a process for making a lightweight structural member or insulating coating uses an initial mixture of hydrated lime , a dispersible additive , rock wool and , optionally an aggregate . this mixture has added to it water , and the water / solids mixture is agitated at high speed to produce a castable slurry having 60 - 80 volume percent air bubbles . the mixing is desirably conducted in a mixer having a cylindrical chamber and using paddles which rotate about their own axis and wherein there is movement of the chamber wall relative thereto , and the blades desirably scrape the chamber wall during such relative rotation .