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The version of record is available online at: http://dx.doi.org/10.1007/s00603-022-02925-5 In this paper, a reduced-order model built from the reduced basis (RB) method is used as a surrogate for an inverse identification problem related to coupled thermo-hydro-mechanical (THM) processes. The RB space—spanned by solutions of the governing THM equations—is constructed using a greedy adaptive procedure guided by an a posteriori error estimator that selects the optimal snapshot points in a given parametric space. The RB model is assessed in terms of accuracy and computational cost reduction for the three-dimensional transient coupled problem described by the ATLAS III small-scale in situ heating test. The substantial system size reduction and the associated significant computational gain result in a surrogate model suitable for parameter identification procedures in the Boom Clay material. The effectiveness of the proposed strategy is demonstrated by performing inverse analysis based on direct-search and genetic algorithm (GA) optimization supported by real sensor measurement data where 800 times faster computational speed-up was achieved. We acknowledge the funding support from the European Commission Education, Audiovisual and Culture Executive Agency (EACEA) under Erasmus Mundus Joint Doctorate Simulation in Engineering and Entrepreneurship Development (SEED), FPA 2013-0043 and SCK CEN for funding the 4th year PhD study of the first author. S. Zlotnik and P. Díez would like to thank the funding from the Generalitat de Catalunya 2017-SGR-1278, the project DPI2017-85139-C2-2-R funded by the Spanish Ministry and the project H2020-RISE MATHROCKS GA no. 777778.

In recent years, the various health benefits of Cyanobacteria microalgae – such as Arthrospira platensis, commonly called Spirulina, an edible blue-green algae – have attracted scientific attention including micro-level examinations of its bioactive components. As a whole food and nutritional supplement, it serves as a plant protein source, which has shown positive effects across a wide range of human health concerns, from malnutrition to metabolic syndrome. Spirulina bioactives, such as essential amino acids, phycocyanin, polysaccharides, carotenoids, and chlorophyll, and essential vitamins and trace minerals, are responsible for its holistic actions against oxidative stress and inflammation, and its antiviral, antibacterial, and immune-modulating effects. Various in vitro, in vivo, and ex vivo experiments have established Spirulina's mechanism of action and its effect on immunity as a proof of concept. The phenolic compounds and extracellular metabolites released from Spirulina whole food after digestion are postulated to strengthen the epithelial lining with antibacterial effects against pathogenic bacteria, adding to its prebiotic effect on the gut microbiota (like Bifidobacterium and Lactobacillus) due to its fiber content. In this study, the digestibility of Spirulina was assessed by the determination of free amino acids and peptide release during the each phase of digestion in a simulated static digestive model system. The hypothesis bridging poor gut health to low-level inflammation and metabolic syndrome, and the potential to address those issues with nutritional supplementation, such as with Spirulina, could also be beneficial in the long run to reduce comorbid illnesses, such as those associated with the currently prevailing coronavirus disease 2019 pandemic.

The problem of calcium leaching kinetics of ordinary Portland cement based mortars is revisited via a mesoscale approach. Based on state-of-the-art lattice-Boltzmann technique, a comprehensive suite of leaching analysis is undertaken to address a number of open questions such as (i) the competing effects of interface transition zone (ITZ) and aggregates, (ii) relative leaching rates of calcium between ITZ and mortar cement paste, and (iii) the influence of different water to cement ratios, volume fraction of aggregates and hence of ITZ on leaching kinetics. The mesoscale model is not only able to correctly reproduce experimentally observed trends but also confirm the commonly accepted hypothesis that ITZ and aggregates counter-balance their effect leading to similar portlandite leaching rates in cement paste and mortars. The model also confirms the applicability of simple scaling approach for upscaling leaching kinetics from pure cement paste to mortar scale under the condition that ITZ is fully percolated.

This paper provides a comprehensive overview of existing experimental and modelling approaches to determine effective diffusion coefficients of water saturated ordinary Portland cement-based materials. A dataset for diffusivity obtained from different experimental techniques have been presented for cement paste, mortar and concrete. For cement paste at low porosities, diffusivity reported by different authors varies up to a factor of five and electrical resistivity measurements for low capillary porosity are up to one order of magnitude higher compared to other techniques. Experimental data of mortar and concrete reveals predominant influence of increasing tortuosity due to aggregates and limited influence of interface transition zone. Hence, a particular emphasis has been placed on assessing predictability of diffusivity models for cement paste on a larger dataset collected in this paper. It has been observed that all predictive models have similar level of accuracy and fail to predict electrical resistivity data at low capillary porosity as these models are not calibrated using electrical resistivity data.

