Magnesium alloys and graphite wastes encapsulated in cementitious materials: reduction of galvanic corrosion using alkali hydroxide activated blast furnace slag

Abstract

Magnesium alloys (Mg-0.5%Zr and Mg-1.2%Mn) and graphite from spent nuclear fuel that has been used in the former French gas cooled reactors, have been stored together in AREVA La Hague plant. The recovery and packaging of these wastes is currently studied and several solutions are under consideration. One of the developed solutions would be to mix these wastes in a grout composed of industrially available cement, e.g. OPC, OPC blended with blast furnace slag or aluminous cement. Within the alkaline pore solution of these matrixes, magnesium alloys are imperfectly protected by a layer of magnesium hydroxide (Mg(OH)2, Brucite) resulting in a slow process of corrosion releasing hydrogen. As the production of this gas must be considered for the storage safety, and the quality of wasteform, it is important to select a cement matrix capable of lowering the corrosion kinetics of magnesium alloys. This is especially true when magnesium alloys are conditioned together with graphite wastes. Indeed, galvanic coupling phenomena may increase early age corrosion of the mixed wastes, as magnesium and graphite will be found in electrical contact in the same electrolyte. Many types of cements have been tested and most of them have caused strong hydrogen production when magnesium alloys and graphite are conditioned together into such cement pastes. Exceptions are geopolymer binder which is already known for that and another binder based on alkali hydroxide activated ground granulated blast furnace slag (AHABFS) which is presented in the present article. First are presented hydrogen production experiments that demonstrate the efficiency of AHABFS towards reduction of corrosion of Mg alloys embedded. In a second part, a formulation of fluid mortar based on this binder is proposed.

Recent progress in low-carbon binders

“…The effective immobilization of cesium in Portland cement-based matrices is well known to be challenging, and so the availability of a cementing system that can restrict its movement is highly desirable. There have been important recent investigations of the potential for compatibility of AAM matrices with complex waste streams containing multiple radioisotopes [265,266], with oily wastes [267,268], with ion exchange media [269,270], and with graphitic or metallic wastes [271][272][273]. AAM matrices have also been demonstrated to show generally good stability under irradiation [274][275][276], and a hydrogen radiolytic yield that depends on water content and pore structure [274].…”

Corrosion Products Formed on MgZr Alloy Embedded in Geopolymer Used as Conditioning Matrix for Nuclear Waste—A Proposition of Interconnected Processes

“…First, the substitution of OH groups of Brucite by F decreases the cell volume of Mg(OH) 2−x F x , as it is reported in the humite system (from the MgO-SiO 2 -H 2 O compounds) [37] and may lead to a possible densification of the Mg(OH) 2−x, F x when x increases [12]. Thus, an increase of the F concentration in the media should go with an increase of the Mg(OH) 2−x F x densification which is the case of the CP layers formed in the GP-NaF sample.…”

“…Indeed, during the geopolymer ageing, cracks and delamination can be formed due to the shrinkage of the geopolymer occurring during the geopolymerization [19] and to a lesser extent during the sample preparation (freezing step) [32,33]. Moreover, the production of hydrogen during the MgZr corrosion may lead to a partial rupture of the CP and/or to CP expansion [12].…”

“…In geopolymer, when the basic poral solution is in contact with MgZr, Brucite Mg(OH) 2 is formed and limits the alloy corrosion [11,12]. Moreover, corrosion inhibitor additives can be considered, such as NaF that favors the formation of fluorine phases, e.g., Mg(OH) 2−x F x , MgF 2 , Na(K)MgF 3 , as proposed or characterized in [13][14][15][16][17], and that not modifies the geopolymer properties.…”