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A silica residue from waste treatment of chlorosilane production was used together with solid sodium aluminate to test its applicability for the production of one-part geopolymers. The blend was activated with water and cured at 70 °C. The degree of reaction and strength were determined after 1, 3, and 7 days. The reaction products were analyzed by XRD and SEM/EDX. Until the third day of curing the degree of reaction of the residue reached 51 % and the strength was 8.9 MPa. The reaction product was identified as geopolymer containing zeolite A. Thus, the results confirmed that the residue may be used in the production of geopolymers. However, after 3 days of curing no further progress of reaction was observed and the strength slightly decreased, which was attributed to changes in the structure of the geopolymeric gel. It was further observed that even harsh vacuum drying left some water (presumably zeolitic water and surface hydroxyl groups) in the geopolymer.
Trotz der zahlreichen Möglichkeiten zur Beeinflussung der Mörteleigenschaften durch Mischungsmodifikationen ist es nahezu unmöglich, einzelne Eigenschaften unabhängig von anderen zu verändern. Bei einer Mischungsoptimierung muss im Einzelfall immer entschieden werden, welcher der gewünschten Eigenschaften die höchste Priorität zukommt. Anhand von Parameterstudien wird im vorliegenden Artikel verdeutlicht, wie es für bestimmte Anwendungen sinnvoll sein kann, die Verschlechterung einzelner Eigenschaften zu akzeptieren, wenn dadurch eine für den speziellen Einsatzfall essenzielle Eigenschaft verbessert werden kann. Die komplexen Wechselwirkungen der einzelnen Bestandteile fuhren dazu, dass auch scheinbar widersprüchliche Kombinationen von Zusatzmitteln im Einzelfall erforderlich werden können. Zur Anpassung der meisten Mörteleigenschaften gibt es verschiedene Möglichkeiten der Zielerreichung mit jeweils unterschiedlichen Nebeneffekten. Frisch- und Festmörteleigenschaften sind dabei gleichermaßen zu beachten.
Derzeit erfolgt in mehreren Arbeitsgruppen unter Einbindung interessierter Kreise die Überarbeitung der 2001 erschienenen Richtlinie 'Schutz und Instandsetzung von Betonbauteilen' (Instandsetzungsrichtlinie) des Deutschen Ausschusses für Stahlbeton e.V. (DAfStb). Hierüber wird in dem nachfolgenden Beitrag berichtet, um der Fachwelt frühzeitig darzustellen, in welche Richtung die Fortschreibung des Regelwerkes geplant ist.
Die geplante neue Richtlinie wird nach derzeitigem Beratungsstand eine deutliche inhaltliche Erweiterung erfahren. Daher wird sie in 'Richtlinie für die Instandhaltung von Betonbauteilen' (Instandhaltungsrichtlinie) umbenannt und wird Festlegungen zu der grundsätzlichen Vorgehensweise bei der Instandhaltung unter Berücksichtigung der Komponenten Wartung, Inspektion, Instandsetzung und Verbesserung in Anlehnung an DIN 31051 enthalten. Da diese umfassenderen Aufgaben deutlich präziser beschrieben werden, konkretisieren sich auch die Anforderungen an die erforderliche Qualifikation des sachkundigen Planers, der zukünftig verstärkt die 'Restnutzungsdauer' bei seiner Planung beachten und in entsprechende Maßnahmen umsetzen muss.
Mit den Ergänzungen und Änderungen wird die Richtlinie an die aktuellen Entwicklungen und Regelungen im Betonbau angepasst. Gleichzeitig gewinnt die Planungsleistung erheblich an Bedeutung. Für Instandsetzungsprodukte gemäß DIN EN 1504 werden, die Beseitigung derzeit noch enthaltener Unklarheiten und technischer Defizite auf europäischer Ebene vorausgesetzt, Verwendbarkeitsmöglichkeiten aufgezeigt. Die Ausführung von Instandhaltungsmaßnahmen wird durch die Einführung von Überwachungsklassen mit zugehörigen Anforderungen an die durchzuführenden Prüfungen spezifiziert und damit konkretisiert.
