Frequently Asked Questions

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, climate, type of building, method of construction, quality of workmanship, detailing, the effect on adjacent materials, etc. The BLF recommends professional advice is sought for answers to site-specific queries. The Building Limes Forum is not an accrediting organisation for the manufacture, supply or use of lime and therefore cannot recommend suppliers, craftsmen or professionals. See Sources of Information for websites that do supply 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 questions.

If you have a question, or can provide other answers, please email us.

Lime Production and Supply

Q: Are old, unopened sacks of hydrated lime of any use?
A After four years in even a dry garage some carbonation will have taken place. It would be a risk to use the hydrated lime for building. However, hydrated lime has other uses less dependent on the level of carbonation. For instance, you could donate it to a local gardening club if your area has clayey or acid soil.
Q: Why is hydrated lime more than double the price of Ordinary Portland Cement?
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.
Q: Can you still get for Blue Lias Hydraulic Lime in the UK (2013)?
A As far as we are aware you won’t find any Blue Lias Hydraulic Lime.   It has been many years since it has been commercially available, and bagged hydrate would have a limited shelf life.
Q: What age can you buy lime in the UK?
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! 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.

Masonry Construction

Q: Can you plaster and limewash linseed treated masonry?
A Linseed oil and lime are compatible but elementary preparation is necessary for success. Remaining oil on the surface will affect bonding between the lime and the masonry and also stain through on the finish. Therefore gentle cleaning with water and soft soap, thoroughly rinsed off afterwards, is recommended. Residual oil soaked into the masonry will reduce thevapour permeability of the lime coating, if this is excessive consider applying the soft soap solution as a poultice to draw out excess oil, again rinsing clean afterwards.
Q: Anyone know how the pink mortar colour was derived?
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 theBookstall) 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.
Q: How can we match a black ash mortar?
A Try using a black, or nearly black sand or other aggregate. Graphite powder may be added to achieve colour (if not texture). Only a small proportion is needed to turn a lime mortar very dark. Use sieved coal ash (4-8 mm sieve) to achieve was the texture of black bits in a whitish lime.
Q: Has anyone experience of brick pointing using lime mixed with coal-derived fuel-ash?
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.
Q: My clients have requested that I find a “healthy” concrete.
A We produced some simple concrete mix designs for a customer based on 25 kg bags of NHL. 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 20 litres of sand and 40 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 2 28 days = 2.5 N/mm2; 56 days = 3.5 N/mm2; at 91 days = 4 N/mm2; and at 1 year estimated 6 N/mm2. NHL 3.5 28 days = 5 N/mm2; 56 days = 7 N/mm2; at 91 days = 9 N/mm2; and at 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 at 1 year estimated 16 N/mm2. Diluting mixes to 1 bag NHL to 30 litres of sand and 60 litres of gravel reduced strengths by 33% of the 20 litres / 40 litres mix. Diluting mixes to 1 bag NHL to 40 litres of sand and 70 litres of gravel reduced strengths by 50% of the 20 litres / 40 litres mix. 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.
Q: Lime mortar for new build: where is it at now?
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.
Q: Why dampen bricks before laying?
by Dr Gerard Lynch It has long been recognised, within the bricklayer’s craft, the benefits of pre-wetting, or dampening, absorbent bricks before laying, as advocated by J. Moxon in his ‘Mechanik Exercises or the Doctrine of Handy-Works applied to the Art of Bricklaying’ (1703, 259), who states: ‘If you lay bricks in hot dry weather, and be it some small piece of work that you would have very strong, dip every brick you lay, all over in a pale of water, which will make the wall much stronger than if the bricks were laid dry…’ Various types of bricks We must, however, be mindful of the nature of historic bricks up until the first quarter of the nineteenth century. These were generally made from the topmost, younger geological brickearth and clays that were hand-moulded and fired – often using wood as the fuel – at relatively low temperatures; that typically averaged 850 to 950°C. After this time the Industrial Revolution dramatically impacted on brickmaking and brick types. Steam-powered machines excavated to greater depths than possible by hand the older geological materials of harder Shale, Marl, as well as Boulder and Lias clays; and, where necessary, utilised mechanical pumps to keep out invasive groundwater. A variety of patented machines were developed that could either press or extrude and wirecut bricks to the desired size and shape, and the vast majority of these were fired with industrial grades of coal within increasingly sophisticated kilns that produced harder bricks, of greater density and lower water absorption than ever seen previously; and these characteristics