Artificial weathering of Spanish granites subjected to salt crystallization tests: 
Surface roughness quantification 
P. Lopez-Arce a.*, M.J. Varas-Muriel a.c, B. Fernandez-Revuelta b, M. Alvarez de Buergo a, 
R. Fort a, C. Perez-Soba c 
• Grnpo de Petrologfa ap/icada a la Conservaci6n del Patrimonio, Instituto de Geologfa Econ6mica (CSIC-UCM), Jose Antonio Navais 2, 28040, Madrid, Spain 

b Laboratorio de Piedra Natural, Instituto Ge%gico y Minero de Espaiia (fGME), Calera 1, 28760 Tres Cantos, Madrid, Spain 

C Departamento de Petro[ogfa y Geoqufmica, Facultad de Geologfa, Universidad Complutense de Madrid (UCM), Jose Antonio Novr'iis 2, 28040, Madrid, Spain 

Keywords: 

Granite 

Salt crystallization tests 

Weathering 

Surface roughness indexes 

Durability 

Natural stone 

1. Introduction 

ABSTRACT 

For hundreds of years, two types of granite (Zarzalejo and Alpedrete) from the Madrid region, Spain, have 

been extensively used as building stones. Fresh specimens of both stone types have been sampled from their 

respective quarries and subjected to sodium sulphate salt crystallization test (SeT). The resulting physical 

and chemical weathering patterns have been characterized by polarized light optical and environmental 

scanning electron microscopy. Water absorption under vacuum conditions and mercury intrusion 

porosimetry techniques were used to determine the pre- and post-SeT porosity and pore size distribution. 

The following non-destructive techniques were performed to assess stone durability and decay: ultrasound 

velocity (US) and surface roughness determination (SR) of intra- and inter-granular quartz, feldspar and 

biotite minerals at the centre as well as at the corners and edges of specimen surfaces. Before the sa, US 

values were lower and SR values higher in Zarzalejo (ZAR) than Alpedrete (ALP) granite. After sa, the US 

values declined while SR rose in both types of granites, with greater average differences in ZAR than ALP for 

both parameters. Feldspar and biotite and their inter-granular contacts were found to be the weakest and 

therefore the most decay-prone areas of the stone. 

The initial SR parameters were generally higher and rose more steeply after SeT at the corners and around 

the edges of the specimens. 

While behaviour was found to be similar in the two types of granite, variations were greater in ZAR, the less 

durable and more decay-prone of the two. Surface roughness measurement of mineral grains in granite 

stones is a very useful, in situ, non-destructive technique for quantifying salt crystallization-mediated 

physical and chemical weathering. The resulting quantification of decay and of related durability provides 

insight into the future behaviour of this type of stone, commonly used in historic buildings. 

Granite is a stone traditionally used as a building material in central 

Spain, where it is known as Berroquefia stone, a name derived from the 

word "berrueco", a local term used to designate the spheroid granitic 

boulders generated by natural weathering (R0yne et al., 2008). The 

materials mostly used in traditional Madrilenian architecture are 

granite, limestone and brick. Granite has been and continues to be the 

rock most intensively quarried in the Spanish capital. Some of the 

region's granitoids, medium-grained and displaying frequently mafic 

micro-granular enclaves, would very likely never have been used as 

building materials if it was not for their local availability in quarries so 

close to the city of Madrid, a large and demanding consumer. 

A number of studies have been published on granite weathering, 

and, more specifically, on the role that intra-granularmicrotexture and 

microstructure play in chemical and physical weathering processes 

and on the comparison between experimental and natural weathering 

in alkali feldspars (Lee and Parsons, 1995; Lee et al., 1998); the surface 

chemistry, etch pits and mineral-water reactions of minerals and the 

dissolution rate of quartz and (Lasaga and Blum, 1986; Blum et al., 

1990), decay patterns in monumental granite (Matias and Alves, 

2001); nature and decay effects of urban soiling on granitic building 

stones (Schiavon et al., 1995) or kaolinization processes on granitic 

building stone in urban environments (Schiavon, 2007). There are also 

works on biodeterioration of granite (Schiavon, 2002). Physical or 

mechanical weathering has also been explored in depth, with research 

focusing on areas such as: the role of micro cracking in shear-fracture 

propagation (Moore and Lockner, 1995); the micro-effects of fire on 

granite, namely the generation of new and the growth of pre-existing 

fissures (G6mez-Heras et al., 2006); the influence of rift and bedding 
* Corresponding author. 

E-mail address:plopezar@geo.ucm.es (P. L6pez-Arce). 



planes on the physical-mechanical properties of granitic rocks and the 

implications for granite monument decay (Rivas et al., 2000); 

microscopic observations and contact stress analyses of granite 

subjected to compression stress (SeD et al., 2002); the effect of 

foliation on the textural strength of granitic rocks (Akesson et al., 

2003); the role of microfracture and porosity in the physical­

mechanical properties and weathering of ornamental granite (Sousa 

et al, 2005); natural microcrack networks in granite affected by a fault 

zone (Onishi and Shimizu, 2005); and the use of fracture indexes to 

check the suitability of granite outcrops for quarrying (Sousa, 2007). 
Few studies have been conducted, however, on the evaluation of 

granite durability when used as a building stone and the selection of 

the most suitable parameters for this purpose. Haneef et al (1993) 

conducted a laboratory simulation of degradation of granite by dry and 

wet deposition processes. A method for determining granite durability 

is artificial weathering via salt crystallization cycles (Rivas et al., 2008), 

but visual quantification of the weathering observed remains elusive. 

The pressure generated by salt crystal growth in confined spaces in 

porous building materials such as stone, brick, and concrete is generally 

acknowledged to be a major cause of damage in both ancient 

monuments and modem buildings (Goudie arxl Viles, 1997; Doehne, 

2002). Inasmuch as sodium sulphate is generally regarded to be 

particularly harmful, it is frequently used in accelerated weathering 

tests. Spanish and European standard UNE-EN 12370:1999 on natural 

stone test methods specifies that testing should be performed on 

materials with a porosity of over 5%. Nonetheless, the method may also 

be useful in lower porosity stones such as granite, where salt may cause 

adverse effects due to the appearance of new or the widening of existing 

cracks (Alonso et al., 2008). A chemical replacement mechanism of 

granite minerals by gypsum has been claimed by Schiavon et al (1995). 

Weathering or durability is difficult to quantify in granite, however, 

since both are usually based on visual aspects or weight loss, which are 

particularly difficult to measure because the low open porosity of this 

stone determines a low salt solution uptake and penetration and 

therefore scant visual decay. Besides weight loss, other physical 

properties such as wave propagation, colour and surface roughness 

have been used to quantify the durability of ornamental granites 

affected by salt crystallization (Alonso et al., 2008). Hodson et al. 

(1997) determined surface roughness in unweathered alkali feldspar 

grains expressed as the ratio of specific surface (found by applying the 

BET isotherm to gas adsorption data) to the geometric area of mineral 

grains in a given size fraction. G6mez-Heras et al. (2006) used image 

processing to find the roughness index (RI(fs)) of burnt and unburnt 

granites based on parameters determined from segmented binary 

images. This RI(fs), which is defined as the ratio between fissure length 

and area, is useful to distinguish between open (RI<l) and closed 

(RI = 1) fracture system patterns. 

Nonetheless, very few natural stone studies based on surface 

roughness measurements have been conducted, and even fewer have 

addressed the use of such measurements to quantify durability. 

Furthermore, as information on the standard tests used to measure 

surface roughness, as well as on measuring conditions (such as the 

choice of the filters) and calculations, is missing in many papers, 

comparison of the results to the present findings is difficult. 

Since the 1930s, when the first roughness metres appeared, surface 

texture measurement has been based on two-dimensional profilometry 

and contact gauges. The standardization and formalization of 3D texture 

analyses is presently underway (Blateyron, 20(0). The stylus for 20 

profilometry with a contact gauge is still in use today, and while it 

delivers lower precision and is more time-consuming, it is also more 

affordable. Further to the DIN EN ISO 3274:1996 standard test, in two­

dimensional surface tracing the stylus is dragged at a constant speed 

across the surface to be measured in a position normal to it Furthermore, 

physical properties such as surface roughness can be used to quantify the 

physical-chemical weathering patterns that may or may not be 

observable under polarized optical or scanning electron microscopy. 

The main aim of the present study was to assess decay patterns 

occurring after artificial weathering induced by Na2S04 crystallization 

cycles in the two types of granite most commonly used for building 

construction in the city of Madrid, Spain: the Zarzalejo and Alpedrete 

granites. Decay assessment focused on the relationship between 

physical-chemical weathering features characterized under polarized 

light optical microscopy, environmental scanning electron microsco­

py, mercury intrusion porosimetry, water absorption and ultrasound 

propagation on the one hand, and the variation in surface roughness 

in mineral grains located on the surface edges, corners and centres of 
granite specimens on the other, as a method for quantifying decay and 

consequently durability of the two varieties of granite. 

2. Materials and methods 

2.1. Granites 

2.1.1. Geology 

The two types of granite used are quarried primarily in the south­

western region of the Guadarrama MOlll1tains, a range in the eastern 

part of the Spanish Central System (SCS) shown in Fig. 1. The SCS is a 

Hercynian orogenic belt located in central Spain, bounded on the north 

and south by two Tertiary river basins, the Duero and the Tagus (Tajo), 

respectively. The SCS batholith comprises large volumes of peralumi­

nous granites, which were emplaced onto high- to medium-grade 
metamorphic rocks during the late Hercynian orogeny (325 to 285 Ma; 

Villaseca et aI., 1998; VillaseGl and Herreros, 2000; Bea et aI., 2004; 

ViUaseca, 2003). The composition of these plutonic rocks is predomi­

nantly monzogranitic to leucogranitic, with minor basic to intermediate 

rocks (GonzaJez-Casado et al., 1996; Villaseca et aL, 1998). 

Although both are compositionally classified as monzogranites, 

the Alpedrete and Zarzalejo varieties are texturally and mineralogi­

cally different The Zarzalejo granite pluton is located about 60 km 

northwest of the city of Madrid, at 1104 m above sea level. Two 

texturally different units can be distinguished: a relatively equigra­

nular, medium- to coarse-grained, grey unit, and a porphyritic unit 

characterized by the presence of K-feldspar megacrysts in a medium­

grained matrix. Microgranular mafic enclaves and xenoliths are 

occasionally found: They are usually sub-rounded or ellipsoidal and 

display a tonalitic composition. The major components are quartz, 

plagioclase, K-feldspar, and biotite (Fig. 2a); muscovite may occur as a 

secondary accessory mineral, along with a number of micaceous 

microcrystalline aggregates indicative of the transformation of 

cordierite. 

