The Influence of the Substrate and the Environmental Conditions on the Performance of Lime-Based Repair Mortars: Paimogo Fort, a Case Study

In order to revitalize and preserve the Paimogo Fort, a Portuguese coastline military fortification built in 1674 and classified as of public interest since 1957, several lime-based repair rendering mortars were developed, considering the compatibility requirements with the original ones. In this investigation, the different lime-based mortar compositions proposed are briefly described and their main physical and mechanical characteristics are analyzed at successive ages (28, 90 and 180 days). Furthermore, some applications of the same mortars’ compositions on different porous substrates were carried out and their performances in laboratory and in situ conditions were compared. Finally, the possible degradation mechanisms and the impact of the composition, interaction with the substrate and climatic and environmental conditions on the durability of the mortars are discussed. The main outcomes show that mortars with some content of quicklime result in a balanced solution for the restoration work of the fort; they show an increase of more than 50% of strength compared to slaked air lime mortars, namely when applied on a medium-absorbent substrate. When applied on very absorbent substrates, although improving the compressive strength and porosity, all lime-based mortar compositions suffer a decrease in their modulus of elasticity and adhesion to the substrate. Air lime mortar compositions applied on a very absorbent and porous substrate generally show an increase in their mechanical strength when subjected to the severe marine environment of the fort.