Mixing cement

Well, I thought 3 parts sand and 1 part cement would be okay, but no. It's too sandy by far. I'm sure I put in 3 levels of cement and 9 of sand.

I had to crush some of the portland cement because it's a bit old and some of has gone hard, but not rock hard. I used a rolling pin.

I'm having to chisel out the concrete now and make up a new mix. I notice that when I'm chiseling the concrete out that I made, it easy turns to powder. My house has concrete/pebble walls and I must chisel out and plug with concrete for shelving.

I'm miffed and not slightly confused why the mix I made turned out so poor. Next time I'll use 2 sand, 1 cement.

Reply to
Rich
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Use NEW cement FFS!

Reply to
RW

Does cement "go off?

I mean, I can crush these clumped up bits to powder, because they are fairly soft. But is the cement now practically useless even when ground down?

Reply to
Rich

3:1 is a very strong mix. There aren't many applications where that would be suitable.

That means the cement has partially set, so you actually have a cement bag containing a mixture of inert dust and cement, the proportions of which are unknown. Just because you broke it apart doesn't mean it's capable of setting a second time.

Where does the concrete come into it? You described making mortar, not concrete.

Because your cement has gone off, and/or because you didn't allow enough time for it to set fully (about 6 weeks).

1) Chuck out the bag of gone-off cemnent. 2) Describe the wall construction in more detail. 3) Specify age of house, and location, so we have some idea how it might have been constructed.

Then people can help you by suggesting what you should be using (which might not be cement at all, but certainly won't be a 2:1 or 3:1 mortar mix).

Reply to
Andrew Gabriel

sort of. Essentially half of that ground up crap has set. So its not cement, its a weak sand. Which is why you needed more..

Reply to
The Natural Philosopher

Yes

Now. what were you doing with a 1:3 mix of cement and sand?

As this is not classed as a concrete mix but a mortar.

A concrete mix would be 1:3:4.

I.E. 1 part cement, 3 parts sand and 4 parts chippings - of varying grades dependant upon the job in hand.

Also, the amount of water used will have an effect on the strength - as will the water content of the sand. Too much water = a weak mix of concrete

Tanner-'op

Reply to
Tanner-'op

3:1 is far too strong for most things.

You have your answer. Cement goes off with age - and is so cheap it's not worth keeping.

Reply to
Dave Plowman (News)

Ok - I've been reading this thread - know

Reply to
dave

For example. If you use too strong a mix when bricklaying any movement will cause the bricks to crack.

Reply to
Dave Plowman (News)

Mortar should be weaker than brick, then any movement breaks the mortar not bricks.

NT

Reply to
meow2222

Actually strength reduces with pure portland cement.

The ideal water resistant mix is where the holes between the grains of sand are completely filled with fine cement. Thats a 'strong' mix and is used underground and below damp according to my brickies, so that it wont split with frost: instead the bricks will....haha. Nothing to do with the mortar though. porous bricks below damp (or in driving rain)+ frost = spalling.

Weaker than that between 2:1 and about 5:1 the mortar becomes less full of cement, and air pockets replace the cement between the sand grains.

Enough to make it quite permeable. Like most bricks are, so its reckoned to be no worse than the bricks in terms of general waterproofness. The texture tends proggresivly from 'hard set muck' towards 'crunchy sand' as you go up the scale reaching crunchy sand with very little strength at about 7:1

Which is sort of what you might lay e.g. patio slabs on. The cement only being there to stop the sand washing away, without reducing drainage.

I've got a few cracks in my wall that I built where I didn't use ties. The cement is rock hard, but no bricks cracked by the way.

I have never seen a brick split by too strong cement yet. Maybe old, soft or very cheap bricks will tho.

The general reason to not use too strong a mix is firstly that it gets weaker beyond the sort of optimal 2:1 and secondly cement, whilst cheap, is still a whole lot more expensive than sand.

To be honest, there is a lot of merit in the view for general bricklaying, that the cement is there to stop the sand washing out, and the sand is there to keep the bricks apart ;-)

In this weather you mortar wont fully set for 2 days as far as general feel goes. After that it will set a bit harder in a few weeks.

Reply to
The Natural Philosopher

Also, good mixing is important, especially for weaker mixes. If you are trying for 5:1 for bricklaying and its not mixed well, you could have parts at 7:1 which is too weak and / or will take too long to set. Simon.

Reply to
sm_jamieson

Butting in here, as I too find the ratios in practice a bit hit and miss, and have to fight a strong tendency to add a bit more cement 'just to make sure':

Other comments seem to be hitting on about 5:1 sand to cement for mortar, but your ratio for concrete is essentially that same 'spoonful' of cement now extended to cover 7 'spoonfuls' of other stuff. Is that really reliably enough? (before I resume trying to remake a large number of steps in the - very steep - garden, that are taking me a long time because of the surprisingly large quantities of materials involved... I would not want to sit back and watch them all get gradually washed away afterwards!)

Cheers, S

Reply to
Spamlet

I always want to add a bit. Doesn't seem feasible it will set sound with standard ratios.

Isn't it very largely the surface area of the 'other stuff' together with how the packing works? That is, the cement fills holes betwen sand particles; the sand/cement fills holes between the aggregate particles. And there is just enough 'binding' power in the cement for the huge area of sand surface plus the much smaller area of aggregate surface.

Posting in the hope that someone will tell me where I am wrong - if I am. That is just how I seem to have absorbed infromation over the years.

The experiemnt I have always wanted to do is to mix in a load of jelly (whether real jelly, carragheenan, agar or something similar). Seems to me that you might manage a very weak mixture but surprisingly strong final substance as the gelling could stop the cement particles flowing out. Or maybe it would work to allow very sloppy mixtures to set?

Reply to
Rod

I think the ballast in concrete (e.g. stones) is large enough simply to take up volume in the mix. But a general strong concrete mix is

1-2-4, so the cement to sand ratio is 1 to 2, more than you would use for a mortar where 1-3 would be the max (with engineering bricks). When I was using all-in ballast (sand and stones), I thought the equivalent to 1-2-4 is now 1-6 which sounded quite weak, but it set very solid, since the stones are really just bulking up the volume, as well as stopping cracks propogating etc. Simon.
Reply to
sm_jamieson

Beginning to see light dawning here. So really we might see the thing as the ballast being equivalent to a number of very small bricks, mortared together by the sand and cement.

Suppose one just has to watch out for any more absorbent or soluble 'stones' that might creep in (bits of old plaster board come to mind, as I consider the bags of rubble from our bathroom conversion, as possible ballast material for my steps - duly sieved of course.).

All very interesting,

Cheers, S

Reply to
Spamlet

Also strong mixes shrink more when curing. Using the weakest suitable mix helps avoid shrinkage problems and saves money.

Reply to
Andy Wade

Don't use secondhand rubble to mix concrete...like anything else, it's only as strong as it's weakest link, and if that link is lumps of lime plaster and bits of plasterboard, then it's going to diseappear fairly quickly - frost is the main enemy of concrete, and given that these highly absorbent particles will soak up water like a sponge, the frost will then expand these, effectively blasting apart the concrete until it comes away in thin sheets. Limestone, other solid stone chippings or pebbles are the only decent quality ballast that should be used with sand and cement for high quality concrete.

Reply to
Phil L

Are the plastery bits any use for anything? Hmm, lots of blackboard 'chalk'...

Could go on the allotment I suppose, but that is real chalk already.

S
Reply to
Spamlet

I have read that it (gypsum) helps to make clay soil workable.

Reply to
Rod

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