Important source material to consider this question is [0] "Lebanon blast: Ammonium nitrate behind many industrial accidents" Basically the AN is an oxidizer a

Important source material to consider this question is [0] "Lebanon blast: Ammonium nitrate behind many industrial accidents" Basically the AN is an oxidizer and as such a chemical that "intensifies combustion" and so must not be combined with fuel or oil. Since it was contaminated, the contaminants are potentially combustable themselves. The warehouse was also likely filled with partially combusted gases from the tyres, fireworks or other burning material. So the explosion that did occur was not just the Ammonium Nitrate, but a mixture. As other writers have discussed, the blast would not have fully involved all the material either - some would be lost and some would remain unburnt. [0] https://www.aljazeera.com/news/2020/8/5/lebanon-blast-ammonium-nitrate-behind-many-industrial-accidents https://www.aljazeera.com/news/2020/8/5/lebanon-blast-ammoni...