KimaCell® HEMC MH40M, a type of cellulose ether, serves as a commonly employed viscosity modifier for concrete. It finds extensive application in self-compacting concrete and grout materials. This additive enhances the cohesiveness of cement-based substances and the uniformity of their solidified structure. The impact of HEMC as a viscosity modifier on the fresh characteristics, mechanical attributes, and drying shrinkage of mortar was examined. The mechanism of action was investigated based on non-evaporative water content and pore structure. Furthermore, the beneficial effect of HEMC modification in iron tailings sand cement mortar was demonstrated. HEMC notably raises the viscosity of mortar. When used in conjunction with a water-reducing agent, it achieves controlled fluidity within an appropriate range and diminishes the rate of mortar bleeding. As the HEMC content increases, the compressive strength of mortar specimens at 7 days and 28 days exhibits an initial rise followed by a decline, reaching its peak when the HEMC content reaches 0.3%.