robvdv
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Moulding calcium carbonate
I'm trying to make a calcium carbonate structure shaped somewhat like a golf tee. At the moment I'm sieving together calcium hydroxide and sodium
bicarbonate, moistening it and putting it in the oven.
It seems to be producing calcium carbonate, water and sodium hydroxide. I've tested this scientifically by poking it and it obligingly removed the
fingerprint on my index finger.
Does anyone have a better way of doing this (making the plug, not removing my skin)? I've tried casting pure calcium hydroxide but the centre of the
moulded plug still hasn't set after two months. I've tried to speed up the cure by immersing plugs in a yeast-produced carbon dioxide "bath", but this
still takes a couple of weeks to set.
Ultimately this is going to be used as a plug on which to grow coral fragments for my reef aquarium. The sodium hydroxide will be flushed before then.
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12AX7
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Try with more chalk in situ. You're making something out of 100% cement, when you want a concrete with like 10% cement.
Using a non-pH-neutral process may be very troublesome. It's obviously hard to wash all that stuff out of something that, though porous, is solid.
Tim
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Mr. Wizard
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Try using limestone or marble chips (calcium carbonate) mixed with sodium silicate (waterglass). When this mixture is exposed to a CO2 bath it is
supposed to harden up. I haven't tried it, but the mixture of silica sand and sodium silicate is supposed to set up when exposed to a CO2 bath, and is
used to make quick hardening sand molds for metal casting. The action of the CO2 on the sodium silicate converts it to Silicic Acid, which is a gell
that will dehydrate to silicon dioxide , binding the material together. A corresponding amount of sodium carbonate will be liberated, but you say you
are going to wash this out. You may have to experiment with the mixture of marble and limestone chips to get a smooth or porous mixture. You could
even add powdered chalk to thicken it up or make it smoother.
Another method would be just to use a plain brick mortar mix with sand and masonry lime, letting it harden from the air. Obviously it does set up
slowly, but it is time tested. If silica or sand is a problem you don't want to use either of these suggestions.
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Xenoid
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Powdered calcium carbonate or chalk is available from a number of sources, for example, it is sold by pottery suppliers as "whiting", you could try
blending this with a minimal amount of cement powder to make a sort of "morter". You could add a little sand, for a more realistic appearance.
Blending the "whiting" or chalk with polyester resin or epoxy resin is another possibility they are available in bulk from fiberglassing and boat
building shops, use a minimum amount.
Many "crack filler" compositions from hardware shops are calcium carbonate based, they will set hard, get one designed for external use, careful, some
are cellulose based.
I believe there are commercial products, similar to what I have outlined, they are designed for artists, sculpters etc. You may find something at an
art shop.
Something like this; http://www.sculpturesupply.com/sscpdfs/marble.pdf
Depending where you live, you could possibly go out to the countryside and find some limestone or marble. Check on a geological map!
BTW, why does this structure have to be made of calcium carbonate, or even contain calcium carbonate. Corals are not plants, they do not send down
roots to extract calcium carbonate in order to grow. The individual polyps secrete calcite (or aragonite) from calcium carbonate extracted from
sea-water. You just need plenty of shells in the tank so that the water is saturated with calcium carbonate (bicarbonate). Individual polyps will grow
wherever they find a suitable substrate, whether it be fibreglass with sand embedded in it or another dead or alive coral or some other natural
material.
Best wishes,
Xenoid
[Edited on 21-8-2007 by Xenoid]
[Edited on 21-8-2007 by Xenoid]
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robvdv
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Thanks very much for the responses, folks. Sorry it's taken me so long to come back.
Xenoid. you're absolutely right. Corals can be grown on more or less any firm substratum, including the aquarium glass. However, reefkeepers are kinda
fussy about what goes into their tanks. The sodium silicate Mr. Wizard suggested may result in extra silicone oxide added to the tank. This will
accelerate diatom growth (a bad thing). Aragonite is regarded as a neutral thing to add.
12AX7's suggestion was good. I've had better results with sugar sized aragonite sand mixed with calcium hydroxide. The air reaches the interior better
when it's a porous structure and thus sets the calcium hydroxide better. The end result isn't nice and smooth, though. I've also tried sodium
bicarbonate and calcium chloride, but got a lumpy mush.
Much of my frustration stems from the fact that I know it _can_ be done. Here's an onling merchant selling 'em: http://cam-net.biz/shop/index.php?main_page=product_info&... I've seen the plugs and it's clear they're cast in a mold (air bubbles in the
structure).
I guess it's back to the drawing board.
[Edited on 24-8-2007 by robvdv]
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Xenoid
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I had a look at the image!
When you mentioned "golf tee" SHAPED, I didn't realise you meant golf tee SIZED as well! I was imagining something about 30-40 cm tall, and about 30
cm across the top. 
