and is not very clear yet. In this paper the microcracks in MDF cement and its influence on the mecha- nical behaviours have been studied by SEM and mechanical testing.The toughen- ing mechanism of the microcrack was discussed. 1 Experimental MDF cements were prepared according to the method described by Birchall et alts1.The composites were prepared from ordinary cement/polymer/water rate of 100/5/14 (weight parts).After mixed and twin roll-milled,the comp)- sites were formed into sheet of approximately 4mm in thick,then divided into two groups cured by two kinds of condition.The flexural strength was determined by three-point bending,each group has ten specimens.The results showa were the average value of these specimens.The fracture energy was determined by dividing the area under the stress-strain curve by nominal cross sectional area of the beam.The critical stress intensity factor was measured following the method described by ASTM E399 (1983)with a notch/depth rate of 0.3.The polished and fracture surface of MDF cement was examined by back-scattered electron image (BEI)and secondary electron image (SEI)in SEM. 2 Results and Discussion In our experiment,microcracks were introduced by certain curing condition. Group A were the specimens which do not have microcracks (Fig.1-a)and group B were the specimens which have microcracks (Fig 1-b).Fig.(1-a)and Fig.(1-b)are typical micrographs of these two kinds of specimens.We can see from Fig.(1-b)that the largest size of a microcrack could exceed 100um. The microcracks were lying in the matrix (hydrated cement)or along the bou- ndary of clinker.These microcracks may be caused by the shrinkage of hydrated cement. In Table the mechanical behaviors of these two groups of specimens are listed.We can see that though there exist microcracks in group B the values of flexural strength are not inferior than those in group A,the fracture energy of notched and un-notched specimens and the critical stress intensity factor of group B are better than group A.Only the Young's modulus of group B is lower than that of group A.Based on the comparision of above behaviors we conclude that the microcracks do not only act as a weakness source of cement, the microcracks may improve the toughness of MDF cement.The decrease of Young's modulus must result from the existing microcracks in the matrix. Davidget1 has suggested the probability of applying the microcrack toughe- ning mechanism of ceramic into cement.For ceramic,microcracks are always 586宜 过 。 “ 。 一 ， 。 ， 一 ， 皿 一 了 。 爪 一 丫 · ， 五 一 一 一 一 · 一 一 叫 拼 。 · 一 ， 一 · ， ， · ’ 。 · ， 月浮 洲口