Effect of Sulfate Ion on the Performance of Polycarboxylate Comb- Like Copolymers in Cement Suspensions

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1 Effect of Sulfate Ion on the Performance of Polycarboxylate Comb-Like Copolymers in Cement Suspensions Effect of Sulfate Ion on the Performance of Polycarboxylate Comb- Like Copolymers in Cement Suspensions Qianping Ran 1,2*, Min Qiao 2, Yinhui Yu 2, and Jiaping Liu 1,2* 1 State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Research Institute of Building Science, Nanjing, , China 2 Jiangsu Bote New Materials Co. Ltd, Nanjing, , China Received: 27 April 2013, Accepted: 5 August 2013 SUMMARY Polycarboxylate comb-like copolymer (PCE) is a kind of polyelectrolyte, which is easily affected by the ions in cement suspensions. In this work, the effects of sulfate ion on the surface charge, adsorption and dispersion in cement system with PCE as dispersant were investigated in detail. The addition of sulfate ion apparently decreased the positive charge on the surface of cement particles. The sulfate ion reduced the adsorption and dispersibility of PCE in cement suspensions, which was due to the competition between sulfate ions and PCE in the adsorption behaviour on the surface of cement particles. Keywords: Polycarboxylate comb-like copolymer; Sulfate ion; Cement; Adsorption; Dispersion 1. INTRODUCTION Comb-like copolymers are new and efficient types of cement dispersants with a very flexible chemical structure 1-5. It is easy to modify and adapt their molecular structure in order to maximize the targeted properties and also meet specific properties for different applications. Thus they have attracted significant attention and have been extensively studied in the last 20 years 1-7. These comb-like copolymers are composed of a polyelectrolyte backbone and non-ionic polyethylene glycol (PEG)- based teeth side chains grafted onto the backbone at frequent intervals. It is considered that, in comb-like copolymers the carboxylate ions in the backbone mainly act as the driver of the adsorption because of the electrostatic attraction between Smithers Information Ltd., 2014 anionic polycarboxylate backbone and cationic surface of cement particles, meanwhile the hydrophilic PEG side chains extend into the solution and provide the steric repulsion, which is the main driver for the dispersion of cement particles 5,6. The polycarboxylate comb-like copolymer (hereinafter called PCE) is a kind of polyelectrolyte, which is easily affected by the ions in the solution [8]. In the suspensions of cement, the type and amount of the ions is complicated, which will certainly affect the interactions between polycarboxylate comb-like copolymers and cement particles. Among these ions, sulfate ion (SO 4 ) is one of the most important types of ions, because that negative ion can be adsorbed onto the positive surface of cement particles 9. In this case, the surface potential of cement is reduced, which is disadvantageous Corresponding author: Qianping Ran, professor, ranqianping@cnjsjk.cn; Fax: : Phone: Jiaping Liu, Professor, liujiaping@cnjsjk.cn; Fax: ; Phone: for comb-like copolymers to adsorb onto the cement particles 10. But different types of cement have different contents of sulfate ion, so polycarboxylate comb-like copolymers present poor adaptability for different cements. To reveal the roles of the relationship between sulfate ion and polycarboxylate comb-like copolymers, this study aims to contribute to a precise understanding of the impact of the sulfate ion on the performance of polycarboxylate comblike copolymers. First, we synthesised and characterized a series of PCE with different charge densities and the same length of side chain, as shown in Figure 1. Then the effects of sulfate ion on the surface charge, adsorption and dispersion in cement system with PCE as dispersant are investigated in detail. 2. MATERIALS AND METHODS 2.1 Cement An ordinary Portland cement (42.5, Lafarge Cement Co., China) which Polymers & Polymer Composites, Vol. 22, No. 6,

