This presentation was delivered as part of a two day Lime Kiln repairs and “Hot” lime mix Workshop on the 1st & 2nd May 2013, at Russborough House, near Blessington, Co. Wicklow. Feedback and comment from the discussions and experiences shared during the two day workshop have been added to the presentation. In line with the aims and objectives of The Building Limes Forum, the purpose of the training day and this presentation was to share understanding and encourage expertise in the use of building limes.

1. Hot Lime Mortars
Building Limes Forum Ireland
Russborough Lime Kiln
1st & 2nd May 2013

2. Quicklime
• Calcium Oxide: CaO (CaCO3 - CO2)
• Has an affinity for water: will take water from the air, from
damp sand, or from mix water.
• When in contact with water, will;
– Produce heat (quite a lot!)
– Expand (approx. twice the volume)
www.frewconservation.co.uk

3. The ‘hot lime’ method
• Where quicklime is slaked while mixing the sand and water.
• The mortar can be applied ‘hot’ whilst still slaking, still warm
or applied cold.
• Was typically used ‘hot’ for masonry bedding, grouting and
sometimes harling.
www.frewconservation.co.uk

4. The ‘hot lime’ method
• This technology (hot lime mix) was used in masonry mortar, where
the expansion of lime between the stones or brick improved the
bond between them and consequently produced high-strength
masonry (Margalha G 2011)
• Hot mixed mortars possess a better bond between the aggregate
and binder as a result of etching of the surface of the aggregate
grains (Jedrzejewska, 1967)
• The hot mixing method had been used in the past to produce high
strength and durable lime mortars (Callebaut et. al. 2000)
www.frewconservation.co.uk

5. Slaking lime with aggregate
• Where quicklime was covered over with damp sand and left
slake to a dry hydrate lime: sand mix prior to mixing with
water. Often this was left overnight or longer.
• The resultant ‘dry’ mortar could then easily be screened to
remove unburnt /overburnt lumps and oversized aggregates.
• Was typically used for brickwork, finer stonework and
plasters/ renders.
www.frewconservation.co.uk

6. Slaking lime with aggregate
• Slaking lime with wet sand was also used in render and plaster
mortars. Here the mortar was only applied after 3-30 days of
maturation to allow complete hydration of the hot lime….they
(Portugese masons) say that the effect of this….was to increase
strength due to better bonding of the lime and sand grains…
(Margalha G 2011)
• Experiments on a medieval castle in Sweden using quicklime and
wet sand mixes for renders showed higher mechanical strength and
lower porosity after 1 year compared with a lime putty mortar…
(Margalha G 2011)
www.frewconservation.co.uk

7. ‘Perceived’ advantages of
‘hot lime’ mortars
• Workability: nice and sticky particularly for impervious stones.
• Ease of mixing: can mix quite wet as mix stiffens quickly.
• Cost: quicklime is relatively cheap and approximately doubles in volume when slaked
(i.e. 1 part quicklime = 2 parts putty).
• Strength: good mechanical bond strength between the sand and binder due to hot
caustic lime etching the surface of the sand..
• Pore structure: generation of steam during slaking affects pore structure and
connectivity, acting as an ‘air entrainer’, possibly leading to improved frost resistance.
• If hydraulic components are present, the initial strength development will be greater as
the rate of hydration increases as temperature increases.
www.frewconservation.co.uk

8. Hot limes in cold weather
From Holmes S (1993) Hot limes in a cold climate, Lime News
www.frewconservation.co.uk

9. Hot lime grouting
Nicholson P (1823) The New Practical Builder…
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10. Russborough Lime Kiln
mortar sample
The presence of white rounded lime lumps (lime
inclusions) which can be associated with the hot
lime mixing method (Leslie & Gibbons 2000)
www.frewconservation.co.uk

11. Russborough Lime Kiln
mortar sample
Analysis of two samples of mortar from Russborough
Lime Kiln was carried out by Hugh Dorrian of
Stoneware Studios (SS).
Further analysis of a mortar sample was carried out,
using thin section and petrographic examination, by Bill
Revie of Construction Materials Consultants (CMC).
There were slight variations between the three
samples analysed but generally, all were identified as
having been mixed and placed as ‘hot lime’ mortars.
There were no specific hydraulic components
identified, however the presence of ash and other kiln
by-products may have altered the binder slightly.
Images from CMC
www.frewconservation.co.uk

