New lime mortar

[KelvinM]

Hi there

We are at the tail end of having some rather major remedial work done on our c300 year old non-listed cottage. Basically we purchased the place and round that the timber soleplate sitting on top of some brick plinths had rotted. After getting specialist advice we opted to remove a c.1970 concrete floor that was pushing damp into the walls, re-instate a suspended timber floor (with adequate ventilation), re-built the brick plinths with 2 dpcs (one at the base between the what was left of the old plinths, and one slate dpc one course below where the new oak soleplate was to sit), and then a new soleplate sitting on top of some lead. On top of that we have re-plastered as there was considerable damage to the studs. All of the plastering and brickworks has been done in time to prevent issues with trapped moisture in the future.

We picked a local specialist in old buildings who came highly recommended and we have been largely very pleased with the work. I do however have one concern. The brickwork was done between July and August and I am noticing that there are still some areas where the lime mortar joints are very crumbly (but dry); I would describe it almost like sand as even if you brush areas lightly you get small piles of granulated mortar on the ground. Is this to be expected? I realise that lime takes a long time to dry, but I was not expecting crumbling, even localised. My understanding is that they used a 1:2:1 gauge (lime: sharp: sand)

What has added to my concern is the following: separately I have had a fireplace repointed using a ready mix lime mortar from a well known supplier in Devon. This work was done later and there is no crumbling on the surface of the mortar joints.

I am going to speak to the contractor about the issue some more, but I just wanted to get a sense from this forum for if this is an issue and what could have caused it.

Thanks

 

[TheForge]

Lime mortar shouldn’t crumble if well mixed as you’ve seen from the Mike Wye pre mixed. The sand may have been too fine or has it been exposed to frost or heavy rain which may have washed out the lime binder?

The ratio you quoted is essentially 1:1 binder to aggregate which is very lime rich but this again shouldn’t cause crumbling in itself, in fact the opposite. It would normally be expected in an overly weak/sandy mix.

Was the mortar lime putty or NHL? Was it applied in very warm weather or exposed to direct sun? Did it dry too quickly without suitable tending or protection? Are there shrinkage cracks as well as the crumbling?

Photos would be helpful.

 

[paulc]

The brickwork was done between July and August and I am noticing that there are still some areas where the lime mortar joints are very crumbly (but dry)

July and August have been exceptionally hot compared to other recent years. But if the brickwork had been sufficiently wetted down first and kept covered with damp hessian, I wouldn't have expected it to be crumbly.

I did a little bit of brickwork myself at the beginning of August. The bricks were left to soak in a tub of water for several days, and the mortar mixed from NHL3.5 and sifted shard sand at a ratio of 1:2 - Probably a bit more lime than was absolutely needed. Didn't bother with covering the brickwork as it was at the base of a north facing wall, even so, the mortar has hardened quite well and isn't crumbly as you describe.

 

[KelvinM]

Thank you for the replies so far. My understanding is that NHL 3.5 was used in the mix, not lime putty. I do not recall any damp hessian being used to preserve the moisture and the brickwork was completed when the weather was very hot. There is evidence of some minimal cracking, mainly in the top courses that I think may be driven by the weight of the oak soleplate and timbers studs.

My other concern is that there are 3-4 courses of brickwork below the floor and under the courses where I can see the problem. What I don't want to happen is for the building to be compromised by weak lime mortar joints.

When using the Mike Wye pre-mix, we were very careful to ensure that there was sufficient moisture in the brick and mortar joints, and made sure we kept the work damp for at least 72 hours after the work was done using a water spray. And that work was done in far cooler weather.

I will post photographs tonight. Thanks again.

 

[noj]

If the problem was the hot weather/drying speed, then the stuff under the floor should be fine since it won't have been exposed in the same way and will have been quietly carbonating away in it's cooler, moister micro-climate - it's probably still a bit damp...

 

[malcolm]

It was a bad summer for lime. I gave up and went inside to do some linseed paint - it was a good summer for that!

Lime does need to stay damp for a few days. As has been said it is good to water it to keep it damp and then cover it with hessian or an old bed sheet so the sun doesn't get at it. Where mine has dried too fast it has a different colour.

The lime inside the wall will have had a better chance. If you scrape away the loose stuff with a screwdriver how far is it before you get to something solid?

It has been really dry for months. My chemistry isn't good and I don't know if the crumbly lime would get better if you hose the wall with water a couple of times.

 

[livias]

No Malcolm you cannot regenerate lime by hosing it down

The main use for the mortar between the bricks is to space the bricks apart, so it doesnt really matter if it has not set too well. The other use of the mortar is for pointing and that does need to be well set so it sounds to me as though it will need repointing. Too late to do it this year so I suggest you have a chat with your specialist and get them to schedule it in for the spring. It should be ok until then