An empirical method for the estimation of permeability in natural hydraulic lime mortars

A mortar used for the restoration of stone needs to be compatible with the original material. Permeability is a primary parameter for compatibility, since moisture is almost always present in building materials, in which it can be the inducer or catalyst of decay. If the permeability of lime mortars could be estimated before they are actually produced, time, cost and mortar misfits could be reduced to a minimum. At present however, there is no method available to estimate lime mortar permeability solely based on the mortar’s ingredients—the only elements available for investigation in that stage of production. This paper presents the development of an estimation method for the permeability of natural hydraulic lime mortar at 90 days of age. More precisely, the gas permeability is estimated using a frequently used estimation equation f × D2, where f is porosity and D the critical pore diameter. Both open porosity and pore diameter were estimated using information which can be obtained without making the mortar: the water-binder ratio, properties of the mortar ingredients such as the aggregate grain size distribution, the mean grain size of the binder and the composition of the binder. The open porosity and modal pore radii were estimated based on available estimation methods in literature. This estimation takes the various elements of which a mortar consists into account: the aggregates, the bulk paste and the interfacial transition zone between aggregates and bulk paste. Gas permeability estimations using this method were then compared with experimental data available in literature and with results from experimental lime mortar research. A link could be established between the empirical method and the measured water permeability.