The 350-km2, irregular Alpedrete monzogranite pluton is located 

45 km north of the city of Madrid, at 919 m above sea level. 

Petrographically, it is an equigranular, fine- to medium-grained, dark 

grey monzogranite to granodiorite pluton, although porphyritic 

varieties may also occur locally. The main petrographic characteristic 

of this variety is the abundance of microgranuiar mafic enclaves 

present in all the outcrops: their mineralogy consists chiefly of quartz, 

plagioclase, K-feldspar and biotite, with variable amounts of accessory 

cordierite (Fig. 2b). Other accessory minerals commonly found include 

ilmenite, apatite, zircon and monacite (ITGE, 1990). 

2.1.2. Architectural use 

The granites used for testing were selected for their role in 

traditional construction in the region of Madrid, Spain. This study 

focused on two varieties of granites from abandoned quarries at 

Zarzalejo and Alpedrete, both located in the province of Madrid. These 

two materials are generally found in good state of conservation on 

many monuments and historical buildings in the city of Madrid and its 

surrounding region. Due to the wide temperature fluctuations 

between winter and summer typical of continental climates, flaking 

and granular disintegration are, though, sometimes visible in the 

granite on ground-level parts of the buildings (Menduifia et al., 2005). 



Biotitic Monzogranites of medium grain size. Zarzalejo-Valdemorillo type 

I�;�I Porphyric te=;j Equigranular 

Biotitic Monzogranites of coarse-medium grain size. Atalaya Real type 

I�:��I Porphyric t==;j Equigranular 

Biotitic Monzogranites of coarse-medium grain size with cordierite. 

IfiM Alpedrete type 

Leucogranite of two micas of medium grain size. 

:;t:!:;� Machotas type 

Leucogneis and glandular Ortogneis 

� 
Sands, clays, granite and gneis boulder and pebbles 

D 
Active quarry 

� 
Ancient quarry 

� 

Town 

:>; 
... ,. -:,.1 

Fig. 1. Geological map of the area surrounding the Alpedrete and ZarzaJejo granite quarries (modified from ITGE (1990) and Menduif'ia et al. (2005)). a) Alpedrete granite in the 

quarry and b) used as a building stone for Madrid Royal Palace. c) ZarzaJejo granite in the quarry and d) used as building material for the Roman VaJdemaqueda bridge. 

Today what was known in the past as Zarzalejo granite is quarried 

and marketed lll1der the trade names Blanco Rafaela (Rafaela White) or 

Gris Escorial (Escorial Grey) (Menduiiia et al., 2005; Garda del Cura et 

al., 2008). This granite was used to build many monuments of the region 

of Madrid from the 15th to the 17th centuries (Perez-Monserrat and 

Fort, 2004). Stone on heritage monuments has likewise been replaced 

with Zarzalejo granite in recent years (Fort et al., 1996). Fig. la shows 

the Alpedrete quarry; Fig. 1 b shows a monument built with this type of 



Fig. 2. Alpedrete and ZarzaJejo granites: a) macroscopic photograph of ZarzaJejo granite; b) macroscopic photograph of Alpedrete granite; c) thin section of ZarzaJejo granite 

under PlDM; d) thin section of Alpedrete granite under PlDM. Bt: biotite; Qtz: quartz; Fs: feJdspar. 

granite (Royal Palace atMadrid); Fig. lc shows the Zarzalejo quarry and 

Fig. ld shows a Roman monument built with this granite (Valdema­

queda bridge). 

2.1.3. Granite samples 

Fifty five cubic (50 ± 5-mm) specimens (rather than the 40 ± 1-

mm cubes prescribed in the RIlEM recommendations on dimensions 

for durability standard test) were obtained for each type of stone. The 

specimens were cut with a Diamant Boart, Scut Mixed Granite 

diamond blade saw, fitted with a 350-mm diameter disc having 

40 mm wide, 3 mm thick and 10 mm high segments with micron­

scale diamonds. 

The cubic specimens were used for both the salt crystallization 

ageing test and to determine the petrophysical properties of the stone, 

before and after Na2S04 exposure. 

2.2. Salt crystallization test (Sa) 

The selection of samples was based on anisotropy indexes 

following the criteria set out in Fort et al. (2008) for the salt 

crystallization-induced accelerated ageing test and according to 

Spanish and European standard UNE-EN 12370:1999. In this test, 

specimens were immersed in a sodium sulphate solution (14% 

decahydrate, density = 1.055 g/cm3) for 2 h at 20.0 ± 0.5 cC and 

oven-dried for at least 16 h at 105 ± 5 0c. They were then allowed 

to cool to room temperature for 2 h before starting the next cycle. 

Pursuant to standard test requirements, the specimens were initially 

subjected to 15 such salt crystallization cycles. Due to the low open 

porosity of granite and the scant weight loss and decay recorded in 

this first series, however, the specimens were subjected to a second 

round of 15 cycles. Consequently, the test specimens were subjected 

to a total of30 salt crystallization cycles prior to their characterization. 

The samples were rinsed daily after the cycles to eliminate all salt, as 

defined by conductivity values declining below the 20 J.IS/m mark. 

2.3. Characterization analyses 

2.3.1. Polarized light optical microscopy (PLOM) 

The surface of the specimens was studied before and after sa 
under polarized light optical microscopy (PLOM) to characterize 

mineralogy and physical-chemical weathering patterns. Grain sizes 

and percentage volumes were roughly estimated. Thin sections of 

stone were studied with an Olympus BX51 polarized light microscope 

fitted with an Olympus DP 12 (6 Vj2.5 A) digital camera. 

2.3.2. Environmental scanning electron microscopy (ESEM) 

Eight-millimetre cubes were cut from the corners of the specimens 

to study granite fractures and fissures in a near-natural state with an 

Inspect FEI environmental scanning electron microscope (ESEM) with 

an Oxford Instrument Analytical 7509 energy dispersive X-ray 

spectroscope. Environmental microscopy is used to analyze roughness 

non-destructively, for it eliminates the need for more destructive rock 

polishing. The physical-chemical weathering patterns were observed 

before and after the sa cycles. 

2.3.3. Surface roughness testing (SR) 

Surface roughness (SR) was measured with a stylus instrument, 

2D profilometry with contact gauge Mitutoyo Surf test S]-201P tester 

fitted with a 2-J.Ull diamond tip stylus, at an applied load of 0.75 mN 

and a measuring speed of 0.5 mm/so The maximum measuring range 

along the Z axis was 350 J.Ull and along the X axis, 12.5 mm. A total 

of 1152 roughness profiles were obtained from four fresh and four 

salt-weathered specimens of each type of granite. Seventy-two 

profiles were obtained for each specimen on any of their surfaces. 

Thirty-six profiles were based on intra-granular and the other thirty­

six on inter-granular measurements (Fig. 3). 

Each primary (or P-) profile was filtered to obtain the mean line 

with which to find the surface roughness parameters. The filtered 

profile (roughness or R-profile), whose total length is called the 

evaluation length, was divided into a number of sampling lengths, 

depending on the length of another profile filter. This latter filter, 



pm , " 
Srn "'jib Srnl 

10 

5 

0 

-5 

-10 

Ra ""; t,1Y1i 

2 

® 

l' l' 
Rz -sf. Yp! + sf, Yvl 

3 4mm 

,m 

20 Smmax 

10 

0 

-10 

-20 

-30 

-40 Rvmax 

-50 
0 0,1 0,2 0,3 0,4 O.5mm 

Fig. 3. Example sketches ofroughness profiles showing the surface roughness parameters: a) sketch of intra-granular surface roughness profile taken on quartz (Qtz), feldspar (Fs) 

and biotite (Bt) grains: b) sketch of inter-granular surface roughness profile taken at contacts between minerals: (biotite-quartz, quartz-feldspar and biotite-feJdspar). The 

measurements were taken at the centre, corners and edges of the surface of granite specimens (top left image). 

known as the cut-off, was applied to delimit wavelengths denoting 

roughness and those denoting waviness. 

Thirty-six 4-mm profiles (with a standard O.8-mm cut-off) were 

obtained from the intra-granular measurements relative to the following 

three main mineral grains: quartz, feldspar and biotite. Some of the 
biotite profiles (particularly in Alpedrete granite, which has smaller grain 

sizes) were 2.4 mm long with a standard 0.8-mm cut -off. The other thirty­

six profiles, 0.5 mm long (with a standard 0.2s-mm cut-off), were 

obtained from the measurements taken at the inter -granular contacts 

between minerals: biotite-quartz (Bt-Qtz), quartz-feldspar (Qtz-Fs) and 

biotite-feldspar (Bt-Fs). In each case, four surface measurements were 

taken at the corners, four on the edges and four in the centre of the faces of 

the granite spedmen. The roughness parameters analyzed in the intra­

granular study (Ftg. 3a) were calculated and defined as stipulated in 

standard ISO 4287:1984. (i) Ra is the arithmetic mean of the absolute 

values of profile deviations from the mean. (ii) Rzis the sum of the vertical 

distances between the five highest peal<s and the five deepest valleys 

within the sampling length (although in some profiles with few peaks, Rz 
was calculated with less than five peal<s). (iii) Sm is the mean spadng 

between profile irregularities. The roughness parameters analyzed in the 

inter-granular study (Fig. 3b), in turn, were: Rvmax, the absolute value of 

the minimum value of the profile deviation from the mean line, i.e., the 

deepest point on the roughness profile (R-profile); and Smmax, the 

absolute value of the maximum spadng between profile irregularities. 

2.3.4. Ultrasonic velocity (US) 

Ultrasonic velocity (Vp or P-wave velocity) was measured to 

evaluate decay non-destructively before and after sa. P-wave 

propagation time was measured to a precision of 0.1 J.IS with a PUNDIT 

eNS Electronics US instrument. Standard recommendations 

were followed, except for: (i) specimen shape and size (cubic, with 

50 ± s-mm sides instead of the prismatic specimens prescribed in 

Spanish and European standard UNE-EN 14579: 2005 on sound speed 

propagation); and (ii), the frequency of the transducers used, which 

was 1 MHz. The diameter of the flat contact area on these transducers 

was 11.82 mm. The bond between the transducers and the surface of 

the specimens was secured with a water and carboxymethylcellulose 

paste (Sichozell Kleister, Henkel). Measurements were taken in direct 

transmission/reception mode, across opposite parallel sides of the 

cubic specimens in all three spatial directions. 

