Degradation of Cement Pastes Subjected to Combined Carbonation and Leaching

Carbonation and leaching are likely to be the primary concrete degradation mechanisms for concrete utilized in geological repositories when the concrete is fully saturated and in contact with host rock pore water containing increased CO2 content. This work describes the changes in the microstructure and transport properties of cement pastes caused by coupled chemical degradation processes that occur at the cement paste-clay interface. In the laboratory, an experimental program was designed to accelerate the interface interaction under carbonation and leaching by bringing highly porous cement pastes (to mimic backfill materials used in geological disposal) and Boom Clay into contact in either accelerated percolation or batch-type experiments. Mercury intrusion porosimetry (MIP) was used to investigate the microstructural alteration. Inductively coupled plasma optical emission spectrometer (ICP-OES) and permeability, diffusion measurements were used to determine the evolution of chemical and transport properties, respectively. The results of the porosity change at the interface demonstrate an increase in porosity of the cement paste interface due to Ca leaching, which overwhelms the carbonation. As a result, the transport properties increase. This suggests that clogging of the cementitious material side is unlikely

Report of RILEM TC 281-CCC: outcomes of a round robin on the resistance to accelerated carbonation of Portland, Portland-fly ash and blast-furnace blended cements

Many (inter)national standards exist to evaluate the resistance of mortar and concrete to carbonation. When a carbonation coefficient is used for performance comparison of mixtures or service life prediction, the applied boundary conditions during curing, preconditioning and carbonation play a crucial role, specifically when using latent hydraulic or pozzolanic supplementary cementitious materials (SCMs). An extensive interlaboratory test (ILT) with twenty two participating laboratories was set up in the framework of RILEM TC 281-CCC ‘Carbonation of Concrete with SCMs’. The carbonation depths and coefficients determined by following several (inter)national standards for three cement types (CEM I, CEM II/B-V, CEM III/B) both on mortar and concrete scale were statistically compared. The outcomes of this study showed that the carbonation rate based on the carbonation depths after 91 days exposure, compared to 56 days or less exposure duration, best approximates the slope of the linear regression and those 91 days carbonation depths can therefore be considered as a good estimate of the potential resistance to carbonation. All standards evaluated in this study ranked the three cement types in the same order of carbonation resistance. Unfortunately, large variations within and between laboratories complicate to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions on the carbonation performance of the specimens tested. Nevertheless, it was identified that fresh and hardened state properties alone cannot be used to infer carbonation resistance of the mortars or concretes tested. It was also found that sealed curing results in larger carbonation depths compared to water curing. However, when water curing was reduced from 28 to 3 or 7 days, higher carbonation depths compared to sealed curing were observed. This increase is more pronounced for CEM I compared to CEM III mixes. The variation between laboratories is larger than the potential effect of raising the CO2 concentration from 1 to 4%. Finally, concrete, for which the aggregate-to-cement factor was increased by 1.79 in comparison with mortar, had a carbonation coefficient 1.18 times the one of mortar

An advanced mineralogical study of the clay mineral fraction of the Boom Clay

In Belgium, the Boom Clay is studied aso ne of the potential host rocks for the geological disposal of radioactive waste. The clay minerals within the Boom Clay contribute significantly to the retardation of the release of radionuclides from the potential repository to the geosphere and biosphere due to their sorption capacity. The major processes responsible for this behaviour are cation exchange in the interlayer of (mostly) swelling clays like smectites, and surface complexation on the broken edges of clay platelets (Baeyens and Bradbury, 1997; Bradbury and Baeyens, 1997; 2000). Due to these processes, clay minerals also dictate to a high extent the pore water chemistry of argillaceous rocks (Bradbury and Baeyens, 1998). Besides a high sorption capacity, clay minerals may equally possess substantial reduction-oxidation potential, depending on their nature and Fe content (Neumann et al., 2011). Several naturally-occurring mechanisms have already been identified which are able to alter the Fe(II)/Fe(III) content of clay minerals (Schaefer et al., 2011), thereby changing both their redox behaviour as well as their layer charge and as such their total adsorptive capacity (Gorski et al., 2013). Novel electrochemical methods were developed to probe the total electron-accepting and electron-donating capacity of clay minerals, based on kinetic probes (Neumann et al., 2008; Gorski et al., 2011). The detailed study of the clay mineral characteristics is therefore fundamental in our understanding of the Boom Clay composition and geochemical processes, and forms the base for further experimental work and modelling. The objectives of this PhD are to study the clay mineral characteristics of various Boom Clay samples, both qualitatively and quantitatively, and to couple this information to geochemical parameters and processes. This coupling would allow to gain a better insight into the retention properties of Boom Clay towards radionuclide dispersion starting from baseline mineralogical information.status: publishe

