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Questions & Answers
Below is a selection of queries received by the BLF over the years and answers from correspondents. The answers are those of the correspondent and not those of the BLF. Information about lime technology is growing and changing, so an answer given below might not be the answer that would be given today.
Most importantly, information about materials and their application should not be included in specifications without taking into account the appropriate use for location, quality of workmanship, climate, type of building, method of construction, the effect on adjacent materials and detailing etc. The BLF recommends professional advice is sought particularly for answers to site-specific queries. The Building Limes Forum has no commercial ties to trade or professions specializing in the manufacture, supply or use of lime and therefore does not recommend suppliers, craftsmen or professionals. See Sources of Information for websites that do supply some of this information. You may also find it helpful to refer to the Bookstall and Courses in Lime on this website which may well provide the source of further relevant information and answers to your questions.
- Lime Production and Supply
- Standards, Codes and Building Regulations
- Masonry Construction
- Floor Construction
- External Finishes
- Internal Finishes
- Historical Reference
If you have a question, or can provide other answers, please email us.
Lime Production and Supply
Aged Lime
Q What age can you buy lime in the UK? In Italy you can buy lime 3 yrs old plus (the older the better). C
A It is possible to get lime putty up to five years old in the UK from specialist suppliers. Most people who use it professionally, however, either slake it or buy it young (and cheaper) and age it themselves. Paying other people to store it for five years is not very good economics - only recommended in emergencies! IB
A Collecting lime putty and keeping it for 5 years is not really the handy way for getting an old one. The dealers are storing hundreds of kilos of lime putty under good conditions, and it's safer for children if you buy it when you need it. OL
The Cost of Lime
Q I notice, at B&Q, that the price of a 25kg bag of Ordinary Portland Cement is £2.48 and of Hydrated Lime is £5.81. Is not the manufacture simpler and energy cost lower in the case of the lime? Why is it more than double the price? BF
A It's simply a matter of the different scales of production - the sheer quantity of cement produced by the major manufacturers and the scale of their production facilities and distribution networks. Of course, the more people use lime, the less difference there will be...we hope. IB
Standards, Codes and Building Regulations
Code of Practice
Q Is there a Code of Practice on the way? There are
rumours of one. Strength and strength-gain characteristics are important
to the engineering community and perhaps to Building Control. T
A The Forum has been in contact with the British Standards
Institute regarding the updating of standards to include hydraulic lime,
but the timescale for producing such documents is long, especially as
they now need to be agreed Europe-wide. We hope to be able to prepare
and issue a "Technical Note" covering the subject to be appended
nationally in the UK, as this is quicker, and ensures the subject is considered
when the next round of EU Norm reviews happens, but, again, this is some
way off at present.
The best I can say is that the rumours you have heard are correct, but actually producing the documents is a frustratingly long process. We are working on it! IB
Building Control Officers & Lime Mortars
Q Has anyone persuaded BCO's who insist on adding waterproofer to re-pointing (such as Sika1) that this is unwise in a stone/lime mortar building? We are currently restoring/ converting a granite random stone/lime mortar stable block. Because it has change of use dated 2003 then the Building Regs for that date apply, currently the BCO is insisting on waterproofing the pointing and using the dreaded Portland cement, which will create huge condensation problems internally. Is there any written advice I can point him towards other than the English Heritage guidance on Part L that may persuade him this is NOT a Good Idea? The building is not listed and not in a conservation area nor is it noted as being of specific architectural interest so the BCO just views it like any other re-build. We intend to re-point with an H2 hydraulic lime but the BCO won't agree to this. T
A It sounds like you are dealing with a BCO who simply doesn't understand the materials as well as you do. It is often helpful to arrange a meeting on site between the BCO and the materials supplier you propose in order to allow them to discuss the matter between them. The BCO is more likely to accept their advice/information about their product directly than through somebody who is not obviously an expert. BCO's do have to deal with some real fools sometimes, and he probably needs reassurance that you know what you are talking about. Although your building is not listed or in a conservation area, it might be worth talking to your local conservation officer too, who may well be aware of very similar work carried out locally, and should know the issues. Failing that, you may have to pay for expert advice from a consultant (who is often a supplier anyway in this instance!), but I would hope that is not actually necessary. It's worth fighting your corner, as you are right about the bad effects of the BCO's proposals. Take it up to his superior if you have to! Don't be tempted to use too weak a hydraulic lime, though: granite is mechanically and physically strong, and effectively impervious, but can be degraded chemically by too much free lime, which turns the mica (I think: or is it the feldspar?) slowly into kaolin. A long-term problem, and perhaps not all that significant, but worth bearing in mind all the same. IB
