Ammonium nitrate is an industrial chemical commonly used around the world as an agricultural fertilizer, and in explosives for mining.
It has also been used as a key component in improvised explosives, notably in the 1995 Oklahoma City bombing, in the 2002 bombings in Bali, Indonesia, and by far-right Norwegian terrorist Anders Behring Breivik in his 2011 shooting and bombing attack.
In this case, according to Lebanese officials, about 2,750 metric tons of ammonium nitrate had been stockpiled at a Beirut port warehouse, just a few minutes' walk from the city's shopping and nightlife districts, since it was confiscated in 2014.
Prime Minister Hassan Diab said the chemical had been stored for the past six years "without preventive measures," and promised an investigation.
It's not yet clear what caused the stockpile in Beirut's port to ignite, with such deadly results, on Tuesday evening.
"Ammonium nitrate is ... relatively safe by itself, although a strong oxidant, but highly dangerous when contaminated by any kind of fuel, such as oil or organic material, even in just a few per cent," Roger W. Read, Honorary Associate Professor at the University of New South Wales' School of Chemistry, told the Science Media Centre.
"In the presence of heat, such a mixture can easily lead to catastrophic outcomes," Read added.
Ammonium nitrate is not flammable in itself, Associate Professor Stewart Walker, from the school of Forensic, Environmental and Analytical Chemistry at Flinders University in Adelaide, Australia, told CNN.
"In this instance, it appears that there was a fire and that fire has caused the ammonium nitrate that had been stockpiled to combust, and when it's in a confined space, it releases a lot of hot gas," he said.
"Because the gas takes up a higher volume than the solid, there's a build-up of pressure and because of the heat released, the hot gas is higher in volume, so you get to the point that when it's confined it will suddenly explode and will release that pressure in a shockwave," Walker added.
Video footage of Tuesday's explosion shows that "the fire starts with a grey-white cloud and then, at the time of the explosion, there is a large column of reddy-orange-brown smoke and a large white 'mushroom cloud,' which is the shockwave," said Walker.
The red-orange-brown smoke is characteristic of nitrous oxide, a toxic gas released from the ammonium nitrate, he said.
Has this happened before?
It's not the first time that ammonium nitrate, which is reasonably cheap to manufacture, has been implicated in deadly industrial explosions.
Perhaps the closest comparison, in terms of scale, is a blast in Texas City in 1947. The fire caused an explosion and additional fires that damaged more than 1,000 buildings and killed nearly 400 people, according to the website of the Texas Historical Association.
For perspective, that explosion was triggered by 2,300 US tons (about 2,087 metric tons) of ammonium nitrate, according to US H
