Recent developments in macro-defect-free (MDF) cements

ReviewRecent developments in macro-defect-free (MDF)cementsS.Donatello a ,M.Tyrer b ,C.R.Cheeseman a,*a Department of Civil and Environmental Engineering,Imperial College,London SW72AZ,UK bDepartment of Materials,Imperial College,London SW72AZ,UKa r t i c l e i n f o Article history:Received 30October 2006Received in revised form 3September 2008Accepted 4September 2008Available online 29October 2008Keywords:Macro-defect-free Polyvinyl alcoholCalcium aluminate cement High strengthMoisture resistancea b s t r a c tDuring the early 1980s macro-defect-free (MDF)cements were developed which had properties compli-mentary to those of conventional ceramics,plastics and metals.The research completed on MDF cements,and particularly the polyvinyl-alcohol/calcium aluminate cement (PVA/CAC)system has been reviewed and the advantages,limitations and potential applications of MDF cements are outlined.The microstruc-tural features of the PVA–CAC MDF systems and the influence this has on physical properties such as flex-ural strength and moisture resistance are discussed.Possible solutions to the poor moisture resistance of MDF cements are examined critically and alternatives to PVA–CAC described.The recent development of calcium aluminate phenol resin (CAPR)MDF cements that have particularly promising properties is pre-sented.MDF cement technology has the potential to produce more sustainable materials that can com-pete with ceramics,plastics and metals in a range of applications and key research issues that need to be addressed if MDF cements are to become commercially viable are highlighted.Ó2008Elsevier Ltd.All rights reserved.Contents1.Introduction ........................................................................................................17622.The MDF cement manufacturing process .................................................................................17622.1.Materials .....................................................................................................17622.2.Mixing paction ...................................................................................................17632.4.Setting and curing..............................................................................................17633.Microstructure ......................................................................................................posite structure ............................................................................................17633.2.Unhydrated cement grains .......................................................................................17633.3.Bulk polymer phase.............................................................................................17633.4.Interphase region ..............................................................................................17634.Properties ..........................................................................................................17635.Limitations of MDF cement technology ..................................................................................17645.1.Moisture resistance.............................................................................................17645.2.Shrinkage.....................................................................................................17646.CAPR –an alternative MDF composite ...................................................................................17657.Applications ........................................................................................................17668.Sustainability and MDF cement.........................................................................................17669.Discussion..........................................................................................................176610.Conclusions.........................................................................................................1766Acknowledgement...................................................................................................1767References (1767)0950-0618/$-see front matter Ó2008Elsevier Ltd.All rights reserved.doi:10.1016/j.conbuildmat.2008.09.001*Corresponding author.Tel.:+442075945971.E-mail address:c.cheeseman@ (C.R.Cheeseman).Construction and Building Materials 23(2009)1761–1767Contents lists available at ScienceDirectConstruction and Building Materialsjournal homepage:w w w.e l s e v i e r.c o m /l oc a t e /c o n b u i l d m at1.IntroductionMacro-defect-free(MDF)cements were developed following work by the research group led by Birchall at ICI during the late 1970s and early1980s[1,2].The manufacture of MDF cement in-volves high shear mixing of selected polymers and hydraulic ce-ments at low w/c ratios typically between0.08and0.20.Most research has used the preparation guidelines provided by Birchall [3]and later optimised by