Synthesis of Quaternized Lignin and Its Clay-Tolerance Properties in

Research Article pubs.acs.org/journal/ascecg

Synthesis of Quaternized Lignin and Its Clay-Tolerance Properties in Montmorillonite-Containing Cement Paste Tao Zheng,† Dafeng Zheng,*,† Xiaokang Li,† Cheng Cai,† Hongming Lou,†,‡ Weifeng Liu,† and Xueqing Qiu*,†,‡ †

College of Chemistry and Chemical Engineering and ‡State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road, Guangzhou 510640, China ABSTRACT: To improve the dispersion capacity of poly(carboxylate ether) (PCE) in clay-containing cement paste, lignosulfonate (LS) was quaternized with 3-chloro2-hydroxypropyl trimethylammonium chloride (CHPTAC), and the product quaternized lignosulfonate (QL) was characterized by FT-IR spectroscopy, element analysis, and zeta potential. The fluidity of the montmorillonite- (MMT-) containing cement paste was clearly improved upon the addition of 0.010 wt % QL, as the yield stress and the rheological behavior index of the paste were reduced. XRD analysis indicated that the adsorption of QL on MMT was driven by a strong electrostatic attraction, rather than intercalation, which was much stronger than that for LS. DLS also showed that the steric effect of QL was much stronger than that of PCE. As a result, when QL and PCE were added to the MMT-containing cement paste, QL adsorbed on the MMT surface preferentially and hindered the adsorption of PCE. KEYWORDS: Clay tolerance, MMT-containing cement paste, Fluidity, Rheological properties, Adsorption

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INTRODUCTION Concrete is the most widely used building material in modern society. To prepare high-performance concrete, the most convenient approach is the use of a superplasticizer.1,2 Among commercial superplasticizers, poly(carboxylate ether) (PCE) is the most commonly used. Generally, the PCE molecule is composed of the polycarboxylate main chain and plenty of poly(ethylene oxide) (PEO) side chains.3 When PCE is used in concrete, the main chain is adsorbed on the cement particles, whereas the side chains stretch out into the solution, leading to enhanced dispersion of the cementitious system through electrostatic repulsion (generated by the carboxylate groups) and steric hindrance (generated by the side chains) .4 As a result, PCE can significantly improve the dispersion performance of fresh concrete and strengthen the durability of hardened concrete. However, with the rapid development of the construction industry, the majority of the supply of high-quality gravel has been exhausted, and increasin amounts of high-clay-content sand is used instead. As a result, clay can weaken the dispersibility of PCE,5,6 seriously damaging the workability and the late strength of concrete. It is known that clay contains a sandwich lattice structure consisting of two silicon tetrahedrons and one aluminum oxide octahedron. The PEO side chains can easily insert into the sandwich structure,7 resulting in serious destruction in the dispersibility of the cement particles.8,9 Thus, the compatibility between PCE and clay has attracted a great deal of attention. To alleviate the negative effect of clay on the performance of PCE, three alternative methods have commonly been studied. © 2017 American Chemical Society

One method is the synthesis of new PCE molecules. For instance, Lei and Plank synthesized a series of PCEs with hydroxyalkyl monomers, including 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, and 4-hydroxybutyl vinyl ether.10,11 These new PCEs exhibited suitable clay tolerance. However, compared with the traditional PCE, the steric effects of these PCEs were obviously weaker, so the dispersion ability decreased. The second method is increasing the dosage of PCE, which increases the production cost of the concrete at the same time. Another method is the use of a clay-tolerance sacrificial agent. For instance, Ng and Plank tried to use poly(ethylene glycol)s as sacrificial agents to improve the compatibility of PCE and clay.12 They found that the use of glycols as sacrificial agents was generally suitable for PCEs with higher side-chain densities. By far, the use of sacrificial agent is thought to be the easiest and most convenient method of improving the compatibility of PCE and clay. Researchers have reported that cationic surfactants can be adsorbed on clay particles by substituting the metal cations, such as K+, Na+, and Ca2+, in the lattice structure of the clay or by forming hydrogen bonds.13 When adsorbed on the clay, the hydrophobic tail of the cationic surfactant extends out into the solution and forms a hydrophobic adsorption layer, further hindering the adsorption of PCE on clay. Thus, cationic surfactants can also be used as clay-tolerance sacrificial agents.14 Received: April 19, 2017 Revised: July 17, 2017 Published: August 2, 2017 7743

