12/09/2016

1

Present-day lime binders

September 2016

Prof Phil Banfill P.F.G.Banfill@hw.ac.uk

Talk overview

1. What is lime mortar?

2. Why use lime?

3. Traditional vs modern production

4. Selected properties and results

5. Conclusions

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Background to lime mortar 1

• Burn limestone: CaCO3 → CaO + CO2

• Slake the quicklime: CaO + H2O → Ca(OH)2

• Mix with sand (either hot or cold, dry or as ‘putty’)

• Mortar sets by drying and by reaction with air (i.e. CO2) or water

to complete the ‘lime cycle’

CaCO3

limestone

CaO

quicklime

Ca(OH)2

lime putty

firing

slaking

carbonation

Background to lime mortar 2

• ‘Air lime’ (CL90) is made from high purity limestone and hardens

only with CO2: Ca(OH)2 + CO2 → CaCO3. Also known as hydrated

lime, building lime, fat lime, lean lime and non-hydraulic lime

• ‘Hydraulic lime’ is a blend of CL90 with pozzolanic material,

hardens by reaction between Ca(OH)2 and SiO2 in pozzolan

• ‘Natural hydraulic lime’ (NHL2, NHL3.5, NHL5) is made from

limestone with silicon or clay impurities which form C2S in the

kiln. It hardens both by carbonation by CO2 and by hydraulic

reaction: C2S + H2O → C-S-H. Also known (in the past) as feebly,

moderately and eminently hydraulic lime.

• ‘Hybrid limes’ are NHLs with added pozzolan or cement to

increase strength (e.g. NHL3.5-Z).

• ‘Lime mortar’ also commonly describes a cement mortar

containing some hydrated lime.

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Issues driving current interest in limes

• Environmental: binder has lower embodied energy / CO2

emissions than cement due to lower burning temperature

• Technical: allows masonry to breathe by permitting passage of water vapour

• Structural: weaker/softer mortar allows masonry to accommodate movement/settlement without cracking. Expansion joints not needed.

• Heritage: repairs in cement-based mortar have caused extensive damage to masonry, so the conservation sector encourages use of CL90 or NHLs.

Embodied energy of production

This shows the

effect of burning

temperature

kWh/tonne

of binder

120

60

0

cement

CL90

NHL2:CL90 25:75

NHL5NHL3.5

Data from

www.stastier.co.uk

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CO2 emissions of binders

As produced After carbonation

in service

kgCO2/tonne

of binder

900

600

300

cement

CL90NHL2:CL90 25:75

NHL5NHL3.5

uncarb.

carb.

CO2 emissions of mortars (1:3 by volume)

After carbonation in service

kgCO2/tonne

of mortar

150

75

0

cement

NHL2:CL90 25:75

NHL5

NHL3.5

cement:CL90 1:1:6

Calculations are not

simple because of

density differences

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Porous mortar lets walls breathe

Title

Lime mortar is flexible

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Damage to masonry

• text

Traditional limes 1

• A locally produced material – limestone, coal, wood

• Using a range of kiln types – clamp, chamber, flare kilns

• Hand packed material – sizes graded to permit air flow

• Experimental replication difficult at suitable scale (20 tonnes)

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Traditional limes 2

• A wide range of raw materials in use, sourced locally

• Heterogeneous product

• Unburnt material always present – can be useful for archaeological / forensic investigation

• Product quality achieved by application of processing and experience of the work force.

Charlestown Lime Works, Fife

Modern limes

• Use pure high calcium resources

• Centralised production, with output tailored to major users – iron/steel and agriculture

• Product consistency is achieved by quality assurance and processing, eg. grinding

• Universal, easy to use products are preferred, eg. ready-mixed mortar.

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Portland

cement

Manufacture

Firing

temp

°C

0

1500

50 75 100 %CaCO3

50 25 0 %clay

NHLs CL90

Natural

cement

Composition (schematic) (Avenier et al, 2007)

There is always some unburnt (inert) material in the product

100

0

Content

%

Increasing clay content

Ca(OH)2

C2S

C3S

aluminatesunburnt

other phases

CL NHL2 NHL3.5 NHL5 natural

cement

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Cementation Index

�� =2.8�� + 1.1 �� + 0.7���

�� + 1.4��

CI = 0 – 0.3 CL90, air lime, hydrated lime

CI = 0.3 – 0.9 NHL2 – NHL5

CI > 0.9 Natural / Roman cement

Strength classes (2, 3.5, 5) refer to a standard (1:1.5) mortar mix.

