Construction and Building Materials 44 (2013) 87–91

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Construction and Building Materials

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Influence of superplasticizer on composition and pore structureof C–S–H

0950-0618/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.03.045

⇑ Corresponding author. Tel.: +86 15807165207; fax: +86 27 87210782.E-mail address: dp19851128@sina.com (P. Duan).

Ping Duan a,⇑, Zhonghe Shui a,b, Wei Chen a, Chunhua Shen c

a State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, Chinab Engineering Research Center of Green Building Materials, Wuhan University of Technology, Wuhan 430070, Chinac Center of Materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China

h i g h l i g h t s

� Pore structure parameters and fractal dimension of C–S–H were calculated.� Influence of organic component on composition and structure of C–S–H were studied.� Polycarboxylic organic macromolecule (B) contributes more to form smaller mesopore.� Aliphatic organic macromolecule (C) contributes more to form larger mesopore.� Fractal dimension is in the sequence: pure C–S–H > C doped C–S–H > B doped C–S–H.

a r t i c l e i n f o

Article history:Received 16 January 2013Received in revised form 12 March 2013Accepted 15 March 2013Available online 9 April 2013

Keywords:Pore structureCalcium silicate hydrateSuperplasticizerAdsorptionSpecific surface area

a b s t r a c t

Calcium Silicate Hydrates (C–S–H) is the main products of cement hydration, influence of differentorganic component (organic macromolecular or polymers) on composition and structure of calciumsilicate hydrate has important implications. Pore structure parameters of pure C–S–H with specific sur-face area of 43.67 m2/g and average diameter of 9.75 nm (A), polycarboxylic organic macromoleculardoped C–S–H with specific surface area of 139.86 m2/g and average diameter of 9.81 nm (B) and aliphaticorganic macromolecular doped C–S–H with specific surface area of 114.96 m2/g and average diameter of14.87 nm (C) were calculated based on nitrogen isotherm of adsorption and desorption, and the fractaldimension was calculated using two types of model (FHH and Neimark). The results show that comparedwith aliphatic organic macromolecule, polycarboxylic organic macromolecule is apt to form mesoporewith smaller pore diameter, which contribute more to the total pore volume and have little effect on largepore. Aliphatic organic macromolecule contributes more to form mesopore with larger pore diameter andhave little effect on smaller pore. Two kinds of organic macromolecule have effects on surface fractalcharacteristics, and the sequence of fractal dimension is A > C > B, which shows that the roughness orirregular degree of C–S–H becomes weak gradually, surface of samples becomes smooth. Differencebetween the surface fractal dimension of samples A and C derived from FHH and Neimark equation isnot obvious compared to sample B.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Pore structure of concrete has important implications on deter-mining the mechanical and transmission characteristics of it [1–3].Specifically, pore structure parameters including the total porosity,pore size distribution and average pore size are progressivelyemployed to evaluate microstructure, it strongly affect permeabil-ity, frost resistance, carbonation resistance and physical strength ofconcrete [4].

The concept of Fractal dimension, proposed by the Mandelbrot,was widely used to describe the irregular disorganized behaviorand phenomenon [5]. Later, it was used for research onnon-uniform surface structure of solid materials so as to providenew tools and methods for materials analysis [6]. According tofractal theory, the surface of the most solid materials has fractalcharacteristics at molecular scale, which means irregular degreeor defects of the surface are similar at different spatial scales [7].Fractal surface can be described by fractal dimension (between 2and 3), which reflects irregular or rough degree of the surface.When the value equals 2, it indicates that the surface is regularand smooth, if the value is nearly 3, it indicates that the surfaceis the irregular and rough.

0

50

100

150

200

250

300

350

0.0 0.2 0.4 0.6 0.8 1.0

Relative pressure p/p 0

Vol

ume

(cm

3 /g)

A B C

Fig. 1. N2 adsorption–desorption isotherms of samples A, B and C.

88 P. Duan et al. / Construction and Building Materials 44 (2013) 87–91

The fractal dimension can be determined by advanced testingmethods, such as nitrogen (N2) adsorption, small angle X-ray scat-tering (SAXS), nuclear magnetic resonance method (NMR) andmercury intrusion porosimetry (MIP), in which N2 adsorptionmethod is relatively widely used. N2 adsorption is based on nitro-gen isotherm of adsorption and desorption, specific surface area ofsample can be calculated using Brunauer–Emmett–Teller (BET) for-mula. Micropore analysis method (MP), Dubinin–Radushkevich(DR) and Horvath–Kawazoe (HK) method, which are suitable formicropore analysis (diameter less than 2 nm) as well as Barret–Joyner–Hallenda (BJH), which is suitable for mesopore analysis[8] (diameter is between 2 and 50 nm) are employed to calculateother pore structure parameters. Fractal dimension is calculatedby Frenkel–Halsey–Hill (FHH) model and Neimark model.

