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Die Hauptfunktion von Verschlussbauwerken für End- und Zwischenlager von radioaktiven Abfällen besteht in der Erhaltung der geologischen Barriere. Vor allem soll ein potenzieller Stofftransport durch eine möglichst geringe Permeabilität des Bauwerks verhindert bzw. auf vernachlässigbare Werte herabgesetzt werden. Die in-situ-Permeabilität des Verschlussbauwerks steht dabei in direktem Zusammenhang mit dessen Poren- und Makrostruktur, welche bei Bauwerken auf Basis von Beton insbesondere durch Schwind-verformungen und durch thermisch induzierte Rissbildung infolge der Reaktionswärmeentwicklung des Bindemittels gestört werden kann. Die Betone müssen daher neben einer hohen chemischen Langzeitstabilität auch eine geringe bzw. langsame Wärmeentwicklung während der Erhärtungsreaktionen aufweisen. Als hybride Zemente werden Mischungen aus Portlandzement, Betonzusatzstoffen und einem alkalischen Aktivator bezeichnet. Dabei können u. a. Alkalisulfate, Alkalicarbonate, Alkalisilicate und Alkalihydroxide als Aktivatoren zum Einsatz kommen. Aufgrund ihrer hohen chemischen Stabilität im salinaren Milieu sind Betone aus solchen Zementen potenziell besonders gut als Verfüllmaterial für End- und Zwischenlager im Steinsalz geeignet. In der vorliegenden Studie wurden daher hybride Zemente hinsichtlich ihrer Wärmeentwicklung in einem isothermen Kalorimeter sowie hinsichtlich Phasenbestand und Festigkeiten untersucht. Hybride Zementleime wurden auf Basis von Portlandklinker, Hüttensandmehl, Flugasche und Natriumsulfat hergestellt und die Zusammensetzungen systematisch variiert, um den Einfluss der Komponenten auf Wärmeentwicklung, Phasenbestand und mechanische Eigenschaften der Leime zu untersuchen; zusätzlich wurden zu Vergleichszwecken ein Zementleim auf Basis der Betonrezeptur M2 sowie eine alkalisch aktivierte Flugasche untersucht.
One-part geopolymers offer advantages over conventional geopolymers with regard to handling and storage of feedstocks. However, they often suffer from a low degree of reaction, a high amount of crystalline byproducts, and consequently low strength. In this study, one-part geopolymers were produced from rice husk ash (RHA) and sodium aluminate, and investigated by XRD, ATR-FTIR, SEM and compressive strength testing. The compressive strength of the material was 30 MPa, i.e. significantly higher than for comparable one-part geopolymers. This is attributed to an almost complete reaction of the RHA and the absence of crystalline byproducts (zeolites) in the hardened geopolymer.
Geopolymers are promising alkali activated materials with a wide range of possible applications like heat-resistant coatings and adhesives or concretes with lower C02 emissions than conventional Portland cement-based systems. Conventional ("two-part") geopolymers are synthesized by an activation of aluminosilicate feedstocks like metakaolin or blast furnace slags with highly alkaline solutions like sodium hydroxide solutions and/or water glass solutions.
A more recent approach are so called one-part geopolymers, where the alkaline activator is provided in solid form, so that only water has to be added to initiate the geopolymerisation reaction. The handling of alkaline solutions is avoided, which possesses several advantages compared to conventional geopolymers, in particular regarding safety and health issues, and, thus, it improves economic and social acceptance of these binders. However, the new formulations are less exhaustively studied and the knowledge about conventional geopolymers cannot simply be transferred to this new class of binders.
This contribution presents the results of structural and compressive strength investigations on cured andhigh-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens weresynthesized from two different silica sources, sodium aluminate and water. The phase content as well asthe compressive strength of the cured composites varied depending on the starting mix-design and thesilica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reactionproducts. One of the silica feedstocks yielded significantly higher compressive strength (19 MPa), whilethe other one appears to cause less variation in phase content. Strength testing indicated an improvementon heating up to
200–400 °C (28 MPa) followed by a moderate decrease up to 700 °C. Above 700 °C the sys-tems underwent new phase formation and shrinkage (volume decrease) deformations. After exposureat 1000 °C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexago-nal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the hightemperature phase transformations varied.
