- Email: [email protected]
Removal of salts from granite by sepiolite
Applied Clay Science 9 ( 1995) 459-463
Short Communication
Removal of salts from granite by sepiolite Raquel Trujillano a, Jacinta Garcia-Talegbn a, Adolf0 C. Ifiigo a, Maria A. Vicente a, Vicente Rives b**, Eloy Molina ’ aInstituto de Recursos Naturales y Agrobiologia, C.S.I.C., c/Cordel de Merinas, s/n, Salamanca, Spain I’Departamento de Quimica Inorgdnica, Universidad de Salamunca, Facultad de Farmacia, Av. Camp0 Charro, s/n, Salamanca, Spain ’Departamento de Geologia, Universidad de Salamanca, Facultad de Ciencias, Plaza de 10s Caidos, s/n, Salamanca, Spain
Received 3 August 1994;acceptedafter revision 8 March 1995
Abstract A study has been carried out on the extraction of salts from different types of granite using patent sepiolite formulation. The ability to remove salts greatly depends on the mineralogical compositions of the granites. Two types of equilibria are important: the absorption and/or adsorption of salts on the stone, and the partition of ions constituting the salt in the adsorbed state between the clay minerals in the rocks and that used as an extracting agent.
1. Introduction Among the different processes invoked to be responsible for stone decay, salt crystallization/precipitation inside the pores of the stone is one of the most important (Evans, 1970; Arnold and Zehnder, 1985; Lazzarini and Laurenzi Tabasso, 1986; Vicente and Brufau, 1986; Arnold and Zehnder, 1989; Vicente et al., 1993; Puertas et al., 1994). This problem is not specific for given weather conditions, but has been observed under many different circumstances. Accumulation of salts can lead to total destruction of the stones, and decrease the efficacy of preservation treatments. To avoid salt absorption is not always possible; therefore, to find a cheap, non-aggressive, easy and effective way to remove salts is one of the challenges for conservation of Cultural Heritage built in stone. In the present work, we report aInthe use of a special clay composition as a method that fulfils the above requirements for salts removal from the surface and subsurface of stones. -* Correspondingauthor. 0169-131~7/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO169-1317(95)00005-4
460
R. Trujillano et al. /Applied Clay Science 9 (1995) 459-83
2. Experimental method Five types of granite from the quarries near Avila (central Spain) have been investigated. These materials have been used to build the Cathedral of Avila (XIII-XVI century) and some other buildings in the area. Their petrophysical properties have been reported elsewhere (Ifiigo et al., 1994). Briefly, two non-porous, sound types - hereafter called coarsegrain grey (CG) and fine-grain grey (FG) -, a highly porous, smectite-containing ochreous facies (O), a medium-porous, red coloured facies (R) containing opal, kaolinite and iron oxohydroxides, and a white (W) facies, similar to facies R, but without iron oxides. Specimens were cut in 5 X 5 X 5 cm cubes. Artificial weathering and salt precipitation were performed following the NORMAL ( 1980) V. la method. The extracting agent was Supermold, a composition based mainly on sepiolite clay, cellulose fibre and other additives, from TOLSA, S.A. (Madrid, Spain). The specimens were first gently brushed to remove salts precipitated on their external faces, and then a mould of 5 X 5 X 1 cm was placed on the top face of the cube and filled with the extracting agent. A second extraction was also applied to the sample, in order to check if more salts were extracted. In both cases, after drying at room temperature, the paste was removed, dried at 110°C in an oven and analyzed for salts content: The dried paste was soaked for 3 days at 50°C with water (5 g dried paste per 100 ml water) and this was analyzed for sulphate and sodium content in a Dionex ionic exchange chromatograph coupled to an SP4400 integrator, and a Varian AA- 1475 atomic absorption instrument. The conductivity of the solution was also determined (conductimeter WTW mod. LS911 with electrode WTW/KLEl/T).
