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

Construction and Building Materials 44 (2013) 87–91

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Influence of superplasticizer on composition and pore structure of C–S–H 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, China Engineering Research Center of Green Building Materials, Wuhan University of Technology, Wuhan 430070, China c Center of Materials Research and Analysis, Wuhan University of Technology, Wuhan 430070, China b

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 2013 Received in revised form 12 March 2013 Accepted 15 March 2013 Available online 9 April 2013 Keywords: Pore structure Calcium silicate hydrate Superplasticizer Adsorption Specific 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 different organic component (organic macromolecular or polymers) on composition and structure of calcium silicate hydrate has important implications. Pore structure parameters of pure C–S–H with specific surface area of 43.67 m2/g and average diameter of 9.75 nm (A), polycarboxylic organic macromolecular doped C–S–H with specific surface area of 139.86 m2/g and average diameter of 9.81 nm (B) and aliphatic organic macromolecular doped C–S–H with specific surface area of 114.96 m2/g and average diameter of 14.87 nm (C) were calculated based on nitrogen isotherm of adsorption and desorption, and the fractal dimension was calculated using two types of model (FHH and Neimark). The results show that compared with aliphatic organic macromolecule, polycarboxylic organic macromolecule is apt to form mesopore with smaller pore diameter, which contribute more to the total pore volume and have little effect on large pore. Aliphatic organic macromolecule contributes more to form mesopore with larger pore diameter and have little effect on smaller pore. Two kinds of organic macromolecule have effects on surface fractal characteristics, and the sequence of fractal dimension is A > C > B, which shows that the roughness or irregular degree of C–S–H becomes weak gradually, surface of samples becomes smooth. Difference between the surface fractal dimension of samples A and C derived from FHH and Neimark equation is not obvious compared to sample B. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction Pore structure of concrete has important implications on determining 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 progressively employed to evaluate microstructure, it strongly affect permeability, frost resistance, carbonation resistance and physical strength of concrete [4].

⇑ Corresponding author. Tel.: +86 15807165207; fax: +86 27 87210782. E-mail address: [email protected] (P. Duan).

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

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

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

2. Experimental 2.1. Materials Calcium nitrate, Ca(NO3)24H2O (Analytical reagent), sodium silicate, Na2SiO39H2O (nNa2 O:SiO2 ¼ 1) and sodium hydroxide, NaOH (Analytical reagent) were used to synthesize pure C–S–H. Superplasticizer included polycarboxylates and aliphatics, and solid content were 27% and 30% respectively. Deionized water was also used in the synthesis process. 2.2. Sample preparation A certain amount of Na2SiO39H2O (189.33 g) dissolved in deionized water, mixed with 1 mol/L Ca(NO3)24H2O, the solid precipitate was collected by filtration and was characteristics using X-ray photoelectron spectroscopy analysis (XPS) until

calcium/silicon (C/S) molar ratio = 1.5. Polycarboxylic organic macromolecular superplasticizer (15 g/L) and aliphatic organic macromolecular superplasticizer (15 g/L) were used to synthesize polycarboxylic organic macromolecular doped C–S–H and aliphatic organic macromolecular doped C–S–H, respectively. Values of pH should always be determined during the synthesis process and adjusted by 1 mol/L NaOH solution to make the pH above 13.0 so as to maintain mole ratio of calcium/silicon (C/S) = 1.5 [9]. The mixed solution mentioned above was stirred continuously at 70 °C for 7 days in 3-neck flask under nitrogen gas flow, after which, the solid precipitate was collected by filtration and rinsed repeatedly using deionized water to eliminate Na+, NO 3 and residual organic components. Rinsed repeatedly with anhydrous alcohol (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 macromolecular doped C–S–H are namely A, B and C, respectively.

2.3. Testing procedure Nitrogen isotherms of adsorption and desorption, specific surface and pore structure of samples A, B and C were obtained at 77.35 K by CHEMBET3000 adsorption 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 are given in Fig. 1. As shown in the figure the adsorption–desorption isotherms of all tested samples presented ‘‘magnetic hysteresis’’ caused by capillary pore condensation. It indicates that there exist connected mesopore and micropore in the three types of C–S–H. The amount adsorbed sample A in relative pressure range is smaller than that of samples B and C and its ‘‘magnetic hysteresis’’ phenomenon is unobvious when compared with samples B and C. The results above stem from the differences between pore structure of samples A, B and C, which indicate that there exists reaction between polycarboxylic organic macromolecular, aliphatic organic macromolecular and pure C–S–H and it may change the pore structure 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 more remarkable when compared with samples A and C, and the amount adsorbed of sample B is higher than that of samples A and C. Around the saturated vapor pressure, adsorption curve of sample C rises rapidly, and the amount adsorbed is higher than that of sample B. The results show that in the range of relatively medium pressure, mesopore with smaller pore diameter of sample B contributes more to the amount adsorbed than samples A and C. However, in the range of relatively high pressure, mesopore with larger pore diameter of sample C contributes more significant to the

