Lime Mortar: What Ratio?

[Roger H]

Hello all.

There appears a bit of vagueness about the ratio of lime mortar that should be used for external pointing. The Collin's/English Heritage Period House book says portland cement/lime/sand at 1:3:9. I suppose all here would recommend against the portland cement. What mix of lime to sand then?

I have seen 1:3 lime putty to sand suggested. But also 1:1 for lime from the bag to sand, then add water. I am guessing I am ok to mix up some hydrated lime with water to make lime putty, leave for a few days, then mix it with sand at 1:3. Does that sound about right? The project is repointing a soft sandstone wall some of which is north-facing.

Thanks for any advice.

Roger

 

[MdB]

I'm using ready mixed 1:3 putty sand for repointing as recommended by Mike Wye. Somebody else stated that I should be using 1:2 in another thread.

Cement + Lime mixtures used to be popular but have been discredited recently.

I've almost run out of the stuff I've bought so I was thinking of getting some hydrated lime and mixing it up myself. Advice on the best way of doing this would be appriciated.

I also assume that the ratios are by volume and not by weight. (Lime putty is about half as dense as lime mortar so it is important to get this right)

 

[biffvernon]

If you mix 1:3 dry hydrated lime and sand it's not far different to 1:3 putty to sand. Stange but true. When you add water to dry lime it's volume doesn't change much - it's just that the air gets replaced by water.

I wouldn't be too precious about the exactness of it all. I never use anything more sophisticated to measure than a bucket and shovel.

 

[Nemesis]

Roger - was about to email you with thoughts on this when logged on and read your post! Tardy I know... apologies.

Personal experience...

Stick dry lime in a mixer, slowly add water to mix a putty. Stick in buckets for a few days. Use a 3:1 sand and putty mix.

Mostly however we've used bought stuff so have only tried this in limited amounts for plaster, but so far very good.

Don't use Portland - not needed, not good, now rather discredited unless you are Tynedale's Planning dept and it's what they require people to use...

:roll:

Say no more!

 

[Gervase]

1:3 lime to sand by volume works for putty or powder; but to echo what's already been said - no cement whatsoever. And no need for any Feb, wetting agent, detergent or the like - they were all introduced to try to achieve some of the benefits of guaging cement compo with lime.

 

[Andrew Bodenham]

As everybody else has said, about 1:3, but you really don't need to be too precise about it. I've found a good gritty sharp sand to be best for pointing, though you should be guided by what's there already.

For mixing a big, slow drill with a mixing paddle is ideal for small quantities. I use a regular drum mixer for larger amounts, but you need to let it run longer than you would for OPC mortar.

Some people get a bit mystical about making putty and leaving it to mature for a bit, which is fine, but I've had good results just mixing in one shot and using straightaway.

One of the great things about using mortar made with hydrate is that you can keep it for ages if it's airtight. I found this partly by accident when I left a half used bucket outside and it filled up with rain. 4 months later I poured the water off, mixed it up and finished using it - it was perfect!

 

[Roger H]

Great, thanks for advice, that seems a lot clearer. I will batch up some hydrated lime with water this weekend and leave it in a tub for a week or so. Is normal water ok? (Only kidding).

Enjoy your projects,

Roger.

 

[MdB]

Snap - I think I'll be mixing some up as well.

Am I correct in saying it is better to add the hydrated lime to water rather than the other way around for safety reasons? (just in case it starts getting hot and spits).

I guess also that it is better to use too much water rather than too little as the putty will just sink to the bottom and the excess water can be drained off later?

 

[biffvernon]

Hot and spitty? No, that's what happens with quicklime. You'll be ok with hydrated.

 

[andrew gayton]

1:2 1:2 1:2 1:2 1:2

 

[Nemesis]

I knew you'd come along and say that...

We used the 1:3 mix for plastering and it was fine, and have used a 1:3 ready made putty mix in the past which has been fine.

Do explain why you think it should be 1:2...


I add that aggregate should be carefully chosen to match existing etc etc. where possible, although in our case that's also a great deal of lumps of lime and various other odds and sods in with the original pointing.

 

[andrew gayton]

the 1:3 mix has come from the 'historic' use of mortars made on site using quicklime and aggregate at the ratio of 1:3..... as soon as the quicklime has slaked, it can almost double in volume hence my advice to go for a 1:2. close inspection of 'historic' mortar shows that it is invariably laess sandy than a 1:3...................

 

[Gervase]

At the risk of being a pain in the neck, I have to disagree. Most of the older lime mortar I come across was clearly made as a hot mix, and I know from anecdotal evidence that such was invariably the way it was done - a cart full of quicklime would turn up and the lime would be shovelled into a pile of damp sand and mixed, with the slaking taking place in the sand. The whole steaming pile, once thoroughly mixed, would be covered with tarpaulins, and as each day's quantity of mortar was required, the tarps would be drawn back and so many barrow loads taken out to a spot board where final mixing took place using a gadget that looked like a draw hoe.
Looking at the resulting mortar one can see that (a) its structure is quite different to a putty or hydrated lime mortar, in that there are significant lumps of free lime, and (b) the ratio of lime to aggregate is nevertheless around 1:3.
You can guage the ideal ratio if you have a measuring cylinder. Fill it up to a measured point with your dry aggregate, and then add enough water to come just up to the surface of the aggregate - so that each grain is completely surrounded by water.
How much hve you added? Say you had 300ml of sand, my guess is that, for the average sharp sand, you'll have poured in around 100ml of water. That's certainly what I get. So, to ensure that each grain is encapsulated in lime (which occupies the same volume as water), you'd be working with a 1:3 ratio.
One could work backwards and take a known volume of old mortar, steep it in brick acid and then wash and dry the results and see how much the volume has gone down. I'm willing to wager it will have dropped by one quarter rather than one third.
I have tried using a 1:2, and for me it seems to be far more prone to cracking - particularly with the coarse aggregate that I use from here. I imagine such a mix would work better with a finer sand, the way a 1:1 finish plaster works best with silver sand (and when applied thinly).

 

[biffvernon]

For strength, the grains of quartz (which are hard and strong) need to just touch each other, transferring loads from one grain to the next. The purpose of the lime is to hold the quartz grains steady so they can do their work. You want just enough lime to fill all the gaps but no more. Too much lime gives a weaker structure because some of the loads are born by the much weaker and softer calcite crystals.

For a finishing plaster, load strength is not important so the ease of getting a smooth finish with lots of lime predominates.

Therefore, Gervase is right.

 

[skier-hughes]

We had our lime analysed and they came back to us with a 1.3 mix.

 

[worms]

To come back to RogerH's original query...

I've now pointed with all manner of really dreadful mixes of sand, putty (and water!) and the amazing thing is it is all so forgiving! For pointing, I wouldn't get too hung up on the recipe! Since I only make up a bucket load at a time, I take the view that if any particular batch fails, then all I have wasted is an hour or two's work. So far none has failed on me and at the very least I have stabilised parts of the wall.

At the end of last summer, I put the final bucket load back in the garage in a plastic bag, after pouring some of the fluid off the putty bin over it. Yesterday I dug it out and mixed it up and so far it looks like it is setting.

 

[andrew gayton]

we will have to agree to disagree.

 

[Nemesis]

Why do you think 1:2 is preferable?

 

[andrew gayton]

in my experience it is a better match for repairs. having used it myself (1 part lime 2 parts [very] sharp sand) for flintwork repointing, i had no shrinkage problems.

 

[Manny]

Good information, thanks very much, very useful.

May I ask where one procures a bag or two of ready mixed lime suitable for a little repointing?