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In the present study, the chemical composition, mineralogy, and mechanisms of alteration of a cementitious grout based on a CEM III/C with addition of smectite, hydrotalcite, and silica fume, are studied using a combination of chemical and physical methods. This material was designed in the context of geological repository of radioactive wastes, with a twofold aim: first, to fill the technical voids left by drilling operations at the interface between the geological formation and the disposal galleries. Second, to neutralize a potential acidic transient due to pyrite oxidation, and to create an environment that favors low corrosion rates of carbon steels. The grout is mainly composed of calcium silicate hydrates having a Ca/Si ratio of ~0.8, incorporating Al in the bridging site of the Si chains (C-A-S-H), and accounting for 29–36 wt.% of the sample. It also contains silica fume (38–48 wt.%), smectite with interlayer Na (11–17 wt.%), hydrotalcite with interlayer CO32− (3–4 wt.%), and lower amounts of portlandite, calcite, and possibly gibbsite and gypsum. Upon alteration by water in a flow-through reactor, the main modifications affecting the sample are calcite and gypsum dissolution, hence releasing aqueous Ca2+ that is adsorbed in smectite interlayer by replacing Na+, and stoichiometric C-A-S-H dissolution. The evolution of solution chemistry and of the solid phase composition are reproduced successfully using a thermokinetic model. [ABSTRACT FROM AUTHOR]

In France, deep geological disposal is considered for the storage of high and intermediate‐level long‐lived radioactive wastes. For aluminium, the possibility to encapsulate the wastes in a cement‐based matrix is studied. However, cement being an alkaline environment, aluminium can lose its passivity, starts to corrode leading to hydrogen evolution in the infrastructures and generate a possible explosive hazard after decades of storage if hydrogen can accumulate somewhere in the facility. It is therefore necessary to study the corrosion behaviour of aluminium in the different cements considered for the encapsulation to estimate the possible amount of hydrogen that could be generated through corrosion and design the cement capsules accordingly. This work mainly focused on the reaction occurring at the aluminium‐cement interface. Raman spectroscopy did not highlight significant differences in the nature of the corrosion products forming at the cement/aluminium interface, leading to the conclusion that it is not the chemistry of the cement that is the key factor controlling the corrosion rate but rather the physical properties of the cement matrix. [ABSTRACT FROM AUTHOR]

Excavated soils and rocks are materials obtained in construction works that could represent an ecological issue if a durable and efficient reuse process is not set. The radioactive waste disposal planned by the French National Radioactive Waste Management Agency will generate large quantities of excavated soil (mainly as Callovo-Oxfordian argillite). The re-use of excavated soils is a recent question. There is a lack in the literature concerning the recycling of such materials. Therefore, this paper aims to investigate the possibility of using Callovo-Oxfordian argillite (COx argillite from the French URL) as a raw material for Portland cement clinker production. COx argillite was first characterized by X-ray diffraction (XRD) and X-ray fluorescence (XRF) then a Portland cement clinker was synthesized at laboratory scale. The produced clinker was characterized to verify the chemical and mineralogical composition. After adding gypsum, the reactivity of the resulting cement was assessed by setting time and isothermal calorimetry measurements. The compressive strength was assessed on standard mortar prisms at 1, 14 and 28 days. The results show that a Portland cement clinker containing 64% C3S, 14% C2S, 10% C4AF, 7% C3A and 1% CaO can be produced when 22.24% of raw meal was substituted by the COx argillite. The setting time and isothermal calorimetry results show that the produced cement shows an equivalent reactivity to conventional ordinary Portland cement. The compressive strength at 28 days is 56 MPa, showing that the produced cement can be considered as CEM I 52.5 N Portland cement. [ABSTRACT FROM AUTHOR]

The Callovo-Oxfordian (COx) claystone is chosen as a potential geological barrier for the geological repository of radioactive waste in France, and high-performance concrete is used as a sealing plug and lining in repository facilities. In this study, the shear strength of the interface between the concrete and claystone is investigated in particular. Ultra-high-performance (UHP) concrete is directly cast and cured beside a COx claystone semi-cylinder to make a cylindrical specimen including an interface. First, a microscopic analysis of the interface surfaces using scanned electronic microscope images is presented. It is shown that the adhesion of the UHP concrete to the COx claystone is of good quality and there is almost no initial interface aperture between the two materials. By using a home-designed original device, a series of direct shear tests is performed on cylinder specimens with interface under different confining stresses. The interface deformation and shear strength are quantitatively analysed. It is found that the interface has a bonded cohesion and the shear strength can be approximated by a non-linear function of normal stress. In most cases, the shear deformation does not generate an opening but rather a closure of the interface. The morphology of the sheared interface is also analysed by optical microscopic scanning images. [ABSTRACT FROM AUTHOR]

This study concerns the project of high and intermediate level radioactive waste disposal (Cigéo project for Centre Industriel de stockage GEOlogique). Millions of tons of argillite will be excavated during the construction of the site. Therefore, the aim of this work is to reuse this argillite in the formulation of an injectable geopolymer grout and study its evolution in different storage conditions (T = 20 and 80 °C). To achieve this challenge, a metakaolin and calcined Callovo-Oxfordian (COx) argillite were used with potassium silicate solutions. The grouts were successfully obtained from both metakaolin and COx argillite in both temperatures 20 and 80 °C. The range of viscosity of several mixtures respected the required specifications (viscosity ≤5 Pa s and setting time superior to 6 h). The metakaolin-based geopolymers didn't display any major changes no matter the storage temperature, except for the pore size, a coalescence phenomenon was observed with the increase of temperature. In contrast, for argillite or argillite-metakaolin based samples, a decrease of pore size as well as in pH values were observed with the increase of temperature. This behaviour was attributed to the dissolution of argillite's impurities such as illite, providing more oligomers to allow the formation of different geopolymer networks. [ABSTRACT FROM AUTHOR]

