Durability of magnesium silicate cements

Magnesium silicate hydrate (M-S-H) based cements are a new type of cement that exploits the strength generated when magnesium oxide is made to react with a soluble silica source to form a mostly amorphous hydrated gel of magnesium silicate (M-S-H gel). It is a perfect candidate for reducing the volume of waste from the fuel ponds in Sellafield as the solids in these ponds are mostly magnesium hydroxide, which can be converted to M-S-H gel under the right circumstances. Therefore, the waste would effectively be a large component of the binder in which it is immobilised.

Hydrated magnesium silicate gels have been known for a long time as a by-product of magnesium sulphate attack on conventional Portland cement-based materials. However, much less is known about the durability and long-term behaviour of M-S-H gel. Therefore the project would explore (i) how sludges should be pre-treated to enable the production of a suitable wasteform as too much water will reduce the mechanical properties substantially and (ii) the durability, i.e. long term stability of the wasteform made from such cements to ascertain whether it is likely to remain stable especially during the initial above ground storage of the waste.

This project will first involve pre-treating simulated waste sludges and processing them to form M-S-H based binders. It is envisaged that several types/formulations of waste sludges will be considered as each may require a different processing approach. Selected products from the most promising approaches will then be characterised using a range of techniques available at the Department of Materials and Department of Civil & Environmental Engineering. The workability, setting behaviour and the heat of hydration (isothermal calorimetry) will be determined. The hardened wasteform will be cured and conditioned to several regimes and then tested as a function of age for mechanical properties (e.g. strength, stiffness), dimensional stability (shrinkage), mass transport properties and resistance against degradation processes such as leaching/frost. The composition, phase assemblage and microstructure will be investigated using electron microscopy, X-ray microanalysis and thermal analyses. XRD will be used to characterise any crystalline phases formed. The pH and composition of pore solution and leachate will be analysed using ICP-OES.

Academic Lead: Luc Vandeperre
Researcher: Mercedes Baxter-Chinery
Location: Imperial College