A A building control officer has no legal right to request this kind of information. Building control remit does not extend to items as basic as pointing repairs. The BS on masonry is full of information about the correct types of materials to use for new construction and they clearly state the most appropriate mortars for various types of materials, they also show the higher the free lime content of a cement mix the more effective they are in excluding rain from buildings, that's just and aside, tell the Building Control officer to show the client his remit and where it states they have any jurisdiction in this matter, they don't. DJ
Masonry Construction
Lime in New Build
Q Lime mortar for new build: where is it at now? T
A The Foresight Project in the UK has produced the book
"Hydraulic Lime Mortar for Stone, Brick and Block Masonry",
published by Donhead in 2003, which is probably the best reference on
the subject to date - you can order it through the Bookstall
page on this site. The Foresight team are continuing their research, and
are working on structural uses and behaviour of hydraulic lime mortars,
but this is not yet complete. IB
Lime Concrete
Q I am designing for a hurricane area for clients who has requested that I find a "healthy" concrete (they don't like regular concrete). The building system will be a hollow block with fiberglass rebar and concrete infill. The walls will be placed on a monolithic concrete slab foundation. Is it possible to use lime concrete for such structural demands? Is lime concrete more human-nervous-system-friendly than regular concrete? MB
A We produced some simple concrete mix designs for a
customer based on bags of NHL 3.5 and 5. Taking a normal concrete mix
of 2 parts stone to 1 part sand we checked the voidage, as per normal
mortar design. This resulted in a mix of 1 bag NHL to 10 litres of sand
and 20 litres of gravel, this would vary depending on the particular sand
or stone. The water demand for a normal concrete consistency was only
slightly higher than would be required for a cement concrete, i.e. around
water: lime ratio of 0.6 giving a density of 2220 kg/m3. Strengths were
as predicted from mortar adjusted for the water content at:
NHL 3.5 @ 28 days = 5 N/mm2; @ 56 days = 7 N/mm2; at 91 days = 9 N/mm2;
and a 1 year estimated 12 N/mm2.
NHL 5 @ 28 days = 7 N/mm2; @ 56 days = 11 N/mm2; at 91 days = 13 N/mm2;
and a 1 year estimated 16 N/mm2.
Using a similar design method it should be possible to arrive at a mix
suitable for the engineering requirements of this, or any other, job.
PL
Hydraulic Lime and Pulversied Fuel Ash
Q We are carrying out works to Barry Port Hydraulic Pumphouse. Analysis of the very hard brick pointing suggests lime based with a coal derived fuel ash of some description. The use (harbour works), date (1890), area (all coal powered), location (next to the S Glamorgan Blue Lias quarries) suggests a Hydraulic lime and 'fly ash' or PFA. No other aggregate was found. Has anyone experience of such mixes?
A It was common practice in Victorian times to use ‘black ash’ mortars in urban and industrial areas. These incorporated ash from factory boilers saving the cost of dumping and replacing at least some of the sand required. The ash, often in the form of large clinker that had to be crushed, had a certain pozzolanic activity and the hydraulicity of the lime would depend on the location. In this query it is probable that it was hydraulic whereas in my main experience from the Manchester / Liverpool area the lime was purer and had only a little hydraulicity. Current equivalent would be a NHL 3.5 / Fly ash / sand mix although the use of Metakaolin instead of fly ash would speed up set and strength gain thereby reducing protection periods in tidal exposure situations. Castle research shows the pozzolanic strength of fly ash as being above that of brick dust and natural pozzolana but below those of granulated slag, Metakaolin and silica fume. Paul Livesey, Technical Manager, Castle Cement Ltd.
Coloured Mortar
Q Anyone know how the pink mortar colour was derived? It was probably a natural feature due to local materials and procurement methods but I am finding it difficult to match existing with new work. I have tried brick dust, ash from coal and wood, soil and blood ! We also have limewash which is a dark crimson colour and believed to be a feature to West Wales only. C
A You are probably correct in saying that the original colour may have derived from natural materials, particularly the sand. This the best way of achieving the colour if you can find a sand but it would be a mistake to choose a sand just for its colour - its grading is much more important. Another suggestion is to use a natural pigment - possibly red oxide. This could either be mixed in with the mortar or 'painted' on afterwards. But ... Technical Advisory Note 1, written for Historic Scotland by The Scottish Lime Centre (available through the Bookstall) contains a comment that there is evidence that using pigment mixed in the mortar may affect the pore structure of the mortar. Also obtaining a consistent colour may be difficult and requires very thorough mixing - but then variation in colour is a feature of natural materials. RJS
Floor Construction
Lime for Bedding
Q I have used a specified mix of 3 parts sharp sand
(with no less than 40% quartz chippings) to one part Hydraulic lime on
a medieval building in Oxford and have come up against a problem with
the floor.
