Characterization of waterproof-covering mortars on Ottoman monuments of “Ghar El Melh” (Tunisia)

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Construction and Building

MATERIALS

Construction and Building Materials 23 (2009) 423–433

www.elsevier.com/locate/conbuildmat

Characterization of waterproof-covering mortars on Ottoman monuments of ‘‘Ghar El Melh” (Tunisia) Mohamed Riadh Labiadh a, Mongi Ben Ouezdou a,*, Besma Trojet Hajjem b, Rachid Mensi a b

a Civil Engineering Laboratory, National Engineering School of Tunis, ENIT, BP 37, 1002 Tunis Belve´de`re, Tunisia National Institute of Research and Physicochemical Analysis, Technological Pole of Sidi Thabet, 2020 Sidi Thabet, Tunisia

Received 4 February 2007; received in revised form 23 October 2007; accepted 14 November 2007 Available online 3 January 2008

Abstract This study is devoted to the characterization of two historic waterproof-coating mortars taken from the ottomans monuments of ‘‘Ghar El Melh” in, northeast Tunisia. The first waterproof mortar was recovered from the aqueduct channel, which was used to supply boats with water. The second type came from the terrace cupola of ‘‘Sidi Ali El Mekki” fortress. To characterize each mortar, physical, mineralogical, and chemical analyses were performed. These revealed that the two samples were mainly made of air-hardening lime mixed with pozzolanic additions. The binder/aggregate ratio was 0.6 for the aqueduct coating and 0.3 for the cupola. The relative presence of gypsum in the mortar of the aqueduct indicated its sulfatic alteration, but the two coatings were free of ettringite. Although these were in permanent contact with corrosive conditions, the two waterproof coatings showed good durability. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Waterproof-covering mortars; Lime; Coatings; Pozzolan; Ottoman monuments; Compositions; Alterations

1. Introduction Since the start of research by Seamon in the 18th century and by Vicat in the early 19th century, which led to the discovery of artificial hydraulic binders, air lime was increasingly abandoned in building works [1]. Presently, one realizes increasingly that there is a quasi-total loss of lime practice, thus complicating restoration work on historic buildings in accordance with the principle of authenticity defined by ICOMOS (International Council on Monuments and Sites) Venice Charter 203 [2]. To understand lime making, it is instructive to study the lime-pozzolana mortars that date to the 19th and even to the 18th centuries. These mortars represent the evolution of knowledge regarding this binder for thousands of years [3]. Depending on the time and region, various pozzolanic additions were used in the formulation of rustic coatings to enhance properties such as insolubility in water, setting in *

Corresponding author. Tel.: +216 71 874 700; fax: +216 71 872 729. E-mail address: [email protected] (M. Ben Ouezdou).

0950-0618/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.11.007

the presence of water, and good mechanical performance. These improvements were due to the formation of stable hydrated calcium silicates and calcium aluminates which fix the portlandite Ca(OH)2. With the above in mind, various Ottoman monuments of the coastal city of ‘‘Ghar El Melh”, which was established in the 17th century, were targeted. Due to its geographic location on the left bank of the Mediterranean and just opposite to Sicily, Ghar El Melh, which was known in the past by the names of Rusucmona and Porto Farina, was a crossroad of several civilizations (Punic, Roman, Arab, Spanish, and Ottoman), such that many construction techniques, for buildings in particular, were developed [4]. The choice of these monuments was deemed appropriate since they involve a diversity of well preserved lime-pozzolana coatings of different colors. The most interesting of these types of mortars are the waterproof coatings, which have to permanently resist water and, while being impermeable to water, be porous to promote the diffusion of CO2. This last element is necessary to allow carbonation, according to the following equations [5]:

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Fig. 1. Ghar El Melh monuments: (A) aqueduct and (B) cupola.

Fig. 2. Restoration failures of the aqueduct wall.

