Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were employed to evaluate the influence of pig hair fibers on the gel pore volume and cement microstructure of concrete aged one year. Cubic concrete samples (15 cm per side) were subjected to three different external curing conditions. The samples were subsequently sectioned into three layers based on the moisture diffusion profile imposed during curing: layer A, exposed to the highest moisture gradient, and layers B and C, subjected to progressively lower moisture gradients. MIP results indicated an increase in gel pore volume (<25 nm) in layer A of the fiber-reinforced concrete compared to plain concrete cured for one year at 50% relative humidity (RH). Furthermore, a progressive increase in gel pore volume was observed in samples pre-saturated for 77 and 156 days before being cured at 50% RH for the remainder of the year. These findings demonstrate that internal curing of concrete can be effectively promoted using natural fibers.