Supplementary cementitious materials (SCMs) have been widely used not only in civil engineering concrete components, but also in (nuclear) waste treatment engineering because of their beneficial effects on microstructure, engineering properties and durability of concrete. A comprehensive experimental campaign has been undertaken to investigate the effects of SCMs, which includes silica fume (SF) and blast furnace slag (BFS) in combination with ordinary Portland cement (OPC) on the behaviour of hydration heat evolution during early ages of mortars. The samples with different water/cement (w/c) ratios (0.5, 0.7, 0.9) and replacement ratios of SF (10%, 20%, 30%) and BFS (30%, 50%, 70%) were subjected to isothermal calorimetry tests at various temperatures (20, 30, 40 and 50 °C) in order to assess the effects of SCMs on the rate of hydration heat, cumulative heat release, activation energy and setting times of blended mortars. Knowledge obtained from these blended systems was then applied for cementation of a heavy metal containing waste sludge simulant, where the potential for thermal cracking and delayed ettringite formation due to hydration heat generation is of great concern. Results show that both BFS and SF increase the hydration rate but reduce cumulative heat release compared to pure OPC mortar. The ternary system (OPC:BFS:SF) exhibits different hydration characteristics compared to the binary system (OPC:BFS(or SF)) and there is a slight interaction between BFS and SF. The presence of sludge in the matrix significantly accelerates the hydration process and reduces the apparent activation energy. The role of temperature is more important for mortars containing BFS rather than SF, and less pronounced for the system containing sludge. Estimating the setting times based on the isothermal calorimetry data is more accurate for the final setting time rather than for initial setting time and an overestimation of 10% might occur or even more for the system containing sludge, which is still acceptable taking into account the measurement uncertainty.

Near-surface cement-based disposal systems for hazardous materials such as radioactive waste will undergo chemical alterations due to interaction with the surrounding environment. One of the most relevant long-term geochemical alteration processes is decalcification or leaching of the cement phases by percolating water. Consequently, the cementitious components of the disposal system will evolve through different chemical degradation states, also altering physical material parameters such as porosity and bulk density and chemical parameter relevant to solute migration such as the solid–liquid partition coefficient or distribution ratio. This paper presents a novel approach in which geochemical modelling serves as a fundamental basis for assessing the evolution of geochemical conditions within a cement-based near-surface disposal facility. On one hand, geochemical modelling is used to quantify uncertainties related to the infiltrating water composition and C–S–H degradation model, both of which allow for various conceptualisations of the evolution of retardation factor. On the other hand, the concept of mixed tank reactor is used to represent cement degradation within the entire disposal system. This paves the way to establish a link between the evolution of the geochemical conditions and the evolution of the retardation factor via the knowledge of amount of percolated water through the system. The usefulness of the approach is demonstrated via a number of case studies concerning leaching of radionuclides ( 14 C and 94 Nb) from a cementitious near-surface disposal facility. The studies reveal that there is a large effect of the conceptualisation on calculated fluxes from the disposal facility, depending on the type of radionuclide. A crucial factor is the amount of radionuclide mass present in the disposal system when large changes in the retardation factor occur, for instance, when different retardation factors exist in different chemical degradation states.

Analysis of the effect of fractures in porous media on fluid flow and mass transport is of great interest in many fields including geotechnical, petroleum, hydrogeology and waste management. This paper presents sensitivity analyses examining the effect of various hypothetical fracture networks on the performance of a planned near surface disposal facility in terms of radionuclide transport behaviour. As it is impossible to predict the initiation and evolution of fracture networks and their characteristics in concrete structures over time scales of interest, several fracture networks have been postulated to test the sensitivity of radionuclide release from a disposal facility. Fluid flow through concrete matrix and fracture networks are modelled via Darcy's law. A single species radionuclide transport equation is employed for both matrix and fracture networks, which include the processes advection, diffusion, dispersion, sorption/desorption and radioactive decay. The sensitivity study evaluates variations in fracture network configuration and fracture width together with different sorption/desorption characteristics of radionuclides in a cement matrix, radioactive decay constants and matrix dispersivity. The effect of the fractures is illustrated via radionuclide breakthrough curves, magnitude and time of peak mass flux, cumulative mass flux and concentration profiles. For the investigated system, radionuclide properties and the imposed water flow boundary conditions, results demonstrate that: (i) magnitude of peak radionuclide fluxes is less sensitive to the fracture network geometry, (ii) timing of the peak radionuclide fluxes is possibly sensitive to the fracture networks, (iii) a uniform flow model represents a limiting case of a porous medium with large number of fine fractures, (iv) the effect of fracture width on the radionuclide flux depends on the ratio of fracture to matrix conductivity and is less sensitive if the ratio is large and the width is higher than a certain critical size, and (v) increased dispersivity in fractured media influences the transport behaviour differently compared to non-fractured media; in particular, the behaviour depends on the nature of fracture network.

