Alkali-activated slag is used as a cement substitute to stabilise poured earth materials, aiming to enhance their mechanical properties and decrease their water sensitivity. However, the interaction between electrically charged clay particles contained in the earth and the alkaline solution can modify the consistency and, more significantly, lead to the absorption of chemical elements. This can have a detrimental effect on the stabilisation efficiency. In order to optimise slag activation, it is essential to understand the interaction between the clay and the alkaline medium, and its effect on slag hydration. This study examines the microstructural effects of the interaction between the alkaline solution and clays on slag hydration, through microstructural characterization (SEM, FTIR, and DTA-TGA analyses). The impact of three distinct types of earth and two levels of sodium concentration are investigated. Clayey activity causes changes in alkaline conditions, leading to the formation of hydrates that are typically produced at very high sodium concentrations, such as hydrotalcite and depolymerised C-A-S-H. Only formulations with a sodium concentration higher than 1.6 M provide robustness with respect to the three types of raw earth studied. This alkaline context has a limited effect on the microstructure of the clay particles.