Sources: Federal Highway Administration; National Ready Mixed Concrete Association, Alexandria, Va.; CP staff
The latest Federal Highway Administration TechNote sheds critical light on a metric that concrete practitioners will weigh as the supplementary cementitious materials universe expands and market conditions compel a lower portland cement clinker factor in mix designs.
FHWA HRT-25-050 Reactivity of Supplementary Cementitious Materials authors assess the chemical reactions when SCM encounter Type I/II or Type IL cement in fresh concrete. “The hydration reactions between portland cement and water have been studied for more than a century. Well-known tools and standardized test methods capture the rate or degree of reaction and impact on setting time and compressive strength development,” they write. “SCMs participate in chemical reactions when combined with portland cement and water, but because they are more variable in nature and have more complex chemistry than portland cement, the reactions are variable, and the tools and test methods for measuring the reactions are not nearly as well defined.” In contrast to portland cement hydraulic reactions, they add, SCM reactions are considered pozzolanic or “latent” hydraulic, the latter hinging on the presence of a calcium-rich or alkaline solution.
SUPPLEMENTARY CEMENTITIOUS MATERIAL REACTIVITY LEVELS
Authors stress the critical nature of measuring and benchmarking reactivity given novel SCMs’ emergence for public or private project concrete specifications. They also discuss the limited reactivity indicators to be gleaned from ASTM C618, C989, C1240 and C1697—respective test methods for fly ash and natural pozzolans, slag cement, silica fume and blended SCMs. One answer to established methods’ limitations, authors note, is ASTM C1897-20, Standard Test Methods for Measuring the Reactivity of Supplementary Cementitious Materials by Isothermal Calorimetry and Bound Water Measurements. Its methods are based on the (rapid, relevant, reliable) R3 test, a tool to which researchers and industry stakeholders increasingly turn as they validate and commercialize new SCM.
Posted here, the 24-page Reactivity of SCM is based on FHWA Office of Infrastructure Research and Development work at the Turner-Fairbank Highway Research Center in McLean, Va. TechNote authors are University of Texas, Austin Professor Maria Juenger, newly elected American Concrete Institute president; Michelle Cooper and Erin Stewartson of the FHWA Office of Infrastructure and Research and Development; and, Prannoy Suraneni of Miami Concrete Consulting.