The chemical compound in question is ammonium nitrate, which Winston-Salem Fire Chief Trey Mayo on Tuesday called “somewhat unpredictable.” It’s used to make fertilizer and can fuel fires in certain conditions, including heat exposure. There was an estimated 500 tons of it at the site, Mayo said, and an additional 100 tons in a rail car next to the facility. That’s far more than the amount of ammonium nitrate that caused an explosion at a fertilizer plant in West, Tex., in 2013, he said. That blast killed 15 people, injured 252, and damaged or destroyed 500 buildings.