Construction

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Construction and Building Materials 21 (2007) 539–545

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MATERIALS

Influence of nano-SiO2 addition on properties of hardened cementpaste as compared with silica fume

Ye Qing a,b,*, Zhang Zenan c, Kong Deyu a, Chen Rongshen a

a College of Architecture and Civil Engineering, Zhejiang University of Technology, 310014 Hangzhou, PR Chinab Department of Civil Engineering, Quzhou College, 324006 Quzhou, PR China

c Department of Physics, Zhejiang University of Technology, 310014 Hangzhou, PR China

Received 4 March 2005; received in revised form 5 September 2005; accepted 7 September 2005Available online 24 October 2005

Abstract

The influence of nano-SiO2 (NS) addition on properties of hardened cement paste (hcp) as compared with silica fume (SF) has beenstudied through measurement of compressive and bond strengths of hcp, and by XRD and SEM analysis. Results indicated that theinfluence of NS and SF on consistency and setting time of fresh cement paste showed different. NS made cement paste thicker andNS accelerated the cement hydration process. Compressive strengths of hcp and bond strengths of paste–aggregate interface incorporat-ing NS were obviously higher than those incorporating SF, especially at early ages. And with increasing the NS content, the rate of bondstrength increase was more than that of their compressive strength increase. With 3% NS added, NS digested calcium hydroxide (CH)crystals, decreased the orientation of CH crystals, reduced the crystal size of CH gathered at the interface and improved the interfacemore effectively than SF. The results suggest that with a small amount of added NS, the CH crystals at the interface between hcpand aggregate at early ages may be effectively absorbed in high performance concrete (HPC).� 2005 Elsevier Ltd. All rights reserved.

Keywords: Hardened cement paste; Interface; Nano-SiO2; Mechanical property; Silica fume

1. Introduction

In this new century, the technology of nano-structuredmaterial is developing at an astonishing speed and will beapplied extensively with many materials. Although cementis a common building material, its main hydrate C–S–H gelis a natural nano-structured material [1–5].

The durability and mechanical properties of HPC aremainly dependent on the gradually refining structure ofhcp and the gradually improving paste–aggregate interfaceincorporating additions and admixtures. Many researchershave applied slag, fly-ash and silica fume (SF) to improvingcement-based materials, and have achieved great successes,such as HPC and reactive powder concrete and so on. SF

0950-0618/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2005.09.001

* Corresponding author. Tel.: +86 571 85816061; fax: +86 57188320124.

E-mail address: zjutyeqing@163.com (Q. Ye).

belongs to the category of highly pozzolanic materials be-cause it consists essentially of silica in non-crystalline formwith a high specific surface, and thus exhibits great pozzo-lanic activity. But the activity of SF at early ages is lowaccording to the literature [6–9]. Mitchell et al. [6] reportedthat the XRD pattern of SF, put into saturated calciumhydroxide (CH) solution, changed little up to 7 days andextensive C–S–H formation had occurred after 120 days.Li et al. [9] showed that only 75% of SF was consumedin a cement paste after 90 days of hydration.

The aggregate–paste interface is regarded as the mostsensitive area within the structure of concrete, where anumber of defects exist and the concrete failure processcommences easily. Through SEM and XRD examinationsof the interfacial transition zone in pastes cast against glassslides, Barnes et al. [10] and Grandet and Ollivier [11] re-ported the occurrence of duplex films, comprising a layerof CH crystals in contact with the glass and oriented with

540 Q. Ye et al. / Construction and Building Materials 21 (2007) 539–545

their c-axes normal to its surface, and a layer of C–S–Hfurther out into the paste. With additions of pozzolanicmaterials such as slag, fly-ash and silica fume added, theinterface structure has been improved, especially in HPC[12–15].

In this paper, using the experience of nano-technologyin ceramics and polymer for reference, the influence ofNS on properties of hcp was studied as compared with sil-ica fume, in order to improve the microstructure of hcp,thus to enhance the durability and mechanical propertiesof cement-based materials. Furthermore, for HPC contain-ing SF or fly-ash or slag, with the addition of 1–3% NS, wehope to produce a new high performance concrete withmuch better properties.

