The characteristics and quantities of hollow shells (Hadley grains) in the microstructure of Portland cement paste have been studied. It is shown that at relatively early hydration stages, the hydration largely occurs in a manner resulting in hollow shells. At later ages, the "hollow-shell hydration" mode becomes much less prominent as "inner products" are increasingly formed. The intrinsic porosity associated with hollow shells can be significant at later ages, between about 1% and 9%, depending on the mix composition and curing regime. The presence of silica fume increases the amount of hollow-shell porosity considerably, while the hollow-shell porosity appears to decrease with decreasing water/binder ratio.