ORIGINAL ARTICLE

Porosity and mineralogy evolution during the decay processinvolved in the Chellah monument stones

Abderrahim Samaouali Æ Larbi Laanab ÆMohamed Boukalouch Æ Yves Geraud

Received: 29 June 2007 / Accepted: 11 February 2009 / Published online: 6 March 2009

� Springer-Verlag 2009

Abstract The objective of this work is to study the decay

process involved in the historical Roman Chellah located in

the Rabat city (Morocco). This monument is made up of

porous calcarenite stone. Several samples, taken from

altered and unaltered blocks, were analyzed by the water

saturation, the mercury intrusion porosimetry techniques

and using the scanning electron microscopy coupled to

energy dispersive X-ray spectrometer (SEM–EDX). To

perform a reliable chemical analysis, some samples were

also analyzed by inductively coupled plasma-atomic emis-

sion spectroscopy (ICP-AES). The mercury porosimetry

results show a bimodal porous network for this porous

material, the deterioration process of these stones involved

an increase in porous volume of about 2%. The lowest

porosity observed in the unaltered block is connected to the

presence of sparitic cement which causes a partial inter-

granular porosity clogging. The highest porosity of the

deteriorated block without crust is due to the increase in

inter-granular space. SEM photographs of the unaltered

sample show the presence of the porous primary grains, of

ovoid forms and millimeter-length sizes, and of the sec-

ondary grains, of rhombohedric forms and micro size.

Porosity is essentially located between the primary grains

and can be completely clogged by secondary precipitations.

Various forms of deterioration are observed on the altered

samples such as the dissolution of the secondary grains

edges, wells of dissolution and also the presence of argil-

laceous residues on the surface. This last was also detected

by the ICP-AES and EDX analysis which show an increase

of the silicon and aluminum contents toward the surface.

Keywords Calcarenite rocks decay � Moroccan historic

building � Porosity measurements � Dissolution �Clay precipitation

Introduction

The citadel of Chellah was built by the Romans, 23 cen-

turies ago, on a hill near the Bouregreg River valley

(Fig. 1). It is located approximately 2 km from Rabat

center (Morocco) and 3 km from the Atlantic Ocean. This

site, re-used by the Almohade dynasties, is now protected

by the Moroccan authorities who seized all the historical

and tourist interests. The building materials (cobble stones

with cobs), used in this site subjected to a subhumid

Mediterranean climate under strong oceanic influence,

undergo an important deterioration process. The question is

to determine the importance, the origin, the mechanisms

and the kinetics.

Several studies (Zaouia et al. 2005; Azeroual et al.

2005) were carried out on other sites in the city center, the

ramparts and the old medina doors. These sites are con-

stituted of calcareous stones similar to those which are the

A. Samaouali (&) � M. Boukalouch

Laboratoire de Thermodynamique, Departement de Physique,

Faculte des Sciences, Mohammed V University, B. P. 1014,

Rabat, Morocco

e-mail: samouali@fsr.ac.ma

L. Laanab

Centre de Microscopie, Departement de Physique,

Faculte des Sciences, Mohammed V University,

B. P. 1014, Rabat, Morocco

Y. Geraud (&)

Ecole et Observatoire des Sciences de la Terre,

Institut de Physique du Globe Strasbourg UMR 7516,

Universite Louis Pasteur, 1 rue de Blessing,

67084 Strasbourg Cedex, France

e-mail: yves.geraud@eost.u-strasbourg.fr

123

Environ Earth Sci (2010) 59:1171–1181

DOI 10.1007/s12665-009-0106-5

subject of our study. The authors studied the surface

deterioration of these monuments and showed that the

sodium chloride formation (halite) constitutes the main

factor of deterioration of these stones located very close to

the ocean. Moreover, several pollution compounds were

detected on the black crust of these monuments such as

SO4-, NO3, Mg2?, Pb2?. The stone decay caused by salt

crystallization was the subject of many studies (Goudie and

Viles 1997; Benavente et al. 2004; Rijniers et al. 2005;

Genkinger and Putnis 2006; Van et al. 2007), and is widely

recognized as an important cause of porous building

materials damage.

