PART 1: 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 1 of a three-part series.

A substantial fraction of fly ash co-mingled with flue gas desulphurisation (FGD) products are discarded as off-specification materials due to exceeding the 5.0% SO₃ content limit specified in ASTM C618. As such, it is expected that fly ash harvested from some landfills and ponds will have high SO₃ content. The behavior of these fly ashes in cementitious systems varies significantly depending on the type of sulfur present in the ash including calcium sulphate, calcium sulphite, and sodium sulphate.

As such, a single SO₃ content limit specified in ASTM C618 is not sufficient to capture their complexity or performance. Four off-specification FGD co-mingled fly ash products were investigated in this study. This included evaluating their impact on workability, setting time, pore solution pH, compressive strength, and expansion in lime water using paste and mortar mixtures at 20% cement replacement level. Advanced characterisation techniques such as X-ray diffraction, isothermal calorimetry, and pore solution analysis using inductively coupled plasma atomic emission spectroscopy were also used to study the hydration behavior of blended systems containing these ashes. The results of this study have been used to identify appropriate beneficiation options and potential modifications to the specifications.

This underutilised material, often overlooked due to its sulphur content, presents significant opportunities and challenges in the construction industry.

The insights and recommendations from this study aim to inform concrete industry practices and pave the way for further advancements in utilising high-sulphur fly ash effectively in construction applications.

Speaker 1 elucidated the characteristics and origins of high-sulphur fly ash. Typically commingled with flue gas desulphurisation products, this type of fly ash is derived from power plants burning high-sulphur coal. The shift in energy production trends, notably towards natural gas, has led to a decline in the availability of traditional low-sulfur fly ash, thereby necessitating exploration into alternative sources.

The presentation underscored the dual nature of high-sulphur fly ash in concrete applications. On one hand, it offers potential advantages such as enhanced durability and reduced carbon dioxide emissions, aligning with global sustainability goals. Conversely, its elevated sulphur content introduces complexities, including the propensity for etching and set retardation in concrete mixes.

A significant portion of the discussion revolved around overcoming technical obstacles associated with high-sulphur fly ash. Strategies were proposed to mitigate issues like calcium sulphide management in cement materials, crucial for optimising concrete performance and longevity. These insights are pivotal for engineers and practitioners seeking to integrate high-sulphur fly ash into their concrete formulations effectively.

Moreover, the presentation highlighted various beneficiation pathways that could enhance the suitability of high-sulphur fly ash for concrete production. These pathways aim to refine the material’s properties, ensuring it meets stringent performance standards while capitalising on its abundance in impounded and landfill sources.

The imperative to reduce clinker content in concrete remains central to sustainability efforts within the construction sector. High-sulphur fly ash emerges as a viable supplementary cementitious material (SCM), capable of bolstering concrete’s environmental footprint through substantial clinker replacement.

By leveraging innovative approaches and technological advancements, stakeholders can harness the full potential of this abundant yet underutilised SCM. The ongoing research and development efforts underscore a promising trajectory towards sustainable concrete practices.

Exploring the origin and management of sulphur in fly ash

Sulphur dioxide in fly ash predominantly originates from the combustion of coal, where sulphur compounds like pyrite, gypsum, or organic sulphur are oxidised during the burning process within power plant boilers. This oxidation results in the emission of SO₂ or SO₃ gases, which are then subject to stringent environmental regulations due to their contribution to acid rain and other environmental concerns.

To mitigate environmental impact, power plants employ various methods of flue gas desulphurisation (FGD). The presentation highlighted three primary FGD techniques:

• Wet FGD process: This involves scrubbing the flue gas with a lime slurry (calcium hydroxide), which reacts with SO₂ and converts it into calcium sulphide (CaSO₃). This method effectively removes sulphur from the flue gas, yielding cleaner emissions and producing a slurry residue.
• Dry FGD process: Here, a lime slurry or sodium carbonate solution is sprayed directly into the flue gas. The heat from the gas evaporates moisture, allowing the lime or calcium to react with sulphur and form particles like CaSO₃ or sodium sulphate. Unlike wet FGD, this method produces sulphur-bearing particles that must be subsequently removed, potentially contaminating fly ash.
• Fluidised bed combustion (FBC) boilers: In FBC boilers, calcium carbonate (calcite) is injected directly into the boiler, where it decomposes into calcium oxide (CaO). This reactive process helps in reducing sulphur emissions directly during combustion.

Continued in Part 2…