With so much information available online, it is almost common knowledge that lime is the predominant ingredient in conservation mortar. Brought to the British Isles by the Romans, its origins can be traced as far back as the Egyptian Dynastic era (circa 3400 BC).
However, it became virtually extinct in the building trade as it was superseded by the widespread use of much cheaper and less labour intensive cement mortar. Although there has been a resurgence in the use of lime over the last 20 years, there is still much to be discovered about its true nature.
A number of preferences and beliefs have thus emerged out of the ever-growing body of information concerning historic restoration and maintenance. One of the most prolific is that lime putty is superior to natural hydraulic lime; others are that all old houses were built with lime. Another popular myth is that that washed sand is a precondition for good lime mortar and that a 1:3 mix of lime to sand is the most suitable. I will begin by addressing lime putty.
Lime putty (non-hydraulic lime).
Limestone, chalk or seashells, when burnt at high temperature, will all form quicklime. This comes out of the kiln as a very dry and highly reactive white compound called calcium oxide. The addition of water rehydrates (slakes) the lime which can lead to a violent exothermic reaction (depending on the brand/type of quicklime) causing that which is being slaked to hiss, spit and boil as it reaches temperatures of up to 300 degrees. Although not without risk, the reward should be a good quality, sticky calcium hydroxide.
When the heat dies down, the cooling mixture forms a white putty which must be laid down (stored) for a minimum of 4 months before it can be mixed with sand to make mortar. As long as it's not exposed to air, it will last indefinitely. The longer it matures, the better it becomes.
If the quicklime has the sand added to it during the slaking process then the reaction is often less volatile with temperatures only reaching about 100 degrees. The freshly mixed putty mortar can then be used almost straight away. The heat generated during the slaking process means it's commonly referred to as a hot mix. The resultant chemical reaction causes the volume of quicklime to actually double
History reveals that original recipes from master builders and architects specified a ratio of 1 part quicklime to 3 parts sand (1:3). However, mortar analysis has revealed that a hot-mixed and therefore expanded quicklime actually translates a higher volume. Knowledge of this has been gradually filtering down over the years and it now means that many of us who specialise in lime mortar are using the same historic ratios.
As the expansion only takes place during hot mixing, it means that it will not occur when lime putty or natural hydraulic lime is ordinarily mixed with sand in the same manner as a conventional mortar. Mortars mixed this way have now been found to be a little too weak for repointing and it's currently recommended that anyone who works with lime should try and emulate historic ratios of lime to sand. Examples include 1:2, 1:2.5 or 2:3
It's sometimes difficult for those in the construction industry to change their ways which means it might be a while before everyone reverts to a mix more in keeping with the lime rich mortars of 200 or 300 years ago. When instructing a contractor it's always wise to check with him that his mortar reflects traditional ways and values. If it does then you can rest assured you're getting a good, strong, flexible quality mortar mixed the same way as it would have been back in the 16th or 17th century.
It may come as a surprise but there are now quite a number of contractors who are slaking their own lime or hot-mixing on site.
This is partly to do with how easy it is to obtain product. Next day deliveries of quality high calcium bagged quicklime now make it pretty straight forward for those who specialise in lime mortar to order bulk consignments. Some, however, have taken it to the next level and are experimentally burning their own shells in order to obtain quicklime as they want to be absolutely certain that their putty is manufactured to their own personal and exacting standards.
Production doesn’t even merit cottage industry status and it may just be a fad. But it’s happening. Partly born out of experience gained by many at college but mainly because of the belief that factory lime burning practises today fly in the face of traditional manufacturing processes. If it gathers momentum and becomes as popular as slaking on-site then at some point we may find suppliers offering bags of crushed shells as part of their product range.
In comparison to yesteryear, firing techniques employed today are so very different. Refining plants are at present super-efficient and highly technical clinical environments where limestone is burnt on an industrial scale using exhaust gases from the kiln.
This is a far cry from the wood or coal fired lime kilns which were only partly successful when it came to an even burn of their charge. Furthermore, as the limestone to be burnt was built up in layers between combustible materials such as wood or coal, it meant that the quicklime came out complete with impurities. One might be generous in describing the process as resulting in the production a somewhat impure calcium oxide. Nevertheless, this would be no bad thing as this antiquated calcination process came complete with clinker coal ash, coal dust and lumps of unburnt and partially burnt limestone which would - surprisingly - have been of benefit to the subsequent mortar.
The lime kiln at Arkleside bridge in the district of Coverdale in the Yorkshire Dales. Although currently used as a bin store, it’s actually grade 11 listed. Unlike many common mortared properties in England, the houses and barns of the Yorkshire Dales were actually built and pointed with lime.
You see, modern manufacturing techniques now mean that limestone rock is subject to equal amounts of heat and there are none of the aforementioned contaminants. The result is that a very pure quicklime is produced. The new reality is that any defects which were inherently part of traditionally kilned limes are no longer present due to the efficacy and efficiency of modern clean methods of production. This could be seen as lamentable because superfluous substances intrinsic to the burning procedure would have given those original putties a slight pozzolanic element. In all likelihood this would have made them stronger and more quick to cure.
