Lime mortar or not? Will cement render be ok?

31 May 2012
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United Kingdom
Hi there, was after some advice. I have 2 patches of blown plaster internally, and outside in the corresponding places. I have removed some of the external render to find I have 2 damp proof courses, 1 is 2 inches above the external ground height and the other 2 courses above that, the mortar is very soft (can rub it out with a finger) is it lime or very weak cement, it is damp. Can I rake some out and repoint with cement or does it need to be lime as I intend to Re-render the exterior after, and does the render need to be something special or can I use hard wall/cement? The house is an ex local authority semi built in the 1930's, with red brick and then rendered, as is the whole street.
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We use 4 plastering sand 1cement, feb and waterproofer for scratch coat.
Also 1 shovel of lime in the mix.
Float coat 5 sand, 1 cement, feb and 1 lime in the mix.
The render maybe a lime mortar and would need to be replace with lime. No cement or any other additive is needed. NHL lime replaced the bad practise of using cement with lime and has been proved to fail.

I would sort out the water problem first because if you have soft mortar you have a bad water problem.
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Your problem is that you have a failed damp course. Once that is remedied it's immaterial what you use. Before that - it is immaterial what you use because neither will work until you sort your damp issues.

You are looking at the effect of a problem - not the problem itself.
Thanks for all your advice guys. I have now stripped the render off the wall upto 1 meter high, on the outside, and all the mortar above the dpc is soft but under the dpc is solid. If it is dpc broken down should I re-point before treating or after? Could I treat this wall now and do the other walls later or do they have to be done together inside and out.
Thanks for all your advice guys. I have now stripped the render off the wall upto 1 meter high, on the outside, and all the mortar above the dpc is soft but under the dpc is solid. If it is dpc broken down should I re-point before treating or after? Could I treat this wall now and do the other walls later or do they have to be done together inside and out.

not the dpc but I will leave it to joe to give out the wrong advice again. if you want a solution pm me.
There is little historical precedent. This practice is likely to be a development of the Portland cement / lime hybrid mortars typified by the 1:1:6 and 1:2:9 mixes used in the 20th century. Lime in these mixes was added as a plasticiser to improve workability, although in a 1:2:9 it was presumably expected that the lime would carbonate and assist the setting process. Analysis of these hybrids regularly finds lime still present after 50 years. The setting cement impedes the carbonation of lime.

It is a dangerous assumption that the addition of, for example, lime putty to an eminently hydraulic hydrate will produce a moderately hydraulic lime. The chemistry of hydraulic lime is complex and the setting processes are delicate. Data available on this1 is preliminary and suggests significantly reduced compressive strength, increased watervapour permeability, and far worse performance in salt crystallisation tests. It is extremely important to define this practice. The blending of non-hydraulic and hydraulic materials can range from a hydraulic hydrate with five per cent putty added to a putty with five per cent hydraulic hydrate added, and every variable between. The end-product will have varying properties.

This blending is now not necessary. We are fortunate to have a wide range of hydraulic hydrates, lime putty and pozzolanic additives available in the UK, and there is no application in historic building repair where a blend is likely to outperform the correct grade of hydraulic hydrate, or pozzolanic lime. The only possible exception to this is the addition of less than eight per cent by volume of lime putty to a hydraulic hydrate to improve plasticity. This will improve workability, hopefully not at the expense of durability, although thorough mixing of the hydrate mortar often makes the addition of putty unnecessary. In the UK, specialists first became aware of this practice when Jura-kalk, an eminently hydraulic hydrate from Switzerland became available. It was advised that this material had been routinely blended with lime putty in equal proportions in Denmark and elsewhere in Europe. Jura-kalk is a binder containing very little lime2,3 and is a complex blend of compounds of calcium and silica (principally C2S), calcium and alumina (principally C3A), calcium, alumina and silica (C2AS), calcium, alumina and iron (C4AF) and calcium carbonate. The addition of lime to this is likely to be different to the addition of lime to a material that does contain lime. Many hydraulic hydrates notably from England and France have significant lime content.
For many years those specialising in historic building repairs have known the dangers of using hard, cement-based mortars. But the specialist world has been split between those who advocated the use of small amounts of Portland cement as an additive to a lime mortar and those who rejected all cement additives. New evidence sheds light on the controversy, with some radical conclusions.

