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Journal of Cultural Heritage 12 (2011) 346–355

riginal article

he use of hydroxyapatite as a new inorganic consolidant for damagedarbonate stones

nrico Sassonia,1, Sonia Naidub,2, George W. Schererc,∗

Dipartimento di Ingegneria Civile, Ambientale e dei Materiali, Università di Bologna, via Terracini 28, 40131 Bologna, ItalyDepartment of Chemical Engineering, Princeton University, Eng. Quad. E-226, Princeton, NJ 08544, USADepartment of Civil and Environmental Engineering, Princeton University, Eng. Quad. E-319, Princeton, NJ 08544, USA

r t i c l e i n f o

rticle history:eceived 2 September 2010ccepted 23 February 2011vailable online 9 April 2011

a b s t r a c t

The feasibility and the effectiveness of using hydroxyapatite (HAP) formed by reacting limestone with asolution of diammonium hydrogen phosphate (DAP) in mild conditions, as a consolidant for carbonatestones were investigated. Firstly, a novel method for predamaging limestone was developed. Then, theeffects of DAP solution concentration and reaction duration were evaluated to define the best treatment

eywords:ydroxyapatiteonsolidationarbonate stonesynamic elastic modulusensile strength

conditions, and the strengthening effect was evaluated on artificially damaged Indiana Limestone sam-ples. Treated samples exhibit a significant increase in the dynamic elastic modulus and tensile strength,which is attributed to microcrack reduction and pore filling consequent to formation of calcium phosphatephases at grain boundaries, as assessed by SEM/EDS and ESEM/EBSD. Consequent to a slight reduction ofcoarser pores, as revealed by MIP, the sorptivity of treated samples is only slightly reduced, so that waterand water vapor exchanges with the environment are not significantly blocked.

© 2011 Elsevier Masson SAS. All rights reserved.

. Introduction

Carbonate stones, such as limestone and marble, have been usedince ancient times in architecture and sculpture. As such lithotypesre subject to several weathering mechanisms, whose harmful-ess depends on stone tensile strength (e.g., salt crystallization,

reezing-thawing cycles, clay swelling, heating-cooling cycles), theevelopment of effective consolidants for carbonate stones is a keyoal in cultural heritage conservation [1]. The immediate goal ofonsolidation is to restore the mechanical integrity of weatheredtone [2,3], and that is the focus of the present study. Of course, its ideal if the same treatment provides protection against furtheramage, but that usually requires treatments in addition to con-olidation [1,4,5]. In fact, the treatment described here is expectedo provide corrosion resistance, but that aspect will be exploredn a future publication. The main consolidants currently used fortrengthening carbonate stones include lime-based, polymeric, and

ilicate consolidants [6,7].

Lime-based consolidants, such as lime milk and lime water,im to introduce lime (Ca(OH)2) inside the pores of the stone. As

∗ Corresponding author. Tel.: +1 609 258 5680; fax: +1 609 258 1563.E-mail addresses: enrico.sassoni2@unibo.it (E. Sassoni), snaidu@princeton.edu

S. Naidu), scherer@princeton.edu (G.W. Scherer).1 Tel.: +39 051 2090363; fax: +39 051 2090322.2 Tel.: +1 609 258 9089; fax: +1 609 258 1563.

296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved.oi:10.1016/j.culher.2011.02.005

Ca(OH)2 reacts with atmospheric carbonic dioxide (CO2), calciumcarbonate (CaCO3) is produced. Newly formed CaCO3 is chemicallycompatible with the calcitic substrate and therefore can bond tostone grains and strengthen the stone. In spite of the chemicalcompatibility, lime-based treatments are characterized by somelimitations, such as the reduced penetration depth, the extremelyslow rate of conversion of Ca(OH)2 into CaCO3 and the limitedsolubility of lime in water, causing chromatic alteration of stonesurfaces [6,8]. To avoid these limitations, studies on the use of nano-limes (i.e., lime particles with submicrometric dimensions) haverecently been undertaken [8,9].

Silicate consolidants, such as tetraethoxysilane (TEOS), aimto introduce silicon-based compounds (e.g., Si(OC2H5)4) into thepores [1,10]. Such compounds, in contact with atmospheric mois-ture or liquid water, undergo a hydrolysis reaction, so that ethoxygroups (OC2H5) are progressively replaced by hydroxyl groups (OH)[6,11]. When hydroxyl groups of different molecules start to react,the molecules undergo a condensation reaction and form a gel. Insilicate stones, the deposited silica gel can bond covalently to thegrain surfaces, because the grains are covered with silanol groupsthat can react with the silica gel, but in carbonate stones the bond-ing between the silica gel and the grains is merely physical, as grainsurfaces lack hydroxyl groups. As a consequence, silicate consoli-

dants are much less effective on carbonate stones than on silicatestones [1,12]. To improve the effectiveness of silicate consolidantson carbonate stones, numerous coupling agents–having an anchorgroup on one end that bonds to calcitic grains and a hydroxyl group

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E. Sassoni et al. / Journal of Cu

n the other end that bonds to silica gel–have been studied (e.g.,artaric acid, organoalkoxysilanes, etc.) [1,10,13].

Polymeric consolidants, including acrylics, silicones, vinyls,olyesters, urethanes and epoxies, aim to strengthen the stone by

ntroducing macromolecules that solidify when the solvent evap-rates (thermoplastic polymers) or when curing agents cross-linkith the resin (thermosetting polymers), hence creating a solid net-ork that can bond the grains together. Polymeric consolidantsave been widely used in the past, as they produce consolidationnd water repellency at the same time. However, the use of poly-eric consolidants may lead to several problems, mainly related to

enetration depth, yellowing by ultraviolet rays, and biodeteriora-ion owing to bacterial and fungal growth [6,14–16].

Considering the limitations of the above-mentioned consol-dants in strengthening carbonate stones, in this paper theffectiveness of hydroxyapatite (HAP) as an inorganic consoli-ant for carbonate stones was investigated. The current study was

nspired by the work of Matteini et al. [17], who suggested that mar-le could be protected by coating it with a layer of calcium oxalate.mild chemical reaction, where calcite is exposed to a solution of

mmonium oxalate in water, results in formation of calcium oxalateonohydrate (whewellite, CaC2O4·H2O) on the surface of the stone.

ield studies [18] indicate that this treatment is helpful in delay-ng damage, but the effect is not as dramatic as had been hoped.o understand why the oxalate treatment is not more effective, weompared the crystal structures of calcite and oxalate, and foundhat they are not compatible. That is, one cannot make a coher-nt (epitaxial) layer of one on top of the other, because the atomsre arranged with different symmetries. This is important, becauselayer that is porous or patchy can be undercut and removed by

he corrosion. Moreover, whewellite is not much less soluble thanalcite. In contrast, the mineral HAP, Ca10(PO4)6(OH)2, which con-titutes the inorganic component of our teeth and bones, is veryurable, and there is a mild chemical reaction that can convert cal-ite to HAP. Moreover, the crystal structures of those two mineralsre quite similar, so it is likely that a coherent layer of apatite cane formed on the surface of marble and limestone.

