Lime mortar

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stuckinthemud

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I have to re-point a small section of my terraced house pine end wall. Its a black mortar fill, the house dates to early 20th century, so I'm guessing its a lime based mortar? Where do I get some, or how do I make it? Any ideas?
 
Apparently cement with added lime can take up to 10 times more load than standard cement?
 
Apparently cement with added lime can take up to 10 times more load than standard cement?
I thought adding lime to cement mortar, usualy hydrated rather than hydraulic or fat lime, increased flexibility rather than strength.
I have always taken the view that the stuff between the bricks, or stone for that matter, was to keep them apart rather than stick them together.
 
I have to re-point a small section of my terraced house pine end wall. Its a black mortar fill, the house dates to early 20th century, so I'm guessing its a lime based mortar? Where do I get some, or how do I make it? Any ideas?
I'm no expert but I have never seen dyed lime mortar in an early 20C brick building. Pointing should be sacrificial, ie softer than the brick so that it fails before the brick. Hydraulic lime is relatively easy to use but will need protecting from cold weather and keeping damp for the first few days to a week.
 
Forget all you know about mixing cement mortar, because it's not relevant to lime mortar. That's that out of the way.

Put your gloves, dust mask and glasses on, as you don't want to get it in your eye or breathe the NHL in.

NHL 3.5 as already specified and mix it 2 parts sharp sand to 1 part lime. Gauge the lime and sand using a bucket and don't tamp it down, as you'll use loads of lime. Don't use building sand or yellow soft sand as it's too fine. you want a bit of grit.

Put it in a belle mixer, if you have one and mix the dry ingredients for a while. Then add some water, you want enough water for it to start balling up and no more.

Then add a couple of rocks the size of skittle balls (heafty, but not massive), granite works well because it doesn't crush up. Lime mortar likes to be chopped and crushed, which is what the stones do.

Keep mixing it and add a little water, not too much as it's easy to make it too wet and runny. Stiff and sticky is what you want and it should stick to a trowel even when it's upside down.

You'll have to put your colour in at the beginning, but I don't have any experience with that, so I'm not going to comment on it.

The lime mortar wants to be sticky enough so that it hangs together in the mixer and the mixer drum is not caked with clumps of dry mix at the bottom. If it's too wet, youll have to add more lime, but this will screw the colour up and it won't be consistent between mixes. So gauge everything.

Once it's mixed, dump it in a barrow and get to work.

Lime mortar has two sets, an initial set where it goes stiff quite quickly and a hydraulic set when it starts to absorb CO2 and turn into calcium carbonate. The initial set can be broken by smashing the mix with a lump hammer. Once you've done that it becomes plastic again and takes a while to set, as it's relying on the hydraulic set this time and that can take a month to harden or longer if it's cold.

Don't add cement, ever, as that turns it into a cement mortar and defeats the object.

Using a finger trowel and a plasterers hawk or tyrolean plate, point your wall and overfill the joints. Leave it proud and wait overnight before finishing the joint, as you'll end up with white or black bricks if you do it earlier. If your pointing goes over a few days, cover the mix in the barrow with some plastic and knock it up again in the morning. Don't add water unless it starts to go a bit dry and won't stick together.

Protect from the sun by covering it with hessian and misting down if it's hot and don't point when it's frosty.

That's about it. It's not rocket science although it can get sciency if you want it too and you can also make it overcomplicated if you feel like it, too.
 
Forget all you know about mixing cement mortar, because it's not relevant to lime mortar. That's that out of the way.

Put your gloves, dust mask and glasses on, as you don't want to get it in your eye or breathe the NHL in.

NHL 3.5 as already specified and mix it 2 parts sharp sand to 1 part lime. Gauge the lime and sand using a bucket and don't tamp it down, as you'll use loads of lime. Don't use building sand or yellow soft sand as it's too fine. you want a bit of grit.

Put it in a belle mixer, if you have one and mix the dry ingredients for a while. Then add some water, you want enough water for it to start balling up and no more.

Then add a couple of rocks the size of skittle balls (heafty, but not massive), granite works well because it doesn't crush up. Lime mortar likes to be chopped and crushed, which is what the stones do.

Keep mixing it and add a little water, not too much as it's easy to make it too wet and runny. Stiff and sticky is what you want and it should stick to a trowel even when it's upside down.

You'll have to put your colour in at the beginning, but I don't have any experience with that, so I'm not going to comment on it.

