Variation of sodium coordination during the hydration processes of layered sodium disilicates as studied by 23Na MQMAS and 1H↔23Na CP/MAS NMR spectroscopyElectronic supplementary information (ESI) available: XRD patterns of SKS-5 and SKS-6 samples after different hydration times, and 1H–1H NOESY spectra of SKS-5 and SKS-6 hydrated for six months. See http://www.rsc.org/suppdata/jm/b2/b209578e/