D0I:10.13374/i.issn1001-053x.1989.06.032 第11卷第6期 北京料技大学学报 Vol.11 No.6 1989年11月 Journal of University of Science and Technology Beijing Nov.1989 Microcracks in MDF Cement' Shang Chengjia(尚成嘉)，Zhi Rentao(职任涛)，Zhu Fengwu(朱逢吾)， Zhou Zhengqian(周政谦)，Xiao Jimei(肖纪美)* 八 ABSTRACT:The relationspip between the microcracks and the mechanical behaviors and the characteristics of fracture of MDF cement have been studied. The results show that most of the microcracks lie and propagate in the matrix or along the boundary of the clinkers.The microcracks improve the toughness of MDF cement.The Griffith's equation is invalid to describe the fracture of MDF cement. KEY WORDS:cement,microcrack,MDF MDF cement was firstly reported in 1981 by Birchall et al.1.Since than, interests have been raised to discover the strengthening mechanism of MDF cement. Some authors t2s,1 considered that the high strength of 70-150 MPa is due to the reduction of the pore size,which is about 1.0mm in hardened cement paste (HCP)and less than 20gm in MDF cement.They proposed to apply Griffith's equation to explain the relationship between flexural strength and pore size.But when the pore size is less than 100um,the flexural strength dose not further increase with the decrease of pore size.They guessed that the flaws other than pores were becoming important.e.g.the large angle cement grain- boundaries and cement-polymer interfaces would play the dominent part in deter- mining strength. However,Eden and Baileyts showed the flexural strength can only be increased to as high as 20MPa when the pore size was reduced to less then 100um by mixing under vacuum with a small addition of a plasticizer.So they suggested that the principle role of the polymer was to act as an adhesive which assisted the strength developed by the hydrated cement.Moreover,Sindair and Grovest61 and Rodger et al.t71 have showed there are chemical reactions between polymer and cement.In a word,the strengthening mechanism of MDF cement is complex Manuscript Received July 17,1989 ··Dept,of Material Physics 585
第 卷第 期 年 月 北 京 科 技 大 学 学 报 一 已 。 ’ 尚 成 嘉 ， 职 任 涛 ， 脚 朱 逢吾 夕 周政 谦 ， ， 肖纪 美广 尹、 。 ， ， ， 叹 、 ， · ’ ， 一 卜 ， 。 拼 · ’ · 拼 ， · · · 一 ， 拼 · 皿 · ， 。 · ， ， ， 二 DOI ：10．13374／j ．issn1001－053x．1989．06．032
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
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introuduced on certain condition to increase the toughness.From Table,it cad see that the microcracks do have toughening effect on MDF cement.This result not only shows the method of improving the toughness of MDF cement,but also predicts the fracture concepts in ceramic may be used to describe the fracture behaviors of MDF cement,and the Griffith's theory is invalid to explain MDF cement. 19 012 19009 (a) (b) Fig.1 BEI of polished surface of two group specimens (a)group A (b)group B Table Group A Gaoup B Flextural strength (MPa) 70.8 73.5 Young's modulus (GPa) 35.3 32.8 Un-notched fracture energy (J/m2) 647 736 Strain (% 0.200 0.224 Critical stress intensity factor (a/d=0.3)(MN/m2) 3.2 4.1 Notched fracture energy (a/d=0.3) (J|m2) 176 261 The characteristics of crack propagation path were also studied by SEM. Fig.2 shows the crack propagation path of a notched specimen which have been bent to failure.From Fig.2 we can see the crack propagate almost entirely in the matrix,the crack propagation path is tortuous and the crack is always aro- und the clinker.In addition there were large branches along the main crack (Fig.2-c),there were a series of microcracks coalescence with the main fracture crack (Fig.2-b). Fig.3 shows SEM examination of fracture surface of MDF cement.There exist a great amount of microcracks on the fracture surface.This also shows that there are a lot of branches along the fracture path. 587
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50:红 1m Fig.2 The crack propagation path of a notched specimen which have been bent to failure Fig.3 SEl of fracture surface Fig.4 The steady crack propagation of a pre-notched specimen under certain stress In Table it can see that the fracture energy is notch sensitive,this pheno- menon is also obtained in HCPt10).This suggests that,in addition to surface energy,there are other sources which consume energy in fracture process.From Fig.2 and Fig.3 We can see the tortuosity and the branches of the fracture crack may be the main source of energy consumption,in addition to surface energy, when the fracture takes place.So,when there exist the microcracks in the matrix with the suitable sizes and density,there will be more branches in the 588
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fracture crack,and the crack path will be more tortuous.More energy will be required to make the crack propagate.This will be the main reason of micro- crack toughening mechanism in MDF cement. The fracture mechanism in cement is cement is complex and it is different from other materials.There is a great deal of controversy as to which theory or model fit to cement.The fictitious crack model (FCM)11!has been intro- duced to describe the fracture of cement.By this model,the fracture zone ahead of crack tip has been considered,the microcracks in this zone will be coalesence under certain stress.So,more energy will be consumed in this zone besides the surface energy.It has observed the steady crack propagation in MDF cement,as is shown in Fig.4.The length of the steady crack propagation is at least about 1.0mm.This suggests the fracture zone exist in MDF cement.The detail of the fracture mechanism of MDF cement will be discussed in the paper to be published. The Griffith's equation fails to explain the observed fracture process and the influence of microcracks on the toughness and crack propagation characteri- stics of MDF cement. 3 Conclusions (1)Under certain conditions,microcracks can be introduced into MDF cement.The microcracks lie in the matrix or along the boundary of the clinker. The microcracks do not redued the strength of MDF cement,yet can improve the toughness of MDF cement. (2)The crack propagates in the matrix or along the boundary of the clinker, and there are branches along the crack,the crack path is tortuous. (3)It is invalid to use Griffith's equation to describe the MDF cement.The fracture zone must be considered. REFERENCES 1 Birchall J D,Howard A J.Nafure,1981:289:388 2 Alford N MeN.Cem.Concr.Res.,1981:11:605 3 Alford N MeN,Groves G W.Cem.Concr.Res.,1982;12:349 4 Kendall K,Howard A J.Phil.Trans.R.Soc.,1983;A301:31 5 Eden N B,Bailey J E.J.Mater.Sci.,1984;19:150 6 Sinclair W,Groves G W.J.Mater.Sci.,1985;20:2846 7 Rodger S A,Sinclair W.J.Mater.Sci.,1985;20:2853 8 Birchall J D,Howard A J.Patent Pub.No.0021682,1981 9 Davidge R W,Phil.Trans.R.Sci.1983;A310:113 10 Watson K L,Cem.Cocr.Res.1978;8:651 11 Hillerborg A,in Fracture Mechanics of Concrete,eds.by F.H.Wittmann, Elseviver,Amsterdam,1983,223. 589
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