117,292 research outputs found
Role of Glucose as Retarding Agent of Magnesium Phosphate Cement
No full textMagnesium phosphate cements (MPCs) find application as alternative inorganic binders in construction, for crack repair and recycling of hazardous wastes. For the most common formulation, setting occurs through the reaction in water between magnesium oxide and potassium dihydrogen phosphate. The products include MgKPO4·6H2O (MKP) and an amorphous phase. Their use is somehow limited by the short working time and excessive release of heat. In this work, glucose has been introduced in the formulation of MPC to extend the setting time and modulate the rate of heat evolution. This can be considered an inexpensive and sustainable solution. The mechanism of action of the additive has been studied by investigating the reaction with isothermal conduction calorimetry, whereas the microstructure and phase composition of the obtained cements have been studied with scanning electron microscopy and X-ray powder diffraction, respectively. Results indicated that the additive influenced the reaction path thanks to the interaction at the molecular level with the dissolution process of magnesium oxide, as well as with the nucleation and growth of MKP. This has been confirmed by the changes induced in the size and shape of MKP crystals observed after the experiments conducted on diluted systems
Properties enhancement of magnesium phosphate cement by cross-linked polyvinyl alcohol
No full textA novel ceramic composite has been obtained by introducing polyvinyl alcohol in magnesium-based chemically-bonded ceramics and exploiting the cross-link reaction with glutaraldehyde during setting. The properties of the obtained material and the interaction of the polymer with the cement reaction have been investigated. Several beneficial effects were observed. Namely, an extension of the working time, a reduction in the water sensitivity, a decrease in the rate of the heat evolution during hardening. The latter has been ascribed to the intervention of surface adsorption processes, which hindered the dissolution of MgO, as well as the nucleation and growth of the magnesium phosphate products, in concert with the film forming ability of the polymer. Thanks to the complementarity of their mechanical properties, the cement and the additive operated in a synergistic fashion, allowing for the obtainment of a material possessing higher strength and better elastic properties. Three-dimensional quantitative image analysis from synchrotron X-ray microcomputed tomography evidenced the development of a more compact microstructure, comprising a higher number of crystals of smaller size. As a consequence, the pore network exhibited a higher fraction of small pores and lower pore connectivity. These characteristics contributed to hinder the water absorption, as confirmed by the fluid transport simulations within the volume. The observed correlation between the solution pH and the polymer cross-link density offers the way to effectively modulate the material performance by acting on the chemical environment thanks to its compatibility with the cross-link reaction
Mechanism of magnesium phosphate cement retardation by citric acid
No full textCitric acid modulates the release of heat and the rates of reaction in magnesium phosphate cements, chemically-bonded ceramics employed in biomaterials, for the encapsulation of nuclear wastes and in civil engineering. To gain knowledge on the mechanism of action of citric acid and, therefore, help in the effective material design, the reaction was studied in-situ to address molecular issues. The results indicated that citric acid enhances dissolution of MgO by promoting surface ligand-exchange reaction which leads to a net acceleration of the first reaction step. The Mg2+ ions released in solution are complexed by citrates. The degree of supersaturation is therefore reduced, delaying the nucleation of phosphates. The growth of stable nuclei, the crystal growth, and the amorphous-to-crystalline transformation are hindered due to citrate adsorption. The formed surface complexes are prevalently inner-sphere complexes exhibiting the combined coordination of hydroxyl and carboxylate groups. The mutating chemical environment dictates the coordination modes of citrate, the competition with phosphates, and the stable forms of phosphate products
Organic-inorganic composites based on magnesium phosphate cement and acrylic latexes: Role of functional groups
No full textThe role of carboxyl functional groups in acrylic latex employed to fabricate an organic-inorganic composite material based on magnesium phosphate cement has been investigated. The acidic nature of the latex aqueous medium enhanced the dissolution of the magnesium oxide in the first stages of the cement reaction. The following increase in pH promoted the deprotonation of the carboxyl groups, which became involved in surface adsorption effects. Adsorption processes were found to control the nucleation and growth of the reaction products. The resulting overall hindering effect slowed down the reaction rates and delayed the precipitation of the solid phosphates with beneficial consequences, namely, the retardation of setting time and the modulation of the heat released. Modification in the morphology of the formed crystals, with the prevalence of platelet-like over prismatic habit, along with a decrease in their average size, was obtained. The crystals formed in higher amounts with respect to the neat cement because the reaction proceeds closer to equilibrium. The obtained microstructure is strengthened because of a more effective intermingling between crystals and the amorphous phase. Furthermore, the synergistic combination of polymer and phosphate cement improved the elastic properties, and reduced the water absorption, impacting positively on the durability of the composite
Aqueous polyacrylate latex nanodispersions used as consolidation agents to improve mechanical properties of Prague sandstone