Although the evaporation of moisture does help a lime putty to set, it actually gains its strength by pulling carbon dioxide from the atmosphere. This is known as carbonation. It takes time for a lime to fully carbonate and doubt can now be cast as to whether today's lime putty mortar - even when mixed at the emulatory ratio of 2:3 - is actually robust enough to stand the test of time as a repointing mortar. As it's now a different product to what it was hundreds of years ago; it won't carbonate as quickly as it used to and sufficient strength may not be achieved before winter. The fact is, these super-soft putties are a worry to many tradesmen so as a matter of course they now advocate the addition of a pozzolan as common protocol.
However, if you don't like the idea of sifting through the array of different products then you could skip on the whole idea of pozzolana and simply enrich your lime putty by gauging in 10% NHL3.5. This would go some way to insure the now absent and yet absolutely necessary potency through impurities is reintroduced.
Although many involved at the specifying end of conservation may disagree, that which has been described above may actually form a case to ditch the use of pure plant produced lime putty all together - except in the case of sheltered southern environments or for work on cob or loose masonry.
It may be worth mentioning - at this point - that I only know of one supplier who actually makes a lime putty containing a small proportion of <3mm particles of flint and grains of unburnt chalk. It would be nice to see more.
This is a later addition to the built environment and was simply a development of its time. It is considered harder than putty because it contains clays which facilitate a set in the presence of moisture whilst also aiding it to reach a supposedly greater compressive strength. Its properties also allow users to enjoy the benefit of its relatively fast setting time in colder weather.
Its strength is derived from the amount of time the limestone is left to burn in the kiln. For example, NHL2 is the weakest because it is in transit for the least amount of time. It also has a smaller proportion of argillaceous or siliceous material present before burning. What comes out of the kiln has also had less time in the presence of reactive silica and is therefore less cementitious.
In direct contrast, NHL5 not only spends more time in transit with impurities but there is also a greater amount which increases hydraulicity and helps to create that familiar grey colour.
In sum: the compressive strength of NHL is directly proportional to the amount of lime present in the kiln in tandem with the amount of clay and silica which remain after burning.
The upshot of less time in the kiln translates to higher flexural strength. Water vapour transmission rates increase also. That is to say, mortar made with a hydraulic lime which is lower in newtons per square millimeter (N/mm2 ) becomes more ductile. Whereas an NHL5 - which is burnt for the longest – is the hardest. What this means is that the best NHLs are made with silicas so shopping around for a good product might actually mean you’ll get a much better lime as well as increased value for money.
Modern production processes and not necessarily a good thing.
They are now highly (overly) efficient meaning the burnt lime reaches a level of super-heated clinical cleanliness which is so elevated that many of the traditional impurities and practises inherent in traditional kilning processes are removed. A large number of contractors have now concluded that NHLs are way stronger than they were ever meant to be. So much so in fact that scores of those on the tools switched years ago from NHL3.5 in favour of a much more yielding NHL2 - whenever circumstances allow. The reason for this is that the lower clay content means it’s softer, more flexible and has greater porosity. This should lead to more successful carbonation as carbon dioxide absorption is improved.
N.B. As a point of information, NHL2 was initially seen as only suitable for internal work as its compressive strength was measured and recorded as lower than NHL3.5. However, this was only after 28 days. It’s known now that the strength of NHL2 mortar increases over time albeit not to that of a 3.5. It’s now considered a fact that the 28 day curing interval bears no resemblance to the eventual strength of a mortar which has been carbonating for years.
If one allows oneself to be influenced by the above, one may be less likely to dispute that the addition of NHL2 (10% for example) would make a 3.5 based mortar marginally more suitable for repointing. The much lower clay content of the NHL2 should, by rights, give a 3.5 a superior level of ductility and permeability. Please note though: compressive strength has only a certain bearing on ductility.
If you want evidence that NHL3.5 is a little unnecessary for normal conservation work then please be advised that one reputable supplier categorically states on their website that NHL should never be used to plaster over cob as it’s simply too hard and therefore more likely to sheet off. A favourite Welsh supplier suggests that NHL3.5 should always be substituted for NHL2 wherever possible as its strength is now likened to the original 3.5.
In sum: when lime was burned in stone kilns, impurities such as charcoal, bits of twigs, unburned limestone, clinker from burnt coal and poor quality limestone were all part of a bygone production process which was much more natural.
Natural hydraulic lime (3.5) may just not be as natural as was originally intended which means we now need to be sensible in our approach to conservation otherwise we run the risk of emulating those in the building trade who today show a preference for tough and impervious cement mortars just because they have become common protocol.
