Lime and Cement Block 1

8/12/2019 Lime and Cement Block 1

1/14

&

&

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


2/14

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


3/14

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.


4/14

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.


5/14

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


6/14

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

&

&


7/14

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


8/14

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


9/14

&

&

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


10/14

Historic buildings are frequently constructed with relatively soft masonry

units, & minor movement in such buildings is quite common due to thenature of the foundations.


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.



11/14

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'.



12/14

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.


13/14

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


14/14

Lime and Cement Block 1

Documents

Transcript of Lime and Cement Block 1