The superplasticizers contain much smaller numbers of ionicgroups (weaker polyelectrolytes) and their spatial structures differdue to the presence of side chains. The novel superplasticizers arederivatives of acrylic, methacrylic and maleic acids like polycarb-oxylates (PCs), acrylic acid and acrylic ester copolymers (CAE),cross-linked acrylic polymers (CLAPs) and polyacrylic esters (PAEs).The functional mechanism of the ‘‘new-generation’’ superplasticiz-ers is based on both the electrostatic repulsion of electric charges– which appear on the surfaces of cement particles due to superp-lasticizer adsorption – and a steric hindrance effect which resultsfrom the presence of long poly (oxyethylene) side chains in the plas-ticizer’s structure. Generally speaking, a superplasticizer’s func-tional efficiency is believed to increase with the length andnumber of oxyethylated side chains.

Superplasticizers are widely used in different industrial fields toimprove the rheological properties of particle suspensions. Espe-cially in cement application, their addition allows a reduction ofthe water-to-cement (w/c) ratio, thus strongly increasing theworkability of the fresh mixtures and the performances of thehardened pastes, mortars or concretes. Despite their widespreadutilization, these polymers are currently still the subject of manystudies, because details about their working principles lack of a fullunderstanding.

Calcium Silicate Hydrates (C–S–H) is the main products of ce-ment hydration. In recent years, with the concrete admixtureswidely used, the influence of different organic component (organicmacromolecular or polymers) on composition and structure of cal-cium silicate hydrate has important implications. It will providetheoretical guidance for mechanism and effect of admixture onproperties of concrete. In this study, pore structure parameters ofpure C–S–H, polycarboxylic organic macromolecular and aliphaticorganic macromolecular doped C–S–H were calculated using Bru-nauer–Emmett–Teller (BET) formula as well as Barret–Joyner–Hallenda (BJH) method based on nitrogen isotherm of adsorptionand desorption, and the fractal dimension was calculated usingFrenkel–Halsey–Hill (FHH) model and Neimark model. Meanwhile,effects of organic macromolecules on pore structure and surfacefractal characteristics of C–S–H were discussed.

2. Experimental

2.1. Materials

Calcium nitrate, Ca(NO3)2�4H2O (Analytical reagent), sodium silicate, Na2SiO3-

�9H2O (nNa2 O:SiO2 ¼ 1) and sodium hydroxide, NaOH (Analytical reagent) were usedto synthesize pure C–S–H. Superplasticizer included polycarboxylates and aliphat-ics, and solid content were 27% and 30% respectively. Deionized water was alsoused in the synthesis process.

2.2. Sample preparation

A certain amount of Na2SiO3�9H2O (189.33 g) dissolved in deionized water,mixed with 1 mol/L Ca(NO3)2�4H2O, the solid precipitate was collected by filtrationand was characteristics using X-ray photoelectron spectroscopy analysis (XPS) until

calcium/silicon (C/S) molar ratio = 1.5. Polycarboxylic organic macromolecularsuperplasticizer (15 g/L) and aliphatic organic macromolecular superplasticizer(15 g/L) were used to synthesize polycarboxylic organic macromolecular dopedC–S–H and aliphatic organic macromolecular doped C–S–H, respectively. Valuesof pH should always be determined during the synthesis process and adjusted by1 mol/L NaOH solution to make the pH above 13.0 so as to maintain mole ratio ofcalcium/silicon (C/S) = 1.5 [9].

The mixed solution mentioned above was stirred continuously at 70 �C for7 days in 3-neck flask under nitrogen gas flow, after which, the solid precipitatewas collected by filtration and rinsed repeatedly using deionized water to eliminateNa+, NO�3 and residual organic components. Rinsed repeatedly with anhydrous alco-hol (Analytical reagent), the samples were finally dried at 60 �C in the vacuum oven.The pure C–S–H, polycarboxylic organic macromolecular and aliphatic organic mac-romolecular doped C–S–H are namely A, B and C, respectively.

2.3. Testing procedure

Nitrogen isotherms of adsorption and desorption, specific surface and porestructure of samples A, B and C were obtained at 77.35 K by CHEMBET3000 adsorp-tion instrument produced by QUANTACHROME company, US.

