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I would like to dissolve $\ce{CaSO4}$ in a hardened condition (e.g. gypsum board) as fast as possible. I have seen in posts about $\ce{CaSO3}$ that vinegar and sulfamic acid are able to dissolve it.

However, I would appreciate leads to reactants that are a little more active. Basically I am looking for a drop of something that would instantly create a hole.

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    $\begingroup$ Lead moving at 1000 fps will create a hole quickly. $\endgroup$
    – MaxW
    Commented Jun 7, 2019 at 0:01
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    $\begingroup$ A little known substance called drillium, maybe? $\endgroup$
    – Oscar Lanzi
    Commented Jun 7, 2019 at 0:26
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    $\begingroup$ Substances instantly creating a hole in CaSO4 instantly create a hole in most of other stuff too. Like ClF3 or RDX or 239Pu. $\endgroup$
    – Poutnik
    Commented Jun 7, 2019 at 1:08
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    $\begingroup$ Thanks to the commenters above, they are useful as they answer the question in a creative way. $\endgroup$
    – Joe
    Commented Jun 8, 2019 at 14:48
  • $\begingroup$ Hi that is kind of a common industrial problem, however, some companies invented some effective Calcium Sulfate scale remover. May be the combination ATMP and other chemicals. $\endgroup$
    – patty
    Commented Dec 15, 2019 at 19:52

2 Answers 2

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Unfortunately calcium sulphate, gypsum, $\ce{CaSO4}$, is depressingly stable and nothing non-deadly will dissolve it.

The other posts you have seen are about calcium carbonate, chalk/limestone/marble, $\ce{CaCO3}$, which acids will dissolve, although people do not in practice use that fact to make holes in it. One of the first difficulties would be to get the application of a liquid focused enough to make a hole-shaped hole.

If you ever did want to dissolve marble, I'd use hydrochloric acid, $\ce{HCl}$, because calcium chloride is nice and soluble. But check beforehand that it doesn't dissolve the gloves you intend to use.

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Mechanical methods and extreme chemistry aside, there is no instant method of dissolving bulky calcium sulfate (or its hydrates: $\ce{CaSO4 · 0.5 H2O}$ and gypsum $\ce{CaSO4 · 2H2O}$).

Soda-acid treatment

Since $\ce{CaSO4}$ is notably one of the reasons for permanent water hardness, industrial scale machinery using water heat exchangers or boilers are typically liberated from calcium sulfate by washing the insides with concentrated hot sodium carbonate solution for several hours, converting sulfate to less soluble carbonate, which is subsequently dissolved in inhibited acid solution (the process might take half a day or longer):

$$ \begin{align} \ce{CaSO4(s) + Na2CO3(aq) &<=>> CaCO3(s) + Na2SO4(aq)}\\ \ce{CaCO3(s) + HCl(aq) &-> CaCl2(aq) + H2O(l) + CO2(g)} \end{align} $$

Complexing

There are numerous commercially available mixes for cleaning molds used for gypsum casting within hours. Exact composition is usually classified as a trade secret; the majority of the manufacturers would list two main components:

  • Alkaline cleansing agents. To put it simply, any alkali, e.g. $\ce{NaOH}$.
  • Complexing agents. Most likely, these are disodium edetate $\ce{Na2H2EDTA}$ and sodium citrate $\ce{Na3C6H5O7}$.

The idea behind using EDTA salts is pretty simple: it makes insoluble metal salts soluble by making chelate complexes such as $\ce{CaNa2EDTA}$ which are stable (and well-soluble) in water. Citrate may also complexate calcium, and alkaline medium promotes complex formation. This method is widely used in analytical chemistry for the qualitative and quantitative determination of numerous cations (transition and rare-earth metals, actinoids).

Both of the above-mentioned methods may be significantly accelerated if the diffusion barrier is lowered, e.g. by using ultrasonic bath.

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