Papers by Keyword: Structure Formation

Abstract: The processes of structure formation in hardening disperse systems are considered from the standpoint of synergetics and catastrophe theory. It is proposed to use a topological approach to identify general patterns of behavior of such systems, based on the possibility of modeling the transition of smooth quantitative changes into radical qualitative ones. Based on the analysis of literary data, a group of curves of the structure formation kinetics is identified, the extreme form of which reproduces the geometry of the simplest catastrophe «fold».Spatial models describing the kinetics of dispersion hardening are constructed and analyzed. It is shown that the agreement between the experimental and model kinetic curves is expressed not only in the external similarity of the nature of the dependencies, but also in their logical generalization.It is noted that the canonical «fold» model can also be effectively applied in the study of anomalousrheological behavior of dispersed systems.

Abstract: The paper discusses the results of studying the hardening process of a gypsum-based composite material using a thermal imager. Thermal imaging research involves periodically obtaining thermal images of hardening material with pronounced hydration exotherm. It is assumed that the intensity of hydration and heat release depend on the state of the heterogeneous system, the structure of the forming material and the degree of aggregation of particles of hydrating components. The resulting images contain both visible and hidden information about the physical and chemical processes occurring in the material. To fully obtain such information, computer image processing methods were used. Intensity histograms were constructed and analyzed, for which regular changes were observed in the process of structure formation. As a generalization of the observed patterns, a working hypothesis is proposed about the filling of scales of physicochemical characteristics and, in particular, large-scale structural scales, in the process of structure formation. An image processing algorithm has been developed that makes it possible to construct isothermal cells‒areas of material with the same temperature. The geometric characteristics of the resulting areas, forming a partition of the study area, were studied by an automated method and reflected using histograms. The interpretation of the influence of a changing temperature distribution on the properties of a material is based on the idea of an approximate correspondence between a network of temperature cells and a Voronoi network that defines regions of a disordered structure. The spatiotemporal features of thermal processes are considered, indicating a possible decrease in the strength characteristics of the material.

Abstract: Some features of the rheological behavior and morphology of systems which contain liquid crystalline polymers are considered from the standpoint of the theory of catastrophes. It is noted that a decrease in the mixing time of melts of polysulfone mixtures, including a liquid crystal component, significantly changes the configuration of the rheological curves and the morphology of the flow. It was found that the appearance of characteristic N-shaped breaks on the rheological curves of such mixtures is informatively described and analyzed on the basis of the standard «fold» model. It was revealed that the concentration dependences of the viscosity of poly-p-benzamide samples of various molecular weights have a «cusp»-type feature inherent in the projection of a «ruffle» type catastrophe on the plane of control parameters. The possibility of a visual illustration of the general picture of the transition of these solutions into the liquid-crystalline state on the basis of this canonical model was shown.

Abstract: The relevance of the use of nanosized particles in the structuring of cement pastes is due to the ability at the micro level to control the processes occurring during the hardening of cement. And it also provides the ability to purposefully control the composition of hydrated neoplasms. Plasticizer-stabilized aqueous suspensions of nanosized particles of non-hydraulically active materials in a cement matrix cause forced crystallization of hydrated neoplasms from a supersaturated liquid phase. And they act as seeds introduced from the outside, on the surface of which neoplasms are concentrated, that subsequently combine into conglomerates that make up the structure of a cement stone. The formation of the structure of the cement matrix occurs without the formation of large the portlandite fields, which are typical for no additive portland cement. The structure of a cement stone with nanosized particles is represented by poorly crystallized flaky calcium hydrosilicates. It was found that the degree of hydration of cements with suspensions of nanoparticles is 15-18% higher. The efficiency of using nanosized particles in structuring cement pastes is giving the cement paste mobility in the initial period of hydration and the possibility of plasticizing it without blocking the surface of cement particles with surfactants. On the other hand, it makes it possible to obtain dense and durable structures of a cement matrix with improved physical and mechanical characteristics. The porosity of cements with suspensions is lower on 15 - 20%, and after 28 days of hardening it is lower on 20 - 35% than that of non-additive cement. And the strength of the samples after 24 hours increased 1.5 - 1.8 times (from 19.5 to 27 - 34 MPa), at the brand age by 15 - 24% (from 64 to 75 - 80 MPa)

Abstract: A method for producing cellular materials on a glass-crystalline matrix has been developed. The formation of a porous structure is ensured by the process of gas formation, combined with a vibration effect on the raw mass. The possibility of gaining the plastic strength of the raw mass, sufficient for fixing the cellular structure of the raw material without adding binders, has been established. The main component of the raw material mixture is a finely ground filler, obtained after grinding glass-crystalline frit of a specially selected composition. For its formation pre-compacted raw masses, containing natural silica and glass-forming additives, are heated. There was investigated the joint influence of the compositions formulation and the roasting parameters on the properties indicators of the glass composite.

