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Effect of polyacrylic acid emulsion on fluidity of cement paste
Colloids and Surfaces A 535 (2017) 139–148
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Research paper
Effect of polyacrylic acid emulsion on fluidity of cement paste a,⁎
b
b,c
c,⁎
a
MARK b
Yanfei Guo , Baoguo Ma , Zhenzhen Zhi , Hongbo Tan , Muyu Liu , Shouwei Jian , Yulin Guob a b c
Hubei Key Laboratory of Roadway Bridge and Structure Engineering, Wuhan University of Technology, Wuhan 430070, PR China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, PR China
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyacrylic acid emulsion Fluidity Electrostatic effect Dispersion
Polymer-modified cement-based materials (PMC) have been widely applied in civil construction and municipal projects. In order to obtain deeper insight into the workability of PMC, the effect of polyacrylic acid emulsion (PAE) on fluidity of cement paste was investigated in this study. The fluidity was assessed with mini slump, and the change of Ca2+ concentration in pore solution was tested with inductive coupled plasma emission spectrometer. Zeta potential and adsorption behavior were characterized to reveal the mechanism behind the fluidity results. The results show that the effect of PAE on fluidity depends on the added dosage: dosage less than 5.0% reduces the fluidity, while the opposite is true with dosage more than 5%, which can also be indicated from the change of Ca2+ concentration in pore solution. Conductivity and X-ray photoelectron spectrometer (XPS) results demonstrate the chemical adsorption of PAE on the surface of cement particles. Dynamic light scattering (DLS) results strongly prove that the surface group of the PAE used is SO3− rather than COO−. Finally, the dispersion model was proposed to illustrate mechanism behind: with a small amount of PAE, the decline in zeta potential caused by adsorption of negatively charged PAE and the agglomeration of cement particles caused by electrostatic attraction of PAE are responsible for the reduced fluidity; while with a great amount of PAE, the negatively increased zeta potential, lubrication effect provided by non-adsorbed PAE, and filling effect of PAE are the main
⁎
Corresponding author. E-mail address: [email protected] (H. Tan).
http://dx.doi.org/10.1016/j.colsurfa.2017.09.039 Received 21 July 2017; Received in revised form 19 September 2017; Accepted 21 September 2017 Available online 22 September 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
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reason for the increased fluidity. Such results would give deeper understanding about the effect of polymer emulsion on rheology of cement paste.
1. Introduction
results. Zeta potential was measured to characterize the interface performance of cement particles. Adsorption behavior was discussed with total organic carbon analyzer (TOC), X-ray photoelectron spectrometer (XPS), dynamic light scattering (DLS) and conductivity; and then the surface structure of PAE was deduced. Finally, a dispersion model was proposed to illustrate the mechanism behind. Such results would show deeper understanding about the interaction between polymers and cement particles.
In recent years, polymer-modified cement-based materials (PMC) have been widely applied in civil construction and municipal projects, and increasingly attracting the attention in research area and industrial area in civil engineering [1]. The main reason for this is due to its excellent performance, such as impermeability, workability, adhesive strength, and toughness [2–4]. Many attempts have been made in the literatures to clarify the mechanism behind the improvement of polymers in performance of cement-based material [5]. Three-step model, proposed by Ohama, has been accepted as the most popular one in these reported studies [6]. As illustrated in this model, first of all, polymer particles would be dispersed in cement suspension; and then in the process of cement hydration, a continuous close-packed layer, mainly composed of polymer particles, should be formed on the surface of cement particles; finally, all these layers would coalesce into a continuous film, and more importance is that this film should be uninterrupted in order to interpenetrate throughout the cement hydrates. It is inferred that the first step, namely dispersing polymer particles in cement suspension, is of great importance, and it would further affect the following two steps. These results would help us understand the interaction between polymer particles and cement particles in PMC [7]. In order to achieve the aim of the first step in Ohama model, these nano polymer particles should be efficiently dispersed. However, cement hydrates may exert negative effect on dispersing the polymer particles. As reported, polyacrylate latex could chemically react with Ca2+ in cement hydration process, and this interaction could provide crosslinking, namely the combination between Ca2+ and COO−, to enhance the basic performance of PMC [8]. Even though this phenomenon was found in hardened paste, cement hydrates formed at the very beginning may negatively affect the dispersion process of polymer particles [9,10], with negative influence on the achievement of the first step in Ohama model. It is worth noting that polymer particles can quickly adsorb onto the surface of the cement particles, and the adsorption may significantly affect the rheology of the cement paste [11,12]. To be more precise, these polymer particles adsorbed can alter the structure of interface between liquid and these particles, thereby affecting the interface performance, such as zeta potential and surficial structure. The interface performance would significantly influence the rheology of the cement paste, and further determine the pumpability, self-compacting, and self-leveling [13], being similar to that in cement paste plasticized by superplasticizer [14]. Taking superplasticizer for example, superplasticizer adsorbed on the surface of cement particles can change zeta potential to improve the fluidity [15]. Actually, in real engineering practice, workability is one of the most important performances for cement-based materials, which determines the application in some sense. Therefore, to obtain deeper insight into the effect of polymer particles on rheological properties of the cementitious materials is of both scientific and practical importance. Polyacrylic acid emulsion (PAE) is one of the most popular polymers used for PMC. With addition of PAE, on the one hand, the carboxyl group would be combined with Ca2+ in pore solution and Ca2+ on the surface of cement particles; on the other hand, PAE would adsorb on the surface of cement particle. Both these processes most likely affect the dispersion of polymers, and also influence the rheology performance of PMC. In this study, the effect of PAE on fluidity of cement paste was investigated. The fluidity of the paste was assessed with mini slump. Ca2+ in pore solution was tested with inductive coupled plasma (ICP) emission as supplementary evidence to further clarify the fluidity
2. Experimental 2.1. Materials 2.1.1. Cement An ordinary Portland cement (P.I 42.5, supplied by China United Cement Co., Ltd., China), in according with Chinese standard GB1752007, was used in this study. The chemical compositions obtained with X-ray fluorescence spectrometer (Axios advanced) is shown in Table 1, and the basic information obtained from the company is shown in Table 2. 2.1.2. Polyacrylic acid emulsion Commercially available polyacrylic acid emulsion (PAE), a white and milky liquid, was used in this study. The physical properties of PAE are shown in Table 3. The pH value of PAE was tested with a pH meter. PAE (1.0 g/L) was obtained from the raw PAE, and then the particle size was characterized with Dynamic light scattering (DLS, Zetasizer Nano, made by Malvern instrument Ltd., UK). The solid content was tested in accordance with the Chinese standard GB 8077-2012. Additionally, raw PAE (0.5 g) was added into deionized water (49.5 g) and mixed, and the zeta potential was then tested with a zeta potential analyzer (Zetasizer Nano ZS, made by Malvern instrument Ltd., UK). PAE used in the experiment was marked as solid content. 2.2. Test methods 2.2.1. Fluidity of the cement paste PAE modified cement pastes were prepared with a water/cement ratio (w/c = 0.50) and addition of various dosages of PAE (0%, 2.5%, 5.0%, 7.5%, 10.0%, 20.0% of cement). The initial fluidity (within 5 min) was measured with a truncated cone mold (height: 60 mm; top diameter: 36 mm; bottom diameter: 60 mm) in accordance with the Chinese standard of GB/T 8077-2012. The truncated cone was filled with the paste on a glass plate, and then removed slowly. The maximum diameter of the spread sample and the maximum width perpendicular to that diameter were measured. The average of these two results was defined as the fluidity value. 2.2.2. Adsorption amount PAE (0–12.0 g/L), diluted by deionized water from raw PAE, was prepared in advance. Cement (1.0 g) was then mixed these emulsions (10.0 g) by magnetic stirrer. After stirred for 5 min, the mixture was separated by centrifugation at 3500 r/min for 5 min. The upper solution was prepared for the TOC measurement. Table 1 Chemical composition of cement (wt%).
140
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2 O
Na2O
Loss
21.402
5.277
3.017
61.917
2.459
2.689
0.758
0.056
1.913
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Table 2 Basic illustration of cement. C3S
C2S
C3A wt%
C4AF
CaSO4
Specific surface area m2/kg
49.61
23.93
8.87
9.17
4.2
350
Table 3 The physical properties of PAE. pH value
Particle size (nm)
Zeta potential (eV)
Solid content (%)
8.0
174.0
−51.4
52.5
Fig. 3. Zeta potential of the cement paste with PAE.
Fig. 1. Effect of PAE on fluidity of cement paste.
Fig. 4. Adsorption amount of PAE on cement particle.
Fig. 2. Effect of PAE on concentration of Ca2+.
Fig. 5. Ca2p XPS spectrum of cement and PAE modified cement particle surface.