DOI: 10.1021/acssuschemeng.7b01217 ACS Sustainable Chem. Eng. 2017, 5, 7743−7750

Research Article

ACS Sustainable Chemistry & Engineering

adjusted to 12 with NaOH. Then, the LS solution was poured into a 500 mL three-necked flask equipped with a motor stirrer and heated with a water bath. The stirring rate was 200 rpm, and the temperature of the water bath was set to 85 °C. When the temperature rose to 80 °C, 43.1 g of 65 wt % CHPTAC solution was added dropwise into the flask at a rate of 6.4 mL/min by a peristaltic pump. After 5 min, 14.9 g of 20 wt % NaOH solution was added as well to ensure that the solution remained alkaline. After the addition of the CHPTAC solution was completed, the reaction was allowed to proceed for 4 h at 85 °C. After the reaction solution had cooled, it was dialyzed with a dialyzer (with a molecular-weight cutoff of 1000) in NaOH solution (pH 12) to remove the residual CHPTAC and small-molecule impurities. Then, the alkaline solution was dialyzed against deionized water to remove Na+ and OH− to yield pure QL concentrate. The product was obtained by rotary evaporation. The reaction process is depicted in Scheme 1.

As is well-known, lignin is the second most abundant and renewable resource on Earth after cellulose.15 Lignosulfonate (LS), a main industrial lignin, is widely used as a water reducer, pesticide dispersant, and dye dispersant, among other uses.16,17 As a water reducer, LS exhibits favorable water retention and workability maintenance for fresh concrete, because it has a three-dimensional network structure and air-entraining properties.18 If LS were modified as a clay-tolerance sacrificial agent, the use of lignin would be extended widely. In this study, LS was modified to prepare a quaternized lignin (QL). The dispersion ability of QL in montmorillonite(MMT-) containing cement paste was evaluated in terms of fluidity and rheological tests. Furthermore, the adsorption behavior, X-ray diffraction (XRD), and dynamic light scattering (DLS) were studied to determine the clay-tolerance mechanism of QL. This work promoted the development of a claytolerance sacrificial agent in PCE research and also broadened the application of industrial lignin.

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Scheme 1. Synthesis Reaction of QL

MATERIALS AND EXPERIMENTS

Materials. Lignosulfonate (LS) that had been obtained through the acidic pulping process from pinewood was purchased from Shixian Paper Co., Ltd. (Jilin Province, China). The LS was purified by filtration after being dissolved in water. The relative molecule weight (Mw) of the LS was 12000 g/mol, and the sulfonation degree was 2.67 mmol·g−1. Poly(carboxylate ether) (PCE) with Mw = 66000 g/mol was provided by Guangdong Aoke Chemical Co., Ltd. (Maoming, China). It was synthesized by free-radical polymerization from acrylic acid and isopentenyl poly(ethylene glycol) (Mw = 2400). The chemical structure of PCE is shown in Figure 1.