2. Experimental procedure

2.1. Raw materials

A commercial Chinese ordinary Portland cement (C)(42.5 grade, Blaine specific surface 310 m2/kg) complyingwith Chinese standard (GB 175) was used. Its composition(wt%) is 81% clinker, 13% slag and 6% gypsum, and theBogue composition of the clinker is 56% C3S, 18% C2S,8% C3A and 12% C4AF. Both NS and SF in this studywere commercially available undensified materials, sup-plied by Mingri Nano-material Ltd. and Zhunyi FerroalloyLtd. in China, respectively. A superplasticizer (SM) usedwas a commercial sulphonated melamine formaldehydepolymer (liquid solution, water content 70%) with specialgravity 1.2 g/cm3 and water reduction up to 20%. Tapwater (W) was used in all experiments. Chemical composi-tions and physical properties of clinker, slag, gypsum, SFand NS are shown in Table 1.

2.2. Experimental programs

(1) Preparation of paste specimen. Cement pastes incor-porating NS or SF were prepared at (near) standardconsistency using a planetary mixer (ISO). For allthe pastes, a cement plus addition:water:superplasti-cizer ratio of 1:0.22:0.025 was used, as shown in Table2. Cement with NS or SF was fully mixed under thecondition of dry process beforehand. The mixing con-sists of a sequence of mixings that involve a total of

Table 1Chemical compositions and physical properties of clinker, slag, gypsum, silica

Chemical composition (wt%)

SiO2 Al2O3 Fe2O3 CaO MgO SO3

Clinker 21.05 5.56 4.03 64.27 1.02 0.75Slag 31.55 13.95 1.08 41.42 8.20 –Gypsum 4.96 0.46 0.26 29.61 1.45 37.56SF 92.10 2.04 1.08 0.45 0.58 0.44NS 99.90 – – – – –

2.0 min at a paddle speed of both 62 rpm (revolution)and 140 rpm (rotation), a 15 s stop and a total of2.0 min at a speed of both 125 rpm (revolution) and285 rpm (rotation).

(2) Test of consistency and setting time of fresh pastes.The consistency and the setting time of fresh pasteswere tested according to ISO 9597:1989. The consis-tency was ascertained by putting the paste in amould consisting of a steel ring (40 mm in height)on a sheet of glass and by determining the penetra-tion depth of a plunger applied to the top surface ofthe paste specimen. The initial and the final settingtime were determined with the needle of the Vicatapparatus.

(3) Test of paste compressive strength. The fresh paste wascast into cubic molds (25 · 25 · 25 mm) to preparespecimens on a vibrating table for a measurement ofcompressive strength. Three cubes were tested for eachsample at the given age, by a hydraulic press with100 kN capacity and 0.5 MPa/s loading speed. Eachstrength value was an average of three specimens.

(4) Test of bond strength of paste–aggregate interface (or

paste flexural strength set with glass plate). The freshpaste was cast into square-bar moulds 40 · 40 · 160mm on a vibrating table, and then a glass plate(39.8 · 39.8 · 3 mm) was set into the middle of thesquare-bar specimen with the plate parallel to thebar cross-section. With ISO method for cement flex-ural strength test for reference, three specimens weretested for each sample at the given age. The span forflexural strength test and the compressed area forcompressive strength are 100 and 40 · 40 mm, respec-tively. Namely, the bond strength is represented byflexural strength.

(5) Preparation of paste–aggregate interface specimen for

microstructure analysis. At first, glass plate(19.8 · 19.8 · 3 mm) was prepared, and the platewas set into the bottom of cubic mold(20 · 20 · 20 mm). Then the fresh paste was cast intothe cubic molds on a vibrating table.

(6) Microstructure analysis at interface. At the given age,when glass plate was split apart between hcp andglass, the fracture surface (interface) on hcp was ana-lysed by XRD and SEM immediately, in order todetermine the degree of interaction between CH and

fume and nano-SiO2

Physical properties

Average of diameter(nm)

Specific surface(m2/g)

Density(g/cm3)

Loose density(g/cm3)

180 21.5 2.22 0.2115 160 2.12 0.15

Fig. 1. XRD powder pattern of nano-SiO2 and silica fume.

Table 2Mix proportions, compressive and bond strengths of pastes made out of cement and NS or SF

Sample Mix proportion in mass Consistencya

(mm)Setting time Compressive strength (MPa) (%) Bond strength (MPa)

(%)

