Alkali-activated binders are considered a promising and more environmentally favorable alternative to Portland cement; however, their large-scale application still requires validation in terms of long-term durability, standardization, and industrial feasibility. This study investigates the mechanical and microstructural behavior of mortars formulated with three calcined clays of distinct geological origin: two clays from Mexico and one from Ecuador. The precursors were characterized with emphasis on SiO₂/Al₂O₃ molar ratios and CaO content, as indicators of gel formation type (N-A-S-H vs. hybrid N-A-S-H/C-A-S-H). Alkali activation was performed using a sodium silicate–sodium hydroxide system, optimized for SiO₂/Na₂O and solid-to-liquid ratios to balance workability and compressive strength. The Ecuadorian clay-based binder exhibited the highest compressive strength at 28 days (67.9 MPa), attributed to its favorable Si/Al ratio (3.27) and low CaO content, classifying it as a true geopolymer. In contrast, one of the mexican clays showed balanced performance that likely formed a hybrid gel due to moderate calcium content, while the other mexican clay displayed limited reactivity, with a high Si/Al ratio (4.90) and low Al₂O₃, resulting in underdeveloped gel structure and poor strength. Complementary SEM–EDS analysis supported the correlation between precursor chemistry, gel formation, and mechanical response. While the term “geopolymer” is often used generically, this study emphasizes the need to distinguish between true geopolymeric systems and broader alkali-activated materials, depending on precursor composition. These findings highlight the potential of selected low-calcium clays for developing high-performance, low-carbon binders cured at ambient temperature.