The NMR spectra of the slag samples were obtained on a Bruker 500WB Avance II spectrometer (11.74 T) equipped with magic angle spinning probes (MAS NMR) of the same manufacturer. For the 29 Si spectra, the samples were placed in a 7 mm zirconia rotor, at a rotation speed of 6 kHz and a Larmor frequency of 99. angle. The chemical shift was calibrated externally with a TMS solution for the 29 Si spectra, a 0.1 M Al(NO3)3 solution for the 27 Al spectra, and kaolinite for the crosspolarized experiments. The spectral intensities were normalized according to the integrated signal between -50 and -120 ppm for 29 Si and between 150 and -90 ppm for 27 Al. They were eventually fitted using the DmFit program . All the parameters used for the fits are reported in the Supplementary Materials.