Errichtet aus Glas und Beton prägen heute unzählige Sakralbauten der 1950er- und 60er-Jahre unsere Stadtbilder. Nicht selten sind in diesen Kirchen erhebliche Schäden an den Betonglasfenstern zu verzeichnen. Diese fachgemäß zu erhalten, erfordert spezielles technisches und denkmalpflegerisches Know-how. An der Entwicklung geeigneter Instandsetzungsstrategien arbeitet derzeit ein interdisziplinäres Projektteam.
Wärmebehandlung und Autoklavierung von UHPC - Neue Perspektiven für die Betonfertigteilindustrie
(2011)
Das Gefüge von UHPC ist durch eine dicht gepackte und fehlstellenarme Matrix gekennzeichnet. Eine zusätzliche Optimierung ist durch eine Wärmebehandlung bzw. hydrothermale Nachbehandlung (Autoklavierung) unter einem Wasserdampfsättigungsdruck möglich. Die Wärmebehandlung von Beton ist ein in der Betonfertigteilindustrie geläufiges Verfahren, um die Erstarrungszeit zu verkürzen und höhere Frühfestigkeiten zu erreichen.
Although being a very promising area of concrete technology, computational modeling of fresh concrete flow is a comprehensive and time consuming task. The complexity and required computation time are additionally increased when simulating casting of heavily reinforced sections, where each single reinforcement bar has to be modeled. In order to improve the computation speed and to get closer to a practical tool for simulation of casting processes, an innovative approach to model reinforced sections is proposed here. The basic idea of this approach is to treat the reinforcement zone as a porous medium in which a concrete is propagating. In the present paper, the numerical implementation of this concept is described. A methodology allowing for the computation of the equivalent permeability of the steel bars network is suggested. Finally, this numerical technique efficiency is evaluated by a comparison with experimental results of model fluids casting in model formworks.
Reaction products and strength development of wastepaper sludge ash and the influence of alkalis
(2014)
Wastepaper sludge ash (WSA) from a newsprint paper mill was investigated for its mineralogical composition and its reaction products and strength development after activation with water and sodium and potassium hydroxide solutions. The results showed the WSA to consist of calcite, free lime, gehlenite, tricalcium aluminate, belite, talc, quartz and probably a glassy phase. The principle reaction product was monocarboaluminate (CO3–AFm) for the water- as well as for the alkali-activated WSA. Formation of monocarboaluminate and strength gain was more rapid for the alkali-activated WSA until 1 day of curing. However, afterwards reactions proceeded much slower when alkali solutions were used, leading to an about twice as high compressive strength for water-activated WSA mortars after 28 days of curing. The observed behavior is tentatively ascribed to a less uniform microstructure of the alkali-activated WSA. Significant differences between NaOH- and the KOH-activated WSA were not observed.
The thermal gravimetric analysis (TG) is a common method for the examination of the carbonation progress of cement-based materials. Unfortunately, the thermal properties of some components complicate the evaluation of TG results. Various hydrate phases, like ettringite (AFt), C-S-H and AFm decompose almost simultaneously in the temperature range up to 200 °C. Additionally, physical bound water is released in the same temperature range. In the temperature range between 450 °C and 600 °C the decomposition of calcium hydroxide and amorphous or weakly bound carbonates takes place simultaneously. Carbonates, like calcite, from limestone powder or other additives may be already contained in the noncarbonated sample material. For this research an attempt was made to minimise the influence of these effects. Therefore, differential curves from DTG-results of non-carbonated areas and areas with various states of carbonation of the same sample material were calculated and evaluated. Concretes based on three different types of cement were produced and stored under accelerated carbonation conditions (1 % CO2 in air). The required sample material was obtained by cutting slices from various depth of previously CO2-treated specimen and subsequent grinding. During the sample preparation, a special attention was paid that no additional carbonation processes took place. As reference method for the determination of the carbonation depth the sprayed application of phenolphthalein solution was carried out. Microscopic analysis where examined to confirm the assumptions made previously. Furthermore, the observed effect of encapsulation of calcium hydroxide by carbonates caused by the accelerated carbonation conditions was examined more closely.
