Durability of geopolymers and magnesium phosphate cements for the conditioning of reactive metallic waste in a cementitious environment

by Guillaume CAMPS (ANDRA)

Tuesday 5 Dec 2017, 14:00 → 14:45 Europe/Zurich

774/R-013 (CERN)

Abstract:

Aluminium and magnesium metallic waste are reactive in cementitious materials, which are the main conditioning matrix for ILL-SL radioactive waste in the Centre de Stockage de l’Aube in France. The high corrosion rate of these metallic waste leads to the production of a high amount of dihydrogen. This production must be controlled and limited because of the explosion risk, the damaging of the confining waste package and to manage the hydraulic-gas transient phase. These problems had led Andra to limit the quantity of reactive waste in each waste package to be sure that they could fulfill their role during the monitoring period (300 years).

A solution to limit the H2 production is the use of alternative materials that can keep the metallic waste in a passive state. Thus, the corrosion rate can be limited and it gives the possibility to fill the waste packages with higher quantity of reactive waste.

Magnesium phosphate cements (MKP) are considered for aluminium alloys. Aluminium is in a passive state in a range of pH from 4 and 9 and MKP matrixes are in a range from 5 to 8. These conditions allow the formation of a protective layer on aluminium surface. Geopolymers (MGeo), an aluminate-silicate mineral binder with a very low quantity of calcium, are considered for magnesium because the medium keeps the metal in a passive state with a very low corrosion rate.

The use of both of these matrixes seems very interesting to optimize the waste disposal. Nevertheless, Andra has several specifications with criteria that must be respected to ensure the durability of the performances of the waste packages during the monitoring period.

The packages are going to be disposed in cementitious vaults. With time, they are going to be resaturated and the water that will be in contact with the new matrix will be a cementitious water (pH ≈ 13.2). This water will lead to chemical evolutions in the geopolymer or in the magnesium phosphate cement. These evolutions must be studied to be sure that they won’t lead to a change in the behaviour of the matrix.

An ongoing project deals with the study of the durability of MGeo and MKP with 3 main topics:

• The chemical evolutions in the matrix, resaturated with cementitious water and the consequences on mineral phases, microstructure, mechanical and transfer properties;

• The corrosion of aluminium, magnesium and steel in sound matrix and cementitious water resaturated matrix, and the galvanic corrosion;

• The durability of the performances of corrosion inhibitors that are used in both materials.

The purpose is to verify the compliance with Andra’s requirements from the operating period up to the end of the monitoring period.