Slurries can behave very differently from regular water. This calculator allows the user to estimate the losses in head pressure and efficiency of a pump, when said pump is used for slurry transport.
The nomograph that can be seen below was extracted from the Weir Slurry Pumping Manual First edition, where you can also find more information.
The resuts provided by this calculator are ratios, meaning that you should multiply the duty point values for your selected pump by these ratios to obtain an approximation of how the pump would behave with the selected slurry.