CaO þ H2 O ! CaðOHÞ2

ð1aÞ

CaðOHÞ2 þ CO2 þ nH2 O ! CaCo3 þ ðn þ 1ÞH2 O

ð1bÞ

In this paper, the authors report the results of a study of samples of the ancient waterproof mortars collected from two monuments of Ghar El Melh: the aqueduct and the castle (A and B in Fig. 1). This study was based on physicochemical analyses, which revealed the information on compositions and durability, thus making it possible to formulate restoration coatings compatible with the monuments of ‘‘Ghar El Melh” and end the successive failures of past restoration efforts in the region [6]. Indeed, this work introduces, for the first time in Tunisia, a scientific approach for the restoration of historical buildings. 2. Failure of repairs Due to a lack of understanding of old masonry functioning, past attempts at restoring historical building sites in Tunisia often resulted in irreversible deterioration which not only disfigured the monuments but also sometimes put them in ruin. For some sites, the use of relatively rigid non-breathing coatings caused these deteriorations. For other sites, it was inappropriate proportioning and errone-

Fig. 3. Blistering of the hydraulic coating.

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425

cement and natural hydraulic lime with high Vicat index created hard points. These rigidities did not permit any movement, which led to the delamination of the coating (Fig. 4) and to crevices in the wall (Fig. 5). These failures imposed the necessity of finding a compatible mortar through studying the characteristics of the historic monuments of ‘‘Ghar El Melh”. 3. Materials and methods 3.1. Sampling

Fig. 4. Effects of an impermeable coating on an old wall (Ghar El Melh): phantom and salt crystallization.

ous choices for the various mortar compounds that facilitated damage. In some cases, the implementation was the cause of accelerated deterioration. Among the problems noted on the historic site of Ghar El Melh, after a recent restoration, was the collapse of several parts of the aqueduct wall. This damage was caused by application of a coating made up of layers containing hydraulic binders (Fig. 2) on this wall. Because of their low permeabilities to water vapor, the hydraulic mortars increased capillary aspiration, which was already high in the old walls. This mortar impermeability caused water retention that dissolved the bonding (Fig. 2) and contributed to the presence of salts and phantoms (Fig. 3). Furthermore, the use of

Samples of two mortars, A and B, were taken from two different monuments on the ‘‘Ghar El Melh” site (Fig. 1). The first one (A) was from the aqueduct (Fig. 6) and the second one (B) came from the cupola of the terrace of Sidi Ali Mekki fortress (Fig. 7). The coating samples were taken from the supports using a hammer and a stone chisel. Various analyses were carried out separately on these samples, some of which, such as chemical and granulometric analyses, were performed by various methods to best ensure the accuracy of the results. Sample A had been applied on a stone support and had a smooth surface. It did not undergo any restoration. Sample B, on the other hand, which protected the cob cupola, had a rough surface. 3.2. Mineralogical and morphological analyses The mineralogical phases were identified by X-ray diffraction (XRD) using a PANalytical X’Pert-PRO powder diffractometer with a tube Cu anode (45 kV, 30 mA).

Fig. 5. Crevice in a wall restored recently.

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Ca, Mg, Fe, Al, K, Na, and Ti was carried out by dissolving mortar samples in a 50% HNO3–20% HCl solution. For silica, we employed the alkaline fusion method with soda. Gypsum proportioning was carried out by gravimetry. Here, the coating samples were treated with a boiling ammonium carbonate solution. The filtrate was acidified with HCl, to which we added a hot barium chloride solution which precipitated the sulfate ions. Calcium carbonate was determined by the Dietrich Fruhling gaz volumetric method. Loss of ignition was determined for two samples by heating to 1050 °C. 3.4. Thermal analyses Differential thermal and thermogravimetric analyses are suitable to establish the characteristics of the ancient mortars, since the main components, the nature of the aggregate, and other aspects can be defined even with a small quantity of sample. Because of the dynamic character of thermogravimetry, one can distinguish from slope variations the release of CO2 resulting from carbonation in contrast to that emerging from the natural calcite. Simultaneous DTA/TGA/DSC analyses were obtained using a SATARAM instrument. DTA/TGA was performed in a helium atmosphere at a 10 °C/min heating rate from ambient temperature to 1400 °C. 3.5. Physical tests The physical analysis of the two samples was performed to determine the apparent bulk density by hydrostatic weighing and the absolute density by using a pycnometer according to the French Standard NFT20-053. Total porosity was deduced from the two previous tests. The water absorption coefficient was determined by complete immersion in distilled water of small samples of the two mortars for a 24 h period. The coefficient of water absorption, CA, was calculated by the equation CA ¼ Fig. 6. An aqueduct portion: (a) Top view; (b) Cross-section.

