Because they want to finish quickly and get paid? Historic England would have a blue fit If I ever did that. We didnt need it on the last building I was working on, if you need a quicker initial set then pozzolans are the correct way.
1 cup in 4 buckets will make no difference whatsoever in the long term with this, the artisans who work on the chateau’s around here obviously don’t think so either.
They work for a restoration company owned by the chateau owner so it’s not a case of doing it to get finished early and get paid, horses for courses.
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Lime mélanges … really depend on both the type of lime and the effect that you want to achieve. The mixture habits artisans, conservators form are not necessarily born out by research. Lime mortars have particular advantages in construction, less carbon impact than cement in manufacture, and are much more breathable and flexible, so more suitable to older building construction. France generally manufactures much more lime in bigger factories at volume than the UK so the carbon impact of French lime is generally lower.
Air-lime/ Slaked lime in dry powdered form (Calcium hydroxide) or quick-lime/”hot-lime” (calcium oxide) based lime putty is the ‘traditional’ form generally used in old masonry construction, stickier, longer to set, and flexible over many years. However the mix traditionally used is probably not what artisans assume it to be today. (cf The Myth in the Mix: The 1:3 ratio of lime to sand) It’s so sticky you can slap it on by glove, and smooth it over challenging surfaces. Masonry walls can settle over centuries and the mortar remains breathable and flexible and ‘stuck’. Adding cement would speed the initial set, but reduce these particular qualities.
Natural Hydraulic Limes (NHL) often contain a lot of impurities (clay etc) already in the ground rock, and set much faster and harder, even being usable below ground. Some products are already mixed with cement and pozzolans. - In France I can readily find NHL 3.5 in local builders merchants but rarely find Air Lime/Lime Putty. (I had little difficulty finding good lime putty in Wales from specialist local suppliers.) NHL 5 has been compared to cement. You should have little or no reason to mix cement with NHL normally.
Hemp-lime blocks, on which more below, generally use blends of air-lime, about two-thirds, with NHL lime, Cement, and Pozzalans, in secret recipes.
I have successfully used an SPAB recipe for limewash, consisting of lime putty with skimmed milk, applied in very thin translucent coats to build up over a number of layers, with both natural colouring (eg tea) and mineral colourants. The tea leaches out if exposed to weather, and people often add tallow to improve weather resistance.
I am mulling over whether, and by how much to adulterate NHL lime for tampable hemp-lime insulation. No cement perhaps for inaccessible areas, since it is then most breathable, but less mechanically strong – some cement for areas where I need greater mechanical strength. So I apologise for the length of this post, but its provided me with a means of crystallising my thinking.
I started learning about lime mortar and conservation (from an elderly builder friend in Montacute, Somerset, in the 1970’s). He used stonedust from the local quarry in preference to sand, since it matched the local golden hamstone. He and his workers would slake quick-lime and mix it with the stonedust into a heap on site, 3 months before work started. (This is now referred to as hot-lime). 3 months later, they would remix the heap and start its use. The key benefits for conservation were a very close material and colour match that had been used through history, a mortar that flexed and coped with settlement, and remained sticky over many years of slow carbonation. (Historically often with hair, dirt and pozzolans added as often inadvertent binders and catalysts). It also made demolition and reuse easy, since the mortar cleaned off the masonry without issues. I have done a fair amount of cleaning off old lime over the years, and it’s very easy compared to old cement.
“A hot-mixed mortar offers a very workable material. Due to its high lime content, it is very sticky and this can help stop the mortar being washed out. It offers good water retention that can be controlled during the mixing process. And it can be a cost-effective option; the purchase price of quicklime is usually less than putty or NHL and cheaper, dirtier sands can be used.
From testing it has also been proven that hot-mixed mortars offer extremely high vapour permeability. Under lab conditions a hot-mixed mortar at 1:3 can offer vapour permeability up to 1.36kg/m2sPa, whereas a comparable NHL 3.5 mortar at 1:3 will be around 0.69kg/ m2sPa. This has yet to be proven in the field or over long enough periods as carbonation is a very slow process. Nevertheless, it seems likely that a hot-mixed mortar could be twice as breathable as an NHL3.5 mortar, offering significant advantages to historic fabrics in many situations.” Hot-mixed Mortars: Advantages and Limitations
I am currently more focused on Hemp-Lime as precast blocks and as ready-mix for poured insulation. I have made precast blocks with both air-lime and pozzolans, and with NHL lime. Both have worked well and gained adequate strength in a week, using a blower technique developed by Lime Technologies Ltd. This involves casting on a perforated mould mounted on a low berm, with constant air pressure applied over 6 or 7 days, before turning out (plastic/kraft paper sheet linings help). (Lime Technologies Ltd mount the material in permanent shuttering/moulds as insulating panels, so then the mould material itself needs to be more breathable). We received funding for this from SBRI, but the similar panels chosen were from a competing team written up as effective breathable internal wall insulation at Developing a Non-hydraulic Setting Air Lime .
