SENFF Et Al (2010) - Effect of Nanosilica and Microsilica on Microestructure and Hardened Properties of Cement Pastes and Mortars

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Effect of nanosilica and microsilica onmicrostructure and hardened properties ofcement pastes and mortars

L. Senff*1, D. Hotza1, W. L. Repette2, V. M. Ferreira3 and J. A. Labrincha4

This paper reports the effects of nanosilica (nS), microsilica (silica fume, SF) and their

simultaneous use (nSzSF) on ‘‘both’’ the microstructure of cement pastes and the mechanical

properties of mortars. After water curing at 21uC for 7, 28 and 90 days, samples with water/binder

w/b ratio of 0?35 were characterised by thermogravimetric analysis, X-ray diffraction, scanning

electron microscopy and compressive strength test. Single or mixed mineral additions did not

generate ‘‘any’’ distinct hydration phases compared to the reference material without additives. A

decrease in the calcium hydroxide contents in later curing ages indicated a pozzolanic effect of

nS and SF. The chemical action promoted by nS together with the physical effect due to the small

particle size distribution given by SF result in higher compressive strength and better hardened

properties, suggesting the synergistic action of nSzSF compared with single additions.

Keywords: Nanosilica, Microsilica, Cement, Pastes, Mortars

IntroductionThe hydration of Portland cement is a chemical processthat is responsible for the formation of the hydratedphases, where tricalcium silicate (3CaO.SiO2 or C3S) anddicalcium silicate (2CaO.SiO2 or C2S) produce calciumsilicate hydrate (CaOx.SiO2.H2Oy or C–S–H), whichcauses hardening in the early and late ages. C–S–H isthe most important phase of a cement paste and is alwaysaccompanied by calcium hydroxide [Ca(OH)2 or CH],also known as portlandite (P). When Ca2z and SO2{

4

ions derived from the dissolution of gypsum(CaSO4.2H2O) react with tricalcium aluminate(3CaO.Al2O3 or C3A), calcium trisulphoaluminatehydrate (CaO.3Al2O3.3CaSO4.32H2O or C6AS3H32 orAFt), known as ettringite, is formed.1,2 Gypsum is addedin a small percentage in order to avoid rapid setting. Onthe other hand, aluminates are responsible for the firstsetting. In contrast to C–S–H, CH is a compound with adefined stoichiometry, appearing as large crystals withdistinctive hexagonal prismatic morphology. Comparedwith C–S–H, the strength contributing potential of CHdue to van der Waals forces is limited as a result of aconsiderably lower surface area. Owing to high solubility

of CH, in the presence of acidic and sulphate waters, thedurability of cement materials is reduced.3

When amorphous silica is incorporated in cementpastes or mortars and concrete, a pozzolanic reactionwith CH is expected. This reaction produces C–S–H thatis responsible for an additional increase in strength andchemical resistance and decrease in water absorption.4

In addition, when a low water content is used, economicaladvantages and higher durability are expected. However,when mortars with nanosilica (nS) and microsilica (alsoknown as silica fume, SF) are produced using low watercontent,5 the resulting material has inadequate work-ability for most applications. In this case, adding extraamount of water seems obvious, but the benefits ofmineral additions on the hardened state properties wouldbe minimised. The use of organic based dispersants, alsoknown as plasticisers and superplasticisers (SP), is alwaysdesirable to improve the rheological properties withoutthe need for addition of extra water.

On the surface of the colloidal silica, silanol groups(5Si–OH) are responsible for the adsorption of ionicand/or organic materials.6 The particles fineness andtheir high specific surface area improve their reactivity,7

which is responsible for stronger chemical interactions atthe interfaces,8 when compared to the other mineraladditions.

In aqueous medium, the amorphous silica also showsa high tendency to dissolution, and its stability,aggregation and settling are modified.9 Siloxane groups(Si–O–Si) are formed, and a three-dimensional rigidcrosslinked polymeric structure or gel is produced.10 Therelative rates of hydrolysis and condensation determinethe final structure of the gel11 that normally presentshigh viscosity. According to Sellergren,11 the pH of the

