Synthesis and Performance of HPEG-AA-AMPS-HPA Polycarboxylate Superplasticizer

An clay-resistance and slump-retaining polycarboxylate superplasticizer (PCE-RC) was synthesized using macro-monomer HPEG with different molecular weight, acrylic acid (AA), 2-acrylamide-2-methacrylic acid (AMPS) and 2-hydroxy-1-methylethyl acrylate (HPA) via H2O2-NaHSO3 redox initiator system. Effects of the acid-ether ratio, HPEG molecular weight, the amount of AMPS and HPA on the properties of PCE-RC in cement were discussed and the optimal molar ratio of HPEG3000, AA, AMPS and HPA was 1:4.5:0.1:0.1. As-synthesized polycarboxylate superplasticizer PCE-RC was characterized by GPC and FTIR. The performance of the cement paste doped with 2.0wt% montmorillonite (MMT) and concrete with cement containing MMT were tested in the presence of PCE. Results showed that cement doped with MMT performed superior fluidity and fluidity retention with the addition of PCE-RC. Compared with commercially available superplasticizer AS-KN, PCE-RC in concrete containing MMT presented better clay resistance and slump retaining performance due to the introduction of AMPS and HPA. Herein the negative charged sulfonic groups provided by AMPS could promote more PCE-RC to adsorb on MMT surface, resulting in the decrease of intercalated PCE-RC amount, which is in favor of clay resistance. While the usage of HPA monomer is beneficial for the slump-retaining ability of PCE-RC in cement due to the introduction of ester group.


Introduction
With the development of concrete technology, superplasticizer (SP) has become an essential component of concrete, besides cementitious materials, sand, gravel and water [1], because SP can improve the physical, mechanical and durable performance of concrete [2].In particular, as the third generation of SP, the polycarboxylate superplasticizer (PCE), with the main chain of carboxylate groups and the side chain composed of polyethylene oxide groups, has been widely used to improve the workability of cement and concrete.However, it is found that PCE is sensitive to clay such as montmorillonite, kaolinite and muscovite in sand [3], by weakening its dispersibility in concrete [4].Generally, Montmorillonite (MMT) is more harmful to the performance of PCE than other clay mineral [5], causing the rapid loss of dispersing capacity of concrete [6].MMT is consisted of stacked octahedral aluminate layers sandwiched between two tetrahedral silicate layers, allowing intercalation and swelling of PCE with polyethylene oxide side chains due to its expanding lattices [3], and then reduces the dispersing capacity of PCE [7].This effect could affect negatively the workability and the strength of concrete [8].
In order to improve the clay resistance ability of PCE, the introduction of functional monomer is an effective way from the point view of molecular structure design.It is reported by Xu et al. that a novel PCE for enhancing clay tolerance was prepared by using β-Cyclodextrin (β-CD) as pendant groups.Because of the steric hindrance brought by β-CD, the adsorbed amount of this kind of PCE on MMT was reduced, leading to good clay resistance [9].Chen et al. synthesized a clay-resistance PCE (SAE-PCE) with modified polyether through esterification between hexahydrophthalic anhydride and isobutyl alcohol polyoxyethylene ether (TPEG).The adsorption of SAE-PCE on MMT was decreased because of the electronegative carboxyl groups in the side chain, improving the clay-resistance of PCE [6].Another type of PCE reported was synthesized by copolymerization of a quaternary ammonium salt oligomer, TPEG, AA, nitrilotrimethylene triphosphonic acid and 2-hydroxy-1-methylethyl acrylate (HPA).The experimental results showed that this type of PCE had good cement performance and excellent clay compatibility [10].However, the functional monomers mentioned above should be prepared before the PCE synthesis, complicating the synthesis process.Feng et al. synthesized a clayresistant PCE by using TPEG, AA, unsaturated binary carboxylic acid and a small functional monomer.The functional monomer was introduced into the structure of PCE to enhance the slumploss and clay-resistance [11].However, the structure of the small functional monomer was unknown.Using [2-(methacryloyloxy)ethyl]trimethylammonium chloride solution (MAEAC) as a modified monomer, Ren et al. prepared a kind of PCE with a new anchoring group.The cationic group in the PCE improved its adsorption ability on MMT and clay tolerance [12].However, the high price of MAEAC limited its application further.2-acrylamide-2-methacrylic acid (AMPS) can be used as a monomer for PCE to achieve good clay resistance, because of its acylamino group and sulfonic group [13].Wu et al. found that the introduction of AMPS to the structure of PCE was beneficial to the clay resistance [14].Nevertheless, owing to its poor calcium complexing ability, the slump loss of concrete would be serious.
This study synthesized a clay-resistance and slump-retaining type PCE (PCE-RC) with methylallylpolyoxyethylene ether (HPEG) of different molecular weight, AA, AMPS and HPA through a redox-initialized polymerization.Owing to the high ion character, the introduction of AMPS into PCE-RC is beneficial to the dispersibility of cement paste containing MMT, but disadvantageous to the slump-retaining ability.While the usage of HPA could solve this problem well, because the ester group introduced into PCE molecules by HPA would release carboxyl groups which could be adsorbed on the cement particle and improve the slump-retaining ability.This study can provide a direction for further development of PCE to enhance the clay resistance.