Characterization of QL. The Fourier transform infrared (FT-IR) spectrum was recorded on a Nexus spectrometer (Thermo Nicolet Corporation, Madison, WI). For these measurements, dried samples were embedded in KBr pellets at a concentration of about 1 mg/100 mg. The range was between 400 and 4000 cm−1 with a resolution of 4 cm−1, and 32 scans were recorded. The FT-IR spectrometer was first calibrated with KBr for background signal scanning prior to the measurement. The contents of C, H, N, and S elements in QL were measured with a Vario EL III element analyzer (Elementar Analysensystem GmbH, Langenselbold, Germany). The zeta potentials of a 0.05 g/L QL solution at different pH values were measured with a Zetasizer Nano Series (Malvern Instruments Co. Ltd., Malvern, UK). The structure of LS was also characterized as a reference. Cement Paste Test. The fluidity of cement paste was measured by the mini-slump test according to Chinese national standard “methods for testing the uniformity of concrete admixture” (GB/T 8077-2012). The test method of the Chinese national standards is similar to ASTM standards for the preparation of cement paste and fluidity testing. The water/cement (w/c) ratio was 0.29. The dosage of PCE was 0.10 wt % (by weight of cement) to give the pure cement paste a spread flow of 27 ± 0.5 cm. A modified mini-slump test was designed by substituting 1.0 wt % cement with MMT to evaluate the clay-tolerance properties of QL. During tests of the fluidity retention, the MMT-containing cement paste was covered with a wet towel and kept in a standard curing box. At time points 30 and 60 min after the paste had been prepared, the spread flow value of the paste was measured again. The decrease of the spread value was used to evaluate the fluidity retention of the MMT-containing paste. Rheological Behavior. The rheological properties of the MMTcontaining cement pastes were tested in a commercial rheometer (Haake MARSIII, Thermo Fisher Scientific Co. Ltd.), with rotor CC35 and plate CC35. First, cement paste was prepared according to Chinese national standard GB/T 8077-2012 at a w/c ratio of 0.29. Then, the rheological test was conducted 0, 30, and 60 min after the paste had been prepared. The rheological parameters were obtained by fitting with the Herschel−Bulkley model.19,20 The Herschel−Bulkley model indicates that the paste starts to flow when τ exceeds a critical value τ0. The strain gradient increment (τ − τ0) exhibits a power-law increase of the form

Figure 1. Chemical structure of PCE. 3-Chloro-2-hydroxypropyl trimethylammonium chloride (CHPTAC) was purchased from Aladdin Chemical Agent Co., Ltd. (Shanghai, China). Sodium hydroxide was purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). All chemicals were of analytical grade and were used as received. Deionized water was used for the preparation of all solutions. The cement was PII42.5R ordinary Portland cement, purchased from the Wuyang Cement Factory (Guangzhou, China), comprising 52.7 wt % tricalcium silicate (C3S), 23.2 wt % dicalcium silicate (C2S), 5.5 wt % tricalcium aluminate (C3A), 10.3 wt % tetracalcium aliumoferite (C4AF), and minor components (e.g., CaO, CaSO4xH2O, Na2O and K2O). MMT, provided by Aotai Mineral Products Factory (Hebei, China), was sodium montmorillonite of industrial grade comprising 16.54 wt % Al2O3, 4.65 wt % MgO, and 50.95 wt % SiO2. The particle size (d50) was about 8.56 μm. Experiments. Synthesis of QL. LS (70.0 g) was dissolved in deionized water to prepare a 20 wt % solution, and the pH value was 7744


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ACS Sustainable Chemistry & Engineering τ = τ0 + kγ n

(ALV/CGS-3, ALV GmbH, Langen, Hessen, Germany) that was equipped with a multidigital time correlator (ALV-7004) and a solidstate He−Ne laser (JDS-Uniphase, output power = 22 mW, 632.80 nm) .

(1)

where τ0 is the yield stress, Pa; k is the consistency coefficient, Pa·s; and n is the rheological behavior index. In the Herschel−Bulkley model, the yield stress τ0 is responsible for the initial slurry flow resistance, as well as k and n. In particular, n reflects the degree of shear thickening of the cement-based composite.21 Adsorption Behavior of QL on MMT. The amount of QL adsorbed on MMT was measured by the depletion method. The nonadsorbed portion of QL remaining in the solution at equilibrium was determined by UV/vis spectrometry (UNICAM Co., Thaxted, Essex, U.K.) at the wavelength of 280 nm. In a typical adsorption kinetics experiment, 0.10 g of MMT was added to 30 mL of a QL solution with an initial concentration of 100 mg/L. Then, the solution was poured into a 50 mL centrifuge tube and shaken in a wobbler for a certain time at a speed of 200 rpm, after which it was centrifuged for 10 min at 10000 rpm. The supernatant was carefully retrieved using a syringe, diluted with deionized water to obtain a certain volume, and then tested by UV/vis spectrometer. The amount of QL adsorbed was calculated as Γ=