One-part alkali-activated (geopolymer) mortars based on three different silica-rich starting materials and sodium aluminate, with and without ground granulated blast furnace slag (GGBFS) addition, were tested regarding sulfuric acid resistance according to DIN 19573:2016-03 (70 days at pH = 1). Corresponding pastes were characterized by XRD, SEM, chemical analysis, 29Si MAS NMR and 1H-29Si CPMAS NMR after water storage and after acid exposure. The mortars exhibited a high resistance against sulfuric acid attack, with the best ones conforming to the requirements of DIN 19573:2016-03. The analytical results showed that this was due to precipitation of silica gel at the acid-mortar interface, which formed a mechanically stable layer that protected the subjacent mortar and thus inhibited further degradation. The addition of GGBFS decreased the acid resistance via formation of expansive calcium sulfate phases.
While calcined clays in general have been credited with a great potential to mitigate CO2 emissions related to cement production and consumption, calcined brick clays are currently understudied in this regard. In the present work, two brick clays, a low-grade kaolinitic clay, and a mixed clay composed of 50% brick clay and 50% low-grade kaolinitic clay were studied regarding transformations on calcination, and strength and durability performance as well as pore structure of mortars made with the blended cements. All calcined clays exhibited pozzolanic reactivity, with the performance of the brick clays inferior to the low-grade kaolinitic clay. However, the mixed clay performed very similar to the low-grade kaolinitic clay, which points to a viable option for optimal use of brick clays in cementitious systems. The carbonation resistance of the blended cement mortars was generally worse than that of the plain Portland cement mortar, as expected, but the former exhibited a significantly improved chloride penetration resistance. The latter improvement was due to pore structure refinement in the blended cement mortars, compared to the Portland cement mortar.
Two brick clays (rich in 2:1 clay minerals) and a low-grade kaolinitic clay were studied regarding their transformations during calcination and their performance in blended cement mortars. The mortars with calcined clays exhibited decreased workability (slump flow), but this effect could be mitigated by employment of a conventional superplasticizer; however, compressive strength of the hardened mortar was lowered in some cases. While the kaolinitic clay generally yielded the highest strength, the performance of a brick clay could be increased by grinding to higher fineness and by mixing it with the kaolinitic clay.
Additive manufacturing of alkali-activated materials currently attracts a lot of attention, because of the possibility to produce customized high-performance elements for a range of applications, potentially being more resource-efficient than conventionally produced parts. Here, we describe a new additive manufacturing process for alkali-activated materials that is based on selective laser-heating of lithium aluminate/microsilica slurries. The new process-material combination allows to manufacture elements with complex geometries at high building rates and high accuracy. The process is versatile and transferrable to structures of sizes differing by orders of magnitude. The mechanical strength of the obtained materials was in the range of values reported for conventional metakaolin-based geopolymers, and superior to what has been hitherto reported for alkali-activated materials produced by additive manufacturing. This mechanical performance was obtained despite the fact that the degree of reaction of the lithium aluminate and the microsilica was low, suggesting that significant reactions took place only at the surface of the microsilica particles.
Einkomponentige alkaliaktivierte Bindemittel (AAB), d. h. AAB, bei denen der Aktivator als Feststoff im Bindemittel vorliegt, weisen Vorteile hinsichtlich der praktischen Anwendung im Bauwesen gegenüber konventionellen AAB auf. Einkomponentige AAB lassen sich als Gemische aus reaktiver Silica und Natriumaluminat herstellen, wobei die Reaktionsprodukte von der eingesetzten Silica und den Verhältnissen Na2O/Al2O3/SiO2/H2O abhängen: Während Mikrosilica und ähnliche Reststoffe zu zeolithreichen Produkten führen, erhält man aus Reisschalenasche vollständig amorphe Gele (sog. «Geopolymere »). Mörtel auf Basis solcher Bindemittel entsprechen den Anforderungen von DIN 19573:2016-03 hinsichtlich des Widerstands gegen Schwefelsäureangriff (Prüfung bei pH = 1 für 70 d) und sind damit potentiell als Reparatursysteme für Abwasserinfrastruktur geeignet. Untersuchungen mittels 29Si-MAS-NMR und 1H-29Si-Kreuzpolarisations(CP)-MAS-NMR sowie REM-Aufnahmen von erhärteten und dem Schwefelsäureangriff ausgesetzten AAB zeigen, dass der hohe Säurewiderstand v. a. auf die Fällung von Silicagel an der Grenzfläche Mörtel/Säure zurückzuführen ist, wodurch der Korrosionsfortschritt in darunterliegenden Schichten verlangsamt wird.
