Chapter 1
Challenges of Technology of Dispersed Composite Materials

This chapter considers the significant features and obstacles hindering the synthesis of one of the most widespread and widely applied among worldwide dispersed materials: cement and asphaltic concretes with a high level of structural-mechanical properties - as a demonstrative example illustrating the necessity of implementation of the approach substantiated in the Preface and in the introduction toward solution of problems of technology of dispersed systems and materials on the basis of physical chemistry of dispersed systems and physicochemical dynamics.

The further chapters also pay a great deal of attention to materials based on mineral binders and bitumens, as these materials are typical representatives of various highly filled solid phases of multicomponent dispersed composites.

By now, significant progress has been achieved in the technology of obtaining various dispersed composite materials including materials used in construction, for example, in the building of roads, bridges, and airdromes.

The increased requirements for these materials and the constructions made using them led to an increase in freight traffic density; and, accordingly, values of static, dynamic, temperature, and chemical exposure of constructions and facilities, in their turn, impose increased demands toward strength, deformation properties of dispersed composites, and their service life.

At the same time, significant importance has been attached to the increased requirements for technical and economic indices of materials in the course of operation of constructions using them.

Progress within the conventional approach to the technology of obtaining various dispersed materials, and primarily concretes, based on mineral and organic binders achieved in the recent years is related chiefly to application of cements with improved characteristics, new types of plastifiers, and modifying agents. However, transition to a qualitatively new, higher level in composite materials science under the conditions of such a conventional, and, as pointed out above, to a certain degree empirical, approach is limited by the possibilities of this conventional technology.

What are these limitations and what are the ways of overcoming them? Let us give several examples to answer these important questions.

A vivid example illustrating these limitations is the technology of obtaining cement concretes implemented at present. In particular, the design of their compositions according to the existing standards is carried out taking into account the options of the available equipment as regards the mixing, transport, casting, formation, and compaction of concrete mixtures. These parameters determine the placeability of mixtures as related to these, that is, their rheological properties (viscosity and fluidity) and, accordingly, water-cement ratio and water content.

This limitation results in a significant (in some cases, by several times) increase in the water content of concrete mixtures (up to W/Cem = 0.4-0.5) as compared to that required for full hydration of cement (W/Cem ~ 0.2) [1, 2].

At the same time, cement hydration approaching 100% during the standard 28 days of normal concrete hardening is possible only in the cases when the size of cement particles does not exceed approximately 15 µm [1, 2]. The average grain size in commercial cements is considerably higher than the stated value (d = 20-25 µm and more) [2]. Therefore, the hydration degree of standard Portland cements by the concrete age of 28 days usually does not exceed 50-60% and can reach 80% only in the case of fine quick-hardening cements. Herewith, as the time period since preparation of concrete mixes and start of interaction between cement and water until concrete placing and compaction is usually not more than 1-3 h, the hydration degree of cement during this period does not exceed several percent.

One can assume on the basis of the effect of the surface of solid phases on the properties of thin water layers [3, 4] that the thickness of these layers with changed properties does not exceed the size of 2-3 molecules of H2O, that is, does not exceed 10 Å (1 nm). Thicker layers of 3-5 water molecules and more correspond to bulk water by their properties (viscosity, freezing point, etc.).

The amount of water covering grains of cement, sand, and chippings and characterized by its bulk properties does not exceed 1% of the given water content, that is, it is admittedly below the maximum amount required for full hydration. This means that mobility and placeability of concrete mixtures, especially during the first hours after concrete preparation, even at W/Cem ~ 0.2, would apparently be necessarily achieved.

However, in reality, the minimum W/Cem ratio in concrete mixtures is usually at least 0.3-0.35 even when plastifier additives are applied. And this means that excess water content determined by the requirements of placeability causes a significant increase in residual concrete porosity, decrease in its water impermeability and freezing resistance, and increase in shrinkage and creep.

Besides, it is necessary to increase cement consumption as a result of increased water content to provide the given strength of concrete. At the same time, the consequence of increased water content of mixtures predetermined by the necessity of providing the given mobility (placeability) is the further segregation of excess water, especially in the course of their transportation to the site of concrete placement and also during the first hours after placement and compaction (Figure 1.1a).

Figure 1.1 (a) Scheme characterizing water segregation in concrete mixtures: V0 is the initial volume of the concrete mixture, V1 is the volume of water segregated under static conditions, V2 is the volume of water segregated in the course of transportation to the site of placement, Vt is the final volume of the concrete mixture after placement and compaction. (b) Scheme of formation of a water "lens" under coarse filler grains as a result of sedimentation: (1) grains of chippings or gravel, (2) cement solution, and (3) water "lens" under filler grains.

It is this circumstance, particularly due to sedimentation of the binder and water segregation under coarse grains (Figure 1.1b) that, to a great extent, explains reduced frost resistance and water impermeability of concretes.

However, an attempt to pass to harsh mixes with lower water content and castability without allowing for the achievement of the maximum uniformity of mixtures under mixing and formation and compaction to the required density results in the fact that the hardness of concrete calculated according to the conventional dependencies, for example, according to Equation 1.1, cannot be implemented when the critical value of given (Cem/W) is exceeded.

As seen in Figure 1.2, a drastic decrease in strength is observed above this value, though it should grow according to Equation 1.1 [5]:

where Rconcr is the strength of concrete at the age of 28 days of normal-humidity storage; Rcem is the activity of cement in mPa; A is a parameter accounting for the shape of filler particles (chippings, gravel) and hardness of mixtures; C is an empirical correction of ~0.5.

Figure 1.2 Dependence of strength of concrete on the cement/water ratio. Curves 1 and 2 correspond to concretes made on usual (2) and quick-setting, highly dispersed (1) cements (S is the specific surface area of cements). Curve 3 is the dependence of Rconcrete on Cem/W according to Equation 1.1. Arrows point to the Cem/W limitation relation to loss of placeability of concrete mixtures.

The following questions arise as related to the above material:

1. 1. Why is it necessary, under actual conditions of concrete technology, to increase the water content of concrete mixtures considerably to reach the specified fluidity to the detriment of the properties of concrete and its technical-economic indicators?
2. 2. What is the mechanism of achieving the required plasticity (placeability) and, ultimately, fluidity of concrete mixtures from the perspective of physical chemistry that results in such a considerable increase in the required amount of water?

This problem becomes even more complicated in the case of finely ground quick-setting cements, as the amount of water required for concrete mixtures based on these cements becomes even greater.

And, finally, the most significant issue is

1. 1. What are the ways of resolving the contradiction between the necessity of decreasing water content in the mixtures, that is, increasing Cem/W and, moreover, advisability of application of finely ground (including quick-setting) and fully hydratable cements, on the one hand, and, in this connection, sharply rising viscosity, problems related to mixing, placing, and compaction of mixtures and loss of their placeability due to this cause?

Similar issues and problems arise in the technology of synthesis of other dispersed materials: of cement according to the wet method, of silicate materials, asbestos cement, concreting paper, and so on. The excessive water content in many of the above examples is due to the necessity of carrying out the...