Meanwhile, other kinds of alteration were observed, like

precipitation of products on the surface or inside the rock,

cracking, disintegration, dissolution or biological coloni-

zation (Samaouali et al. 2005). Thus, various decay

mechanisms are to be brought into account to explain the

obtained degradation of implemented materials. One can

quickly state the processes of secondary phase precipitation

inside or outside the blocks. This process enhanced by

freezing phenomena which occurs if stone water content

reaches a critical value. So, when there is enough porosity

and water uptake in the porous network, the volume

expansion at the time of the transition water freezes (*9%)

exerts a destructive pressure on the stone (Prick 1995).

These decay phenomena involve black crusts formation if

precipitation takes place on the blocks, and causes cracking

when precipitation or freezing occur inside. They are

associated to the water transfer by capillarity and the

evaporation process (Hammecker 1995; Alves et al. 1996);

both depend on the porous network geometry. Among the

other mechanisms of deterioration, one can quote the bio-

logical activity related to the action of micro-organisms

like the bacteria, or plants such as moss. In fact, different

kinds of organisms were observed at the surface of these

monument stones. They can grow using the mineral com-

ponents of stones and its superficial deposits. The main

consequence of their metabolic activities, such as the

excretion of enzymes, inorganic and organic acids and of

complex forming substances, is the minerals dissolution of

the stone. Moreover, the roots or biochemical attack of the

plants, mainly, mosses and lichens induce physical stresses

Fig. 1 The red arrow indicates

the geographical location of

Chellah monument in the Rabat

area (extracted from Morocco

map). 275 9 275 mm

(96 9 96 DPI)

1172 Environ Earth Sci (2010) 59:1171–1181

123

and mechanical breaks (Tiano 1994). A last factor deter-

mining, under these climatic conditions, is the action of

wind charged with sand and aerosols, thus causing

important surface erosions.

Finally, stones deteriorate continuously as a result of

physical, chemical and biological processes, depending on,

e.g.,: air constituents, relative humidity, temperature, wind

velocity, solar radiation, frequency and intensity of rain,

sea spray, composition of the soil, living organisms

(Camuffo 1995).

The aim of this work is to study the decay process of

limestone blocs (called calcarenite) on Chellah monument.

Samples, taken from the second shop wall of the Roman

part (see Fig. 2), were analyzed in order to determinate the

porous network structure, the content and the nature of

clays formed during the deterioration process. These results

are compared to those obtained on samples taken from the

national library quarry (Rabat area), considered as a cal-

carenite material in unaltered state and not subjected to the

meteoric action. The first results allow us to determine

the consequences of the 2000 years action of exposure to

the atmospheric conditions on these materials.

Materials

The materials used in this archeological site are numerous;

one finds various limestones, granite, red bricks of various

periods as well as cob. In this study, we were interested in

materials used mainly in the Roman part for the current

buildings and in the Islamic part for the monumental doors.

It is a coarse bioclastic limestone resulting from a littoral

drawstring of plio-quaternary age (Akil 1990; Azouaoui

et al. 2000; Zaouia et al. 2005). This material corresponds

to an accumulation of shelly elements, biological remains

(urchin) or alga clusters. According to the bibliography, the

porosity of limestone varies between 18 and 47%

(Azouaoui et al. 2000; Benboughaba 2001; Samaouali et al.

2005). These authors studied physical characteristics

(porosity) of different limestone quarries in the regions of

Rabat, Sale (10 km from Rabat), Kenitra (40 km from

Rabat) and Casablanca (80 km from Rabat). Results show

that the porosity varies mainly from a quarry to the other.

But in the same quarry, there is no remarkable porosity

variation. More specifically, for the quarry of Rabat, the

measurements, performed on 18 samples as presented in

this work, show that the porosity ranges between 30.33 and

33.84% with an average of 32.04%. Meanwhile, this

material is characterized by good mechanical loads

(stresses and shocks) (Benboughaba 2001).

Different cylindrical samples were cut from better

weathered and unaltered blocks. The deteriorated block

presents faces with black crusts (Fig. 2) and faces in con-

tact with other elements (stones) of the wall, without crusts.

For that we carried out cores on the two types of faces. The

samples cored in the unaltered block, are referred as Unaltr.

The samples taken from the altered block are referred as

Altr_no_cr and Altr_cr, respectively, for those without and

with black crust.

