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

Author(s): Gopakumar Kaladharan, Moe (Mohammadreza) Sharbaf, Farshad Rajabipour

Publication: Symposium Paper

Volume: 362

Issue:

Appears on pages(s): 682-700

Keywords: flue gas desulfurization, low carbon concrete, off-specification fly ash, setting time, calcium sulfite

DOI: 10.14359/51742002

Date: 6/17/2024

Abstract:
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 are 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.