When nanomaterials are added to cementitious materials, their microstructure and mechanical properties are affected. The most targeted effect of their use is the improvement of the composites’ mechanical performance. The present investigation brings the characterization of a cement paste reinforced with two different cellulosic inclusions: microcrystalline cellulose (MCC) and nanofibrillated cellulose (NFC), added in the contents of 0.050%, 0.075%, and 0.100%, in relation to cement weight. First, the morphology, crystallinity, surface area, and thermal decomposition of MCC and NFC were analyzed. Then, the composites’ microstructure was characterized by thermogravimetric analysis, X-ray diffraction, and porosity measurements. This analysis was made
considering the development of hydration between 3 and 28 days. Furthermore, their mechanical performance was investigated by compressive, flexural, and direct tensile tests at 28 days of hydration. Despite the differences regarding the morphologies and physical aspects between MCC and NFC, their inclusions resulted in positive effects on the flexural and tensile strengths. The cement pastes reinforced with 0.075% of MCC and NFC had their tensile strength enhanced by over 70% compared to the reference. Nevertheless, TGA and XRD analyses showed that the addition of MCC and NFC decreased the formation of all hydration products, except the calcium carbonate. Therefore, the combination of mechanical and microstructure analyses indicates that the tensile strength improvement obtained with the MCC and NFC addition was most likely due to their reinforcing capability.