Internal rendering/Plastering with Sand/Cement/Lime Mix

[Andy Hide]

I have previously used a mix of 5:1 Sand/Cement to patch up blown areas of plaster in my c1910 house. I have then gone over this with multi-finish. Some of these patches have developed hairline cracks and it was suggested that adding lime to the mix would help allow for any slight movement of the house. It was built with lime mortar.

I have now changed to a 6:1:1 Sand/Cement/Lime with the sand being Wickes (red bag) "for use in bricklaying and internal rendering". Is this the same as plastering sand ? Would plastering sand be better in this case ?

Having left the new mix to dry out overnight I come back this morning to find large cracks have appeared throughout as it has dried out. Things are so bad I think I am going to have to start again. Cracks are worse at the edges where the wall meets the door frame (weakest point ?).

A few thoughts on what I think may be going wrong:

1) I'm using hydrated lime which is white and very fine powder. Did I get the right stuff from the BM ? 2) At first I thought this was the problem of the wall sucking out the water from the render too quickly thus it drying and cracking - but having throughly soaked the wall to the point where it wouldn't suck in more water I don't think this is the problem. Perhaps it's too wet now ? 3) Is the 6:1:1 mix to weak ? 4) Have I overmixed ? I'm mixing up in a large bucket with a whisk attached to a drill. I guess you would normally use a cement mixer but as I am dealing with relatively small amounts I thought this would be OK. I'm adding the water first followed by cement, lime and then sand. Adding the lime certainly makes the mix more fluffy and easy to work with. Perhaps it's now too "bouncy" and full of air and as it dries out it's shrinking ? Mixing with a whisk is probably a tad more violent than a cement mixer. 5) I've been applying to about 10mm thickness. I have found the thicker I apply it the worse the cracks are but never had any problems with this thickness before adding the lime.

Any thoughts on this would be very much appreciated.

Andy.

[Anna Kettle]

Adding lime to a cement mix wont do that cos you get a cement set, not a lime set (the cement inhibits the lime set) What it will do is make the mix fluffier and easier to work

Yes

My immediate reaction to the 'cracks are worse ...' is that you have not killed the suction enough where dry materials meet the mortar. But then you say no. Could you try wetting the wall (and especially the edges) well then allow time for that lot of water to soak in before wetting again - say three sprayings in 20 minutes? Too wet a wall does not give that problem, the plaster just won't stick to it.

I'd never come across this suggestion until a few weeks ago when it came up in a thread here. Written on someones bag of readymix were the words 'do not overmix'. So it is possible it appears

That should be OK. No thicker though

Anna ~~ Anna Kettle, Suffolk, England |""""| ~ Lime plaster repairs / ^^ \ // Freehand modelling in lime: overmantels, pargeting etc |____|

01359 230642

[Andrew Gabriel]

Be careful with thermal blocks. They expand when they get wet, and contract when they dry out (blowing the plaster and possibly cracking the blocks), so don't soak them, just wet the surface. Use PVA to limit the suction by sealing the surface in that case rather than lots of water.

Another cause for large cracks can be applying too thickly. Plaster shrinks as it sets. A thin coat shrinks by getting thinner which doesn't notice, but a thick coat will shrink in all directions, resulting in cracking.

Hairline cracks in gypsom plaster used in a lime mortar building are pretty inevitable. Such buildings routinely move/flex, and gypsom plaster doesn't. You just have to put up with it or switch to lime plaster.

[N. Thornton]

not best practice. Old places move and crack, and when soft brick meets hard mortar the bricks sometimes end up broken.

dont confuse 1:1:6 with lime mortar though, very different.

yup

more likely it waited till you left then sucked more water. About a pint per brick they reckon.

no, its as strong as one should ever use on soft old bricks.

I dont know, though I doubt it. Bucket mixing is very quick, just do about a third of a bucket full, and to mix it just lift the bucket up sharpish and move it to the side as well, mixes the lot in 20 seconds. I cant imagine a drill thingy being nearly as quick.

would be a better bet, ask them.

NT

[the yorkshire dalesman]

IMHO your plaster is too hard/brittle (assuming the wall - brick? - is completely stable: does it move if someone walks on the floor above or shuts a door hard?). Soften it by doubling the lime and halving the cement content - eg 6:0.5:1.5 - keeping the 3:1 overall ratio, you could possibly increase the amount of sand to an overall 4:1.

I could mix by spade.

Minor cracks in the background shouldn't maatter too much as the finish coat will cover them up.

make sure you trowel over, preferably with a wooden trowel to get a smooth sandy finish then rake over it to get a good scratch scrolling finish to key for the finish plaster.

Your finish coat should ideally go on about 24-36 hours after the background is put on. It's much harder to finish plaster onto a dry sand/lime/cement background.

Is

IMHE almost any building sand will do the job. That's unlikely to be your problem.

Are you certain the wall is not vibrating or moving?

that's the right stuff

not sure on effect of wetting here: that may be a major part of your problem.

The lime and to some extent the cement in the mix should make the mix adhere to the wall by suction, so you don't normally want to wet the wall. It may be the prior wetting is making the mix move on the wall during the set & hence leaving cracks.

When you *do* wet the wall is when you finish plastering on to a sand/lime/cement background that has been allowed to become too dry.

Try putting the background onto a dry wall.

HTH

[Andy Hide]

Can you explain the difference ? This is not 1 part sand, 1 cement and

6 hydrated lime is it ? Lime mortar uses a different type of lime?

Is it possible to create a mix the same as what was originally used ?

[Andy Hide]

Having hacked off my original work I agree that this is the most likely reason for failure.

I took a photo so you can see the cracks before I removed it:

(Hi-Res)

The cracks ran right through the whole coat that I applied.

I have since re-applied a thinner coat (4mm instead of 10mm) which I will then go over again. This has given a much improved result with no cracks but using the same mix.

Any advice on building up in layers? Should the first coat be a stronger mix than subsequent coats or can it all be of the same strength ?

[G&M]

Why not just get rid of the cement and use a 4:1 sharp sand:lime mix. This will be flexible enough for any circumstance and you can smooth it out for days to come if cracks appear. Obviously a real PITA for professional work but fine for DIY.

[Andrew Gabriel]

I didn't realise you meant the scratch coat. I don't think this matters much -- the cracks give some extra key to the finish coat. May be an issue if the Sand/Cement/Lime is being used for its waterproofing properties, as the finish coat in the cracks would bridge the waterproof layer.

I think the same mix. You should wait at least 24 hours to recoat sand/cement (with plaster or more sand/cement), but not long enough for it to dry out (around 48 hours for 10mm IME). If it does dry out, you'll have to PVA it. In this weather, you'll probably need to dampen surface a bit anyway.

Can't tell the scale of your photos, but I usually do more devil floating (scoring of the surface) than I think you have done.

[Andrew Gabriel]

Main difference is it contains no cement. The hydrated lime will be in putty form. The powder form you have has been dried, and it's not quite the same just mixing it with water again and using it instantly. Lime putty has to be left in water (but sealed against Carbon Dioxide from the air) for some weeks before it's used. Apparently you have to do the same with the dried hydrated lime, but it doesn't work as well (not tried myself though).

Certainly, but probably not quite the same with convenience products like bags of dried hydrated lime. What do you want to use it for? Also, sand was not graded like it is today -- for a more realistic mix you probably want to mix some different types of sand together, but that makes it more difficult to use for some things.