The current trend is to use clean sand i.e. aggregates which have been washed. This is good for appearance as it means the sand is dirt-free so the finish is better than it would be if a second rate building sand had been used. Nevertheless, uncontaminated sands may actually bear very little relation to their historic counterparts as even river sands would have contained impurities. Basically, much sand back in the day would have been unwashed pit dug aggregate which was full of silica and clay. Anyone who’s used this kind of sand will testify to the irksome habit of the impurities rising to the surface – particularly at the base of a property. But the apparent failure of manufacturers who make lime too pure might actually signal the necessity to make a return to dirty sand. The reason for this is that pollutants in sand (particularly sand with a high clay content) could act as pozzolana and speed up the curing time of lime putty mortar. It may also marginally increase its compressive strength.
In the case of NHL, a dirty sand could actually weaken it perhaps making a 3.5 a little more like it was originally intended.
Ideally though, a building sand which is low in impurities but which has a high clay content would be preferable. And these are rare.
As mentioned earlier, the origin of lime mortar can be traced back to Dynastic Egypt (circa 3400 BC). However, earth mortar goes back as far as 8000 BC.
Although many may baulk at its use simply because it’s only humble soil and clay (and therefore weak, right?), it’s actually a strong and healthy building material and has a nil carbon footprint as the materials are readily available on-site. The oldest example of an earth-built house in the UK is a cob house built in 1410 in Devon. The fact that millions of people on the planet see out their daily lives in earth mortared buildings – some as tall as 9 stories – should inspire one’s confidence where sympathetic repairs using it are concerned.
During my many years of experience, I’ve dug into a multitude of period properties in Oxfordshire, Northamptonshire and Buckinghamshire only to discover that they were pointed with lime but not actually built with it. Common mortar for building is usually a soft, dark, crumbly, feeble mortar. Known as earth mortar, analysis reveals that they mostly originated from sub-soil and clay whilst others were hot mixed with quicklime. Although great for bedding, as they maintain compressive strength whilst being extremely flexible and vapour permeable, they were simply not robust enough to be exposed to the elements. Therefore, the joints were pointed with a rich lime mortar.
This C1650 previously thatched cottage just south of Oxford city centre was built using common mortar and pointed with lime. The use of common mortar (clay and sand, with perhaps the addition of quicklime, was used extensively up until around 1890.
Whilst pointing remains intact and maintains its porosity it will wick away moisture but when its shelf life is over the single most worst thing anyone could ever do is to repoint using Ordinary Portland Cement mortar, or a hard lime mortar, as this means any moisture trying to make its way out of a wall through vapour open joints will be held back or restricted, respectively.
The issue is compounded by the difference in thermal movement. As cement is not a natural product and heats up at a different rate to stone and brick, the end result often looks like the building has physically rejected the cement mortar. Then there’s the issue of soluble salts in rainwater. When cement mortar surrounds brick and stone there’s no lime mortar to pull away moisture leaving damaging soluble salts behind after evaporation. It’s all very well builders simply gauging a little cement into lime mortar to “send it off” but all this does is pollute a mix with the worst case scenario being failure of the bond. The rule always should be lime plaster on the inside, lime pointing on the outside. Failure to adhere to this principle will result in only one outcome: damp, mould, energy inefficiency, fungus, rotten stone, high repair bills, a musty old house smell, unhealthy inhabitants, condensation and a potentially increasingly difficult property to sell.
In sum, there’s a lot to be said for gauging impurities into industry standard NHL3.5s or simply switching to a good NHL2 (when and where conditions allow). There’s also much to be said in favour of dirty sand and pozzolans – especially towards the end of summer when filling large joints with putty mortar which will take some time to carbonate properly. For future best practise though it may be worth more manufacturers offering bagged quicklime with a higher impurity content. Suppliers could choose to offer consumers a choice of sand ratios in premixed putties i.e. 1:2, 1:2.5 or 1:3. I currently only know one who offers this amount of inclusivity.
With brand names for quicklime becoming more prolific and consumers already offered the choice of kibble or powder it may be assumed that the industry is changing.
Moreover, although a rarity, some contractors are already producing their own quicklime - complete with bird's nests and weasel tails. These are clear signs that the industry is morphing in a more hands on direction. Customer demand, specifier requirements and contractor preferences will dictate the pace.
N.B.1. Although NHL is sometimes used as an additive to lime putty mortar because it has a pozzolanic affect, there is evidence that doing it the other way round i.e. adding non-hydraulic to hydraulic lime means you could risk a catastrophic failure. Therefore keep the amount to no more than 5 – 10% and bear in mind that the addition of putty to NHL will reduce its strength.
N.B.2 The Scottish Lime Centre Trust has suggested lime putty be banned north of the border because it is no longer appropriate.
With special thanks to:
David Wadham, Sean Mackenzie, Damon Mackins (Kentish Lime), Andrew DeGruchy, Adam Brown, Nigel Copsey, Buffy McClelland, Adrian Finley (Finlay Construction) and Edward Byrne.