The analysis with the Scanning Electron Microscope (SEM) enabled us to characterize the morphology and microstructure of the two coatings. This task was performed with a Philips XL30 microscope. Polished sections of each of the two mortars, coated by a fine layer of nanolm of gold–palladium, were used for this optical analysis. 3.3. Chemical analyses A type Novaa 400, Analytik jna AG Atomic Absorption Spectrometer (AAS) was used to identify the different chemical elements in the two samples. Quantification of

ðM H  M S Þ  100% MS

ð2Þ

where MH is saturation mass sample after 24 h and MS is dry mass after drying at 105 °C. 3.6. Grain size distribution The granular distribution of all the constitution of the mortar was determined by two methods: separation of binder from aggregate and SEM grain size measurement. 3.6.1. Separation of binder from aggregate The mortar samples were placed in a muffle furnace at a temperature of 800 °C for 2 h, at which dissociation of the binder from aggregates took place, as reported by Baronio and Binda [7]. A Calgon solution, considered as a strong dispersant, was then added to the obtained aggregates

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Fig. 7. Top view of the cupola.

and agitated for 24 h [8]. The product, after drying in an oven at 105 °C, was filtered through a series of sieves (ISO565). The percentages of fine grained soils (620 lm) present in the two samples were determined by the Robinson pipette test based on Stokes law. This method could be used with positive results since the aggregate was not mainly calcareous. Furthermore, separation of binder/aggregate using a hot HCl treatment was discounted because the aggregates were partially carbonated [9]. The granular fraction lower than 0.063 mm was considered to be mainly binder; however, it was assumed to have inert grains within this fraction [10] or to have binder particles, certainly in small quantities, of size higher than this value. The granulometric analysis allowed the estimation of the binder/aggregate ratio.

Table 1 XRD identification of sample A and B Sample

Calcite (cc)

Aragonite (a)

Quartz (Q)

Gehlenite (G)

Hatrurite (H)

Gypsum (g)

A B

+++ +++

 ++

+ ++

+ +

 +

+ 

+++: dominantly present; ++: present; +: present in low quantities; : not present.

3.6.2. SEM grain size measurement The second method was by measurement using the Scanning Electron Microscope (SEM) of the maximum diameter of each aggregate present in the sample sections. These various measurements allowed the determination of the average diameter and standard deviation based on a statistical investigation. 4. Results and discussion 4.1. XRD analyses The mineralogical characterization by means of XRD of the samples (A) from the aqueduct and (B) from the cupola is summarized in Table 1. This analysis (Fig. 8) shows a predominant presence of calcium carbonate (CaCO3) and, to a lesser extent, of quartz. The calcium carbonate

Fig. 8. XRD patterns of mortars A and B.

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coexisted in two forms, calcite and aragonite, in sample B, and only as calcite in sample A. The high intensity of the calcite peaks and the absence of portlandite Ca(OH)2 indicate that the two mortars are made of binder containing air lime mixed with pozzolana, which served to promote carbonation and form stable hydrates. Gehlenite (C2AS) was present in small quantities in the two samples, whereas hatrurite (C3S) was detected in very small quantity only in sample B (Fig. 8b). Knowing that gehlenite is a characteristic of natural hydraulic lime, its weak presence confirmed that the two mortars contain fat lime with slightly hydraulic components, these being obtained at low heating temperature. The presence of gypsum in sample A (Fig. 8a) is attributed to the process of coating deterioration by sulfating rather than the use of a lime/plaster bastard mortar. The sulfatic elements, which came from the environment, were deposited on the surface and reacted with CaCO3 present in the mortar according to Eqs. 3a and 3b. The mechanism of sulfating has not resulted in the formation of ettringite because of the absence of ‘‘calcium aluminate hydrate” (3CaO Al2O36H2O) [11]. CaCO3 þ SO2 þ 0:5H2 O ! CaSO3  0:5H2 O þ CO2

