Abstract Nanosilica particles (nS) were added to lime (L) and lime-pozzolana (LP) pastes to study the effect of nS as a pozzolanic admixture in L and to synergistically improve the pozzolanic reactivity of LP. Relationships between microstructure and mechanical properties of the pastes were examined. The macroporosity of both pastes decreased, and the compressive strength increased. SEM and X-ray μ-CT analysis accounted for explaining the inconsistent results between the porosity obtained by MIP and density by He-pycnometry. The strong pozzolanic reaction in LPnS explained the high consumption of mixing water, increment of density, and pores assigned to CSH. The SEM analysis also showed that BET and BJH can give erroneous results regarding the adsorption/desorption isotherms, thus affecting the values of the specific surface area and nanoporosity.