C NS SF W SM Initial Final Dt 1d 3d 28d 60d 7d 28d

CO 100 0 0 22 2.5 34 2h57m 4h23m 1h26m 48.9 (100) 61.1 (100) 79.2 (100) 94.9 (100) 5.1 (100) 5.8 (100)A1 99 1 0 22 2.5 34 2h57m 4h05m 1h08m 49.2 (101) 71.6 (117) 94.7 (120) 101.6 (107) 5.9 (116) 7.3 (126)A2 98 2 0 22 2.5 33 2h55m 3h50m 0h55m 49.8 (102) 72.6 (119) 95.8 (121) 102.5 (108) 6.2 (122) 8.3 (143)A3 97 3 0 22 2.5 33 2h48m 3h40m 0h52m 52.0 (106) 82.2 (135) 97.6 (123) 105.8 (111) 6.6 (129) 10.0 (172)A5 95 5 0 22 2.5 32 2h16m 3h06m 0h50m 53.0 (108) 86.1 (141) 98.8 (125) 108.8 (115) 7.3 (143) 10.9 (188)B2 98 0 2 22 2.5 35 3h50m 4h45m 0h55m 47.5 (97) 61.0 (100) 84.2 (106) 101.5 (107) 5.2 (102) 6.3 (109)B3 97 0 3 22 2.5 35 4h35m 5h20m 0h45m 47.3 (97) 60.4 (99) 92.0 (116) 104.3 (110) 5.0 (98) 6.7 (116)B5 95 0 5 22 2.5 36 4h45m 5h28m 0h43m 47.0 (96) 60.0 (98) 95.3 (120) 106.9 (113) 4.9 (96) 7.1 (122)

a Penetration depth.

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NS or SF, and to observe the orientation, size andmorphology of CH crystals or other hydration prod-ucts at the interface.

(7) Curing conditions for specimens. All paste specimens,which were cured at (20 ± 2) �C and above 90% r.h.moisture, were demolded after 24 h, and then storedin water at (20 ± 1) �C.

The above consistency, setting time, compressive andbond strength tests were repeated three times.

2.3. Equipment and test conditions

X-ray diffraction analyzer used is Rigaku D/Max-3Btype with the following conditions: Cu Ka radiation, tubeelectric current 40 mA and tube voltage 40 kV, and scan-ning speed 1�/min. Scanning electron microscope used isHitachi S570 type.

3. Results

3.1. Degree of crystallinity of NS and SF

Fig. 1 shows XRD powder patterns of NS and SF. Strongbroad peaks of NS and SF were centered on 23� and 22�(2h), respectively, which was in keeping with the strongbroad peak of a characteristic of amorphous SiO2. The re-sults show that both NS and SF are in an amorphous state.

3.2. Consistency and setting time of fresh pastes

The influence of NS or SF addition on consistency andsetting time of fresh pastes is presented in Table 2. Withincreasing the NS content, fresh pastes for sample A-seriesgrew thicker gradually and their penetration depths (con-sistency value) decreased gently as compared with that ofcontrol sample CO. While with increasing the SF content,the pastes for sample B-series grew thinner and their depthsincreased. The setting of fresh pastes (sample A-series) wasslightly accelerated but the difference between the initialand the final time decreased with increasing the NS con-tent. While the setting of fresh pastes (sample B-series)was obviously retarded and the difference was also de-

creased with increasing the SF content. The results indicatethat the influence of both NS and SF on consistency andsetting shows different, and NS makes cement paste thickerand accelerates the cement setting process as comparedwith SF.

3.3. Compressive strength of pastes

Table 2 also gives the variation of the compressivestrength development of hcp with NS or SF added. It isfound that all paste strengths of sample A-series were obvi-ously higher than those of control sample CO especially at3 days. Furthermore, with increasing the NS content, pastestrengths increased. For example, compared with sampleCO, the strengths of sample A3 increased by 6%, 35%,23% and 11% at ages of 1 day, 3 days, 28 days and 60 days,respectively.

However, with increasing the SF content, all pastestrengths of sample B-series were slightly lower than thoseof sample CO at ages of 1 day and 3 days. But at ages of 28days and 60 days, strengths with increasing the SF contentwere obviously higher than those of sample CO, and thestrengths were slightly lower than those of sample A-serieswith the same content of addition. For example, as com-pared with sample CO, the strengths of sample B3 de-creased by 3% and 1% at ages of 1 day and 3 days, andincreased by 16% and 10% at ages of 28 days and 60 days,respectively.

These results show that the compressive strength of hcpis enhanced with NS addition added, especially at earlyages, and the pozzolanic activity of NS is much greaterthan that of SF.

Fig. 2. Variation of interaction between nano-SiO2 and Ca(OH)2 at theinterface between paste and aggregate with time determined by XRD ascompared with silica fume.