The addition of cement to lime mortars is a widespread, almost traditional practice, but few consider why it is done or the consequences. There is also confusion over the substances and chemistry involved.

Non hydraulic lime hardens by a slow process of carbonation, reacting with atmospheric carbon dioxide over a period of weeks. Hydraulic limes and cements set rapidly by reacting with water in a matter of hours. A non hydraulic lime can be made to set much more rapidly by the addition of an hydraulic or 'pozzolanic' additive. This practice is known as 'gauging'. The additives include finely crushed brick powder, PFA, HTI, pozzolana, trass or cement (white or OPC). These all contain finely divided and therefore highly reactive silica and/or alumina, which are the constituents necessary to obtain a rapid chemical set by reaction with water. Of these, cement is by far the most widely used in this country, and the cheapest. Typical proportions, commonly in use, are 1:1:6 (cement: non hydraulic lime: aggregate) and 1:2:9.

There are, as one would expect, both advantages and disadvantages in gauging non hydraulic mortars with cement to make them hydraulic.
I would investigate the cavity wall insulation installed after you have eliminated gutters etc

Some reading that may explain the damp above the dpc.

What makes me think it may be the cavity insulation causing the damp issue is that it's above the dpc, cavity insulation would not fill below the dpc inside the cavity because the dpc will block it.

You need to investigate the cause of the damp before you re plaster.

Internal walls have been skimmed with gypsum and probably bonded with PVA, this will fail when damp, blister and fall off.

The outside wall is starting to dry out, you can see this because salts are evident. This happens when the walls dry.
thanks for that peaps2, my wifes late grandfather (old time builder) said that we could have problems in the future because of the insulation, we bought the house anyway. I did have a problem at the back of the house with guttering but that was sorted last year, the side of the house is sea facing and we are 1 street back from the sea, so probably a lot of salt in the air & rain. As the worst affected area is under a window could i remove the insulation from just this area? I know the other area is at the bottom of a 7 meter high wall and that makes it tricky to remove, could i remove a few bricks and remove it just from the lower area or will this just cause the same problem higher up?
Just read all that info. CWI doesnt seem to be a good idea, causes as many problems as it solves.
Ahh, sea facing wall.. Ok I have alot to do for the local village today so if you give me a few hour I will post my solution to the problem.

I would be inclined to remove the insulation as you stated. I have a cost affective solution for you but some work to do all the same..

No easy long term fix.

1. you have quite a few issues here that have not been addressed:perhaps we were all too distracted with looking for signs of life: breathing.

2. The extn (external) render goes below the too high pathway, capillary action will pull moisture up the brickface. Neither should render bridge the DPC's.
A bell cast bead is reqd. fitted just above the upper DPC.

3. you might have to keep going higher, testing the render for damp and failure.

4. what type of "paint" is on the render?

5. No scratch coat was used on the extn wall.

6. the 1m "rule" is for internal walls only.

7. Are the internal floors solid or suspended?

8. Your skirtings are modern, indicating, perhaps, previous remedial work - why not remove some skirting and examine the back for rot? A rusty fixing can be seen in one pic.

9. Internal back wall pic: what is the black stuff/effect?

10. how do you know that CWI is present?

11. peaps: 1. how would dpc material block the cavity?
2. salts are specific to all contaminated surfaces, wet or dry.I have seen them on weeping wet wallpaper.

Image may help with why insulation would not fill the void below the dpc. Some times it covers the cavity, depending.

Insulation is my guess here, causes many problems.

The black plaster may be ash mortar.

Salts show after wet when drying out.

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