The synthesis of a refractory geopolymer with excellent thermal and mechanical properties to develop materials that resist temperatures above 1000 °C has become a challenge in recent years. The production of a refractory material involves the combination of clays and alkaline substances, whose formulation produces an improvement in the final properties of the refractory material. In this work, we report a Taguchi L18 design to synthesize a refractory geopolymer that exhibits the highest compressive strength, based on two natural clays from Mexico, seashells, sodium silicate and water. The prepared mixture was molded to form mini bricks, which were cured for 24 hours at room temperature. Subsequently, the bricks were dried for 4 days, and we evaluated the effect of temperature at 40 °C, 60 °C and 80 °C. Subsequently, the samples were subjected to calcination between 900 °C and 1200 °C to determine the resistance to both thermal and mechanical fracture via compression. The highest thermal resistance and compressive strength were obtained with a geopolymer formulated with 7.5 % seashells, 38.2 % Tepozán‐Bauwer clay, 30.0 % Kaolin clay, 20 % commercial sodium silicate and 4.3 % water, which was dried at 40 °C. The compressive strength was 7.28 MPa (74.2 kgf /cm2), and the thermal resistance was up to 1200 °C. In the FTIR analysis of the geopolymers calcined at 1200 °C, the influence of calcium was observed by the bands of the 800–1200 cm−1 region, where there was a greater distribution since the Si−O− bond was added. Ca was found at 950 cm−1. By adding CaCO3 in the form of seashells, the presence of the anorthite mineral phase (CaAl2Si2O8) can be identified, which originates from the degradation of calcium carbonate crystals (calcite, vaterite and aragonite) of the identified seashells using XRD. The synthesized geopolymer has potential use in the refractory and ceramic materials industry.