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A polycarboxylate as a superplasticizer for montmorillonite clay in cement: Adsorption and tolerance studies
⁎Corresponding author at: School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China. 452736102@qq.com (Jiaheng Lei)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
A novel polycarboxylate superplasticizer (PCE) with a long polyoxyethylene (PEO) chain and a terminal carboxylic group was synthesized from a modified polyether (SAE-IPEG) to increase its performance in cement. The molecular structure of the PCE was characterized by infrared spectroscopy and 1H nuclear magnetic resonance (NMR) spectroscopy. The performance of synthesized PCE in cement was studied in the absence and the presence of montmorillonite (Mmt) clay. It was found that the PCE disperses in cement uniformly without aggregation, which is different significantly from the conventional PCEs. Adsorption measurements and X-ray diffraction analysis revealed that the synthesized PCE only interacted with Mmt via surface adsorption, whereas the conventional PCEs interact with the clay through the surface adsorption and the chemical intercalation. Such dramatic change could be ascribed to the introduction of an electronegative carboxylic acid group as a terminal group into the long polyoxyethylene chain of PCE, which reduced the adsorption and enhanced tolerance of PCE on Mmt.
Keywords
Adsorption
Tolerance
Polycarboxylate
Clay
Cement
1 Introduction
As a new generation of high-performance water-reducer in cement, polycarboxylate superplasticizer (PCE) has attracted considerable interests in academia and industry, compared with naphthalene sulfonate-formaldehyde superplasticizer (NFS) (Yoshioka et al., 1997; Halim et al., 2017). The NFS is a linear polymer, while PCE is a comb-type polymer, which is usually prepared by grafting long side chain with a polyoxyethylene (PEO) group (Yamada et al., 2000; Rahman et al., 2017). The side chain on the PCE provides an effective barrier to the aggregation of cement particles that are suspended in water (Winnefeld et al., 2007 De’nan et al., 2017). Therefore, excellent dispersion performance of PCE was observed. Considering the environmental friendliness, high water reducing rate, long slump retention rate, and easy structural modification, PCE has gradually become an alternative to NFS as a concrete water-reducer (Habbaba et al., 2013; Alonso et al., 2007; Singh et al., 2012; Liu et al., 2012; Kong et al., 2016; Dalas et al., 2015).
Polycarboxylate superplasticizer also exhibited some limitations. A major concern is its poor tolerance to clay minerals in cement (Hassan and Ismail, 2017). The PCE cannot effectively prevent the clay minerals in concrete from forming aggregates (Ismail and Hanafiah, 2017). This is nontrivial as it will lead to a dramatic decrease of dispersion of clay minerals in concrete and its stability. For practical application, it is desirable for clay to disperse in concrete uniformly without forming aggregates in a certain period of time (Atarashi et al., 2004; Aziz and Hanafiah, 2017; Khan et al., 2017). The main clay minerals in concrete include montmorillonite (Mmt), illite and kaolin. The Mmt has a stratified structure consisting of silica tetrahedron and alumina octahedral in a 2:1 ratio (Norvell et al., 2007; Aslam et al., 2017). The adsorption of PCE on Mmt is approximately 100 times larger than that of Portland cement (Ng and Plank, 2012; Razali et al., 2017a). Strong affinity of PCE through side chain to Mmt enables an effective insertion of PCE into the interlayer of Mmt, thus inhibiting the performance of PCE in fresh concrete (Lei and Plank, 2014a,b; Razali et al., 2017b; Nordin et al., 2017). Some researcher reported that intercalation adsorption is the dominant mechanism for Mmt-PCE adsorption (Ng and Plank, 2012; Lei and Plank, 2014a,b). When the concrete aggregate contains clay minerals, the aggregates often cause the rapid loss of the fluidity of the concrete, therefore, the increased amount of PCE used to meet the requirements of concrete. Such a phenomenon is often referred to as poor clay tolerance of PCE (Roslan et al., 2017). Under such circumstance, the tolerance of PCE to clay minerals in cement needs to be improved to increase the fluidity of the cement.
