The improvement of soils that are not suitable for a particular purpose through techniques that combine densification and the incorporation of portland cement have been extensively done for decades in spite of the poor environmental performance related to such material. In this context, the development of alternative binders through the use of industrial byproducts may be an alternative to mitigate environment-related issues. Therefore, the current study aims to evaluate the performance of a gap-graded soil via incorporation of a binder composed of coal fly ash and hydrated lime. For this, strength, stiffness, and durability tests were carried out on compacted soil–coal fly ash–hydrated lime specimens molded at different porosities and with distinct coal fly ash and hydrated lime contents. Moreover, the stiffness of the specimens was also assessed during wetting-drying cycles. Results of all mechanical tests have shown the significant influence exerted by porosity, followed by the relatively minor impact of the amounts of binder (here composed of lime plus fly ash). Hence, good correlations between the response variables and the adjusted porosity/binder index () were obtained. Furthermore, an innovative approach to replacing the laborious durability test is proposed.