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The nanolimes in Cultural Heritage conservation: Characterisation and analysis of the carbonatation process
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Journal of Cultural Heritage 9 (2008) 294e301 http://france.elsevier.com/direct/CULHER/
Original article
The nanolimes in Cultural Heritage conservation: Characterisation and analysis of the carbonatation process Valeria Daniele, Giuliana Taglieri*, Raimondo Quaresima Department of Chemistry, Chemical Engineering and Materials, University of L’Aquila, Monteluco di Roio, I-67040 L’Aquila, Italy Received 28 June 2007; accepted 18 October 2007
Abstract Water and milk of lime are usually adopted for conservative surfaces treatments, thanks to the conversion of lime into calcium carbonate. Calcium carbonate is, as a matter of fact, very compatible with many carbonatic lithotypes and architectonic surfaces, because its characteristics are very similar to those of the materials to be restored. But there are some limiting aspects to treatments effectiveness: the reduced penetration depth, the binder concentration and the incompleteness carbonatation process. In order to improve lime treatments, Ca(OH)2 particles with submicrometric dimensions (nanolimes) are recently introduced in Cultural Heritage conservation. Lime nanoparticles are typically produced by a chemical precipitation process in supersaturated aqueous solutions of the reactants (calcium chloride and sodium hydroxide). The aim of the present work is to analyse the nanolime carbonatation process in relation to some parameters, like time and the relative humidity conditions. For this scope, lime nanoparticles are therefore synthesised and characterised by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEMeTEM), electron diffraction measurements (ED) and dark field images (DFI). The possibility to improve the nanolime carbonatation process is investigated using an alcoholic suspension and by adding a baking soda solution in order to disaggregate particles and to increase CO2 content in the suspension respectively. The efficiency of the nanolime carbonatation process is reported too. After that the lime nanoparticles are applied on natural lithotypes (‘‘Estoril’’ and ‘‘Pietra Serena’’) and some tests are performed in order to estimate the superficial consolidating and protective effect of the treatment: ‘‘Scotch Tape Test’’, capillarity and imbibition tests. SEM analyses are performed to evaluate penetration depth and surface adhesion of nanolime treatments. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Calcium hydroxide; Consolidation; Lime; Nanoparticles; Protection
1. Introduction The use of lime (Ca(OH)2), in building industry as in Cultural Heritage conservation, is based on the well-known carbonation reaction and on the characteristics of calcium carbonate (CaCO3) obtained. The low solubility and the compatibility between the latter compound and material substrates offer a favourable use in many lime-based conservative treatments (preconsolidation [1], cleaning [2], consolidation and protection [3]). The applications employ lime solutions (lime milk or lime water). Lime water consolidation is generally obtained by spraying the lime solution on the cleaned surface. To reach a good * Corresponding author. Tel.: þ39 0862 434212; fax: þ39 0862 434203. E-mail address: [email protected] (G. Taglieri). 1296-2074/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.10.007
penetration, the treatment is repeated several times until the surface is able to absorb lime water [4]; some authors indicate that it could be necessary to repeat the application for 30e40 times [5]. Lime milk is used on the same basis as lime water [6]. Compared with lime water, lime milk treatments involve greater amounts of lime with the same water volume; this represents an advantage due to a reduced water percentage brought to the stone. Lime water and lime milk treatments are characterised by some limitations due to: - the incomplete conversion of lime into calcium carbonate that leaves free particles on the surfaces [6]; - the binder concentration due to the low water solubility of lime, giving chromatic alteration to stone surfaces; - a reduced penetration depth [4].
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
In order to obviate to these limitations, Ca(OH)2 particles with submicrometric dimensions (nanolimes) are synthesised [7e11]; the innovative nanolime treatments constitute a valid alternative to the lime traditional ones [12e18]. The particles are synthesised at high temperature in supersaturated aqueous solutions of reactants; the precipitate phase is then dispersed into an alcoholic medium to improve particles disagglomeration and stability [19]. Concerning the above limitation due to the conversion of lime, this paper could represent a contribution in analysing the carbonatation process and in finding a way to promote the completeness of the reaction itself. According to the mentioned literature, a 2-propanol alcoholic nanolime suspension is prepared [20]. The conversion of lime into calcium carbonate is estimated by taking into account parameters like the time you need to complete the reaction, the dispersing medium (water or alcohol), and the relative humidity conditions. To complete the carbonatation reaction of the alcoholic nanolime produced, the addition of a baking soda (NaHCO3) solution is considered. The completeness of the carbonatation reaction is, as a matter of fact, a relevant issue in order to enhance the effectiveness of the application of a pure, crystalline and nanometric lime to be used in Cultural Heritage applications. To correlate the nanolime produced to its properties, morphological and microstructural characterisation are performed by X-ray diffraction (XRD) and profile analysis, electron
295
Fig. 1. TEM micrograph of the lime nanoparticles agglomerate (WS sample).
microscopy (SEM and TEM), electron diffraction (ED) and dark field images (DFI). The produced lime nanoparticles are applied on two natural lithotypes (Estoril, Pietra Serena) in order to evaluate the consolidating and protective effectiveness of the treatment, and some tests are performed: ‘‘Scotch Tape Test’’ [21], capillarity [22] and imbibition [23] tests. With scanning electron microscopy (SEM), the penetration depth of the treatment is evaluated too.