HAP has the formula Ca5(PO4)3(OH), but is usually written asa10(PO4)6(OH)2, to denote that the crystal unit cell comprises two

ormula units. HAP is expected to be an effective consolidatinggent for carbonate stones since it:

is notably less soluble than calcite (the solubility products at 25 ◦Cbeing Ksp = 1.6·10−117 for HAP [19] and Ksp = 3.4·10−9 for calcite[20]); since the formula for HAP contains 18 ions and that of cal-cite contains 2, the solubility of the “molecule” is cHAP = Ksp

1/18

≈ 3.25 × 10−7 M and cCalcite ≈ 5.83 × 10−5 M (or, in terms ofgrams/liter or moles of Ca2+ dissolved in a liter of solution, HAPis about 18 times less soluble than calcite);has a dissolution rate [21–23] about 4 orders of magnitude lowerthan that of calcite [24];has a crystal structure similar to calcite (the unit cell beinghexagonal for HAP [25] and rhombohedral for calcite, but oftendescribed as hexagonal [26]);has lattice parameters close to calcite (respectively, a = b = 9.43 Aand c = 6.88 A for HAP [27] and a = b = 9.96 A and c = 17.07 A forcalcite, considering two molecules per unit cell [28]).

Thus, the lattice parameters of HAP and calcite differ by only5%, indicating compatibility of the structures sufficient to permitpitaxial growth. The good lattice match between calcite and HAPavors nucleation of the phosphate layer on the surface of marble

r limestone, and encourages strong bonding of the layer to theubstrate.

Indeed, apatite coatings have been found on ancient mon-ments, apparently formed by slow weathering of ancient

Heritage 12 (2011) 346–355 347

treatments, such as milk-based coatings [29,30]. The fact that theseapatite layers still exist, and are not being deposited by currentlyactive processes (such as microbial activity), implies that they aredurable and coherent. One goal of our research is to develop a prac-tical method for applying such coatings, and to evaluate their abilityto protect carbonate stones from corrosion. A secondary benefit isto restore the mechanical integrity of weathered stone, and that isthe focus of the present paper.

Numerous techniques, divisible into solid-state and wet meth-ods, have been developed in bioceramic and biomineralisationstudies for HAP preparation [19,31–35]. Solution-deposited phos-phates have also been used to consolidate loose sand [36,37]and to preserve frescoes [38]. Even among wet methods (suchas precipitation, hydrothermal, and hydrolysis processes), manyare not applicable in the case of monument restoration, as theyinvolve high temperatures, hazardous chemicals or extreme pHvalues. The most suitable method for an application in situ forstone strengthening is the one [39] involving the reaction of cal-cite and diammonium hydrogen phosphate (DAP, (NH4)2HPO4) toform carbonate-containing HAP at temperature close to room tem-perature (40 ◦C). According to Kamiya et al., HAP is formed by thefollowing chemical reaction:

10CaCO3 + 5(NH4)2HPO4→ Ca10(PO4, CO3)6(OH, CO3)2

+ 5(NH4)2CO3 + 3CO2 + 2H2O,

where PO43− and OH− can be partially replaced by CO3

2−.The resulting HAP is typically non-stoichiometric, as it contains

carbonate ions, and its precipitation is expected to be precededby the formation of several intermediate metastable phases, suchas monocalcium phosphate monohydrate and anhydrous (MCPMand MCPA, respectively), dicalcium phosphate dihydrate and anhy-drous (DCPD and DCPA, respectively), octacalcium phosphate(OCP), amorphous calcium phosphate (ACP) and/or calcium-deficient HAP (CDHA). Such precursor phases, whose formationdepends on reaction conditions (e.g., degree of supersaturation,temperature, pH, presence of foreign ions, etc.), are expected totransform eventually into HAP by dissolution and reprecipitationprocesses [27,40].

Both cationic and anionic substitutions can be made in the HAPstructure. The substitution of fluoride ions into HAP is known toimprove its chemical durability slightly [41], but the fluoride pre-cursors are relatively dangerous to handle on the scale that wouldbe required for treating a monument. Replacing calcium cation withstrontium or barium cations in HAP improves the lattice matchwith calcite [42] at the cost of a small increase in dissolution rate[43], so doping with those cations will be investigated in the future.Improvement in the lattice mismatch would favor heterogeneousnucleation of the HAP layer on calcite and thereby facilitate forma-tion of a hermetic coating.

2. Experimental procedure

2.1. Materials

Indiana Limestone (IL) was used to test the HAP consolidationeffect. IL is a porous carbonate stone (porosity ∼14%), mainly madeof calcite (> 97 wt%) and a small amount of other components (e.g.,Al2O3, SiO2 and MgCO3) [44]. Some of our samples showed tracesof Mg by EDS, but no magnesite was detected by XRD. IL mainlyconsists of calcite cemented oolites, even though a small amountof sparry calcite crystals may be also present. Cubic samples (5 cm

side) and cylindrical samples (2 cm diameter) were respectively cutand core-drilled from 5 cm thick IL plates.

The DAP was purchased from Fisher Scientific (assay ≥ 98.0%,reagent grade) and used as received. All water used was de-ionized.

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48 E. Sassoni et al. / Journal of Cu

.2. Artificial damaging

To test the consolidation effectiveness on weathered samplesith uniform characteristics, for reproducibility’s sake, IL samplesere artificially damaged by heating, following a suggestion by

latt [45]. To assess the heating temperature effect, 10 cubic sam-les (two for each temperature) were heated to 100 ◦C, 200 ◦C,00 ◦C, 400 ◦C and 500 ◦C for 1 hour. To assess the heating durationffect, nine cubic samples (three for each duration) were heated forhour, 4 hours and 16 hours at 300 ◦C.

The damaging effect was evaluated by comparing the dynamiclastic modulus (Ed) of the samples before and after heating. Theodulus was calculated according to the formula Ed = �V2 whereis the density and V the pulse velocity, measured using a com-ercial instrument (PUNDIT) with 54 kHz transducers. For cubic

amples, the Ed was calculated as the average for the three valueseasured in the three directions.The tensile strength (�t) variation was also evaluated by com-

arison between three core-drilled samples heated to 300 ◦Cor 1 hour and three unheated core-drilled samples. The tensiletrength was measured by performing the Brazil test.

.3. Definition of treatment conditions

The effects of DAP solution concentration and treatment dura-ion were evaluated on cubic samples artificially damaged byeating to 300 ◦C for 1 hour.

.3.1. Concentration effectFour DAP solutions were prepared: 0.1 M, 0.5 M, 1.0 M and 4.4 M

the last corresponding to the saturation concentration). All treat-ents were performed at room temperature. In the following tests,

he DAP was introduced by immersion of the samples; however,ther tests were performed by brushing on the solution until itas rejected, and those samples performed at least as well.

