The launch of Spacex Starship on April 20 2023 looked pretty normal on first sight, apart from some engines not running, and some concrete flying around.
The huge exhaust cloud looked normal, because it is normal for a rocket using solid propellant. But Starship runs on methane and oxygen. Which burns to mainly carbon dioxide and water, and maybe some soot.
The huge plume was not black, not even dark, so it was not soot.
Does that mean the whole large cloud was almost exclusively material from the foundation of the launchpad*, in a very wide range of particle sizes?
When seeing it, I assumed the cloud was just a big normal exhaust cloud, with surprisingly many big pieces of concrete in it. But now I suspect this cloud was 100% concrete and sand from under it.
Note that there is no water deluge system, that creates a huge cloud of water vapor with a clean burning rocket like one burning hydrogen, and also no foam isolation that partially burns creating some dark smoke, as seen when a Delta IV Heavy starts.
* I want to express my respect for the engineering of the launchpad and point out that the launchpad actually worked, even after loosing much of its total mass.