3. Results Conductivity data for all the granite samples and for the reference (unused sepiolite extracting agent) are given in Fig. 1. In the case of facies FG and CG the conductivity after the first extraction is close to 100 &S/cm; after the second one it is similar to that shown by the reference material (ca. 80 pS/cm). So, it should be concluded that all removable salts have been removed from samples FG and CG after the first extraction treatment. The values for samples 0, R, and W are noticeable higher; in these three cases, the conductivity after the second extraction has markedly decreased, but is still larger than the values for the reference material. So, not all salts have been removed after the second treatment. Results of the analysis for sodium and sulphate after the first and the second extraction treatments are given in Figs. 2 and 3. As it can be seen in these figures, the results for conductivity and sodium and sulphate show similar trends. At first sight, it would be expected that the behaviour shown by these five samples should follow patterns similar to changes in specific surface area and/or porosity. However, this is not the case, as, although samples FG and CG have low porosities (as measured by water absorption, NF, 1973)) lower than l%, the values for the other samples are 28% (sample 0) ,20% (R) , and 21% (W) (Iiiigo et al., 1994). Of course, the larger the porosity of the
R. Tncjillano et al. /Applied Clay Science 9 (1995) 45W63
461
4o04
300 -
E 2 v) 5
200-
100
0
rl
FG
CG
0
R
W
Fig. 1. Conductivity of the water soaked with successive extractions of sepiolite from different samples of granite: FG: fine gmy; CG: coarse grey; 0: ochreous; R: red; W: white. Solid bars: first extraction; empty bars: second extraction. rl: reference.
sample, the larger the capacity for salt accumulation (i.e., higher salt concentration would be expected) ; in such a case, the trend expected would be FG = CG -SER = W < 0. However, it should be noted that the samples also have a different mineralogical composition. The grey granite is formed by well crystallized, low reactive species with no surface activity and so they will absorb (and hence desorb) only very small amounts of salts. The smectite content was very much higher in sample 0 (that also contains a low percentage of goethite)
20 -
15E Q P lo-
rl
FG
CG
0
R
W
Fig. 2. Chemical analysis for Na+ in the water soaked with successive extractions of sepiolite from different samples of granite: FG: fine grey; CG: coarse grey; 0: ochreous; R: red, W: white. Solid bars: first extraction; empty bars: second extraction. rl: reference.
R. Trujillano et al. /Applied Clay Science 9 (1995) 459-463
462
60
1 1
20
0
rl
FG
CG
0
R
W
Fig. 3. Chemical analysis for SO:- in the water soaked with successive extractions of sepiolite from different samples of granite: FG: fine grey; CG: coarse grey; 0: ochreous; R: red; W: white. Solid bars: first extraction; empty bars: second extraction. rl: reference.
than in the other samples. Then, although its larger porosity would explain a larger retention of ions, these ions would be strongly adsorbed by the smectite (both internal and external), and would be difficult to be removed on application of the sepiolite paste. On the other hand, sample R contains large amounts of iron oxides and oxohydroxides (responsible for its intense colour), mainly hematite, goethite and amorphous compounds, with strong absorbing properties and thus would absorb large amounts of salt; however, their ability to retain the ions is less pronounced than that shown by smectite (one of the major components of sample 0). Therefore, although both samples 0 and R could show similar absorbing capacities, when put in contact with the extracting agent larger amounts of salt would be desorbed from sample R than from sample 0. Finally, the mineralogical composition of sample W is rather different: its pores are filled with amorphous opal and kaolinite, and does not contain iron oxides; its absorbing/desorbing properties will then be intermediate between those shown by grey granite, on one side, and samples 0 and W on the other side. So, it should be concluded that the ability of agents to remove salts from the pore network of stones greatly depends on the mineralogical composition of such stones. Two sorts of equilibria seem to be important: first, the absorption of salts by the stone, large if it contains clay minerals and/or amorphous materials, and secondly, the partition of ions constituting the salt in the adsorbed state between the clay forming the stones and that used in the extracting agent.
Acknowledgements The authors wish to thank to A. Alvarez and I. Duch, from the R&D Department of TOLSA, for the useful comments and discussion. Thanks to the company in providing samples and information concerning their products and methods.