The initial Ed was evaluated for eight cubic samples. The sam-les were then water-saturated by immersing them in water up tone half of the height, letting them soak up the water until theyppeared completely wet (∼60 minutes) and then adding water upo 5 mm below the upper face of the cubes (the same procedure wasollowed to solution-saturate all the samples described later in theaper). After 24 hours, the samples were taken out of the water,he surface gently dried with a towel to remove excess water andhe increase in the dynamic elastic modulus owing to water fill-ng the pores evaluated. The samples were then dried under a fant room temperature until constant weight (the same procedureas followed to dry all the samples described later in the paper).

his procedure was used to avoid damage from heating. Once com-letely dry, two samples for each concentration were immersed inhe DAP solutions. To avoid water evaporation and hence concen-ration changes, all solution containers were sealed with parafilm.

The consolidation progress through time was monitored after, 2, 4, and 8 days by taking the samples out of the DAP solutions,emoving the excess solution from the surface, measuring Ed andhen putting the samples back in the solutions. After 8 days, all theamples were removed from the solutions and immersed in watero remove the unreacted DAP from inside the pores. After washingor 3 days (the water being completely renewed every day), theamples were finally dried and Ed measured again. The washingrocedure was used to insure that changes in Ed were not causedy residual reactants or byproducts.

.3.2. Time effectThe initial Ed was measured for eight cubic samples. The samples

ere then immersed in a 1.0 M DAP solution and extracted, twot a time, after 1, 2, 4, and 8 days. Upon extraction, the samples

Heritage 12 (2011) 346–355

were water-immersed for 3 days and then dried. Then Ed was finallymeasured again.

2.4. Evaluation of consolidation effects

The effects of the consolidation treatment were evaluated onsamples artificially damaged by heating to 300 ◦C for 1 hour andthen treated by immersion in a 1.0 M DAP solution for 2 days, thenextracted, water-immersed for 3 days and dried. For comparison,some samples were treated by brushing on the solution of DAPuntil it was rejected.

2.4.1. Mechanical propertiesThe improvement of the mechanical properties of the stone was

evaluated by comparing (i) the dynamic elastic modulus of 10 core-drilled samples before and after treating; (ii) the tensile strengthof the same 10 samples, after treating, with the tensile strength of10 untreated samples. To further investigate the role of treatmentduration, the dynamic elastic modulus and the tensile strength of10 core-drilled samples treated for 4 days were also measured.

2.4.2. Formation of calcium phosphate phasesThe calcium phosphate phases formed after treatment were

observed in a scanning electron microscope (SEM, Philips XL30Field-Emission-Gun). The samples for the observation wereobtained by hammer fracturing of a 2 cm diameter, 5 cm lengthcylindrical sample which had been core-drilled from the center ofa cubic specimen. In this way, the penetration depth of the treat-ment could be also estimated. The elemental composition of theobserved phases was assessed by energy dispersive X-ray spec-trometry (EDS) performed on the scanning electron microscope.To identify the calcium phosphate phases, electron back-scattereddiffraction (EBSD) was performed in an environmental scanningelectron microscope (FEI Quanta 200 ESEM).

2.4.3. Resistance to wetting/drying cyclesThe resistance to wetting/drying cycles (w/d cycles) of the con-

solidating calcium phosphate phases was evaluated by comparingthe dynamic elastic modulus of 10 core-drilled samples, treatedwith the DAP solution, before and after each of five w/d cycles. Thew/d cycles were performed by water-saturating the core-drilledsamples for 24 hours (following the procedure described in §2.3.1)and then drying them under a fan until constant weight.

After five w/d cycles, the tensile strength of the samples wasmeasured and compared to that of untreated samples and treatedsamples not subjected to w/d cycles.

2.4.4. Pore size distributionThe modification of the pore size distribution was evaluated

by performing mercury intrusion porosimetry (MIP, Micromet-rics 9410) on fractured samples obtained from various depths(0÷5 mm, 10÷15 mm and 20÷25 mm) of a treated cubic specimen.

2.4.5. SorptivityThe effect of HAP treatment on sorptivity (or, rate of capillary

absorption) was evaluated by comparing the sorption rates of atreated sample and an untreated one. The sample (2.5 × 1.5 × 5 cm)was suspended below a balance, and a large dish of water was raiseduntil it just came into contact with the bottom of the sample. Thegain in weight was continuously recorded by a computer.

2.4.6. Color changeThe color changes between untreated samples, treated samples

and treated samples subjected to w/d cycles were evaluated by

E. Sassoni et al. / Journal of Cultural Heritage 12 (2011) 346–355 349

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Fig. 2. Dynamic elastic modulus of IL cubic samples treated with DAP solutions atvarious concentrations versus duration of treatment; open symbols represent theEd of dry samples (before and after the treatment) while solid symbols represent

TD

ig. 1. Dynamic elastic modulus of IL cubic samples versus heating temperaturevalues are averages for two samples).

easuring the CIE 1976 L*a*b* color parameters and then calculat-ng the total color difference �E* = (�L*2 + �a*2 + �b*2)1/2, using aommercial colorimeter (X-rite).

.4.7. Effectiveness for different weathering levelsThe effectiveness of the consolidating treatment on samples

eathered to different extents was evaluated by measuring theariation of the dynamic elastic modulus after consolidation forhe same samples that had been used to evaluate the effect of heat-ng temperature on the artificial damage process (i.e., on sampleseated to 100, 200, 300, 400, and 500 ◦C for 1 hour).

. Results

.1. Artificial damaging

The samples exhibited a decrease in Ed proportional to theeating temperature (Fig. 1). For samples heated at the same tem-erature for different periods (1, 4, 16 hours), no difference in thed decrease was found. For samples heated to 300 ◦C for 1 hour,hose Ed was reduced by 43.4% after heating, the tensile strengthas 27.1% lower than that of the unheated samples (Table 1).

.2. Definition of treatment conditions

.2.1. Concentration effectAfter water saturation for 24 hours, all the samples showed an

ncrease in the Ed, compared to dry samples. This is a result of theigher density of the wet sample, and does not indicate stiffening.o demonstrate an increase in stiffness, the DAP-saturated sam-les must show E higher than the water-saturated stone. Once

dried again and then solution-saturated, the samples exhibited an

ncrease in Ed that rose with solution concentration up to 1 M, buthanged little between 1 and 4.4 M (Fig. 2).

able 1ynamic elastic modulus and tensile strength of IL samples unheated and heated to 300 ◦

Samples Dynamic elastic modulus (Ed)

Ed [GPa] Ed

Unheated 37.3 (±0.3) –Heated to 300◦C for 1 hour 21.1 (±0.0) 43Unheated 33.9 (±1.5) –Heated to 300◦C for 4 hours 20.0 (±0.7) 40Unheated 35.0 (±1.5) –Heated to 300◦C for 16 hours 19.8 (±0.6) 43

the Ed of water- and solution-saturated samples (during the treatment); the finalvariation �Ed,dry was calculated comparing the Ed values in the dry condition beforeand after treatment (all values are averages for two samples).

3.2.2. Time effectSamples treated for increasing periods exhibited progressive Ed

increases, as indicated in Table 2. Notably, the increase in Ed mea-sured after 2 days was already 91% of that measured after 8 days.

