October 18, 2023
There’s no lack of awareness that challenges exist in the energy sector today. For the built environment, and concrete specifically, these challenges represent promising opportunity. Opportunity for the concrete industry to overcome challenges of its own, while providing added value to the energy sector. Along with increasing pressure to decommission coal-fired power plants in the U.S., electric utilities need to meet mandated closures of landfills and retention ponds that store an enormous amount of coal ash. These mandates translate into solutions for the concrete industry’s need to source reliable supplies of an alternative cementitious ingredient for formulating lower-carbon building materials.
The higher demand for, and lower supply of, fly ash driven by the decline in coal-fueled power generation is driving a growing trend of harvesting previously disposed coal ash for reuse in concrete. It is a demanding undertaking that requires extensive laboratory testing and analysis techniques to determine the unique ash characteristics within a disposal site, the quantities of coal-combustion products (CCPs) that can be recovered, and the beneficiation processes needed to ensure the harvested ash meets stringent specification standards for use in concrete.
Beneficiation companies work with electric utilities to identify potential harvesting locations and conduct upfront studies to determine if the mining and beneficiation of legacy ash is technically and economically feasible. Boring samples are taken at multiple locations and depths in the storage site and laboratory testing and analysis by scientists reveal the quantities of coal combustion residuals (CCRs) for delineating harvesting
Sample material from potential CCR harvesting location.
The test method for delineating the CCR content within an ash-retention pond relies on microscopy examinations and colorimetry analyses. The results then help to create reference curves and a compositional mapping of the site. Coal ash and clean sand samples from the pond are initially evaluated microscopically to determine the homogeneity and characteristics of the ash samples. In addition, the evaluation will reveal if the sand is free of ash contamination. After a portion of each sample is dried and pulverized, reference mixtures are created by blending the ash and sand.
Microscopy examinations and colorimetry analyses are used for delineating the CCR content within an ash retention pond. The figure above shows Clay, iron oxides, quartz particles, and 3 to 5% CCR (dark particles).
The color of the mixtures is then measured with a spectrophotometer. From here, unique corresponding values are calculated and reference curves created for each sample. Ongoing testing and analysis of field samples by material scientists determine the amount of coal ash material that still needs to be recovered by work crews to ensure a certified clean pond closure.
Remaining portions of the ash samples are dried at specific temperatures for additional testing and analysis in accordance with ASTM C618. This specification for coal fly ash and raw or calcined natural pozzolan use in concrete prescribes the chemical composition and physical requirements that the material must meet. It also provides a consistent framework for producing a final laboratory certification report on the results of tests performed and compliance to the applicable limits established by the ASTM C618 standard.
ASTM C311, “Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete”, contains the required laboratory tests and procedures to develop data for comparison with the property limits detailed in ASTM C618. This includes chemical-composition testing to determine free moisture content, LOI, and percentages of different mineral oxides. Physical-property assessments include density, fineness, strength activity index at 7 and 28 days, and water requirement.
Reach out to our team for more information on test methods for harvesting ash and to learn the ways CTLGroup can help. Our scientists are actively engaged in ASTM and AASHTO standards committees and can walk you through each step of the testing process. With more than a century of laboratory experience and an array of sophisticated physical and chemical testing equipment at hand, CTLGroup has the experience and advanced capabilities to help you succeed with your current or future harvested ash projects.
