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

Abstract

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 × 5 × 5 and 5 × 5 × 20 cm) and soaked first with ultrapure H 2 O and second with different concentrations (0.1, 0.5 and 1 mol l) of a saline solution of NaCl and Na 2 SO 4 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.

The role of moisture and salt distribution in the weathering of the medieval cave town of Uplistsikhe, Georgia

“…PPP are patches of wet cellulose bres that are placed on the stone as a non-destructive salt sampling method. The poultices allow to extract salts from the near-surface stonework and thus to assess the spatial distribution and type of salts [21,22]. We used the cellulose bre material Arbocell BC1000 [23] and formed each poultice from 4 g of cellulose bres and 20 ml of deionised water (pulp-to-water weight ratio about 1:5).…”

“…Martinho et al 2012). The electrical conductivity of the resulting saline solution was measured with a conductivity sensor (WTW Cond 315i; measurement accuracy: 0.5% at 0… 35°C) which provides a very good proxy for the total salt content of the sample [21].…”

Approaches to the Study of Salt Weathering of Geological Materials

“…Field studies also frequently include the characterization of the salts, namely in terms of types, amounts, and sources, using diverse mineralogical and chemical techniques, including advanced analytical techniques such as isotopic analyses in the search for the sources of the salts (Hosono et al [38]; Siedel et al [39]; Rivas et al [40]; Kloppmann et al [41]). The involved sampling mostly consisted of collecting salt concentrations (efflorescences) and loose stone portions (which are considered as already compromised by the action of the soluble salts) but could include contact by extraction by paper pulp poultices (Egartner & Sass [42]) and more invasive procedures such as microdrilling (Ruiz-Agudo et al [36]; Siedel et al [39]). There are some cases where it was possible to collect laboratory specimens from discarded blocks (but these are rare situations that generally cannot be replicated for other study places so they will not be specifically considered here).…”