Combined U-Pb isotopic signatures of U mill tailings from France and Gabon: A new potential tracer to assess their fingerprint on the environment.

Uranium milling activities have produced high volumes of long-lived radioactive processed wastes stored worldwide in near surface environment. The aim of this study is to highlight relevant tracers that can be used for environmental impact assessment studies involving U mill tailings. A multi-tracer study involving elemental content, 238U decay products disequilibria and stable Pb isotopes was performed in different types of U mill tailings (alkaline, acid, neutralized acid) collected from five Tailings Management Facilities in France (Le Bosc, L'Ecarpière, Le Bernardan, and Bellezane) and Gabon (Mounana). Our results showed that U and Pb concentrations range between 30 and 594 ppm and 66-805 ppm, respectively. These tailings have a strong disequilibrium of (234U/238U) and (230Th/238U) activity ratios (1.27-1.87 and 6-65, respectively), as well as higher 206Pb/207Pb (1.86-7.15) and lower 208Pb/207Pb (0.22-2.39) compared to geochemical background ((234U/238U) and (230Th/238U) equal to unity; 206Pb/207Pb = 1.20; 208Pb/207Pb = 2.47). In situ analyzes (SEM, SIMS) showed that Pb-bearing phases with high 206Pb/207Pb are related to remaining U-rich phases, S-rich phases and potentially clay minerals or oxyhydroxides. We suggest that the combination of the 206Pb/207Pb with the (234U/238U) ratio is a relevant tool for the fingerprinting of the impact of U milling activities on the environment.

Chemical and Microstructural Properties of Designed Cohesive M-S-H Pastes.

Concretes can be exposed to a magnesium attack in several environments leading to the formation of magnesium silicate hydrates (M-S-H) and brucite (MH). The formation of M-S-H is likely to alter the properties of the cement matrix because it is linked to the decalcification of C-S-H. However, relatively few data on M-S-H exist in the literature. In order to characterize, physically and mechanically, the M-S-H phase, pure M-S-H cohesive pastes are needed. This work studies the formation of cohesive M-S-H pastes made with MgO-to-SiO2 atomic ratios of 0.78, 1 and 1.3, from two types of silica (silica fume or colloidal silica) and under 20 °C and 50 °C thermal curing. X-ray diffraction and thermogravimetric analyses confirmed that the consumption of brucite and the formation of M-S-H were quicker with a 50 °C curing. Energy-dispersive X-ray spectroscopy and microtomography showed that colloidal silica enabled a better distribution of the particles than silica fume. Microstructural characterizations were conducted under the protocol with colloidal silica and 50 °C thermal curing. Porosity investigations allowed to describe the M-S-H pastes as highly porous materials with a low content of micropores in comparison with mesopores. The type of mixing influenced the mesopore size distribution.