We use rock physics theories to estimate elastic mineral properties from published laboratory measurements of compacting clay-water composites at increasing vertical stress. By using differential effective medium theory (DEM) with multiple aspect ratio pore spectra we were able to model the compaction of a kaolinite-water composite between 1 -50 MPa of effective vertical stress. Assuming this model is a plausible idealization of the evolution of the composite microgeometry during compaction we bounded the elastic properties of clay between upper and lower bounds. Then by averaging them we estimated the bulk and shear moduli of kaolinite to be K = 19 GPa and G = 5 GPa, respectively.