are typical of the majority of bricks manufactured in the UK today. Porosity and water absorption of bricks BS EN 771-1 requires the average of ten sample bricks to be tested when checking for water absorbency, and the brick manufacturer should declare it. Under the old masonry structural code BS 5628: Part 1: 1992 (replaced with EC6 and PAS 6697) the categories for Water Absorption - expressed as a percentage increase in weight - are given as: - Less than 7% - 7% to 12% - Greater than 12% The majority of dense wirecuts fall into this first category, whilst typical handmade and stock bricks are within the third, as would the vast majority of handmade historic bricks; many of which are often found to have an average porosity value of around 35%. It is important not to confuse ‘porosity’ with ‘permeability’, as they are not the same. Porosity is a measure of the available pore space within a brick. Permeability, however, is a measure of the extent to which air, water, or other fluid can pass through a brick, and depends on the pore structure and degree to which these pores constitute a means of transporting from the face to the rear of the brick. A brick can be highly porous, yet impermeable, because if its pores are not interconnected then no water falling on its face can pass through them to the back.  Why wet bricks prior to laying them? With highly porous bricks, of greater than 12% water absorption, there is a danger that they might rapidly absorb moisture from the bedding mortar (particularly in warm weather) causing it to stiffen quickly. This would result in it losing the all-important characteristic of plasticity that would inhibit correct and accurate positioning to line and face-plane and the provision of a secure bedding, leading to poor adhesion with attendant negative consequences on aspects of compressive and flexural strengths of the overall walling.  Further, there is the additional problem arising from the detrimental effects of the loss of moisture out of the mortar into the porous bricks that was necessary to successfully complete the full chemical setting action of its hydraulic lime or OPC binder, resulting in a final mortar performance much weaker than specified. With some porous bricks this problem can be overcome by having a slight increase in the moisture content of the bedding mortar that is agreed with the designer. A more favoured procedure specified on good sites, particularly with highly porous bricks, is to dampen, or wet, the bricks in clean, ‘potable’, water when laying them. On small areas of brickwork, ‘docking’ individual bricks into an adjacent container of water ready for laying into position, can be adopted, but it is essential to wear rubber gloves to prevent the skin softening from repeated immersion and becoming sore and even cracked from recurrent chaffing against the surfaces of the dampened bricks. Better practice is to employ a hosepipe to evenly spray a stack of bricks, continually removing the sufficiently dampened topmost bricks to allow the water to be equally distributed and absorbed throughout the entire stack and prevent areas of saturation. Spraying should always commence immediately prior to loading-out for bricklaying, which must commence shortly afterwards, and before they can begin to dry-out. The amount of water required to sufficiently dampen the bricks and reduce their absorbency to a level ideal for bricklaying comes with experience, but a brick that has been sufficiently dampened should not leave the hand wet when held. There is a big difference between a brick that is ‘soaked’ and one that is ‘saturated’. A brick that has been soaked has a high percentage of moisture content, but retains sufficient available pore space to still provide the all-essential water uptake, or suction, necessary for it to be properly bedded into, and adhering onto, the fresh bedding mortar. A brick that has been saturated, however, has had all available pore space filled with water. In such a case there is no longer an ability for water uptake, with seriously reduced adhesion, or suction, so the brick ‘floats’ on the mortar; and it can even begin to shed its excess moisture into the bedding mortar that can result in it leaking out of the joint and staining the facework immediately below. With some new bricks, a further possible problem is that saturation can liberate any integral soluble salts into solution resulting in aesthetically disfiguring efflorescence as they emerge and crystallise on the face of the bricks. Should one wet all types of bricks prior to bricklaying? Not all bricks types require dampening before laying them. The pressed or extruded bricks of low porosity should never be wetted prior to bricklaying as they naturally have a significantly reduced water uptake (and almost zero with a Class A engineering brick) that, if wetted, would result in the brick retaining a thin film of water on all its surfaces and this would cause it to ‘swim’ on the bedding mortar; and that invariably leads to it both sliding out of face line and sinking out of level. In such an instance it is best practice to adjust the water content of the mortar so that it is used as stiff as possible. In this respect, as they have greater plasticity and far more workability with the reduced water content of a stiff mortar, traditional lime-based mortars have the advantage over mortars based on OPC. Best practice during the winter months In the