Fly ash from a power plant is very heavy. It also acts as an insulator, and after several weeks the ash under it will still be so hot it will melt the soles of your shoes.
Çoal ash from Wyoming can be mixed with water and will harden upa like concrete.
I believe the Romans built the harbor at Caesaria with such ash from Mt Etna.
Wow, ash is a gift to rebuild our infrastructure!
God, he’s the man!