3.3. Evaluation of consolidation effects

3.3.1. Mechanical propertiesTreated samples exhibited a significant Ed increase, amount-

ing to +96.0% after 2 days and +96.8% after 4 days of treatment(Table 2). Analogously, the tensile strength of samples treated for2 days and 4 days was, respectively, 24.5% and 28.8% higher thanthat of untreated samples (Table 2).

3.3.2. Calcium phosphate phase formationAs can be seen in the SEM images in Fig. 3, calcium phosphate

phases formed on the surface of the pores and at grain boundaries.According to the EDS results (Fig. 4), calcium, carbon, oxygen andphosphorus are present. No nitrogen was detected, so there was noresidual DAP. During the reaction, there was an odor of ammonia,indicating that the ammonium carbonate by-product evaporated.Further investigation of the calcium phosphate phases by EBSDidentified:

(i) HAP, Ca10(PO4)6(OH)2 (Fig. 5a);(ii) octacalcium dihydrogen phosphate esa(phosphate),

Ca8H2(PO4)6, which corresponds to the general formulaCa (HPO ) (PO ) (OH) (with 0 ≤ x ≤ 2), used in the

10−x 4 x 4 6−x 2−xliterature to describe calcium-deficient HAPs [35], and which,therefore, in the rest of the paper is referred to as CDHAP(Fig. 5b);

C for 1, 4, and 16 hours (values are averages for three samples).

Tensile strength (�t)

decrease [%] �t [MPa] �t decrease [%]

4.8 (±0.5) –.4 3.5 (±0.1) 27.1

– –.9 – –

– –.4 – –

350 E. Sassoni et al. / Journal of Cultural Heritage 12 (2011) 346–355

Table 2Dynamic elastic modulus of IL cubic samples before and after treatment with 1.0 M DAP for 1, 2, 4 and 8 days (values are averages for two samples); dynamic elastic modulusand tensile strength of IL cylindrical samples before treatment and after 2 days and 4 days treatment with 1.0 M DAP and after 2 days treatment with 1.0 M DAP and then fivewetting/drying cycles (values are averages for 10 samples).

Treatment on cubic samples Dynamic elastic modulus [GPa] Tensile strength [MPa]

Before treatment After treatment Increase [%] Untreated Treated Increase [%]

1.0 M DAP for 1 day 23.1 ( ± 0.8) 35.6 ( ± 0.9) 54.5 ( ± 1.4) – – –1.0 M DAP for 2 days 22.3 ( ± 0.4) 35.8 ( ± 0.8) 60.7 ( ± 0.7) – – –1.0 M DAP for 4 days 22.6 ( ± 0.2) 37.2 ( ± 0.3) 64.4 ( ± 2.9) – – –1.0 M DAP for 8 days 21.3 ( ± 1.1) 35.4 ( ± 0.5) 66.8 ( ± 6.3) – – –

Treatment on cylindrical samples

1.0 M DAP for 2 days 18.6 ( ± 0.8) 36.5 ( ± 1.2) 96.0 ( ± 5.8) 3.3 ( ± 0.4) 4.1 ( ± 0.7) 24.51.0 M DAP for 4 days 18.8 ( ± 1.0) 37.0 ( ± 1.4) 96.8 ( ± 6.0) 3.3 ( ± 0.4) 4.2 ( ± 0.4) 28.81.0 M DAP for 2 days + 5 w/d cycles 18.8 ( ± 0.8) 33.9 ( ± 1.0) 80.7 ( ± 5.8) 3.3 ( ± 0.4) 3.8 ( ± 0.2) 16.8

Fig. 3. SEM images of fracture surfaces of IL samples treated with 1.0 M DAP for 2 days; the arrows indicate the points where EDS analysis was performed (EDS results arereported in Fig. 4).

ectra,

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Fig. 4. EDS spectra of calcium phosphate phases observed in Fig. 3; the EDS sp

iii) octacalcium dihydrogen esa(phosphate) monohydrate,Ca8H2(PO4)6·H2O, which differs from OCP described in theliterature [19] by being a monohydrate, rather than a pentahy-drate, and which, therefore, in the rest of the paper is calledoctacalcium phosphate monohydrate (OCPM) (Fig. 5c);

iv) calcium dihydrogen phosphate monohydrate,Ca(H2PO4)2·H2O, which is described in the literature [19]as monocalcium phosphate monohydrate (MCPM) (Fig. 5d);

As the above mentioned phases were detected in samples com-ng from different depths (0–5 mm and 15–20 mm) of a treatedubic specimen, the treatment penetration depth can be assesseds greater than 2 cm.

.3.3. Resistance to wetting/drying cyclesAs illustrated in Fig. 6, treated and dried samples that were sub-

ected to wetting/drying cycles experienced a 13.9% loss of the Ed

(a) and (b) respectively, were taken in the points indicated by arrows in Fig. 3.

improvement, which decreased from +93.8% (before the w/d cycles)to +80.7% (after w/d cycles) (Table 2). The tensile strength com-parison between untreated samples, treated samples and treatedsamples subjected to five w/d cycles shows a reduction of 31.4% ofthe �t improvement, which was +24.5% for samples not subjected tow/d cycles and +16.8% for samples subjected to w/d cycles (Table 2).

3.3.4. Pore size distributionThe two samples that had been heated to 300 ◦C, but not treated

with DAP (labeled “Untreated”) show good reproducibility (Fig. 7).After the treatment, there is a slight reduction of total open poros-ity that is more significant within 5 mm of the surface. There isa decrease in the fraction of coarser pores (having radius larger

than 1 �m) and an increase in the percentage of finer pores (havingradius smaller than 0.1 �m). The latter effect is caused in part bythe phosphate layer reducing the size of the pores, and in part bythe porosity within that layer.

E. Sassoni et al. / Journal of Cultural Heritage 12 (2011) 346–355 351

F ts for the four phosphate phases identified: (a) HAP; (b) calcium-deficient hydroxyapatite( phosphate monohydrate (MCPM).

3

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ig. 5. Unit cell, chemical formula, SEM image of the analyzed point and EBSD resulCDHAP); (c) octacalcium phosphate monohydrate (OCPM); (d) calcium dihydrogen

.3.5. SorptivityThe sorptivity comparison between untreated and treated sam-

les (Fig. 8) shows that the latter experienced a reduction of 44.0% ofhe sorptivity, which decreased from 0.0452 g/cm2·min1/2, beforehe treatment, to 0.0253 g/cm2·min1/2, after the treatment.

.3.6. Color changeCompared to untreated samples, treated samples and treated

amples subjected to w/d cycles exhibited a �E* = 7.65 andE* = 6.45, respectively (values are averages for 15 measurements).

n most of the cases, the total color change is mainly due to a changen the sample lightness (L* parameters), which generally increasedfter the treatment.

ig. 6. Dynamic elastic modulus of IL core-drilled samples before treating (openquare), after treating with 1.0 M DAP for 2 days and after w/d cycles (solid squares)values are averages for 10 samples).

