Unreacted cement content in macro-defect-free composites: impact on processing-structureproperty

Transcription

1 JOURNAL OF MATERIALS SCIENCE 29 (1994) Unreacted cement cntent in macr-defect-free cmpsites: impact n prcessing-structureprperty relatins P. G. DESAI*, J. A. LEWIS*, D. P. BENTZ* NSF Science and Technlgy Center fr Advanced Cement Based Materials, *and als Department f Materials Science and Engineering, University f Illinis at Urbana-Champaign, Urban& IL 61801, USA, rand als Natinal Institute f Standards and Technlgy, Gaithersburg, MD 20899, USA The effect f unreacted cement cntent n the prcessing, structure, and prperties f macr-defect-free (MDF) cmpsites fabricated frm calcium aluminate cement (CAC), ~-alumina (AI203), and plyvinyl alchl-acetate (PVAA) has been investigated. Three systems were frmed with initial CAC: AI203 ratis f 50:50, 35:65, and 25:75 by vlume in their respective frmulatins. The amunt f unreacted cement was reduced frm 68.1 vl % which is present in standard (100% CAC) MDF cement, t 14.9 vl% fr cmpsites with an initial CAC: AI203 rati f 25:75, while the hydratin prduct cntent was reduced frm 18.1 vl % t 11.4 vl % fr these respective systems. A hard cre/sft shell cntinuum perclatin mdel was used t determine that alumina substitutin did nt significantly affect the perclative nature f the interphase and bulk plymer regins. Hwever, experiments shwed that the reductin in unreacted cement cntent thrugh AI203 substitutin affected bth the prcessing and micrstructural features f these cmpsites. The misture absrptin kinetics and flexural strength f cmpsites expsed t 100% relative humidity were als evaluated, and it was fund that their misture sensitivity imprved with decreasing unreacted cement cntent. A hypthesis is presented t explain the rle f unreacted cement n the misture sensitivity f these materials. 1. Intrductin Macr-defect-free (MDF) cement was develped by Birchall et al. [13 using a nvel prcessing methd. This prcess cnsists f mixing a hydraulic cement, a water-sluble plymer, and water under high shear, fllwed by a lw-pressure and lw-temperature curing prcess. When calcium aluminate cement (CAC ~) and plyvinyl alchl-acetate (PVAN I ) are emplyed as these cnstituents, the material exhibits superir mechanical prperties [3]. The plymer perfrms three functins in this material [4] : (1) it acts as a rhelgical aid, cating individual cement particles, thereby reducing interparticle frictin during prcessing, (2) it acts as a pre filler, in vids between unreacted cement grains, and (3) it interacts chemically with the cement hydratin prducts t frm an integral micrstructural feature referred t as the interphase regin [5]. The MDF cement micrstructure cntains unreacted cement grains, interphase regins which cat individual cement grains, and bulk rganic regins (plasticized PVAL ~) as illustrated schematical- ly in Fig. 1. The interphase regins cntain bth crystalline (Ca2A12Os.SHeO) and amrphus (AI(OH)3 and PVAL) phases mixed n a nansized level. The fllwing hydratin reactin has been prpsed by Ppla etal. [5], based n their highreslutin electrn micrscpy (HREM) bservatins, t accunt fr the presence f these phases CaAI20, + 5.5H20 --* 0.5CazA12Os"8H20 + AI(OH) 3 (1) The cnversin f the metastable Ca2A12Os.8H20 phase int the thermdynamically stable Ca3Al206. 6H20 phase is believed t be suppressed by PVAL present in these regins [5-]. The cmbinatin f high flexural strength and fracture energy as well as lw cst makes MDF cements particularly attractive fr several applicatins [4]. Hwever, these prperties degrade significantly when MDF cement is expsed t misture [7-11]. T address this issue, ne must develp a clear understanding f hw the individual micrstructural elements The abbreviatin CAC has been used t dente calcium aluminate cements which cntain ther phases in additin t the principal CaAI204 phase. IIIUPAC recmmended ntatins used [2]: PVAA = partially hydrlysed plyvinyl alchl, als knwn as plyvinyl alchl-acetate cplymer; PVAL = fully hydrlysed plyvinyl alchl; PVA has been used here when the term is applicable bth PVAA and PVAL. '~ During prcessing, cement hydratin is accmpanied by a rise in ph (l ). This results in hydrlysis f PVAA t frm PVAL [6] Chapman & Hall 6445

