Filtern
Dokumenttyp
- Beitrag zu einem Tagungsband (19) (entfernen)
Schlagworte
- Cement (5)
- Superplasticizer (4)
- Cement hydration (3)
- Rheology (3)
- Structural build-up (3)
- Thixotropy (3)
- Admixtures (2)
- Concrete (2)
- Early hydration (2)
- Polycarboxylate ether (PCE) (2)
Organisationseinheit der BAM
- 7.4 Baustofftechnologie (19) (entfernen)
Influence of the hydroxypropylation of starch on its performance as viscosity modifying agent
(2022)
Synopsis: Starch is a commonly used viscosity modifying agent (VMA). The performance of starch as VMA depends on its origin (e.g. potato, corn, cassava, etc.) and corresponding molecular properties, such as molecular weight, ratio between amylose and amylopectin etc. Depending upon the application, the efficiency of starch can be enhanced by hydroxypropylation. The maximum degree of substitution (DoS) cannot be greater than 3.0, which is the number of hydroxy groups per glucose monomer in the polymer. In the current research three potato starches exhibiting the DoS of 0.4, 0.6 and 0.8 were utilised. The influence of the modified starch on the rheological properties and hydration of cement paste, as well as the viscosity of the pore solution were investigated. Our findings show that the starch with the highest DoS increases the dynamic yield stress the most, while the plastic viscosity is less dependent on the DoS. Additionally, starch with the highest DoS retards hydration to lower degree than other starches.
Synopsis: The paper presents the results of a study on the influence of agitation on the structural build-up of fresh cement pastes using a penetration test. It first presents results about the influence of the penetrating shape’s geometry on the influence on the cement paste specimen. Then, results are shown for the load-deflection curves depending upon time and agitation before testing. Based on the observation of the force required to penetrate the sample, conclusions on the structural build-up can be made. The observations were made over the course of time with samples that were left at rest and partly agitated before testing at different time steps. The setup allows to identify the contribution of the chemical reaction to the structural build-up process. The presence of superplasticizer obviously reduces the load that cement paste can withstand, while the loss of thixotropy is found notably lower than that of neat cement pates
Grouting is a universal repair and strengthening technique, which is constantly used for structural remediation of concrete components, trenches, mine subsidence, dam joints, restoration of masonry structures, and geological stabilizations. Having an extremely small particle size of only few microns, ultrafine cements are ideal for grouting applications due to their superior permeability and compressive strength properties of the hardened cement paste compared to that of the less-expensive, but coarser ordinary Portland cements. Supplementary cementitious materials (SCMs) are often used to replace ultrafine cement in order to modify certain properties and to reduce costs. The aim of this experimental study is to investigate the effect of three supplementary materials: microsilica (MS), fly ash (FA), and metakaolin (MK) on the workability, and mechanical properties of an ultrafine cement based grout with a constant water-binder ratio and constant superplasticizer content. Maximum percentages of replacement with ultrafine cement were 6% by volume of cement for MS and 16% for FA, and MK. In general, results suggest that the workability is improved by addition of FA, whereas is reduced, when modified with MS and MK. The compressive strength of grout after cement replacement remains comparable to that of pure cement grout. However, there is a tendency of the MS to positively affect the compressive strength opposite to FA, whereas flexural strength is positively affected by FA. Based on the results, it is evident that grouts with Hägerman cone flow more than 500 mm and compressive strength of more than 90 MPa after 28 days can be produced.
Superplasticizer and Shrinkage Reducing Admixture Dosages for Microfine Cement in Grout Systems
(2019)
Grouts have numerous applications including crack repair as maintenance in construction industries. Microfine cements are intensively used for high strength mortar and grout products. They are ideal for injection grouting in structural repair. Such grouts should have suitable rheological properties to be injectable, especially those used in repair and rehabilitation. The use of superplasticizers (SP) in these products is thus becoming increasingly crucial to achieve favorable workability and viscosity properties. A difficulty in such grouts is the plastic shrinkage due to finer particles used. It is thus necessary to determine optimum SP and shrinkage reducing admixture (SRA) dosages for a microfine cement based grout. In this study, a saturation dosage was decided from two Polycarboxylate ether (PCE) based SPs in relation to neat cement using slump flow and rheological parameters. A range of grout mixtures was formulated containing micro silica (MS) and fly ash (FA), and tested for suitable rheological and mechanical parameters. Based on the results, a grout mixture with MS and FA was selected to determine optimum SRA content. According to the results, a SP dosage of 3% by weight of neat cement is sufficient to achieve saturation. The grout material including MS and FA can produce comparable properties to neat cement grout. MS is found to improve compressive strength within the range considered, whereas a higher FA content provides favourable rheological properties. Finally, a SRA dosage of 4%, which could reduce the shrinkage by about 43% after 28d days, is determined for the grout system.
