Characterizing Hydraulic Properties of Drying Poultices Using Soil Physics Measurement Methods

Abstract

Salt contamination is a major cause of deterioration in masonry and historical buildings. Drying poultices constitute an established, straightforward method for extracting salt from porous substrates. Applied in a wet state, their composition and pore size enable salt removal via evaporation-driven advection, which slows and ceases as the poultice dries.
However, characterizing the poultices, particularly their hydraulic properties and pore size distribution, is challenging due to the material's dynamic behavior during drying. Previous studies have primarily focused on the dry state or indirect measurements, neglecting the critical phase during evaporation when salt removal occurs.
In this work, we have applied measurement techniques traditionally used in soil physics to determine the water retention and the hydraulic conductivity curve across the full drying range of the poultice.
Hyprop, KSAT and WP4C devices reveal poultice hydraulic properties throughout the process of drying. The resulting data are subsequently fitted to established mathematical models. In this study, particular emphasis is placed on the constrained bimodal van Genuchten-PDI (VG-PDI) model.
We show that the hydraulic properties of various mixtures of drying poultices can be measured precisely. Even small changes in the formulation (different materials, various clay content) become clear, particularly in the water retention curve. Nonetheless, we discuss the weakness of the measurement procedure and offer suggestions for improvement.
Insights gained will support poultice optimization and potentially enable substrate-specific poultice tailoring, laying the foundation for more time- and cost-effective salt removal in masonry.
This is a contribution to SWBSS2026 Conference.