Grafting tertiary amine groups into the molecular structures of polycarboxylate superplasticizers lowers their clay sensitivity

Abstract

A monomer containing a tertiary amine group was employed to synthesize polycarboxylate superplasticizers (PCEs) preparing from acrylic acid (AA), itaconic acid (IA) and 2-(dimethylamino) ethyl methacrylate (DMAM). The terpolymers were characterized and tested for their dispersion performance in the absence and presence of montmorillonite (Mmt). These novel PCEs displayed better dispersion performances in cement in the presence of Mmt than did conventional PCE. Adsorption measurements and X-ray diffraction analysis suggested that these novel PCEs interacted with Mmt only via surface adsorption, in contrast to the interaction of conventional PCE with Mmt by both surface adsorption and chemical intercalation.

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1. Introduction

Polycarboxylate superplasticizers (PCEs) are new-generation superplasticizers introduced in the 1980s.¹ PCEs display several outstanding features such as a high water reducing ratio of 25–50%,² long slump retention times, designable molecular structures,³ observable improvement of the mechanical strength of the concrete⁴ and environmental friendliness.⁵ PCEs are therefore replacing the other types of plasticizers. Many kinds of common PCEs have been produced, including MPEG-type,⁶ APEG-type,⁷ HPEG-type,⁸ TPEG-type (also known as IPEG-type),⁹ and VPEG-type¹⁰ PCEs. The TPEG-type PCE has in recent years been gradually taking the place of other types of PCEs because of its excellent performance. It has been used in industry widely, especially in China.⁹ All of these PCEs share certain structural characteristics. They all have a comb-like molecular structure and polyethylene oxide (PEO) side chains grafted to the backbone, which provide the steric hindrance effect.

Despite the excellent performance features of these PCEs, they also suffer from certain drawbacks. One of these problems is clay sensitivity, which results from the sensitivity of the PEO to clay.¹¹ The impacts of three different clay minerals, namely montmorillonite (Mmt), kaolinite and muscovite, on the dispersion force of the superplasticizer have been studied. It was found¹² that the conventional PCEs were negatively affected by each of the clays, with the extent of the effect being far greater for Mmt than the two other clays, whereas PEO-free PCEs were only slightly affected by the three clays, with the results of this study suggesting no serious problem posed by the contamination of concrete with clay impurities as long as Mmt was not present. In further research¹³ the adsorption of the conventional PCEs was found to depend both on the architecture of the PCE molecules and the nature of cations located on the interlayer exchange sites of the Mmt. Whatever the conventional PCE, a larger amount of it was adsorbed on Na–Mmt than on Mg–Mmt or Ca–Mmt. The interaction mechanisms between PCEs and Mmt was that because the Mmt having expandable lattices, which allowed intercalation, swelling and cation exchange, and then the intercalation of the PEO side chains into the aluminosilicate layers of the clays to occur. Also, physisorption of PCEs on the clay surfaces occurred, but only to a slight extent.¹¹ Na–Mmt clay has been found to be especially harmful to conventional PCEs.

The chemical structure of the PCEs should be modified to enhance their clay tolerance. Lei and Plank¹⁴ synthetized modified PCEs from methacrylic acid and hydroxyl-alkyl methacrylate esters and tested their dispersion performance in cement in the absence and presence of Na–Mmt. This new type of PCE was found to disperse cement well when Na–Mmt was present. A mechanistic study including adsorption and XRD experiments revealed that the new PCE only adsorbed on the surface of Na–Mmt and did not intercalate into its layered structure, and this behavior explained the tolerance of this PCE against clay contamination. Their further research¹⁵ found that terpolymers (MA–monoalkyl–HBVE) synthesized from maleic anhydride (MA), 4-hydroxy butyl vinyl ether (HBVE) and maleic monoalkyl ester were little affected by Na–Mmt. XRD experiment suggested that this novel PCE interacted with Na–Mmt only via surface adsorption, whereas conventional PCEs became incorporated chemically into the interlayer of the aluminosilicates. Hanjun Xu et al.¹⁶ synthesized a novel PCE that employed β-cyclodextrin as pendant groups and their dispersion performance was tested in the absence and presence of Ca–Mmt. They found that this novel PCE exhibited enhanced clay tolerance, which was ascribed to the steric hindrance induced by β-cyclodextrin, and exhibited decreased adsorption onto the surface of the clay. Yinwen Li et al.¹⁷ prepared a novel amphiphilic polycarboxylate copolymer (APC) that employed the zwitterionic (ZI) polymer 3-(2-(methacryloyloxy) ethyl) dimethylammonio-propane-1-sulfonate (DMAPS) and found that these APCs could effectively inhibit their own adsorption into clay, which was attributed to the APCs with ZI groups being more easily adsorbed on the surface of clay but not incorporated into the layer structure of the clay.

