Cement render problem

[MDE]

Our (stone rubble) gable end wall was covered in cement render and was cracked and damp; also the roof over it was in poor condition, exposing the top of the wall. We had the roof re-slated and the builder arrived yesterday to re-render the wall in lime. Today we hit a problem. Although the render comes away easily in places (particularly lower down the wall), in some places (particularly higher up) it is stuck quite hard to the stone. In removing the render, a lot of small stone was coming out of the wall - so much so that the builder did not want to continue for fear of damaging the wall. Any ideas what to do next?
My first thoughts are to just remove the render where it comes away easily and the wall is obviously damp - say about a six foot high strip along the bottom. I would then have (mostly) cement render at the top and lime at the bottom. Then there is the problem of making the wall look consistent. The cement render is painted in some sort of external emulsion, so perhaps I could cover that with some of Mike Wye's DG27 and then apply a lime harl over the whole lot and limewash it. Or would it be better to cover it all in a "breathable" emulsion or a silicate paint? Why is life never simple? (Don't bother answering that last question, but any ideas on the render problem would be gratefully received.)

 

[Gervase]

I know it's a pain, but it's best to get rid of all the render. Mixing lime and cement on the same wall isn't a good idea, as it seems to speed up the failure of the cement, leading to water getting trapped behind it.

It's quite common to lose some stone when removing render, particularly if you've got a rubble stone wall with galleting or 'pins' in the courses - raking out the pointing to provide a key for the cement often leaves these little stones very exposed. Just stick 'em back in when the wall is dubbed out prior to putting on the scratch coat.

But do, if possible, get rid of the cement.
What methods is your builder using to remove it?

 

[MDE]

The builder is using an SDS drill and a hand chisel. I had previously taken off a section towards the bottom of the wall which came off reasonably easily by hand. He then started at the top of the wall, where the render was stuck a lot harder (and the wall is thinner) and was worried about damaging the wall. We then agreed that he would start at the bottom and point that before moving up to the next section. Today he removed a six foot high strip along the bottom and pointed it, with quite a bit of galleting. On Monday he'll move up to the next section and see what that is like. Working bottom-up like this will (hopefully) give a stronger wall below when he's working on the high sections. I think that his problem is partly that there is more galleting than he planned as the old ones come out with the render, the old (300+ yrs) lime mortar being quite soft and crumbly.
By the way, he is using a mixture of non-hydraulic mortar (made hot with fresh lime, then matured as a mortar) and NHL 3.5 (in a 3:1 ratio). He says this has given him the best results in the past. I haven't come across this before.
Also, in removing this render, we have exposed the edge of the front wall which I had been told was a lime render. It looks pretty cementitious to me. Any ideas how to find out what it is made of? All the renders used on the house contain some lime and therefore give off bubbles when placed in vinegar. However, some seem to have worked better than others.

 

[Gervase]

There's lime and there's lime, of course. The Victorians, particularly, were spoiled for choice with different varieties of lime, ranging from fat thorugh lean and the varying degrees of hydraulicity up to natural cement. Different ones were chosen depending on whether the render was common, rough, trowelled or bastard - and also depending on cost and availability.

Gauging a fat lime with a hydraulic lime is controversial. I've done in in window reveals and it seems to have survived a few winters, but there are some, including Peter Ellis of Rose of Jericho who counsel against it.

 

[middi]

Bastard lime finish application.

Would that be a type of plaster that is prepaired by a professional and applied by someone like myself ?. ( I don't do plastering).

It must be, because all I keep saying during the odd bit of plastering I do:,"what a bastard".

 

[Sue Wilkinson]

I wonder if he's just whacking the chisel a lot harder than you did and therefore removing more wall. Can you get up higher to one of the problem areas and have a go yourself? If I'm right you have to tell him to take more time and care over the removal. The SDS drill will definately do more to dislodge the smaller stones. However, I've taken chunks out of walls and then put them back together with no ill effect, so I'd not worry too much.

From experience I can say that the joins between lime plaster and gypsum soon crack and move apart. I'd expect the same to be true of render.

Sue

 

[MDE]

Thanks for your various comments. I look forward to today's discussions with the builder :? . It's hard enough to find a builder who understands that lime is not just something you add to cement. Discussing Peter Ellis's paper might present a challenge. One point I am slightly confused on is that Peter Ellis says of gauged limes "Analysis of these hybrids regularly finds lime still present after 50 years. The setting cement impedes the carbonation of lime." However, to quote from another post by NT

When moved to breaking point, instead of forming a single break, lime
tends to form lots of microcracks. Lime then reacts with the CO2 in the
air to grow hard crystals across these microcracks, and thus rebonds
itself

Surely in that case, the presence of residual uncarbonated lime is a Good Thing and will help protect if cracks arise after many years.
Also Peter Ellis says "Certain compounds, notably the calcium aluminates, but also any di-calcium silicates present will hydrate or begin hydration in the lime pit. This explains why the ‘hot mixes’ where quicklime is mixed with water and aggregate on site have different properties."
I'm not clear what "different properties" he is talking about. My builder is using a local hot mix mortar plus moderately hydraulic lime, which he says is what he has found to work best over time. My only options are to go along with him or abandon! I have no idea how the hot mix would work without the NHL.

 

[NT]

Discussing Peter Ellis's paper might present a challenge. One point I am slightly confused on is that Peter Ellis says of gauged limes "Analysis of these hybrids regularly finds lime still present after 50 years. The setting cement impedes the carbonation of lime." However, to quote from another post by NT

When moved to breaking point, instead of forming a single break, lime
tends to form lots of microcracks. Lime then reacts with the CO2 in the
air to grow hard crystals across these microcracks, and thus rebonds
itself

Surely in that case, the presence of residual uncarbonated lime is a Good Thing and will help protect if cracks arise after many years.

I'm not clear why this needs discussing before work, as we already know what works fine for stone walls.

My builder is using a local hot mix mortar plus moderately hydraulic lime, which he says is what he has found to work best over time. My only options are to go along with him or abandon! I have no idea how the hot mix would work without the NHL.

Thats a novel idea to me. If he says it works though and he's got experience I doubt I'd argue.


NT

 

[Penners]

all I keep saying during the odd bit of plastering I do:,"what a bastard".

By that criterion, pretty well all of the work that I've done in my house has been of dubious parentage.