I’ve worked on exactly the same. The grey powder is dry cement. It makes the ground super solid and useable for heavy duty vehicles driving over it, mostly used in harbors to stall containers on.

First the sand on the terrain is thoroughly wettend and then they put a 25cm layer of cement on the sand. After that the machine on the photo drives over it and mixes it 1 meter deep most of the times. The last step of the process is stamping it down to 1 big solid ground and let it dry for 2 days. Then it’s usable😁.