In large cement-based structures such as a near surface disposal facility for radioactive waste voids and cracks are inevitable. However, the pattern and nature of cracks are very difficult to predict reliably. Cracks facilitate preferential water flow through the facility because their saturated hydraulic conductivity is generally higher than the conductivity of the cementitious matrix. Moreover, sorption within the crack is expected to be lower than in the matrix and hence cracks in engineered barriers can act as a bypass for radionuclides. Consequently, understanding the effects of crack characteristics on contaminant fluxes from the facility is of utmost importance in a safety assessment. In this paper we numerically studied radionuclide leaching from a crack-containing cementitious containment system. First, the effect of cracks on radionuclide fluxes is assessed for a single repository component which contains a radionuclide source (i.e. conditioned radwaste). These analyses reveal the influence of cracks on radionuclide release from the source. The second set of calculations deals with the safety assessment results for the planned near-surface disposal facility for low-level radioactive waste in Dessel (Belgium); our focus is on the analysis of total system behaviour in regards to release of radionuclide fluxes from the facility. Simulation results are interpreted through a complementary safety indicator (radiotoxicity flux). We discuss the possible consequences from different scenarios of cracks and voids.

The paper aims to highlight recent developments at the Belgian Nuclear Research Centre SCK•CEN in experimental and numerical study of the coupled physical-chemical behaviour of concrete subject to chemical degradation. The discussion mainly focusses on three interlinked research projects covering novel experimental methods to study the alteration of hydraulic and transport properties during carbonation and calcium leaching, a pore scale numerical model to capture microstructural changes due to the above degradation processes and a generic multiscale model aimed at determining evolution of the properties of a macrostructure over the long term. The paper also describes supplementary continuum scale numerical studies concerning concrete-clay interactions and geochemical impact on the physical structure of concrete. Preliminary findings from these studies show encouraging results such as the development of novel leaching, water permeability and diffusion apparatus, a robust pore scale model based on Lattice-Boltzmann method and a mesoscale study focused on the importance of interfacial transition zones on the effective diffusivity for linear and nonlinear diffusion problems.

Long-term safety of radioactive waste disposal facilities relies on the longevity of natural or engineered barriers designed to minimize the migration of contaminants from the facility into the environment. Especially near surface disposal facilities, such as planned by ONDRAF/NIRAS for the Dessel site in Belgium, long-term safety relies almost exclusively on the containment ability of the engineered barriers (EB) with concrete being the most important EB material used. Concrete is preferred over other materials mainly due to its favourable chemical properties resulting in a high chemical retention capacity, and owing to its good hydraulic isolation properties. However, due to the long time frames typically involved in safety assessment, the chemical, physical and mechanical properties of concrete evolve in time. The alterations in concrete mineralogy also cause changes in pH and sorption behaviour for many radionuclides during chemical degradation processes. Application of dynamic sorption of concrete requires an adequate knowledge of long-term concrete degradation processes, knowledge of the effect of changing mineralogy to sorption of radionuclides and knowledge of large-scale system behaviour over time. Moreover, when applied to safety assessment models, special attention is required to assure robustness and transparency of the implementation. The discussion in this paper focuses on the sorption properties of concrete; selection of data, rescaling issues and on the hypotheses used to build a robust and yet transparent dynamic model for large-scale concrete structures for assessing the long-term performance.

In this chapter the water flow and contaminant transport processes in the unsaturated or vadose zone are described. These processes include water retention and hydraulic conductivity, evapotranspiration, preferential flow, root water uptake (water flow) and diffusion, dispersion, advection and volatilization (contaminant transport). The equation governing transport of dissolved contaminants in the vadose zone is obtained by combining the contaminant mass balance with equations defining the total concentration of the contaminant and the contaminant flux density. Further attention is this chapter is given to nonequilibrium transport, stochastic models, multicomponent reactive solute transport, multiphase flow and transport. Mathematical models should be critical components of any effort to understand and predict site-specific subsurface water flow and contaminant transport processes. Generally, models range from relatively simple analytical approaches for analyzing contaminant transport problems during one-dimensional steady-state flow, to sophisticated numerical models for addressing multi-dimensional variably-saturated flow and contaminant transport problems at the field scale. An overview is given of several existing analytical and numerical models. Moreover, several applications to unsaturated flow and geochemical transport modeling are presented in this chapter.