Repointing a stone wall in lime, costs.

[GRW]

I admit I don't really understand Lime Mortar. I think regular lime for rendering mixes is the hydrated stuff, & the other stuff for lime rendering is the hydraulic stuff?
Also I imagine you can make your own "Lime Putty" if you obtain the correct hydraulic lime? Perhaps cheaper & you can mix up what you need as you need it (I.e. to do sections of pointing at a time)?
I saw that @sako243 mixed up his own Limewash using Hydraulic Lime to coat over his stone walls, but I don't think his needed pointing??
Like the OP though, I have a very large (guessing maybe 40m2 wall of typical higgledy-piddeldy very random rural farmhouse construction walling to try and weather proof. The old cement render has been hacked off months ago to dry the wall out - my quandary is the best treatment to have a dry interior and a good exterior.... my meeting with @sako243 had my thinking Lime pointing, followed by several spray coats of Limewash - but I don't know if that is what's best.

 

[sako243]

@GRW I haven't used hydraulic lime anywhere. My understanding is unless you need it for historical reasons or are particularly exposed to coastal weather then you can just use hydrated lime.

The limewash was made with quicklime and is I guess a very weak hydraulic lime.

 

[GRW]

@GRW I haven't used hydraulic lime anywhere. My understanding is unless you need it for historical reasons or are particularly exposed to coastal weather then you can just use hydrated lime.

The limewash was made with quicklime and is I guess a very weak hydraulic lime.

Ah, OK. I get confused with which is what .
So just the regular builders merchants Lime is the stuff? (I thought you'd used other stiff you sourced from DP's near you that came from TML in Brecon)

 

[sako243]

Right I've tried to do some more research to firm up my understanding of it (bearing in mind this is all self-taught rather rapidly starting a couple of years ago) so I'm not speaking complete and utter codswallop but I think I've just confused myself even more but clarified a lot of things.

This is going to be long but a bit of a brain dump.

I think I may have been wrong when I was referring to hydrated lime - it's a bit of a misnomer (in that I think all lime becomes hydrated lime when added to water). I think it's easier to refer to it as hydraulic or non-hydraulic lime. Basically everything starts from limestone (calcium carbonate - CaCO3) where it's crushed, burnt and slaked producing quicklime (calcium oxide - CaO).

Now this is where my understanding was wrong. There isn't really hydrated lime and hydraulic lime - it's either hydraulic or it's not but builders refer to NHL5 as hydraulic lime and I presumed that hydrated lime was <NHL5. Hydrated lime is simply lime that's been hydrated (had water added).

Apologies - this is likely to now get confusing in terms of back-and-forth.

How hydraulic a lime is is a sort of "measure" of how well it sets in the prescence of water, the more hydraulic a lime the better it sets in water, it also sets harder but takes longer to do so. There are varying degrees and they are typically NHL2, NHL3.5 and NHL5 (the NHL stands for Natural Hydraulic Lime - the natural referring to the fact there are no additions such as Pozzolans or cements), NHL5 being the most hydraulic. What governs how hydraulic a lime is is the ratio of silicates, aluminates and clay in the original mix. The more clay the higher the hydraulic rating. The additions allow the mortar to harden in the prescence of water.

Non-hydraulic lime is made using quicklime in an abundance of water, as long as it stays in the water it will not set, hence forming a putty. The lime putty can / should be left to mature if used for pointing - basically left under water for a minimum of 3 months in order for all the calcium oxide to react with the water and make calcium hydroxide Ca(OH)2. This putty is recommended for old building restorations etc., where permeability is required. Once you've got the putty you can water it down to make limewash etc., but you also don't need to let it mature for limewash (at least that was my understanding) so you can just mix a lot of water in and crack on.

So what's available and what did we use:
- NHL3.5 is readily available in builders merchants and is what the builders used for re-pointing the house (basically the lime powder, water and sand)
- NHL2 and 5 I haven't seen as readily available but they can be sourced fairly easily I think, NHL2 is more used internally I think, 5 is typically coastal properties.
- Quicklime I'm not sure is readily available from builders merchants because unless it's kept dry it'll produce quite an exothermic reaction. DP's (local builders merchant) happened to have some because Dorian (the boss) had bought some for a restoration project many years ago and they got a pallet of it. This actually came from Tarmac but was what I'd been looking at getting from Ty-Mawr.
- Lime Putty (non-hydraulic lime) this is the stuff that I think typically comes from specialists such as Ty-Mawr and various other sources. It has to be shipped in water I think in order to prevent it going off.

In summary:
- NHL3.5 for most exterior re-pointing work, best speak with someone who's not self-taught about whether NHL5 should be used lower down near ground level.
- Quicklime can be sourced fairly easily to make limewash if you wish, it can also be used to make putties for work but that needs time.

 

[colnerov]

Tim from the youtube channel The Restoration Couple did a Q&A with a lime supplier, if it's any help.

