TYPES OF LIME?
Building limes are broadly divided into hydraulic and non-hydraulic. ‘Hydraulic’ with regard to
mortars refers to setting and hardening once mixed with water. ‘Non-hydraulic’ instead set by carbonation; reacting with carbon dioxide in the air.
NATURAL HYDRAULIC LIME OR NHL
LNatural Hydraulic Lime (NHL) is a lime containing minerals which react with water and begin to set
once mixed. A Natural Hydraulic Lime can contain no additives, all the minerals must be present in
the limestone rock from which it is made. All NHLs contain a certain amount of ‘hydraulic’
component and a remainder of ‘non-hydraulic’ component, often referred to as free lime. The
hydraulic part will set first, beginning within a few days and lasting several weeks. The free lime will
then continue to carbonate slowly, usually taking several years to cure completely. There are
different grades of NHL; NHL2, NHL3.5 and NHL5. The strongest, NHL5, has the highest percentage
of hydraulic component and the lowest percentage of free lime. This will produce a stronger, faster
setting lime mortar while the weaker NHL2 will produce a slower setting, fatter, more flexible lime
HYDRAULIC OR FORMULATED LIMES
Hydraulic Limes (HL) or Formulated Limes (FL) are similar to NHL but containing additives such as
pozzolans or cement rather than naturally occurring hydraulic minerals. A Formulated Lime (FL) must
declare what ingredients are present, a Hydraulic Lime (HL) is not required to declare its full
Non-hydraulic lime is a pure lime which does not contain active minerals to impart a set. It is
variously referred to as air lime, fat lime or free lime. It can be supplied as either a dry hydrated lime,
lime putty or quicklime.
Hydrated lime is supplied in dry bag form similar to NHL (most commonly in Ireland under the brand
name White Rhino). It can be mixed sand to produce a lime mortar. Pozzolans can be added to
impart a set, it is also often to added to cement mortars as a plasticiser or to reduce the strength of
the cement mortar.
Lime putty is hydrated lime which has been soaked in an excess of water and left to mature for
several months where it forms a putty. This will produce a very fat and workable lime ideal for fine
plastering or finishing work and also ideal for thin lime washes. Left to mature the putty will
continue to become finer in particle size, lasting indefinitely and becoming better quality the older it
is, though it must be protected from frost and sub-zero temperatures.
Quicklime is lime unslaked lime direct from the kiln. When mixed with water it produces a very
vigorous, hot reaction, producing what is referred to as hot lime mortar. Hot lime mortars are
extremely fat and workable. Pozzolans can be added or they can be gauged with NHL produce
hydraulic mortar if required. Quicklime is supplied in either granulated or powder form. The powder
form will slake more quickly but the potential for reactive dust means granulated is often a safer
option on site. The nature of the vigorous, hot reaction means additional health and safety concerns
above other types of lime are involved when using quicklime, extreme care must be taken on site if