Effect of unburnt carbon on the corrosion performance of fly ash cement mortar

The increase in activated carbon content accelerated the corrosion of rebar (steel) in ordinary Portland cement (OPC) mortar containing fly ash and different percentages of carbon. The alkalinity of the cement was greatly affected with increased carbon contents, and when the quantity of carbon was increased, cement lost its characteristic colour. For the exposure period of one year, more than 60% of the surface area was rusted when the carbon content was increased beyond 8%. Corrosion rates comparable to OPC were obtained for up to 6% carbon level. Therefore it was concluded that the upper limit of replacement for various admixed carbon systems, under aggressive alternate wetting and drying conditions with 3% NaCl, was 6-8%.