The present work explores the fundamental mechanism behind the increased surface area and pore volume of CaO after hydration. First, a widely believed mechanism, the "physical attrition theory", is experimentally examined and is found to have limitations in explaining this phenomenon. Next, to explain the improvement of morphological properties by hydration, a typical water hydration process is examined by dividing the process into four independent chemical and physical substeps. The morphological changes of Ca(OH)(2) and its derived CaO by each substep are measured by Brunauer-Emmett-Teller (BET) analysis. During the first step, the intrinsic chemical conversion from CaO to Ca(OH)(2), the formed Ca(OH)(2) product layer disintegrates because of its low tensile strength and weak crack resistance, which explains the increases in surface area and pore volume by steam/moisture hydration as well as the rapid heat release during hydration. The physical interaction with water (the second step) slightly decreases the surface area and pore volume, possibly by lodging microparticles into the porous structure of bigger particles and inducing stronger particle agglomeration. The Ca(OH)(2) solid can further chemically bond water molecules (the third step), which significantly enlarges the solid volume during water-bonding and consequently generates a more porous structure during dehydration. The final precipitation of the dissolved Ca(OH)(2) (the fourth step) decreases the solid's surface area and pore volume. This decrease is attributed to the formed microparticles from solution, which can plug some surface pores on the larger particles during the drying process.