Evaluation of Fly Ash Residue after Rare-Earth Element Extraction as a Potential Ingredient for Cementitious Composite Mixtures

Abstract Fly ash has been identified as a potential source for extracting rare-earth elements (REEs), which are instrumental in the production of many technological devices and equipment. However, it is currently unknown whether the fly ash residue obtained after REE extraction can be used beneficially in cementitious paste, mortar, and concrete mixtures. It is also unclear how the inclusion of the fly ash residue into said cementitious mixtures affects their behavior relative to the inclusion of traditional fly ash. An experimental study is conducted to: (1) characterize the physiochemical properties of the residue compared to Class F fly ash; (2) evaluate cementitious paste soundness by considering samples that feature portland cement (PC), Class F fly ash, and the residue; (3) quantify water requirements after the inclusion of the residue at the cementitious paste and mortar level; (4) determine strength activity index of mortar cubes containing the residue; and (5) characterize plastic and hardened properties of concrete that contains the residue as a partial replacement of PC. Pozzolanic activity is inferred by observing changes in chemical composition and increases in mortar and concrete compressive strength. It is determined that the fly ash residue features a 36% reduction in density compared to PC and a 15% reduction compared to Class F fly ash. The particles in the fly ash residue feature a 22% reduction in fineness compared to PC and an 18% reduction compared to Class F fly ash. Soundness testing of cementitious paste containing the residue suggests stable volume retention by featuring an autoclave expansion of 0.42%. Water requirements to attain normal consistency for cementitious paste and mortar specimens containing the residue increased by 128% compared to the control mixtures. This phenomenon is linked to coarser particle sizes in the residue leading to reduced packing ability, greater void structure, larger filling water demand, and hygroscopic material response through chloride enrichment. A 10% replacement by mass of PC with the residue and another 10% replacement with Class F fly ash in concrete leads to an 82% reduction in slump, a 50% increase in air content, a 2% decrease in density, a 4% decrease in temperature, comparable compressive strength and modulus of elasticity, and a 47% increase in ultimate drying shrinkage and creep compared to control samples. It is concluded that the fly ash residue may be used as a 20% replacement by mass of PC at the cementitious paste and mortar level, and a 10% replacement by mass at the concrete level because the measured changes in material properties compared to control samples are manageable and appropriate for various applications.