1Graduate Programme on Materials Science and Engineering, FederalUniversity of Santa Catarina, 88040-900 Florianopolis, SC, Brazil2Department of Civil Engineering, Federal University of Santa Catarina,88040-900 Florianopolis, SC, Brazil3Department of Civil Engineering/Centre for Research in Ceramics andComposite Materials, University of Aveiro, 3810-193 Aveiro, Portugal4Department of Ceramics and Glass Engineering/Centre for Research inCeramics and Composite Materials, University of Aveiro, 3810-193 Aveiro,Portugal

*Corresponding author, email [email protected]

� 2010 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the Institute

Received 24 April 2009; accepted 15 August 2009104 Advances in Applied Ceramics 2010 VOL 109 NO 2 DOI 10.1179/174367509X12502621261659

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solution determines whether the rate limiting step for thereaction of silanes in the sol is hydrolysis or condensa-tion. At low pH, the polymer growth is favoured overcrosslinking, while at high pH, the growing polymerchains tend to crosslink rapidly and form particulatesthat aggregate, producing heterogeneous structures. Inaddition, the porosity of the dried gel is influenced bythe drying conditions.12

The ultrafine particles in solution have a strongertendency to agglomerate13 due to van der Waals andelectrostatic forces, decreasing the free surface area ofthe particles.14,15 In this situation, it is necessary to addan SP (chemical dispersant), which will be adsorbed onthe surface of the mineral additions modifying itsinteraction with the liquid medium and improving theparticle packing. Achieving the maximum packingdensity on the consolidated product has been the goalof the researchers who wish to produce high strengthmaterials. Higher density is obtained when the arrange-ments of solid particles are improved. In addition, thedensification and the mechanical properties in thehardened state are improved when, in the fresh state, ahigher level of points of contact of the ‘‘solids’’ particlesis obtained. According to Yang et al.,16 the arrangementof structures is influenced by the particles size and thenetwork becomes more porous when the mean particlessize decreases from 10 to 1 mm.

This paper reports on the effects of nano- andmicrosized amorphous silica on the chemical andmicrostructural features of Portland cement pastes andmortars. Mixtures with water/binder ratio (w/b50?35)were prepared and characterised by thermogravimetricanalysis (TGA), X-ray diffraction (XRD), scanningelectron microscopy (SEM) and compressive strengthafter 7, 28 and 90 days of curing.

ExperimentalThe chemical compositions of the starting materials usedin this study are listed in Table 1. The Portland cementused was CEM I-52?5R (Portugal), classified by the EN197-1 standard,17 with Blaine specific area of0?43 m2 g21, density of 3?1 g cm23, particle size ranging

from 0?5 to 48 mm (average, 12 mm). The admixture wasa polycarboxylic acid based SP (Glenium 51, BASF,Germany) with density between 1?067 and 1?107 g cm23

and solids content between 28?5 and 31?5 wt-%. The nSslurry (Levasil 300/30%, Germany) contained 30 wt-%solids corresponding to a 1?21 g cm23 density.Nanosilica particles had an average size of 9 nm andspecific surface area of 300 m2 g21. The SF (920D,Elkem, Norway) had a specific surface area of18?41 m2 g21 (Brunauer–Emmett–Teller), density of2?2 g cm23, particles ranging from 0?07 to 1?3 mm(average, 0?15 mm). The aggregate used in the mortarmixes was a commercially available sand, composed byfour particle size fractions (1?2, 0?6, 0?3 and 0?15 mm),each one corresponding to 25 wt-% of the total sand.

The cement paste (Table 2) was characterised throughXRD, TGA and SEM, while the mortars (Table 3) wereinvestigated by compressive strength. All samples weretested after 7, 28 and 90 days of curing.

For XRD and TGA, the samples were fragmentedinto a powder, and the material passing through a 75 mmsieve was used. X-ray diffraction was conducted in aRigaku Geigerflex diffractometer with a Cu Ka radiationsource at a scanning speed of 0?02u s21. For TGA(Shimadzu, TGA-50), the temperature of the test rangedbetween 27 and 1000uC, the heating rate was10uC min21, using N2 as atmosphere at a flowrate of40 cm3 min21.