R. Trujillano et al. /Applied Clay Science 9 (1995) 459-463
463
References Arnold, A. and Zehnder, K., 1985. Crystallization and habits of salt efflorescences on walls, part II: Condition of crystalliz.ation. 5th Int. Congr. Deterior. Conserv. Stone, Lausanne, pp. 269-277. Arnold, A. and Zehnder, K., 1989. Salt weathering on monuments. In: F. Zezza (Editor), The Conservation of Monuments in the Mediterranean Basin. Grafo Edizioni, Brescia, pp. 31-58. Evans, IS., 1970. Salt crystallization and rock weathering: a review. Rev. Gtomorphol. Dynam., 19: 153-177. hiigo, A.C., Garcia-Taleg6n, J., Vicente, M.A., Vargas, M., Perez-Rodrfguez, J.L. and Molina, E., 1994. Granites employed in Avila-Spain. II. Petrophysical Characteristics. Mat. Construcci6n, 44: 23-37. Lazzarini, L. and Laurenzi Tabasso, M., 1986. 11 Restauro della Pietra. CEDAM (Casa Editrice Dott Antonio Milani), Padova. NF, 1973, Norme Francaise Homologuee: Mesures de la porosite, de la masse volumique reelle et de la masse volumique apparente, B 10-503. NORMAL, 1980, Normativa Manufatti Lapidei, Test no. V. la. Puertas, F., 13lanco-Varela, M.T., Palomo, A., Ariiio, X., Ortega-Calvo, J.J. and Saiz-Jimenez, C., 1994. Characterization of mortars from the mosaics of Itfilica: causes of deterioration. In: V. Fassina, H. Ott and F. Zezza (Editors), The Conservation of Monuments in the Mediterranean Basin. La Photograph-Albignasego, Padova, pp. 577-,584. Vicente, M..4. and Brufau, A., 1986. Weathering of the Villamayor Arkosic Sandstone used in building, under continental semi-arid climate. Appl. Clay Sci., 1: 265-272. Vicente, M.A., Garcia-Taleg6n, J., Igigo, A.C., Rives, V. and Molina, E., 1993. Weathering mechanisms of silicated rocks in continental environments. In: M.-J. Ehiel (Editor), Proc. Int. RILEM/UNESCO Congr. on Conservation of Stone and Other Materials: Research-Industry-Media. Vol. 1. E and F N Spon, London, pp. 320-327.
Short Communication
Removal of salts from granite by sepiolite Raquel Trujillano a, Jacinta Garcia-Talegbn a, Adolf0 C. Ifiigo a, Maria A. Vicente a, Vicente Rives b**, Eloy Molina ’ aInstituto de Recursos Naturales y Agrobiologia, C.S.I.C., c/Cordel de Merinas, s/n, Salamanca, Spain I’Departamento de Quimica Inorgdnica, Universidad de Salamunca, Facultad de Farmacia, Av. Camp0 Charro, s/n, Salamanca, Spain ’Departamento de Geologia, Universidad de Salamanca, Facultad de Ciencias, Plaza de 10s Caidos, s/n, Salamanca, Spain
Received 3 August 1994;acceptedafter revision 8 March 1995
Abstract A study has been carried out on the extraction of salts from different types of granite using patent sepiolite formulation. The ability to remove salts greatly depends on the mineralogical compositions of the granites. Two types of equilibria are important: the absorption and/or adsorption of salts on the stone, and the partition of ions constituting the salt in the adsorbed state between the clay minerals in the rocks and that used as an extracting agent.
