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Properties of alkali-activated mortars with salt aggregate for sealing structures in evaporite rock
(2021)
Concrete structures for sealing of tunnels in the host rock are an essential part of systems for nuclear waste storage. However, concretes based on blended cements or magnesium oxychloride cements, which are commonly considered for this application, can deteriorate severely due to a significant heat of hydration and associated deformation and cracking. Alkali-activated materials (AAMs) offer a potential solution to this problem because of their low heat release during hardening. To explore their suitability for the construction of sealing structures in evaporite rock, various AAMs with salt aggregate were studied regarding fresh properties, heat release, mechanical properties and microstructure. The heat of reaction of the AAMs was up to 55% lower than that of a blended cement designed for sealing structures, indicating significant benefits for the intended application. Other relevant properties such as mechanical strength and permeability depended strongly on the mix-design of the AAMs and curing conditions.
Within the project SealWasteSafe, we advance construction materials and monitoring concepts of sealing structures applied for underground disposal of nuclear or toxic waste. As these engineered barriers have high demands concerning integrity, an innovative alkali-activated material (AAM) is improved and tested on various laboratory scales. This AAM has low reaction kinetics related to a preferential slow release of the heat of reaction in comparison to alternative salt concretes based on Portland cement or magnesium oxychloride cements. Hence, crack formation due to thermally induced strain is reduced. After successful laboratory scale analysis (Sturm et al., 2021), the AAM is characterised on a larger scale by manufacturing test specimens (100–300 L).
Conventional salt concrete (DBE, 2004) and the newly developed AAM are compared using two specimen geometries, i.e. cylindrical and cuboid. A comprehensive multisensor monitoring scheme is developed to compare the setting process of AAM and salt concrete for these manufactured specimens. The analysed parameters include temperature and humidity of the material, acoustic emissions, and strain variations. Passive sensor systems based on radiofrequency identification technology (RFID) embedded in the concrete, enable wireless access to temperature and humidity measurements and are compared to conventional cabled systems. Additionally, fibre-optic sensors (FOS) are embedded to record strain, but also have potential to record temperature and moisture conditions. Part of this project aims at demonstrating the high reliability of sensors and also their resistance to highly alkaline environments and to water intrusion along cables or at sensor locations. Further technical improvements were implemented so that first results clearly show the scalability of the setting process from previous small-scale AAM experiments and particularly the high potential of the newly developed approaches.
Furthermore, ultrasonic methods are used for quality assurance to detect obstacles, potential cracks and delamination. On the one hand, both active and passive ultrasonic measurements complement the results obtained from the multisensor monitoring scheme for the produced specimens. On the other hand, the unique large aperture ultrasonic system (LAUS) provides great depth penetration (up to nearly 10 m) and can thus be applied at in situ sealing structures built as a test site in Morsleben by the Federal Company for Radioactive Waste Disposal (Bundesgesellschaft für Endlagerung, BGE) as shown by Effner et al. (2021). An optimised field lay-out identified from forward modelling studies and advanced imaging techniques applied to the measured data will further improve the obtained results. To characterise the inside of the test engineered barrier and achieve a proof-of-concept, an ultrasonic borehole probe is developed to enable phased arrays that can further improve the detection of potential cracks. Modelling results and first analysis of semispherical specimens confirmed the reliability of the directional response caused by the phased arrays of the newly constructed ultrasonic borehole probe.
Overall, the project SealWasteSafe improves the construction material, multisensor monitoring concepts and ultrasonics for quality assurance. This will help to develop safe sealing structures for nuclear waste disposal. The outcomes are particularly valuable for salt as a host rock but partly also transferrable to alternative conditions.
In this work, engineered recycled mineral admixtures were obtained from four different industrial residues: basic oxygen furnace slag, iron ore tailings, quartz mining tailings and quartzite mining tailings.
The grinding performance was evaluated in two different programs and the characterization included chemical and mineralogical composition, particle morphology, and physical properties. Performance evaluations were carried out in blended pastes and mortars, including flow properties, hydration kinetics, soundness, pozzolanic activity, and compressive strength. Coarser-than-cement admixtures allowed better flow performance, greater dimensional stability and more economical production, while finer-thancementn admixtures improved mechanical performance by both filler effect and cementing activity.
One of the major tasks in nuclear waste management is to design safe and reliable sealing structures for radioactive waste repositories. Particularly for salt as a host rock, engineered barrier systems for underground waste disposal must be constructed of well-understood (cementitious) materials that will sustain environmental conditions and ensure high durability.
Within the SealWasteSafe project, we studied two materials with a potential for sealing structures for nuclear waste repositories: an innovative alkali-activated material (AAM) and standard salt concrete (SC). To analyze the development of microstructural changes within the materials, we monitored microseismicity (aka acoustic emission monitoring) occurring during the hardening and setting period in two 340-liter-cubic specimens for up to ~250 days.
The monitoring results showed, that in the first 61 days, fewer events occurred in AAM compared to SC. After this time the number of events in AAM sharply increased and significantly exceeded those observed in SC. However, the source localization analysis revealed that the increase of microseismicity in AAM material was not related to the formation of macro-cracks within the material but was mainly caused by the surface effects. Accompanying analysis of the test specimens with additional methods (such as e.g., X-ray CT) proved that no macro-cracks were observed inside the AAM specimen. Further analysis in the time and frequency domains helped to characterize the tested materials and to estimate their potential to be used for engineered barriers in nuclear waste repositories.
Overall, our study shows the potential of microseismicity monitoring for feasibility studies and quality assurance in a broad range of applications, also in structural health monitoring.
The present work addresses powder bed binder jetting additive manufacturing by selective magnesium phosphate cement activation. Despite the potential of this technology to aid the digitalization of the construction industry, the effect of processing parameters on the mechanical performance of printed materials has not yet been studied to generate a guideline for the further development of the technology. Statistical methodologies were used to screen the effect of four printing process parameters (printing speed, layer thickness, raster angle, and build direction on flexural and compressive strength). As the exploited technology works with constant fluid pressure, the physical interpretation of the effect of each factor can be considered taking into account the interactions between the binder materials in the powder bed. Analysis of variance (ANOVA) indicated that printing speed and layer thickness significantly affect mechanical performances. Furthermore, the layout of samples for the printing process is preferable to be parallel the printhead movement. An anisotropic behavior was observed, and the samples subjected to compressive forces parallel to the layer plane possessed lower strength values. This effect can be interpreted as a result of a weak area of low density in between layers, leading to a pronounced delamination under compression. Even though the strength of the printed material is not suitable for a structural concrete, it can be marginally improved by design of experiment and optimized for non-structural applications, such as for porous artificial stone. Design of experiment coupled with ANOVA methods can be used in the future to support the development of novel material mixtures, thus expanding the fields of application of this novel additive manufacturing technology.