The lime mortar wants to be sticky enough so that it hangs together in the mixer and the mixer drum is not caked with clumps of dry mix at the bottom. If it's too wet, youll have to add more lime, but this will screw the colour up and it won't be consistent between mixes. So gauge everything.

Once it's mixed, dump it in a barrow and get to work.

Lime mortar has two sets, an initial set where it goes stiff quite quickly and a hydraulic set when it starts to absorb CO2 and turn into calcium carbonate. The initial set can be broken by smashing the mix with a lump hammer. Once you've done that it becomes plastic again and takes a while to set, as it's relying on the hydraulic set this time and that can take a month to harden or longer if it's cold.

Don't add cement, ever, as that turns it into a cement mortar and defeats the object.

Using a finger trowel and a plasterers hawk or tyrolean plate, point your wall and overfill the joints. Leave it proud and wait overnight before finishing the joint, as you'll end up with white or black bricks if you do it earlier. If your pointing goes over a few days, cover the mix in the barrow with some plastic and knock it up again in the morning. Don't add water unless it starts to go a bit dry and won't stick together.

Protect from the sun by covering it with hessian and misting down if it's hot and don't point when it's frosty.

That's about it. It's not rocket science although it can get sciency if you want it too and you can also make it overcomplicated if you feel like it, too.
I'm not sure the OP knows yet if the mortar is cement or lime based.
As you quote NHL 3.5 I asume you referring to bagged hydraulic lime.
Personally I would refer to the manufacturers instructions with regard to ratios, I have mostly used 3 to 1 sand to lime. Each manufacturer will be different, having more or less pozzolans in the mix. This will depend to some extent on the brick/stone and how soft or hard it is. You could use a weaker lime, NHL 2 if the brick is particularly soft.
The rocks would certainly help if you were mixing a fat lime mortar in a mini belle type mixer, I was lucky enough to find an inexpensive roller pan, I'm not sure they are strictly neccessary with hydraulic lime.
The mortar should be mixed for a long time, certainly more than cement mortar, as it tends to get softer/wetter/fattier the longer you mix. Be careful to add the water slowly as you don't want it overly wet.
I'm not sure I would agree with the 2 sets when it comes to hydraulic lime. Yes, once it starts to harden you can knock it back and it will soften but I believe this will weaken the eventual mortar. I am certainly guilty of knocking back hydraulic lime mortar that has started to set up.
The mortar will take a long time to full set, probably months. Initial protection from frost/cold for a week or so with damp hessian a good idea.
Different limes will go off at different rates. A bit of experimentation is usualy a good idea to know when to go back and start to clean up the joints. Temp will affect set too.
Try and match the sand too if using lime or cement. In the past sand was sourced locally and can have varying degrees of size of particles and sharpness.
 
or you could just buy the ready mixed lime mortar to the colour you want from the likes of Masons Mortar....just add water and away you go,,,
 
or you could just buy the ready mixed lime mortar to the colour you want from the likes of Masons Mortar....just add water and away you go,,,
Excellent company, although I haven't dealt with them for many years.
Although there were times I cursed the day Douglas introduced me to lime mortars.
 
I'm not sure the OP knows yet if the mortar is cement or lime based.
As you quote NHL 3.5 I asume you referring to bagged hydraulic lime.
Personally I would refer to the manufacturers instructions with regard to ratios, I have mostly used 3 to 1 sand to lime. Each manufacturer will be different, having more or less pozzolans in the mix. This will depend to some extent on the brick/stone and how soft or hard it is. You could use a weaker lime, NHL 2 if the brick is particularly soft.
The rocks would certainly help if you were mixing a fat lime mortar in a mini belle type mixer, I was lucky enough to find an inexpensive roller pan, I'm not sure they are strictly neccessary with hydraulic lime.
The mortar should be mixed for a long time, certainly more than cement mortar, as it tends to get softer/wetter/fattier the longer you mix. Be careful to add the water slowly as you don't want it overly wet.
I'm not sure I would agree with the 2 sets when it comes to hydraulic lime. Yes, once it starts to harden you can knock it back and it will soften but I believe this will weaken the eventual mortar. I am certainly guilty of knocking back hydraulic lime mortar that has started to set up.
The mortar will take a long time to full set, probably months. Initial protection from frost/cold for a week or so with damp hessian a good idea.
Different limes will go off at different rates. A bit of experimentation is usualy a good idea to know when to go back and start to clean up the joints. Temp will affect set too.
Try and match the sand too if using lime or cement. In the past sand was sourced locally and can have varying degrees of size of particles and sharpness.

