There's the general perception of an "explosion" then there is the legal (BATFE) definition of an "explosive" (burn rate) then probably a scientific definition also. If a grainry "blows up" because of grain dust being ignited, the news media, the fire department, and even cops will call it an "explosion" even though we all know wheat grain is not an explosive nor is the dust. I just don't see the reason for debating the word "explosion" or "explosive" for a general health and safety discussion.
https://www.osha.gov/dsg/combustibledust/
Grain dust ABSOLUTELY IS an explosive, and a grainery blowing up IS an explosion. The burn rate is more than sufficient to satisfy this definition.
POWDERED aluminum is also explosive, when at the right concentration (it is about as bad as an explosive dust can get, except for perhaps powdered magnesium). On that note, a mixture of the two is what is found in "flash powder", with the smallest amount of gunpowder added to start the ignition.
But grinding does not generally produce powders fine enough to get these sort of explosive results. The fineness of the powder determines the surface area per unit mass, which then determines the reaction speed.
Back to thermite. Thermite CAN potentially be explosive, but again, the reaction rate is controlled by the grain size, and a fine enough powder to create an explosion is not possible to create in a shop setting. The issue as I see it, is that aluminum rapidly forms an oxide coating in air. This happens especially rapidly to grinding dust, because it comes off the grinding wheel at near the melting temperature (and high temperatures speed the oxidation reaction). At grinding temperatures, the oxide layer is going to form a stable thickness that is greater than the radius of a powder grain (i.e. if your grinding dust were fine enough, you might only be producing aluminum oxide dust). The relatively coarse dust created from most grinding isn't going to support an explosive thermite reaction rate (though it can still react violently). Flash powder has to be produced in an inert environment (something not found in a shop).