(C0 − Cr)V m

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RESULTS AND DISCUSSION Synthesis and Characterization of QL. Nucleophilic substitution can take place on LS at an appropriate concentration under alkaline conditions. As a result, quaternary ammonium ions were grafted onto the phenolic hydroxyl groups of LS to obtain the amphoteric surfactant QL, as shown in Scheme 1. NaOH was used to adjust the pH of the reaction solution to 12. Because 1 mol of CHPTAC generates 1 mol of HCl molecules in the reaction, the amount of NaOH added was calculated as one-half of the CHPTAC mass. The mass of CHPTAC used was 40 wt % LS. The structure of QL and LS were characterized by FT-IR spectroscopy, as shown in Figure 3.

(2)

where Γ is the amount of QL adsorbed, mg/g; C0 is the initial concentration of QL in the solution, mg/L; Cr is the residual concentration of QL in the solution, mg/L; V is the volume of the QL solution, L; and m is the mass of MMT, g. In a typical adsorption thermodynamics experiment, the process was the same as in the literature.22 The UV absorption standard curve was determined from purified QL and is shown in Figure 2.

Figure 3. FT-IR spectra of QL and LS.

In the FT-IR spectrum of LS, the band at 1505 cm−1 is the characteristic absorption of aromatic skeletal vibrations.23 The band at 1138 cm−1 is the bending vibration of phenolic hydroxyl groups, and that at 1032 cm−1 is the symmetrical stretching vibration of SO, which is the characteristic absorption peak of sulfonate groups. Compared with the spectrum of LS, a sharp absorption at 1625 cm−1 appeared in the spectrum of QL, which corresponds to the NC vibration of quaternary ammonium groups. Meanwhile, the absorption at 1138 cm−1 became weaker in QL, showing that phenolic hydroxyl groups were consumed in the reaction. The results imply that the quaternary ammonium groups were introduced into the LS through a substitution reaction. The nitrogen content of QL can indicate the degree of the substitution reaction. The elemental contents of QL and LS were obtained by elemental analyses and are reported in Table 1. According to Table 1, the nitrogen content in QL was 8.53 wt %, whereas that in LS was only 0.60 wt %. The increase in the nitrogen content indicates that lignosulfonate was quaternized successfully.

Figure 2. UV absorption standard curve of QL. From Figure 2, the correlation coefficient (R2) of the linear fitting result was 0.999, indicating that the concentration of QL solution could be calculated according to the UV absorbance. XRD Analysis. Precisely 1.0 g of MMT and 29.0 g of 1.0 wt % polymer solution were added to a 50 mL centrifuge tube, shaken in a wobbler for 5 min at 8000 rpm, and then centrifuged for 10 min at 10000 rpm. The solid residues were dried overnight at 80 °C and gruond to an average particle size of 10 μm for XRD analysis. XRD scans of all samples were taken at room temperature on a D8 Advance Bruker AXS instrument with Bragg−Brentano geometry. The measurement conditions were as follows: step size 0.15 s/step, scan spin at a revolution time of 4 s, nickel filter as the incident beam, aperture slit of 0.3°, and scan range from 2° to 20°. DLS Measurements. A cement paste was prepared at a w/c ratio of 0.29 and centrifuged to obtain the cement pore solution. PCE, QL, or a mixture of PCE and QL was dissolved in the pore solution to a concentration of 1.0 g/L. The hydrodynamic diameters of PCE, QL, and the mixture of PCE and QL in the cement pore solution were measured by DLS with a commercial light scattering instrument

Table 1. Element Compositions of QL and LS

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sample

N (wt %)

C (wt %)

H (wt %)

S (wt %)

