The impact of polydispersity index (PDI) values and doses of two sodium polyacrylates on grinding limits and particle size distributions of highly concentrated limestone suspensions was studied. The results indicated that viscosity and agglomerate particle size, and thus the apparent grinding limit, depended on the sodium polyacrylate dose as well as PDI value of sodium polyacrylate. The higher the sodium polyacrylate dose, the lower the viscosity and the smaller the apparent grinding limit were. The smallest primary particle size, the smallest apparent grinding limit, the narrowest agglomerate particle size distribution, and the lowest viscosity curve were obtained with sodium polyacrylate having a low PDI.