Characterization techniques

The total porosity analysis was carried out using water

saturation and mercury intrusion porosimetry techniques.

The samples were also characterized by scanning electron

microscopy (SEM), coupled to an EDX spectrometer, and

by inductively coupled plasma-atomic emission spectros-

copy (ICP-AES) technique.

Water total porosity technique

The water total porosity is measured after water saturation

following the standard recommended by the AFPC-AF-

REM (1997) which consists of drying the samples at a

temperature of 60�C for 48 h, until their mass becomes

constant. After a degasification step, under a primary

vacuum for 24 h, the samples were submerged in water

until the saturation. The samples are weighed dry, after

saturation and in hydrostatic condition. Total porosity, Nt,

is calculated as:

Nt ¼M2 �MS

M2 �M1

� 100

Where M1 is the hydraulic weight of the sample, M2 is

the weight of the sample saturated with water, MS is the

weight of the dry sample.

Fig. 2 Photograph showing the zone where a block on the wall of the

second shop of the Roman part was taken. This block presents faces

with black crusts. 191 9 143 mm (96 9 96 DPI)

Environ Earth Sci (2010) 59:1171–1181 1173

123

Water total porosity measurements were taken on a set

of eighteen cylinders of 40 mm in diameter and 55 mm in

length. This technique gives access to the connected porous

volume of the sample. The weighing is made with a pre-

cision of 0.001 g; however the porosity measurements

precision is estimated at about 0.4%.

Mercury intrusion porosimetry technique

This technique consists in injecting the non-wetting fluid

(mercury), under various pressures in previously desiccated

and degassed samples (Van Brakel et al. 1981; Gueguen

and Palciauskas 1992). It allows porosity and pore size to

be estimated by measuring the volume of injected mercury

and the injection pressure. The applied pressure is con-

nected to the threshold access of the pore by the Young-

Laplace equation:

P ¼ 2r cos hR

where P is the mercury pressure, r is the air-mercury

interfacial tension (0.486 Nm-1 at 25�C), R is the radius of

the capillary tube (Washburn 1921) or the distance of the

pore walls in slit-shaped pore (Lenormand et al. 1983), h is

the contact angle between mercury and solid, h = 140� for

the non-wetting fluid (Fripiat et al. 1971).

These porosimetry measurements are performed using

an apparatus Micromeritics Pore Sizer 9320 which makes

it possible to inject mercury with pressures ranging

between 0.001 and 300 MPa. So the access threshold

ranges between 400 and 0.003 lm. This technique

determines the connected porous volume and its distri-

bution according to the injection pressure and the

thresholds access.

Cylindrical samples, of 25-mm length and 20 mm in

diameter, were dried at 60�C, weighed and placed in an

injection cell. After a degasification step under a 50-lm

mercury depression, the injection cell is filled with

mercury, and then the vacuum is broken gradually until

atmospheric pressure. The intrusion measurement, i.e.,

the volume of mercury injected into the sample, is made

for low pressures (between 0.001 and 0.15 MPa) and for

high pressures (between 0.15 and 300 MPa). The pres-

sure rises are carried out in stages; after each stage, the

injected mercury volume is measured. From these data, it

Table 1 Statistical results of

water total porosity

measurements

Number

of samples

Minimal

value (%)

Maximal

value (%)

Mean

(%)

Standard

deviation (%)

Unaltered 18 30.33 33.84 32.04 0.88

Altered 18 33.75 35.60 34.67 0.56

Fig. 3 a Mercury porosimetry curves obtained for the unaltered

sample (Unaltr), the altered one without crust (Altr_no_cr) and with

crust (Altr_cr). b Mercury porosimetry curves, giving the increment-

ing of the injected mercury volume versus pores access ray of the

unaltered sample (Unaltr), the altered one without crust (Altr_no_cr)

and with crust (Altr_cr). 313 9 228 mm (96 9 96 DPI)

1174 Environ Earth Sci (2010) 59:1171–1181

123

is possible to determine the saturation curve according to

the injection pressure. The precision of the measurements

calculated by Carrio-Schaffhauser (1987) is about 4%.

