The natural stone industry generates large amounts of industrial waste every year. Limestone powder produced by the activity of this industry is dumped into landfills, generating an environmental impact that could be reduced by using this waste as a binder in building materials. In the present work, the use of this by-product as an addition for soil improvement of clayey soils has been studied. The tested natural soil is a soft clay from southeastern Spain, which has been mixed by adding 5, 10, 15, 20, and 25% of dry limestone dust by total dry weight of the soil. The natural soil and the additive have been characterized, in addition to the common geotechnical tests, by means of X-Ray diffraction and X-Ray Fluorescence. The improvement of the geotechnical properties of the mixed soil has been evaluated by means of the change in the Atterberg limits, free swell index, unconfined compressive strength, and one-dimensional consolidation test. The change in the microstructure of the mixed soil has been studied by scanning electron microscopy. In general, the results obtained show an increase in the strength of the soil and a reduction of its deformability when limestone powder is added.