Russell[4]and Fig.1illustrates the key stages involved in the MDF manufacturing process.MDF cements have unusual and superior properties compared to traditional cement pastes and concretes,and are often referred to as chemically bonded ceramics or MDF composites[5].How-ever,in the25years since they werefirst patented,the incorpo-ration of MDF materials into commercial processes has been very limited.This is because despite the identification of many potential applications,MDF cements have certain limitations, and particularly loss of strength on exposure to moisture or humidity[6].Modifications to the MDF manufacturing process and the selection of the polymer used have overcome many of these problems.The major obstacle to MDF cements in many applications is the economics of manufacturing on a large scale and their overall commercial viability compared to existing materials.The properties of MDF cements and details of the manufactur-ing process are described.Problems encountered and research into overcoming poor moisture resistance are summarised.New MDF cements with superior properties have recently been developed and these are discussed.Possible applications for MDF cements, the energy requirements compared to other materials,future re-search needs and the potential for MDF cement technology to pro-vide low energy sustainable materials are discussed.2.The MDF cement manufacturing process2.1.MaterialsThe most widely used cement in MDF formulations is calcium aluminate cement(CAC)and particularly Secar71,manufactured by Lafarge[7-9]combined with polyvinyl alcohol(PVA),such as Gohsenol KH-17s manufactured by Nippon[7,9,10].Glycerol is of-ten added as a plasticiser.Examples of other cement/polymer sys-tems used to manufacture MDF cements are given in Table1.2.2.MixingInitial premixing of the CAC,PVA glycerol and water at low w/s ratio is followed by high shear mixing.This is essential to achieveTable1Examples of polymer/cement combinations used in MDF manufactureAuthor(Ref)Year Polymer Hydraulic cement w/c RatioMojumdar[11]2006BA/AN,starch,soluble Poly-P OPC:SAFB:Al2O3blends0.2Lewis et al.[12]1994PVAA with0.1ratio of glycerol CA(Secar71)0.1067Mojumdar[28]2004PBA,SACP,poly-P OPC:SAFB:Al2O3blends0.08–0.2Zhihong et al.[33]2003AAM–monomer and modifiers OPC,SAC,Slag$0.195Drabik et al.[27]2001HPMC,poly-P(aq),poly-P(s)SAFB:OPC85:150.2Chandrashekhar and Shafer[32]1989PVA,Novolac epoxies CA0.115+heat treatment Titchell[24]1991PVA,HPMC OPC,CAC–Pushpalal et al.[30]1997Phenol resin precursor and modifier HAC0.01aBA/AN,butylacrylate/acrylonitrile;PC,Portland cement;CAC,calcium aluminate cement;HAC,high alumina cement;SACP,styrene/acrylonitrile copolymer;SAC,sulfoa-luminate cement;SAFB,sulfoaluminate–ferrite–belite;PBA,poly(butyl acrylate);soluble Poly-P,aqueous solution of sodium polyphosphate;PVAA,polyvinyl alcohol–acetate copolymer.a Actually no water added,is generated in situ and estimated to be1%by weight of cement.1762S.Donatello et al./Construction and Building Materials23(2009)1761–1767intimate contact between the polymer,water and cement.A great-er degree of mixing produces stronger polymer cross linking dur-ing cement hydration.Most MDF production has used a twin roll mill to produce high shear and this allows a satisfactory homoge-nous mix to form in less than5min[11].pactionAfter the MDF cement sheet is removed from the twin roll mill relatively low pressure compaction is required during curing.Pres-sures applied are typically paction allows the MDF particles to form and retain a dense packing arrangement (0.6–0.7solid volume fraction)[5].This is advantageous in terms of moisture resistance as it causes a reduction in the diameter of the bulk polymer phase.Hot pressing has also been used,and this speeds up the curing process.2.4.Setting and curingThe polymer is considered to have different roles which vary during the manufacturing process[12].Initially the polymer acts as a rheological aid,significantly improving the workability of the very dry cement paste and reducing particle friction during mixing and particularly the roll milling process.The polymer also fills voids between unreacted cement grains and allows them to come together in a close packed arrangement during compaction. The functional groups of the polymer are reported to interact chemically with the cement hydration products and the ions re-leased during cement hydration.This further contributes to in-creased strength development during curing.Examination of differential thermal analysis(DTA)curves and infra-red(IR)spectra of PVA and hydrated CAC phases and PVA/hy-drated CAC MDF cement mixes have shown that the polymer prop-erties are completely unaltered by interaction with the hydrating cement phases[13].The PVA changes the CAC hydration reactions as it inhibits the formation of the C3AH6phase[14].3.Microstructureposite structureMDF cements consist of three distinct regions.These are:(i) unhydrated cement