12/09/2016
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Present-day lime binders
September 2016
Prof Phil Banfill [email protected]
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 - [email protected]
School of Energy, Geoscience, Infrastructure & Society, Heriot-Watt University, Edinburgh, EH14 4AS, UK