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I mistakenly applied a patio cleaner containing acetic acid to my porous concrete facing brick thinking it was biocide. The concentrate solution has a pH of 3 and I diluted it to one part concentrate and three parts water.

About three hours later, having realised my mistake, I flushed it with water. I repeated this process the following morning and also on the morning after that. The bricks now appear to be dry. I have been told that the pore structure of the type of bricks I have is vital in protecting them from frost attack.

I am very concerned that the acid has damaged the brick and mortar joints. I am wondering if the acid has now completely evaporated — one day after the last rinse. Is there anything more I can do to neutralise any residual acid? I would be very grateful for any information for someone who has little understanding of chemistry.

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  • $\begingroup$ Usually bricks are made of impure aluminum silicate. This substance is not attacked by acids, even if the acid is stronger than the acetic acid. The only condition for them to be attacked by an acid is the calcium carbonate (chalk) that they may contain as an impurity. But you will soon know that it is the case, if you observe the surface of your brick after having applied the acid. If there is some foam or froth on the brick, it means that there is some chalk in the brick. But don't worry. The effect will soon disappear if you solution had a pH 3, or even more. The result is negligible. $\endgroup$
    – Maurice
    Commented Apr 26, 2020 at 12:39
  • $\begingroup$ I've heard that vinegar is a mold killer, so maybe you did your bricks a good thing. If they were susceptible to acetic acid, I suppose acid rain would be terrible! $\endgroup$
    – James Gaidis
    Commented Apr 26, 2020 at 20:20
  • $\begingroup$ Maurice, is calcium carbonate not one of the ingredients of Portland cement, which is used in making concrete bricks and the mortar in which they are bedded? $\endgroup$
    – Brendan Little
    Commented Apr 26, 2020 at 22:29
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    $\begingroup$ @Brendan Little. Calcium carbonate is used to produce Portland cement. But when heated with clay, it loses its Carbon atoms as $\ce{CO2}$ and is transformed into a calcium aluminosilicate. So there is no calcium carbonate any more in the cement $\endgroup$
    – Maurice
    Commented May 22, 2021 at 7:38
  • $\begingroup$ The quantity of acetic acid will dissolve very little brick before it is consumed. An application or two will dissolve very little brick. $\endgroup$
    – blacksmith37
    Commented Sep 19, 2021 at 16:54

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Per Wikipedia on Fired Bricks, to quote their composition;

Normally, bricks contain the following ingredients:[21]

Silica (sand) – 50% to 60% by weight

Alumina (clay) – 20% to 30% by weight

Lime – 2 to 5% by weight

Iron oxide – ≤ 7% by weight

Magnesia – less than 1% by weight

I would expect slowly, with time, that the acetic acid could attack all of the above with the exception of Silica.

Flushing with water would reduce the magnitude of the attack, but I would not categorically claim that the brick structure is completely sound.

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  • $\begingroup$ Is acetic acid not totally miscible with water and, therefore, would evaporate at roughly the same rate? Even if the acid evaporate a a slightly slower rate, I understand that this type of acid evaporates completely leaving no residue or crystals. Surly copious dilution with water would cause its corrosive effects to be negligible during the evaporation precess? $\endgroup$
    – Brendan Little
    Commented Apr 28, 2020 at 10:51
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    $\begingroup$ "Contain" is a wrong word to use. These things could have been taken initially, but when heated together, they form something different. $\endgroup$
    – Ivan Neretin
    Commented Sep 19, 2021 at 7:41
  • $\begingroup$ Upvote for your last sentence. Muriatic acid is used to clean bricks, but acid readily attacks mortar. Try adding baking soda to the water flush, and rinse until no "fizzing" is observed. $\endgroup$
    – Robert DiGiovanni
    Commented Oct 16, 2022 at 9:49

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