Could have been amonium nitrate (fertilizer) like was used in Oklahoma City.
http://www.freerepublic.com/focus/f-bloggers/3871476/posts
Al Arabiya: Beirut Explosion was a Hezbollah Weapons Depot
from link:
But Al Arabiya reported late Tuesday afternoon that the warehouse was actually an arms depot for the terrorist group Hezbollah, which unfortunately essentially runs the national government.