SL QL

0.60 8.53

51.22 53.92

5.52 6.29

6.50 5.85


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From Figure 5, it can be seen that the initial fluidities of the control paste (without any QL or LS), paste with QL, and paste with LS were almost the same, indicating that QL and LS had no obviously positive effects on the dispersion capacity of PCE. That is, the clay-tolerance properties of QL (if any) in MMTcontaining paste cannot be attributed to its effect on the dispersion capacity of PCE. On the other hand, the fluidity loss of the paste with QL or LS at 60 min was less than that of the control, for the reason that lignin has a strong retarding effect on cement hydration. The fluidity experiments also indicated a favorable compatibility between QL and PCE. In a comparison study, 1.0 wt % cement was substituted by an equal mass of MMT to prepare an MMT-containing cement paste. When 0.1 wt % PCE was added to the MMT-containing paste, the initial fluidity of the control sample was only 190 mm, because of the strong adsorption of PCE on MMT, which resulted in the reduction of the actual concentration of PCE in the cement paste. After the addition of PCE, a 0.01 wt % admixture (QL, LS, or CHPTAC) was added to the MMTcontaining cement paste. The effects of these admixtures on the fluidity of the MMT-containing pastes were studied, and the results are shown in Figure 6.

The zeta potentials of 0.05 g/L QL and LS solutions under different pH condition are presented in Figure 4. As LS

Figure 4. Zeta potentials of QL and LS solutions as functions of pH, c = 0.05 g/L.

contains sulfonate anions, the zeta potential increased from −39.5 to −21.7 mV when the pH value was varied from 12 to 1. For QL, the zeta potential increased from −17.0 to 4.9 mV as the pH decreased. When the zeta potential was zero, the isoelectric point was 2.5. This is because, when the pH value decreased, the sulfonate anions were gradually neutralized by hydrogen ions. In addition, the quaternary ammonium groups exhibited a positive charge. As a result, the zeta potential of QL gradually changed from a negative value to a positive value. These results are consistent with Dilling et al.’s patent, according to which amino groups can neutralize the negatively charged sulfonic acid groups when lignosulfonate is aminated.24 All of the results presented above confirm that quaternary ammonium groups were introduced into LS. Effect of QL on the Fluidity of MMT-Containing Cement Paste. To evaluate the clay tolerance of QL, the effect of QL on the dispersion capacity of PCE was first tested in pure cement paste. In pure cement paste, the w/c ratio was determined to be 0.29; the dosage of PCE used in the paste was 0.10 wt % (by weight of cement) in every sample. The effect of 0.01 wt % QL on the dispersion capacity of PCE in shown in Figure 5, where LS is included for comparison.

Figure 6. Effects of different admixtures on the fluidity of MMTcontaining cement paste.

Compared with the control, it was obvious that, when 0.01 wt % QL was added, the initial fluidity of the paste increased by 25 mm, which indicated the the addition of QL reduced the adsorption of PCE on MMT. In another case, 0.01 wt % LS did not increase the initial fluidity of the paste, nor did CHPTAC, which further implies that the clay-tolerance function of QL is not just the result of simple mixing of LS and CHPTAC. After 60 min, the fluidity of the control paste was 135 mm, while that of the paste with 0.01 wt % QL was 145 mm, showing that the fluidity retention of the MMT-containing paste was also improved by QL. In the comparative experiments, it was surprised that the paste with 0.01 wt % LS or CHPTAC showed the fluidity of 115 mm or 95 mm, indicating 0.01 wt % LS or CHPTAC accelerated the fluidity loss of the MMT-containing paste instead. The exact reason needed to be further explored in the later study. To further illustrate the clay-tolerance properties of QL, the effect of QL dosage on the initial fluidity of the MMT-

Figure 5. Effects of QL and LS on the dispersion capacity of PCE in pure cement paste. 7746


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ACS Sustainable Chemistry & Engineering containing paste was studied, and the results were shown in Figure 7. LS and CHPTAC were also used as contrast.