Fig. 2. (a) TEM image of a group of nanolime particles (WS sample); (b) electron diffraction image on A particle; (c and d) TEM dark field images for two different crystalline planes orientation of A particle.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
296
Fig. 3. (a) TEM micrograph of a small single particle (A0 ); (b) electron diffraction images on A0 particle; (c) A0 particle dark field image.
2. Experimental 2.1. Materials Calcium chloride (CaCl2), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3) and 2-propanol pro analysis products, supplied by Merck, are used without further purification. Water is purified by a Millipore Organex system (R 18 MU cm). 2.2. Synthesis of the particles To obtain about 20 g of Ca(OH)2 nanoparticles, two different aqueous solutions of 900 ml, containing 0.3 mol/l of CaCl2
and 0.6 mol/l of NaOH, respectively, are prepared. The NaOH alkaline solution (used as precipitator) is added dropwise into the CaCl2 solution (speed z 4 ml/min, temperature of 90 C). After about 24 h, two distinct phases are observed: a limpid supernatant solution and a white precipitated phase. In order to remove the produced NaCl, several deionised water washings are performed; the chloride content is measured by means of a UV spectrophotometer (Dr. Lange Mod. CASAS 50). Aqueous nanolime suspension (with a concentration of 15 mg/ml) is defined as sample WS. An alcoholic nanolime suspension is prepared in rotavapor at a temperature of 80 C at low pressure (100 mbar). When a ratio of Ca(OH)2/H2O z 0.6 is reached, 1000 ml of
° Ca(OH)2 * CaCO3 ° Intensity (a.u.)
*
° * 25
30
35
*
* 40
45
2-theta
Fig. 4. SEM micrograph of the aqueous nanolime suspension (sample WS).
Fig. 5. XRD pattern of the aqueous nanolime suspension (sample WS, concentration of 15 mg/ml) after 30 min of air exposition time.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
Intensity (a.u.)
Intensity (a.u.)
°
° *
* 25
35
30
° Ca(OH)2 * CaCO3
*
° Ca(OH)2 * CaCO3
*
297
*
*
° 40
45
2-theta Fig. 6. XRD pattern of the alcoholic nanolime suspension (sample ALS).
2-propanol is added in order to obtain a final suspension concentration of 15 mg/ml (sample ALS). Sample ALS is then exposed, at room temperature, to different relative humidity conditions (RH 40%, 70%, 90%), depositing 0.2 ml of suspension on a silica sample holder in a climatic chamber. ALBKS sample is prepared by adding to ALS sample an aqueous solution (0.16 mol/l) of NaHCO3, corresponding to a concentration of 15 mg/ml. In a preliminary instance, the ratio between lime and baking soda solution is considered 1:1; this addiction doesn’t seem to alter the dispersion stability.
25
*
° 30
35
*
* 40
Fig. 7. XRD pattern of the alcoholic nanolime suspension mixed with the baking soda solution (sample ALBKS).
performed in order to quantify the humidity influence on nanolime carbonatation process. Each experimental diffraction spectrum is elaborated by a Profile Fit Software (Philips PROFIT v.1.0). The ratio between the CaCO3 peak area and the spectrum total area is assumed as the carbonatation process efficiency (yield); the ratio between CaCO3 and Ca(OH)2 areas is indicated as ICaCO3 =CaðOHÞ2 . All peak profile fittings are carried out retaining the Ka2 wavelength.
2.3. Morphological and microstructural characterisation To correlate the characteristics of the obtained nanolime to the carbonatation process, the suspensions are characterised by TEM (Philips CM200) and SEM (Philips XL30CP) techniques. In order to analyse morphology, particles’ dimensions and crystallinity, 0.2 ml of WS sample is deposited on a grid. The carbonatation process in air is investigated by XRD (Philips X’Pert PW 1830). XRD measurements allow, as a matter of fact, to determine the crystalline phases (data from JCPDS), to estimate crystallites dimension (D) (Scherrer equation), and to understand the completeness of the carbonatation reaction. The sample is prepared by depositing 0.2 ml of the nanolime suspension on a silica sample holder; the measures are performed on dry samples. In particular, WS and ALS samples are left in the laboratory environmental conditions (T ¼ 20 C and RH ¼ 40%), and investigated after the following times: 30 min, 1 day and 24 h, 30 days. After that the samples are left 24 h in relative humidity (RH) conditions of 70% and 90% (ALS70 and ALS90, respectively), and tests are
Table 1 Relative humidity influence on the carbonatation process Sample
RH (%)
Yield (%)
ALS ALS70 ALS90
40 70 90
65 80 93
45
2-theta
Fig. 8. Porosimetric distribution of: (a) Estoril; (b) Pietra Serena.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
298 Table 2 Porosimetric data Lithotypes
Total pore volume (mm3/g)
Average pore radius (mm)
Total porosity (%)
Estoril Pietra Serena
61.54 34.43
0.46 6.15
14.95 7.05
