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Preparation of Lignin-Based Superplasticizer by Graft Sulfonation and Investigation of the Dispersive Performance and Mechanism in a Cementitious System Hongming Lou, Huanran Lai, Mengxia Wang, Yuxia Pang, Dongjie Yang, Xueqing Qiu,* Bin Wang, and Haibin Zhang School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510641, China S Supporting Information *

ABSTRACT: A practical graft sulfonation process was developed to synthesize a lignin-based superplasticizer using acidprecipitated lignin from wheat straw black liquor as raw material. The graft-sulfonated lignin, prepared in optimal reaction conditions, was named GSL. The properties of GSL in a cementitious system were investigated. The adsorption isotherms of GSL fractions separated by ultrafiltration and the thickness of their absorbed films on cement particles were measured to reveal the dispersion mechanism. Also, it was found that the molecular weight of graft-sulfonated lignin increased with the dosage of acetone and formaldehyde, but having too high a molecular weight reduced its dispersive performance. High sulfonic group content in graft-sulfonated lignin severely inhibited the increase of molecular weight, resulting in a decrease of dispersive performance. GSL has stronger compressive strength enhancement in concrete and lower hydration heat temperature than the commercial naphthalene-sulfonated formaldehyde superplasticizer. Moreover, the strong dispersion of GSL with high molecular weight is mainly attributed to strong steric hindrance among cement particles.

1. INTRODUCTION Superplasticizer is an indispensable component in modern concrete that is used to improve the workability of concrete or reduce the amount of mixing water.1,2 Additionally, superplasticizer can enhance the strength and durability of concrete structures.3,4 The annual consumption of superplasticizer in China is more than 4 800 000 tons,5 including polycarboxylate superplasticizer liquor with a solid content of 20%. Production keeps its rapid annual growing by more than 20%.5 The superplasticizers include lignosulfonates (or modified lignosulfonate),6 naphthalene-sulfonated formaldehyde condensate,2,7 sulfonated acetone formaldehyde condensation,8,9 sulfanilic acid-phenol-formaldehyde condensate,10 sulfonated melamine formaldehyde condensate,11 and polycarboxylate polymer.4,12 Among these superplasticizers, lignosulfonate is the only one from nature polymer that is environmental friendly and renewable.13 Industrial interest in environmentally friendly materials has driven research in the modification and application of lignin.7,14 Lignin is one of the three main components of plant cell wall, accounting for one-third of their net weight.15 Industrial lignins come from waste pulping liquor of papermaking industry. Lignosulfonate is the coproduct of acidic sulfite pulping; alkali lignin is the major component of black liquor of alkaline pulping, which accounts for more than 90% produced in China. Lignosulfonate is an amphiphilic polymer surfactant with a hydrophobic phenylpropanoid backbone linked to a hydrophilic sulfonic group.13 Alkali lignin can be converted to lignosulfonate by sulfonation. Lignosulfonate has been widely used in concrete as dispersants because of its surface activity and adsorption action.7,16 It is usually used as a normal water © 2013 American Chemical Society

reducer, because its water reducing ratio in concrete is 8−12%, which is only half of superplasticizers (e.g., naphthalenesulfonated formaldehyde condensate) and one-third of polycarboxylate superplasticizer.7,17 Therefore, lignosulfonates are limited to be used in low grade concrete or in the combination with superplasticizers. The preparation and modification of lignin-based superplasticizer have recently attracted more and more attention.7,14,16,18,19 Our previous studies have found that the molecular weight and sulfonic group content are the dominant factors to affect the dispersive performance of lignosulfonates in concrete.7,8,10,16 Lignosulfonate fraction with higher molecular weight has stronger dispersion.7 Sulfonation increases the adsorption and dispersive performances of calcium lignosulfonate.16 Kamoun et al. reported the preparation of a sulfonated esparto grass lignin (SEL) with sodium sulfite and formaldehyde and its performance as a water reducing agent in cement−water systems.20 The average molecular weight of SEL is about 10 000 Da, and the sulfonic group content is 0.8 mmol/g. The water reducing ratio of SEL is 7−12% at the dosage of 0.4−0.6%. However, it is difficult to synthesize a lignin-based dispersant with both high molecular weight and high sulfonic group content. The difference of performance between grafting and blending modifications of lignosulfonatemodified sulfanilic acid−phenol−formaldehyde condensates were investigated by Ji et al.18 The results suggested that Received: Revised: Accepted: Published: 16101