grains,(ii)the bulk polymer phase and(iii)a complex inter-phase region which includes hydrated cement parti-cles and polymer chains.The microstructure has been described as close packed unhydrated cement grains within a three dimensional PVA network,with coatings of hydrated cement on the surface of grains interacting with the bulk polymer via chemical cross-link-ages[7].3.2.Unhydrated cement grainsDue to the low w/c ratio there is a stoichiometric deficit of water that prevents all the cement from fully hydrating.The stability of a material that contains a significant volume of unhydrated cement during subsequent exposure to moisture has always been a con-cern.A study which substituted up to75%of the original cement with inert Al2O3showed improved moisture resistance with decreasing unhydrated cement content[15].This research indi-cated that unhydrated cement acts as a potential store for moisture when exposed to high humidity.The locking of free moisture into newly hydrated cement phases maintains a positive gradient allow-ing more moisture to enter.It was suggested that a combination of solubilisation of the polymer phase and in situ cement hydration were responsible for loss of strength on exposure to moisture.3.3.Bulk polymer phaseThe bulk polymer phase consists of cross-linked PVA.This cre-ates an intricate and continuous three dimensional network of pathways through the material.A major factor in the poor mois-ture resistance of MDF cements is the hygroscopic nature of PVA.The PVA and interphase regions have been observed to absorb moisture when exposed to humidity using in situ transmission electron microscopy(TEM)[16].Transport of moisture through the structure is most likely via the network of the bulk polymer phase[12].The use of a hard core-soft shell percolation model coupled with mercury intrusion porosimetry(MIP)analysis of MDF cements showed that selective removal of the bulk PVA phase left pores of around30nm in diameter,whereas removal of the interphase region left pores of around5nm[12].Therefore it was concluded that water can pass readily through the bulk polymer phase. Increasing the cross-linking density of PVA within the bulk poly-mer was suggested as a possible way to improve moisture resis-tance,as this would reduce the diameter of pathways through the MDF cement[12].An organo-titanate cross-linking agent has been used and found to be effective[9].3.4.Interphase regionCharacterisation of the interphase region is difficult as it is amorphous.A number of different techniques have been employed to understand the nature of the interphase chemistry and struc-ture.13C nuclear magnetic resonance(NMR)in combination with hydrogen relaxation time studies showed that the polymer mole-cules were affected by interaction with inorganic atoms within 30nm[10].The interaction of the polymer phase with aluminium ions via an ester group and/or acetate ions has been postulated [17,18].Observation of the organic–inorganic interface region with electron microscopy showed an intimate mix of polymer and hy-drated cement phases were present on the surface of unhydrated cement grains[19].The calcium and aluminium ions released by CAC hydration are thought to have a major role in forming chemical links to dissoci-ated alcohol(ROÀ)and carboxylic groups(ROOÀ)formed from the PVA polymer.In the high pH environment that exists at the surface of hydrating cement grains,Ca2+ions will be present as Ca(OH)2 and Al3+ions present as AlðOHÞÀ4.It has been suggested that Ca can form a hemihydrate calcium acetate following interaction with the polymer[10],and Al ions rather than Ca ions are thought to have a role in forming the cross-linking interphase[7,16,20].The nature of the cement hydration products is affected by the presence of the polymer phase[16].This is due to partial interca-lation of the polymer chains within the CAC hydrate lattice.The ef-fects of organic molecules on CAC hydration products in normal cement systems have been reported[21].4.PropertiesMDF cements have significantly improved physical properties compared to normal hydraulic cement pastes.A comparison of some properties is given in Table2.This shows that although MDF cements and normal Portland cement(PC)pastes have similar densities,MDF cements show compressive andflexural strengths that are typically an order of magnitude higher.MDF cements have almost double the Young’s modulus of PC and the increasedflex-ural strength of MDF has been attributed to the absence of macro-pores[22].Typical microstructures of MDF cement and normal concrete are shown in Figs.2and3a.The absence of macroporesS.Donatello et al./Construction and Building Materials23(2009)1761–17671763(>200l m)and the intimate interaction between the polymer phase and the cement hydration products contribute to high strength.The degree of intimacy between polymer and hydrated cement is illustrated in Fig.3b.5.Limitations of MDF cement