SHIH Et Al (2006) - Effect of Nanosilica on Characterization of Portland Cement Composite

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Materials Science and Engineering A 424 (2006) 266–274

Effect of nanosilica on characterization of Portland cement composite

Jeng-Ywan Shih, Ta-Peng Chang ∗, Tien-Chin Hsiao

Department of Construction Engineering, National Taiwan University of Science and Technology, Taipei 10672, Taiwan, ROC

Received 16 November 2005; accepted 4 March 2006

Abstract

Both the filling effect and the pozzolanic reaction make siliceous materials as one of major ingredients of high-performance Portland cement-

based composites. Hence, the introduction of nanosilica with finer particle size and larger silicon dioxide to the composite becomes a great deal

of interest in recent years. In this study, a liquid-form of nanosilica particle with a spherical diameter of about 20 nm was incorporated into the

Portland cement paste at five different dosages and analyzed at four different ages to identify the nanosizing effects on the microstructures andmaterial properties of composite cement paste. Experimental results show that the Portland cement composite with 0.60% of added nanosilica by

weight of cement has an optimum compressive strength, in which the increase of compressive strength is about 43.8%. Moreover, the corresponding

nanosilica paste of one portion of water mixed with nanosilica of 1.08wt.% of water has the maximum absolute value of zeta potential of 41.3 mV.

Properties through the analyses of NMR, BETand MIPalso indicate that the microstructure of Portland cement composite with nanosilica evidently

has a more solid, dense and stable bonding framework.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanosilica; Cement paste; Compressive strength; Zeta potential

1. Introduction

Due to results of the filling effect to reduce porosity byuniform distribution of particle size and the pozzolanic reac-

tion that consumes calcium hydroxides (Ca(OH2)) to yield

calcium silicate hydrates (C–S–H), the silica fume, a con-

ventional supplementary cementitious material, is one of the

very important admixtures for producing the high-performance

Portland cement-based concrete with enhancing strength and

abrasion resistance along with reducing permeability and dry

shrinkage [1–4]. In order to obtain further improvements of

Portland cement-based composite (or alternatively called as

cement composite for simplicity when appropriate hereinafter),

siliceous materials of higher purity and finer size are intro-

duced. In the past decade, novel properties of nanoparticles with

1–100 nm scale have attracted enormous attention [5]. Among

these nanoparticles, nanosilica is commonly used for reinforce-

ment of polymer to increase the hardness, modulus, weatherabil-

ity, flammability, and so on [6–10]. Likewise, some efforts on

excellent mechanical properties and microstructure of cement

composites with nanosilica have been also reported [11,12], in

∗ Corresponding author. Tel.: +886 2 2737 6577; fax: +886 2 2737 6606.

E-mail address: [email protected] (T.-P. Chang).

which a water reducing agent was always applied to aid the

dispersion of nanosilica during the process of specimen prepa-

ration. Although, the water reducing agent may help improvethe material properties of cement composites, some unpredicted

interaction among water, water reducing agent, nanosilica and

cement is anticipated to complicate the study on the chemical

mechanism of such composite material. For this reason, in order

to alleviate the complex interaction of state variables and to

properly evaluate the influencing effects of nanosilica on cement

composites, this study does not include water reducing agent as

one of the ingredients. Rather, only the addition of nanosilica

in liquid form was included to investigate the characteristics of

corresponding cement composite.

On the other hand, the zeta potential is the electric potential

around the particle on the slip surface within an electric double

layer formed at the particle–liquid interface. In the slip sur-

face, the double layer is divided into two parts: the inner region

named stern layer and the outer region called diffusive layer.

Because the zeta potential performs the quantitative measure on

the effective charge of particles, it is related to the stability and

agglomeration processes. In other words, if the repulsive force

between approaching particles becomes high enough, then they

will repel each other to make them separable. On the contrary if

the repulsive force is sufficiently weak, then the particles will get

together and ambient particles may also be seized in the growing

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2006.03.010

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J.-Y. Shih et al. / Materials Science and Engineering A 424 (2006) 266–274 267

agglomeration. So, when the absolute value of the zeta poten-

tial gets over a specific electric charge, the system will keep in

a dispersing state. Therefore, when silica particles in the fresh

Portland cement paste are under such spreading condition, the

opportunity of producing calcium silicate hydrates, which plays

a vital role of strength development, can rise apparently.

