Binding Mechanism of CSA Cement on Premixed ${\mathrm{Cl}}^{-}$ and Its Governing Parameters

Calcium sulfoaluminate (CSA) cement showed a good binding capacity on the external ${\mathrm{Cl}}^{-}$ transported into concretes from its surrounding environment. However, it is unclear whether the same binding mechanism can still apply if the ${\mathrm{Cl}}^{-}$ ions were introduced during a concrete mixing stage when marine resources, without being properly processed, are used as fine aggregates. Experimental work on CSA mixes, with different water/powder, gypsum content, and existences of premixed ${\mathrm{Cl}}^{-}$, has been conducted and showed that the highest binding capacity on premixed ${\mathrm{Cl}}^{-}$ was obtained in the mixes with a higher water/powder and without gypsum. Mineralogy and ${\mathrm{Cl}}^{-}$ binding results of the tested CSA mixes proved that except for the calcium monosulfoaluminate hydrate (Ms), which has been well established with a good ${\mathrm{Cl}}^{-}$ binding capacity, the ${\mathrm{CAH}}_{10}$ produced should also contribute to the binding on premixed ${\mathrm{Cl}}^{-}$. The premixed ${\mathrm{Cl}}^{-}$ in the CSA cement led to a conversion of Ms and ${\mathrm{CAH}}_{10}$ to Friedel’s salt and, meanwhile, shortened the setting time, altered the heat release process, prohibited the generation of microcracks, and resulted in a more densified microstructure.