Sources: National Institute of Science and Technology (NIST), Gaithersburg, Md.; CP staff
Adding limestone powder to fly ash-rich mixtures can improve finished concrete performance when measured against more conventional formulations, note NIST and Federal Highway Administration researchers in an August report, “Ternary blends for controlling cost and carbon content.”
Writing in the American Concrete Institute’s Concrete International, they cite promising laboratory results potentially leading to greater use of fly ash in concrete, coupled with sizable reductions in greenhouse gas emissions, energy use, construction costs and landfill volumes. Fly ash accounts on average for about 15 percent of the binder in ready mixed concrete. To produce a more green concrete, NIST is researching new material combinations and procedures that could help the industry justify fly ash to replace 40–50 percent of ordinary portland cement.
Because of delays in setting times and questions about its strength in the first few days after placement, “Green concrete has been a tough sell in large parts of the construction industry,” says NIST chemical engineer and report co-author Dale Bentz. He and FHWA colleagues find that a “judicious combination of fine limestone powder” can help to put these concerns to rest. So-called high-volume fly ash “ternary” mixtures (including some limestone) that replace between 40 percent and 60 percent of the cement portion set at rates comparable to those for typical concrete and are superior in terms of key properties, they contend.
In initial NIST/FHWA testing, the strength of the green concrete mixtures after 28 days slightly lagged that of concrete without any fly ash. However, the team was able to tweak fly ash-limestone-portland cement mixture to overcome the gap, primarily by lowering the water-to-powder ratio and switching from an ASTM C150 Type I to Type III product. Greater use of high-volume fly ash mixtures could significantly reduce a global cement production environmental burden and reduce costs for concrete construction, notes Bentz, who along with colleagues will test limestone-enhanced mixtures in the field, where curing conditions can vary.