Rheological behaviour of fresh mortars is a key characteristic since it determines workability, compaction and segregation properties of the material and greatly influences the final characteristics of the hardened product. Substituting Portland cement with pulverised lamp glass waste (incandescent light bulb borosilicate glass waste cullet and fluorescent lamp tube glass waste cullet) in concrete has a significant effect on its workability and strength. Present study evaluates the effectiveness of lamp glass waste cullet at a level of 30% as cement component in mortar and determines the grinding time for optimal physical and mechanical properties of mortars. Viscosity, electrical conductivity, ultrasound pulse velocity and compressive strength of such cement mortars were determined. The results showed that the finer the incandescent light bulb borosilicate glass waste cullet powder particles were, the lower were the viscosity, ultrasound pulse velocity and electrical conductivity. However, the finer the fluorescent lamp tube glass waste cullet powder particles were, the higher were the viscosity and electrical conductivity. An increase in compressive strength was observed for cement mortars with incandescent light bulb borosilicate glass waste cullet powder.