technologyThere have been some significant problems and limitations associated with early generation MDF cements and these have included:Low moisture resistance. Shrinkage.Difficulties with processing on a commercial scale.5.1.Moisture resistanceProblems with loss of strength when MDF cements are exposed to moisture have been widely reported.There has been consider-able research to understand the process of moisture ingression and the resulting chemical changes that occur.These cause loss of compressive and flexural strength [6,9,11,12,15,16,23–30].A 55%loss in flexural strength was observed after 24h immersion in water,although most of this was found to be reversible on dry-ing at 80°C [14].In the widely used CAC–PVA system the PVA polymer phase is associated with moisture ingress [12,15,16].Moisture enters via the polymer network,causing the polymer to swell.This has a sig-nificant effect on the nature of cross links in the polymer–cement interphase region eventually initiating in situ hydration reactions of unhydrated cement grains [15,19].The rate of moisture uptake is thought to be diffusion controlled [26].A common method of assessing moisture resistance is to place MDF cement samples in a container with controlled relative humidity and measure the sample mass at regular intervals.Sam-ples are then dried and reweighing at regular intervals [11,25,28].This type of exposure shows the reversible and irreversible extent of moisture absorption.The reversible changes in properties are generally attributed to polymer swelling whereas irreversible changes are linked with carbonation and the reaction of unhydrat-ed cement [16].Materials high in moisture resistance should exhibit a low mass increase under these test conditions.A noticeable feature of mois-ture attack is the formation of new CaCO 3detectable by thermo-gravimetric (TG)/DTA analysis in the temperature range 500–750°C.This occurs by the following reaction:CaO þH 2O !Ca ðOH Þ2þCO 2!CaCO 3þH 2Oð1ÞThe effect of different cement phases on moisture resistance has been investigated.For example,using sulphoaluminate–ferrite–belite (SAFB)and Portland cement (PC)blends instead of CAC pro-duced irreversible moisture absorption of 0.5–10%of initial weight [6,11,20,25,27,28].Changing the polymer phase has also been investigated.PVA with different degrees of hydrolysis and average molecular weights have been used as well as sodium polyphosphate (poly-P),hydroxypropylmethyl cellulose (HPMC)and butylacrylate/acrylo-nitrile (BA/AN)[6,11,20,25,27,28,31].Another approach has been to remove the moisture sensitive bulk polymer phase by heat treatment at 500°C followed by incor-poration of a heat resistant polymer into the structure [32].These materials showed good moisture resistance.For example,CAC–PVA cement heated to 500°C before being partially rehydrated showed zero loss of strength after 48h immersion in water at 20°C [24].The effect of cross-linking agents to cause the bulk polymer pathway to become narrower and less accessible and percolative to moisture has been investigated [9].The research has used an organotitanate linking agent to increase cross-linking with PVA.Unfortunately the workability of the paste was reduced before the manufacturing process had finished due to the extensive cross-linking and it remains unclear if lower additions of cross-linking agent would have the desired effect.Using a monomer and activation agent instead of a polymer,so that polymerisation occurs in situ during MDF cement hydration has been investigated [33].Using in situ polymerisation of acryl-amide monomer (AAM)with a PC/sulphoaluminate and slag blend,the MDF cement formed showed no decrease in flexural strength after 4months immersion in water.Reducing the quantity of unreacted cement in MDF materials by substituting CAC for inert fillers prior to pre-mixing has also been shown to result in improvements in moisture resistance [15].Changing the curing conditions by either delayed drying during curing or prolonged curing in ambient water followed by oven cur-ing have both been shown to improve moisture resistance [25,30].This is believed to be due to either the formation or more stable ce-ment hydration products or to a greater extent of hydrated cement phases being produced.5.2.ShrinkageThe precise natures of hydration reactions are very difficult to ascertain in a composite matrix where hydration is incomplete.During CAC–PVA MDF cement curing,a characteristic shrinkage of about 10%volume occurs which may be due to dehydration of the polymer as moisture is removed by reaction with unhydrated cement grains [5].PVA therefore affects the hydration reactions that would otherwise occur in pure CA cement systems [16].Table 2Comparison of material properties [42]Material Density (g/cm 3)Flexuralstrength (MPa)Youngsmodulus (GPa)Fractureenergy (J/m 2)OPC2.35–1020–2520MDF cement 2.3–2.5>15040–45300–1000Aluminium 2.7150–400701,00,000Glass 2.5707010Wood1.01001010,000Fig.2.Backscattered SEM image of a typical concrete microstructure [43].1764S.Donatello et al./Construction and Building Materials 23(2009)1761–1767The hydration reactions in PVA–CAC systems are the same as in CAC except that they occur to different extents [34]:CA þ10H !CAH 10T <15 Cð2Þ2CA þ11H !C 2AH 8þAH 315C <T <30C ð3Þ3CA þ12H !C 3AH 6þ2AH 3T >30Cð4Þ2CAH 10!C 2AH 8þAH 3þ9Hð5Þ3C 2AH 8!2C 3AH 6þAH 3þ9Hð6Þwhere cement chemistry notation used so that C =CaO,A =Al 2O 3,H =H 2O.The reactions will thermodynamically proceed towards the products of Eq.