Fig. 7. Pore size distribution of IL samples untreated (grey lines) and treated (blacklines) with 1.0 M DAP for 2 days (� = open porosity [%]; r > 1 �m = % of pores havingradius > 1 �m; r < 0.1 �m = % of pores having radius < 0.1 �m).

3.3.7. Effectiveness for different weathering levelsFig. 9 shows the improvement in Ed for samples subjected to

heat treatment at increasing temperature. For treatment tempera-tures up to about 200 ◦C, the treated samples have a higher Ed thanthe virgin stone. After treatment above 300 ◦C, the modulus is notfully restored, but the fractional increase is greater. As indicated inFig. 10, for a sample heated to 500 ◦C, the treatment produces anincrease of more than 120% in the modulus of the damaged stone.

4. Discussion

4.1. Artificial damaging

The decreases in dynamic elastic modulus and tensile strengthexhibited by the heat-treated samples (Table 1) are to be ascribed

352 E. Sassoni et al. / Journal of Cultural

Fig. 8. Sorptivity of IL samples untreated (gray line) and treated with 1.0 M DAP for2 days (black line).

Fig. 9. Dynamic elastic modulus of IL cubic samples unheated (open bars), heated(gray bars) and treated with 1.0 M DAP for 2 days (black bars) at different heatingtemperatures (values are averages for two samples).

Ftf

teabit

achieved after 2 days of treatment, the latter duration–also corre-

ig. 10. Dynamic elastic modulus increase of IL cubic samples heated and thenreated with 1.0 M DAP for 2 days versus heating temperature (values are averagesor two samples).

o the anisotropic thermal expansion of the calcite crystal, whichxpands parallel and contracts normal to the crystallographic c-xis [46,47]. As a consequence, stresses are generated at grain

oundaries, resulting in microcracks responsible for the decreases

n Ed and �t. While the decrease in Ed proved to be a linear func-ion of heating temperature (Fig. 1), consistent with the thermal

Heritage 12 (2011) 346–355

strain being proportional to temperature, the heating durationproved to have no effect on Ed. Evidently, as soon as the stonespecimen reaches the heating temperature, stresses arise andmicrocracks open; afterwards, sustaining the same temperaturefor prolonged periods does not cause any further damage, andnew microcracks are generated only if the temperature is furtherincreased.

The decrease in Ed assessed for actual stoneworks exposed tonatural weathering in the field was found to be ∼36% [48], so a heat-ing temperature of 300 ◦C, responsible for an Ed decrease of ∼43%,was chosen to artificially damage the IL samples for the prosecutionof the study and the evaluation of treatment effectiveness.

Heating thus proved to be a very efficient and controllablemethod to artificially damage limestone samples to a desired“weathering” level, ensuring significant constancy of the final spec-imen properties. Studies of the microstructural changes caused bythis treatment in limestone and marble are underway.

4.2. Definition of treatment conditions

The increase in Ed observed after water saturation for 24 hours(shown at Time = −10 in Fig. 2), is owing to water filling the pores,which allows the acoustic pulse to travel faster inside the stonespecimen, so that an increase in the elastic dynamic modulusis recorded. After drying and resaturating with DAP solution for24 hours, the Ed increased compared to the sample containing onlywater. The difference between the modulus in solution and that inwater rose in proportion to the solution concentration (Time = 1in Fig. 2). Afterwards, as the reaction between calcite and DAPproceeds, further Ed increases are achieved after 2, 4 and 8 days(Time = 2, 4 and 8 in Fig. 2, respectively).

Unexpectedly, after 8 days of reaction both the samplesimmersed in the 4.4 M DAP solution exhibited an Ed decrease(Time = 8 in Fig. 2) and then a new increase after drying (Time = 21in Fig. 2). Since all the other samples experienced a reduction ofthe Ed after drying, as a consequence of the evaporation of waterfilling the pores, it is reasonable to assume that the samples treatedwith 4.4 M DAP solution also underwent such a reduction in Ed andthat, therefore, the Ed value measured after 8 days of treatment wasunderestimated. The explanation for this underestimation may befound considering that, during the treatment period, some superfi-cial deposits formed on the specimens treated with the 4.4 M DAPsolution (whose concentration corresponds to the saturation con-centration). Since the pulse velocity measurement highly dependson the contact between the specimen and the instrument, thesurface deposits presumably impeded a perfect contact with thetransducers, resulting in an underestimation of the elastic modu-lus. Once the samples were removed from the solutions, the surfacewas gently washed and left in water for 3 days, so the superficialdeposits were mostly removed and thus the real Ed value could beestimated for the dry samples.

Since treating the samples with a 1.0 M DAP concentration for8 days produced an increase in Ed of 62% and since samples treatedwith a 4.4 M DAP concentration, even though exhibiting a higher Edincrease, also exhibited surface deposits, the 1.0 M DAP concentra-tion was chosen for the prosecution of the study and the evaluationof treatment effectiveness.

To confirm that the temporal trend of increase in Ed, exhibited bysolution-saturated samples, actually corresponds to the temporaltrend experienced by dried samples, the time effect was evaluatedon dried samples treated with a 1.0 M DAP concentration for differ-ent periods. Since the 91% of the 8-day increase in Ed was already

sponding to a feasible treatment period in the field–was chosen forthe prosecution of the study and the evaluation of consolidationeffectiveness.

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.3. Evaluation of consolidation effects

The core-drilled samples used to evaluate the mechanicalroperties exhibited significant increases in dynamic elastic mod-lus and tensile strength, comparable to the strengthening effectchieved by silicate consolidants applied on silicate stones [49].

The core-drilled samples exhibited an Ed increase substantiallyigher than the cubes treated in the same conditions (i.e., 1.0 MAP concentration for 2 days): indeed, the average Ed increasemounted to 96.0% for the cylinders and 60.7% for the cubes. Tond the explanation for such a difference one should firstly con-ider that the Ed increase calculated for each cube is the averageor the three values calculated for the three sides of the cube. Now,he Ed increase measured for the cube side that corresponds to theylinders’ height (and that also corresponds to the original thick-ess of the plate the specimens were cut from) is actually muchloser to the Ed increase found for the cylinders (on the average, itmounts to 82.0%). As the Ed increase measured for the other twoides of the cubes are lower (on the average, 53.0%), the overall Edncrease, calculated by averaging the values for the three sides, isubstantially lower. The reason for such anisotropic behavior maye found in some “bedding plane” effects that have been describedor IL: even if no bedding plane is discernible and the change inhysical properties with direction is small, relatively large differ-nces in the energy necessary to remove a unit volume of rock haveeen assessed for different directions [50].

The observed improvement in mechanical properties is to bescribed to the deposition of calcium phosphate phases on thenternal surfaces of the pores and at grain boundaries (Fig. 3). Inact, the deposition of calcium phosphate phases at the contactoints between grains led to a reduction of the microcracks thatad opened during heating, resulting in a greater resistance to crackropagation and new crack formation.