In addition to using the compressive strength and zeta poten-

tial as two major indices to evaluate the effects of added nanosil-

ica on the features of cement composite, other material proper-

ties such as degree of hydration, bonding pattern, and fractal

dimension are also addressed.

2. Experimental program

2.1. Experimental techniques and facilities for

microstructural properties

The examination of microstructural properties of cement

composite and nanosilica was conducted by one or more of the

following methods and corresponding facilities, whichever suitsthe requirements:

(1) Zeta potential measured by electrophoresis apparatus using

Malvern Instruments Zetasizer 2000.

(2) Degree of hydration assessed by nuclear magnetic resonance

(NMR) using a Varian 400 MHz (one-pulse pattern, pulse

delay 180 s.).

(3) Specific surface measured by nitrogen adsorption according

to the Brunauer, Emmett, and Teller (BET) method using a

Micromeritics ASAP 200C.

(4) Porosity analyzed by Mercury Intrusion Porosimeter (MIP)

using a Micromeritics AutoPore IV 9520 (from atmosphericpressure to 414 MPa).

2.2. Materials

Three major ingredients of cement composite were used in

this study: (1) Type I Portland cement complying with ASTM

C150 standard; (2) ordinary distilled water and (3) nanosilica

(NS) in liquid form used as the admixture. The chemical compo-

sitions and physical properties of ASTM Type I Portland cement

are given in Table 1. Basic material properties of nanosilica are

given in Table 2.

2.3. Experimental variables, procedure and specimen preparation

At the beginning, five different water/cement (W / C ) ratios of

0.25, 0.35, 0.45, 0.55 and 0.65 were used to mix the cement

composite whose performance was carefully examined accord-

ingly. Finally, the optimum water/cement (W / C ) ratio was fixed

at 0.55. Five different dosages of nanosilica, 0.0, 0.2, 0.4, 0.6

and 0.8 wt.% of cement were added to the Portland cement paste

to cast the specimens of cement composite. The mix proportions

of these five cement composites are given in Table 3.

Cylindrical nanocomposite specimens of Ø25 mm×50mm

were used for the compressive test. Each experiment was per-

Table 1

Chemical compositions and physical properties of Type I Portland cement used

in this study

Chemical composition (mass%)

SiO2 20.31

Al2O3 5.05

Fe2O3 3.16

CaO 62.43MgO 3.81

SO3 2.48

Free CaO 0.4-78

LOI 1.49

Physical properties

Fineness (m2/kg) 349.0

Specific weight 3.15

Initial setting (min) 185.0

Final setting (min) 292.0

Table 2

Basic material properties of nanosilica (NS) (in liquid form)

Composition (mass%) SiO2 (40.6%), H2O (58.8%), others (0.6%)

Dimension 20 nm (spherical shape)

pH value 10.1

formed at four ages of 7, 14, 28 and 56 days using three speci-

mens prepared from same batch of mixture. All the specimens

of cement composite were demoulded 24 h after they were cast

and then immersed in the saturated limewater basin until 1 day

before the test and then cured in the ambient temperature for

24 h. The test of compressive strength of cylindrical nanocom-

posite specimen was performed by a 10 tonnes material testing

machine following the procedure of ASTM C39 standard.The zeta potential can be calculated from the velocity per

unit electrical field strength, which is the electrophoretic mobil-

ity. Thus the specimenusedin the zetapotential test is inthe form

of a dilute suspension. The procedures of suspension prepara-

tion and zeta potential measurement are briefly described as

follows. After entering into deionized water, nanosilica was

ultrasonically dispersed for 180 s in the solution to obtain the

aqueous suspension. Then, the sample suspension was injected

into the electrophoresis apparatus to determine the zeta poten-

tial of nanosilica. The average was decided as the experimental

consequence by measuring the same suspension for five times.

