I made a slate cave based on this article: http://www.plantgeek.net/article_viewer.php?id=6
I did not build up a planting area on the top - mine is just the sides with a top. Getting the sides right is a bit like a jigsaw puzzle, so I "dry stacked" mine and only siliconed it when I was completely happy. I made sure there were no sharp edges on the slate by checking with my finger. My cory cat loves it!
In addition to this, Quikcrete will also get weak and crumble after some time in the tank as the limestone is dissolved. The acidic ph of most planted tanks will eat it away.You could but I think the problem with concrete products is that they may increase the KH of your water. If you want caves for soft water species using concrete may not be a good idea but if you are using it for hard water species, it may be OK.