Mixing cement

I've chucked loads of gypsum slop-ends over the lawn and the beds, and things that grow there seem to like it. Did the same with cement in one particular flower bed, and it developed quite a strong crust. The bluebells all managed to break through it though, giving their usual dense display of blue.

I wouldn't use these anywhere where you might grow veg though; there are some nasty things in cement. Probably wasn't a good idea to dump cement mortar slops into the flower bed in the first place.

Reply to
Andrew Gabriel
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Alkaline though. You could always use New Improved B&Q Soil Improver (aka sharp sand!)

Reply to
stuart noble

Crushed dead cement is still good for oil spills on the garage floor.

Andy

Reply to
Andy Champ

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.

Reply to
The Natural Philosopher

but most domestic concrete des not need to be high quality.

And limestine is never used in concrtete to my current knowledge: its usually flint (silica), or possibly granite.

Limestone is soft and permeable.

Reply to
The Natural Philosopher

I did wonder about that. I've never seen it in mixed aggregate. Not that I've knocked up much.

Reply to
Dave Plowman (News)

It must depend on where you live in the UK then because around here it's almost all limestone, even the readymixed crowd use it, and almost all BM's stock it, along with B&Q etc. Flint is a rarity, except for decorative purposes and granite is used, but in the form of 'granno' - IE sharp green grit.

Reply to
Phil L

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Reply to
Phil L

I asked this

And my cement was fluffy and did not need to be broken up by a rolling pin.

Adam

Reply to
ARWadworth

Glad you posted that refresher. I contributed there and said I had used some old cement for a fence post. It set. And a few days, maybe a week or two, later I realised that I needed to 'adjust' it. Bolster. Lump hammer. And by gosh it was tough.

For critical work - fresh, in date, dry. Never even consider anything else.

For probably-strong-enough - slightly old, still looking good, dry can be adequate.

What OP of this thread said, he'd need to send it to a cement factory for reprocessing to stand a chance.

Reply to
Rod

well well well. Round here its never ever used. gravel (flint) only..

Reply to
The Natural Philosopher

In my experience, limestone and alluvial gravels are by far the two most frequently used aggregates for concrete. Flint is something of a rarity - I don't think I have ever seen a project that used flint for a concrete aggregate.

Where gravel is available, it tends to be cheaper. However, away from south east England, gravel is used less and limestone dominates the market. There are some localised areas where granite is quarried in large quantities for roadstone, and some of it finds its way into local readymix plants, but granite is always quite expensive because of the much higher cost of crushing it - it wears out crushers four or five times faster than limestone does.

Even in south east England, limestone has taken an increasing share of the market. That is largely because of the efforts of ARC and Foster Yeoman, who ship limestone from the Mendip Hills of Somerset principally into Acton, West London, using their own fleet of trains.

There is nothing inferior about concrete made with limestone. Many large bridges, landmark office developments and other major structures are built with it. Most of the nuclear reactor pressure vessels in the UK's nuclear power stations are made of pre-stressed concrete made with limestone aggregate. So it is a top quality product.

[This is not in any way a criticism of your postings about concrete in this thread, which are otherwise factually correct and make an interesting read - thanks.] ** Posted from
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Reply to
Bruce

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
Reply to
Spamlet

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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Reply to
Bruce

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

Reply to
The Natural Philosopher

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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Reply to
Bruce

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.

Reply to
Rod

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.

Reply to
The Natural Philosopher

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`?

Reply to
The Natural Philosopher

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.

Reply to
The Natural Philosopher

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