SALT CRYSTALLIZATION TESTS IN BUILDING STONE MATERIALS

Transcription

1 SALT CRYSTALLIZATION TESTS IN BUILDING STONE MATERIALS Aikaterini I. Vavouraki* and Petros G. Koutsoukos Chemical Engineering Department, University of Patras, Karatheodori 1, Rio, 26500, Patras Institute of Chemical Engineering and High Temperature Chemical Processes, Stadiou Str., Platani, P.O. Box 1414, GR Patras, Greece Tel./ Fax: , Keywords: Soluble salts, Inhibitors, Limestone, Sandstone ABSTRACT Accelerated degradation of porous rocks of Granada s limestone and Czech sandstone specimens were conducted. Tested materials were exposed and impregnated in concentrated solutions of soluble salts (i.e. sodium sulfate, magnesium sulfate and sodium chloride). Immersed limestone type material showed susceptibility to sodium sulfate and sandstone type material to sodium chloride salt solution. Different pre-treatments of limestone specimens with organophosphonate and ferrocyanide compounds resulted in limiting material chemical damage from salts of sodium sulfate and sodium chloride, respectively. The use of such organic compounds may direct towards a potential implication of conserving building frameworks. Suspended limestone rods were subjected to sodium sulfate spray chamber. Pre-treatment of limestone rods with organophosphonate compounds of HEDP (1-hydroxy ethane-1, 1-diphosphonic) were completed. Again applications of organophosphonate compounds to exposure of limestone material in salt spray chamber may work towards a case of preventing porous material from salt damage and protect building stones. INTRODUCTION The combination of physical, chemical or/ and biological factors may result in the deterioration of stone in monuments and building structures. A variety of factors including topoclimatic environmental conditions, stone type, its preservation state and its location on the monument play a significant role in the degradation of stone material. Water and salts present in stones contribute to their deterioration because of chemical processes and the concomitant mechanical effects [1] Long before crystallization pressure may cause physical damage to a building material stone chemical damage may have already weakened the stone pore network. Salt weathering causes decay and damage to a wide range of materials, not least in buildings that are important to Europe s cultural heritage and civil frameworks. Salt occurs into stone pores and, in the worst cases it may cause complete structural deterioration. Structural damage and/ or material loss include granular disintegration, crumbling, flaking or contour scaling. Soluble salts (i.e. sodium and magnesium sulfate, sodium chloride) have been recognized as the most destructive salts in rock disintegration related to salt weathering. These salts occur in natural environments (e.g. coastal areas, arid and desert regions, and Antarctica) and may affect stone, brick and mortars used in the sculptural and building heritage. However salt formation may be controlled through the industrial experience where crystallization inhibitors are commonly used. Inhibitors of salt crystal growth may control efficiently the corresponding crystallization processes in porous building materials and may be applied in conservation practice. The present study is focused in the investigation of the mechanism of damage of two different natural porous rocks of calcarenite (Granada, Spain) and sandstone (Prague, Czech Republic) 1/7