3. Results and discussion TEM micrographs, obtained on WS sample, are reported. Fig. 1 shows a typical Ca(OH)2 nanoparticles’ agglomerate, where the particles range from 50 to 600 nm; in Fig. 2a a particle with a hexagonal and a regular shape is shown (particle A), while a smaller irregular single particle is reported in Fig. 3a (particle A0 ). As concerns their crystallinity, these particles reveal typically crystalline features (circular symmetrylike), with spots and few marked circles (Fig. 2b) or with a spot size distribution (Fig. 3b). The difference can be related to the particles dimension: smaller (<100 nm) or greater (600 nm) particles seem to reveal a single or several crystallites, respectively. Dark field images confirm these considerations. By tilting the incident beam, regions corresponding to crystalline grains with planes oriented along the chosen direction become light; thus, the variation or not in bright marks the presence of different crystallites (Fig. 2c and d) or of a single crystal (Fig. 3c). SEM micrograph (Fig. 4) shows the effect of the carbonatation process on Ca(OH)2 particles in WS samples; it is
possible to observe only a partial growth of CaCO3 crystallites on lime particles surfaces. The partial CaCO3 growth can be a result of the incomplete carbonatation process and/or of the SEM technique itself. As a matter of fact, the liquid phase is quickly eliminated during the specimen gold sputtering (metallization), realised under vacuum conditions. XRD measurement on WS sample, performed after 30 min, is reported in Fig. 5. It is possible to recognize Ca(OH)2 and CaCO3 phases (84-1276 and 85-1108 JCPDS patterns, respectively). The parameter ICaCO3 =CaðOHÞ2 is 1.2. For this sample, I ratio remains the same also for the spectra recorded after 1 h, 24 h and 30 days: the carbonatation process, in the environmental experimental conditions, stops after 30 min. The corresponding WS yield is about 50%. XRD pattern of ALS sample, still obtained after 30 min, is shown in Fig. 6. It is possible to note a light improvement in the carbonatation process corresponding to an ICaCO3 =CaðOHÞ2 of 1.9 and a yield of 65%. In Table 1 the influence of the relative humidity conditions in the carbonatation process is shown. It is possible to note the expected improvement of the yield, but the completeness of the reaction can be reached only at the limiting conditions. In Fig. 7 the XRD result, obtained on ALBKS sample after 30 min of air exposure time, is shown. In this case the main crystalline phase is CaCO3 and Ca(OH)2. A very weak signal at about 32.3 could be attributed to the presence of Na2CO3$2H2O (08-0448 JCPDS pattern) as a reaction product. In this case the ICaCO3 =CaðOHÞ2 ratio is 18.7 while the yield reaches the value of 95% already in the first 30 min of air exposure time.
Fig. 9. SEM micrographs of Estoril sample: (a) untreated material surface; (b) material surface after nanolime treatment; (c) penetration depth of the treatment in a typical cross-section.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
299
Table 3 Scotch Tape Test (STT): experimental results Lithotypes
Untreated materials (mg/cm2)
Treated materials (mg/cm2)
DV (%)
Estoril Pietra Serena
0.54 0.60
0.15 0.40
70% 35%
SEM analysis on the Pietra Serena stone is reported in Fig. 10. In particular, Fig. 10a shows that the nanolime treatment seems to reach a penetration depth of about 1 mm. This result could be attributed to the pore size distribution and the high porosity value of this lithotype (see Table 2). In Fig. 10b the adhesion of the treatment on grains surface is reported too. In Table 3 the STT results, in terms of the materials removed from the surface before and after the treatment, are reported; the percentage variation is indicated as DV. The treatment seems to be effective, relative to the superficial consolidation properties of the considered materials. The best
Fig. 10. SEM micrographs of Pietra Serena stone: (a) penetration depth; (b) adhesion of the treatment on grains’ surface.
4. Nanolimes treatments on natural stones The obtained nanolimes, together with the baking soda solution, are then applied on some natural stones (Estoril and Pietra Serena), in order to evaluate the protective effectiveness of the treatment. The typical porosimetric distribution graphs, obtained by a Porosimeter 2000 Series e Carlo Erba, are shown in Fig. 8, and in Table 2 the quantitative results are summarised. The pore size distribution data relative to Estoril sample exhibit an average pore radius of 0.46 mm and a total porosity of about 15%. On the contrary, Pietra Serena lithotype appears very different as in the pore size distribution as in porosity. The nanolime treatment is realised, with a brush, by applying the alcoholic suspension on the dry and clean stone surface. SEM analyses are performed in order to evaluate the reached penetration depth and the surface adhesion of nanolime treatments. To investigate the superficial consolidation of the treated stones, a ‘‘Scotch Tape Test’’ (STT) is performed. Capillarity and imbibition tests, according to literature procedures, are finally executed, before and after the treatment, to understand the material behaviour towards water. In Fig. 9 SEM micrographs of Estoril sample are reported: it is possible to note the partial filling of the pores due to the treatment (Fig. 9b) and a penetration depth of about 30 mm (Fig. 9c).
Fig. 11. Capillarity curves: untreated stones (continued line), treated stones (dots line).