The microstructural imaging was performed on aHitachi SU-70 model SEM; the samples(30630650 mm) were dried at 100uC for 24 h andthen sputtered with a thin film of gold. The mortarspecimens with 406406160 mm were prepared accord-ing to the EN 196-1 standard.18

For the compressive strength tests, after demoulding,the mortars were immersed in water and tested (7, 28and 90 days) according to EN 1015-11 standard.19

The lower and upper limits of nS, SF and SP amountsused in this study were established based on fresh state

Table 1 Compositions (in wt-%) of Portland cement (CEMI-52?5R), nanosilica (nS), and microsilica (SF)

Component,wt-%

Portlandcement

Nanosilica(NS)

Microsilica(SF)

SiO2 20.6 99.4 91.3CaO 62.79 … …Al2O3 4.84 0.075 …Fe2O3 3.15 … …SO3 3.55 ,0.1 …MgO 1.93 … …K2O 0.7 … …Na2O 0.9 0.45 1.4Loss on ignition 3.2 … 2.5C3S 55 … …C2S 19 … …C3A 10 … …C4AF 7 … …Ca, ppm … 10 …Fe, ppm … 25 …Zn, Pb and Cu, ppm … ,0.1 …

Standard cement chemistry notations: C, CaO; S, SiO2; A, Al2O3;F, Fe2O3

Table 2 Formulations of cement pastes

Mixture

Components

Water,mL

Cement,g

SF,g

nS,g

SP,g

SF (0) 35 100 … … 1.2SF (20) 35 80 20 … 1.2nS (0) 35 100 … … 3nS (3.5) 35 96.5 … 3.5 3nSzSF (0z12.2) 35 87.8 12.2 … 1.2nSzSF (2z10.2) 35 87.8 10.2 2 1.2

Table 3 Formulations of mortars

Mixture

Components

Water,mL

Cement,g

Sand,g

SF,g

nS,g

SP,g

SF (0) 203 580 1160 … … 7SF (10) 203 522 1160 58 … 7SF (20) 203 464 1160 116 … 7nS (3.5) 203 559.7 1160 … 20.3 17.4nSzSF (0z12.2) 203 509.2 1160 70.8 … 7nSzSF (2z10.2) 203 509.2 1160 59.2 11.6 7

Senff et al. Effect of nS and SF on properties of cement pastes and mortars

Advances in Applied Ceramics 2010 VOL 109 NO 2 105

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properties, obtained by rheology and flow analysisreported elsewhere.5

Results and discussion

Compressive strengthThe combined effect of mineral addition and lower w/bratio results in higher compressive strength of themortars. Figure 1 shows that all samples with mineraladditions reached higher strength (except for the mixwith 20%SF at seven days) when compared with sampleswithout mineral additions. Also, the mortars withnSzSF showed better results than those of theindividual nS or SF additions. The maximum compres-sive strength of nSzSF, SF and nS were 103, 97 and89 MPa respectively after 90 days curing. When singleadditions are compared, nS showed better results in thefirst ages (seven days) while SF at 28 and 90 days. In theearly ages, higher surface areas available for reactionseem to contribute to the evolution of compressivestrength. However, after 28 and 90 days, this effectseems to be insufficient to reach the benefits achieved bySF addition. Probably, the continuous size distributionof SF may have contributed to improve the densificationof material, giving origin to a more compact matrix. FornS, the strong tendency to produce agglomerates maydecrease the packing of the particles in the matrix. Inaddition, the faster formation of the gel (due to thehigher percentage of nS) influences significantly therheological properties of mortars5 and hinders theirworkability. The compressive strength of mortars with20%SF at seven days decreased 5% in relation to 0%SF.The higher cement substitution is responsible for thisresult. The precise amount that ensures maximummechanical strength was not defined, since the amountswere defined based on workability tests.5

Thermogravimetric analysisThermal analysis is an important experimental procedureused as a measure of the cement hydration andconsumption of CH due to pozzolanic reactions.20 Inaddition, various products can be detected by differentialthermal analysis and TGA (Table 4).2 The amount of CHin a cement paste can be used as an indication to theevolution of the hydration. According to Taylor,2 thisestimation is erroneous if it is calculated, assuming theweight of starting or dry samples as reference for theweight loss determination. In particular, the moisturecontent of samples may vary significantly. Therefore, the

weight of the calcined samples was used as a reference forthis determination.

