I scouted several options like brick wallpapers (yuck!), real bricks (expensive), fake fiberglass bricks (also expensive)... but then, at the office I saw many old foam core boards (KAPA Line), I had salvaged from other projects.
These boards are 5mm thick with coated paper on the front and the back. One side had a print on it and as I peeled it of, it resulted in a nice rough paper surface. So that was, what I was going with.
I started by drawing some lines on the wall using a level to guide me. Then I glued the tiles on the wall using tiling adhesive. Since the tiles are very light, they stuck pretty good and didn't come off 'til the glue cured.
Using my spacers I kept adding row after row, randomly picking tiles, but tried to arrange them so they would look realistic. This took about two hours.
The trick is not to paint two neighboring tiles the same way so I jumped rows and colums and then mixed a new batch of color so it started to look natural.
After that was done, I painted the groves grey.
Then I added some weathering with the same grey, some mossy green, yellow and white using a sponge, a brush and my fingers.
Finally I sprayed some grey paint dust around.
That's pretty much it - this resulted in a more drastic redecoration of my hallway, which I will cover in an other 'ible.