The use of paraffin as heat accumulators in building envelopes is difficult due to the rather high temperature of their phase transition, and low coefficient of thermal conductivity. Besides, traditional insulation systems using heat-insulating material in energy-active structures are ineffective. To improve the energy efficiency of residential buildings, it becomes necessary to develop new systems and structures of fencing, which is active in nature and can use renewable energy sources for the needs of buildings. It will also reimburse part of the heat spent on heating the building. For this, it is necessary to study the heat exchange processes in the building structures. Тhe article presents the results of studying the processes of phase transition of heat storage material in enclosing structures. The article considers processes of phase transition of heat storage material in enclosing structures, considering the prerequisites and assumptions. They are necessary to simplify the mathematical model of the phase transition process. The authors have been developed a mathematical model of heat transfer during the phase transition of a heat storage material, which considers the thermophysical properties of the material, the thickness of the phase transition layer and the time of its formation, as well as the temperature difference between the temperature of the initial state and the temperature of the phase transition of the heat storage material. It was found that the heat flux density varies in many parameters from the initial moment of time to the time of the process of transition to a new phase and it is determined that the average heat flux density during the melting of a layer of heat-accumulating material is two times higher than the heat flux density at the moment of time, at which the phase boundary will move a distance.

Improving the durability of reinforced concrete sleepers is essential for railway infrastructure exposed to dynamic loading, moisture, and repeated freeze–thaw action. This study proposes a material-level modification approach for heavy concrete for type 2 reinforced concrete sleepers based on the combined use of activated microsilica, a ferroalloy-production byproduct, electrolyzed mixing water, and a polycarboxylate superplasticizer. The novelty of the work lies in the preliminary electrochemical activation of microsilica in an alkaline medium and in the optimization of its joint use with KN-5 by means of second-order experimental design. The concrete was evaluated by compressive and bending strength tests, scanning electron microscopy (SEM), water-penetration testing, and freeze–thaw resistance testing. All modified mixtures outperformed the reference concrete. The highest 28-day compressive strength reached 67.0 MPa, while bending strength reached 7.26 MPa. SEM observations showed a denser and more homogeneous cement matrix with reduced capillary porosity and improved interfacial transition zones. Water resistance improved from W8 for the reference mixture to W10–W14 for the modified concretes. Most modified mixtures achieved a frost resistance grade of F500, and the composition containing 15% activated microsilica and 1.0% superplasticizer reached F550. The proposed approach is effective at the material level for producing heavy concrete with enhanced strength and durability characteristics for reinforced concrete sleeper applications.

This article explores the use of waste from polymetallic combines in South Kazakhstan, specifically tailings from the Achisay and Ansay deposits, as aggregates (crushed stone, sand) and mineral additives (dispersed barite powder) for producing concrete with specified operational properties. These secondary raw materials are now abundant in relation to their use, which makes them an affordable and accessible alternative for the manufacturing of concrete while also promoting environmental sustainability. X-ray diffraction, differential thermal analysis, and scanning electron microscopy of enriched barite ores in these tailings revealed valuable components, such as calcite, quartzite, and dolomite, suitable for use as aggregates and mineral additives. The calcite and quartzite content in the Ansay samples exceeds that in the Achisay samples. Concrete mixes with various proportions of crushed stone and sand from these tailings were prepared, and their working characteristics were analyzed. The impacts of filler content and grain composition on the characteristics of concrete mixtures were identified, and the requirements for optimizing aggregate grain composition to produce heavy concrete with desired qualities were determined. Heavy concrete with densities from 2300 to 2839 kg/m³ and compressive strengths from 41.6 to 58.2 MPa was developed. Physical and mechanical properties, including density, water absorption, frost resistance, and compressive strength, were also evaluated, confirming the feasibility of using technogenic waste in composite heavy concrete production.