2 tems, their micrstructural develpment, and their sensitivity t misture. Interphase regin (b) 5 nm... 9 Ca2AIzOs.8H20 AI(OH) 3 Figure 1 Schematic representatin f the standard (100% CAC) MDF micrstructure based n direct bservatins f MDF cement by Ppla et al. [5] using electrn micrscpy: (a) the general micrstrueture and (b) an expanded view f interphase regins. affect perfrmance prperties. In previus wrk [10], this fundamental understanding was develped fr the binder phase (defined here as bth the bulk plymer and interphase regins). We attribute the superir mechanical prperties f MDF cement t the absence f large vids and t the chemistry and cnnectivity f the interphase regins [4, 10], and the absrptin f misture t the bulk plymer phase due t its hygrscpicity [12] and perclative nature [-10]. In this wrk, we have investigated the effects f the unreacted cement cntent n the prcessing, structure, and prperties (namely misture sensitivity) f MDF cmpsites in which increasing amunts f the starting calcium aluminate cement (CAC) phase were replaced by an inert filler phase (~-A1203). The term MDF cmpsite is used here t define thse systems which cntain bth cementitius and inert inrganic phases in their initial cmpsitin. In cntrast, the term "standard" MDF cement is used t define thse systems that nly cntain cementitius inrganic phases. The three-dimensinal cmpsite micrstructures have been mdelled using the hard cre/sft shell cntinuum perclatin mdel [13] t investigate hw alumina substitutin, apart frm reducing the vlume fractin f unreacted cement, affects the perclative nature f the bulk plymer and interphase regins. Thrugh this understanding, the effects f unreacted cement n MDF perfrmance prperties can be reslved independently. In additin, experiments have been perfrmed t characterize the paste rhelgy and prcessability f these cmpsite sys- 2. Experimental prcedure 2.1. Materials system, fabricatin, and characterizatin Samples were fabricated using the prcess develped by Birchall et al. [3] and ptimized by Russell [11]. The individual cnstituents fr each batch are given in Table I. Previusly, ther investigatrs [-14] demnstrated that alumina-substituted MDF cmpsites culd be fabricated. Hwever, these substitutins were made n a weight basis, which alters the vlume fractins f the respective micrstructural elements cnsiderably. Therefre, in the present wrk, these substitutins have been made n a vlume basis t avid such cmplicatins. The individual cnstituents given in Table I were premixed in a planetary mixer. (Certain cmmercial prducts are identified in this paper in rder t specify adequately the experimental materials and prcedures. In n case des such identificatin imply recmmendatin r endrsement by the Natinal Institute f Standards and Technlgy, nr des it imply that the prduct is necessarily the best available fr the purpse.) This was fllwed by high shear mixing n a twin-rll mill t frm sheets whse apprximate dimensins were 300 mmx 150 mmx 2 mm. The sheets were then pressed between plates at 5 MPa fr 10 rain and subsequently cured at 80 ~ in a frced-air ven fr 24 h. Discs (30 mm in diameter) were cre drilled frm these as-cured sheets and subjected t further evaluatin. The particle-size distributin f the starting inrganic cnstituents used in each batch was measured TABLE I Batch cmpsitin f MDF cement and cmpsite frmulatins Cmpnent CAC : A120 3 (vlume rati) 100:0 50:50 35:65 25:75 CAC (Secar 71) (vl%) AlzO 3 (vl %) Mean diameter gm (Alca A-10) (vl %) Mean diameter 0.5 pm (Alca A-16) (vl %) PVAA (Ghsenl KH-17S) (vl%) Glycerl (reagent grade) (vl %) Deinized water (vl %) Secar 71, Lafarge Calcium Aluminates, Inc., Chesapeake, VA, USA. Alca A-10 and A-16, Aluminum Cmpany f America, Inc., Pittsburg, PA, USA. Ghsenl KH-17S, Nippn Ghsei, Inc., Otsaka, Japan. 6446

