GENERAL THEORY OF HARDENING MATRIX SUBSTANCES IN ISK...

GENERAL THE THEORY OF CURING MATRIX SUBSTANCES IN A CLAIM

Hardening is a complex process of transition of the matrix substance of ICS from a liquid or liquid (viscous-plastic) state to a solid state. In non-burnt conglomerates, the astringent exhibits the first signs of solidification even at the stage of its mixing, when there are associations of molecules or chemical compounds that accompany structure formation at subsequent stages of the technology. The avalanche nature of the increase

The symptoms of the beginning and developing solidification are characteristic for the special treatment stage. In firing conglomerates, the curing process takes, as a rule, a shorter period of time compared to the non-fired ones, and passes, mainly, when the products molded or molded completely or partially from the melt, as well as sintered during firing. But even here the formation of individual structural elements and chemical compounds occurs even at the stage of melts with the transition to solidified alloy.

Each kind of inorganic and organic binders (for more details, see chapter 3) cures under the influence of specific factors. All binders harden under the influence of a number of common factors, which gives the solidification process a regular character, allows it to be directed and managed in a general way. The formed solid body is characterized by the stability of the structure and the fixed position of the particles in it at sufficiently small distances from each other.

In the complex process of solidification of binders that make up the matrix part of the conglomerate structure, two stages can be distinguished, characterized by directly opposite changes in the curing system: dispersion - in the first stage, condensation and consolidation - in the second stage. Theoretically, the second stage in time follows the first, but it is practically impossible to draw a clear boundary between them, since many phenomena characteristic of the second stage often accompany the first, and vice versa. Both stages are superimposed on each other, although they have distinct distinctive features.

The first stage of the solidification process is characterized by a mass transition of a solid or solid substance entering the components of the matrix part of the ICS into a state of high dispersity to the size of molecules, atoms, ions or larger macromolecules, associates of atoms, aggregates, etc. Such dispersing favors transferring particles in the system to the least stable, metastable and at the same time into the most energetically active state. These conditions contribute to the free movement of particles with the inevitable thermal motion of them in the environment, the formation of collisions under the action of activation energy of previously absent compounds, associations and aggregates, new phases and other microstructural elements. Neoplasms often appear so quickly that they appear and accumulate in the system in the first stage of mass dispersion.

The transition of substances to the state of high dispersity during the technological period of ICS production occurs under the influence of various factors: chemical (hydrolysis), mechanical, thermal, physico-chemical (peptization), electrical, etc. The most characteristic for binders are: dissolution in a liquid medium, melting at high temperature, mechanical grinding (for example, in colloid mills). With all methods, the transfer of matter to a new aggregate state is usually accompanied by the expenditure of energy from an external source and its partial absorption by the emerging new disperse system. This system becomes more energetically active with increasing unbalance of all states. Such highly disperse systems are formed as true and colloidal solutions, suspensions or suspensions, homogeneous and heterogeneous melts, sometimes emulsions and emulsoids, as well as foams. Even greater importance for the first stage is not the difference in the aggregate state of the particles, but the nature and intensity of their interaction with the molecules of the dispersion medium.

The second stage of solidification is basic and is characterized by a gradual or accelerating process of transition of an unbalanced system into a new qualitative state - a solid stone-like product with a relatively stable and ordered microstructure with a partial formation of the crystalline phase. As the structure is ordered with microparticle enlargement to macroscopic size, the free energy of the system decreases. With a stable crystalline state of the solidified matrix material, it becomes minimal, remaining more significant in the amorphous substance. But the desire of the system to minimize the free energy in them due to the transition to the crystalline state does not always remain realized in the technological conditions.

Below, we briefly discuss the main highly disperse systems and their behavior in both solidification stages.

System clean medium. Such systems include dispersions, in the medium of which, during the first solidification stage, there was no dissolution of the solid phase. Typical representatives of such systems are water or an aqueous suspension consisting entirely of insoluble solids in it; metal, free from impurities, is in a liquid, molten state. Curing of such systems occurs with a decrease in temperature. In this case, the water jumps to ice crystals in an abrupt way, and together with an inert solid component (in the case of an aqueous suspension), ice forms into a kind of artificial conglomerate with a crystalline matrix. The metallic crystal lattice is inherent in elementary metals. In these systems the microstructure changes from liquid to solid and completely ordered - crystalline.

