1,720,985 research outputs found
Polyamorphism and frustrated crystallization in the acid-base reaction of magnesium potassium phosphate cements
No full textMagnesium potassium phosphate cements are a class of acid-base cements for bioengineering and civil engineering applications. The kinetics of their chemical reaction was investigated in situ with isothermal conduction calorimetry and powder X-ray diffraction, quantifying amorphous and crystalline products. The first reaction step, dissolution of MgO, with an apparent activation energy of 71 kJ mol-1, dictates the time evolution of two amorphous intermediate precursors and the crystalline product. The early crystallization of the latter has been described with an Avrami equation with an apparent activation energy of 81 kJ mol-1, pointing to a mechanism of deceleratory nucleation and growth in one direction, compatible with the acicular crystal habit observed with electron microscopy. The observed polyamorph transformation is controlled by a complex interplay between kinetic and thermodynamic factors, in which the changes in the chemical environment (increase in pH) driven by the MgO dissolution, play a crucial role. It is proposed that the onset of the amorphous-amorphous transformation hinders crystallization by decreasing ion mobility, raising the energy barriers to structural reorganization. The rate of MgO dissolution depends on the reactivity of the powder and the parameters of the mix (such as the amount of liquid) and influences the reaction pathways, impacting on material performance
Investigation of setting reaction in magnesium potassium phosphate ceramics with time resolved infrared spectroscopy
No full textCrucial information on reaction products in magnesium potassium phosphate ceramics, essential for material design, is lacking. Setting reaction has been followed with time resolved infrared spectroscopy, supported by 2D correlation spectroscopy. We found evidence of a first amorphous orthophosphate precursor, possibly MgKPO4·H2O, forming early, and of a second intermediate amorphous phase, more structurally similar to MgKPO4·6H2O, forming later. Crystallization of MgKPO4·6H2O occurs from this last phase. Presence of amorphous Mg(OH)2 and magnesium phosphates identified as precipitates from diluted suspensions, was excluded. This complex set of parallel reactions, bears analogies with reaction of zinc phosphate cements and is consistent with recent NMR results
Amorphous-crystalline transformation control on the microstructural evolution of magnesium phosphate cements
No full textMagnesium phosphate cements are chemically-bonded ceramics exhibiting high strength and good adhesive properties. In-situ investigation of the reaction of Mg-K phosphate cements, indicated that microstructural evolution is controlled by a cement densification process entailing amorphous-to-crystalline transformation during hardening. Owing to the replacement of the amorphous fraction with a material of higher density, possessing elongated crystal habit, an increase in porosity with time occurs, at variance with previous models. The beneficial effect provided by the developed interlocked lath-shaped microstructure, constituted by crystals embedded in the amorphous matrix, prevails over the detrimental role of the increased porosity, explaining the increase in mechanical strength
Investigation of amorphous and crystalline phosphates in magnesium phosphate ceramics with solid-state 1H and 31P NMR spectroscopy
No full textMagnesium phosphate ceramics are chemically-bonded ceramics for bioengineering and civil engineering applications. Information about the nature of the reaction products, especially the amorphous fraction, and their role in the setting reaction, essential for linking reaction mechanisms to microstructure and performance of ceramics, is lacking. By exploiting 1H and 31P magic-angle spinning nuclear magnetic resonance spectroscopy, in addition to the crystalline reaction product (MKP), two amorphous phases, characterized by two distinct proton and phosphorous environments, have been identified. The results pointed to amorphous hydrated orthophosphate compounds which are the precursors of MKP. They show different 1H spin-lattice relaxation dynamics, and, higher water mobility with respect to MKP. Although these amorphous precursors should not be crypto-structural variants of MKP, they likely host similar structural units. Conversion into MKP is thought to occur through relatively minor rearrangements, as in Ca phosphate hydrates. The new information provided in this work allows us to propose a model for the setting reaction based on an existing theory involving a densification process, analogue to sol-gel processing of ceramics, coherent with results from small angle neutron scattering and mechanisms predicted by reaction kinetics analysis
Nucleation of amorphous precursor in magnesium phosphate cements: Clues to the reaction pathway
No full textIn-situ X-ray techniques indicated that in its early stages, the reaction pathway of magnesium phosphate cements involves the formation of an amorphous precursor, first detected as nanoparticles which organize themselves into larger domains. Crystallization occurs from this precursor (already containing the structural motives of the crystalline counterpart) by increasing the long-range order through relatively minor structural rearrangements. The process is compatible with multi-steps nucleation involving non-classical mechanisms, as observed in Ca-carbonates and phosphates, therefore, indicating ways to control the transformation to improve cement design
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
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
Physical-chemical-mechanical quantitative assessment of the microstructural evolution in Portland-limestone cement pastes exposed to magnesium sulfate attack at low temperature
No full textThe changes in structural integrity and microstructure of Portland-limestone cement pastes were investigated in the course of magnesium sulfate attack at low temperature. A deterioration front, consisting of three distinct layers (brucite, gypsum, leached cement matrix), swelled in time due to the expansive nature of the deterioration products, generating cracks and subsequently detaching from the sound cement matrix, continuously promoting the process. Gypsum and thaumasite characterized the leached matrix, which experienced extensive cross-linking of the aluminosilicate structures, as a result of decalcification and dealumination of the calcium silicate hydrates (C[sbnd](A[sbnd])S[sbnd]H), impairing the overall mechanical performance. C[sbnd]S[sbnd]H of low packing density was most severely affected by the process, as confirmed by the significant drop in nano-mechanical properties. The increased rate of deterioration with limestone content was tentatively attributed to the prevalent morphology of the C[sbnd]S[sbnd]H phase. Results were validated by thermodynamic simulations, indicating that the real systems did not reach equilibrium
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
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