PART 3: HARNESSING THE POTENTIAL OF HIGH SULPHUR FLY ASH IN CONCRETE PRODUCTION

By Eamonn Ryan, derived from the ACI podcast

In a recent online presentation, the American Concrete Institute explored innovative avenues in concrete production, focusing on the utilisation of high-sulphur fly ash, presented by Farshad Rajabipour, Pennsylvania State University. This is Part 3 of a three-part series.

…continued from Part 2.

Key findings recap

Hydration behaviour and setting time: High sulphur fly ashes, especially those with elevated calcium sulphide content, exhibit significant retardation in setting times due to the interaction with C₃A (tricalcium aluminate) and delayed gypsum consumption. Accelerators prove effective in mitigating these setting delays, offering a practical solution for applications where setting time is critical.

Volume stability and expansion: ASTM C1038 testing showed that none of the tested high-sulphur ashes exceeded the specified 0.02% expansion limit at 14 days. However, prolonged testing revealed potential concerns with gradual expansion, particularly notable in FBC ashes.

Mineralogical analysis: X-ray diffraction data highlighted the persistence of gypsum in controlling sulphate interactions within the pore solution. This dynamic affects both early hydration rates and long-term durability of concrete mixes.

Particle size and calcium sulphide impact: The particle size of calcium sulphide within fly ashes significantly influences setting times. Finer particles correlate with more pronounced set retardation, emphasising the need for particle size control or removal strategies.

Recommendations

Performance testing and standards: Reassessing ASTM C618 limits of 5% SO₃ as potentially conservative, the recommendation is to conduct comprehensive performance testing. This includes flow, setting time, strength development, and expansion characteristics to accurately assess suitability for specific applications.

Mitigation strategies:

• Utilisation of accelerators and fine limestone powders offers practical means to mitigate set retardation issues associated with high-sulphur fly ashes.
• Acid washing of fly ashes, albeit challenging due to SO₂ emissions, represents a viable method to reduce calcium sulphide content.
• Screening processes to selectively remove fine calcium sulphide particles can enhance the quality and applicability of fly ashes in concrete production.

Future directions

Long-term durability studies: Further research is crucial to understand the extended performance and durability of concrete incorporating high-sulphur fly ashes. Focus should expand beyond early-age hydration to encompass long-term behaviours, including resistance to sulphate attack and durability under varied environmental conditions.

Optimisation and innovation: Continued innovation in processing technologies and concrete mix designs will optimise the use of high-sulphur fly ashes, supporting sustainable construction practices and reducing environmental impact.

High-sulphur fly ashes, particularly FBC ashes and HSFAs, offer promising opportunities as SCMs in concrete production. Their utilisation supports sustainable construction goals by reducing cement clinker content and mitigating environmental impact. However, careful consideration of their specific properties and performance characteristics is essential to ensure compatibility with desired concrete properties and long-term durability requirements.