This article focuses on the study of the self-reactivity of a 800 degrees C calcined palygorskite-bearing marlstone which is a complex multi-phase system (including CaO and MgO) whose reactivity in water must be studied before being used as Supplementary Cementitious Material (SCM). The sample was hydrated (w/s = 0.8) for 7, 14, 28 and 180 days and then investigated using X-ray diffraction (XRD), Solid State Nuclear Magnetic Resonance (MAS NMR), and Scanning Electron Microscope (SEM). Crossing the results of XRD and MAS NMR analyses (in particular the phase quantification by spectral integration of Si-29 MAS NMR spectra) have revealed the reactivity of the calcined clay phases with the portlandite which leads to the formation of C-(A)-S-H. It also highlighted the reactivity of C2S (neoformed during calcination) within 7 days of hydration. Hydration of this sample showed high self-reactivity (mostly pozzolanic) making it a promising sample for use as SCM.