Turning Waste into Structure: Clay-Rich Materials for Poured Earth Construction

Abstract

Clayey residues such as washing sludges and excavation material are commonly treated as waste. Washing sludges – mainly clayrich by-products from aggregate production – accumulate at approximately 50 million tons annually in central Europe, while excavation material from construction sites amounts to about 44 million tons in Austria in 2022 alone. This study explores their potential reuse as resources for pourable construction materials. Three industrial sludges, one laboratory-designed sludge, and one excavation clay were formulated with a constant clay-to-aggregate ratio of 30 vol-% sludge and 70 vol-% aggregate. No stabilizers were added, in order to preserve recyclability and maintain a low CO₂ footprint. A constant dosage of 0.3 wt-% sodium hexametaphosphate was used as a dispersing agent. All four formulations were investigated using slump flow and mechanical testing. All tested sludge mixes achieved compressive strengths of approximately 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 was observed: higher water contents improved consistency but reduced compressive strength. Carbonate-rich sludges required less water to reach 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 utilized as low-emission building materials. Their use reduces stockpiling and supports more sustainable and circular construction practices, whereas the excavation material tested in this study showed only moderate performance