2.3.5. Water absorption under varuum 

In addition to the variation in sample weight, water absorption, 

pre- and post-Sa open porosity and bulk density were found for the 

s-cm cubic specimens, as described in Spanish and European standard 

UNE-EN 1936: 1999 on natural stone test methods, determination of 

real density and apparent density and total and open porosity. 

Although this standard has now been replaced by UNE-EN 1936: 

2007, the 1999 edition was in effect when the trials were begun and 

therefore was the one used throughout the experiment. 

2.3.6. Mercury intrusion porosimetry (MIP) 

Mercury intrusion porosimetry (MIP) was applied to surface 

sections before and after sa to assess sample pore structure, i.e., total 

porosity (P) and pore size distribution (PSD). Readings were taken in 

pore diameters of 0.005 to 400 J.Ull under measuring conditions 



ranging from atmospheric pressure to 60,000 psia (228 MPa). Sample 

cores 10 mm in diameter and at least 30 mm high were cut and 

analyzed with a Micromeritics Autopore IV 9500 MIP. 

3. Results 

3.1. Mineralogy and physical-chemical weathering under PLOM and 

ESEM 

Both natural (pre-Scr) and salt crystallization-induced (post-scr) 

physical-chemical weathering of the two granite samples were examined 

on thin sections under polarized light optical microscopy (PLOM) and 

environmental scanning electron microscopy (ESEM). The pre-Scr thin 

sections showed that Zarzalejo monzogranite had a coarse- to medium­

grained inequigranular texture (Fig. 2a and c), with sporadic K-feldspar 

phenocrysts (1-3 cm). The components of this grey rock are plagioclase 

(35 vol.%), quartz (2-5 mm and 30 vol.%), K-feldspar (2-8 mm and 

20 vol.%), and biotite (2-4 nun and 15 vol.%). The main accessory 

minerals are apatite, ilmenite and zircon. 

The natural chemical weathering of Zarzalejo monzogranite showed 

some transformation of biotite into chlorite, and extensive plagioclase 

and minor K-feldspar crystal seritization. Brittle deformation, mainly in 

quartz crystals which displayed considerable undulatory extinction, 

sutured intra-granular edges and frequent recrystallization into 

polycrystalline aggregates, the presence of V-shaped polysynthetic 
twins in many plagioclase crystals and local kink-bands and exfoliation 

dislocations in biotite are signs of natural physical weathering. 

Numerous intra-granular and inter-granular microfractures were 

present in feldspars and quartz. 

Alpedrete monzogranite proved to be a medium- to fine-grained, 

equigranular, dark grey rock (Fig. 2b and d) consisting of interlocking 

plagioclase aggregates (30 vol.%), quartz (2-5 mm and 25 vol.%), K­

feldspar (1-4 cm and 35 vol.%) and biotite (2 mm and 10 vol.%). Other 

accessory minerals include ilmenite, apatite and zircon. Evidence for 

natural physical weathering was found primarily in quartz crystals, 

which exhibited undulatory extinction and sutured intra-granular 

edges, V-shaped twins in many plagioclase crystals and occasional 

kink-bands in biotite crystals. An abundance of microfractures was 

observed in quartz and feldspar crystals. 

No significant mineralogical changes were found in both Alpedrete 
and Zarzalejo monzogranite samples after the salt crystallization test. 

However, the natural physical weathering is increased after the salt 

crystallization test due to the increase of intra-granular and inter­

granular microfractures, especially in feldspar crystals and the opening 

of cleavage planes in biotite crystals (Fig. 4a and e). In light of the risk of 

microfractures and micro fissures production during thin section 

preparation, in the present study environmental scanning electron 

microscopy (ESEM) was used only to acquire visual information on 

these features in weathered granite samples, while decay quantification 

was performed on the basis of surface roughness trials. ESEM studies 

were, then, conducted on sample corners (8-nun cubes), as the most 

visibly weathered part of the 5-cm cubic specimens. Fresh samples of 

both ZAR and AlP were barely weathered, with only a few intra- or 

inter-granular fissures due to natural weathering. After the scr, the 

number of intra- and trans-granular fissures increased, particularly in 

the case of ZAR samples: fissures were mainly generated at and 

developed along the basal cleavage planes of biotite, to subsequently 

expand by circling quartz and cutting across feldspar grains (Fig. 4a and 

b). Radial fissures were also observed in the potassium feldspars 

surrounding biotite grains (Fig. 4b). This suggests that the opening of 

biotite planes during the salt crystallization test could generate stress 

forces within the crystal structure, resulting in the development of 

cutting across fissures. Physical weathering patterns induced by sodium 

sulphate crystallization between mineral grains are shown in Fig. 4c and 

d. Mineral fragments (10-12 J.Ull) were observed at the ZAR inter­

granular contacts. Fracturing would seem to have been induced by the 

salt crystallization process, for the fragments were detected in the post­

scr samples but not in fresh ZAR samples; none of the foregoing was 

observed in the AlP sample before or after the scr. The EDS analyses of 

these micro-fragments revealed the presence of Si, Al, K, Ca and Na, 

consistent with potassium feldspars and plagioclase composition. Pits and 

other signs of corrosion (Fig. 4c and f) attributable to natural weathering 

were also observed in plagioclase, but these developments may well have 

been intensified as a result of sodium sulphate crystallization. 

3.2. Intra- and inter-granular surface roughness 

3.2.1. Intra-granular surface roughness 

3.2.1.1. ZAR granite. The variation in the roughness parameters (Ra, Rz 

and Sm) relating to the three selected mineral grains (quartz, feldspar 

and biotite) and measured respectively at the centre, corners and edges 

of the surface of the four ZAR specimens before and after scr is given 

in Table 1. In the mineral grains located at the centre of the fresh 

specimen (pre-Scr) surface, the lowest values of Ra, or the arithmetic 

mean of the profile deviation from the mean, were found for biotite 

(3.73 ± 1.22 j.Ull), followed very closely by feldspar (3.94 ± 0.35 j.Ull); 

quartz values (5.99± 1.41 J.Ull) were somewhat distant. In fresh 

samples, the mean deviation was slightly higher for the corner and 

edge locations as compared with the centre ones. Considerable 

differences were found between the pre- and post-scr (weathered 
sample) values: the lowest Ra values in the centre of the samples were 

recorded for feldspar (6.33 ± 0.22 J.Ull, up by 61%), followed by biotite 

(8.06± 2.16j.Ull, 116% higher) and quartz (9.46± 2.14j.Ull, 58% over 

the fresh sample values). The difference between the values for the 

corners and edges of fresh and weathered samples was even greater 

(138% increase in biotite, 124% in feldspars and 47% in quartz). No 

distinction could be drawn between results for corners and edges: in 

some cases, the differences between fresh and weathered samples 

were larger in the former while in others the inverse relationship 

applied. Some examples of pre- and post-SCT intra-granular rough­

ness profiles for the three types of mineral grains at the corners and 

edges of ZAR specimens are shown in Fig. 5. 

The results for parameter Rz are given in Table 1, which show that 

the lowest pre-Scr values in the ZAR specimen were recorded for 

feldspar grains (average of 15.1 ± 1.66 J.Ull), followed by biotite (17.18 ± 

3.9711m) and quartz (23.46± 4.14J.Ull). The Rz values were very 

similar in the centre and on the edges for all the minerals and at the 

corners for feldspar, whereas the corner biotite and quartz grains 

exhibited higher values. After the SCT, the biotite and quartz values 

were very similar in all three locations (average of 34.67 ± 6.46 J.Ull 

for biotite and 34.87 ± 8.01 J.Ull for quartz), although the average 

increase in Rz was smaller for quartz (around 49% compared to 102% 

for biotite). The Rz values for feldspar rose from 15.38 ± 1.59 J.Ull to 

22.76 ± 0.89 Jlm (up 48%) in central grains, from 15.78 ± 2.18 Jlm to 

29.42 ± 3.67 Jlm (up 86%) at the corners, and from 14.14 ± 1.2 J.Ull to 

33.66± 7.84J.Ull (a 138% rise) along the edges of the specimens. 

Table 1 shows the Sm values i.e., the mean spacing between profile 

irregularities obtained in ZAR sample. The highest Sm values before 

and after SCT were recorded for biotite, which exhibited similar 

values in the centre and on the corners and edges of samples, 

although the values were somewhat higher in the latter location. The 

mean pre-Scr Sm value for biotite was 0.23 ± 0.02 mm. After the SCT, 

this value rose by around 36%. The average pre-Scr Sm for quartz 

grains was 0.21 ± 0.02 mm. After the test, this value rose (19%) up to 

0.24 ± 0.04 mm. While the lowest Sm values before and after the scr 

were found for feldspar grains, they were very similar to findings for 

the quartz grains. The meanSm value for feldspar prior to the SCTwas 

0.18 ± 0.02 mm. After the scr this value rose to 0.24 ± 0.02 mm. 

3.2.1.2. ALP granite. The variation in roughness parameter Ra for the 

three selected mineral grains measured at the centre, corners and 



i 

'if'� 'J�[ 
.J·II.:rtt-

liM 

Ag. 4. Environmental scanning electron micrographs (ESEM) of granite samples after the salt crystallization test. a) fissures across ZAR granite minerals (Bt: biotite, showing the 

opening of cleavage planes: Qtz: quartz and FsK: potassium feldspar showing trans-granular fissures; b) radial fissures on feldspar grains surrounding biotite grains in ALP granite: c) 

signs of chemical and physical weathering on ZAR granite (feldspar corrosion and mineral disaggregation): d) fragmentation offeJdspar caused by salt crystallization in a fissure in 

ZAR granite: e) detail of biotite showing the opening of cleavage planes: f) corrosion pits in feldspar grain. 

edges of the surface of ALP granite before and after sa is given in 

Table 2. The lowest Ra (profile deviation from the mean) values for the 

mineral grains located at the centre of the fresh specimen (pre-Scr) 

surface were found for feldspar (2.90 ± 0.56 J.Ull), followed by biotite 

(3.31 ± 1.34 J.Ull) and quartz (3.66 ± 0.55 J.Ull). The values measured 

on the corners and edges were very similar. The most significant 

difference in Ra values taken at the centre of the ALP sample before 

and after the ageing test was recorded for feldspar (with a post-test 

value of 5.89 ± 2.32 J.Ull, up by approximately 103%with respect to the 

fresh sample). The post-scr Ra value for biotite in the centre of the 

specimen was 5.46 ± 0.54 J.Ull, up to approximately 65% over the fresh 

specimen. The greatest increase for the corner and edge grains 

was found for biotite (with pre-Scr values of 3.14 ± 0.39 J.Ull at 

the corners and 3.25 ± 0.47 J.Ull on the edges, and post-scr means of 

6.83 ± 1.74 J.Ull at the corners and 7.57 ± 1.67 J.Ull on the edges, 

corresponding to an increase of approximately 117% and 133%, 

respectively). The increase in the Ra value for feldspar located on 

specimen corners and edges was only slightly lower than that for 

biotite (with before scr values of 2.74± 0.61 J.Ull at the corners and 

2.98 ± 0.48 J.Ull on the edges, and after scr means of 5.97 ± 1.10 J.Ull at 

the corners and 5.45 ± 0.54 J.Ull on the edges, corresponding to an 

increase of approximately 118% and 83%, respectively). For quartz 

grains in turn, the rise in Ra values after the scr was very similar for all 

three locations (with an average value of 5.75± 1.18J.Ull and an 

average increase of 52%). 