The calcium phosphate phases include, alongside HAP, sev-ral intermediate phases that are usually expected to precedetoichiometric HAP formation (Fig. 5). Such precursor phases areharacterized by a Ca/P ratio lower than that of HAP: whereasa/P = 1.67 for HAP, Ca/P = 0.5 for MCPM and Ca/P = 1.33 for OCPMnd CDHAP [19]. Thus, formation of MCPM, OCPM and CDHAP maye favored over HAP, since a lower amount of Ca2+ ions, com-

ng from calcite dissolution, is needed. As a consequence of thealcium-deficiency, the intermediate phases are thermodynami-ally less stable and more soluble than HAP: while the solubility ofAP at 25 ◦C is ∼0.0003 g/l, it is approximately ∼0.0081–0.0094 g/l

or OCPM and CDHAP and ∼18 g/l for MCPM [19].Considering the high solubility of MCPM, potentially detrimen-

al to the consolidation treatment effectiveness after exposuref the stone to water, the resistance of the treatment to wet-ing/drying cycles was tested. As shown in Fig. 6, after the first w/dycle the samples exhibited an Ed decrease of about 13.9% (from93.8% to +80.7%) and then, for the next 4 w/d cycles, Ed remainedonstant. The tensile strength was then tested. Compared to that ofamples treated but not subject to w/d cycles, the tensile strengthfter five w/d cycles was 31.4% lower (on the average, 3.8 MPa, theverage tensile strength of untreated and treated samples being.3 MPa and 4.1 MPa, respectively, Table 2).

The partial loss of the improvement in mechanical propertieschieved after the w/d cycle is related to the weight loss the sam-les experienced. Indeed, after the treatment with the 1.0 M DAPolution, the core-drilled samples exhibited an average weight gainf 0.1 g and an increase in Ed. Then, after the first w/d cycle, an aver-ge weight loss of 0.03 g occurred and, correspondingly, a decrease

n Ed was recorded. Afterwards, the weight remained substantiallytable and so did Ed.

As for the weight gain after the treatment, it is reasonable toscribe it mainly to the formation of calcium phosphate phases.

Heritage 12 (2011) 346–355 353

Indeed, even though stoichiometric HAP is supposed to formwith no significant weight increase–as 10 moles of CaCO3 (totallyweighing 1000 g) are expected to dissolve to form 1 mole ofHAP (weighing 988 g)–the three intermediate calcium phosphatephases detected are expected to form with a weight increase.Assuming that a number of CaCO3 moles corresponding to the num-ber of calcium atoms has to dissolve to form 1 mole of each of theintermediate calcium phosphate phases, 1 mole of CaCO3 (weigh-ing 100 g) is needed to form 1 mole of MCPM (weighing 252 g) and8 moles of CaCO3 (totally weighing 800 g) are needed to form 1mole of MCPM (weighing 910 g) and 1 mole of CDHAP (weighing892 g). In addition, the HAP and the precursor phases formed bythe reaction of calcite with DAP may likely contain CO3

2− ions par-tially replacing PO4

3− and OH− ions (cf. §1), which, in the lattercase, would bring a further increase in weight. Another possiblereason for the weight increase might be the presence of some unre-acted DAP inside the stone pores, notwithstanding the three-dayperiod of washing after the treatment. However, the time requiredfor unreacted ions to diffuse from the samples (assuming a diffu-sion coefficient D = 4·10−6 cm2/s for ions in water solutions insideof stone pores [51] and the radius of the samples being r = 1 cm)should amount to t ≈ r2/D = 2.9 days, which actually corresponds tothe time the samples were left in D.I. water after treatment. In addi-tion, in the EDS spectra no nitrogen peak, owing to unreacted DAPstill present inside the pores, was detected (Fig. 4). Nevertheless,the most likely cause of the weight increase seems to be the for-mation of the intermediate calcium phosphate phases, particularlythe MCPM.

The main reason for the weight loss after the first w/d cycle maybe found in the high solubility of MCPM, that dissolves in contactwith water, hence reducing the mechanical property improvementachieved after treatment. However, after the first w/d cycle, sampleweight and Ed remained constant, suggesting that all the MCPMformed was removed after the first contact with water.

The effects of a prolonged exposure to environmental weather-ing agents need to be tested on some treated specimens exposedto outdoor conditions. Indeed, after prolonged contact with rain-water, the intermediate calcium phosphate phases may tend todissolve and reprecipitate. This could even lead to the formation ofmore stable phases, such as HAP, and hence to a further strengthen-ing of the stone; on the other hand, the deposition of new phasesmay cause cracking and detachments, thus damaging the stone.Therefore, field testing of the treated stones is essential.

The formation of HAP, rather than less stable precursor phases,during the reaction between stone calcite and DAP might be pro-moted by providing calcium ions externally. Indeed, as the onlycalcium ions participating to the reaction come from calcite disso-lution in contact with the DAP solution [52], providing additionalcalcium ions (for instance, adding calcium chloride to the DAP solu-tion) could favor the formation of phosphate phases with a highercalcium content, closer to HAP’s one. Recent experiments showthat this is indeed the case, resulting in faster growth and bettercoverage of phosphate phases on calcite [53]. Tests are underwayto verify whether reduced porosity in the HAP deposit leads toenhanced consolidation and reduced solubility.

As a consequence of the deposition of calcium phosphate phaseson internal surfaces of the pores, a slight reduction of the totalopen porosity was observed (Fig. 7). Different variations of totalopen porosity and pore size distribution were detected at differ-ent depths of a treated specimen, the greatest pore reduction beingobserved for the most superficial sample (0–5 mm depth). As thesamples were treated by partial immersion and capillary rise, uni-

form solution penetration and reaction between the solution andthe stone can be expected. During the drying phase, the solutionmoves towards the surface where evaporation occurs. As the solu-tion may still contain unreacted DAP and/or ions coming from

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issolution of the most soluble phase (i.e., MCMP), higher phos-hate ion concentrations may occur near the stone surface, leadingo more abundant precipitation of calcium phosphate phases andence to a larger reduction of pore size. A relatively dense zone athe surface can lead to damage, if salts accumulate behind it, buthat is not expected in the present case, owing to the small changen sorptivity caused by the treatment.

The average pore volume is 0.0624 cm3/g for the untreatedamples and 0.0611 cm3/g for the treated samples, for an averageore volume reduction of 0.0013 cm3/g, or ∼1%. This should corre-pond to the weight gain consequent to calcium phosphate phaseeposition, which amounts, on the average, to 0.0030 g/g. As theensities of the calcium phosphate phases range from 2.23 g/cm3

for the MCMP [19]) to 3.16 g/cm3 (for the HAP [19]), the weightain related to pore volume reduction is expected to amount to.0029÷0.0041 g/g. As the measured weight gain lies close to the

ower bound, an abundance of the phase with the lowest densityviz., MCMP) seems to be indicated, as suggested by thermody-amic considerations.

