The effect of wood ash on the properties and durability of lime mortar for repointing damp historic buildings

Abstract Historical evidence shows that wood ash has been used in lime mortar to help absorb moisture from masonry walls. In this study, an experimental programme was designed to assess the impact that varying the content of wood ash has on a range of properties of lime mortar and to draw conclusions about the reasons for these impacts. Biomass wood ash was added to mortars made with natural hydraulic lime (NHL) and air lime (AL) as an aggregate replacement at different concentrations: 0% (control mix), 10%, 20%, 30%, 40%, 70% and 100%. Compressive strength, strength activity index, open and total porosity, pore size distribution, water absorption by capillarity, desorption, water vapour permeability, and freeze-thaw durability were assessed after 90 days. The results indicate that fine particles of wood ash induce a higher proportion of pores in the capillary range and a higher open porosity. The tight structure of lime and wood ash increases the compressive strength. The fine particles given from the wood ash could also be responsible for the potential pozzolanic activity. Furthermore, by being hygroscopic, wood ash gives lime mortars a capacity to hold more water, resulting in a delayed capillary absorption, while retaining good desorption. The research findings imply that, used at its best potential in medium amounts (20–40%), adding wood ash in lime mortar gives a potentially good mortar mix for repointing masonry joints, especially in damp environments. However, in high amounts (70–100%) some negative effects are seen, such as high drying shrinkage.