2 specimens, typically used for the construction of European monuments, due to the crystal growth of sodium sulfate (mirabilite). More specifically, the effect of phosphonic acid and ferrocyanide inhibitors of mirabilite crystal growth was tested. Macroscale crystallization tests of immersion and salt spray chamber were performed. Accelerated laboratory experiments were performed to investigate the effect of the formation of mirabilite on the two stone types at identical experimental conditions. Granada limestone and Prague sandstone specimens were subjected to immersion in saturated solutions of soluble salts. Specimens of the limestone were also pre-soaked in inhibitor solution (i.e. phosphonates, ferrocyanides) following which, they were immersed in in sodium sulfate, sodium chloride and magnesium sulfate saturated solutions and via versa. Phosphoric acid salts have been suggested as efficient metal corrosion inhibitors. It is believed that their activity is possibly due to the formation of protective metal-phosphonate films [2] while ferrocyanides promote NaCl efflorescence growth as opposed to subflorescence growth in porous stones [3]. The presence of inhibitors carried in the interior of the stone pore system by the aqueous medium in which they are dissolved, results in the retardation or the cancellation of the formation of salts, allowing them to form a harmless efflorescence at the stone surface. The experimental measurements have shown that sodium sulfate did not have the same deterioration effect to both types of rock tested. Sodium sulfate perfectly outranked sodium chloride and magnesium sulfate in disintegrating Granada limestone whereas sodium chloride crystallized inside the porous Prague sandstone. These results have been attributed to the formation of crystals of the respective salts through a crystal growth process. Phosphonate compounds, present in the salt solutions in contact with the stone specimens resulted in the partial protection of limestone. Limestone specimens suspended in sodium sulfate spray conditions resulted in material disintegration-dissolution whereas specimens pre-treated with 1-hydroxy ethane-1, 1-diphosphonic (HEDP) compound limited limestone disintegration. The use of phosphonate inhibitors is recommended as a possible means to mitigate damage of calcitic building stone material due to dissolution accelerated by sodium sulfate. EXPERIMENTAL Cubic shaped specimens of each rock type of limestone and sandstone measuring 1x 1x 1 cm were subjected to immersion. Typical glass tubes (2 x2 x 10 cm) were used for immersion case studies. Prior to salt impregnation, samples were immersed in distilled water for 24 h followed by drying in an oven at 50 C. Each specimen was impregnated in a separate glass tube and their initial mass was measured. The glass tubes contained 5 ml of sodium sulfate solutions supersaturated with respect to mirabilite. Impregnation lasted 24 h for each cycle followed by 24 h drying at 80 o C. The impregnated specimens were removed from the oven and they were weighted immediately until constant weight. The respective mass values were recorded. Two sets of material immersion cycle experiments were conducted in different solutions. Limestone and sandstone specimens were immersed in in concentrated solutions of 9.6 m NaCl, 2.8 m MgSO 4 and 1.7 m Na 2SO 4. Seven total immersion were completed, with the exception of sodium sulfate where pieces of materials were detached before the completion of the immersion. In order to test the efficiency of materials pre-treatment, the impregnation tests were done following two types of treatment. In treatment A, the specimens were immersed initially in an inhibitor solution following salt solution immersion and in treatment B, the specimens were immersed in the salt solution following inhibitor solutions immersion. The inhibitors solutions used in the tests were 0.1% w/w HEDP for specimens immersed in in sulfate salts and 10-3 M, K 3Fe(CN) 6 for specimens immersed in in NaCl. All solutions were 2/7

3 prepared from the respective crystalline solids (Merck, pro analisi) followed by filtration through membrane filters (0.22 µm, Millipore). In addition to the immersion, accelerated tests were conducted in a salt spray chamber in which Granada limestone (size of 1x1x10 cm) specimens were exposed to a salt spray of a sodium sulfate solution in dispersed in small droplets with the aid of a glass nebuliser. The specimens were exposed at constant temperature (30 ºC) to a mist of 0.1 M Na 2SO 4 solution for six days (one cycle). The exposed specimens included both untreated and treated calcarenite specimens. The treatment consisted in the specimens impregnation in HEDP solutions of concentration 0.2-1% w/w, at ph= 5 and 10. The duration of the exposure of the calcarenite samples in the sodium sulfate spray chamber was a total of three (18 days). Before any analysis specimens were left to dry (80 o C) resulting in sodium sulfate crystallization in the porous limestone material. RESULTS & DISCUSSION Figure 1 presents the mass loss of limestone samples impregnated in in saline solutions. Limestone specimens immersed in in concentrated magnesium sulfate solutions did not show any damage and mass loss remained relatively constant. Limestone specimens immersed in in sodium chloride solutions exhibited a trend of increasing mass 5% of the initial specimen weight. This behavior may be explained by the dry stage during which the soluble salt solution fills the pores. However immersion of limestone specimens in in sodium sulfate solution (even after the completion of the second cycle) led to material loss. Sodium sulfate promoted severe damage to limestone porosity more than the corresponding in the already mentioned soluble salts of sodium chloride and magnesium sulfate. Sandstone specimens were also immersed in in salt solutions of the same concentrations. Macroporous materials, such as sandstone were more susceptible to aqueous solution immersion in comparison to limestone. The prevalence of macroporosity in sandstone resulted in fluctuating mass loss measurements. Following the second immersion cycle in NaCl solutions, the sandstone specimens mass increased whereas the subsequent immersion showed relatively constant increase in comparison with the first immersion cycle. Sandstone specimens immersion in in sulfate (Na 2SO 4 and MgSO 4) solutions, showed constant but lower mass loss values in comparison with the specimens immersed in water. Following drying, the sandstone specimens were weighted. Immersion of the sandstone specimens in in the sulfate solutions (MgSO 4, Na 2SO 4) did not resulted in mass loss of the material. However, sandstone specimens immersed in in sodium chloride showed mass increase because of sodium chloride crystallization inside material pores. Comparison of the material resistance of sandstone to limestone specimens immersed in in concentrated salt solutions, the latter appeared to be relatively weak, resulting in material loss even after the completion of the fourth immersion cycle. In high-porosity (limestone porosity ~24%) the weight change of stones depended on the salt or on the mixture of salts of the immersion solutions. Following A and B treatment, the integrity of the limestone specimens was preserved to a large extent (Fig. 2). Zero cycle corresponded to immersions completed before the successive immersion to the same solution in repeated either it was a salt solution (treatment A) followed by 3/7

