Organosilanes can be used as coupling agents between carbon nanotubes (CNTs) and the major hydration product of Portland Cement, calcium silicate hydrate, overcoming the low interfacial interaction of these nanomaterials with the cementitious matrix. Nevertheless, the modification of CNTs with silane has not yet been extensively investigated for cementitious composites application. Thus, this work investigated the CNTs functionalization using four different organosilanes (APTES, AEAPTMS, GPTMS and TEOS) for application in cement pastes. The results showed that only APTES and AEAPTMS effectively functionalized CNTs surface. Cement pastes were produced by adding 0.1% (w/w) of non-silanized and silane-functionalized CNTs and evaluated through rotational rheometry, isothermal calorimetry, compressive strength, and water absorption. CNTs silanization with APTES and AEAPTMS reduced the agglomeration trend of nanotubes at pH 12 as suggested by DLS results, which reduced the dynamic yield stress, equivalent viscosity, and hysteresis area compared to non-silanized CNTs cementitious composite. Furthermore, cement pastes with functionalized nanotubes increased the 28-day compressive strength by 13.3% and 20.2% and reduced the volume of permeable voids by 1.8% and 2.7% when using APTES and AEAPTMS, respectively, compared to plain cement paste. Overall, silanization can reduce the agglomeration trend of carbon nanotubes and improve the interface with cementitious matrix, improving the fresh and hardened properties of cement-based materials compared to non-silanized CNTs.