Fly ash has been identified as a potential source for extracting rare earth elements (REE), which although relatively plentiful in Earth’s crust, are not often found concentrated in large ore deposits. It is unclear, however, whether the fly ash residue created as a result of REE extraction can still serve as a cementitious material or as an ingredient of cementitious composite mixtures, which are widely used in the construction industry. Two concrete mixtures that feature identical dosages of recycled and traditional fly ash are characterized in terms their properties in plastic (unhardened) and hardened states. Plastic concrete tests include the determination of slump, air content, temperature, time of setting, and density. Hardened concrete tests include the quantification of compressive strength, modulus of rupture, and splitting tensile strength. A comparison of these fundamental properties between the control specimens and those that feature the beneficiated fly ash is conducted to determine the impact of fly ash residue as a new potential ingredient in cementitious composite mixtures and to identify avenues of application. Differences between the rate of compressive strength gain in the two mixtures are presented including a discussion on the need of higher amounts of Portland cement and accelerating admixtures when using the beneficiated fly ash. The validity of established empirical models that relate compressive strength to modulus of elasticity for traditional concrete mixtures is investigated in terms of their applicability to mixtures that feature the beneficiated fly ash. Similar discussions are presented for the relationship between compressive strength, modulus of rupture, and splitting tensile strength, including the impact that the beneficiated fly ash has on the likelihood of crack formation in concrete structures.

Evaluation of beneficiated fly ash as a potential ingredient for cementitious composite mixtures

ACS Spring 2022

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