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Organisationseinheit der BAM
Besides their plasticizing effect, superplasticizers (SPs) are known to retard the hydration of inorganic systems such as cement. Despite their frequent use, the understanding of these highly complex systems is still limited and the relevant parameters, which control the interaction between SPs, and cement components and reaction products are in the focus of ongoing research activities.[1] Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules. The potential of these methods to study processes at the interface between (hydrated) particles and the fluid phase at a very early stage of concrete formation could reveal possible mechanisms of interaction.
This investigation focuses on the study of organic/inorganic mixtures consisting of cement (CEM) and cement phases (C3S and C3A) in the presence of polycarboxylate ether and organic dyes in aqueous solution (particularly alkali resistant dyes) at a water to powder ratio of 1. Diffuse reflectance as well as steady state and time resolved fluorescence spectroscopy of the above mentioned mixtures were evaluated. Based upon changes of the intensity of the reflectance and fluorescence signal and spectral changes of the dye, acting as optical reporter, a model for the interactions of dye, PCE and cement (including different cement phases) was derived which describes the very first stage of cement hydration.
Organic/inorganic mixtures were prepared from ordinary Portland cement (OPC), water (w/c 0.22), a fluorescent dye in aqueous solution (stable at alkaline pH; BAM-I), and two different comb shape polycarboxylates (PCEs), i.e., high charge (PCE-HC) and low charge (PCE-LC), respectively. Rheological and calorimetric measurements were performed prior to optical studies in order to select PCE concentrations. Absorption and fluorescence spectroscopy of the system OPC + BAM-I (CBAM-I) revealed maxima of dye BAM-I located at 645 nm and 663 nm, respectively. In presence of PCE-HC and PCE-LC, these mixtures displayed a small red shift in reflectance and a faster decrease in intensity compared to studies with CBAM-I; however, only slight differences were observed between the different PCEs. With time, all systems exhibited a decrease in intensity of BAM-I in absorption/reflectance and emission. This could be caused by dye adsorption and possibly decomposition when in contact with cement particles or hydration products.
In order to observe the effect of the aggregate phases between 2 mm and 16 mm without overlap with rheological effects induced by the cement hy-dration and without interactions with a threshold fine sand particle size that affects both, paste and aggregates, rheological experiments were conducted on a limestone filler based paste mixed with aggregates up to 16 mm. Vari-ous aggregate fractions were blended and mixed with the replacement paste in different volumetric ratios.
The dry aggregates’ flow coefficients were determined and compared to yield stress and plastic viscosity values at different aggregate volume fractions. The results indicated that the flow coefficient is not a suitable parameter to predict the performance of the aggregates in the paste. It was shown that the yield stress of pastes is largely determined by the blend of different aggregate fractions, while the plastic viscosity to large extend depends upon the coars-est aggregate fraction.
Based on the results, ideal aggregate composition ranges for minimised yield stress are presented. For the plastic viscosity no such grading curves to achieve minimum values could be found, but high viscosity curves are identified.
Die meisten Einflüsse auf die Rheologie von Beton wirken auf sehr kleinen Skalenebenen. Einflussgrößen im Millimeter oder Zentimeterbereich sind im Wesentlichen auf den Sand und die Gesteinskörnung beschränkt. Letztere machen aber in den meisten Betonen 50 %-70 % des Gesamtvolumens aus, was bei Untersuchungen zur Steuerung der Verarbeitungseigenschaften von Beton häufig vernachlässigt wird. Während Betonzusatzmittel und Zusatzstoffe bei geeigneter Wahl die Rheologie sehr gezielt beeinflussen können, eignen sich Sand und Gesteinskörnung weniger gut zur Steuerung der Rheologie, aber sie tragen zur Rheologie des Gesamtsystems bei. Hierbei können Effekte aus Sand und Gesteinskörnung die Effekte von Zusatzmitteln und Zusatzstoffen überlagern und im ungünstigsten Falle sogar nahezu unwirksam machen. Deshalb sollten Ergebnisse zur Verarbeitbarkeit von Bindemittelsystemen nur mit großer Sorgfalt auf Beton übertragen werden. Es ist wichtig, den Einfluss des Sandes und der Gesteinskörnung besser zu verstehen, um diese so abzustimmen, dass sie Effekte aus Fließmitteln positiv ergänzen anstatt diesen entgegenzuwirken.
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Most factors acting on concrete rheology work at an extremely small-scale level. Influencing factors in the millimetre or centimetre area are essentially restricted to sand and aggregates. The latter, however, make up 50 to 70% of the total volume of most concretes – a fact often ignored in research on controlling concrete processing properties.