(6).However the formation of the stable C 3AH 6phase is inhibited by the presence of PVA [16].As a result an inter-mediate hydration product CAH 10or C 2AH 8is formed during curing and these phases persist longer than would be the case for pure CAC.Eventually C 3AH 6conversion will occur,but it is accompanied by approximately 10%shrinkage during curing which is at a time when the stresses on the material can cause significant weakening.Ideally this shrinkage should occur during compaction or the for-mation of C 3AH 6should be so rapid that the shrinkage is not noticeable.6.CAPR –an alternative MDF compositeAn totally new MDF type of material referred to as CAPR com-posites (calcium aluminate phenol resin)have been produced using processing similar to that required for MDF cement manufac-ture [35,36].However,the manufacture of CAPR is considered suf-ficiently different to conventional MDF cements that it is protected by a separate patent [37].In CAPR composites high alumina ce-ment (or other hydraulic cement)is mixed with a phenol resin pre-cursor.A modifier is used to control cross linking density and glycerol is added as a plasticiser [30].The process involves no addi-tion of water and the cement is hydrated only by water given off by the phenol precursor during the condensation polymerisation reaction that occurs in situ.This can lead to CAPR composites with an effective w/c ratio as low as 0.01[30].A summary of this pro-cess is outlined in Fig.4.CAPR composites that have been immersed in water at 20°C for one year showed only 0.82%increase in mass and 0.12%expansion and a loss in flexural strength of 9%[30].An ‘immunisation proce-dure’has been developed for CAPR composites in which they are cured in hot water prior to heat curing in order to produce the sta-ble phase C 3AH 6instead of metastable CAH 10and alumina gel.‘Immunised’samples showed virtually no loss in flexural strength after 1year immersion in water at 20°C [30].The nature of the interaction between the phenol and cement phases in CAPR composites has been studied [35].A cross-linking mechanism involving initially Ca ions at the roll-milling stage and then Al ions during heat curing has been proposed.The fracture toughness of CAPR composites has been reported [38].Crack propagation was found to occur through the phenol re-sin matrix and the cement particles were believed to act as reinforc-ing particles.However increasing the number of reinforcing cement particles in CAPR composites produced cements with inferior prop-erties,probably due to poorer particle lubrication duringformation.Fig.3.Electron micrographs of MDF cement microstructure [12]:(a)bright-field TEM image (arrows indicate interphase region);(b)interface region between the cement grain and PVA region.S.Donatello et al./Construction and Building Materials 23(2009)1761–176717657.ApplicationsSuggestions of applications for MDF cements have been extre-mely wide ranging.These have included roofing tiles,fire resis-tant doors,sewage pipe,airport bridges,window shutters, plastic moulds,printing rollers,thermal insulators,tube exhaust, oil tanks,corrosion resistant tanks,cable duct covers,electric gen-erator propellers,electrical parts,boat decking,brake lining,body armour,pallets,toys,cryogenic vessels,signboard,cooler box, sound insulators and electromagnetic interference screening [6,39].Many of these applications currently employ metals,ceramics or plastics.Metals normally have considerably higher elastic mod-ulus but do not have the potential corrosion resistance of MDF ce-ments.Plastics can be made to high strengths and can be easily shaped but do not have the range of temperature stability poten-tially offered by MDF cements.Ceramics are corrosion resistant,hard and temperature resis-tant but are difficult to machine into complex shapes.CAPR MDF cements are reported to be stable up to250°C and they are lighter than most ceramics and can be formed into complex shapes.Factors preventing MDF cements being used on a commercial basis are lack of confidence in durability and stability under various environmental conditions.They also need to be econom-ically competitive with existing materials in particular applications.8.Sustainability and MDF cementThe energy requirements and CO2emission associated with the manufacture of materials is becoming increasingly important