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 ²⁹Si and ²⁵Mg 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.