engleski

Optimization of an ammonium oxalate treatment for the protection of monumental limestone and marble surfaces

Stone samples have been treated with two application methods (poultice and brushing) using a solution of ammonium oxalate (AmOx) and varying treatment time. Limestone and marble model samples with different porosity were selected, and attention was specifically focused to study brushing treatment, which is economically more feasible on large surfaces than the poultice method for creating an acid-resistant calcium oxalate (CaOx) layer. The distribution/contrast between calcite and the two Ca-oxalate forms (weddellite and whewellite) on treated stone samples was visualized using synchrotron-based micro analytical techniques (SR-µXRD in transmission geometry with 10.5 × 6.5 µm2 (hor. × vert.) probe size and SR-µFTIR with 10 × 10 µm2 probe size). SR-µXRD provided information on the layer thickness of the two oxalate forms while SR-µFTIR was used to visualize the distribution of calcium oxalate on the stone surface. SR-µXRD results of two brushing treatments (1 and 3 hours treatment time) on two different stones showed that on the less porous (2%) Carrara marble there is only a small difference between the shorter and longer treatment. The oxalate layer thickness, consisting of both weddellite and whewellite, is between 30 and 40 µm. The measured diffraction intensity of both calcium oxalates is approximately the same for both treatments. On the more porous (7%) Veselje limestone the difference between the shorter and longer brushing treatment is more visible. The thickness of the weddellite layer is approximately the same for both treatment times (roughly 50 µm). Whewellite reaches a depth of around 400 µm with the 3h treatment compared to 150 µm for the 1h treatment. The measured intensity of both calcium oxalates is higher on stones treated with the longer treatment time. SR-µFTIR comparison of the CaOx distribution on surfaces treated by brushing on two model stones showed that on a less porous Carrara marble there is no difference between shorter and longer treatments. This difference is more visible on more porous Veselje limestone, as the surface of the stones after longer brushing treatments shows a higher coverage with oxalate. It seems that on the less porous stone after 1 h of brushing treatment the layer of CaOx is more homogeneous, so there is no more CaCO3 left on the surface to react with AmOx. On the more porous stone after one hour of treatment, there is still some CaCO3 on the surface that reacts during subsequent application of AmOx. Both analytical methods confirmed that on less porous stones (<2%) there is actually no difference between different application methods and treatment times, since a protective CaOx layer has been homogeneously formed after 1 hour of brushing or ten hours of poultice treatment. Stones that are more porous need a longer brushing treatment to create a similar CaOx layer.

ammonium oxalate ; synchrotrone ; limestone

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