Fluorescence and scanning electron microscopy were used to investigate hydroxide-activated slag-silica fume-based AAM mixes containing PCE superplasticizers. By applying two distinct fluorescent markers, the competitive adsorption of two PCEs with differing charge densities was observed. The results indicate that each PCE exhibits a location-specific affinity for certain topographical features of the precursor surface, as adsorption on the amorphous material is heterogeneous and varies significantly between the superplasticizers. Through the location-dependent investigations, the PCE on particle coverage levels of the particles were approximated at specific times and correlated with rheological measurements. A high charge density causes a high initial dispersion of the paste with a rapidly decreasing effect, while a lower charge density has a weaker but more persistent effect effect. A comparison of the spatially resolved adsorption derived from the fluorescence signal with the obtained scanning electron microscopy investigations shows that the tested PCEs used attach primarily to the early formed reaction products.

Alkali activated materials (AAM) represent an alternative to building materials using Ordinary Portland Cement (OPC). There is a great variability of AAM, especially in slag-based systems. The aim of this study is to show how carbonation of ground granulated blast furnace slag (BFS) changes the fresh and hardened concrete properties as well as the effectiveness of polycarboxylate ether superplasticizers (PCE). The carbonation of slag is a process that occurs naturally with the aging and storage of slag but can also be applied artificially. The results show that the latent hydraulic BFS exhibits significant changes in setting time, compressive strength and PCE efficacy depending on the state of carbonation. The uptake of CO2 into the BFS was estimated through elemental and thermogravimetric analysis. It was concluded that calcium carbonates are formed. The formation of calcium carbonate leads to less calcium dissolving from the BFS at the beginning of the reaction, which leads to an extension of the setting time as well as a lower PCE efficacy while increasing the compressive strength.

Abstrakt Ein wesentlicher Baustein zur Erreichung der UN‐Nachhaltigkeitsziele bzgl. der Kreislaufwirtschaft in der Baubranche kann die Nutzung von Betonabbruch für die Herstellung von neuem Beton sein. Grobe Fraktionen können bereits als RC‐Zuschlagstoffe verwendet werden, aber insbesondere die Fraktionen <2 mm sind als Feinzuschlagstoffe nicht geeignet. Seit 2023 dürfen feine RC‐Fraktionen laut DIN EN 197‐6 als Füllstoff in Zementen verwendet und als inerter Stoff angerechnet werden. In dieser Studie wird das Reaktionspotenzial von RCP (rezykliertes Betonmehl) untersucht und ob dieses durch Carbonatisierung erhöht werden kann. Ferner wird der Einfluss der Carbonatisierung des RCP auf die Verarbeitbarkeit und Druckfestigkeit des resultierenden Betons ermittelt und diskutiert. Drei verschiedene Recyclingbetonmehle wurden phasenanalytisch mittels Röntgendiffraktometrie (XRD), thermogravimetrischer Analyse (TGA) und mikroskopischer Methoden charakterisiert. Isotherme Wärmeflusskalorimetrie lieferte Erkenntnisse über die Reaktivität des mit RCP gemischten Zements und Frisch‐ und Festbetonuntersuchungen zeigten, dass der Einfluss der Carbonatisierung von RCP sich in einer Verbesserung der rheologischen Eigenschaften und der Steigerung der Druckfestigkeit bemerkbar macht.

Ultra-high performance concrete for the repair and strengthening of bridges – Part 2: Producing UHPC, application on-site, and quality assurance An increasing number of road bridges in Germany shows deficits in terms of structural safety and durability. Replacing these structures by new ones is cost-intensive and requires considerable material and human resources, which are hardly available to the necessary extent given the size of the task to be accomplished. A look at neighbouring countries shows that the repair or strengthening of bridge superstructures by means of thin reinforced cover layers made of ultra-high performance concrete (UHPC) represents a cost-effective and durable alternative to reconstruction. The special suitability of UHPC for this field of application results from the impermeability of its microstructure and its resistance to all forms of chemical and physical attack, so that UHPC can also be used for bridge decks without further protective measures. In this aspect as well as regarding its mechanical properties, UHPC differs fundamentally from normal and high strength concrete. In the first part of this article, the various possibilities of applying UHPC with existing bridges are presented and the special material properties are explained. The present second part of the article deals with the production of UHPFRC, the application on-site and quality assurance measures. As with part 1, the information can be used, for example, as a basis for project-related approvals, for developing or supplementing technical guidelines and for drafting the construction contract.