A variety of functionalized PCEs have been reported and their performance in concrete was also studied. In other studies, a group researcher stated that synthesized two PCEs without a PEO long side chain (Lei and Plank, 2012; Lei and Plank, 2014a,b). The first type of PCE was synthesized using methacrylic acid and hydroxyalkyl methacrylate, while the other type of PCE was synthesized using maleic anhydride, maleic monoalkyl ester, 4-hydroxy-butyl-ethylene ether as materials. The dispersion capacity of two PCEs is less affected when Mmt content is 1%. XRD test shows that PCEs did not intercalate into Mmt, and only a small amount of PCEs are observed. The absence of a chain prevents the PCE from intercalating into Mmt. Other researchers also conducted numerous studies on novel PCE without PEO long side chain. Other researcher prepared amphiphilic polycarboxylic acid copolymer (APCs) using 3-2-(methacryloyloxy) ethyl (ZI) and dimethylammonio-propane-1-sulfonate (DMAPS) as amphiphilic monomers and found that that the APCs with the ZI group was more likely to be adsorbed on clay surface, but not penetrate into clay layer structure (Li et al., 2016). A scientist also prepared a PCE from a reaction of 2-(dimethylamino)ethylmethacrylate (DMAM), acrylic acid and itaconic acid (Xing et al., 2016). They found the synthesized PCE had sound dispersion capacity in the presence of clay due to the weak adsorption of PCE into the inner layer of clay, however, some PCEs still adsorbed on the clay surface. Since intercalation adsorption can be avoided by removing the PEO long side chain in PCE, grafted β-cyclodextrin as an anti-clay group into PCE molecular structure under the condition of maintaining a PEO long side chain (Xu et al., 2015, 2016). An improved tolerance to clay was observed, which was ascribed to the reduced adsorption of PCE on the Mmt due to the bulky structure of β-cyclodextrin. These studies indicated that the performance of PCEs can be tuned by structural modification.
Recent studies showed that surface of the Mmt is electrically charged. Some researcher found that under the alkaline condition the surface and edge of a laminated structure of Mmt particle were negatively charged (Tombacz and Szekeres, 2004). During the course of a study between clay and a polymer, a researcher also found that the negatively charged polymers did not intercalate into the inter layers of clay (Theng, 1982). When Mmt interacts with surfactant, Sanchez-Martin found that the quantity of adsorbed non-ionic surfactant and cationic surfactant on Mmt was much larger than that of anionic surfactant (Martin et al., 2008). These studies indicated clearly that performance of PCEs could be tuned by charge plasticizers. To this end, we prepared a new type of PCE. We used succinic anhydride to esterify and modify the long-chain terminal group of isobutenyl polyoxyethylene ether (IPEG), making the long-chain terminal group negatively charged. Our results revealed that the prepared PCEs exhibited much better tolerance, low adsorption properties, and the resulting concrete is quite uniform.
2 Experimental
2.1 Materials
2.1.1 Chemicals
Isobutenyl polyoxyethylene ether (IPEG, ≥99% purity, Wuhan Oxiranchem Co., Ltd., China, the degree of polymerization is about 48–52), succinic anhydride (SAE), acrylic acid (AA), mercaptopropionic acid, Vitamin C (both ≥99% purity, Aladdin Industrial Corporation, China), hydrogen peroxide (30% aqueous solution, Aladdin Industrial Corporation, China), and polyacrylic acid (PAA, Mw, ∼2000, Aladdin Industrial Corporation, China) were used without further purification. All chemicals were used as received without further purification.
2.1.2 Cement
The cement used in this study was an ordinary Portland cement (PO 42.5R, supplied by China United Cement Group Co., Ltd), and the chemical compositions are shown in Table 1. The average particle size (d50 value, determined by laser granulometry) was found at 11.6 μm. Its density was 3.20 g/cm3 (powdered Lee pycnometer method).
Oxides
SiO2
Fe2O3
Al2O3
CaO
MgO
SO3
Na2O
f-CaO
LOSS
Other
wt%
21.53
2.81
4.41
63.04
1.74
2.94
0.55
0.57
1.51
0.90
2.1.3 Clay
The clay sample used in this study was a sodium Mmt supplied under the trade name of PGW by Nanocor (Chicago, America). This clay mineral is a naturally occurring sodium bentonite possessing a specific surface area (BET method, N2 adsorption) of 36.50 m2/g and was used as obtained. Table 2 provides its oxide composition as determined by X-ray fluorescence (XRF). LOIa = loss of ignition.
