Use of High SO₃ Coal Ash as Supplementary Cementitious Materials in Concrete

A large volume of freshly produced and harvested coal ash (from landfills and ponds) contains SO3 content above 5.0%. This exceeds the allowable limit in major SCM specifications (e.g., in ASTM C618), and disallows the use of such coal ashes in concrete. This presentation explores the properties and performance of these coal ashes as SCM. It is shown that SO3 may be present in ash in the form of CaSO3, CaSO4, or Na2SO4 (including solid solutions of alkali sulfates and carbonates), and that the form of SO3 has a large impact on the performance of coal ash in concrete. For example, while ashes containing CaSO3 may cause extensive set retardation, those containing CaSO4 may increase the risk of internal sulfate attack and deleterious expansion. In this work, the mechanisms responsible for each behavior is explored, the allowable SO3 limits are better defined, and strategies for beneficiation of coal ashes that exceed such limits are introduced. The outcome is facilitating the safe and efficient use of large volumes of currently off-spec coal ash to produce durable and low-CO₂ concrete.