1.2. Submicrostructure
The submicroscopic level concern the structures of materials, which are formed by colloid-size particles between 10–9 and. 10–7m and can be distinguished in an ordinary light microscope. Most construction materials belong to the heterogeneous disperse systems which consist of two and more phases. In the disperse systems one or several substances (disperse phase) are fine particles or pores, distributed in a disperse medium. Dispersity of a system is characterized by the specific surface area(= ratio of general area F of disperse phase to its general volume) and the mass.
It is also possible to distinguish micro-heterogeneous systems, where disperse phase contains particles over 10–7 m in size , and colloid systems with the size of particles of disperse phase measuring between 10–9 and 10–7 m. Disperse systems are divided into three basic groups which differ by phase composition of disperse environment - solid, liquid and gaseous. Thus disperse phases in the systems of every group also can be in three aggregate states.
In construction materials which belong to the disperse systems, a disperse phase consists of solid particles more frequently. There are various powders, suspensions, pastes, yielding liquid-like mixtures, binders, plastics, paint-and-lacquer materials, ceramic masses, mortars and concrete mixtures, melts of glassy substances, etc. In some materials a disperse phase can be liquid (polymer emulsions) or gaseous (porous rocks, cellular concretes, foam glass, foam plastics, etc.).
For the colloid systems it is possible to apply a series of molecular kinetic theory positions. In particular, in colloid solutions (sols) the same as in true ones, disperse particles are able to take part in thermal motion. At the same time sizes of colloid particles, considerably larger than ordinary molecules sizes, predetermine the insignificant osmotic pressure of colloid substances, their slow diffusion. All the colloid systems are resistant to sedimentation, because the gravity in them becomes balanced diffusion.
Molecules of surface layer are under the action of molecular pressure, that is why the remains of energy forms at the phase interface, amount of energy (Е), which is per 1 сm2 of surface area (f) is called surface tension ():
. (1.2)
Minimization of free energy and transition of the system in the thermodynamic stable state become possible due to diminishing of the phase interface, which is achieved by the arbitrary coagulation or agglomeration of particles in the colloid systems. Free energy can decrease due to the surface tension decline at active substances absorption on the phase interface – adsorption. This process can be described by Gibbs equation:
, (1.3)
where G - adsorption, mol/l; - surface tension, J/cm2; С - solution concentration, mol/l; R - gas constant, J/mol∙K; Т - absolute temperature °К.
M
ost
organic materials, containing both polar and non-polar groups in
molecules are in the group of surface-active substances (SAS). This
peculiarity of SAS molecules structure explains their ability to
adsorb at the `phase interface and
orient in such manner that polar groups (ON, COOH,
NH2
and others) are directed to the polar phases of the system (for
example, to the water molecules), and non-polar (hydrocarbon chain)
are directed to the non-polar phase (e.g, air). In technology of
construction materials the phenomenon of SAS adsorption on solid
surfaces is widely used, for the improvement of wettability
of solid surface by the liquids and also reduction of their hardness
(Rebinder effect), plasticity improvements and other property
changes.
Adsorption on solid surfaces (adsorbents) of nondissociated or slightly dissociated substances (molecular adsorption) is inversed and decreases with temperature increase. The process of adsorption of strong electrolytes from water solutions (ionic adsorption) is irreversible and its intensity can increase with increasing temperature.
Colloid particles have certain charges and move in the electric field to the oppositely charged electrode (electrophoresis). Because of the external difference of potentials a liquid phase in the colloid system is able to move in relation to an immobile hard porous environment (electroosmosis) (Fig. 1.9). The electrokinetic phenomena, which are characteristic of colloids, are used in the technology of construction materials. Thus, by electrophoresis it is possible to prepare ceramic masses for forming of porcelain and faiences, to separate fine particles, to get rubber products from latex. An electroosmosis is used for wood dehydration and in other technologies of industrial treatment of various porous materials.