Yes, the NHL bit refers to natural hydraulic lime and mostly it comes in 20kg bags in the UK. The pozzolan is added to the mix at the time of mixing, not contained in the bag of NHL.

To the OP, don't worry about adding pozzolan for pointing bricks, it's not necessarry.

The two sets have been well documented and in 22 years of solely building in lime, I've never noticed a weakened mortar from knocking up, and as far as I know there is no documented evidence of it happening. If you have any and post a link, I'd be more than interested in reading it.
 
Yes, the NHL bit refers to natural hydraulic lime and mostly it comes in 20kg bags in the UK. The pozzolan is added to the mix at the time of mixing, not contained in the bag of NHL.

To the OP, don't worry about adding pozzolan for pointing bricks, it's not necessarry.

The two sets have been well documented and in 22 years of solely building in lime, I've never noticed a weakened mortar from knocking up, and as far as I know there is no documented evidence of it happening. If you have any and post a link, I'd be more than interested in reading it.
I've been building with lime, originally fat lime mortars which I made myself using 1-1.5 tons of quicklime per batch by the hot mix method since the early 90's. This was occasionaly gauged with hydraulic lime, St Astier 3.5 mostly as it was the only one available.
I haven't lived in the UK since the early 90's and back then, lime mortars, both hydraulic and fat, were hard to come by. I imagine they are more common now.
I have no evidence but this is what I was taught by respected members of the lime forum.
The plasterer who plastered my house had the contract to plaster Stirling castle, we didn't get on very well. I remember he used to wear suede brogues to work, very old school.
There are numerous premixed hydraulic lime mortars here in Italy, some I believe now available in the UK. Given how easy they are to use I suspect they have pozzolans already added.
 
I've been building with lime, originally fat lime mortars which I made myself using 1-1.5 tons of quicklime per batch by the hot mix method since the early 90's. This was occasionaly gauged with hydraulic lime, St Astier 3.5 mostly as it was the only one available.
I haven't lived in the UK since the early 90's and back then, lime mortars, both hydraulic and fat, were hard to come by. I imagine they are more common now.
I have no evidence but this is what I was taught by respected members of the lime forum.
The plasterer who plastered my house had the contract to plaster Stirling castle, we didn't get on very well. I remember he used to wear suede brogues to work, very old school.
There are numerous premixed hydraulic lime mortars here in Italy, some I believe now available in the UK. Given how easy they are to use I suspect they have pozzolans already added.

I get my lime from Breckweg in Germany, it's a very pure form of NHL and has a lovely cream colour and sets much slower than St. Astier. The reason I use it is because of the beautiful colour, as I found St Astier to be too grey and also for the fat lime rich mix that I can achieve with it.

I was initially taught by Dr. Gerard Lynch, who's a master mason specialising in gauged brickwork as well as being an academic. I choose to listen to his reasoning that historic mortars would have included impurities, such as clay, present in the lime at the time of burning and were by their very nature naturaly hydraulic to some extent. This is also why I favour a lime rich NHL mix over a lean one, as historic lime mortars contained a much higher amount of lime than modern NHL mixes do.

There's a lot of nonsense spouted about NHL mortars and how they can set as hard as an OPC mortar and damage masonry, which is clearly rubbish, as an NHL 3.5 mortar (3.5-10N/mm2) will never achive the crushing strength of OPC mortar (33-53N/mm2 ) or Caen limestone (43N/mm2 ).

I was at Canterbury Cathedral a couple of days ago, where they are using modern hot mix mortars, made from a very pure type of quicklime, for repairs to high level masonry on the west towers. The masons there were adamant that they were following the correct method by using this feeble hot mix, but they didn't seem to be allowing for the exposed position where the masonry was subject to driving rain and frost.

I'm not sure that it was such a good idea, but I guess time will tell. My NHL mortars are happy in exposed positions down to -25ºC and don't damage soft red hand made brickwork.
 
Now that must be quite a rarity Adam!
I'm curious what started the interest? Your own house?
A love of historic architecture from when I was a Barking Tec. (aged16) and discovered Lutyens' Munstead Wood.

What's not to like.....I still want to live there.

Munstead Wood.jpg
 
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