Chloride binding in concrete containing PFA, gbs or silica fume under sea water exposure

Abstract

The paper reports an experimental investigation on different mix proportions of normal concrete and on concretes incorporating various fractions of cement replacement by pulverized fly ash (PFA), ground blastfurnace slag (gbs) or silica fume. The chloride binding capacity of each matrix was determined after immersion in sea water for up to 270 days, after exposure to laboratory marine cycles (up to 520 cycles) and after exposure to tidal cycles in the North Sea (up to 7.5 years). Extensive data on acid-soluble chloride concentration (total chloride), free chloride concentration in the pore fluid and the evaporable water content of hardened concrete were obtained in order to determine the chloride-binding capacity of the various concrete matrices. Corrosion of rebar electrodes embedded in these matrices was also monitored. The results show that the bound chloride and the chloride concentration decrease with increasing depth into concrete. Higher water-cement ratios or cement contents result in increased bound chloride. The chloride binding near the surface of concrete is not significantly affected by the incorporation of PFA or gbs. At greater depths, the use of PFA or gbs results in a reduction of Cl- bound in the cementitious binder. Replacement of cement with silica fume results in greatly reduced contents of bound chloride. Accelerated penetration of chloride in concrete results in lower binding than under natural exposure conditions. It is also shown that the free Cl- concentration of pore fluid correlates with the corrosion rates of reinforcement embedded in concretes made with various cementitious binders, while the Cl--OH- ratio has no correlation.