In this study, we aimed to synthesize a new type of PCE with PEO-free side chains to enhance its Na–Mmt tolerance. In general, PCEs consist of two necessary components: one that plays a role of adsorbing to cement particles containing carboxyl functional groups, and the other that results in a steric hindrance effect and/or electrostatic repulsion and causes deflocculation.¹⁸ Acrylic acid (AA), methacrylic acid (MAA) and maleic anhydride (MA) are usually used as the adsorption component, with the dicarboxylate MA yielding a greater competitive adsorption on cement than the “classical” carboxylate AA or MAA.¹⁹ Itaconic acid (IA) was found²⁰ to display a better performance than MA when used for PCEs; note that IA has been shown to consist of two carboxyls with one of them not connected to the alkene directly, in contrast to the direct connection mode of MA. In our research, we chose to use IA instead of AA, partially because of the inferior solubility of IA in water. Also, 2-(dimethylamino) ethyl methacrylate (DMAM) is a widely used reactive monomer, whose structure has no PEO units but does have tertiary amine groups that can generate cationic ammonium in aqueous solution, and has been used as a clay swelling inhibitor in drilling fluids used for oil exploration.^21–23 DMAM was chosen as the steric hindrance component of the PCE with the aim of lowering the clay sensitivity of the PCE.

2. Experimental

2.1. Materials

2.1.1. Chemicals. 2-(Dimethylamino)ethyl methacrylate (DMAM) and itaconic acid (IA) (both ≥99% purity, Aladdin Industrial Corporation, China), 3-mercaptopropionic acid (98% purity, Aladdin Industrial Corporation, China), and acrylic acid (AA), ammonium persulfate, sodium chloride, calcium sulfate dihydrate, sodium sulfate anhydrous, potassium sulfate, potassium hydroxide (all ≥98% purity, Sinopharm Chemical Reagent, China) were used without further purification.

2.1.2. Cement and clay. The cement used in this research was an ordinary Portland cement (CONCH p.o 42.5, Anhui Conch Cement Corporation, China). The clay used in this research was a naturally mined Mmt clay that was treated with a five-fold excess of 1 N NaCl to ensure that the interlayer exchangeable cations were Na⁺, then washed with deionized (DI) water until chloride ions were not detected with AgNO₃, and finally dried, ground and sieved with a 200-mesh sieve. The chemical compositions, average particle sizes, density values, and loss on ignition (LOI) values of the cement and modified clay were characterized. Their chemical compositions were tested by using X-ray fluorescence (XRF). Their average particle sizes (d₅₀, μm) were determined using laser granulometry. Their density values were determined via helium pycnometry. The LOI (%) value of each sample was determined by heating the sample at 150 °C for 2 h, subtracting its post-ignition weight (g) from its pre-ignition weight (g), and then dividing this difference by the pre-ignition weight (g). The chemical composition and physical properties of the cement and the modified clay are shown in Table 1.

Table 1 The chemical compositions and physical properties of the cement and modified clay

	Cement	Clay
SiO2 (%)	23.4	67.8
Al2O3 (%)	9.0	14.0
CaO (%)	55.7	1.2
MgO (%)	1.8	2.7
Fe2O3 (%)	4.4	3.3
Na2O (%)	0.4	4.9
K2O (%)	1.5	2.8
TiO2 (%)	1.1	0.2
LOI (%)	1.8	2.1
Particle size (d50, μm)	12.6	25.4
Density (g cm^−3)	3.2	2.1

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2.1.3. SNF polycondensate and TPEG PCE. The industrial first-generation superplasticizer sulfonated naphthalene formaldehyde (SNF) and the industrial TPEG-type PCE Ahtq® TCD-8H (40 wt%) were used as samples for comparison.

