites) that contribute to the waterabsorption and thus to the premature damage of thepavement.

� The absorbed water that comes from the mortar and cement–glue will bring dissolved salts that will remain on the stratifiedareas. These zones are visibly weakness points formed by clayminerals like Nacrite, which is a hydrous aluminium phyllosili-cate with a structure similar to the micas and therefore formsflat hexagonal sheets. Nacrite is also identified as a low temper-ature hydrothermal alteration product which indicates thatthese areas can absorb water.

� The reaction of the minerals in the stratified areas along with thewater will cause the appearance of a white coated layer on thetiles surface and the consequent premature mechanical damageof the rock.

� Results show that if no waterproof treatments are performed onBA tiles, water will arise by capillarity (until it reaches the equi-librium between evaporation and capillarity) from the pave-ment substrate and will reduce the rock strength.

� The general conclusion of this work is that all fabric and petro-physical rock properties must be considered in order to evaluate,predict and understand the vulnerability to water action and saltweathering in carbonate natural stones.

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