The dependence of fracture mechanics terms (gammaT Kc. Gc and Jc) on relative humidity and porosity is determined for hardened portland cement paste (HCP). The cardinal fractures of the fracture term/humidity curves and fracture term/porosity curves are discussed. Large hysteresis in fracture term/relative humidity curves is observed, the magnitude depending on the drying history of ECP. Meniscus effects, aging and moisture assisted stress corrosion procsses are thought to contribute to fracture of HCP. Unhydrated portland cement, Ca(OH)2 particles, and pores with specific size distributions in the HCP system possibly act as crack arrestors. Analysis of portland cement can aid in its conservation.