Actual mortar strengths at 28 days are about half the standard

but the high C2S content means that strength continues to

increase by 3x up to one year.

Example compound composition

St Astier – data from www.stastier.co.uk

NHL2 NHL3.5 NHL5

C2S 17 35 43

C3A 0.4 0.5 0.7

C4AF 0.4 0.5 0.7

C2AS 0.8 1.0 1.3

CaSO4 0.5 0.8 0.7

Free Ca(OH)2 58 25 22

Insoluble 8 9.6 5.6

Unburnt CaCO3 13 25 23

C.I. 0.29 0.62 0.74

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Fresh properties

• Site workers adjust water content to get desired rheology.

• W/B ratio needed for equivalent rheology covers 2-fold range.

• NHL3.5: Otterbein needs 40-60% higher W/B than SingBirch,

so strength is potentially halved. Also other hardened

properties – behaves more like a NHL2?

Yie

ld s

tre

ss

Banfill & Shimizu 2016

Water retention – fresh mortar

• Mortar loses water to an absorbent substrate and this may

be detrimental to adhesion and durability in service.

• Collier et al, 2007:�

��= �

��+ �

��

A = transfer sorptivity, R = desorptivity of mortar, S = sorptivity of substrate

• Desorptivity increases with hydraulicity. 1:3 CL90:sand has

excellent water-retaining characteristics. Cement:sand’s

desorptivity is 4x higher.

• Desorptivity increases with W/B ratio.

• All this supports practical experience.

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Bond strength

• Bond strength governs the ability of masonry to resist lateral

or eccentric loading

• Barr et al, 2015: Bond-wrench test on sandstone masonry

• Bond strength is proportional to compressive strength

• Bond strength of NHL3.5 and NHL5 mortar is similar to that

of low strength cement mortar

• Bond strength is reduced by pre-wetting the stone

Carbonation

Carbonation rate depends on porosity, decreases sharply with

increasing RH but is independent of CO2 pressure. (Van Balen, Van

Gemert, 1994, 2005)

In practical mortars carbonation rate at 20°C/90%RH:

Very low Cement/CL90Low NHL5 1:2, 1:3.

NHL3.5 1:1, 1:1.5, 1:2.NHL3.5 1:3 with CL90, slag, FA, MK or MS

Moderate NHL3 1:4NHL3.5 1:2.5, 1:3NHL3.5 1:3 with brickdust or natural pozzolanaNHL5 1:4CL90 with 30% MK

High NHL2 1:2, 1:3NHL3.5 1:4, 1:6CL90 with 10% MK

Very high CL90 (Allen et al, 2003)

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MicrostructuresSEM of NHL3.5 paste before … and after carbonation at 97%RH

CSH

Ca(OH)2

CaCO3

CaCO3

Ca(OH)2

El-Turki et al, 2009

Ageing of lime putty (Mascolo et al, 2010)

Continuous dissolution and

recrystallization of

portlandite:

• PrismaNc → platelike

• Reduces total surface

energy

• Broadens particle size

distribution

• Improves rheology

“Age lime putty for >36

months” (Roman Building

Laws)

3 months

66 months

66 months

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Durability and lime leaching

1. Leaching calcium from the binder

reduces strength, increases

porosity and sorptivity.

2. Resistance to leaching increases

(CL90)<NHL2<NHL3.5<NHL5 and

uncarbonated<carbonated.

3. Possible implications from climate

change and moisture build-up in

dwellings.

Forster et al, 2014, 2016

Conclusions

1. There is plenty of interesting science in lime

binders.

2. Strong links to the cultural heritage field – history,

archaeology.

3. It’s a challenge to apply science to a craft

technology.

4. And also to work within a suitable standardisation

framework.

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Thank you for listening

Contact details:Prof Phil Banfill - P.F.G.Banfill@hw.ac.uk

School of Energy, Geoscience, Infrastructure & Society, Heriot-Watt University, Edinburgh, EH14 4AS, UK