Interaction of aluminum alloys with MKPC and Portland-based cements on the metal-matrix interface

Abstract

Cementitious matrices are considered for the immobilisation of radioactive aluminium. The processes occurring at the interface between both materials are relevant for the long-term stability of the repository. The present study compares Ordinary Portland cement (OPC), CEM I-type, with alternative Portland blended cement (CEM IV and CEM I + 50% silica fume) and magnesium potassium phosphate cement (MKPC). Al A1050 and Al AA5754, with 3.5% Mg, have been used as reactive metal alloys. Two exposure conditions were employed: (1) water immersion and (2) isolated in sealed plastic films. Long-term corrosion monitoring (E_corr, i_corr and V_corr) was evaluated to understand the effect of Al reactivity in the matrix. The associated H₂ release was quantified to understand the changes at the metal/matrix interface. Furthermore, pore ion concentrations and the pore microstructure were evaluated. The metal/matrix interface alterations were analysed at the end of the tests. The study revealed that after 300 days of water immersion, the CEM I + 50% of silica fume matrix had reduced the pore solution pH down to 10.5 compared to CEM I, which remained high alkaline (pH 12.9), and CEM IV with no significant decreases (pH 12.3). In contrast, MKPC showed the lowest pH (7–9.8). Low Al corrosion rates were found with MKPC, followed by CEM I + 50% SF according to their lower pH. A corrosion product layer of 90-50 μm thickness was observed at the Al/CEM I matrix interface constituted of aluminium and oxygen. Furthermore, enrichment in Al was detected in the matrix (around 1 mm depth), causing the formation of ettringite nodules. Voids were detected at the interface level associated with the high volume of H₂ released. The MKPC matrix showed no alteration at the metal/matrix interface due to the lower reactivity of the matrix and lower H₂ release. A homogeneous and dense region (30 μm) rich in phosphorous, potassium and magnesium was observed near the metal surface.