Preparation of PCE
PCE was synthesized by following the synthetic route listed in figure 1.The agitator, constant pressure funnel and thermometer were equipped on the four-neck round-bottom flask.Then a certain amount of water and HPEG macro-monomer were placed into the flask.The mixture was bubbled under nitrogen to remove oxygen while stirring and the initiator H2O2 was added after slowly heating up to 60 ℃.Consequently, both solutions of I and II were added dropwise into the reaction flask for 3.0 and 3.5 h, respectively.Herein, the solution I was composed of AA, AMPS or HPA and water.The solution II was consisted of water, NaHSO3 and HE.Then the mixture in the flask was stirred for 60 min.After cooling to room temperature, NaOH was added to adjust the pH=7.0 of the product, and a solid content of 40 wt% PCE was obtained.The monomer ratios used in the reaction were shown in table 1.The amount of HE and H2O2 are 1.8 wt % and 1.5 wt % of the total monomer mass, respectively, and the mass ratio of H2O2 and NaHSO3 is 1:0.8.

Table 1. Monomer ratios for PCE synthesis
Note: a, b and c are the mole amount of AA, AMPS and HPA, respectively.Fourier transform infrared spectroscopy.The PCE-RC was placed in a semi-permeable dialysis bag with 5000 Da, and the deionized water was changed every 6 hours for a total of 48 h.The refined PCE-RC powder sample was obtained by vacuum drying at 60 ℃ for 24 h after dialysis.After grinding with potassium bromide and pressing tablets, infrared spectral analysis (FTIR) was performed in the wavelength range of 400~4000 cm-1 using Vertex 70 infrared spectrometer of Brock, Germany.

2.3.3.Fluidity of cement paste.
Reference cement and MMT were used as cementitious materials, in which MMT replacement ratio of cement was 2.0 wt %.The fluidity of cement paste was tested in accordance with GB/T 8077-2012.The dosage of PCE was 0.15 % (solid content) based on the weight of cement and MMT.

2.3.4.Performances of PCE in concrete.
The performance of PCE in concrete was tested according to GB 8076-2008.The reference cement and the continuous graded gravel of 5-20 mm were used, and the MMT was equal to 2 wt% of the cement, and PCE dosage was 0.20 wt % (solid content) based on the weight of cement and MMT.

2.3.5.Adsorption measurement.
A Shimadzu TOC-5000A total organic carbon analyzer (TOC) was used to detect the adsorption capacity of PCE on the surface of cement or MMT. 10 mL of PCE-B or PCE-RC solution with concentration of 2 g/L were mixed with 0.2 g of MMT (or 9.8 g of cement) and stirred for 30 min.Then sample was processed using high-speed centrifugation at 8500 rpm for 10 min, and the supernatant was filtrated with a disposable filter.Then, the filtrate was diluted to a certain concentration for detection.PCE-B or PCR-RC solution with the same concentration before adsorption were used as blank samples, respectively.