Oxide
SiO2
Al2O3
CaO
Na2O
Fe2O3
MgO
K2O
LOIa total
wt%
60.78
13.72
3.26
1.10
1.16
4.61
0.49
13.598.62
2.2 Synthesis
2.2.1 Synthesis of the AA-IPEG copolymer (IPEG-PCE)
In a 250 mL four-neck round-bottom flask, deionized (DI) water (52 mL) was added. The flask was equipped with a condenser and a stirrer. After the water was purged with nitrogen for 30 min, IPEG (70.0 g, 0.029 mol) was then added slowly while the temperature was maintained at 30 °C. Subsequently, aqueous H2O2 (30%, 0.4 g) was added into the mixture to initiate the reaction. Then an aqueous solution of acrylic acid (6.9 g, 0.096 mol) in H2O (20 mL), vitamin C (0.113 g, 0.642 mmol) and mercaptopropionic acid (0.236 g, 0.002 mol) in H2O (26.4 mL) was added simultaneously over 150 min under nitrogen. The reaction temperature was carefully kept at 30 °C throughout all additions. Afterward, the mixture was stirred for an additional 90 min under nitrogen. The final product designated as IPEG-PCE was obtained a light yellowish aqueous solution (∼44.5 wt%).
2.2.2 Succinic anhydride ester monomers (SAE-IPEG)
To a 250 mL three-neck flask was added succinic anhydride (5.8 g, 0.058 mol) and 0.029 mol of IPEG (70.0 g). The resulting mixture was heated at 65 °C for two hours under stirring. Then DI water (75.8 g) was added. The resulting solution was cooled to 20 °C and coded as SAE-IPEG solution. The solution was used directly for reactions without further purification.
2.2.3 Synthesis of the AA–IPEG–co-SAE-IPEG copolymer (SAE-PCE)
The copolymer was prepared in a similar procedure as described for AA-IPEG. Briefly, to a 250 mL four-neck round-bottom flask equipped with a stirrer, a reflux condenser, and two separate inlets was added DI water (45 mL). After the water had been purged with N2 for 30 min, IPEG (63 g, 26.25 mmol) and SAE-IPEG solution (14 g, 2.92 mmol) were added slowly at 30 °C. After 30% aqueous H2O2 (0.4 g) was added to initiate the reaction, acrylic acid (6.9 g, 0.096 mol) in 20 mL of H2O, and the vitamin C (0.113 g, 0.642 mmol) and mercaptopropionic acid (0.235 g, 0.002 mol) in H2O (26.4 mL) in 150 min while maintaining the temperature at 30 °C. Afterward, the mixture was stirred for an additional 1.5 h, a light yellow aqueous solution was obtained. The final product was coded as SAE-PCE-10 (44.5 wt%). The solution was used directly without further purification. The SAE-PCE-20 and SAE-PCE-30 were prepared following the procedure described above except that 28 g (5.83 mmol) and 42 g (8.75 mmol) of the aqueous solution of SAE-IPEG were used to replace IPEG. The sample was exhibited a solid content of 44.5 wt%.
2.3 Characterization of polymers
2.3.1 Size exclusion chromatography (SEC)
Molar masses (Mw and Mn) and the polydispersity index (PDI) of the superplasticizer samples were determined via size exclusion chromatography (commonly known as gel permeation chromatography) on an Agilent 1260 Chromatography System with UV and RID Dual Detector. Separation of the polymer fractions was achieved by using the chromatographic column (PL aquagel-OH MIXED 8 μm, 4.6 × 250 mm). The boric acid-sodium borate buffer solution (pH = 8.5, KCl was added to adjust the ionic strength to 0.3 mol/L) was used as the carrying phase with a flow rate of 0.350 mL/min. A series of sodium polyacrylate polymers with known molar masses were used as the standard samples.
2.3.2 FTIR and 1H NMR spectroscopy analysis
The synthesized copolymers were purified by using ethyl acetate (the unreacted polyether monomer could be removed) and anion exchange resin (the unreacted carboxylic acid monomer could be removed), and were vacuum dried overnight at 80 °C. Fourier transform infrared (FTIR) spectra between 400 and 4000 cm−1 were acquired using an AVATAR370 spectrometer (Thermo Nicolet, USA) with a KBr sample holder. The 1HNMR spectrum was recorded by using a superconducting-magnet NMR spectrometer (Bruker AVANCE III HD 500 MHz) with D2O as the solvent.
