I'd try that too, brushing & hosing to exposeaggregate. But I dont
think you need any retarder for that, concrete has very little
strength when first set, it takes a few days to have serious strength,
so you could do it after 24 hours or sooner.

NT

I have created bricks by casting concrete in wooden forms/molds, and I
found that 24 hours was too soon to de-mold them. The edges and
corners were still too fragile, and ended up chipping off during the
extraction from the mold. At that point, there is no way at all that I
could have exposed any of the aggregate without extremely vigorous
brushing and spraying. I think people use the retardent for a reason.
I have never seen the stuff for sale at any home centre style of
retailer. I think you would have to go to a wholesaler that
specializes in serving the concrete contracting industry.
--- rod.