I have no knowledge of chemistry, but I heard the blast could have been caused in part by the water used to put out the chemical fire?
You don’t need water and AN (Ammonium Nitrate) does not need oxygen because it contains more than enough oxygen to burn on it’s own. All you need is sufficient heat and some compression to allow the material to transition from deflagration (burning) to detonation (explosion) and once a small part detonates all the material in proximity will detonate.
Here is a wiki page on fertilizer disasters - quite a few of them and again makes one wonder why they would build dwellings and a school around such a facility.
http://en.wikipedia.org/wiki/Ammonium_nitrate_disasters
This one (in Texas in 1947) was the ship that exploded and killed 581 people.
http://www.history.com/this-day-in-history/fertilizer-explosion-kills-581-in-texas
Ammonium nitrate.
In it’s base form, just nitrogen, hydrogen, and oxygen, all gases.
Because of the chemistry, it doesn’t need an oxidizer, it’s inherently unstable, causes a high pressure low velocity explosion.
I’ve worked with the commercial, prilled form use in mining.
Friend of mine has a small gold mine.
Kinda cool. Getting to play with blasting caps, fuses, etc.