Total Organic Carbon Analyzer (TOC, Multi N/C 2100, made in Jena, Germany) was used to test the carbon content of organics in upper solution. Measurement was generally repeated three times and the average was the result. The residual concentration of PAE was inferred from the results of TOC, and the adsorption amount (mg/g-cement) was calculated as follow:
2.2.3. Concentration of Ca2+ in pore solution PAE with difference in concentration (0–5.0%) was diluted with deionized water from raw PAE in advance. Cement (1.0 g) was then mixed with the emulsion (10.0 g) and stirred for 5 min. And then the suspension was processed as the same process with 2.2.2. The concentration of Ca2+ in upper solution was tested with inductive coupled plasma-optical emission spectrometer (ICP, Prodigy 7, made by LEEMAN LABS INC., USA). Measurement was generally repeated three times and the average was the result.
Adsorption amount = V (C0 − C)/m Where C0 is the initial concentration (g/L) of PAE before adsorption; C is the residual concentration (g/L) after adsorption; V is volume of the solution (L); m is the mass of the cement (g).
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Fig. 8. ATR-FTIR spectra of PAE. (a) TG-DSC (b) MS.
Table 5 ATR-FTIR characteristic bonds of PAE (cm−1). Polymer
eCH3
PAE
2958, 2956, 2869
a
C]O
eCH2e
eCeOeCe
1728
1450
1182,1157
a
ReSO3e
eCH3
1064
760, 700
b
Remark: a asymmetric stretching vibration. b Out-of-plane deformation vibration.
2.2.4. Surface performance of cement particle with PAE PAE (5.0%, 20.0%), diluted from raw PAE, was prepared in advance. Cement (1.0 g) was then mixed these emulsions (10.0 g) by magnetic stirrer. After stirred for 5 min, the mixture was separated by centrifugation at 3500 r/min for 5 min. The solid was dried in a vacuum drier at 60 °C. The dried solid was grinded by hand, and these powders with particle size less than 200-mesh were prepared for XPS measurement. The blank sample was prepared with cement (1.0 g) and deionized water (10.0 g) as the same process. The binding energy of Ca2p in powder samples was assessed with XPS (X-ray photoelectron spectroscopy, VG Multilab 2000X). Aluminum was used as an anode target (hν = 1486.6 eV); energy resolution was 0.10 eV. If chemical adsorption took place, new type of calcium-based bond would be formed on the surface of the cement particles, with obvious difference from the reference. Based on this analysis, the reaction between PAE and surficial Ca2+ can be indicated from XPS results as supplementary evidence to prove the chemical adsorption of PAE onto the surface of cement particles surface.
Fig. 6. Conductivity of PAE with addition of CH solution. (a) PAE 1.0%, (b) PAE 2.0%, (c) PAE 5.0%.
Table 4 Proportion between PAE and Ca2+ in the conductivity test. Content of PAE (%)
Initial conductivity (μS/cm)
Ca(OH)2 (g) (Inflection point)
Mass ratio of g-PAE/ g-CH (Inflection point)
0 1.0 2.0 5.0
0 103.5 196.6 456
– 0.008 0.0177 0.0535
– 187.5 169.5 140.0
2.2.5. Conductivity PAE with difference in concentration (1.0%, 2.0% and 5.0%) was diluted with deionized water from the raw PAE, respectively, and then the conductivity of these three diluted PAE (150.0 g) and deionized water (i.e. reference, 150.0 g) with the increasing dosage of calcium hydroxide solution (CH, 1.0 g/L) was carried out with an electrical conductivity meter (Seven Compact S230, made by Mettler Toledo, Switzerland). The measured liquid was stirred with magnetic stirrer at 350 rpm. The difference in conductivity between PAE and the reference (deionized water) could indicate the reaction between PAE and Ca2+. All operations were carried out at 25 °C. 2.2.6. Zeta potential and size distribution Zeta potential is generally used to measure the electrostatic charge on the surface of particles in suspension, and it can also characterize the stability of the suspension [16]. The variation of zeta potential also
Fig. 7. Schematic diagram of the structure of PAE.
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Fig. 9. TG-DSC-MS spectra of PAE.
Fig. 10. The complexation between RCOO− and Ca2+. Fig. 11. Reaction between COO− and Ca2+ in solution.
provides evidence to prove adsorption of charged polymers. The samples for the measurement of zeta potential were prepared as follows: PAE with difference in concentration (0–1.0%) was diluted by deionized water from raw PAE in advance. Cement (1.0 g) was then mixed with the diluted emulsions (10.0 g) and stirred for one minute, and then one gram of the suspension was added into 9.0 g deionized water and
stirred. The diluted suspension was tested with the instrument (Zetasizer Nano ZS, made by Malvern instrument Ltd., UK). The value of the zeta potential was computed from the average of three dependent measurements. Each test should be finished within 5 min. 143
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minerals and liquid would be one of the most important factors affecting this release. The agglomerated particles would reduce this contact area, and the agglomeration degree of particles would determine the release of Ca2+ in some sense. Based on this, better fluidity would result in greater contact area between minerals and water molecules, which would make the release of Ca2+ easier. As a consequence, the change trend of Ca2+, in consistency with the fluidity, offers further evidence to confirm the effect of PAE on fluidity of cement paste. As reported, the fluidity of cement paste is closely related to the surface performance of particles [20]. These polymers added can probably alter the interface between particles and liquid. In cement paste plasticized by superplasticizer, adsorption of polymer can change the surface performance of the particles to disperse the agglomerated particles, resulting in the release of free water wrapped by these agglomerated particles, thereby significantly increasing the fluidity. Taking naphthalene-based superplasticizer for example, this kind of superplasticizer can adsorb onto the surface of cement particles to increase the zeta potential of the system [21]. It is this negative zeta potential that provides the dispersion force, which has been proved in many studies [22]. Another example is that polycarboxylate superplasticizer can also adsorb onto the surface of cement particles not only to increase the zeta potential but also to offer steric hindrance as the main dispersion force, and these two aspects are responsible for its excellent plasticizing effect [23]. Based on discussion above, the effect of PAE on fluidity should be related to variation of surface performance of cement particles, being involved in zeta potential and adsorption behavior.
Fig. 12. Size distribution of PAE in the presence of Ca2+.
PAE (10.0 g/L) prepared in 2.2.2 and calcium hydroxide solution (CH, 1.0 g/L) prepared in 2.2.5 were used here. With these two liquids and deionized water, PAE-CH mixture (PAE: 1.0 g/L; CH: 0 mg/L, 3.0 mg/L, 6.0 mg/L, 10.0 mg/L) was prepared, respectively, and then the particle size of the mixture was characterized with dynamic light scattering (DLS, Zetasizer Nano, made by Malvern instrument Ltd., UK). This result can indicate the size distribution of particles or polymer coils in a liquid medium [17]. Based on this result, the effect of Ca2+ in pore solution on dispersing PAE particles can be revealed. All measurements were conducted at a constant temperature of 25 °C.
3.2. Zeta potential 2.2.7. Characterization of chemical structure of PAE PAE was dried in a vacuum at 25 °C, and the solid was characterized with Fourier-transform infrared spectroscopy (FTIR, Nexus, made by Thermo Nicolet, USA). CH3, CH2, CH, SO3−, eCOe, and CeOeCe groups could be indicated from the FTIR spectra [18]. And the solid was also characterized with Thermogravimetry Differential Scanning Calorimetry Mass Spectrometry (TG-DSC-MS). With the result, the presence of some elements (e.g. sulfur) in PAE could be indicated.
Zeta potential is one of the most important permeters for the fludity of cement paste, and for example, naphthalene-sulfonic-based superplasticizer and amino-sulfonic-based superplasticizer exert the dispersion via strong negative zeta potential caused by adsorption [24,25]. In this section, the effect of PAE on zeta potential was discussed. As shown in Fig. 3, the zeta potential of the pure cement suspension was +5.0 eV, in agreement with other researchers. The main reason for this positive zeta potential is ascribed to the fast hydartion of tricalcium aluminate (i.e. C3A) [26,27]. Nevertheless, with increasing dosage of PAE, the zeta potential is reduced obviously. It becomes zero with the dosage of 0.20%, and with the dosage more than 0.20%, the zeta potential is negatively increased. With the dosage more than 8.0%, no obvious increase in zeta potential can be found. Actually, the absolute value of the zeta potential is reduced firstly and then increased, showing the same change trend with the fluidity and Ca2+ concentration. Based on the theory that greater zeta potential results in stronger dispersion, it is deduced that the effect of PAE on zeta potential should be one of the reasons for the change of fluidity. And the reason for the change of zeta potentail can be revealed as follows: The zeta potential of PAE is −51.4 eV, and this should be attributed to the negatively charged groups either R-COO− or R-SO3−. It is beasue of this negative zeta potentail that PAE can easily adsorb on the surface of cement particles via electrostatic attraction. With the increasing dosage of PAE, the positive zeta potential would be neutralized to be zero. With further increasing the dosage, the excessly adsorbed PAE,
3. Results and discussion 3.1. Effect of PAE on fluidity The initial fluidity (within 5 min) of the cement paste (w/c = 0.50) with various dosages of PAE was assessed with mini slump, and the results are shown in Fig. 1. It can be seen that with the increasing dosage of PAE, the initial fluidity is decreased when the dosage less than 5.0%, and surprisingly, the opposite tendency can be observed when the dosage more than 5.0%. This result illustrates that the effect of PAE on fluidity of cement paste depends on added dosage of PAE [19]. The concentration of Ca2+ in PAE modified cement suspension with various dosages of PAE is shown in Fig. 2. An interesting phenomenon can be seen that the change trend was extremely similar to that of the fluidity. In cement suspension, water molecules contact the cement minerals, and the chemical reaction takes place, resulting in the release of Ca2+ into solution. It is inferred that the contact area between
Fig. 13. Reaction between SO3− and Ca2+ in solution.