300
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
morphological and microstructural characterisations show that crystalline and regularly shaped particles, with dimensions ranging between 50 and 600 nm, are obtained. An aqueous nanolime suspension of 15 mg/ml concentration stops its carbonatation within 30 min, reaching a carbonatation yield of about 50%, while the use of 2-propanol alcohol, as de-agglomerating medium, produces a yield of 65%. An attempt to complete the carbonation process by adding an external CO2 source is then realised using a baking soda solution (NaHCO3) as a promoter of the process itself: a conversion factor of 95% is so reached. After that the lime nanoparticles, together with the baking soda solution, are applied on two natural stones (Estoril and Pietra Serena) and three different tests seem to confirm the protective and consolidating effectiveness of the treatment: ‘‘Scotch Tape Test’’, capillarity and imbibition tests. With scanning electron microscopy (SEM), the penetration depth of the treatment is evaluated too. Further and more complete study on better promoter agents should be carried out in order to avoid surfaces’ alteration on treated stones. As a matter of fact, the introduction of sodium salt within the porous structure of the stones can produce efflorescence phenomena. Acknowledgments The authors thank Eng. Giovanna Di Tommaso and Dr. Lorenzo Arrizza and Dr. Maria Gianmatteo, of the InterDepartmental Centre of Electron Microscopy of the University of L’Aquila, for their collaboration. A special thanks is directed, last but not least, to Prof. Luigi Dei (University of Florence) for his very kind and helpful cooperation. References
Fig. 12. Imbibition curves: untreated stones (continued line), treated stones (dots line).
result is obtained for Estoril (DV z 70%), while Pietra Serena reached a DV value of about 35%. Capillarity and imbibition tests are reported in Fig. 11 and Fig. 12, respectively. The treatment effectiveness on water absorption in both the samples is evident. Results are particularly interesting for Pietra Serena stone, probably due to a partial filling of macropores and their transformation into smaller pores, as shown in SEM investigation. 5. Conclusions In this work the carbonatation of Ca(OH)2 nanoparticles, synthesised starting from stoichiometric ratio of the initial reactants at a synthesis temperature of 90 C, is analysed. The
[1] G.G. Amoroso, M. Camaiti, Trattamento alla calce, in: Scienza dei Materiali e restauro. La pietra: dalle mani degli artisti e degli scalpellini a quelle dei chimici macromolecolari, Alinea Editrice, Firenze, 1997, pp. 165e167. [2] N. Ashurst, Cleaning Historic Buildings, Donhead, London, 1994. [3] C. Arcolao, Trattamento all’acqua di calce, In: Tecniche di restauro architettonico, tomo II, P.B. Torsello, UTET, Torino, 1996, 698e699. [4] M.G. Vinardi, M.H. Cully, A. Brunetto, La reversibilita` nel restauro. Riflessioni, esperienze, percorsi di ricerca, In: XI National Congress Scienza e Beni Culturali: ‘‘La Pulitura delle Superfici dell’Architettura’’, Bressanone, 1e4 Luglio, vol. XIX (2003) 399e405. [5] P. Sten, Lime Water Consolidation, ICCROM, 1981, pp. 53e61. [6] R. Turriziani, I leganti ed il calcestruzzo, Edizioni Sistema, Roma, 1972, pp 70e82. [7] R. Giorgi, L. Dei, P. Baglioni, A new method for consolidating wall paintings based on dispersion of lime in alcohol, Studies in Conservation 45 (2000) 154e161. [8] B. Salvadori, L. Dei, Synthesis of Ca(OH)2 nanoparticles from diols, Langmuir 17 (2001) 2371e2374. [9] P. Baglioni, C. Cesari, L. Dei, R. Giorgi, R. Grassi, M. Lorenzetti, M. Mauro, D. Pinzani, P. Ruffo, G. Schonhaut, Stucco forte veneziano con ‘‘Tecniche della Tradizione’’: proprieta` chimico-fisiche, degrado in ambiente lagunare e conservazione mediante nanofasi cristalline di Ca(OH)2, In: National Congress Scienza e Beni Culturali: ‘‘Lo Stucco. Cultura, Tecnologia, Conoscenza’’, Bressanone, 10e13 Luglio, vol. XVII (2001) 289e298.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301 [10] M. Ambrosi, L. Dei, R. Giorgi, C. Neto, P. Baglioni, Colloidal particles of Ca(OH)2: properties and applications to restoration of frescoes, Langmuir 17 (2001) 4251e4255. [11] A. Nanni, L. Dei, Ca(OH)2 nanoparticles from W/O microemulsions, Langmuir 19 (2003) 933e938. [12] M. Ambrosi, L. Dei, R. Giorgi, C. Neto, P. Baglioni, Stable dispersions of Ca(OH)2 in aliphatic alcohols: properties and application in cultural heritage conservation, Progress in Colloid and Polymer Science 118 (2001) 68e72. [13] R. Giorgi, L. Dei, M. Ceccato, C. Schettino, P. Baglioni, Nanotechnologies for conservation of cultural heritage: paper and canvas deacidification, Langmuir 18 (2002) 8198e8203. [14] M. Ambrosi, P. Baglioni, P.R. David, L. Dei, R. Giorgi, C. Lalli, G. Lanterna, A. Mairani, M. Matteini, M. Rizzi, G. Schonhaut, Inorganic Consolidants and Protectives for Architectonic Surfaces: Experimental Tests on Santa Prisca Church Apse in Rome, In: Second International Congress on ‘‘Science and Technology for the safeguard of Cultural Heritage in the Mediterranean Basin’’, vol. 2 (1992) 873e877. [15] L. Dei, B. Salvadori, E. Arlango, F. Pietropoli, C. Scardellato, Gli affreschi del XIII e XIV secolo nella Cripta di San Zeno a Verona: la sperimentazione della nanocalce dispersa in alcol iso-propilico durante l’intervento conservative, In: International Congress Scienza e Beni Culturali: ‘‘Sulle pitture murali. Riflessioni, conoscenze, interventi’’, Bressanone, 12e15 Luglio, vol. XXI (2005) 293e302. [16] L. Dei, B. Radicati, B. Salvadori, Sperimentazione di un consolidante a base di idrossido di calcio nanofasico sugli affreschi della Cappella
[17]
[18] [19]
