STRUCTURE OF BUILDING MATERIALS AND PRODUCTS - Building materials science. T 1

STRUCTURE OF BUILDING MATERIALS AND PRODUCTS

By the structure or internal structure of building materials, as well as other physical bodies, we mean the spatial arrangement of particles of different degrees of dispersion, located in stable mutual bonds (primary or secondary) with a certain order of adhesion between them themselves. In the concept of the structure includes, in addition, the size and arrangement of pores, capillaries, phase interfaces, microcracks and other elements. In the ICS structure, there are microdispersed and macrodispersed parts.

By a microstructure is meant the location, relationship and interrelation of different or identical in size atoms, ions and molecules, from the totality of which the substances are composed in certain aggregate states. The formed atomic-molecular structure, located in a relatively stable equilibrium, predetermines the macroscopic features of the material. At a macroscopic level, the stable location, interconnection and order of coupling of macromolecules, micelles, crystals, crystal fragments and intergrowths, amorphous and other relatively large particles and elements composing materials, as well as the ratio of components, phases and interfaces to more complex material system - a conglomerate (composite material).

The main shape of the arrangement of microparticles in space is the crystal lattice. Each type of connection corresponds to its characteristic type of crystal lattice, namely: the ionic lattice; Molecular, or polarization grating, formed with the help of Van-dsr-Waals forces; atomic with a pronounced covalent bond; metal; lattice with hydrogen bonds. A feature of solids is the interdependence, or correlation, of the positions of neighboring atoms with short-range and long-range orders. In crystalline lattices, the long-range order extends to large regions, and the short-range order to the environment of a given atom. Under real conditions, crystals usually have deviations from an ideal geometric shape due to a number of side effects in solidification processes.

Solids, which have a crystalline structure, are amorphous. The most common representative of amorphous bodies is glass. The disordered arrangement of atoms and molecules in amorphous bodies complicates their structure. It is often judged on some indirect indicators. So, for example, amorphous substances, when heated, unlike crystalline materials, are able to melt gradually, without having a certain melting point; they have isotropy, i.e., identical properties in all directions. The ordering of the arrangement of particles is observed only in small volume elements (short-range order). In these zones, the structure is referred to as a crystalline structure: between the crystals occupying microvolumes there are layers of a completely amorphous substance.

Crystalline solids have very significant features: a fixed melting point - a complete transition to the liquid state; a certain geometric shape of the crystals, which remains characteristic of the substance; anisotropy, expressed in unequal properties in different directions. The thermal effect of crystallization is the main criterion for this phase transformation.

Crystalline and amorphous structures can be inherent in the same substance, for example, crystalline quartz (cristobalite) and quartz glass have a common chemical composition Si0 2 . One and the same crystalline substance can be in various forms (modifications) of the existence of crystals, which is known as polymorphism. Polymorphism causes a change in properties while maintaining a constant composition of matter, which once again points to the crucial role of structures in the development of the quality of the material. So, for example, diamond and graphite, being crystalline carbon modifications, have different

Hardness: diamond is used for drilling solid rock, graphite is soft and is used as a lubricant or pencil rod. Another example is the crystalline modifications of iron produced by heating and cooling: in the high-temperature form of the crystals, iron is capable of dissolving a relatively large amount of carbon, while at a low temperature, carbon is practically insoluble, and when the iron is cooled from the dissolved state, carbon becomes a mechanical impurity. When other crystalline bodies are modified, other properties change.

The structure does not remain unchanged, frozen. & quot ;. It continuously undergoes changes in space and time. This is facilitated, in particular, by the constant movement of elementary particles, the interaction of material with the environment, the transition of matter from one state to another under the influence of redistribution of bonds between atoms in molecules, changes in the structure of molecules and other chemical forms of motion of elementary particles. Relative stability of the structure and external shape of macroscopic bodies is due to certain relationships and relationships of structural elements, and the forms of changes and transitions of their states are manifested in unavoidable thermal, heat and mass exchange phenomena, crystallization processes, etc.

The microstructure and kinetics of its change are studied using optical methods, electron microscopy, differential thermal analysis, X-ray diffraction, etc. A relatively simple measurement made on the observation plane is determined by calculation of the content of some key element of the structure in the volume of the material. >

Depending on the nature of the bonds of the particles being contacted, homogeneous microstructures are divided into coagulation, condensation and crystallization.

Coagulation refers to structures in the formation of which relatively weak forces of molecular interaction between particles are involved - van der Waals cohesion forces acting through interlayers of the liquid medium.

