Maybe not? … Harvested fly ash, previously disposed of as waste, has been demonstrated to match the chemical and physical properties of freshly produced fly ash, i.e. Harvested fly ash has potential to provide a consistent and reliable alternative fly ash supply for the foreseeable future!
Coal fly ash is the most widely used SCM in concrete. Class F fly ashes, usually replacing 15%–25% of the total cementitious material, are pozzolans. Class C fly ashes, usually replaced at the higher 15%–40% dosage rate, are both hydraulic and pozzolanic. Fly ash (ASTM C618/AASHTO M 295)
Magnified scanning electron microscope micrographs illustrating the
similarities between freshly produced fly ash (left) and harvested fly ash (right) from the
same power generation station (Larry Sutter in Tritsch, et al. 2021). Click to Enlarge.
Fly ash characteristically impacts concrete as follows:
- Less water is normally required to achieve a given level of workability
- Setting time may be delayed
- Early strengths may be decreased but later strengths increase (because fly ash reaction rates are initially slower but continue longer)
- Heat of hydration is reduced
- Resistance to alkali-silica reaction (ASR) and sulfate attack is improved when the appropriate fly ash substitution rate is used
- Permeability is reduced
Video Presentation: “Fly Ash Utilization, Appliations, & Supply Logistics”
Click on image on the right to go to the ISCP post / (article) of the VIDEO.
Historically, fly ash has been obtained from coal-fired power plants directly following its production. However, with the ongoing shift away from coal-fired power, the supply of freshly produced fly ash has decreased while demand has increased.
After processing, harvested fly ash from fly ash previously disposed of as waste has been shown to match the chemical and physical properties of freshly produced fly ash and therefore to have capacity to provide a consistent and reliable alternative fly ash supply for the foreseeable future.
For the CP Tech Center‘s new “Cementitious Materials” Resources Page, please go to:
https://cptechcenter.org/cementitious-materials/#FlyAsh
For the VIDEO Presentation “Fly Ash Utilization, Appliations, & Supply Logistics”,
click on image on the right above, or go to: www.concretepavements.org/2022/03/11/video-reclaimed-harvested-fly-ash-utilization-applications-supply-logistics/