How lime works

Lime is composed of calcite crystals. These crystals are anisotropic. Anisotropic means that when the mortar is heated the crystals expand in different amounts in different directions, and when subsequently cooled, the crystals cannot return to their original position because they interlock. This phenomenon often results in a permanent expansion of the mortar, increased absorption rate, and an accompanying strength loss. With large thermal cycles, some mortar can transform into very fine sugar like particles through granulation.

However the platelet crystals formed through a variety of means such as maturing putty, tempering or souring mortar can drastically reduce the likelihood of this happening. Or at least the effects caused by it. The other factor to consider is free lime content as this is the source of calcium that is required for the mortar to autogenously repair micro fissures caused by not only this but also movement of the building and the units its constructed with. 

By far the best method of producing platelet crystals is through hot mixing or at the very least by slaking the lime with a minimum of water and not through immersion. 

My personal preference for a basic mortar of lime and fairly inert aggregtes is to slake the lime in a mill whilst constantly turning it and gradually adding water until a paste is formed and then mix that paste with the aggregate without the addition of more water. However for rapid setting it would be advisable to mix everything together and maximise the advantages the heat affords by producing the mortar faster, in this case the aggregates would be better not inert. The first method described allows for greater compaction to be applied so would be advised for exterior pure/near pure works. The second is better suited to applications which aren't subject to a lot of water ingress but rather regular water ingress such as plaster which is subject to daily humidity. I would also add that when producing a hydraulic pozzalonic hot mixed mortar that it would be best practice to also produce a wetter mortar, although not as essential as is commonly thought. 

https://www.youtube.com/watch?v=9LDG9cnGlDo&list=PLmJDknXr4_-JbgqQA5-kFdFkD4DL34XM4&index=10

Calcite, Vaterite and Aragonite: the three crystals types found in most lime mortars and plasters. Each has a different shape and each is formed for different reasons. 

There are a wide variety of other crystals which can be formed, especially when hot mixing. They all have different properties and therefore can easily have an impact on the mortars performance. Eg. adding sulfates in the form of wood ash can create gypsum. Which is far more hygroscopic than lime yet lacking the same level of drying action but its harder. It is more common to use lime mortars with a gypsum content such as NHl's in countries like Portugal which aren't perpetually damp but do suffer from more extreme weather such as higher temperatures and winds. These more severe wet/dry periods are more exacting upon the mortar and some of the softer, fast drying mortars we use in the UK due to our damp climate would erode very quickly. Vice versa, the use of a mortar with a higher level of gypsum in the UK can often result in perpetually damp mortar. This is oversimplifying the use of NHL's versus air limes. 

Nitrates, silicates...to be continued.

Organic additives. There are a number of reasons for adding organic additives and they perform a variety of functions. 

Bacteria that self heals is very common and there are essentially two types, aerobic and anaerobic. I have yet to come across one with both but I'm happy to be corrected on this... The self healing nature of lime is not just something which is relevant long term but also for the ease of curing the mortars during initial application. 

Both types of bacteria increase the compressive strength and water resistance of the mortars as well as carbonating faster, thus setting faster and with reduced shrinkage. They are essentially calcite forming in a similar manner to the build up witnessed around hot springs, so we're trying to replicate that reaction using specific materials and mixing methods. Hot and complicated...shocking I know. 

Lime is binder, aggregates(filler), gauging water, mineral additives, organic additives and mechanical action. 

Binder can contain: minerals(calcium), nitrates, sulfates, silicates, 

Aggregates can contain: minerals, sulfates, nitrates, silicates. CO2

Gauging water can contain: bacteria, silicates, nitrates, CO2

Mineral additives can contain: minerals, silicates, nitrates, bacteria. C02

Organic additives can contain: bacteria, nano reinforcement, nitrates, silicates, minerals, CO2

Mechanical action: TBC. Does the folding and compressing action of a pan mixer increase the amount of CO2 as its heavier than air and often produced during early stages. Spirit of the lime kept under control so it falls back into the pan and gets incorporated? How could that be tested? Digital canaries? Is CO2 visible in certain spectrums and/or with airborne additives. Comparative mixing could never show the difference in production levels of CO2 due to differing mixing methods, so a redundant test?  Probably not tbh, but an interesting thought expounded from steam slaking by trapping the 'spirit'. 

I've simplified this for ease of understanding. But I've explained limecrete floors. cement problems, stonemasonry and a few other bits if anyone is interested.

Limecrete floors:

Not breathable, they do not release moisture off their internal surface. They drain downwards internally, like a wall heart. If they are releasing moisture internally it will be an insignificant amount and isn't beneficial.

The goal in a old property with a floor is something which can get wet and then dry out, whilst having insulative properties if possible. It is beneficial not to have the floor drive moisture into the wall, not that that's what actually happens. Its capillary action, old walls have lots of lime in, the amount of lime affects the capillary rate. EG. 1:1 will pull moisture from 1:2.