Thus, the zeta potential tests of four various added dosages

of nanosilica, 0.36, 0.72, 1.08, and 1.44 wt.% of water, in which

these four mix proportions were corresponding to the mix tags

of C-02, C-04, C-06 and C-08 of cement composite, were also

Table 3

Mix proportion of Portland cement composite

Designation Cement:water:NS (by mass)

C-00 1:0.55:0.000

C-02 1:0.55:0.002

C-04 1:0.55:0.004

C-06 1:0.55:0.006

C-08 1:0.55:0.008



268 J.-Y. Shih et al. / Materials Science and Engineering A 424 (2006) 266–274

investigated to gain the optimal addition. Finally, the test results

of compressive strengths and zeta potentials were used as two

performance indices to select the optimal nanocomposite from

which its microstructural properties were examined.

For the MIP analysis, the cylindrical disk specimens of

Ø20 mm× 10 mm were dried in an oven at 105 ◦C for 24h to

reach a constant weight, and then were loaded into a penetrom-

eter. After specimen was installed and evacuated to the required

vacuum level, the penetrometer was backfilled with mercury. As

the pressure was increasing, the mercury was gradually entering

the pores of specimens. Thus the volume of mercury deducted

in the penetrometer is related to the volume of the pores filled.

Hence, the volume of mercury penetrating into the pores can

be obtained from the applied pressure from which the pore size

distribution and pore volume of specimen can also estimated.

3. Results and discussion

3.1. Compressive strengths

All the test data of compressive strengths of cement com-

posite with five dosages of NS at four ages are given in Fig. 1.

Fig. 1 shows the development of mean values of compressive

strength of test data with corresponding errors bars at four ages

versus five different added percentages of NS represented by

five vertical lines. Basically, the compressive strength at four

ages increases with the increase of the amount of NS until it

reaches an optimal amount of 0.6% and then drops to some

lower values at 0.8% addition. The curve line of C-06 with the

addition of NS of 0.6% shows the highest compressive strengths

among all ages with the highest value of 65.62 MPa at age of 56

days. At least two likely mechanisms can be deduced to con-tribute to the increase of compressive strengths of hardened

cement paste due to the addition of NS. The first strengthen-

ing mechanism is the packing effect of small NS acted as filler

Fig.1. Compressive strengths of Portland cementcompositeat various additions

of NS and ages.

to fill into the interstitial spaces inside the skeleton of hard-

ened microstructure of cement paste to increase its density as

well as the strength. The second strengthening mechanism is

the pozzolanic effect that combines glass-like silicon elements

in NS with the lime elements of calcium oxide and hydroxide in

cement to add the bonding strength and solid volume, resulting

in higher compressive strength of hardened cement paste. Most

pozzolanic reaction between the calcium hydroxide and amor-

phous silica (silicon dioxide) normally reacts slowly during a

prolonged period of moist curing. Since the spherical particles

of NS have an average particle diameter of about 20 nm which

is about 1000-times finer than average cement particle of 20m

resulting in an extremely large surface area, the NS reacts very

rapidly with the calcium hydroxide to form calcium silicate in

an alkaline environment such as the pore solution of fresh Port-

land cement paste. For this reason, the contribution of added NS

to the increase of strength of hardened cement paste becomes

apparent as early as 14 days after hydration. These strengthen-

ing mechanisms of cement paste attributed to the addition of NS

will be justified by the following microstructural examination inthe following sections.

3.2. Zeta potentials

The interactions between the NS particles with Portland

cement are assessed using the Zeta potential measurements in a

rheological state to determine the mechanism of dispersion. The

zeta potential is the electro kinetic potential of a particle in an

aqueous solution as determined by its electrophoretic mobility

between particle and solution in an electric field. By measuring

the charge density of particles on a surface, the zeta potential

is the electric magnitude of the repulsion or attraction betweencolloidal particles. Thus, the zeta potential is a major influential

parameter to study the dispersion mechanism of NS particles

in an alkaline solution of Portland cement paste. Since the zeta

potential of a particle is an indication of its electrostatic surface

charge, usually, the zeta potential is expressed in the level of

energy barrier preventing the proximity of particles [12]. Each

particle has a natural electrostatic surface charge at different pH

values of the ambient environment. When the values of pH of

the environment vary from 2 to 12, zeta potentials will change

from positive at low pH to negative at high pH. The isoelectric

point (IEP) of each particle indicates the pH at which the natu-

ral surface charge is zero. Therefore, to enhance the dispersion

behavior, the higher the absolute value of zeta potential is; the

better the dispersion of particles will be. The ranges of IEPs of

SiO2 are between pHs 2.0 and 2.5 [13]. If the pH value exceeds

the corresponding pH value of IEP, the process of reaction takes

place through Eq. (1) [14]. Hence, the zeta potential becomes

negative because Si(O−) forms at the surface of SiO2:

Si(OH) + OH−→ Si(O−) + H2O (1)

The ratios of weights between NS and water (NS/water) for

four mix proportions of C-02, C-04, C-06 and C-08 in Table 3

can be calculated as 0.36, 0.72, 1.08 and 1.44%, respectively.