3 by a sedimentatin technique (X-ray Sedigraph mdel 5000E, Micrmeritics, Inc., Nrcrss, GA) and the vlume per cent distributins were btained by accunting fr the density difference between CAC and A1203. Althugh tile median particle diameters f CAC and A-10 alumina are almst identical, A-10 alumina has a narrwer size distributin. Thus, a finegrained alumina (A-.16) was blended with A-10 alumina in a rati f 1:4 t braden the A1203 particlesize distributin and imprve the prcessability f these cmpsites. The rhelgy f pastes frmed frm the cmpsitins given in Table I was characterized using a Banbury mixer (Haake Rhecrd System 90 and Rhemix 600, Fiscns Instruments, Paramus, NJ), the details f which are discussed elsewhere [15]. Selected as-cured MDF cmpsite samples were heated t either 500 r 1000 ~ at a rate f 1 ~ min- i in air. The measured weight lss upn heating t 1000 ~ was used t determine their final cmpsitin. This weight lss resulted frm the remval f vlatile decmpsitin prducts prduced by PVAA and glycerl and by the hydratin prducts (yielding water) present in the interphase regins. The rati f rganic (PVAA and glycerl) t inrganic (CAC and A1203) species shuld be unaffected during prcessing. Therefre, after accunting fr the lss f rganics, the remainder f the weight lss was attributed t lss f water. The final cmpsitin f each system given in Table II was then calculated assuming that the sle hydratin reactin is given by Equatin 1. The cmpsite samples heated t 500 ~ were characterized by mercury intrusin prsimetry (Micrmeritrics Autpre II 9220, Nrcrss, Ga) t determine hw the distributin f binder-filled pres was affected by increasing alumina cntent. Finally, weight gain and flexural strengths f several representative samples were measured after expsure t 100% relative humidity (r.h.) fr varius perids f time. Strength measurements (Instrn Mdel 4502, Cannn, MA) were cnducted using the biaxial flexural testing methd [16]. MDF cements, and t study the perclative nature f the bulk plymer and interphase regins. A cmplete descriptin f this mdel is given elsewhere [10]. Sme mdificatins t this mdel were made in the present wrk t accunt fr the inert A1203 phase present in the MDF cmpsite micrstructures. First, the system size was reduced frm its riginal 150 gm t 50 gm per side cube due t the finer size distributin f the alumina particles (see Fig. 2). The reductin in system size prduced systems with apprximately particles, which were cmputatinally tractable and nearly identical t the particle cntent used in previus wrk [10]. Secndly, tw types f hard cres were used in these systems, as shwn in Fig. 3: ne representing the CAC particles and the ther representing the A1203 particles. Hard spherical cres f bth types were placed in each system randmly 100 g 80 ~ E ~"% 25:75 CAC:AI203 '.,~\\\ -- 35:65 CAC:AI203,~ 50:50 CAC:AI20 a ~~.. 100:0 CAC:AI203 X Increasing :~cntent, , 1,. Equivalent spherical diameter (pm) Figure 2 Particle-size distributins f starting inrgamc cnstituents fr each system Mdel descriptin The hard cre/sft shell cntinuum perclatin mdel has been used previusly [10] t mdel the threedimensinal micrstructure f standard (100% CAC) TABLE II As-cured MDF cement and cmpsite cmpsitins Cmpnent CAC:AIzO 3 (vlume rati) 100:0 50:50 35:65 25:75 Unreacted cement (vl%) AI20 3 (vl%) Ca2A12Os.SH20 (vl %) AI(OH)3 (vl %) Plasticized-PVA (vl %) Nnvlatiles (vl %) Vlatiles (vl %) Plasticized-PVA (vl%) H20 (vl %) Figure 3 A crss-sectinal view f the mdel micrstructure generated fr the 25 : 75 CAC:A120 3 system, where the black hard cres represent inert alumina grains, the grey hard cres represent unreacted cement grains, the light grey sft shells represent interphase regins, and white space represents the bulk plymer regins. 6447