[20] [21] [22]
[23]
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del Podesta` al Museo del Bargello di Firenze: aspetti chimico-fisici e prove di colore, In: International Congress Scienza e Beni Culturali: ‘‘Sulle pitture murali. Riflessioni, conoscenze, interventi’’; Bressanone, 12-15 Luglio, vol. XXI (2005) 99e108. L. Dei, M. Favaro, R. Giorgi, R. Portieri, Gli stucchi della Cappella della Madonna di Lourdes nella Chiesa di S.Giovanni Evangelista in Venezia: caratterizzazione del degrado e pre-consolidamento mediante dispersioni alcoliche di calce, , In: National Congress Scienza e Beni Culturali: ‘‘Lo Stucco. Cultura, Tecnologia, Conoscenza’’; Bressanone, 10e13 Luglio, vol. XVII (2001) 557e566. P. Baglioni, R. Giorgi, Soft and hard nanomaterials for restoration and conservation of cultural heritage, Soft Matter 2 (2006) 293e303. M. Ambrosi, R. Giorgi, B. Salvadori, L. Dei, Nanotecnologie per la conservazione di affreschi e materiali lapidei a base carbonatica, in: First International Workshop on Science, Technology and Cultural Heritage, Venezia, 29 Giugnoe1 Luglio (2004), pp. 29e33. R.T. Morrison, R.N. Boyd, Chimica organica, Casa Editrice Ambrosiana, 1997, p. 243. ASTM D3359-02: Standard Test Methods for Measuring Adhesion by Tape Test, ASTM International, 10 August, 2002. Commissione ‘‘Beni Culturali e NORMAL: ‘‘Materiali lapidei naturali ed artificiali. Determinazione dell’assorbimento d’acqua per capillarita`’’ (2000) UNI 10859. Commissione ‘‘Beni Culturali e NORMAL’’: ‘‘Materiali lapidei naturali ed artificiali. Determinazione dell’assorbimento d’acqua per immersione totale e capacita` di imbibizione’’ (1981) NORMAL 7/81.
Journal of Cultural Heritage 9 (2008) 294e301 http://france.elsevier.com/direct/CULHER/
Original article
The nanolimes in Cultural Heritage conservation: Characterisation and analysis of the carbonatation process Valeria Daniele, Giuliana Taglieri*, Raimondo Quaresima Department of Chemistry, Chemical Engineering and Materials, University of L’Aquila, Monteluco di Roio, I-67040 L’Aquila, Italy Received 28 June 2007; accepted 18 October 2007
Abstract Water and milk of lime are usually adopted for conservative surfaces treatments, thanks to the conversion of lime into calcium carbonate. Calcium carbonate is, as a matter of fact, very compatible with many carbonatic lithotypes and architectonic surfaces, because its characteristics are very similar to those of the materials to be restored. But there are some limiting aspects to treatments effectiveness: the reduced penetration depth, the binder concentration and the incompleteness carbonatation process. In order to improve lime treatments, Ca(OH)2 particles with submicrometric dimensions (nanolimes) are recently introduced in Cultural Heritage conservation. Lime nanoparticles are typically produced by a chemical precipitation process in supersaturated aqueous solutions of the reactants (calcium chloride and sodium hydroxide). The aim of the present work is to analyse the nanolime carbonatation process in relation to some parameters, like time and the relative humidity conditions. For this scope, lime nanoparticles are therefore synthesised and characterised by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEMeTEM), electron diffraction measurements (ED) and dark field images (DFI). The possibility to improve the nanolime carbonatation process is investigated using an alcoholic suspension and by adding a baking soda solution in order to disaggregate particles and to increase CO2 content in the suspension respectively. The efficiency of the nanolime carbonatation process is reported too. After that the lime nanoparticles are applied on natural lithotypes (‘‘Estoril’’ and ‘‘Pietra Serena’’) and some tests are performed in order to estimate the superficial consolidating and protective effect of the treatment: ‘‘Scotch Tape Test’’, capillarity and imbibition tests. SEM analyses are performed to evaluate penetration depth and surface adhesion of nanolime treatments. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Calcium hydroxide; Consolidation; Lime; Nanoparticles; Protection
1. Introduction The use of lime (Ca(OH)2), in building industry as in Cultural Heritage conservation, is based on the well-known carbonation reaction and on the characteristics of calcium carbonate (CaCO3) obtained. The low solubility and the compatibility between the latter compound and material substrates offer a favourable use in many lime-based conservative treatments (preconsolidation [1], cleaning [2], consolidation and protection [3]). The applications employ lime solutions (lime milk or lime water). Lime water consolidation is generally obtained by spraying the lime solution on the cleaned surface. To reach a good * Corresponding author. Tel.: þ39 0862 434212; fax: þ39 0862 434203. E-mail address: [email protected] (G. Taglieri). 1296-2074/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2007.10.007
penetration, the treatment is repeated several times until the surface is able to absorb lime water [4]; some authors indicate that it could be necessary to repeat the application for 30e40 times [5]. Lime milk is used on the same basis as lime water [6]. Compared with lime water, lime milk treatments involve greater amounts of lime with the same water volume; this represents an advantage due to a reduced water percentage brought to the stone. Lime water and lime milk treatments are characterised by some limitations due to: - the incomplete conversion of lime into calcium carbonate that leaves free particles on the surfaces [6]; - the binder concentration due to the low water solubility of lime, giving chromatic alteration to stone surfaces; - a reduced penetration depth [4].