The zeta potentials for each mix proportion were measured five




Fig. 2. Zeta potential by various additions.

times and the test results of each individual data and average

value are presented in Fig. 2. As described in Section 3.1, the

mix proportion of C-06 has the highest compressive strengths

among all ages. The experimental data of zeta potential also

indicates an optimum value of −41.3 mV for the NS/water of

1.08% which corresponds to the mix proportion of C-06. Both

the compressive strength and zeta potential verify an optimum

added amount of NS of 0.6%. Due to alkalinity of the cement

hydration products of C–S–H and Ca(OH)2, the relationship of

zeta potentials and pH values from 7 to 12 investigated at 1.08%

of NS/water ratio is shown in Fig. 3. The absolute values of zeta

potential gradually increase with the increase of pH values. As

a result, under alkali environment, the dispersion mechanism of

nanosilica performs quite well.

Fig. 3. Zeta potential at various pH values.

3.3. Microstructural examination on performance of

nanosilica in cement composite

To justify the optimum macro-behavior of strengthen mech-

anism of hardened cement composite in terms of compressive

strengths due to the addition of NS with an optimum added

amount of 0.6 wt.% of cement, several microstructural exam-

inations such as NMR, BET and MIP were also conducted in

this study. For the purpose of comparison, both the specimens of

harden cement composite with 0.6% addition of NS and without

addition of NS at age of 56 days were used.

3.3.1. NMR examination

The non-destructive and non-invasive measurement of

nuclear magnetic resonance (NMR) measured on 29Si nuclei

has been used to study the behaviors of Portland cement pow-

der, pure hardened Portland cement paste and hardened Portland

cement composite with addition of 0.6% of NS. In a 29Si NMR

spectroscopy, the quantity of chemical shift of a nucleus, nor-

mally expressed in ppm, is the difference between the resonancefrequency of the nucleus and the standard of tetramethylsilane,

Si(CH3)4, abbreviated TMS. Let Qn represent a SiO4 tetrahe-

dron with n bridging oxygens. The intensities of five separate

peaks appear around−70,−80,−88,−98 and−110 ppm in the

NMR curve, denoted as Q0, Q1, Q2, Q3 and Q4, can be used as

a base to characterize quantitatively the degree of hydration of

tested sample through the curve given either by Lorentzian func-

tion or Gaussian function to fit the experimental data of NMR

curve [15]. All the test results of NMR experiments are shown

in Figs. 4 and 5a and b, respectively. Fig. 4 is the NMR curve

for pure Portland cement that only has a single peak of Q0. Both

Fig. 5a (hardened pure cement paste) and b (hardened cementcomposite with NS) show one sharp peak of Q0 and another

wider peak containing both Q1 and Q2. These two figures do not

have either Q3 peak or Q4 peak.

After some preliminary assessment on the suitability of the

fitted equation to fit the experimental data, the Lorentzian func-

Fig. 4. NMR curve of Portland cement.




Fig. 5. NMR curves of hardened Portland cement composite: (a) without NS and (b) with NS.

tion was adopted. A general Lorentzian function y( x ) can be

expressed as Eq. (2) [16]:

y(x) = y0 +2A

π

W

4(x− xc)2+W 2

(2)

where x is the chemical shift (ppm) in NMR, y( x ) the intensity,

y0 the datum of the peak, A the area covered by the curve, x cthe location of peak and W is the full-width-at-half-maximum

(FWHM). The values of these four parameters, y0, A, x c and

W , for three NMR curves in Figs. 4 and 5a and b, can be

obtained through the least-square curve-fitting technique, and

listed in Table 4. The coefficients of determination of R2 for

these three curves are 0.972, 0.989 and 0.992, respectively, indi-

cating a relatively satisfactory strength of the linear association

between independent variable x and dependent variable y. Thelocations of peak intensities, Q0, Q1 and Q2 in Table 4 are close

to those values of −70, −80 and −88 ppm. The fitted curves

of Lorentzian function are also drawn in Figs. 4 and 5a and b.

