DEVELOPMENT OF COMPATIBLE AND SUSTAINABLE RENDER SYSTEMS FOR SALT-CONTAMINATED BRICK MASONRIES

In presence of salty water and marine aerosol, historic brick walls are subjected to a particularly aggressive environment due to capillary rise phenomena, transport and deposition of salts that cause disaggregation, efflorescence, fractures and fissures of the surfaces. One of the possible protection measures, intensively investigated since the 1990s, is the use of render systems with suitable composition, able to adjust the water flow and the migration of salts from the substrate towards the render. In this study, double-layer renders were formulated considering the following objectives: compatibility with ancient masonries, environmental sustainability, and effectiveness in ensuring the salt migration. Thus, natural hydraulic lime as binder (lower carbon footprint in comparison to cement) and a partial replacement of the aggregates with recycled render wastes were selected. Moreover, an air entraining agent in the inner layer and different percentages of water-repellent admixtures in the outer layer were added in order to adju st the render structure. The systems were applied on bricks and subjected to rising damp evaporation cycles, in order to investigate their behaviour and properties in respect to water flow and salt migration. The water vapour permeability, the capillary absorption, the drying behaviour and the compressive strength of each formulation were evaluated on mono -material render specimens according to EU standard tests. Two layers of render with different composition were applied on bricks, cured for 28 days a nd subjected to drying-wetting cycles of salty water (NaCl 3%) by capillary rise. Fundamental parameters such as the adhesion of the plaster to the substrate and the resistance to the action of salts were evaluated by visual and optical microscope observat ions, colorimetric and sclerometric measurements. In order to assess the salt transport and distribution within each system, the salt profile was determined by conductivity measurements, whilst the disruptive effect of salts on the render microstructure was estimated by mercury intrusion porosimetry. Moreover, the renders were separated from the bricks at the end of the cycles and the water vapour permeability, the capillary absorption and drying behaviour of the double-layer systems were determined. In the present contribution, selected results concerning the more interesting systems in relation to the salt transport behaviour will be reported. In general, different behaviours were observed: deposition of salts within the lower layer or at the lower-upper layer interface (sacrificial plaster); rapid transport of salts to the surface (fast salt -transporting permeable plasters); slow transport and permanence of salt at the brick-render interface (salt blocking renders). The first two behaviours led to positiv e responses with high salt transport towards the render and without render failure, and were observed whenever two layers with different microstructure and permeability properties were present. The third behaviour caused render detachment and matrix disruption.