ta

Coll

n behavior of nano-metakaolin cement mortars.ressive and exural strengths at 250 C.ease apetakaometaka

then dried at 105 5 C for 24 h and then exposed to 250, 450, 600 and800 C for 2 h. The compressive and exural strengths were measured for blended cement mortar and

density and its appearance [25]. One of the most important phys-ical deterioration processes that inuence the durability of con-crete structures is high temperature. Nevertheless, it is possibleto minimize the harmful effects of high temperatures on concreteby taking preventive measures, such as choosing the appropriatematerials. Material properties, such as properties of aggregate,cement paste and aggregatecement paste bond, and thermal

is also found to be useful [1820].The benecial effect of mineral admixtures, such as y ash, sil-

ica fume, silica our, metakaolin and ground granulated blast-fur-nace slag, with respect to high-temperature resistance arises fromthe stabilization of Ca(OH)2 released during cement hydration bymeans of pozzolanic reaction. The detrimental effects of Ca(OH)2are that, the micro-cracks appear rst in the areas of Ca(OH)2 con-centration at about 300 C [5], due to the decomposition of calciumhydroxide into lime and water vapor above 350 C. Decompositionof calcium hydroxide is not critical in terms of strength loss duringheating. However, it may lead to serious damage due to lime

Corresponding author.

Construction and Building Materials 35 (2012) 900905

Contents lists available at

B

evE-mail address: msmorsy@yahoo.com (M.S. Morsy).Elevated temperatureNano-metakaolinMortarMicrostructureMechanical propertiesThermal analysis

compared with the strength of pure OPC mortar. It was found that after an initial increase in compressivestrength at 250 C for the mortar specimens, the strength decreased considerably at higher temperatures.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Concrete is well known for its capacity to endure high temper-atures and res owing to its low thermal conductivity and highspecic heat [1]. However, it does not mean that re, or high tem-peratures, do not affect concrete at all. High temperature may re-sult in color changes along with signicantly affecting theconcretes compressive strength, modulus of elasticity, concrete

compatibility between aggregate and cement paste, greatly affectthe high-temperature behavior of concrete [610].

Therefore, many researchers have recently become increasinglyinterested in the possibility of developing concrete that has betterre resistance which can be improved in various ways as dis-cussed. The replacement of cement with pozzolanic materials suchas slag, silica fume or y ash, for example, is a very efcient mea-sure [1117]. The addition of polypropylene bers to concrete mixKeywords:blended cement: sand ratiunder water for 28 days;" Studied effects of high temperature o" There was an initial increase in comp" Compressive & exural strengths decr" Replacement of cement by 5% nano-m" Replacement of cement by 15% nano-

a r t i c l e i n f o

Article history:Received 3 March 2012Received in revised form 23 April 2012Accepted 29 April 2012Available online 15 June 20120950-0618/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.04.099preciably at temperatures above 250 C.lin at 25 C leads to optimal mortar.olin at above 250 C gives good results.

a b s t r a c t

The effects of high temperatures up to 800 C on the mechanical properties and microstructure of nano-metakaolin cement mortars were investigated in this study. The blended cement used in this investiga-tion is ordinary Portland cement (OPC) containing nano-metakaolin (NMK). The nano-metakaolin wasprepared by thermal activation of nano kaolin clay at 750 C for 2 h. The mortar was prepared using

o of 1:3 and water/binder ratio of 0.6. The cement mortar pastes were curedh i g h l i g h t sBehavior of blended cement mortars conelevated temperatures

M.S. Morsy , Y.A. Al-Salloum, H. Abbas, S.H. AlsayedSpecialty Units for Safety & Preservation of Structures, Department of Civil Engineering,

Construction and

journal homepage: www.elsll rights reserved.ining nano-metakaolin at

ege of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia

SciVerse ScienceDirect

uilding Materials

ier .com/locate /conbui ldmat

tered Cu-Ka radiation at 40 kV and 20 mA were used throughout in a Philips PW

Fig. 1. TEM micrograph of NMK.

Fig. 2. XRD patterns of kaolin and metakaolin.

Table 1Chemical composition of materials in percent by weight.

Oxide composition OPC (%) Nano Kaolin (%)

CaO 63.85 0.01SiO2 19.83 48.00Al2O2 5.29 36.50Fe2O3 3.53 0.20MgO 0.52 0.02SO3 2.43 Na2O 0.21 0.03K2O 0.07 0.07TiO2 1.30Total 95.73 86.13Ignition loss 4. 27 13.87

Builexpansion during the cooling period [21]. Such damage can also beobserved during re extinguishing process, where CaO reacts withwater and turns to Ca(OH)2 with a signicant expansion.