Do they have to be made of aragonite, I don't know whether corals secrete this or not!
Finding aragonite may be your biggest problem!
Best wishes, Xenoid
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Eclectic
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Maybe a mix of crushed coral and calcium hydroxide? Is there any problem with using some portland cement in the mix? It's calcium aluminum silicate.
If you don't want portland cement, you might try a bit of phosphoric acid as a binder. It will react with the calcium hydroxide to form
hydroxyapatite (bone), but I don't know how fast the reaction will be.
[Edited on 8-24-2007 by Eclectic]
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robvdv
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Thanks Xenoid. Aragonite is easy to get hold of. Many reefkeeping stores sell it in various grades. It's the first crystal calcium carbonate makes
when it forms before reverting to calcite over time (take this with a pinch of salt -> I'm not that clued up in this area).
Thanks, Eclectic. I've used that "recipe" before successfully. The extra porosity of the crushed coral allowed carbon dioxide to penetrate deeply
quickly. Unfortunately, quickly in this context means "around a month".
I've used regular Portland cement and white cement to make structures before. Both of these binders require two or three months of flushing in fresh
water before they become pH neutral enough to put in a reef tank.
Phosphoric acid is a clever idea, but unfortunately phosphates are an infamous algal accelerant in reef tanks. Many reefers use ferric oxide to adsorb
PO4 out of the water.
This is going to be more of an issue in the future. It's likely wild collected corals will become harder to obtain due to (welcome, in my opinion)
legislation. Cultivated corals will need to be instantly recognisable as such. An obviously man-made base is one of the best ways of doing this. The
most universally acceptable substance that this base can be made out of is aragonite.
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Xenoid
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| Quote: | Originally posted by robvdv
It's the first crystal calcium carbonate makes when it forms before reverting to calcite over time (take this with a pinch of salt -> I'm not that
clued up in this area).
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When calcium carbonate crystallises above 30 oC, aragonite (orthorhombic) forms, below that temperature, calcite (trigonal) forms. Aragonite existing
below 30 oC is "metastable" and slowly (very slowly) reverts to calcite. All, really old (we're talking hundreds of millions of years here) calcareous
fossils are calcite, but some still exhibit the crystalline shape of aragonite.
Some marine creatures secrete aragonite, others calcite. Some, such as the cuttlefish, both.
Regards, Xenoid
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unionised
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I realise that if you want lots then this is a non starter, but can't you just carve one from real chalk (not blackboard chalk)?
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Eclectic
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You won't have any free soluble phosphate after it reacts completely with the calcium hydroxide. I think coral is also largely hydroxyapatite?
You may be able to speed up a CO2 "cure" and alkali neutralization with a wet CO2 bath (carbonated water).
[Edited on 8-26-2007 by Eclectic]
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JohnWW
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| Quote: | Originally posted by Xenoid
| Quote: | Originally posted by robvdv
It's the first crystal calcium carbonate makes when it forms before reverting to calcite over time (take this with a pinch of salt -> I'm not that
clued up in this area).
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When calcium carbonate crystallises above 30 oC, aragonite (orthorhombic) forms, below that temperature, calcite (trigonal) forms. Aragonite existing
below 30 oC is "metastable" and slowly (very slowly) reverts to calcite. All, really old (we're talking hundreds of millions of years here) calcareous
fossils are calcite, but some still exhibit the crystalline shape of aragonite. Some marine creatures secrete aragonite, others calcite. Some, such as
the cuttlefish, both. Regards, Xenoid |
How would that temperature be affected by the presence of either magnesium or strontium, substituting for part of the calcium on the crystal lattice?
Also, limestone/chalk, on continued contact with sea-water, undergoes gradual partial replacement of the Ca with Mg, which may lead eventually to the
formation of dolomite (a mixed Ca-Mg carbonate) and magnesite (MgCO3). The insoluble carbonates of other divalent metals may also occur in it as
impurities.
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Xenoid
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Aragonite tends to be a very pure mineral with very little substitution by divalent metals.
Sr, because of its similar ionic radius, is the main exception, and to a lesser extent, Pb.
The presence of Sr, Pb, Ba and CaSO4 in solution favour aragonite formation.
It is interesting to note that in some corals that secrete both calcite and aragonite, the aragonite parts contain as much as twice the amount of Sr
as the calcite. The presence of Sr also apparently inhibits the conversion of aragonite to calcite under natural conditions.
Aragonite formation is favoured by high pressure, at say, 20oC aragonite forms above about 4 kbar. When looking at the aragonite - calcite equilibrium
curve, in theory, aragonite should not form at all at atmospheric pressure, but in natural systems it does.