2.4 Dispersing performance in cement
A mini slump test was performed to evaluate the dispersion property of cement according to the Chinese standard GB/T 8077-2012. The cement paste with a water−cement ratio of 0.29 was prepared at 25 °C, and the amount of water contained in the polymer solution was subtracted from the mixture. The cement (300 g) was added to the mixer of an aqueous solution of copolymers and slowly agitated for 2 min. After the mixture was rested for 15 s without stirring and then was stirred quickly for 2 min. The cement paste was then immediately poured into a mini slump cone (height 60 mm, upper diameter 36 mm, bottom diameter 60 mm) placed on a glass plate and the cone was vertically removed. After waiting for 30 s, the spread of the paste was measured twice, the second measurement being vertical to the first and averaged to give the final spread value. To observe the cement paste flow in the presence of clay, different amounts of Mmt were added to the mixture as described in the procedure above. The amount of Mmt was based on the weight of cement (300 g).
2.5 Sorption on clay
The adsorbed amounts of superplasticizer samples on Mmt were determined according to the depletion method. In a typical experiment, 1.0 g of Mmt and 100 g of an aqueous solution of copolymer were added into a 100-mL centrifuge tube. After shaken in a shale shaker for 10 min, the suspension was centrifuged at 4000 rpm for 3 min. The supernatant was filtered through a 0.45 μm membrane under vacuum and then diluted with DI water. The concentration of superplasticizer in the filtrate was determined by GPC method, the chromatographic conditions were the same as Section 2.3.1. The calibration curve of peak area and concentration was established by using superplasticizer solution with known concentration as the standard, and the superplasticizer concentration in the sample was calculated. The amount of superplasticizer sample adsorbed was calculated by subtracting the concentration of the superplasticizer found in the centrifugate from that contained in the reference sample.
2.6 XRD analysis
To investigate whether the superplasticizers had intercalated into the structures of clay or not, XRD analysis was performed on Mmt samples which were hydrated for 2 h in the presence and absence of superplasticizer samples. In a typical experiment, a mass of 0.2 g of clay was dissolved in 10.0 g of an aqueous solution with/without 0.25 g 40.0 wt% of the tested superplasticizer (w/clay = 50). The mixture was stirred for 10 min and centrifuged for 15 min at 9000 rpm. The solid part collected from the centrifuge tube was dried overnight at 80 °C and ground. XRD patterns were obtained on a D8 Advance, Bruker AXS instrument (Bruker, Karlsruhe, Germany) utilizing Bragg−Bretano geometry. Samples were placed on a front mounted plastic sample holder. The measuring conditions were as follows: step size 0.15 s/step, nickel filter as incident beam, aperture slit 0.3° and scan range from 0.5° to 10° 2θ.
3 Results and discussion
3.1 Characterization of SAE-PCE polymers
The free radical copolymerization process in an aqueous solution for IPEG, SAE-IPEG and AA was detailed in the experimental section. According to the SEC data listed in Table 3, the synthesized PCEs had Mw values between 36,400 and 38,100 and PDI values between 2.28 and 2.36, indicating a narrow molecular weight distribution of the synthesized polymers. Compared with the conventional IPEG-PCE, which has an Mw of 35,100 and PDI of 2.16, the Mw and PDI value of the synthesized SAE-PCEs did not change too much, indicating the similar reaction pathway for PCEs.
Polymer sample
Molar ratio (SAE-IPEG: IPEG)
Mw (g/mol)
Mn (g/mol)
PDI (Mw/Mn)
IPEG-PCE
0.0:1.0
35,100
16,250
2.16
SAE-PCE-10
0.1:0.9
36,400
15,960
2.28
SAE-PCE-20
0.2:0.8
37,300
16,150
2.31
SAE-PCE-30
0.3:0.7
38,100
16,140
2.36
The synthesized polymers were characterized by the FT-IR spectroscopy. Fig. 1(a) shows the infrared spectrum of regular IPEG, while Fig. 1(b) showed the infrared spectrum of IPEG esterified by succinic anhydride. Through a comparison, a peak corresponding to the C⚌O stretching vibration was observed at 1729 cm−1, which indicated the successful esterification of IPEG by succinic anhydride. The infrared spectra of two types of PCEs (IPEG-PCE and SAE-PCE) were shown in Fig. 1(c) and (d), respectively. The C⚌O stretching vibration was observed at 1729 cm−1, and several peaks around 1567 cm−1 were also observed, which can be ascribed to C—O vibration indicating the existence of hydroxyl, methyl, carboxyl, methylene, polyoxyethylene, esters, groups. The 1HNMR spectrum of prepared PCE along with IPCE were shown in Fig. 2. The peak at δ = 2.0–2.5 ppm can be assigned to —CH— of the main chain, while the peak at δ = 1.5～2.0 ppm is from —CH2— of the main chain. The peak at 3.4–3.8 ppm was assigned to —CH2—CH2—O— of polyoxyethylene ether. A peak at 2.60 ppm (peak A) was observed for SAE-PCE and SAE-IPEG, which can be assigned to ROOC—CH2—CH2—COOR of succinic anhydride. This observation further confirmed the esterification of IPEG by succinic anhydride.