In ALP sample, the lowest pre-Scr Rz values (Ta.ble 2) were 

also recorded for feldspar grains (average of 11.65 ± 1.77 J.Ull), 

followed by biotite (average of 14.01 ± 3.2 J.Ull) and quartz (average of 

15.64± 2.7 J.Ull). The central, edge and corner values were very similar 

for all the mineral grains both before and after the salt crystallization 



Table 1 
Variation in roughness parameters measured on the intra-granular surface of the three mineral grains at the centre, corners and edges of four ZAR specimens before and after salt 

crystallization test Surface roughness parameters: (i) Ra is the arithmetic mean of the absolute values of profile deviations from the mean: (H) Rz is the sum of the vertical distances 

between the five highest peaks and the five deepest valleys within the sampling length: and (Hi) Srn is the mean spacing between profile irregularities, 

RP Ra (Jl11l) Rz (,un) Srn [10,000%1 (mm) 

Mineral Before After Before After Before After 

Biotite 

Centre 3,73 ± 1,22 8.06±2,16 16,24±4,70 31,02± 7,80 0,24± 0,02 0,26 ±0,05 

Corners 4.39 ± 1,80 959 ± 1.77 20,46±4,8 38.29 ±3,73 0,22 ± 0,02 0,29 ±0,08 

Edges 3,49 ± 0,75 9,18 ±2.39 14,84± 2,42 34,71 ± 7,86 0,24± 0,03 0.35 ±0,07 

Quartz 

Centre 5,99 ± 1,41 9,46 ±2,14 22,66±4,73 34,73±6,25 0,19 ± 0,02 0,25 ±0,04 

Corners 7,49 ± 0,91 8.92 ±2,24 2755 ±3,49 32,49±9,46 0,25 ± 0,04 0,24 ±0.03 

Edges 552 ± 1.37 10.23 ±2,72 20,19 ±4,20 37.38±8.33 0,19± 0,01 0,24±0,04 

Feldspar 

Centre 3,94± 0.35 6.33 ±0,22 15.38 ± 159 22,76 ±0,89 0,18 ± 0,02 0,21 ±0,01 

Corners 3,94± 0,69 7,98 ± 1,07 15,78±2,18 29,42 ±3,67 0,18 ± 0,02 0,25 ±O.D2 

Edges 3,71 ± 0,28 9,16± 1,49 14,14± 1,20 33,66 ± 7,84 0,18 ± 0,02 0,26± 0,03 

test, with the exception of the post-Sa corner and edge biotite, which 

showed a higher Rz, Quartz and feldspar exhibited very similar post-Sa 

values (means of 21.38 ± 4,03 J.Ull and 21,41 ± 5,00 J.Ull, respectively), 

but the average Rz increase was lower for quartz (about 37% and 

84% for feldspar), The Rz values for biotite rose from 14.33 ±6,0 J.Ull 
to 24,71 ± 3,8 J.Ull (up 72%) in central grains, from 12,91 ± 1.0 J.Ull to 

28.41 ± 6.3 J.Ull (up 120%) at the corners, and from 13.89 ± 2.17 J.Ull 
to 31.86 ± 8.75 J.Ull (for a rise of 129%) along the edges of the specimens, 

Table 2 show the Srn values in ALP sample, The highest Srn values, 

before and after the sa, were recorded for biotite, which exhibited 

similar values in the centre and on the corners and edges of samples, 

although the values were somewhat higher in the latter location, The 

.m 
15 

10 
5 

o 
-5 

Before salt crystallization test 

-10 Ra:02.3 pm 
-15 Rz=8.9 �Lm 

@ 

Sm=O. 18 mm Biotite -20±-=--=-,-__ ---,, __ ---,:---=:.c:.::.::., 
o 2 3 4 mm 

f·m 
15 

10 
5 
o 
-5 

-10 
Ra=;3.91L 

-15 RZ=;15,3 pm 
�0�S� m���o.�1 6�m� m

c
-_________________ F�e� l�d�sp�a�r" 

o 2 3 4 mm 
,m 
15 

-15 
Ra=5.8 )Lm 
Rz"'20.5 )lm 

-20 Sm.::O.20 mm Quartz 
0 2 3 4 

mm 

mean pre-Sa Srn value for biotite was 0.21 ± 0.04 mm. After the sa, 

these values rose by around 36%. The average pre-Sa Srn for quartz 

grains was 0.18 ± 0.03mm. After the test, this value rose 19% up to 

0.21 ± 0.02 mm. While the lowest Srn values, before and after the sa, 

were found for feldspar grains, they were very similar to findings for 

the quartz grains. The mean Srn value for feldspar was 0.17 ± 0.02 mm. 

After the sa this value rose to 0.22 ± O.04mm, a 28% increase. 

3.2.2. Inter-granular surface roughness 

Examples of inter-granular roughness profiles for the three 

selected types of mineral grains at the corners and edges of ALP 

specimens before and after the sa are shown in Fig. 6. The variation 

After salt c rystallization test 

-40 
-60 Ra=;9.7 )Lm 

Rz=32.6 )Lm 
Sm=O.24 mm Biotite - 80!:----,------,,-----,,------, 0 2 3 4 

Ra=16.0 )Lm -60 Rz=62.8 )Lm 

mm 

�0, <_-----"-S�
m�

�O�
.

�43�m�m"-__ ------�--F�e�
l

�d�s�a�r 
o 2 3 4 mm 

40 
-60 Ra=13.0 �Lm 

Rz=48.4 pm 
-80 Sm=0.25 mm Qua rtz 

0 2 3 4 
mm 

Fig. 5. Examples of intra-granular surface roughness single profiles for Zarzalejo granite minerals before and after the salt crystallization test (SIT). a) biotite on corner before SIT: 

b) biotite on edge after SIT: c) feldspar on corner before SIT: d) feldspar on edge after SIT: e) quartz on edge before SIT: f) quartz on edge after SCT. Surface roughness parameters: 

(i) Ra is the arithmetic mean of the absolute values of profile deviations from the mean: (H) Rz is the sum of the vertical distances between the five highest peaks and the five deepest 

valleys within the sampling length: and (Hi) Srn is the mean spacing between profile irregularities. 



Table 2 
Variation in roughness parameters measured on the intra-granular surface of the three mineral grains at the centre, corners and edges of ALP specimens before and after salt 

crystallization test 

RP Ra (Jlffi) Rz (Jlffi) Sm [10,000%1 (mm) 

Minerals Before After Before After Before After 

Biotite 

Centre 3.31 ± 1.34 5,46± 054 14.33 ± 5,98 24,71 ±3,81 0,22±0,07 0,29 ± 0,04 

Corners 3,14± 0.39 6,83± 1,74 12,91 ± 1,01 28,41 ±6,29 0,19±0,01 0,27 ± 0,01 

Edges 3,25 ± 0,47 757 ± 1,67 13.89 ±2,17 31,86±8.75 0,22±0,03 0,28 ± om 
Quartz 

Centre 3,66± 055 5,96± 01,22 14,57 ± 1,66 20,26±3,07 0,17±0,02 0,20± 0,01 

Corners 4,06± 0,95 5,71 ± 1.35 16,29 ±3,92 22,95 ±3,88 0,18±0,01 0,21 ± 0,03 

Edges 3,62±0,41 5,97 ± 1,48 15,13 ± 1,75 2253 ± 55 0,18±0,05 0,22 ± 0,03 

Feldspar 

Centre 2,90±056 5,89 ± 2.32 11,60± 1,61 21,82 ± 7,8 0,17 ±0,02 0,22 ± 0,04 

Corners 2,74±0,61 5,97 ± 1,1O 11,05 ± 1.84 22.32 ±4,9 0,18 ±0,02 0,24± 0,04 

Edges 2,98 ±0,48 5,45 ± 054 12,29 ± 1.94 20,09 ± 2.3 0,17 ±0,02 0,19± 0,04 

in roughness parameter Rvmax before and after SCT as measured on 

inter-granular contacts (Bt-Fs: biotite-feldspar; Bt-Qtz: biotite­

quartz; Qtz-Fs: quartz-feldspar) in the ZAR and ALP granites is 

given in Table 3. 

As the table shows, the pre-SCT Rvmax and Smmax values were 

higher in ZAR than in ALP granite and these values rose in greater 

proportion in the former stone after the test. Before the SCT, the 

Rvmaxvalues for the three mineral grain contacts were very similar in 

ZAR granite, although somewhat lower for Bt-Fs (mean 14.29 ± 

3.90J.Ull) than for Bt-Qtz (mean 14.74 ± 4.34J.Ull) and Qtz-Fs (16.10 ± 

3.29J.-Ull). The values were slightly higher on the corners and edges, 

particularly for the Qtz-Fs contact. After the SCT, the Rvmax values 

"m 
15 

Before salt crystallization test 

10 

5 

o 
., 

-10 

-15 

-20 

Rvmax=10,8 )!m 
Smmax:O,09 mm 

0 0.1 

"m 
15 

10 

5 

0 

., 

-10 

-15 Rvmax=15.7 )lm 

-20 
Smmax"'O. 1 7 mm 

0 0.1 

"m 
15 

10 

5 

0 

., 

-10 

-15 Rvmax"'9.5 )lm 

-20 
Smmax=O.10 mm 

0 0.1 

Biotite-Feldspar 
0.2 0.3 0.4 0.5 

mm 

© 

Biotite-Quartz 
0.2 0.3 0 .4 0.5 

mm 

® 

Quartz-Felds ar 
0.2 0.3 0.4 0.5 

mm 

rose most in Bt-Fs (up by 18% in all, on average), followed by Bt-Qtz 

(10%) and Qtz-Fs (2%). The rises were greater in the centre of the 

specimens (especially in the case of Bt-Fs (average increase of 50%)). 