ð3aÞ

2CaCO3  0:5H2 O þ O2 þ 3H2 O ! 2CaSO4  2H2 O

ð3bÞ

The fact that the quartz peaks in the XRD spectra were greater for sample B than for sample A (Fig. 8) indicates that this component was more prevalent in the former. 4.2. Morphological analyses Low magnification SEM viewing of sample A revealed a smooth and well preserved surface (Fig. 9a). Nevertheless,

the presence of microscopic cracks was noticed. These cracks were the result of shrinkage of the mortar [12]. Pores, corresponding rather to the instances of grain pullout, were also observed. Some organic remnants (rootlets and stems) were present at the surface. The sample showed surface pellicular deterioration with traces of amorphous calcite reprecipitation (Fig. 9b). This chemical deterioration was due primarily to the dissolution capacity of pure water similar to the dissolution of limestone by acid water rich in CO2, according to the equation [12] Ca2þ þ 2HCO 3 $ CaCO3 þ CO2 þ H2 O

ð4Þ

The body of the coating was very porous and spongy. The binder still retained its adhesion with the aggregates although the voids that resulted from dissolution tended to compromise the coating thickness (Fig. 9c). The authors also observed the presence of plates and microcrystal spearheads, which indicates the existence of a gypsum fraction (Fig. 9d), as confirmed by the XRD. Furthermore, the absence of ettringite within the mortar was confirmed and checked by the absence of fibrous prismatic crystals, as indicated by Sabbioni et al. [11]. Sample B surfaces appeared clearly less preserved than those of the aqueduct (Fig. 10a), as indicated by the rough surface and traces of dissolution (deterioration). These microfacies of deterioration appeared in the form of a porosity synonymous with a differential attack that particularly affected the calcic phase. The crushed aggregates were not very faded. They have in zone I a jointed and smoothed appearance (Fig. 10a). In zone II, they were separated and showed subround and oblong forms. Slits of desiccations, observed at the aggregate-binder interface, showed that the coating expanded and contracted in response to hygrometrical and temperature variations

Fig. 9. SEM micrographic of the structure of Sample A at different magnifications: a: 173, b: 500, c: 1000 and d: 2500.

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Fig. 10. SEM micrographic of the structure of Sample A at different magnifications: a: 173, b: 500, c: 1000 and d: 2500.

(Fig. 10b). The surface, which consisted primarily of extinct lime, appeared in the SEM to have a very porous morphology (Fig. 10c). The whitish calcite particles had an amorphous appearance and a very variable size in relation to the dissolution/reprecipitation cycle (Fig. 10d), which typically characterizes this type of material under Mediterranean climatic conditions (seasonal variations) [3]. 4.3. Chemical analyses The chemical analysis results for samples A and B are presented in Table 2. High percentages of Ca were found in the two samples. A relatively large amount of Ca was present in the CaCO3 and the remaining Ca was combined with SiO2, Al2O3, and SO2 to give Hatrurite, Gehlenite, and gypsum. This result was also confirmed by the XRD analyses. The large proportion of CaCO3 and the small quantities of Al2O3, Fe2O3, and SiO2 indicate the use of air hardening lime and the presence of a pozzolanic addition in the mixture. These high proportions of CaCO3 resulted in carbonation due to the fixing of CO2 by Ca(OH)2, which explains the high loss of the ignition values due primarily to the CO2 departure, as well as the reduction of pH to below 12.8 by carbonation [12]. Indeed, when the fresh water became more or less rich in carbon dioxide from contact with air, carbonic acid was formed,

which caused a drop in pH [13]. The CO2 in the two mortars was distributed mainly between CaCO3 and MgCO3. Furthermore, the amount of SiO2 was lower in sample B in comparison to sample A; however, XRD analyses indicated greater quartz quantities in sample B than in sample A. This contradiction can be explained by taking into account that only crystallized SiO2 and not the amorphous contributed to the XRD peaks (Fig. 8). This observation also suggested the presence of natural pozzolanic materials (amorphous silica) in mixture A. The fact that only small quantities of alkaline oxides (KOH and NaOH) were present in the mortar of the aqueduct and the cupola precluded alkaline reactions with the aggregates. Accordingly, their setting accelerator effect was limited. 4.4. Thermal analysis Differential thermal (DTA) and thermogravimetric (TG) analyses were necessary to establish the characteristics of the ancient mortars. It was easy to detect the main components, the nature of the aggregate, and other aspects with a relatively small sample [14]. DTA/TG/DSC thermoanalytical investigations indicated, for both mortars, a notable effect of calcite decomposition at about 800 °C, this phase having resulted from the carbonation of the original lime binder (Figs. 11 and

