Mixing up a Lime mortar correctly

[purplemonkey]

hi,

Over the last few weeks I've been reading up on how to use Lime products, but it mostly leaves my head spinning. Based on the available information I can see why people simply stick to cement! however I am starting to repair some failed brickwork in my property and I want to use lime mortar for this job.

so here's my scenario, I bought a 25kg bag of NHL 3.5 (powder) and a 20kg tub of Lime putty (a white putty). my thinking being I'd try them both out, to see which I prefer.

my question though, how do I mix them up correctly.

First off I'd try the the bag of dry NHL 3.5

1. I put one measure of nhl 3.5 into a bucket
2. added little water and mixed till it was a malleable ball of putty
3. then I added 2 measures of sharp sand to it, mixed it up, and it looked like a good mortar.
4. I've put one brick down with it and left it over night, and its looking pretty good (to my lime untrained eye).

Using this method, I am assuming the lime putty would put me at stage 3 instead?

The main question that keeps reoccurring in my head is I keep reading bits of info saying you should mix the lime/water and leave for at least 24 hours?

Thanks in advance for any advice you can offer.

 

[JanJonWestWales]

hi...Im not an expert, but have done lots of liming on our project (Putty, NHL 2, 3.5 and 5)
My process is somewhat larger as I use a cement mixer, but essentially...

For Putty, Only really used for indoor walls and ceilings. The mix is 3:1. Sand in first, then putty, leave to mix for a good 20mins to ensure an even mix, then add water slowly (if needed ...it often isnt) until you get the consistency for the job....

For external pointing etc. Ive used 3.5 powder (and 5/5Pozzolan for hard surfaces...limecrete floors, sills etc)
Mix is similar 3:1, sand in first, add the powder, let it mix thorougly (only takes 5 mins for powder), then add water to consistency required ( I would assume faily dry for pointing), and use straight away...no waiting.

Remember the wall/surface needs to be wet before hand, and covered after to ensure the lime doesnt dry too quickly ( I use old hessian sacking). It can take a week+ to dry properly, depending on situation, thickness and conditions.

good luck, and hope this helps

http://blaencwmbychan.blogspot.co.uk/

 

[agbagb]

It sounds like you have a bit of information overload. You don't need to mix NHL in to putty before adding aggregate. 3:1 sharp sand to NHL is a good all round mix. 2:1:1 sharp sand : Soft Sand : NHL can be a bit easier consistency to work with. Just bung it all in bucket, mixer, board, add water and mix. So it's like using cement although you should mix it for longer or leave it for a cup tea before using.

The difference comes using it, first the brick or joints need wetting down, then when you've laid the bricks or applied the pointing - when the mortar has stiffened due to the water being drawn out, it will have shrunk so the surface has to be compressed firmly with your trowel or other tool.
Next you have to make sure it doesn't dry out and crack further until it has started its initial set (will just be a few hours). On visible mortar I would then brush the surface for mainly aesthetic reasons.

I've put together some notes on how I do all this here

Using lime putty is similar to the above but requires more "after care" as it doesn't have an initial set. It can continue to shrink for days so you have to keep an eye on how its drying out and keep it moist if it needs it.

I'm sure others will add to this.

You've got plenty of materials for practising with so just have a go, see what happens, you'll soon get the hang of it.

 

[plasticpigeon]

I would add by saying sharp sand from a builders yard makes mortar very hard to work as the gritty bits are often very large, up to 6mm. You might get demoralised if you keep on trying to use builders yard sharp sand. Plastering sand is sharp but finer, and soft sand can be used for colour. I add some hydrated to my NHL to make a 1:1:6 mortar, just like a 1:1:6 cement mortar but substituting NHL for cement. Just mix dry and add enough water for the right consistency, and I'll second leaving it a bit and having a cup of tea before reworking it and using. Hydrated is cheaper than NHL and it makes the mix more workable and it is easily as strong as the original mortar.

 

[livias]

NHL mixes are better used asap, so don't leave them overnight
Putty mixes can be left as long as you like (years...) but cover them with water so they don't start to set

I don't know where you live ...
JanJonWestWales is perfectly correct that only NHL mixes are used outdoors - in Wild West Wales. Here in Sunny Suffolk we use putty mixes both indoors and out

 

[malcolm]

I've put together some notes on how I do all this here

That's a really useful site if a week late. It's good to learn from other's experiences rather than work it out myself.

 

[88v8]

NHL mixes are better used asap, so don't leave them overnight

Errrrm, I think if you knock them back they can be kept all day, left as long as you want.... I had some dried mix in a pot, about 14 month old, was NHL5+sharp, I bashed it with a mallet until it was powder then added water and used it last week.
It's set quite happily.

After all, if you knock off lime render, you can powder it, add water and re-use.

Funny stuff, lime.

Unlike cement mix, which after 2/3 hours should be thrown away and not knocked back to continue using it.

Ivor

 

[jackpoint]

Here's a quote from a wise man on this very forum. ...

" I still think the wine analogy is good. You can read thousands of books, spend zillions of pounds and wax lyrical with adjectives conceived for quite different purposes.
Or you can go to the nearest supermarket and grab a couple of bottles of whatever is in the bin by the door. I can't usually tell the difference.

