No 1-2 (2024)

In modern construction concrete recycling is one of the promising directions of utilization of multi-tonnage concrete and reinforced concrete waste. Most often concrete crushing products are reused as coarse and fine aggregate. It is shown in the paper that reuse of the dusty fraction of cement-sand stone as an active colloidal additive obtained by its alkaline mechanical-chemical activation allows to partially replace cement in commercial concrete and hydraulically hardening compositions without loss of their strength. This paper presents the results of the effect of colloidal admixture on technological and physical-mechanical properties of concrete mixture.

This article discusses issues related to the development of technological solutions designed to obtain protective paint and varnish coatings with high operational resistance for building metal structures. The developed technological map and scheme for creating nanostructured paint and varnish coatings on the surfaces of building metal structures are presented. These proposals complement the comprehensive approach to the development of protective paint and varnish coatings, consisting of methodological, technological and formulation solutions, and presented in other studies by the authors, which allows for its effective use in paint and varnish industry enterprises. The research results, based on the application of the developed solutions in laboratory and operational conditions, confirm the high efficiency of the developed integrated approach. Assessment of the quality of coatings in laboratory conditions showed that coating samples after 100 hours of thermal exposure retain high properties that meet the operating conditions of building metal structures. As part of production testing, it was established that the service life of nanostructured coatings increases by 1.5–2.6 times compared to traditional coatings. In this regard, the costs of repair and restoration of coatings are reduced, and their service life between repairs is increased. The introduction of the developed solutions into production will solve the problem of low operational durability of protective paint and varnish coatings of steel building metal structures operated under various influences.

When GOST 17608 and GOST 6665 were republished, the authors prohibited the use of slag fillers, so manufacturers of products produced according to these standards have a problem: the use of only natural inert materials will lead to an increase in the cost of the final product. At the same time, however, this should lead to an improvement in the quality of manufactured products. But is it so? This article presents a comparative analysis of standards for fillers of natural origin and from metallurgical production waste in order to identify the reasons that do not allow the use of slag aggregates in concrete products for road construction. It was determined that most of the requirements for aggregates of both natural and man-made origin are the same, i.e. if the manufacturers observe and fulfill the requirements of the standards for slag aggregates, there should be no risk of reducing the quality of the final product. Then why the developers of GOST 17608 and GOST 6665, excluding from the list of aggregates for concrete materials according to GOST 3344–83 do not give an alternative in the form of aggregates according to GOST 5578–2019. The authors of the article have tried to reveal the answer to this question.

The properties of fine-grained concrete containing 20–80% granulated blast furnace slag and bacteria species Bacillus Subtilis have been studied. An assessment was made of changes in strength, self-healing of cracks using optical and electron microscopy and measuring the speed of ultrasound propagation perpendicular to the crack plane; composition and characteristics of the healing agent in cracks using X-ray analysis methods. Self-healing of cracks in concrete without bacteria occurred due to calcite deposition as a result of carbonation of portlandite during 50–65 cycles of humidification-drying, and in the presence of Bacillus Subtilis bacteria due to calcite deposition during their vital activity in 10–15 cycles. It is shown that the addition of granulated blast furnace slag slows down the crystallization of calcite, which forms a healing substance in the crack. It is assumed that the combined use of granulated blast furnace slag in dosages of 40–80% and Bacillus Subtilis bacteria in concrete structures operating under conditions of variable humidification can ensure the process of self-healing cracks and maintaining the strength of concrete in the long term due to simultaneous processes of strengthening the structure due to prolonged hydration of slag minerals and calcite deposition in cracks due to the vital activity of Bacillus Subtilis bacteria.

Cellular phosphate concretes are used as an insulating material for some high-temperature aggregates due to their high temperature resistance, fire resistance and residual strength at the level of values after drying. The use of industrial waste in phosphate cellular concrete technology made it possible to improve some properties without reducing the application temperature. The paper shows that dispersed aluminosilicate waste from refractory production has sufficient activity (porization ability) to obtain a phosphate binder. The features of the hardening of an aluminosilicophosphate binder cured with dispersed metallic aluminum have been studied; the change in the phase composition of the cured binder after firing at different temperatures has been studied by differential thermal and X-ray phase analysis. It has been established that the developed aluminosilicophosphate binder makes it possible to obtain fireclay cellular concrete with an application temperature of up to 1400^оC. A comparison of the changes in the phase composition for the developed aluminosilicophosphate composition and a pure aluminophosphate binder is performed. A shift in the temperature of the processes is noted in an upward direction for the aluminosilicophosphate binder, which is explained by the fact that silicon ions do not form independent phosphate compounds, but are embedded in the crystal lattice of aluminophosphates, changing their properties and shifting the intervals of phase transitions.