FTIR analysis
The FTIR spectrum of PCE-RC is shown in figure 2. As can be seen, the stretching absorption peak of C-H bond in PCE-RC appears at 2919 cm -1 , while the absorption peak at 1570 cm -1 is attributed to the stretching vibration peak of carboxyl group C=O.The characteristic absorption peak of ether bond C-O-C in the side chain of ethylene oxide appears at 1110 cm -1 [15].A characteristic absorption peak of sulfonic group is found at 1361cm −1 [16].According to FTIR spectra, carboxyl, ester, sulfonic and polyoxyethylene groups successfully polymerizes into the PCE-RC molecular structure.

3.2.GPC characterization
The molecular weight and molecular weight distribution of PCE-RC are tested by GPC, as shown in figure 3. Figure 3 shows that there is a main peak appeared when the retention time is about 12.5 min in the GPC spectrum, classified as PCE-RC.It signifies that the copolymer appears to be quite pure.The weight average molecular weight Mw is 81281 g/mol and the number average molecular weight Mn is 48386 g/mol, with the molecular weight distribution index (PDI) 1.68.It shows that the molecular weight of PCE-RC is concentrated and distributed evenly, proving that the PCE-RC is synthesized efficiently.

Effect of acid-ether ratio of PCE-B monomer on fluidity of cement paste containing MMT
The effect of molar ratio of HPEG to AA (1:a) on the dispersion property of PCE-B in cement paste is investigated by using HPEG2400 as macro-monomer and AA as small monomer.Figure 4 shows that MMT evidently presents an adverse effect on the workability of cement, resulting in a significant reduction in the fluidity.This is mainly because MMT consumes a large amount of PCE containing PEO side chains through surface physical adsorption and intercalation adsorption of PCE on MMT particles [3], which reduces the PCE dose for dispersing cement particles and thus diminishes the fluidity.
With the increase of acid-ether ratio in synthesis of PCE-B, the addition of PCE-B makes the fluidity of cement paste with and without MMT increase at first and then decrease, while the loss of fluidity induced by the existence of MMT decreases all the way.When the acid-ether ratio for PCE-B is increased, more carboxyl groups are introduced into the molecular structure of PCE-B and result in the greater adsorption capacity on the cement paste and the better dispersion property [17].At the same time, the higher the ratio of acid-ether is, the lower the side chain density of PCE-B would be.As a result, there will be a smaller amount of PCE molecules being intercalated by MMT.Thereby, more PCE are used to disperse cement particles, resulting in a smaller fluidity loss rate of cement paste containing MMT. Accordingly, the molar ratio of AA and HPEG was determined to be 4.5:1.

Effect of molecular weight of HPEG on the fluidity of cement paste containing MMT
When AA is selected as unsaturated acid monomer and n(AA): n(HPEG) is 4.5:1, the effect of molecular weight of HPEG on the dispersion property of PCE in cement is investigated.As can be seen from figure 5, the higher the molecular weight of HPEG is, the greater the fluidity of the cement paste with and without MMT would be.In the case of the same acid-ether ratio, the larger molecular weight of HPEG means the longer the side chain in PCE, leading to the greater steric hindrance between cement particles.This is helpful for dispersing cement particles and improve the dispersion performance of cement paste.This trend is the same as the fluidity of cement containing MMT.When using HPEG with molecular weight of 1200, 2400 and 3000 to synthesize PCE, the fluidity loss rate of cement paste with and without MMT is 47.0%, 42.9% and 40.8%, respectively.The reason is that PCE with the longer side chain will be consumed less by the intercalation adsorption on MMT [18], and more PCE can be adsorbed on the cement particles to improve the dispersing property, resulting in the smaller loss rate of cement paste fluidity.Therefore, HPEG3000 is determined as the macro-monomer in the polymerization reaction of PCE.Consequently, PCE-B was synthesized under the condition of n(HPEG3000):n(AA)=1:4.5.

