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Lime mortaris a type of mortar composed of:
Lime, an Aggregate (e.g. sand), & Water.
It is one of the oldest known types of mortar, dating back to the 4 th century BC
and widely used in Ancient Rome & Greece,
Introduction to Hydraulic & Non-Hydraulic Lime
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Hydraulic Limes set under water. To produce hydraulic lime mortars, the lime isderived from limestone containing impurities.
Non-hydraulic limes need air to carbonate & therefore set. A non hydraulic lime
can be produced from high purity calcium limestones.
Introduction to Hydraulic & Non-Hydraulic Lime
High calcium limes are derived from 95-100% pure CaCO3.
Magensian limes are derived from limestone a 5-40% content of magnesium carbonate.
Dolomitic limes contain over 40% magnesium carbonate.
Definitions:
Magnified calcium
carbonate crystals
Limestone quarry in Brnny, Norway
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Dolomite is formed when water containing dissolved magnesium seeps into sediments that are
largely calcium carbonate. The magnesium then replaces some of the calcium..
Non-hydraulic lime is
produced by first heating of
sufficiently pure limestone
(CaCO3) to between 954 &
1066C, driving off CO2
, to
produce quicklime (CaO).
This process of heating
limestone so that it
decomposes to lime & CO2 is
called calcination. This is
done in a lime kiln.
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Early Lime kilns
A common feature of early kilns was an egg-cup shaped burning chamber, with
an air inlet at the base (the "eye"), constructed of brick. Limestone was crushed (often by hand) to fairly uniform 20-60mm lumps.
Successive dome-shaped layers of
coal & limestone were built up in the
kiln on grate bars across the eye.
When loading was complete, the kiln
was kindled at the bottom, & the fire
gradually spread upwards through the
charge.
When burnt through, the lime wascooled & raked out through the base.
Fine coal ash dropped out & was
rejected.
Only lump stone could be used, because the charge needed to "breathe" during
firing. This also limited the size of kilns.
Above a certain diameter, the half-burned charge would be likely to collapse
under its own weight, extinguishing the fire. So kilns always made 25-30 tonnes of
lime in a batch.
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Typically a kiln took 1 day to load,
3 days to fire, 2 days to cool and 1day to unload. So a 1 week
turnaround was normal.
The degree of burning was
controlled by trial & error from batch
to batch by varying the amount of
fuel used.
Because there were large
temperature differences between
the centre of the charge & the
material close to the wall, a mixture
of under-burned, well-burned &
dead-burned lime was normally
produced.
Early Lime kilns
Typical fuel efficiency was low, with 0.5 tonnes or more of coal being used per tonneof finished lime (15 MJ/kg).
A preserved lime kiln in Burgess Park, London
Once the quicklime has been taken out of the kiln, it must be slaked,
which is the action of combining moisture with the quicklime.
When quicklime & water are combined, an exothermic reaction takes place
as the quicklime begins to swell & crumble away, producing a calcium
hydroxide.
Lime Slaking
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Lime putties that are run directly from quicklime areconsidered better when applications require superior
plasticity & carbonation.
Also, as it is in a wet state there is a greatly reduced risk
of the material carbonating.
If slaking is done with a minimum amount of
water, the quicklime breaks down to a powder,or a hydrated lime (Ca(OH)2).
If slaking is done with an excess of water, the
material can be worked down to form milk of
lime. This milk of lime, over time, will settle out
& a lime putty will be formed.
The material continues to slake, & the
particles break down, dissolve, & then
precipitate, causing the particle sizes to
become finer & richer.
Lime Slaking
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Left to Right: Natural hydraulic Lime, Lime putty, Ash, Terracotta (clay), clean Sand/Gravel
Do not confuse the terms hydraulic and hydrated:
Hydrated lime is any lime other than quicklime. Thus, hydrated lime can refer to either hydraulic (hardens underwater) or non-
hydraulic (doesn't harden underwater) lime.
Stored lime putty is always non-hydraulic & as the name suggests, lime putty is
in the form of a putty made from just lime & water.
Non-hydraulic lime mortar sets/hardens by reaction with atmospheric CO2(carbonation).
Natural Hydraulic Lime mortars set due to both their hydraulic properties &
carbonation (the amount of hydraulic set v.s. carbonation is dependant on the
degree of hydraulicity).