If you buy a bag of 'lime' from the nearest builders merchant and pretend it is cement it works just fine. Mix it dry, one part of the dry white powder and three parts of sand as dirty as you like (I fish the larger leaves and twigs out), add water to taste as use straight away. I can't usually tell the difference.

Unlike cement, if you add too much water it doesn't matter - just leave it overnight and then pour off the water that has settled out on top.
Unlike cement, if you've made too much and don't use it all, stick it in a container and add an inch or two of water to cover it. It will keep forever. "

It's worked well so far

 

[Keithj]

... and above all, if you're doing repairs and the wall moves a bit, the lime flexes where concrete cracks.

 

[purplemonkey]

Thanks for all the comments. Its good to know I've been using it as most have suggested, and I've found it to be very pleasant to use, more so then a cement mix.

 

[Andrew Bodenham]

Errrrm, I think if you knock them back they can be kept all day, left as long as you want.... I had some dried mix in a pot, about 14 month old, was NHL5+sharp, I bashed it with a mallet until it was powder then added water and used it last week.
It's set quite happily.

After all, if you knock off lime render, you can powder it, add water and re-use.

Funny stuff, lime.

Unlike cement mix, which after 2/3 hours should be thrown away and not knocked back to continue using it.

Sorry, but that's just wrong information. I'm not going to explain why in great detail as there is good and extensive information about the use of building limes online and in numerous books for anybody who can be bothered to do a little research. But I don't like to see people who are new to the material being given spurious advice.

NHL limes achieve a chemical set due to their initial reaction with water, as well as undergoing a longer-term carbonation process. Attempting to re-use them by re-mixing with water after the initial set has taken place will, at best, result in a weak mortar.

Lime render, once carbonated, cannot be reconstituted - again the result, if it sets at all, will have a fraction of the strength of the orginal render.

Purplemonkey: Now that you've had some initial experience I'd do some more reading - maybe attend a lime course if you're serious about learning about the material and techniques. Most plastering / masonry work is time consuming and expensive, so it's false economy to fudge the job by using the wrong materials or wrong methods. Lime-based materials should be good for decades of service when correctly applied.
Essentially, as others have said, you can think of NHL limes as analagous to cement powder, whilst putty is simply mixed with the sand and the resulting mortar can be stored (with air excluded under a layer of water) indefinitely. They are two quite different materials. In both cases the nature of the sand, and the mix ratio, can have a big effect on the perfomance of the mortar / plaster.

 

[stuart45]

Lime mortar was used at College for brickwork as the work could be knocked down and the mortar thrown back in the mixer and reused over and over again, sometimes a bit more lime was added. Although the mortar would still go hard after a few days the final strength would be much lower than lime that hadn't been allowed to carbonate and remixed.
When mixing NHL it needs about 20 minutes in the mixer, left for 20 to fatten up and then mixed again for a couple of minutes. Be carefull not to add too much water at the start, as after about 10 minutes it will start to get wetter as air gets into the mix.

 

[88v8]

Errrrm, I think if you knock them back they can be kept all day, left as long as you want.... I had some dried mix in a pot, about 14 month old, was NHL5+sharp, I bashed it with a mallet until it was powder then added water and used it last week.
It's set quite happily.

After all, if you knock off lime render, you can powder it, add water and re-use.

Sorry, but that's just wrong information.
Lime render, once carbonated, cannot be reconstituted - again the result, if it sets at all, will have a fraction of the strength of the orginal render.

Thankyou. I sit corrected. It seems one can pick up bad practice, even from those who regularly use the material.
All I can say about the strength is that it does set.
But then, knocked-back cement also sets and yet that certainly is wrong.

Ivor

 

[Flyfisher]

It may be bad practice, but it may also work - well, a little bit.

Lime (calcium hydroxide) sets by absorbing carbon dioxide from the air, turning back into the calcium carbonate it was made from. Lime mortar on the outside of any brickwork will set first as it is exposed to the air, but on the inside it will set more slowly as carbon dioxide has to seep through the mortar.

Andrew is right that when fully set lime mortar cannot be reused, because calcium carbonate is stable and will not react any more. However, full carbonation can take a long time, possibly many years. it's therefore easy to see that Ivor could also be right about reusing a lime mortar if it has not yet fully carbonated and still contains some active lime. But it will contain less than the original mix and so the reused mortar will be weaker when sets - if it sets. The actual results will be unpredictable, hence it being bad practice.


For a more detailed discussion, this is a good thread from 2010: http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=11166&p=139299

Among other things it discusses the use of phenolphthalein as an indicator that carbonation has taken place (by turning a dark pink colour). In particular:

A while back I broke open a lump of 3:1 fat lime mortar the size of a cricket ball that I'd made two years earlier, and the core was still completely uncarbonated, turning dark pink with the phenolphthalein (showing that the pH was still around 10 or 11).

 

[plasticpigeon]

However a hydraulic lime has a chemical set as well as carbonation, so I believe that remixing this after more than a few hours would break the chemical set and make the mortar weaker than it would otherwise be even if the majority of the mix was uncarbonated.

 

[Flyfisher]

Yes, you'd be relying on any uncarbonated lime to give a setting effect and it would be a completely unknown quantity.

I don't think it's worth the risk.

 

[Keithj]

Didn't someone tell us that when they were excavating the old London Wall, they found some Roman lime that hadn't carbonated in the middle (but it was about six feet thick)?