Is this chemistry right? CO2 and Calcium Hydroxide fun

[Electrobes]

I am unfortunately not trained in the chemistry arts and am persistant in trying to figure out this method (and test it) of the following... it's been driving me nuts:

The (And correct me if I am wrong) majority of cement is calcium oxide, made by heating calcium carbonate to certain temps and creating calcium oxide and CO2 as products.

When you add water to cement, more importantly to the calcium oxide, you get calcium hydroxide. When cement hardens (In simple terms) the calcium hydroxide is being used, but since we'd like to use the rock before it fully hardens (Because it takes a while for the process to fully complete), we prematurely limit the hydration process which causes calcium hydroxide to remain unused.

Now we have calcium hydroxide being forced out of the rock, and it causes the pH in our waters to go sky high, far past for any marine animal to live in.

Now we need to do one of two things: 1) We somehow remove the calcium hydroxide from the rocks (Typically done by flushing it with water) or 2) we make the calcium hydroxide react to form calcium carbonate.

I have tried numerous ways to accomplish option one but it pains me to use so much water to achieve that goal. I've also tried muriatic acid dosing to the water but don't think it worked well.

So I am trying to make option 2 work. This is where my chemistry, in particular, fails the heck outta me. But I'll try none the less.

The goal is to convert the calcium hydroxide to calcium carbonate:

Ca(OH)2 ==> CaCO3

From reading around I see what is shown in the wiki for calcium carbonate:

Ca(OH)2 + CO2 ==> CaCO3 + H2O

What confuses me is the way this equation is to be executed to get the resulting products. If I created some rocks, that is precipitating out calcium hydroxide... how to I introduce CO2 to get it to form into calcium carbonate?

My guess is to simply add CO2 gas to a container filled with rock (At room temp?) and the results should be calcium carbonate and water. Since CO2 is heavier than air, the CO2 would "push" the air out leaving just CO2 in the closed container.

If the above is true, the other question would be how fast is the CO2 gas used up (Being used in the reactions).. within minutes or hours, or something more like a day or days?

What do y'all think?

[Percula9]

What is the point of this? Even if you make the calcium carbonate, it is not soluble at seawater pH.

[Electrobes]

The point of all this is to make the leftover calcium hydroxide something other than itself, as it drives the pH too high in seawater.

I'd rather have a small amount of the rock be insoluble calcium carbonate than calcium hydroxide that shoots the pH in that same seawater sky high.

[Randy Holmes-Farley]

I see no strong reason to think that the acid reaction with carbon dioxide (carbonic acid in water) will be any faster or more effective than reaction with other acids, like muriatic acid (HCl). The products are different (calcium carbonate in the former, calcium chloride in the latter), but what is the advantage of the CO2?

Thinking maybe the calcium carbonate will form an impermeable coating on the rock surface that the calcium chloride (very soluble) does not?

That's possible, I suppose.

You can soak the rocks in seltzer water if you want to see if it is different than other acids, but be careful to not drop the pH too low or even the calcium carbonate will dissolve (like in a CaCO3/CO2 reactor). That will be way faster than using gas.

[Electrobes]

Okay so I wrote out a response but then I realized what y'all mean... I love discovering the "doh" moment. I thought the reactions of CO2 in water and CO2 in plain ol' dry environment were more different than they are:

CO2 gassing the calcium hydroxide without an aqueous environment:

Ca(OH)2 + CO2 ==> CaCO3 + H2O


CO2 bubbled in water reacting to the calcium hydroxide:

Ca(OH)2 + H2CO3 = CaCO3 + 2H2O


Randy - When I was thinking and trying this, I was just gassing the rocks in a dry environment... meaning the rocks sat in a vat (With a lid) and I pumped CO2 gas into the vat until it was filled. I wasn't pumping CO2 underwater (Like a calcium reactor).

But in short, I was trying to make an impenetrable wall using the leftover calcium hydroxide in the rocks. I was trying to see which would be faster... using CO2 (In open air) to create calcium carbonate or using muriatic acid to create calcium chloride. I still am trying to figure that out. CO2 in open air uses no water, which is a major plus as I'd like to use as little water as possible when making rocks.

How fast would the CO2 reactions with calcium hydroxide take place, be it via water or in an open (non-aqueous) environment? It seems to me that it would take place rather quickly, no?

Haha, man I usually hate chemistry but this is oddly fun! :lol:

[Randy Holmes-Farley]

The reaction of pure calcium hydroxide and CO2 likely requires some moisture in the air, at least to proceed at a reasonable rate. Some moisture will make a wet interfacial layer where the reacts can take place. The pure gas phase/solid reaction may be pretty slow. But it will proceed. A container of calcium hydroxide will eventually go bad by reacting with CO2, but I had a bucket I drew from frequently for 10 years without loosing all its potency.

But DIY rock is different, and one may be limited by the rate of diffusion of ions through the wet solid, not the rate of surface reaction. Whether one can really do that at all in CO2 gas, I do not know.

I don't see much reason to use gas and not aqueous CO2, but if you try either, let us know what happens. :)

[Electrobes]

Hmm.. do you suspect evaporating water in the container while pumping CO2 would help?

I'll definitely keep y'all updated on the progress.

[Randy Holmes-Farley]

I'd keep it humid, yes. Maybe water on the bottom is adequate. :)

How do you intend to add CO2?

[Electrobes]

Well I am currently on my second run, and I just use a CO2 can with a regulator/flow meter on it, plus a hose.

I calculated how much CO2 I needed to fill the vat's volume (minus the volume of the rocks), open the valve at a steady flow and keep a timer on it.

In the vat I also have a 5G bucket with water, a heater, and a pump. This produces the evaporation to keep the environment hot and humid.

After the timer's gone off (Meaning the vat is full of CO2) I simply close off the valve.

The only thing I am not sure of for this experiment is how often I should refill the vat with the CO2 gas and when. I am not sure about how much CO2 it would take (On average) to get most of the calcium hydroxide to react. Currently I am just filling the vat once a day, with the gas. Any suggestions?

[Randy Holmes-Farley]

I can't know how much or how fast one needs to deliver acid to cure the rock. It's going to be guesswork.