Abstract
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.
Highlights
Lime mortar has been used since before Roman times on various constructions and under many different environmental conditions
After 40% of wood ash content in both mortar types, results obtained show a significant decrease in total porosity, likely related to misreading and are not presented here
In comparison to the control mortar, the total porosity of natural-hydraulic lime (NHL) mortars is lower while the open porosity is higher, whereas in air lime (AL) mortars total porosity slightly increases while the open porosity increases in comparison to the control mortars
Summary
Lime mortar has been used since before Roman times on various constructions and under many different environmental conditions. A leading French architectural theoretician, points out in his Art de Bâtir in 1803, that the good performance of Roman mortar results from good workmanship and care as well as the use of appropriate materials [1]. The optimum combination of materials (that is, a binder, an aggregate and potentially an additive) comes from understanding the compatibility requirements of the mortar within this structural and environmental context such as for instance, similar or increased permeability, porosity higher than the surrounding stones, ability to set in specific environmental conditions, and relative resistance to frost and salts [4].
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