July 9, 2013 August 7, 2013 October 19, 2013 October 20, 2013 dx.doi.org/10.1021/ie402169g | Ind. Eng. Chem. Res. 2013, 52, 16101−16109

Industrial & Engineering Chemistry Research

Article

Table 1. Composition and Properties of Portland Cement Chemical composition (wt%) SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

K2O

Na2O

L.O.I

Density (kg/m3)

Specific surface area (m2/kg)

19.87

6.11

4.46

64.73

1.06

2.79

0.66

0.10

0.32

3100

340

After feeding of the formaldehyde solution, the temperature was raised to 98 °C and the reaction continued for 3 h. The obtained GSL condensate, according to the above procedure, is crimson solution after cooling. 2.3. Preparation of GSL Fractions by Ultrafiltration. GSL was separated into four fractions using poly(ether sulfone) (PES) ultrafiltration membranes with the cutoff molecular weights of 2500, 10 000, 50 000 Da, and >50 000 Da (Wuxi Saipu Membrane Technology Corp., China). The fractions were named GSL-A, GSL-B, GSL-C, and GSL-D, respectively, with the increase of molecular weight. 2.4. Intrinsic Viscosity Measurement. The inherent viscosity of the graft-sulfonated lignin samples is measured using an Ubbelohde viscometer. The dilution and extrapolation method is used for theoretical calculations.22 2.5. IR Analysis. IR spectra were recorded between 4000 and 400 cm−1 with 32 scans on a Nexus spectrometer (Thermo Nicolet, USA). Disks were prepared by mixing 2 mg of dried sample with 200 mg of KBr (for spectroscopy) in an agate mortar. The resulting mixture was successively pressed at 12 MPa for 3 min. 2.6. Molecular Weight Distribution Measurement. The molecular weight was determined by Waters 1515 gel permeation chromatography (Waters, USA), using Ultrahydrogel 120 and 250 columns, 0.10 mol/L of sodium nitrate as a mobile phase with a flow rate of 0.50 mL/min, and sodium polystyrene sulfonates with different molecular weights as standards. 2.7. Sulfonic Group Content Measurement. The potentiometric titration method is used to measure the sulfonic group content of samples through an automatic potentiometric titrator (809 Titrando, Metrohm Corporation, Switzerland). Before titration, the GSL samples were ion-exchanged through the anion exchange resin and cation exchange resin to remove salts and other impurities. A NaOH standard solution of 0.05− 0.10 mol/L was used as the titrant. The titration end point was determined by the peak of the first-order derivative of the titration curve. The titration temperature was 20−30 °C. The sulfonic group content can be calculated as follows:23

grafting modification made the resultant possessing higher charge density and greater steric hindrance than blending modification, resulting in stronger dispersive performance. In this work, we developed a novel approach to prepare lignin-based superplasticizer by graft sulfonation using acid precipitated lignin from black liquor of wheat straw as the raw material. The effects of material ratio on the molecular weight and dispersive performance of graft-sulfonated lignin were evaluated. Interaction between molecular weight and sulfonic group content in graft-sulfonated lignin were discussed. The dispersive performance of GSL in cement paste, mortar, and concrete were evaluated as well as a commercial naphthalenesulfonated formaldehyde condensate that was used as a reference. The adsorption and dispersive performance of GSL fractions prepared by ultrafiltration were also investigated to understand the dispersion mechanism of GSL.

