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The hydration process of Portland cement in a cementitious system is crucial for development of the high-quality cementbased construction material. Complementary experiments of Xray diffraction analysis (XRD), scanning electron microscopy (SEM) and time-resolved laser fluorescence spectroscopy (TRLFS) using europium (Eu(III)) as an optical probe are used to analyse the hydration process of two cement systems in the absence and presence of different organic admixtures.
We Show that different analysed admixtures and the used sulphate carriers in each cement system have a significant influence on the hydration process, namely on the time-dependence in the formation of different hydrate phases of cement. Moreover, the effect of a particular admixture is related to the type of sulphate carrier used.
The quantitative information on the amounts of the crystalline cement paste components is accessible via XRD analysis. Distinctly different morphologies of ettringite and calcium-silicate-hydrates (C-S-H) determined by SEM allow visual conclusions about formation of these phases at particular ageing times. The TRLFS data provides information about the admixture influence on the course of the silicate reaction. The dip in the dependence of the luminescence decay times on the hydration time indicates the change in the structure of C-S-H in the early hydration period. Complementary information from XRD, SEM and TRLFS provides detailed information on distinct periods of the cement hydration process.
Porous ceramic diesel particulate filters (DPFs) are extruded products that possess macroscopic anisotropic mechanical and thermal properties. This anisotropy is caused by both morphological features (mostly the orientation of porosity) and crystallographic texture. We systematically studied those two aspects in two aluminum titanate ceramic materials of different porosity using mercury porosimetry, gas adsorption, electron microscopy, X-ray diffraction, and X-ray refraction radiography. We found that a lower porosity content implies a larger isotropy of both the crystal texture and the porosity orientation. We also found that, analogous to cordierite, crystallites do align with their axis of negative thermal expansion along the extrusion direction. However, unlike what found for cordierite, the aluminium titanate crystallite form is such that a more pronounced (0 0 2) texture along the extrusion direction implies porosity aligned perpendicular to it.
Fresh cement paste is a suspension consisting of a hydraulic binder (cement), water, and numerous minor components – admixtures. Addition of admixtures aims at specific modification of properties of the fresh cement paste or hardened cementitious building material. Specific admixtures, so-called superplasticizers (SP), are used to improve the flowability of the fresh cement paste with reduced water content. The latter is the starting material for the high-strength concrete. Thus, SPs are essential for the ambitious construction projects.
However, uncontrollable retardation of the setting time in presence of SPs is occasionally observed. Obviously, SPs influence early products of the cement hydration leading to changes in the microstructure development. The hardening is thus delayed, and the quality of the resulting building material suffers. The mechanisms of the admixture action during the hydration process are still intensively investigated [1-7]. A detailed understanding of the admixture effects during the early hydration stage is the key to control and individual adjustment of the cement-based construction materials.
We use the unique combination of the wall-free sample holder and the time-resolved X-ray scattering analysis to achieve the full information about the hydrate phases formed under the influence of admixtures. We use ultrasonic levitator to start the cement hydration in levitated cement pellets [8, 9]. The sample levitation allows collection of the unimpaired information about cement hydrate phases. The most beneficial is the avoiding of the contributions of the sample holder material to the data signal.
We induce the cement hydration by adding water to unhydrated Portland cement during the data acquisition. The full phase composition of the hydrating cementitious system can be gathered in situ using wide angle X-ray scattering (WAXS). During the hydration of cement both crystalline and amorphous hydrate phases are formed. WAXS data contain the information about crystalline phases behind the Bragg reflections, whereas the amorphous hydrates influence the appearance of the background. Application of the data analysis specific for crystalline or amorphous phases is needed. The data quantification by the Rietveld method allows to conclude about the changes of the phase amounts due to the presence of admixture. The calculation of the pair distribution functions allows analysis of the amorphous hydrates. Based on this information, the SP effects and the extent of their involvement into the ongoing reactions can be concluded. A detailed understanding of the complex cement hydration process is envisioned.
Chemical admixtures are widely used to adjust properties of the cementitious mixtures. Different superplasticizers (SP) are typically added to improve the flowability of the fresh cement paste. Their working period is the initial stage of the cement hydration from its beginning until stiffening. Currently, polycarboxylate-based SP (PCE-SP) are the most effective ones. They can act beyond their aimed function causing changes in the hydration course, e. g. retardation effects. The exact mechanisms and influencing factors are still under investigation. However, the occurrence of the retardation effect indicates the involvement of PCE in the hydration reactions. Some observations indicate, that the retardation depends on the type, molecular structure and concentration of PCE, and the presence of other admixtures (stabiliser, ST). These factors influence further the working time of admixture and its amount related efficacy in the flow improvement.