Üblicherweise werden Werkzeuge für den industriellen Einsatz aus hochlegierten Chromstählen hergestellt. Für den Einsatz in explosionsgefährdeten Bereichen werden oft Werkzeuge aus speziellen Nichteisen-Metalllegierungen (NE-Metall) verwendet. Für diese sogenannten funkenarmen Werkstoffe und deren mögliche Schlagpartner gibt es bisher keine umfassenden Untersuchungen mit Aussagen zur Zündwahrscheinlichkeit.
Die TRGS 723, Abschn. 5.15, fordert den Nachweis der Funkenfreiheit der verwendeten Werkstoffpaarung. Jedoch sind Werkzeuge keine Geräte oder Schutzsysteme im Sinne der Richtlinie 2014/34/EU. Deshalb ist es nicht möglich, Werkzeuge in Übereinstimmung mit dieser Richtlinie zu zertifizieren. Der Nachweis der Erfüllung dieser Anforderungen ist durch Zertifizierungen im sogenannten "gesetzlich nicht geregelten" Bereich möglich. Die BAM bietet diese Zertifizierung für "Funkenarme Werkzeuge" im Rahmen ihres Zertifizierungsprogrammes an.
Die bisherigen Ergebnisse der hier vorgestellten Untersuchung haben gezeigt, dass es keine funkenfreien, metallischen Werkzeuge (non-sparking Tools) gibt. Entscheidend ist im Hinblick auf die TRGS 723, ob zündfähige Funken entstehen oder nicht. Es ist immer die Frage, worauf die Werkzeuge im praktischen Anwendungsfall schlagen oder fallen gelassen werden können, d.h. aus welchem Material der mögliche Schlagpartner besteht. Dies können metallische Schlagpartner, aber auch Betonoberflächen sein. Unterschiedliche Kombinationen von Werkstoffen ergeben bei Schlagbeanspruchung im explosionsgefährdeten Bereich jeweils unterschiedliche Zündwahrscheinlichkeiten. Aber auch die Höhe der beim Einsatz der Werkzeuge möglichen kinetischen Schlagenergie und der Brenngas/Luft-Atmosphäre im betreffenden explosionsgefährdeten Bereich sind wichtige Einflussgrößen.
Grouted connections are intensively used in offshore rigs, platforms as well as jacket and monopile offshore wind turbine structures. Being located in remote offshore conditions, these connections can experience considerable adverse loading during their lifetimes. Degradation was reported inside similar connections, which were installed in the last three decades. Grouting in the offshore sites may often be proven difficult, which eventually leads to reduced load-bearing capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs. In this study, scaled grouted connections were manufactured using a novel mould, whose integrity were monitored using digital image correlation (DIC). The connections were loaded under static load to visualize the main failure pattern using distributed fibre optic sensors and acoustic emission (AE) analysis. Grouted connections were then repaired using a cementitious injectable grout. The effectiveness of the grout injection was monitored using dye penetration technique. Finally, specimens are reloaded to identify the potential of such repair for grouted connections.
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.
