Lime Putties and Mortars

Based upon a scientific understanding of the reasons different properties develop, slaked lime putty, aged slaked lime putty (which undergoes time-dependent changes upon storage in water) and putty from hydrated lime (i.e., soaked dry hydrate) are discussed, along with mortars made with them. The results of recent research on lime mortars are reviewed and interpreted focusing on the implications for the appropriate or inappropriate use of high-calcium lime mortar types in architectural conservation. The nanostructure and irreversible colloid nature of calcium hydroxide crystals in slaked lime putty, which determine the desirable working properties of putties, such as plasticity, density and viscosity, are modified through crystal coarsening triggered by drying. These properties are not fully regained upon subsequent reintroduction of water (e.g., putty preparation with hydrated lime such as commercially available dry powder). The calcium hydroxide crystals in slaked lime putty may be further modified upon storage in water for extended time periods, also affecting the working properties and density. The nature of the crystals also influences the carbonation, and development of porosity and compressive strength of the mortar. Additionally, the results of tri-axial compressive strength testing, as opposed to uni-axial compression, suggest that the limited strength of high-calcium lime mortar contributes to the durability of masonry.