2. EXPERIMENTAL MATERIALS AND METHODS 2.1. Materials. The commercial products of acetone, formaldehyde solution (37%), and sodium sulfite were used as raw materials of the chemical synthesis. Lignin supplied by Quanlin Paper-Making Co., (Shandong Province, China) was separated and purified from wheat straw Kraft pulping black liquor by acidification with H2SO4. Yuexiu Brand PII42.5R Portland cement was produced by Zhujiang Cement Factory (Guangzhou, China). Its composition and properties are shown in Table1. FDN, a commercial naphthalene dispersant formaldehyde condensate produced by Zhanjiang Additive Company (Guangdong Province, China), was used as superplasticizer reference. The content of sodium sulfate in FDN is less than 5%. The mass average molecular weight is 5200 Da, and the sulfonic group content is 1.77 mmol/g.21 SAF, a pure acetone formaldehyde sulfite condensate, was prepared in our laboratory.8 The molar ratio of sodium sulfite and acetone is 0.62, and the molar ratio of formaldehyde and acetone is 2.03. The mass average molecular weight is 23 100 Da, and the sulfonic group content is 2.12 mmol/g.8 The fractions with the cutoff molecular weight of 2500−10 000 Da, 10 000−50 000 Da, and >50 000 Da were named SAF-B, SAFC, and SAF-D, respectively.8 2.2. Preparation of Graft-Sulfonated Lignin. Sodium sulfite was dissolved in water in a reactor flask equipped with a temperature-controlling electric heating device, a motor stirrer, a thermometer, a dropping funnel, and a reflux condenser. The temperature of the solution was maintained at 45 °C during the dissolution. When the solution became clear, the acetone was added, and then the reaction was performed at 45 °C for 30 min. Then, the lignin powder was added into the reactor; the temperature was raised to 55 °C and maintained for an additional 60 min. The formaldehyde aqueous solution of 37% concentration was fed into the reactor by means of a dropping funnel. During the feeding, the temperature of the solution will increase automatically and should be controlled below 75 °C. A strong reflux condenser was used to reduce the loss of acetone and intermediates with low molecular weight.

S=

C NaOHVNaOH m

(1)

where S is the sulfonic group content (mmol/g), CNaOH is the molar concentration of NaOH (mmol/L), VNaOH is the volume of NaOH solution used (L), and m is the weight of sample (g). 2.8. Test of the Performance of Cement Paste, Mortar, and Concrete. The test of the spread of cement paste was conducted according to the Chinese national standard GB/T 8077−2000. Cement (300 g) and desired amount of water plus the desired amount of superplasticizer were mixed at a low speed for 2 min and then at a high speed for 2 min. The cement paste was put into a small flow cone mold (base diameter of 6.0 cm, top diameter of 3.6 cm, and height of 6.0 cm), and no external vibrations were used to consolidate the fresh cement paste. Once the test cone was lifted, the fresh cement paste collapsed and spread. The maximum diameter of the spread d1 and the diameter perpendicular to it d2 were measured. The 16102

dx.doi.org/10.1021/ie402169g | Ind. Eng. Chem. Res. 2013, 52, 16101−16109


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mean value of the diameters (d1 + d2)/2 was defined as the spread of cement paste. The water reducing ratio of superplasticizer in mortar was conducted according to the Chinese national standard GB/T 2419−2005 (test method for fluidity of cement mortar) and GB/T 17671−1999 (method of testing cementsdetermination of strength, similar to ISO679−1989). Cement (400 g) and the desired amounts of water and superplasticizer were mixed in the mixer for 30 s at a low speed. Then 1200 g of river sand was added in the mixer and mixed for 30 s at a low speed and 30 s at a high speed. The residue on the vane and wall was scraped into the center of the mixer during the time-out of 90 s. The mortar was achieved after the next mixing for 60 s at a high speed. The mortar was put into a cone mold (base diameter of 10.0 cm, top diameter of 7.0 cm, and height of 6.0 cm) by twice. The mold was first filled to the two-thirds mark with mortar, and it was paddled by a knife for 5 times along two perpendicular directions, respectively. Then it was tamped down by a tamper with the diameter of 2.0 cm 15 times from boundary to center of the mold. The second mortar layer was put into the mold and each paddled 5 times along two perpendicular directions and then tamped down for 10 times. The extra mortar was scraped off by a knife. Once the test cone was lifted, the fresh mortar began to spread by an external vibration for 25 times, with 1 time/s. The mean value of the mortar diameters along two perpendicular directions was defined as the spread of mortar. The difference of water dosage between in the presence and in the absence of superplasticizer to achieve a constant spread of 19−20 cm was used to calculate the water reducing ratio in mortar. The test was finished within 6 min from adding water to measuring the spread. The slump, air content, water reducing ratio, and compressive strength of the concrete were determined according to the Chinese national standard GB 8076−2008 (Similar to ISO 1920−2:2005). The proportion of cement, sand, small stone (diameter: 5−10 mm), large stone (diameter: 10−20 mm) is 1:2.20:1.46:2.19. Cement dosage is 330 kg/m3. The initial concrete slumps were kept in 20−22 cm. The value of compressive strength was the average of the three specimens with variations of less than 5%. 2.9. Determination of the Hydration Heat Curve of the Cement Paste. After being mixed for 3 min, the cement paste containing 500 g of cement, 200 g of water, and the desired amount of superplasticizer was wrapped with a plastic bag and placed into Dewar vessel immediately; then, a thermocouple sensor was inserted into the center of the cement paste. The vessel was well-sealed, and the variation of temperature was recorded by a computer. 2.10. Determination of Adsorption Amount on the Surface of Cement Particles. Cement (0.6 g) was placed in a beaker and the GSL aqueous solution with a given concentration (0.1−1.5 g/L) was added into it, and the mass ratio of GSL aqueous solution and cement was 50:3. After the mixture was stirred for 120 min at 30 °C, 5 mL suspension was centrifuged. The concentration of supernatant was determined using a UV/vis spectrometer (UV-2450, Shimadzu Corp., Japan) by comparing the measured absorbance peak height with standard curves of absorbance versus GSL concentration at the wavelength of 280 nm. The adsorption amount of GSL on the surface of cement particles ns was calculated as follows:

ns =

V (C 0 − C ) m

(2)

Where C0 and C are the initial and the residual concentrations of GSL solution (mg/mL), V is the volume of the initial GSL solution (L), and m is the weight of cement (g). 2.11. Measurement of Thickness of Adsorbed Dispersant Film. The thickness of the adsorbed dispersant film of the cement surface was measured by XPS using a Ultra Axis DLD XPS (Kratos Analytical Ltd., UK) with Kα characteristic ray radiographic source of aluminum and photoelectron energy of 1486.8 eV. As the Si element is contained in the mineral matter of cement but not contained in dispersant molecule, the Si element can be used as the eigen element for the measurement of thickness of adsorbed dispersant film. The weakening degree of the Si 2p photoelectron that passes through the adsorbed dispersant film of cement surface is used to approximately calculate the thickness of adsorbed GSL film.8,24,25 Before the XPS measurement, 6 g of cement powder was dispersed in 100 mL of dispersant solution with the mass concentration of 1.5 g/L, which is higher than the concentration required to reach saturated adsorption. Then, the suspension was stirred for 2 h at a temperature of 30 °C. After filtration, the obtained lower filter cake was dried in vacuum and is used for XPS measurement.

3. RESULTS AND DISCUSSION 3.1. Process Optimization of Graft-Sulfonation of Lignin. 3.1.1. Effects of Acetone Dosage and Reactant Concentration on the Spread of Cement Paste. Molecular weight and the sulfonic group content play dominant roles in the dispersive performance of lignosulfonate-based superplasticizers.7,8,10,16 For the graft sulfonation of lignin, the acetone dosage and reactant concentration controlled by water dosage are key factors affecting the molecular weight of resultant. The average molecular weight of polymer is directly proportional to the intrinsic viscosity, denoted by [η]. The quantitative relationship between average molecular weight M and [η] can be described through Mark−Houwink equation: [η]=KMα, where K and α are constants specific to the solvent and temperature used in the measurements.26 The molecular weight of lignosulfonate measured by GPC using sodium polystyrene sulfonate as the standard is different from that measured by light scattering measurements.27 The separation process would have a serious impact on the molecular weight of lignosulfonate. Therefore, it is difficult to get the real molecular weight of lignosulfonate. In this work, the change of intrinsic viscosity was used to represent the change of molecular weight of graft-sulfonated lignin. The effects of acetone dosage and reactant concentration on intrinsic viscosity of the graft-sulfonated lignin, and their spreads of cement pastes are shown in Figure 1a. The water/ cement ratio (W/C) and the dosage in the measurements were 0.29 and 0.5%, respectively. The intrinsic viscosity of graft-sulfonated lignin increased significantly with an increase of reactant concentration from 37% to 46% when the mass ratio of acetone to lignin (mA/mL) was 0.62, 0.79, or 1.11. It indicates that higher concentration benefits condensation of sulfonated lignin. However, the spread profiles of each of the three acetone dosages had a peak during the conducted concentration range. The maximum spreads of cement paste were 238, 240, and 204 mm, respectively, when 16103



