Foamed concrete utilizing excavated soil and fly ash for urban underground space backfilling: Physical properties, mechanical properties, and microstructure

Backfilling abandoned urban underground spaces with foamed concrete utilizing excavated soil and fly ash is an environmentally sustainable method. However, a comprehensive evaluation of backfilling-related performance is required. This study develops a foamed concrete utilizing excavated soil and fly ash and examines the physical properties, mechanical properties, and microscopic properties. The results show that the water-solid ratio and soil content have a significant impact on workability and overall stability. Excessive foam and soil will reduce the overall stability of foamed concrete. The UCS dropped as foam, soil, and fly ash content increased. The stress-strain behavior is closely related to Young's modulus but has nothing to do with the UCS. However, the crack initiation and propagation are closely related to the UCS and ingredients, but not to Young's modulus. The lower UCS specimens have compression damage, but the higher UCS specimens frequently suffer deadly piercing and additional splitting damage. Increasing foam, soil, and fly ash content typically improves ductility, which is favorable for guaranteeing the safety of backfilling projects. A proper amount of fly ash can lower pore size and improve pore roundness, according to microstructural development, but an increase in foam content has the opposite effect. The results also imply a significant correlation between the microstructure and macroscopic mechanical strength when the foam content and water-to-solid ratio varied. However, there was a poor correlation between microstructure and macroscopic mechanical strength as soil content and fly ash content changed. Finally, a mechanism of strength and failure development was proposed.