FTIR spectra of IPEG (a), SAE-IPEG (b), IPEG-PCE (c), and SAE-PCE (d).
1HNMR spectra of IPEG (a), SAE-IPEG (b), PCE (c), and SAE-PCE (d).
3.2 Sorption on clay
Sorption measurement were performed to quantify the interaction between individual PCE samples and Mmt. Fig. 3 shows the adsorption profiles of IPEG, SAE-IPEG, PAA by Mmt under room temperature. When IPEG content was 8.00 g/L, the saturated adsorption amounts of IPEG and SAE-IPEG on Mmt surface were about 66 mg/g and 46 mg/g, respectively. The adsorption amount of polyacrylic acid (PAA) on Mmt surface was lower than 10 mg/g. It was found that Mmt was less likely to adsorb negatively charged PAA main chain, but more likely to adsorb polyether containing polyoxyethylene (PEO) long side chain. Results showed that the adsorption amount of negatively charged PAA on Mmt was far lower than that of polyether monomer containing PEO long side chain. Moreover, the adsorption amount of SAE-IPEG on Mmt was significantly lower that of IPEG.
Sorption isotherms for IPEG, SAE-IPEG, PAA samples on Mmt dispersed in water solution (w:c = 100).
Fig. 4 shows the adsorption amounts of PCE and SAE-PCEs on Mmt under different concentrations of PCE. When PCE content was 2.00 g/L, the adsorption amount of SAE-PCE-100 on Mmt is 6.55 mg/g, which is the lowest among all samples and is also much lower than the conventional PCE (12.74 mg/g). This was probably due to the introduction of negatively charged carboxyl group onto a long side chain. The electrostatic repulsion between PCE and Mmt (also negatively charged) was enhanced, therefore the adsorption amount of PCE on Mmt was reduced. Therefore, the adsorption amount of SAE-PCE and Mmt was lower than that of conventional PCE. In addition, it was found that the adsorption amount of SAE-PCE decreased with the increase of substitution proportion of SAE-IPEG.
Sorption isotherms for various superplasticizer samples on Mmt dispersed in water solution (w:c = 100).
3.3 Cement dispersion
The effect of synthesized PCEs on dispersing was also studied. When PCE was set at the w/c ratio of 0.29, the dosages that are required to achieve a cement paste spread of 26 ± 0.5 cm were determined. A shown in Fig. 5, the synthesized PCE fluidized the paste at a dosage that is comparable to conventional PCE. The SAE-PCE-10 and SAE-PCE-20 showed the lower dosage than conventional PCEs. In a mini slump test that was carried out over 60 min, the slump retention over time was recorded and the spreading flow vs the time was shown in Fig. 6. It was found that the dispersion-retaining capacity of cement pastes with all four types of plasticizers decreased with the time. However, the decrease of cements with conventional PCE decreased much faster than SAE-PCEs during the hydration. The dispersion-retaining capacity of cement paste with SAE-PCE was better than others with the SAE-PCE-20 the best. This could be due to the hydroxyl groups on the side chain.
Dosages of superplasticizers required to achieve a cement paste spread of 26 ± 0.5 cm (w/c = 0.29).
Spread flow of cement pastes (w/c = 0.29) containing synthesized copolymers over a period of 1 h.
3.4 Cement dispersing power in the presence of clay
The spread flow of cement paste was also measured with different contents of clay. In this test, different dosages of clay bwoc was added to the cement, and the total amount of cement and clay remains constant. To compare the slump loss changes with the increasing content of clay more vividly, the same initial spread flow of cement paste was obtained, even though the dosages of superplasticizer was different. The quantity of plasticizer in the cement was also fixed. It was found that the initial spread flow of cement paste was quite similar to each other under different dosages of superplasticizer. As shown in Fig. 7, the dispersing effectiveness of conventional PCE on cement dispersion was strongly affected by the quantity of clay. When the content of clay increased to 3.0 wt% bwoc, the spread flow of cement paste decreased by 52.0%, indicating significant loss of PCE dispersing capability. In contrast, the spread flows of SAE-PCE-10, SAE-PCE-20 and SAE-PCE-30 were only decreased by 38.4%, 32.0% and 39.9%, respectively, in the presence of 3.0 wt% (bwoc) clay. These results demonstrated that terminal carboxyl in the PEO chain enhanced dispersing ability of cement toward clay dramatically due to presence of terminal hydroxyl group compared to conventional PCEs with terminal hydroxyl groups. Fig. 8 shows the dispersion retaining capacities of PCE and three types of SAE-PCE to cement paste containing 3.0 wt% Mmt. After PCE interacts with Mmt containing cement paste for 60 min, the fluidities of cement paste containing PCE, SAE-PCE-10, SAE-PCE-20, SAE-PCE-30 were respectively decreased by 48.21%, 35.48%, 29.71%, 37.14%. Results showed that under tested conditions, the dispersion capacities of four water reducers were all decreased with SAE-PCE being the most, indicating a better tolerance of SAE-PCE to Mmt compared to conventional PCEs.
