Evaluating the Risk of Internal Sulfate Attack in Concrete Containing High-SO₃ Coal Ashes: A Path Toward Low-Carbon Concrete

High-SO₃ fly ashes (FA), including reclaimed landfilled and ponded ashes, present a significant opportunity to decarbonize concrete through clinker substitution. However, their use is restricted by ASTM C618, which limits SO₃ content to 5.0% due to concerns over internal sulfate attack. This study evaluates the expansion risk in concrete containing high-SO₃ fly ashes (both real and doped) and investigates the underlying expansion mechanisms. The results demonstrate that fly ashes exceeding 5.0% SO₃ can be safely used if they pass the ASTM C1038 lime water expansion test at the intended dosage. Specifically, fly ashes with up to 12.0% SO₃ at 20% replacement levels did not exceed the 0.02% expansion limit, as shown in Figure 1. Furthermore, total SO₃ content in the binder (cement + fly ash) was found to be a more reliable predictor of expansion risk than fly ash SO₃ content alone. A maximum limit of 5.0% SO₃ in the binder is recommended to mitigate expansion risk, as supported by Figure 2. To further investigate sulfate-induced expansion, quantitative X-ray diffraction (QXRD) and pore solution analysis were performed on a paste containing 5.6% SO₃ in the binder. The results revealed that ettringite formation—the main driver of expansion—persists until sulfate in the pore solution is depleted, even though solid gypsum depletes within 24 hours (Figure 3). These findings indicate that sulfate availability in the pore solution, rather than cement C₃A content, controls expansion, supporting the safe use of high-SO₃ fly ashes under appropriate dosage conditions.