2 Qianping Ran, Min Qiao, Yinhui Yu, and Jiaping Liu meets the requirements of GB8076 standard was used in this study. The cement composition was determined by x-ray fluorescence and Bogue analysis. The particle size analysis using a Helos-Sucell Laser particle size analyzer (SYMPATEC Instruments, German) showed the average mean particle size was about 25 μm. The BET surface area was m 2.g -1. The characteristics and composition of the sample are given in Table 1. Figure 1. the illustration of the chemical structure of comb-like copolymers with different charge densities Table 1. Phase composition and physical properties of the cement Phase composition/% Percent volume diameters/ μm BET surface C 3 S C 2 S C 3 A C 4 AF d 10 d 50 d 90 VMD area/m 2.g -1 The content of SO 3 / mass% Here, C 3 S, C 2 S, C3A and C 4 AF represent dicalcium silicate, tricalcium aluminate, tricalcium silicate and tetracalcium aluminate ferrite, respectively. d 10, d 50 and d 90 represent the particle size below which the volume percent is 10%, 50% and 90% respectively. VMD is the average mean particle size Table 2. Molecular characteristics of comb copolymer dispersants (4,7) Molar composition /% a M n M b w μmol COO - / g x y polymer PCE PCE PCE PCE a M n =number average molecular weight, b M w =weight average molecular weight, c DPw is the degree of polymerization which is calculated according to M w 2.2 Comb-like Copolymer Dispersants The comb-like polymers dispersant (PCE) composed of sodium polyacrylate backbone and uncharged polyethylene glycol (PEG) teeth as shown in Figure 1 were prepared in our laboratory. Details of the preparation procedure and characterization methods are described elsewhere 4,7. In this study, a series of comb-like copolymers with different charge densities have been chosen and tested. The composition of PCE has been estimated classically by 1 H-NMR spectroscopy. The molecular weight of PCE was determined using a Wyatt Technology minidawn static three-angle laser light scattering detector (MALLS) equipped with TSK-GELSW (TOSOH) columns. The used PCE architectures and their molecular characteristics are listed in Table 2. All the PCE were used as sodium salts. 2.3 Zeta Potential Measurements Zeta Potential of cement suspensions was tested in 0.01 mol/l KCl solution as background. 1 wt.% cement suspensions were prepared by adding 1 part of cement powders to 99 parts of 0.01 mol/l KCl solution. The prepared cement suspensions had a ph value in range of 12.5~12.8. The sodium sulfate is added to the suspensions. Then Zeta Potential of cement suspensions is determined at 20±2 C using a Colloidal Dynamics electroacoustic-based Zeta Probe analyzer (USA). 2.4 Adsorption Measurements The amounts of PCE comb-like copolymers adsorbed were determined by means of a total organic carbon analyzer, Multi N/C3100 (Analytikjene AG, Germany). 20 g of solution containing various amount of PCE and 10 g of cement were mixed by a magnetic stirrer for 5 minutes at 20 C. The sample solution was separated by a suction filter. The aqueous phase was 528 Polymers & Polymer Composites, Vol. 22, No. 6, 2014

3 Effect of Sulfate Ion on the Performance of Polycarboxylate Comb-Like Copolymers in Cement Suspensions separated by centrifuging at 13,000 rpm for 5 minutes. The supernatant was immediately decanted and dilute with deionized water for TOC. Several aliquots of each sample were measured. The difference in the concentration before and after contact with the cement was assumed to be adsorbed polymer. 2.5 Dispersion Behaviour Cement pastes were mixed at 20 C and at water to cement ratio (w/c) =0.23, using a Hobart mixer. First, PCE and water were weighed into a bowl. Then 300 g of cement was added and mixed for 1 min at low speed and a further 2 min at high speed. The amount of PCE added was expressed as a percentage of dry solid with respect to the mass of cement. The cement paste was used to measure fluidity and rheological behaviour at the same time. The fluidity of cement paste was evaluated by a flow test. The paste flow was measured at 20 C by pullout spread of cement paste from a cone (upper inner diameter 36 mm, lower inner diameter 60 mm and height 60 mm), in accordance with GB surface of cement particles due to electrostatic attraction. The zeta potential data indicated that the addition of sulfate ion could influence the surface charge of cement particles, so a series of experiments was held to reveal the effect of sulfate ion on the adsorption of PCE as shown in Figure 3. It could be clearly seen that the adsorption ratio of PCE decreased as the addition of sulfate ion increased. But it is interesting that the PCE with higher contents of carboxyl suffered less decrease in adsorption with the addition of sulfate ion (the sequence of PCE in content of carboxyl: PCE1<PCE2<PCE3<PCE4). So it could be considered that sulfate ion and PCE are competitive in the adsorption process, as shown in Figure 4. On the surface of cement particle, the Ca ions give the surface a positive charge. Figure 2. Zeta potential curve of cement suspensions with different additions of sulfate ion 3. RESULTS AND DISCUSSION 3.1 Zeta Potential Test As shown in Figure 2, after adding sulfate ion, the potential of the cement particle surfaces decreased rapidly from 1.2 mv to 0 when the content of sulfate ion was increased to 160 mmol/l. When increasing the content of sulfate ion to more than 160 mmol/l, the potential of particle surface reversed to become negative. The results indicated that adsorption of sulfate ion on the surface of cement particles had place, and the surface charge of cement particles had therefore been influenced, which is also proven by L. Divet 11. Figure 3. Influence of SO 4 concentration on the adsorption of PCE onto cement particles 3.2 Adsorption Behaviour The negatively charged PCE molecule is easy to adsorb on the positive Polymers & Polymer Composites, Vol. 22, No. 6,

enhancing the addition of sulfate ion (The sequence of PCE in content of carboxyl: PCE1<PCE2<PCE3<PCE4), which was consistent with the result of adsorption behaviour. 4. Conclusions Figure 5.