12. Russborough Lime Kiln
mortar sample
Components
Total
Quicklime
Effective Lime
in binder
Quartz 28.1 28.1
Chert 0.7 0.7
Quartzite 1.8 1.8
Sandstone 2.7 2.7
Shale 5.0 5.0
Limestone Particles 7.2 7.2
Burnt Particles 3.8
Lime inclusions 9.6
Total Aggregate 45.5 58.9
Burnt Particles 3.8
Lime inclusions 9.6
Binder 41.1 41.1
Total Binder 54.5 41.1
Secondary minerals (Calcite) 0 0
Total Constituents 100 100
voids 15.1 15.1
Cracks 1.6 1.6
Cracks/Voids 16.7 16.7
Binder : Aggregate Ratio 1.0 : 0.83 1.0 : 1.43
Determination of mix composition by
modal analysis (from point count on
thin section).
The results are reported both where
the total proportion of the quicklime is
taken as binder and where the
effective binder is taken as the fully
slaked quicklime, i.e. putty in the mix.
In the latter case the burnt lime and
inclusions are counted as aggregate
particles, as this is the function they
are performing in the mix.
Table from CMC
www.frewconservation.co.uk

13. Russborough Lime Kiln
mortar sample
www.frewconservation.co.uk
Image from CMC

14. Historic hot lime (Russborough Lime Kiln) CL90 lime putty: sand mortar
NHL3.5: sand mortar OPC: sand mortar
www.frewconservation.co.uk
All images from CMC

15. Hot lime mortars
The Scottish experience
• The following examples are provided to illustrate a range of situations where hot lime mortars
may be well suited.
• These projects were undertaken during the authors time working with the Scottish Lime Centre
Trust (SLCT), Laing Traditional Masonry Group (LTM) and more recently Craig Frew Building
Conservation (CFBC).
• In all but one example, they illustrate completed work.
• Most projects are located in moderately or severely exposed sites.
• The mortar mix specifications are indicative and building specific. It should not be inferred that
similar mixes will necessarily be suitable for other buildings.
• The variation in the mix ratios between the following examples is to achieve optimum workability
using a range of sands with different grading and ‘sharpness’. In some cases mix ratios are
adjusted to alter the mortar performance characteristics.
www.frewconservation.co.uk

16. Lime re-harling (2008-9)
Craigievar Castle, Aberdeenshire
Flushing out (used hot):
2 parts Roundtower NHL 5 :
1 part non-hydraulic quicklime:
6 parts sharp sand (used hot)
Tack/pricking up coat (used hot):
2 parts Roundtower NHL 5:
1 part non-hydraulic quicklime:
7 parts sharp sand (used hot)
Finish harling (used cold):
2 parts Roundtower NHL 5:
1 part non hydraulic quicklime:
8 parts sharp sand
N.B. all specifications are indicative and building specific
www.frewconservation.co.uk

17. Lime re-pointing (2013-14)
Drum Tower, Aberdeenshire
Rubble pointing mortar
(to be used hot)
2 parts NHL 5:
1 part non hydraulic quicklime:
8 parts sharp sand (used hot)
Parapet wall pointing:
1 part NHL 5:
2 parts sharp sand:
Magnesium Stearate waterproofer
N.B. all specifications are indicative and building specific
www.frewconservation.co.uk

18. Consolidation & re-pointing (2004)
Ardvreck Castle & Calda House, N/W Sutherland
Pointing mortar (used hot):
2 parts St Astier NHL 5:
1 part non hydraulic quicklime:
10 parts sharp sand
Wallhead/ rough-racking mortar:
1 part St Astier NHL 5:
2.5 parts sharp sand
N.B. all specifications are indicative and building specific
www.frewconservation.co.uk

19. New-build(2009-10)
Private Chapel, Scottish Highlands
Photos courtesy of LTM Group
N.B. all specifications are indicative and building specific
Concrete blockwork construction:
1 part Singleton Birch NHL2
2.5 parts sharp sand
Granite masonry construction (used hot):
1 part Singleton Birch NHL3.5 quicklime
3 parts sharp sand
www.frewconservation.co.uk
Photos courtesy of LTM Group

20. Lime re-harling (2002)
Scalloway Old Haa, Shetland
Pointing and harling mortar (used hot):
1 part St Astier NHL 5:
1 part non hydraulic quicklime:
4 parts sharp sand
0.5 parts crushed shell N.B. all specifications are indicative and building specific
www.frewconservation.co.uk

