Training Group Rhineland Palatinate” (Max- von-Laue Institute of Advanced Ceramic Material Properties Studies, CerMaProS) of the Rhineland-Palatinate Ministry for Science, Further Education and Culture and by the Alexander Tutsek-Foundation, Munich. We also thank the German Research Foundation (INST 366/6-1) for the purchase of the NMR spectrometer. REFERENCES 1. Ko YC, Chan CF. Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000–1500°C. J Eur Ceram Soc. 1999 19(15):2633-2639. https://doi.org/ 10.1016/S0955-2219(99)00042-4. 2. Wang H, Glaser B, Sichen D. Improvement of resistance of MgO-based refractory to slag penetration by in situ spinel formation. Metall Mater Trans B. 2015 46B:749-757. https://doi.org/10.1007/s11663-014-0277-7. 3. Seko A, Yuge K, Oba F, Kuwabara A, Tanaka I. Prediction of ground-state structures and order-disorder phase transitions in II-III spinel oxides: A combined cluster-expansion method and first-principles study. Phys Rev B. 2006 73(18):184117. https://doi.org/10.1103/PhysRevB.73.184117. 4. Redfern SAT. Harrison RJ, O'Neill H, Wood DRR. Thermodynamics and kinetics of cation ordering in MgAl2O4 spinel up to 1600 degrees C from in situ neutron diffraction. Am Mineral. 1999 84(3):299-310. https://doi.org/10.2138/am-1999-0313. 5. Podworny J, Wojsa J, Piotrowski J. High temperature behaviour of MgAl2O4, AmCr2O4 and MgFe2O4 spinels in relation to their structure. Applied Crystallography. 2004:403- 406. https://doi.org/10.1142/9789812702913 _0080. 6. Wagh AS. Chemically bonded phosphate ceramics. 2. Auflage, Elsevier, Amsterdam 2016. https://doi.org/10.1016/B978-0-08- 044505-2.X5000-5. 7. Kalyoncu RS. Chemically bonded refractories – a review of the state of the art, U.S. Dept. of the Interior, Washington, D.C. 1982. 8. Kingery WD. Fundamental study of phosphate bonding in refractories. J Am Ceram Soc. 1950 33(8):239-250. https://doi.org/10.11 11/j.1151-2916.1950.tb14171.x. 9. Morris J, et al. Interaction between aluminium dihydrogen phosphate and quartz. J Appl Chem Biotechnol. 1976 26(1):385-390. https://doi.org/10.1002/jctb.5020260157. 10. Hahn D, Alavi AM, Quirmbach P. Powder XRD and 31 P and 27 Al solid state MAS NMR investigations of phase transformations in aluminium phosphate bonded Al2O3-MgAl2O4 refractories. Mater Chem Phys. 2021 267(124663). https://doi.ord/10.1016/j. matchemphys.2021.124663. 11. Hahn D, Alavi AM, Hopp V, Quirmbach P. Phase development of phosphate bonded Al2O3-MgAl2O4 high-temperature ceramics: XRD and solid-state NMR investigations. J Am Ceram Soc. 2021 104(12):6625-6642. https://doi.org/10.1111/jace.18022. 12. Hopp V, Alavi AM, Hahn D, Quirmbach P. Structure-property functions of inorganic chemical binders for refractories. Materials. 2021 14(4636). https://doi.org/10.3390ma1416 4636. 13. Wegner S, van Wüllen L, Tricot G. The structure of aluminophosphate glasses revisited: Application of modern solid state NMR strategies to determine structural motifs on intermediate length scales. J Non-Cryst Solids. 2008 354(15-16):1703-1714. https:/ doi.org/10.1016/j.jnoncrysol.2007.10.034.