Basic types of crystalline lattices of cement clinker minerals

Mineral	Grate
Tricalcium Silicate	Triclinic
 - Dicalcium Silicate	Rhombic
Tricalcium aluminate	Cubic
Tetracalcium aluminoferrite	Rhombic
Dicalcium ferrite	Monoclinic

Distances a, b, c between the centers of neighboring atoms in an elementary cell are called the lattice periods and are measured in nanometers (1 nm = 10^-9 m).

The density of cristallyne lattice is characterised by a coordination number (i.e the amount of atoms), nearest to the current atom. Half of the minimal distance between atoms or ions in crystal lattice is called the atomic(ionic) radius. When the coordination number increases, the atomic radius reduces because the distance between atoms increases.

T he most stable state of crystalline structure of a substance is achieved, when cation come in contact with all the surrounding anions. In other cases the elements of the lattice reorder, forming an unstable structure with the diminished coordination number. Unstable structures with a low co-ordination number have greater chemical activity. For example, in metakaolin - a product of the process of dehydration of the clay mineral kaolinite, the coordination number for Al³⁺ ion is 4, whereas for the original kaolinite, the coordination number is 6. Consequently unlike kaolinite, metakaolin can chemically react at normal conditions, and also during autoclaving treatment with calcium and magnesium hydrosilicates.

The structure of crystals is divided into five classes by the parameters of their interatomic distance into: coordinative, isle, linked, layer and frame structures.

In coordinating structures which are representative of oxides, salts, etc, the distance between basic units (atoms, ions) is equal in order. In isle structures, there are groups of the separated atoms which form "islands" (calcite, pyrite, most of organic compositions). Linked structures are characterized by separated atomic groups which form continuous links (wolastonite CaSiО₃, diopside CaMgSi₂О₆, etc.). The layer structures have endless two-dimensional atomic layers (graphite, talc, etc.). Framework structures consist of volumetric three-dimensional anion frame of coordinating type and neutralizing "filling" types from cations or atomic groups. KAlSi₃О₈ exemplifies such orthoclase structures.

B ecause of the different density of ions or atoms in different planes and directions crystals properties depending on direction are different. This feature of crystalline solids is called anisotropy. It is not a characteristic of amorphous solids (glass, polymers and others like that). Anisotropy reflects mostly in linked and layer structures.

Mineral construction materials and metals are mostly in the polycrystalline state, including most of the chaotically oriented crystals which decreases anisotropy. Some treatment methods (e.g., cold hammering of metals) can be accompanied by the spatial orientation of crystals and can cause the anisotropy of such materials.

Depending on the chemical bond type between structural elements, it is possible to distinguish between ionic, atomic (covalent), metallic, and molecular bond (Fig. 1.3) as well as crystals with hydrogen bond.

I onic bond is typical for crystals which considerably vary in electro-negativity of their structural elements.

Most cations are smaller in size than anions, and the crystalline lattice of ionic compounds form as a result of placing cations in the voids between anions. The characteristic properties of ionic crystals are low conductivity, heat-conducting, brittleness and high temperature of melting.

In the atomic crystalline lattice points there are neutral atoms, bounded by covalent bond. These bonds are very strong. Consequently, substances which have such lattices, are hard, refractory and practically insoluble (diamond, silicon, compounds of some elements, with a carbon and silicon - carbides and silicides). Covalent crystals are formed of atoms which have nearest values of electronegativity. If the difference in electronegativity of elements increases, the degree of transition of the covalent bond into ionic grows.

Typical bond of silicates Si - О is half ionic and half covalent. Oxide compounds of silicone, containing siloxane bond (Si - О), occupy an important place in modern developments in construction materials. Silica (SiО₂) is the most spread and stable silicone compound, it occurs naturally and exist mostly in crystalline state. According to modern conceptions silica consists of tetrahedron [SiО₄]^–4, bonded by points into separate complexes in such a way that every atom of oxygen is common to two neighboring tetrahedrons and is connected with two atoms of silicone (Fig. 1.4).

Silica tetrahedron is a basic unit of all of natural and artificial silicates. Complexes of silica tetrahedrons form a closed annulus or endless links. Combinations of links form endless strips which can form layers by turn. Known examples of stratified silicates are talc, mica and kaolinite. Part of the Si⁴⁺ ions can be substituted by the Ai³⁺ ions.

I n metallic crystals, electrons play an outstanding role. They move freely among the atoms. Positive metal ions oscillate in the points of such crystals, and valent electrons move through the lattice in different directions. The totality of free electrons is sometimes called electron gas. Such lattice structure cause high heat and electric conductance and plasticity of metals. Mechanical deformation of crystalline lattice within certain limits does not causes crystals destruction since the ions, composing them, as if float in the electronic gas cloud.

The groups of atoms or molecules, bonded between themselves by Van der Waals forces and dipole interactions, are placed in the points of molecular crystalline lattices. Van der Waals forces grow when the amount of atoms in a molecule and their polarity increase. Molecular forces are comparatively weak, therefore molecular crystals, characterized by row of organic matters, fusible, and volatiles, have low hardness. For example, the crystals of paraffin with a molecular lattice are very soft, although covalent bonds С - С between atoms of hydrocarbon molecules are strong enough.

