By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 21 2024A recent article published in Applied Sciences explored the potential of a new course of fly ash as a cement replacement material. The examined fly ash was sourced from the 1985 to 1995 deposits on a coal-fired power plant site in Ireland.
Concrete compressive strength results. Image Credit: https://www.mdpi.com/2076-3417/14/10/4128
Background
Electricity generation by burning coal was planned to be phased out in Ireland by 2025 when the only coal-fired power plant in Ireland (located at Moneypoint) was due for decommissioning. However, due to the current energy crisis, its operation will continue as a backup until 2029. The plant, generating up to 915 MW, has been operational since 1987 and has several million tons of fly ash deposited over a 25-acre field.
Blending Portland cement with suitable supplementary cementitious materials (SCMs), such as fly ash, offers a significant opportunity to reduce CO2 emissions per ton of cement produced.
Current cement standards recognize two types of siliceous fly ash blends. The first type, classified as calcium-enriched mixture (CEM II/A-V), allows for up to 20 % cement replacement. The second type, CEM II/B-V, permits up to 35 % replacement. Utilizing the latter blend could cut Ireland's total carbon emissions by up to 2 % and repurpose as much as 750,000 tons of Moneypoint fly ash annually.
Thus, the researchers investigated the characteristics and hydration behavior of the Moneypoint ash from 1985 to 1995 as a suitable cement replacement in concrete.
Methods
Four ash samples (each 5 kg) were collected (from a depth of 1 meter from the surface) from each of the three sections, reflecting ash deposits of different operational phases within the ash field. Subsequently, different experiments were performed using commercially procured CEM I cement with and without fly ash at different replacement levels (0 %, 10 %, 25 %, and 35 %).
Scanning electron microscopy (SEM) was used to examine the particle-size distribution and surface morphology of the ash taken from the site. In addition, the chemical compositions of the samples were identified by X-Ray fluorescence (XRF), while X-Ray diffraction (XRD) was used to determine the change in mineralogical composition over time with hydration.
The moisture content of the fly ash was determined through weight measurements before and after drying each of the 12 samples in an oven at 105 °C for 24 hours. Furthermore, the compressive strength of concrete samples was assessed using 100 mm cubes to evaluate the ash’s performance in concrete.
Thermodynamic calculations were performed using PHREEQC (pH redox equilibrium program written in C) geochemical software. The cemdata18 thermodynamic database was then used to model the predicted hydration products, pore solution chemistry, and pH over time. The amorphous/glass phase of the ash was modeled using phase equations. These phases were added to PHREEQC for an accurate representation of the behavior of the amorphous phases rather than adding molar concentrations of the oxides present in the glass.
Results and Discussion
The XRD analysis revealed a unique mineral composition of the fly ash with promising characteristics for enhancing the strength and durability of concrete. The reactive silica and alumina present in the ash are mainly responsible for its reactivity. Additionally, the glass/amorphous content was calculated to be 81 % using a Rietveld analysis.
The average moisture content was around 11.5 %, which was an important consideration during the concrete production to achieve the design strengths. Additionally, the compressive strength of all samples containing the fly ash reached a similar level in 56 days. However, the 35 % sample took longer to reach similar strengths to CEM I and the other fly ash replacement levels.
Another key finding with fly ash-blended cement is the reduction in portlandite, indicated by a lower Ca:Si ratio, as a result of pozzolanic activity over time. This alteration likely reduces ettringite formation while increasing the AFm (alumina-ferric oxide-mono-substituted) phase compared to traditional 100 % CEM I cement. These changes can lead to a reduction in the overall volume of cement and potentially lower strength. Consequently, cement standards limit the content of fly ash to a maximum of 35 %.
Thermodynamic modeling has played a crucial role in predicting changes in the solid phase assemblage over time for fly ash-blended cement. The model successfully forecasted the formation of a stable ettringite phase alongside mono-carbonate, aligning with experimental observations. Notably, the volume of mono-carbonate increased as the fly ash content rose, attributed to the release and subsequent precipitation of additional calcium and aluminates into the solution.
Additionally, the model revealed a significant reduction in portlandite, which occurs as it reacts with silicates and alumina to form additional calcium-aluminum-silicate-hydrates (C-A-S-H). This reduction was so pronounced that portlandite was completely exhausted before 1000 days in blends containing over 10 % fly ash, indicating the high reactivity of the ash source used in the study. This finding was corroborated by XRD data, which showed a consistent decrease in the intensity of portlandite peaks over time across all levels of fly ash replacement.
Conclusion
The ash being studied has been confirmed as Class F according to the American Society for Testing and Materials standards and also meets European standards, making it suitable for use as fly ash in structural concrete. Its reactive pozzolanic properties contribute to the solid phase assemblage in concrete, enhancing its structural integrity beyond merely acting as a pore filler.
The combined approach of experimentation and thermodynamic modeling employed in this research can be applied to represent the glass phase in most supplementary cementitious materials (SCMs), predict their hydration behavior over time, and determine the optimal replacement levels. The researchers intend to further investigate whether Moneypoint ash from subsequent periods (1995-2005 and 2005 to present) also qualifies as an effective SCM in concrete.
Journal Reference
Shaji, N., Holmes, N., & Tyrer, M. (2024). Early Age Assessment of a New Course of Irish Fly Ash as a Cement Replacement. Applied Sciences, 14(10), 4128. https://doi.org/10.3390/app14104128, https://www.mdpi.com/2076-3417/14/10/4128
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