Scanning electron microscopy and EDX analysis

Samples were observed, after a metallization step, using a

JEOL JSM 840. This technique, based on electron beam

interaction with sample’s atoms, leads to X-ray, secondary

and backscattered electrons emission. Each signal is pro-

cessed to extract more information about the analyzed

sample. To perform reliable EDX analysis an environ-

mental scanning electron microscopy (ESEM) Quanta 200,

coupled to an energy dispersive X-ray spectrometer (EDX)

was used. In the low vacuum mode, samples were analyzed

without a metallization step.

Inductively coupled plasma-atomic emission

spectroscopy (ICP-AES)

ICP-AES is an emission spectrophotometric technique, in

which excited electrons emit energy at a given wavelength

as they return to ground state. The characteristic of this

process is that each element emits energy at specific

wavelengths. The intensity of the energy emitted at the

chosen wavelength is proportional to the amount of that

element in the analyzed sample. Thus, by determining

which wavelengths are emitted by a sample and by deter-

mining their intensities, the analyst can quantify the

elemental composition of the given sample relative to a

reference standard. Since ICP-AES analysis requires a

sample to be in solution, samples were dissolved by a

combined attack employing HNO3 and HCl acids.

Results and discussions

Two set of 18 samples were analyzed by water porosity

technique for the altered and the unaltered blocks. Results

presented in Table 1 show that the values vary between

30.33 and 33.84% with an average value of 32.04% and a

standard deviation of 0.88% for the unaltered samples.

Meanwhile, the connected porosity varies between 33.75

and 35.60% with an average porosity of 34.67% and a

standard deviation of 0.56%, for the altered samples. These

results show that the decay process of these monument

stones lasted about 20 centuries, involved a porous volume

increase of about 2% only.

Figure 3a, b presents the results of mercury porosimetry

analysis performed on the altered and unaltered samples.

These curves present a bimodal porous network. The

access to the porous volume of the unaltered sample is

controlled by access thresholds ranging from 300 to 10 lm

for 90% of the total porous volume which constitutes a

macro porous network.

Moreover, these curves also highlight a microporous

network (about 10%) corresponding to access thresholds

ranging between 10 and 0.005 lm. In order to clarify the

variation of porosity according to the sample state, various

mercury porosity values (NHg), deduced from Fig. 3 for

various access rays intervals, are presented on the Table 2. It

shows that total porosities of the samples Unaltr, Altr_cr and

Altr_no_cr are, respectively, 32.80, 34.37 and 34.90%. It is

obvious that the unaltered sample (Unaltr), at uncertainties

close, has the lowest porosity compared to the two other

samples in both microporosity and macroporosity fields.

Nevertheless, a constant porosity was observed for the

three types of samples along the access rays ranging from 0.2

to 2 lm and over 200 lm. Meanwhile, a light reduction of

Table 3 Mercury porosimetry results obtained at different access ray intervals for Unaltr, Altr_8 cm and Altr_surf samples

Access ray intervals (lm) [0.001, 0.2] [0.2, 20] [2, 40] [40, 100] [100, 200] Over 200 Total porosity (%)

NHg (%) Unaltr 2.36 %0 6.18 3.55 15.65 5.06 32.80

NHg (%) Altr_8 cm 2.39 %0 6.24 3.54 15.69 5.01 32.87

NHg (%) Altr_surf 3.40 %0 2.92 3.60 19.29 5.15 34.36

Kind of porosity Microporosity Interval change Macroporosity

Table 2 Mercury porosimetry results for the three samples obtained at different access ray intervals

Access ray intervals (lm) [0.001, 0.2] [0.2, 2] [2, 40] [40, 100] [100, 200] Over 200 Total porosity (%)

NHg (%) Unaltr 2.36 %0 6.18 3.55 15.65 5.06 32.80

NHg (%) Altr_cr 3.40 %0 2.92 3.60 19.29 5.15 34.36

NHg (%) Altr_no_cr 3.48 %0 2.83 3.73 19.76 5.10 34.90

Kind of porosity Microporosity Interval change Macroporosity

Environ Earth Sci (2010) 59:1171–1181 1175

123

porosity was observed in the altered sample with crust; that

may be caused by a partial cementing of the inter-granular

space. This important result was also observed by Beck

(2006) on the white tuffeau taken from a site in the Paris area.