4 using their experimentally determined particle-size distributins. Peridic bundaries were emplyed t eliminate the edge effects assciated with inefficient particle packing in these regins. Three mdel systems were develped crrespnding t initial CAC:AI20 3 ratis f 50: 50, 35:65, and 25:75 by vlume. The CAC hard cres were reduced in vlume by 11, 16, and 20 vl % in these respective systems t accunt fr their varying extent f hydratin (refer t Table II). Finally, sft shells f cnstant thickness, which represented the interphase regins, were added t the CAC hard cres. The inert A1203 hard cres were left unaltered. The simulated three-dimensinal micrstructures crrespnding t each system (50:50, 35:65, and 25:75 CAC:AI203) were first studied t determine their ptimum sft-shell thicknesses [10]. These thicknesses were btained by cmparing the cmputed vlume fractins f the resulting interphase regins t thse btained experimentally. The cmpsitin f these regins was assumed t be the same as determined previusly [10]. The micrstructures crrespnding t these "best" shell thicknesses were then evaluated t determine the perclatin characteristics f the bulk plymer and interphase regins as described previusly [ Results and discussin 3.1. Phase cmpsitin and distributin in MDF cmpsites The cmpsitins f the as-cured MDF cmpsite samples are given in Table II. Fr cmparisn, the cmpsitin f standard MDF cement is als included. An increase in the amunt f alumina substitutin resulted in a decrease in the unreacted cement cntent in the as-cured cmpsites frm 68.1 vl % t 14.9 vl%. Hwever, as expected, their hydratin prduct cntent did nt decrease t the same extent due t the variatin in their initial cmpsitins (e.g. water:cement rati). Furthermre, the amunt f cement hydrated in these cmpsites increased frm 7 vl % fr standard MDF cement [10] t 11, 16, and 20 vl % fr initial CAC: A120 3 ratis f 50: 50, 35 : 65, and 25:75 by vlume, respectively. The hydrated cement cntent was calculated by taking a rati f the vlume fractin f reacted cement t the vlume fractin f the initial cement in these systems, Cnsequently, ne wuld expect the average thickness f the interphase regins t vary in these samples. Fig. 4 depicts the calculated "best" thickness f the sft shells, r interphase regins, as a functin f initial cement cntent (refer t Table I) as determined by cmputer simulatins. Fr cmparisn, a plt f the hydrated cement cntent as a functin f initial CAC cntent is als included in Fig. 4. As the CAC cntent is reduced thrugh increasing A120 3 substitutin, the interphase thickness increases. This increase in shell thickness is related t their hydrated cement cntent as expected. While MDF cmpsites fabricated frm initial CAC :A1203 ratis f 50:50 by vlume have abut the same shell thickness as the standard MDF cement [10], thse with greater A120 a substitutins 6448 c (D c IE O ,, i,,,,, i,, i,, 2O Initial cement cntent (vl %) 25 > 20 c E 15~ 0" E Z3 >. 5 3: 70 Figure 4 Cmputer simulatin results which predict the variatin f () interphase thickness as a functin f initial CAC cntent (vl%) in the starting frmulatins. A plt f hydrated cement cntent ([]) versus initial CAC cntent is als shwn here t illustrate the similarities between these tw trends, have significantly larger interphase thicknesses. In additin, the fractin f interphase regins cnnected within these mdel micrstructures is 1.0, 0.99, 0.94, and 0.87 fr CAC:A120 3 ratis f 100:0, 50:50, 35 : 65, and 25 : 75, respectively. The perclative nature f the bulk plymer regins was evaluated fr the simulated three-dimensinal cmpsite micrstructures crrespnding t the "best" sft-shell thickness fr each system. T reslve the cnnectivity f these regins, ne must examine these mdel micrstructures at a minimum reslutin crrespnding t the diameter f the largest pres cntaining bulk plymer (i.e. maximum pre diameter). In previus wrk [10], it was fund that a reslutin f 500 nm was required t detect the perclatin f bulk plymer in standard MDF. This value crrelated well t the maximum pre diameter bserved by mercury intrusin prsimetry f actual samples. In the present study, a reslutin f 250 nm was required t detect perclatin f these regins in each cmpsite system. This reslutin is significantly higher than that required fr standard MDF cement, and suggests that these cmpsites cntain a finer binder-filled pre-size distributin. The fractin f the bulk plymer cnnected in these simulated threedimensinal micrstructures was determined t be 0.927, 0.942, and fr CAC:A120 3 ratis f 50 : 50, 35 : 65, and 25 : 75 by vlume, respectively Paste rhelgy Fig. 2 shws the particle-size distributin f the starting inrganic cnstituents used t fabricate each cmpsite system. Fr cmparisn, the particle-size distributin f pure CAC (100:0) is als given. The particle-size distributin narrws with increasing alumina substitutin in the size range abve the median particle diameter, while it bradens significantly belw this diameter due t the incrpratin f finer A1103 particles (Alca A-16). As. discussed belw, these differences influence bth the prcessing and micrstructure f these cmpsites. Fig. 5a shws the rhelgical behaviur f MDF pastes with varius CAC:A1203 ratis, where trque