The African cement, concrete and construction business is growing at rapid pace. The cement sales are expected to grow rapidly until 2050. The number of newly built cement plants increases dramatically and in addition more cements are being imported from outside the continent, e.g. from Turkey, Pakistan, Indonesia, and China, driven by overcapacities in the countries of origin. This causes a high number of potentials and challenges at the same time. Newly built cement plants can operate directly at best technological state of the art and thus incorporate more sustainable technologies as well as produce new and more sustainable products such as cements blended with sustainable supplementary cementitious materials such as calcined clays, and industrial or agricultural by products. At the same time the new variety of binding agent as well as the international imports, which are driven by price considerations, make the cement market prone to quality scatter. This puts pressure on the quality control regulations and institutions to ensure safety of construction, healthy application, and environmental safety for the population. The paper presents possible solutions to build up the rapidly increasing African cement production more sustainably than in the rest of the world as well as the related challenges and obstacles that need to be overcome. Based on experiences with a series of pan-African cement testing laboratory proficiency schemes conclusions are made on technical, regulative and political level.
Während für die Bauwirtschaft in den meisten Ländern der nördlichen Halbkugel die Instandhaltung bestehender Strukturen und Infrastruktur im Vordergrund steht, geht es in vielen Ländern der Südhalbkugel in der Hauptsache um die Schaffung von Bauwerken und Infrastruktur zur Verwirklichung angemessener Lebensbedingungen, was wiederum das schnelle Wirtschaftswachstum weiter befeuert.
Beton ist als Massenbaustoff, trotz landläufig anderer Bewertung, aufgrund seines im Vergleich zu anderen Baustoffen günstigen CO2-Footprints und der globalen Verfügbarkeit hierbei aktuell ohne Alternative. Allerdings ist Beton nach Wasser das am zweithäufigsten genutzte Produkt auf Erden [1] und die globale Nachfrage und Produktion steigt dramatisch. Dies hat Konsequenzen für das globale Klima, denn gerade die als Bindemittel eingesetzten Portlandzemente verursachen bei ihrer Herstellung selbst unter modernsten Produktionsbedingungen große Mengen an CO2.
Deshalb müssen für zukünftige nachhaltigere Technologien neuartige Betone entwickelt werden, die bei gleicher oder verbesserter Leistungsfähigkeit, einen geringeren Bedarf an Portlandzement aufweisen.
Um den CO2-Ausstoß, der mit der Betonherstellung verbunden ist zu minimieren, müssen Anteile an Portlandzement im Bindemittel reduziert und durch nachhaltigere Ersatzstoffe ausgetauscht werden. Darüber hinaus muss das eingesetzte Bindemittel im Beton so effizient wie möglich ausgenutzt werden. Das heißt, ein Bauteil oder Bauwerk sollte gerade so viel Bindemittel enthalten, wie für die Tragfähigkeit und Dauerhaftigkeit erforderlich ist. Hierfür erscheint der Einsatz von leistungsfähigen bauchemischen Produkten unumgänglich. Diese können zum einen die veränderten Verarbeitungseigenschaften, die Zementersatzstoffe mit sich bringen, ausgleichen und gleichzeitig helfen, den Gesamtwassergehalt zu minimieren, so dass das Bindemittel seine maximale Leistungsfähigkeit erzielt. Fließmittel sind aktuell die wichtigsten Zusatzmittel für Beton. Sie reduzieren den Wasserbedarf und ermöglichen, Zement im Beton effizienter auszunutzen. Darüber hinaus sind Stabilisierer wichtig zur Erhöhung der Robustheit, insbesondere im Hinblick auf die bautechnischen Herausforderungen der Zukunft – vom Pumpen über weite Distanzen, dem Spritzen, dem Betonieren bei extremen Klimabedingungen bis hin zum 3D-Druck.