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Fig. 14. Cement suspension without PAE.
Fig. 15. Cement suspension with a small amount of PAE.
groups via combination with surficial Ca2+ (i.e. Ca2+ on the surface of the cement particles) [30]. If so, the chemical environment of surficial Ca2+ on cement particles would be significantly different from the reference (i.e. without PAE). In order to clarify this, binding energy of Ca2p on surface of the cement particles was assessed with XPS, and the results are shown in Fig. 5. It can be seen that without PAE, the binding energy of Ca2p is 349.88 eV and 346.38 eV, and 349.68 eV and 346.28 eV for addition of PAE 5.0%. By contrast, a noticeable peak shift of 0.20 eV and 0.10 eV can be seen. With PAE 20.0%, the binding energy is 349.48 eV and 345.88 eV, with peak shift of 0.40 eV and 0.50 eV. This result demonstrates that in first few minutes, PAE can be reacted with Ca2+ to form a new type calcium-based compound on the surface of the cement particles, and this provides sufficient evidence to prove that PAE can chemically adsorb onto the surface of the cement particles. It is this combination that can provide the main adsorption force to make PAE continuously adsorb onto the surface of cement particles. As a result, the adsorption force is involved in electrostatic attraction and combination. With a very small amount of PAE, when adsorption balance is reached, the amount of PAE adsorbed is not great enough to make the cement particles negatively charged. In this case, the adsorption force is
resulting from combination between negatively charged groups and surficial Ca2+ which has been proved in following text, could make the particles show negative zeta potential. 3.3. Adsorption behavior of PAE in cement suspension In order to further prove the reason for the change of zeta potential, the adsorption of PAE was studied with TOC, and the results are shown in Fig. 4. It can be seen that PAE can adsorb onto the surface of cement particles, in consistency with the results of other researchers [28,29]. With the increasing dosage of PAE, the adsorption amount is increased, and further increase would make the adsorption to reach the saturation. As shown in 3.2, the zeta potential of the cement particle without PAE is positive while the zeta potential of PAE is negative, and accordingly, PAE can easily adsorb onto the surface of cement particles via electrostatic attraction. It implies that electrostatic attraction is one of the reasons for adsorption. In adsorbing process, the question is: what motivates PAE to continuously adsorb onto the cement surface and make the zeta potential to be negatively increased, after the zeta potential of cement particle become zero. In the literature, SO3− groups or COO− groups on the surface of PAE have been proved as the adsorption 145
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Fig. 16. Cement suspension with a large amount of PAE.
that all PAE have been depleted by CH. Additionally, among these PAE, difference in concentration brings about difference in amount of CH at inflection point, as shown in Table 4. This result indicates that with the increasing dosage of PAE, the amount of PAE combined with Ca2+ would be increased. The increased combination results in the increased adsorption of PAE and negatively increased zeta potential.
attributed to both electrostatic attraction and combination. However, with a large amount of PAE, in a very short time, the adsorption of PAE can make cement particles negatively charged via electrostatic attraction and combination, and at this time, the adsorption balance has not been reached. It is worth noting that electrostatic repulsion would hinder the adsorption because cement particles and polymer particles bring the same kind of charge (i.e. negative charge). In the following adsorption process, the combination should result in the main adsorption force. Even though the electrostatic repulsion between cement particles and polymer particles can hinder the PAE to approach to cement particles, combination would provide much stronger adsorption force than that of the repulsion. In this case, the adsorption takes place continuously. With time going on, more and more amount of PAE adsorb onto the cement surface, and electrostatic repulsion would be increased rapidly, resulting in the increase in ability to hinder adsorption of PAE. This may be one of the reasons for the slower increase with dosage more than 8.0 g/L. With the further increase in adsorption of PAE, surficial Ca2+ available for the combination would become less and less, and the electrostatic repulsion would be bigger and bigger. In this case, the balance would be reached, showing the saturated adsorption. Based on discussion in this section, it is concluded that the combination between surficial Ca2+ and the surface groups of PAE is the main adsorption force, and the adsorption of the negatively charged PAE is inferred responsible for the negatively increased zeta potential.
3.5. Analysis of the structure of PAE In the literatures, three kinds of surface structures, with difference in surfical groups, can be recognized, as shown in Fig. 7 [34]. For structure (a), COO− groups on surface can be seen, and both COO− groups and SO3− groups for structure (b) and only SO3− groups for structure (c). Firstly, the structure of PAE was characterized with FTIR and TGDSC-MS, and the results are shown in Fig. 8, Table 5, and Fig. 9. As shown in Fig. 8 and Table 5, CH3, CH2, CH, SO3−, eCOe, and CeOeCe groups can be clearly seen in FTIR spectra [18]. This result demonstrates the presence of COO− groups and SO3− groups in PAE. Furthermore, as shown in Fig. 9, SO, SO2, and SO3 can be seen clearly, and this result definitely indicates the presence of SO3− groups in PAE used. As reported, SO3− groups and COO− groups in PAE would result in significant difference in the surface property of polymer particles, thereby considerably affecting the dispersion of polymer latex in cement suspension [30]. However, based on the results of FTIR and TDDSC-MS, the surficial structure of PAE used cannot be confirmed here. Furthermore, as reported, COO− can be easily combined with Ca2+ in pore solution, namely complexation, shown in Fig. 10. One Ca2+ ion can connect with two or four COO− groups as a cross-linking, resulting in agglomeration of these organics [35,36]. The conformation size of comb type copolymer known as superplasticizer can be increased by more than 10–200 times, for example, mainly because of this complexation [32]. Based on this, it can be inferred that PAE with structure (a) would be combined with Ca2+ in pore solution as shown in Fig. 11, resulting in agglmoration. In order to verify the surface structure, the particle size distribution of PAE in the presence of Ca2+ was characterized with DLS to illustrate the effect of Ca2+ on agglomeration of PAE, and the results are shown in Fig. 12. It can be seen that almost no difference in particle size distribution of PAE with different dosages of Ca2+. This result indicates that Ca2+ has nearly no effect on conformation size of PAE in solution.
3.4. Interaction between PAE and Ca2+ in pore solution Both carboxyl groups (COO−) and sulfonic groups (SO3−) could be combined with surficial Ca2+ as the adsorption force. This combination can also take place in pore solution, which has been proved with conductivity measurements in the literatures [31–33]. As shown in Fig. 6, obviously, the conductivity of the deionized water is increased with the increasing dosage of CH solution (1.0 g/L), while that for PAE is reduced firstly and then increased. An inflection point and difference in increasing tendency from the reference can also be seen clearly. This indicates the interaction between PAE and Ca2+. Furthermore, with the dosage of CH less than the inflection point, this interaction between polymer and CH takes place; with the dosage more than the inflection point, the increasing tendency is almost the same as the reference, indicating that nearly no interaction between PAE and CH, which implies 146
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firstly and then increased, depending on the added dosage of PAE. The reason for the decline is due to the reduced zeta potential and electrostatic attraction of PAE, and the increase is due to the increase in zeta potential, lubrication of PAE, and filling effect of PAE.
Thus, PAE cannot be agglomerated in pore solution, with execellent dispersion in mixing process. This demonstrates that no COO− group exists on the surface of PAE particles, and probably, PAE used in this study should be in aggreement with the structure (c). Additionllly, as shown in Fig. 13, PAE with SO3− groups can also be reacted with Ca2+, and this reaction is probably attributed to electrostatic attraction rather than complexation. And this can avoid the agglomeration of PAE and benefit the dispersion of PAE in cement paste, which can perfectly obtain the first step in Ohama model. Based on discussion above, it can be inferred that large amount of SO3− groups rather than COO− groups on surface of PAE particles, and it is these surficial groups that cause the combination of Ca2+, resulting in adsorption of PAE and the increased zeta potential.
