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 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 trap water. 

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 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 possibly beaten with a churn brush depending on the aesthetic choice. 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 then rewetted.

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 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 binder rather than the material as one compacted homogeneous mass. Air limes mortars are a mass of different crystals, your goal os 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 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.

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.