As a result of redistribution of electric charge at the interphase boundary of two different chemical compositions there is a double electric layer which consists of two parts: denser internal and diffusive external. The difference of potentials between two parts of double electric layer is called electrokinetic, or ζ -potential (ζ -zeta). Potential is determined by speed of electro-osmosis or electrophoresis. It is a very important influence of the colloid systems, in particular, and represents their stability. The SAS and electrolytes additives make a significant influence on an index and sign of colloid solutions ζ-potential. If ζ- potential is equal to zero (isoelectric state), the system is unable to have electrokinetic characteristics. If ζ- potential is 25...30 mV, there is coagulation, i.e. aggregation of colloid particles. As a result of cohesion of disorderedly distributed solid particles of disperse phase in suspensions and colloid solutions, a coagulation structure is formed. Formation of such structures is typical for row of the materials, in particular materials based on mineral binders at the first period of their hardening. The distinguished feature of their hardening and coagulation structure formation is the presence of reverse contacts, i.e. freely renewable after destruction contacts (Fig. 1.10). Strength of these contacts is caused by weak Van der Waals molecular forces of adhesion through thinnest interlayers of dispersion medium, thickness of which correlates to minimal surface energy.
C
oagulation
structures
sometimes are
called gels.
Gelatinization
is transition of
colloid solution
from the
freely dispersed state
(sol) to
the bound dispersed (gel).
Different factors make influence on
gelatinization,
that is coagulation process, in particular form of particles,
concentration of disperse phase, temperature of mixture, types of
mechanical actions (mixing, vibration). Coagulation is caused by
electrolytes which contain the ions of opposite sign in relation to
colloid solution. Coagulation force of ion-coagulator is related to
its charge. For univalent cations it is approximately 350 times
weaker, than for trivalent cations.
A process, which is the reverse coagulation, (i.e., the passing of aggregated particles to the initial colloid state), is called peptization. It can take place under the influence of substances - which promote disaggregation of sediments (for example, additions of electrolytes, SAS). Thus, clay slurry at the ceramics manufacturing are peptized, that is diluted using alkalis. The effect of peptization by SAS is applied for dilution of raw pulp at the cement clinker manufacturing. The peptization mechanism consists in extraction from the sediments of coagulating ions or formation of double electric layers in the colloid particles as a result of peptizer adsorption by them.
Coagulation structures are rarefied also under mechanical actions such as mixing, shaking or vibration. This isothermal process which flows as a type gel - sol is called thixotropy. The phenomenon of thixotropy proves that structure-forming in the coagulation colloid systems takes place due to Van der Waals forces. After mechanical actions stops, bonds broken in the process of coagulation structure formation, become renewed. A thixotropy is widely applied in technologies of construction materials, for example, for the vibratory compacting of concrete mixture.
I
t
is possible to connect properties of colloidal solutions also with
their micellar structure. Micelle
is the minimum of colloid, which is
a complex formation in which particles of disperse phase (nucleus)
are in certain physical and chemical bound with solution through the
ionic double electric layer (Fig. 1.11).
Micellar structure impacts significantly the properties of construction materials. E.g., solid disperse phase of bitumen – asphaltenes form nuclei, covered by shell of liquid medium - from heavy resins to comparatively light oils. In the case of liquid medium excess micelle does not make contact with each other and move freely under the influence of Brownian movement. Such a structure is characteristic of liquid bitumen. On heating viscous bitumen, the gel type colloid solution destroys, but if the micelle concentration increases, the bitumen gains the gel structure.
Coagulation structures for many construction materials, in particular binders based, are primary. They transform into condensation crystallization structure in the course of time. The network of chemical bonds develop during formation of such structures (for example, during spatial polymerization, formation of gel-like silicic acid in water, burning-out of ceramic and other products). Condensation crystallization structures with characteristic irreversible contacts have high strength, low plasticity and do not renew after mechanical destruction.
Along with coagulation and condensation crystallization structures there can be structures of intermediate type. For example, if composition of solid phase and accordingly strength of coagulation structure exceeds some limit, its mechanical destruction becomes irreversible. Dried ceramic pastes, pressed by dry method moulding powders are among such objects.