So groundwater rises, the slab and wall bases get wet. The walls drain downwards internally, hence why DPC's are massive dicks and make it look like rising damp. Falling damp in reality. And hygroscopic salts. The slab does the same thing, much slower, so if it has contact with the wall bases, ie. no cork edge, it will transfer water into the wall, which dries quicker. Salty water. 

Lime only draws water out of masonry for 100mm so any deeper than that and it stays until there is enough for it to become a liquid instead of a vapour, at which point it drains downwards. Or not in some cases, there are ancient structures with uncured, still moist lime mortar from when they were originally built. But lime, theoretically doesn't need to cure to be structural, just to gain resistance to erosion and for drying action. Not the way 'lime specialists' use it but traditionally, certainly.

Limecrete AKA NHL5 with sharp sand does drain, cement to a much lesser extent. Vapour can get into the micro pores but doesn't have a 'route' to get deeper. Air limes do not do well in damp environments which is why it's NHL5. As to why it's specifically that instead of NHL2, NHL3,5, Natural Cement or Hourdex etc. is just down to suppliers and specifiers turning shit up to 11 to cover their arses and maximising profit. I would avoid using much wood ash as it can form water soluble gypsum. This is also the reason why black mortar stopped being used. Its often eroded along the edges where water runs and not only that but the gypsum the water dissolves can then be redeposited on the surface of the bricks or stone, often resulting in the face delaminating. Which is a shame as its actually a good mortar other than that. I've seen it's successful use in internal walls fairly extensively especially with chimneys as it doesn't seem to deteriorate the same way as air lime does.  

'Salt': reacts with lime to create minerals which not only prevent breathability but actively attract water. Salt is present in the ground, cement, NHL, bricks, stone etc.

And when I say 'salt', I actually mean a wide range of harmful stuff that is too tedious to list.

NHL5 and recycled aggregates is far from a good idea. Those agg's are full of salt which will use up the freelime, which people think is allowing limecrete to breathe, creating minerals. Which will then prevent its ability to drain downwards or 'breathe'. It will also increase the strength of the slab dramatically. Not that this is an issue but if it puts people off doing, great.

Strength is often seen as a problem when its usually not...nasty, horrible cement which is too strong. Lol. Nope, I have only ever seen one issue where the strength of the mortar caused damage and that was sheering as you'd expect, not erosion. Which is weird and illogical; "the strength of an adjacent material causes another to deteriorate by erosion," of course it doesn't. I would even go as far to say that's impossible, unless you're counting the shape or texture of the material and implying that it wouldn't keep its form unless it was strong. Which I would argue is also a stretch to blame the strength in that scenario. (the shape or form increases air flow speed or similar) Cement is bad because it stops the passage of water and has less freelime in it than the original mortar. Meaning water gets drawn backwards into the wall. It also creates a strong liquid alkaline which can damage the binder in bricks and stone. And it has a lot of 'salt' in, which then gets pushed into the surrounding masonry and damages it, in a variety of ways. It's 'strength' is almost immaterial barring the difference in comparison to the original mortar as it expands and contracts due to thermal dilation. 

A badly made lime mortar can do the same thing and lime suppliers do not make the best mortar. I would go into detail about why the main lime suppliers are a bunch of utter twats and how much they are being sued but I don't want to get cease and desist letters or suffer the consequences of them trying to undermine my reputation as they are wont to do. Eden Hot lime and Chalk Down Lime are pretty good though. Tim at Eden keeps it simple and Declan at CDL actually seems to give a shit about getting it right to the extent that they alter what they do as information becomes available.

If people listen to nothing else, hear this; internal works are a piece of piss in comparison to external or damp areas (including floors) and the methods and materials should be very different and far more tailored to each application, environment and building material. By all means plaster a house but leave rendering and external works to more experienced professionals. And for the love of God have some bloody respect for Stonemasonry, its far, far more complicated than is implied in some groups. The stupid shit I hear truly amazes me; rendering Granite to protect it from exposure, using putty mortars with Sandstones, limewashing is good for all sandstone. Adding aggregates for aesthetic reasons to a pointing mortar for stone, oh yes that's an effing spectacularly professional thing to do, adding a highly reactive material for its look. Or using a lot of ash with porous stone. Using a different type of stone dust with stone. Or my very favourite, using a mortar with a lower ratio of lime than the original. That last one is some professionally warmed up idiot sauce unless you have tested it in situ and for a good reason, an argumnet could be made for a bonder lean mortar, which are actually more porous on some cases, for use with stones that degrade from water percolation. 

Just to reiterate that last point "I know that lime has an active drying action but the amount of lime has no affect on how effective that is."  This is no different than saying "I know wood makes fire hot but how much wood won't affect the temperature." Surprisingly enough this isn't true. Except when it is...lol. The wrong sort of porosity would perhaps be more accurate. 

https://heritagesciencejournal.springeropen.com/articles/10.1186/s40494-021-00546-9