In the Bt-Qtz and Qtz-Fs contacts, Rvmax declined slightly on 

specimen corners and edges. While prior to the SCT, the Bt-Qtz 

Smmaxwas substantially higher than the values for the other contacts 

in ZAR, after the test the values were very similar for all three types of 

contacts. The Bt-Fs and Qtz-Fs values declined slightly, by 1% and 5%, 

respectively, while the dip was steeper for Bt-Qtz (33%). 

The pre-Scr Rvmax values for the three mineral grain contacts 

were very similar in AlP granite, but slightly lower for Qtz-Fs 

(mean 8.17 ± 1.55 J.Ull) than for Bt-Fs (mean 10.58 ± 2.31 J.Ull) and Bt-

"m 
15 

After salt crystallization test 

10 

5 

o 
., 

-10 

-15 

-20 
0 

"m 
15 

10 

5 

0 

., 

-15 

-20 
0 

"m 

15 

., 

··10 

··15 

·-20 

Rvmax"'15,3 )lm 
Smmax=O.14 mm 

0.1 02 0.3 

Rvmax=10.3 )J.m 
Smmax=O. 16 mm 

01 02 0 3  

Rvmax=20.9 )!m 
Smmax=O.08 mm 

0 0.1 02 0.3 

Biotite-Feldspar 
0.4 0.5 

mm 

@ 

Biotite-Quartz 
0.4 0.5 

mm 

CD 

Quartz-Felds ar 
0. 4 0.5 

mm 

Fig. 6. Examples of inter -granular surface roughness single profiles for Alpedrete granite minerals before and after the salt crystallization test (SIT). a) biotite-feldspar on corner 

before SIT; b) biotite-feldspar on edge after SIT; c) biotite-quartz on edge before SIT; d) biotite-quartz on corner after SIT; e) quartz-feldspar on corner before SCT; f) quartz­

feldspar on edge after SIT. Surface roughness parameters: (i) Rvmax, absolute value of the minimum value of the profile deviation from the mean line; (ii) Smmax, absolute value of 

the maximum spacing between profile irregularities. 



Table 3 
Variation in roughness parameters (RP) measured at inter-granular contacts (Bt-Fs: biotite-feldspar: Bt-Qtz: biotite-quartz: Qtz-Fs: quartz-feldspar) on ZAR and AlP granites, 

before and after salt crystallization test 

RP Rvmax (Jl11l) ZAR Rvmax (Jl11l) AlP Smmax [10,000%1 (mm) ZAR Smmax [10,000%1 (mm) ALP 

Minerals Before After Before After Before After Before After 

Bt-Fs 

Centre 

Corners 

Edges 

13.37 ± 5,22 

13.17± 2,28 

16.35±4,19 

19.55 ±4,94 

14,73 ± 2,20 

15,70± 2,82 

9,87 ±2,17 

11,76± 1.56 

10,10± 1,64 

11,89 ± 1.58 

15,09 ± 1,80 

14.55 ±3.54 

0,12 ±0,04 

0,12 ±0.30 

0,13 ±0,03 

0,12 ±0,03 

0,13 ±0,01 

0,13 ±0,03 

0,10±0.D1 

0,14 ±0,04 

0,11 ±0,02 

0,10±0,02 

0,13 ±0,01 

0,13 ±0,02 

Bt-Qtz 

Centre 

Corners 

Edges 

14,06±3,15 

13.87±4,10 

16.30±5,78 

18.32±3,75 

14,80±4,23 

15,25 ±4,68 

12.35±4,18 

11,88 ± 2.59 

11,61 ± 1,98 

13.92 ±2,74 

12,63 ±2,75 

13.28 ±2,72 

0,12 ±0,05 

0,14±0,01 

0,13 ±0,03 

0,12 ±0,01 

0,12 ±0,03 

0,11 ±0,01 

0,12 ±0,04 

0,11 ±0,04 

0,10 ±0.D1 

0,13 ±0,01 

0,13 ±0,02 

0,12 ±0,02 

Qtz-Fs 

Centre 

Corners 

Edges 

15,09±3,71 

16,20± 1,22 

17.30±4,93 

15,77±2,18 

15,23 ±2,16 

16.49 ±4,28 

9,16± 1,77 

7,62± 0,70 

7,72 ±2,16 

12.47 ±3,1O 

13.53 ±3,28 

12.51 ±2,83 

0,13 ±0,03 

0,14±0,01 

0,14 ±0,05 

0,13 ±0,01 

0,12 ±0,03 

0,14 ±0,02 

0,1O±0,02 

0,12 ±0,02 

0,1O±0,Q3 

0,11 ±0,03 

0,1O±0,04 

0,11 ±0,03 

Qtz (11.95 ± 2,92J.Ull), Scant differences were found between the 

centre, corner and edge values, After the sa, the Rvmax values rose in 

all the mineral grain contacts and at all specimen locations, The 

highest rise was recorded for Qtz-Fs (up by 60% in all, on average), 

followed by Bt-Fs (31%) and Qtz-Fs (11%). 

In ALP granite, Smmax before and after the sa was very similar in 

the three mineral grain contacts, although it rose after the test, by 16% 

in Bt-Qtz, 8% in Qtz-Fs and 7% in Bt-Fs, 

33. Ultrasonic velocity 

ZAR and ALP granite ultrasound velocity results are given in 

Table 4. Measurements were taken along the three axes of the cubes 

and averaged to establish post-Sa rises or declines. Velocity declined 

in all ZAR samples after the sa. The mean pre-Sa velocity in the 

selected samples, 3234 m/s, dropped to 2998 m/s after the test, for an 

average decline of around 9%. In ALP granite also, all the specimens 

had a higher velocity before (44% higher than in ZAR) than after the 

sa. Before the sa, the selected samples showed a mean velocity of 

4670 m/s, which declined to 4446 m/s after the test, i.e., by around 5%. 

3.4. Mass variation, density and open porosity 

Table 5 shows the mass variation, density and open porosity values 

obtained by water absorption in vacuum, before and after the sa. 

The minus sign preceding the number in the weight variation (%) 

column indicates weight loss, whereas the plus sign indicates weight 

gain. The weight gains shown in the weight variation column may 

have been due to premature completion of the salt rinsing process. 

Table 4 
Ultrasonic velocity values for Zar and AlP granites, before and after salt crystallization 

test (SIT), 

Granite 

samples 

ZAR AlP 

10 9 

16 15 

19 16 

26 17 

34 21 

35 28 

SCT Average 

velocity for 3 

axes (m/s) 

ZAR 
Before 3103 

After 3161 

Before 3116 

After 2808 

Before 3052 

After 2868 

Before 3174 

After 2957 

Before 3452 

After 3097 

Before 3506 

After 3097 

Variation 

velocity 

/).Vp (%) 

- 2  

1 0  

6 

7 

10 

12  

Average 

velocity for 3 

axes (m/s) 

ALP 
4821 

4272 

4761 

4535 

4469 

4288 

4615 

4373 

4556 

4462 

4798 

4743 

Variation 

velocity 

/).Vp (%) 

11  

5 

4 

5 

2 

The mean variation in the mass of the six ZAR specimens tested was 

- 0.14%. Such a slight variation after the first 15 cycles according to 

the standard UNE-EN 12370: 1999 led to repetition of the test After 

30 cycles, a somewhat greater weight increment was found: - 0.85%. 

The mean increment in the six ALP specimens after 30 cycles was 

- 0.18%. The bulk density, before and after the sa, was very similar in 

both granites. The average open porosity in the ZAR sample rose from 

1.7% before to 1.9% after the sa, for a 12% increase. At 0.9%, the average 
pre-Sa open porosity was lower in the AlP than in the ZAR sample. 

It rose to 1 % after the test, for an 11 % increase. Mean water absorption 

in ZAR samples was 0.6% before and 0.7% after the sa, while in ALP 

samples, the pre-Sa mean was 0.3% before and the post-mean 0.4%. 

In other words, while open porosity was slightly higher in ZAR, the 

rise in water absorption was about the same in the two granites. Open 

porosity and water absorption increased in all the ZAR samples. 

3.5. Total porosity and pore size distribution (PSD) by MIP 

The MIP results for the granite samples are given in Table 6 and the 

PSD curves are shown in Fig. 7. Total porosity was observed to rise 

after the sa in both Zarzalejo and Alpedrete granites: in ZAR, from 

1.4% in the fresh sample to 2.11% in the weathered specimen; and in 

ALP, from 0.5% before to 1.01% after the test The mean pore diameter 

was around 0.2 J.Ull in both fresh granites (0.19 J.Ull in ALP and slightly 

higher, 0.24 J.Ull, in ZAR). This value rose slightly in the ALP specimen 

(to 0.21 J.Ull) after weathering. Mean pore diameter increased 

somewhat more in ZAR (to 0.31 J.Ull) than in ALP. 

The PSD results showed that neither of the stones exhibited pores 

below 0.01 J.Ull either before or after the test Pores with diameters 

Table 5 
Mass variation, density and open porosity found by water absorption in vacuum, before 

and after the UNE-EN 12370:1999 salt crystallization test 

Samples First Second Apparent 

weight weight density 

variation variation (kg/m3) 

Open 

porosity 

(%) 

Water 

absorption 

(%) 