After 2 years of the floor being laid, the stone floor slabs have become
loose and unbonded and the mixture beneath is soft and crumbling. The
floor has been subject to under-floor heating and I am assuming this would
be the probable cause for the breakdown in the bedding. The floor was
laid (40 - 50mm thick bedding) and the specification was to prevent heat
being introduced for 7 days, and then to gradually introduce heat after
the 7-day period but not to raise the heat above 19 degrees for the next
month to achieve initial set. I believe the floor was subjected to heat
sooner than the 7-day mark at temperatures higher than 19 degrees and
this allowed the lime mortar to dry out before carbonation took place,
affecting the integrity of the mixes strength. Since then I have carried
out some remedial work and was able to leave the heat off the floor for
approximately one month. Added to this I included the potzolan additive
Metastar to aide in the curing time because of the buildings need for
public use the month after the floor was laid.The remedial work has shown
no signs of movement and appears to have worked.
In a professional capacity, is it fair to say the heating is the cause
for the breakdown in the strength?
The client is thinking of investigating the mixture but because I have
changed the mix by adding metastar, do you think conclusions will be met
by testing the mix and what should be expected from the investigation
outcome?
Can they tell me if the mix was correctly mixed?
Can they tell me if the Lime was suitable in the mix?
Can they tell me if the lime has carbonated or not by testing it?
I would be grateful if somebody could explain what can be done to conclude
the mystery of why the mix has failed in this manner. JB
A It is not really possible to give definitive answers
to your questions without further investigations, but you are correct
in saying that the heat would cause excessively quick drying of lime and
thus prevent carbonation. However, there are one or two other points you
need to take into account:
Laying stone slabs over a lime bed will also prevent carbonation, as the
stone will only allow carbon dioxide to penetrate through the joints:
almost certainly not enough to permit carbonation of any more than a tiny
fraction of the bedding mortar. Assume therefore that an effective set
based on carbonation cannot in practice happen in this instance.
In this situation it is therefore correct to use a hydraulic lime, which
will set in the absence of CO2. You do not say which type of hydraulic
lime you use, but a 1:3 mix of hydraulic lime is slightly weak in itself.
Your addition of pozzolan will have increased the effective strength of
the hydraulic set by combining with the free lime which in this instance
is probably not carbonating anyway. Even with hydraulic lime, the longer
you can allow for curing the better, and the application of any artificial
heat is usually detrimental.
What can be discovered through testing depends on the tests applied, some
of which can be very sophisticated/expensive, especially when analysing
hydraulicity. Interpretation of results will still be necessary. IB
A Other possible causes of the problem encountered are:
Inadequate wetting of the surface onto which the mortar was laid and inadequate
wetting of the slabs prior to laying. It would normally be desirable to
thoroughly wet the whole base a few hours prior to laying with a further
wetting of smaller areas and the slabs immediately prior to laying. Without
this the suction of the dry base and slabs would cause the mortar to dry
out far too quickly leading to its failure. The application of heat would
accentuate this. Even with the wetting as above it may still be necessary
to further wet the slabs after laying.
How was the mortar mixed? If a drum mixer was used it is recommended that
20 minutes should be allowed to ensure a thorough mix; preferably mixing
the sand and lime dry before adding any water.
As to the application of heat, I would expect any within the 30 days after
laying to be detrimental, and it ideally should be left longer.