As a result of the reduction in coarser pores, which allow a largeuantity of water to quickly enter the stone, after the treatment aeduction of stone sorptivity was found (Fig. 8). In particular, thereated sample exhibited an initial sorption rate markedly lowerhan the untreated sample, as a consequence of the higher poreystem modification that occurred near the surface. Afterwards,he treated sample’s sorptivity was 44.0% lower than that of thentreated sample, until about 800 minutes, when the sorptivity ofhe two samples approaches the same value. The modest reductionn sorptivity means that water and water vapor exchanges betweenhe stone and the environment are not significantly blocked.

As a consequence of the formation of calcium phosphate phases,olor changes were detected for treated samples (�E* = 7.65) thatere slightly reduced following w/d cycles (�E* = 6.45). Even after

he w/d cycles, the change still remains slightly higher than thehreshold (�E* ≤ 5.00) generally accepted for stones subjectedo conservation treatments [49]. Nevertheless, it is noteworthyhat a higher color difference (amounting to �E* = 11.52, averageor 10 measurements), was assessed between different untreatedamples, suggesting that color differences due to consolidatingreatment are much less pronounced than natural differenceswing to stone color variability.

For stone that has been weathered to the point that Ed decreasesy half, the DAP treatment can fully restore the modulus. Forore seriously damaged stone, although the original properties

re not obtained following treatment, the relative improvementncreases.

. Conclusions

The use of HAP and HAP precursor phases, formed by reaction ofDAP solution with calcite in limestone, seems to be a promising

onsolidating technique for carbonate stones. Significant increasesn the mechanical properties can be achieved after the treatment,

hich has the advantage of consisting of a non-hazardous aqueousolution of DAP, able to penetrate deeply into the stone (> 2 cm) andring significant strengthening after just 2 days of reaction. Thisuration of exposure can be achieved in the field by covering thereated surface to prevent evaporation. In the lab, the performancef the treatment is equally good, whether the solution is brushednto the surface to the point of refusal, or introduced by immersion.

The treatment proved to be fairly resistant to wetting/drying

ycles, and the loss that did occur could be attributed to dissolutionf an unstable phosphate compound. The stability of the treatments expected to increase if process modifications, such as addition ofalcium salts, result in formation of more stable phases.

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Heritage 12 (2011) 346–355

Acknowledgements

The authors thank the Kress Foundation and the Getty Conser-vation Institute for financial support. The authors also acknowledgethe usage of PRISM Imaging and Analysis Center, which is supportedin part by the NSF MRSEC program through the Princeton Centerfor Complex Materials (grant DMR-0819860). We are also indebtedto Dr. George Wheeler (Metropolitan Museum of Art) for the use ofthe colorimeter.

References

[1] G.W. Scherer, G.S. Wheeler, Silicate consolidants for stone, Key Eng. Mater. 391(2009) 1–25.

[2] G.G. Amoroso, V. Fassina, Stone decay and conservation, Elsevier, New York,1983.

[3] L. Lazzarini, M. Laurenzi Tabasso, Il restauro della pietra, CEDAM, Padua, 1986.[4] E. Wendler, D.D. Klemm, and R. Snethlage, Consolidation and hydrophobic

treatment of natural stone, pp. 203-212 in Proc. 5th Int. Conf. on Durability ofBuilding Materials and Components, eds. J.M. Baker, P.J. Nixon, A.J. Majumdar,and H. Davies (Chapman & Hall, London, 1991).

[5] J. Houck, G.W. Scherer, Controlling stress from salt crystallization, in: S.K.Kourkoulis (Ed.), Fracture and Failure of Natural Building Stones, Springer,Dordrecht, 2006, pp. 299–312.

[6] E.M. Winkler, Stone in architecture – Properties durability, Springer, New York,1997.

[7] K.L. Gauri, J.K. Bandyopadhyay, Carbonate stone: chemical behavior, durabilityand conservation, Wiley Interscience, New York, 1999.

[8] V. Daniele, G. Taglieri, R. Quaresima, The nanolimes in Cultural Heritage con-servation: characterization and analysis of the carbonatation process, J. Cult.Herit. 9 (2008) 294–301.

[9] V. Daniele, G. Taglieri, Nanolime suspensions applied on natural lithotypes: theinfluence of concentration and residual water content on carbonatation processand on treatment effectiveness, J. Cult. Herit. 11 (2010) 102–106.

10] G. Wheeler, Alkoxysilanes and the Consolidation of Stone, Getty ConservationInstitute, Los Angeles, 2005.

11] C.J. Brinker, G.W. Scherer, Sol-Gel Science, Academic Press, New York, 1990(Chp. 3).

12] R. Zárraga, J. Cervantes, C. Salazar-Hernandez, G. Wheeler, Effect of the additionof hydroxyl-terminated polydimethylsiloxane to TEOS-based stone consoli-dants, J. Cult. Herit. 11 (2010) 138–144.

13] G. Wheeler, J. Mendez-Vivar, E.S. Goins, S.A. Fleming, C.J. Brinker, Evaluationof alkoxysilane coupling agents in the consolidation of limestone, in: Proceed-ings of 9th Int. Con. on the Deterioration and Conservation of Stone, Elsevier,Amsterdam, 2000, pp. 541–545.

14] M. Favaro, R. Mendichi, F. Ossola, U. Russo, S. Simon, P. Tomasin, P.A. Vigato,Evaluation of polymers for conservation treatments of outdoor exposed stonemonuments. Part I: photo-oxidative weathering, Polym. Degrad. Stabil. 91(2006) 3083–3096.

15] S. Vicini, S. Margutti, G. Moggi, E. Pedemonte, In situ copolymerisation of ethyl-methacrylate and methylacrylate for the restoration of stone artefacts, J. Cult.Herit. 2 (2001) 143–147.

16] F. Cappitelli, P. Principi, R. Pedrazzani, L. Toniolo, C. Sorlini, Bacterial and fungaldeterioration of the Milan Cathedral marble treated with protective syntheticresins, Sci. Total Environ. 385 (2007) 172–181.

17] M. Matteini, A. Moles, and S. Giovannoni, Calcium oxalate as a protectivemineral system for wall paintings: methodology and analyses, III Int. Symp.Conservation of Monuments in the Mediterranean Basin, ed. V. Fassina, H. Ott,F. Zezza, 1994.

18] B. Doherty, M. Pamplona, M. Costanza Miliani, A. Matteini, B. Sgamellotti,Brunetti, Durability of the artificial calcium oxalate protective on two Floren-tine monuments, J. Cultur. Herit. 8 (2007) 186–192.

19] S.V. Dorozhkin, Calcium orthophosphates in nature, biology and medicine,Materials 2 (2009) 399–498.

20] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, The Chemical RubberCo, Boca Raton, FL, 1999.

21] N. Harouiya, C. Chaïrat, S.J. Köhler, R. Gout, E.H. Oelkers, The dissolutionkinetics and apparent solubility of natural apatite in closed reactors at tem-peratures from 5 to 50 ◦C and pH from 1 to 6, Chem. Geol. 244 (2007)554–568.

22] D.G.A. Nelson, J.D.B. Featherstone, J.F. Duncan, T.W. Cutress, Effect of carbonateand fluoride on the dissolution behavior of synthetic apatites, Caries Res. 17(1983) 200–211.

