Chloride binding mechanism and free chloride reduction method of alkali-activated slag/fly ash mixed with seawater

Abstract

The chloride binding mechanism of alkali-activated materials (AAM) under various granulated blast furnace slag (GBFS)/fly ash (FA) ratios is investigated in this work. Two types of layered double hydroxides (LDH), i.e., MgAl-LDH and CaAl-LDH, and active magnesium oxide (MgO) were added to reduce the concentration of free chloride in AAM. The results demonstrate that the ratio of GBFS to FA has non-negligible influence on the AAM’s ability to bind chloride. The content of bound chloride increased by 3.78%-17.42% as the FA content in seawater-based alkali-activated materials (SAAM) raised. The chloride binding forms of SAAM were mainly chemical binding by the hydrated calcium aluminate (AFm) phase and physical adsorption by calcium (alumino) silicate hydration (C-(A)-S-H) gels when the FA dosage is less than 20% in SAAM. When the FA mass ratio in SAAM was larger than 20%, the chloride binding form of SAAM was mainly the physical adsorption by calcium (sodium)alumino-silicatehydrate ((C,N)-A-S-H) gels. Among the three additive materials, CaAl-LDH was the most effective in reducing the free chlorine content by 11.34%. While for MgAl-LDH, the chloride binding capacity was compromised by the less strength loss. In this work, seawater was used as the mixing water to introduce chloride ions, the results of which could provide a reference for the application of seawater in future projects.