1.3. Microstructure
The elements of materials structure which can be seen by use of an optical or electronic microscope are investigated at the microscopic level. Their size range is between 10–4 and 10–7 m, and they are characteristic of elements of the microheterogeneous systems. With reference to the concretes, they are characteristic of the structure of cement stone and contact layer, for ceramics they are the crystalline and glassy phases, and for metals they are linear, superficial and evident in volume defects, different phases, etc.
The typical microheterogeneous systems are powders, suspensions, emulsions and foams. For the microheterogeneous systems unlike colloid ones Brownian movement is not typical. Particles in such systems move under weight influence, therefore these systems are sedimentationally unstable.
Powders which are widely applied in construction materials can be considered as disperse systems, where air is a dispersion medium. Powders are obtained mainly, using the different methods of grinding. Dispersity of powders is controlled by the specific surface area (or just – by a specific surface) and grain distribution. Among the methods of specific surface determination of construction powders are methods based on measuring of layer resistance of the material under research to the air, passing through this layer is widely used. Blowing of air the heavier, the finer powder is. For porous powders an adsorption method is applied, which is based on dependence expressed by the following equation:
,
(1.4)
where Ads is adsorption on the powder surface (for example, amount of nitrogen which is adsorbed on the cement particles surface; N– Avogadro constant; So - surface area which is covered by one molecule of adsorbable matter.
For determination of grain distribution of powder screen and sedimentation analyses are used. Sedimentation analysis is related to speed of particles setting in a liquid medium changes depending on their sizes.
Diminishing of grains sizes in powders below critical level causes their agglutination and granulation. Granulation of powders takes place due to diminishing of surface energy of the system at particles agglutination. The wettability of solid phase surface by a liquid favors the activation of this process. It provides formation of a layer with raised viscosity which increases adhesive interaction at the interface.
Grains forms in powder materials can be distinguished as: isometric (spherical, polyhedral) and anisomeric (fibrous or needle-like, lamellar, etc.) grains. There are a lot of transitional forms of grains. The Anisometry of grains influences their location in space and results in anisotropy of powders properties.
Suspensions and emulsions are micro-heterogeneous systems where solid or liquid dispersed phases are allocated in liquid disperse medium. Suspensions are widely used in construction materials production at obtaining raw sludges, slurries, and mortars. Emulsions are applied particularly as paint-and-lacquer materials. Concentrated suspensions are called pastes. Microstructure of bitumen paste is shown in Fig. 1.12. For aggregate stability of suspensions and emulsions, that is for preventing coagulation (agglutination of emulsion drops, is called coalescence), it is required, that their particles should be covered by the shells made of disperse medium molecules (solvate shells). It is possible when disperse medium wets the disperse phase. Wettability of particles can be improved by SAS application. Formation of double electrical layer of ions around mineral particles facilitates system stabilization.
E
mulsions
can be direct and inverse (Fig.
1.13). Disperse
phase in
direct or
first type
emulsions is
nonpolar or
weakly polar
liquid (e.g.,
oil), and
disperse medium
is nonpolar.
Water soluble
emulsifiers
promote formation
of oil
in water
type emulsions
(O/W),
and nonsoluble
– water in
oil type
(W/O).
High-molecular
compositions and
soaps are
representative emulsifiers.
Powders which
are well-wetted
by disperse
medium and
have
substantially smaller grains
sizes than
disperse phase
particles can
also be the emulgators. Practically
in some
cases there
should be
induced
accelerated decay of emulsion.
For this
purpose
substances, which
have high
surface activity
and meanwhile
do not
form strong
films in
adsorbed layers
(demulsification agents)
are applied.
There are widely used emulsions based on the organic binders - bitumen, tar, polymers, etc. as construction materials
Bituminous emulsions are anionic and cationic. For preparation of the first type the anionic SAS are used (they are mainly high molecular-weight organic acids or their alkaline soaps). For the obtaining the second type emulsions emulsifiers of cationic type - salts of amines, amine-amide soaps, etc. can be used. For the acceleration or decay adjusting of emulsions 0.5... 1% potassium-aluminium alum, salts of chrome, magnesium, potassium are used.
It is considerably simpler to prepare bitumen pastes, which are the concentrated disperse systems consisting of bitumen, water and solid emulsifier, in the capacity of lime, cement or plastic clay.