350 300 250 A

3

The fractal dimension can be determined by advanced testing methods, such as nitrogen (N2) adsorption, small angle X-ray scattering (SAXS), nuclear magnetic resonance method (NMR) and mercury intrusion porosimetry (MIP), in which N2 adsorption method is relatively widely used. N2 adsorption is based on nitrogen isotherm of adsorption and desorption, specific surface area of sample can be calculated using Brunauer–Emmett–Teller (BET) formula. Micropore analysis method (MP), Dubinin–Radushkevich (DR) and Horvath–Kawazoe (HK) method, which are suitable for micropore 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 calculate other pore structure parameters. Fractal dimension is calculated by Frenkel–Halsey–Hill (FHH) model and Neimark model. The superplasticizers contain much smaller numbers of ionic groups (weaker polyelectrolytes) and their spatial structures differ due to the presence of side chains. The novel superplasticizers are derivatives of acrylic, methacrylic and maleic acids like polycarboxylates (PCs), acrylic acid and acrylic ester copolymers (CAE), cross-linked acrylic polymers (CLAPs) and polyacrylic esters (PAEs). The functional mechanism of the ‘‘new-generation’’ superplasticizers is based on both the electrostatic repulsion of electric charges – which appear on the surfaces of cement particles due to superplasticizer adsorption – and a steric hindrance effect which results from the presence of long poly (oxyethylene) side chains in the plasticizer’s structure. Generally speaking, a superplasticizer’s functional efficiency is believed to increase with the length and number of oxyethylated side chains. Superplasticizers are widely used in different industrial fields to improve the rheological properties of particle suspensions. Especially in cement application, their addition allows a reduction of the water-to-cement (w/c) ratio, thus strongly increasing the workability of the fresh mixtures and the performances of the hardened pastes, mortars or concretes. Despite their widespread utilization, these polymers are currently still the subject of many studies, because details about their working principles lack of a full understanding. Calcium Silicate Hydrates (C–S–H) is the main products of cement hydration. In recent years, with the concrete admixtures widely used, the influence of different organic component (organic macromolecular or polymers) on composition and structure of calcium silicate hydrate has important implications. It will provide theoretical guidance for mechanism and effect of admixture on properties of concrete. In this study, pore structure parameters of pure C–S–H, polycarboxylic organic macromolecular and aliphatic organic macromolecular doped C–S–H were calculated using Brunauer–Emmett–Teller (BET) formula as well as Barret–Joyner– Hallenda (BJH) method based on nitrogen isotherm of adsorption and desorption, and the fractal dimension was calculated using Frenkel–Halsey–Hill (FHH) model and Neimark model. Meanwhile, effects of organic macromolecules on pore structure and surface fractal characteristics of C–S–H were discussed.

Volume (cm /g)

88

B

C

200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

Relative pressure p/p 0 Fig. 1. N2 adsorption–desorption isotherms of samples A, B and C.

1.0

89

3.3. Pore structure by BJH

0.3 0.25 A B C

0.2 0.15 0.1 0.05

3

0 0

5

10

15

20

25

30

35

40

45

Pore diameter (nm) Fig. 3. Cumulative pore volume of C–S–H.

0.100

0.014 0.012

0.080

B

A

C

0.010

0.060

0.008

0.040

0.006 0.004

0.020

0.002

0.000

The result provided in Fig. 3 shows that the cumulative pore volume of C–S–H with organic macromolecules increases obviously. In the range of pore diameters < 10 nm, the cumulative pore volume of sample B is larger than that of sample C and curve of B rises sharply, however, the cumulative pore volume of sample C rises gently and the trend is nearly the same with sample A. The results indicate that the content of mesopore with smaller pore diameter in polycarboxylic organic macromolecule doped C–S–H are higher than that of samples A and C and aliphatic organic macromolecule causes little effect on pore structure in the relative size range. Although the cumulative pore volume of sample B is higher

0.000 2

3

4

5

6

7

8

9 12 17 26 33 38 44

Pore diameter (nm) Fig. 4. Pore size distributions of C–S–H.

