Pore-related properties of natural hydraulic lime mortars: an experimental study

Restoration of historic buildings is often executed using lime mortars. A restoration mortar is considered compatible with the original building materials when it answers to certain criteria, such as the facilitation of moisture transport. This can be determined by studying porosity, permeability, and the pore size distribution of both materials. Since NHL mortars are used in restoration of the exterior, where water and moisture are ever-present, a good characterization in terms of their pore-related properties is necessary. This study therefore researched characteristics of pure NHL mortars, without metakaolin or pozzolan additives. In this study, 16 different mortar recipes were made with NHL 5, the most hydraulic NHL type. The mortars had variations in spread, sand type and grain size. The sand types varied from coarse-grained to very fine, in order to be able to study the influence of the largest and smallest grains. The spread was adapted from 120 to 180 mm by changing the water:binder ratio. This allows understanding the influence of binding water on these pore-related properties. The measured pore-related properties were linked to compressive strength measurements, so as to characterize these eminently hydraulic limes also on a mechanical level. Nearly 180 samples were tested after 90 days of curing. All mortars show a bimodal pore size distribution. Depending on the sand type and the mortar spread, the mechanical strength varied from 1 to 10 MPa and pore sizes varied from the larger to smaller pore region. This demonstrates the large influence aggregates and water have on the properties of the hardened mortar. In general, the influence of the variation in water:binder ratio was less apparent, although an excess of binding water was translated into a change towards smaller pores, higher porosity and lower compressive strength. This study demonstrates that NHL 5 lime can be a good binder for restoration mortars, in which the properties of the mortar can be adapted through variation of aggregate grain size and spread. This allows researching a possible estimation of properties before making the mortars.