Condensation is called a structure that arises from direct interaction of particles or under the influence of chemical compounds in accordance with the valence of the contacting atoms or under the influence of ionic and covalent bonds.

Crystallization (or crystalline) refers to structures formed by the crystallization of a solid phase from a melt or solution and the subsequent direct fusion of individual crystals into a solid aggregate, including under the influence of chemical bonds.

Academician P.A. Rebinder, who divided the microstructures into these three varieties, noted the possible, and even more typical, formation of mixed structures as aggregates of two or three homogeneous, for example, crystallization-coagulation, etc. Under certain conditions, a spontaneous transition with a different speed of the coagulation structure in condensation-crystallization, etc. The real character of microstructures is related to a certain extent to representations about their qualitative characteristics. So, for example, with coagulation structures, a reduced strength of a substance is almost always observed, the ability to thixotropic restoration of a structure destroyed under the influence of mechanical action, for example, vibration. Condensation and, especially, crystallization structures give the substance an increased strength, but at the same time increase brittleness and reduce thixotropy. Some modifications of crystals of one substance can have a low (for example, graphite) and very high (for example, diamond) hardness and strength.

The microstructure in the ISK extends to the knitting part. To give the astringent of the required quality, additional active ingredients are added-additives. The sizes of their particles are commensurable with the sizes of particles of initial binders and emerging neoplasms, therefore they are an element of the microstructure of ICS.

Sometimes a very significant volume in the microstructure is occupied by the pores - closed and communicating, or both of them of different origin at the same time, which depends on the variety of the cementing substance. The pores are small, for example, up to (1-2) -10 cm, as a rule, closed, resulting from shrinkage phenomena; larger after, for example, evaporation of capillary moisture (capillary pores) with a size in a cross-section up to 5-10 "cm, open or communicating among themselves; even larger (macropores), conventionally taken in a spherical shape, ranging in size from 50-100 microns to 2-5 mm. The number of large pores depends on how they originated in the binding agent: involuntarily or intentionally. With the involuntary involvement of air during the preparation of a mixture, their quantity is usually small (2-5%). If they arise under the influence of specially introduced air-entraining or pore-forming additives, then in a porous binder, up to 50% or more of the volume of spherical (cellular) pores can be concentrated, most often closed.

In microstructures, there may be other types of looseness. They are usually classified as microstructure defects that adversely affect the quality of the material. Among them: crystal lattice defects in the form of so-called vacancies caused by "evaporation" atom from the lattice site, or in the form of dislocated atoms, i.e., displaced in the interstitial space of the crystal lattice, or as impurities in the crystal lattice with a significant distortion in the quality of the substance compared to pure and ultrapure substances. Especially dangerous are defects in the form of microcracks, capable of growing under the load and moving into a macrocrack or a main crack that grasps the macrovolumes of crystalline aggregates and bodies.

In addition to the binder, microfine structure also has near-surface layers or contact zones in the material that separate the astringent from the surface of the other component, for example, aggregate grains, phases from each other. The composition and structure of thin contact layers (mono- and polymolecular) differ from the main binder. It differs from other volumes of material and the quality of these layers, since it depends on the boundary defects of the structure, the adhesion strength of the substances to be contacted, etc. The difference in the quality of the near-surface layer and the remaining volume of the binder is not spasmodic but rather smooth.

Layout microparticles occurs compactly, according to the known physicochemical principle of the most dense packing. This principle is characterized by the fact that when laying atoms, ions or molecules in the crystal, the smallest residual, free space arises. However, such a principle is not applicable to all types of crystals, since a denser packing may result in a less stable equilibrium, which depends on the direction of the valences of the particles being contacted.

The macrostructure of ICS is discernible to the naked eye. It is formed under the influence of the cementitious ability of the binding agent, due to which the polygrain or other forms of aggregate particles - fibrous, lamellar, angular, spherical, etc. - are fastened together into a common monolith. The macrostructure also contains a capillary-porous part, and macrodispersed pores and capillaries in cellular concrete in the form of closed cells filled with a gas or air medium are, as it were, a kind of filler.

Like a compact package of discrete particles in the microstructure of binders, a mixture of coarse aggregates is selected with the smallest volume of intergranular voids, which saves on the consumption of binders as the most expensive components and reduces the average thickness of the continual layer of binding material in the conglomerate.

