A cementitious system for the immobilisation of magnesium rich Magnox sludge was produced by blending an Mg(OH)2 slurry with silica fume and an inorganic phosphate dispersant. The Mg(OH)2 was fully consumed after 28 days of curing, producing a disordered magnesium silicate hydrate (M-S-H) with cementitious properties. The structural characterisation of this M-S-H phase by (29)Si and (25)Mg MAS NMR showed clearly that it has strong nanostructural similarities to a disordered form of lizardite, and does not take on the talc-like structure as has been proposed in the past for M-S-H gels. The addition of sodium hexametaphosphate (NaPO3)6 as a dispersant enabled the material to be produced at a much lower water/solids ratio, while still maintaining the fluidity which is essential in practical applications, and producing a solid monolith. Significant retardation of M-S-H formation was observed with larger additions of phosphate, however the use of 1 wt% (NaPO3)6 was beneficial in increasing fluidity without a deleterious effect on M-S-H formation. This work has demonstrated the feasibility of using M-S-H as binder to structurally immobilise Magnox sludge, enabling the conversion of a waste into a cementitious binder with potentially very high waste loadings, and providing the first detailed nanostructural description of the material thus formed.