4 inhibitor cycle solution or an additive solution (treatment B) followed by salt cycle solution. The HEDP concentration was 0.1% w/w. The results obtained suggested that the presence of HEDP in the solutions resulted in the prevention of mass loss. For the specimens with the treatment A the drying process resulted to the mass increase of the sodium-chloride-treated limestone specimens, possibly due to the crystallization of sodium chloride in the pore network. Limestone specimens with treatment B did not exhibit any significant difference in mass loss. 1,5 1,0... H 2 O NaCl Na 2 SO 4 MgSO 4 0,5 mass loss dry process (g) 0,0-0,5-1,0-1, Figure 1: Mass loss evolution following drying of limestone specimens immersed in in concentrated solutions of 9.6 m NaCl, 2.8 m MgSO 4 and 1.7 m Na 2SO 4. Mass loss is observed after immersion in sodium sulfate solution after the forth cycle of immersion ,5 1.0 A Na 2 SO 4 A NaCl A MgSO 4 1,0 B Na 2 SO 4 B NaCl B MgSO ,5 mass loss dry process (g) mass loss dry process (g) 0,0-0, , , (a) (b) Figure 2: Mass loss evolution past drying of limestone specimens a) with treatment A, in which the specimens were immersed in an inhibitor solution following immersion in salt solutions and b) with treatment B, in which the specimens were immersed initially in the salt solution following immersion in inhibitor solutions. In the case of treatment A repeated immersion of specimens in salt solution resulted in sodium chloride crystallization in the porous limestone material. 4/7

