Lime Mixes - Weight or Volume?

[Flyfisher]

Seems like I've had my first lime failure.

I've laid a limecrete slab in an inglenook fireplace in preparation for installing a boiler stove. The base was laid on compacted soil/hardcore and was a 6:2:1 mix of leca : sharp sand : NHL2 by volume. The thickness varied between about 6 to 8 inches. It was firm to the touch the morning after laying and all seemed ok, though I left it another day before laying the final screed.

The screed was a 2:1 mix of sand : NHL2 but I was running short of sharp sand so mixed it 1:1:1 - sharp:soft:NHL2. I also read that it should be mixed by weight instead of volume and should be 6 buckets (15 litres/bucket) to a 25kg bag of lime. So I mixed 3 buckets sharp sand, 3 buckets soft sand to 1 bag of NHL2.

The day after laying, the resulting 50-60mm screed was still soft to the touch. No problem, I thought, it is quite cool (though my max/min thermometer didn't record anything below 7C) so just leave it for a while.

One week later and the screed is still soft, a bit like plasticine, so it looks like I'll have to take it up and try again.

Any ideas about what might have gone wrong? Should I have simply mixed by volume as I usually do? Is the soft sand the culprit?

At least it should be easy to rip up . . . or roll up more likely. :lol:

 

[skier-hughes]

I thought it was by volume, does it weigh more if it is wetter?

 

[Flyfisher]

I guess so, and the sand was fairly wet. However, I sort of mixed it by volume because a 25kg bag of lime must be about 3 bucketfuls plus six buckets of sand still gives about 2:1 by volume. Even a 3:1 mix shouldn't remain soft after a week should it? Or could it be too much lime?

 

[plasticpigeon]

Hello FF

Why did you use NHL2? Wouldn't a stronger hydraulic lime be better for a base for stove. I used NHL5 for the limecrete in my kitchen floor, and the mortar that I used to stick the tiles down. It did harden up quite quickly but after a few days it was still a bit cheesy. (I know this as I pulled up a quarry tile that was not bedded properly and had to dig the mortar out and it was still reasonably easy to do so). If you are happy with NHL2 perhaps it just needs a bit more time>?

 

[Flyfisher]

Good question. I used NHL2 because I wanted the new slab to have maximum breathability. I didn't use hydrated lime as I was concerned about the cool temperature inhibiting the setting. I wasn't overly concerned about the strength of the slab as it is about two square metres and roughly nine inches thick and is inside an inglenook with only a stove to support and I reckoned NHL2 would easily manage that, so I felt it was better to err on the 'soft' side so as not to disturb the existing original chimney structure . . . . only I wasn't expecting the screed to be so soft that it wouldn't actually set at all!!

As I mentioned, the NHL2 limecrete mix seemed to set with no problem.

I suppose it could be 'just one of those things', though I tend not to believe that sort of thing so I'm sure I've made a mistake somewhere, I just don't know where.

 

[plasticpigeon]

Perhaps it is the soft sand not interlocking and therefore not giving strength to the mix. It might still harden up and be strong enough, who knows!!!

 

[88v8]

Soft sand would weaken it, as the grains don't interlock, but it should still set.
If the sand was not dry then that could exacerbate the problem as the lime wouldn't mix well.
And perhaps overall it was a bit on the wet side? But it should still set.
If the temp is low, that will slow the set. When our limecrete floor was laid, I put a fan heater in there with a stat set to about 60F.

The construction industry doesn't mix in volume, it uses weight.
Example:
http://www.ce.memphis.edu/1101/notes/concrete/cylinder_concrete_mix_proportations.pdf
Volume tends to be a DIY method, less accurate but it's how most of us mix I guess, and one should still get a set.
Our builders have tended to mix in vague shovel-fulls. Weight or volume? Errrm.

Not casting any light here, I'm afraid.
Just be thankful it wasn't a whole room. :?

Ivor

 

[chuckey]

Where's that document from? - they still use lbs !. As said how much water in the sand?, builders merchants or builders don't sheet over their big bags of sand and ballast, and they have not compensated for different density of sands. In general mortar and concrete are used in volume, their weight is just what they weigh, so they should be mixed by volume. (unless you are casting counter weights!).
Frank

 

[stuart45]

It's an American document, that's why it's in Imperial.

 

[Flyfisher]

Soft sand would weaken it, as the grains don't interlock, but it should still set.
If the sand was not dry then that could exacerbate the problem as the lime wouldn't mix well.
And perhaps overall it was a bit on the wet side? But it should still set.
If the temp is low, that will slow the set. When our limecrete floor was laid, I put a fan heater in there with a stat set to about 60F.

Yes, it's the setting - or rather the lack of - that has confused me. I can see now that soft sand wasn't a great idea and I was prepared for the low temperature to slow things, but after the limecrete mix behaved itself in the same conditions, the screed failure was something of a surprise.

The sand was certainly wet (it had been in the rain and uncovered) so your comment about it not mixing well was interesting, but it looked fine in the mixer and I didn't add any water until it looked thoroughly 'dry mixed'. Even then, I didn't over-wet the mix - just enough to 'ball' when clenched in my hand.

Still, as you rightly said, at least it wasn't an entire floor and it won't be a big job to pull it up and lay a new screed - just an annoying one!

 

[skier-hughes]

NHL2 is as breathable as NHL5, just that after a month it has only set to a certain strength, whereas NHL5 after a month will be a lot stronger.

 

[Zebra]

Oh really? I don't question your greater knowledge, but I had understood from the lime plastering course I went on, that there's a kind of spectrum, a sliding scale, from "fat" lime on one end to cement on the other, with the NHLs in between; and with every gain in strength and speed of set as you progress along the scale, goes a loss of breathability. Is that not true?

 

[skier-hughes]

And I got it from a course I went on
Which said that all limes are breathable. There may be a slight difference, but it's not perceivable in everyday life, so not something your average Joe should worry about.
Whereas hardness for things like pointing is something to concern yourself about.
I bet we could both google and fine articles to support our cases, but I'm having a nice coffee and piece of toast with yummy local honey and well......................

 

[Flyfisher]

Update time.

I pulled up the screed (not exactly difficult!) and relaid it yesterday. I've read various sources suggesting 2:1 or 3:1 so I decided to go with 2.5:1 by volume, sharp sand and NHL2.

I had some of the mix left over so decided to finish an outstanding job of filling the gap above a window I fitted earlier in the year (six months proccrastination on that one - not bad eh? :wink: ).

This morning, the new screed is still soft to the touch but the brickwork mix is much harder to the touch.

So I'm now wondering if the previous screed mix might not have failed as such but was just going to take a long time to set. Too early to decide about the new screed yet but it's looking as if the fireplace, and therefore the boiler stove, is not going to be ready before Christmas.

Perhaps I should have used NHL5 for a quicker set?

Ho hum.