Table 2. Rheological Parameters of MMT-Containing Cement Pastes with Different Dosages of QL QL dosage (wt %) 0

0.01

time elapsed (min)

τ0 (Pa)

k (Pa·s)

n

R2

0 30 60 0 30 60

9.625 26.32 39.88 11.37 11.45 25.25

1.9410 0.2858 0.2834 0.9919 1.8040 0.6216

0.9048 1.3860 1.8530 0.9006 0.9053 1.4600

0.9958 0.9995 0.9969 0.9994 0.9985 0.9958

Without the addition of QL, MMT-containing paste showed a trend of shear thickening as n was larger than 1.0000 at the very beginning. When 0.01 wt % QL was added to the paste, n decreased sharply. Especially after 30 min, the n value of the paste with 0.01 wt % QL was still less than 1.0000, exhibiting shear thinning behavior. Even after 60 min, the n value with 0.01 wt % QL was 1.4600, close to that of the control paste (without QL) at 30 min. It is known that the larger the value of n, the stronger the shear thickening.25 Thus, the paste with QL had more advantage in retaining the workability than the control paste, which was consistent with the results of the fluidity experiments. According to Table 2, with the hydration of cement, the τ0 value of the control paste increased over time, especially from 30 to 60 min. The addition of QL reduced τ0 obviously, further showing that QL maintained the workability of the cement paste. When the dosage of QL was 0.01 wt %, the τ0 value of the paste at 30 min was 11.45 Pa, much less than that of the control paste (26.32 Pa), indicating that QL reduced the yield stress by 56.5% at this point. In another experiment, the τ0 value of the paste with 0.11 wt % PCE at 30 min was 12.12 Pa, larger than 11.45 Pa. These results strongly support the conclusion that the workability of MMT-containing cement paste with PCE and QL was better than that of the paste with an equal mass of PCE only. Adsorption Behaviors. To further investigate the interaction between QL and MMT, the adsorption behavior of QL on MMT was studied, and the results areshown in Figure 9. For comparison, the adsorption of LS was studied as well. According to the adsorption kinetic curves (Figure 9a), QL absorbed on MMT more quickly than LS. The adsorption kinetic parameters were well-fitted by the pseudo-second-order t 1 t kinetics model ( q = ), as shown in Table 3. When 2 + q

Figure 7. Effects of QL dosage on the initial fluidity of MMTcontaining cement past. Data for LS and CHPTAC are included for comparison.

In a certain range of dosage, the initial fluidity of MMTcontaining paste increased significantly with the addition of QL. When the dosage of QL was 0.02 wt %, the fluidity reached 225 mm, 35 mm larger than that of the control. However, when the dosage exceeded 0.02 wt %, the initial fluidity began to decrease. That meant the excessive use of QL would also reduce the dispersion capacity of PCE, because the excessive part of QL would occupy some surface of cement particles. In the contrast experiment, the clay-tolerance effect of LS was weaker than QL, and the maximum fluidity of the MMTcontaining paste with LS was 205 mm. It was obvious that CHPTAC had little clay-tolerance capacity in this study, as there was no long alkyl chain in the molecule structure, so it could not hinder the adsorption of PCE on MMT. In a word, QL was superior to LS and CHPTAC in the clay-tolerance property. Summarily, the dosage of QL used as a clay-tolerance agent in the MMT-containing paste was no more than 0.02 wt %. Rheological Performance. MMT-containing cement pastes were prepared for the rheological behavior test, which was conducted at 25 °C with 0.1 wt % PCE and different dosages of QL. The rheological curves of the pastes 0, 30, and 60 min are shown in Figure 8. The rheological parameters were fitted by the Herschel− Bulkley equation, and the results are reported in Table 2. From Table 2, it can be concluded that the rheological behavior is consistent with the Herschel−Bulkley model (R2 > 0.99).

k ′ qe

e

the initial concentration of the solution was 100 mg/L, the amounts of QL and LS adsorbed on MMT at equilibrium were 22.19 and 6.87 mg/g, respectively, indicating that the

Figure 8. Rheological curves of MMT-containing pastes with different dosages of QL. 7747


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Figure 9. Adsorption behaviors of QL and LS on MMT: (a) Adsorption kinetics curves (100 mg/L) and (b) adsorption thermodynamics curves.

