sand cement render

Hi

Ive tried looking in the archives and the FAQ but cant find a definitive answer.

I was taught to use builders sand and cement (and water obviously) to mix a render suitable for then skimming with plaster.

In this context is there really any or much difference between builders sand and sharp sand ? or are both suitable.

Thanks

Tim

Reply to
Tim Smith
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I recently put a base layer of render on couple of walls in my cellar with a bit of waterproofer added, prior to plastering and used cement to sharp sand in a mix of 1:4. Seemed to go on pretty well.

Alistair

Reply to
Ali Mac

Sharp sand is grittier and tends not to slide about so much. I don;t think its any more than that.

Ive used both, and buiolder sand makes a smoher slip[perier mortar, but once dry there is leiitle difference,

However a screed of sharp sand with low cement content seems to be more porous and permeable than builders sand.

Reply to
The Natural Philosopher

You may find the odd larger stones in sharp sand annoying with a coat of under 10mm thickness. Plastering sand is better and easier, but more expansive and is best to use for a finish render.

-- Mark§ This is an automatic signature of unknown origin

Reply to
Mark

Sand varies all over the country. Some building sand is very fatty and some dry when mixed with cement, the latter may require additives to make it usable. Most building sand is quarried straight from the ground. Sharp sand or sometimes known as washed sand comes from a water course, either a river or gravel pit course. Its the very small partials of the graded gravel. Sharp sand or washed sand is usually used for path's, floor screeds and the finer sharp/ washed sand for the backing coat for plastering. It can also be mixed with building sand, Hydraulic lime and cement for bricklaying. This mortar mix is very sticky and pliable.

Reply to
keith_765

Reply to
keith_765

Oh dear, oh dear. When will people stop rendering with cement and plastering with gypsum. Its too impermeable and too brittle and will end in tears. Use lime putty and sand 1:3 mix with a little horse or goat hair for the base coat. Then lime putty and a fine sand at 1:2 for the top coat. 1:1 if you want a really high class finish. Your wall will then be breathable (so long as you don't then ruin it with an impermeable paint). Sharp sand has a variety of grain size and is stronger when grains are angluar. Good for bricklaying. The spaces between the large grains are partly filled with small grains so you need slightly less lime than when the grains are all about the same size. Angular grains interlock and don't slip past each other as easily as the rounded grains typical of 'soft' sand so develops a higher load strength. For plastering soft sand is better but it is not very critical. Just make sure that the sand is as fine as possible for the top coat of lime plaster.

Reply to
Biff

Is there any evidence (preferably not anacdotal) that gypsum is less permiable to water vapour than lime? I mean it passes through 10mm gypsum plasterboard like its not there. Surely the addition of 9" brick would make any difference between 10mm of lime or gypsum irreleevent?

Reply to
basil

Gypsum is very permiable, but passes water through in liquid form, which then evaporates from the surface. This is why you will see salts deposited on the surface. It actually sucks water up like a sponge. Prolonged exposure will wreck the gypsum though.

Raw sand and cement is much less susceptable to this, and it can be reduced to pretty insignificant levels with the addition of a waterproofer into the mix. If the mixture is weak as normally used for renedring and mortar, then it will allow some breathing (gas permiable so water vapour can pass through), though not as much as lime.

Neither gypsum nor sand and cement will move, so if you use them in a lime mortar house, at best you'll get hairline cracks forming where the house is routinely moving (at worse, bits will fall off). This will not happen with lime and horse hair plaster, which will move in sympathy without any such problem. The lime plaster is not damaged by moisture directly. However, damage can occur on either side; many modern decorations are not sympathetic to breathability, and over the years, any salts which are deposited on the underlying brickwork surface due to continued moisture traversal can cause the lime plaster to lose key.

Reply to
Andrew Gabriel

Gypsum is very permeable to water. Problem is when it dries out it is weakened whereas lime dries out at its original strength.

Reply to
Mike

Good post!

On the subject of waterproofer for sand and cement would wickes's waterproof PVA be the correct thing? What do you use? I'm missing a DPC on one small area of internal wall.

Reply to
basil

No, I don't think that's what the 'waterproof' in waterproof PVA means. (I think it just means the PVA won't degrade inside damp sand/cement when the PVA is used as a bonding additive.)

You can buy a waterproof additive for mortar mix. The one I use is also combined with plasticiser, but you can get it separately. I think it came from a large B&Q, but I can't remember the make (may be their own brand).

If you have a measurably damp brick wall, a sand and cement scratch (base) coat with waterproofer (and I use lime too -- makes the mix very nice to handle) will protect a gypsum finish coat. I've done this a number of times, and whilst the underlying wall is still damp, the gypsum remains bone dry. However it probably isn't a substitute for tanking, where you have water being forced through the wall due to being below the water table for example (at least I've never tried, and I wouldn't expect it to work).

The general sequence for doing this is... PVA the wall just as you would before any plastering -- if it's very absorbent, a 1:5 dilution with water soaked in to dry to seal the surface. Then apply a 1:3 dilution and allow it to get nearly dry before applying the sand/cement. I use the standard 1:1:6 mix of cement:lime:sand plus waterproofer (plus plasticiser as that's mixed with my waterproofer, but the lime acts as a plasticiser anyway so that's not necessary). You want to apply the gypsum finish coat after the sand/cement has gone off but before it dries; this means waiting at least 24 hours, but probably not more than

48 hours. If you leave it longer and the sand/cement does dry out before skimming, then you'll probably want to PVA it as as though you were doing a reskim.
Reply to
Andrew Gabriel

I've just got some!

Reply to
basil

SOunds like the one I got from Wickes £2.99 / 5ltr

I'll try this now. I dont think its serious enough to warant tanking, there are some stains and the dust at the base of the wall looks damp and I can smell the BG "plaster finish" which I applied several months ago as if its stll damp.

Thanks!

Reply to
basil

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