Which mortar mix to use for repointing lime?

[perky416]

Hi guys.

I have some repointing to do (will do the whole house eventually) but im struggling to figure out what mortar to use.

The house is a 1957 traditional 3 bed semi, when i purchased it a structural engineer came out as old repointing had cracked, the report said this was likely due to the mortar used being incompatible with lime mortar. I also tested a peice with vinegar and there was some minor bubbling. So i know i have lime in my mortar, heres where it gets tricky.

Ive done a lot of reading and concluded that at the time of build cement mortar was mainly being used, but so was lime still. I cant figure out whether i have proper lime mortar or a sand/cement/hydrated lime mix.

I have tapped a large sample with a screwdriver and the noise was low pitched, however ive only ever done cement pointing so never had to do this before so this test is uncertain.

Please take a look at the pictures attached, one is of a sample of my mortar, the other is of the darker brown mortar that was used to repoint in the past and has cracked.

What would you guys do? Go with a 5 parts sand/1 part cement/2 parts hydrated lime or start looking at Natural Hydrated Lime mixes? It looks like im fine to go with the former if the repointing cracked because the water cant escape, but if it was because of movement NHL would be the way to go. I just cant figure out what to do.

All help greatly appreciated.

 

[Bonni]

Where's the crack? In the middle of nowhere or from a window or door area?

 

[^woody^]

There's an old bloke on here who knows alot about this bricklaying and lime lark. I think he finishes at 4 on Fridays, so may be along later.

 

[perky416]

Where's the crack? In the middle of nowhere or from a window or door area?

Its below the bay window, similar cracks appear below the bedroom windows and above the kitchen window. Its not what im re-pointing at the moment, it was just an example of the previous repair.

There's an old bloke on here who knows alot about this bricklaying and lime lark. I think he finishes at 4 on Fridays, so may be along later.

Thanks hopefully he will be along to help out

 

[stuart45]

The repointing might have been done in a really strong mortar like 3/1 sand/OPC. The original is most likely to be a weaker mix of sand/lime/OPC.
A common mix was either 6/1/1 or 9/2/1 sand/lime/OPC.
You can always pay to have your mortar analysed.