Ref. Deer, Howie and Zussman - An Introduction to the Rock Forming Minerals. 2nd. Ed. 1992
Regards, Xenoid
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JohnWW
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Thanks, Xenoid. I wonder if that reference is available as a PDF or DJVU download. By "insoluble carbonates of other divalent metals", besides Ca, Mg,
Sr, as impurities, I was thinking of those of Fe(II), Zn, Cu(II), Co(II), Ni(II), Mn(II), Pb(II), Ba, Cd, in about that order.
[Edited on 27-8-07 by JohnWW]
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Xenoid
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@ JohnWW Re. the reference, I doubt it, Deer, Howie & Zussman has been pretty much a universal standard text book on mineralogy for University
Geology courses since the 1960's.
Mg in dolomite can be replaced by Fe all the way to ankerite (Ca(Fe,Mg,Mn)(CO3)2), with the term dolomite being restricted to Mg/Fe > 4. Pure
CaFe(CO3)2 does not apparently occur naturally. Similarly dolomites with a few % Mn exist and there is probably a complete series through to
kutnohorite (CaMn(CO3)2).
Zn and Pb dolomites are also known! The stability of dolomite type compounds follows the trend Mg>>Mn>Zn>Fe>Co>Ni. Many if not all
the divalent metals you list occur as natural carbonates, especially those associated with mineral deposits, and will substitute for Ca in calcite in
at least trace amounts. Those with ionic radii close to Ca, will obviously substitute more readily.
Incidentally, most calcite is actually quite pure, but those with Mn, Zn and Cu impurities make the best fluorescent specimens for UV displays.
All this is getting pretty much away from the thread topic... 
Regards, Xenoid
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robvdv
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Heheh. Yeah, I feel a bit like I've bitten off more than I can chew asking you folks. On the subject of fluorescence, though, corals are quite
impressive, producing a large number of fluorescent compounds. There's quite a bit of conjecture as to the purpose of the fluorescence in coral.
Interestingly, it appears as though fluorescence is used as both protection in that it reflects light away and also for maximising low light
conditions by bouncing the light back into the tissues.
Strontium is often added as a trace element to reef tanks. There's a fair bit of talk about it being a simple substitute for calcium and not being
particularly biologically important. There's a lot of snake oil in this industry.
Magnesium carbonate would be one of the few acceptable non CaC03 substances to add to the coral plugs. Dolomite is often added to calcium reactors to
add magnesium to the water.
I understand that coral skeletons are predominantly calcium carbonate. Some corals, notably the Mediterranean red coral used for jewelry, are mostly
calcite while the bulk of reef building corals (staghorn, elkhorn types) are aragonite.
I've cut through some coral skeletons with a Dremel tool and smelt that distinctive "burning bone" smell but for the most part not.
I find this quote surprising:
"When looking at the aragonite - calcite equilibrium curve, in theory, aragonite should not form at all at atmospheric pressure, but in natural
systems it does."
Does this mean that the lime plugs I've made so far consist of a crystal other than aragonite?
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Xenoid
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| Quote: | Originally posted by robvdv
Strontium is often added as a trace element to reef tanks. There's a fair bit of talk about it being a simple substitute for calcium and not being
particularly biologically important. There's a lot of snake oil in this industry.
I find this quote surprising:
"When looking at the aragonite - calcite equilibrium curve, in theory, aragonite should not form at all at atmospheric pressure, but in natural
systems it does."
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I wonder if the coral polyps (wonderful little creatures that they are) are able to extract Sr from seawater, to enable them to selectively secrete
aragonite. I don't know what the advantage or otherwise would be for this (aragonite vs. calcite for skeleton construction). One would imagine calcite
would be better. With regard to Sr, it may just be a case of "which came first, the chicken or the egg".
I'm not really sure how relevant the aragonite - calcite equilibrium curve is in this case. It is determined by subjecting pure CaCO3 to high
pressures and temperatures in a steel "bomb". Impurities and crystallisation from solutions and coral polyps doesn't come into it.
Actually I just noticed the following statement in the book reference cited above;
""Aragonite can be readily synthesised by mixing carbonate solutions with solutions containing calcium ions under conditions of controlled temperature
and ageing. With the addition of sodium polyphosphate, CaCO3 precipitated from sodium chloride and carbonate solutions is largely or wholly aragonite.
In general the crystallisation of aragonite is favoured by temperatures of 50 - 80 oC.""
So, I guess if you mixed say CaCl2 with Na2CO3 at say 70 oC, the resulting CaCO3 precipitated would be in the form of fine aragonite crystals.
Regards, Xenoid
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robvdv
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Thanks for the suggestions, folks.
Xenoid, you may be interested to know there's an interesting genus of coral known as the Pulsing Xenia. They're quite mesmerising. Here's a video: http://www.youtube.com/watch?v=chDXsuRlYoA
Cheers,
Rob
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