Black ash mortar repointing problem (picture included)

[LimeFan]

Hello,

I'm looking to repoint my 1880s built mid terrace (sheffield) and am struggling to find what to repoint it with. The bricks look like they were originally laid with black ash mortar and most of the house has a thick layer of black coal/ash plaster/render internally as a base coat. Spoken with multiple lime works and online groups, all with differing opinions on what to use. I am not concerned about matching the colour of the mortar to the original at all really, its more that I want to ensure that the new mortar I use is of a suitable consistency for the original black ash mortar. If I use the wrong mix, it could end up being as useless as using sand and cement and spall my bricks further and retain water/moisture more. Somone has tried to repoint with sand and cement at some point in the past and i'm pulling all of that off quite easily as its only about 3-5mm thick.

I change my mind between using NHL or quicklime/putty every time I speak to a different limeworks' advice. although my latest idea was to use LaFarge NHL 3.5 (due to high lime content) with some sharp sand 0-4mm, using a ratio of - 1 : 2.5. Given that its my first time repointing, i've been told that NHL is more forgiving despite being less effective at moisture wicking.

You can see from the photo below however that the original mortar looks like it almost had no sand in it at all, just looks like coal ash dust and quicklime. Does anyone know if my mix of Lafarge 3.5 with sharp sand would be ok to point over the original?

Thanks

 

[malcolm]

I've used NHL3.5 throughout mixed with sharp sand and builders sand to make it easier to use. The mix is 2.5 sand to lime, and the sand is 50% sharp sand and 50% builders sand.

In general NHL3.5 is fine for bricks after about 1800. The exceptions are soft reds that spall when you look at them, soft stone like sandstone, and buildings that are on the move and need the most flexible mortar. For those a putty mix is best. But there is normally no real advantage to NHL3.5 over putty apart from being easier to mix.

The original mix will likely have used materials easily available at the time rather than being a chemistry master plan. Any other lime mix should work with it.

 

[LimeFan]

I've used NHL3.5 throughout mixed with sharp sand and builders sand to make it easier to use. The mix is 2.5 sand to lime, and the sand is 50% sharp sand and 50% builders sand.

In general NHL3.5 is fine for bricks after about 1800. The exceptions are soft reds that spall when you look at them, soft stone like sandstone, and buildings that are on the move and need the most flexible mortar. For those a putty mix is best. But there is normally no real advantage to NHL3.5 over putty apart from being easier to mix.

The original mix will likely have used materials easily available at the time rather than being a chemistry master plan. Any other lime mix should work with it.

Thank you Malcolm

I've read everywhere that builders sand shouldn't be used with NHL as the shape of the grain makes the mortar shrink too fast and crack, only washed sands like plastering sand. Have you had any issues using builders sand in your mix?

 

[malcolm]

I mixed in builders sand to match the colour of the existing pointing but also found it makes the mortar easier to use. Wouldn't use 100% builders sand but 50% builders sand and sharp sand is fine. I've had no problems pointing, bricklaying and rendering with the mix.

Washed sands don't tend to work well as they wash away the fines. The sharp sand should ideally be a mix of sizes from fine up to maybe 4mm.

 

[stuart45]

The size of the grains depends on the size of the brick joints. For example with really large joints in some stone walls I've used a really coarse 6mm down sand, because you need a bit more body in the mix. Finer sands would tend to slump. However with smaller joints, a larger grain size means the mortar won't fill correctly. In this case a finer sand is needed. I've used 0.5mm down on some jobs. On really tight joints with gauged brickwork a pure lime putty might be needed. Some of the joints done years ago were built with white lead and Shellac.
You are correct that soft building sand is more likely to cause shrinkage issues, although a lot of people mix it with sharp for a more workable mix.
When using NHL, the method of mixing is important. Mix it dry for 5 minutes in the mixer, and then add just enough water so it's dryer than what you want. If it starts to ball up add a bit more water. Let it mix for around 15 minutes, and as more air gets into the mix you'll notice it looks wetter and more workable. Let it rest for about 20 minutes to fatten up, and then mix for another 5 minutes. Even with only sharp sand this will make a workable mix.

 

[LimeFan]

The size of the grains depends on the size of the brick joints. For example with really large joints in some stone walls I've used a really coarse 6mm down sand, because you need a bit more body in the mix. Finer sands would tend to slump. However with smaller joints, a larger grain size means the mortar won't fill correctly. In this case a finer sand is needed. I've used 0.5mm down on some jobs. On really tight joints with gauged brickwork a pure lime putty might be needed. Some of the joints done years ago were built with white lead and Shellac.
You are correct that soft building sand is more likely to cause shrinkage issues, although a lot of people mix it with sharp for a more workable mix.
When using NHL, the method of mixing is important. Mix it dry for 5 minutes in the mixer, and then add just enough water so it's dryer than what you want. If it starts to ball up add a bit more water. Let it mix for around 15 minutes, and as more air gets into the mix you'll notice it looks wetter and more workable. Let it rest for about 20 minutes to fatten up, and then mix for another 5 minutes. Even with only sharp sand this will make a workable mix.

Thank you so much Stuart.

I appreciate that all guidance for sands point toward sharp sands when using lime as a binder, however if you look at the photo of the original mortar, one limeworks pointed out that it looks like it has 0% sand content, just pure coal ash and quicklime slaked on site most likely, i'm based in south yorkshire, house was built around 1880. Another lime pointer recommended using only fine sands like plastering sand and if needed a little bit of builders sand. If I was to use sharp sand 0 - 4mm, wouldnt the original mortar wick the moisture inward from the new mortar?

Here's the mix i'm thinking of using right now, i've hyperlinked each of the below directly to the site's product page.

Lafarge 3.5 NHL
0 - 2mm Sand
Fine Silica Sand

With a ratio of 1 : 2 : 0.5
1 NHL : 2 2mm San : 0.5 Fine Sand

Any thoughts? I'm hoping this will get closer to the correct material to repoint with. NHL has been known to be quite strong, you think this would still slump?

 

[stuart45]

If the size of your joints are around 10 mm, then 2mm down sand is about right. A rough guide for the grain size is about 1/5 or a 1/4 of the joint size, but 1/3 is the absolute maximum. So 4mm would need at least a 12mm joint.
The strength of your mortar depends on how soft the bricks are. NHL3.5 has been found to be harder and stronger after a number of years than was previously thought.

 

[malcolm]

My sharp sand occasionally includes pebbles (10mm to 20mm). It's only a real issue when bricklaying when you have to pull the brick off, ping the pedal away, then put new mortar on and try again. For pointing anything that fits in the gap is fine.

Moisture tends to wick to the outside. Wind blows past and takes moisture away from a breathable wall. Even in Yorkshire that system works.

 

[stuart45]

Having a grain size that just fits the gap isn't really using the correct sand. It'll go in, but the binder won't work as well with the aggregate.