Chloride ion binding effect and corrosion resistance of geopolymer materials prepared with seawater and coral sand

• The chloride binding properties of geopolymer and Portland cement were compared. • The physical adsorption and chemical binding mechanism of geopolymer were revealed. • The evaluation of resistance to chloride ion diffusion of geopolymer was discussed. In this work, the chloride binding capacities and chloride diffusion resistance of alkali-activated fly ash and slag powder geopolymer systems prepared with seawater and coral sand are investigated. Through the macro tests (e.g., chloride ion binding ratio, electric flux, etc.), stripping C-S-H, C/N-A-S-H from anhydrous particles and a variety of microscopic tests (e.g., MIP, XRD, SEM-EDS), the chloride binding mechanism of ordinary Portland cement (OPC) and geopolymer are analyzed and compared. The result shows that the chloride binding capacity of geopolymer samples under standard curing is better than that of OPC samples under the same condition (for instance, the 28-day chloride binding coefficient of geopolymer (FS1): 83.372%, OPC (R1): 73.223%). And the binding of chloride ion can be improved to a certain extent by increasing the initial chloride concentration. Geopolymer bounds the chloride ions through physical absorption or chemical reaction with cement compositions. The porous structure of C/N-(A)-S-H gel and high content of Al element promote the chloride binding capacity. The additional porous coral sand can reduce the chloride resistance capacity of specimens. But with slag proportion increasing, the geopolymer shows better chloride resistance capacity (for example the chloride diffusion coefficient decreases by 98% from 152.61 m 2 /s to 2.86 m 2 /s when slag proportion increased from 10% to 90%). The chloride diffusivity of geopolymers measured by the electric flux method has a large deviation, which is not suitable for direct evaluating of resistance to chloride penetration of geopolymer system.