Table 2 Chemical analyses of the samples Sample

Ca

Mg

Al

Fe

Si

Na

K

Ti

CaCO3 (%)

CaSO4 (%)

Loss of ignition (%)

pH

A B

30.97 40.38

1.19 0.75

0.72 1.6

0.62 1.68

4.57 1.3

0.231 0.06

0.023 0.036

0.15 0

59.2 53.4

10.7 0.18

38.4 29.6

9.55 9.97

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12). The percentages of CaCO3 in the two samples are estimated at 55% for the aqueduct and at 50% for the cupola (Figs. 11a and 12a). These values were very close to those found by the volumetric DIETRICH Fru¨hling test. This is a calcic and nondolomitic lime because no significant amounts of MgCO3 were detected in any samples and only low weight losses occurred between 550 °C and 610 °C. The losses of mass for the two mortars between 20 °C and 100 °C corresponded to the loss of water, which is normal for very porous materials containing air lime (Figs. 11b and 12b). Calcite decomposition in the DTA curves of sample A seemed to occur continuously and without steps (Fig. 11a). This was probably due to the absence of calcic

aggregate [14] and was not the case with the DTA of sample B, which indicated a weak endothermic transformation around 850 °C (Fig. 12a). This peak corresponded to the dissociation of aragonite, which is the second form of CaCO3 present in this coating, as shown by DRX analysis. Between this dehydration and the decomposition of calcite, losses of mass continued to occur in the two curves of TG. This was an indication of the presence of hydraulic elements. These losses were due to residual hydration water loss and dehydroxyliation [14,15]. For the DTA curve obtained of sample B, an endothermic peak was observed at 570 °C (Fig. 12a). Because there was no associated weight loss, this peak was probably related to the transformation of the quartz from the a to

Heat Flow/µV

TG/mg

a

Exo

2

4 2

0

0 -2

-2

-4

-4

-6 -8

-6

-10

-8

-12 -14

-10

-16

-12

-18 -20

-14 200

400

600

800

1000

1200

Temperature ˚C -22

b Heat Flow/mW 4

Exo 2 0 -2 -4 -6 -8 -10 -12 50

100

150

200

250

300

350

Temperature ˚C

Fig. 11. Thermal curves of Sample A; a: DTA/TG patterns and b: DSC patterns.

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the b form [16]. For this sample (B), the small endothermic effect, detected at about 370 °C, was attributed to aluminium and iron hydrated oxides. The DSC of sample A (Fig. 12b) revealed the presence of two endothermic transformations between 150 °C and 200 °C, which were due to dehumidification in two steps of gypsum. At about 150 °C, hemihydrates (CaSO4  ½H2O) were apparent according to

due to the combination of silica with calcium oxide, thus forming monocalcic (CS) and dicalcium (C2S) silicates [16]. The DTA results were the basis of the choice to heat the samples at 800 °C before they underwent any dispersion and were analyzed by sifting.

CaSO4  2H2 O ! CaSO4  1=2H2 O þ 3=2H2 O

The physical characterization is presented in Table 3. The two types of mortars had a very high scatter in grain size, which was formed by grains less than 1.25 mm in size (Fig. 13). The small dimensions of the aggregates contributed to the presence of many small pores, which increased the singular pressure drop inside the mortar and thus

ð5Þ

Depending on the atmosphere, at about 200 °C the hemihydrates (CaSO4  1/2H2O) completely released its water to form the soluble anhydrite (CaSO4 III). The transformations recorded for the two mortars around 1300 °C were

4.5. Physical analysis

Heat Flow/µV

TG/mg

a Exo

2

4 2

0

0 -2

-2

-4 -4

-6 -8

-6

-10 -8

-12 -14

-10

-16 -12 200

400

600

800

1000

1200

-18

Temperature ˚C

Heat Flow/mW

b Exo 2

0

-2

-4

-6

-8

50

100

150

200

250

300

350

Temperature ˚C

Fig. 12. Thermal curves of Sample B; a: DTA/TG patterns and b: DSC patterns.

