Air bubbles in cement/concrete

[Spinal]

Howdy! A TOTALLY off-topic one here;

I'm trying to make some "fake" rocks, but I need the make them light and porous. I've tried just about everything I can think of; sand, coarse salt, pasta, rice, etc. My mix is:
-3 parts crushed oyster shell
-1 part portland cement

To which I add anything that can make it lighter.

My next idea is to add some lava rock/pumice pebbles to the mix - but I'm having trouble sourcing them.

Then, I remembered something from my childhood - we had these large white slabs of cement that weighed next-to nothing. They were (to the best of my knowledge) white cement with lots of tiny air bubbles trapped inside.

Any ideas how thats made? A quick search of wikipedia seems to make me add some surfactants to the mix, but it doesn't mention any names...

Ideas?

Thanks,
Michele

 

[Mudster]

You're probably thinking of Breeze Block or some thing similar.

I'd say pumice was the obvious material to source, as it floats in water.

Pumice is used in the jewellery trade as a polishing compound (at least I think that's what it's for), try some jewellery trade suppliers, they may be able to point you in the right direction.

 

[Mudster]

Here's an example of a supplier selling pumice powder:-

http://thepolishingshop.co.uk/acatalog/Abrasive_Powder___Grit.html

The Romans used this to make their concrete lighter so you're on the right lines, get googling and you'll probably find some Roman recipes.

 

[Will]

What are the 'light' rocks for?

Could you not coat some polystyrene or something with the mix - ie, partially 'hollow' in effect?

Will

 

[television]

Why not cut them out of polystyrene and then shape them, then roll them in wet cement, add a little plaster if too coarse. the special cement mix used for drains is very smooth, add some colouring dye to the mix

 

[Spinal]

Wow! Talk about a quick reply !

I was googling some more and I found that:
'air-entrained concrete'
is what I had seen in my childhood; but it seems like I can't just buy the additive to make it.

The rocks will be used in an aquarium/fishtank (after a 6 month curing process to allow the cement to stop leeching).

Mudster: Thanks for the link; but wow is that expensive! At £5 for a 500gr tub, I'de be looking at £500 worth for 50l!

Polystyrene rocks: I actually didn't think of this! What happens to polystyrene if left in marine/salty water for long periods of time? Does it break down into anything harmfull to marine life?

Michele

EDIT: Found this on the CAA website

Foamed Concrete / CLSM
Powerful surfactants that allow very large amounts of air to be formed in the concrete, producing a low strength, light weight material, particularly suitable for filling large voids where strength is not an issue.

Any idea where I can find this stuff?

 

[nick mercedes]

Polystyrene rocks: I actually didn't think of this! What happens to polystyrene if left in marine/salty water for long periods of time? Does it break down into anything harmfull to marine life?

Won't they float??

 

[Spinal]

Won't they float??

Not if there is enough cement on them I'm thinking polystyrene pieces (like the packing stuff) mixed in with the cement... or maybe even the small round ones...

 

[Mudster]

Wow! Talk about a quick reply !

I was googling some more and I found that:
'air-entrained concrete'
is what I had seen in my childhood; but it seems like I can't just buy the additive to make it.

The rocks will be used in an aquarium/fishtank (after a 6 month curing process to allow the cement to stop leeching).

Mudster: Thanks for the link; but wow is that expensive! At £5 for a 500gr tub, I'de be looking at £500 worth for 50l!

Polystyrene rocks: I actually didn't think of this! What happens to polystyrene if left in marine/salty water for long periods of time? Does it break down into anything harmfull to marine life?

Michele

EDIT: Found this on the CAA website

Any idea where I can find this stuff?

Take another look at the link, they are selling pumice rocks for 50p each.

 

[jamesmc]

Vemiculite is what sprung to my mind. Not too expensive at £10.76 (including Vat) per large bag.

Micafil ( Hoben ) Large Bag Course Grain Vermiculite


Check out..... http://www.blanchford.co.uk/acatalog/Micafil_Vermiculite.html

for more info & a picture.