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plasticizer is the main group to adsorb on the cement surface by electrostatic attraction. Higher sulfonic group content made superplasticizer on the cement surface more negatively charged. Both higher adsorption amounts and more negative charges can improve the electrostatic repulsion among cement particles and consequently the dispersion of superplasticizer. Therefore, the effects of the sulfonic group content adjusted by sulfonating agent dosage on intrinsic viscosity and spread of cement paste were studied (Figure 1b). It was clearly shown in Figure 1b that the sulfonic group contents of graft-sulfonated lignins were in a range of 1.5 to 3.7, higher than that of lignosulfonate from the acidic sulfite pulping of wood (1.0−1.2 mmol/g).28 The intrinsic viscosity of the graft-sulfonated lignin reduced with the increase of sulfonic group content (or sulfonating agent dosage in reaction) when mA/mL = 0.79 and 0.62. The result indicates the sulfonic group in lignin inhibits the following condensation to some extent. There exists a peak in each spread profile of two acetone dosages in Figure 1b. Too high a sulfonic group content of graft-sulfonated lignin reduced the dispersive performance to cement particles, which may be due to the reduction of molecular weight. In order to understand the influence of acetone dosage on the sulfonation, we compared the sulfonating efficiency at two acetone dosages. When the sulfonic group content was 2.1 mmol/g, the sulfite dosage of mA/mL = 0.79 is 0.829 g/(g of lignin), which is lower than that of mA/mL = 0.62 by 12.6%. It suggests that acetone facilitates a higher sulfonic group content of the graft-sulfonated lignin. However, too high acetone dosage may lead to gel in the condition of low sulfonating agent dosage, resulting in poor dispersive performance in cement paste, such as the leftmost point in curve of mA/mL = 0.79. 3.1.3. Effect of Formaldehyde Dosage on Spread of Cement Paste. Both sulfonation and condensation in graft sulfonation of lignin are strongly dependent on formaldehyde. The effect of formaldehyde dosage on intrinsic viscosity of resultants and their spread of cement paste were studied under the condition of fixed other material ration and process parameters. As shown in Figure 1c, the intrinsic viscosity of graft-sulfonated lignin increased with the molar ratio of formaldehyde to acetone (nF/nA) from 2.0 to 2.4. It indicates formaldehyde facilitates the condensation of sulfonated lignin. However, the optimal nF/nA of maximum spread of cement paste is 2.1. The results seem to show that too a high molecular weight decreases the dispersive performance of graft-sulfonated lignin, in agreement with the results in Figures 1a and 1b. 3.2. Molecular Structure of GSL. The graft-sulfonated lignin was prepared in the following optimal conditions: mA/mL = 0.62, mS/mL = 0.48, nF/nA = 2.1, reactant concentration = 44%, sulfonation temperature = 55 °C, and condensation temperature = 95 °C. The optimal resultant is called GSL. Lignin is a physically and chemically heterogeneous material consisting of three representative phenylpropanes. It is an amorphous and three-dimensional polymer. Thus, lignin does not have a regular chemical structure like cellulose. Most technical lignins are produced by pulping the industry in a large scale. The lignin in plant cell walls is removed and dissolved in alkaline pulping liquor. Therefore, it is more difficult to elucidate the chemical structure of technical lignin. Figure 2 represents the FTIR spectra of acid-precipitated lignin and its graft-sulfonated resultant (GSL). Compared to lignin, GSL purified by preparative column chromatography has strong absorbance of SO and SO stretch at 1043 cm−1 and

Figure 1. Effects of ratio of acetone to lignin (a), reactant concentration (a), sulfonic group content (b), and ratio of formaldehyde to acetone (c) on the intrinsic viscosity of graftsulfonated lignin and its dispersion performance in cement paste.

