**1. Introduction**

As is known, high-strength concrete currently includes concrete with a compressive strength of 60–100 MPa and higher at the age of 28 days [1,2]. With the use of classic technology, obtaining high-strength concrete is possible by ensuring the high quality of raw materials, low values of the water-cement ratio, and sufficient compaction of the concrete mixture. In practice, these requirements are achieved by reducing the water content of the concrete mixture when using stiff mixtures, or by introducing plasticizing additives, increasing the activity and specific surface of cement, introducing hardening accelerators, reducing water consumption, and optimizing the grain composition of aggregates with high physical and mechanical properties [1–6].

The technology of high-strength concrete of the new generation involves the introduction of superplasticizer additives with a high (20–40%) water-reducing effect and dispersed active mineral fillers into the composition of concrete mixtures. These components provide both important individual effects, namely achieving extremely low values of the watercement ratio while maintaining ease of installation, an increase in the volume of hydration products, the degree of cement hydration, and a significant synergistic effect. With the simultaneous introduction of superplasticizer additives and a dispersed active filler, the rheological potential of the plasticizing additive is fully realized on the one hand, whereas on the other hand, in the conditions of a limited water-cement ratio, the positive effect of the dispersed active filler on the structure formation of cement stone and concrete is manifested [7–10].

In hydration systems, the physical interaction of cement particles and the filler is significantly affected by the so-called "compressed conditions" [11], which are characterized by a sharp increase in the concentration of the solid phase and the transition of part of the volume water into film water. At the same time, the change in free surface energy

**Citation:** Dvorkin, L.; Zhitkovsky, V.; Marchuk, V.; Makarenko, R. High-Strength Concrete Using Ash and Slag Cements. *Mater. Proc.* **2023**, *13*, 16. https://doi.org/10.3390/ materproc2023013016

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 14 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

between the solid and liquid phases contributes to the intensive formation of crystal nuclei of hydration products, which is in accordance with the Gibbs–Volmer theory [12]. With the optimal concentration and dispersion of the active additive, a fine-grained structure of the binder is formed, which has a positive effect on the technical properties of the artificial stone.

The structuring effect of dispersed additives in concrete increases when the physicochemical activity of their surface increases. Methods of mechanical activation of additives include grinding them with cement in the presence of plasticizers and, if necessary, other chemical admixtures. This method of activation is the basis for obtaining composite cement with low water consumption [13].

An important indicator of the quality of concrete as a structural material is the specific consumption of cement per unit of concrete strength. In traditional concretes, this indicator is on average close to 10 kg/MPa, in new generation concretes it decreases two or more times [7–9].

The best active fillers for high-strength concrete, as demonstrated by many studies and by practice, are microsilica additives [14,15]. At the same time, the use of microsilica is complicated due to the need for its granulation or briquetting for transportation and dosing, as well as to significant fluctuations in its composition and properties.

As regards active fillers for high-strength concrete, the use of dispersed siliceous and aluminosilicous materials based on local raw materials and industrial waste is a practical interest [14,16]. Among such materials, the most common is coal fly ash and granulated blast furnace slag. The main task of our work was to figure out the technological parameters for obtaining high-strength concrete using composite ash and slag-containing cement and to research their complex structure and technical properties.
