Abstract: In recent years, the use of graphene and its derivatives to improve the properties of cementitious composites have received much attention. However, there are few reports on the effect of graphene surface properties on the performance of cement-based materials. Graphene oxide (GO) was converted to reduced graphene oxide (rGO) using different concentrations of L-ascorbic acid (10wt%, 20wt%, 30wt%, 50wt% and 70wt%) and reduction time (15 min, 30 min, 45 min and 60 min) which was then added to the cement mortar composites at the same dosing level 0.05% (by weight cement). The effects of different degrees of reduced rGO on the mechanical properties of cement mortar were investigated. The test results show that the incorporation of rGO prepared by 50wt% L-ascorbic acid reduction 30 min increases the 28 days compressive strength and flexural strength of cement mortar by 36.84% and 43.24%, respectively, compared to the normal specimens. SEM and other analyses show that both GO and rGO with different degrees of reduction could promote the crystallization of Ca(OH)₂ and the formation of silica tetrahedra in hydrated calcium silicate gels (C-S-H) to form dense microstructures. However, an optimal threshold exists (i.e., 30 min reduction by 50wt% L-ascorbic acid). At this threshold, the binding of rGO surface functional groups to hydration products is favored.