For my ready-mix to pour, I am still undecided which lime binder to use. There is an interesting PhD thesis at Bath Uni -
https://purehost.bath.ac.uk/ws/portalfiles/portal/187941867/Thesis_final_Cristiano_Figueiredo.pdf. This research shows that depending on the binder/aggregate proportions in some cases NHL2 mortar will end up after a year with greater compressive strength than NHL5. It also compared cement-lime mortar and lime-metakolin mortar mixes with a lime-putty version. It points out that the type of cement used is critical, and that “higher quantities of cement lead to higher compressive strength and faster set, but reduced porosity and pore diameter”. ‘Some cements can make a mortar that is too strong and not permeable enough for some conservation applications.’ It also concluded that some lime putty based mortars could be stronger than lime mixes containing 5% metakolin (MK), but the latter could end up with bigger pores. Another PhD thesis tested different blends of lime based binders with hemp at https://researchportal.bath.ac.uk/files/187944938/UnivBath_PhD_2013_E_Hirst.pdf. This concluded that the compressive strength of NHL5 based mixes was generally worse than those of the more specialist mortars such as Batichanvre from St Astier, but the thesis only tested NHL5 and did not explore the difference in binder/aggregate. Tradical lime based binders had better strength than either. Batichanvre and Tradical are believed to contain about 70% air-lime, and 30% other elements including pozzolans, possibly cement, and hydraulic lime, however they have tested up to 70% of non-air-lime components.
For hemp-lime the type of mixer is fairly critical. For our test insulation panels we used a drill with a stirrer in a bucket. The best mechanical mixers are the horizontal drums used in art schools for mixing plaster. However I am tempted by the wheelie-bin approach. Mixing - Neighbourhood Construction
In conclusion, recent research on the Pantheon also indicates that the right mix of lime with pozzolans and impurities can produce a mix that will last centuries with less wear and tear than cement based concrete. Cf Researchers uncover the secrets behind the insane durability of Roman concrete)
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Without going to the full expense of a forced action mixer, I bought the belle tub mixer which are actually made and sold in Italy under various names, belle just have them in orange with stickers on them does a good lime mix.
I contacted hemplime ( Isohemp) in Belgium the other week, other than the auto response no contact so I will probably use parpang blocks, biggest issue with trying to go green is the cost and supply of materials hence why many projects dont.
NHL5 normally used on sea defences as it tolerates salt unlike most cements.
Farmers in the Doubs (fairly local to me) are now using hemp as part of a rotation of crops, I tried really hard to get a delivery of loose hemp chivs, however there appear to be a limited number of organisations actually processing the chivs. I ended up buying 70 x 200 litre plastic sacks from www.materiaux-naturels.fr. These were delivered on tall pallets a couple of weeks after. As mentioned, I still have not decided on the lime side, but will probably use air-lime, and add pozzolans etc as needed. I previously bought 7 palettes of blocs Multichanvre, from Vielle Materiaux in the Doubs… ZAC de la Croix de Pierre - 1 Chemin des Planches - 25580 Étalans. Fabricant de blocs de chanvre Étalans : blocs de chanvre, produits béton – Vieille Matériaux (vieille-materiaux.com). I did get one quote slightly cheaper, but from Brittany, and the food miles put me off. The Multichanvre blocs were eventually delivered, at a markup and VAT added which irritated me a bit, by Gedimat. I did go to the factory and inspect the blocks and manufacture first. I am very happy with them. They are not particularly fragile. They manufacture at 300mm thick then use a saw to cut them thinner, so should be a little cheaper in the 300mm thickness. I have more time now, hence my plan to do it myself, and save money.
Using a translator it said cement in difference to lime. Lime evaporates water (breaths) cement does not so would hasten any rotting of the hemp?
Likewise the use of hair in lime, if it gets damp any caustic remnants can destroy the hair, much rather use polyfibres these days.
I suppose nowadays the use of polyfibers wouldn’t be seen as very environmentally friendly surely, where hair is.
Splitting hairs there a bit 
The fibres are trapped so not harming the environment and last hundreds of years so not really an issue against people who buy bottled water for instance
If the Chateau next door who are renovating their large chapel had used polyfibers it would be going into the environment at the moment, they have replaced all the horse hair lime mix with new due to going back to the original features, it had only been up 50 years ago, I take it renovations do this all the time throughout France so no guarantee it’s going to stay there for centuries.
You can make a lot of fibres from just one plastic bottle. Its a more reliable and arguably safer way of repairing. Large ceilings in historic London buildings failed as horse hair dissolved from the lime plaster. Considering the huge amount of plastics used everyday compared to the amount of fibres used its the least worrying part of humans existence. The larger amount of energy and CO2 used to make your favorite cement is not a worry when we can build better from lime in most cases.
