Using paper pulp poultices in the field and laboratory to analyse salt distribution in building limestones

We investigated the behaviour of salt solutions during the drying process in porous stone and the extraction of salts by paper pulp poultices (PPP). The investigations were carried out at a 300 year old boundary wall of the Worcester College in Oxford, UK, as well as under laboratory conditions. The laboratory blocks of porous, bioclastic limestone (Aflenzer stone, Austria) were cut into stone cubes and prisms (5 x 5 x 5 and 5 x 5 x 20 cm) and soaked first with ultrapure H2O and second with different concentrations (0.1, 0.5 and 1 mol l(-1)) of a saline solution of NaCl and Na2SO4 to determine their behaviour during the dehydration process under different conditions. After determining the capillary absorption coefficient and the capillary rise behaviour of the stone cubes, we sampled material by PPP and by drilling. The samples were immersed in ultrapure water and the salt ion content was measured via conductivity and ion chromatography (IC). The main aim of the laboratory analysis was to investigate the effectivity of the PPP in extracting salts from the stone and to derive a semi-quantitative calibration (PPP results vs. original salt content). The field work at the college boundary wall comprised sampling (PPP and stone surface material) at surfaces of different weathering features. Finally, the field work results were compared to the laboratory work. The lab experiment results show that approximately 24% of the salts within a 100% saturated stone cube moved into the poultice within the 1 h of application. At 75% and lower saturation degrees, only 14-11% of the salts moved into the respective poultices. For the field setting, the amount of anions extracted by PPP is 6-63% (mean 20.7%) of the amount found in the stone samples. The spatial distribution of salt ion concentrations, collected by poultices, corresponds with the 'hot spots' of weathering.