The use of calcined clays in cementitious systems has lately gained extensive attention, especially regarding the current context of environmental issues. Several studies focused on the hydration aspects of cement binders in the presence of pure calcined clays, but few are the studies that were conducted on the mechanical aspects of these same systems with low-grade calcined clays. In this work, four cementitious systems incorporating slag, low-grade and high-grade calcined clay are studied, and their hydration and mechanical properties are compared. The incorporation of slag enabled highlighting the synergetic effect between slag and low-grade calcined clay and the ability of this ternary binder to be equivalent to the binary binder with high-grade calcined clay in terms of: 1-hydration speed between 7 and 28 days and 2-mechanical properties at 28 days. Finally, a novel deconvolution method of grid nanoindentation results is presented, coupling XRD/Rietveld analysis and classic Gaussian Mixture Model deconvolution method; and the advantages and disadvantages of this method are discussed.