... void space, normally, its filtering velocity is 15-20 m3/h, less loss water-head, and suitable to ... it can save the back-flush water more than 30-50%;3. High surface area, large pore volume, ... ,%≤0.5- Compressive strength, N30-150- Water permeability,t/m2.h2-4- Gas permeability,m3/m2h.h1500-2000- ... Thermal resistance,≥1000- Acid resistance,99.5Mixed in 30% HCl+50% H2SO4 solution for 14 days Alkali ...