4 Qianping Ran, Min Qiao, Yinhui Yu, and Jiaping Liu Figure 4. The competition between SO 4 and PCE in the process of adsorption on the surface of cement particles in Figure 4, the PCE with higher content of carboxyl had less decrease in dispersion when enhancing the addition of sulfate ion (The sequence of PCE in content of carboxyl: PCE1<PCE2<PCE3<PCE4), which was consistent with the result of adsorption behaviour. 4. Conclusions Figure 5. Influence of SO 4 concentration on dispersion of cement pastes containing PCE dispersant (the dosage of PCE was 0.2% of the weight of cement) Polycarboxylate comb-like copolymer (PCE) is a kind of polyelectrolyte, which is easily affected by the ions in the ambient solution. Among these ions, sulfate (SO 4 ) is the most important type of ion. It has been found that the addition of sulfate ion can decrease the positive charge on the surface of cement particles. The sulfate ion reduced the adsorption and dispersibility of PCE in cement suspensions. But increasing the charge density in copolymers can weaken the influence of sulfate ion. It is considered that the dispersibility of PCE depends on both of the adsorption and steric repulsion. In this paper, it was shown that the molecular structure of PCE could be optimized to weaken the influence of sulfate ions in cement suspensions. PCE with high charge density was less sensitive to the sulfate ions in cement suspensions. When added to the suspensions, SO 4 anions have strong ionic bonding with Ca 2+, which makes the sulfate ions more easily adsorbed; hence the adsorption of PCE is suppressed. When the charge density of PCE is low, the driving force for adsorption is weak, which means that in the competitive adsorption process sulfate ion could easily displace the PCE. So the adsorption ratio of PCE1 decreases rapidly when the amount of added sulfate ion is enhanced. When the charge density of PCE is high, the drive force for adsorption is strong, which meant that in the competitive adsorption process PCE is able to defeat sulfate ion. So the adsorption ratio of PCE1 had no distinct changes when the amount of added sulfate ion is enhanced. 3.3 Dispersion Behaviour It is considered that the dispersibility of comb-like copolymers is dependent on the adsorption and steric repulsion. Comb-like copolymers with higher adsorption activity and stronger steric repulsion would have better dispersibility. The dispersion behaviour of PCE in cement paste is shown in Figure 5. Because of the same length of side chain, the 4 PCEs would have similar steric repulsion. So the adsorption will play an important role for the dispersion. As shown Acknowledgments This study is financially supported by National Basic Research Program of China (973 Program) (Grant No. 2009CB623205) and National Fund for Natural Sciences (Grant No ). References 1. Yoshioka K., Sakai E., Daimon M., and Kitahara A., Role of Steric Hindrance in the Performance of Superplasticizers for Concrete. J. Am. Ceram. Soc. 80 (1997) Yamada K., Hanehara S., and Honma K., Effect of the chemical structure on the properties of polycarboxylate type superplasticizers, Cement Concrete Res., 30 (2000) Polymers & Polymer Composites, Vol. 22, No. 6, 2014

5 Effect of Sulfate Ion on the Performance of Polycarboxylate Comb-Like Copolymers in Cement Suspensions 3. Yoshioka K., Tazawa E., Kawai K., and Enohata T., Adsorption characteristics of superplasticizers on cement component minerals, Cement Concrete Res., 32 (2002) Ran Q.P. and Somasundaran P., Miao C.W., Liu J.P., Wu S.S., Shen J., Adsorption Mechanism of Comb Polymer Dispersants at the Cement/ Water Interface, J. Disper. Sci. Technol., 31 (2010) Plank J., Vlad D., Brandl A., and Chatziagorastou P., Colloidal chemistry examination of the steric effect of polycarboxylate superplasticizers. Cement Int., 3 (2005) Uchikawa H., Hanehara S., and Sawaki D., The role of steric repulsive force in the dispersion of cement particles in fresh paste prepared with organic admixture. Cement Concrete Res., 27 (1997) Ran Q.P., Somasundaran P., Miao C.W., Liu J.P., Wu S.S., and Shen J., Effect of the length of the side chains of comb-like copolymer dispersants on dispersion and rheological properties of concentrated cement suspensions, J. Colloid Interf. Sci., 336 (2009) Solis F. and Cruz M.O., Collapse of flexible polyelectrolytes in multivalent salt solutions. J. Chem. Phys., 112(4) (2000) Veiga K.K. and Gastaldini A.L.G., Sulfate attack on a white Portland cement with activated slag. Constr. Build. Mater., 34 (2012) Yamada K., Ogama S., and Hanehara S., Controlling of the adsorption and dispersing force of polycarboxylatetype superplasticizer by sulfate ion concentration in aqueous phase. Cem. Concr. Res., 31 (2001) Divet L. and Randriambololona R., Delayed ettringite formation: the effect of temperature and basicity on the interaction of sulphate and C-S-H phase. Cem. Concr. Res., 28(3) (1998) Polymers & Polymer Composites, Vol. 22, No. 6,

6 Qianping Ran, Min Qiao, Yinhui Yu, and Jiaping Liu 532 Polymers & Polymer Composites, Vol. 22, No. 6, 2014

New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties

New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties New Generation Amphoteric Comb-like Copolymer Superplasticizer and Its Properties Changwen Miao,2, Qianping Ran,2*, Jiaping