Lime mortar is a type of mortar composed of lime and an aggregate such as sand, mixed
with water. It is one of the oldest known types of mortar, dating back to the 4th century
BC and widely used in Ancient Rome and Greece, when it largely replaced the clay and gypsum
mortars common to Ancient Egyptian construction. With the introduction of ordinary portland
cement during the 19th century the use of lime mortar in new constructions gradually
declined, largely due to portland's ease of use, quick setting, and high compressive strength.
However the soft, porous properties of lime mortar provide certain advantages when working
with softer building materials such as natural stone and terracotta. For this reason, while
OPC continues to be commonly used in brick and concrete construction, in the repair and
restoration of brick and stone-built structures originally built using lime mortar, the use
of OPC has largely been discredited. Despite its enduring utility over many centuries,
lime mortar's effectiveness as a building material has not been well understood; time-honoured
practices were based on tradition, folklore and trade knowledge, vindicated by the vast
number of old buildings that remain standing. Only during the last few decades has empirical
testing provided a scientific understanding of its remarkable durability.
Etymology Lime comes from Old English lim "'sticky substance,
birdlime, mortar, cement, gluten,'" and is related to Latin limus "'slime, mud, mire,'"
and linere "'to smear;'" Mortar is a mixture with cement and comes from Old French mortier
"'builder's mortar, plaster; bowl for mixing'" in the late 13th century and Latin mortarium
"'mortar,'" Lime is a cement which is a binder or glue which holds things together but cement
is often reserved for Portland cement such as the common term lime-Portland cement.
Uses Lime mortar today is primarily used in the
conservation of buildings originally built using lime mortar, but may be used as an alternative
to ordinary portland cement. It is made principally of lime, water and an aggregate such as sand.
Portland cement has proven to be incompatible with lime mortar because it is harder, less
flexible, and impermeable. These qualities lead to premature deterioration of soft, historic
bricks so the traditionally, low temperature fired, lime mortars are recommended for use
with existing mortar of a similar type or reconstruction of buildings using historically
correct methods. Lime is commonly available in the United States
as Type S hydrated mason's lime. However, this lime product is intended to be added
to Portland cement to improve workability and several other reasons. Type S lime is
not reliable as a sole cement because some manufacturers fire the lime above 1,100°
C which begins to cause vitrification of impurities in the lime. This over-burned lime is sometimes
called dead-burned because lime fired at these temperatures loses reactivity leading to poor
or no bond strength and will not hold up to freeze-thaw weather cycles.
Hydraulic and non-hydraulic lime Hydraulic lime sets by hydration so it can
set under water. Non-hydraulic lime sets by carbonatation and so needs exposure to carbon
dioxide in the air and cannot set under water or inside a thick wall. For natural hydraulic
lime mortars, the lime is obtained from lime stone naturally containing a sufficient percentage
of silica and/or alumina. Artificial hydraulic lime is produced by introducing specific types
and quantities of additives to the source of lime during the burning process, or adding
a pozzolana to non-hydraulic lime. Non-hydraulic lime is produced from high purity source of
calcium carbonate such as limestone or oyster shells.
Non-hydraulic lime Non-hydraulic lime is primarily composed of
calcium hydroxide. Non-hydraulic lime is produced by first heating sufficiently pure calcium
carbonate to between 954° and 1066°C, driving off carbon dioxide to produce quicklime. This
is done in a lime kiln. The quicklime is then slaked – hydrated by being thoroughly mixed
with enough water to form a slurry, or with less water to produce dry powder — a hydrated
lime. Hydrated lime naturally turns back into calcium carbonate, the entire process called
the lime cycle. The slaking process involved in creating a
lime putty is an exothermic reaction which initially creates a liquid of a creamy consistency.
This is then matured for 2 to 3 months - depending upon environmental conditions - to allow time
for it to condense and mature into a lime putty.
A matured lime putty is "thixotropic", meaning that when a lime putty is agitated it changes
from a putty into a more liquid state. This aids its use for mortars as it makes a mortar
easier to work with. If left to stand following agitation a lime putty will slowly revert
from a thick liquid to a putty state. It is advised that a lime mortar should be "knocked
up" prior to its use. As well as calcium-based limestone, dolomitic
limes can be produced which are based on calcium magnesium carbonate.
A frequent source of confusion regarding lime mortar stems from the similarity of the terms
hydraulic and hydrated. Hydrated lime is any lime other than quicklime,
and can refer to either hydraulic or non-hydraulic lime.
