1.5. Basic processes of structure forming

Formation of natural and artificial construction materials occurs as a result of complex physical and chemical processes. The main ones are the processes of dissolution, hydration, coagulation, polymerization, crystallization and sintering. Either of these processes are decisive depending on the type of materials. For example, artificial stone materials are obtained by hardening of mineral binders, mainly by hydration and crystallization processes; but organic binders are obtained by polymerization and polycondensation processes.

Depending on the character of binder hardening, the processes which take place may be divided (Table 1.3) into three groups: hydration, coagulation and polycondensation (polymerization).

Ceramic materials formation is determined by the reactions at sintering processes, for metals by the processes of melting and crystallization.

Dissolution of solid in a liquid is destruction under the action of dissolvent for formation of solution - homogeneous system, which consists of dissolvent and molecules or ions which have passed into solution. If there is no chemical interaction, there can be reverse crystallization of dissolved substance. Substance ability to dissolve in contact with a solvent depends on the change of Gibbs energy ΔZ. The only substances that are soluble are those in which ΔZ<0. The measure of solubility is a concentration of the saturated solution. Solubility of most substances grows with temperature increase but it can diminish also (Fig. 1.17). There are also supersaturated solutions which are unstable, as they are not in thermodynamic equilibrium with a solid phase.

T rue or colloid solutions can form at dissolution of solids. True solution is characterized by the homogeneous distribution of molecular or ionic size particles. The systems with the size of disperse phase particles between 1 and.100 m are considered to be colloid solutions. The requirements for colloid solutions formation is low enough solubility of disperse phase and presence of substances stabilizing colloid particles. Colloid particles can also appear due to polymerization or polycondensation on the basis of particles of ionic or molecular size.

Hydrationbinding materials processes in accordance with modern ideas take place mainly as a result of the chemical interaction of dissolved substance with water (hydration through solution).

Hydration is thermodynamically possible also in a solid phase without previous dissolution of initial substance (topochemical process).

Dissolution and hydration are the major processes which predetermine hardening of mineral binding materials. The mechanism of hydration processes depends primarily on water-solid ratio. The particles size of hydration products varies from colloids to the crystals, which can be observed with microscope. Formation of mainly colloid hydration products is a characteristic for Portland cement, autoclaved materials, crystalline products - for gypsum binders, magnesia cements.

Spatial coagulation structures form in highly-concentrated disperse systems with developed interface (Fig. 1.18). Such structures are intermediate for hydration and polycondensation or polymerization binders, for binders of coagulation type they are basic. Thus, in clay suspensions dispersible clay particles – micelle - are negatively charged which prevents their cohesion. After the loss of charge (drying out) clay particle cohere and coagulate. The gels formed during coagulation lose water with time and are subjected to recrystallization.

Table 1.3

Classification of binding materials

Hydration		Coagulation		Polycondensation (polymerization)
Air	Hydraulic	Inorganic	Organic	Inorganic	Organic	Elemental-organic
Gypsum Air lime Magnesia	Hydraulic lime Portland cement Roman cement Pozzolanic cements Slag cements Alumina cement Expanding cement Autoclaved binding materials	Clay	Bitumen Tar	Waterglass Sulphuric cement Phosphoric acid cements	Phenol formaldehyde resins Furane resins Polyester resins Epoxy resins	Organic-silicon Hydrolyzate Ethyl siliconate

Organic binders – bitumens - also form micellar colloidal structures. The solid part of (asphaltenes), is surrounded by protecting bodies (resins), forms micelle, suspended in oils. Stability of the system depends on surface interaction between micelle and oil medium.

Coagulation structure - gel, predefined by raised content of asphaltenes is characteristic of solid bitumens at the temperature range 20...25°С. For the bitumens of liquid consistency with high resins and oil content, the structure of colloid solution – sol is characteristic. The structure of bitumens under the action of temperature, changes inversely. Under the action of air oxygen oxidization (polymerization and polycondensation) occur in bitumen which results in its ductile-to-brittle transition (ageing).

P olymerization and polycondensation are underlying processes of forming polymer materials. The process of bonding of low-molecular products (monomers) without formation of by-products is called polymerization. Monomers which enter into polymerization reaction, are more frequently the compounds with multiple bonds (double, triple) or cyclic compounds. In the first case polymerization takes place as a result of multiple bonds opening under the influence of any power action, in the second one - as a result of cycles opening.

Polycondensation is the formation of polymers, which is accompanied by release of low-molecular substances (water, ammonia, hydrogen chloride, etc.). Thus mass of polymer obtained unlike the polymerization process is smaller than the mass of original substances and its elemental composition does not coincide with the elemental composition of compounds entering into reaction.

An example of polycondensation reaction for obtaining polyethers is as follows:

(1.7)

Polycondensation takes place stepwise. It can be conducted in melt, solution, emulsion, suspension, solid phase both with catalysts and without them.

Processes of crystallization occur in the supersaturated solutions of mineral binding materials. They can also pass as structure forming polymer compounds. The process of crystallization in Polymers is determined by their chemical composition. The degree of crystallization of most polymers varies from 10 to 90 %. Crystallization processes may be divided into elementary process of formation of crystal nucleus and a secondary process of their growth.

The increase of mechanical strength of binders and concrete during hardening process is predefined formation of crystalline aggregates. The hardening structure develops in two stages:

1) Formation of crystalline structure framework with formation of intergrowth contacts.

2) Framework creation without new contact formation.

Crystalline contacts between particles form, if they approach each other at thermal motion and diffusion at a distance not greater than the thickness of the doubled hydrate molecules adsorption layer. The value and kinetics of supersaturated liquid phase deeply influences the retained strength of hardening materials.

Crystallization processes take place not only in the supersaturated solutions of hardening binders but also during formation of different materials from the fusions - rocks, glass, slags, and metals. Crystallization from fusions flows at a temperature lower than equilibrium one.

Fusion hardens in the glassy state if between temperature of intensive formation of crystals nuclei and temperature of crystals growth is wide difference. Usually, the glassy state becomes at rapid cooling of silicate melts. Glass can be considered as the supercooled liquid, it is more similar to melts, than to the solids by its structure. Glass is the unstable form of a state and at certain regimes of heat treatment it can be crystallized. The process of glass crystallization is exothermic. Crystallization ability of glass depends on its chemical composition. For example, the presence of small amounts of MgO and Al₂O₃ simultaneously interferes with the crystallization of silicate glass, but the introduction of fluorine fastens the process.

Obtaining the glass-ceramic materials (pyrocerams) which have particularly high mechanical properties and resistance is based on the crystallization ability of the glass of certain composition.

For ceramic and some other fired material structure forms at sintering( -strengthening and compacting of a material, taking place at high temperatures and accompanied by shrinkage). The essence of sintering consists in slow filling with a substance caused by increasing of elements mobility of its lattice at the high temperatures in free space inside the grains and between them. The amount of unbalanced grains is also increased in the sintering process, the defects of their lattice diminish and the stresses occurring at the bonding areas of a material are also relaxed. It is normal to distinguish sintering in a solid phase and sintering in liquid phase. Sintering can be also mixed (so-called solid-liquid).

The basic factors which influence sintering are the radius of grains and temperature. The surface area of grains in mutual contact also influences the sintering rate. Surface area is higher for polydisperse powders than for monodisperse ones.

The most common sintering of materials often takes place in the presence of liquid phase. The rate of liquid-phase sintering is proportional to the radius of grains and viscosity of the liquid phase. As the grain size diminishes from 50 to 0.5 mkm, sintering speed increases a hundredfold.