If the mortar is kept from air (either stored in containers, or mounded up &
covered with damp hessian & plastic), it can be stored for extended periods
of time.
Setting of hydraulic & non-hydraulic limes
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Setting of hydraulic & non-hydraulic limes
After the lime mortar has been applied (through repointing, laying, plastering, or
other uses), CO2 dissolves in the water to form carbonic acid (H2CO3), which
combines with the dissolved Ca(OH)2 to form CaCO3.
In simple terms; Ca(OH)2 + CO2 CaCO3 + H2O
It is very important that the mortar does not dry too quickly, which could inhibit the
absorption of this carbonic acid, but it is also important that the mortar is not
oversaturated with water, which could also inhibit carbonation.
The setting process is much slower than in (OPC). Depending on the thickness of
the mortar & climate conditions, an initial skin hardening may take from a couple of
hours to several days to gain an initial hardness. The full drying & hardeningprocess can continue, for many years (at a slower rate) with the mortar continuing
to gain strength.
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.
Properties of lime mortar
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Under cracking conditions, OPC breaks, whereas lime often produces
numerous micro-cracks if the amount of movement is small. These micro-cracks then re-crystallise on exposure to air, effectively self-healing.
Properties of Lime Mortar
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Historic buildings are frequently constructed with relatively soft masonry
units, & minor movement in such buildings is quite common due to thenature of the foundations.
Properties of Lime Mortar
This movement breaks the weakest part
of the wall, & with OPC mortar this is
usually the masonry. When lime mortar is
used, the lime is the weaker element, &
the mortar cracks in preference to the
masonry. This results in much lessdamage, & is relatively simple to repair.
Properties of Lime Mortar
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Badly eroded cement mortar pointing
Pointing is the name given to the sacrificial
mortar that fills the first 20mm of the vertical &horizontal gap around the bricks.
The pointing will eventually erode & fall off
requiring repointing. If the mortar pointing is not
replaced, the mortar joints keeping the bricks
apart will deteriorate & eventually the bricks will
start to become loose as illustrated in photo
opposite.
If your brick is 50 years old or less, you can
probably repoint it safely with modern OPC
based mortar. But if your house was built
before World War II, the mortar is likely a mix oflime putty & sand, and you should try to match
it.
Mortar pointing
Lime mortar is more porous than
cement mortars, & 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 & thus saving
the masonry. Cement on the other hand evaporates
water less than soft brick, so damp
issues are liable to cause salt formation
on brick surfaces & consequent
disintegration of bricks. This damp
evaporation ability is widely referred to
as 'breathability'.
Properties of Lime Mortar
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Example: The soft stone of the
house has been eroded away
because the hard cement pointing
has not allowed moisture to
evaporate through the mortar. The
only way the moisture can escape is
through the stone itself. This has led
to frost damage over a period of time
& is always worse lower down the
wall (due to rising moisture & no
damp proof course).
Using lime mortar for repointing
will allow the bricks to dry out. Lime
mortar will tend to suck moisture outof the brick & in turn the air will dry
out the mortar.
Hydraulic lime
In the context of lime or cement, the term
'hydraulic' means to 'harden under water'.
Hydraulic lime can be considered, in terms
both of properties & manufacture, as part-way
between non-hydraulic lime & OPC.
The limestone used contains sufficient
quantities of clay and/or silica.
The resultant product will contain dicalciumsilicate (Belite) but unlike OPC not tricalcium
silicate (Alite).
It is slaked enough to convert the CaO to Ca(OH)2 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 & magnesium also produce a hydraulic set, & some pozzolans contain
these elements.
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Researchers have found that construction workers in ancient China ( 1,500 years ago)developed sticky-rice mortar by mixing sticky rice soup with slaked lime. The paste was used to
bind & fill gaps between bricks & stone blocks. The mortar was stronger & more resistant to
water than pure lime mortar.
Researchers identified amylopectin, a soluble type of polysaccharide (complex carbohydrate
found in rice & other starchy foods), as the 'secret ingredient'.
Sticky-rice mortar was probably the world's first composite mortar, made with both inorganic
(CaCO3) & organic (amylopectin) components.
It was found that amylopectin in the mortar acted as an inhibitor: the growth of the CaCO3crystal was controlled, & a compact microstructure was produced, which should be the cause of
the good performance of this kind of mortar.
The great wall of china
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