5 O "E25 z 20 G ~_15 10 Prcessing_ - windw > 1:75 CAC:AI203 35:65 CAC:AI203 50:50 CAC:AI203 ~'[ I,[I ]]I 0 r (a) Time (min),( Matrix degradatin which are a cnsequence f the increased trque values Pre structure f MDF cmpsites Mercury intrusin prsimetry (MIP) was used t analyse the pre develpment f representative MDF cmpsite samples heat treated t 500~ Fig. 6 shws the incremental prsity as a functin f pre diameter fr these cmpsites. T accunt fr the differences in sample density resulting frm their differing AlzO3 cntent, we cnverted the raw MIP data t vlume f prsity intruded, using Equatin 2 belw. 12,, ~..., 2tandar d Prcessing windw '~ ' /'I' ~ MDF cement g J z : ~t i 8 ~- degradatin- b-4 j, 2~ii n n1 IV O 9 ' ' (b) ~ Time (min) Figure 5 The rhelgical behaviur, as given by trque as a functin f time, fr pastes frmed frm: (a) the MDF cmpsite systems and (b) standard MDF cement prcessed at 100 r.p.m. is pltted as a functin f time. T aid ur interpretatin, we cmpared these bservatins t thse reprted by McHugh and Tan [15] n standard MDF pastes shwn in Fig. 5b. The rhelgical behaviur f these pastes is cmplex and can be divided int fur regins. Regins I and II, which are readily identifiable fr standard MDF pastes, are cmbined int ne brad peak in the trques curves btained n the CAC:A1203 cmpsite pastes. The magnitude f this brad peak increases substantially with increasing alumina cntent, which we believe results partly frm an increase in the interparticle frictin. Furthermre, regin III als differs; significantly fr these pastes as cmpared t standard MDF cement. McHugh and Tan [15] hypthesized that the slw rise in trque bserved in this regin resulted frm the develpment f crss-linked netwrk structure in the paste. Beynd this secnd trque maxima, the standard paste breaks dwn int a friable mass (designated as regin IV). Althugh n rise in trque was bserved fr ur pastes, beynd the plateau in regin III these pastes als brke dwn (regin IV). It is pssible that the verall trque increase, resulting frm the alumina cntent in these pastes, has masked these subtle features in regin III. Alternatively, the extent f netwrk frmatin may be less in ur systems as a result f the reductin in hydratin prduct cntent. Finally, we believe the narrwer prcessing windws bserved in ur systems result frm higher paste temperatures, Tin t where Pint is the amunt f prsity intruded (%) at a given pressure, Vint is the vlume f mercury intruded (cm 3 g- 1) at a given pressure, and Papp is the apparent density (gcm- 3) measured by MIP fr a given sample. Nte, the apparent density des nt include pre vlume fr clsed pres r very fine pres ( < 3 nm diameter). The bimdal pre-size distributins shwn in Fig. 6 develp upn the remval f vlatile cnstituents frm these samples. The larger pres are assciated with the remval f bulk plymer, whereas the finer pres are assciated with the remval f matter frm the interphase regins, as illustrated fr standard MDF [10]. A characteristic pre size, defined as the maximum f the incremental intrusin curve, was determined fr the prsity that develps fr each f these binder regins. The characteristic pre size fr the larger pres decreases frm apprximately 170 nm t 80 nm with increasing alumina substitutins, which is expected based n their respective particle-size distributins. Hwever, the characteristic size f the smaller pres ( g 15 nm) was fund t be independent f alumina substitutin. This, t, is expected, because alumina is inert with respect t hydratin. The maximum pre diameter, D... f these cmpsites was determined by MIP t be apprximately nm. The nset f intrusin is much lwer 2.5 ~2,0 v t~ 1.5 E 1. E P ~ :75 CAC:AI Pre diameter (/Jm) ~[ I 50:50 I t ~, CAC:AI203 Figure 6 Incremental prsity as a functin f pre diameter fr MDF cmpsites heat treated t 500 ~ at a rate f i ~ min

