Mixing cement


wrote:

Er: in the SE, where I come from: 'gravel' generally = small bits of flint + the odd oyster and belemnite fossil, as it comes from Chalk-derived clay with flints washed and sorted by rivers. [Take a look at a Google Earth of Rickmansworth to get an idea of the vast amount that has already been removed along most of the river valleys in the region. Quite scary I find.]
S
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I have lived most of my life in SE England, and worked in the concrete industry for many years. The correct term for what you are describing as "flint" is "crushed gravel".
Perhaps it is colloquially known as flint, but in geological terms flint consists of hard inclusions within sedimentary rocks such as chalk. If the chalk is eroded, and the flint particles are washed down rivers, those particles are then termed alluvial gravel - but this could take hundreds of thousands or millions of years, and the washing and sorting process significantly changes the shape of the particles. Alluvial gravels are very rounded compared to flints.
I'm sorry to be pedantic but the point of the discussion is to understand which rocks are used for concrete aggregate, and which are not. Flint generally isn't, except as an imprecise term for something rather different, which is correctly called gravel.
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Bruce wrote:

Flint is the name of the material as a mineral type. Gravel or shingle is a generic term for sedimentary particles larger than sand grain size.
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Bruce wrote:

What did you think alluvial gravel WAS if not flint? Small and smashed up..but its still flint.

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No, that's what it was. An incredible amount of changes are made from flint being eroded out of chalk to becoming alluvial or sea-dredged gravel. Ask a geologist. My partner is a geologist, and she is highly amused that people should describe alluvial gravel as "flint".
That's like describing coal or oil as "trees".
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Bruce wrote:

Long ago, I have bought a bag of fine ready mix concrete in which was something that looked like flint. Of the order of quarter inch pieces and most edges very square and sharp (some very sharp). At the time, I assumed some sort of crushed flint. Clearly not alluvial gravel as that would have its edges smoothed off at least a bit.
--
Rod

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Rod wrote:

Glaicial
The way it works is that ice carrues racks that smash other rocks in te way, and the whole lot got carried darn sarth, at which point global warming was just enough to melt the glaciers, and as they melted they left behind huge deposits of crushed rock, mainly flint, but few other types as well: the softer rocks like sandstones became more mud like and formed the clays. Largely thats is what you buy as sharp sand and gravel. Its full of jaggedy bits of smashed stones.
That is although its been deposited by water, unlike sea stuff which gets constantly pounded until the edges are knocked off, the vast majority of gravels and sharp sands only made one trip in the water, before being left where they are found today.
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The Natural Philosopher wrote:

Up to a point.
Alluvium is from running water, n'est pas? I was thinking if it was from a glacier it would be moraine. But then I tried looking it up and found the word 'till'.
I can't help feeling that most river deposits are somewhat rounded (but not, I agree, as rounded as sea stuff). Where the stuff I mentioned was truly sharp.
But you are probably right.
--
Rod

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Rod wrote:

moraines are made of till.
I live on one.
Clay here, with flint and chalk well down underneath.
Up the road, gravel pit, where they sieve out the sand (very smashed flint), and a fair amount of less smashed flint, which is called gravel or shingle.
The difference between chert, flint, and most sand is one of context: all are basically silica.
Gravel/shingle/sand is a loose term that really boils down to fragment size..however in the building and gardening trade, flint gravel is just called 'gravel'. and you tend to get e.g. 'granite shingle' used to describe smashed up granite chippings..
although technically 'limestone gravel' exists, its not called that here: Its called MOT type I.. ;-)

Really it may or may not be. If you hit e.g. N walees you will find scree slopes of very sharp stuff indeed. It only needs a storm to carry a bit away in water, and lay it down for it to become a shingle or gravel bed.
it takes a lot longer to weather to rounded pebbles or 'soft sand'

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wrote:

How rounded pebbles and grains are, depends on how many times around the sedimentary cycle from beach to rock and back again they have endured. For hard materials like silica the time scales involved are almost unimaginable. Real experts can look at your sand/gravel deposit and work out exactly where it came from and how it got there and just how long ago the individual grains were created, and indeed where they to were made. For a perfect natural concrete look at some Hertfordshire 'pudding stone', the individual rounded beach pebbles look as if they are separate from the matrix, but the whole breaks clean across - that is, if you are strong enough to break it at all. There is some in my garden...
Ah the heady days of Rutley, Read and Watson.
S
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Probably crushed alluvial gravel.
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Bruce wrote:

Then your partner is being silly. Its only been smashed up, not metamorphosed, and in the gravel in my drive there are plenty of intact small flints to be seen, the rtest being spmewaht smashed.
I suppose that he/she is equally amused by roadstone being called 'granite'. since its no longer bloody great lumps of igneous rock, but small stome chippings.
Or limestone being called limestone, since its crushed into a fine powder`?

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She thinks you're hilarious. She's right. ;-)
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wrote:

An experiment gainsayers might like to try before getting any more cherty:
In this Wiki entry: http://en.wikipedia.org/wiki/Flint
Where the picture caption reads:
"Pebble beach made up of flint nodules eroded out of the nearby chalk cliffs"
Try editing it to read: "Pebble beach made up of *gravel* eroded out of the nearby chalk cliffs."
Let us know how you get on.
Anyhow, all irrelevant to an overall very handy thread.
Cheers, S (One time geologist. Anybody in the market for my old rock samples? Ah: don't tell me: ballast!)
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Spamlet wrote:

Yes. Concrete is strong in compresssion, relatively weak in shear, and very weak in tension.
Essentially conctrete is packed stones. To increase shear strength we add sand to stop them sliding about and we add cement to stop the sand grains sliding about. We then add steel to give tensile strength.
The cement merely increase the shear strength by adding a reasonably high shear matrix between the sand grains.

I dont even bother to do that, TBH. large limps of crap embedded in mortar work pretty well.
concrete will work at prettty low cement ratios, but it becomes porous, liable to frost damage, aand easily damaged by sharp onbjects.

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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).
--
Andrew Gabriel
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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.
--
*I used to have an open mind but my brains kept falling out *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On Tue, 15 Jul 2008 00:23:03 +0100, "Dave Plowman (News)"

Ok - I've been reading this thread - know <0 about cement/mortar, but trying to learn a bit. This has got 2B a stupid question, but how can the mix be "*too strong* for most things". Too weak I could understand - but too strong! Please explain.
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For example. If you use too strong a mix when bricklaying any movement will cause the bricks to crack.
--
*I never drink anything stronger than gin before breakfast *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

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