The mineralogical composition of fine aggregates plays an important role when used in mortar production because they generally effect the strength and durability of mortar structures. In this work, some mechanical aspects are highlighted concerning the use of fine aggregates containing Na-feldspar, K-feldspar, mica and some clay minerals such as kaolinite, illite and montmorillonite. With in this scope, total of there series of mortar mixtures were made with different water/cement ratio. First mixture was referance mortar prepared by using standard sand. In second mixtures, standard sand was replaced with 20 % Na-feldspar, K-feldspar and mica. Third mixtures contained 4 % of kaolinite, illite and montmorillonite as partial replacement of standard sand. According to the results obtained, addition of mica and clay minerals such as illite and montmorillonite with different percentages in standard sand causes to increase water demand in mortar and therefore reduce the strength of mortar.