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S. Ahmad et alii, Frattura ed Integrità Strutturale, 34 (2015) 524-533; DOI: 10.3221/IGF-ESIS.34.58 530 enhancement in maximum CMOD is that the ductility and the fracture energy of the composites has improved substantially as depicted in the Fig. 8. Figure 8 : Toughness of the cement composites samples with various amounts of PC & PCA particles inclusions. Typical fracture surfaces of the notched cement composite samples tested in flexure are presented in Fig. 9. The fracture surfaces of the control mix samples are smooth and sharp as compared to the cement composites containing various amounts of inert carbonized particles. The FESEM observations of the fracture surfaces revealed that the inert micro particles acted as an obstacle in the crack growth path, therefore the growing cracks have to deflect and contour around the inclusion. The most interesting thing about this modified cracking mechanism is that all the phenomenon of crack pinning, trapping, deflection and contouring is happening at the micro level, thus resulting in substantially improved performance in terms of energy absorption capacity (see Fig. 10, 11 and 12). The inert carbonized particles affect the cement matrix in two ways, one is that they provide the nucleation sites for the deposition of the calcium hydrated products and results in stronger matrix [36]; the second factor, which is more pronounced is that the inert carbonized particles affects the growing crack path. In pure cement matrix after the initiation of the crack, it propagates rapidly resulting in sudden failure while in other case the crack is trapped by the inert carbonized particles so the cracks have to divert their path involving more material in the failure process. Fig. 10, 11 and 12 shows the typical fracture surfaces of cement composites containing pyrolyzed coconut shell particles. The diversion of cracks due the toughening mechanisms led by the inclusion of inert carbonaceous particles results in the development of three-dimensional fracture surface of the cement composites. The development of torturous crack surfaces indicates that more energy is required by the composite for the growth of cracks. In FESEM analysis, it was also observed that sometimes the cracks propagated through the large particles inclusions but always contoured the small particles and the crack branching phenomena was also observed due the presence of inert pyrolyzed particles (see Fig. 11). Figure 9 : Typical fracture surfaces of the cement composites tested in flexure