repointing brickwork

[N. Thornton]

I've been warned not to do that as the bricks tend to become absorbent and turn green. The accumulated muck keeps the water out apparently.

If I understood it right it works but is weak, and will fail early. Consider that decent cement or lime mixes should last over 50 years at least. So 'early' might not be such a big problem.

I gather the best mix is lime and sand plus a pozzolan. Brick dust is a pozzolan, I dont remember what the others are.

I wonder if you have undercooked bricks, rejects. Old bricks are soft, but they shouldnt be that bad. Usually the rejects were used on the inner layer of wall, so Vic houses tend to look a right eyesore if the plaster's stripped off, with broken, misshapen, burnt and undercooked bricks galore. Perhaps an inexperienced builder was involved. Who knows.

Regards, NT

[Christian McArdle]

Well, the bricks I've seen drilled have all been inside, mostly in the fireplace, as elsewhere they are covered in plaster, so I can't see what's going on. About a quarter are black (but probably because they formed the flue, although there isn't much of a pattern), most are cracked.

Christian.

[Christian McArdle]

I should mention that the brick front facade is excellent with a mixture of grey, yellow and red brick in the local Reading brick fashion. It's a shame the presumably original roof slates haven't lasted as well.

Christian.

[stuart noble]

Movement and self repair in regard to pointing amounts to something that will wash away with the first rainfall. If the mortar *and* the pointing were the same, maybe....

[Andrew Gabriel]

In article , "stuart noble"

[stuart noble]

I think a small amount of cement is just another pozzolan, but I wonder if non-hydraulic lime alone has ever been successful externally.

[Jerry Built]

No it has not. See below.

Erm, NT, I think you've become confused. This is doing people a big dis-service. There is nothing wrong with 1:1:6. For a brief and easy-to-understand look at the subject, please see:

Please note carefully that above I said "approach 1:2:9".

A LOT of mis-information has recently been spouted by those with vested interests in the lime production/use fields. 1:1:6 has been around for a long time - about 150 years.

J.B.

[Jerry Built]

What is your source of information? It's fine to clean external brickwork by non-destructive means. Things like sandblasting or scabbling are right out though! You don't really want a build-up of moss etc. on your house!

See other post. I'm not trying to be nasty!

Cement is a pozzolan, too!

I had a job on Wed/Thu fixing skirting/picture rail, some to a party wall. On the party wall I didn't use "hammer", just an ordinary drill with a newly-sharpened masonry bit. No problem at all (LBC commons). Not too noisy, either.

Do you mean rejects, or regrades? Rejects are junk, cracked or mis- shaped, used for hardcore etc. Regrades are bricks that are fine except for slight damage and colour problems. You wouldn't want to use them for facing. There are/were many, many types, textures and colours of bricks. There is likely to be a "brick museum" or a "brick library" at a BMs near you - for fun, 'phone up a few to see if anyone knows where (or say approx. where you are & I'll see what I can do). Obvious variations are from Staffordshire blues and similar which are as hard as hell, to Baxi decorative bricks used in fireplaces, which aren't as hard as your thumbnail.

[Jerry Built]

A problem with the fixings and corrosion?

J.B. --

[Christian McArdle]

Not that I can tell. Most of the slates are still present, they've just rotted away at the edges. They are 100 years old, though.

Christian.

[N. Thornton]

OK so if I understand correctly, according to this, 1:1:6 is fine, but some cement lime mixes arent. I know little about lime mortars, I read some articles and paper a year or so ago, some from English Heritage, and the concensus of them all was that lime cement mixes should be avoided. I guess, as the article you refer to says, there are differences of opinion. I expect I'll be using 1:1:6 then, thanks for the info.

Regards, NT

[Anna Kettle]

I shall stick my head above the parapet and call myself an expert ...

There is already mortar between the bricks, so the pointing is not there to stick the bricks together. It is there as a sacrificial layer.

Rain (water, salt, acid ...) is driven into the brick surface and then when the weather improves, leaves by the easiest escape route. If the pointing is weaker than the brick, then the easier route is through the pointing. Over the (many) years this causes the pointing to wear away and eventually the bricks will need repointing. This is a much better situation to be in than if the pointing is stronger than the bricks because then the surface of the brick wears away leaving the soft core exposed which wears away quickly. OUCH!

Consider three situations:

Old soft bricks, pointed with lime mortar. Lime mortar is weak, even with a pozzolan added, so the bricks will be OK.

