Environmental and geotechnical properties of lightweight aggregates made of reused solid wastes

In the present research, a comprehensive experimental investigation was carried out for the geotechnical characterisation of new synthetic lightweight aggregates (SLAs) composed of reused biomass fly ash and waste plastics (i.e. high-density and low-density polyethylene, i.e. HDPE and LDPE). Aggregate fly ash-to-plastic ratio, by weight, was 50:50. Both aggregates were characterised by low specific gravity values, i.e. 0.98. The physical, chemical and mechanical properties (i.e. one-dimensional compressibility, creep properties, compaction features and stress-strain-strength behavior) of lightweight aggregates were evaluated together with hydraulic conductivity and water absorbability. Based on one-dimensional compression test results, lightweight aggregates made of fly ash and HDPE resulted to be less compressible upon loading compared to the companion material made of fly ash and LDPE and exhibited an overall response similar to that of traditional aggregates of expanded clay. Moreover, the analysis of the time-dependent compressive behavior of the two synthetic lightweight aggregates showed that the creep rate depends on applied stress in the range of 50 kPa to 400 kPa with values of creep coefficients C αε slightly higher than those of compacted sand, but comparable, or even lower, than those of other recycled materials. Shear strength parameters, including peak friction angle (ϕ′) and cohesion intercept (c′), determined by isotropically consolidated drained triaxial compression tests, were very satisfactory for both aggregates. The ϕ′ is within the range of 42.3 to 46.3°, and c′ is in the range of 1.3 to 8.6 kPa, with the highest values concerning the fly ash-HDPE aggregate. Constant head permeability tests provided values of the hydraulic conductivity (k)in the range 3÷4·10−4 m/s evidencing favourable permeability properties of both investigated synthetic aggregates. Leaching tests and thermal stability analyses proved the sustainability of both aggregates from an environmental point of view. Finally, the results obtained in the present study demonstrate the suitability of the investigated lightweight synthetic aggregates for valuable use in many geotechnical applications.