The first UK civil nuclear reactors used natural uranium fuel surrounded by a magnesium non-oxidising alloy cladding (Magnox). The Magnox waste was stored under water, alongside uranium, aluminium, and other metals, and has evolved into a magnesium hydroxide (Mg(OH)2) rich sludge, requiring encapsulation for long-term storage in a geological disposal facility. The current system (composite cement) is unsuitable for the Magnox sludge, as at the cement pH (13), aluminium would corrode. A mortar pH of 9 – 11 would limit the corrosion (and hydrogen production) of any unreacted aluminium and magnesium.

Magnesium-silicate-hydrate (M-S-H) is formed from the hydration of magnesium and silica sources. An M-S-H mortar using magnesium oxide (MgO) and microsilica had a final pH of 10 – 11, porosity of 9.2 %, and achieved strengths of 40 MPa by 90 days. Using an M-S-H mortar as an encapsulation matrix could allow for the Magnox sludge to be included as a component in the mortar, significantly reducing waste volume.

Two mortars, using Mg(OH)2-based sources (powder and sludge (CMgS)), were optimised for waste use, maximising the proportion of included Mg(OH)2 whilst still meeting the set criteria. Mg(OH)2 has a low reactivity, so MgO was added to promote early M-S-H formation. Using CMgS reduced the mortar fluidity, requiring an increased water/solid ratio. The properties of these mortars (and a reference MgO-based mortar) were studied for up to 1 year. The mortars met the encapsulation criteria. The CMgS-based mortar achieved a compressive strength of 25 MPa, connected porosity of 8.8 % by 365 days, a pH between 10 – 11 and exhibited the greatest resistance to chemical ingress.

If the CMgS-based M-S-H mortar was used as the encapsulation matrix, with Magnox sludge as the Mg(OH)2 component, the waste package volume would be 4, 000 m3, reducing the required storage space by 52 %.