A system with a completely dissolved solid of the electrolyte type (true solutions). In true (molecular-dispersed) solutions, the particles are represented by atoms, ions, molecules of dissolved or associated astringents, which include salts - electrolytes and grounds. For true solutions, the most typical solvent is water (or aqueous solutions of certain chemicals) in which the particles of the solute are distributed evenly and constitute one phase, ie, a homogeneous system is formed. The solubility of solid particles in the medium increases with increasing temperature, and the dissolution process itself must be endothermic and accompanied by energy absorption. Under real conditions, however, when certain binders are converted to a true solution, the energy effect is often observed, which is expressed, for example, in the increase in the temperature of the solution. This indicates that not only aggregate transformations occur in the system, but also chemical interaction of binders with a solvent (water). The increase in temperature causes an increase in the randomness of the thermal motion of the particles in the solution and promotes a new interaction between the dispersed particles themselves, as well as between the latter and the solvent. There are new compounds and phases, which, although appearing in the first stage of solidification, are more typical of the second. The chemical compounds and phases that are formed are characterized by different types of bonding, which depends on the composition of the dissolved substance and the medium reacting with it. The most typical for these systems are ionic and covalent bonds with the formation of appropriate crystalline lattices during crystallization. The crystalline phase is formed gradually. The process of its formation begins with the appearance of micro-embryos as reaction centers and their development with an increase in size until the finer crystallites are isolated at a later stage. On the surface of these crystals or grains, as a kind of substrate, new crystalline formations of the same or another chemical composition arise. The regular growth of crystalline matter on the substrate is known as the manifestation of so-called epitaxy.

Another option for the formation of a stable microstructure is crystallization from supersaturated solutions. The allocation of crystals is preceded by the stage of appearance of embryos in the form of an ordered accumulation of a small number of atoms and associations of molecules that become centers from supersaturated solutions.

When cooling liquid solutions, crystals are formed, in the lattice sites of which there are alternately particles (ions, atoms, molecules) of various dissolved substances. When the crystal components of the substances are similar, they are soluble in each other in the solid state (solid solutions). In the case of incomplete mutual dissolution of substances in the solid state, an inhomogeneous conglomerate of two or more solids occurs,

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Fig. 2.10. Transitions of liquid into crystal and glass:

/- with rapid cooling, 2 - with slow cooling

In addition to the crystalline phase, solid phases can be formed with a less ordered or completely unordered arrangement of microparticles (glass phase). In them an excess reserve of free energy is preserved and, consequently, there is a tendency for further ordering of the microstructure under favorable conditions (Figure 2.10).

A system like colloidal solutions. These solutions are often called sols, and in the presence of an aqueous medium - hydrosols. They consist of particles up to 210 ~ m in size and are micro-heterogeneous systems with a huge total particle surface. The surface serves as an interface with the solvent and causes the development of adsorption processes inside the system. Characteristic for colloidal solutions is the interaction of their particles with molecules of the liquid medium. In the process of dispersing the binder to colloidal sizes in a liquid medium, coagulation is possible, which is more typical for the second solidification step. It means spontaneous aggregation of particles into aggregates, as a result of which a certain overlap of the processes of the first stage of solidification takes place in the processes of the second stage, just as it is observed in true solutions.

Condensation of these systems is associated with a constant tendency to reduce a large reserve of free surface energy due to the aggregation of particles through mainly molecular (van der Waals) coupling forces. Naturally, this process of condensation occurs spontaneously and is accompanied by energy release. The cause of the solidification of the colloidal solution can be caused by a decrease in the temperature at which a supersaturation of the solution appears, embryos appear in the form of amorphous microparticles. They move more or less rapidly into a relatively ordered (cryptocrystalline) position. The growth of particles of the new phase is accompanied by the fact that the medium becomes supersaturated with respect to them, as a result of which the speed and intensity of coarsening of the structuring particles of the whole system increase.

It should be noted that the coagulation mentioned above, also spontaneously flowing in this system, favors the coarsening (with decreasing energy) and the formation of a gel-like structure. In this gelatinous product, a significant part of the dispersion medium is retained, and sometimes the gel completely binds it. But it can again spontaneously isolate the medium (the phenomenon is called syneresis) with the compaction of the gel and a decrease in its volume. In the case of a non-uniform colloidal solution, the crystalline phase can be released during the formation of the gel. In general, the system is relatively stable and solidifies. Gels in their properties are divided into fragile and elastic.