Effect of the amount of AMPS on the fluidity of cement paste containing MMT
With HPEG3000 as macro-monomer, AA and AMPS as small monomer, under condition of AA: AMPS: HPEG3000= As shown in figure 6, with the increase of AMPS dosage, the initial and 1 h fluidity of cement paste doped with MMT increases at first and then decline.When the molar ratio of AMPS to HPEG3000 is 0.1:1, the value of fluidity reaches to a plateau.As we know, sulfonic group has the higher electronegativity than carboxyl group [13].The introduction of sulfonic group into the molecular structure of PCE-RC enhances its adsorption on MMT and cement particles, resulting in good fluidity of cement containing MMT.However, the complexing ability of sulfonic group with calcium ions is weaker than that of carboxyl group, which is harmful to the retardation of cement hydration and more amount of PCE would be consumed during the process of cement hydration, leading to the fluidity loss of cement.When the amount of AMPS is too large, the fluidity loss of cement paste containing MMT can be obviously observed from figure 6.According to the results, the molar ratio of AMPS to HPEG3000 is determined to be 0.1:1.

Effect of the amount of HPA on the fluidity of cement paste containing MMT
With HPEG3000 as macro-monomer, AA and AMPS as small monomer, under condition of AA: AMPS: HPA: HPEG3000=(4.5-c):0.1:c:1, the influence of HPA dosage c on the dispersing property of PCE-RC in cement is explored.
As can be seen from figure 7, when PCE-RC is added into cement, with the amount of HPA for synthesizing PCE-RC becoming higher, the fluidity of cement paste doped with MMT declines gradually but the 1 h fluidity increases.This is because that the introduction of HPA into the molecular structure of PCE-RC decreases the amount of anion anchoring group, which is harmful to the adsorption of PCE-RC on cement particle and its initial fluidity.However, with time elapsing, carboxyl groups would be released by the ester group in PCE-RC in the alkali environment of cement paste and be adsorbed on the cement particles to disperse them once more.Therefore, the 1 h fluidity of cement containing MMT increases.Consequently, the molar ratio of HPA to HPEG3000 is determined to be 0.1:1.Based on the experimental data above mentioned, the optimum molar ratio of materials for synthesizing PCE is n(HPEG3000):n(AA):n(AMPS):n(HPA) is 1:4.5:0.1:0.1

Surface tension of PCE
In order to investigate the surface activity of PCE, the surface tension of aqueous solution of superplasticizer is measured.The curves in figure 8 show the surface tensions of PCE-RC and PCE-B in water at different concentrations.As can be seen, the surface tension values of these two PCE solutions show a downward trend with the increase of PCE concentration.When the concentrations of both PCE solutions are less than 0.1g /L, their surface tensions decline sharply with the increase of the concentration, and then the decrease gradually slows down.At the same concentration, PCE-RC solution exhibits the lower surface tension value than that of PCE-B, indicating that the surface activity of PCE-RC is higher.When the concentration of PCE solution is 50 g/L, the surface tension of PCE-B and PCE-RC is 35.15 mN/m and 33.92 mN/m, respectively.Generally speaking, a higher surface activity will contribute to a stronger adsorption capacity of PCE on cement particles and consequently lead to a better flow properties of cement paste [19] .Therefore when compared with PCE-B, PCE-RC exhibits the higher surface activity, meaning a better dispersing ability in cement.