6 than that bserved fr standard MDF cement (Dmax,,~ 510 nm) [10], and is in gd agreement with the values predicted by cmputer mdelling ( 250 nm). This result supprts the use f this mdified versin f the hard cre/sft shell cntinuum perclatin mdel t simulate the three-dimensinal micrstructure f these cmpsites. A v t- t- X 80 \\ 60 ~ ~ 35: Prperties f MDF cmpsites Fig. 7 demnstrates the weight gain as a functin f time fr samples expsed t 100% relative humidity (r.h.). Their rate f misture absrptin was clearly affected by their unreacted cement cntent. Because the vlume fractin and cnnectivity f bulk plymer regins are nt significantly different as discussed previusly, the dminant pathway fr misture transprt is similar in all three cmpsites. Thus, ver shrt time perids (24 h), there is little difference in their weight gain. At lnger times, the unreacted cement cntent influences the misture transprt prcess by affecting the cncentratin gradient f water between the sample surface and its interir. The unreacted cement phase serves as a misture getter, because free water cmbines with this phase t prduce additinal hydratin prducts. Therefre, as the cntent f this phase increases there is less free water in the sample interir, r a large H20 cncentratin gradient persists between the envirnment and the interir during misture expsure. In the absence f this phase, ne wuld expect this cncentratin gradient t disappear ver time, thereby minimizing further diffusin. Thus, we expect even lwer amunts f misture absrptin fr cmpsites that d nt cntain any unreacted cement. The nrmalized strength f MDF cmpsites as a functin f time during expsure t 100% r.h. is shwn in Fig. 8. Their initial strengths were nt strngly affected by their respective cmpsitins, and their average values ranged between 230 and 260 MPa. Hwever, their strength degradatin upn expsure t misture was clearly affected by their unreacted cement cntent, with the 25:75 CAC:A120 3 cmpsite exhibiting superir misture 4'01 35:65 CAC:AI :50 / _. ~ / ~ ~ 3'01 C A C ~ Time (days) Figure 7 A plt f weight gain as a functin f time fr MDF cmpsite samples expsed t 100% r.h. Unreacted cement: (A) 32.8 vl %, (e) 21.7 vl %, (11) 14.9 vl %. " z 20~ L "3 0 i i i I I i i ~ i I ; L I ~ i I I I I I ;-I I I, ~ Time (days) Figure 8 A plt f nrmalized flexural strength as a functin f time fr MDF cmpsite samples expsed t 100% r.h. Unreacted cement: (Ik) 32.8 vl %, (e) 21.7 vl %, (HI) 14.9 vl %. resistivity. It appears that ver lnger perids f expsure, the strength f these cmpsites may apprach the same final value, althugh the rate f this apprach is clearly reduced with decreasing unreacted cement cntent. Based n the hypthesis described belw, we expect that the ttal absence f unreacted cement may lead t far superir perfrmance under identical expsure cnditins. We prpse the fllwing hypthesis t explain the misture absrptin prcess and its effects, based n the abve results and the existing knwledge [4-11, 17-25] f PVA and MDF. cement. First, the hygrscpic bulk plymer regins absrb misture and swell, thereby creating free vlume. Secndly, the availability f free vlume and water in the sample interir allws unreacted cement grains t underg further hydratin. This results in a basic envirnment and the frmatin f further hydratin prducts. Hydratin reactins can prceed as lng as free vlume exists within the cmpnent, and the necessary reactants (e.g. cement and water) remain available. Thirdly, as the plymer absrbs misture it sftens, because water is a well-knwn plasticizer fr PVA [12]. This prcess alne, hwever, des nt necessarily lead t strength degradatin, therwise these materials wuld cntinuusly decrease in strength upn initial expsure t misture. This was nt bserved in the 25:75 CAC:A1203 samples r in ther studies [11, 26, 27] n MDF cements. Yet at lnger times, strength degradatin is bserved under these cnditins fr all systems. These bservatins can be explained by the fact that the interphase regins serve as the main lad-bearing and transferring regins in these systems. Either the bulk plymer r interphase regins culd carry and transfer lad, because bth frm perclative pathways thrugh the micrstructure. Hwever, it is expected that the interphase regins wuld display superir mechanical prperties due t their highly filled nansized cmpsite structure (refer t Fig. 1). In additin, because these regins are strngly bnded t the unreacted cement grains, they may transfer lad mre efficiently. Finally, the basic envirnment created as a result f further hydratin is believed t accelerate the degradatin f 6450