3. Results and discussion

3.1. Nitrogen adsorption isotherms

The adsorption–desorption isotherms of samples A, B and C aregiven in Fig. 1. As shown in the figure the adsorption–desorptionisotherms of all tested samples presented ‘‘magnetic hysteresis’’caused by capillary pore condensation. It indicates that there existconnected mesopore and micropore in the three types of C–S–H.The amount adsorbed sample A in relative pressure range is smal-ler than that of samples B and C and its ‘‘magnetic hysteresis’’ phe-nomenon is unobvious when compared with samples B and C. Theresults above stem from the differences between pore structure ofsamples A, B and C, which indicate that there exists reaction be-tween polycarboxylic organic macromolecular, aliphatic organicmacromolecular and pure C–S–H and it may change the pore struc-ture of C–S–H [10].

In the range of relatively medium pressure (0.4 < p/p0 < 0.9), the‘‘Magnetic hysteresis’’ phenomenon of sample B is relatively moreremarkable when compared with samples A and C, and the amountadsorbed of sample B is higher than that of samples A and C.Around the saturated vapor pressure, adsorption curve of sampleC rises rapidly, and the amount adsorbed is higher than that ofsample B. The results show that in the range of relatively mediumpressure, mesopore with smaller pore diameter of sample B con-tributes more to the amount adsorbed than samples A and C. How-ever, in the range of relatively high pressure, mesopore with largerpore diameter of sample C contributes more significant to the

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 5 10 15 20 25 30 35 40 45

Pore diameter (nm)

Cum

ulat

ive

pore

vol

ume

(cm

3/g

)

ABC

Fig. 3. Cumulative pore volume of C–S–H.

0.000

0.020

0.040

0.060

0.080

0.100

2 3 4 5 6 7 8 9 12 17 26 33 38 44

Pore diameter (nm)

dV/d

D (c

m3 •g-1

•nm

)

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

B A C

Fig. 4. Pore size distributions of C–S–H.

5.0

5.5

6.0

6.5

ABC

P. Duan et al. / Construction and Building Materials 44 (2013) 87–91 89

amount adsorbed than sample B. The changes could be related topore size distribution of samples, which indicate that the two typesof organic macromolecular causes different effects on pore struc-ture of C–S–H. In the range of relatively low pressure (0.05 < p/p0 < 0.15), in which micropore filling effect reflects specific surfacearea, the amount adsorbed (specific surface area) of the sample isin the following sequence: sample B > sample C > sample A. Theamount adsorbed is related to existing status of organic macromol-ecules in the layer structure of C–S–H.

3.2. Specific surface area by BET

The result provided in Fig. 2 presents BET curve of samples A, Band C, specific surface area aBET and adsorption constants cBET canbe calculated by slope and intercept of these straight line graphs (Vmeans volume of the adsorbed nitrogen). As shown in Table 1,polycarboxylic organic macromolecular and aliphatic organic mac-romolecular exhibit significant effects on specific surface area of C–S–H, and the specific surface area increases up to 2 and 1.5 timesthe original value, respectively, and the conclusions are consistentwith results of adsorption–desorption isotherms. cBET can also re-flect adsorption characteristics of samples to certain extent. LargercBET indicates that it will be easier and faster to form a thinneradsorption layer on surface of samples [11], vice versa.

3.3. Pore structure by BJH

The result provided in Fig. 3 shows that the cumulative porevolume of C–S–H with organic macromolecules increases obvi-ously. In the range of pore diameters < 10 nm, the cumulative porevolume of sample B is larger than that of sample C and curve of Brises sharply, however, the cumulative pore volume of sample Crises gently and the trend is nearly the same with sample A. Theresults indicate that the content of mesopore with smaller porediameter in polycarboxylic organic macromolecule doped C–S–Hare higher than that of samples A and C and aliphatic organic mac-romolecule causes little effect on pore structure in the relative sizerange. Although the cumulative pore volume of sample B is higher

Table 1Pore structure characteristics of samples.

Sample aBET (m2/g) cBET Total volume (cm3/g) Average diameter(nm)

A 43.67 191.64 0.1136 9.75B 139.86 47.21 0.3490 9.81C 114.96 118.66 0.4201 14.87

0

5

10

15

20

25

0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Relative pressure p/p0

1/[V

(p0 /

p-1)

]

A B C

Fig. 2. Multipoint BET graphs of three types of C–S–H.