21. Situations where ‘hot limes’ (in a form we would
manufacture them today) may not be suitable
• Coping stone pointing
• Open wallhead pointing
• Rough racking
• Plasters and flat renders
• Fine brickwork and ashlar pointing
www.frewconservation.co.uk

22. Situations where ‘hot limes’ (in a form we would
manufacture them today) could be beneficial
• Building rubble masonry walling
• Pointing rubble masonry
• Harling rubble masonry
The benefits are greater when working with hard impervious stones.
www.frewconservation.co.uk

23. Russborough Lime Kiln repair
summary and notes from the training days
www.frewconservation.co.uk

24. Russborough Lime Kiln repair
Pat McAfee’s notes on mortar preparation…
• The mix was 1 part NHL5 (St Astier) : 1 part quicklime (Clogrennane powder) : 5 parts coarse sand (Co
Wexford) : 0.5 parts limestone dust. Gauging was by (level) bucket, and mixed on boards by hand.
• The above materials were mixed dry and then water added. Other than some steam the mix did not
behave violently as occurs when using lump quicklime.
• The mix was slightly over-wetted which facilitated easier mixing; by the time the mix was loaded in the
barrow it was the right consistency.
• A large wide, solid bladed hoe caused splashing of participants on more than one occasion and had to be
taken out of use. It occurred when the hoe was being pushed rather than pulled. Some traditional mortar
hoes have two holes in the blade which would have partly alleviated this problem. Mixing by shovel
caused no problems.
• The powdered quicklime heated quicker and produced a faster mix than the small sample bucket of
pelleted (7-8mm granules) quicklime.
• The powdered quicklime became dustier further down the sack and caused a slight hazard when being
extracted and being mixed dry.
www.frewconservation.co.uk

25. The masons view…..
(Pat McAfee)
• The hot lime mix produced cleaner work with no runs of mortar down the face of
the work
• Produced solid, full joints and a better quality of work
• Allowed the wall to be built higher than normal without leaks occurring down the
face of the work and mortar squeezing out through overhead weight
• Increased production
• Prevented slumping of joints which then have to be tidied up
www.frewconservation.co.uk

26. The masons view…..
(Pat McAfee)
• Prevented leaching of lime from the mortar leading to weaker mix at the face of
the work
Also, in situations where the stones and sand are wet…
• Allow wet stones to be laid and stabilised without their subsequent movement; a
common problem with most mortars that are used cold.
• Very wet sand could be used without adverse effects on the final consistency of
the mix because of the massive absorption of water by the quicklime.
www.frewconservation.co.uk

27. Moving forward…..
• Feedback from masons on the use of hot lime mortar was positive.
• When compared with the quicklime ‘dust’, the quicklime granules appeared slower to react.
Further investigation, discussions with supplier, and possibly tests required to ascertain
whether the single bucket of 7/8mm granules was representative.
• In terms of site H&S, the granular quicklime with less dust would be advantageous – this is
the most commonly used form in Scotland for site mixing.
• The granular quicklime requires to be tested against the building lime standard. The supplier
is confident that their product will meet the necessary requirements.
• On-going monitoring of the works at Russborough Lime Kiln required to assess performance.
• Comparative testing of hot limes required against alternative lime mixes inc. NHL: sand mixes,
to provide a ‘benchmark’ for specifiers and users.
www.frewconservation.co.uk

28. For assistance with their technical input in compiling
this presentation and for use of photographs
Thank-you to…
Stoneware Studios http://www.stonewarestudios.com
Construction Materials Consultants http://www.cmcstirling.co.uk
Pat McAfee http://homepage.eircom.net/~mcafee
Laing Traditional Masonry Group http://www.ltm-group.co.uk
Scottish Lime Centre Trust http://www.scotlime.org
http://www.frewconservation.co.uk

29. Russborough Lime Kiln and Hot Mix Workshop
And a BIG thank-you to…
Russborough House http://www.russboroughhouse.ie/
Building Limes Forum Ireland http://www.buildinglimesforumireland.com/
The Traditional Lime Company http://www.traditionallime.com/
Clogrennane Lime Works http://www.irishlime.com/
Wexford beach pebble http://www.wexfordbeachpebble.com/
Tallis and Company Ltd