The use of fly ash in high strength concrete mix design

In this paper the use of fly ash for particular High Strength Concrete (HSC) realizations is investigated. The use of fly ash as a partial replacement of Portland cement in HSC seems a valid solution in particular for a sustainable construction design, considering the interesting HSC performances and the economic aspects. Obviously the advantages demonstrated for the underground constructions can be also evaluated for the structures in elevation. In this paper the most important technical regulations about the use of fly ash for the concrete mix design are discussed and it’s shown the increase of the mechanical resistance and the better protection against the chemical agents attacks and against the freezing and thawing cycles using fly ash. Moreover, in comparison to the fresh concrete, it’s presented the benefits related to the minor ratio water/cement (defined by w/c) in the same conditions of workability. Additional considerations are carried out about the hydration process: the mixture produces less heat for the lowering of the C3A and C3S percentages using fly ash. Finally, a greater concrete impermeability obtained using fly ash, due to the presence of a lower hydrolysis content, which leads to an increment of the cement paste porosity, has been evidenced