Pozzolanic soils: a natural resource for soil improvement techniques

Soil improvement techniques with binders are used in several geotechnical engineering applications, as a sustainable solution for the reuse of waste soils. An experimental research started some years ago on lime-treated medium/coarse-grained pyroclastic soils from volcanic districts of Central Italy, whose geotechnical characterization was previously investigated in the past. The suitability to lime stabilisation was broadly investigated and verified through laboratory geotechnical testing. The mechanical improvement induced by lime addition highlighted the development of a structured behavior linked to the chemo-mineralogical evolution of the system, while showing at the same time the effectiveness of the improvement technique to pozzolanic soils. While for clayey soils, the fine fraction is mainly responsible for ion exchange and pozzolanic reactions induced by lime, for the investigated pyroclastic soils pozzolanic reactions are dominant processes due to low quantity of fine fraction along with abundance of aluminates and silicates. The amorphous phase is found to play a fundamental role in the development of pozzolanic reactions and in the formation of new cementitious compounds. The research has been performed following a multi-scale approach i) involving some special experimental techniques in order to observe the chemo-physical evolution of the system over time, ii) looking at the microstructure of the solid skeleton through mercury porosimeter tests and scanning electron microscopy, iii) aimed at investigating the mechanical behavior of lime-treated materials, by means of more conventional geotechnical laboratory techniques. The mechanical response of raw and lime-treated pozzolanic soils has been investigated in terms of 1D compressibility and shear strain behaviour. Through this multiscale and multidisciplinary approach - from mineralogy to geochemistry to geotechnics - it has been possible to explore the link between the phenomena detected at the microscale level and the macroscopic behaviour of the treated materials. The research aims also to lay the basis for reconsidering the use of pozzolanic soils as resources and innovative materials. Due to amorphous structure and abundance of silicon and aluminium, pozzolanic soils can be considered suitable natural materials to act as precursors for alkaline activated binders in soil treatment.