winter months the need to dampen porous bricks is no longer necessary as the ambient atmosphere is normally sufficient to raise their moisture content, and then the concern transfers to the need to actually keeping the bricks dry and to protect newly constructed brickwork from the damaging effects of frost. All photographs by Gerard Lynch. From top: - Soaking, or 'docking' a highly porous brick that has been cut out from a Victorian wall, in clean potable water prior to re-laying brick into the wall. - Spraying the indent to ensure that the brickwork does not draw out the moisture from the bedding mortar prior to re-laying the brick. - The dampened brick bring carefully positioned back into the wall surrounded by fresh mortar on all joints to ensure a secure bedding. - The re-set brick, with joints finished to match surrounding brickwork.
Q: Why doesn't sulfate attack affect hydraulic lime mortar?
A The aluminate phases in natural hydraulic lime are different to those in Portland cement due to the lower burning temperature. Lime is burnt with minimal liquid phase which would generally require a temperature approaching 1300oC. At this lower temperature any aluminate forms gehlenite, C2AS, which has low reactivity to hydration or further reaction. Very little of the more reactive tri-calcium aluminate, C3A, forms below 1300oC, certainly less than would be present in sulfate-resisting Portland cement. In the cement kiln the alumina goes into the melt above 1300oC and C3A then forms as the liquid phase solidifies on cooling. Ettringite, calcium-aluminate-sulfate-hydrate, forms from reactive C3A, hence the minimal quantity in hydrating hydraulic lime systems exposed to sulfate ions. Thaumasite, calcium-silicate-sulfate-carbonate-hydrate, although not containing aluminate, has a structure almost identical to that of ettringite and it is thought that ettringite is an intermediary, or at least has a seeding effect, in the formation of thaumasite (Taylor, Cement Chemistry). Hence little thaumasite will be found in hydrating hydraulic lime systems
Q: Has anyone else been caught by last year's frost?
A Analysis of the weather patterns in the North of England over the last two years can be compared with laboratory data for frost resistance. The data show that moderately hydraulic lime mortars moist cured at +15oC developed reasonable frost resistance after 14 days. Those cured at 10oC took 28 days and those cured at 5-10oC took 56 days. In this region a first frost usually occurs in late October but a dangerous persistent frost by late November. Comparing average temperatures with frost resistance development indicates that mortar placed by 1st October would have some resistance by mid-October; mortar placed by 7th October would take until end-October and mortar placed after mid-October would have little resistance until late November. Of course this varies with precise location and degree of protection. 2009 was kind in that the first frost did not come until 1st December and persistent frost until the end-December. 2010 had a first frost late-November and persistent frost soon after so any mortar placed after early October in this region could have been at risk.
Q: Can you supply any guidance on re-pointing historic canal walls of brick and stone construction using lime/sand mortars?
A Every job will vary so these are just general guidelines. There are no specific mixes just purely for repair work to canal walls. It will depend on a few factors: the strength of the existing fabric, the time of the year in which the work will take place, the location of the repointing in relation to wetting and drying and freeze thaw cycles, how long will be allowed to do the work, the availability of good craftsmen and good materials. If it's a Listed structure the Conservation Officer will usually want an input. In addition, once the canal is drained, if there is any water in or behind the brick or stone face, i.e. within the wall, then time will possibly be needed to allow for this water to exit once the existing pointing is chopped out. Otherwise any new mortar will not have time to set before the water is put back into the canal. That said the choice will be limited to a hydraulic lime; non hydraulic lime putty mortars are rarely used as they will not achieve carbonation before the water is put back into the canal. If a well graded sand is used alongside a NHL 3.5 hydraulic lime and the mix is used in a "stiff" state then that mortar should be suitable for repointing. The newly placed repointingshould be compacted as it stiffens, this will ensure it will resist the weather and contact with water. I would suggest a 1 part NHL 3.5 to 2.5 parts aggregate. (The aggregate could be made up of 1.5 parts soft sand to 1 part sharp sand, but this is only a guide as aggregate sizes will vary.) Trial mixes should be made and approved. NHL 5 might be an alternative lime: it will gain a quicker set, but it will achieve a greater set than the NHL 3.5 which might not always be advisable. Lastly, if very deep repointing is required (i.e. if the joints are hollow and maybe 30-150mm in depth) then a stiffer mix should be used to compact into the back of the joints. This mix could possibly have a structural role so it is important to make sure it is well compacted with a pointing iron or similar tool. Once the back of the joint is repointed and has set then the front 25mm should be repointed in a separate operation.

