Effect of slowly released barium ions on chloride binding in Portland cement paste

The sulfate from gypsum in Portland cement can weaken the chloride binding. In this study, barium ions (Ba2+) were loaded in expanded perlite (EP) to prepare Ba/EP composites and then added to the Portland cement paste. The chloride ion binding capacity and the setting time of the cement-Ba/EP composite system were verified. The diffusion of Ba2+ in the cement paste was observed by SEM-EDS, and the phase assemblage was examined by XRD, TG-DTG, and NMR. The results showed that the Ba2+ loading capacity of the Ba/EP composites reached 59.3 wt%, and only 14.6 wt% Ba2+ was released in the first 3 h, demonstrating the slow release of Ba2+. The addition of Ba/EP composites reduced the negative effect on the setting time of the Portland cement paste compared to the direct addition of barium salt. The 3 wt% Ba/EP composites could increase the chloride binding capacity by 16 %, 18 %, and 32 % at 7 d, 28 d, and 240 d, respectively. Due to the precipitation of Ba2+ on SO4 2-, the migration of Ba2+ simultaneously caused the migration of SO42-in the opposite direction. The Ba2+ released from the Ba/EP composites promoted the stability of Friedel's salt at the late age, which improved the chloride binding of Portland cement paste in the long term.