Closer to the true than colloidal solutions are systems with very large, but not the same size molecules, mainly linear form. When these systems solidify, the linear structure is ordered. In addition to covalent, as the most common strong bond, hydrogen and molecular (for example, dispersion) bonds are represented in it. The ordering of such a structure, often of a local nature, occurs before the formation of crystals, which is expressed in the strictly regular arrangement of linear molecules.

Suspension type system. Suspensions are more coarsely dispersed than colloidal solutions. In them, the solid phase - the dispersed particles - remains in the solid state in the form of small crystals, and more often - fragments of crystalline and amorphous substances, practically insoluble or hardly soluble in a liquid medium. Concentrated suspensions are usually called pastes or dough.

The solid phase of the suspensions can precipitate in the liquid medium at a faster rate, the lower the concentration of the suspension, the larger the particle size, the lower the density and viscosity of the medium, the higher the temperature. The phenomenon of precipitation of the solid phase is known as sedimentation, which can be prevented by stirring. If the solid particles are represented by inhomogeneous aggregates, then their selective (ie selective) dissolution in a medium with the decomposition of aggregates into composite small parts is possible, which is accompanied by their transition to a molecularly disperse (true) or colloidal state with subsequent electrolytic dissociation of the molecules into ions .

The structure of the suspension as a dispersed system becomes more complicated in this case, since colloidal and molecular dispersed particles, as well as ions of decaying molecules, appear in addition to solid insoluble particles. As the particles are grinded (usually due to thermal, vibrational and other external factors), the Brownian motion increases, the number of collisions of particles per unit of time increases with the formation of new phases and aggregates, which in mass amounts is observed, however, in the second solidification stage. The process of interaction of dissolved particles with each other and with molecules of the dispersion medium (usually water) is intensified by heat and moisture treatment, autoclaving, and other kinds of technological influences on the curing slurry (paste).

Under the influence of external factors and to a greater extent spontaneously avalanche develops a complex of chemical reactions with the formation of new compounds and phases. Crystalline (through the reactions and supersaturation of the solution) and amorphous substances arise, which together form a solidified microconglomerate. It usually presents various neoplasms in the form of crystals and gel with their specific mass ratios. The latter depends on the initial binder, the concentration of the slurry (paste), the external conditions, etc. A transition of some of the initial solids to the microconglomerate as a matrix of the conglomerate without a noticeable change in composition is possible. It should be noted that Portland cement and its varieties are typical representatives of substances curing under this scheme.

The melts are liquids obtained by high-temperature heating of silicates, aluminosilicates, phosphates or other initial solids with the transition to another aggregate state of either their total mass or only its fusible part.

Heating and melting of raw materials leads to thermal dissociation of molecular compounds, radicals and other particles into simpler ones. They acquire increased activity to the subsequent interaction with each other with the formation of new compounds and phases.

The viscosity of the melt, which depends on the composition of the raw material and the temperature, plays an important role. As the viscosity decreases, the melt loses the initial ordering of the structure to an increasing extent and, at the same time, the movement of the microparticles is accelerated in it. The melt becomes a high-temperature binder; in it the level of free and surface energies rises.

With subsequent lowering of temperature in the melt, more stable compounds arise, from which a crystalline phase is formed, the mass formation of which is related to the second stage of solidification.

The crystallization of the cooling melt begins at a certain temperature corresponding to the melting point of this substance and the appearance of the largest number of micro-embryos. From the melt, in the first place, an excess component is released, which, spontaneously dropping this excess as a new crystalline phase, brings its residue in the melt closer with a further gradual decrease in temperature to the composition of the eutectic. In eutectic points, simultaneous crystallization of two or three phases or more is possible. First, substances containing high-valence ions with small radii that ensure the most dense packing in the crystal lattices crystallize.

When melt solidifies, the eutectic law manifests itself: the desire for such a mixture, which is ensured by its transition to a hard alloy at the lowest (eutectic) temperature.

