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.]
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.
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".
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.
Hypothyroidism is a seriously debilitating condition with an insidious
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.
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
But you are probably right.
Hypothyroidism is a seriously debilitating condition with an insidious
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
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
it takes a lot longer to weather to rounded pebbles or 'soft sand'
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.
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`?
An experiment gainsayers might like to try before getting any more cherty:
In this Wiki entry:
Where the picture caption reads:
"Pebble beach made up of flint nodules eroded out of the nearby chalk
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.
(One time geologist. Anybody in the market for my old rock samples? Ah:
don't tell me: ballast!)
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.
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).
[email address is not usable -- followup in the newsgroup]
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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