mA/mL was 0.62, 0.79, and 1.11. The spread profile indicates too high an intrinsic viscosity may reduce the dispersive performance of graft-sulfonated lignin in cement paste. In addition, the intrinsic viscosity was found to increase with the mA/mL value from 0.62, 0.79, to 1.11, and the opposite was observed for the optimal concentration at maximum spreads, which was 43.9%, 42.0% and 38.5%, respectively. The results indicated that the dosage of acetone played an important role in the condensation of graft-sulfonated of lignin. 3.1.2. Effect of Sulfonating Agent Dosage on Spread of Cement Paste. Sulfonic group of lignosulfonate-based super16104



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Table 2. Structure Characters of Lignin and GSL sample

sulfonic group content (mmol/g)

GSL lignin

2.48 0

intrinsic viscosity (mL/g−1) 7.26

Mn (Da)

Mw (Da)

P

8 200 1 000

25 700 1 900

3.13 1.90

mmol/g) by approximately 120%.28 The Mw and Mn of GSL are 25 700 and 8200 Da, respectively, which are more than that of sodium lignosulfonate (11 200 Da) by approximately 130%.28 It indicates that graft sulfonation is an effective approach to synthesize a lignin-based dispersant with high molecular weight and a high sulfonic group content. 3.3. Dispersive Performance of GSL in Cementitious System. 3.3.1. Effects of GSL, FDN, and SAF on the Spread of Cement Paste. The effects of GSL, FDN, and SAF on the spread of cement paste at W/C=0.29 are shown in Figure 4a. When the dosage is 0.5%, the spread of GSL, FDN, and SAF is 213 mm, 224 mm and 236 mm, respectively. A spread larger than 200 mm indicates that GSL has strong dispersion in cement paste and can be used as superplasticizer. The spread of GSL in cement paste is slightly less than FDN and SAF, probably due to the strong air entrainment of lignosulfonate to reduce the spread of cement paste.6 3.3.2. Water Reducing Ratio of GSL and FDN in Mortar. The water reducing ratio in mortar is a key indicator to evaluate the dispersion of superplasticizer. The effect of dosage on the water reducibility of GSL and FDN in mortar are shown in Figure 4b. The water reducing ratio of GSL in mortar increased from 10.0% to 29.2% when the dosage increased from 0.2% to 1.0%, which is more than that of FND by 1.5−5.5%. It suggests that GSL has higher dispersion than FDN in mortar. 3.3.3. Effects of GSL and FDN on Compressive Strength of Concrete. The water reducibility, compressive strength enhancement, and effect on concrete workability are the critical indicators to evaluate a superplasticizer. Table 3 showed GSL had higher water reducing ratio in concrete than FDN by approximately 1.5% at the dosage of 0.6%. The concrete slump of GSL remains 19 cm, which is far more than that of FDN (10 cm). It indicates that GSL has strong ability to control concrete slump loss. This may be caused by two things: (1) GSL can delay the hydration of cement like lignosulfonates,30 and (2) GSL can increase the air content in concrete, as seen in Table 3. GSL increased the concrete compressive strength at 28 days by 32%, higher than that of FDN (24%). The enhancement of GSL on concrete compressive strength at 3 days is lower than that at 28 days, probably due to its delay in cement hydration and air entrainment. Therefore, GSL is a superplasticizer with strong water reduction and compressive strength enhancement and moderate hydration delay and air entrainment in mortar and concrete. 3.3.4. Effects of GSL and FDN on Hydration Temperature of Cement Paste. The hydration of Portland cement is exothermic. Therefore, the temperature of cement paste will rise in an adiabatic container. The hydration heat and its release are relevant to the late compressive strengthen and durability of concrete.31 Figure 4c illustrates the temperature of cement paste with GSL and FDN in semiadiabatic calorimeters in relation to the hydration time (mixing time). The temperature of cement pastes with FND reached a maximum at a hydration time of 9 min, as did the control, but the maximum temperature of FND is 60.2 °C, which is higher than the control by 1.9 °C. However, cement paste with GSL reached a

Figure 2. FTIR spectra of lignin and GSL.

