R-curve behaviour in a macro-defect-free cement paste

The fracture resistance of a macro-defect-free cement paste has been studied in terms of the crack growth resistance (K_R) curve. It is shown that, in contrast to common belief, a single fracture parameter, such as the critical stress intensity factor (K_c) or the critical potential energy release rate (G_c), cannot be used to characterize the critical failure event in this material. Using double-cantilever-beams together with both optical and scanning electron microscopy, the toughening mechanisms giving rise to the K_R-curve behaviour were identified and found to occur behind the advancing crack tip. The predominant mechanism was due to unbroken cement ligaments bridging across the crack faces with other less significant contributions due to frictional interlocking of adjacent grains on the fracture plane and tearing of polymer fibrils. Approximate solution for the crack resistance curve was given in accordance with the Mai-Lawn crack-bridging model originally developed for alumina ceramics.