Many melts of silicates, aluminosilicates, borates and others are capable of supercooling, passing into solid vitreous substances. The higher the cooling rate, the faster the state of the supercooled liquid (glass). Approximately at a viscosity of 10-10 Pa, the vitreous material becomes brittle, which corresponds to the glass transition temperature. With such a huge viscosity, the glass does not change its amorphous structure. The process of solidification (crystallization and supercooling to glass phase) can be accelerated, for example, by vibration, by the introduction of catalysts, by irradiation with radioactive substances, (3-rays.

In the melts, as in solutions, the gas phase can be present as the main or by-product of chemical reactions. It may also appear under the influence of pore-forming additives, evaporation, etc. In these cases, the pores of the binder are more or less filled with gas, which can be accompanied by new chemical reactions with the release of new phases.

The system of binding contact hardening. To this system are astringent amorphous and unstable crystal structures that are able to condense at the time of the contact between particles when they approach them at a distance of the surface forces of attraction . The fossilization of these binders is not related to chemical processes and changes in the volume of the solid phase. Provision of stronger contacts between particles of the binder is achieved by applying external pressure. At low pressures, the presence of very small amounts of a liquid medium as a kind of lubricant is useful in the system. The most important for this system is the preparation of a substance in unstable, crystalline or amorphous states. Therefore, in the first solidification stage, technological operations are performed to ensure the formation of a disordered structure. To this end, depending on the type of feedstock, heat treatment is applied prior to removal of the crystallization water and maximum amorphization of the substance, deep hydration without the formation of a crystalline phase, etc. The hardening (or more accurately the petrification) of the powdery binder occurs at the time of the formation of strong bonds between the particles of the amorphous substance and the ordering of the structure along the contact boundaries with the transfer of the metastable state to the stable one.

In the second stage of solidification of the matrix substance, in all possible systems, which include real binders in micro- and macrostructural building conglomerates, the processes result in more or less ordering, lower entropy, the transition of the system to a relatively more stable, if possible, crystallization state. But the second stage does not end only with the condensation of substances; At this stage, there have also been processes of consolidation - consolidation, reinforcement of the newly formed structure at the micro and macro levels. The process of the second solidification stage is a consequence of the continuous qualitative and quantitative changes of the liquid medium (c) and the solid phase (F) in the system.

At the final stage of solidification, the amount of liquid medium in the system becomes minimal, and the amount of solid phase - the maximum, ie, the value of the ratio c/f gradually decreases, approaching some optimal value. Significantly, their quality characteristics also change. Part of the liquid medium (c) from the free state passes into the chemically bound, colloidal-solvated, supercooled (glassy) state, the vapor-gas phase, etc. Some of the remaining liquid in the free state dissolves the lyophilic ingredients of the mixture, becoming a metastable solution. The solid phase (F) changes its molecular composition and microstructure, with the transition, as a rule, to other types of coupling as compared to the original solid. It can exist in various systems in crystalline, crystalline, amorphous, glassy or gel-like states. It is sometimes possible to remove some of the solid matter from the system through sublimation (sublimation). Qualitative and quantitative changes lead to an increase in the concentration of the solid phase, a decrease in the average distances between the particles, consolidation and consolidation of the structure, ie, consolidation of the hardening agent.

Hardened matrix substances, that have passed into a stone-like state, for example, cement stone, gypsum stone, asphalt cement, filled polymer, cements of high temperatures - ceramics, glass, slags, stone casting, etc., occupy a certain part of the structure in the corresponding artificial conglomerates, performing in them the function of a cementing bundle. Coarse-grained or otherwise, a mineral or organic mixture that forms a much larger part of the ICS volume and functions as a filler in it, is cemented together, forming a single monolith with an astringent part as a matrix. A small proportion of the binder directly adjoins the surface of large and small aggregate grains, forming a thin contact layer, called the adsorption-solvate shell. It has an increased density and hardness in comparison with the rest (bulk) matrix part. The contact layer forms in the structure of ICS a continuous spatial grid of the binder, or a matrix of the conglomerate. Boles details of the structures formed as a result of the solidification of molded and compacted products and parts are set forth in the next chapter. In conclusion of the general theory of solidification, it should be noted that in practice, a compulsory dispersion technique has been developed, including the mechano-chemical treatment of the initial mixture. The smaller technological art has manifested itself so far in the realization of the second stage of solidification - the condensation and consolidation of solidifying systems, although chemical, thermal, heat and moisture, autoclave, porous and other methods are used for this purpose.

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