An increasing number of road bridges in Germany shows deficits in terms of structural safety and durability. Replacing these structures by new ones is cost-intensive and requires considerable material and human resources, which are hardly available to the necessary extent given the size of the task to be accomplished. A look at neighbouring countries shows that the repair or strengthening of bridge superstructures by means of thin reinforced cover layers made of ultra-high performance concrete (UHPC) represents a cost-effective and durable alternative to reconstruction. The special suitability of UHPC for this field of application results from the impermeability of its microstructure and its resistance to all forms of chemical and physical attack, so that UHPC can also be used for bridge decks without further protective measures. In this aspect as well as regarding its mechanical properties, UHPC differs fundamentally from normal and high strength concrete. In the first part of this article, the various possibilities of applying UHPC with existing bridges are presented and the special material properties are explained. The present second part of the article deals with the production of UHPFRC, the application on-site and quality assurance measures. As with part 1, the information can be used, for example, as a basis for project-related approvals, for developing or supplementing technical guidelines and for drafting the construction contract.

The initial investigation evaluates the feasibility of ultra high performance concrete (UHPC) as a material for reusable molds in aluminum casting. Two specific UHPC formulations were investigated: one based on ordinary Portland cement (OPC) and another utilizing alkali-activated materials (AAM). The study focused on investigating the surface through roughness measurements and the thermal durability through repeated casting cycles. The thermal stability of the molds was investigated by thermogravimetric analysis, mercury intrusion porosimetry, crack segmentation, optical microscopy, and electron microscopy. Results indicate that molds fabricated from AAM-UHPC exhibit relatively better performance in terms of maintaining structural integrity and surface quality over repeated uses. AAM-UHPC molds were able to withstand up to ten casting cycles with acceptable surface degradation and no significant failure, while OPC-UHPC molds exhibited a faster degradation under similar conditions. Microstructural changes and the interaction of UHPC materials with molten aluminum were investigated, highlighting the low adhesion and defect formation. Additionally, the molds demonstrated sound casting quality, with a grain size comparable to that achieved using traditional steel molds (~ 90 µm), underscoring the potential of UHPC materials for enhancing casting quality and efficiency. The study concludes that UHPC, particularly with alkali-activated formulations, shows promise for low-pressure casting environments.

The spatial-dependent adsorption of polycarboxylate ethers (PCE) in alkali-activated materials was analysed using fluorescence microscopy after coupling PCE with a fluorophore marker. The adsorption has been derived from the fluorescence with the consideration of the pH-dependence of the fluorophore marker using two different PCE superplasticizers and two different fluorescent markers. The synthetic route of a staining reaction was successfully applied to IPEG PCE and APEG PCE superplasticizers, verified and quantified by different spectroscopic methods. The derived adsorption values were compared with the flowability by mini-slump tests and total organic carbon measurements. It was shown that low activator concentrations can increase the adsorption of a superplasticizer on particles of different precursors. Additionally, it was shown that in alkali-silicate activated systems PCE adsorb on particles while the functionality is lost.

Incineration bottom ash (IBA) is the main solid residue from municipal solid waste incineration. IBA mostly contains minerals that can be used as secondary construction materials in unbound applications as well as concrete after appropriate treatment. Major challenges, in particular for its utilization in concrete, are residual metal contents, soluble salts, as well as the high porosity of the material. The goal of this study was to investigate the processing of the fine (0-2 mm) and medium (2-8 mm) IBA fractions with respect to their utilization as a partial substitute for binder and aggregates in concrete. Therefore, the IBA was treated in a two-stage process and the material fractions produced were utilized as secondary aggregates and partial cement replacement in concrete paving stones. The processing led to a reduction in residual metal concentrations, e.g. the metallic aluminum was reduced by 67 % and 60 % in the processed fine and medium fractions, respectively. Soluble metals and salts could be reduced to some extent, but remaining salts (up to 1800 mg/l and 2300 mg/l for chloride and sulfate, respectively) hinder the use of the IBA in reinforced concrete. The results demonstrate that IBA has promising potential for use in concrete paving stones, but further optimization is needed to meet requirements such as tensile splitting strength and weathering resistance (the tensile splitting strength was approx. 20 % lower and the frost resistance was about 4 times higher). Overall, partial replacement of binder and aggregates can effectively contribute to reducing the environmental footprint of concrete products.

