Grout as concrete

Just earlier today I was using Wall spackle ( the white stuff you fill in screw holes and stuff in walls with) and it gave a nice concrete look, especially once painted.

I would follow Ian's recommendation.  The grout is too grainy and will be impossible to file and shape once it is dry.  I always use wall spackle for my groundwork and walls.  It is easy to work with either wet or dry and takes paint well.  You can also make small holes in it for bullet holes and they look great since the plaster is soft and workable with a white under color.

OK guys thanks for your input. I'll pick up a small tub of wall spackle then.

If you get the premixed, sanded grout, the best thing it will make is a neatly-troweled sidewalk.  The acrylic-modified grout will give a smooth, precast, sort of look; but you need to get a whole bag, then you need a bucket, and then you need to be able to "goose" the mix with either more "hot" or less "hot" so that it does not set up on you too fast.

But, that's just my experience having grouted one too many surfaces in my time as a remodeler (and I'm never having a tiled surface in my own house, I love grout so much <g> {or, she'll need to be a rich, beautiful, and generous woman <g>}).

I prefer a medium drywall "mud" over "spackle" per se, it has a bit more working time.  But, "drywall repair" mud works wonders, too.

plaster of paris