(%) (%) 
Before After Before After Before After 

ZAR 10 -0,10 

16 -0,08 

19 -0,08 

26 -0,15 

34 -0,23 

35 -0,16 

AlP 9 

15 

16 

17 

21 

28 

0,03 

0,05 

0,05 

0,05 

0,04 

0,04 

- 0,71 

- 0,60 

- 0,67 

- 0,74 

- 1.35 

- 1,03 

-0,17 

-0,15 

- 0,28 

-0,18 

-0,17 

- 0,14 

2656 2636 1,8 

2643 2629 1,7 

2624 2624 1,8 

2641 2638 1,7 

2646 2643 1.5 

2689 2643 1,6 

2665 2660 0,8 

2647 2660 0,9 

2647 2650 1,0 

2650 2666 0,8 

2663 2659 0,8 

2648 2665 0,8 

2.0 

1.9 

2.0 

1.9 

1.8 

1.9 

0.6 

0.6 

0.6 

0.6 

0.5 

0.5 

1,0 0.3 

1,0 0.4 

1.2 0.4 

1,0 0.3 

1,0 0.3 

0.8 0.3 

0.7 

0.7 

0.7 

0.7 

0.7 

0.6 

0.4 

0.4 

0.4 

0.4 

0.3 

0.3 



Table 6 
Surface area, density, open porosity and pore size distribution of ZAR and ALP granite 

samples obtained by MIP before and after the salt crystallization test 

Granites ZAR AlP 

Before After Before After 

Surface area (m2jg) 0,088 0,104 0,039 0,074 

Median pore diameter volume (Jl11l3) 0,893 2,237 0.309 0,705 

Mean pore diameter (Jl11l) 0,24 0.31 0,19 0,21 

Apparent density at 1,14 psia (gjcm3) 2,65 2,62 2,67 2,65 

Real density (gjcm3) 2,69 2,68 2,68 2,67 

Porosity (%) 1.40 2,11 051 1,01 

Micro<5 Jl11l (%) 75,7 62 715 635 

Macro>5 Jl11l (%) 24.3 38 285 365 

Pore size distribution 

<0,01 Jl11l (%) 0,00 0,00 0,00 0,00 

0,01-0,1 Jl11l (%) 12,68 9,76 15.43 1557 

0,1-1 Jl11l (%) 39,15 29.34 5350 43.39 

1-10Jl11l (%) 27,17 31,06 7,20 5.64 

10-100Jl11l (%) 6,15 6.37 8.86 11,79 

>100 Jl11l n%) 14,85 23.46 15,01 23,61 

ranging from 0,01 to 0,1 J.Ull prevailed in both ZAR and ALP, although 

the percentage declined slightly after salt weathering in both granites 

(in the AlP sample, however, the percentage of 0,01 to 0,03-J.Ull pores 

rose and the percentage in the 0,03 to 0,05-J.Ull range declined), In 

ZAR, the number of pores from 0,1 to 1 J.Ull and 1 to 3 J.Ull declined 

slightly, while the percentage of those measuring from 3 to 10 J.Ull 

increased, Moreover, pores with diameters between 5 and 10 J.Ull, 

non-existent in the fresh samples, appeared in the weathered ZAR 

specimens, The percentage of pores with diameters over 100 J.Ull also 

rose (23.6%). 

The percentage of these over 100 J.Ull pores grew in ALP as well, 

along with pores between 0.5 and 2 J.Ull, while pores with diameters 

ranging from 0,1 to 0.5 J.Ull declined in number and no pores were 

detected in the 2-10 J.Ull range, The proportion of pores in the 0,01-

0,05 J.Ull and 10-100 J.Ull ranges was similar in both fresh and 

c ,. 
• 

1 2  • 
• 
� 
u 

10 E 
• E 8 , 
:g 

6 c 
0 
•• 

• 2 
E 

2 m 
:z: 

0 
0,001 

C 
20 

• 18 
• 
• 16 � 
u 1. E 
• 1 2  E , 10 

:g 8 
c 
0 

6 •• 
2 • 
E 

2 m 
:z: 

8.001 

0,010 

--'-J 

0,100 

[ ZAR-Weathered sample (after seT) 
-lAR-Fresh sample (before SCT) 

, 
\ 
, 

1 ,000 10,000 100,000 

Pore diameter hIm) 

, 
\, 
\ 

�AlP-Weathered sample (after seT) I -ALP-Fresh sample (before SCT) 

0,010 0,100 1 ,000 10,000 100.000 

Pore diameter (Ilm) 

Fig, 7, Open porosity and pore size distribution determined by MIP for ZAR and ALP 

granites. 

weathered ALP and ZAR specimens, The main difference was found 

in the 0.5-10 J.Ull range, 

Before the scr, ALP granite had a high real density (2,68 gjcm3) 

and low total porosity (0.5%), The latter is defined to be the sum of the 

pores <5 J.Ull (71.5%) and the pores >5 J.Ull (28.5%), The PSD was found 

to be bimodal, with one mode in the 0,1-0.5 J.Ull interval (over 50% of 

the total porosity in the sample) and the other in the >100 J.Ull 

intervaL After the scr, real density declined slightly (to 2,67 gjcm3), 

while total porosity increased considerably (to 1,01%), 

Before the scr, ZAR granite had a similar real density (2,69 gjml) 

and a higher total porosity (1,4%) than the Alpedrete stone, The 

percentage of <5 J.Ull pores (75,7%) trebled the >51.lm value (243%), 

Pore size was concentrated primarily in three intervals: O , l-l l.lm 

(over 39% of total porosity), 1-5 J.Ull (over 25%) and > 1 00 J.Ull, After 

scr weathering, real density declined slightly in ZAR (from 2,69 to 

2,68 gjcm3), while total porosity rose somewhat (from 1,4% to 2,1%), 

The proportion of pores >5 J.Ull grew (to one third of the initial total 

porosity), with the major share (23.5%) measuring> 100 J.Ull, 

The three other parameters determined with MIP were specific 

surface (m2jg), pore volume (J.Ull3) and mean pore diameter (J.Ull) 

(Table 6), Both the initial values of these parameters and their pattern 

of change after the scr differed in the two granites, 

After the scr, ZAR pore diameter and specific surface grew slightly 

(from 0.241 J.Ull to 0.311 J.Ull and from 0.088 m2/g to 0.104 m2/g, 

respectively), while pore volume rose to over double the initial value 

(from 0,893 J.Ull3 to 2,237 J.Ull3), In ALP, in turn, pore diameter 

increased slightly (from 0,193 J.lm to 0,206 J.Ull), but pore volume 

and specific surface jumped to double the initial values (from 

0309 J.lm3 to 0,705 J.Ull3 and 0,039 m2jg to 0,074 m2jg, respectively), 

4_ Discussion 

4.1. Intra-granular and inter-granular surface roughness 

According to the results, in fresh (pre-Scr) ZAR samples, biotite had 

the lowest Ra values (defined as curve deviations from the mean line), 

followed by feldspar and then quartz, (Fig. 5 and Table 1). After the scr, 

central, corner and edge quartz Ra, and central feldspar Ra increased by 

about50% of the initial (pre-Scr) value. The Ra rose by more than double 

for biotite on specimen centres, corners and edges and feldspar on the 

corners and edges. The steep rises in roughness parameter Ra values for 

biotite after the scrwere due the opening of mica cleavage planes (Chen 

et aL, 2000; Alonso etaL, 2008). Parameter Rzfollowed the same pattern 

as Ra in both AlP and ZAR. Biotite Ra and Rz roughness measurements 

were observed to also depend on the orientation of the mineraL When 

measurements were taken on the basal planes, the Ra and Rz values 

were much lower; this would explain the higher standard deviation for 

this mineral, particularly for post-Sa Rz. However, biotite exhibited no 

prevalent orientation in these granites and the number of measure­

ments (48 for each type of granite: four in the centre, four at the corners 

and four on the edges of each of four specimens) was regarded to be 

large enough to accommodate the possible differences in orientation in 

the samples analyzed. 

In feldspars, significant variation was observed in the surface 

roughness of fresh alkali feldspar grains of the same size and from the 

same source rock, with some relatively high values denoting grains 

rich in patch perthite and lower values reflecting the presence of 

grains in which cryptoperthite and lamellar microperthite prevailed 

(Lee and Parsons, 1995). Surface roughness values obtained for freshly 

ground and washed alkali feldspar as the ratio of the specific surface 

(obtained by the BIT gas adsorption technique) to the geometric area 

(obtained by direct measurements of grain size and shape) ranged 

from 3.0 to lOA (Holdren and Speyer, 1985, 1987). In a number of 

naturally weathered alkali feldspars from soils, by comparison, this 

parameter ranged from 83 to 653. The value of 2 obtained under 

atomic force microscopy (Blum, 1994), in turn, indicated that much of 



the additional specific surface was internal to the crystals and that the 

irregularity of cleavage surfaces may have made a significant 

contribution to surface roughness. This provides a further explanation 

for the fairly high standard deviation found in the intra-granular 

surface roughness values, particularly the post-Sa Rz, for feldspar 

grains in both types of granites (Tables 1 and 2). 

While the location of feldspar and quartz on specimen surfaces had 

no effect on Srn values, this parameter did vary with location for 

biotite, which showed higher values on the corners and edges after 

the sa, especially in ZAR. The higher Srn values (defined to be the 
mean spacing between profile irregularities) for biotite explain and 

quantify the post-Sa opening of biotite basal planes. The Srn values 

for all three mineral grains were slightly higher in ZAR than ALP. The 

highest values were recorded for biotite, both before and after the sa, 

with larger post-test increases at corners and edges. The lowest values 

and smallest rise in Srn were found for quartz, with feldspar in an 

intermediate position. 

The effect of mineral location in more (corners and edges) or less 

(centre) exposed areas was visible in ZAR granite after the sa. This 

was especially the case for feldspar, as, while in biotite the Srn rose by 

more than double and in quartz by around 50%, regardless of location, 

in corner and edge feldspar it grew by more than double, but by only 

around half of the initial value when the mineral was located in the 

centre of the sample. On the contrary, specimen cutting with a 

diamond blade saw would appear to have little effect on roughness 

parameters (in the case of intra-granular study), in light of the scant 

difference observed between the central and corner/edge values in 

the fresh specimens (before the Sa). 

In AlP granite before the sa, feldspar had the lowest Ra values, 

followed by biotite and then quartz (Table 2). After weathering (Sa), 

the value of this parameter nearly doubled in feldspar regardless of 

location and grew by about 50% in central, and more than 100% in 

corner and edge, biotite. As in ZAR, quartz Ra grew in ALP (after the 

Sa) by less than 50% regardless of the location of the mineral. 

Initially, before the sa, the Rz roughness parameter was higher in 

the ZAR than in the AlP samples with respect to all selected mineral 

grains at every location (by 30% in feldspar, 23% in biotite and 50% in 

quartz). After the test, values increased at a similar rate in the two 

samples: quartz rose by around 50% (slightly less in ALP); the figure 

for biotite was double the initial value at all locations in ZAR granite, 

and more than double at corners and edges in the ALP sample; in 

feldspar the increase was 50% in the centre and more or less 100% at 

the corners and edges of the ZAR granite specimens, and nearly 

double at all locations in the ALP specimens. In other words, the Rz 

parameter followed the same pattern as Ra in both ALP and ZAR 

Before the sa, the values of roughness parameters Ra, Rz and Srn 

were higher in the ZAR than in the ALP samples, particularly as 

regards quartz grains (whose Ra showed a 64% increase). In feldspar 

the difference was on the order of 35% while in biotite of only 16%. The 

average post-Sa increase in Ra was also greater in the ZAR sample, 

especially for biotite (which initially had the lowest Ra value), whose 

values grew by more than double regardless of its location. In the AlP 

sample, in turn, biotite Ra rose by more than double at the corners and 

edges and by around 50% in the centre of the sample. In ZAR, the post­

sa value for feldspar was double the initial value at the corners and 

edges only, with the figure for the central grains increasing by 20%. In 

the ALP samples feldspar weathered evenly (with increases of 

approximately 100%) regardless of its location. In both the ZAR and 

AlP specimens, the increase in quartz grain roughness (Ra) was 

around 50% of the initial value, regardless of location. 