Polymers that help tailoring rheological properties during the casting process have become inevitable constituents for all kinds of high-performance concrete technologies. Due to lacking industries, these typically crude-oil based admixtures are not readily available in many parts of the world, which limits the implementation of more sustainable high-performance construction technologies in these regions. Alternative polymers, which often demand for less processing, can be derived from local plant-based resources. The paper provides experimental data of flow tests of cement pastes with polysaccharides from Triumfetta pendrata A. Rich, acacia gum and cassava without and in the presence of polycarboxylate ether superplasticizer. The flow tests are amended by observations of the zeta potentials and the hydrodynamic diameters in the presence of and without calcium ions in the dispersion medium. The results show that in the presence of and without calcium ions all polysaccharides provide negative zeta potentials, yet, they affect flowability and thixotropy in different ways. Cassava starch, acacia gum, and the gum of Triumfetta pendrata A. Rich qualified well for robustness improvement, strong stiffening, and additive manufacturing, respectively. The reason for the different effects can be found in their average sizes and size distribution. Due to the promising results, a flow chart for local value chains is derived on the example of yet unused cassava wastes, which can be converted in parallel.
With ongoing innovation in process technology, the challenges of concrete technology are more and more focused on the rheological optimisation for these processes, since improper mixture stability or poor compaction ability negatively affect the concrete homogeneity and quality. However, along with the increasing complexity of today’s concrete mixture compositions, concrete becomes more prone to failure regarding the casting process. Variable properties of the raw materials typically cause changing workability. The reasons can be found among others in scattering water contents, physical or chemical properties of the cement or varying environmental temperatures. Robustness in the delicately adjusted rheology, however, is of utmost importance for modern and future process technology, from sprayed concrete over pumpable concrete towards 3D-printing, with regard to the long-term strength, the function and the durability. Typically, material induced changes cannot be identified easily due to the complex interactions of concrete constituents. Therefore, a precise and prompt counteraction is impossible. However, it is known that the yield stress can be controlled by addition of supplementary superplasticizer or stabilising agent. In combination with computerized process observation tools that can rapidly interpret and react on changes in the rheology, it is therefore thinkable, that only these two admixture types can adjust the rheology steadily and permanently, regardless of the actual root cause for observed macroscopic rheology change. The presentation will firstly give a comprehensive overview of effects at the interface between pore solution, particles and hydrates, which affect the rheology of fresh concrete. Secondly, ways are recommended how the rheology can be actively manipulated before eventually computerized methods are demonstrated that help to actively and rapidly assess and counteract performance scatter during steady casting processes.
In the 21st century, adequate habitat and functioning infrastructure are critical for global societal and economic stability. In addition, growing urbanisation and environmental pollution cause challenges to societies.
With increasing velocity, humanity faces that the current way of living is not sustainable. Thus, habitat,infrastructure, urbanisation, environment and sustainability are definitively among the most striking challenges of the 21st century.
By consulting, planning, building, maintaining, exploiting and processing of global resources, civil engineers contribute significantly to the existence of these challenges. This is a high responsibility, but due to the heavy involvement, together with adjacent disciplines such as architecture, geosciences, chemistry,
physics, environmental sciences and economics, civil engineers also hold the key to mitigate these challenges and provide a brighter global future.
Solutions towards greener, more sustainable and economically viable materials do exist, and there is ongoing research on how greener technologies can contribute to better livelihood and economic growth, but their level of implementation is limited, a major reason for which is that these approaches require more fundamental understanding rather than standard application.
Structural build-up describes the stability and early-age strength development of fresh mortar used in 3D printing. lt is influenced by several factors, i.e. the composition of the print able material, the printing regime, and the ambient conditions. The existing modelling approaches for structural build-up usually define the model parameters for a specific material composition with out considering the influence of the ambient conditions. The goal of this contribution is to explicitly include the temperature dependency in the modelling approach. Temperature changes have signifi cant impact on the structural build-up process: an increase of the temperature leads to a faster dissol ution of cement phases and accelerates hydration. The proposed extended model includes temperature dependency using the Arrhenius theory. The new model parameters are successfully calibrated based on Viskomat measurement data using Bayesian inference. Furthermore, a higher impact of the temperature in the re-flocculation as in the structuration stage is observed.