There's no evidence that it was ordnance and explosions of ammo dumps don't tend to go up like that, all in one explosion. Hunt for some video of acknowledged explosions of ammo dumps and you'll typically see many separate detonation events and something that goes on for quite some time.
Here's one:
There's no real reason to believe that it was anything other than the 2,750 tonnes of ammonium nitrate known to be stored there. That's the equivalent in explosive energy to 1.155 kilotonnes of TNT (about 1/13 of the Hiroshima blast). How and possibly why it got detonated is open to debate, but there's a simple and highly plausible explanation of what got detonated - it's been in common use as a basic/the basis for fertilizers for a very long time.
Totally agree, it was not ammo dump going, the visible colour of the smoke are all hallmarks of it being Ammonium Nitrate, there are loads of examples of this going up all over the internet. Looking closely at the various pieces of footage of the sad incident it would seem that the warehouse or a building close to it was on fire, then if look closely, you can see the fireworks going as bright-coloured twinkles, e.g., rockets going off and then a few moments later the massive blast. That certainly also supports the early news that its was either a fireworks factory or warehouse used for storing them that was on fire. Then maybe, some fireworks landed on the Ammonium Nitrate, setting that off.
There is zero evidence currently of there being any secondary and ongoing explosions the searing heat ignited things like RPG's, artillery shells, bullets etc sending them skywards as can be seen the given example. Either way I feel deeply saddened for those got up in all of this, many have lost their lives and many thousands injured and homeless right now, and they need massive international help in rescuing others from the piles of rubble and massive amount of relief aid needs to be pouring into help many victims find some form of shelter and food etc.