 

[Dragnet]

Builders' merchant hydrated lime is chemically the same, but behaviourally different. It should not be used as if it were lime from a specialist supplier. As soon as it is made, it starts to carbonate in the bag. It's supplied as a plasticiser for portland cement, and I would not countenance it being used on any project I were involved in as a substitute for a lime putty (I'm an architect). In certain non-critical circumstances it could be, but the mortar would not be of the same quality, one would have to be certain the bags were extremely fresh, and an experienced eye would be a good idea. A slaked lime putty by contrast improves with age, and a lime putty mortar kept wet can be knocked up and be ready to use almost indefinitely. Lime putty itself is wonderfully versatile, and beautiful to use - it's rather like mascarpone and brilliant white. It can be used for paints, plasters, mortars, surface finishes, pigments... I love the stuff.

In the Olden Days, lime had some classifications which were feebly, moderately and eminently hydraulic. These corresponded to percentages of silicate of alumina, or clay. The greater the clay content, the greater the hydraulicity. Up to 10% clay; feebly hydraulic. Up to 20%; moderately and up to 30% eminently. It was also mostly done on site, with lime burned as required (if you look, you'll see kilns near almost every building project from the Victorian period or earlier). Unfortunately, the decline of craft experience to gauge the mortars requires Standards, and these old terms have become associated with the modern NHL standards of 2, 3.5, and 5 when they are not at all equivalent.

In strength terms, the NHL limes are greatly in excess of the earlier ones, and are almost entirely unnecessary, particularly 5 which is as bad as concrete. The English Heritage book 'Mortars, Renders and Plasters' is an excellent resource, and they call NHLs "...a useful addition to the palette of materials available for conservation" but are concerned about their being too strong for repairing masonry. Much masonry, particularly historic, is very soft and an NHL will damage it.

For building, a straightforward lime putty mortar is what you need most of the time. You can add pozzolan (originally clay from Pozzuola, a village on the slopes of Vesuvius where the Romans got their clay for hydraulic mortar) in various amounts to increase the hydraulic set. This is useful in external work, where at least a slight hydraulic set is recommended; exposed locations and thick walls, where the ability of the lime to carbonate from the CO2 in the air is limited since the mortar in the depths of the wall is far from the surface. It could be that even in a very old building, lime in the middle is uncarbonated. Pozzolans can be almost anything with the silicate content. Old mortars have been found with brick dust, earth, ashes etc. in their composition. For my parents' place, this green man carving by my brother was set into the gable using mortar mixed with my grandparents' ashes, making the build a real family affair. The whole north-facing wall is pre-mixed lime putty with pozzolan added during knocking up.

However, the greater the hydraulic set and the hardness of the mortar, the lower the vapour permeability. An NHL lime can be as bad as a cement in this respect. You want the mortar to be softer than the surrounding masonry to avoid spalling and weathering problems. The vapour permeability is important to maintain, too.

Up until the 1960s, mortar was made on site, slaking quicklime as it was needed. Exciting and dangerous, it also made a very good mortar which boffins are investigating to see how the properties differ from lime putty mortars. Good to work with, apparently, though I've not tried it myself.

Internal work should be lime putty. I've just finished some three coat haired lime plastering with this. The main reason lime has fallen out of favour is time, and today's rush. It's much more pleasant to work with than portland cement, and more forgiving. It looks better, too. One of my books from about 1901 talks about types of lime, and feebly hydraulic 'stone lime' "...will set in still water owing to its containing 5-12% of clay, is firm in fifteen to twenty days and in twelve months is as hard as soap. Stone lime is suitable for building ordinary walls of brick or stone; it is feebly hydraulic, of a light buff colour, and when made into mortar with two parts of sand will sensibly resist the fingernail at a month old." I can't see Persimmon being pleased with that timescale.

For normal external pointing I'd usually specify a fat lime putty mortar with gauged pozzolan dependent on site conditions.

An example of internal work is attached. This wall is half ground, with neighbour's ground level where the large stones are, presumably laid as footings. The rest is the frangible shale the house is made from. Good in compression, but can be broken by hand. This had been covered in bitumen, and a plastered blockwork wall built in front on a concrete slab cast inside the original solid stone walls. This room was damp. We have removed the blockwork wall, dug out the concrete and the ground beneath for a couple of feet, and refilled with foamed glass, a gravel levelling layer, woodfibre boards, Lithotherm underfloor heating tiles with floorboards going on top. The bitumen has been removed from the wall, and the finish is going to be a lime putty plaster with some small amount of pozzolan to maintain maximum vapour permeability while ensuring carbonation in the humid Cornish climate. With the bitumen and concrete removed, the room is now drying and the damp problem looks to be solved. Covering the wall in an NHL mortar would simply retain any moisture in the wall, cool the plaster and create condensation on the surface. Salts would be drawn through and the mortar damaged. They would attract more water and the room would be considered 'damp'.

You may see I'm not keen on excessively hydraulic limes, but then I'm a purist.