Mechanistic–Experimental Evaluation of Sugarcane Molasses as a Sustainable Stabilizer for Granular Subbase Materials

The use of sugarcane molasses (SCM) as a sustainable stabilizing agent for geomaterials, particularly granular subbase soils, has recently attracted growing attention as an alternative to traditional stabilization methods employing cement, lime, or bitumen, which are often associated with high costs and environmental concerns. This study presents a mechanistic–experimental evaluation of SCM for stabilizing granular subbase materials at dosages of 2.5%, 5%, 7.5%, and 10% by weight of dry granular material. A comprehensive testing program was conducted, including compaction characteristics, California Bearing Ratio (CBR), resilient modulus (Mr), permanent deformation under repeated loading, optical microscopy, and FTIR spectroscopy. In addition, multilayer elastic analysis using KENLAYER was performed to assess pavement structural performance in terms of critical strains and service life. The results showed that SCM significantly improved subbase performance within an optimum dosage range. The mixture containing 5% sugarcane molasses (5SCM) exhibited the highest overall performance, increasing CBR from approximately 26% to 34% and reducing accumulated permanent strain by approximately 45% compared with the control mixture. Optical microscopy and FTIR analyses supported the proposed stabilization mechanism, indicating improved particle contact at moderate SCM contents, whereas excessive SCM contents adversely affected performance due to lubrication and excessive particle-coating effects. Mechanistic analysis demonstrated that 5SCM improved pavement durability, increasing allowable load repetitions from 9.54 × 105 to 1.14 × 106 and extending pavement service life by approximately 20%. A durability–cost assessment further identified 5SCM as the most efficient dosage from both engineering and economic perspectives. Water immersion assessment indicated that SCM stabilization is suitable for pavement structures with effective drainage, whereas its application in continuously saturated or flood-prone environments is not recommended. Overall, SCM demonstrates strong potential as an environmentally sustainable stabilizer for granular subbase materials, with an optimum dosage of approximately 5% for enhancing both material performance and pavement durability.