Improvement of fracture toughness of green concrete as a result of addition of coal fly ash. Characterization of fly ash microstructure

Abstract Composites with the addition of coal fly ash (CFA) can be included in the sustainable and green concrete. Effective promotion of green concrete incorporating CFA is necessary to minimize the threat to the environment posed by CFA waste disposal and to reduce cement consumption thus cutting CO2 emissions. This study investigates the influence of the curing time on the fracture toughness of concrete produced with different concentrations of CFA. Concentrations of 20% – CFA-20 and 30% – CFA-30 of CFA were used and the results were compared with a reference mixture with 100% Ordinary Portland Cement (OPC) – CFA-00. Compressive strength – fcm and fracture toughness under mode I – KIcS (tension at bending), were determined after: 3, 7, 28, 90, 180 and 365 days. The results obtained lead to the conclusion that, it is possible to make green concrete containing CFA with high fracture toughness. Furthermore, this is one of the ways to reduce cement industry CO2 emissions. 20% additive of CFA guarantees high fracture toughness in mature concretes, whereas concrete with 30% CFA additive is characterized by highest dynamic increase of the parameter KIcS. The experimental program was completed by the analysis of microstructure of CFA by using SEM. These studies showed that the main morphological forms in the CFA are single grains, such as: pyrospheres, cenospheres, plerospheres, multispheres, ferrospheres, grains of irregular shapes, and amorphous grains. Grains of CFA may also occur in larger quantities, as: jointed grains, clusters and agglomerates.

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