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3.4. Bond strength of paste–aggregate interface

The variation of bond strength of paste–aggregate inter-face made from cement and NS or SF with time is shown inTable 2. Similarly to the compressive strengths, all bondstrengths of sample A-series were obviously higher thanthose of control sample CO and than those of sampleB-series. Moreover, with increasing the NS content, therate of bond strength increase was more than that of theircompressive strength increase. At the ages of 7 and 28 daysbond strengths of sample A3 were, respectively, 6.6 and10.0 MPa and were, respectively, 29% and 72% higher thanthat of sample CO, and were, respectively, 32% and 49%higher than that of sample B3. The results show that thebond strength at the interface between aggregate and hcpincorporating NS increases obviously and NS additioncan improve the interface structure more effectively thanSF.

3.5. Consumption of CH crystals gathered at the paste–

aggregate interface

Fig. 2 shows the variation of interaction between CHand NS or SF at the interface with time determined byXRD pattern. The consumption of CH (crystals) contentat the interface between glass plate and paste containingNS or SF can be demonstrated approximately by intensitychanges of main diffraction peaks of CH crystals, such as(001) crystal face, as well as (100) and (101) crystal faces(d = 0.490, 0.310 and 0.263 nm, respectively), neglectingthe effect of orientation on the intensity. At the same ages,diffraction peak intensities of (100) and (101) crystal facesof CH at the interface of sample A3 were almost equal tothose of sample CO, but the intensity of (00 1) crystal facewas much less than that of sample CO. For example, ascompared with sample CO, the intensities of (001) crystalface at the interface of sample A3 decreased by 67%, 61%and 64% at ages of 1 day, 7 days and 28 days, respectively.

Similarly to sample A3, intensities of (100) and (101)crystal faces of CH at the interface of sample B3 were al-most equal to those of sample CO, but the intensity of(001) crystal face was less than that of sample CO. For in-stance, as compared with sample CO, the intensities ofsample B3 decreased by 29%, 19% and 12%, respectively,at the same above ages.

The results indicate that NS can consume more CHcrystals at the interface and can improve the interfacestructure more effectively than SF.

3.6. CH orientation at the paste–aggregate interface

The influence of NS or SF on CH orientation at thepaste–aggregate interface can be determined by XRD pat-tern as shown in Fig. 2. The orientation in this paper is de-fined as R = 1.35I(0 0 1)/I(1 0 1), where I(0 0 1) and I(1 0 1) is(001) and (101) crystal face peak intensity, respectively,and R is orientation index. At the interface between paste

and glass plate, orientation indexes of control sample COwere very great and were 3.9, 3.6 and 3.6 at ages of 1day, 7 days and 28 days, respectively, and the indexes ofsample B3 were 3.4, 3.1 and 3.3. But those of sample A3

Q. Ye et al. / Construction and Building Materials 21 (2007) 539–545 543

were very small and were just 1.8, 1.6 and 1.5 at the sameabove ages. The results reveal that NS can decrease the ori-entation of CH crystals more effectively than SF.

3.7. CH micrographs at the paste–aggregate interface

The variation of SEM micrographs of CH crystals gath-ered at the paste–aggregate interface with NS and SFadded is presented in Fig. 3. Fig. 3(a) shows that, for sam-ple CO, hexagonal plates of CH crystals occurred withtheir c-axes perpendicular to the glass surface. Edges ofhexagonal plates were clear and the size of the biggest crys-tal CH was up to 10 lm in the SEM micrograph.

CH micrographs at the interface of sample A3 areshown in Fig. 3(b). It can be seen that the big CH platesoccurred with their c-axes also normal to the glass surface.But edges of hexagonal plates with eroded phenomenonwere unclear, and the size of the biggest crystal CH wasup to 4 lm in the SEM micrographs.

Fig. 3. SEM micrographs of Ca(OH)2 crystals at the interface between aggrega(b) with 3% NS; (c) with 3% SF.

As shown in Fig. 3(c), for sample B3, CH plates also oc-curred with their c-axes perpendicular to the glass surface.Edges of hexagonal plates were clear similarly to sampleCO and the size of the biggest crystal CH reached 7 lm.

The results show that NS can reduce the size of CH crys-tals at the interface more effectively than SF.

4. Discussion

It is common knowledge that SF has a high pozzolanicactivity and is normally considered as the best mineraladdition used for concrete up to now. When SF is addedto cement or concrete, it acts both as a chemical inert filler,improving the physical structure and providing nucleationsites for hydration products, and as a pozzolan, reactingchemically with CH formed during hydration of cement,which will probably improve the paste–aggregate bond[15,16]. However, the pozzolanic activity of SF at earlyages is low according to the aforementioned literature

te and paste made from cement and nano-SiO2 at 28 days: (a) without NS;

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[8–11,16]. It is reported that the pozzolanic reaction be-tween SF and CH formed during cement hydration beginsto occur after 3 days of hydration.

