OK, there's a lot of misnomers here.
1. RDX is not the most common demolition agent. It is used primarily by the military. About 95% of all explosives now are ANFO (ammonium nitrate fuel oil) explosives. When doing controlled demolitions of buildings, usually a gel is used: nitroglycerin dissolved in nitrocellulose.
2. A very small amound of explosive is actually used in the demolition of a building. They are placed at strategic points and gravity is used to collapse the building.
3. The dust cloud you see has little, if anything, to do with the energy left behind by the explosive. It is, as Borek said, just parts of the building. By the time you see the dust from the explosion, the shockwave has already long since passed that way.
4. Explosives do not necessairly form gas when they detonate. All explosives release a tremendous amount of energy, this is true, and most release gas, but not all. The expansion is from the relase of energy instantaneously. Take cuprous acetylide, for instance: you only get copper and carbon out as detonation products, carbon as solid, copper as solid or liquid. No gas is produced, but it is a brisant explosive. The power comes from the fact that most high explosives release enough energy to locally heat the area to 4500K or higher and generate an overpressure (above atmospheric pressure) of around 210,000 atm. This forms the shockwave from the compressed medium around it and on average is about 0.2mm thick and travels between 4000-10,000 m/s.
The website you have listed is interesting and has some good information to get you started. It neglects a lot of very important things if you really want to calculate volume of gas, etc., though. You'll need more than a video to do it, too.
It sounds like you are into the physics of explosives. I would strongly recommend one title to you for this: Explosives Engineering by Paul W. Cooper. You can get it at Amazon. As long as you have a little calculus, the book will be perfect. You don't really need the calc, but it helps to have it so you're not scared off by the math. It goes over the physics, calculations you would want to make, the thermodynamics, the decomposition products, etc. I think you'll really get into it.
There is no simple answer to what you're trying to do. The fact is that if you try and calculate based on the dust cloud, you might as well guess. I suppose if you wanted a very rough estimate of volume, you could use the ideal gas law, but you're so far from STP at explosive conditions, you'd really be grasping at straws. The relation:
Doesn't hold up so well under extremely non-ideal conditions, but perhaps you could get a vague ballpark of volume by inserting the average numbers I quoted above.