but seriously... BE CAREFUL!!! this is how they make some fiberglass forms (such as speaker boxes) and when you light it... It is nearly explosive... There will be a puff kinda like when you light a grill that has lighter fluid on it... You can loose eyebrows...
Actually there are people that make base rock for their saltwater aquariums with cement. GARF is probably the most well known organization that makes base rock with concrete, they call it Aragocrete. Check out http://www.garf.org/class.html
They suggest soaking it for 6-8weeks in a large rubber bin. Doing a water change on it daily to allow the rock to cure so it doesn't mess up your Ph levels.
My only advise is to do some careful measuring. I helped my brother out making a concrete (or cement, i forget) shelf using pieces of that blue foam sheeting. He covered it and cured it and everything and when we went to put it in his 55 the structure was too big to fit in
Also watch out for coloring. We found some red cement dye that he wanted to used, but it's coloring agent was ferrous oxide (rust), which would have been very bad for his fish.
Also I suggest not to use a torch to take out the foam. A little foam will make no different and a torch is a bad idea. Burns the plastic, makes gases, releases chemicals, could possibly kill your fish, although im not sure exactly what chemicals are present. I would suggest just cutting as much out as you can..
FYI...Cement is a powder. Sidewalks, roads, buildings, etc are all made of concrete. Lots of people speak of a cement sidewalk... No such thing. It is concrete. Cement is one of the ingredients used to make concrete.