More on lime and water...oh the joy!

Water, repointing and carbonation...

"The loss of mass is also correlated with the amount of mixing water. The loss of mass represented about 59% of the mass of the mixing water  (Fig. 7). This indicated that shrinkage was mainly due to drying and governed by the mixing water content." From a study on brick dust pozzolana.

This was for a hydraulic lime mortar made with brick dust and hydrated lime, the results are closer to 100% with air lime mortars AKA hot mixed, putty and hydrated lime mortars because they don't require water to cure. Very little anyway.

This is shrinkage and you want it to occur otherwise you can create larger voids within the mortars matrix which trap water. It can be too porous. This and the use of fibres is not a good thing IMO. Fibres induce water to travel along their length, introducing more water at a greater depth and interrupting the pore structure. 

So why am I saying this? Because the other thing about air limes is that they need keeping wet...or do they? This is because if the surface dries too quickly the mortar cannot carbonate at depth, it requires water to drive CO2 into it. This is a modern mistake from using wet mortars which people are doing silly things to compensate for.

So what happens is this:

People add too much water to make it easier to mix.

Mortar is struck or scraped and then beaten back with a brush but not tooled.

These mortars are then kept wet and covered for a prolonged period, effectively slow curing them to ensure carbonation at depth.

Or suppliers and lime specialists add pozzalons and fibres to prevent shrinkage and induce faster curing.

This results in the mortar NOT shrinking back and therefore not being compact enough to effectively move water if they cure at depth at all. The pore structure has large voids in which can trap water and prevent the efficient movement of water. 

This is also further exacerbated by the use of a mature putty which will cure on the face very quickly making it also dry very quickly. 

What should happen;

Minimum amount of water and a roller pan or a lot of elbow grease is used along with other methods such as tempering and organic additives to reduce water content.

Mixing methods and materials used to increase water retention such as sugar, higher fines content, milling etc.

Mortar is struck, tooled back firmly when green with a pointing tool, not a plasterers leaf or small tool which is too weak to be very effective and then possibly beaten with a churn brush depending on the aesthetic choice. This is then left exposed to the air as it has good water retention from being made correctly and will not prevent carbonation at depth if allowed to dry and subsequently rewetted.

It should take physical effort to compact or densify an air lime mortar. You could not use a plasterers leaf to do this effectively as it isn't rigid enough and to use a brush you would need to catch it at a very specific time. the action of beating back with a churn brush is to close up bedding mortar not pointing mortar. 

A personal observation; there is very little information about repointing even though it will have been undertaken regularly. When this is the case it normally means something is easy so doesn't require instructions. This says to me that the current putty/wet mortar method which requires lots of spraying down and covering up is definitely not how it was done. This and the effort involved made me look at other options, in particular the stiffness and water content to combat poor carbonation and excessive shrinkage cracks. I was right. there are numerous visual and written descriptions of obviously not fluid lime mortars. most in fact say stiff this or that and the least amount of water etc.

Masonry hosed down for two minutes on each section to fully saturate the wall at depth. Twice a week for up to six weeks. (This is from a study on carbonation and is very effective from my observations.) CO2 settles on the surface of the wall and is carried into the back by water. Using a little water will not do this, keeping it damp or covered will also impair this. Spraying the surface is just making the problem worse as you further carbonate the surface, it needs absolutely drenching. You want the back wet. 

Finished with a method which prevents too much water egress such as magnesium tooled or burnished. Steel will close the surface and prevent too much water from going in or out. Other materials have a different level of effect. Beating with a churn brush will dry it out the fastest.

Using a hot mix to bed with requires a different approach to be taken when compacting the joints. This should be done much earlier and with a stiff bristled brush. It should also be done in conjunction with much firmer bedding in. The bricks or stone should be so well bedded in and compacted that no further compaction or settling can ever take place. This is a very serious problem with amateurs who do not compact well enough and use weak air limes which cannot support a structure in the same manner that cement based mortars do: relying almost entirely on the strength of the binders chemical bond rather than the materials physicality as one compacted homogeneous mass. Air limes mortars are a mass of different crystals, your goal is to compact them together as much as possible. They will interlock and not spread sideways when compacted if you have good mortar. This is criminally overlooked, ignored or unknown by a disturbing number of lime specialists and suppliers who actively try and fight this. And colloidal shrinkage, augmenting binder strength with pozzolana and fibres. They rarely specify that products require intense compaction instead interpreting this craft skill as consolidating or emulsifying the surface with sand floats and brushes. 

Results;

A faster, stronger and more functionally effective mortar which cures quicker and is far more resistant to erosion. This will create a denser material whose strength is based upon that rather than the strength of the binder or fibres as it should be.

Why? Because buildings move around a lot and when that happens the mortar is ground to dust and then settles. It can heal but if it has voids it creates even more as it falls to the lowest point it can reach, flowing through the wall as dust. If the masonry has a lot of empty vertical joints this is likely what has happened.

However...hot mixed mortars used very fresh are an entirely different beast. See notes above. 

An additional note on porosity; modern masonry and mortars work differently to traditional. They use voids in conjunction with binder strength to resist damage by salt or frost, effectively allowing them to expand into the extra space. They also use the additional space to create an area that water doesn't pass through easily interrupting the flow and preventing water penetration. But this is not the only method or even preferred with a lot of solid wall constructed buildings as it has its limits. A dense water resistant surface can be created with lime which will prevent water ingress and allow egress which can be backed a more porous substrate to manage wind driven rain, condensation and frost etc. 

Essentially this balance between micro, mesa and macro pores is one of the key issues with exterior lime applications. Determining what you want and how you get there is not as simple as it may seem. I should add that this is a personal view of mine and that currently the industry is advocating the use of mesa/macro porous coatings for the full depth of renders rather than the above layering that I believe is higher functioning. As with most of what I state, it has historical precedent with clear examples of this type of layering and the theory is sound in principle. IE. hot mixed porous base coats topped with very thick coatings of limewash. 

Lime render should be mesa and macro porous with interconnected voids and have strength of 1-3N/mm2

Lime Wash should be micro porous with no interconnected voids and have a strength of 2-5N/mm2

i suspect that the majority of the reasons for limewash requiring replacement are down to delamination from salt cementing and expansion in the pores. The lime system draws the salt out in a poulticing action and then it dries in the limewash or perhaps just behind it depending on the size???. 

So should mineral silicate paint be used on salt laden substrates, probably not. fresh well cured render is fine but perhaps not at wall bases. 

There is currently a lot of confusion around these two principles. And neither is necessarily correct or incorrect, nor are they as black and white as all that. Eg. you can have a water resistant coating over lime render that has a large void content. Or you could have a layer with a large amount of voids against a damp wall with a porous layer to prevent water transfer after that and a dense water ingress resistant layer over the top for a cellar. 

Again this is case by case and there are no generalised rules to follow. Eg. a wooden frame in the SE will not like a West coast of Scotland render for a sandstone building. The former requiring a much more flexible material which is far less subject to wind driven rain whilst the stone render will be very hard, dense and heavy. 

There is a current trend for the use of chalk in all types of plasters and mortars across all areas. This is as big a mistake as the liberal use of hemp in damp areas as chalk will hold a lot more water than a good quality sand. Potentially struggling to dry out leading to damp and mould. It is entirely acceptable to utilise it in the SE but not everywhere.