Corrosion behaviour of steel in fly-ash geopolymer mortars exposed to chloride solutions

In this research different fly ash geopolymer mortars were prepared by varying the sodium oxide/silicon oxide ratios in the activating solutions. At decreasing ratios from 0.16 to 0.12, the porosity decreased, though always remaining higher than that of the Portland cement mortar prepared with the same liquid to solid ratio. Reinforced slabs obtained with geopolymer and traditional mortars were continuously exposed to either distilled water or 3.5% NaCl solution. The exposure conditions permitted chlorides to penetrate by a combination of capillary suction and diffusion. Corrosion was monitored by periodic corrosion potential and polarization resistance measurements. Chloride content at the rebar level was measured on unreinforced slabs exposed to the same environmental conditions. The results showed that a quick chloride penetration occurred and corrosion started within one day in Portland cement mortar and in the geopolymer mortar with the highest sodium oxide/silicon oxide ratio and the highest porosity. On the contrary, corrosion took one month to start in the other more compact geopolymer mortars. The results were interpreted by taking into consideration the mortar pH values and the total chloride concentrations at the rebar levels.