FePO4 battery waste as set retarder and insights into the reaction products

Global electric vehicles (EVs) are projected to increase more than tenfold by 2035, raising significant concerns about battery disposal. This paper explores valorizing FePO₄-dominated waste (FPW) from the LiFePO₄ battery recycling industry as a set retarder, offering insights into the phases derived from its reaction with hydration products. The addition of 3–5% FPW meets BS EN 934–2 requirements for setting time and compressive strength in retarding admixtures. Investigations on cement pastes concluded that FePO₄ (FP) reacted over time, as indicated by the decrease in FP and the reduction in CH content compared to the control group, though no new phases were detected. Through selective dissolution of cement pastes and studies on FPW in simulated reaction conditions, experimental evidence confirmed the generation of siliceous hydrogarnet (Si–Hg) and hydroxyapatite (HAp) from FPW, consistent with thermodynamic predictions. The addition of 5 % FPW increased Si–Hg by 1.45 % at 1 day and 4.36 % at 28 days, and produced HAp at 1.72 % and 4.03 % over the same intervals. EDS analysis of partially reacted FP lumps consistently suggested the presence of HAp and Si–Hg. Inside the FP lumps, P was partially leached out, and Fe–Si–Hg was detected. Near the edges, a pronounced reaction region of typically 5–8 μm displayed increased Al/Fe–Si–Hg and mixed hydration phases such as C–S–H. Beyond its retarding effects, FPW demonstrates potential through rapid reactions with CH to generate HAp with cementitious properties, occurring much faster than the pozzolanic reaction of glass powder.