2.0

2.5

3.0

3.5

4.0

4.5

-4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5

ln (-ln p/p0)

ln V

Fig. 5. Fractal analysis by FHH equation for C–S–H.

than that of sample C with increasing pore diameter, its curve ris-ing trend becomes slow and parallels with sample A in the end. Thecumulative pore volume of sample C rises significantly and isnearly the same as the one of sample B in the end. In the rangeof pore diameter > 36 nm, accumulative pore volume of sample Cis larger than that of sample B, which indicates that aliphatic or-ganic macromolecule doped C–S–H has a higher ratio of mesoporewith smaller pore diameter, however, polycarboxylic organic mac-romolecule causes little effect on pore structure in the relative sizerange [12].

The results provided in Fig. 4 and Table 1 demonstrate the dif-ferences between the effects of the two kinds of organic macromo-lecular on pore structure of C–S–H. As shown in Fig. 5 sample B hashigher ratio of mesopore with smaller pore diameter (<10 nm)compared to samples A and C. Sample C has a higher ratio of mes-opore with larger pore diameter (15–40 nm) compared to samples

0.5

1.5

2.5

3.5

4.5

5.5

6.5

-1.5 -0.5 0.5 1.5 2.5 3.5 4.5

ln r

ln S

ABC

90 P. Duan et al. / Construction and Building Materials 44 (2013) 87–91

A and B. Sample B has the same ratio of mesopore with larger porediameter (>20 nm) with sample A, while sample C has the samepore size distribution of mesopore with smaller pore diameter(<10 nm) with sample A.

As shown in Table 1, the total pore volume of polycarboxylic or-ganic macromolecule doped C–S–H increases to three times theoriginal value, however, the average pore diameter increases onlyby 0.06 nm; the total pore volume of aliphatic organic macromol-ecule doped C–S–H increases to approximately four times the ori-ginal value, correspondingly the average pore diameter increasesby 5 nm. Therefore, compared with the aliphatic organic macro-molecule, polycarboxylic organic macromolecule is apt to formmesopore with smaller pore diameter, which is the main contribu-tor for the total pore volume and have little effects on average porediameter. Compared to polycarboxylic organic macromolecule, ali-phatic organic macromolecule is apt to form mesopore with largerpore diameter, which is the main contributor for total pore volumeand average pore diameter. The above conclusions validate resultsfrom the isothermal lines.

The difference between effects of the two types of organic mac-romolecules on pore structure of C–S–H can be related to the struc-ture. Polycarboxylates superplasticizer contains sulfonic acid, andcarboxyl functional group as well as methyl and long chain alkylside chain, however, aliphatics superplasticizer contains only sul-fonic acid functional group. Therefore, the reactions between poly-carboxylates superplasticizer and C–S–H occur intensely, and thelonger side chain make it easy to form numerous small pore in lay-ered products, and the aliphatics superplasticizer contributes moreto form larger mesopore. This may explain relatively higher BETspecific surface area, smaller total pore volume and average porediameter of sample B compared with sample C [13].

3.4. Surface fractal dimension from FHH model

The FHH equation was derived on the basis of the adsorptionpotential energy on the solid surface by Frenkle, Halsey and Hillwas not homogeneous.

ln V ¼ C þ h lnð� ln p=p0Þ ð1Þ

where V is the amount adsorbed; C is a constant, and a ordinateintercept for curve lnV vs ln(�lnp/p0); h is the slope; after pressure,p and p0 are the equilibrium and saturated pressure of nitrogen,respectively, p/p0 is the relative pressure.

When van der Waals is the main bonding forces between solidsurface and absorbent film [14]:

h ¼ ðD� 3Þ=3 ð2Þ

where D is the surface fractal dimension, 3 > D P 2, 0 > h P �1/3.When the surface tension between the liquid and the gas is the

main bonding force between absorbent and adsorption:

h ¼ D� 3 ð3Þ

where 0 > h P �1.

3.5. Surface fractal dimension from Neimark model

Neimark developed another equation to calculate the surfacefractal dimension on the basis of thermodynamic:

ln S ¼ Aþ ð2� DÞ ln r ð4Þ

where S is the area of liquid–gas interface; A is a constant, ordinateintercept for curve ln S vs lnr; r is the average curvature radius ofthe interface, D is the surface fractal dimension:

S ¼ RTc

Z Nmax

N� ln

pp0

� �dN ð5Þ

r ¼ 2cVm

RTð� ln p=p0Þð6Þ

where Nmax is the biggest amount adsorbed; N equilibrium amountadsorbed; c surface tension; Vm molar volume; R universal gas con-stant; T temperature.

