PART 2: 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 2 of a three-part series.

…continued from Part 1.

Regulatory considerations and ASTM standards

The discussion then turned to ASTM C618, a standard governing the use of fly ash in concrete production, which imposes a maximum limit of 5% on sulphur dioxide content. The rationale behind this limit was questioned, as no documented evidence supporting this threshold was identified. Anecdotal evidence suggested concerns over volume instability and potential alkali-silica reaction (ASR) risks if sulphur content exceeded this limit.

The presentation challenged conventional wisdom by examining whether all high-sulphur fly ashes pose similar risks. It emphasised the need for nuanced assessment and potentially revisiting sulphur content limits in ASTM standards based on empirical data and performance criteria. Furthermore, the exploration of beneficiation pathways emerged as critical, aiming to refine high-sulphur fly ash into a viable SCM without compromising concrete quality or performance.

Integrating high-sulfur fly ash effectively into concrete mixes promises to reduce clinker content, mitigate environmental impact, and enhance material durability. Ongoing research and collaboration within the industry are essential to unlocking the full potential of these materials and advancing sustainable construction practices.

The presentation provided detailed insights into the composition and mineralogy of various high-sulfur fly ashes. These included:

• FBC fly ash: Characterised by anhydrite (calcium sulphate) particles and amorphous phases, alongside quartz and clay minerals.
• High-sulphur fly ash (HSFA): These variants contained significant amounts of calcium sulphide (CaSO₃), ranging from 17% to 21%, alongside amorphous materials and inert phases.
• Trona fly ash: Notably rich in sodium sulphate and sodium carbonate (trona), with approximately 11% of the ash composed of these minerals, alongside amorphous components and other phases.

The evaluation focused on concrete mixes where 20% of ordinary Portland cement was replaced with fly ash. Key performance metrics analysed included:

• Setting time: Some retardation was observed, particularly notable in HSFA variants and doped ashes with high-sulphur content.
• Workability and flow: Generally unaffected across most variants, indicating good flow characteristics in mortars.
• Volume stability: Critical findings suggested that volume expansion concerns were minimal within ASTM’s 5% sulphur dioxide content limit. Issues only surfaced notably with sulphur content exceeding 11%, aligning with anecdotal evidence about volume instability.
• Compressive strength: Showed promising results across all variants, suggesting no significant compromise in strength properties.
• Pore (defined as the alkaline solution present in the pores of hardened concrete) fluid pH and alkalinity: A notable challenge was observed with trona fly ash, where high alkali content affected pore fluid pH, potentially impacting alkali-silica reaction (ASR) mitigation effectiveness.

Implications for ASTM standards and future research

The findings raised pertinent questions about existing ASTM C618 standards, particularly the 5% sulphur dioxide content limit. It highlighted that while this limit generally ensures concrete performance stability, certain high-sulphur fly ashes can perform acceptably even beyond this threshold, provided other properties are carefully managed.

FBC ashes are derived from boilers burning coal at approximately 800°C, significantly lower than traditional pulverised coal power plants. This lower temperature preserves the particle morphology of minerals like clays, maintaining their porosity and angularity. However, this also increases water demand and can potentially reduce workability in concrete mixes. Key observations include:

Setting time and acceleration: FBC ashes exhibited delayed setting times compared to standard Portland cement mixes, attributed to the presence of gypsum (calcium sulphate) which delays the hydration of C₃A (tricalcium aluminate). Accelerators were effective in mitigating these delays.

Volume stability: Under ASTM C1038 testing conditions, none of the tested high-sulphur ashes, including FBC ashes, exhibited volume expansion exceeding 0.02% at 14 days. However, FBC ash showed a gradual increase in expansion over extended periods, questioning the adequacy of the current standard in predicting long-term performance risks related to delayed ettringite formation (DEF).

Further analysis included X-ray diffraction (XRD) data to track mineral phases and hydration reactions over time:

• C₃A hydration and sulphate absorption: Control pastes with fly ash and FBC ash showed delayed C₃A hydration due to gypsum presence, persisting up to 24 hours. Despite gypsum depletion, sulphate presence in pore solution continued due to interactions with calcium silicate hydrate (CSH), influencing long-term hydration kinetics.
• Calcium sulphide (CaSO₃) content: High calcium sulphide content in some fly ashes significantly delayed setting times. This retardation was the primary performance concern, impacting early-stage setting properties that required compensation through accelerators.
• Calorimetry analysis: Calorimetry data illustrated initial delays in heat evolution and strength development for high-sulphur ashes. However, by three days, these ashes caught up and often surpassed standard fly ash mixes, indicating robust long-term performance potential despite initial setbacks.

The presenter highlighted a need for:

• Reassessment of standards: Revisiting ASTM standards to better align with the specific characteristics and performance profiles of high-sulphur fly ashes, ensuring accurate prediction of long-term durability and stability.
• Optimisation strategies: Developing tailored strategies to optimise the use of high-sulphur fly ashes in concrete mixes, balancing setting time control, workability, and long-term performance metrics.
• Further research: Future research endeavours were proposed to refine understanding of sulphate interactions with CSH, validate testing standards for delayed ettringite formation risk, and explore innovative approaches to enhance the utility of high-sulphur fly ashes in sustainable concrete construction.

Continued in Part 3…