Table 3. Kinetic Parameters for the Adsorptions of QL and LS sample

k′

qe (mg/g)

R2

QL LS

0.0976 0.0684

22.19 6.87

0.9040 0.9766

adsorption of QL on MMT was much stronger than that of LS. The adsorption rate constant k′ of QL was larger than that of LS, further indicating that QL adsorbed more easilty on MMT. From the adsorption kinetics curves (Figure 9a), the adsorption of QL on MMT reached the equilibrium state at about 40 min, which was much faster than LS. The adsorption thermodynamic curves are shown in Figure 9b. The adsorption parameters were fitted according to the Freundlich adsorption model (Γ = k′cn), as shown in Table 4. It

Figure 10. DLS analysis of PCE, QL, and an admixture of PCE and QL.

molecules mainly appeared at the smaller size in the DLS measurement. Interestingly, when the PCE/QL (w/w = 10:1) mixture was added to the cement pore solution, the hydrodynamic diameter distribution of the mixture was centered at about 56.1 nm. The diameter distribution might lead to a different adsorption configuration from that of Lei and Plank’s study.28 The reason for the larger diameter of PCE/QL might be the aggregation of QL and PCE, which would happen because of opposite charge attraction, forming larger particles than PCE and QL alone. As can also be seen in Figure 10, the volume of a QL molecule is much larger than that of a PCE molecule. Once QL was adsorbed on the surface of MMT, the steric effect hindering the adsorption of PCE on MMT was much stronger. This result further shows that QL is more suitable for clay tolerance. XRD Analysis of MMT. To understand the adsorption mode of QL, the structure of MMT was tested by XRD. Before the test, MMT was mixed with PCE solution, QL solution, or the PCE/QL (w/w = 5:5) hybrid solution. Then, the MMT mixtures were dried overnight and ground to an average particle size of 10 μm for the XRD tests. The results are shown in Figure 11. According to Figure 11, the control MMT (without any admixture) showed a d spacing (distance between the alumosilicate layers) of 1.25 nm. This spacing is the consequence of the uptake of water between the negatively charged alumosilicate layers. When MMT was mixed with conventional PCE, a shift in the d spacing from 1.25 to 1.72 nm was detected. This spacing shift is characteristic for MMT intercalated by polyglycols.26 It was reported that the PEO side chains in the conventional PCE molecule also exhibited a

Table 4. Adsorption Thermodynamic Parameters Fitted by the Freundlich Model sample

c

n

R2

QL LS

3.9529 0.8226

0.4476 0.4381

0.9545 0.9950

can be seen that the adsorption thermodynamics of both QL and LS on MMT followed the Freundlich model (R2 > 0.95). As shown in Figure 9b and Table 4, the amount of QL adsorbed increased significant;y with the initial concentration, which was 178.9 mg/g when the equilibrium concentration was 5000 mg/L, whereas the saturated adsorption amount of LS was about 34.3 mg/g. These results further confirmed that the adsorption of QL on MMT was much stronger, so that it could be used as a clay-tolerance agent. DLS Measurements. The hydrodynamic diameters of PCE, QL, and the PCE/QL mixture in the cement pore solution were also measured. From Figure 10, the distribution of hydrodynamic diameters of PCE in the cement pore solution was mainly centered at 9.2 nm, whereas that of QL was centered at 34.9 nm. Another diameter distribution of PCE also appeared at about 258.2 nm, that is thought to be the result of molecule aggregation. It is obvious that the light intensity of the larger diameter was stronger than that of the smaller diameter. The main reason for this difference is that the volume of the particles with larger diameters was much larger than that of particles with smaller diameters, leading to the result of stronger reflection light intensity. On the other hand, the number of the aggregates would be much lower, so it is generally believed that the diameter distribution of the 7748


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positive charge could create an electrostatic attraction with the negative charge on the cement particles. Because of the huge steric hindrance, it can also hinder the adsorption of PCE when QL is adsorbed onto MMT. On the other hand, QL could bind with PCE because of the electrostatic attraction, showing a competition with the adsorption of PCE on MMT. Because of the weak interaction between PCE and QL, the PCE bound with QL would be released again when QL was adsorbed on MMT, so that the released PCE could effectively disperse the cement particles. Generally, QL can be used as an efficient claytolerance sacrificial agent.

