Tortuosity as a Key Parameter of Chloride Diffusion in LC³ Systems

In the context of climate changes, limestone calcined clay cement (LC³) represents a very promising alternative to ordinary Portland cement (OPC), not only to reduce by up to 30% the CO₂ emissions but also to dramatically increase service life. Notably, the resistance to chloride ingress is significantly higher in LC³ systems, although understanding and quantifying the mechanisms responsible remain a challenge. In this study, the effect of porosity on chloride diffusion was investigated by comparing at water-to-binder ratios w/b = 0.3–0.5, OPC pastes and LC³-50 pastes (50% clinker, 30% calcined clay, 15% limestone and 5% gypsum). Chloride diffusion was measured using a mini-migration test developed for paste samples. The microstructure was further characterized in terms of porosity and diffusion tortuosity as obtained from the formation factor. The results showed how increasing the tortuosity of the porous network (by changing the binder type) led to a higher reduction of the chloride diffusion coefficient compared to decreasing the total porosity with lower water-to-binder ratios. These results strengthened the importance of the tortuosity as one of the key microstructure features to explain and model the chloride diffusion in alternative cementitious systems (along with the chloride binding and the composition of the pore solution).