Strength and Microstructure Characteristics of Metakaolin-Based Geopolymer Mortars with High Water-to-Binder Ratios

Geopolymers and other alkali-activated materials were investigated in detail as alternatives to ordinary Portland cement because of their reduced CO2 emissions, high (radionuclide) binding capacities, and low permeabilities. The last two properties make them potential materials for the immobilization of several types of chemical waste. In this context, the direct immobilization of liquid waste streams would be a useful application. This study aimed to develop geopolymers with high water-to-binder ratios, but with good mechanical strengths, while elucidating the parameters that dictate the strengths. Three potential metakaolin geopolymer recipes were cast and cured for 28 days, after which their strengths, mineralogy, and microstructures were determined. The results show that it is possible to attain acceptable mechanical strengths at water-to-binder ratios that vary from 0.75 to 0.95, which is a significant increase from the ratio of 0.55 that is commonly used in the literature. It was found that the most important parameter that governs the mechanical strength is the dilution of the activating solution, which is represented by the H2O/Na2O ratio, while the microstructure was found to benefit from a high SiO2/Al2O3 ratio

Evaluation of a Long-Term Thermal Load on the Sealing Characteristics of Potential Sediments for a Deep Radioactive Waste Disposal

An in situ and a batch heating experiment were applied on the fine-grained sediments of the Opalinus Clay from Mont Terri (Switzerland) and the Boom Clay of Mol (Belgium), both being currently studied as potential host formations for deep nuclear waste disposal. The purpose was here to test the impact of a 100 °C temperature rise that is expected to be produced by nuclear waste in deep repositories. The experiment on the Opalinus Clay mimicked real conditions with 8-months operating heating devices stored in core drillings into the rock. The comparison of the major, trace, rare-earth elemental contents and of the whole-rock K-Ar data before and after heating shows only a few variations beyond analytical uncertainty. However, the necessary drillings for collecting control samples after the experiment added an unexpected uncertainty to the analyses due to the natural heterogeneity of the rock formation, even if very limited. To overcome this aspect, Boom Clay ground material was subjected to a batch experiment in sealed containers during several years. The drawback being here the fact that controls were limited with, however, similar reproducible results that also suggest limited elemental transfers from rock size into that of the <2 μm material, unless the whole rocks lost more elements than the fine fractions. The analyses generated by the two experiments point to identical conclusions: a visible degassing and dewatering of the minerals that did not induce a visible alteration/degradation of the host-rock safety characteristics after the short-term temperature increase

Diffusive Transport of Dissolved Gases in Potential Concretes for Nuclear Waste Disposal

In many countries, the preferred option for the long-term management of high- and intermediate level radioactive waste and spent fuel is final disposal in a geological repository. In this geological repository, the generation of gas will be unavoidable. In order to make a correct balance between gas generation and dissipation by diffusion, knowledge of the diffusion coefficients of gases in the host rock and the engineered barriers is essential. Currently, diffusion coefficients for the Boom Clay, a potential Belgian host rock, are available, but the diffusion coefficients for gases in the engineered concrete barriers are still lacking. Therefore, diffusion experiments with dissolved gases were performed on two concrete-based barrier materials considered in the current Belgian disposal concept, by using the double through-diffusion technique for dissolved gases, which was developed in 2008 by SCK CEN. Diffusion measurements were performed with four gases including helium, neon, methane and ethane. Information on the microstructure of the materials (e.g., pore size distribution) was obtained by combining N2-adsorption, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and water sorptivity measurements. A comparison was made with data obtained from cement-based samples (intact and degraded), and the validity of existing predictive models was investigated

Immobilization of cesium and strontium-based waste by metakaolin geopolymer : effect of waste loading and water-binder ratio on the properties of the host matrix

