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This study was conducted to investigate the internal curing effect of the colloidal nano silica (CNS) in concrete. Various dosages of the CNS at 0%, 0.3%, 0.6%, and 1.0% were designed to be incorporated in concrete, mortar, and cement paste for discussion. The internal humidity of prepared concrete was monitored; a concrete mix with the lightweight aggregate (conventional internal curing agent) was also prepared for comparison. The drying shrinkage of the mortar samples was tested to verify the effectiveness of the internal curing. Sorptivity analysis was then carried out to investigate the influence of the CNS on the pore structure of the concrete. Afterward, the water retention properties of the mixtures were evaluated to study the internal curing mechanism of the CNS. To further evaluate the effect of the CNS on hydration, isothermal calorimetry (IC), thermal gravimetric analysis (TGA), and a scanning electron microscope were used. The result of this study reveals that CNS possesses the internal curing effect, which could improve the curing quality and hydration of the cement. Compared to conventional internal curing aggregates, CNS as an internal curing agent not only improves the curing quality, but also requires less storage space and a more straightforward mixing procedure.