1. Introduction Among the different processes invoked to be responsible for stone decay, salt crystallization/precipitation inside the pores of the stone is one of the most important (Evans, 1970; Arnold and Zehnder, 1985; Lazzarini and Laurenzi Tabasso, 1986; Vicente and Brufau, 1986; Arnold and Zehnder, 1989; Vicente et al., 1993; Puertas et al., 1994). This problem is not specific for given weather conditions, but has been observed under many different circumstances. Accumulation of salts can lead to total destruction of the stones, and decrease the efficacy of preservation treatments. To avoid salt absorption is not always possible; therefore, to find a cheap, non-aggressive, easy and effective way to remove salts is one of the challenges for conservation of Cultural Heritage built in stone. In the present work, we report aInthe use of a special clay composition as a method that fulfils the above requirements for salts removal from the surface and subsurface of stones. -* Correspondingauthor. 0169-131~7/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO169-1317(95)00005-4
460
R. Trujillano et al. /Applied Clay Science 9 (1995) 459-83
2. Experimental method Five types of granite from the quarries near Avila (central Spain) have been investigated. These materials have been used to build the Cathedral of Avila (XIII-XVI century) and some other buildings in the area. Their petrophysical properties have been reported elsewhere (Ifiigo et al., 1994). Briefly, two non-porous, sound types - hereafter called coarsegrain grey (CG) and fine-grain grey (FG) -, a highly porous, smectite-containing ochreous facies (O), a medium-porous, red coloured facies (R) containing opal, kaolinite and iron oxohydroxides, and a white (W) facies, similar to facies R, but without iron oxides. Specimens were cut in 5 X 5 X 5 cm cubes. Artificial weathering and salt precipitation were performed following the NORMAL ( 1980) V. la method. The extracting agent was Supermold, a composition based mainly on sepiolite clay, cellulose fibre and other additives, from TOLSA, S.A. (Madrid, Spain). The specimens were first gently brushed to remove salts precipitated on their external faces, and then a mould of 5 X 5 X 1 cm was placed on the top face of the cube and filled with the extracting agent. A second extraction was also applied to the sample, in order to check if more salts were extracted. In both cases, after drying at room temperature, the paste was removed, dried at 110°C in an oven and analyzed for salts content: The dried paste was soaked for 3 days at 50°C with water (5 g dried paste per 100 ml water) and this was analyzed for sulphate and sodium content in a Dionex ionic exchange chromatograph coupled to an SP4400 integrator, and a Varian AA- 1475 atomic absorption instrument. The conductivity of the solution was also determined (conductimeter WTW mod. LS911 with electrode WTW/KLEl/T).
3. Results Conductivity data for all the granite samples and for the reference (unused sepiolite extracting agent) are given in Fig. 1. In the case of facies FG and CG the conductivity after the first extraction is close to 100 &S/cm; after the second one it is similar to that shown by the reference material (ca. 80 pS/cm). So, it should be concluded that all removable salts have been removed from samples FG and CG after the first extraction treatment. The values for samples 0, R, and W are noticeable higher; in these three cases, the conductivity after the second extraction has markedly decreased, but is still larger than the values for the reference material. So, not all salts have been removed after the second treatment. Results of the analysis for sodium and sulphate after the first and the second extraction treatments are given in Figs. 2 and 3. As it can be seen in these figures, the results for conductivity and sodium and sulphate show similar trends. At first sight, it would be expected that the behaviour shown by these five samples should follow patterns similar to changes in specific surface area and/or porosity. However, this is not the case, as, although samples FG and CG have low porosities (as measured by water absorption, NF, 1973)) lower than l%, the values for the other samples are 28% (sample 0) ,20% (R) , and 21% (W) (Iiiigo et al., 1994). Of course, the larger the porosity of the
R. Tncjillano et al. /Applied Clay Science 9 (1995) 45W63
461
4o04
300 -
E 2 v) 5
200-
100
0
rl
FG
CG
0
R
W
Fig. 1. Conductivity of the water soaked with successive extractions of sepiolite from different samples of granite: FG: fine gmy; CG: coarse grey; 0: ochreous; R: red; W: white. Solid bars: first extraction; empty bars: second extraction. rl: reference.
sample, the larger the capacity for salt accumulation (i.e., higher salt concentration would be expected) ; in such a case, the trend expected would be FG = CG -SER = W < 0. However, it should be noted that the samples also have a different mineralogical composition. The grey granite is formed by well crystallized, low reactive species with no surface activity and so they will absorb (and hence desorb) only very small amounts of salts. The smectite content was very much higher in sample 0 (that also contains a low percentage of goethite)
20 -
15E Q P lo-
rl
FG
CG
0
R
W
Fig. 2. Chemical analysis for Na+ in the water soaked with successive extractions of sepiolite from different samples of granite: FG: fine grey; CG: coarse grey; 0: ochreous; R: red, W: white. Solid bars: first extraction; empty bars: second extraction. rl: reference.