I’ve really just started playing with HA and it’s a real time vampire, but fascinating. Really I should just have bought two thermostats for the electric radiators
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We will agree to disagree, if there is a natural way to do something you should take it, every little helps and the it’s only doing a little damage compared with other things mounts up.
With regards to building and renovation work it just always smacks slightly of hepocrasy.
Not disagreeing at all, used hair plenty of times but having seen building failures and the guys I use now having quite a catalogue showing the degredation there is an alternative, thats all I was pointing out. On the other hand you seem to like adding cement to the mix so I can agree to disagree on that as its not a natural way of doing it as cement is a modern invention by lime standards. You accept that but reject anything else that is actually a positive to the construction. Its not important enough for me to get upset over.
One up of cement maybe twice a year, right, if it makes you feel better 
Hi. Your concise questions gave me food for thought! When is cement not portland cement? When is NHL too close to cement to have the qualities we want for hemp-lime? I think for me a basic question is when is the hemp (or timber frame) sufficiently coated with caustic lime to be protected, without being changed or made less breathable? Our internal wall insulations were thoroughly wet and coated, almost filling the interstices with a lime plaster. We got a density of about 465.5 kg/m³ (higher than is usual even in France.) Thermal conductivity of 0.09874 W·m-1·K-1 when measured dry in a lab, although that is a poor measure of the actual performance. The 50mm tiles internally improved the U value from the original 1.55 w sq m deg C to 0.8237 w sq m deg C, and made the room feel warm, dry and comfortable. However when air dried under shelter on site, they were still fragile after a couple of months, although could be removed carefully from moulds and fixed to the interior of the cold/damp stone wall. When I used the blower approach, they were biscuit hard and strong, not fragile at all, in a few days, and still wet enough that some slurry dropped through the vent holes and formed neat white tablets on the floor of the barn.
Its clear that NHL limits the hygroscopic performance much more than air-lime. So then its a question of density versus hygroscopic performance. The stronger the binder, the less dense can be the blocks, so permeable in other ways.
NHL 5 can be up to 30% clay, so only 70% lime. So a further question is how far does NHL 5 differ from Portland Cement? The Vicat Natural Cement used in the manufacture of the Vieille Materiaux Multichanvre blocks is not Portland Cement, but akin to NHL 5, ie clay blended limestone with a possibly slightly-greater clay content (20% to 35% depending on the use -cf Tout savoir sur le ciment naturel Prompt Vicat | Ciment Prompt Vicat).
“• No admixture is used, just a single and unique stone is fired, hence use of the qualifier “natural”.
• Raw materials are selected from very specific geological beds in order to obtain an optimal and
consistent chemical composition; NHL stem from either siliceous or clayey limestone, while PNC are
derived from a clayey limestone
• Firing procedures are identical, i.e. same temperatures of less than 1.200°C, in a shaft kiln.” (https://naturalbuildingstore.com/wp-content/uploads/2021/04/Vicat-Tech_Spec_Prompt_NHL.pdf)
It seems that each precast hemp-“soi-disant-lime” block supplier is compromising between speed of set, and lightness of weight for transport etc., rather than complete encasement in caustic lime. The admixture of cements and pozzolans and NHL in the Tradical, Batichanvre, and I think Isohemp indicate the same compromise.
I found an interesting site in Canada, where they usefully provide measurements in buckets, and mixed 50/50 air-lime with metakaolin - Hempcrete developments - Endeavour Centre
As to Hair, my ex partner in practice had a giggle. He read a spec photocopied and sent with contracts to local builders for many years. The section on hair admixture said “and the hair shall be plucked from the tail of a living ox”. No-one had every challenged or raised a question about this.
I suspect also that the level of caustic lime protection is also the issue with the hair. Research says that the air based caustic lime and the natural oils in the hair preserve it for centuries, and that it can even be removed from laths and rolled up like a carpet. However recent research says most hair supplied comes from 3rd world abattoirs, and due to anthrax fears in the EU/UK is now chemically treated, so destroying the natural oils.
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From what I was taught and believe to be true, most lime is fired in kilns at lower temperatures 900-950c with the 1200c reserved for cements as it brings out the aluminate content and hence why sea and cements do not work.
They do with Roman concrete, seawater strengthens it.
Precisely my point, concrete and cement are not the modern portland cements we use today.
Lime and its minerals consolidates over time to limestone, this we already know from ancient sites.
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We are all basically singing of the same hymn sheet, using one cup of cement to make a batch go off a lot quicker once in a while is hardly the greatest environmental catastrophe, use old techniques when you can or newer technology if it supplements the older ones, anything that can get the use of materials that damage the environment due to their manufacturer out of the equation is good.
Yes JoCo’s post is interesting reading, some of the links I have studdied before but confusion over the terms for modern products vs the ancient ones.