Highly-concentrated systems in which gas is a disperse phase and liquid is disperse medium are foams. As construction materials, mainly heat-insulating ones, solid foams, where straps between gas bubbles are in solid phase (foam plastics, foam glass, gas- and foam concrete) are used.
To obtain stable foams foaming agents as highly molecular substances, soap and other compositions which have high activity of surface are used. The basic indexes of foams are multiplicity, dispersity and stableness. The multiplicity of foams refers to foam volume ratio to the volume of liquid or solid phase formed at sides of bubbles. Stableness of the foam is measured by the term of its existence and depends on the durability of films. Solid foams have unlimited stableness practically.
Foams, as well as other disperse systems, are obtained by two ways: by condensation - the association of ultrafine bubbles in larger ones; by dispersion - grinding of large bubbles and occluded gas.
A series of construction materials, in particular those based on mineral binders and aggregates, form conglomerate type of structure. Term "conglomerate" (latine conglomeratus) means mechanical aggregation of different components. The classic rocks which consist of rolled fragments of mountain origin, consolidated by clay, ferric oxide, silica, etc. is also called conglomerate (Fig. 1.14).
At micro structural level there is a well-known binding part of conglomerates. It can be considered as the original microdispersed conglomerate where cementing substances and pore space are in. Cementing substances from the binders, which harden due to the chemical reaction with water or water solutions, are given hydrated new formations, and from synthetic organic based binders - by hardened polymers.
T
he
peculiarities of materials microstructure substantially depend on the
amount of fillers, their dispersity and physical and chemical
activity of the surface. Fillers
are fine-grained materials
components, which do not form the hardening structure independently,
but actively interfere in its forming jointly with cementitious
substances. For mineral binders based construction materials fillers
form primary adhesive contacts
at the stage of coagulation structure forming, that transform with
hydration in the irreversible contacts of intergrowth, durability and
structure of which are determined by efficiency of filler. Fillers,
diminishing energy at the, fasten crystallization of new formations
in the same time. They also can intereact with the products of binder
hydration and to increase, thus, volume of new formations.
Transition of binders in the filling systems from volume state to thin-film state gives the possibility to improve substantially their technical properties and decrease expenses.
The major elements of materials microstructure which determine their properties are pores. Finest pores (ultramicropores) appear as a result of anisotropy of properties of crystals and particles of condensation structures, and also their random orientation in space in growth process. The examples of such pores are pores in the particles of the hydrated cement (so-called gel pores), the size of which is (15...30) 10–8 m. Water in the pores is under the strong action of the field of forces of pores walls. Because of this most of water properties (density, viscosity, heat-conductance, etc.) have anomalous character. Larger pores of artificial materials are mostly technological by origin. They appear as a result of loose mixture placing, air entrapping, evaporation of excess water, destructive processes of leaching, dehydration or weathering, etc.
Pores can be divided into two groups: capillary and noncapillary. In capillary pores the surface of liquid acquires the form, caused by forces of surface tension and distorted a little due to weight. For capillaries with the radius (r) is representative the height of raising of liquid in the capillary (h), which is determined after the formula of Zhyuren:
(1.5)
where is surface tension; is an angle of wettability; g is free fall acceleration; р – density of a liquid.
Microcapillaries (r < 0.1 mkm) as the result capillary condensation effect can be fully filled with liquid due to absorption of its vapors from environment.
Macrocapillaries (1.0 > r >0.1 mkm) can be filled with liquid only in the case of direct contact. Besides that, the peculiarity of macrocapillaries is that they not only adsorb the water from the air, but also vice-versa loss moisture into the atmosphere.
To estimate influence of structure on materials properties, the concept of porosity(- relation of pores volume to the general volume of material) is applies. The integral parameters of porous space are true (or total), open (apparent), conditionally closed porosity, etc. Thus it is important to distinguish pores by their sizes, form and character. The types of porous structures of cement stone are shown in Fig. 1.15.
T
here
are a series of methods for porosity analysis and pore-structure
assessment. For determination of ultramicropores for example, the
method of water and helium adsorption is applied. For micropores
there are methods involving electron microscopy, adsorption of
nitrogen and methanol,
or mercury intrusion porosimetry.