Still confused by lime! Help!

[Stevers]

Whilst it dried out my basement chimney breast, it is true that I can't drive a picture nail into the 'plaster'. Elsewhere I've been able to chop it out with a bolster chisel just as I would expect. Chemically I suspect it does the job, but the end result will possibly be less accommodating of movement - something my house doesn't suffer from since it's built on bedrock. In my case NHL2 might have been a better choice for internal finishes, and that could have been as simple as using a 50:50 mix of NHL3.5 and Hydralime as the binder.

 

[stuart45]

So, would I be right in saying NHL has its place, but that’s ideally not on the inside of single redbrick walls?

Red bricks can vary in strength greatly. An Accrington Nori is red and one of the hardest bricks in the world, and red rubbers used for gauged brickwork are one of the softest.
However, a rough rule of thumb is that softer air limes are used internally and harder NHL is used externally.

 

[Kearn]

So, would I be right in saying NHL has its place, but that’s ideally not on the inside of single redbrick walls?

Yep

 

[Lacan07]

So, would I be right in saying NHL has its place, but that’s ideally not on the inside of single redbrick walls?

The issue is it can set harder than its original grade which might cause issues for houses that structurally move through the seasons or have soft red bricks. For internal work and repointing its perhaps best to try and use non hydrated lime if you can. Its important to note however that NHL's are specified on many projects and abundant form many outlets. If you have to use an NHL St Astier is considered the best.

 

[Flyfisher]

Does the lime/sand mix make a lot of difference to the mortar 'softness', or is that yet another characteristic to consider?

EG, what would be the resulting difference between a mortar mixed at, say 2:1, 3:1 and 4:1 ratio of sand:NHL2 ?

 

[Kearn]

The lime is only acting as the binder, so yes ratio is key, but softer isn’t better - more than a 3:1 would be seen as way too weak a mix, not enough binder and therefore a crumbly sandy mix. Less than 2.5:1 for a bedding mortar and you start to lose any integrity as there’s no aggregate to provide strength.

 

[stuart45]

The size of the grain makes a difference to the mix as well. Generally for most building sands a 3/1 mix is about the point when there's enough binder to completely surround every aggregate particle. However, for finer sands the binder ratio has to be increased to do this, so a 2/1 or 1/1 might be used.

 

[Maria D]

Thanks to everyone for the replies to this thread. They've been enlightening! I've now come up with a plan and found supplies locally.