1,721,022 research outputs found
Impact of particle size on interaction forces between ettringite and dispersing comb-polymers in various electrolyte solutions
No full textThe inter-particle forces play a fundamental role for the flow properties of a particle suspension in response to shear stresses. In concrete applications, cement admixtures based on comb-polymers like polycarboxylate-ether-based superplasticizer (PCE) are used to control the rheological behavior of the fresh mixtures, as it is negatively impacted by certain early hydration products, like the mineral ettringite.In this work, dispersion forces due to PCE were measured directly at the surface of ettringite crystals in different electrolyte solutions by the means of atomic force microscopy (AFM) applying spherical and sharp silicon dioxide tips.Results show an effective repulsion between ettringite surface and AFM tips for solutions above the IEP of ettringite (pH~12) and significant attraction in solution at lower pH. The addition of polyelectrolytes in solution provides dispersion forces exclusively between the sharp tips (radius. ≈. 10. nm) and the ettringite surface, whereas the polymer layer at the ettringite surface results to be unable to disperse large colloidal probes (radius. ≈. 10. μm).A simple modeling of the inter-particle forces explains that, for large particles, the steric hindrance of the studied PCE molecules is not high enough to compensate for the Van der Waals and the attractive electrostatic contributions. Therefore, in cement suspensions the impact of ettringite on rheology is probably not only related to the particle charge, but also related to the involved particle sizes. © 2013 Elsevier Inc
Reaction of clinker surfaces investigated with atomic force microscopy
No full textThe application of microscopy to investigate cement hydration has widely spread in the last decades. For instance, scanning electron microscopy (SEM) is of primary importance to detect the formation of microstructures and detect their chemical composition. In this work, atomic force microscopy (AFM), which is a relatively less diffused technique, is applied to illustrate and estimate changes of the surface roughness of a clinker substrate, treated with different electrolytic solutions, with and without superplasticizer, at different times of hydration. Furthermore, SEM images are collected to characterize the chemical composition of hydration products formed on the clinker surface. It is shown that surface reaction increases drastically with the increase of the pH, and that surface roughness changes occur mainly in the first minutes of hydration. Moreover, the formation of hydration products is reduced when the clinker is treated with solutions containing polycarboxylate ether-based superplasticizer. Additionally, AFM images collected in tapping mode revealed the presence of nano-structures on calcium silicate phase. © 2012 Elsevier Ltd. All rights reserved
Interaction of cement model systems with superplasticizers investigated by atomic force microscopy, zeta potential, and adsorption measurements
No full textPolyelectrolyte-based dispersants are commonly used in a wide range of industrial applications to provide specific workability to colloidal suspensions. Their working mechanism is based on adsorption onto the surfaces of the suspended particles. The adsorbed polymer layer can exercise an electrostatic and/or a steric effect which is responsible for achieving dispersion. This study is focused on the dispersion forces induced by polycarboxylate ether-based superplasticizers (PCEs) commonly used in concrete. They are investigated by atomic force microscopy (AFM) applying standard silicon nitride tips exposed to solutions with different ionic compositions in a wet cell. Adsorption isotherms and zeta potential analysis were performed to characterize polymer displacement in the AFM system on nonreactive model substrates (quartz, mica, calcite, and magnesium oxide) in order to avoid the complexity of cement hydration products. The results show that PCE is strongly adsorbed by positively charged materials. This fact reveals that, being silicon nitride naturally positively charged, in most cases the superplasticizer adsorbs preferably on the silicon nitride tip than on the AFM substrate. However, the force-distance curves displayed repulsive interactions between tip and substrates even when polymer was poorly adsorbed on both. These observations allow us to conclude that the dispersion due to PCE strongly depends on the particle charge. It differs between colloids adsorbing and not adsorbing PCE, and leads to different forces acting between the particles. © 2010 Elsevier Inc
Force measurements by AFM on clinker surfaces and model systems in aqueous solutions containing superplasticizer
No full textLow-pH cements based on blends of MgO and hydromagnesite
No full textMost of the alternative low CO2-cements that have been proposed are based on
calcium silicates and aluminates. Their main calcium source is limestone, even though the
amounts required may be somewhat lower than for Portland cement clinker.
MgO can be used to make a variety of hydraulic binders as well as binders that harden
by carbonation. Potential raw materials are Magnesium silicates or Mg-containing brines,
which could be carbonated and partially calcined afterwards to obtain mixture of MgO
and basic magnesium carbonates. If the final composition of the hardened binder were
sufficiently rich in carbonate the CO2 sequestered by it could fully compensate for the CO2
emitted by the production of the energy needed to drive the manufacturing process, we
could talk of a truly carbon-neutral binder. We could even envisage carbon-negative binders
for which there would be net consumption of CO2 during manufacture and use. Besides their
ability to bind CO2, such binders have a low pH (around 10-11), which could be of potential
interest regarding waste encapsulation.