With ultrafine particle size reducing to nano-structuredone, such as SF particle size to that of NS, specific surfaceand the number of atoms in the surface increase rapidly.Owing to atoms in surface of nano-meter particles notbeing full of other coordinate atoms, there exit a lot offree bonds and unsaturated bonds or residual valenceforces in the surface which lead the particles to be unstablestate of thermodynamics. On the other hand, with the par-ticle size reducing, there exit many uneven atom stepswhich increase chemical reaction area [17]. For this rea-son, the nano-meter particles, such as NS, have high sur-face energy; and atoms in the surface have a high activity,which leads the atoms to react on outer ones easily. Con-sequently, the pozzolanic activity of NS at early ages ishigher than that of SF.

The mechanisms that addition of NS has different ef-fects on the properties of the cement paste, as comparedwith addition of SF, can be illustrated as follows. Whena material with high specific surface is added to cementor concrete, it acts as the micro-filler of the cement parti-cles, which can reduce the amount of water that filled inthe void of the blending materials. However, replacing ce-ment with a high specific surface material would increasethe wettable surface area and the amount of water ad-sorbed. Thus, the final water requirement will depend onwhich of the two above-mentioned factors will be supe-rior. With replacement of less than 5% in this paper, theformer factor may be superior for SF. But the later factormay be superior for NS, due to the very high specific sur-face area of the nano-meter sized particles, which wouldincrease greatly the wettable surface area and the amountof water adsorbed. Thus addition of NS can make cementpaste thicker.

Because the specific surface of NS with 160 m2/g is muchgreater than that of SF with 21.5 m2/g, the reaction be-tween SiO2 and Ca(OH)2 shows differences between the ce-ment paste incorporating NS and that incorporating SF.For NS having many unsaturated bonds „Si–O– and„Si– in the surface, the reaction process between SiO2

and Ca(OH)2 may be as follows:

BSi–O–þH–OH! BSi–OH ðreact quicklyÞ ð1ÞBSi–þOH! BSi–OH ðreact quicklyÞ ð2ÞBSi–OHþ CaðOHÞ2 ! C–S–H ð3Þ

For SF having many saturated bonds „Si–O–Si„ and alittle unsaturated bonds in the surface, the process maybe as follows:

BSi–O–SiBþH–OH! BSi–OH ðreact slowlyÞ ð4ÞBSi–OHþ CaðOHÞ2 ! C–S–H ð5Þ

In the cement–water system, the bond Si–O of SF is noteasy to break down owing to its higher bond energy. Thefirst step reaction (Eq. 4) for SF is slower than that (Eq.

1) for NS [17]. So, NS can accelerate the cement settingprocess and the hydration process.

It is the surface effects of nano-structured particles thatcause the superiorities of cement paste incorporating NS tothat incorporating SF. For this reason, NS can providemuch more nucleation sites for hydration products thanSF at early ages, and NS has a higher pozzolanic activitythan SF. Therefore, incorporating NS can increase com-pressive strengths of hcp and bond strengths of paste–aggregate interface, especially at early ages, and canimprove the interface structure more effectively than incor-porating SF. So, using a small amount of NS can enhancethe durability and the mechanical properties of cement-based materials.

5. Conclusions

The following conclusions may be drawn from the ob-tained experimental data:

(1) Both nano-SiO2 and silica fume are in the amorphousstate.

(2) The influence of nano-SiO2 and silica fume on consis-tency and setting time are different. Nano-SiO2 makescement paste thicker and accelerates the cementhydration process.

(3) Compressive strengths of hcp increase with increasingthe nano-SiO2 content, especially at early ages. How-ever the strengths of hcp decrease slightly withincreasing the silica fume content at early ages, butincrease at later ages.

(4) Bond strengths of paste–aggregate interface incorpo-rating NS are higher than those of control sampleCO and than those incorporating SF. With increasingthe NS content, the rate of bond strength increase ismore than that of their compressive strength increase.

(5) The pozzolanic activity of nano-SiO2 is much greaterthan that of silica fume. Nano-SiO2 consumes CHcrystals, decreases the orientation of CH crystals,reduces the size of CH crystals at the interface andimproves the interface structure more effectively thansilica fume.

Acknowledgements

The authors wish to thank the Center of Science Re-search, Zhejiang University of Technology of China forfinancial support.

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