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CONCLUSIONS Quaternized lignosulfonate (QL) was successfully synthesized from LS and CHPTAC by a substitution reaction, and its claytolerance properties in MMT-containing cement paste were studied. When 0.010 wt % QL was added to the MMTcontaining cement paste (w/c = 0.29, PCE content = 0.10 wt %), the fluidity of the paste was 215 mm, 25 mm larger than that of the control paste. The fluidity retention was also improved by QL. The rheological behavior of the MMTcontaining paste could be described with the Herschel−Bulkley model. With the addition of QL, the rheological behavior index n and the yield stress τ0 generally decreased. The adsorption thermodynamics of QL on MMT followed the Freundlich adsorption model, and the amount adsorbed was 178.9 mg/g when the equilibrium concentration of QL was 5000 mg/L, much larger than the saturated adsorption amount of LS. XRD analysis showed the d spacing of MMT mixed with QL to be the same as that of the control, indicating that QL was adsorbed on the surface of the MMT by strong electrostatic attraction, rather than intercalation. DLS showed that the volume of the QL molecule was much larger than that of the PCE molecule, so that the steric effect of QL was strong enough to hinder the adsorption of PCE on MMT. It was thus successfully demonstrated that QL can be used as an efficient clay-tolerance sacrificial agent.

Figure 11. XRD spectra of MMT mixed with PCE, QL, and PCE/QL.

similar tendency as polyglycols to intercalate into alumosilicate layers.27 This interaction is the reason for the strong adsorption of PCE by clays, resulting in the poor performance of PCE in clay-containing concrete. When MMT was mixed with QL, the d spacing did not change, indicating that QL was adsorbed on the surface of the MMT by strong electrostatic attractions, rather than intercalation between the alumosilicate layers. Moreover, the network structure of the lignin molecule seals the interlayer space of MMT, thus hindering the adsorption of PCE on MMT. Consequently, when the PCE/QL mixture was added to the MMT-containing cement paste, QL was preferentially adsorbed on MMT, enhancing the clay tolerance of PCE. Clay-Tolerance Mechanism of QL. Based on the above results, a clay-tolerance model of QL can be proposed, as illustrated in Figure 12. When PCE and QL were applied together in the MMT-containing cement paste, QL hindered the intercalation of PCE into MMT. On one hand, QL contains quaternary ammonium groups in its molecule, and part of the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 020-87114722. *E-mail: [email protected]. Tel.: +86 020-87114722. ORCID

Dafeng Zheng: 0000-0001-8993-9388 Hongming Lou: 0000-0003-3941-7287 Xueqing Qiu: 0000-0001-8765-7061 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are deeply thankful for the National Natural Science Foundation of China (Nos. 21436004 and 21376100).

Figure 12. Clay-tolerance mechanism of QL on MMT. 7749

ABBREVIATIONS C3A, tricalcium aluminate C4AF, tetracalcium aliumoferite C2S, dicalcium silicate C3S, tricalcium silicate CHPTAC, 3-chloro-2-hydroxypropyl trimethylammonium chloride DLS, dynamic light scattering LS, lignosulfonate DOI: 10.1021/acssuschemeng.7b01217 ACS Sustainable Chem. Eng. 2017, 5, 7743−7750

Research Article


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Mw, relative molecule weight MMT, montmorillonite PCE, poly(carboxylate ether) PEO, poly(ethylene oxide) QL, quaternized lignin R2, correlation coefficient XRD, X-ray diffraction Γe, equilibrium adsorption capacity τ0, yield stress

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Synthesis of Quaternized Lignin and Its Clay-Tolerance Properties in

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