R. Trujillano et al. /Applied Clay Science 9 (1995) 459-463
462
60
1 1
20
0
rl
FG
CG
0
R
W
Fig. 3. Chemical analysis for SO:- in the water soaked with successive extractions of sepiolite from different samples of granite: FG: fine grey; CG: coarse grey; 0: ochreous; R: red; W: white. Solid bars: first extraction; empty bars: second extraction. rl: reference.
than in the other samples. Then, although its larger porosity would explain a larger retention of ions, these ions would be strongly adsorbed by the smectite (both internal and external), and would be difficult to be removed on application of the sepiolite paste. On the other hand, sample R contains large amounts of iron oxides and oxohydroxides (responsible for its intense colour), mainly hematite, goethite and amorphous compounds, with strong absorbing properties and thus would absorb large amounts of salt; however, their ability to retain the ions is less pronounced than that shown by smectite (one of the major components of sample 0). Therefore, although both samples 0 and R could show similar absorbing capacities, when put in contact with the extracting agent larger amounts of salt would be desorbed from sample R than from sample 0. Finally, the mineralogical composition of sample W is rather different: its pores are filled with amorphous opal and kaolinite, and does not contain iron oxides; its absorbing/desorbing properties will then be intermediate between those shown by grey granite, on one side, and samples 0 and W on the other side. So, it should be concluded that the ability of agents to remove salts from the pore network of stones greatly depends on the mineralogical composition of such stones. Two sorts of equilibria seem to be important: first, the absorption of salts by the stone, large if it contains clay minerals and/or amorphous materials, and secondly, the partition of ions constituting the salt in the adsorbed state between the clay forming the stones and that used in the extracting agent.
Acknowledgements The authors wish to thank to A. Alvarez and I. Duch, from the R&D Department of TOLSA, for the useful comments and discussion. Thanks to the company in providing samples and information concerning their products and methods.
R. Trujillano et al. /Applied Clay Science 9 (1995) 459-463
463
References Arnold, A. and Zehnder, K., 1985. Crystallization and habits of salt efflorescences on walls, part II: Condition of crystalliz.ation. 5th Int. Congr. Deterior. Conserv. Stone, Lausanne, pp. 269-277. Arnold, A. and Zehnder, K., 1989. Salt weathering on monuments. In: F. Zezza (Editor), The Conservation of Monuments in the Mediterranean Basin. Grafo Edizioni, Brescia, pp. 31-58. Evans, IS., 1970. Salt crystallization and rock weathering: a review. Rev. Gtomorphol. Dynam., 19: 153-177. hiigo, A.C., Garcia-Taleg6n, J., Vicente, M.A., Vargas, M., Perez-Rodrfguez, J.L. and Molina, E., 1994. Granites employed in Avila-Spain. II. Petrophysical Characteristics. Mat. Construcci6n, 44: 23-37. Lazzarini, L. and Laurenzi Tabasso, M., 1986. 11 Restauro della Pietra. CEDAM (Casa Editrice Dott Antonio Milani), Padova. NF, 1973, Norme Francaise Homologuee: Mesures de la porosite, de la masse volumique reelle et de la masse volumique apparente, B 10-503. NORMAL, 1980, Normativa Manufatti Lapidei, Test no. V. la. Puertas, F., 13lanco-Varela, M.T., Palomo, A., Ariiio, X., Ortega-Calvo, J.J. and Saiz-Jimenez, C., 1994. Characterization of mortars from the mosaics of Itfilica: causes of deterioration. In: V. Fassina, H. Ott and F. Zezza (Editors), The Conservation of Monuments in the Mediterranean Basin. La Photograph-Albignasego, Padova, pp. 577-,584. Vicente, M..4. and Brufau, A., 1986. Weathering of the Villamayor Arkosic Sandstone used in building, under continental semi-arid climate. Appl. Clay Sci., 1: 265-272. Vicente, M.A., Garcia-Taleg6n, J., Igigo, A.C., Rives, V. and Molina, E., 1993. Weathering mechanisms of silicated rocks in continental environments. In: M.-J. Ehiel (Editor), Proc. Int. RILEM/UNESCO Congr. on Conservation of Stone and Other Materials: Research-Industry-Media. Vol. 1. E and F N Spon, London, pp. 320-327.