I'm glad this thread has come back to life. My personal life has gotten in the way of my hobby life a lot the last year, so I've been pretty absent.
My own original thoughts were for making images in concrete countertops. Like for kitchens and toilets. So clearly the image needs to be sealed. But as pointed out, concrete is the wrong pH despite its available iron.
And from a previous life I know that jerking around the pH can have detrimental results to the concrete.
A conundrum to say the least.