1185 cm−1. It indicates that a lot of sulfonic groups have been induced in GSL by graft sulfonation. Additionally, GSL has strong absorbance band at 1646 cm−1, which is the stretch of CO. Compared with the peak at 1710 cm−1 of nonconjugated carbonyl group, the shift up to lower frequency at ∼60 cm−1 is possibly due to intra- and intermolecular hydrogen bonding between the carbonyl group and the adjacent hydroxyl group.29 The band at 526 cm−1 is due to the COC stretch of GSL. In addition, there are strong absorbances at 3400 cm−1 and 3443 cm−1 in the FTIR spectra of lignin and GSL, respectively, which are the stretches of phenolic and aliphatic hydroxyl groups. Both of them move forward to a low wavenumber due to intermolecular or intramolecular hydrogen bond formation among phenolic and aliphatic hydroxyl groups in lignins, compared to the wavenumber of 3610−3640 cm−1 of the OH without a hydrogen bond. The higher absorbance wavenumber of OH in GSL than that in lignin suggests that there are fewer aggregates among GSL molecules than those among lignin molecules by hydrogen bond. Combined with the phenomena in the reaction including effects of process parameters on structure and dispersion, a possible chemical structure was proposed and shown in Figure 3, where L

Figure 3. Chemical structure schematic diagram of GSL.

represents lignin molecule; m and n are the polymerization degrees of two types of constitutional unit. The macromolecule of GSL is composed of lignin and polymer side chain. The sulfonic group may be linked to aromatic ring or side chain. The end of the side chain may be a hydroxyl group or a link to another lignin through an ether bond. The molecular structure parameters of GSL and lignin, including sulfonic group content, intrinsic viscosity, and average molecular weight, are listed in Table 2. The sulfonic group content of GSL is 2.48 mmol/g, which is higher than that of sodium lignosulfonate produced by acidic sulfite pulping (1.13 16105



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maximum temperature at a hydration time of 11 min, and the maximum temperature of GSL is 57 °C, which is lower than the control by 1.3 °C. It validates that GSL can delay the hydration of cement and consequently be used to control the release rate of hydration heat in mass concrete construction. 3.3.5. Effect of Molecular Weight on Dispersive Performance of GSL and the Dispersion Mechanism. It is an effective way to understand the effect of molecular weight on dispersion of GSL and its dispersion mechanism by studying the adsorption and dispersive performance of GSL fraction with different molecular weight.8 GSL are classified into four fractions using ultrafiltration membranes with the cutoff molecular weight of 2500 Da, 10 000, and 50 000 Da. As molecular weight increased, the fractions were named respectively GSL-A, GSL-B, GSL-C and GSL-D. It is difficult to measure the structure features of GSL-A (below 2500 Da), because it contains too much inorganic salt. The molecular weight distribution and sulfonic group content of GSL fractions were shown in Figure 5 and Table 4.

Figure 5. Molecular weight distribution of GSL fractions measured by GPC.

Figure 6a represents the effects of GSL fractions on the spread of cement paste at W/C = 0.29. The spread of cement paste increased with the increase of GSL fraction molecular weight. And GSL-D has stronger dispersion than FDN by 30− 70 mm for spread of cement paste. It indicates GSL with higher molecular weight has stronger dispersion. However, the results in Figures 1a, 1b, and 1c showed graft-sulfonated lignin with too high an intrinsic viscosity decreased the spread of cement paste. What is the difference between them? It was found in Figures 1a, 1b, and 1c that the optimal intrinsic viscosities for maximum spread of cement paste were all more than 7.5 mL/g. The higher intrinsic viscosity is caused by the gelling of graftsulfonated lignin, which leads to decrease of the spread of cement paste. However, there is little gel in GSL-D because the

Figure 4. Effects of GSL and FDN on the spread of cement paste (a), on the water reducing ratio in mortar (b) and on the temperature of cement paste hydration (c).

Table 3. Effects of Superplasticizers on the Concrete Properties compressive strength (MPa) /relative compressive strength (%) superplasticizer control GSL FDN

dosage (%)

initial slump (cm)

slump at 1h (cm)

0.6 0.6

22 22 22

5 19 10

water reducing ratio (%)

air content in concrete (%)

3d

28d

23.5 22.0

2.0 2.5 1.9

18.3/100 22.5/123 22.8/125

29.2/100 38.4/132 36.1/124

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Table 4. Molecular Weight and Sulfonic Group Content of GSL Fractions sulfonic group content GSL fractions

cut-off molecular weight

intrinsic viscosity(mL/g)

Mw

Mn

P

(mmol/g)

(mol/mol)

