Microstructural characterisation of foam-induced porosity in lightweight cemented soils using X-ray micro-tomography

Lightweight Cemented Soils (LWCS), produced by mixing natural soil, water, cement and air foam, are characterised by high workability, good mechanical properties and reduced unit weight. Their microstructure is complex and consists of large foam-induced voids embedded within a cemented porous matrix. The matrix has been recently studied, whereas there is a lack of knowledge about the distribution, size and stability of foam-induced voids during the chemo-physical evolution of the system. In this study, a novel microstructural investigation has been developed by performing X-ray microtomography on LWCS samples lightened with 40 % of foam at increasing curing time. The use of this technique allows quantitative analysis on the evolution of the foam-induced voids, not achievable by other conventional experimental techniques (e.g., Mercury Intrusion Porosimetry). Image analysis of X-ray microtomography scans shows that the foam-induced porosity remains stable (i.e., without collapse or coalescence) over curing time, whereas shrinkage fractures due to cement hydration lead to a slight increase of the porosity. Moreover, the frequency of largest voids decreases slightly due to precipitation of new compounds. The hydraulic conductivity of LWCS is estimated for the first time through a Pore Network Model, based on the real microstructure of the material, obtained from X-ray microtomography scans. The computed hydraulic conductivity is compared with the permeability of the matrix (i.e. cemented sample without foam) derived from Mercury Intrusion Porosimetry test and with the hydraulic conductivity estimated from experimental tests. The numerical result shows a good agreement with the experimental data (the values are of the same order of magnitude i.e., 10−10 m/s), highlighting that, for the considered foam content, hydraulic conductivity of LWCS is primarily controlled by the permeability of the matrix, as air voids and shrinkage fractures are isolated and accessible only through the matrix.