Fly debris doesn't adjust the hydric properties of the blocks yet it makes them lighter. Indeed, every one of the blocks with fly debris have a lower thickness. Fly debris blocks show less harm than customary blocks when presented to salt crystallization cycles. This improvement is because of the decrease of the surface space of the blocks, for example the decrease of the volume of the littlest pores, the ones that cause the most harm to the blocks because of solvent salt crystallization.