For this purpose, the granular aggregates are preliminarily divided into fractions in size, and then the content of each fraction in a dense mixture of ICS filler is found, either experimentally or by calculation, which follow either continuously after each other and thus obtain a continuous granulometric composition, or excluding certain fractions and obtaining an intermittent granulometric composition of the mixture. A unique method of selecting the granulometric composition of a dense mixture, suitable for aggregates of non-burning and firing ICS, was proposed by P.I. Bozhenovym. This method is based on intermittent granulometry. He established that the ratio of the grain sizes of any two adjacent aggregate fractions in the ideal case (with the spherical shape of the grains) is:

where di is the average grain diameter of any ith fraction, mm; dt * is the average grain diameter of the adjacent fraction, mm.

The number of largest grains of the fraction Pi entering the aggregate is numerically equal to its average density in the condensed state: Pi = y 0 bi • V, where y & lt; , бі - average density, kg/m; V is the volume of the fraction, m.

The number of all fractions following the largest (for placement in the same volume V), is determined by the formula:

Here: P, is the mass of the/th fraction, kg; y is the density of the material, kg/m;

2 , & lt; p3, ..., φ/are the filling factors for the volume of intergranular voids.

There are other formulas for calculating the amount of fraction y (%) passing through a sieve with cells of size dt (mm) with the size of the largest fraction of the mixture D (mm). Thus, according to Andresen's formula y - ( dJD ) '• 100; for example, for the fraction d, = (0.2-0.1) mm and for D = 3 mm y = (0, 15/3.00) - • 100 = 22%. For refractory materials [49], a close packing is achieved using the formula: y = [a + (1-a) ( dilD)] ■ 100 (%), where a - coefficient depending on the properties of the mass and the content of the fine-milled component (within the range 0 n is the exponent characterizing the distribution of narrow fractions inside the coarse-grained and fine-grained constituents in the mixture, equal to 0.5-0.9. For example, if D = 3.0 mm, di = 0.06 mm, and the coefficient a = 0.31, then y = 0.31 • (1-0.3) • (0.6/3.00) '= 42%. The optimal value of the quantities a and n for a given D is found experimentally.

Fig. 2.11. Schemes for the location of large particles in structures when determining the packing coefficients

If large particles, for example gravel and gravel, are brought together in such a way that they contact directly with each other or through thin interlayers of binding material, then the formed structure is called a contact structure. If the particles are separated by interlayers of an astringent of considerable average thickness, then the macrostructure is usually called porphyry. In the first approximation, the structure can be estimated from the packing factor. The coefficient of packing Ku is understood as the value obtained by dividing the size of the projection of the distance (Figure 2.11) between adjacent large grains (particles) on the plane to their diameters: K y = (l-d) ld, where/is the projection of the distance between the centers of neighboring grains (particles); d is the diameter of the particles (grains) for which this coefficient is calculated, and if the particles (grains) have different diameters, then d - n + r, where r and ry are the radii of neighboring particles (grains). Positive values ​​of the packing ratio characterize the porphyry structure, and negative - the contact structure. When ЛГ У = 0, the particles are in contact with each other without engagement, i.e. without entering them one by one.

With a maximum tight packing of particles of spherical shape and one diameter with an amount of 74% by volume, the packing factor is -0.1. An increase in the number of particles in the conglomerate (for example, crushed stone) leads to a further increase in the negative value of the packing ratio, i.e., to a greater engagement or entry of the rubble with each other, which characterizes the end-to-end structure. But the value of the packing factor is affected not only by the quantity, but also by the size of coarse fragment particles. The larger the grain, the smaller the amount of crushed stone is required to ensure that the packing factor is negative.

When evaluating the nature of the structure, instead of the packing ratio, one can use the numerical value of the ratio of the volume of the filling part to the volume of the conglomerate. For relationships close to unity, the structure is contact.

In addition to the aggregate, a powdery material is often added to the mixture, the particles of which are commensurable with the particle sizes of the astringent used in ISK, and neoplasms - crystalline, amorphous, crystalline, etc. They are called fillers.

Fillers and fillers can be active, inactive or inactive. The active ones are those that, when

The addition to the astringent enhances the strength of the ICS of the optimal structure for at least one type of stress - compression, stretching, shearing, etc.

Strengthening of the binder with the use of active filler (filler) occurs under the influence of additional physicochemical or chemical interactions of the substances to be contacted, or due to the reinforcing effect (for example, in fibrous varieties of aggregate or filler).