We investigate the cement hydration process in situ in levitated droplets. High resolution synchrotron X-ray diffraction allows gathering temporal course of the hydration reactions. The measurement setup is based on an ultrasonic levitator allowing in situ investigation of the temporal changes of the phase composition in the hydrating cementitious system. Further, the hydration can be followed ab initio avoiding the time loss for the sample preparation and filling in the sample holder. The collected data allows to conclude about the mechanisms of the admixture action and their involvement into the ongoing hydration process.
Chemical admixtures are widely used to adjust properties of the cementitious mixtures. Different superplasticizers (SP) are typically added to improve the flowability of the fresh cement paste. Their working period is the initial stage of the cement hydration from its beginning until stiffening. Currently, polycarboxylate-based SP (PCE-SP) are the most effective ones. They can act beyond their aimed function causing changes in the hydration course, e. g. retardation effects. The exact mechanisms and influencing factors are still under investigation. However, the occurrence of the retardation effect indicates the involvement of PCE in the hydration reactions. Some observations indicate, that the retardation depends on the type, molecular structure and concentration of PCE, and the presence of other admixtures (stabiliser, ST). These factors influence further the working time of admixture and its amount related efficacy in the flow improvement.
We investigate the cement hydration process in situ in levitated droplets. High resolution synchrotron X-ray diffraction allows gathering temporal course of the hydration reactions. The measurement setup is based on an ultrasonic levitator allowing in situ investigation of the temporal changes of the phase composition in the hydrating cementitious system. Further, the hydration can be followed ab initio avoiding the time loss for the sample preparation and filling in the sample holder. The collected data allows to conclude about the mechanisms of the admixture action and their involvement into the ongoing hydration process.
Building materials consist of cement, water and chemical admixtures, which adjust cement paste properties. They can act beyond their aimed functions causing changes in hydration course. The mechanisms are still under investigation. We investigate cement hydration in levitated droplets using an ultrasonic levitator. Hydrate phase formation is followed in situ by synchrotron XRD. The data allows detailed conclusions about the mechanisms of the admixture action in the ongoing hydration reactions.
The exact mechanisms of the phase transitions caused by a combined sulphate-chloride attack are discussed controversially. The main points concern the mutual influences of sulphate and chloride ions during the secondary binding processes of these anions within cement hydrate phases. We simulated combined sulphate-chloride attack under laboratory conditions using solutions containing NaCl and Na2SO4 in different concentrations. Three sample compositions were used for the preparation of the specimens. In two of them, 30% of Portland cement was replaced by supplementary cementitious materials (fly ash, slag). The phase distribution in the samples was determined using synchrotron X-ray diffraction. The analysis with high spatial resolution allows the localisation of the secondary phase formation in the microstructural profile of the sample. A mechanism of the phase developments under combined sulphate-chloride attack is derived.
High resolution in situ monitoring of the initial cement hydration influenced by organic admixtures
(2015)
Numerous admixtures are used in the building practice to customize the properties of the cement paste during application. The influences of admixtures on the course of cement hydration and formation of hydrate phases have to be considered. Polycarboxylate ether (PCE) based polymeric superplasticizers (SPs) are known to retard the setting of the cement paste. The extent of the retardation differs depending on the molecular structure of the SP. Additionally, the presence of a stabilizing agent (SA) in the cement paste has a retarding side effect on the setting. The initial cement hydration processes and the detailed mechanisms of the retardation influenced by PCEs, as well as their interactions with particular SAs, are insufficiently understood. Up to now, only the results of phenomenological studies were taken into account to describe this retardation process. A detailed structure analysis monitoring the change of the phase composition during the hydration was never applied. Both SP and SA affect the adsorption of the sulphate ions on the clinker particles, causing changes in the formation of ettringite during the initial hydration, and are therefore a crucial part of the setting process itself. Here, the initial hydration of cement influenced by the interaction of SP and SA was monitored in situ by synchrotron X-ray diffraction. The high time resolution of the measurements allowed a continuous detection of the hydrates formed. The hydration was followed from the starting point of water addition and for couple of hours afterwards. The hydration of the levitated cement pellets containing starch as SA was initialized by adding aqueous solutions of different commercial SPs. Changes in the ettringite formation were detected in comparison to the reference hydration of pure cement.
In diesem Vortrag werden hochauflösende analytische Methoden für die Untersuchungen von zementgebundenen Baustoffen vorgestellt. Zum einen werden frühe Hydratationsprozesse in situ und mit hoher Zeitauflösung untersucht und die Einflüsse von polymeren Fließmittel und Stabilisierer auf die Erstarrungsverzögerung beobachtet. Zum anderen werden Phasenübergänge unter schädigenden Angriffen im Gefügeprofil auf µm-Skala verfolgt. Die Möglichkeit die Auswirkungen der niedrigen realen Konzentrationen von schädigenden Agenten zu identifizieren stellt ein großes Potential für die Schädigungsanalyse unter realen Bedingungen dar. Insgesamt haben die vorgestelletn analytischen Techniken ein großes Potential für die Analyse der Phasenentwicklung in zementgebundenen Baustoffen.