Poorly-graded sandy soils require effective stabilization due to their low stability and bearing capacity. Stabilizing such soils with conventional cementitious binders like lime and portland cement raises construction costs and emits high levels of carbon dioxide. Geopolymers, sustainable binders derived from aluminosilicate sources, offer an effective and ‌

eco-friendly alternative. This study investigates the mechanical and microstructural behavior of sandy soil from Varzaneh, Isfahan, stabilized with a geopolymer based on recycled brick powder (RBP). The experimental program comprised three binder contents (10%, 20%, and 30% by dry soil weight) and four activator-to-binder ratios (0.5, 0.75, 1.0, 1.5). The alkaline activator was prepared by mixing 8 M sodium hydroxide with sodium silicate solution. Specimens were cured for 7, 14 and 28 days and tested for unconfined compressive strength and microstructural characteristics using SEM and XRD. Results showed that the compressive strength of samples stabilized by RBP-based geopolymer improved compared with the untreated soil; the mixture containing 20% RBP with an activator-to-binder ratio of 1 cured for 28 days achieved the highest strength of 15.6 Mpa, which is approximately twice the maximum strength obtained from specimens stabilized with 11% Portland cement, demonstrating the superior mechanical performance of the geopolymer system. The A/B ratio equal to 1 established an appropriate balance in the system’s alkalinity, enabling effective dissolution of aluminosilicate phases and promoting the formation of a denser N–A–S–H gel matrix. Microstructural analysis results also confirmed the formation of this gel and the reduction in porosity. Based on the findings, geopolymers derived from construction and demolition waste may serve as sustainable, environmentally friendly alternatives to conventional stabilizers in geotechnical and civil engineering applications.