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Cut-off walls built using self-hardening slurries are an important tool for modern engineering pursuing Sustainable Development Goals. Much like cement concrete, this material is affected by the challenges posed by the increasing human pressure on the environment, although it is used significantly less widely than concrete; for this reason, relatively little comprehensive literature data is available describing the interaction of self-hardening slurries with the environment. This article provides a review that complements the current state of knowledge on self-hardening slurries in this area, with a particular focus on the durability of the material and its pollutant immobilization capabilities. To provide context, the material’s operating conditions, properties and components are briefly characterized. The resistance of self-hardening slurries to environmental aggression is described extensively, as it is a key factor in ensuring the durability of the material. A sample analysis of the material’s carbon footprint in several representative composition variants is presented. The subject of pollutant immobilization by self-hardening slurries is outlined. Lines of further research are proposed to fill gaps in the available knowledge.

Cut-off walls built using self-hardening slurries are an important tool for modern engineering pursuing Sustainable Development Goals. Much like cement concrete, this material is affected by the challenges posed by the increasing human pressure on the environment, although it is used significantly less widely than concrete; for this reason, relatively little comprehensive literature data is available describing the interaction of self-hardening slurries with the environment. This article provides a review that complements the current state of knowledge on self-hardening slurries in this area, with a particular focus on the durability of the material and its pollutant immobilization capabilities. To provide context, the material's operating conditions, properties and components are briefly characterized. The resistance of self-hardening slurries to environmental aggression is described extensively, as it is a key factor in ensuring the durability of the material. A sample analysis of the material's carbon footprint in several representative composition variants is presented. The subject of pollutant immobilization by self-hardening slurries is outlined. Lines of further research are proposed to fill gaps in the available knowledge. [ABSTRACT FROM AUTHOR]

This paper examines the effects of multicycles of desorption/adsorption on the water isotherms of a typical cement–bentonite mixture. These water isotherms were evaluated over a relative humidity (RH) range of 90% and 0.3%. This study shows that, after the first adsorption–desorption cycle, the subsequent cycle shifted the water isotherm curves upward, particularly at RH < 70%, with a high degree of hysteresis. In contrast, at RH > 70%, the water isotherm curves shifted slightly downward with a low degree of hysteresis. The overall degree of hysteresis slightly decreased with the subsequent cycles. It is postulated that multi-adsorption–desorption cycles led to microstructural changes in the CB porous system as the CB material's water retention ability was enhanced by increasing isotherm cycles. [ABSTRACT FROM AUTHOR]

The paper discusses the results of an experimental research on the short and long term hydraulic performance and sorption of cement-bentonite mixtures in saline sulphate solutions. Batch tests at different curing times and permeation tests were performed on three cement-bentonite mixtures with solutions of K2SO4. The results show that permeation can adversely affect the hydraulic performance of cement-bentonite mixtures depending on the sulphate concentration and mixture composition and that the choice of a very low permeable mixture is essential to limit adverse effects of interaction with SO42- on the hydraulic performance.Sorption of K+ was found to be significant, well described by a linear isotherm in a wide range of concentration and nearly constant with curing. Sorption of SO42 was found to decreases with mixture porosity. Since sorption of SO42- means negative interaction, batch tests were found to be useful for a rapid estimation of possible negative effect of SO42- on the hydraulic behaviour in the long term, when selecting a mixture in the design phase.All the results point out the importance of selecting a mixture to be used for cut-off walls with a low hydraulic conductivity already at brief curing and of carefully considering interaction with solute pollutants and sorption capacity.

Despite the extensive use of cement–bentonite in contaminated land containment applications, there are still many challenges related to durability. The development of self-healing cement–bentonite materials could provide more resilient, sustainable and reliable cut-off walls with significantly enhanced durability, reduced maintenance costs, enhanced safety and protection against sudden or undetected failure. The objective of this study was to develop self-healing cement–bentonite cut-off wall materials incorporating microcapsules. Microencapsulated sodium silicate, as a healing agent, would be released into cracks when the microcapsules rupture as a result of any damage incurred, and would react with the cement–bentonite matrix to fill and heal the cracks. Novel microcapsules with switchable mechanical properties developed for self-healing cement applications were employed here. The results demonstrated the enhanced average crack mouth healing and recovered permeability performance provided by the microcapsules compared to control cement–bentonite samples. X-ray microcomputed tomography and scanning electron microscopy were applied to investigate the self-healing process of the microcapsule-containing cement–bentonite system. The microstructural analysis confirmed the survivability, uniform dispersion and crack-triggered rupture of the microcapsules, as well as the release of the healing agent and the generation of hydration products within the cracks. These are promising results for the application of the microcapsule-based system for self-healing cement–bentonite cut-off wall materials. [ABSTRACT FROM AUTHOR]

