I'd not heard that old concrete was PH neutral, so I can't honestly answer that question. I wonder how I'd go about testing this new information?

One of the problems could be that by adding PH adjusters to the growing media such as (or especially) vinegar that the vinegar, or acidifying agent would cause the concrete to break down more quickly, and there may even be a noticeable chemical reaction (??)

I've got some old and new concrete here I'll have to moisten with distilled water and test the PH and see what results I get.

Interesting question, and by the way I don't generally recommend concrete because in addition to the PH, minerals will leach out and a true bog is naturally exceedingly low in not only nutrients but also dissolved minerals (unlike a fen which often has high amounts of dissolved minerals such as calcium)

Thanks. I was going to try litmus paper when I can find some.

'Pup