The Fig. 3a represents the porosity variation according

to the pores access ray for the three studied samples. It

clearly confirms the total volume increase of the pores,

noted by the water saturation technique. However, these

curves show a macropore volume increase of about 1% (for

access thresholds ranging from 200 to 2 lm) and also a

micropore volume increase of about 1% (for the access

thresholds lower than 0.2 lm). The porosity variations are

comparable in the samples covered and not covered by

black crust. This porosity increase can be due to a cavities

widening or/and to an increase in cavities number. Mean-

while, no porosity variation was observed for access

thresholds ranging from 0.2 to 2 lm that can be attributed

to the absence of the porous network in this interval. The

development of deterioration in the samples with and

without crust results in the increase of macroporosity as

well as of microporosity. The macroporosity is induced by

partial cement dissolution, whereas microporosity, lower

than 0.5 lm, tends to be intra-granular and develops in the

primary grains as well as in the secondary grains. Although

the difference between the porosity curves of samples with

and without crust is in the error margin of the technique,

the variations could be attributed to a partial cementing of

the porous network by secondary products as could be

observed by Beck (2006) on the Touraine tuffeau.

In order to determine if the slight porosity increase in

the altered sample is linked to the natural inhomogeneities

of the stones, two small fragments of altered sample, cored,

respectively, at the surface and at the core (8 cm below the

surface) were analyzed by mercury intrusion porosimetry

technique. Results are shown in Fig. 3c. We also present in

this figure the results previously obtained on the unaltered

sample. As it was checked by SEM observations, we make

the hypothesis that the alteration did not occur deeply in

the core of the altered stone (or only slightly). In order to

show the variation of the porosity according to the sample

state, various mercury porosity values (NHg), deduced from

Fig. 3c for various access rays intervals, are presented on

the Table 3. It shows that total porosities of the samples

Unaltr, Altr_8 cm and Altr_surf are, respectively, 32.80,

32.87 and 34.36%. It is obvious that the Unaltr and

Alter_8-cm samples have the same behavior, at uncer-

tainties close and exhibit the lowest porosity compared to

Altr_surf sample, in both microporosity and macroporosity

fields. These results clearly show that the slight porosity

increase is mainly due to the alteration process and not

strongly linked to the stones’ inhomogeneities.

Comparison between both kinds of measurements, per-

formed on samples taken on the same altered bloc or on

different blocs, shows that the alteration process is clearly

associated to the porosity development for the high

threshold volumes and for the lowest ones.

Fig. 4 SEM photographs of various areas a, b, c of the unaltered

sample surface. It shows the presence of primary grains (a),

surrounded by a sparitic layer (b), and secondary grains (c) of

rhombohedric forms. 60 9 175 mm (96 9 96 DPI)

1176 Environ Earth Sci (2010) 59:1171–1181

123

Scanning electron microscopy (SEM)

Surface of the unaltered sample

Figure 4 presents SEM micrographs of fragments taken on

the surface of the unaltered sample. It shows the presence

of two families of calcite grains: on one hand, porous

primary grains (a), of ovoid forms and millimeter-length

sizes, which are coated with a precipitated calcite layer (b)

of approximately 20 lm of thickness; the other hand,

secondary grains of rhombohedra forms (c) and micro-

metric sizes (between 50 and 5 lm), which precipitated

around the primary grains and lead to cement formation. In

addition to the calcite peripheral layer, the primary grains

interior is empty (dissolved) or made up of a smaller

crystalline material (1–2 lm). Porosity, of relatively

lengthened form, is located primarily between the primary

grains and can be completely clogged by secondary pre-

cipitation phases.

Surface of the altered sample

SEM micrographs of four fragments taken on different

areas of the altered block surface are presented on Fig. 5.

Various decay forms are visible. Image (Fig. 5a) shows

the presence of altered calcite grains testifying to an

unbalanced medium favorable to the dissolution which

preferentially affects the secondary calcite grains in par-

ticular on their edges. Meanwhile, the primary grains, little

or not altered, preserve their initial forms. Secondary cal-

cite grains, corroded by well dissolution development (a),

are visible on the images (Fig. 5a, b, d). This phenomenon

involves the local development of a high porosity. More-

over, the image (Fig. 5d) shows a zone of high dissolution

privileging the low index crystallographic orientations of

the calcite. In addition, these images show the presence of

an argillaceous deposit (b and u) on the surfaces; that tends

to reduce porous space.

