Melted Guns....

[William Seger]

6. So you're okay with jumping to conclusions?

Crushed concrete doesn't react with water to make new concrete, but it can become loosely stuck together when soaked and dried, and there isn't any way to tell from that one photo how solid that stuff is. Also, in addition to concrete, there should be a lot of drywall gypsum in the dust, which can make a chalk-like accretion. But gypsum also has much lower melting point than sand -- only 262-325 ° F -- so melted and/or sintered gypsum should be expected if it was exposed to fire. The point is, you are speculating about extreme temperatures without any real evidence of melting and without ruling out mundane explanations.