Washing sludges – mainly clay-rich residues from aggregate production – are largely traded as waste, with approximately 50 million tons generated annually in central Europe alone. This study explores their reuse as a resource for pourable construction materials. Three industrial and one artificially designed pourable sludge were investigated through combined rheological and mechanical testing. Results show that all tested materials achieve compressive strengths around 3 MPa, comparable to unstabilized rammed earth. Workability, assessed with slump tests, varied strongly with water demand, which was linked to differences in mineralogy rather than particle size distribution. A clear trade-off emerged: higher water content improved consistency but reduced compressive strength. The carbonate-rich sludges required less water for target consistency than the siliceous sludge, while the designed material achieved the best balance of workability and strength. These findings demonstrate that aggregate washing sludges, both natural and engineered, can be effectively valorized as low-emission building materials. By reducing stockpiling, their use offers a sustainable pathway toward circular construction practices.