Microstructure and mechanical behaviour of cemented soils lightened by foam

The management of large amounts of excavated soil is a primary problem in civil engineering. However, if the excavated soil can be reused as a material in the same construction project (or in a different one) for backfilling, trench reinstatement, soil embankments or substituting quarrying material in productive processes, it can be qualified as by-product with clear advantages in terms of environmental and economic costs. A suitable solution for reuse of excavated soil is the addition of cement and foam to produce lightweight cemented soils (LWCS). Lightweight cemented soil is prepared by mixing soil with water, cement and an air foam. The aim of this technique is to obtain a material with high workability in the fresh state (so that it can be transferred by pumping from batch plant to the construction site and poured) improved mechanical properties of the hardened paste given by the binding agent (as cement) and a specific low density (varying from 6 to 15 kN/m3) thanks to the addition of a foam. The fresh paste is self-levelling and no compaction is required, thus reducing construction time. In this experimental study, the influence of addition of cement and foam to soil on mineralogical and microstructural features is presented. Time dependent mineralogical and microstructural changes have been monitored at increasing curing time by means of X Ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP). Mechanical behaviour of treated soil has been investigated by means of oedometric and direct shear tests. Chemo-physical evolution induced by cement addition is the major responsible for mechanical improvement shown by treated samples. Porosity of samples induced by foam addition plays a key role in the mechanical response of lightweight cemented samples, inducing a transition of stress-strain behaviour from brittle and dilative to ductile and contractile as foam content is increased. The Mohr Coulomb criterion was adopted to describe the failure surface of cemented and lightweight cemented soils. A unique failure surface which takes account of cement factor, curing time and amount of foam was determined.