Influence of Unburnt Carbon in the Performance of Concrete Mixtures

Abstract

In many standard specifications there is a limit for the maximum amount of unburnt carbon of fly ashes aften referred to as LOI. In particular, according to the European norm EN 450, this limit is 5% on the continental basis of the European Unity, or 7% on the domestic national basis. Therefore, fly ashes with LOI over 7% should be rejected as a supplementary cementitious material in concrete mixtures. Four fly ashes from coal-fired electric generating plants, with LOI content of about 4,7,9,and 11%, were used to manufacture concrete mixtures. They had the water-cement (w/c) ratio of 0.68, corresponding to a water-binder ratio of 0.48 and a fly ash/binder ratio of 0.30. A small amount of superpasticizer (0.3-0.4% by cement mass) was required to compensate the slump decrease caused by fly ash with higher LOI (>=7%). Two reference concrete mixtures, without fly ash, were also produced with a w/c of 0.68 and 0.48. The performance of all these concrete mixtures was assessed in terms of compressive strength at early and later ages (1-180 days), water permeability, chloride diffusion, and carbonation rate. There was no evidence available which indicated that the LOI content of the fly ash affected negatively any of the properties studied. In particular, due perhaps to its peculiar pozzolanic activity, the fly ash with the highest LOI content (11.30%) performed better than that with the smallest amount of LOI material (4.19%). This occurred in terms of higher compressive strength, lower water-permeability, slower chloride diffusion, and decreased carbonation rate in the corresponding concretes. Therefore, the conformity criteria adopted by some standard specifications in rejecting fly ashes only on the basis of the relatively high LOI content, without determining the corresponding concrete performance in terms of strength and durability, appear to be technologically inadequate.