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.

it was suggested that adding lime to the mix would help allow for any
slight movement of the house.

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

1) I'm using hydrated lime which is white and very fine powder. Did I
get the right stuff from the BM ?

Yes

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 ?

Cracks are worse at the edges where the wall meets the door frame

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.

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.

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

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.

That should be OK. No thicker though

Anna
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repairs
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk 01359 230642

In article ,
(Anna Kettle) writes:

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.

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.

--
Andrew Gabriel

(Andy Hide) wrote in message . com...
I have previously used a mix of 5:1 Sand/Cement to patch up blown
areas of plaster in my c1910 house.

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

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 ?

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

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 ?

yup

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 ?

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

3) Is the 6:1:1 mix to weak ?

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

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.

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.

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.

http://www.periodproperty.co.uk/cgi-...orum.pl?page=1
Lime would be a better bet, ask them.

NT

(Andy Hide) wrote in message ...

I have previously used a mix of 5:1 Sand/Cement to patch up blown
areas of plaster in my c1910 house.

snip

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".

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
this the same as plastering sand ? Would plastering sand be better in
this case ?

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?

snip

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 ?

that's the right stuff

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 ?

not sure on effect of wetting he 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

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 ?

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

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 ?

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.

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:

http://www.andy.hide.dsl.pipex.com/IMGP9763a.jpg
http://www.andy.hide.dsl.pipex.com/IMGP9763b.jpg
http://www.andy.hide.dsl.pipex.com/IMGP9763.JPG (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 ?

"Andy Hide" wrote in message
om...
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 ?).


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.

In article ,
(Andy Hide) writes:
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.

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:

http://www.andy.hide.dsl.pipex.com/IMGP9763a.jpg
http://www.andy.hide.dsl.pipex.com/IMGP9763b.jpg
http://www.andy.hide.dsl.pipex.com/IMGP9763.JPG (Hi-Res)

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

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 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 ?

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

In article ,
(Andy Hide) writes:
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 ?

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

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?

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).

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

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.

--
Andrew Gabriel