Depending on the lifetime and level of radioactivity of radioactive wastes, different disposal facilities are considered. Though low‐ and intermediate‐level short‐lived waste can be disposed in surface disposal facilities, deep geological disposal is considered for high‐ and intermediate‐level long‐lived waste. In France and Belgium, long‐term disposal is studied in clay host rock media. For aluminum, the disposal concept is based on encapsulation of the waste in a cement‐based matrix. It is also well‐known that aluminum is prone to severe corrosion in sufficiently alkaline environments leading to possible hydrogen production. To ensure the safety of the disposal facilities and the integrity of the cement capsules, the amount of aluminum that is disposed in each waste package must be specified and is limited to mitigate the level of hydrogen production by aluminum corrosion. In the present study, the corrosion resistance of an aluminum alloy (grade EN‐AW‐5754/H111) in two different cement matrices was studied in different configurations at room temperature. In each case, the evolution of hydrogen production was monitored to address the corrosion rate variation versus time. [ABSTRACT FROM AUTHOR]

An experimental campaign was carried out to study the passivity of steel embedded in blended cement paste. Mockup samples made of steel coupons (milled or pre-corroded) embedded in cement paste were tested under controlled relative humidity or in a solution simulating clay underground water and under aerated and de-aerated conditions, during up to 3 years. The results of gravimetric measurements show that the average corrosion rates determined on milled coupons are lower than 1 µm/year and vary from 0.2 to 2.7 µm/year for pre-corroded coupons. An apparent passive corrosion state is reached rapidly whatever the environmental conditions. Moreover, solid characterisations (Raman spectroscopy and SEM-EDS) point to a stable state in terms of the nature of the corrosion products and of the composition of the steel/cement paste interface. These results are similar to the ones previously obtained for steel coupons embedded in Portland cement mortar. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems. [ABSTRACT FROM AUTHOR]

Abstract: Cement based materials play an important role in the design and performance of future deep radioactive waste disposals. The type of materials selected for engineered barrier systems (EBS) is an important issue: protection of waste packages against corrosion and chemical degradation may be greatly enhanced by a proper choice and dimensioning. The long-term geochemical performance of two concrete types, CEM-I (pure Portland cement) and CEM-V (blended Portland, fly ash, blast furnace slag cement) was evaluated using reactive transport simulations of the interactions between the Callovo-Oxfordian mudrock (argillite) and concrete EBS over periods up to 106 years at 25°C. Simulations have been run with cylindrical geometry using the modeling tool Hytec 3.5 (ENSMP/CIG). Diffusion of solutes in the porewater of the solid media was considered to be the dominant transport process. Different scenarios were studied, including the case of sulfate attack. Various assumptions on the transport properties of the EBS and the argillite medium have been considered and compared. The interactions between concrete, CEM-I or CEM-V, and mudrock are in both cases predicted to lead to a significant clogging in the mudrock, in the vicinity of the interface with the concrete EBS. However, the nature and extent of the EBS/mudrock interface transformations greatly differ for the two materials considered. CEM-V, due to lower diffusion coefficients, keeps longer internal high pH conditions which are favorable to the waste packages from a chemical point of view. In addition, simulations predict that CEM-V material should have a better resistance to sulfate attack than CEM-I based barriers. In the case of CEM-I, the altered clay zone has a greater extent (up to 1.3m in 400,000y) and the drop of porosity is mainly due to zeolites and reprecipitated illite, quartz and calcite. For CEM-V, the altered clay zone is much smaller (less than 0.2m in 400,000y) and secondary smectite and calcite precipitation are predicted to cause porosity reduction. [Copyright &y& Elsevier]

Here, the valorization of Callovo-Oxfordian (COx) argillite from the geological layer focused by the Cigéo project (Industrial Center for Geological Disposal of radioactive waste) is studied by Andra. Its reuse by alkali-activation is studied in order to develop thermally resistant binders. The thermal properties of various argillite-based samples are studied at 800 °C, and they present a good resistance to a slow firing environment but a lower resistance to thermal shock. This article discusses the influence of the sample compositions on the thermal resistance. First, the thermal activation of argillite has a significant impact on the thermal resistance of the alkali-activated samples. Indeed, a higher resistance is reached with the use of furnace-calcined argillite due to the complete dehydroxylation of clay minerals and decomposition of carbonates. However, the use of flash-calcined argillite (partial dehydroxylation and decompositions) leads to different species present in the mixture and therefore may be conducive to higher structural evolutions during sample firing. Then, the influence of the nature of the alkaline silicate solution is highlighted. It appears that the use of sodium-potassium silicate powder-based solution (Si-Na-K) is conducive to lower reactivity and more viscous flow than a liquid-based potassium solution (Si-K). Finally, the addition of filler, sand (SiO 2 ) and calcined clay (Al 2 O 3 -SiO 2 ) allows for better thermal resistance but limits the development of mechanical properties. [ABSTRACT FROM AUTHOR]