In both granites, therefore, location influenced post-Sa mineral 

roughness parameters. Specimen cutting with a diamond blade saw 

initially appeared to have little effect on Ra, however, in light of the 

scant difference between the central and corner/edge values in the 

fresh specimens (before the Sa), with the exception of quartz grains 

located at the corners, which exhibited higher values. In ZAR before 

the sa, the value of Rz was very similar in the centre and on the edges 

and corners for all the minerals, except for corner biotite and quartz, 

which exhibited higher values. These results indicate that the 

diamond blade saw cut did affect this parameter, in quartz grains 

more than the other two minerals, in light of its higher Ra and Rz 

values on edges and corners than in the centre in the fresh (pre-Sa) 

samples. Initial sample surface roughness was caused by the saw and 

depended on the blade used. While this did not affect the present 

results, which compare roughness values before and after the sa, it 

may have had an impact on the differential behaviour of the minerals, 

depending on their hardness or abrasion resistance. The higher post­

sa corner and edge SR values in both granites (compared to the 

centre values) may be explained by their greater exposure to salt 

solution penetration and alteration due to saw cutting, and by the fact 

that volume expansion during salt crystallization is less confined in 

these areas. 

Intra-granular surface roughness values were higher and rose 

more after the sa than the inter-granular surface roughness values, 

in both granites (Figs. 5 and 6 and Tables 1-3). Inter-granular 

measurements showed that ZAR had higher pre-Sa Rvrnax values, 

and steeper rises after the test, but in the centre of the specimens only, 

for the value of this parameter actually declined at the corners and 

edges, particularly in the Qtz-Fs contacts. In ALP granite, Rvrnax rose 

more for contacts located at the corners and edges. Bt-Fs in ZAR and 

Qtz-Fs in ALP were the mineral contacts most affected by the increase 

in SR These findings prove that feldspar and its grain contacts are the 

weakest areas and therefore most prone to decay. 

No differences were observed in the measurements taken in the 

inter-granular contacts at the centre, corners or edges of the pre-Sa 

ALP specimens: in other words, these contacts were not affected by 

cutting. The post-SCR increase in SR values was higher in specimen 

corners and edges than in the centre. 

The pre-Sa values in ZAR, by contrast, did reveal location-based 

differences, particularly for the Qtz-Fs contact, where the higher 

values recorded on specimen corners and edges denoted the effect of 

cutting, similar to the effect observed for Qtz in the intra-granular SR 

study. Such alterations could impact the quartz surrounding minerals 

and generate trans-granular fissures (Fig. 4a). 

In post-Sa ZAR, the inter-granular SR values declined slightly at 

corners and edges. The explanation for this finding is that the ZAR 

specimens were visually eroded in these areas: the sharp, smooth 

corners and edges in pre-Sa samples gave way to rounder, rougher 

surfaces, as these were the most severely weathered parts of the 

specimens, particularly in this granite. Dimensional loss in these 

visibly weathered parts of the stone was confirmed by the 0.6% to 1.4% 

weight loss detected (Table 5) after the sa. This in turn would 

explain why, on the edges and corners where the outer-most surface 

of the granite had worn away, the maximum depth and width of the 

valleys between mineral grains (Rvrnax and Srnrnax, respectively) 

were shallower and narrower, rather than deeper and wider, than in 

both ALP granite and the centre of the ZAR specimens. 

4.2. Relationship between roughness and other results 

4.2.1. PLOM and ESEM 

Further to the study of thin sections, the sa caused barely any 

physical or chemical alterations in the rock-forming minerals in the 

Alpedrete and Zarzalejo monzogranites. Cracks and fissures were 

also difficult to detect, particularly when located between minerals 

(inter-granular cracks). Even intra-granular (cracks inside a mineral) 

and trans-granular (affecting more than one mineral) cracks may be 

hard to see (Sousa et al., 2005). 

The polarized light optical microscopic (PLOM) and environmental 

scanning electron microscopic (ESEM) findings in the samples before 

sa, however, revealed natural dissolution-corrosion processes in 

feldspar due to plagioclase seritization. These weathered areas were 



more readily penetrated by salt solutions, which may explain the 

concomitant post-Sa rise in porosity observed in water absorption 

and MIP analyses. Most physical properties of granites for ornamental 

use are known to be affected primarily by voids in the "pristine rock", 

for weathering processes are known to widen micro fractures and 

therefore the number of pore-shaped voids, particularly in feldspars 

(Sousa et al., 2005). Plagioclase and feldspar alteration on granitic 

stocks from Southern Iberian Massif (Spain), studied by Jimenez­

Espinosa et al. (2007), appears to result from dissolution especially 

along cleavage and fracture planes. 
Greater intra-granular surface roughness (Ra and Rz), especially in 

feldspar grains, may be due to salt solution penetration and 

crystallization during accelerated ageing that affect almost the entire 

mineral grain. Dislocations in alkali feldspars may act as pathways for 

the ingress of relevant amounts of fluid into feldspar crystals (Lee and 

Parsons, 1995). Experiments conducted by these authors on alkali 

feldspars etched with HF acid showed that patch perthites (irregular, 

microporous microcline and albite intergrowths) dissolved. The pits 

enlarged by etching subsequently merged, giving rise to a highly 

porous, honeycomb texture. The heavily pitted areas were believed to 

be albite-rich and the areas with fewer pits to constitute K-feldspars. 

Sousa et al. (2005), using a combination of PLO M and fluorescence 

microscopy, identified intra-granular fissures or cracks to be the most 

frequent type of fissures in naturally weathered granites, accounting 

for 63% to 82% of all the cracks observed on two different types of 

granites with a medium to coarse grain size. This may explain why the 

post-Sa increase (Tables 1 and 2) in intra-granular surface roughness 

was higher than the rise in inter-granular roughness (Table 3). These 

authors also found that inter-granular cracks constituted less than one 

third of all microfractures in all the granites studied and were 

predominantly located at quartz-feldspar boundaries. These findings 

concur with the present ESEM findings (Fig. 4a), as well as inter­

granular surface roughness measurements, where the highest Rvrnax 

values were found between quartz and feldspar grains in ALP granite 

(Table 3), and biotite and feldspar grains in ZAR granite. 

According to intra-granular measurements, biotite underwent the 

greatest increase in roughness after the sa. That, and the fact that 

biotite grain size is larger and less uniform in ZAR (2-4 mm) than in 

ALP (around 2 mm), might explain the higher values for roughness 

parameters Ra and Rz and therefore the greater decay in the former 

stone. It may also be the reason why Ra grew at the same rate (more 

than double) regardless of where biotite grains were located on the 

ZAR specimen surface, but differentially depending on position in the 

ALP sample (50% in the centre and 100% on the corners and edges). A 

study conducted by Matias and Alves (2001) on stones in monuments 

at Braga, in north-western Portugal, showed that the decay patterns of 

biotite-rich, medium- to fine-grained granite were associated with 

quarry weathering, the presence of heterogeneous elements 

(enclaves and phenocrysts) and grain size variations. They also 

found that granular disintegration-mediated decay was more intense 

in coarser grained stones. Granular disintegration consists in grain­

by-grain disaggregation along grain boundaries, cleavages and minute 

fractures, particularly those in the vicinity of altered biotite grains. 

Biotite-rich rocks abrade most readily because they weather most 

readily. Granite has been observed to disintegrate due to biotite 

expansion during weathering. Laboratory and field studies have 

shown that K-ion leaching from the crystal structure and the addition 

of water and other ions causes biotite expansion due to splitting along 

basal cleavages, exposing additional surface area to chemical attack 

(Bradley, 1970; Helmi, 1985). Dissolution and expansion of secondary 

mineral phases along cleavage planes and grain boundaries controlled 

deterioration of the microfabric by causing transgranular cracking and 

the opening of grain boundaries ultimately causing disintegration of 

the rock (Curran et al., 2002). 

Biotite expansion is a physical weathering process: feldspars are 

affected by chemical weathering, namely salt solution-induced rises 

in pH (Schiavon et al., 1995). Nonetheless, the presence of potassium 

feldspar and plagioclases in micro-fragments of Zarzalejo granite 

(Fig. 4c and d) revealed by post-Sa EDS chemical analysis showed 

that salt crystallization caused physical desegregation of these 

minerals. Physical weathering might also have been responsible for 

the observed increase in surface roughness. Salt crystallization 

pressure generates tensile stress on pore surfaces, leading to micro­

cracking in the form of the initiation and propagation of new, or the 

extension and widening of existing, microcracks and pores, which 

cause rock disintegration, detachment and fracturing (Scherer, 1999; 

Nicholson, 2001; Scherer et al., 2001). 

Biotite minerals are also related to the increase in porosity due to 

the salt solution penetration- and crystallization-induced opening of 

basal plane layers, as discussed above. Gypsum crystallization along 

mica basal cleavage planes and extensive feldspar kaolinization has 

been observed in granite surfaces on a church at La Coruna, in north­

western Spain (Schiavon, 2007), where such stone is also frequently 

used as a building material. The opening of biotite planes during the 

salt crystallization test (Fig. 4a and b) may also generate stress in the 

crystals, resulting in the development of fissures running across them 

(trans-granular fissures). The inter-granular contact between biotite 

and quartz, for instance, constitutes a boundary between materials 

with different atomic lattices, types of bond, strength and E-modulus, 

resulting in different microfracture propagation patterns (Moore and 

Lockner, 1995). These authors observed that microcracks in under­

formed rock and in the far-field region of the laboratory sample were 

concentrated within and along the edges of quartz crystals, but near 

the shear fracture they were somewhat more abundant within K­

feldspar crystals. 

On the other hand, quartz grains might also be related to the 

increase in open porosity, pursuant to the cracking and widening of 

pre-existing fissures observed under the polarized light optical 

microscope but not detected during roughness measurements because 

the dimensions fall outside the detection range. This led to a smaller 

increase in the roughness parameter for this mineral because the 

readings were taken on the smooth, solid areas of the grains. 