The degree of hydration of cement-based composite, α, can be

estimated on the base of values of Q0 through the following

equation [17]:

α (%) =

1−

I (Q0)

I 0(Q0)

× 100 (3)

where I (Q0) is the A value of Q0 for the hydrated cement-based

composite and I 0(Q0) the A value of Q0 for the Portland cement

powder. Substituting the A values of 57996.43, 52532.62 and195769.26 in Table 4 into Eq. (3) gives the values of α of 70.38%

and 73.17%, respectively. Thus, at the age of 56 days, the degree

of hydration of hardened cement composite with NS increases

by about 2.8% by comparison with the pure hardened cementpaste. Meanwhile, the ratio of A value of Q2 to that of Q1 can

be served as an index to indicate the relative quality of Port-

land cement composite in which a higher value means better

mechanical properties [18]. The ratio calculated from the values

of A of Q2 and Q1 in Table 4 increases from 0.0806 to 0.2927.

Thus, this result confirms a positive improvement on the com-

pressive strengths of cement composite due to the addition of

NS based on the experimental results as stated in the previous

section. Furthermore, the variation of W (FWHM) values at Qn

peaks can also be used to evaluate the amorphous character of

microstructure of hardened cement paste [19]. After the addition

ofNS, the FWHM valueof Q1 peak in cement composite reduces

substantially by about 29.23%, indicating that the arrangement

order of O–Si–O net seems to become regular with a pattern of

sharing a common oxygen atom. Therefore, both the increases

of α and Q2 / Q1 and the decrease of FWHM indeed confirm the

improvement of microstructures of hydrated cement composite

as a result of the addition of NS.

3.3.2. BET examination

The Brunauer–Emmett–Teller (BET) technique is primarily

used to measure the surface area of a solid from the physical

adsorption of a gas, such as nitrogen N 2, on the solid sur-

face, particularly suitable for materials like hydrated Portland

cement paste with apparent open porosity [20]. The BET equa-tion that computes the number of adsorbed gas molecules from

Table 4

Values of four constants of Lorentzian functions for three NMR curves

Portland cement powder Pure hardened Portland cement paste Hardened Portland cement composite with NS

Qn Q0 Q0 Q1 Q2 Q0 Q1 Q2

y0 0 0 0

x c −71.26 −71.06 −79.48 −84.05 −70.39 −77.92 −82.70

A 195769.3 57996.4 175894.7 14170.4 52532.6 133362.9 39033.9

W 5.15 1.65 9.58 3.12 2.58 6.78 3.98

R2 0.972 0.989 0.992

Note: R2

denotes the coefficient of determinant defined as the ratio of sum of squares due to regression to the total sum of squares.




the adsorption isotherms of nitrogen N2 is given as follows [21]:

P

W (P 0 − P )=

1

W mC+

C − 1

W mCP 0P (4)

where P0 is the saturation pressure of the adsorbate, P the equi-

librium pressure of the adsorbate, W the weight of nitrogen

adsorbed at a given P / P0, W m the weight of a monolayer of adsorbate, and C is the constant related to the heat of adsorp-

tion. A linear relationship between 1/ W [(P0 / P)− 1] and P / P0 is

required to obtain the quantity of nitrogen adsorbed. The slope

a and intercept b are used to determine the quantity of nitrogen

adsorbed in the monolayer, W m, by the following equation:

W m =1

a+ b(5)

Then, the specific surface area of the sample, S , canbe calculated

as

S =W mN AAcs

Mw (6)

where N A is Avogadro’s number (6.023× 1023 molecules/mol),

Acs the molecular cross-sectional area (77◦K nitrogen N2

16.2 A2), M the molecular weight of adsorbate (nitrogen N2

28.0134 g/mol), and w is the sample weight.

