Chloride Penetration in Geopolymer Concrete with Alkali and Reactive Oxide Contents for Precast Application

This study investigates the chloride ion penetration mechanism in alkali-activated low calcium fly ash-based geopolymer for precast applications. Fundamental insights into chloride penetration in geopolymer concrete influenced by diffusion, migration, and binding are obtained from an experimental program that considers the influences of sodium and water contents in the alkali-activated fly ash (AAF) binder pastes. Binder pastes are proportioned based on the reactive silica and alumina in the fly ash for an identical reactive oxide composition of silica and alumina in the activated paste. The influence of sodium content was studied by increasing the sodium content from 20% to 25% of the reactive silica, and the influence of the water content is studied by the decrease in water content from 0.19 to 0.17. Chloride ion diffusion is evaluated, and binding isotherms are established for varying paste compositions. Experimental results demonstrated that the chloride diffusion measures in two-part geopolymers with a solution-to-fly ash ratio equal to the water-to-cement ratio in cement paste are comparable when the sodium content is 20% of the reactive silica in the AAF paste. Binding in geopolymers is, however, three to five times higher than in cement paste. A significant portion of the sodium is in an uncombined form, and it does not contribute to the N-A-S-H content or the compressive strength of the geopolymer. The excess sodium, however, densifies the pore structure, significantly increasing the chloride binding in the geopolymer. Proportioning geopolymers with larger alkalinity enhances the durability in a chloride environment.