Lime putty is always non-hydraulic and will keep indefinitely stored under water and,
as the name suggests, lime putty is in the form of a putty made from just lime and water.
If the quicklime is slaked with an excess of water then putty or slurry is produced.
If just the right quantity of water is used, the result is a dry material. This is ground
to make hydrated lime powder. Hydrated, non-hydraulic lime powder can be
mixed with water to form lime putty. Before use putty is usually left in the absence of
carbon dioxide to mature. Putty can be matured for as little as 24 hours for many years;
an increased maturation time improves the quality of the putty. There is an argument
that a lime putty which has been matured for an extended period becomes so stiff that it
is difficult to work. There is some dispute as to the comparative
quality of putty formed from dry hydrated lime compared with that produced as putty
at the time of slaking. It is generally agreed that the latter is preferable. A hydrated
lime will produce a material which is not as "fatty" and often, due to lengthy and poor
storage, the resulting lime produced by hydrated lime will exhibit longer carbonatation periods
as well as lower compressive strengths. Non-hydraulic lime takes longer to set and
is weaker than hydraulic lime, and should not be allowed to freeze before it is well
set. Although the setting process can be slow, the drying time of a lime mortar must be regulated
at a slow rate to ensure a good final set. A rapidly dried lime mortar will result in
a low-strength, poor-quality final mortar often displaying shrinkage cracks. In practice,
lime mortars are often protected from direct sunlight and wind with damp hessian sheeting
or sprayed with water to control the drying rates. But it also has the quality of autogeneous
healing where some free lime dissolves in water and is redeposited in any tiny cracks
which form. Hydraulic lime
Hydraulic lime sets by reaction with water called hydration.
When a stronger lime mortar is required, such as for external or structural purposes, a
pozzolan can be added, which improves its compressive strength and helps to protect
it from weathering damage. Pozzolans include powdered brick, heat treated clay, silica
fume, fly ash, and volcanic materials. The chemical set imparted ranges from very weak
to almost as strong as Portland cement. This can also assist in creating more regulated
setting times of the mortar as the pozzolan will create a hydraulic set, which can be
of benefit in restoration projects when time scales and ultimately costs need to be monitored
and maintained. Hydraulic lime can be considered, in terms
both of properties and manufacture, as part-way between non-hydraulic lime and OPC. The limestone
used contains sufficient quantities of clay and/or silica. The resultant product will
contain dicalcium silicate but unlike OPC not tricalcium silicate.
It is slaked enough to convert the calcium oxide to calcium hydroxide but not with sufficient
water to react with the dicalcium silicate. It is this dicalcium silicate which in combination
with water provides the setting properties of hydraulic lime.
Aluminium and magnesium also produce a hydraulic set, and some pozzolans contain these elements.
There are three strength grades for natural hydraulic lime, laid down in the European
Norm EN459; NHL2, NHL3.5 and NHL5. The numbers stand for the minimum compressive strength
at 28 days in newtons per square millimeter. For example the NHL 3.5 strength ranges from
3.5 N/mm2 to 10 N/mm2. These are similar to the old classification of feebly hydraulic,
moderately hydraulic and eminently hydraulic, and although different, some people continue
to refer to them interchangeably. The terminology for hydraulic lime mortars was improved by
the skilled French civil engineer Louis Vicat in the 1830s from the older system of water
limes and feebly, moderately and eminently. Vicat published his work following research
of the use of lime mortars whilst building bridges and roads in his work. The French
company Vicat still currently produce natural cements and lime mortars. Names of lime mortars
were so varied and conflicting across the European continent that the reclassification
has greatly improved the understanding and use of lime mortars.