Grouts have numerous applications in construction industry such as joint sealing, structural repair, and connections in precast elements. They are particularly favoured in rehabilitation of structures due to penetrability and convenience of application. Grouts for repair applications typically require high-performance properties such as rapid strength development and superior shrinkage characteristics. Sometimes industrial by-products referred as supplementary cementitious materials (SCM) are used with neat cement due to their capabilities to provide binding properties at delayed stage. Micro silica, fly ash and metakaolin are such SCMs, those can modify and improve properties of cement products. This study aims at investigating long-term mass loss and linear shrinkage along with long-term compressive and flexural strength for grouts produced from ultrafine cement and SCMs. A series of mixtures were formulated to observe the effect of SCMs on these grout properties. Properties were determined after 365 days of curing at 23oC and 55% relative humidity. The effect of SCMs on the properties are characterised by statistical models. Response surfaces were constructed to quantify these properties in relation to SCMs replacement. The results suggested that shrinkage was reduced by metakaolin, while micro silica and fly ash had positive effects on compressive and flexural strength, respectively.
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.
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.
Selbstverdichtender Beton verhält sich unter Temperatureinfluss anders als Normalbeton, da die Rheologie neben der fortschreitenden Hydratation zusätzlich durch die von der Zeit und dem Hydratationsfortschritt abhängige Adsorption von Fließmitteln beeinflusst wird. Anhand rheometrischer Betonversuche an SVB unterschiedlicher Entwurfskonzepte mit variierter anionischer Ladungsdichte im Fließmittel wird verdeutlicht, dass mehlkornreiche SVB bei niedrigen Temperaturen sehr robust sind, während bei hohen Temperaturen mehlkornärmere Entwürfe zu bevorzugen sind. Darüber hinaus wird gezeigt, wie sich unterschiedliche Fließmittelmodifikationen in bestimmten Temperaturbereichen verhalten. Anhand des Wasser-Feststoff-Verhältnisses und des Adsorptionsverhaltens von Fließmitteln werden die maßgeblichen Prozesse erläutert und Möglichkeiten für die Entwicklung robuster Mischungen für individuelle Temperaturbereiche aufgeführt.--------------------------------------------------------------------------------------------------------------------------------------------------------------
Regarding the temperature dependent performance, self-compacting concrete (SCC) distinguishes from normal concrete, since its rheology does not only depend upon the hydration itself but supplementary upon the adsorption of superplasticizers, which is affected by the time and the hydration progress. Based on rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the superplasticizers, it is shown that SCC, which is rich in powder components, shows robust performance at low temperatures, while compositions with lower powder contents are favourable at high temperatures. Furthermore, the performance of different superplasticizer modifications at different temperature ranges is demonstrated. The relevant processes are explained by means of the water to powder ratio as well as the adsorption behaviour of superplasticizers, and options for the development of robust mixture compositions for individual temperature ranges are itemised.
Self-compacting concrete (SCC) differs substantially from normal concrete as regards the quantity of superplasticiser dosage. The functionality of superplasticisers, based on the polycarboxylate ether (PCE) superplasticiser customary with SCC, is nowadays very well understood at construction material research level. However, knowledge concerning pertinent correlations does not always permeate practice to an equal extent. PCEs are extremely polymorphic as opposed to earlier superplasticiser groups, which still possess a considerable proportion of the superplasticiser market. The geometry of polymers can be adjusted individually to performance characteristics required for certain uses. Yet, in selecting a superplasticiser for its specified rheological properties, a very strong influence can equally be exerted simultaneously on the subsequent hydration process. The charge density of the PCE employed also plays a key role in connection with both rheology and early hydration.
While today, engineers can choose from a wide range of rheology modifying admixtures, in some parts of the world, these are difficult to access, due to their complex processing. However, alternatives can be bio‐based polymers such as polysaccharides from various sources. These are easily accessible all over the world, do not demand for complicated processing, and typically they are more sustainable than many established materials, which are crude oil‐based.
The paper presents the effects of acacia gum, cassava starch and the gum of triumfetta pendrata A. Rich on the rheological performance of cementitious systems. It is shown that acacia gum can be as efficient as polycarboxylate based superplasticisers, cassava starch can reduce the yield stress slightly with little effect on the plastic viscosity, and the gum of triumfet ta pendrata A. Rich increases the thixotropy of cement pastes with plasticizing polymers significantly.