Acknowledgment Financial supports from the “13th Five-Year” National Science and Technology Support Program of China (2016YFC0701003-5). References [1] S. Ahmad, A. Elahi, S. Barbhuiya, Y. Farid, Use of polymer modified mortar in controlling cracks in reinforced concrete beams, Constr. Build. Mater. 27 (1) (2012) 91–96. [2] Y. Ohama, Recent progress in concrete-polymer composites, Adv. Cem. Based Mater. 5 (2) (1997) 31–40. [3] J. Gao, C. Qian, B. Wang, K. Morino, Experimental study on properties of polymermodified cement mortars with silica fume, Cem. Concr. Res. 32 (1) (2002) 41–45. [4] H. Ma, Z. Li, Microstructures and mechanical properties of polymer modified mortars under distinct mechanisms, Constr. Build. Mater. 47 (2013) 579–587. [5] A. Beeldens, D. Van Gemert, H. Schorn, Y. Ohama, L. Czarnecki, From microstructure to macrostructure: an integrated model of structure formation in polymermodified concrete, Mater. Struct. 38 (6) (2005) 601–607. [6] Y. Ohama, Handbook of Polymer-modified Concrete and Mortars: Properties and Process Technology, William Andrew, 1995. [7] J. Plank, M. Gretz, Study on the interaction between anionic and cationic latex particles and Portland cement, Colloids Surf. A 330 (2) (2008) 227–233. [8] M. Wang, R. Wang, S. Zheng, S. Farhan, H. Yao, H. Jiang, Research on the chemical mechanism in the polyacrylate latex modified cement system, Cem. Concr. Res. 76 (2015) 62–69. [9] A.A. Bonapasta, F. Buda, P. Colombet, G. Guerrini, Cross-linking of poly (vinyl alcohol) chains by Ca ions in macro-defect-free cements, Chem. Mater. 14 (3) (2002) 1016–1022. [10] A.A. Bonapasta, F. Buda, P. Colombet, Interaction between Ca ions and poly (acrylic acid) chains in macro-defect-free cements: a theoretical study, Chem. Mater. 13 (1) (2001) 64–70. [11] Z. Lu, X. Kong, Q. Zhang, Y. Cai, Y. Zhang, Z. Wang, et al., Influences of styreneacrylate latexes on cement hydration in oil well cement system at different temperatures, Colloids Surf. A 507 (2016) 46–57. [12] H. Atahan, C. Carlos, S. Chae, P. Monteiro, J. Bastacky, The morphology of entrained air voids in hardened cement paste generated with different anionic surfactants, Cem. Concr. Compos. 30 (7) (2008) 566–575. [13] X.-M. Kong, C.-C. Wu, Y.-R. Zhang, J.-L. Li, Polymer-modified mortar with a gradient polymer distribution: preparation, permeability, and mechanical behaviour, Constr. Build. Mater. 38 (2013) 195–203. [14] O. Burgos-Montes, M. Palacios, P. Rivilla, F. Puertas, Compatibility between superplasticizer admixtures and cements with mineral additions, Constr. Build. Mater. 31 (2012) 300–309. [15] H. Tan, F. Zou, B. Ma, M. Liu, X. Li, S. Jian, Effect of sodium tripolyphosphate on adsorbing behavior of polycarboxylate superplasticizer, Constr. Build. Mater. 126 (2016) 617–623. [16] A.S. Dukhin, P.J. Goetz, Acoustic and electroacoustic spectroscopy for characterizing concentrated dispersions and emulsions, Adv. Colloid Interface Sci. 92 (1–3) (2001) 73. [17] W. Hyung, K. Mun, W. Kim, Properties of polymer-modified mortars based on methyl methacrylic latexes with emulsifier contents and various monomer ratios, J. Appl. Polym. Sci. 103 (5) (2007) 3010–3018. [18] D.A. Silva, H.R. Roman, P. Gleize, Evidences of chemical interaction between EVA and hydrating Portland cement, Cem. Concr. Res. 32 (9) (2002) 1383–1390. [19] J. Hot, H. Bessaies-Bey, C. Brumaud, M. Duc, C. Castella, N. Roussel, Adsorbing polymers and viscosity of cement pastes, Cem. Concr. Res. 63 (2014) 12–19. [20] H. Ma, Y. Tian, Z. Li, Interactions between organic and inorganic phases in PA-and PU/PA-modified-cement-based materials, J. Mater. Civ. Eng. 23 (10) (2011) 1412–1421. [21] H. Tan, B. Ma, X. Li, S. Jian, H. Yang, Effect of competitive adsorption between sodium tripolyphosphate and naphthalene superplasticizer on fluidity of cement paste, J. Wuhan Univ. Technol.–Mater. Sci. Ed. 29 (2) (2014) 334–340. [22] J. Plank, C. Hirsch, Impact of zeta potential of early cement hydration phases on superplasticizer adsorption, Cem. Concr. Res. 37 (4) (2007) 537–542. [23] H. Tan, Y. Guo, B. Ma, X. Li, B. Gu, Adsorbing behavior of polycarboxylate superplasticizer in the presence of ester group in side chain, J. Dispers. Sci. Technol. 38 (5) (2017) 743–749. [24] Y.-R. Zhang, X.-M. Kong, Z.-B. Lu, Z.-C. Lu, S.-S. Hou, Effects of the charge characteristics of polycarboxylate superplasticizers on the adsorption and the retardation in cement pastes, Cem. Concr. Res. 67 (2015) 184–196. [25] G. Li, T. He, D. Hu, C. Shi, Effects of two retarders on the fluidity of pastes plasticized with aminosulfonic acid-based superplasticizers, Constr. Build. Mater. 26 (1) (2012) 72–78. [26] M. Yang, C. Neubauer, H. Jennings, Interparticle potential and sedimentation behavior of cement suspensions: review and results from paste, Adv. Cem. Based Mater. 5 (1) (1997) 1–7.
3.6. Mechanistic model Effect of PAE on fluidity of cement paste depends on the added dosage of PAE. Less than 5.0% dosage of PAE has a negative effect on the fluidity, while a positive effect can be found with the dosage more than 5.0%. The reason can be summarized as follow: In cement paste, at the very beginning, because of the hydration of C3A, the positive charged particles can be found, which has been confirmed in 3.2, in agreement with many results in the literatures [26]. It is this positive zeta potential that can provide the electrostatic repulsion among cement particles, as shown in Fig. 14, to disperse the cement particles and slightly plasticize the paste in some sense. Even though this dispersion force is very weak, agglomeration degree of the cement particles would be declined in comparison with these suspensions with less zeta potential. The greater zeta potential at the very beginning would provide stronger contribution to the fluidity. With addtion of a small amount of PAE, as shown in Fig. 15, because of the quick adsorption of the negative charged PAE via electrostatic attraction and combination, the zeta potential would be declined in comparison with that of the paste without PAE, which has been proved in our study. The reduced zeta potential would result in more agglomeration of cement particles. This is considered as one of the main reason for the decline in fluidity in the presence of small amount of PAE (less than 5.0%). Additionally, adsorption of PAE can directly result in agglemoration of the cement particles. As shown in Fig. 15, one negatively charged PAE particle can connect several postively charged cement particles together, and this structure can wrap more amount of free water as well, thereby tending to reduce the fluidity. With a great amount of PAE added, as shown in Fig. 16, a large amount of PAE would adsorb on the surface of cement. On the one hand, the large amount of adsorption can make the cement particles negatively charged, showing very strong negative zeta potential. It is this zeta potentail that can provide strong electrostatic repulsion to disperse the particles. On the other hand, because of the nano spherical structure of PAE, the non-adsorbed PAE in solution would exist among these cement particles to lubricate the movement of the partiles, offering the lubrication effectto reduce the resistance for moving, with contribution to fluidity. Addtionaly, these non-adsorbed PAE existing among cement particles would occupy the space which should be occupied by water molecules, benefiting the release the free water. This can also contribute to the fluidity. 4. Conclusions (1) Because of SO3− groups rather than COO− groups on the surface of the polyacrylic acid emulsion, the agglomeration of PAE in cement paste can be avoided and the polymer particles can be excellently dispersed. (2) Adsorption of PAE on surface of cement particles has been confirmed, and the electrostatic attraction and combination of SO3− groups with surficial Ca2+ was inferred responsible for adsorption. (3) With the increasing dosage of PAE, fluidity of cement paste is reduced 147
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Y. Guo et al. [27] M. Heikal, I. Aiad, M. Shoaib, H. El-Didamony, Physico-chemical characteristics of some polymer cement composites, Mater. Chem. Phys. 71 (1) (2001) 76–83. [28] Q. Ran, P. Somasundaran, C. Miao, J. Liu, S. Wu, J. Shen, Adsorption mechanism of comb polymer dispersants at the cement/water interface, J. Dispers. Sci. Technol. 31 (6) (2010) 790–798. [29] Y. Tian, X.-y Jin, N.-g Jin, R. Zhao, Z.-j Li, H.-y Ma, Research on the microstructure formation of polyacrylate latex modified mortars, Constr. Build. Mater. 47 (2013) 1381–1394. [30] X. Kong, Q. Li, Properties and microstructure of polymer modified mortar based on different acrylate latexes, J. Chin. Ceram. Soc. 37 (1) (2009) 107–114. [31] F. Zou, H. Tan, Y. Guo, B. Ma, X. He, Y. Zhou, Effect of sodium gluconate on dispersion of polycarboxylate superplasticizer with different grafting density in side chain, J. Ind. Eng. Chem. 55 (2017) 91–100. [32] H. Tan, F. Zou, B. Ma, Y. Guo, X. Li, J. Mei, Effect of competitive adsorption
[33]
[34]
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between sodium gluconate and polycarboxylate superplasticizer on rheology of cement paste, Constr. Build. Mater. 144 (2017) 338–346. H. Tan, Y. Guo, F. Zou, S. Jian, B. Ma, Z. Zhi, Effect of borax on rheology of calcium sulphoaluminate cement paste in the presence of polycarboxylate superplasticizer, Constr. Build. Mater. 139 (2017) 277–285. Z. Lu, X. Kong, C. Zhang, F. Xing, Y. Zhang, Effect of colloidal polymers with different surface properties on the rheological property of fresh cement pastes, Colloid Surf. A 520 (2017) 154–165. T. Pavlitschek, M. Gretz, J. Plank, Effect of Ca2+ ions on the film formation of an anionic styrene/n-butylacrylate latexpolymer in cement pore solution, Adv. Mater. Res.: Trans. Tech. Publ. (2013) 322–328. J. Plank, B. Sachsenhauser, Experimental determination of the effective anionic charge density of polycarboxylate superplasticizers in cement pore solution, Cem. Concr. Res. 39 (1) (2009) 1–5.