as the quantities of materials produced increase and existing energy sources become more expensive both economically and environ-mentally.Despite the negative environmental image of industrial scale PC manufacture,it has a much lower energy requirement per volume than many other commonly used materials,as can be seen in Table3.For example the manufacture of1m3of alu-minium requires approximately32times as much energy as 1m3of PC.There is considerable interest and environmental ben-efit to be gained from developing less energy intensive cements [40].However,the development of low energy more sustainable alternative materials to conventional thermally processed ceram-ics,plastics and metals is also of great interest.MDF cement pro-cessing using lower energy cements such as sulphoaluminate–ferrite–belite(SAFB)and sulphoaluminate–belite(SAB)systems offer distinct advantages and recent research has investigated MDF cements produced from a mix of SAFB and PC[27,41].A fur-ther step would be to produce MDF cements using other pozzola-nic waste material as a substitute for some or all of the hydraulic cement used.9.DiscussionMDF cements represent a potentially attractive range of materi-als whose properties lie between those of conventional cements and ceramics.Concerns about their durability in water have lim-ited research activity in recent years,but there appear to be estab-lished routes by which these may be overcome.Thefirst generation of MDF cements was susceptible to dimensional change and loss of strength due to water up-take by the polymer phase.This has been solved through the use of alternative organic compounds which may be cured in situ,to form a hydrophobic polymer.Such polymerisation may involve a condensation reaction in which water is released from the or-ganic phase.This is available for hydration of the inorganic(ce-ment)phase.Formulations of this type have included very low water/solids ratio cements and in these,it is unlikely that the ce-ment clinker minerals will have appreciably hydrated.This allows subsequent hydration in service,with the possibility of further dimensional instability.However,very low water/solids formula-tions are likely to remain a laboratory curiosity and practical MDF cements are more likely to contain sufficient water to ensure a considerable degree of cement hydration.In this case,the com-posite material ceases to be a micro-scale polymer concrete in which the cement is present as a discontinuous phase,but is present as mineral hydrates bridged by siloxane bonds,as in con-ventional cement.In such materials,the intimate mixing of the organic(polymer)phase and the inorganic(cement hydrate) phases produces a truly composite material,exhibiting much of the strength of hydrated cements with high fracture toughness imparted by the polymer.One area which has not been sufficiently researched is that of hybrid chemistry MDF composites.The change of chemical environment from the inorganic,alkaline and hydrophilic do-main of the cement,to the organic,often hydrophobic regime of the polymer introduces a chemical discontinuity at the micro-scopic scale.Despite advances in organo-silicon chemistry,the authors are unaware of any attempts to form such compounds in MDF cement.The possibility of organic–inorganic phase bonding may yield an entirely new group of materials.It is speculated that rather than a brittle polymer resin forming from the monomer,compounds more akin to tough,low molecular weight polymers,or even rubbers may be polymerised in the system.Similarly,pozzolanic cements have received little attention as components in MDF cements.Thefine grain size of ground gran-ulated blast furnace slag and especially silica fume[31]makes their intimate mixing somewhat easier than coarser Portland ce-ment,whilst pulverisedfly ashes containing cenospheres may produce highly workable pastes.The relatively slow hydration kinetics of many pozzolans may be used to advantage in MDF processing as there is potentially more time available for process-ing and shape forming.Moreover,incorporation of pozzolans will reduce the overall cost of MDF products.10.ConclusionsMDF cements deserve further study,in which the range of compositions is broadened considerably.Specifically,an im-proved understanding of the nature of the bonding between the polymer and cement phase will allow new polymer types to be incorporated with increased confidence.Polymerisation of an intimately mixed monomer allows a hydrophobic polymer to be produced in situ,overcoming the problems of moisture up-take whilst offering a route to producing a new family of materials.Table3Total energy content of materials per unit volume relative to Portland cement[5]Material Total energy(volume basis)Portland cement 1.0Flat glass 3.0PVC 3.8LDPE 4.2HDPE 4.4Polystyrene 6.0Steel19.2Stainless steel28.8Aluminium31.8Zinc34.81766S.Donatello et al./Construction and Building Materials23(2009)1761–1767。