Abstract: To this day, there is a large volume collected of the results of experimental studies on structure changes in various dispersions serving as base for production of most construction materials. The analysis of collected information revealed that there is an entire category of stick-slip phenomena, the case history of which is represented by N-and S-type inflections on rheological, kinetic and other curves. We should emphasis that the view of such non-trivial charts is alike with geometry of standard curves of standard conditions. And this alikeness predetermines the possibility of applying topological models of «fold» and «ruffle» types for studying various abnormal effects. We must also note that besides N- and S-types there is a range of other characteristics («flags») pointing to applicability of the methods of catastrophe theory to studying certain processes initiating the apparition of interruptions in system development. Recognition of above-mentioned particularities allows determining the fact and type of catastrophe, the standardised structure of which facilitates finding strict patterns and thus defines directions of optimisation of various situations of research and practical nature. This work shows that pieces of evidence and consistent patters are reliably interpreted within the framework of the proposed concept.

Abstract: The paper describes the methodology of processing of the experimental data from the study of the hardening process of the binder paste, which allows to reveale information hidden in the results and objectively divide the period of structure formation into steps that differ in mechanisms and conditions. The proposed method has a universal character, is designed to study compositions of a heterogeneous structure and is implemented using experimental data on the plastic strength of gypsum binder paste as a simplified model of such a composition. It consists of two stages - the transition to normalized values and coordinate transformation to adequately display the observed processes of structure formation. To determine the type of the corresponding coordinate transformation, Erofeev and Kolmogorov equations were considered, which describe, in particular, crystallization processes. Based on the proposed equations, a physically reasonable choice of coordinate transformations of the re-logarithmic type was carried out. Processing the data on the plastic strength of the hardening gypsum binder paste as a model of the composition of an inhomogeneous structure allows us to identify the main stages of structure formation characterized by linear dependencies and transition stages in which qualitative transformations of the regime of change in plastic strength during hardening are observed. The separation of the total time of structure formation at the step is fixed on the graphs of changes in the corresponding properties, and the transition stages correspond to the points of change in the angular coefficients of the obtained dependencies. It is shown that the stationary step of structure formation corresponding to the stable maintenance of supersaturation is reflected in the general linear section on the graphs. It is shown that the time of occurrence of transition stages recorded during the measurement of plastic strength approximately corresponds to the setting time obtained by regulatory methods. A geometric interpretation of the angular coefficients of the obtained dependences which allows one to determine the structure of growing nuclei of a new phase was considered. The proposed technique can be applied to control the processes of structure formation of heterogeneous composition compositions.

Abstract: A detailed theoretical analysis of the processes occurring in foam concrete mixtures in the period between their laying in the mold and the phase transition “from viscous to solid” completion moment. The stages of the formation of inter-pore septa in foam concrete mixtures, depending on the material nature and shape of the solid dispersed particles of raw materials, are sequentially considered. It is shown that their shape affects the mass transfer rate and the density of inter-pore septa during the structure formation. The reasons for the fluctuations in the elasticity of foam films during the predominance period of viscous bonds between the components of the raw material are described. The gas inclusions factors of influence on the dispersion parameters and, as a consequence, the aggregative stability of foam mixtures, are ranked. The positive dispersed reinforcement effect on the mass transfer processes in mixtures and the hardened concrete strength is substantiated.

Abstract: This paper presents a numerical simulation of powder sintering. The numerical model presented in this paper uses the discrete element method, which suggests that the material can be modeled by a large set of discrete elements (particles) of a spherical shape that interact with each other. A methodology has been developed to determine the DEM parameters of bulk materials based on machine vision and a neural network algorithm. The approach is suitable for obtaining the exact values of the DEM parameters of the investigated bulk material by comparing the visual images of the material’s behavior at the experimental stand in reality and in the model. Simulation of sintering requires an introduction of cohesive interaction between particles representing interparticle sintering forces. Numerical sintering studies were supplemented with experimental studies that provided data for calibration and model validation. The experimental results have shown a significant capability of the designed numerical model in modeling sintering processes. Evolution of microstructure and density during sintering have been studied under the laboratory conditions.

Abstract: One of the effective ways to reduce the material consumption of structures, to save all types of resources due to this, is the transition to porous constructional materials instead of traditional dense materials. In this regard, non-autoclaved gas concrete is of great interest. This material has a high manufacturability and has a good potential for improving the strength properties due to the replacement of traditional portland cement by special composite binders. Available publications confirm the effectiveness and prospects of this direction. An important disadvantage of such works is the fact that the binder indicators and the final cellular concrete characteristics are considered mainly independently. In our opinion, it is more correct to consider at least two structure-forming processes proceeding in parallel and sequentially at different scale levels. Micro level is a stone structure formation based on the composite binder; macro-level is a gas porization of cellular mass concrete. These processes have a great mutual influence on each other, and therefore this article attempts to observe the gassing products effect on the stone hardening based on composite binders, as well as the composite binder makeup on the viscosity change of the molding compound, as an important condition for the formation of high-quality less defective pore structure of cellular concrete.