2.2. Synthesis of the DMAM–AA–IA terpolymers (DAI)

As an example, the synthesis of the DMAM–AA–IA terpolymer with a molar ratio of 1 : 2.65 : 0.35 is described. First, a mass of 10 g of DI water was placed in a 100 mL three-neck round-bottom flask equipped with a stirrer and a reflux condenser while the temperature was kept at 80 °C by heating, and then a mass of 0.9106 g (0.007 mol) of IA was added and the solution was purged with N₂ for 30 min. Subsequently, 3.8192 g (0.053 mol) of AA, 3.1442 g (0.02 mol) of DMAM, and 0.0685 g of 3-mercaptopropionic acid (chain transfer agent) were dissolved in 10 g of DI water, and this solution was designated as solution 1. Separately, a mass of 0.1187 g of ammonium persulfate (initiator) was dissolved in another 10 g of DI water, and this solution was named solution 2. Solution 1 and solution 2 were fed starting at the same time continuously into the reaction vessel over periods of 3.5 h and 5 h respectively at 80 °C using two peristaltic pumps. When additions of both solutions were completed, the mixture was stirred for another 1 h at 80 °C, and then was allowed to cool down to room temperature. The final product was designated DAI-2, and showed a concentration of 20.7 wt% in the aqueous solution, a low viscosity and a pH of 6–7.

The above-described procedure was adapted to synthesize DMAM–AA–IA terpolymers with other molar ratios of the components.

2.3. Characterization of DAI terpolymers

2.3.1. Size-exclusion chromatography (SEC). Weight-average molecular weight (M_w), number-average molecular weight (M_n) and polydispersity index (PDI) values of these novel PCEs were determined using size-exclusion chromatography, also known as gel permeation chromatography (GPC). An SEC instrument (575–2414, Waters, American) equipped with a differential refraction detector and two Ultrahydrogel columns (120 and 250) were used to separate the polymer components. A 0.1 N NaNO₃ solution adjusted to pH 12 with NaOH was utilized as the carrying phase with a flow rate of 1 mL min⁻¹. A series of poly (ethylene glycol) polymers with known molar masses were used as the standard samples.

2.3.2. Elemental analysis, and FTIR and ¹³C NMR spectroscopy analyses. The synthesized terpolymers were purified by using a dialysis bag (BIOSHARP, M_w 3600, America) for 24 h, and were vacuum dried overnight at 80 °C. The elemental analysis was carried out using an elemental analyzer (Vario EL c, Elementar, Germany) with the combustion tube temperature at 1150 °C and the reduction tube temperature at 850 °C. Fourier transform infrared (FTIR) spectra between 500 and 4000 cm⁻¹ were acquired using a Nicolet 67 spectrometer (Thermo Nicolet, USA) with a KBr sample holder. A ¹³C NMR spectrum was recorded by using a 400 MHz superconducting-magnet NMR spectrometer (VNMRS600, Agilent, USA) with D₂O as the solvent.

2.4. Dispersion performance in cement

The spreads of cement pastes containing the various amounts and types of superplasticizer were determined by performing mini slump tests according to the DIN EN 1015 standard. The tests were carried out as follows. At first, the water/cement (w/c) ratio of the neat cement paste to acquire a spread flow of 17 ± 0.5 cm was determined, and was found to be 0.49. At this w/c ratio, the dosages of superplasticizers required to reach a spread of 24 ± 0.5 cm in the absence and presence of 1% by weight of cement (bwoc) of Na–Mmt were determined. A cement-purifying slurry blender (NJ-160, Dinjian Instrument, China) equipped with an iron cup was used to automatically stir the cement paste at a rotation rate of 62 ± 5 rpm for 120 s, followed by no rotation for 15 s, and then a rotation of 125 ± 10 rpm for 120 s. Generally, a mass of 300 g of cement was placed in the iron cup, and then a mixture of a certain amount of water and a certain amount of superplasticizer was added. After being agitated, the cement paste was immediately poured into a Vicat cone (40 mm height, 65 mm top diameter, 75 mm bottom diameter) on a wet glass plate and the cone was removed by displacing it vertically. The resulting spread of the paste was measured twice, with the second measurement carried out perpendicular to the direction of the first measurement, and the two measurements were averaged to give the final spread value. All measurements were repeated five times to give the average value.

