The two types of lime commonly used in traditional construction techniques and building conservation are non-hydraulic (lime putty), and hydraulic lime.
Non-hydraulic lime is made by heating limestone to between 850 and 1300 C˚ which produces calcium oxide or quick lime. The quick lime is then combined with water to produce lime putty. Non-hydraulic lime sets, hardens, cures or “goes off” by absorbing carbon dioxide, a fact that goes some way to reduce the carbon footprint of it’s production. Providing the putty it is kept under water then the putty will not cure- in fact over time the quality of the lime putty will improve.
Hydraulic lime is made from limestone containing fine clay or silica heated to between the same temperatures as non-hydraulic lime. Then a precise amount of water is added to produce a dry powder. Hydraulic lime “goes off” more quickly than non-hydraulic lime and does not need contact with air to cure.
Lime mortar as a material is said to breathe, that is it that allows moisture in the brick, stonework or render to evaporate, reducing dampness and the risk of frost damage. Lime mortar also has a degree of flexibility and will move to an extent as a structure settles and shifts over time. Portland Cement does not share these properties with lime, and as so can cause damage to stone or brick.
Additives may be added to a lime mortar to make it set more quickly and increase its durability. These additives are known as pozzalons after the city of Pozzuoli in the province of Naples, Italy, where the Romans would add volcanic deposits to their lime mortar to enable these qualities. The Romans used such mortars in the creation of their aqueducts as they will even set under water. Today a by-product of coal burning power stations, Pulverised Fuel Ash (PFA), also known as Fly Ash can be added to lime mortar to the same effect.
Two sections of stone wall; Left: pointed with cement based mortar; Right pointed with lime based mortar.
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