Influence of Unburnt Carbon in the Performance of Concrete Mixtures


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Title: Influence of Unburnt Carbon in the Performance of Concrete Mixtures

Author(s): L. Coppola, R. Troli, P. Zaffaroni, G. Belz and M. Collepardi

Publication: Special Publication

Volume: 178


Appears on pages(s): 257-272

Keywords: carbon; cementitious material; compressive strength; concrete; fly ash; slump; superplasticizer.

Date: 6/1/1998

In many standard specifications there is a limit for the maximum amount of unburnt carbon of fly ashes often 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 LO1 over 7% should be rejected as a supplementary cementitious material in concrete mixtures. Four fly ashes from coal-fired electric generating plants, with LO1 content of about 4, 7, 9, and 1 l%, 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 superplasticizer (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 (l-l 80 days), water-permeability, chloride diffusion, and carbonation rate. There was no evidence available which indicated that the LO1 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 LO1 content (11.30%) performed better than that with the smallest amount of LO1 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 LO1 content, without determining the corresponding concrete performance in terms of strength and durability, appear to be technologically inadequate and