The hydration kinetics of Portland cement is known to be relatively insensitive to water-to-cement (w/c) ratio changes through experimental observations such as isothermal calorimetry tests and chemical shrinkage tests.As calcium silicate hydrate (C-S-H) nucleation and growth has been identified as the controlling mechanism during the early stage of Portland cement hydration, the relative insensitivity of Portland hydration to w/c ratio,which directly influences the inter-particle spacing of cement grains (presumably, the available space for C-S-H nucleation and growth to occur), is not well understood.