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
In order to obviate to these limitations, Ca(OH)2 particles with submicrometric dimensions (nanolimes) are synthesised [7e11]; the innovative nanolime treatments constitute a valid alternative to the lime traditional ones [12e18]. The particles are synthesised at high temperature in supersaturated aqueous solutions of reactants; the precipitate phase is then dispersed into an alcoholic medium to improve particles disagglomeration and stability [19]. Concerning the above limitation due to the conversion of lime, this paper could represent a contribution in analysing the carbonatation process and in finding a way to promote the completeness of the reaction itself. According to the mentioned literature, a 2-propanol alcoholic nanolime suspension is prepared [20]. The conversion of lime into calcium carbonate is estimated by taking into account parameters like the time you need to complete the reaction, the dispersing medium (water or alcohol), and the relative humidity conditions. To complete the carbonatation reaction of the alcoholic nanolime produced, the addition of a baking soda (NaHCO3) solution is considered. The completeness of the carbonatation reaction is, as a matter of fact, a relevant issue in order to enhance the effectiveness of the application of a pure, crystalline and nanometric lime to be used in Cultural Heritage applications. To correlate the nanolime produced to its properties, morphological and microstructural characterisation are performed by X-ray diffraction (XRD) and profile analysis, electron
295
Fig. 1. TEM micrograph of the lime nanoparticles agglomerate (WS sample).
microscopy (SEM and TEM), electron diffraction (ED) and dark field images (DFI). The produced lime nanoparticles are applied on two natural lithotypes (Estoril, Pietra Serena) in order to evaluate the consolidating and protective effectiveness of the treatment, and some tests are performed: ‘‘Scotch Tape Test’’ [21], capillarity [22] and imbibition [23] tests. With scanning electron microscopy (SEM), the penetration depth of the treatment is evaluated too.
Fig. 2. (a) TEM image of a group of nanolime particles (WS sample); (b) electron diffraction image on A particle; (c and d) TEM dark field images for two different crystalline planes orientation of A particle.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
296
Fig. 3. (a) TEM micrograph of a small single particle (A0 ); (b) electron diffraction images on A0 particle; (c) A0 particle dark field image.
2. Experimental 2.1. Materials Calcium chloride (CaCl2), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3) and 2-propanol pro analysis products, supplied by Merck, are used without further purification. Water is purified by a Millipore Organex system (R 18 MU cm). 2.2. Synthesis of the particles To obtain about 20 g of Ca(OH)2 nanoparticles, two different aqueous solutions of 900 ml, containing 0.3 mol/l of CaCl2
and 0.6 mol/l of NaOH, respectively, are prepared. The NaOH alkaline solution (used as precipitator) is added dropwise into the CaCl2 solution (speed z 4 ml/min, temperature of 90 C). After about 24 h, two distinct phases are observed: a limpid supernatant solution and a white precipitated phase. In order to remove the produced NaCl, several deionised water washings are performed; the chloride content is measured by means of a UV spectrophotometer (Dr. Lange Mod. CASAS 50). Aqueous nanolime suspension (with a concentration of 15 mg/ml) is defined as sample WS. An alcoholic nanolime suspension is prepared in rotavapor at a temperature of 80 C at low pressure (100 mbar). When a ratio of Ca(OH)2/H2O z 0.6 is reached, 1000 ml of
° Ca(OH)2 * CaCO3 ° Intensity (a.u.)
*
° * 25
30
35
*
* 40
45
2-theta
Fig. 4. SEM micrograph of the aqueous nanolime suspension (sample WS).
Fig. 5. XRD pattern of the aqueous nanolime suspension (sample WS, concentration of 15 mg/ml) after 30 min of air exposition time.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
Intensity (a.u.)
Intensity (a.u.)