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Research paper
Effect of polyacrylic acid emulsion on fluidity of cement paste a,⁎
b
b,c
c,⁎
a
MARK b
Yanfei Guo , Baoguo Ma , Zhenzhen Zhi , Hongbo Tan , Muyu Liu , Shouwei Jian , Yulin Guob a b c
Hubei Key Laboratory of Roadway Bridge and Structure Engineering, Wuhan University of Technology, Wuhan 430070, PR China School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, PR China
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyacrylic acid emulsion Fluidity Electrostatic effect Dispersion
Polymer-modified cement-based materials (PMC) have been widely applied in civil construction and municipal projects. In order to obtain deeper insight into the workability of PMC, the effect of polyacrylic acid emulsion (PAE) on fluidity of cement paste was investigated in this study. The fluidity was assessed with mini slump, and the change of Ca2+ concentration in pore solution was tested with inductive coupled plasma emission spectrometer. Zeta potential and adsorption behavior were characterized to reveal the mechanism behind the fluidity results. The results show that the effect of PAE on fluidity depends on the added dosage: dosage less than 5.0% reduces the fluidity, while the opposite is true with dosage more than 5%, which can also be indicated from the change of Ca2+ concentration in pore solution. Conductivity and X-ray photoelectron spectrometer (XPS) results demonstrate the chemical adsorption of PAE on the surface of cement particles. Dynamic light scattering (DLS) results strongly prove that the surface group of the PAE used is SO3− rather than COO−. Finally, the dispersion model was proposed to illustrate mechanism behind: with a small amount of PAE, the decline in zeta potential caused by adsorption of negatively charged PAE and the agglomeration of cement particles caused by electrostatic attraction of PAE are responsible for the reduced fluidity; while with a great amount of PAE, the negatively increased zeta potential, lubrication effect provided by non-adsorbed PAE, and filling effect of PAE are the main
⁎
Corresponding author. E-mail address: [email protected] (H. Tan).
http://dx.doi.org/10.1016/j.colsurfa.2017.09.039 Received 21 July 2017; Received in revised form 19 September 2017; Accepted 21 September 2017 Available online 22 September 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
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reason for the increased fluidity. Such results would give deeper understanding about the effect of polymer emulsion on rheology of cement paste.
1. Introduction
results. Zeta potential was measured to characterize the interface performance of cement particles. Adsorption behavior was discussed with total organic carbon analyzer (TOC), X-ray photoelectron spectrometer (XPS), dynamic light scattering (DLS) and conductivity; and then the surface structure of PAE was deduced. Finally, a dispersion model was proposed to illustrate the mechanism behind. Such results would show deeper understanding about the interaction between polymers and cement particles.
In recent years, polymer-modified cement-based materials (PMC) have been widely applied in civil construction and municipal projects, and increasingly attracting the attention in research area and industrial area in civil engineering [1]. The main reason for this is due to its excellent performance, such as impermeability, workability, adhesive strength, and toughness [2–4]. Many attempts have been made in the literatures to clarify the mechanism behind the improvement of polymers in performance of cement-based material [5]. Three-step model, proposed by Ohama, has been accepted as the most popular one in these reported studies [6]. As illustrated in this model, first of all, polymer particles would be dispersed in cement suspension; and then in the process of cement hydration, a continuous close-packed layer, mainly composed of polymer particles, should be formed on the surface of cement particles; finally, all these layers would coalesce into a continuous film, and more importance is that this film should be uninterrupted in order to interpenetrate throughout the cement hydrates. It is inferred that the first step, namely dispersing polymer particles in cement suspension, is of great importance, and it would further affect the following two steps. These results would help us understand the interaction between polymer particles and cement particles in PMC [7]. In order to achieve the aim of the first step in Ohama model, these nano polymer particles should be efficiently dispersed. However, cement hydrates may exert negative effect on dispersing the polymer particles. As reported, polyacrylate latex could chemically react with Ca2+ in cement hydration process, and this interaction could provide crosslinking, namely the combination between Ca2+ and COO−, to enhance the basic performance of PMC [8]. Even though this phenomenon was found in hardened paste, cement hydrates formed at the very beginning may negatively affect the dispersion process of polymer particles [9,10], with negative influence on the achievement of the first step in Ohama model. It is worth noting that polymer particles can quickly adsorb onto the surface of the cement particles, and the adsorption may significantly affect the rheology of the cement paste [11,12]. To be more precise, these polymer particles adsorbed can alter the structure of interface between liquid and these particles, thereby affecting the interface performance, such as zeta potential and surficial structure. The interface performance would significantly influence the rheology of the cement paste, and further determine the pumpability, self-compacting, and self-leveling [13], being similar to that in cement paste plasticized by superplasticizer [14]. Taking superplasticizer for example, superplasticizer adsorbed on the surface of cement particles can change zeta potential to improve the fluidity [15]. Actually, in real engineering practice, workability is one of the most important performances for cement-based materials, which determines the application in some sense. Therefore, to obtain deeper insight into the effect of polymer particles on rheological properties of the cementitious materials is of both scientific and practical importance. Polyacrylic acid emulsion (PAE) is one of the most popular polymers used for PMC. With addition of PAE, on the one hand, the carboxyl group would be combined with Ca2+ in pore solution and Ca2+ on the surface of cement particles; on the other hand, PAE would adsorb on the surface of cement particle. Both these processes most likely affect the dispersion of polymers, and also influence the rheology performance of PMC. In this study, the effect of PAE on fluidity of cement paste was investigated. The fluidity of the paste was assessed with mini slump. Ca2+ in pore solution was tested with inductive coupled plasma (ICP) emission as supplementary evidence to further clarify the fluidity
2. Experimental 2.1. Materials 2.1.1. Cement An ordinary Portland cement (P.I 42.5, supplied by China United Cement Co., Ltd., China), in according with Chinese standard GB1752007, was used in this study. The chemical compositions obtained with X-ray fluorescence spectrometer (Axios advanced) is shown in Table 1, and the basic information obtained from the company is shown in Table 2. 2.1.2. Polyacrylic acid emulsion Commercially available polyacrylic acid emulsion (PAE), a white and milky liquid, was used in this study. The physical properties of PAE are shown in Table 3. The pH value of PAE was tested with a pH meter. PAE (1.0 g/L) was obtained from the raw PAE, and then the particle size was characterized with Dynamic light scattering (DLS, Zetasizer Nano, made by Malvern instrument Ltd., UK). The solid content was tested in accordance with the Chinese standard GB 8077-2012. Additionally, raw PAE (0.5 g) was added into deionized water (49.5 g) and mixed, and the zeta potential was then tested with a zeta potential analyzer (Zetasizer Nano ZS, made by Malvern instrument Ltd., UK). PAE used in the experiment was marked as solid content. 2.2. Test methods 2.2.1. Fluidity of the cement paste PAE modified cement pastes were prepared with a water/cement ratio (w/c = 0.50) and addition of various dosages of PAE (0%, 2.5%, 5.0%, 7.5%, 10.0%, 20.0% of cement). The initial fluidity (within 5 min) was measured with a truncated cone mold (height: 60 mm; top diameter: 36 mm; bottom diameter: 60 mm) in accordance with the Chinese standard of GB/T 8077-2012. The truncated cone was filled with the paste on a glass plate, and then removed slowly. The maximum diameter of the spread sample and the maximum width perpendicular to that diameter were measured. The average of these two results was defined as the fluidity value. 2.2.2. Adsorption amount PAE (0–12.0 g/L), diluted by deionized water from raw PAE, was prepared in advance. Cement (1.0 g) was then mixed these emulsions (10.0 g) by magnetic stirrer. After stirred for 5 min, the mixture was separated by centrifugation at 3500 r/min for 5 min. The upper solution was prepared for the TOC measurement. Table 1 Chemical composition of cement (wt%).
