Adding "bits" to lime mortar

[Zebra]

Hi

My original brick mortar has "bits" in it, small stones - I understand the term is aggregate? I have been using ready mixed lime mortar bought in 25k bags for repointing the brickwork that will be covered by render anyway but this has no bits in, and in the fireplace I'd like to match the original more closely. I have read other threads on here about matching the colour so I won't go over that ground again, but just a quick question - is it OK to add "bits" to a lime mortar mix? Will it affect the strength? What kind of ratio? I have a load of leftover lumps of flint and have experimented with smashing it up and seiving it (through the old net curtains!!) and it looks OK but afraid to add too much... I suppose for authenticity I should seive the local topsoil but don't know that I have the patience for that!!
Thanks for any tips!

 

[TheForge]

Hi Zebra, I'm no expert on lime mortars but having spent the last few months matching various mortars (with superb help from a local stone conservator) on our house for repointing I have some experience! I doubt adding anything to a lime mortar will be particularly damaging to its strength (unless in the extreme), in fact some additives (pozzolans) may act as catalysts, causing the mix to set quicker or even slightly harder. As you say many mortars had a large soil content originally anyway. What does the existing mortar look like, a photo might help someone suggest what the aggregate or additive is. Ash was a common additive to lime mortar and can look like little black stones or grit in the mix. If the bits are whiteish they might be chalk or topsoil.

Picture below shows some of the ash lime mortar we have matched - right hand side is old mortar, left hand side is mainly new pointing, with a mix of various sands and ash.

 

[robgil]

I raked out the old mortar , crushed it , added a wee bit of sand and some lime putty and bish bash bosh repointed part of my fire place with a very accurate matching mortar.

 

[middi]

Just add coarse sharp sand to the mix ( 40/50 % of total mix), point brickwork and leave flush with brick face.
12/16 hours later lightly rub pointing with stiff wirebrush. You can add sivved sub soil to the sharp sand and experiment with type of finish but not absolutely necessary. The above will give an excellent aged textured finish.

Middi

 

[Zebra]

Here's a picture of the old stuff....

 

[biffvernon]

Ash was a common additive to lime mortar...

The ash may not have been a deliberate additive but rather an impurity from the lime burning process. Coal and lime are mixed together and fired in the kiln. When the kiln is emptied the stuff at the bottom will tend to have a lot of ash mixed in.

 

[Zebra]

Tried mixing some today with some ash in it off the bonfire. Think it looked good, but was not set when I left, so will have to check it tomorrow.
However, did find that rubbing it down with wire brush afterwards as per Middi's tip made all the difference, even with the pre-mixed mortar I had been using, and even with the pointing I did last week, because it strips off the smooth top layer and the small stones in the mix show through. Good tip!
I find too that the longer it dries the whiter it gets - how long does that continue to happen? Is that why the existing mortar that's hundreds of years old is pure white? Will this stuff, even though it's yellowy when it goes in, will it eventually go white anyway?

 

[Zebra]

Just add coarse sharp sand to the mix ( 40/50 % of total mix), point brickwork and leave flush with brick face.
12/16 hours later lightly rub pointing with stiff wirebrush. You can add sivved sub soil to the sharp sand and experiment with type of finish but not absolutely necessary. The above will give an excellent aged textured finish.

Middi

Tried mixing it like this however, with extra sharp sand in , and it was kind of grey, and less workable. If the sharp sand is 40-50% of the total mix, do I assume the lime is still 25%, so the soft sand would be 25-35%? Tried it in a test patch anyway, and will see tomorrow what it looks like.
Thanks all...

 

[TheForge]

Is that why the existing mortar that's hundreds of years old is pure white?

I think the 'ancient' mortar is white though a combination of the lime gradually forming chalk as it carbonates over time and the fact that the original mix may have been little more than lime and soil. I doubt your mix will go completely white but it will continue to lighten for some weeks or even months!

The brushing down is crucial to reveal the aggregate - I've found that spraying the mortar with a diffuse spray from a hosepipe can have the same effect if done at the right time too. But it's difficult to judge the right time!

 

[MdB]

If you want to find out what colour the mortar is going to go when it is fully dried, shove a small amount in the oven for a while. It won't have carbonated so will have little strength but will show the colour ok.

 

[biffvernon]

Tried mixing some today with some ash in it off the bonfire.

Oh. Ah. Er, how shall I put this? Look, the bonfire ash was probably a bad move. It's not the same as coal ash. Bonfire ash is going to have bits of charcoal, carbon, whereas coal ash is the bits of coal that are not carbon. It's kinda the complete opposite and will behave in a less durable fashion.

Probably won't actually do any harm if there's only a little of it.

 

[Flyfisher]

But, taking your previous example, Biff, of ash in historic lime mortar probably being a by-product of contamination during the lime firing process, wouldn't any such ash have come from wood anyway? At least until coal started to be used in lime kilns?

i guess it's all a matter of how much wood ash is present in the mortar mix.

 

[robgil]

I find that NHL will come out grey but putty will whiten up better, you could add chalk to the mix.

 

[biffvernon]

But, taking your previous example, Biff, of ash in historic lime mortar probably being a by-product of contamination during the lime firing process, wouldn't any such ash have come from wood anyway? At least until coal started to be used in lime kilns?

Ah, but when was coal used in lime kilns? I'm assuming that most of the lime mortar that we see from the 19th century was coal fired and probably quite a lot from the 18th century. It would be easier to get high enough temperatures with coal. But I don't really know. I expect it varied geographically.

 

[biffvernon]

I find that NHL will come out grey but putty will whiten up better, you could add chalk to the mix.

It's the stuff that is not calcium hydroxide that is not white so one should expect NHL to be less white than hydrated lime.

 

[Zebra]

Tried mixing some today with some ash in it off the bonfire.

Oh. Ah. Er, how shall I put this? Look, the bonfire ash was probably a bad move. It's not the same as coal ash. Bonfire ash is going to have bits of charcoal, carbon, whereas coal ash is the bits of coal that are not carbon. It's kinda the complete opposite and will behave in a less durable fashion.
Probably won't actually do any harm if there's only a little of it.

Oh well, it was only a test patch. In fact though, despite all these experiments, I've gone back to using a normal mix, no bits, as advised on this forum - a 2:1:1 soft sand/sharp sand/hydrated lime mix, and although it's a bit creamy/yellowy and not quite white, it looks absolutely great, and I figure that as mortar is supposed to be the sacrificial element, and is designed to be replaced over the lifetime of the building, and as I'm replacing the whole lot everywhere where it'll be seen, I don't actually have to match the original.
I was going to write above: "despite all these experiments, I've gone back to using cement (ONLY KIDDING!!)" and then I thought, no, that's not even funny.....

 

[Zebra]

I raked out the old mortar , crushed it , added a wee bit of sand and some lime putty and bish bash bosh repointed part of my fire place with a very accurate matching mortar.

OH YEAH!!!! Just tried this last night, you can't tell which bits I've done!! It's not very workable, and it dries out a bit quick, so I added a dollop of a ready mixed mortar for strength and workability, but it matches exactly!

This is after trying about ten test patches of various ratios of old mortar with ready mixed, with various ratios of chalk in etc. My God I'm becoming such a lime geek! And Friday nights aren't what they used to be!