3.6. Surface fractal dimension in two models

The results provided in Figs. 5 and 6 (V amount adsorbed, rmean radius of the curvature, S area of the gas–liquid interface)present linear fitting curves according to FHH and Neimark equa-tion, respectively. Data points of three types of C–S–H demonstratelinear relationship, which means specific surface fractal dimension,therefore the surfaces of samples have fractal features. As shown inFigs. 5 and 6, in the range of relative pressure, the data points ofsamples of B and C are discrete than sample A (see linear correla-tion coefficient R in Table 2). Slopes of fitting line of samples B andC are also different with that of sample A, which shows that organ-ic macromolecules influence surface of C–S–H to some extent.

In Table 2 (h slopes of lnV vs ln(�lnp/p0) in Fig. 5 and lnS vs lnrin Fig. 6; D1, D2, D3 fractal dimensions of samples by differentmethods; R correlative coefficient of fitting; p/p0 relative pressure;p, p0 equilibrium and saturation pressures of the nitrogen, respec-tively), slopes of the straight line are less than �l/3 when usingFHH equation to calculate the fractal dimension, it indicates thatthe surface tension is the main bonding force between C–S–Hand liquid nitrogen, which is consistent with the ‘‘magnetic hyster-esis’’ phenomenon of adsorption–desorption isotherms [15]. Inaddition, the fractal dimension derived from Formula (2) is differ-ent from that of Neimark equation, but the fractal dimension de-rived from Formula (3) is nearly the same as that of the Neimarkequation. Formula (3) is more suitable for calculating fractaldimension.

The sequence of fractal dimension derived from Formula (3) ofthe FHH equation is A > C > B, which shows that the roughness orirregular degree of C–S–H is gradually strengthened in the se-quence B, C, A. The surface fractal dimension could also be relatedto pore structure, the higher fractal dimension demonstrates coex-istence of a large number of pore with different diameter. The frac-tal dimension of B is smaller than that of C, which shows that thepore structure of sample B is homogeneous to that of sample C.This can also be validated in Fig. 4, the dominant part of the porestructure of sample B is mesopore (<10 nm), and the dominant partof sample C is mesopore (<10 nm and > 20 nm). The experimentshows that the fractal dimensions do not have a direct relationshipwith other pore structure parameters (such as pore volume oraverage diameter).

The difference between the surface fractal dimension of sam-ples A and C derived from FHH and Neimark equation is not obvi-ous compared to sample B (D2 from FHH is smaller by 0.231 than

Fig. 6. Fractal analysis by Neimark equation for C–S–H.

Table 2Surface fractal dimensions derived from FHH and Neimark equation.

Sample FHH equation Neimark equation

h D1 = 3 + 3�h D2 = 3 + h R h D3 = 2 � h R

A �0.380 1.860 2.620 0.996 �0.644 2.644 0.993B �0.567 1.299 2.433 0.978 �0.664 2.664 0.965C �0.394 1.818 2.606 0.987 �0.559 2.559 0.984

P. Duan et al. / Construction and Building Materials 44 (2013) 87–91 91

D3 from Neimark). The main reason is that the fractal dimensionderived from FHH equation is defined according to adsorption filmvolume or pore size distribution, however, fractal dimension de-rived from Neimark equation is defined according to specific sur-face area of adsorption film, which makes FHH equation moresensitive to pore size distribution, especially small pore [16].

4. Conclusions

From the previous results we can conclude that:

(1) Polycarboxylic organic macromolecule is apt to form meso-pore with smaller pore diameter, which contributes moreto total pore volume and have little effect on larger pore. Ali-phatic organic macromolecule contributes more to formmesopore with larger pore diameter and have little effecton smaller pore.

(2) Fractal dimension calculated according FHH equation indi-cates that the two kinds of organic macromolecule haveeffects on surface fractal characteristics, and the sequenceof fractal dimension is A > C > B, which shows that rough-ness or irregular degree of C–S–H becomes weak gradually,surface of samples becomes smooth.

(3) Difference between surface fractal dimension of samples Aand C derived from FHH and Neimark equation is unobviouscompared with sample B. The main reason is that fractaldimension derived from FHH equation is defined accordingto adsorption film volume or pore size distribution whilefractal dimension derived from Neimark equation is definedaccording to specific surface area of adsorption film, whichmakes FHH equation more sensitive to pore size distribu-tion, especially small pore. And mesopore becomes smallpore and occupy main proportion in sample B.

Acknowledgements

This research has been financially supported by the NationalFundamental Scientific Research Project (PR China), relevant to

‘‘Basic Research in Environmentally Friendly Concrete(2009CB623201)’’, the Youth Chenguang Project of Science andTechnology of Wuhan (Project 201150431086) and the NaturalScience Foundation of Hubei Province (20101j0164).

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