140
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2 O
Na2O
Loss
21.402
5.277
3.017
61.917
2.459
2.689
0.758
0.056
1.913
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Table 2 Basic illustration of cement. C3S
C2S
C3A wt%
C4AF
CaSO4
Specific surface area m2/kg
49.61
23.93
8.87
9.17
4.2
350
Table 3 The physical properties of PAE. pH value
Particle size (nm)
Zeta potential (eV)
Solid content (%)
8.0
174.0
−51.4
52.5
Fig. 3. Zeta potential of the cement paste with PAE.
Fig. 1. Effect of PAE on fluidity of cement paste.
Fig. 4. Adsorption amount of PAE on cement particle.
Fig. 2. Effect of PAE on concentration of Ca2+.
Fig. 5. Ca2p XPS spectrum of cement and PAE modified cement particle surface.
Total Organic Carbon Analyzer (TOC, Multi N/C 2100, made in Jena, Germany) was used to test the carbon content of organics in upper solution. Measurement was generally repeated three times and the average was the result. The residual concentration of PAE was inferred from the results of TOC, and the adsorption amount (mg/g-cement) was calculated as follow:
2.2.3. Concentration of Ca2+ in pore solution PAE with difference in concentration (0–5.0%) was diluted with deionized water from raw PAE in advance. Cement (1.0 g) was then mixed with the emulsion (10.0 g) and stirred for 5 min. And then the suspension was processed as the same process with 2.2.2. The concentration of Ca2+ in upper solution was tested with inductive coupled plasma-optical emission spectrometer (ICP, Prodigy 7, made by LEEMAN LABS INC., USA). Measurement was generally repeated three times and the average was the result.
Adsorption amount = V (C0 − C)/m Where C0 is the initial concentration (g/L) of PAE before adsorption; C is the residual concentration (g/L) after adsorption; V is volume of the solution (L); m is the mass of the cement (g).
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Fig. 8. ATR-FTIR spectra of PAE. (a) TG-DSC (b) MS.
Table 5 ATR-FTIR characteristic bonds of PAE (cm−1). Polymer
eCH3
PAE
2958, 2956, 2869
a
C]O
eCH2e
eCeOeCe
1728
1450
1182,1157
a
ReSO3e
eCH3
1064
760, 700
b
Remark: a asymmetric stretching vibration. b Out-of-plane deformation vibration.
2.2.4. Surface performance of cement particle with PAE PAE (5.0%, 20.0%), diluted from raw PAE, was prepared in advance. Cement (1.0 g) was then mixed these emulsions (10.0 g) by magnetic stirrer. After stirred for 5 min, the mixture was separated by centrifugation at 3500 r/min for 5 min. The solid was dried in a vacuum drier at 60 °C. The dried solid was grinded by hand, and these powders with particle size less than 200-mesh were prepared for XPS measurement. The blank sample was prepared with cement (1.0 g) and deionized water (10.0 g) as the same process. The binding energy of Ca2p in powder samples was assessed with XPS (X-ray photoelectron spectroscopy, VG Multilab 2000X). Aluminum was used as an anode target (hν = 1486.6 eV); energy resolution was 0.10 eV. If chemical adsorption took place, new type of calcium-based bond would be formed on the surface of the cement particles, with obvious difference from the reference. Based on this analysis, the reaction between PAE and surficial Ca2+ can be indicated from XPS results as supplementary evidence to prove the chemical adsorption of PAE onto the surface of cement particles surface.
Fig. 6. Conductivity of PAE with addition of CH solution. (a) PAE 1.0%, (b) PAE 2.0%, (c) PAE 5.0%.
Table 4 Proportion between PAE and Ca2+ in the conductivity test. Content of PAE (%)
Initial conductivity (μS/cm)
Ca(OH)2 (g) (Inflection point)
Mass ratio of g-PAE/ g-CH (Inflection point)
0 1.0 2.0 5.0
0 103.5 196.6 456
– 0.008 0.0177 0.0535
– 187.5 169.5 140.0
2.2.5. Conductivity PAE with difference in concentration (1.0%, 2.0% and 5.0%) was diluted with deionized water from the raw PAE, respectively, and then the conductivity of these three diluted PAE (150.0 g) and deionized water (i.e. reference, 150.0 g) with the increasing dosage of calcium hydroxide solution (CH, 1.0 g/L) was carried out with an electrical conductivity meter (Seven Compact S230, made by Mettler Toledo, Switzerland). The measured liquid was stirred with magnetic stirrer at 350 rpm. The difference in conductivity between PAE and the reference (deionized water) could indicate the reaction between PAE and Ca2+. All operations were carried out at 25 °C. 2.2.6. Zeta potential and size distribution Zeta potential is generally used to measure the electrostatic charge on the surface of particles in suspension, and it can also characterize the stability of the suspension [16]. The variation of zeta potential also
Fig. 7. Schematic diagram of the structure of PAE.
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Fig. 9. TG-DSC-MS spectra of PAE.
Fig. 10. The complexation between RCOO− and Ca2+. Fig. 11. Reaction between COO− and Ca2+ in solution.
provides evidence to prove adsorption of charged polymers. The samples for the measurement of zeta potential were prepared as follows: PAE with difference in concentration (0–1.0%) was diluted by deionized water from raw PAE in advance. Cement (1.0 g) was then mixed with the diluted emulsions (10.0 g) and stirred for one minute, and then one gram of the suspension was added into 9.0 g deionized water and
stirred. The diluted suspension was tested with the instrument (Zetasizer Nano ZS, made by Malvern instrument Ltd., UK). The value of the zeta potential was computed from the average of three dependent measurements. Each test should be finished within 5 min. 143
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minerals and liquid would be one of the most important factors affecting this release. The agglomerated particles would reduce this contact area, and the agglomeration degree of particles would determine the release of Ca2+ in some sense. Based on this, better fluidity would result in greater contact area between minerals and water molecules, which would make the release of Ca2+ easier. As a consequence, the change trend of Ca2+, in consistency with the fluidity, offers further evidence to confirm the effect of PAE on fluidity of cement paste. As reported, the fluidity of cement paste is closely related to the surface performance of particles [20]. These polymers added can probably alter the interface between particles and liquid. In cement paste plasticized by superplasticizer, adsorption of polymer can change the surface performance of the particles to disperse the agglomerated particles, resulting in the release of free water wrapped by these agglomerated particles, thereby significantly increasing the fluidity. Taking naphthalene-based superplasticizer for example, this kind of superplasticizer can adsorb onto the surface of cement particles to increase the zeta potential of the system [21]. It is this negative zeta potential that provides the dispersion force, which has been proved in many studies [22]. Another example is that polycarboxylate superplasticizer can also adsorb onto the surface of cement particles not only to increase the zeta potential but also to offer steric hindrance as the main dispersion force, and these two aspects are responsible for its excellent plasticizing effect [23]. Based on discussion above, the effect of PAE on fluidity should be related to variation of surface performance of cement particles, being involved in zeta potential and adsorption behavior.
Fig. 12. Size distribution of PAE in the presence of Ca2+.
PAE (10.0 g/L) prepared in 2.2.2 and calcium hydroxide solution (CH, 1.0 g/L) prepared in 2.2.5 were used here. With these two liquids and deionized water, PAE-CH mixture (PAE: 1.0 g/L; CH: 0 mg/L, 3.0 mg/L, 6.0 mg/L, 10.0 mg/L) was prepared, respectively, and then the particle size of the mixture was characterized with dynamic light scattering (DLS, Zetasizer Nano, made by Malvern instrument Ltd., UK). This result can indicate the size distribution of particles or polymer coils in a liquid medium [17]. Based on this result, the effect of Ca2+ in pore solution on dispersing PAE particles can be revealed. All measurements were conducted at a constant temperature of 25 °C.
3.2. Zeta potential 2.2.7. Characterization of chemical structure of PAE PAE was dried in a vacuum at 25 °C, and the solid was characterized with Fourier-transform infrared spectroscopy (FTIR, Nexus, made by Thermo Nicolet, USA). CH3, CH2, CH, SO3−, eCOe, and CeOeCe groups could be indicated from the FTIR spectra [18]. And the solid was also characterized with Thermogravimetry Differential Scanning Calorimetry Mass Spectrometry (TG-DSC-MS). With the result, the presence of some elements (e.g. sulfur) in PAE could be indicated.