The intercept a and the slope b for the experimental BET

data in this study were founded to be 1.517 and 147.98 for the

hardened Portland cement paste, and 0.209 and 137.0 for the

hardened cement composite with NS, respectively, as shown in

Fig. 6a and b. From Eq. (6), the specific surface area increases

from 23.30 m2 /g for the hardened Portland cement paste to

25.38 m2 /g for the hardened cement composite with NS by

8.95%.To estimate the surface characteristics of hardened Portland

cement composite, an index often referred to as fractal dimen-

sion D determined from adsorption data was used in this study

[22–24]. One kind of the fractal dimension value D is defined

as given in Eq. (7) [24]:

D = 3+ s (7)

where s is a material parameter. In order to conform to the phys-

ical meaning of a surface, the range of D should be between 2

(smooth surfaces)and 3 (all available porositiesbeingoccupied).

In other words,−1 < s <0 is essential for Eq. (7).

The value of s can be estimated through the

Frenkel–Halsey–Hill equation shown as follows [25]:

logP 0

P

= BV s

(8)

where P0 and P are the same as those in Eq. (4), V the amount

of adsorption, and B is the another constant accounting for the

interaction between the adsorbent and adsorbate. The value of

s can be obtained as the slope of a linear equation in Eq. (9)

transformed from Eq. (8):

log

log

P 0

P

= logB− s logV (9)

plots of log V verses log(log(P0 / P)) for the experimental BET

curves are presented in Fig. 7a and b from which the values of

s for the hardened Portland cement paste and cement composite

with NS were found to be −0.4977 and −0.3805, respectively.

From Eq. (7), the values of fractal dimension D become 2.502

and 2.620, respectively, indicating an improvement on the sur-

face roughness. Therefore, both the surface area and fractal

dimension confirm that the hardened Portland cement composite

with NS has denser microstructures.

3.3.3. MIP examination

Mercury intrusion porosimetry is based on the capillary

law governing liquid penetration into minor pores. This law is

expressed by the modified Washburn equation [26]:

d =−φγ cos θ

p(10)

where φ is the shape factor, d the narrow dimension of pore,

γ the surface tension, θ the contact angle and p is the applied

pressure.

In respect to mercury liquid and oven-dried Portland cement-

based materials, the values of surfacetensionγ and contact angle

Fig. 6. BET plots of hardened Portland cement composite: (a) without NS and (b) with NS.




Fig. 7. FHH plots of hardened Portland cement composite: (a) without NS and (b) with NS.

θ were taken as 117◦ and 0.484 N/m, respectively [27,28]. More-

over, according to the observation of Maage [29] as well as the

report of Cook and Hover [30], the value of shape factor φ is

assumed to be 4 in terms of the circular cross section of cylin-

drical pore. Therefore, Eq. (10) can be rewritten as follows:

d =878.93

p(11)

where d is the pore entry diameter in nm, p the applied pressure

in MPa. Eq. (11) was used to convert a given applied pressure

to a specific pore diameter for the experimental MIP data.

Two types of experimental MIP plots can be obtained. One

plot is the cumulative pore volume verses the pore diameteras shown in Fig. 8a and b, and the other is the distributive

pore volume verses pore diameter as presented in Fig. 9a and

b. Both Fig. 8a and b show that total pore volume decreases

from 0.2435 ml/g for pure hardened cement paste to 0.2382 ml/g

for hardened cement composite with NS by 2.18%. Also its

curvatures of cumulative curve become slightly greater. By com-

parison with Fig. 9a, the pore size distribution shown in Fig. 9b

shifts to finer pore sizes.Therefore, both thedecreaseof pore vol-

ume and smaller pore size distribution indicate that the hardened

Portland cement composite with NS has a more consolidated

microstructure.

A fractal dimension D* based on thepore size distribution can

be computed with the following Eqs. (12) and (13) deducted by

Ji et al. [31]:

logV ∗ = C + k log d (12)

D∗ = 3− k (13)

where V * is the cumulative pore volume in percentage, C and k

are constants to be found, and d is the pore entry diameter in nm

same as in Eq. (11).Using Eqs. (12) and (13), the experimental MIP data are

plotted in Fig. 10a and b. Note that, since the cumulative pore

volume ratio V * at the maximum log V * pore entry diameter is

assumed to be 100, it is the corresponding value of becomes

2 as shown in these two figures. According to the locations

at the changes of slope for the curves in Fig. 10a and b, the

abscissa is roughly divided into three zones A–C, where zone

Fig. 8. Accumulated MIP pore volume of hardened Portland cement composite: (a) without NS and (b) with NS.




