first of all you NEED to chisel out around the pipe an area big enough to glue either a male connector (t/a) or a coupling onto the pipe. period. that is, if you want to be up to code. if you don't you can do as suggested above. with a coupling (not recommended), you will have to find a 1/2" Box Adapter, that will glue into the coupling. to be very watertight, you glue the crap out of the back of it that contacts (and glues to) the back of the box. any weatherproof pvc box will do, use a 7/8" unibit or a hole saw for a 1/2" fitting. (7/8"). i wouldn't recommend this way because you would have to chisel a hole twice as deep into the cement for this coupling to be flush with the wall, than you would for a connector. with the connector the threads have to stick out from the wall so you can screw a metal box onto them.
the better way would be the connector (male t/a). you will have to, in either case, chisel the cement out about an inch back into the wall. you cut off the pipe as close to flush as you can, better if you have a dremmel tool and can cut it about 1/8" inside the wall with a cut off attachment. but even if you can only cut it flush, say with a sawzall, you can still get away with it being an 1/8 or so off the wall as long as you screw it into the cement on two corners (hammer drill will help, but for two one inch deep holes for plastic anchors, you could prolly use a regular drill with masonry bit).
glue the connector on. buy a 1/2" sealing locknut if you are using a plastic box and use it on the inside of the plastic box. if a metal box with a threaded hole, just put any kind of goop on the threads and screw the box on until it's tight and vertical or horizontal. then use plastic anchors to attach the box, add your gfi protected device and a bubble cover approved for extra hard use, and your done. just to clarify: if you use a plastic box, the 'box adapter' and coupling will work OR the male t/a. if you use a metal box you HAVE to use the male t/a only. and i think i said to use the connector and the metal box. that's what i would do. peace out and don't forget to wear eye protection when you are chiseling, ya chizzler!