Recycling seashell waste in building materials is a relevant and attractive solution for eliminating this waste while at the same time reducing its environmental impact. Our study focuses on the use of Mytilus galloprovinciallis shell waste in mortars of cement as a partial replacement of sand. A granular range similar to that of fine sand was used with the extraction of fine powder < 0.08 mm, in order to reduce the effect of organic matter on material properties, we used percentages of 0%, 15%, 20%, 25%, 30%, and 45% according to mass. Physical, thermal and mechanical properties are estimated, and the capillary absorption coefficient is also determined. The two types of sand have an almost identical particle size distribution, the same sand equivalence, and nearly the same density, but other properties such as fineness modulus, specific surface area, and water absorption coefficient are slightly different. However, the morphology of the grains is totally different, which makes the physical, thermal, and mechanical properties of the cement mortars prepared varied while increasing the substitution rate. The results show that replacing natural sand with shell aggregates improves the thermal insulation performance and durability of the mortars, knowing that 45% of the replacement reduces the thermal conductivity and capillary coefficient to 21.7% and 75.21%, respectively, compared with the mortar of reference.