The impact of impurities on salt healing properties is studied through creep tests performed on brine saturated granular salt with various quartz contents. Quartz grains act as shields that reduce dissolution at salt grain contacts and decrease the creep rate. Non-smooth creep curves are obtained for specimens with 50% quartz contents, due to sequential pore collapse. Micro-CT images acquired before creep, after creep, and after unloading, show that pore-to-pore distances decrease with quartz contents and that the creep rate decreases as the mean area of the salt grain contacts increases. Based on grain scale thermodynamic models, we show that creep deformation is controlled by diffusion-not dissolution-precipitation. Permeability evolution is less sensitive to porosity than to void radius and spacing, which control pore connectivity. The proposed modeling framework can be used in any crystalline material to relate microscopic reaction rates to macroscopic deformation rates.