I've got a large North-facing house wall of soft Norfolk red bricks - and a large number have spalled.
It's really an impractically large number to cut out and replace.
I've been trying to patch over them with lime mortar (lime putty, sharp sand and red brick dust).
Mix is good consistency & sticky - but it still isn't bonding very well to the bricks (which have been gone over roughly with a wire brush, brushed down and then wetted).
So I'm thinking - maybe wet down with 10% pva solution?
I suspect that your layer of mortar a bit thin. 6-10mm works best
Consolidate the mortar once it has picked up a bit (ie some of the water has migrated into the brickwork leaving air voids)
Try painting the brick with limewash before plastering. It will soak into the brick a bit so when the lime sets there will be a chemical bond between the brick and the lime mortar
Try drilling a few holes in the brick to give added mechanical key. Hoover out the holes, wet them, limewash them, then plaster
But whatever you do it is still patching and you will never get a good colourmatch. Cutting out and replacing the bricks will look better :-)
I wouldnt add PVA, it impairs the breathability and so will tend to make the spalling worse in the future
and really do wet the wall down well, several times. An exposed wall in midsummer gets very dry. Protect the setting mortar from the sun. You may need to spray it down once or twice as it sets
Dear Dom Without question the best repair will be the lime - which is in effect a lime render shaped to look like bricks. However, if you wish to reconsitute the bricks then one way of doing this that I have done sucessfully with a 20 year (so far) no-problem success rate is to: key the brick and remove all friable material (you don't want expanding salts contaminating the substrate if it needs it screw in some stainless steel eml with stainless screws apply SBR and put in a light coloured sand /brick dust/ cement mortar (you will have to get lots of damaged old bricks)with sbr in the mix and when set but not hard cut out the pointing area later to be pointed once set point in gaps
Thanks for those tips Anna - I think we're going to have a change of tactics today - I'm going to concentrate solely on prepping the spalled bricks and limewashing the damaged area.
Coverage was generally 15-25mm (i.e.fairly deep spalling) and North facing so not getting the sun - but it's 100's of bricks. Having the whole face lime-rendered might be necessary at some point.
Overnight the patches have taken a good set (the ones rubbed down with a wire brush late last night have taken the most natural, blended-in appearance, ones brushed down earlier - less so) - but probably only
50-60% have a good bond.
I think the change of tactics today will reduce any tendency to "get the mortar on" without sufficiently thorough prep.
how are you defining a 'good bond'? One thing I've noticed is that if the layer of material is strong enough in its own right then voids under it cease to be an issue. Whether thats going to apply to an inch of lime is another question.
Good bond is when rubbed down very firmly with a wire brush 24hours later there's no chance of dislodging the patch.
Update - we spent Friday solely on prepping the old spalled faces with wire brushes (have now done about one-third of the wall). We also pulled out all the less than satisfactory patches from Thursday. Most of those still in place are brilliant, strong bond and good surface - they look like pinky bricks mixed in with the reds.
The key factors seem to have been very through wire brushing of the old faces and thorough wetting.
Our plan now is to complete the wire brushing of the damaged faces (and reinspecting them until we're really confident we haven't got sloppy anywhere), then limewash a few, then more thorough wetting, patching - but with more experience of when to brush over (for the surface texture) and when to wire brush (to clear off unwanted excess mortar).
We've been using red brick dust to get the colour (and its pozzolanic effect) - but the proportion has been 25% (i.e 1 part lime putty, 3 parts sharp sand, 1 part brick dust).
Can anyone tell me more about the possible effects of using such a large proportion of brick dust?
Well there's your problem. 24hrs is way too early to mess with lime, its strength is close to zero after so little time.
Its pozzolanic property means it sets quicker and harder, and breathability is reduced a bit. Dont think it has any other effect. Just give it much longer before hassling it.
I dont think there has been any research into the use of pozzolans as an additive to lime putty mortar, but in general it will make it harder, more brittle and possibly less durable
I dont use pozzolans. If I want hydraulicity then I use a hydraulic lime to get a more predictable effect. My clients expect me to get it right first time :-)
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