Mixture Proportioning Options for Improving High Volume Fly Ash Concretes
Abstract: 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.