A brief history of Lime
Lime was produced by burning locally available limestone in a coal or wood fired kiln at a temperature rarely in excess of 900 degrees C. The properties of the lime produced were largely influenced by the chemical composition of the limestone burnt, and limestones containing clay minerals produced a lime with weak hydraulic properties. The hydraulicity of these limes was likely to be weak because of the low firing temperatures, and weakened further if the lime produced was stored as putty. Certain compounds, notably the calcium aluminates, but also any di-‐calcium silicates present will hydrate or begin hydration in the lime pit. This explains why the ‘hot mixes’ where quicklime is mixed with water and aggregate on site have different properties.
The belief that most historic limes were non-‐hydraulic or only very weakly hydraulic is supported by the fact that certain additives have been added historically to alter the performance characteristics. These ‘heated’ materials contained silica, alumina and iron which became reactive towards alkalis including lime.
The earliest mortars analysed from Jericho in the Jordan Valley, and Tel-‐Ramad, Syria4, dating from 7000 BC contain stable end-‐products of pozzolanic reactions although it is not possible to conclude whether the pozzolanic material was deliberately added, or naturally present in the aggregate.
Pozzolanic materials in the form of crushed brick and tile were deliberately added in large quantity in mortars of Minoan Crete of c 1000 BC, ancient Greece, and the Roman period. Evidence suggests that the Romans used crushed brick and tile before they discovered the naturally occurring pozzolanic aggregates from around Vesuvius.
The practice, not so much ‘gauging with pozzolans’ but more the deliberate inclusion of pozzolanic materials as aggregate, was lost in Britain after 400 AD but continued in Europe as demonstrated by the analysis of samples from the Byzantine Empire5, Venetian renders6, Sistine Chapel plasters6, and recently analysed samples from the 13th century Moorish castle in Gibraltar which have the ingredients of ancient mortars -‐ carbonated lime, calcium silicate hydrate, brick particles, quartz sand and limestone particles.
Further evidence that limes were generally non-‐ or only very weakly hydraulic is demonstrated by Vicat7 in early 19th century France, and his frustration with the limes available, and his exhaustive trials to find a binder that would prove durable for hydraulic engineering works. His work, and others’, notably John Smeaton in late 18th century England, led to the recognition of natural hydraulic lime, the manufacture of artificial hydraulic limes, the invention of Parker’s ‘Roman Cement’, and in 1824 to the first Portland cement patent.