Fig. 9. MIP pore distribution of hardened Portland cement composite: (a) without NS and (b) with NS.

Fig. 10. Relationship between log V * and log d of Portland cement composite: (a) without NS and (b) with NS.

A denotes the pore size smaller than 10 nm, zone B the pore

sizes between 10 and 100 nm and zone C the pore size larger

than 100 nm. For each zone, an individual least-square linear

model to fit the test data was conducted to obtain the slope k

in Eq. (12) and thus the value of D* in Eq. (13). These calcu-

lated values are presented in Table 5, where the percentages of

partial volumes between bounds of each individual neighboring

zone computed by the difference of V * are also illustrated. After

adding NS into cement composite, the partial volume ratio V p

at zone A increases by 1.22%, whereas decreases by 1.21% and

0.01% for zones B and C, respectively. On the contrary, the value

of D* decreases from 1.567 to 1.517 for zone A, but increase

from.2.261 to 2.305 for zone B and from 2.991 to 2.993 for

zone C, respectively. These results indicate that the microstruc-

ture of hardened cement composite with NS becomes denser

at the regions where the pore size is larger than 10 nm, but

becomes looser at the regions where the pore size is smaller

than 10 nm.

Table 5

Values of slope k , fractal dimension D* and volume ratio V P at three zones

Pure hardened Portland cement paste Hardened Portland cement composite with NS

Zone A B C A B C

Range <10 nm 10–100 nm >100 nm <10 nm 10–100 nm >100 nm

k 1.433 0.739 0.009 1.483 0.695 0.007

D* 1.567 2.261 2.991 1.517 2.305 2.993

V p 14.84% 79.31% 5.85% 16.06% 78.10% 5.84%

V * V * (<10 nm): 14.84%; V * (<100 nm): 94.15% V * (<10 nm): 16.06%; V * (<100 nm): 94.16%

Note: V *

= accumulated volume ratios.




4. Conclusions

Based on the experimental results and microstructural anal-

yses presented in this study, the following conclusions can be

drawn:

(1) Among four ages and five sets of mix proportions, the opti-

mal mix proportion is the set of cement:water:nanosilica =

1:0.55:0.006 which has the highest compressive strength of

65.62 MPa at age of 56 days. By comparison with the con-

trol set of cement paste without the addition of nanosilica,

the ratio of maximum increase in compressive strength is

about 60.6% at age of 14 days and reduces to 43.8% at age

of 56 days.

(2) Among four sets of mix proportions, the zeta potential for

the optimum mix set of nanosilica/water of 1.08% by weight

has a maximum absolute value of 41.3 mV. This optimal

mixture set obtained by zeta potential test is corresponding

to the mix proportion of cement composite in the compres-

sive test. Thus, the test results of zeta potential suggest thatthis test method may be able to serve as a quick and easy pre-

liminary technique to select quantitatively the optimal mix

proportion of cement composite with addition of nanosilica

simply by using the paste mixing the nanosilica with water.

(3) The observation on the changes of test values for these

indices of α, Q2 / Q1, and FWHM obtained from the NMR

analyses, has shown that the microstructures of Portland

cement composite added with nanosilica become more sta-

ble and stronger bonding.

(4) Both the surface area and the fractal dimension deducted by

BETexperiment also confirma densermicrostructure forthe

hardened cement composite with the addition of nanosilica.(5) The Portland cement composite incorporating nanosilica

measured by MIP technique reveals that the pore vol-

ume increases and the fractal dimension of microstructure

decreases at regions where the pore size smaller than 10 nm,

but the results are opposite for regions where the pore size

larger than 10 nm.

Acknowledgments

The authors thank the National Science Council of Taiwan,

Republic of China forfinancialsupport of this work (GrantNSC-

92-2211-E-011-052). Courtesy of some experimental data from

Mr. Kuo-Ming Yang is also appreciated.

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SHIH Et Al (2006) - Effect of Nanosilica on Characterization of Portland Cement Composite

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Transcript of SHIH Et Al (2006) - Effect of Nanosilica on Characterization of Portland Cement Composite