Supplementary Cementitious Materials: Assessment of Test Methods for New and Blended Materials

In Texas, and most of the U.S., Class F fly ash is the most used supplementary cementitious material (SCM) due to the many benefits it provides to concrete. In recent years, the availability of Class F fly ash has decreased as many coal-fired power plants have shut down. Plants that have not shut down are required to install various emission control systems that can significantly alter the type of fly ash produced. As the face of fly ash production continues to change, the usability of non-traditional fly ashes and fly ash alternatives in concrete must be evaluated. As the number of new SCM sources rises to meet demand, rapid tests are necessary to screen out poor-performing materials and long-term performance testing is needed to qualify promising materials for use in concrete mixtures. In this study, both rapid SCM screening tests and long-term performance of non-traditional, blended fly ashes were examined. For rapid screening, both R3 testing (ASTM C1897) and a lime reactivity test were successful at screening out inert materials. By pairing the R3 test with an extra step in bound water testing, it is also possible to distinguish between pozzolanic and hydraulic reactivity of SCMs. With respect to non-traditional, blended fly ashes, it was determined that fly ashes that do not meet the definition of a Class F fly ash but do meet the chemical and physical property requirements performed comparably to a traditional Class F fly ash in most cases. Most of the performance differences were negligible and can be remedied through the addition of limestone, gypsum, or chemical admixtures, except for sulfate resistance. The poor sulfate resistance of some non-traditional fly ashes can be directly linked to the presence of certain crystalline phases, making the ash perform more like a Class C fly ash than a Class F fly ash. The full report can be downloaded from the University of Texas at Austin Center for Transportation Research at https://library.ctr.utexas.edu/ctr-publications/0-6966-1.pdf