No full textThis investigation aims at assessing the potential of polyacrylate latex nanodispersions as consolidation agents for sandstones. Four different latex types, implementing polymer fluorination and chemical crosslinking, have been synthesized at the scope and fully characterized. The Prague sandstone employed in this study has been selected as an example of highly porous stones used as building materials for many historical monuments. Two different concentration levels of nanodispersions have been adopted. The consolidated stone samples have been tested using a combination of physical-mechanical tests and microscopic observations. Compared to the non-treated samples, significant increments of mechanical properties (e.g. up to 3.3 times higher bending strength) have been detected after the consolidation treatment with concentrated products. Moreover, when 10 times diluted latex nanodispersions have been applied, the improvement of mechanical properties has still been significant, while moisture transport properties, such as water absorption, have been found to be comparable with those of the untreated sample. Fluorinated polymers imparted better hydrophobic properties with a contact angle above 100°. Observations using a scanning electron microscope revealed the good filling and bridging capacity of the applied consolidation agents. As demonstrated, by acting on the polymer structure, that is, by tuning the degree of crosslinking, polymer fluorination and gel content, the properties of these novel polyacrylate latex nanodispersions can be tailored to the specific stone and type of decay in order to improve the effectiveness of the treatments and obtain the desired final properties. The flexibility of their chemistry offers new opportunities for preserving objects of cultural heritage that are also at risk due to the ongoing climate change
The effect of 3D structure design on fire behavior of polyethylene terephthalate glycol containing aluminum hypophosphite and melamine cyanurate
No full textEffect of the shape of 3D printed samples on fire behavior of polyethylene terephthalate glycol (PET-G) and PET-G additivated with a mix of aluminum hypophosphite (AHP) and melamine cyanurate as flame retardant, was investigated. The additives improved fire performance (e.g., maximum average rate of heat emission, total oxygen consumption, heat release rate indices) irrespective of structural complexity, favoring carbonaceous char formation. However, at increasing structural complexity, they promoted higher release of smoke, compared to neat PET-G, because of a change in the prevalent retardation mechanism, which became dominated by the flame inhibition action of AHP. Consequently, the synergistic effect obtained combining the two additives, was hindered. Impact of product design on mechanisms of fire retardation helps in devising engineering solutions aimed at meeting required level of fire-safety performance, which should be tailored to the specific product
Design of polymeric binders to improve the properties of magnesium phosphate cement
No full textIn the context of reducing the environmental impact of cement manufacturing, magnesium phosphate cements raise interest as alternative binders in construction, for immobilization of wastes, and recycling purposes. Their use in applications is somehow limited by short setting time, brittleness and low water resistance; this calls for the use of additives. Two polymer additives were designed adopting emulsion polymerization, an environmentally friendly solution to make available polymers as water-based latex dispersions. The composites containing 5 wt% of polymer, exhibited better elastic behaviour, with up to twice the toughness of the reference sample and of a sample produced with commercial styrene-butadiene rubber latex. Moreover, the additives reduced the apparent porosity, promoted phosphate crystallization, modified the size and shape of crystals, and effectively retarded the reaction, extending working time. The acrylic emulsion developing keto-hydrazide self-crosslinking reaction imparted better properties to the composite, thanks to the synergistic effect with the MPC setting reaction
Study of keto-hydrazide crosslinking effect in acrylic latex applied to Portland cements with respect to physical properties
No full textPolymer-modified Portland-based composites are of interest for specific applications, in reason of their properties. There are different types of commercial additives and waste polymer-based materials applied to cement-based composites, however, their impacts on the environment are debatable. This work has prepared new acrylic latex additives with and without keto-hydrazide crosslinking from standardly available low-cost raw monomers. The influence of their incorporation into Portland cement-based fine-grained mortars has been investigated. The obtained results indicate that the highest effect on heat flow evolution changes has been detected in the case of latexes without crosslinking. The incorporation of both latex types into produced cement composites resulted in a significant increase in open porosity connected with the gradual decrease in mechanical resistance, especially the compressive strength. On the other hand, an important mitigation of liquid water transport properties of latex-modified composites has been achieved, and such properties can be tuned according to the used latex type and its concentration. The developed latex cement-based composites may find utilization as special materials for structures or products for water-loaded constructions or in areas with high concentrations of water-soluble salts or other pollutants
Struvite-K crystal growth inhibition by citric acid: Formation of complexes in solution and surface adsorption effects
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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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