For 7 and 28 days (up to 400uC), samples with 3?5%nSand 2z10?2%nSzSF showed a higher weight loss when

1 Compressive strength of mortars (SF, nS and nSzSF)

after curing at 7, 28 and 90 days

Table 4 Expected temperature regions for endothermicdecomposition of main components in cementpaste2

Phases Temperature, uC

Evaporation of water y100Partial dehydration of C–S–H 115–125Partial dehydration of AFt 120–130Partial dehydration of AFm 180–200Dehydration of CH 450–550Decomposition of CC 800

2 Weight loss (%) attributed to calcium hydroxide (CH)

and carbonate (CC) decomposition in pastes with SF,

nS and nSzSF, for a 7, b 28 and c 90 days curing


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compared to 0%nS and 0z12?2%nSzSF, while for20%SF, it was lower than that of 0%SF. In the first case,the results suggest that these compositions increased thephase formation in relation to samples without mineraladditions, while for 20%SF, the higher substitution ofcement could influence the amount of the phaseformation. For 90 days (up to 400uC), materials with2z10?2%nSzSF and 20%SF showed higher weight lossthan materials without mineral additions, while forsamples between 0% and 3?5%nS, no difference wasdetected. In the last case, poorer dispersion of nS in themixture might have influenced the results. The decom-position of C–S–H and aluminates was not quantifiedindividually because they have similar interval of weightloss. According to Taylor2 and Ramachandran andBeaudoin,21 the endothermal peaks of ettringite may beidentified in the temperature range of 115–130uC, whileC–S–H gel formation occurs under 150uC.

The weight loss estimations of CH and calciumcarbonate are summarised in Fig. 2. In general, theresults showed that the content of CH increased in thecement pastes without mineral additions for longer curingtimes as a consequence of the expected cement hydrationevolution.1 However, when SF and nS were added, theamount of CH decreased in relation to SF5nS50%. By

comparing each single action, it may be concluded that nSpresented a higher effectiveness in the CH consumptionwhen compared to SF. The results showed that carbona-tion occurs in all pastes tested. For 7 and 90 days, sampleswith 0 and 3?5%nS presented an increasing effect, while20%SF decreased and the sample containing 0z12?2%and 2z10?2%nSzSF maintained the same level. Inaddition, for all ages, the sample with 2z10?2%nSzSFshowed lower carbonation in relation to 3?5%nS and0z12?2%SF samples. Probably, the synergistic effectbetween nS and SF improved the resistance to carbona-tion. When the analysis is performed at the same age,samples with 3?5%nS showed more carbonation inrelation to 0 and 20%SF (except for seven days) and0z12?2%nSzSF (except for seven days) and 2z10?2%nSzSF, although for the formulation with3?5%nS, the viscosity increased markedly during mixing,leading to a more inhomogeneous dispersion. When allcarbonation results were analysed, it was not possible toclearly identify a dominant factor (time or mineraladdition) because no trend was straightforward.

Phase formationFigure 3 shows the main crystalline phases identified atall cement pastes studied: ettringite (E),22 calcite (C),23

3 X-ray diffraction of cement pastes with nS, SF and nSzSF after a 7, b 28 and c 90 days curing



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calcium silicate (S)24–26 and portlandite (P).27 C–S–Hwas not identified, as expected from its amorphouscharacter. In addition, alumina peaks (A)28 in the rangeof 25–30u, 35–45u, 55–60u and 65–70u shown in nS5

SF50%, nS53?5% and SF520% with 28 days andnS50% with 90 days are related to the calcined alumina

added in mixtures. Initially, the authors wished to carryout a quantitative analysis of Ca(OH)2, using calcinedalumina as an internal standard. The calcined alumina(10 wt-%) was added in samples after fragmentation ofcement paste (before XRD test). However, the quanti-tative analysis was not performed because, for that, theresults were not accurate enough. In general, the resultsindicated that the substitution of cement by mineraladdition did not cause significant differences in the X-ray spectra, except for the peak intensity of portlandite(P).

Figure 4 shows that nS or SF addition decreased theportlandite peak intensity in relation to the sampleswithout mineral additions for all ages. For the studiedinterval (5–70u), portlandite showed a secondary diffrac-tion peak at 18?008u (Fig. 4a) and a main diffractionpeak at 34?102u (Fig. 4b). For the samples cured forseven days, the addition of nS shows a reduction ofportlandite peak than that achieved using SF. However,a synergistic effect seemed to occur, and maximumattenuation was observed by adding of2z10?2%nSzSF sample. The role of nS was easilyconfirmed by the minor differences between 0% (nS andSF) and 0z10?2%nSzSF samples. After 28 dayscuring, differences in the pozzolanic reaction caused bynS and SF additions were minor, suggesting that kineticaspects were controlling the portlandite consumptionreaction. For later curing ages, the larger SF particleshad enough time to react with the portlandite grains.The same tendency is observed for 90 days curing.