The type of widespread connection in the inorganic crystals is due to hydrogen ion bond, between two anions tightly linked together. Formation of hydrogen bond, involving hydrogen and oxygen atoms, matters very much in the structures of water and many other compounds. Formation of hydrogen bond explains presence of associated molecules (H₂O)_n in water. The doubled molecules (H₂O)₂ are the most strong, and its formation is accompanied by two hydrogen bonds appeal which are the most stable.

According to polymerization theory the molecules of water can exist in the forms of hydrol H₂O, dyhydrol (H₂O)₂, and also trihydrol (H₂O)₃. Ice (Fig.1.5) consists mostly of molecules of trihydrol, which are characterized by the highest volume of voids and therefore the least density for water, and water vapor forms from the molecules of hydrol.

Hydrogen bond explains anomalous properties of water to a great extent: the high dielectric constant, surface tension, capacity for moistening and dissolution of many substances.

Hydrogen bonds predetermine the polymerization of some organic acids and promote the formation of many inorganic polymers. The hydrogen bond formation partly causes hydration of polar groups, and also hydrophilic property at the proper surfaces of materials.

Energy of covalent, ionic and metallic bonds is 126...420 kJ/mol, molecular one - does not exceed 42 kJ/mol, and hydrogen bond is 8.4...42 kJ/mol.

A few types of bonds act in most crystals taking into account character of their estimated theoretical strength and other properties.

There are always different defects of crystalline lattice of real materials. By geometrical features they are divided into point, linear, surface and volume. Point defects can be caused by thermal vibrations in points, influence of radiation and electromagnetic waves (energetic imperfection), changes in electrons distribution on energy levels (electronic defects), displacement of atoms from a midposition, by the presence of admixture atoms, presence of empty points - vacancies (atomic defects).

Among the point defects mentioned above, the atomic ones that increases atoms (ions) mobility in a crystalline lattice, resulting in increase of the diffusion permeability and ionic conductivity in crystals are of great significance.

If a substance is crystallizing from the solution or melt in the presence of foreign atoms, these atoms can enter into the lattice structure of the basic compound and form solid solutions. Foreign atoms penetrate into the lattice of basic crystal in two ways (Fig.1.6): 1) They occupy the key points of crystalline lattice, substituting the particles of basic component (solid solutions of substitution); 2) This takes place in the merithalluss of crystalline lattice (solid solutions of penetration).

Substitution formation of solid solutions (Fig. 1.6, a) is a distinctive feature for obtaining most ceramic materials, cement clinker, etc.

Solutions of penetration (Fig. 1.6, b) belong to the solid solutions which are characterized by changeable composition. As a rule, atoms and ions of small sizes adjusted to the voids of crystalline lattice are able to penetrate through merithallus. Mostly all solutions of penetration occur in metallic materials. Hydrogen, boron, carbon, nitrogen and oxygen form such solutions.

T he basic types of linear defects of crystals are dislocations (Fig. 1.7), along and near-by which order in the location of atomic planes disrupts.

U nder the action of tangential stress dislocations can move, due to plastic deformations in crystals. Dislocations are the sources of internal stresses, areas of crystal close to them are in the plastic-elastic state. Even the negligible quantity of dislocations can reduce strength of materials by several digits.

Threadlike crystals have a small amount of dislocations and therefore extremely high strength close to theoretical one. These crystals can be effective microreinforcing fireproof materials and other elements used for special applications. Occasionally strengthening of materials is achieved by introduction of alloy elements to prevent dislocations movement.

Structure of amorphous materials (Fig 1.8), as well as the structure of liquids is characterized by the so-called short-range order, when the well-organized state is observed only between the neighboring particles of a material. The main peculiarities of amorphous or glassy, structures are derived from the isotropic properties and absence of constant melting temperature.

The absence of crystalline lattice causes smooth variation of amorphous materials properties at the solid-liquid transition. The amorphous substances can be considered as supercooled liquids, but unlike liquids in them there is not a rapid exchange placed between nearby particles, what explains their high viscidity.

X -ray photography methods are the main methods used for the investigation of the structure of materials. In this method, X-rays, passing through crystalline lattices, are exposed to diffraction, as interatomic distances are compatible with length of X-ray waves. Every crystal has an X-rays diffraction pattern with characteristic lines which differ by location and intensity. By defining the distance between planes and relative intensity of lines, it is possible to define phase composition of the probed material by comparing with tabular information, preliminarily compiled for the known substances. Decoding of X-rays diffraction pattern also enables an investigator to define character of defects, type of elementary unit, position of atoms or ions and other peculiarities of atomic-molecular level of material .structure.

Spectroscopic methods for assessing the structure of materials are based on quantum concepts. For a phase analysis infrared spectroscopy which is based on the ability of compounds to absorb the rays in the infrared area of spectrum preferentially is usually applied . The method enables investigation of the character of chemical connections in materials, their valency states and a series of other structural features. It is based on the phenomenon of electronic paramagnetic resonance, which consists in resonance absorption of energy of the radio frequency field in matters which contain paramagnetic particles at the imposition of magnetic-field. X-ray spectroscopy is applied in studying the energy features of atomic-molecular structure and in conducting chemical express-analysis.

Application of electron bunch with wave length in several times less than lengthes of visible light waves is the fundametnal statement of electronic microscopy, which permits to give a possibility to study objects with the sizes 6-100 m at the enlarging up to 200 thousands times.