For the time-dependent flow behavior of the paste, the fluidity was tested at 30, 60, 90, and 120 min after the first test. After each test, the cement paste was transferred back to the cup and covered with a wet towel to avoid drying.

2.5. Adsorption on clay

Total adsorbed amounts of superplasticizers were measured using the depletion method for total organic carbon (TOC). Generally, a mass of 0.5 g of clay was dissolved in 24.5 g of synthetic cement pore solution (prepared by dissolving 1.72 g CaSO₄·2H₂O, 6.956 g Na₂SO₄, 4.76 g K₂SO₄ and 7.12 g KOH in 1 L of DI water¹¹) with one of various amounts of superplasticizer (w/clay = 49), and each suspension was stirred for 15 min and centrifuged for 3 min at 10 000 rpm. The amount of the total organic carbon in each filtrate was measured with a High TOC apparatus (TOC-V CPH, SHIMADZU, Japan) and combustion at 890 °C. The adsorbed amount of superplasticizer for each case was calculated by subtracting the TOC content in the filtrate from the TOC content in the reference sample.

2.6. XRD analysis

X-ray diffraction (XRD) is a convenient method to monitor the interlayer spacing (d-spacing) of the silicate layers of the clay. To investigate the change in d-spacing caused by the intercalation of the superplasticizers, the d-spacings of the clay in the presence and absence of each superplasticizer were measured by using an X'Pert PRO MPD XRD instrument (PANalytical B.V., Holland). In a typical experiment, a mass of 0.5 g of clay was dissolved in 24.5 g of synthetic cement pore solution with/without 1.01 wt% of the tested superplasticizer (w/clay = 49), and the suspension was stirred for 15 min and centrifuged for 3 min at 10 000 rpm. The centrifugate was dried for 48 h at 45 °C, then ground, and finally sieved with a 200-mesh sieve. XRD parameters were as follows: scan range (2θ) from 3° to 25°, and a step size of 0.23 s per step.

3. Results and discussion

3.1. Characterization of DAI terpolymers

The aqueous free radical copolymerization process for DMAM, AA and IA is shown in Fig. 1. According to the SEC data listed in Table 2, the novel PCEs had M_w values of ∼47–73 kDa and PDI values of ∼1.6–2.3, which indicated a molecular weight distribution that was not too wide. Higher relative amounts of IA yielded lower M_w values for the copolymer products, which can be attributed to the lower reactivity of the IA monomer than of AA according to the Q–e scheme developed by Alfrey and Price.²⁴ Due the higher free radical reactivity of the IA monomer, which can easily terminate chain growth by combining with another free radical, we found that when the molar ratio of IA : AA was equal to or greater than 0.7 : 2.3, only oligomers were obtained, so that the polyreaction process was restrained absolutely. The anionic charge density of PCEs can influence its adsorption and is hence an important factor. The anionic charge density of the novel PCEs followed the order: DAI-1 < DAI-2 < DAI-3 according to the eqn 1.²⁵

Fig. 1 Synthetic route for preparation of the novel PCEs.

Table 2 SEC data and N contents of the synthesized copolymers

Polymer	Molar ratio (DMAM : AA : IA)	Mw (g mol^−1)	Mn (g mol^−1)	PDI (Mw/Mn)	N content (%)
					Calculated	Measured
DAI-1	1 : 2.90 : 0.10	73 342	46 856	1.57	3.69	3.76
DAI-2	1 : 2.65 : 0.35	68 586	32 352	2.12	3.56	3.57
DAI-3	1 : 2.5 : 0.5	47 393	20 557	2.31	3.48	3.45

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In eqn (1), M_u is the molecular mass of one repeated unit in the polymer chain, and n_COO⁻ represents the number of the COO⁻ groups in one repeated unit. The novel PCEs have similar theoretical side chain molar density values based on their molar composition. Schematic molecular structures of DAI are shown in Fig. 2.