Modern, hard bricks pointed with cement. So long as the cement mix is fairly weak this should be OK too. I'm not a cement expert so I assume that others know better than me and that a that a

1cement:1lime:6aggregate mix is OK. (The lime is just there as a plasticiser and a red herring)

Old soft bricks pointed with cement. The cement is stronger than the bricks so producing the effect which I can see outside the window as I sit here. An old red brick wall has a grid of cement pointing but the bricks in between the pointing have worn back by about 10mm since it was done. This is a problem which has shown up particularly in the last 50 years, partly because late 20thC builders have no experience of lime mortar, but also because Portland cement today is MUCH stronger than the cement of 100 years ago.

The thing to understand is that a weak mortar is not a poor mortar. The mortar must be weaker than the brick.

I shall create another message about pozzolans which seem to be causing a lot of confusion.

-- ~~ Anna Kettle, Suffolk, England |""""| ~ Lime plasterwork, plaster conservation / ^^ \ // Freehand modelling and pargeting |____|

07976 649862

[Anna Kettle]

If you want to be picky, then the word pozzolan only applies to volcanicash from Pozzolano in Italy. I shall use the word pozzolan in a much more general way and the pozzolan which is normally used today in UK is brick dust.

Pozzolans are impurities which are added to a pure lime mortar to give it some cementitious properties; to make it harder and more impermeable to water. When lime putty mortar is used outdoors, then pozzolans are always added to give it some extra weather resistance.

Hydraulic lime mortar contains natural pozzolans, so is fine for using outdoors without any more additives.

Hydrated lime should be put in a tub of water for a couple of days when lo and behold it becomes lime putty. Pozzolans can then be added for external work.

Once pozzolans have been added to lime putty mortar, it will start to set and so normally is used the same day.

To say that hydraulic lime (or lime putty mortar + pozzolan) have cementitious properties is misleading because todays cement is MUCH stronger than either. I don't have the figures to hand but my dodgy memory thinks that todays cement is about 10 times as strong as cements of 150 years ago and 20 times as strong as hydraulic lime mortar. Castle Cement have done lots of research on this.

Until a few years ago, a little Portland cement was often added to lime mortar to act as a pozzolan but there were too many mortar failures so English Heritage commissioned some research.

The results of the research were published as "The Smeaton Report" and in summary the report says that that cement+lime mixes will succeed if there is enough cement in the mix to get a _cement_ set. The lime is there just to provide plasticity.

What does NOT work is a mix where 'just a little' cement is used as a pozzolan because the cement clogs up the pores of the lime so the air and water which are required for a _lime_ set can't get into the matrix. Mortar failure is likely because there is a poor lime set and not enough cement for a cement set.

-- ~~ Anna Kettle, Suffolk, England |""""| ~ Lime plasterwork, plaster conservation / ^^ \ // Freehand modelling and pargeting |____|

07976 649862

[take away nojunk]

I always use 3 to 1 mix for pointing, any specific reason you have to use lime? Are you matching in to some old pointing? Tel

[Lobster]

Am I allowed to have my house sandblasted (to be followed up by repointing and coating with waterproofer)? It's worrying me; but how else do I remove all the old bits of crud and cement left behind when the render was hacked off, to restore the facade both aesthetically and functionally?

Thanks David

[N. Thornton]

I cant remember Im afraid. Youre saying scrubbing old bricks up wont make them go green? Thinking about it it makes sense. I expect only if they have water sitting or splashing on them theyre likely to green.

I've got that. What's the best action?

Rejects. In Victorian times as many as 50% of bricks made on site were rejects: broken, cracked, funny shaped, undercooked and too soft, etc. Bricks were generally made by itinerant gangs who came to the site, dug up the clay and burnt the bricks on site. Later pressed bricks took over. Aesthetically the hand made bricks are far nicer, but the reject rate was high, and the junk was normally used for the inner skin of the wall.

I just wondered if you might be going into undercooked bricks there.

Theres a good site online, cw lots of piccies, I lost the address though.

Regards, NT

[John Rouse]

In article , stuart noble

[Jerry Built]

To some extent it depends on the brick, but it's not generally a good idea outside! The brickwork can be made much more susceptible to water absorbtion and spalling. The operator's technique and the "sand" used to blast clean are factors too. What sort of bricks is the construction of? As a matter of interest, why was the render removed, and was it an original feature? If you have not too much to clean up, could you use brick cleaner (acid based)?

J.B.