Preliminary Study of the Potential of a Beneficiated Ultrafine Class F Fly Ash

As usable fly ash sources dwindle, beneficiation of previously undesirable fly ash is becoming more important. Utilization of this material could provide many benefits. As a first step, a beneficiated ultrafine Class F fly ash (BUFFA) was compared, in similar mixtures, to a popular Tennessee Class F fly ash (F) Tennessee Department of Transportation (TDOT) Class D (bridge deck) concrete was selected as the application for this preliminary investigation. Two BUFFA mixtures (25% and 35% substitution by weight of cement) were compared to two similar F mixtures from previous research at Tennessee Technological University (TTU). A 100% PC mixture was also generated as a basis for comparison. Six batches of each mixture were tested. Surface resistivity (chloride permeability) was measured at 7, 14, 28, 42, and 56 days. Compressive strength was measured at 28 and 56 days. Absorption after boiling was measured at 56 days. Both BUFFA mixtures were statistically superior to both F mixtures in all tests after 7 days. The 25% BUFFA mixture was equal or superior to 100% PC mixture at 14 and 28 days and superior thereafter. All mixtures met TDOT Class D plastic and hardened property requirements. BUFFA costs more than F. However, 25% BUFFA cementing material cost was less than 20% more than 100% PC. The advantage of BUFFA is excellent use of curing time. After 28 days of curing the 25% BUFFA mixture had surface resistivity and compressive strength similar to the 25% F mixture with 72 and 334 day curing times, respectively. INTRODUCTION AND RESEARCH SIGNIFICANCE The use of fly ash in concrete mixes enhances the concrete’s strength, durability, and sustainability, and reduces its cost of production. The demand for fly ash remains high while its supply is decreasing, or at least is uncertain in the future. Unfortunately, many sources of fly ash do not meet American Society for Testing and Materials (ASTM) or American Association of State Highway and Transportation Officials (AASHTO) requirements for fly ash used in concrete. Typically, loss-on-ignition (LOI), a measure of unburned carbon, is too high in fly ashes which fail to meet these standards. Thermal beneficiation of fly ash typically begins with a fly ash not meeting ASTM or AASHTO standards for LOI and results in a fly ash meeting ASTM and/or AASHTO Class F fly ash requirements for use in concrete. The Tennessee Concrete Association (TCA) strongly favors the use of thermally beneficiated fly ash as a supplementary cementing material (SCM) in concrete. Thermally beneficiated fly ash would help ensure that fly ash is always available for Tennessee concrete producers. Unfortunately, Tennessee does not currently have thermally beneficiated fly ash available. In an effort to validate the use of thermally beneficiated fly ash, TCA helped Tennessee Technological University (TTU) researchers obtain thermally beneficiated ultrafine Class F fly ash. TTU conducted two comparisons of beneficiated ash and a common Tennessee Class F fly ash known to perform well as an SCM in concrete, one on thermally beneficiated Class F fly ash (described in a separate paper) and one on ultrafine thermally beneficiated Class F fly ash (described in this paper). Finer grinding of Class F fly ash should increase surface area of the fly ash, thus increasing the rate at which pozzolanic reactions occur. The two primary target audiences for the comparison are Tennessee concrete producers and the Tennessee Department of Transportation (TDOT).