The first-generation nuclear power plants and reprocessing facilities are approaching the end of their service lives [1]. As a result, spent nuclear fuel (SNF) and radioactive waste, generated by both the normal operations and decommissioning of nuclear power plants will undoubtedly rise significantly [2]. Reprocessing of SNF has long been regarded as a viable solution to address existing disposal issues. Under the Belgian waste management framework; the ASOF (Advanced Separation for Optimal management of spent Fuel) Project is advancing the development of new, innovative processes for the separation of minor actinides (Americium) and fission products Cesium (Cs) and Strontium (Sr) to optimise SNF disposal. In parallel, approaches for conditioning Cs and Sr are being investigated. Alkaliactivated materials (AAMs) have been intensively studied in recent years and are considered as one of the potential alternatives to ordinary Portland cement (OPC) for waste immobilization. Geopolymer, a class of AAMs with a low calcium system and consisting of alkali aluminosilicate gel as the main product; offers a good immobilization and binding capacity of cations due to the pseudo-zeolitic structure of its amorphous network as well as high alkalinity. However, only one study exists to date in the literature concerning the possibility of co-immobilization of Cs and Sr into one single matrix. Li et al (2016 & 2018) investigated the immobilization of Cs and Sr by paper sludge (PS) ash-based geopolymer bearing 1% wt Sr(NO3)2 and CsNO3 to address the issue of Cs and Sr present in contaminated water from Fukushima Daiichi NPP [3,4]. However, the reported mechanical properties were very low (Flexural strength < 1MPa) which potentially limits its wide adoption and application in other countries. This work aims to develop a metakaolin (MK)-based geopolymer mortar bearing high Cs and Sr loading and exhibiting high mechanical properties. The focus was to evaluate the effect of the water-binder (W/B) ratio and simulated Cs and/or Sr waste loading on the properties of MK geopolymer

What does mediated electrochemistry reveal about regional differences in the redox properties of Boom Clay?

The Boom Clay is a potential host rock for geological storage of radioactive waste in the Netherlands and Belgium. The redox properties of the host rock are important in the context of safety assessment as they affect the speciation and thus the mobility of redox sensitive radionuclides. In this study, redox properties of the clay were assessed by mediated electrochemical analyses. The electron donating (EDC) and accepting (EAC) capacities and reduction potential of a suite of Boom Clay samples were determined. Boom Clay samples from various locations in the Netherlands and Belgium were investigated in unaltered form, and after size separation or chemical treatment to relate variations in redox properties to regional differences in diagenetic history or in the assemblage of allogenic minerals. In the investigated samples, the EDC can be attributed to the oxidation of pyrite, FeII in clay minerals and reduced natural organic matter (NOM) while the EAC can be ascribed to the reduction of FeIII in clay minerals and in Fe (oxyhydr)oxides. Combining Na-pyrophosphate extraction, to remove reactive NOM, with mediated electrochemical oxidation (MEO) allowed determining the individual EDC of NOM and FeII in clay minerals. Mediated electrochemical analysis showed systematic differences between samples from two locations in the Netherlands, Zeeland and Limburg. In samples from Zeeland, the reduction potential was higher, the EAC was larger, and the contribution of NOM to the EDC was smaller compared to samples from Limburg. These differences can be attributed to partial oxidation of Boom Clay in Zeeland during its diagenetic history but partial oxidation could also be a storage artefact. The electron yield obtained by pyrite oxidation in samples from Zeeland was larger compared to those from Limburg, which can be explained by a smaller particle size of pyrite in Zeeland. The size of pyrite particles, in turn, can be used as a proxy for the depositional conditions. The electrochemical activity of Fe in clay minerals did not vary systematically between the two locations in the Netherlands. In general, the fraction of electrochemically active Fe in clay minerals increased with the relative content of 2:1 clay minerals. In comparison with samples from the Netherlands, larger fractions of structural Fe in clay minerals were redox-active in samples from Belgium, which had a higher chlorite or glauconite content. This study demonstrates that mediated electrochemical analysis can reveal redox properties of Boom Clay, which might be of relevance for the migration of redox sensitive radionuclides or when assessing the impact of constructing and operating a repository for nuclear waste on the surrounding host rock

Microstructural evolution and its impact on the mechanical strength of typical alkali-activated slag subjected to accelerated carbonation

International audienceThis study aims to comprehensively investigate the evolution of microstructure, mechanical strength, and their correlation in alkali-activated slag (AAS) mortars, designed for application in the immobilization of liquid radioactive waste, under accelerated carbonation conditions (1% CO2, 20 °C and 60% RH). To gain insights into the underlying microstructural changes, CO2 uptake and decalcification of C-A-S-H were analyzed using TGA/DSC and EDS. The pore structure of AASs was systematically assessed across nano- to macro-scales, employing N2-adsorption, MIP, and SEM segmentation. Generally, carbonation led to a decrease in total porosity, primarily attributed to the reduction in meso-macropore volume. However, the pore size distribution of AAS exhibited a complex alteration over varying carbonation durations. Carbonation significantly reduced flexural strength, whereas its effect on compressive strength was comparatively milder. Notably, an evident linear correlation emerged between porosity and compressive strength in both reference and carbonated AASs