However, the presented N2 BET data above were collected ex-situ, while the associated BJH calculations were based on arguable assumptions i.e. the occurrence of only disconnected parallel cylindrical pores. Therefore, to gain a complementary information of the pore structure, in-situ SAXS/WAXS measurements of isothermally heated M-struvite at T = 90°C were performed allowing a view on the temporal evolution of pores during the thermal treatment (Figure ). Characteristic steps in the in-situ SAXS data of the mesoporous Mg-and Ni-system were selected and fitted to determine the pore size distribution. By fitting the SAXS curves of selected time steps, a quantitative comparison of the calculated pore size distribution and a temporal evolution of the mesoporous framework can be obtained. All details concerning the fitting procedure can be found in Methods: SAXS/WAXS (ESI: Supplementary Note 1, Equation S1, S2). All SAXS fits and pore size distributions of selected time steps of heated Mg-, Ni-and NixCo1-x-struvites can be found in the ESI (ESI: Figure , Table and).