Influence of Structure of Lightweight Structural Concretes Based on Waste Polypropylene Containers Aggregate on Their Strength Characteristics

. This paper focuses on the fundamental challenge of polymer waste recycling through the development of structural lightweight concretes based on recycled polypropylene aggregates. The research links the composite’s internal macrostructure and its mechanical performance using a combination of standardized experimental techniques and multifractal theory. The mechanism of “spatial sliding” of polypropylene aggregate grains relative to each other and to a portion of the binder, identified during concrete mix compaction, is attributed to the geometric dimensions and shape of the aggregate grains, surface morphology, near-zero crushability, and ultimate compressibility of the aggregate. This mechanism enables the production of lightweight concrete compositions characterized by maximum aggregate packing density and a minimal yet sufficient volume of binder distributed between the grains. A promising non-destructive method has been developed for predicting the compressive strength of structural lightweight concretes incorporating polypropylene aggregates, based on the multifractal analysis of their macrostructure. The strong correlations (R 2 = 0.78–0.96) established between the multifractal parameters (D 0 , Δ, K, Δf(α)) and compressive strength enable an accurate assessment of mechanical performance directly from internal surface cross-section images, eliminating the need for destructive sample testing. It has been established that compressive strength decreases with the growth of fractal dimension D 0 and heterogeneity Δf(α), while it rises with an increase in ordering Δ and regularity K. This methodology offers a significant reduction in the material and temporal costs associated with quality control processes. These results support the broader application of secondary raw materials in construction and facilitate the use of the resulting structural lightweight concretes with a density of 1625–1805 kg/m 3 and compressive strength up to 25 MPa in the assembly of sustainable load-bearing and environmentally friendly enclosing structures of buildings.