So I have this question. Can you dissolve concrete with acid? I know that there are many types of concrete, but the common and dominant component, as far as I know, is $\ce{Ca3SiO5.H2O}$ (hydrated tricalcium silicate). Will the acid react with it? What kind of acids are best for the job? Specifically I'm interested in nitric acid ($\ce{HNO3}$). If the reaction is possible, I also want to know, will it generate heat and how much?

  • $\begingroup$ From experience, Nitric Acid will happily react with concrete producing NOx. Cannot give the exact reaction and the generated heat, so will leave it to others to answer. $\endgroup$ – Jeppe Nielsen Jun 27 '19 at 1:14
  • $\begingroup$ There is a thing called chlorine trifluoride which is able to ignite sand and concrete. $\endgroup$ – Nilay Ghosh Jun 27 '19 at 3:48
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    $\begingroup$ Well, chlorine trifluoride would ignite most of things, including human body. Not much suitable for home experiments. $\endgroup$ – Poutnik Jun 27 '19 at 4:04
  • $\begingroup$ @Jeppe Nielsen I suppose HNO3 will not dissolve concrete, but at the best will convert it to some silicate / hydrated silica based residue. $\endgroup$ – Poutnik Jun 27 '19 at 8:53

Concrete is frequently treated with dilute hydrochloric acid to etch the surface prior to painting or coating with epoxy. After treatment with acid, good rinsing is suggested. This etch treatment will react with CaCO3 at the surface (Ca(OH)2 produced as a product of the hydration of cement reacts with CO2 in the air to form CaCO3 over a period of months), and it will likely bubble up a little. Then the acid will react with Ca(OH)2 which is deeper in the concrete, and some sand and stones will be loosened. If you brush these away, the acid can go deeper.

Nitric acid offers no better results.

Silica is generally formed after acid treatment, but is so finely divided that it can be rinsed off. If you want to dissolve the silica, you would use hydrofluoric acid, carefully, because of its human health hazard.

The sand and stone will not be affected much at all.

If you have lots of time, you could use mild reagents, like sugar dissolved in a little water. The concrete beneath tanks where sugar is purified suffers degradation as this very weak acid just complexes the calcium into solution. Months, years, maybe, depending on how much sugar and how much rinsing away there is.

Heat generated is fairly small because the reaction is slow with bulk concrete, especially if it is strong, but can get larger if you grind up the concrete first.

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  • $\begingroup$ Hmm, I would think that Nitric Acid would perform significantly better. $\endgroup$ – Jeppe Nielsen Jun 30 '19 at 16:04
  • $\begingroup$ In commercial practice, concentrated muriatic acid (HCl) is diluted to acid wash concrete or mortar. Nitric acid is more expensive, more hazardous when concentrated and turns your skin yellow if it doesn't burn it off. (The rinse-off would be a better fertilizer if it were neutralized enough so as not to kill vegetation!) You might think faster is better, but not only would nitric acid be no faster, a slower etch is often desired for better control. Hydroxyacetic acid is used for slower etch and complexation of multivalent metals to prevent discoloration of concrete. $\endgroup$ – James Gaidis Jul 1 '19 at 12:52

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