Workability of NHL based mortar

[Zebra]

I've bought a bag of NHL 3.5 and am trying to do some brickwork repairs with it.
I'm using sharp sand as recommended by the manufacturer in a 2:1 ratio of sand:lime.
I've worked it with a paddle mixer for 10-15 minutes or so, tried it a bit wetter, tried it a bit drier.
But I can't get the darned stuff to stand up on the pointing trowel! So I can't get it in the joints! It's not becoming workable at all. It's either sloppy or sandy.
What am I doing wrong?!!

 

[skier-hughes]

Our pointers mixed for a good 1/2 an hour, then left it for about the same, and mixed for another 1/2 and hour at least.
So I'd give it plenty more mixing time.

 

[MatthewC]

Just a thought from my little experience: how old is the NHL? I have heard (on this forum possibly?) that it does go off with age and should be used in 3-6 months (I think). I haven't a clue as to exactly what changes chemically (I did fail O Level).

 

[AMc]

My experience with lime mortar was bedding a patio but I was advised to mix it for about an hour with a 10min rest that was using a drum mixer.
It really changes consistency and I had to be very careful adding water as it seemed to reach a tipping point and would go from sandy to slop with a cupful of extra water.
I was using 3:1 with NHL5 and found it stiffened up really fast once I stopped mixing so I'd be tempted to keep it turning over.
Not sure how you'll achieve that with a paddle mixer ... assuming that's what is colloquially known as a "whisk" around here?

 

[Zebra]

I see. So it needs to be mixed for longer. (Not sure how old the bag is, I just bought it, but it was the last one on the pallett. But product literature says it'll keep until opened, then only 2-3 days once opened!)
Yes I guess a paddle mixer is probably what you'd call a "whisk" - like the thing plasterers use in a drill to mix up plaster. I've been mixing it up in a bucket. The quantities I'm doing per day it's not been feasible to get a drum mixer in.
With the putty based mortar I was using before I was able to effectively mix up small batches that way, but perhaps I ought to have been mixing that for much longer too.
I'll have to hire a drum mixer then. I can't stand there for an hour with the paddle in the bucket!!

Nothing's ever easy is it?!
Thanks for your fast advice!

 

[MdB]

A paddle mixer will do a lot better job than a drum mixer for small amounts so it won't take as long and if you are only using a small amount a day then hiring a drum could get rather expensive.

I would either have another go with the paddle mixer just be very careful about adding water when you get to the critical point. Or I would switch back to putty / hydrate, hire a mixer and mix a lot up at once. As hydrate / putty doesn't set when wet, you can store indefinitely as mortar iin a plastic tub with lid good enough to stop it drying out (such as an old 10L plastic paint tubs). I mix up 80L (three batches in the drum mixer - uses up one bag of hydrate) and then store until needed.

Personally I think that an hour in the mixer is longer than is needed. 20 mins to half an hour seems enough to me but it may get better if you leave it longer. I do agree however that you need to remix before use especially if not used for more than a day. I use a paddle mixer in a bucket and bash it about a bit. I makes a big difference to the consistency.

 

[Zebra]

Thanks MdB, I know you said before in another thread that putty/hydrate based mortar should be fine, and while I'm not questioning that, it would seem to be a grey area. Once again I asked the Old House Store, and they said that putty/hydrate based mortars can be fine for external brickwork, but perhaps not on chimneys or in exposed areas. They said that it might not last as long or be as durable as an NHL based mortar. I'm not in a very exposed area but it is for a chimney, which is why for peace of mind I bought a bag of the NHL to try it.

Interestingly, I also asked a professional historic brickworker, who also said that the putty/hydrate based mortar may not fail, I might be lucky, but that Hydraulic has had a lot of failures recently (especially Singleton Birch) so he now uses a putty mortar with Argical or Metastar which is a calcined clay additive, and it gives an excellent mortar. Or I could use the NHL with a little hydrate to fatten it up.

I'm on the point of giving up and getting a professional in, waving my white handkerchief, admitting I'm out of my depth etc. But perhaps (glutton for punishment) I'll just buy a bag of the Argical or Metastar....

 

[AMc]

I see your point about drum mixing being overkill for the amounts you can practically get through in a day doing pointing.

Have you looked into a premixed mortar - it's more expensive kg for kg but the reduction in plant hire and wastage might be worth it?
e.g. http://www.mikewye.co.uk/mikeprices.htm#limemortarunhaired3or3.5/1

My experience of NHL5 was that it was a lot of hard work and 'went off' much quicker than I would have anticipated - faster than cement I've used before.
I was using it to bed sandstone patio flags I was about able to use a "5 buckets" mixer load to create full beds in the time the next load took to mix up. I was lucky to get 5 mixes in a day (starting at 9.30 and finishing at 4 to suit school runs).
The last bag I stored dry for several months then opened up and used to patch repair a few floorbricks in the kitchen, hand mixing in a bucket but not for anything like as long. Obviously it needs little strength in that application and I'm prepared to accept it's probably not that great a mix.
After about 4 days the NHL was clumped together in the bag and I wouldn't expect it to be usable at all much after that.

I'm no expert - just a skint amateur trying my best though

 

[Zebra]

Have you looked into a premixed mortar - it's more expensive kg for kg but the reduction in plant hire and wastage might be worth it?

I've looked into using a ready mixed mortar but all the ready mixed mortars are putty based, and not hydraulic. What I have been advised that I need for this application is a hydraulic mortar, as it is doing a very structural job - replacing all the mortar, the entire depth of the joint, on one entire face of a chimney.
I might try again with mixing a new batch of NHL, but the stuff I did yesterday hasn't set.
Thanks for your help though.

 

[AMc]

One of the Mike Wye mixes is designed for adding pozzolan to get a hydraulic set but my knowledge of that is from the single line on the site.
I emailed them and they were dead helpful with my newbe lime questions.

I might try again with mixing a new batch of NHL, but the stuff I did yesterday hasn't set.

That seems odd to me as the 3:1 sharp/NHL5 I mixed was stiff in a matter of a couple of hours and set in less than a day. Not sure at what point it carbonates (serveral months I believe) but it was impossible to move slabs I'd laid after an hour or so.

 

[Zebra]

I've now bought a bag of ready mixed dry hydraulic mortar - it's got the sand and the lime already, you just add the water - from the Old House Store. Will see tomorrow how that goes.

 

[Flyfisher]

What I have been advised that I need for this application is a hydraulic mortar, as it is doing a very structural job - replacing all the mortar, the entire depth of the joint, on one entire face of a chimney.

I'm afraid I can't really help with the NHL point but I'm interested to know why it was specified for a 'structural' job. Does anyone know why putty/hydrate is not regarded as suitable? Don't they build complete houses with putty/hydrate mortars? That would surely be a structural job.

Good luck with the ready-mix though.

 

[MdB]

Putty / hydrate is really poor in compressive strength - 0.5 to 1 comes to mind - compared to NHL 3.5. I believe its tensile strength is equally poor.

Walls etc generally have roofs or the equivalent pressing down on them which means that the mortar is generally a gap filler and doesn't have to do a lot of work. Chimneys on the other hand have to deal with wind loads and the like without extra support so a stronger mix is required.

 

[Zebra]

I've now bought a bag of ready mixed dry hydraulic mortar - it's got the sand and the lime already, you just add the water - from the Old House Store. Will see tomorrow how that goes.

Since writing the above I've been working on it every day except one, and managed to get it done, the whole chimney, 40 courses on three sides. You've never seen a hooved animal so pleased with a bit of pointing. Darned hard holding the trowel with these hooves though.

 

[MdB]