Die Entwicklung von Instandsetzungssystemen fur denkmalgeschutzte Betonbauwerke stellt sowohl materialtechnisch als auch asthetisch eine Herausforderung dar. Der starke Korrosionsfortschritt an den kunstlerisch gestalteten Beton-Glas-Fensterbandern des Baudenkmals St.-Mauritius-Kirche in Wiesbaden macht eine Instandsetzung unumganglich. Basierend auf Daten zur Bauwerkshistorie und visuellen Inspektionen des Schadigungsgrads wurde ein denkmalkonformer Instandsetzungsansatz entwickelt, um die Lebensdauer der Glaseinfassungen unter den aktuellen Nutzungsbedingungen zu verlangern. Unter Berucksichtigung der Materialvertraglichkeit wurden faserverstarkte Mortel fur die Instandsetzung entwickelt. Eine prazise Abstimmung der Materialien untereinander, mit dem Ziel einer optimierten Packungsdichte, ergab ahnliche Gefugedichten wie bei hochfesten oder ultrahochfesten Betonen, wahrend die bauphysikalischen und mechanischen Eigenschaften dem Altbeton angepasst waren. Insbesondere Druck, Biegezug- und Haftzugfestigkeit sowie der Elastizitatsmodul, das Schwinden, die Temperaturwechselbestandigkeit und die Wasseraufnahme wurden im Vorfeld untersucht. Es konnte experimentell nachgewiesen werden, dass eine Faserbewehrung die Haftzugfestigkeit des Instandsetzungsmortels deutlich verbessert und seine Dauerhaftigkeit erhoht. Restoration of the concrete and glass windows of St. Mauritius Church in accordance with the preservation requirementsThe development of repair systems for use on heritage-listed concrete structures represents a major challenge in terms of materials and aesthetics. The reinforced concrete structures of the St. Mauritius Church in Wiesbaden, in particular the artistically designed concrete and glass window bands, are severely corroded and need to be repaired. Based on historical data collections and visual inspections of the degree of damage to these window bands, a restoration approach was developed to extend the service life of the glass surrounds under current conditions of use. Taking into account the material compatibility, dedicated fiber-reinforced concretes were developed for the repair. Care was taken to ensure that the materials used were matched to each other in order to achieve an optimized packing density. In this way, a similar structural density was achieved for the repair concrete as for high-strength or ultra-high performance concretes. The physical and mechanical properties were adapted to the old concrete; especially compressive strength, flexural strength and adhesive tensile strength as well as the modulus of elasticity, shrinkage, thermal fatigue resistance and water absorption. It was experimentally proven that fiber reinforcement significantly improves the adhesive tensile strength of the repair concrete and increases its durability.

Global efforts to minimise carbon dioxide emissions are also leading to attempts to use calcined clays (CC) as a partial substitute for cement in concrete. While the hydration mechanism of such CC blended cements is now well understood, the range of effective admixtures like polycarboxylate ethers (PCE) is limited. There are PCE types that promise relatively high effectiveness, but the mechanisms of action are not yet sufficiently understood. For a detailed understanding of the adsorption of such PCEs, spatially resolved studies of the binder were performed using a combination of fluorescence and scanning electron microscopy. In a comparison of two superplasticisers, the investigations have shown different sites of preferred adsorption in a CC blended system and the results can be correlated with flow tests and setting behaviour. The investigations have shown that a certain PCE type has a higher adsorption on CC and other components of a blended system in comparison to other types.

The fracture behaviour of concrete is studied in various micro- and macro-damage models. This is important for estimating serviceability and stability of concrete structures. However, a detailed understanding of the material behaviour under load is often not available. In order to better interpret the fracture behaviour and pattern, images of lightweight concrete were taken using a high-resolution computed tomography (mu-CT) scanner. The samples were loaded between the taken images and the load was kept constant during the measurement. This study describes the method used and how the data set was analysed to investigate displacements and cracks. It has been shown that displacements and damage to the concrete structure can be detected prior to failure, allowing conclusions to be drawn about the structural behaviour. In principle, the mu-CT measurement can be used to examine different kinds of concrete as well as other systems with inorganic binders and to compare the fracture behaviour of different systems.

This study aims to develop a material-saving performance prediction model for fast-hardening alkali-activated slag/silica fume blended pastes. The hydration process in the early stage and the microstructural properties after 24 h were analyzed using design of experiments (DoE). The experimental results show that the curing time and the FTIR wavenumber of the Si-O-T (T = Al, Si) bond in the band range of 900–1000 cm−1 after 24 h can be predicted accurately. In detailed investigations, low wavenumbers from FTIR analysis were found to correlate with reduced shrinkage. The activator exerts a quadratic and not a silica modulus-related conditioned linear influence on the performance properties. Consequently, the prediction model based on FTIR measurements proved to be suitable in evaluation tests for predicting the material properties of those binders in the building chemistry sector.

