Traditional air lime-earth mortars: The effect of earth replacing the binder and the aggregate on performance and durability

Air lime-earth mortars remain relatively underexplored despite their promising potential for use in conservation and restoration practices. Traditionally, both air lime and earth were key binders in construction, making them valuable references for heritage preservation. This study explores the combination of hydrated air lime and earth inspired by traditional techniques that mixed clayish earth, extracted from the local soil, with quicklime or lime putty. In the experimental design, earth replaced air lime (up to 50 % by wt.) and fine sand (up to 25 % by wt.) in mortars with a binder-to-sand volumetric ratio of 1:2 and 1:3, respectively. These earth substitutions required more mixing water in production to obtain similar flow, resulting in mortars with lower bulk densities once hardened. The modified mortars showed higher open porosity and lower mechanical strength and thermal conductivity. However, the clay induced-rise in small pore volume improved sorption capacity. Although capillary water absorption increased with clay; the drying was faster mitigating the drawback. Notably, this was the only property observed to exceed the recommended range for conservation mortars, supporting the viability of air lime-earth mortars for conservation applications. When exposed to salts, the mortars showed sodium sulphate resistance, with no collapse by the end of the test confirming the absence of hydrated compounds. Moreover, the higher moisture buffering activity suggests that the partial replacement of raw materials by earth – a potential byproduct of local excavation – not only supports sustainable conservation but can also be applied to enhance indoor air quality and comfort in modern buildings.