23] Y. Zhu, X. Zhanga, Y. Chena, Q. Xiea, J. Lana, M. Qiana, N. He, A comparativestudy on the dissolution and solubility of hydroxylapatite and fluorapatite at25 ◦C and 45 ◦C, Chem. Geol. 268 (2009) 89–96.

24] P.V. Brady, Ch. 4 in Physics and Chemistry of Mineral Surfaces, ed. P.V. Brady

(CRC Press, Boca Raton, FL, 1996).

25] G. Boivin, The hydroxyapatite crystal: a closer look, Medicographia 29 (2007)126–132.

26] A.J. Skinner, J.P. LaFemina, H.J.F. Hansen, Structure and bonding of calcite: atheoretical study, Am. Mineral 79 (1994) 205–214.

ltural

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

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[

[

[

[

[

[

E. Sassoni et al. / Journal of Cu

27] M. Mathew, S. Takagi, Structures of biological minerals in dental research, J.Res. Natl. Inst. Stand. Technol. 106 (2001) 1035–1044.

28] E.N. Maslen, V.A. Streltsov, N.R. Streltsova, N. Ishizawa, Electron density andoptical anisotropy in rhombohedral carbonates. III. Synchroton X-ray studiesof CaCO3, MgCO3 and MnCO3, Acta Crystallogr B51 (1995) 929–939.

29] K. Polikreti, Y. Maniatis, Micromorphology, composition and origin of theorange patina on the marble surfaces of Propylaea (Acropolis, Athens), Sci. TotalEnviron. 308 (2003) 111–119.

30] M. Alvarez de Buergo, R. Fort Gonzalez, Protective patinas applied on stonyfacades of historical buildings in the past, Constr. Build. Mater. 17 (2003) 83–89.

31] C. Liu, Y. Huang, W. Shen, J. Cui, Kinetics of hydroxyapatite precipitation at pH10 to 11, Biomaterials 22 (2001) 301–306.

32] A. Osaka, Y. Miura, K. Takeuchi, M. Asada, K. Takahashi, Calcium apatite pre-pared from calcium hydroxide and orthophosphoric acid, J. Mater. Sci. Mater.Med. 2 (1991) 51–55.

33] T. Brendel, A. Engel, C. Russel, Hydroxyapatite coatings by a polymeric route, J.Mater. Sci. Mater. Med. 3 (1992) 175–179.

34] M. Yoshimura, H. Suda, K. Okamoto, K. Ioku, Hydrothermal synthesis of bio-compatible whiskers, J. Mat. Sci. 29 (1994) 3399–3402.

35] A. Slòsarczyk, E. Stobierska, Z. Paskiewicz, M. Gawlicki, Calcium phosphatematerials prepared from precipitates with various calcium: phosphorous MolarRatios, J. Am. Ceram. Soc. 79 (1996) 2539–2544.

36] C.A. Paraskeva, P.C. Charalambous, L.E. Stokka, G. Pavlos, P.G. Klepetsanis, P.G.Koutsoukos, P. Read, T. Østvold, A.C. Payatakes, Sandbed Consolidation withMineral Precipitation, J. Colloid Interf. Sci. 232 (2000) 326–339.

37] I.T. Hafez, C.A. Paraskeva, A. Toliza, P.G. Klepetsanis, P.G. Koutsoukos, P. Read, T.Østvold, A.C. Payatakes, Calcium phosphate overgrowth on silicate sand, CrystalGrowth Design 6 (3) (2006) 675–683.

38] R. Snethlage, C. Gruber; V. Tucic, W. Wendler, Transforming gypsum intocalcium phosphate - a better way to preserve lime paint layers on naturalstone? Proc. Int. Symp. Stone consolidation in cultural heritage, eds. J. Delgado

Rodrigues and J. Manuel Mimoso (Laboratório Nacional de Engenharia Civil,Lisbon, 2008), ISBN 978-972-49r-r2135-8.

39] M. Kamiya, J. Hatta, E. Shimida, Y. Ikuma, M. Yoshimura, H. Monma, AFM anal-ysis of initial stage of reaction between calcite and phosphate, Mater. Sci. Eng.B 111 (2004) 226–231.

[

Heritage 12 (2011) 346–355 355

40] M.S.A. Johnsson, G.H. Nancollas, The role of brushite and octacalcium phosphatein apatite formation, Crit. Rev. Oral. Bio. M. 3 (1992) 61–82.

41] E.C. Moreno, M. Kresak, R.T. Zahradnik, Fluoridated hydroxyapatite and cariesformation, Nature 247 (1974) 64–65.

42] A. Yasukawa, M. Kidokoro, K. Kandori, T. Ishikawa, Preparation and Character-ization of Barium–Strontium Hydroxyapatites, J. Colloid Interf. Sci. 191 (1997)407–415.

43] J. Christoffersen, M.R. Christoffersen, N. Kolthoff, O. Barenholdt, Effects of stron-tium ions on growth and dissolution of hydroxyapatite and on bone mineraldetection, Bone 20 (1) (1997) 47–54.

44] T. Perry, N.M. Smith, W.J. Wayne, Salem limestone and associated formations insouth-central Indiana, Indiana Department of Conservation, Geological Survey,Field Conference Guidebook 7, 1954.

45] R. Flatt, ETH, 2003, private communication.46] S. Siegesmund, K. Ullemeyer, T. Weiss, E.K. Tschegg, Physical weathering of

marbles caused by anisotropic thermal expansion, Int. J. Earth Sci. 89 (2000)170–182.

47] T. Weiss, S. Siegesmund, E.R. Fuller Jr., Thermal degradation of marble: indica-tions from finite-element modelling, Build. Environ. 38 (2003) 1251–1260.

48] T. Esaki, K. Jiang, Comprehensive study of the weathered condition of weldedtuff from a historic bridge in Kagoshima, Japan, Eng. Geol. 55 (1999) 121–130.

49] C. Miliani, M.L. Velo-Simpson, G.W. Scherer, Particle-modified consolidants: astudy on the effect of particles on sol–gel properties and consolidation effec-tiveness, J. Cult. Herit. 8 (2007) 1–6.

50] R. Benjumea, D.L. Sikarkie, A note on the penetration of a rigid wedge into anonisotropic brittle material, Int. J. Rock Mech. Min. Sci. 6 (1969) 343–352.

51] A. Duguid, M. Radonjic, G.W. Scherer, Degradation at the reservoir/cementinterface from exposure to carbonated brine, Int. J. Greenhouse Gas Control(submitted).

52] A. Kasioptas, C. Perdikouri, C.V. Putnis, A. Putnis, Pseudomorphic replacementof single calcium carbonate crystals by polycrystalline apatite, Mineralog. Mag.

72 (1) (2008) 77–80.

53] S. Naidu, E. Sassoni, G.W. Scherer, New Treatment for Corrosion-Resistant Coat-ings for Marble and Consolidation of Limestone, Proceedings of Jardins dePierres, Conservation of stone in Parks, Gardens and Cemeteries, Paris, 22–24June 2011 (submitted).