When nano particles are used as supplementary cementitiousmaterial (SCM) in concrete, various improvements can be attained,thereby leading to improved permeability and strength. The nanoparticles act as nuclei of hydration, possess pozzolanic behavior[22], and can ll the voids in the cement matrix [2325]. Pozzolansreact chemically with calcium hydroxide liberated during cementhydration to form cementitious compounds [26]. The large surfacearea of nano particles and their abundance due to their small sizecan facilitate the chemical reactions necessary to produce a densecement matrix with more calcium silicate hydrate (CSH) and lesscalcium hydroxide contents. This, in turn, will enhance the overallconcrete performance. Nano particles are, in general, smaller thanthe commonly used SCMs, making them more reactive and effec-tive. Different forms of nano-silica (NS) and nano clays (NC) in ce-ment paste have been shown to increase compressive strength,reduce permeability, and develop a denser microstructure [27].Nano particles can also strengthen the interfacial transition zonebetween the cement paste and the aggregate, which would leadto improved strength and permeability. For nano particles to be asubstitute for other SCMs of larger particle size, equal or better per-formance at lower or equal cost is needed.

The object of this work is to provide experimental data on theresidual mechanical and physical properties of blended cementmortar containing NMK as a pozzolanic material when exposedto heat. These properties are very important for a safe design ofconcrete and in the repair of concrete structures when exposedto elevated temperature.

2. Experimental

2.1. Materials

The ordinary Portland cement (OPC), ASTM C-150 Type I [28] used in this inves-tigation was supplied by Yamama Cement Company, Saudi Arabia.

The Blaine surface area of nano kaolin was 48 m2/g and average dimensions100 50 10 nm and was supplied by the Middle East Mining Investments Com-pany (MEMCO), Cairo, Egypt. The morphology of nano kaolin is shown in Fig. 1.The raw nano kaolin consists of kaolinite, illite and quartz as major mineral phases,whereas NMK contains only illite and quartz. The heat treatment of raw kaolinite at750 C resulted in the disappearance of its crystalline structure and the develop-ment of an amorphous structure as shown in Fig. 2. The oxide composition of nanokaolin and OPC are summarized in Table 1. Commercial local red sand was used as ane aggregate in the mortar.

2.2. Mortar Preparation and Identication

In this investigation OPC was partially substituted by NMK by weight as illus-trated in Table 2. The blended cement mixes were prepared by mixing OPC andNMK in the dry state for successive periods of 5 min until homogeneity wasachieved. The mortar was prepared using blended cement: sand ratio of 1:3 andwater/binder ratio of 0.6.

The mortar pastes were molded into 5 cm cubes for compressive and4 4 16 cm prisms for exural strengths tests. The molds were vibrated forone minute to remove any air bubbles. The samples were kept in molds at 100% rel-ative humidity for 24 h, and then cured in water for 28 days. The hardened cementmortar specimens were removed from water before testing. The compressive andexural tests were performed on wetted control and blended specimens. The hard-ened cement paste was dried at a temperature of 105 5 C for 24 h in an oven toremove the free water. Then, they were kept for 2 h at temperatures 250, 450, 600,and 800 C. Each temperature was maintained for 2 h to achieve the thermal steadystate. The specimens were allowed to cool gradually inside the furnace to roomtemperature. The compressive strength was performed on wetted and heated spec-imens. The crushed samples (wetted and heated) resulting from compressivestrength tests were grinded for thermal analyses, SEM and X-ray diffraction studies.The evaporable water of the hydrated crushed samples was removed using themethod described elsewhere [29].

Differential Thermal Analyses run were conducted using a Shimadzu DTA-50

M.S. Morsy et al. / Construction andthermal analyzer at a heating rate of 20 C/min. The sample chamber was purgedwith nitrogen at a ow rate of 30 ml/min. The crystalline phases present in thehydration products were identied using the X-ray diffraction technique. Nickle-l-ding Materials 35 (2012) 900905 9011390 diffractometer. Scanning speed of 2/min. was used. The scanning electronmicroscope JWEL JSM 6360A was used for identication of the changes occurredin the microstructure of the formed and/or decomposed phases.

