Mixture Proportioning Options for Improving High Volume Fly Ash Concretes

High volume fly ash (HVFA) concretes are one component of creating a more sustainable infrastructure. By replacing 50 % or more of the Portland cement with fly ash, a significant reduction is achieved in the carbon footprint of the in place concrete. While HVFA mixtures can be proportioned to produce equivalent long term performance as conventional (cement-only) mixtures, performance problems are often encountered at early ages, including low early-age strengths, long delays in finishing, and potentially greater susceptibility to curing conditions. In this paper, a variety of mixture proportioning options to mitigate these deficiencies are investigated within the framework of a proposed mixture proportioning methodology. Variables examined in laboratory studies include cement type, fly ash class, the provision of internal curing, and the addition of either calcium hydroxide or a rapid set cement to the binder. Switching from a Type II/V to a Type III cement enhanced one-day compressive strengths by over 50 %. Using a Class C fly ash produced a mixture with a higher calcium-to-silicate ratio than a comparable Class F fly ash and increased the measured 7-day compressive strength. However, in this study, sulfate balance was a problem in the Class C HVFA mixtures, requiring 2 % additional gypsum to provide a proper sulfate balance. Internal curing was found to significantly reduce autogenous deformation by 50 % or more, with a concurrent 13% decrease in compressive strength. Excessive retardations of 3 to 4hrs were observed in both mixtures with the Class C and the Class F fly ashes; powder additions of either a rapid set cement or calcium hydroxide were found to be effective in reducing this retardation (and setting time delays) in pastes and mortars.