The current study aimed at increasing the initial carbonate content of such binders in order
to enhance their effectiveness as a means of reducing CO2 emissions. The hydration of
reactive periclase (MgO) in the presence of hydromagnesite (Mg5(CO3)4(OH)2·4H2O) was
investigated by a variety of physical and chemical techniques. Isothermal calorimetry,
X-ray powder diffraction, thermal analysis, vibrational spectroscopy (infrared and Raman
spectroscopy) and thermodynamic modelling were applied to assess the hydration mechanism.
Isothermal calorimetry showed that partial replacement of the MgO with
hydromagnesite accelerated the hydration of the pure MgO at early ages (i.e. for about the
first half day). The MgO-hydromagnesite blends always produced a significant amount of an
unknown hydrate with a thermal decomposition peak centered on about 100°C as
determined by thermogravimetry. This hydrate, which was not observed in a previous study, formed quickly – it was already present by one day of hydration and did not increase
significantly after that. We hypothesise that this peak represents an amorphous or very
poorly-crystalline phase that leads to cohesive binding in these blends. The other main
difference between the hydrated MgO-hydromagnesite pastes and the pure MgO paste is the
observation of very broad X-ray diffraction peaks for brucite whenever hydromagnesite is
present. Furthermore, reflections of a dypingite-like phase (Mg5(CO3)4(OH)2·5H2O) were
identified by X-ray diffraction. The formation of artinite, which was calculated to be the
thermodynamic stable phase, could not be confirmed, even if a few % of pure artinite were
added as seeds to the mix
Multi-method approach to study influence of superplasticizers on cement suspensions
No full textSuperplasticizers are widely used in concrete processing to increase the rheological properties of hardening pastes. In this study, different techniques (rheology, adsorption, atomic force microscopy-AFM, and ζ-potential) are used to characterize the impact of polycarboxylate-ether based superplasticizer (PCE) on particle suspensions. Results obtained with two cements and two inert powders (MgO and calcite) show that superplasticizer efficiency is strongly influenced by polymer architecture and by the ionic species present in solution. Additionally, experiments performed with AFM and ζ-potential contributed to characterize dispersion forces exerted by superplasticizers at the solid-liquid interface. The application of plateau AFM-tips coated with platinum reveals that dispersion forces depends on the presence of ions in solution, and that multilayer formation occurs with certain superplasticizer types. A further conclusion includes the idea that the PCE has a lubricating effect between adjacent particles and PCE increases surface wettability. © 2011 Elsevier Ltd
Role of Zn in aluminosilicate glasses used as supplementary cementitious materials
No full textThe cement industry relies on the use of supplementary cementitious materials (SCM) to lower the related CO2 emission. SCM are often amorphous materials containing minor elements, which is why it is important to study the impact of these materials on cement reactivity. Two calcium aluminosilicate glass series as model substances of SCM were synthesized and investigated. In both series Ca was partially replaced by Zn, Sr and Mg in different ratios. Moreover, four cement blends made with CEM I 52.5 R and aqueous solutions of Mg(NO3)2·6H2O, Zn(NO3)2·6H2O and Sr(NO3)2 were investigated. Zn retards the reactivity of both glasses and cements system. This retardation effect is suggested to be related both to the formation of the calcium hydroxozincate hydrate, which affects the portlandite formation and, thus, the pH of the system, and to the formation of surface complexes which retards the dissolution and so the reactivity of the glasses
Characterization of polycarboxylate-ether based superplasticizer on cement clinker surfaces
No full textThe application of Ordinary Portland Cement (OPC) based binder is commonly improved by the addition of rheology modifying agents, i.e. superplasticizers. In this work the interaction between polycarboxylate-ether based superplasticizers (PCEs) and the hydration products formed in aqueous solutions, after 30 min on polished OPC clinker surfaces is studied. Energy-dispersive X-ray spectroscopy provides the chemical analysis and the localization of the different phases below the superficial hydration products. Time-of-flight secondary ion mass spectrometry allowed to detect for the first time on which cement phases PCE preferentially adsorbed, and is used to map the distribution of the chemical elements at the surface of the clinker. Force measurements in liquid, performed by atomic force microscopy (AFM) with sharp tips, test the surface-tip interactions in presence/absence of PCE. The results show that superplasticizers interact strongly with K + and SO 42- ions, precipitating in K 2SO 4salts. When PCE traps the sulfate ions, the concentration of this ions specie in the solution is reduced, inhibiting ettringite formation. AFM force measurements allow the distinction between positively and negatively charged hydration products, revealing that the interaction forces between different phases are homogenized by the presence of PCE, providing stability to the suspended particles. © 2012 The American Ceramic Society
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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