3. Results and discussion

The compressive and exural strengths of blended NMK cementmortar hydrated for 28 days under water are shown in Fig. 3. It isclear that, the compressive and exural strengths increase withincreasing replacements up-to 5% and then decrease with its in-crease up-to 15% by weight. Obviously, the replacement of OPCby NMK enhances the compressive and exural strengths up to15% by weight. At 5% NMK replacement the increase in the com-pressive and exural strengths was 1.23 and 1.08 folds than the

investigated samples (M3 and M4). The blended cement mortarcontaining 5% NMK (M2) achieved the maximum gain in compres-sive and exural strengths than its control sample in comparison toother blended mixes at ambient environment (Fig. 3). When a den-ser structure is exposed to elevated temperatures the dehydrationprocess of the formed hydrate is associated with driving out of freewater and the remaining fraction of water leads to formation anddevelopment of micro-cracks. However, the compressive strengthsof blended cement mortars, M3 and M4 exhibited slow decreasethan those of M1 and M2 specimens as the exposure temperatureincreased up to 800 C. Evidently, the increase in compressivestrength up to 250 C may be due to the additional hydration ofunhydrated cement grains as a result of steam effect under thecondition of the so-called internal autoclaving effect. The increasein compressive strength of blended cement mortars may be due tothe pozzolanic reaction of amorphous aluminosilicate

Table 2The dry mix composition of blended cement in percent by weight.

Mix OPC (%) NMK (%)

M1 100 0M2 95 5M3 90 10M4 85 15

902 M.S. Morsy et al. / Construction and Building Materials 35 (2012) 900905control sample respectively. Basically NMK enhances the compres-sive and exural strengths of hardened cement mortar by twomechanisms. The rst mechanism is the physical ller of NMK par-ticles in interstitial spaces inside the skeleton of hardened struc-ture of cement mortar, thereby increasing its density as well asits strength. The second mechanism is the pozzolanic reaction, be-tween NMK and the free CH liberated during OPC hydration, whichproduces additional CSH. The reduction in the compressive andexural strengths at higher NMK replacements may be due tothe decrease of C3S and b-C2S phases in blended cement. Also, athigher ratio replacements the NMK agglomerates around the OPCgrains and hinder the hydration process. Therefore, the amountof hydration product is decreased; leading to the development offewer points of contact which act as binding centers between ce-ment grains.

Fig. 4 illustrates the development of compressive strength forcontrol and blended cement mortars exposed to 250, 450, 600and 800 C for 2 h. It is evident that, the compressive strength in-creases with temperature up to 250 C then decreases as the tem-perature increases up to 800 C. The gain in compressive strengthat 250 C is 28.4%, 5.2%, 33.7%, and 36.6% while the correspondingloss in compressive strength at 800 C is 57.0%, 61.5%, 58.1%, and46.5% for mixes M1, M2, M3 andM4, respectively. Moreover the in-crease of compressive strength of blended cement mortar M2 islimited in comparison to its control sample and the otherFig. 3. Compressive and exural strength of NMK-cement mortar versus NMKproportion.(Al2O32SiO2) present in NMK with CH librated during OPC hydra-tion to form an additional amount of CSH that has low Ca/Si ratiowith high strength [30] and calcium aluminates hydrate (CAH)phases that deposit in the pore system. Furthermore, the additionalhydration products bridge the pore system leading to a decrease ofthe thermal stresses generated around the pores. Cement matrixwith higher contents of gel-like hydration products and lowerCa(OH)2 crystal contents has improved re resistance. This phe-nomenon, leads to low increase in strength and decreases porosity[31]. Also, the decrease in compressive strength with increase intemperature above 250 C may be due to the dehydration of cal-cium hydroxide at about 500 C producing CaO and H2O. Strengthlosses over 600 C, are mainly caused by calcium carbonate disso-ciation and subsequent CO2 escape from CaCO3. Strength losses ofM4 blended cement mortar are less signicant than that of its con-trol sample, in comparison to M1, M2 and M3 cement mortar.

Fig. 5 shows variation in exural strength for control andblended cement mortars exposed to 250, 450, 600 and 800 C for2 h. It is evident that, the exural strengths of blended cementmortars increase with temperature up to 250 C, then decreaseup to 800 C. The gain in exural strength of blended mortar at250 C is 3.4%, 13.9%, and 27% for mixes M2, M3 and M4, respec-tively. However, the increase in exural strength up to 250 Cmay be due to the increased content of hydration products, whichplay the dominant role, especially for concrete at the age of28 days. This process exhibits similar behavior of strength at cer-tain curing treatments (so-called steam curing). Also, due to phys-ical ller of NMK which act as ber in cement structure.Furthermore, the decrease in exural strength with increase intemperature from 250 C to 800 C is due to the formation ofmicro-cracks. Also, the reduction in exural strength can beFig. 4. Compressive strength of NMK-cement mortar versus temperature.