5 Fig. 3 shows the results of nitrogen absorption porosity measurements for limestone specimens impregnated in cycle in organic compound solutions following saline solution immersion (treatment A). Dried specimens were ground in agate mortar and sieved (~500 µm). Porosity measurements for the second set of limestone immersions showed that immersion in in sodium chloride as well as in ferrocyanide solutions did not affect material porosity. However impregnation of limestone specimens in MgSO 4 solutions increased the limestone porosity. Such damage in porosity was limited after treatment A in with solutions containing 0.1% w/w HEDP. Significant limestone specimen porosity was measured past immersion in in sodium sulfate solutions. Nonetheless, even in this case, the immersion of corresponding limestone specimens in solutions containing 0.1% w/w of HEDP resulted in reduced porosity damage. For sandstone specimen immersed in in sulfate solutions their porosity was not changed. However, sodium chloride crystallization inside the pores resulted in the decrease of porosity. 500 average porous diameter (A o ) calcar NaCl (A)NaCl MgSO4 (A)MgSO4 Na2SO4 (A)Na2SO4 Figure 3: Average pore diameter (APD) values of immersed limestone obtained from BET analysis. The APD for untreated limestone is ~70 Å; NaCl, MgSO 4, Na 2SO 4 designate the APD for limestone specimens after immersions in fifth cycle in salt solutions and (A)NaCl, (A)MgSO 4, (A)Na 2SO 4 correspond to specimens with treatment A immersed in cycle in inhibitor solutions following immersion in fifth cycle in salt solutions. Limestone porosity was greater after immersion in cycle in sodium sulfate solution. In all nitrogen porosity measurements, following the seventh immersion cycle (except for Na 2SO 4 - fifth cycle), the increases were likely to be due to the dissolution of limestone as evidenced from the granular disintegration and cracks formation. Sodium chloride was found to deposit within the pore spaces in the limestone samples while crystallization of sulfate salts was also identified. Salt aggregation on submicrometer sized limestone grains and thenardite crystals seemed to fill the pores. Intersecting pits over the limestone surface may lead to total disintegration, while material loss resulting from limestone immersion in sodium sulfate solutions resulted in an easily-crumbled material. SEM images of immersed limestone specimens after treatment A in the presence of additives in cycle showed disperse grains, but a slightly fragmented limestone surface (Fig. 4). The stone surfaces examined did not show deep etches and the original limestone grain morphology was dominant. Fig. 5 shows the calcium analysis of the runoff solutions from the limestone specimens both untreated and treated with organophosphonate. Limestone specimens exposed to 5/7

6 (c) salt spray in, pre-treated with HEDP solutions (1% w/w, ph= 5) showed lower calcium loss in the runoff suggesting amelioration of their resistance to deterioration upon exposure to salt spray conditions. (a) (b) (c) Figure 4: SEM micrographs of limestone specimens after treatment A with inhibitor solution in cycle of (a) 0.1% w/w HEDP, (b) 10-3 M K 3Fe(CN) 6 and (c) 0.1% w/w HEDP following immersion in in salt solutions of MgSO 4, NaCl and Na 2SO 4, respectively. A slightly fragmented limestone surface is observed. 6/7

7 exposed calcarenite 1% HEDP_calcar ph=10 1% HEDP_calcar ph=5 100 (Ca 2+ ) (mg/l) Figure 5: Comparison of the results of total calcium concentrations in the runoff from Granada limestone specimens exposed in salt spray chamber; 0.1 M Na 2SO 4, 30 0 C. Limestone rods pretreated with HEDP solutions (1% w/w, ph= 5) showed lower calcium loss in the runoff despite their exposure to the repeated sodium sulfate mist of 3 (18 days). CONCLUSIONS Accelerated weathering tests on limestone and sandstone specimens in contact with concentrated solutions of soluble salts of Na 2SO 4, NaCl, MgSO 4 showed the destructive impact of these salts on the tested porous materials. Pre-treatment of the limestone specimens by immersion in HEDP and ferrocyanide solutions resulted in significant protection from deterioration due to salt crystallization. Limestone specimens pretreated with a 1% solution of HEDP at ph= 5, when exposed to sodium sulfate salt spray delayed calcitic material (limestone main component) from dissolution, suggesting amelioration of the stability of the respective material in environmental conditions at which soluble salts may crystallize inside the pores. REFERENCES [1] Goudie, A.S., Viles, H.A., Parker, A.G. (1997), Monitoring of rapid salt weathering in the central Namib Desert using limestone blocks, Journal of Arid Environment, Vol. 37, pp [2] Demadis, K.D., Mavredaki, E., Stathoulopoulou, A., Neofotistou, E., Mantzaridis, C. (2007), Industrial water systems: problems, challenges and solutions for the process industries, Desalination Vol. 213, pp [3] Rodriguez-Navarro, C., Fernández, L.L., Doehne, E., Sebastian, E. (2002), Effects of ferrocyanide ions on NaCl crystallization in porous stone, Journal of Crystal Growth Vol. 243, pp /7