‘Reactivating’ old hydraulic lime

[Big Gavin]

i wonder if anyone can offer any advice on the following for me?

around 8 years ago we had our house re-rendered in lime which was delivered in 1 tonne builders bags. It was a dry mix which I believe is therefore a hydraulic lime?

when the project was finished there was around 1 tonne surplus.

I previously successfully used this for repointing work, by crushing the old hardened mortar and mixing with water. The pointing work is aesthetic only as it is on garden walls rather than the house.

recently I tried the same remixing technique bur when the mortar dried it has quickly become crumbly (I kept the drying mortar damp whilst curing using old towels).

could it be that I need to ‘reactivate’ the old mortar mix with fresh lime?

If this is this is correct, what ratio of new lime should I add to the old sand and grit render mix?

 

[MatthewC]

I'm not totally sure of the science as I was never good at chemistry. However, I suspect that using fresh lime to try to reactivate the old stuff could simply be an expensive waste of the fresh lime. I would guess that if it had all hardened and needed crushing, then the chemical reaction had already started. Therefore the new mix will not be as good as it should be, which is why your recent attempt became crumbly (and therefore weak, liable to fall off).

NB Are you sure it was hydaulic lime or could it have been hydrated lime?

 

[RBailey]

If left out it will eventually carbonate from the moisture and CO2 in the air.
Putty can be kept for a long time but only if it is in an air tight container / under a water blanket.

Sorry.

 

[Stevers]

As long as it is well broken up - with a texture between building sand and sharp sand, any stale lime already in it will act as aggregate and you should be able to add new lime in the usual proportions (1:3) to reuse it as mortar. When repointing my garden walls I use the clean sweepings from tooling up and brushing down the joints in just this way with fresh NHL 3.5.

 

[stuart45]

At Tech we used lime mortar. When the work was knocked down the bricks were cleaned up and reused, and the mortar was thrown back into a roller pan mixer for reuse.

 

[Big Gavin]

I'm not totally sure of the science as I was never good at chemistry. However, I suspect that using fresh lime to try to reactivate the old stuff could simply be an expensive waste of the fresh lime. I would guess that if it had all hardened and needed crushing, then the chemical reaction had already started. Therefore the new mix will not be as good as it should be, which is why your recent attempt became crumbly (and therefore weak, liable to fall off).

NB Are you sure it was hydaulic lime or could it have been hydrated lime?

Matthew,
thank you for responding. The chap that did the original render work has advised it was actually a lime putty mix and not hydraulic lime.

I know he said that to me at the times did the original work, that he used to sweep up the old dust from his ‘bag and scrape’ finish and reuse it at home, simply mixing it with water (which is what I successfully did as well some years ago), but my latest attempt failed as noted in my original post. Hence the reason I wondered if the old mix could be ‘
reactivated‘

 

[Big Gavin]

As long as it is well broken up - with a texture between building sand and sharp sand, any stale lime already in it will act as aggregate and you should be able to add new lime in the usual proportions (1:3) to reuse it as mortar. When repointing my garden walls I use the clean sweepings from tooling up and brushing down the joints in just this way with fresh NHL 3

stevers,
thank you for responding. was your lime hydraulic, or non hydraulic? The chap that did the original render work has advised mine was actually a lime putty mix and not hydraulic lime (My original post was incorrect)

I know he said that to me at the time he did the original work, that he used to sweep up the old dust from his ‘bag and scrape’ finish and reuse it at home, simply mixing it with water (which is what I successfully did as well some years ago for repointing some brickwork on a garden wall).

I had hoped to do the same on a new stone wall I’m building, but my latest attempt pointing in some coping stones failed as noted in my original post. Hence the reason I wondered if the old mix could be ‘reactivated‘

can I bash up and mix old non hydraulic lime render and if this is to reactivated what type of lime and mix would be required?

gavin

Gavin

As long as it is well broken up - with a texture between building sand and sharp sand, any stale lime already in it will act as aggregate and you should be able to add new lime in the usual proportions (1:3) to reuse it as mortar. When repointing my garden walls I use the clean sweepings from tooling up and brushing down the joints in just this way with fresh NHL 3.5.

 

[Stevers]

I always use NHL 3.5, at about 1:3, but don't see why it would matter which type of lime. It must have been a non-hydraulic lime putty mix that your builder supplied, and he would definitely have been able to take the sweepings home and just add water to use them, because the slow carbonation process would barely have started at that stage. After a year though, carbonation of the lime in the bagged mix would be pretty much complete, and you would need to add fresh lime of either sort to continue - subject to the required processes and limitations of the chosen type. Wall cappings of course being about the worst case for exposure along with chimneys. It was a wall capping that I completed too late in the year using fresh NHL 3.5 that partially failed that winter.

Due to the myriad shortages after the war, my father used hydrated lime instead of cement to build some cold frames for the family nursery; that I was frequently informed had been a total failure, probably because it wouldn't have been taken through a lime putty stage, and it probably dried out long before it would have set through carbonation. He also taught me as a teenager to add washing up liquid to cement mortars, so the less said about that the better!