BuilM.S. Morsy et al. / Construction andattributed to the driving out of free water and a fraction of hydra-tion water of cement mortar due to high temperatures. Dehydra-tion of concrete causes a decrease in its strength, elastic

Fig. 5. Flexural strength of NMK-cement mortar versus temperature.

Fig. 6. XRD patterns of hydrated cement mortars and NMK-cement mortars: (a)control cement mortar (M1) and (b) blended cement mortar (M4).ding Materials 35 (2012) 900905 903modulus, coefcient of thermal expansion and thermal conductiv-ity [32]. Moreover, the replacement of OPC by 10 and 15% NMK incement mortar resulted in marginal decrease and/or nearly stableexural strength up to 450 C followed by a sharp decrease. In ce-ment mortar containing 10% and 15% NMK, the exposure up to450 C led to further hydrothermal reaction of unhydrated cementgrains as well as pozzolanic reaction of NMK with calcium hydrox-ide being released during the hydration process. Basically, the de-crease in exural strength due to increase of temperature from 450to 550 C is due to dehyroxylation of Ca(OH)2. The increased micro-cracking is the result of high thermal stresses, which are generateddue to the induced temperature gradients up to 800 C.

The XRD diffractograms obtained for the hydrated cement mor-tar samples before and after exposure to elevated temperatures areshown in Fig. 6. The XRD patterns of control cement mortar sampleare illustrated in Fig. 6a. The main phases identied in hydrated ce-ment mortar are CSH gel, unhydrated C3S and b-C2S, calciumhydroxide and calcium carbonate. It was found that the amountsof CSH and CH increased with heating at 250 C. This is becausefurther hydration of unhydrated cements grains. Evidently, thecrystalline phase of CH (portlandite) decreased as the temperatureincreased up to 800 C. This is due to thermal decomposition of cal-cium hydroxide phase at 570 C. The effect of replacement of OPCby 15% NMK is shown in Fig. 6b. The calcium hydroxide phase de-creases as the exposure temperature increases, while the weakpeaks of CAH and calcium aluminates sulfate hydrate (CASH) inblended cement mortars increase. Moreover, it can be concluded

Fig. 7. DTA thermograms of hydrated cement mortars and NMK-cement mortars:(a) control cement mortar M1 and (b) blended cement mortar (M4).

Buil904 M.S. Morsy et al. / Construction andfrom these diagrams that, after duration of 2 h, CSH, CH and cal-cium carbonate all remained persistent at 250 C; both CH and cal-cium carbonate decomposed but CH decomposed at a slower ratethan calcium carbonate, whereas CSH was persistent at 800 C.

The variations of the DTA thermograms of control cement mor-tar, (M1), and blended cement mortar, (M4) at ambient tempera-ture and exposed to 250 and 800 C are shown in Fig. 7.Evidently, there are almost four endothermic peaks. The rst peaklocated at about 100110 C, is mainly due to the decomposition ofthe nearly amorphous calcium silicate hydrates as well as the cal-cium sulpho-aluminate hydrates. The second endothermic peakobserved at about 160 C represents the decomposition of the crys-talline part of CSH. The third endothermic peak at about 470 C isdue to the dissociation of of Ca(OH)2. The fourth endothermic peakwas observed at around 580 C which represents the transforma-tion of quartz. As the exposure temperature increases the peak areaof calcium hydroxide increases up to 250 C and shifts towardslower temperature at 800 C as shown in Fig. 7a. This is due tothe formation of ill-crystals of Ca(OH)2. Also, it is clear that thethermograms of the blended cement mortars made of mix M4 thatthermally treated at 250 C shows a sharp CH endotherm and shiftsto a higher temperature. This is due to the formation of well-crys-tals of Ca(OH)2. Moreover the thermal treatment at 800 C exhibitsa very weak CH endothermic peak as shown in Fig. 7b. It is clear

Fig. 8. SEMmicrographs of control and blended cement mortars subjected to elevated teNMK hydrated at 25 C, (c) control mortar thermally treated at 250 C, (d) blended motreated at 800 C, and (f) blended mortar containing 15% NMK thermally treated at 800ding Materials 35 (2012) 900905that the calcium hydroxide peak area decreases as treatment tem-perature increases. Evidently, this is due to further reaction be-tween NMK and CaO at high temperature up to 800 C.