An excellent booklet is Technical Advice Note 1, 'Preparation and Use
of Lime Mortars' written by The Scottish Lime Centre for Historic Scotland
(available through the Bookstall). RJS
Lime Ash Floor
Q I am interested in learning about installing a lime ash floor on the upper story of my strawbale house in Vermont, USA. would the lime ash be durable with in-floor heat (hot water with oxygen barrier pex tubing)? E
A It is very important to have the right mixture and consistency for the lime-ash and in particular to allow it to harden and dry out naturally before any heat is applied. This should be left for at least one month and ideally longer. Some would say that 'lime-ash' is often a misnomer for old floors, which were often made of gypsum not lime. Hydraulic lime would be safest for the binder, otherwise you need to be very sure of the pozzolanic qualities of any ash, brickdust etc used, as carbonation will only take place in the outer surfaces of the slab. RJS
A I have successfully carried out repairs to a lime ash floor using NHL 3.5 with brick dust, sharp sand and crushed charcoal added to replicate the appearance, but not the mix, of the original. Do you intend it to be left uncovered? If so, you can create a very attractive finish with the traditional mix (including quartz). We have treated ours with hard oil and wax which really brings out the colour and material variation. If you are happy with materials which are currently more easily available, you may want to use LECA (light expanded clay aggregate) in a layer under the heating pipes. This is very light (which will help with any weight issues) and also has good insulation qualities, so will direct heat upwards. We laid the mix onto reed, but straw may be more appropriate for you as you are using straw bales. RJS
External Finishes
Cracking in Cornice
Q In India for restoration works of a 19th century palace, we had broken part of cornice band for roof repairing. Now after five months I am seeing few hairline cracks in the cornice. The cornice is made of bricks which are corbelled out and are plastered with lime brick dust, sand mortar. R
A Plasterers often tell me that it's normally not the plasterwork that fails but movement in the backing to which the plaster is applied. Do you know why the cornice was originally damaged - could it have been movement in the corbelled brickwork, which is still moving? Another possible cause is shrinkage caused by using the plaster too wet. RJS
Internal Finishes
Lime Plaster in a Shower
Q Can hydraulic lime be used as a plaster wall about 1/2" thick in an application such as, an indoor shower that will have tile installed over it? If so, then how long should the wall cure before installing the tile? What type of bonding agent should be used? Can lime putty/paste e used to set the tiles to the wall? I am in Arkansas, USA. J
A Yes you can...but why tile over it? Hydraulic lime is used for showers, bath and pools in the middle east (where my grandfather trained me). You colour your final layers and polish. Use a soap mix to form a layer of calcium stearate and re polish. This will give you a shower that looks as if formed from a single block of marble in any colour. If you must tile it ..cure time depends on the lime you use. Don't use lime putty as a adhesive...however I have used hydraulic lime as one and grout for mosaic work. F
Lime and Chalk Plaster (no sand)
Q Has anyone had experience of using this mix for platering over oak laths? I recently came across this on 1 day lime course and the material was beautiful to work with - is also very light (in weight) when it dries. A lot more hair is required than with sand apparently. Can this mix be limewashed in the usual manner? H
A I have been working with chalky plaster for a few years. An article was published in the 2003 BLF Journal (available as a back issue, see Journal). It will stick very well to oak lath. I know of someone using it over plasterboard successfully. Not a recommended conservation technique! It has all the advantages you mention. There are many examples of work carried out with this material in Suffolk. RMS
Anthrax Spores
Q We are involved in the demolition of some structures which may contain horsehair plaster. Can you suggest any references to standard methods of assessing the risks due to potential contamination with anthrax spores. For example how many tests would be required per 100 sq metres of plaster and what size of sample would be required. HH
A The risk is extremely low; no cases of infection from old plaster are known. Information in the measures to be taken are to be found in the English Heritage Technical Advice Note on Anthrax in Historic Plaster. GS
Guild of Plasterers
Q I am trying to contact the Guild of Plasterers but am unable to find contact details. Can anyone help? LC
A There is The Plasterers Craft Guild, Coleg Sir Gar, Ammanford Campus, Ammanford, Carmarthenshire, SA18 3TA and The Worshipful Company of Plaisterers, 1 London Wall, EC2Y 5JU Web site www.plaistererslivery.co.uk.
Q I am doing research on lime and its history. Any suggestions where I can find references? SK
A For historical information, a comprehensive listing of authors can be found in Some Writers on Lime and Cement, by Charles Spackman, W. Heffer & Sons Ltd, Cambridge, 1929, available for reference in the Concrete Information Limited library, British Cement Association, enquiries@concreteinfo.org and legal deposit libraries. PL
A For more mordern information, there is an extensive bibliography on the Preservation of Lime Mortars and Plasters produced by the Getty Conservation Institute. This is pretty comprehensive but you should bear in mind the snowballing volume of literature since March 2003 when it was compiled. RM
Any More?
If you have a question about building lime, or can provide other answers
to any of the questions above, please email
us.