Cement bentonite (CB) barriers are self-supporting, low permeability, structures used to retard groundwater flow and as such strength and hydraulic conductivity parameters are often stipulated when developing the mixtures. This paper reports an investigation into the deformation and compression behaviour of a CB containing ground granulated blastfurnace slag using the unconfined compressive strength apparatus, triaxial (undrained, unconsolidated) and oedometer. Samples were also exposed to drying and rewetting to investigate possible response to changes in environmental conditions. Cracking was observed prior to peak stress suggesting that the hydraulic conductivity of a barrier may be adversely affected before the shear strength is reached in undrained conditions. The compression response of CB indicates the presence of a threshold stress; once exceeded the magnitude of settlements are significantly greater than those encountered below this threshold. If a barrier experiences localised changes in loading conditions then there is the potential for damage from induced differential settlements; thus it is recommended that the threshold stress should also be considered at the design stage in addition to strength and hydraulic conductivity requirements. The response of the material exposed to drying-rewetting was unexpected and requires further investigation to determine how a barrier will respond to changing environmental conditions. [ABSTRACT FROM AUTHOR]

The paper discusses the results of an experimental research on the short and long term hydraulic performance and sorption of cement-bentonite mixtures in saline sulphate solutions. Batch tests at different curing times and permeation tests were performed on three cement-bentonite mixtures with solutions of K 2 SO 4. The results show that permeation can adversely affect the hydraulic performance of cement-bentonite mixtures depending on the sulphate concentration and mixture composition and that the choice of a very low permeable mixture is essential to limit adverse effects of interaction with SO 4 2- on the hydraulic performance. Sorption of K+ was found to be significant, well described by a linear isotherm in a wide range of concentration and nearly constant with curing. Sorption of SO 4 2 was found to decreases with mixture porosity. Since sorption of SO 4 2- means negative interaction, batch tests were found to be useful for a rapid estimation of possible negative effect of SO 4 2- on the hydraulic behaviour in the long term, when selecting a mixture in the design phase. All the results point out the importance of selecting a mixture to be used for cut-off walls with a low hydraulic conductivity already at brief curing and of carefully considering interaction with solute pollutants and sorption capacity. [ABSTRACT FROM AUTHOR]

Cementitious materials will act as a source of alkaline fluids modifying the mineralogy, pH and physical–chemical properties of bentonite barriers implemented for long-term pollutants retention. A series of reactivity experiments in batch reactors at 175°C were performed for nearly 15months to evaluate the chemical and mineralogical responses of four different bentonites with potential use as buffer barriers against a hyperalkaline K–Na–OH-type solution. This solution simulates the early stage of an ordinary Portland cement leaching in a clay hosted nuclear waste repository. The tested buffer materials were three montmorillonitic bentonites: MX-80 (Wyoming type, USA), FEBEX (Almería, Spain), and a bentonite from Chile (MMT); and a saponitic bentonite from el “Cerro del Águila” (Toledo, Spain). Partial dissolution of smectite and precipitation of secondary phases such as analcime and K-feldspar (sanidine or orthoclase) were found as reaction products in the dioctahedral smectite-based bentonites whereas the trioctahedral character bentonite showed more resistance to alteration. In addition, mixed-layered illite/smectite (I–Sm) interstratification was observed in the four bentonites with different degrees of order. The most heterogeneous octahedral sheet with significant Fe and Mg occupations (FEBEX and MX-80 bentonites) altered to produce R0 random mixed-layered I–Sm (60–40% I), whereas virtually ideal Al–Mg octahedral sheet (MMT bentonite) altered to produce saponite and R≥1 ordered I–Sm (>80% I). The physical–chemical properties (CEC and SSA) were also altered differently in the four bentonites. The methodology used in this work is thought to develop a direct comparison of the performance of potential buffer materials in a nuclear or hazardous waste repository. The evaluation of other basic hydro-mechanical properties should be further evaluated to serve as an indicator of long-term stability of bentonites used in different reactive environments. [ABSTRACT FROM AUTHOR]

In spite of the well-established design and construction approaches of slag–cement–bentonite slurry walls, the materials deteriorate inevitably in contaminated land. The development of effective materials which are sustainable, resilient and self-healing over the lifetime of slurry walls becomes essential. This study, for the first time, adopts a styrene–ethylene/butylene–styrene (SEBS) polymer to modify slag–cement–bentonite materials to enhance mechanical and self-healing performance. The results show that the increase in SEBS dosage results in significantly increased strain at failure, indicating the enhanced ductility thanks to the modification by the deformable polymer. The increased ductility is beneficial as the slurry wall could deform to a greater extent without cracks. After the permeation of liquid paraffin, the SEBS exposed on the crack surface swells and seals the crack, with the post-healing permeability only slightly higher than the undamaged values, which exhibits good self-healing performance. Scanning electron microscopy and micro-computed tomography analyses innovatively reveal the good bonding and homogeneous distribution of SEBS in slag–cement–bentonite. SEBS acts as a binder to protect the slag–cement–bentonite sample from disintegration, and the swollen SEBS particles effectively seal and heal the cracks. These results demonstrate that the SEBS-modified slag–cement–bentonite could provide slurry walls with resilient mechanical properties and enhanced self-healing performance. [ABSTRACT FROM AUTHOR]