Crumbling Cement

[Mark64]

Hi,

Just a little background and if anyone can help with advice or has ha a similar experience it would be great to hear from you.

Three weeks ago we completed building some internal walls on a Bar conversion using a 4:1 mix. It appears the cement is not setting correctly, joints are weak an when it is rubbed it turns to powder.

The cement is standard LaFarge OPC. Storage has been in a barn, nice and dry and away from wind and damp stock rotation has been observed. It has been placed on the floor but on a thick layer of plastic bags.

Walls have been build in the last week with Lafarge Mastercrete and th muck has set perfectly well.

LaFarge technical called out and took a sample of the cement for a Muc mix test. The technical guy agreed there was a serious problem and i all had to come down (600 blocks!).

We also ran exhaustive tests and found the same faulty results ever time even with differant aggregates.

Admittedly the cement we are using to test is now a week or so out o date, it was within date when the walls were built.

When talking to the sales rep on another note she assured that th product being out of date by a few weeks would not affect th performance.

The test has come back from LaFarge and they are claiming the muck i not setting due to the product being out of date and poor storag conditions.

Can anybody help?

Regard

-- Mark64

[Weatherlawyer]

What was the weather like in your neck of the woods 3 weeks ago? There wasn't much bricklaying or concreting done on our barn conversion site beginning in the frost prior to the snow.

[George]

I always thought it was...3:1 ratio? the bloke next door but one has had his outside back yard wall rerendered because they used too much sand in the mix which resulted in the same scenario...you rub it and it will reduce it to dusty particles.

[The Natural Philosopher]

I would say frost damage as well

Ive used really old cement, and it still works sort of..weak, but it sets OK.

[The Natural Philosopher]

You need to be up at 6:1 or 10:1 to get that.

[meow2222]

Has someone actually tried 10:1? I used 7:1 just once when the cement ran out and the job needed doing without delay. It was totaly useless, a finger nail could scrape it away easily. Removing it was a very fast job.

Fraid I cant help with the OP's cement problem. I cant help wondering if Lafarge might be right though, unless the barn's really 100% dry it can be affected.

NT

[Andrew Gabriel]

I've tried 6:1, and that set fine.

1:1:6 is a very commonly used mix too.

You can tell when cement is getting old -- it gradually turns from a fine flour into a more lumpy/crumbly texture as the fine grains have absorbed moisture and stuck the the next fine grain. I suspect that in effect it becomes diluted with set cement. I had a large bag of sand and cement in the shed for about 10 years (probably about 9 years past its use-by date). It was opened, but the top was folded down. In the end it was getting rather lumpy, but it still worked fine, and the odd bits of outdoor brickwork it was used on are still rock solid after a further 10-15 years.

One thing to bear in mind is that mortar takes a long time to set. The first set comes in around 24 hours, but the second stage takes up to 6 weeks, and during that time, the surface will rub off. At only 3 weeks old, I wonder if the OP's cement simply hadn't been given long enough, particularly in cold weather?

[nightjar]

Sounds about right. The bags should have been stored stacked closely together, to minimise air circulation, raised on pallets and closely wrapped in polythene sheet. Rain can still raise the humidity inside an unheated building enough to affect cement that is not properly protected from damp air.

Colin Bignell

[meow2222]

... and rock solid. Which makes me think perhaps the cement was below par in the 7:1 I used. I gave it a few weeks to harden up, to no avail, it just went from sand to softly stuck sand.

NT

[The Natural Philosopher]

Hmm. That is what tends to happen in VERY weak mixes used to mount slabbing.

[George]

Serious I've always mixed at 3:1 when doing brickwork,maybe thats why the guy had trouble knocking the bricks out of the fireplace when he fitted a neighbours inset gas fire. lol

[<me9]

I used 5:1 for bedding some tiles, knowing I would want to remove them (when completing the job two years later). They were too firmly bedded to remove when I came to do it, so I have just left them, still as good as with the 3:1 mix I used for the others. I haven't tried a weaker mix, but a local leisure centre had to be re-built as the walls were built with too weak a mix (mortar could be removed with finger) due to poor quality control as it was being built.

[Andrew Gabriel]

One other thought -- it's probably much more critical in the weaker mixes that the sand and cement is very well mixed, or you probably end up with something which is 3:1 interspersed with areas of just sand, which won't appear to set.

[The Natural Philosopher]

That is true. If the cement starts to crumb you need it mixed in a mixer..all of mine was.

The difference in a weak mix is the cement still sticks the grains together but there is a lot of air in the gaps between the sand grains.

For building walls, its arguable that its just bricks piled on one another on a sand bed..the cement just stops the sand falling out :-)