Inactive or low-active varieties of fillers and fillers not only do not contribute to hardening the optimal structure of ICS as their content increases, but also reduce strength characteristics. The degree of activity of these components of the conglomerate mixture can be controlled by means of solid or liquid activators and surfactants, by enriching, crushing round (eg gravel, gravel) grains, by mineralization (for example, using organic aggregates in combination with inorganic binders), washing, swings, etc. at the stages of preparatory work of the accepted technological process. Below (see Chapter 3), the formula for the strength of the ICS of the optimal structure is given, in which the numerical value of the power exponent n sufficiently fully reflects the qualitative characteristics of the aggregate and the dimensions of the change in the strength characteristics of the conglomerates under the influence of the methods used to activate and compact the mixture. One of the important technological problems at work is the complete reduction of the numerical index n.

It follows from the foregoing that all ICS have micro- and macrostructures. Sometimes, if necessary, the mesostructure is provisionally allocated, i.e., intermediate in particle dispersion between micro- and particulate matter. It goes without saying that such a subdivision of a monolithic ICS structure is purely conditional. In real conditions, it is impossible to isolate such structural elements without destroying the monolithic nature of the entire system. The structure of ICS, the more optimal, constitutes a single, indivisible, whole system. However, the mental separation of its individual parts from a monolithic structure is of great methodological significance, since it allows modeling calculated rheological schemes, corresponding mathematical formulas for determining qualitative indices, which depend on the physical characteristics of these basic structural elements, with some approximation. At the technological stage of manufacturing the ISK, the conditional division of the structure into two or three parts can be used for practical purposes, for example, in performing operations of so-called separate technology, when the prepared astringent of a certain composition is combined by means of combined mixing with a separately prepared filling part. However, regardless of the sequence of operations in the technological period, taking into account all their positive and negative features, the formed structure after solidification of the binding element and ICS as a whole becomes monolithic, unified and indivisible.

Repeated studies have shown that with selective (selective) dissolution of micro- and macrostructural parts, the conglomerate either completely collapsed, or almost completely lost its qualitative indicators. Therefore, sometimes an expression used to characterize a conglomerate (composite material) as a "structure in the structure" can be attributed only to the combined systems mentioned in the ICS classification (see Figure 1.1); they belong to manufactured ICS impregnated in the subsequent stage of the technology with another astringent substance with cavity filling and the formation of a second continuous spatial grid of binder, for example, in cementopolymer concrete. Two interconnected structures are formed, which are possible not only mentally, but also practically separate from each other, although, naturally, with some deterioration of the quality of the combined conglomerate.

The unified and monolithic structure of the conglomerate can be optimal and non-optimal.

Optimum structure is characterized by: a uniform distribution of filler volume, phases, components, pores and other constituent elements; absence or minimal content of defects as voltage concentrators or accumulators of aggressive media; the presence of a continuous spatial grid, or matrix, of an astringent; the minimum value of the ratio of the mass of the medium to the mass of the solid phase, termed conditionally as the phase ratio; The greatest packing density of solid particles in both micro- and macro-structural parts. If there are no binding layers in the material, then one of the conditions for the optimality of the structure is the largest contact surface and the interaction of solid particles or its decrease, if chemical bonds, for example, van der Waals, do not provide effective strengthening of contacts.

The product does not always have the same optimal structure in all its details, for example, the surface layer may differ from its interior. However, the optimal structure is always a reflection of the accepted technological features of its formation in production conditions.

The structures that do not satisfy at least one of the above mandatory conditions for optimality are called non-optimal

With the same technology of manufacturing the mixture and product, other identical conditions, you can get an unlimited number of non-optimal structures, much less - optimal and one or two - rational. The latter include the optimal structures under which the conglomerate fully meets the specified and, at the same time, extreme quality indicators, as well as the actual production parameters. When designing, it is important to focus on the composition, in which the structure formed with this technology and with the adopted modes is not only optimal, but also rational.

Optimal structures correspond to improved quality indicators of materials in comparison with non-optimal structures. This improved quality is due to increased density, minimal amount of liquid medium, increased concentration of solid, for example, crystalline, phase, minimum pore volume in contact areas and a number of other reasons, especially energy. Under optimal structures, the free Gibbs energy and the Helmholtz free energy become minimal, since they transform into more efficient forms of coupling.

In theory, ICS has been developed, and in practice a general method for designing optimal compositions and optimal structures for various non-burning and calcining materials is used (see Chapter 3). With its help, the material obtained from the accepted components satisfying the specified technical requirements with the optimal structure, i.e., extreme quality indicators, is obtained.

The advantage of optimal structures is also in their similarity with each other, which means, in particular, a fundamental commonality of the regularity revealed in relation to any one material. The similarity of materials of the optimal structure is described in 3.3.

Currently, the regulatory parameters of the optimal structures and technical properties are being updated to replace obsolete standards.

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