Zeta potential is one of the most important permeters for the fludity of cement paste, and for example, naphthalene-sulfonic-based superplasticizer and amino-sulfonic-based superplasticizer exert the dispersion via strong negative zeta potential caused by adsorption [24,25]. In this section, the effect of PAE on zeta potential was discussed. As shown in Fig. 3, the zeta potential of the pure cement suspension was +5.0 eV, in agreement with other researchers. The main reason for this positive zeta potential is ascribed to the fast hydartion of tricalcium aluminate (i.e. C3A) [26,27]. Nevertheless, with increasing dosage of PAE, the zeta potential is reduced obviously. It becomes zero with the dosage of 0.20%, and with the dosage more than 0.20%, the zeta potential is negatively increased. With the dosage more than 8.0%, no obvious increase in zeta potential can be found. Actually, the absolute value of the zeta potential is reduced firstly and then increased, showing the same change trend with the fluidity and Ca2+ concentration. Based on the theory that greater zeta potential results in stronger dispersion, it is deduced that the effect of PAE on zeta potential should be one of the reasons for the change of fluidity. And the reason for the change of zeta potentail can be revealed as follows: The zeta potential of PAE is −51.4 eV, and this should be attributed to the negatively charged groups either R-COO− or R-SO3−. It is beasue of this negative zeta potentail that PAE can easily adsorb on the surface of cement particles via electrostatic attraction. With the increasing dosage of PAE, the positive zeta potential would be neutralized to be zero. With further increasing the dosage, the excessly adsorbed PAE,
3. Results and discussion 3.1. Effect of PAE on fluidity The initial fluidity (within 5 min) of the cement paste (w/c = 0.50) with various dosages of PAE was assessed with mini slump, and the results are shown in Fig. 1. It can be seen that with the increasing dosage of PAE, the initial fluidity is decreased when the dosage less than 5.0%, and surprisingly, the opposite tendency can be observed when the dosage more than 5.0%. This result illustrates that the effect of PAE on fluidity of cement paste depends on added dosage of PAE [19]. The concentration of Ca2+ in PAE modified cement suspension with various dosages of PAE is shown in Fig. 2. An interesting phenomenon can be seen that the change trend was extremely similar to that of the fluidity. In cement suspension, water molecules contact the cement minerals, and the chemical reaction takes place, resulting in the release of Ca2+ into solution. It is inferred that the contact area between
Fig. 13. Reaction between SO3− and Ca2+ in solution.
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Fig. 14. Cement suspension without PAE.
Fig. 15. Cement suspension with a small amount of PAE.
groups via combination with surficial Ca2+ (i.e. Ca2+ on the surface of the cement particles) [30]. If so, the chemical environment of surficial Ca2+ on cement particles would be significantly different from the reference (i.e. without PAE). In order to clarify this, binding energy of Ca2p on surface of the cement particles was assessed with XPS, and the results are shown in Fig. 5. It can be seen that without PAE, the binding energy of Ca2p is 349.88 eV and 346.38 eV, and 349.68 eV and 346.28 eV for addition of PAE 5.0%. By contrast, a noticeable peak shift of 0.20 eV and 0.10 eV can be seen. With PAE 20.0%, the binding energy is 349.48 eV and 345.88 eV, with peak shift of 0.40 eV and 0.50 eV. This result demonstrates that in first few minutes, PAE can be reacted with Ca2+ to form a new type calcium-based compound on the surface of the cement particles, and this provides sufficient evidence to prove that PAE can chemically adsorb onto the surface of the cement particles. It is this combination that can provide the main adsorption force to make PAE continuously adsorb onto the surface of cement particles. As a result, the adsorption force is involved in electrostatic attraction and combination. With a very small amount of PAE, when adsorption balance is reached, the amount of PAE adsorbed is not great enough to make the cement particles negatively charged. In this case, the adsorption force is
resulting from combination between negatively charged groups and surficial Ca2+ which has been proved in following text, could make the particles show negative zeta potential. 3.3. Adsorption behavior of PAE in cement suspension In order to further prove the reason for the change of zeta potential, the adsorption of PAE was studied with TOC, and the results are shown in Fig. 4. It can be seen that PAE can adsorb onto the surface of cement particles, in consistency with the results of other researchers [28,29]. With the increasing dosage of PAE, the adsorption amount is increased, and further increase would make the adsorption to reach the saturation. As shown in 3.2, the zeta potential of the cement particle without PAE is positive while the zeta potential of PAE is negative, and accordingly, PAE can easily adsorb onto the surface of cement particles via electrostatic attraction. It implies that electrostatic attraction is one of the reasons for adsorption. In adsorbing process, the question is: what motivates PAE to continuously adsorb onto the cement surface and make the zeta potential to be negatively increased, after the zeta potential of cement particle become zero. In the literature, SO3− groups or COO− groups on the surface of PAE have been proved as the adsorption 145
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Fig. 16. Cement suspension with a large amount of PAE.
that all PAE have been depleted by CH. Additionally, among these PAE, difference in concentration brings about difference in amount of CH at inflection point, as shown in Table 4. This result indicates that with the increasing dosage of PAE, the amount of PAE combined with Ca2+ would be increased. The increased combination results in the increased adsorption of PAE and negatively increased zeta potential.
attributed to both electrostatic attraction and combination. However, with a large amount of PAE, in a very short time, the adsorption of PAE can make cement particles negatively charged via electrostatic attraction and combination, and at this time, the adsorption balance has not been reached. It is worth noting that electrostatic repulsion would hinder the adsorption because cement particles and polymer particles bring the same kind of charge (i.e. negative charge). In the following adsorption process, the combination should result in the main adsorption force. Even though the electrostatic repulsion between cement particles and polymer particles can hinder the PAE to approach to cement particles, combination would provide much stronger adsorption force than that of the repulsion. In this case, the adsorption takes place continuously. With time going on, more and more amount of PAE adsorb onto the cement surface, and electrostatic repulsion would be increased rapidly, resulting in the increase in ability to hinder adsorption of PAE. This may be one of the reasons for the slower increase with dosage more than 8.0 g/L. With the further increase in adsorption of PAE, surficial Ca2+ available for the combination would become less and less, and the electrostatic repulsion would be bigger and bigger. In this case, the balance would be reached, showing the saturated adsorption. Based on discussion in this section, it is concluded that the combination between surficial Ca2+ and the surface groups of PAE is the main adsorption force, and the adsorption of the negatively charged PAE is inferred responsible for the negatively increased zeta potential.
3.5. Analysis of the structure of PAE In the literatures, three kinds of surface structures, with difference in surfical groups, can be recognized, as shown in Fig. 7 [34]. For structure (a), COO− groups on surface can be seen, and both COO− groups and SO3− groups for structure (b) and only SO3− groups for structure (c). Firstly, the structure of PAE was characterized with FTIR and TGDSC-MS, and the results are shown in Fig. 8, Table 5, and Fig. 9. As shown in Fig. 8 and Table 5, CH3, CH2, CH, SO3−, eCOe, and CeOeCe groups can be clearly seen in FTIR spectra [18]. This result demonstrates the presence of COO− groups and SO3− groups in PAE. Furthermore, as shown in Fig. 9, SO, SO2, and SO3 can be seen clearly, and this result definitely indicates the presence of SO3− groups in PAE used. As reported, SO3− groups and COO− groups in PAE would result in significant difference in the surface property of polymer particles, thereby considerably affecting the dispersion of polymer latex in cement suspension [30]. However, based on the results of FTIR and TDDSC-MS, the surficial structure of PAE used cannot be confirmed here. Furthermore, as reported, COO− can be easily combined with Ca2+ in pore solution, namely complexation, shown in Fig. 10. One Ca2+ ion can connect with two or four COO− groups as a cross-linking, resulting in agglomeration of these organics [35,36]. The conformation size of comb type copolymer known as superplasticizer can be increased by more than 10–200 times, for example, mainly because of this complexation [32]. Based on this, it can be inferred that PAE with structure (a) would be combined with Ca2+ in pore solution as shown in Fig. 11, resulting in agglmoration. In order to verify the surface structure, the particle size distribution of PAE in the presence of Ca2+ was characterized with DLS to illustrate the effect of Ca2+ on agglomeration of PAE, and the results are shown in Fig. 12. It can be seen that almost no difference in particle size distribution of PAE with different dosages of Ca2+. This result indicates that Ca2+ has nearly no effect on conformation size of PAE in solution.
3.4. Interaction between PAE and Ca2+ in pore solution Both carboxyl groups (COO−) and sulfonic groups (SO3−) could be combined with surficial Ca2+ as the adsorption force. This combination can also take place in pore solution, which has been proved with conductivity measurements in the literatures [31–33]. As shown in Fig. 6, obviously, the conductivity of the deionized water is increased with the increasing dosage of CH solution (1.0 g/L), while that for PAE is reduced firstly and then increased. An inflection point and difference in increasing tendency from the reference can also be seen clearly. This indicates the interaction between PAE and Ca2+. Furthermore, with the dosage of CH less than the inflection point, this interaction between polymer and CH takes place; with the dosage more than the inflection point, the increasing tendency is almost the same as the reference, indicating that nearly no interaction between PAE and CH, which implies 146
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firstly and then increased, depending on the added dosage of PAE. The reason for the decline is due to the reduced zeta potential and electrostatic attraction of PAE, and the increase is due to the increase in zeta potential, lubrication of PAE, and filling effect of PAE.
