Why Lime instead of Cement Mortar?

[TheScullster]

Noticed in an earlier thread that a poster was advised to use lime instead of cement based mortar on a victorian house. Is this a brick type issue? When did cement based mortars take over?

TIA

Phil

[Christian McArdle]

Yes. Victorian bricks are generally quite soft. It is essential that in any masonry structure that the mortar is softer than the masonry. Otherwise, the masonry will crack or spall. Cement based mortars are very strong and totally unsuited to Victorian brick.

Increasingly common from the early twentieth century, along with other innovations like concrete blocks and plasterboard.

The Victorians did have cement mortar, but it was generally used for its pozzolanic qualities for submerged work, such as sewers and bridges, along with suitably hard bricks.

Christian.

[Andy]

There are other issues of course. Lime mortar is lighter in appearance than cement, so using cement to repoint bits of a lime mortar bulding will look ugly. Also, it is said that it is breathable, whereas cement is not, so a lime mortar render can let out damp that enters via a crack in the render, whereas damp that enters through a crack in cement render is stuck, causing penetrating damp and rot in the walls.

Andy

[Rob Morley]

Old bricks are often soft and porous, while cement mortar is harder and less permeable to moisture. The mortar should be the weak link in a wall or the bricks may start to fail, or sometimes cement pointing will just fall out in large pieces if the wall moves.

Lime was still extensively used up until WWII.

[s--p--o--n--i--x]

Lime based mortar is softer than concrete and allows a certain amount of movement in a wall.

In days gone by, pointing was intended to be a sacrificial material-the intention was that it'd be replaced every 50 years or so.

Usually, modern builders/surveyors see weathered pointing as a fault and replace the lime mortar with hard wearing cement based mortar. This is a mistake.

Often, when walls are re-pointed with concrete you end up with cracks in the softer bricks as the mortar has no 'give'. This leads to moisture getting in, bricks having to be replaced etc..etc.

If you have lime based pointing and it's wearing away then it is doing its job correctly. Have it re-pointed with more lime based mortar and you will get no problems in the furure.

sponix

[Derek ^]

I remember a pair of semis being built across the road from my school entrance and the kids had previously been used to walking across the site as a short cut. There was a big pile of grey powder on the site with a notice that said "This lime will burn your hands". It would have been Ca. 1955 in Leeds

DG

[TheScullster]

Thanks to all respondents for illuminating discussion.

Phil

[Andrew Gabriel]

Portland Cement was too expensive for widespread use in general mortar until 1920's, when it starts appearing near the production sites initially, but took a long time to work its way across the whole country.

[david lang]

|| Usually, modern builders/surveyors see weathered pointing as a fault || and replace the lime mortar with hard wearing cement based mortar. || This is a mistake.

How can you tell which is which? Does it look different?

I have a 50's built semi and the front garden walls (like the rest of the estate) are falling apart. The bricks are sound, but the motar between them seems to have lost it's 'bond'. Its crumbly.

Some of this is due to no capping being used, just bricks on edge so the failure is more pronounced where the weather gets to them.

If I rebuild the crumbling sections, should I use lime or cement?

Dave

[Chris Bacon]

Is the wall still structurally sound? If not, pointing is point-less.

Bricks on *edge*? Do you mean the top course?

[Christian McArdle]

Yes. Cement mortar is much, much harder.

Use what was originally used. 1950s front garden walls could have been lime or cement based.

Christian.

[Andy]

Lime mortar is usually white. It may have weathered a bit but it's still lighter than portland cement. Scrape a bit off and look at an unweathered surface, I've seen it look anywhere from off-white to creamy. Of course, the type of sand or grit used with it might colour it slightly different from locale to locale.

Andy.

[bigcat]

Also there is the question of foundations and movement. 100 yr old houses had very shallow foundations, and movement in these is normal. With cement mortar, the weaker bricks break. With lime, the lime cracks, but unlike cement it microcracks, then self heals. It reacts with CO2 to grow hard crystals across the crack, so it heals itself, with no damage done.

Also cement tends to pull the surfaces off the bricks in time, and this loss of fireskin causes the brick to get wet, and crumble when it freezes. Over the years bricks thus affected will slowly crumble away.

And finally theres damp. Water that gets into the wall, as is inevitbale, evaporates out from lime mortar. It doesnt with cement, so old walls built with porous bricks and no dpc are more prone to damp is cement is used.

Note about cement lime mixes: almost all such mixes fail prematurely. The only one that is ok is 1:1:6.

NT

[Cordless Crazy]

The 'brick on edge' is refered to as the title suggests..

a brick....on its edge!!

Normally found on most 1 brick thick walls to finish it off the top

Walls are not normally terminated on a course of, errr, stretcher bon for example, as the weather will penetrate and get up to its usua business.

Half brick thick walls sometimes have soldiers on the top but thes walls are very flimsy and a quick boot can bring them down, especiall with lime mortar :-

-- Cordless Crazy

[Lobster]

While we're having lessons - what's the difference between Portland and other types of cement? I'm sure Portland is the only type I ever see at my usual suppliers.

David

[Christian McArdle]

Well Ordinary Portland cement is what we would generically call cement, although naturally occuring Pozzolana is also often called cement, especially in historical contexts (i.e. Roman construction). Portland cement is a manufactured material actually based on lime, but mixed up with clay (or sand) and baked, to modify its qualities, making it much quicker setting, non-porous and hydraulic (able to set under water quickly).

Some lime mortars have OPC in them, whilst some have lime only (plus sand, of course). Even with OPC mixed in, they are much more porous and suitable for older buildings, as they allow the building to move and prevent soft bricks from cracking.

Christian.

[david lang]

|| Is the wall still structurally sound? If not, pointing is point-less.

It's the top two or three courses that are unsound, the rest seems OK. I'm talking about rebuilding rather than pointing.

|| Bricks on *edge*? Do you mean the top course?

Yes

||| If I rebuild the crumbling sections, should I use lime or cement? || || Both.

What, cement mortar to rebuild & lime to point?

Dave

[Chris Bacon]

No, 1:1:6! I shouldn't put a "hard hat" on it IIWY. If you use cement mortar to build, it's no use to have lime pointing.

[Chris Bacon]

I would have said "cement pointing is much harder than the lime the wall was originally built with; the excessive force on the edges of the bricks causes spalling, which is exacerbated when water is then allowed into the brick and is subject to the freeze/thaw cycle".