Standards, Codes and Building Regulations

Q: Has anyone persuaded BCO’s that adding waterproofer to re-pointing is unwise in a stone/lime mortar building?
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. 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.
Q: Is there a Code of Practice on the way?
A In conjunction with the NHBC Foundation and the BRE a document has been published entitled NF12. The use of lime-based mortars in new build. An Appendix to this document is a Draft for Development (the first stage to a full Code of Practice) entitled The structural use of unreinforced masonry made with natural hydraulic lime mortars - technical annex for use with BS 5628-1:2005. The NHBC Foundation has kindly made the publication free to download as a pdf file (1.8MB). Although BS 5628-1 is now superseded by BS EN 1996-1 the authoritative technical guidance remains valid.

Floor Construction

Q: Would a lime ash floor be durable with in-floor heating?
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. 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.
Q: Stone floor slabs bedded with a mix of 3 parts sharp sand to one part hydraulic lime have become loose.
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. 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 Trust for Historic Scotland (available through theBookstall).

External Finishes

Q: How long we should expect hair reinforcement to last in lime plasters and renders?
A A possible cause of degradation of hair in lime plaster is that commercially available hair today, which is mostly imported, has been treated to clean it, heated to sterilise it and it has been combed and bundled. Whatever is done appears to render the hair less resistant to the alkalinity of lime, and possibly more susceptible to mechanical damage. Certainly, degradation of hair in lime does appear to be more common nowadays.
Q: Five months after restoring a 19th century plastered brick cornice in India I am seeing hair-line cracks.
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.

Internal Finishes

Q: Are there any references about contamination of horsehair plaster with anthrax spores?I am trying to contact the Guild of Plasterers but am unable to find contact details. Can anyone help?
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
Q: Are there any references about contamination of horsehair plaster with anthrax spores?
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 Historic England Technical Advice Note on Anthrax in Historic Plaster.
Q: Has anyone had experience of using lime and chalk plaster with no sand for plastering over oak laths?
A I have been working with chalky plaster for a few years. An article was published in the 2003 BLF Journal (available to members as a back issue, see BLF Publications). 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.
Q: Can you stop mortar beneath roof tiles from creating dust?
A The standard recommendation for this condition is to brush off loose dust, if necessary using a vacuum to reduce airborne dusting, and to paint with limewash or distemper paint which will retain permeability. It is appreciated that it is more difficult to do this in a confined roof space than on a simple wall.
Q: Can hydraulic lime be used as a plaster wall in an indoor shower that will have tiles installed over it?
A Glazed tiles and modern grouts by their nature are not breathable.  The minute you tile with modern glazed tiles and adhesive it's not breathable. If you tile it, the cure time depends on the lime you use. Don’t use lime putty as a adhesive…hydraulic lime can be used to fix tiles and as grout for mosaic work. You could also consider using different finishes by moving the wet surfaces away from the sides of the room altogether. If breathability is important, you could batten, board and tile but the void needs to be ventilated top and bottom so the wall can breathe behind. Potentially you could use natural stone tiles and lime mortar and create a breathable wall. Hydraulic lime is used for showers, bath and pools in Morocco and the Middle East asTadelakt, a water-proofed lime plaster used in Morocco, but the water-proofing will affect thevapour permeability. 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.