Adsorption amount
As we all know, PCE, as the widely used superplasticizer, has exhibited high performance in cement.This is usually attributed to the effective electrostatic repulsion and steric hindrance effect provided by the PCE molecule being adsorbed on the cement surface.Because the negative charges from the carboxylic group as well as the sulfonic group of as-synthesized PCE can be adsorbed on the positively charged surface of cement minerals via the electrostatic interaction.Therefore, the adsorption behavior of PCE in cement system plays a key role in their dispersion ability.To analyze the interaction between PCE and cement particle, TOC is used to measure the adsorption capacity of PCE on the surface of cement particles and MMT. Figure 9 (a) shows that the adsorption capacity of PCE-RC and PCE-B on the surface of cement paste containing MMT is 1.32 mg/g and 1.26 mg/g, respectively.The larger adsorption capacity of PCE-RC on the particle surface leads to the better fluidity than that of PCE-B.As can be seen in figure 9 (b), the adsorption capacity of PCE-RC on the surface of MMT particles is significantly lower than that of PCE-B.This is because that the sulfonic group in the molecular structure promotes more amount of PCE-RC to be adsorbed on the MMT surface, producing strong repulsive force and hindrance force to prevent PCE closing to MMT and being intercalated by MMT, which is beneficial to improving the clay resistance of PCE-RC.Based on the above experimental data, the clay resistance mechanism of PCE-RC is analyzed as shown in figure 10.There are two adsorption ways of PCE on MMT, surface adsorption and intercalation adsorption.The intercalation adsorption is the main way.As a monomer with high ionic character, AMPS exhibits stronger adsorption character than AA [20].Compared with PCE-B whose structure is the same as conventional PCE, the introduction of AMPS in PCE-RC molecules makes more PCE-RC be adsorbed on the MMT surface, leading to the decrease in the amount of intercalated PCE-RC.On the other hand, the more amount of PCE-RC on the MMT surface produce stronger repulsive force and hindrance force to prevent PCE closing to MMT and intercalating.Reasons for the above two aspects makes less amount of PCE-RC be consumed by MMT and more amount of PCE-RC be used to disperse cement particles.Therefore, PCE-RC performs better clay resistance than PCE-B

3.10.Performances of PCE-RC in concrete
In order to investigate the clay resistance in concrete, the contrast test between PCE-RC and AS-KN is performed.The mix proportion of concrete is m (cement containing MMT):m (sand): m (gravel): m (water)= 380:850:1050:170.The amount of MMT is 2 % based on the weight of cement and PCE dosage is 0.2 % (solid content) based on the weight of cement and MMT.
Table 2.The performance of concrete doped with MMT AS can be seen from table 2, compared with the commercially available clay-resistance type PCE (AS-KN), the fluidity and slump-retaining performance of concrete with PCE-RC is better, as well as the workability.Moreover, the compressive strengths of concrete with PCE-RC after 3 d, 7 d and 28 d, respectively, are all higher.Consequently, PCE-RC behaves better in concrete doped with MMT than AS-KN.
(2) GPC and FTIR spectra show that the carboxyl group, polyethylene group, sulfonic group and ester group are all introduced into the structure of PCE-RC which has a relatively narrow molecular weight distribution.
(3) The test of concrete doped with MMT indicates that PCE-RC exhibits better clay-resistance and slump-retaining performance than that of commercially available AS-KN.
(4) Because of the high ion character, the introduction of AMPS could promote more PCE-RC to be adsorbed on MMT surface, resulting in the decrease of intercalated PCE-RC amount and the improvement of clay resistance.Meanwhile the usage of HPA monomer in the synthesis of PCE is beneficial for the slump-retaining ability of PCE-RC due to the introduction of ester group.

2 .
PCEsn(HPEG):n(AA):n(AMPS):n(HPA) Gel permeation chromatography.The molecular weight and molecular weight distribution of PCE-RC were determined by Agilent 1260 gel chromatograph (GPC).The narrow-distributed polyethylene glycol standard group was used as the standard sample, and the mobile phase was 0.1mol/L NaNO3 solution.

Figure 4 .
Figure 4. Effect of the acid-ether ratio for PCE-B on the initial fluidity of cement paste containing MMT.

Figure 5 .
Figure 5.Effect of HPEG molecular weight for PCE-B on the initial fluidity of cement paste containing MMT

Figure 6 .
Figure 6.Effect of AMPS amount for PCE-R on the initial and 1 h fluidity of cement paste containing MMT.

Figure 7 .
Figure 7. Effect of HPA amount for PCE-RC on the initial and 1 h fluidity of cement paste containing MMT

Figure 8 .
Figure 8.Effect of concentration of PCE-RC and PCE-B on the surface tension of aqueous solutions

Figure 9 .
Figure 9. Adsorbed amount of PCE-RC and PCE-B on (a) cement containing MMT and (b) MMT

Figure 10 .
Figure 10.Action mechanism of PCEs and MMT (a) PCE-B and (b) PCE-RC