7 plymer in bth regins leading t the bserved strength lss f these MDF materials. In this study, the maximum amunt f AI203 that culd be substituted fr CAC was 75 vl %, while still yielding a prcessable paste. Althugh this prduces a final MDF cmpsite with much lwer unreacted cement cntents than bserved in standard MDF materials, their cement cntent is still fairly high relative t the ttal amunt f misture absrbed during expsure t 100% r.h. as calculated belw. The absrbed water culd react with the CAC grains t frm either the CazAlzOs'8H20 phase accrding t Equatin 1 r the thermdynamically stable Ca3AI206.6H20 phase as given by the reactin belw 3CaAI HzO ~ Ca3AlzO6"6HzO + 4Al(OH)3 (3) The amunt f misture absrptin required t cnsume cmpletely the CaAI204 phase present in each cmpsite can be calculated fr either reactin, Equatin 1 r Equatin 3. The unreacted cement grains cntain apprximately 48 wt % CaA1/O 4 [i1]; thus, the required amunt f absrbed water is 4.2, 6.3, and 9.8 wt % accrding t Equatin 1 and 3.0, 4.5, and 7.2 wt % accrding t Equatin 3 fr cmpsites crrespnding t CAC : A1203 ratis f 25 : 75, 35 : 65, and 50:50 by vlume, respectively. Hence, even fr these systems, a significant amunt f water needs t be absrbed befre they apprach equilibrium. These results clearly serve as a guideline fr future design f MDF materials. Althugh this study is the first t demnstrate the deleterius effects f unreacted cement cntent n the misture sensitivity f MDF materials, it als illustrates that much lwer cntents are required t successfully imprve their perfrmance. In summary, tw prcessing strategies have emerged frm ur wrk t imprve the misture sensitivity f MDF cements and cmpsite systems thrugh micrstructural design. First, ne must minimize the unreacted cement cntent present within the threedimensinal micrstructure, while maximizing the interphase regin cnnectivity and cntent. This can be achieved in part by prducing a finer, mre evenly distributed cement phase in the starting materials system which is cmpletely cnsumed during fabricatin. Secndly, ne must mdify either the accessibility (r cnnectivity) f the bulk plymer regins, r their hygrscpic nature. Its accessibility can be minimized by decreasing the bulk plymer t interphase regin rati, whereas its affinity fr water can be altered thrugh chemical mdificatin as suggested previusly [11, 23, 26, Cnclusins The effects f reducing the unreacted cement cntent n prcessing, structure, and misture sensitivity f CAC-PVAA-based MDF cmpsites have been investigated thrugh bth cmputer simulatins and experiments. A mdified hard cre/sft shell cntinuum perclatin mdel was used t successfully mdel the three-dimensinal alumina-substituted MDF cmpsite micrstructures. It was shwn that bth the bulk plymer and interphase regins frm perclative pathways in these micrstructures. Furthermre, the accessibility f the bulk plymer phase was shwn t remain unaltered, while the thickness f the interphase regins was fund t increase as their cement cntent decreased. Accrding t ur hypthesis, the bulk plymer prvides a pathway fr water absrptin and transprt, but this prcess alne is nt respnsible fr the bserved strength lss during misture expsure. We have prpsed that their strength lss is affected by further hydratin f unreacted cement grains, which leads t an accelerated deteriratin f the perfrmance prperties f MDF materials. In additin, these results suggest that the keys t ptimizing the micrstructural design f MDF cements are t minimize their unreacted cement cntent and t eliminate their pathway fr misture transprt. 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