Structure of altered sample substrate

SEM micrographs of three fragments, taken at 8-cm depth

of the same altered block, are presented in Fig. 6. Image

(Fig. 6a) presents a zone made up of micrite surrounded by

a zone of strong sparitic calcite precipitation (a). A dis-

solution phenomenon, in presence of clay, is still observed

on the images (Fig. 6b) and (Fig. 6c). Meanwhile, the

image (Fig. 6c), shows unaltered primary grains (u)

Fig. 5 SEM photographs, of

various areas a, b, c of the

altered sample surface, showing

altered calcite grains (c)

testifying to an imbalance

medium; secondary calcite

grains, corroded with

development of dissolution

wells (a). Meanwhile, the

images b and d show the

presence of an argillaceous

deposit (b and u) on the surface.

129 9 122 mm (96 9 96 DPI)

Environ Earth Sci (2010) 59:1171–1181 1177

123

protected by a sparitic gangue. In-depth, secondary

cements seem to be less abundant, even if that can be

related to the sampling. Moreover, the wall deterioration of

certain primary elements makes it possible to connect

intern porous volumes of these elements with the inter-

granular network (Fig. 6a).

Microanalysis by energy dispersive X-rays (EDX)

and inductively coupled plasma-atomic emission

spectroscopy (ICP-AES)

In order to determine if the porosity variations are linked to

the variations of the chemical composition with the depth,

three fragments of altered samples (a, b and c), cored,

respectively, at the surface and at two different depths (2

and 8 cm), were firstly analyzed by the EDX technique.

The results, presented on Fig. 7, show that the altered

sample, cored at 8-cm depth (Fig. 7b, c), is mainly made up

of high contents of oxygen, carbon, and calcium with low

contents of silicon and aluminum. However, other elements

were detected but with small proportions such as iron,

manganese, potassium, phosphorus and sulphur. Mean-

while, analysis carried out in low depth (2 cm) and on the

surface of the altered sample shows a remarkable increase

in the relative content of silicon and aluminum that can be

connected to a clay precipitation toward the surface. To

further clarify this point, an ICP-AES analysis was per-

formed on samples cored at different depths (8, 2 cm and at

the surface). Results are shown in Table 4. The carbon

ratio was measured using the loss on ignition technique

(LOI).

These results are in perfect agreement with those

obtained by the semi quantitative technique (EDX) which

shows a high content of calcium and a low content of (Al,

Si, Fe, Mg, K, P and S). Also, these results show again,

with more precision, the remarkable increase in the relative

content of silicon and aluminum toward the surface. This

fact can be connected to a clay precipitation phenomenon

in this region, which is experimentally proved by SEM

observations that show high clay content on the surface.

Indeed, on the image of Fig. 8, also taken on the surface of

the deteriorated block, clays cover the entire surface and

mask even the calcite signal. Nevertheless, no remarkable

variation of the Si and Al contents according to the depth

was observed in the case of the unaltered sample.

SEM coupled to EDX and ICP-AES analysis, of the

various unaltered and altered blocks, show important

modifications of the internal structure of altered block. In

the unaltered block one attends to a high sparitisation of

micritic grains, detritic grains (quartz, magnetite) and clay.

Meanwhile, the altered sample shows a depth variation of

Fig. 6 SEM photographs of three fragments taken at 8 cm in depth

from the surface of the altered block. The image a presents a zone

made up of micrite (b) surrounded by a zone of high precipitation of

sparitic calcite (a). The image b shows intra-granular porosity (d).

The image c shows unaltered primary grains (u), protected by a

sparitic gangue. 64 9 185 mm (96 9 96 DPI)

1178 Environ Earth Sci (2010) 59:1171–1181

123

the internal structure and also of the chemical composition.

On the surface, one notes the presence of micritic grains

with gangue in a fine clayey carbonated matrix accompa-

nied by a dissolution of the sparitic calcite grains. In-depth,

the assembly of micritic grains with sparite and gangue is

characteristic of a primarily macroscopic porous space.

