Comparing the diffusive transport of chloride for a soil-cement material at different water-cement ratios

Cement-based solidification/stabilisation of soils is a common risk management technique for contaminated sites. The long-term performance of these monolithic systems and the role of mix design in the diffusion of contaminants from cement-stabilised soil is not well understood. In this study, a silty sand treated with 10% Portland cement and mixed at three different water-cement ratios (1.3, 1.5 and 2.5) was tested to examine how chloride diffused both into and out of the cured monolithic samples. When comparing specimens of the same mix proportions, the diffusive flux out of a soil-cement monolith pre-mixed with chloride was less than the diffusive flux into the uncontaminated monolith. Results showed that the ratio of the inward chloride diffusive flux to the outward chloride diffusive flux is 3.0, 2.5 and 1.4 for a water-cement ratio of 1.3, 1.5 and 2.5, respectively. This is attributed to more interconnected pores (less occluded pores) at higher water-cement ratios. From a design perspective, the findings indicate that using diffusive properties (n_e and D_e) derived from conventional diffusion tests produces conservative estimates of long-term contaminant transport in cement-stabilised soils. However, this conservatism diminishes with increasing water-cement ratio, owing to a reduced fraction of occluded pores.