AbstractSupplementary cementitious materials (SCMs) play a significant role in improving the durability of cement-based materials by restricting moisture and aggressive ion transfer through capillary pores. Phosphorous slag (PS), as a SCM, is a waste product that seems to be capable of enhancing durability and mechanical properties of cement-based materials because of its chemical components. In order to investigate its effects, fresh and hardened cement mortars prepared with various amounts of PS were studied in this research project. Moreover, surface area is a key factor in SCM performance. Therefore, the effects of surface area on PS performance were also investigated. The main results show that various amounts of PS have an insignificant effect on the results of flow table tests, regardless of surface area. However, PS significantly improves the capability of cement mortars to retain their initial workability. Plus, as surface area increases, the capability of PS-incorporated mortars to retain their initial workability improves. Additionally, findings of hardened mortar analysis show that the pozzolanic reaction of PS occurs slowly and it can be argued that the impact of PS on hardened mortars does not depend on surface area. For example, findings show that the effects of the lowest and highest surface areas tested are about the same on compressive strength, electrical resistivity, and chloride diffusivity. Plus, the mechanical and durability properties of PS-incorporated mortars are usually lower than those of reference mixtures after short periods of moist curing. By contrast, PS-incorporated mortars have better mechanical and durability properties in comparison with reference mixtures after longer periods of moist curing. In order to shed light on PS performance, its performance was compared with a well-known SCM, silica fume (SF). The outcomes show that PS is not as active as SF. Interestingly, PS contributes to strength gain after longer periods of moist curing, while SF improves mechanical and durability properties even after short periods of moist curing.