Installations of the mid-twentieth century for testing concrete for creep, as a rule, have significant wear and are not intended for testing new concrete. To carry out testing of modern high-strength concrete, their modernization and retrofitting is required. Due to the lack of standard technical solutions, design and survey work is required in preparation for testing. The purpose of the presented study was to study the nodes and elements of spring installations for determining the creep of concrete and to establish the possibility of their use under increased loads, to modernize installations for the possibility of testing samples of various lengths without complete disassembly, to ensure safety during such tests. To determine the strength and stiffness parameters of the springs, it was necessary to conduct two-stage tests with the development of special equipment. Also, according to the results of the evaluation of the hinge assemblies for axial load transfer to the sample, their replacement with a change in design solutions was required. According to the results of experimental studies, individual springs and thrust hinge elements were fragilely destroyed, in connection with which a hinge assembly of a new design was developed, changes have been made to the design of the installation to ensure the physical protection of maintenance personnel, a modular system of steel spacers with protection against horizontal movement has been developed, which makes it possible to test samples of different lengths on existing installations without changing the configuration of the installations themselves. An important result of the work is the proposed system of double experimental control, when first a random test of individual elements is carried out to assess the possible level of loads on the equipment, then a continuous control of the already assembled test installations is carried out for large loads relative to the planned experiment. Only in this case it is possible to simultaneously ensure high reliability of the results obtained during testing, reliability and durability of the equipment. Moreover, it is impossible to carry out such work only numerically without testing, as practice has shown.

3D concrete printing (3DCP) is an innovative and promising method for constructing buildings and structures. Compositions of fine-grained concrete based on Portland cement are widely used as raw material mixtures in this technology. An alternative to the use of cement binder is the use of gypsum-cement-pozzolan binder, which can significantly reduce the cost of the finished product and, accordingly, increase its competitiveness. The raw material mixtures based on gypsum-cement-pozzolan binder presented on the construction market do not fully meet the requirements of 3DCP. Achieving optimal performance of gypsum-cement-pozzolan mixtures in 3DCP is possible by regulating the content of fine aggregate in the composition of fine-grained concrete, as well as the use of multicomponent modifying additives. The purpose of this work is to develop modified gypsum-cement-pozzolan concretes for 3DCP based on optimization of aggregate content and multifunctional additive, providing optimal rheotechnological properties of raw mixtures and technological characteristics of finished products. The formation of samples during experimental studies was carried out using the layer-by-layer extrusion method on a workshop construction 3D printer “AMT S-6044”. Modified gypsum-cement-pozzolan concretes have been developed for 3DCP with increased ultimate shear stress of the mixture (87.6 Pa), dimensional stability (23 cm), average composite density (1920 g/m3), flexural strength (8.4 MPa) and compression (30.6 MPa) and water resistance (0.85). The possibility of targeted regulation of the structure and properties of gypsum-cement-pozzolan mixtures and concrete due to the synergistic effect of the components of the developed multifunctional complex additive, including an aqueous solution of a polycarboxylate ether, a copolymer based on carboxylic acid esters, and a homogeneous mixture of oligoethoxysiloxanes, has been proven. The results obtained are consistent with the results of the differential thermal analysis of modified gypsum-cement-pozzolanic stone.

One of the authors is a participant in the XVI International Congress on Cement Chemistry (ICCC 2023), which was held in Bangkok (Thailand) on September 18–22, 2023 under the motto “Further decarbonization and recycling production and application of cement and concrete.” Statistical data, thematic areas of the congress are presented and some reports are presented, the content of which may be of most interest to Russian specialists.

The resistance of chloromagnesial composites to prolonged water saturation is determined by the softening coefficient and the tendency to cracking. Technological methods for preventing the cracking of a magnesia binder stone in contact with water have a number of disadvantages, in particular, related to their difficult reproducibility in production conditions. This study is devoted to the search for additives that make it possible to regulate the processes of structure formation of chloro-magnesial composites in order to form predominantly stable phases evenly distributed in volume during prolonged saturation with water. In the course of the work, standard methods were used to study the properties of dough and astringent stone, as well as microcalorimetry. As a result, it was found that the addition of sodium tripolyphosphate significantly slows down the setting time of the chloromagnesial composition, but eliminates the tendency of the artificial stone obtained from it to crack during prolonged water saturation.