°
° *
* 25
35
30
° Ca(OH)2 * CaCO3
*
° Ca(OH)2 * CaCO3
*
297
*
*
° 40
45
2-theta Fig. 6. XRD pattern of the alcoholic nanolime suspension (sample ALS).
2-propanol is added in order to obtain a final suspension concentration of 15 mg/ml (sample ALS). Sample ALS is then exposed, at room temperature, to different relative humidity conditions (RH 40%, 70%, 90%), depositing 0.2 ml of suspension on a silica sample holder in a climatic chamber. ALBKS sample is prepared by adding to ALS sample an aqueous solution (0.16 mol/l) of NaHCO3, corresponding to a concentration of 15 mg/ml. In a preliminary instance, the ratio between lime and baking soda solution is considered 1:1; this addiction doesn’t seem to alter the dispersion stability.
25
*
° 30
35
*
* 40
Fig. 7. XRD pattern of the alcoholic nanolime suspension mixed with the baking soda solution (sample ALBKS).
performed in order to quantify the humidity influence on nanolime carbonatation process. Each experimental diffraction spectrum is elaborated by a Profile Fit Software (Philips PROFIT v.1.0). The ratio between the CaCO3 peak area and the spectrum total area is assumed as the carbonatation process efficiency (yield); the ratio between CaCO3 and Ca(OH)2 areas is indicated as ICaCO3 =CaðOHÞ2 . All peak profile fittings are carried out retaining the Ka2 wavelength.
2.3. Morphological and microstructural characterisation To correlate the characteristics of the obtained nanolime to the carbonatation process, the suspensions are characterised by TEM (Philips CM200) and SEM (Philips XL30CP) techniques. In order to analyse morphology, particles’ dimensions and crystallinity, 0.2 ml of WS sample is deposited on a grid. The carbonatation process in air is investigated by XRD (Philips X’Pert PW 1830). XRD measurements allow, as a matter of fact, to determine the crystalline phases (data from JCPDS), to estimate crystallites dimension (D) (Scherrer equation), and to understand the completeness of the carbonatation reaction. The sample is prepared by depositing 0.2 ml of the nanolime suspension on a silica sample holder; the measures are performed on dry samples. In particular, WS and ALS samples are left in the laboratory environmental conditions (T ¼ 20 C and RH ¼ 40%), and investigated after the following times: 30 min, 1 day and 24 h, 30 days. After that the samples are left 24 h in relative humidity (RH) conditions of 70% and 90% (ALS70 and ALS90, respectively), and tests are
Table 1 Relative humidity influence on the carbonatation process Sample
RH (%)
Yield (%)
ALS ALS70 ALS90
40 70 90
65 80 93
45
2-theta
Fig. 8. Porosimetric distribution of: (a) Estoril; (b) Pietra Serena.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
298 Table 2 Porosimetric data Lithotypes
Total pore volume (mm3/g)
Average pore radius (mm)
Total porosity (%)
Estoril Pietra Serena
61.54 34.43
0.46 6.15
14.95 7.05
3. Results and discussion TEM micrographs, obtained on WS sample, are reported. Fig. 1 shows a typical Ca(OH)2 nanoparticles’ agglomerate, where the particles range from 50 to 600 nm; in Fig. 2a a particle with a hexagonal and a regular shape is shown (particle A), while a smaller irregular single particle is reported in Fig. 3a (particle A0 ). As concerns their crystallinity, these particles reveal typically crystalline features (circular symmetrylike), with spots and few marked circles (Fig. 2b) or with a spot size distribution (Fig. 3b). The difference can be related to the particles dimension: smaller (<100 nm) or greater (600 nm) particles seem to reveal a single or several crystallites, respectively. Dark field images confirm these considerations. By tilting the incident beam, regions corresponding to crystalline grains with planes oriented along the chosen direction become light; thus, the variation or not in bright marks the presence of different crystallites (Fig. 2c and d) or of a single crystal (Fig. 3c). SEM micrograph (Fig. 4) shows the effect of the carbonatation process on Ca(OH)2 particles in WS samples; it is
possible to observe only a partial growth of CaCO3 crystallites on lime particles surfaces. The partial CaCO3 growth can be a result of the incomplete carbonatation process and/or of the SEM technique itself. As a matter of fact, the liquid phase is quickly eliminated during the specimen gold sputtering (metallization), realised under vacuum conditions. XRD measurement on WS sample, performed after 30 min, is reported in Fig. 5. It is possible to recognize Ca(OH)2 and CaCO3 phases (84-1276 and 85-1108 JCPDS patterns, respectively). The parameter ICaCO3 =CaðOHÞ2 is 1.2. For this sample, I ratio remains the same also for the spectra recorded after 1 h, 24 h and 30 days: the carbonatation process, in the environmental experimental conditions, stops after 30 min. The corresponding WS yield is about 50%. XRD pattern of ALS sample, still obtained after 30 min, is shown in Fig. 6. It is possible to note a light improvement in the carbonatation process corresponding to an ICaCO3 =CaðOHÞ2 of 1.9 and a yield of 65%. In Table 1 the influence of the relative humidity conditions in the carbonatation process is shown. It is possible to note the expected improvement of the yield, but the completeness of the reaction can be reached only at the limiting conditions. In Fig. 7 the XRD result, obtained on ALBKS sample after 30 min of air exposure time, is shown. In this case the main crystalline phase is CaCO3 and Ca(OH)2. A very weak signal at about 32.3 could be attributed to the presence of Na2CO3$2H2O (08-0448 JCPDS pattern) as a reaction product. In this case the ICaCO3 =CaðOHÞ2 ratio is 18.7 while the yield reaches the value of 95% already in the first 30 min of air exposure time.
