Thermodynamic equilibria-based modelling of reactive chloride transport in blended cementitious materials

A physico-chemical modelling of multispecies transport through cementitious materials is proposed considering thermodynamic equilibria, diffusion and migration. The model considers seven species profiles (Cl⁻, Na⁺, K⁺, Ca²⁺, SO₄²⁻, Al(OH)₄⁻ and OH⁻) and the dissolution/precipitation rates during multispecies transport under an electrical field. The fluxes are calculated by the Nernst-Planck equation. Case studies were performed simulating the chloride migration test in the steady state and NT Build 492 test on cement pastes based on slag and/or Portland cement. In order to simulate real exposure to seawater, the migration tests were based on synthetic seawater in the upstream compartment and a synthetic pore solution in the downstream. Pore solution extractions and scanning electron microscopy were performed in order to provide input data and to monitor dissolution/precipitation reactions. The proposed modelling highlights a reduction of up to 10% of free chlorides in the material tested compared to the classic Nernst-Planck modelling.