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.

The present study investigates the combined influence of nano-ground quartz sand, microsilica, and a polycarboxylate-based modifier on phase evolution, microstructure development, and mechanical properties in low-clinker cement systems. The main objective is to evaluate whether finely dispersed quartz acts as an inert filler or contributes to hydration processes, and to determine the critical clinker threshold governing system performance. A hybrid modifier composed of microsilica and polycarboxylate (4:1) was introduced into a clinker–gypsum system together with varying amounts of nano-ground quartz sand. The mixtures were subjected to mechanical activation in a ball mill. Chemical composition, phase evolution, and microstructure were analyzed using XRF, XRD, and TEM, while mechanical properties were evaluated through standard tests. The results show that grinding duration and modifier dosage significantly influence particle dispersion and water demand. Optimal performance was achieved at 90 minutes of grinding and 6% modifier content. A nonlinear relationship between quartz content and compressive strength was observed, with a maximum value of 57.1 MPa obtained at 30% quartz and 55% clinker content. XRD analysis confirmed partial portlandite consumption and the formation of additional low-crystalline C–S–H gel, indicating a synergistic interaction between nano-quartz and microsilica. The results demonstrate that nano-ground quartz acts not only as a filler but also as a nucleation-active component with limited pozzolanic contribution. A further reduction in clinker content below 55% leads to insufficient formation of hydration products and a decline in mechanical performance, confirming the existence of a critical clinker threshold. These results provide a scientific basis for the design of low-clinker, low-carbon cementitious materials with enhanced performance.