Fig. 9. SEM micrographs of Estoril sample: (a) untreated material surface; (b) material surface after nanolime treatment; (c) penetration depth of the treatment in a typical cross-section.
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
299
Table 3 Scotch Tape Test (STT): experimental results Lithotypes
Untreated materials (mg/cm2)
Treated materials (mg/cm2)
DV (%)
Estoril Pietra Serena
0.54 0.60
0.15 0.40
70% 35%
SEM analysis on the Pietra Serena stone is reported in Fig. 10. In particular, Fig. 10a shows that the nanolime treatment seems to reach a penetration depth of about 1 mm. This result could be attributed to the pore size distribution and the high porosity value of this lithotype (see Table 2). In Fig. 10b the adhesion of the treatment on grains surface is reported too. In Table 3 the STT results, in terms of the materials removed from the surface before and after the treatment, are reported; the percentage variation is indicated as DV. The treatment seems to be effective, relative to the superficial consolidation properties of the considered materials. The best
Fig. 10. SEM micrographs of Pietra Serena stone: (a) penetration depth; (b) adhesion of the treatment on grains’ surface.
4. Nanolimes treatments on natural stones The obtained nanolimes, together with the baking soda solution, are then applied on some natural stones (Estoril and Pietra Serena), in order to evaluate the protective effectiveness of the treatment. The typical porosimetric distribution graphs, obtained by a Porosimeter 2000 Series e Carlo Erba, are shown in Fig. 8, and in Table 2 the quantitative results are summarised. The pore size distribution data relative to Estoril sample exhibit an average pore radius of 0.46 mm and a total porosity of about 15%. On the contrary, Pietra Serena lithotype appears very different as in the pore size distribution as in porosity. The nanolime treatment is realised, with a brush, by applying the alcoholic suspension on the dry and clean stone surface. SEM analyses are performed in order to evaluate the reached penetration depth and the surface adhesion of nanolime treatments. To investigate the superficial consolidation of the treated stones, a ‘‘Scotch Tape Test’’ (STT) is performed. Capillarity and imbibition tests, according to literature procedures, are finally executed, before and after the treatment, to understand the material behaviour towards water. In Fig. 9 SEM micrographs of Estoril sample are reported: it is possible to note the partial filling of the pores due to the treatment (Fig. 9b) and a penetration depth of about 30 mm (Fig. 9c).
Fig. 11. Capillarity curves: untreated stones (continued line), treated stones (dots line).
300
V. Daniele et al. / Journal of Cultural Heritage 9 (2008) 294e301
morphological and microstructural characterisations show that crystalline and regularly shaped particles, with dimensions ranging between 50 and 600 nm, are obtained. An aqueous nanolime suspension of 15 mg/ml concentration stops its carbonatation within 30 min, reaching a carbonatation yield of about 50%, while the use of 2-propanol alcohol, as de-agglomerating medium, produces a yield of 65%. An attempt to complete the carbonation process by adding an external CO2 source is then realised using a baking soda solution (NaHCO3) as a promoter of the process itself: a conversion factor of 95% is so reached. After that the lime nanoparticles, together with the baking soda solution, are applied on two natural stones (Estoril and Pietra Serena) and three different tests seem to confirm the protective and consolidating effectiveness of the treatment: ‘‘Scotch Tape Test’’, capillarity and imbibition tests. With scanning electron microscopy (SEM), the penetration depth of the treatment is evaluated too. Further and more complete study on better promoter agents should be carried out in order to avoid surfaces’ alteration on treated stones. As a matter of fact, the introduction of sodium salt within the porous structure of the stones can produce efflorescence phenomena. Acknowledgments The authors thank Eng. Giovanna Di Tommaso and Dr. Lorenzo Arrizza and Dr. Maria Gianmatteo, of the InterDepartmental Centre of Electron Microscopy of the University of L’Aquila, for their collaboration. A special thanks is directed, last but not least, to Prof. Luigi Dei (University of Florence) for his very kind and helpful cooperation. References
Fig. 12. Imbibition curves: untreated stones (continued line), treated stones (dots line).
result is obtained for Estoril (DV z 70%), while Pietra Serena reached a DV value of about 35%. Capillarity and imbibition tests are reported in Fig. 11 and Fig. 12, respectively. The treatment effectiveness on water absorption in both the samples is evident. Results are particularly interesting for Pietra Serena stone, probably due to a partial filling of macropores and their transformation into smaller pores, as shown in SEM investigation. 5. Conclusions In this work the carbonatation of Ca(OH)2 nanoparticles, synthesised starting from stoichiometric ratio of the initial reactants at a synthesis temperature of 90 C, is analysed. The
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