Microstructural aspectsWhen the low w/b content ratio is used to prepare amortar or a concrete, a denser material is obtained andhigher compressive strength is expected. However, the

4 Evolution of portlandite (P) peaks for cement pastes

with nS, SF and nSzSF after 7, 28 and 90 days located

at a 18?008u and b 34?102u

5 Image (SEM) showing a poor dispersion of solid grains in mortar 0%SF, b size distribution of SF (nSzSF52z10?2%),

c discrete size distribution of nS53?5% and d different sizes between SF and nS (nSzSF52z10?2%)



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internal friction between surface grains and agglomera-tion of particles are intense as well. As a consequence,the homogenous dispersion of solids grains in thehardened state of mortar (0%SF) was poor (Fig. 5a).When round aggregates are used in mixtures, theyimproved the workability, while the irregular shapedaggregates develop stronger bonds with the cementpaste.29 When the grains having concave shape areadded in mixture, the fulfillment of intergranular spacebetween the grains becomes more difficult, and theparticle packing diminishes.30 In this case, the additionof spherical particles, such as nS or SF, can act as alubricant, decreasing the internal friction betweengrains. In addition, the particle packing is higher forisotropic structures formed from spherical grains due toeasier orientation or settlement.30 Figure 5b and c showsthe particle size distribution of SF in mixtures with2z10?2%nSzSF and the nS discrete particle sizedistribution (3?5%nS) respectively. Figure 5d shows awhite circle enlarged from Fig. 5b, where the remarkablesize difference between SF and nS can be observed. ForSF, the continuous particle size distribution mayimprove the packing density when compared to discretesize distributed nS particles. According to Furnas,31 themaximum packing of particles occurs when the finerparticles fill the empty space within the larger particles.This behaviour agrees with the results of compressivestrength tests. The nS particles enhance the cementreactions in the fresh state and increase the mechanicalstrength in the early ages (Fig. 1). However, theagglomeration effect for these particles was veryintense.32 Therefore, the beneficial effects of nS werelower for later ages. According to Yang et al.,16 theagglomeration and reduction of particle arrangement isa consequence of van der Waals and electrostatics forcesat small distances. Between single additions (nS or SF),the results are poorer than those reached by theircombination (nSzSF blend).

ConclusionsCement pastes and mortars containing nS, SF andnSzSF additions were prepared with low water/binderratio (0?35) and were characterised by TGA, XRD,SEM and compressive strength tests after curing for 7,28 and 90 days.

Thermogravimetric analysis showed that CH contentsincrease in the cement pastes without mineral additionsfor longer curing time, while the amount of CHdecreased for the cement paste with nS and SF. Inaddition, 2z10?2%nSzSF showed less carbonation inrelation to 3?5%nS and 20%SF samples, indicating thatthe synergistic effect between nS and SF enhanced thepaste carbonation resistance.

X-ray diffraction revealed that the addition of nS andSF did not induce the formation of distinct hydrationphases when compared to the paste without mineraladdition. The only detectable difference corresponded tothe portlandite peak intensity, which tended to diminishfor all curing ages with addition of nS, SF and nSzSF.The consumption of portlandite corresponds to theknown pozzolanic reaction, which was more effectivewith nS or the combination of 2z10?2%nSzSFsamples.

Scanning electron microscopy showed that using alow water/solids content ratio increased the density of

solids particles in the mixtures, and in some cases, thedispersion was highly hindered. This effect was moresevere with nS. In this case, the arrangement andpacking of particles were poorer. As a consequence, thecompressive strength tended to decrease, but it was stillsuperior to the values obtained for samples withoutmineral additions. Higher compressive strength wasobtained with nSzSF mixtures, suggesting a combinedaction in terms of chemical and physical effects.

Acknowledgements

The authors acknowledge the financial support of theBrazilian agency Coordination for the Improvement ofHigher Education Personnel (CAPES). The authors alsothank Weber-Portugal, BASF, Elkem, H.C. StarckEmpowering High Tech Materials and Secil for provid-ing raw materials.

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Documents

SENFF Et Al (2010) - Effect of Nanosilica and Microsilica on Microestructure and Hardened Properties of Cement Pastes and Mortars