Fig. 2 Schematic molecular structures of DAI.

It was important to ascertain that the terpolymer indeed contained the DMAM subunit. According to the N content analysis shown in Table 2, not only was the element N detected, but its content also matched well with calculated values, which indicated that these reactive monomers copolymerized nearly according to their molar ratio. The chemical structures of DAI were confirmed by the FTIR and ¹³C NMR spectra. The FTIR spectrum is shown in Fig. 3, and includes characteristic peaks at 1723 cm⁻¹ (–COOH vibration), 1170 cm⁻¹ (C–N vibration), 2962 cm⁻¹ (–CH₃ vibration), and 1465 cm⁻¹ (–CH₂– vibration). No peak was observed in the range 1610–1670 cm⁻¹ (C=C vibration), indicating that there was no C=C in the sample. This result affirmed the success of the dialysis purification process.

Fig. 3 FTIR spectra of DAI.

To further characterize the molecular structures of the synthetic polymers, NMR spectra were acquired. The variety of ways to connect the monomers, the presence of active hydrogens (–NH⁺–, –COOH) in the polymers, and the narrow chemical shifts (∼0–20 ppm) of their ¹H NMR spectra made it challenging to assign the proton signals. ¹³C NMR, however, is an effective method to determine the molecular structure of synthetic polymers since it is possible to eliminate the ¹³C–¹H chemical couplings and ignore the ¹³C–¹³C chemical couplings, and due to the wide chemical shifts (∼0–250 ppm) of their ¹³C NMR spectra. The ¹³C NMR spectrum of DAI is shown in Fig. 4. ¹³C NMR δ (400 MHz, D₂O, ppm): 183.45–175.08 (–COOH), 61.81 (O–CH₂–CH₂–N), 58.22 (O–CH₂–CH₂–N), 37.23 (–N–(CH₃)₂). No peak was observed in the range 150–110 ppm, indicative of the absence C=C bonds in the sample.

Fig. 4 ¹³C NMR spectra of DAI.

3.2. Cement dispersion

To evaluate the dispersion performances of the superplasticizers at the w/c ratio of 0.49, the dosages of these superplasticizers required to reach a cement paste spread of 24 ± 0.5 cm were determined as shown in Fig. 5. The TPEG PCE exhibited the best dispersion performance in cement, which was attributed to the high steric hindrance effect of the long PEO side chains. The novel PCEs still displayed a better dispersion performance in cement than did SNF, owing to the novel PCEs having a high density of anionic carboxyl functional groups on the backbone; these groups are ionized in the alkaline pH of the cement suspension, and thus adsorb onto the cement surface, which is positively charged due to the strong affinity of Ca²⁺ with the surface.⁵ The side chains of these terpolymers with tertiary amine groups provided a steric hindrance force that overcame the attractive interparticle forces and dispersed the cement particles. The most effective of the novel PCEs was DAI-2. The lower relative amount of IA in DAI-1 did not allow the effective function of IA to be prominent. In DAI-3, with its relatively high amount of IA but low molecular weight, the latter was the main determinant of its dispersion performance, as only PCEs with appropriate molecular weights show good dispersion properties.²⁶

Fig. 5 Dosages of superplasticizers required to achieve a cement paste spread of 24 ± 0.5 cm (w/c = 0.49).

Flow retention performance was tested over a period of 2 h as shown in Fig. 6. According to these data, the novel PCEs showed a remarkable increase of fluidity within 30 min, and this result confirmed that the IA provided a better slump retention as previously reported,²⁰ especially for DAI-2 which showed excellent flow retention within 30–120 min. All of these PCEs showed a better retention performance than did polycondensate SNF.

Fig. 6 Low retention performances of the various superplasticizers (w/c = 0.49).

3.3. Dispersion performance in the presence of clay

To investigate the clay sensitivity levels of the superplasticizers at the w/c ratio of 0.49, the dosages of superplasticizers that achieved a cement paste spread of 24 ± 0.5 cm in the presence of 1% bwoc of clay were measured, as shown in Fig. 7. According to the results, the novel PCEs were more effective than conventional PCE and polycondensate. The superior dispersion performances of the novel PCEs in concrete when clay was present in the aggregate combined with their weaker performances in the absence of clay indicated the lower clay sensitivity levels of the novel PCEs than of conventional PCE.

