This article is devoted to studying the feasibility of using Moroccan diatomite as replacement of sand in mortars. The principal objective is to value the use of this type of lightweight aggregates in insulating concrete. The diatomite is characterized by means of physical and chemical analyses, to determine its potential use in mortars, and then seven formulations of mortar with different amounts of diatomite are tested. The effect of this type of aggregates on the density, porosity, thermal conductivity and diffusivity, thermal effusivity and capacity, compressive strength, flexural strength and water absorption, is presented herein. The thermal characterization shows that the incorporation of diatomite in mortar improves significantly its thermal insulation potential. Thus, the lightning results in a net reduction of thermal conductivity and diffusivity. In the mechanical characterization, it is noted that despite the decrease in the compressive and flexural strengths with the addition of diatomite, the results obtained are satisfactory to use this type of mortar as insulating concrete. The water absorption test brings to light the increase of hygroscopic aspect with increasing diatomite ration.