4.2.2. US, open porosity and water absorption 

As noted above, salt crystallization pressure generates tensile 

stress across pore surfaces (Scherer, 1999) and microcracking, with 

the initiation and propagation of new cracks or the extension and 

widening of existing microcracks and pores. Determination of open or 

free porosity on granite from Oporto (Portugal) is considered by 

Begonha and Sequeira Braga (2002) as the property most strongly 

influenced by weathering. 

post-sa declines in ultrasonic velocity have been attributed to 

this widening of existing cracks or the appearance of new cracks 

(Alonso et al., 2008) and the development of new (Ruiz de Argandona 

et al., 1988; Suzuki et al., 1995) and the widening of existing (Suzuki 

et al., 1998) inter-granular cracks. The greater post-Sa decline in 

velocity observed for ZAR than ALP granite denotes the development 

of more fissures in the former. This is consistent with the initially 

higher open porosity and water absorption values in ZAR specimens 

and the steeper rise in these parameters after sa. An inverse 

relationship between ultrasound velocity and open porosity has been 

reported by other authors for different stones (Calleja et al., 1989; 

Jermy and Bel� 1998; Marques and Vargas, 1998; Sousa et al., 2005). 

In ZAR granite, the post-Sa cracks and intra-granular and trans­

granular fissures observed in quartz and feldspars under ESEM and 

the opening of biotite basal planes detected with roughness 

measurements (Srn) and observed under ESEM (Fig. 4a), can be 

related to this decline in ultrasound velocity and rise in open porosity. 

423. MIP 

Mercury intrusion porosimetry (MIP) was performed on 1-cm0 x 3-

cm cylinders removed from the specimen surface, while open porosity 



was measured by water absorption in vacuum on 5-cm cube specimens. 

These differences and the higher pressures involved in MIP explain the 

variations in both the pre- and post-Sa porosity data. 

In both ZAR and ALP, pores ranging in diameter from 0.01 to 0.1 J.lm 

were the most abundant, but their percentage declined slightly with 

salt weathering (Fig. 7). This concurs with Winkler (1997) that pores 

with diameters of under 0.1 J.UTI are scantly affected by decay because 

they cannot absorb moisture and that the weathering process 

primarily affects larger pores with the gradual break-up of the grains, 

much the same as in crystallization tests. 
The appearance of 0.5-2 J.lm pores in weathered ALP samples may 

possibly be attributed to the opening of intra-granular pores and 

fissures, while the rise in the percentage of pores of over 100 J.UTI in 

both granites might be the outcome of the development and widening 

of inter-granular and trans-granular fissures (as observed under 

ESEM). Mosquera et al. (2000) using MIP, obtained clearly bimodal 

pore size distributions for granite (as observed here for ALP) and 

defined a group of macro fissures as trans-granular fissures and 

similarly sized microfissures as inter- and intra-granular fissures. 

The stress generated by salts in pores leads to greater pore size and 

porosity and a decline in rock strength (Winkler, 1997; Benavente 

et al., 1999; Nicholson, 2001). The percentage of 1-3-J.UTI pores 

declined slightly in ZAR, while the proportion of 3-1 O-J.UTI pores rose 

and a new group of pores, with diameters of 5 to 10 J.UTI, appeared 

(a development that may be related to the existence of intra-granular 

pores and fissures). This increase in post-Sa microporosity can be 

associated with the rise in biotite and feldspar surface roughness. 

In ZAR the percentage of pores >5 J.UTI grew (accounting for one 

third of the initial porosity) after the sa, with a prevalence of the 

over 100 J.UTI size class (23.5%), perhaps as a result of the generation of 

inter- and trans-granular fissures. Macroporosity may be associated 

with intra-granular cracking in quartz grains and the trans-granular 

cracks propagated to surrounding minerals. 

Variations in specific surface (m2jg), pore volume (J.UTI3) and mean 

pore diameter (J.UTI) denote changes in the size and morphology of the 

pores (Meyer et al.. 1994). 

The post-Sa growth in ZAR pore diameter and specific surface 

(from 0.241 J.Ull to 0.311 J.Ull and from 0.088 m2/g to 0.104 m2/g. 

respectively) and the rise in pore volume to over double the initial 

value (from 0.893 J.UTI3 to 2.237 J.UTI3) suggest a change in pore 

morphology to larger and possibly interconnected pores. In ALP, in 

turn, pore diameter increased slightly (from 0.193 J.UTI to 0.206 J.UTI), 

but pore volume and specific surface jumped to double the initial 
values (from 0.309 J.UTI3 to 0.705 J.UTI3 and 0.039 m2jg to 0.074 m2jg, 

respectively). These findings infer that the pores were smaller and 

more irregularly shaped than in ZAR, and perhaps also interconnected. 

Using resin impregnation and SEM, Lee and Parsons (1995) 

observed: a three-dimensional network of corroded dislocations 

extending > 15 J.UTI beneath the surface of slightly weathered alkali 

feldspars; submillimetre to millimetre sized alkali feldspar micropores 

with some turbid regions (comprising orthoclase and albite-rich 

feldspar intergrowths), typically containing 0.65-0.70 angular micro­

poresjJ.lm2, for microporosities of approximately 1-2%; patch 

perthites (irregular microporous microcline and albite intergrowths) 

characterized by large irregular corrosion pits and continuous channels 

delineating subgrain boundaries; and on microperthite cleavage surface 

001, the enlargement of initially trapezoidal or hexagonal corrosion pits 

into near-equilateral triangles. 

Sousa. et a.l. (2005) observed that since salt crystallization is 

conditioned by voids or porosity, the damage caused by ageing tests in 

granites with open porosities of under 1.5% was practically nil after 

100 salt crystallization cycles; with increasing porosity, however, 

material loss grew to significant levels. This would explain, in part, 

why ZAR (with an open porosity of 1.4%) was more weathered after 

sa than ALP (with an open porosity of 0.5%) and why its intra­

granular roughness was higher but its inter-granular roughness at the 

corners and edges was lower. Walker (1990) and Walter (1991) 

found that fresh unweathered alkali feldspars had 0.25-4.75 vol.% 

porosity (average 1.45%) and that their micropores were often 

connected. The foregoing means that in ALP and ZAR granites, 

feldspar plays a significant role in salt solution absorption, while 

salt crystallization in its pores leads to greater porosity, crack and 

fissure development, the breakdown of mineral grains and mass loss. 

5. Conclusions 

The natural weathering in Zarzalejo (ZAR) and Alpedrete (ALP) 

granites from Madrid, Spain, observable under polarized light optical 

(PLOM) and environmental scanning electron (ESEM) microscopes, 

mainly involves feldspar grains. Before the sodium sulphate salt 

crystallization test (Sa), the ultrasound velocity (UV) values were 

lower, while open porosity, water absorption and surface roughness 

(SR) values were higher in ZAR than ALP granite. While post-Sa 

physical-chemical weathering detected under PLOM and ESEM was 

scant in both granites, the mean decline in US, the rise in SR 

parameters and weight loss were also greater in ZAR than ALP. 

Surface roughness measurement of mineral grains in granite 

stones is a very useful, in situ, non-destructive technique for 

quantifying salt crystallization-mediated physical and chemical 

weathering, as well as decay in and durability of this type of stone, 

typically used in historic buildings. 

1. The intra-granular SR study showed that the pre-Sa Ra, Rz and Srn 

values were higher in ZAR than in ALP and rose more in the former 

after the test. Bt and Fs were the minerals with the lowest initial 

values in ZAR and ALP, respectively. After the sa, the Ra and Rz for 

Bt rose by more than double regardless of its location on ZAR 

specimens, as well as on the corners and edges of ALP specimens, 

but only by 50% at centre ALP locations. The Ra and Rz values for Fs 

doubled at all locations in ALP specimens but only on the corners 

and edges of ZAR specimens, where the values for centrally located 

Fs grew by 50%. Qtz was the mineral with the highest initial Ra and 

Rz values in both granites, but since they only rose by 50% after the 

sa regardless of the location of the grains, the final values were 

similar to the Bt and Fs values. 

2. The intra-granular study showed that Bt and Fs were the minerals 

most affected by the increase in SR in both granites. According to 

the inter-granular study, Bt-Fs in ZAR, and Qtz-Fs in ALP were the 

mineral contacts most affected by the increase in SR. The initial 

values and the post-SCT increases were higher on specimen 

corners and edges. In other words, feldspar and biotite and their 

inter-granular contacts were found to be the weakest and hence 

most decay-prone areas. 

3. Physical weathering caused by salt crystallization and the opening 

of cleavage planes with concomitant increases in microporosity 

were observed in Bt, along with intra-granular fissures that 

developed into trans-granular fissures. The physical-chemical 

weathering due to salt solution reactions and crystallization 

observed in Fs led to increases in microporosity and intra-, inter­

and trans-granular fissures. The physical weathering exhibited by 

Qtz, induced primarily by sawing and salt crystallization, intensi­

fied inter-granular fissuring and prompted the appearance of 

trans-granular fissures. The initial SR parameters were generally 

higher and rose more steeply after sa at the corners and around 

the edges of the specimens. 

4. While behaviour was similar in the two types of granite, post sa 

variations in the parameters studied were greater in ZAR. The 

larger biotite and feldspar grain sizes in this granite and their 

concomitantly larger specific surface allowed greater salt solution 

penetration and crystallization between biotite cleavage planes 

and inside natural feldspar micropores. This Zarzalejo variety was 

found to be a less durable, more decay-prone granite than ALP. 



While stylus instrument, 20 profilometry and contact gauge 

measurement of surface roughness is more time-consuming and less 

precise than modem surface roughness instrument techniques, it is 

less expensive and a sufficiently accurate, highly useful and generally 

affordable method of quantifying material decay and durability. 

This study allows researchers to lll1derstand the physical changes 

that occur in bedrock due to chemical and physical salt weathering 

processes. These are mainly changes in porosity and density of the 

samples with increased weathering, which are similar to the processes 

important for the development of soils. It leads to the changing 
hydrologic conditions that result in increased rate of weathering with 

time due to intrinsic changes in the properties of the exposed bedrock. 

Acknowledgements 

This study was funded by the Government of the Community of 

Madrid under the MATERNAS project (Ourability and conservation of 

traditional natural materials in heritage architecture) (MATERNAS CM 

0505jMATj0094) and the Spanish Ministry of Science and Innovation 

as part of the Consolider-Ingenio 2010 programme (CS02007-0058). 

Thank you to the JAE-Ooc CSIC contract for supporting P. Lopez-Arce 

to develop this work. The authors are grateful to Laura Tormo of the 

Natural Science Museum (CSIC) for providing the ESEM photographs 

and analyses. Special thanks go to Inmaculada Ruiz and Ivan Serrano, 

IGE (Institute of Economic Geology) petrophysics laboratory techni­

cians, for their work, suggestions, ideas, and support. We also thank 

Margaret Clark for the English review of the manuscript, and to Nick 

Schiavon for his comments and for having greatly improved the 

manuscript. We also appreciate the help of two anonymous referees 

for the reviewing process that also has improved the present research. 

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