The effects of adding different nano-MgO dosages (0, 1, 2, 3, and 4 wt.% with respect to cement) on microstructural, compressive strength, and phase evaluation of concrete were investigated. Two different post-treatment conditions with water and CO2 gas were used to study the processing method on the samples. The specimens were characterized via SEM and XRD analysis. The mechanical properties of the samples were also investigated. The results showed that compressive strength significantly improved after the addition of magnesium oxide nanoparticles. However, this improvement was more remarkable in the case of post-treatment with CO2 compared to the samples fabricated with water. SEM results showed that the samples treated under CO2 gas had irregular and needle-like morphology. The samples prepared by normal processing had CaCO3 and SiO2 phases, whereas the ones fabricated under CO2 gas contained CaCO3, SiO2, and Ca(OH)2. With the addition of nano-MgO, the density of concrete decreases in the samples post-treated with water, whereas it increases for the samples post-treatment with CO2 gas. Adding 4 wt.% nano-MgO to concrete and further post-treatment with CO2 for 45 days could increase the mechanical properties from ~ 23 MPa to ~ 55 MPa. Manuscript profile