Original abstract: The experimental study investigates the feasibility of using two extensively waste (wastewater treatment sludge and oak wood ash) as raw materials in the manufacture of clay-free bricks. The wastes were characterized by particle-size distribution, chemical composition (EDX), X-ray diffraction (XRD), thermal analysis, and scanning electron microscopy (SEM). The oak wood ash replaced the water treatment sludge in different amounts (10–30 wt.%) in brick manufacture. The properties of clay-free green building brick were compared with the conventional bricks (obtained by standard methods). The properties depended of waste proportion. The 30 wt.% oak wood ash content achieved increase apparent porosity, with direct effect over water absorption and decreasing compressive strength. By replacing the water treatment sludge with oak wood ash, the CaO content increased, with negative effect over efflorescence on the surface; however, for 30 wt.% oak wood ash is below the imposed limits. Amounts of heavy metals in the leachates of fired samples are observed below the specified limits of the EPA (Environment Protection Authority). Additionally, the clay-free green bricks showed properties similar to the commercial bricks and improved thermal conductivity. The bricks containing 80 wt.% wastewater treatment sludge and 20 wt.% oak wood ash fulfilled standard requirements for clay masonry materials.