Ultrafine GGBS and calcium nitrate as concrete admixtures for improved mechanical properties and corrosion resistance

This work reports the modification of ordinary Portland cement with ultrafine ground granulated blast furnace slag (GGBS) as a mineral admixture and calcium nitrate as a chemical admixture and examines how mechanical and corrosion properties are improved by this modification. Ultrafine GGBS with average particle size of 4–6 μM was introduced as a replacement mineral admixture (10%) to ordinary Portland cement while calcium nitrate was introduced as a chemical admixture at 2% amount of cementitious material, in the preparation of concrete. X-ray diffraction studies on powdered concrete showed that the amount of silica in the concrete increases with the introduction of GGBS. Calcium hydroxide was converted to calcium silicates. Ultrafine GGBS reduced the workability and water absorption and increased the compressive strength of the concrete (18%) and the bond strength of the steel rebar (45%). Adding calcium nitrate further reduced water absorption of the concrete but improved workability, compressive strength (32%) and bond strength (131%). The pH of the concrete powdered solution became more alkaline with the replacement of ultrafine GGBS and addition of calcium nitrate. Free chloride content dropped by 39% and 65%, respectively, with the introduction of GGBS and nitrate. Corrosion behaviour of the concrete specimens were studied using measurement of open circuit potentials, linear polarization resistance and Tafel polarization in an accelerated corrosion medium of 3.5% NaCl and 1 M sulphuric acid. Corrosion potential and current of the control specimens decreased with time for 40 days after which an increase was observed. Ultrafine GGBS shifted the corrosion potential in the cathodic direction, indicating retardation of the cathodic reaction (ex. oxygen reduction). Calcium nitrate, on the other hand, shifted the corrosion potential anodically by promoting the formation of a passive film of iron(III) hydroxide on the steel surface. Corrosion currents in GGBS and nitrate-modified concrete decreased by 200-fold compared to the control specimen on the first day, and by 480-fold on the 50th day (150-times smaller than the specimen modified with GGBS alone). Finally, scanning electron microscopy images of the corroded rebar at the end of 50th day indicate that pitting and intergranular corrosion occurs, with its extent reduced significantly by the introduction of admixtures. These results demonstrate that ultrafine GGBS and calcium nitrate as admixtures enhance the mechanical properties of concrete and reduce the corrosion of rebar.