Fig. 7 Dosages of superplasticizers required to achieve a cement paste spread of 24 ± 0.5 cm with 1% bwoc of clay (w/c = 0.49).

To check whether just the DMAM monomer played a role in enhancing the clay tolerance of the novel PCEs in the cement, a simple test involving blending the TPEG PCE with an additional amount of DMAM monomer was carried out. This test yielded a worse performance. This result indicated that the lower clay sensitivity of the novel PCEs in cement arose from the success of the copolymerization of the selected monomers.

3.4. Adsorption on clay

Superplasticizers are probably absorbed on the interlayers and/or the surface of the clay. The total adsorbed amounts of superplasticizers were measured using the depletion method, which was carried out for TOC as shown in Fig. 8. The highest saturated adsorbed amount was ∼200 mg g⁻¹ clay, and was observed for TPEG PCE. This value was ∼7 times that of the ∼30 mg g⁻¹ clay value for SNF. However, DAI showed a slightly higher value of ∼30–35 mg g⁻¹ clay, and showed that the clay consumed much less DAI than TPEG PCE. The adsorbed amounts of novel PCEs on the clay followed the order DAI-1 < DAI-2 < DAI-3, and this result was attributed to the physical adsorption (which was the only adsorption mode characterized for the novel PCEs) being associated with the anionic charge density of PCEs.²⁵ Although the adsorbed amounts of DAI-1 on clay were slightly lower than that of DAI-2, the DAI-2 PCE showed a more effective dispersion performance in the presence of clay, which was attributed to its ability to disperse the cement in the absence of clay. It can be found that when the adsorbed amounts of superplasticizers on clay are approximate, the dispersion performance of superplasticizers when the clay is present is the dispersion capacity of superplasticizers when the clay is absent dependent.

Fig. 8 Adsorption isotherms of superplasticizers on clay in the synthetic cement pore solution (w/clay = 49).

3.5. Mode of interaction with clay

As shown in Fig. 9, we monitored the interlayer spacing (d-spacing) of the silicate layers (d₀₀₁) of the clay, which can provide specific information on the intercalation between superplasticizers and clay in the synthetic cement pore solution. Hydrated clay exhibited a d-spacing of 1.23 nm, which is a characteristic d-spacing of Na–Mmt.²⁷ When the conventional TPEG PCE containing PEO side chains was used, the d-spacing value shifted to 1.40 nm, which implied that the PEO side chains of the polymer intercalated into the aluminosilicate sheets.¹¹ However, when the other superplasticizers were added, the d-spacing remained constant, suggesting the lack of intercalation of these superplasticizers into the alumsilicate sheets. Therefore, the novel PCEs and polycondensate SNF are slightly consumed by clay, and only by surface adsorption. It is well known that Mmt is composed of thin alumsilicate sheets that contain two tetrahedral sheets and sandwich one central octahedral sheet, and that the alumsilicate sheets have a permanent negative charge on the faces but pH-dependent charge on the edges.²⁸ When Mmt is present in the high pH (∼13) environment of cement paste, negative charges result from the deprotonation of these sites on the edges and then cations become absorbed onto these sites as well as onto the faces of the alumsilicate sheets. As a result, electrostatic interactions are generated between these cations and the anionic PCE backbone.¹¹ In addition, these conventional PCEs become intercalated into the alumsilicate sheets, hence explaining why the conventional PCEs are more sensitive to clay than are the novel PCEs.

Fig. 9 XRD patterns of clay in a synthetic cement pore solution and in the absence or presence of various superplasticizers (w/clay = 49).

4. Conclusions

A monomer containing a tertiary amine group was employed to synthesize novel PCEs preparing from AA, IA and DMAM by free radical copolymerization. The novel PCEs exhibited an effective dispersion performance in the presence of clay, which we attributed to its weak interaction with the clay as was demonstrated by TOC and XRD measurements. Our study demonstrates that modifying the side chains of the PCE by introducing tertiary amine groups is an available way to enhance clay tolerance.

Acknowledgements

The authors thank the University of Science and Technology of China for providing us with facilities to carry out the analysis and testing work.

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