Chloride induced corrosion in different fly ash based geopolymer concretes

This study investigates the corrosion of reinforcement in geopolymer concrete manufactured from three different low calcium fly ashes from Australian power plants. The long term corrosion condition of embedded rebar in fly ash geopolymer concretes containing cast in chlorides (0–5%) subjected to wet-dry cycles, together with specimens exposed to ponding in 3% NaCl were examined. Half-cell potential and linear polarisation resistance techniques were used to measure corrosion up to 540 days of age, and compared with results of a similar binder content PC concrete. Increased levels of corrosion were observed in the cast-in chloride geopolymer specimens compared with the equivalent PC concretes. However, in the case of the ponded specimens the reinforcement in the geopolymer concrete specimens displayed lower corrosion levels than the PC concrete. The higher corrosion rate in the cast-in specimens is attributed to a lower pH in the geopolymer specimens resulting in a higher Cl−/OH− ratio. In the ponded specimens the formation of three-dimensional N-A-S-H and C-A-S-H cross linking in the gel matrix reduces chloride diffusion to rebar depth, resulting in a lower corrosion rate being observed for ponded geopolymer specimens compared to the PC concrete