 

[Benzowner]

Won't they float??

Isn't that what all the floating marinas are using? 2400 x 1200 x 600mm polystyrene blocks coated in cement? Thicker cement will eventually sink the blocks, but you have to get the blocks denser than water, so 240 x 120 x 60 divided by 1000 approx 1700 litres or 3500lb of water. Archimedes did something on this

 

[Howard]

Michele,

Read these, the marine fishkeepers among us do this all the time ...

http://www.reef-eden.net/diy_9.htm

http://www.reefsuk.org/forum/viewtopic.php?t=14125

You have to be super careful what you put in it , and it needs to cure in a container with a pump to ensure movement ...

And yes, we use poly blocks covered in concrete to keep our marinas afloat .... very good it is at doing it too !!

 

[DITTRICH]

If you make the rocks more or less where they're going to end up then surely the weight issue doesn't enter into it?
Having read the other thread I now understand a little bit more.
Les

 

[Spinal]

Thanks for all the advice!

I had a look at some other DIY live rock threads before; but no-one has a good idea regarding additives... (that I can find)

I've just come back from the garden center (went to buy some worms and daphnia for the fish) and found some "LECA" (balls of clay with air bubbles in them. They seem VERY light, and at £4 for a 5l bag its cheapish. I also bough a 25l bag of pond-filter rocks for £21-something (a porous medium that allows bacteria to grow on them).

John, I just read your link in more depth as I really liked their consistency - I realised they used dishwasher salt which I didn't think of. Will try that as soon as yesterday's batch is hard enough to move.

That said, I'm using grey portland cement; wickes didn't have the white one. I'll need to find white one!

Les; the reason you want the rocks to be light as possible is that bacteria and little critters live in the tiny little cavities. Also, they displace less water which means you can fit a bit more water in the tank.

As for curing, I'm buying an inflatable swimming pool off ebay (£10) and dumping it in the shed with 2 powerheads and water. I'm not planning to use vinegar (partly because the acidity will weaken the cement and partly because I don't want vinegar to infuse the rocks...)

Michele
Michele

 

[DITTRICH]

Now I understand its to do with fish tanks...

 

[Howard]

Some of the reef guys are now making aquascapes out of expanding foam , coated in sand .....

Have a read of this build , its a long one, but start from page 11 onwards , and look how it ends up ....

Impressive ...

http://www.ultimatereef.net/forums/showthread.php?t=211033&highlight=wave

 

[david55amg]

you could always use lemonade instead of water

 

[TonyVianoAmbiente]

Howdy! A TOTALLY off-topic one here;

I'm trying to make some "fake" rocks, but I need the make them light and porous. I've tried just about everything I can think of; sand, coarse salt, pasta, rice, etc. My mix is:
-3 parts crushed oyster shell
-1 part portland cement

To which I add anything that can make it lighter.

My next idea is to add some lava rock/pumice pebbles to the mix - but I'm having trouble sourcing them.

Then, I remembered something from my childhood - we had these large white slabs of cement that weighed next-to nothing. They were (to the best of my knowledge) white cement with lots of tiny air bubbles trapped inside.

Any ideas how thats made? A quick search of wikipedia seems to make me add some surfactants to the mix, but it doesn't mention any names...

What you need is "foamed concrete". It is a near-normal concrete mix with a foaming agent added. Getting the proportions right is important, and mixing it with a mechanical mixer is essential to get consistent results.

Concrete normally weighs 2.4 tonnes/cubic metre. Depending how much foaming agent you add, you can get foamed concrete down to 1.1 tonnes/cubic metre. But you may not need to go that low.

For information about the foaming agent, do a Google search on "foamed concrete" and you will find all the information you need.

During my career as a civil engineer I was responsible for many thousands of cubic metres of concrete in many types of structures. Alas, I have no personal experience of foamed concrete but I hope that the above information is enough to point you in approximately the right direction.