Thus, PAE cannot be agglomerated in pore solution, with execellent dispersion in mixing process. This demonstrates that no COO− group exists on the surface of PAE particles, and probably, PAE used in this study should be in aggreement with the structure (c). Additionllly, as shown in Fig. 13, PAE with SO3− groups can also be reacted with Ca2+, and this reaction is probably attributed to electrostatic attraction rather than complexation. And this can avoid the agglomeration of PAE and benefit the dispersion of PAE in cement paste, which can perfectly obtain the first step in Ohama model. Based on discussion above, it can be inferred that large amount of SO3− groups rather than COO− groups on surface of PAE particles, and it is these surficial groups that cause the combination of Ca2+, resulting in adsorption of PAE and the increased zeta potential.
Acknowledgment Financial supports from the “13th Five-Year” National Science and Technology Support Program of China (2016YFC0701003-5). References [1] S. Ahmad, A. Elahi, S. Barbhuiya, Y. Farid, Use of polymer modified mortar in controlling cracks in reinforced concrete beams, Constr. Build. Mater. 27 (1) (2012) 91–96. [2] Y. Ohama, Recent progress in concrete-polymer composites, Adv. Cem. Based Mater. 5 (2) (1997) 31–40. [3] J. Gao, C. Qian, B. Wang, K. Morino, Experimental study on properties of polymermodified cement mortars with silica fume, Cem. Concr. Res. 32 (1) (2002) 41–45. [4] H. Ma, Z. Li, Microstructures and mechanical properties of polymer modified mortars under distinct mechanisms, Constr. Build. Mater. 47 (2013) 579–587. [5] A. Beeldens, D. Van Gemert, H. Schorn, Y. Ohama, L. Czarnecki, From microstructure to macrostructure: an integrated model of structure formation in polymermodified concrete, Mater. Struct. 38 (6) (2005) 601–607. [6] Y. Ohama, Handbook of Polymer-modified Concrete and Mortars: Properties and Process Technology, William Andrew, 1995. [7] J. Plank, M. Gretz, Study on the interaction between anionic and cationic latex particles and Portland cement, Colloids Surf. A 330 (2) (2008) 227–233. [8] M. Wang, R. Wang, S. Zheng, S. Farhan, H. Yao, H. Jiang, Research on the chemical mechanism in the polyacrylate latex modified cement system, Cem. Concr. Res. 76 (2015) 62–69. [9] A.A. Bonapasta, F. Buda, P. Colombet, G. Guerrini, Cross-linking of poly (vinyl alcohol) chains by Ca ions in macro-defect-free cements, Chem. Mater. 14 (3) (2002) 1016–1022. [10] A.A. Bonapasta, F. Buda, P. Colombet, Interaction between Ca ions and poly (acrylic acid) chains in macro-defect-free cements: a theoretical study, Chem. Mater. 13 (1) (2001) 64–70. [11] Z. Lu, X. Kong, Q. Zhang, Y. Cai, Y. Zhang, Z. Wang, et al., Influences of styreneacrylate latexes on cement hydration in oil well cement system at different temperatures, Colloids Surf. A 507 (2016) 46–57. [12] H. Atahan, C. Carlos, S. Chae, P. Monteiro, J. Bastacky, The morphology of entrained air voids in hardened cement paste generated with different anionic surfactants, Cem. Concr. Compos. 30 (7) (2008) 566–575. [13] X.-M. Kong, C.-C. Wu, Y.-R. Zhang, J.-L. Li, Polymer-modified mortar with a gradient polymer distribution: preparation, permeability, and mechanical behaviour, Constr. Build. Mater. 38 (2013) 195–203. [14] O. Burgos-Montes, M. Palacios, P. Rivilla, F. Puertas, Compatibility between superplasticizer admixtures and cements with mineral additions, Constr. Build. Mater. 31 (2012) 300–309. [15] H. Tan, F. Zou, B. Ma, M. Liu, X. Li, S. Jian, Effect of sodium tripolyphosphate on adsorbing behavior of polycarboxylate superplasticizer, Constr. Build. Mater. 126 (2016) 617–623. [16] A.S. Dukhin, P.J. Goetz, Acoustic and electroacoustic spectroscopy for characterizing concentrated dispersions and emulsions, Adv. Colloid Interface Sci. 92 (1–3) (2001) 73. [17] W. Hyung, K. Mun, W. Kim, Properties of polymer-modified mortars based on methyl methacrylic latexes with emulsifier contents and various monomer ratios, J. Appl. Polym. Sci. 103 (5) (2007) 3010–3018. [18] D.A. Silva, H.R. Roman, P. Gleize, Evidences of chemical interaction between EVA and hydrating Portland cement, Cem. Concr. Res. 32 (9) (2002) 1383–1390. [19] J. Hot, H. Bessaies-Bey, C. Brumaud, M. Duc, C. Castella, N. Roussel, Adsorbing polymers and viscosity of cement pastes, Cem. Concr. Res. 63 (2014) 12–19. [20] H. Ma, Y. Tian, Z. Li, Interactions between organic and inorganic phases in PA-and PU/PA-modified-cement-based materials, J. Mater. Civ. Eng. 23 (10) (2011) 1412–1421. [21] H. Tan, B. Ma, X. Li, S. Jian, H. Yang, Effect of competitive adsorption between sodium tripolyphosphate and naphthalene superplasticizer on fluidity of cement paste, J. Wuhan Univ. Technol.–Mater. Sci. Ed. 29 (2) (2014) 334–340. [22] J. Plank, C. Hirsch, Impact of zeta potential of early cement hydration phases on superplasticizer adsorption, Cem. Concr. Res. 37 (4) (2007) 537–542. [23] H. Tan, Y. Guo, B. Ma, X. Li, B. Gu, Adsorbing behavior of polycarboxylate superplasticizer in the presence of ester group in side chain, J. Dispers. Sci. Technol. 38 (5) (2017) 743–749. [24] Y.-R. Zhang, X.-M. Kong, Z.-B. Lu, Z.-C. Lu, S.-S. Hou, Effects of the charge characteristics of polycarboxylate superplasticizers on the adsorption and the retardation in cement pastes, Cem. Concr. Res. 67 (2015) 184–196. [25] G. Li, T. He, D. Hu, C. Shi, Effects of two retarders on the fluidity of pastes plasticized with aminosulfonic acid-based superplasticizers, Constr. Build. Mater. 26 (1) (2012) 72–78. [26] M. Yang, C. Neubauer, H. Jennings, Interparticle potential and sedimentation behavior of cement suspensions: review and results from paste, Adv. Cem. Based Mater. 5 (1) (1997) 1–7.
3.6. Mechanistic model Effect of PAE on fluidity of cement paste depends on the added dosage of PAE. Less than 5.0% dosage of PAE has a negative effect on the fluidity, while a positive effect can be found with the dosage more than 5.0%. The reason can be summarized as follow: In cement paste, at the very beginning, because of the hydration of C3A, the positive charged particles can be found, which has been confirmed in 3.2, in agreement with many results in the literatures [26]. It is this positive zeta potential that can provide the electrostatic repulsion among cement particles, as shown in Fig. 14, to disperse the cement particles and slightly plasticize the paste in some sense. Even though this dispersion force is very weak, agglomeration degree of the cement particles would be declined in comparison with these suspensions with less zeta potential. The greater zeta potential at the very beginning would provide stronger contribution to the fluidity. With addtion of a small amount of PAE, as shown in Fig. 15, because of the quick adsorption of the negative charged PAE via electrostatic attraction and combination, the zeta potential would be declined in comparison with that of the paste without PAE, which has been proved in our study. The reduced zeta potential would result in more agglomeration of cement particles. This is considered as one of the main reason for the decline in fluidity in the presence of small amount of PAE (less than 5.0%). Additionally, adsorption of PAE can directly result in agglemoration of the cement particles. As shown in Fig. 15, one negatively charged PAE particle can connect several postively charged cement particles together, and this structure can wrap more amount of free water as well, thereby tending to reduce the fluidity. With a great amount of PAE added, as shown in Fig. 16, a large amount of PAE would adsorb on the surface of cement. On the one hand, the large amount of adsorption can make the cement particles negatively charged, showing very strong negative zeta potential. It is this zeta potentail that can provide strong electrostatic repulsion to disperse the particles. On the other hand, because of the nano spherical structure of PAE, the non-adsorbed PAE in solution would exist among these cement particles to lubricate the movement of the partiles, offering the lubrication effectto reduce the resistance for moving, with contribution to fluidity. Addtionaly, these non-adsorbed PAE existing among cement particles would occupy the space which should be occupied by water molecules, benefiting the release the free water. This can also contribute to the fluidity. 4. Conclusions (1) Because of SO3− groups rather than COO− groups on the surface of the polyacrylic acid emulsion, the agglomeration of PAE in cement paste can be avoided and the polymer particles can be excellently dispersed. (2) Adsorption of PAE on surface of cement particles has been confirmed, and the electrostatic attraction and combination of SO3− groups with surficial Ca2+ was inferred responsible for adsorption. (3) With the increasing dosage of PAE, fluidity of cement paste is reduced 147
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