Effect of Glass Cullet Content on the Mechanical and Compaction Behavior of Cement-Bound Granular Mixtures for Road Base/Subbase Applications

The growing accumulation of glass waste and the limited availability of natural aggregates present major challenges for sustainable road construction. This study aimed to evaluate the influence of the glass cullet content (GC) in the range of 0–30% on the mechanical and compaction properties of cement-bound granular mixtures (CBGM 31.5 mm, R_c class C_5/6) intended for the road base and subbase layers. Laboratory tests were carried out to analyze the effect of GC on the optimum moisture content (OMC), the maximum dry density (ρ_d,max), and the compressive strength after 7 and 28 days (R₇, R₂₈). The results showed a systematic decrease in OMC and ρ_d,max with increasing GC content, by approximately 18% and 2.8%, respectively, for the mixture containing 30% glass. All CBGM mixtures met the strength requirements for class C_5/6 (R_c = 6–10 MPa), with the highest value of R28 obtained for the mixture containing 20% GC (9.4 MPa), representing a 24% increase compared to the reference mix. The relationship between GC content and compressive strength was best described by a second-degree polynomial function (R² = 0.60–0.65), indicating an optimum within the 10–20% range. Strength enhancement was attributed to synergistic effects of physical mechanisms (filler effect and improved particle packing) and chemical activity (pozzolanic reactivity of fine glass fractions). The 30% GC mixture provided the minimum required strength while achieving the highest level of waste utilization and environmental benefit. Therefore, the optimal GC content should be determined as a balance between mechanical performance and sustainable use of secondary materials in the temperate climatic conditions of Central Europe.