The use of calcined clays as supplementary cementitious materials (SCMs) has been identified as a viable option to decrease the CO2 emissions related to cement production. However, while extensive data is available about kaolinitic clays in this context, other clays such as illitic clays appear to be under-studied. Therefore, in the present study, two illitic clays were compared to two low-grade kaolinitic clays in terms of transformations in the calcination temperature range 650–900 °C, and performance of the calcined clays in blended cement pastes as measured by strength evolution, heat release, hydrated phase formation and portlandite consumption. The illitic clays required a higher calcination temperature for complete dehydroxylation of their illite than what is necessary for dehydroxylation of kaolinite. These higher calcination temperatures also led to particle sintering, significantly decreasing the specific surface area of the illitic clays, particularly for the clay with the higher Fe2O3 content. Nevertheless, while the kaolinitic clays generally exhibited the best performance as SCM, the illitic clay with lower Fe2O3 content performed similar to the kaolinitic clays when calcined at optimum temperature and applied at a moderate substitution rate. These findings demonstrate that several different clays have the potential to be used as SCM and indicate possible routes to identify suitable deposits for this purpose.
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.
Mixing and curing of geopolymers and related alkali-activated materials without storage and handling of highly alkaline solutions possesses advantages regarding safety and economic viability. One possible approach is to produce these materials from solid silica feedstocks and solid sodium aluminate, and subsequent mixing with water. We present a comparison between geopolymers and aluminosilicate gel-zeolite composites synthesized by this route from different silica feedstocks (by-product silica from chlorosilane production, microsilica, rice husk ash) and with different SiO2/Al2O3 ratios, using results from XRD, NMR, SEM, thermal analysis, mechanical and acid resistance testing. The use of rice husk ash favors formation of a fully amorphous geopolymer with high strength. Utilization of the other silica feedstocks leads to formation of aluminosilicate gel-zeolite composites, the amount and kind of zeolites depending on the feedstock and the SiO2/Al2O3 ratio. These composites show beneficial dehydration behavior, viz. no distinct dehydration step of thermal strain, with the phase assemblage after heating to 1000°C controlled by the starting composition. Mortars produced from both, the geopolymers as well as the composites, exhibit high resistance to sulfuric acid attack, making them promising materials for the construction and the repair of industrial and sewer structures.
One-part geopolymers, synthesized from three different silica sources and sodium aluminate, were investigated regarding their microstructure and the evolution of their compressive strength on curing at slightly elevated temperatures. The effect of thermal treatment was studied up to 1000 °C and for the investigation of the chemical durability specific samples were treated with sulfuric acid (H2SO4, pH = 1).
Depending on the silica feedstock, different degrees of reaction were observed. One of the silicas had a significantly higher reactivity. For two of the silicas, significant amounts of zeolites occurred as reaction products besides geopolymeric gel, whereas the more reactive silica source lead to the formation of a rather fully condensed geopolymeric network.
The composites indicated promising behavior on heating in so far as no distinct shrinkage step occurred in the temperature range of dehydration. Up to 400 °C the residual strength of those mixes increased. Above 800 °C the samples underwent new phase formation. After exposure to sulfuric acid (pH 1; 70 d) specimens showed a residual compressive strength of about 77 % of the reference, indicating high acid resistance.
Mortars for application on concrete, e.g. repair mortars or protective coatings, need to have a durable bond to the substrate. This bond is determined by the adhesion between the two materials and by the differential deformations of the mortar and the substrate. In the present contribution, the hygric deformations (shrinkage/expansion) of novel one-part alkali-activated mortars and their bond to concrete substrates are studied. Shrinkage of the mortars was studied at 50 % r.H., while expansion was studied on mortars stored over an open water surface (> 99% r.H.). The bond behaviour was studied by pull-off tests according to DIN EN 1542 and by optical microscopy.
The alkali-activated mortars exhibit hygric deformations much lower than the deformations of an established, commercial mortar for sewer maintenance that was tested as reference in parallel with the alkali-activated mortars. The bond behaviour of the alkali-activated mortars depends strongly on their mix-design and curing. Optical microscopy showed that in the
mortars with lower bond strength, cracks developed in the mortar during curing. Mortars with appropriate mix-design and curing did not exhibit cracking, and their pull-off strength (up to >3 MPa) conformed to the requirements of relevant standards.
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.