Mix Traditional lime mortar is a combination of
lime putty and aggregate. A typical modern lime mortar mix would be 1 part lime putty
to 3 parts washed, well graded, sharp sand. Other materials have been used as aggregate
instead of sand. The theory is that the voids of empty space between the sand particles
account for a 1/3 of the volume of the sand. The lime putty when mixed at a 1 to 3 ratio,
fill these voids to create a compact mortar. Analysis of mortar samples from historic buildings
typically indicates a higher ratio of around 1 part lime to 2 part aggregate/sand was commonly
used. A traditional coarse plaster mix also had horse hair added for reinforcing and control
of shrinkage, important when plastering to wooden laths and for base coats onto uneven
surfaces such as stone walls where the mortar is often applied in thicker coats to compensate
for the irregular surface levels. If shrinkage and cracking of the lime mortar
does occur this can be as a result of either The sand being poorly graded or with a particle
size that is too small The mortar being applied too thickly
Too much suction from the substrate High air temperatures or direct sunlight which
force dry the mortar High water content in the lime mortar mix
Poor quality or unmatured lime putty Hair reinforcement
Hair reinforcement is not found in lime mortars, but is common in lime plaster and many types
of hair and other organic fibres can be found in historic plasters. However, organic material
in lime will degrade in damp environments particularly on damp external renders. This
problem has given rise to the use of polypropylene fibres in new lime renders
Properties Lime mortar is not as strong in compression
as OPC mortar, but both are sufficiently strong for construction of non-high-rise domestic
properties. Lime mortar does not adhere as strongly to
masonry as OPC. This is an advantage with softer types of masonry, where use of cement
in many cases eventually results in cement pulling away some masonry material when it
reaches the end of its life. The mortar is a sacrificial element which should be weaker
than the bricks so it will crack before the bricks. It is less expensive to replace cracked
mortar than cracked bricks. Under cracking conditions, OPC breaks, whereas
lime often produces numerous microcracks if the amount of movement is small. These microcracks
recrystallise through the action of 'free lime' effectively self-healing the affected
area. Historic buildings are frequently constructed
with relatively soft masonry units, and minor movement in such buildings is quite common
due to the nature of the foundations. This movement breaks the weakest part of the wall,
and with OPC mortar this is usually the masonry. When lime mortar is used, the lime is the
weaker element, and the mortar cracks in preference to the masonry. This results in much less
damage, and is relatively simple to repair. Lime mortar is more porous than cement mortars,
and it wicks any dampness in the wall to the surface where it evaporates. Thus any salt
content in the water crystallises on the lime, damaging the lime and thus saving the masonry.
Cement on the other hand evaporates water less than soft brick, so damp issues are liable
to cause salt formation and spalling on brick surfaces and consequent disintegration of
bricks. This damp evaporation ability is widely referred to as 'breathability'.
Lime mortar should not be used below temperatures of 5 °C and takes longer to set so it should
be protected from freezing for three months. Usually any dampness in the wall will cause
the lime mortar to change colour, indicating the presence of moisture. The effect will
create an often mottled appearance of a limewashed wall. As the moisture levels within a wall
alter, so will the shade of a limewash. The darker the shade of limewash, the more pronounced
this effect will become. A load of mixed lime mortar may be allowed
to sit as a lump for some time, without it drying out. When ready to use, this lump may
be remixed again and then used. Traditionally on building sites, prior to the use of mechanical
mixers, the lime putty was mixed with sand by a labourer who would "beat and ram" the
mix with a "larry". This was then covered with sand and allowed to sit for a while - a
process known as 'banking'. This lump was then remixed and used as necessary. This process
cannot be done with OPC. Lime with cement
Addition of lime to cement mortar makes the mixture thicker and stickier while wet. Addition
of cement to lime mortar acts as a pozzolan giving some degree of quick set, but this
comes at a price, and is not recommended. Three cement-sand-lime mixes have been widely
used. 1:1:6 is a satisfactory mortar mix for various
purposes, but behaves as cement-based mortar. 1:2:9 has been found to be prone to premature
failure, and is no longer recommended 3:1 + 5% cement makes a lime mortar that firms
up quickly, avoiding delays to work, but the addition of cement has been found to be associated
with premature failures, and the practice is no longer recommended.
1:2:9 and 1:1:6 are frequently mistaken for lime mortars, but are considered cement mortars,
as although they contain substantial amounts of lime, their properties are primarily those
of cement mortar. 1:1:4 is the usual mix for the experienced builder. Note: this calculation
is based on weight, not volume. See also
Energetically modified cement Plastering
Whitewash References
Burnell, George Rowdon; Rudimentary Treatise on Limes, Cements, Mortars, Concretes, Mastics,
Plastering, Etc. Dibdin, William Joseph; Lime Mortar & Cement:
Their Characteristics and Analyses. With an Account of Artificial Stone and Asphalt
Gilmore, Quincy A.; Limes Hydraulic Cement and Mortars
Hodgson, Fred T.; Concrete, Cements, Mortars, Artificial Marbles, Plasters and Stucco: How
to Use and How to Prepare Them Lazell, Ellis Warren; Lime Mortar & Cement :
Their Characteristics and Analyses. With an Account of Artificial Stone and Asphalt
External links The following are mid-19th-century technical
articles on the respective subjects: lime mortar, cement making on a small scale, cement
making on a large scale and mortar. Gerard Lynch, 'The Myth in the Mix: The 1:3
Ratio of Lime to Sand', The Building Conservation Directory, 2007