**Abstract**

The paper reviews the theoretical framework of nano-modification principles of building composites and experimental verification of these principles. The concepts of nano-modification of building composite structures are studied. The paper also suggests the conceptual model of the nano-modification from the point of view of the evolutionary model of a solid-phase formation depending on the kinetics of heterogeneous processes. The technological tools of nano-modification of building composites are substantiated. Cement-based materials are shown as an object of nano-modification. Their hydration kinetics, phase composition, microstructure, and strength characteristics are investigated. The results are analyzed based on the special performance criteria which confirm that the efficiency of cement-based materials technological cycle can be improved by nano-modification.

**Keywords:** cement system, nano-additives, hydration, hardening, structure, strength

## **1. Introduction**

The scientific substantiation of the principles of structure management at the nanoscale level is the key point in the development of high-performance and ultra-high-performance concrete [1, 2]. It is the class of materials, which includes the nanostructured elements as part of their structure. Moreover, to regulate their structure and properties, it seems logical to use nanoscale various types of particles. Nowadays, the problems of synthesis and construction of high-performance and ultra-high-performance concrete structures in building technologies more actively rely on nano-concepts and nano-approaches. It is relevant to identify the potential possibilities and limitations of the application of nanotechnological approaches in solving specific problems of the technology of building materials.

### **2. Theoretical framework**

While applying the technologies of building materials and stages of transitions in the processes of forming the structures of their solid-phase state, it is necessary to keep in mind hydration (lime, gypsum, cement), hydrothermal-synthesis (silicate autoclave) hardening systems, and thermal-synthesis (ceramics, glass) composites (**Table 1**) [1].

The fixed assets of the "nano tool," implemented in construction material technologies, include the influence and effect of "**top-down**" and "**bottom-up**" nanotechnological approaches.


#### **Table 1.**

*Hardening systems (HS) for design and synthesis structures of building composites.*

The "nano" "top-down" approach is based on the use of the phenomenon of changes in the energy state of the structure and, accordingly, the physical and/or physicochemical activity of surface and internal volumes of particles of a solid as they are crushed by mechanical or physical methods.

The effectiveness of the principle is determined not only by the nano- and micro-size of particles but also by the measure of the amorphization of their surface volumes. Amorphization is expressed in the "saturation" of volumes with structural defects of the nano-size range. The principle is the basis of the mechanochemical activation of the components of raw mixtures in the technologies of building materials.

The "**nano**" "**bottom-up**" principle is based on the management of condensation phenomena in the evolutionary route of the formation of a solid-phase state of a substance in hydration, hydrothermal-synthesis, and thermal-synthesis hardening systems. The effectiveness of the principle is determined by the possibilities of influencing the development of stages, transitions, selections in the evolutionary route, and, in general, the kinetics of heterogeneous processes of hydration, colloidation, and crystallization in systems (mixtures) of basic and acid oxides upon receipt of building materials.

"Nano tool," based on "top-down" principles, includes mechanochemical activation of the substance during the dispersion of the initial components of the raw mixes, physical and chemical activations during the high-temperature processing of the mixture components, compulsory compaction of particles of components of a mixture, etc. "Nano tool," implemented in the principles of "bottom-up," includes the means of controlling the solubility of components and the degree of saturation of the system with cations and anions; methods of changing the rate of condensation and crystallization through thermal or other physical effects; methods of changing the spatial and geometric environment in the system by introducing additives of nano-modifiers, for example, nanoparticles with both related and nonrelated crystal-chemical structure of the synthesized compounds; and techniques for changing the state of the surface of solid particles when using additives of surface-active substances, etc.

The "top-down" and "bottom-up" principles act in a systemic unity, and their presence in nanotechnology of building materials is an objective fact. The principles operate within the framework of their own laws. However, it happens within the single process of structure formation and the formation of a solid state of materials. The effects of the above principles in nanotechnology are articulated and superimposed based on the principle of "top-down," which is expressed in the role and influence of the phenomenon of mechanochemical metamorphism of the structure of the initial components (according to Avvakumov, Hodakov, etc.) and from the "bottom-up" principle (the phenomenon of stereochemical and topochemical memory according to Bokiy, Belov, Mamedov, Oleinikov, etc.). The principle of "top-down" has a precursor (preparatory) role, ensuring the activation of the initial components of raw mixes in nanotechnology. The evolutionary route of the formation of the nanostructure of the solid phase on the principle of "bottom-up" is developed in the framework of the action of this role.

In the compositional structure of conglomerate building materials, two types of structures are distinguished according to their origin ("genesis"). The first, related to macro-, meso- and microstructure, is the result of the addition of ready-made granular particles. The second one, related to the micro-, submicro-, and nanostructure, is the result of the synthesis of particles in the evolutionary route of their formation. Both types of structures in their genesis are integrated in the composite. The genesis (formation) of the first type of the structure is carried out according to the concept and bases of the mechanics of granular media; at the same time, the nanotechnological top-down principle with the corresponding nano-modifying effects can be realized at the same time. The genesis of the second type of structure is carried out according to the concepts and bases of the condensation mechanisms, which is implemented in the framework of the action of the nanotechnological principle "bottom-up," which predetermines the effects of nano-modification in the evolutionary route of the formation of a solid state. Thus, the development of the technology of nano-modification of the hardening system structures and construction composites should proceed from the systemic unity of the "top-down" and "bottom-up" principles.
