The porosity-to-cement index ($\mathsf{\eta }$/C_iv) has been extensively applied to study the evolution of different types of soil stabilization. However, this index has still not been used to characterize soils cemented with crushed limestone waste (CLW). In this sense, this paper sought to analyze the applicability of the porosity-to-cement index over the unconfined compressive strength (${\mathrm{q}}_{\mathrm{u}}$) and initial stiffness at small deformations ($G_{\mathrm{o}}$) of clayey soil improved with CLW and Portland cement. In addition, a microstructural analysis (SEM and EDX tests) was also conducted. CLW addition increased soil strength and stiffness over time. Moreover, ${\mathrm{q}}_{\mathrm{u}}$ and $G_{\mathrm{o}}$ compacted mixtures containing CLW have established a distinctive correlation. Chemical microanalyses have uncovered a complex interfacial interaction between the soil, cement, and fine CLW particles, leading to a notable reduction in porosity.