Moreover, in the altered sample with crust, compared to the

deteriorated sample without crust, the porosity reduction is

allotted to a partial inter-granular space cementation. ICP-

AES and EDX analysis clearly show an increase in the Si

and Al contents in the vicinity of the surface. From these

observations, one can note that the decay mechanism in

these stones is schematized by an increasing clay

concentration at the surface, accompanied by remarkable

calcite dissolution. The increasing clay content on the

surface is an effect of a clay precipitation enhanced by the

modification of the pH conditions (weather). It can also be

attributed to the accumulation of ‘‘detritic’’ clays by dust

and wind. These lead to the formation of a thick crust that

is responsible of inter-granular porosity clogging.

Conclusion

Water saturation and mercury intrusion porosimetry ana-

lysis show that the decay process of Chellah monument

Fig. 7 Energy dispersive

X-rays analysis of three

fragments of the altered sample

a, b, c cored, respectively, on

the surface and at two different

depths (2 and 8 cm). The

analysis performed at the

surface and at shallow depth

(2 cm) shows a remarkable

increase in the relative content

of silicon and aluminum this

area; that can be connected to a

presence of clays.

313 9 211 mm (96 9 96 DPI)

Environ Earth Sci (2010) 59:1171–1181 1179

123

stones lasted more then 20 centuries, involve a porous

volume increase of about 2% only. The porous network

is bimodal where two families of access thresholds to

the pores coexist. The modifications of porosity lead to a

different distribution of porous volume as a function of the

access diameter. The access to macroporous volume (90%)

is controlled by thresholds ranging from 300 to 10 lm;

meanwhile, microporous volume (10%) is controlled by

access thresholds ranging between 10 and 0.005 lm. The

relative low porosity of the unaltered block is connected to

the presence of sparitic cement which is responsible of

inter-granular porosity clogging; while that of the deterio-

rated block without crust, highest, is due to the increase in

inter-granular space. The decay development in the sam-

ples with and without crust results in an increase as well of

the intra-granular microporosity as of the macroporosity;

this last is induced by partial cement dissolution. Whereas

for the sample with crust, the reduction in porosity com-

pared to the sample without crust is due particularly to the

clay precipitations.

Acknowledgments This work was supported by the French-

Moroccan cooperation within the project named (PAIVolubilis)

No. MA/07/168. The authors would like to thank H. Ouaddari

from CNRST for his technical assistance to perform ICP-AES

analysis.

References

AFPC-AFREM (1997) Determination de la masse volumique appar-

ente et de la porosite accessible a l’eau. Methodes recommandees

pour la mesure des grandeurs associees a la durabilite (Determi-

nation of apparent density and water porosity. Methods

recommended for the sustainability measurements). Compte

–rendu des Journees techniques. 11–12 Decembre Toulouse,

France, pp 121–124

Akil M (1990) Les depots quaternaires littoraux entre Casablanca et

cap Beddouza: Etudes Geomorphologiques et Sedimentologi-

ques (Littoral quaternary deposits between casablanca and

beddouza: geomorphological and sedimentological studies)

These de Doctorat Es- Science, Universite Mohamed V Rabat,

Maroc, p 419

Alves C, Sequeira Braga MA, Hammecker C (1996) Water transfer

and decay of granitic stones in monuments. CR Acad Sci Paris

t.323, series II a, pp 397

Azeroual M, Bouamar B, Bounakhla M, Doukkali A, ELwartiti M

(2005) Action de la pollution atmospherique sur les monuments

historiques: caracterisation et estimation de l’etat de degradation

des remparts de la medina de Sale (Action of the atmospheric

polution on the historical buildings : characterization and estimate

of the medina ramparts degradation). 3eme Journees des geo-

sciences de l’Environnement. El Jadida 8, 9 et 10 Juin, Maroc

Azouaoui H, El Hatimi N, El Yamine N (2000) Plio-Quaternary

formations of the Casablanca area (Morocco): sedimentological

and geotechnical aspects. Bull Eng Geol Environ 59:59–74

Beck K (2006) Etude des proprietes hydriques et des mecanismes

d’alteration de pierres calcaires a forte porosite (Studies of

hydrous properties and the mechanisms of porous limestones

Table 4 ICP-AES analysis

performed on three altered

samples cored at different

depths (8, 2 cm and at the

surface)