Fast and durable repair of asphalt roads is important for a functioning infrastructure.An important method is the milling of the old asphalt layer in order to place a new layer on top ("hot on cold" paving).A bitumen emulsion is used as an adhesion primer.The adhesion of the materials due to the emulsion at the layer boundary is intended to improve the bond and thus dissipate the stresses that occur due to traffic loading [1]- [3].For this purpose, it is necessary to choose the correct quantity of spray [4]-[5] and to distribute it uniformly.Bitumen emulsion consists of bitumen, water and an emulsifier.The emulsifier causes the fine bitumen droplets to spread in a stable state in the water [6].After spraying, the bitumen emulsion breaks and the water evaporates [7].To determine the location of bitumen emulsion in this study, a C40 B5-S (40-wt.%content of bitumen) was sprayed onto the different bases using a specially designed spray-on system (spray-on quantities: 225 g/m 2 ; 450 g/m 2 ).The surfaces of the two variants differ in the texture of the surface and in the void content on the surface (dense surface (milled); open-pore surface (asphalt texture)).The aim of this study was to determine the position of bitumen emulsion on theses surfaces using the X-ray computed micro tomography (µ-CT) technique.Tis technique allows 3D imaging of structures by measuring different densities [8].For the analysis of the thin layers of the bitumen emulsion, a highdensity tracer (barium sulfate) was added, which had no interfering effect on the viscosity of the bitumen emulsion.This made it possible to identify the bitumen emulsion in the subsequent analysis due to the difference in density.[9].The (sprayed) samples were measured using a high-resolution µ-CT (type: Zeiss Xradia 520 Versa).The measurements were performed with a voltage of 140 kV and a power of 10 watts.The spatial resolution was 34.5 µm.The results of the measurements with the µ-CT showed that the distribution of the bitumen emulsion on a milled surface is uneven.In the deepening's of the structure, an emulsion thickness of 917 µm could be measured.Further there was only a small amount of bitumen emulsion on the crests and slopes compared to the deepening's (measured thickness: 217 µm).For the samples with the asphalt surface the bitumen emulsion flows into the voids and was collected there.Almost no emulsion was detected at the actual layer boundary.From the results it can be concluded that the distribution is not optimal and thus no optimal bonding is achieved.For this reason, in further studies the bitumen emulsion will be adjusted in terms of viscosity to achieve a more uniform distribution.This study showed that a new understanding of the material can be obtained with 3D imaging, allowing optimization steps to be applied that lead to better material behavior.

Ultra-high performance concrete (UHPC) is characterised by a high compressive strength, high durability, and a dense microstructure. The latter causes UHPC to fail in a brittle and sometimes explosive manner. For this reason, UHPC is reinforced with microfibre reinforcement. On the one hand, these lead to increased tensile and bending loads being able to be absorbed, but above all to ductile post-fracture behaviour. The fibres used are mostly steel fibres. An essential aspect of the fibre reinforcement is the bond strength between UHPC and metallic fibre. The bond is divided into chemical-adhesive bond, form bond and friction bond. Depending on the shape, material and surface condition of the fibre, the individual types of bond have different effects on the concrete. To quantify these effects, fibre pull-out tests are often carried out. These provide information about the bond strength between the fibre and the concrete. However, results of various studies show that the bond strength does not automatically correlate with the actual influence on the resulting tensile and flexural strengths of concrete components.

Knowledge of the exact composition of building materials (aggregate, binder, air voids, etc.) is essential for the further development of more resistant and sustainable building materials. In numerous scientific studies, the material behavior of asphalt is tested using mechanical methods. Here, the overall material behavior is determined (bitumen, air voids, aggregate). With the advent of imaging techniques, it is becoming possible to determine the individual constituents separately and perform a more detailed analysis of their location, shape and composition. Three-dimensional and two-dimensional methods are available for this purpose. For this study, two different types of asphalt (porous asphalt and asphalt concrete) were analyzed using 3D X-ray computed tomography and asphalt petrology as 2D methods; the results of both investigations are compared. The objective of this study is to determine whether the 2D method provides suitable results for the microstructural analysis of asphalt samples and how the results differ from those studied by the 3D method. The comparison shows that both methods can be used to analyze voids in asphalt samples. The 2D method provides valuable insight into the distribution of voids in a sample. In addition to the distribution of voids within a 2D section, the 2D method can also be used to make some structural statements about the location and structure of the voids in the 2D plane. The X-ray computed tomography method allows more complex analyses of the pore structure because of the third direction (3D). In addition, the 3D method provides more data, so that the pore structure can be described even more precisely, and the pore size (length, width, height) can be mapped and analyzed with a high degree of accuracy.

