They are 285 Kg/m2 for 15 cm thickness. (Take care, they also exist in a light version)
By comparison, the concrete blocks used in the study were 235 Kg/m2 for 190 mm thickness.
That is about half of real concrete.
Silka is quite heavy (especially the last row at 3m height, when your head is against the upper slab at 3m 25 so you have to lift them above your shoulders...), and, much less resonant when you tap it than a comparable brick or concrete structure.
But then, my mastering room thread ended with an open invitation to have lots of beer, so If you want to knock on the walls, be my guest