Hot mixed air lime mortars, with or without small volumes of pozzolanic addition, were the ubiquitous lime mortar of tra...
Hot mixed air lime mortars, with or without small volumes of pozzolanic addition, were the ubiquitous lime mortar of traditional construction around the world, only falling from general use as the 20thC wore on, when the use of Portland cement or hybrid Portland cement and industrially produced dry hydrated air lime mortars displaced them from most building sites. This shift coincided with the growing disempowerment of masons and other crafts in the industry. Previously, mortar design had been the domain of the craftsmen, and wherever they were in the world, they chose to use hot mixed air lime mortars.
These mortars were the most economically made; they were quickly and efficiently prepared (by a variety of simple methods) and delivered mortars of eminent workability and usefulness. They enjoyed unparalleled water retentivity and excellent bond, and displayed maximum cohesiveness and adhesiveness.
All of these properties were shared with traditional earth-lime mortars, a mortar of similar ubiquity, which had dominated masonry construction throughout Europe and the Middle East before the 18thC and which were still commonly used for bedding mortars and base-coat plasters across the Americas even into the 20thC. Hot mixed lime and traditionally slaked lime putties were always used in conjunction with these.
Hot mixed air lime mortars, with suitable pozzolanic addition according to purpose, are eminently breathable – they have a vapour permeability more than twice that of a typical NHL mortar; they are much more effectively porous than other mortars and keep building fabric dry. They are durable and frost resilient in a properly detailed traditional building and are easy to prepare and deploy.
Hot mixed lime mortars are truly like-for-like and are compatible with all traditional fabric in ways that modern Natural Hydraulic Lime simply is not. Natural Hydraulic Limes were not much used for building in the air before the later 19thC – they were used, if at all, for water and underground works and for concretes. They were not available in North America unless imported from Europe, although natural cement was produced and was used for similarly wet situations, or as a gauge to otherwise hot mixed mortars in thick-walled military construction during the 19thC.
