The escalating demand for durable, high-performance cementitious systems has driven the incorporation of two-dimensional nanomaterials into concrete. This study presents a comparative experimental investigation into the influence of graphene oxide (GO) on the mechanical, durability and microstructural behaviour of conventional concrete (CC, M40) and high-strength self-compacting concrete (HSSCC, M60). GO synthesised via a modified Hummers' method was ultrasonically dispersed and introduced at dosages of 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05% by weight of cement. Fresh-state characterisation confirmed that GO-modified HSSCC retained self-compactability within EFNARC limits up to 0.03% GO. Hardened-property evaluation covered compressive, split-tensile and flexural strength at 7, 28 and 56 days, while durability was assessed through water absorption, capillary sorptivity, rapid chloride penetration (RCPT), water-permeability depth and acid resistance. A well-defined optimum of 0.03% GO was identified for both systems: 28-day compressive strength increased by 23.5% (CC) and 19.5% (HSSCC), while chloride charge passed and water-permeability depth decreased by up to 45% and 55%, respectively. Beyond 0.03%, agglomeration reversed the gains. Scanning-electron and thermogravimetric interpretation attributes the enhancement to GO-templated nucleation of calcium-silicate-hydrate (C-S-H), pore refinement and micro-crack bridging. HSSCC consistently outperformed CC in absolute terms, whereas CC showed a larger relative gain, indicating GO is most beneficial where the baseline matrix is more porous. The findings quantify the dosage-dependent, matrix-dependent efficacy of GO and support 0.03% as a design-optimal nano-reinforcement level.