Oxides/elements (%) Sample cored

at 8 cm

Sample cored

at 2 cm

Sample cored

at the surface

CaO 58.99 60.62 50.31

CO2 36.05 32.68 39.10

SiO2 2.04 3.67 5.81

SO3 0.55 0.57 0.66

Al2O3 0.55 0.63 1.45

Fe2O3 0.90 0.96 1.44

MgO 0.39 0.39 0.45

P2O5 0.45 0.44 0.63

K2O 0.09 0.01 0.26

Fig. 8 Energy dispersive

X-rays analysis of an area

located on the surface of the

altered block. Clays cover the

entire surface and mask even the

calcite signal. 313 9 200 mm

(96 9 96 DPI)

1180 Environ Earth Sci (2010) 59:1171–1181

123

deterioration). These de Doctorat, l’Universite d’Orleans, Paris,

France

Benavente D, Garcia del Cura MA, Garcia-Guinea J, Sanchez-Moral

S, Ordonez S (2004) Role of pore structure in salt crystallisation

in unsaturated porous stone. J Cryst Growth 260:532–544

Benboughaba R (2001) Etude Sedimentologique et Geophysique des

formations littorales plio-quaternaires de l’axe Rabat-Kenitra

(Maroc) (Sedimentological and geophysical studies of the plio-

quaternary littoral formations of the Rabat-Kenitra axis). These

de doctorat, Universite Mohammed V Agdal, Rabat, Maroc,

p 233

Camuffo D (1995) Physical weathering of stones. Sci Total Environ

167:1–14

Carrio-Schaffhauser E (1987) Evolution des proprietes petro phy-

siques d’un calcaire: le role de la dissolution cristallisation dans

une deformation cassante (Evolution of the petrophysical

properties of a limestone : the role of dissolution crystallization

in a breakable deformation). These de l’Universite de Aix-

Marseille I, pp 291

Fripiat J, Chzaussidon J, Jelli A (1971) Chimie physique des

phenomenes de Surface (Physical chemistry of surface phenom-

ena). Edition Masson, Paris, p 387

Genkinger S, Putnis A (2006) Crystallisation of sodium sulfate:

supersaturation and metastable phases. Environ Geol 52:329–

337. doi:10.1007/s00254-006-0565-x

Goudie AS, Viles H (1997) Salt weathering hazards. Wiley,

Chichester, p 241

Gueguen Y, Palciauskas V (1992) Introduction a la physique des

roches (Introduction to rock physics). Hermann (ed) pp 299

Hammecker C (1995) The Importance of the petrophysical properties

and external factors in the stone decay on monuments. Pure Appl

Geophys 145(2):337–361

Lenormand R, Zarcone C, Sarr A (1983) Mechanisms of displacement

of one fluid by another in a network of capillary ducts. J Fluid

Mech 135:337–353

Prick A (1995) Dilatometrical behaviour of porous calcareous rock

samples subjected to freeze-thaw cycles. Catena 25:7–20

Rijniers LA, Huinink HP, Pel L, Kopinga K (2005) Salt crystalliza-

tion as damage mechanism in porous building materials—a

nuclear magnetic resonance study. Magn Reson Imaging

23:273–276

Samaouali A, Laanab L, Geraud Y, Nounah H, Boukalouch M (2005)

Chemical alteration of stones of monuments Chellah Roman.

Paper presented at the first international meeting on Mediterra-

nean Architectural Culture Heritage, in Collaboration with

CICRP (Marseille, France). 26–28 September, Meknes, Morocco

Tiano P (1994) Biodeterioration of stone monuments. A critical

review. In: Garg KL, Garg N, Mukerji KG (eds) Bio deterio-

ration of cultural heritage, vol I. Naya Prokash, Calcutta,

pp 301–321

Van Brakel JG, Modry S, Svata M (1981) Mercury porosimetry: state

of the art. Powder Technol 29:1–12

Van TT, Beck K, Al-Mukhtar M (2007) Accelerated weathering tests

on two highly porous limestones. Environ Geol 52:283–292. doi:

10.1007/s00254-006-0532-6

Washburn EW (1921) Note on method of determining the distribution

of pores sizes in a porous material. Proc Nat Acad Sci 7:115–116

Zaouia N, ELwartiti M, Baghdad B (2005) Superficial alteration and

soluble salts in the calcarenite weathering. Case study ofAlmohade monuments in Rabat: Morocco. Environ Geol

48:742–747

Environ Earth Sci (2010) 59:1171–1181 1181

123