Spread flow of cement pastes (w/c = 0.29) containing synthesized copolymer measured in the presence of different clay contents by weight of cement (bwoc) (PCEs dosage 0.10%).
Spread flow of cement pastes (w/c = 0.29) containing synthesized copolymer measured in the presence of 3% clay contents bwoc (PCEs dosage 0.18%).
3.5 Mode of interaction with clay
To probe the interaction between individual PCE samples and Mmt, XRD analysis was performed for IPEG-PCE+Mmt, SAE-PCE+Mmt and hydrated Mmt. As shown in Fig. 9, hydrated clay exhibited a d-spacing of 1.21 nm. When the conventional IPEG-PCE with terminal hydroxy group was present, the d-spacing of Mmt shifted from 1.21 nm to 1.70 nm, indicating that the intercalation of the polymer into the alumosilicate sheets has occurred. This chemisorption was caused by the high affinity of PEO side chains to the alumosilicate layers in the Mmt structure. For the SAE-PCEs with terminal carboxyl group, the d- spacing of Mmt remained constant as 1.21 nm. It is well-known that Mmt has a thin alumsilicate sheet structure. It is composed of two tetrahedral sheets sandwiching a central octahedral sheet. Since the high-priced cations in the layered structures are substituted by low-priced cations, the alumsilicate sheets exhibit a permanent negative charge on the faces; however, the charge states on the edges are pH-dependent as shown in Fig. 10 (Tombacz and Szekeres, 2004). Under high pH (∼13), the Mmt in cement paste will also generate negative charges from the deprotonation of these sites on the edges. These faces and edges will experience a strong repulsion from anions. For SAE-PCE that was treated by a base has already been deprotonated, therefore its backbone and side chains are negatively charged. When it was mixed with cement, it produced a repulsion due to the counter charge interactions with the clay. However, due to the introduction of side chains SAE-PCE will experience high steric hindrance for interaction inside the layers. They will mainly interact with Mmt through edges. As a result, d-spacing of clay did not change, which is sufficient to enhance the dispersion capacity but not decrease the spreading flow of cement as we observed. However, when the PCE was used in the mixture of cement and Mmt, the competitive adsorption between cement and Mmt should be also considered. Fully esterifying of long side chain terminal group into carboxyl, the negative charge amount of PCE would be significantly changed, thus leading to a big impact on its adsorption condition on Mmt. Based on fluidity test of cement paste, it is quite clear that only the long side chain terminal group of PCE was partially esterified, which can make the PCE molecule having anti-clay property while maintaining good dispersion capacity simultaneously. Therefore, the substitution proportion of SAE-IPEG for IPEG should be controlled within 10–20%.
XRD patterns of Mmt hydrated in solution holding different superplasticizers (w/c = 50).
Schematic illustration of the adsorption of SAE-PCEs containing carboxyl terminal group on clay particles.
4 Conclusions
Polycarboxylate superplasticizers (PCE) with terminal carboxyl groups have been synthesized successfully from free radical copolymerization reactions. Synthesized PCEs were characterized by FT-IR and 1H NMR spectroscopy. The PCEs possess high cement dispersing ability and good clay tolerance. XRD experiments suggest that the newly synthesized terpolymers undergo only weak interaction with clay, opposite to conventional PCE products. When a larger quantity of esterified macromonomer was used during the synthesis, more carboxyl groups will be introduced to the PCE backbone, which led to the smaller the adsorption amount of PCE on Mmt. The adsorbed quantity of PCE on Mmt can be reduced by negatively charged carboxyl terminal group. As a result, the cement dispersing power was increased due to the electrostatic repulsion between carboxy groups and negatively charged clay surfaces.
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