A major concern in the modern cement industry is considering how to minimize the CO2 footprint. Thus, cements based on belite, an impure clinker mineral (CaO)2SiO2 (C2S in cement chemistry notation), which forms at lower temperatures, is a promising solution to develop eco-efficient and sustainable cement-based materials, used in enormous quantities. The slow reactivity of belite plays a critical role, but the dissolution mechanisms and kinetic rates at the atomistic scale are not known completely yet. This work aims to understand the dissolution behavior of different facets of β-C2S providing missing input data and an upscaling modeling approach to connect the atomistic scale to the sub-micro scale. First, a combined ReaxFF and metadynamics-based molecular dynamic approach are applied to compute the atomistic forward reaction rates (RD) of calcium (Ca) and silicate species of (100) facet of β-C2S considering the influence of crystal facets and crystal defects. To minimize the huge number of atomistic events possibilities, a generalized approach is proposed, based on the systematic removal of nearest neighbors' crystal sites. This enables us to tabulate data on the forward reaction rates of most important atomistic scenarios, which are needed as input parameters to implement the Kinetic Monte Carlo (KMC) computational upscaling approach. The reason for the higher reactivity of the (100) facet compared to the (010) is explained.

Prestressing of concrete is a commonly used technique in civil engineering to achieve long spans, reduced structural thicknesses, and resource savings. However, in terms of application, complex tensioning devices are necessary, and prestress losses due to shrinkage and creep of the concrete are unfavourable in terms of sustainability. In this work, a prestressing method using novel Fe-Mn-Al-Ni shape memory alloy rebars as a tensioning system in UHPC is investigated. A generated stress of about 130 MPa was measured for the shape memory alloy rebars. For the application in UHPC, the rebars are prestrained prior to the manufacturing process of the concrete samples. After sufficient hardening of the concrete, the specimens are heated inside an oven to activate the shape memory effect and, thus, to introduce the prestress into the surrounding UHPC. It is clearly shown that an improvement in maximum flexural strength and rigidity is achieved due to the thermal activation of the shape memory alloy rebars compared to non-activated rebars. Future research will have to focus on the design of the shape memory alloy rebars in relation to construction applications and the investigation of the long-term performance of the prestressing system.

Light metal die casting is usually performed using steel molds. However, these lead to a reduced quality of the casting due to the occurrence of metal corrosion on the surface and the incorporation of hydrogen into the casting as a result of required process chemistry. Ultra‐high performance concrete based on alkali‐activated slag can be used to produce mineral molds for aluminum casting. The use of reusable mineral molds not only enables the production of various thin‐walled geometries. The risk of metal corrosion is eliminated and the concrete molds can withstand multiple cycles due to their thermal stability and high strength, making them potentially superior to the already common lost mineral molds.

Considering the ambitious greenhouse gas emission reduction and efficient use of resource targets set by the Sustainable Development Goals and the importance of concrete structures to achieve these goals, there is an increasing need to study the environmental performance of different concrete production alternatives. Cement is one of the main building materials that contribute significantly to global warming; therefore, studying the environmental performance of innovative binders that can substitute the use of cement is highly recommended. This article investigates the climate, material, energy, and water footprints of four innovative mixtures of ultra-high-performance concrete (UHPC) with a binder made of alkali-activated materials in comparison with the one made of Portland cement. Footprint analysis is carried out within cradle-to-grave life cycle assessment boundaries. Within the life cycle assessment, the functional unit defines the quantification of the final product or service. The functional units of the UHPC were adapted for the comparability of concrete mixtures with different compressive strengths. The results show that UHPC made with an alkali-activated material has 32%–45% better performance in terms of a climate footprint and 19%–33% better performance in terms of material footprints, whereas a trade-off can be seen regarding 44%–83% higher energy footprints and 75%–146% higher water footprints. The disadvantages in energy and water footprints are caused by waterglass. When allocation is considered, mixtures with high silica fume content have higher environmental footprints.