Lime plasters, mortars and concretes have been used in construction for over a thousand years and are in many ways superior today's products. The term lime plasters and mortars are closely related, the difference being the aggregates added to lime putty. Lime plaster has the advantage over modern gypsum plaster in that it can be use outside as well as in. The disadvantage of lime putty based mixtures is the time needed for them to reach substantial hardness. A minimum curing period of 28 days is required, during that time the lime plaster must be kept damp and protected from freezing.
The four week cure is the final stage in the ‘lime cycle.’ It begins with roasting limestone (CaCO3) to drive off carbon dioxide and water thus creating quicklime (CaO). Quicklime is hydrated in a highly exothermic reaction known as slaking, often accomplished in a clay lined shallow pit near a town or large job site. Once allowed to rest while covered with water the resulting putty can close the loop by absorbing atmospheric carbon dioxide to become limestone once again.
The various tradesmen erecting what is now a historic building often used putty from the same pit throughout construction. A lime concrete was used for the foundation followed by the laying of brick or stone for walls, mixing in sand rather than gravel as the aggregate. The walls were then stuccoed on the outside (referred to as renders in England) with coarse lime plaster while interior walls were coated with fiber reinforced finer stuff. The great advantage of this system is that it allows moisture to move inside and outside without trapping it in walls. Lime plaster is also more flexible so a structure move without fracturing its skin.
If a structure was built prior to 1900, it probably was constructed using lime plasters and mortars. Modern portland cements and gypsum plasters should never be used to repair lime plaster walls, especially exterior applications. The cement will trap moisture in the walls causing more damage that if the repair were not undertaken. This has been revealed by studies conducted by England's Ministry of Building Works (Ancient Monuments Division). It was shown that repairs carried out in the 20's and 30's using portland cements resulted in more damage after 20 to 30 years than had occurred in the previous 100 yrs. of neglect.
Hydraulic lime is another type of lime cement, one variant used by the
ancient Romans is unsurpassed in its resistance to water thanks to unique
naturally occurring contaminants. For centuries deposits around the world
were mined, crushed and prepared for sale based on their particular
qualities. Hydraulic limes come in several grades NHL 2, NHL3.5 and NHL 5.
The higher number indicating greater hardness. Once used in Europe on sea
walls, these cements have outlasted modern portland varieties. These
products are not available at the average building supply and must be
sought out when needed. There is a growing number of suppliers in the
United States who produce or import these various NHLs. An internet search
will yield several of these enterprises as well as organizations devoted
to promoting of the use of lime plaster.