Permeability in lime plaster in relation to durability of covered materials

Paper in proceeding, 2004

In restoration of old buildings, lime plasters are used for reasons of authenticity and for technical reasons as well. In many traditional constructions, there is wood behind the plasters or there is old lime mortar in the masonry. For the durability of both these underlying materials, it is important that the relative humidity, RH is not too high under a long time of exposure. This can lead to an accelerated degradation process of wood and leaching of lime in mortars. To prevent this, the plaster must have a proper permeability. The permeability of a material determines its ability to transport gas and moisture. In lime plasters, permeability affects hardening, carbonation as well as durability of the underlying materials. In the present paper, eleven mixtures of lime mortar were studied. The binders used were lime slaked in four different ways, hydraulic lime and cement. The following measurements were made to analyse the permeability and the pore structure of the samples. Gas permeability and draining curves give an indication of rate of carbonation. Vapour permeability shows the ability of plaster to transmit moisture at different RH. Thin sections were made for microscopically investigations of the samples. The investigations show that both the vapour permeability and gas permeability decreases with an increased amount of hydraulic binder in the lime plasters. Even small amounts of hydraulic binder have a significant effect. By connecting these results with examples from reality, the picture becomes clearer. In the medieval town Visby in Sweden, there are several examples of both rotten wood and disintegrated lime mortars. It is constructions from 13th to 19th centuries and low permeable cementitious plasters have covered them all, during the middle of the 20th century.