Sustainable Recycling of Excavated Soil Waste into Unfired Bricks: Mechanical and Microstructural Performance

Although soil stabilization has been widely studied, the mechanical behaviour and microstructural characteristics of unfired bricks produced from excavated soil waste have received limited systematic investigation, particularly across soils with different properties and stabilizer combinations. This study investigates the feasibility of producing unfired bricks from four excavated soils using cement, lime, and fly ash stabilization. Mechanical performance was evaluated through unconfined compressive strength tests and supported by microstructural observations. The 28-day UCS reached approximately 0.4–0.6 MPa for silty clay, 1.0–2.0 MPa for silt, and 0.8–1.6 MPa for silty sand when cement was the primary stabilizer. A simplified model was further developed to predict the strength performance based on soil properties and stabilizer composition. The results indicate that stabilization effectiveness strongly depends on soil characteristics, and different stabilizer combinations show different advantages for various soil conditions. Furthermore, life-cycle assessment and economic evaluation demonstrate that the proposed unfired bricks can reduce environmental impacts compared with conventional fired clay bricks by eliminating the high-temperature firing process and utilizing waste soil as a resource. The unfired brick showed a GWP of 332–394 kg CO2-eq/m3, compared with 503–721 kg CO2-eq/m3 for fired clay bricks. This study provides a practical approach for converting excavated soil waste into sustainable construction materials and supports the development of low-carbon and circular construction practices.