The influence of the pore structure on the moisture transport in lime plaster-brick systems as studied by NMR

- NMR is a powerful method for simultaneously analysing the porous structure and the moisture transport in masonry systems.
- The water-repellent plaster inhibited the drying of the brick.
- The inhibition of drying by the water-repellent mortar was assigned to the interruption of liquid transport.
- The NMR-T2 relaxation technique distinguished different pore size geometries in the specimens at a precise position.

This paper presents an experimental analysis of the porous structure and drying kinetics of lime-based plasters and plaster-brick systems using different methods. The effect of adding a water-repellent admixture (linseed oil) to the plasters was also evaluated. Nuclear Magnetic Resonance (NMR) was used to measure moisture profiles during drying of mono-material plaster specimens and of specimens constituted by the same plasters applied on fired-clay brick. NMR transverse relaxation time was used to examine the pore water distribution. The NMR moisture profiles showed that the drying behaviour of the water-repellent mono-plasters differs slightly from the reference, but when applied on brick, it significantly hinders the drying of the substrate. The results indicated that the addition of linseed oil to the lime plaster has negatively influenced the bonding process and created an hydraulic resistivity for the moisture to flow from the brick to the plaster during drying. This phenomena was mainly atributed to three factors: i) hydrophobization of part of the mortar pores; ii) the development of air bubbles related to entrained air during the mixing process; and iii) the mismatch between the physical-chemical properties of both systems, namely the different pore network and surface energy, which seems to affect the interpenetration of both porous layers.