6.5 6.0

A B C

5.5 5.0

ln V

25

0.35

-1

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

3

3.2. Specific surface area by BET

0.4

dV/dD (cm •g •nm)

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

Cumulative pore volume (cm /g)

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

4.5 4.0 3.5 3.0

20

1/[V(p 0 /p-1)]

A

B

2.5

C

2.0 -4.5

15

-3.5

-2.5

-1.5

-0.5

0.5

1.5

ln (-ln p/p 0 ) 10 Fig. 5. Fractal analysis by FHH equation for C–S–H.

5

0 0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Relative pressure p/p 0 Fig. 2. Multipoint BET graphs of three types of C–S–H.

Table 1 Pore structure characteristics of samples. Sample

aBET (m2/g)

cBET

Total volume (cm3/g)

Average diameter (nm)

A B C

43.67 139.86 114.96

191.64 47.21 118.66

0.1136 0.3490 0.4201

9.75 9.81 14.87

than that of sample C with increasing pore diameter, its curve rising trend becomes slow and parallels with sample A in the end. The cumulative pore volume of sample C rises significantly and is nearly the same as the one of sample B in the end. In the range of pore diameter > 36 nm, accumulative pore volume of sample C is larger than that of sample B, which indicates that aliphatic organic macromolecule doped C–S–H has a higher ratio of mesopore with smaller pore diameter, however, polycarboxylic organic macromolecule causes little effect on pore structure in the relative size range [12]. The results provided in Fig. 4 and Table 1 demonstrate the differences between the effects of the two kinds of organic macromolecular on pore structure of C–S–H. As shown in Fig. 5 sample B has higher ratio of mesopore with smaller pore diameter (<10 nm) compared to samples A and C. Sample C has a higher ratio of mesopore with larger pore diameter (15–40 nm) compared to samples

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 pore diameter (>20 nm) with sample A, while sample C has the same pore size distribution of mesopore with smaller pore diameter (<10 nm) with sample A. As shown in Table 1, the total pore volume of polycarboxylic organic macromolecule doped C–S–H increases to three times the original value, however, the average pore diameter increases only by 0.06 nm; the total pore volume of aliphatic organic macromolecule doped C–S–H increases to approximately four times the original value, correspondingly the average pore diameter increases by 5 nm. Therefore, compared with the aliphatic organic macromolecule, polycarboxylic organic macromolecule is apt to form mesopore with smaller pore diameter, which is the main contributor for the total pore volume and have little effects on average pore diameter. Compared to polycarboxylic organic macromolecule, aliphatic organic macromolecule is apt to form mesopore with larger pore diameter, which is the main contributor for total pore volume and average pore diameter. The above conclusions validate results from the isothermal lines. The difference between effects of the two types of organic macromolecules on pore structure of C–S–H can be related to the structure. Polycarboxylates superplasticizer contains sulfonic acid, and carboxyl functional group as well as methyl and long chain alkyl side chain, however, aliphatics superplasticizer contains only sulfonic acid functional group. Therefore, the reactions between polycarboxylates superplasticizer and C–S–H occur intensely, and the longer side chain make it easy to form numerous small pore in layered products, and the aliphatics superplasticizer contributes more to form larger mesopore. This may explain relatively higher BET specific surface area, smaller total pore volume and average pore diameter 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 adsorption potential energy on the solid surface by Frenkle, Halsey and Hill was not homogeneous.

ln V ¼ C þ h lnð ln p=p0 Þ

ð1Þ

where V is the amount adsorbed; C is a constant, and a ordinate intercept for curve lnV vs ln(ln p/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 solid surface 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:

h¼D3

r¼

2cV m RTð ln p=p0 Þ

ð6Þ

where Nmax is the biggest amount adsorbed; N equilibrium amount adsorbed; c surface tension; Vm molar volume; R universal gas constant; T temperature. 3.6. Surface fractal dimension in two models The results provided in Figs. 5 and 6 (V amount adsorbed, r mean radius of the curvature, S area of the gas–liquid interface) present linear fitting curves according to FHH and Neimark equation, respectively. Data points of three types of C–S–H demonstrate linear relationship, which means specific surface fractal dimension, therefore the surfaces of samples have fractal features. As shown in Figs. 5 and 6, in the range of relative pressure, the data points of samples of B and C are discrete than sample A (see linear correlation coefficient R in Table 2). Slopes of fitting line of samples B and C are also different with that of sample A, which shows that organic macromolecules influence surface of C–S–H to some extent. In Table 2 (h slopes of lnV vs ln(ln p/p0) in Fig. 5 and lnS vs ln r in Fig. 6; D1, D2, D3 fractal dimensions of samples by different methods; R correlative coefficient of fitting; p/p0 relative pressure; p, p0 equilibrium and saturation pressures of the nitrogen, respectively), slopes of the straight line are less than l/3 when using FHH equation to calculate the fractal dimension, it indicates that the surface tension is the main bonding force between C–S–H and liquid nitrogen, which is consistent with the ‘‘magnetic hysteresis’’ phenomenon of adsorption–desorption isotherms [15]. In addition, the fractal dimension derived from Formula (2) is different from that of Neimark equation, but the fractal dimension derived from Formula (3) is nearly the same as that of the Neimark equation. Formula (3) is more suitable for calculating fractal dimension. The sequence of fractal dimension derived from Formula (3) of the FHH equation is A > C > B, which shows that the roughness or irregular degree of C–S–H is gradually strengthened in the sequence B, C, A. The surface fractal dimension could also be related to pore structure, the higher fractal dimension demonstrates coexistence of a large number of pore with different diameter. The fractal dimension of B is smaller than that of C, which shows that the pore structure of sample B is homogeneous to that of sample C. This can also be validated in Fig. 4, the dominant part of the pore structure of sample B is mesopore (<10 nm), and the dominant part of sample C is mesopore (<10 nm and > 20 nm). The experiment shows that the fractal dimensions do not have a direct relationship with other pore structure parameters (such as pore volume or average diameter). The difference between the surface fractal dimension of samples A and C derived from FHH and Neimark equation is not obvious compared to sample B (D2 from FHH is smaller by 0.231 than

ð3Þ 6.5

where 0 > h P 1.

A B C

5.5

3.5. Surface fractal dimension from Neimark model

ln S ¼ A þ ð2  DÞ ln r

ln S

4.5

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

3.5

ð4Þ

2.5

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

1.5

  RT Nmax p dN S¼  ln p0 c N Z

ð5Þ

0.5 -1.5

-0.5

0.5

1.5

2.5

3.5

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

4.5

91

P. Duan et al. / Construction and Building Materials 44 (2013) 87–91 Table 2 Surface fractal dimensions derived from FHH and Neimark equation. Sample

A B C

FHH equation

Neimark equation

h

D1 = 3 + 3h

D2 = 3 + h

R

h

D3 = 2  h

R

0.380 0.567 0.394

1.860 1.299 1.818

2.620 2.433 2.606

0.996 0.978 0.987

0.644 0.664 0.559

2.644 2.664 2.559

0.993 0.965 0.984

D3 from Neimark). The main reason is that the fractal dimension derived from FHH equation is defined according to adsorption film volume or pore size distribution, however, fractal dimension derived from Neimark equation is defined according to specific surface area of adsorption film, which makes FHH equation more sensitive to pore size distribution, especially small pore [16].

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

4. Conclusions

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From the previous results we can conclude that: (1) Polycarboxylic organic macromolecule is apt to form mesopore with smaller pore diameter, which contributes more to total pore volume and have little effect on larger pore. Aliphatic organic macromolecule contributes more to form mesopore with larger pore diameter and have little effect on smaller pore. (2) Fractal dimension calculated according FHH equation indicates that the two kinds of organic macromolecule have effects on surface fractal characteristics, and the sequence of fractal dimension is A > C > B, which shows that roughness or irregular degree of C–S–H becomes weak gradually, surface of samples becomes smooth. (3) Difference between surface fractal dimension of samples A and C derived from FHH and Neimark equation is unobvious compared with sample B. The main reason is that fractal dimension derived from FHH equation is defined according to adsorption film volume or pore size distribution while fractal dimension derived from Neimark equation is defined according to specific surface area of adsorption film, which makes FHH equation more sensitive to pore size distribution, especially small pore. And mesopore becomes small pore and occupy main proportion in sample B. Acknowledgements This research has been financially supported by the National Fundamental Scientific Research Project (PR China), relevant to

References