Whereas suitably chosen concrete admixtures and additives can influence rheology in a very targeted manner, sand and aggregates are less suitable for controlling rheology but nonetheless contribute to the rheology of the Overall system. The actions of sand and aggregate can impose themselves upon the actions of admixtures and additives
and, in unfavourable circumstances, even render them redundant. For this reason, any results concerning the processability of binding agent systems can only be transferred to concrete with great care. It is important to better understand the action of sand and aggregates in order to be able to harmonise them in such a way that they complement the action of superplasticisers positively, instead of
working against them. Savings on costs can also be made by this targeted fine-tuning.
Organic/inorganic mixtures were prepared from ordinary Portland cement (OPC), water (w/c 0.22), a fluorescent dye in aqueous solution (stable at alkaline pH; BAM-I), and two different comb shape polycarboxylates (PCEs), i.e., high charge (PCE-HC) and low charge (PCE-LC), respectively. Rheological and calorimetric measurements were performed prior to optical studies in order to select PCE concentrations. Absorption and fluorescence spectroscopy of the system OPC + BAM-I (CBAM-I) revealed maxima of dye BAM-I located at 645 nm and 663 nm, respectively. In presence of PCE-HC and PCE-LC, these mixtures displayed a small red shift in reflectance and a faster decrease in intensity compared to studies with CBAM-I; however, only slight differences were observed between the different PCEs. With time, all systems exhibited a decrease in intensity of BAM-I in absorption/reflectance and emission. This could be caused by dye adsorption and possibly decomposition when in contact with cement particles or hydration products.
The Effect of Low Charge Polycarboxylate on C3A Passivation Monitored by Optical Spectroscopy
(2019)
Tricalcium aluminate (C3A) is less than 10 wt.% of the total cement composition; however, during hydration, the soluble C3A plays an important role in cement setting when mixed with the appropriate amount of sulfate.1 A good understanding about the balance of these components is therefore crucial to follow with the rapid growth of substitution materials and the rising levels of aluminate clinker. The aim of this investigation is the use of optical spectroscopy and in-situ X-ray diffraction utilizing a water-soluble organic dye (dye-S) to monitor early hydration of calcium aluminate (C3A) in the presence of 26 wt.% CaS04.2H2O (G) and PCE polymers with different charge densities (PCE-LC and PCE-HC). Phase characterization and optical evaluation were performed using in-situ X-ray diffraction and steady-state fluorescence and diffuse reflectance spectroscopy. Fluorescence spectroscopy of the reference C3A + dye-S revealed a fast decay in fluorescence intensity. However, in the presence of 26 wt.% G (C3A + dye-S + 26 wt.% G), a gradual increase in fluorescence intensity was observed in the first hours of reaction followed by a plateau that subsequently dropped in intensity after eight hours. The addition of PCE-LC and PCE HC to the mixture exhibited changes in the intensity threshold and overall a higher fluorescence intensity. Dye changes during hydration and structural changes will be further discussed.
Chemical admixtures like superplasticizers or stabilizing agents are of ever increasing importance for modern concrete technology. Although such admixtures have meanwhile become common practice in many applications of concrete technology, the understanding of these highly complex systems is still limited and the relevant parameters, which predominantly control the interaction between the superplasticizer and the cement components, have not been identified yet.
Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules. This encouraged us to assess the potential of these methods, and particularly reflectance and fluorescence measurements, for the study of the interactions that occur at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation. Special emphasis is dedicated to search for and identify differences between commonly used superplasticizers.
Here, we focus on hydration effects using commercial comb shape polycarboxylate ethers (PCEs) with different charge densities, which are known to allow a very low water/cement ratio (w/c of 0.20 or less) while maintaining good workability. Based upon changes of the intensity of the reflectance and fluorescence signal and spectral effects of a dye, acting as optical reporter, a model for the interactions of dye, PCE molecules and cement nanoparticles in the very first phase of cement hydration is derived
Nowadays, superplasticizers (SPs) are widely used to increase fluidity and reduce water content in concrete; thus, allowing better workability for final applications. The present study will focus on the hydration effect using comb shape polycarboxylates (PCEs), which are known to allow a very low water/cement ratio (w/c of 0.20) or less.Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules.This encouraged us to assess the potential of these methods, and particularly fluorescence, for the investigation of the interactions that occur at the interface between hydrate surfaces of cement particles and PCE at a very early stage of concrete formation and to differentiate between the impact of PCE’s molecular structures on such interactions.