White powdery substance in basement?

In our walk-out basement, we can see the frost wall due to the fact that our concrete man did not pour the floor all the way to the wall. Between where the floor stops and the frost wall starts, we see a white powdery substance that almost looks like frost, but does not melt like frost. Anyone have an idea about this? It does not have an odor to it. We want to finish our basement, but would like to know what this is first. Thanks.