The scanning electron micrograph of control mortar (M1) andblended cement mortar containing 15% NMK (M4) and thermallytreated at 25, 250 and 800 C are shown in Fig. 8. Evidently, themicrostructure of the control mortar at 25 C displays the existenceof microcrystalline and nearly amorphous, CSH; in addition to largecrystals of calcium hydroxide as shown in Fig. 8a. Furthermore, themicrograph of blended cement mortar containing 15% NMK dis-played the presence of a nearly amorphous CSH and dense micro-structure as shown in Fig. 8b. It was clear that, the microstructureof OPC-NMKmortar, thermally treated at 250 C, was perfectly sta-ble for thermal treatment and illustrates a dense structure of hy-drated products as shown in Fig. 8d. This can be clearlyunderstood from the microstructure of the hardened blended ce-ment mortar (M4) after thermal treatment at 250 C which dis-played the existence of CSH and calcium hydroxide (CH).Therefore, the replacement of OPC by 15% of NMK resulted in animprovement of the thermal stability of the hardened blended ce-ment mortar made of mix M4 as indicated from the SEM micro-graph shown in Fig. 8d. Thermal treatment of mix M1 at 800 Cdisplays decomposition of the hydration products with the forma-tion of wide micro-cracks as shown in Fig. 8e. Furthermore, the

mperatures, (a) control mortar hydrated at 25 C, (b) blended mortar containing 15%rtar containing 15% NMK thermally treated at 250 C, (e) control mortar thermallyC.

cement paste was separated from the aggregate thereby creatinggaps. Moreover, the thermal treatment of blended mortar M4exhibited all hydrated phases including CSH and CH appearing asamorphous structures losing their characteristic crystalline struc-

[3] Morsy MS, Rashad AM, Shebl SS. Effect of elevated temperature oncompressive strength of blended cement mortar. Build Res J 2008;56:17385.

[4] Morsy MS, Othman TA, Abo-El-Enein SA. Effect of re on physiothermalproperties of admixed cement. Disasters Management and Safety of Buildings

M.S. Morsy et al. / Construction and Building Materials 35 (2012) 900905 905ture due to less formation of narrow micro-cracks as shown inFig. 8f. Poor microstructure is associated with generation of unde-sirable conguration of CSH crystals, and increased cracking athigh temperature. Generally, the CSH crystals grow long, thin/nar-row and occupy less space in the matrix at high temperatures,thereby resulting in decreased densication of the microstructure[33]. The increased micro- cracking is the result of high thermalstresses that are generated due to the induced temperaturegradients.

4. Conclusions

In this study, the effect of experimental parameters, namelyheating degree and NMK, on the compressive and exuralstrengths is investigated experimentally; the research results in-clude the following:

1. The replacement of OPC by 5% NMK in cement mortar hydratedat ambient temperature increases the compressive and exuralstrengths of cement mortar.

2. It is found experimentally that the higher the compressive andexural strengths results were obtained from the specimenscontaining 15% NMK, as compared with other replacement per-centages considered in this study for all thermal treatment,with increase of temperature from 250 to 800 C.

3. The test results showed that the compressive and exuralstrengths increases up to 250 C and then decreases as theexposure temperature increases up to 800 C. The reduction incompressive and exural strengths may be due to the occur-rence of micro- and macro-cracks in mortars because of hightemperatures.

4. The blended cement mortar M4 can be applied as a re resis-tance binding material.

5. The SEM micrograph of control mortar thermally treated at800 C possesses a formation and growth of micro-cracks thanblended cement mortar containing 15% NMK.

6. Thermal treatment of blended cement mortar containing 15%NMK at 250 C indicate a formation of well crystalline CH.

Acknowledgments

The authors extend their appreciation to the Deanship of Scien-tic Research at King Saud University for funding the work throughthe research group project No. RGP-VPP-064. Thanks are also ex-tended to the MMB Chair for Research and Studies in Strengthen-ing and Rehabilitation of Structures, at the Department of CivilEngineering, King Saud University for providing technical support.

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Behavior of blended cement mortars containing nano-metakaolin at elevated temperatures1 Introduction2 Experimental2.1 Materials2.2 Mortar Preparation and Identification

3 Results and discussion4 ConclusionsAcknowledgmentsReferences