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Table 3 Physical characterization of the samples Sample

Source

MB (Kg/m3)

MT (Kg/m3)

Total Porosity (%)

CA (%)

vu

vC

Width (cm)

Polychromy

A B

Canal aqueduct External cupola

1.54 1.67

2.54 2.58

39.37 35.27

29 23.6

23 13

0.9 0.2

2–8 3

Grey layer with black pigments Red layer

MB: bulk density; MT: true density; CA: absorption coefficient; vu: Hazen coefficient; vC: curve coefficient.

100

Cum. Passing (%)

90 80 70 60 50

Sample A

40

Sample B

30 20 10 0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Sieve Apertume [mm] Fig. 13. Granulometric behavior of the examined samples A and B.

0.3 for sample B. These ratios were close to those recommended in the old writings. Thus, Vitruve advised to mix three parts of cellar sand or two parts of sea or stream sand to one of lime [5]. Measurements of granulometric dimensions by SEM gave for sample A (resp. B) an average diameter D 50% (A) = 319 lm (resp. D 50% (B) = 280 lm) with a standard deviation of 162 lm for A (resp. 192 lm for B). This measurement method is appropriate for aggregates; however, it is ineffective for very small dimension particles, such as flexible binders or for agglomerates of binder/aggregates. This largely explains the differences between the results of the grain size analysis, which was performed after heat treatment, and those based upon SEM. Furthermore, heating of the mortar at 800 °C may reduce the dimensions of the aggregates and mainly the calcic ones. 5. Conclusion

blocked water infiltration through the coating. Moreover, the relative high porosity of the two coatings facilitated diffusion of the CO2 and, therefore, accelerated the carbonation reaction. Sample A had a variable thickness (2–8 cm) to match the support irregularities. The thickness of these layers was relatively significant for the two samples since it decreased the risk of percolation. The two coatings were very absorbent and behaved like sponges. While freeze– thaw resistance is not relevant in Tunisia, where the climate is mild even in winter, accessible pores do promote carbonation, since high porosity allows an interchange surface assimilating the CO2 and the humidity. Such mortars are suitable for such circumstances and in cases where waterproofing is important [17]. Sample B had a continuous granulometric curve, which was not the case for sample A with three points of inflection (Fig. 13). This result can be explained by a ternary composition (tri modal) of this mortar. Indeed, the heating treatment of sample A emphasized the presence of both fine white sand aggregates similar to those of the ‘‘Ghar El Melh” beach and small quantities of ferrous black pigments, which were attracted to the magnetic agitator (see cross section in Fig. 6b). These pigments were metallurgical slags coming from the forging mills as indicated by Biston [5]. Furthermore, red terra cotta was obtained after the heating treatment of sample B. It was similar to the historical ‘‘cocciopesto” mortars employed generally in the Mediterranean area. By considering that the particles of diameter l < 0.063 mm primarily form the binder, the binder/aggregate ratios for the two samples were 0.6 for sample A and

From the results of the various analyses of the coatings, it was determined that these consisted primarily of strongly carbonated air lime to which pozzolanic aggregates had been added. These aggregates were formed by metal slags and quartzose sand for sample A and by crushed terra cotta for sample B. These materials were at the origin of the gehlenite in the two samples and hatrurite in sample B, thus giving rise to the hydraulic properties of the two coatings. These aggregates were as much reactive as they were relatively fine (less than 1.25 mm). This fine granulometry, as well as the relatively large thicknesses, caused these two coatings to behave like sponges and to retain water without letting it filter through. Based on the assumption that the aggregates diameters were higher than 0.063 mm, the two mortars had a binder/aggregate ratio of 0.6 for A and 0.3 for B. Furthermore, the two mortars were superficially deteriorated by dissolution–precipitation cycles of calcite. The presence of gypsum in sample A was the result of its sulfatation. However, notwithstanding that the mortars are exposed to corrosive conditions (fresh water and marine spray), the two coatings remain in a satisfactory state. Acknowledgements The authors address their thanks to Slama Abdelhakim, archaeologist at the National Heritage Institute of Tunisia and the conservator of the historic buildings at Ghar El Melh. They also thank the responsible engineers at Tunisian SIKA for their help in performing some tests.

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