GSL-B GSL-C GSL-D

2500−10 000 10 000−50 000 >50 000

4.2 8.3 14.5

8 200 14 500 30 200

3 100 7 200 16 200

2.65 2.01 1.86

2.72 2.57 2.03

22.3 37.3 61.3

Table 5. Results from Linear Regressions of GSL Fraction Adsorption Isotherms by Cement Particles Using the Langmuir Model GSL fractions

maximum adsorption amount, Γm (mg/g)

K (L/mg)

R2

GSL-B GSL-C GSL-D

6.6 5.2 4.4

0.0066 0.0368 0.2321

0.9915 0.9557 0.9063

The electrostatic attraction between sulfonic group and mineral cations on cement particles is the dominant driving force of adsorption. Therefore,the GSL fraction of higher molecular weight adsorbs more firmly on the cement particles to result in stronger dispersion in cementitious system. However, steric hindrance among GSL fraction molecules adsorbed on cement surface increased with molecular weight, resulting in the decrease of adsorption amount with high molecular weight. Figure 6b illustrates the Si 2p XPS spectrum of cement surface with or without GSL fractions. The relative strength of Si 2p XPS spectrum with or without GSL fractions can be used to calculate the thickness of adsorbed film of GSL fractions on cement surface.24 The calculated results are listed in Table 6. Table 6. Thickness of Absorbed Film of GSL Fractions on Cement Surface GSL fractions

Mw

thickness of absorbed film (nm)

GSL-B GSL-C GSL-D

9 400 15 600 33 200

11.2 13.4 19.8

The adsorption layer thickness of GSL-B, GSL-C, and GSL-D is 11.2 nm, 13.4 nm, and 19.8 nm, respectively. In the same conditions, the adsorption layer thickness of SAF-B, SAF-C, and SAF-D is 3.91 nm, 5.50 nm, and 7.02 nm, respectively.8 The result indicates that the large three-dimensional structure of lignin makes the adsorption layer of GSL higher than SAF. Table 6 shows the thickness of adsorbed films of GSL fractions increase with the increasing of molecular weight. Because these are linear chains linked between the threedimensional lignin units in GSL macromolecule, it is softer and easier to change the adsorption conformation than in lignin or lignosulfonate. The GSL-D with high molecular weight is more likely to twist and be adsorbed on cement surface through “loop and tail” adsorption, resulting in a thick adsorption film.8 Also, the GSL-B molecule with low molecular weight is adsorbed on the cement surface through “flat train” adsorption, resulting in a thin adsorption film.

Figure 6. Spread of cement paste (a) and XPS scanning spectrum of cement particles (b) with GSL fractions.

intrinsic viscosity of GSL is only 7.26 mL/g. Therefore, increasing the molecular weight of superplasticizer except of gelling is a new idea to improve its dispersion. In order to understand the dispersion mechanism of GSL, the adsorption natures of GSL fractions with different molecular weight were studied in this work. The adsorption isotherms of GSL fractions on cement particle (25 °C) are fitted by the Langmuir equation, and the obtained parameters are listed in Table 5. The coefficient R2 of all GSL fractions are more than 0.90, so the Langmuir monolayer adsorption model is suitable for the adsorption isotherms of GSL fractions on cement. The maximum adsorption amount (Γm) of GSL fractions decreases with the increase of molecular weight, whereas the opposite is observed for the adsorption equilibrium constant (K) of GSL. K increases with the number of sulfonic groups of each molecule listed in Table 4, which suggests that adsorption force between GSL fractions and cement particles is determined by the sulfonic group content of each molecule.

4. CONCLUSIONS (1) Graft sulfonation is a practical approach to prepare a superplasticizer using lignin from Wheat Straw Kraft pulping black liquor as raw material. Graft-sulfonated lignin with moderate molecular weight and sulfonic 16107



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group content has excellent dispersion in a cementitious system. (2) The molecular weight of graft-sulfonated lignin increased with the reactant concentration and the dosage of acetone and formaldehyde, whereas the opposite was observed with the increase of the dosage of sulfonating agent (e.g., sulfite). The sulfonic group content of graftsulfonated lignin increased with the dosage of sulfonating agent. (3) The Mw and sulfonic group content of GSL prepared in optimal reaction conditions are 25 700 Da and 2.48 mmol/g, both of which are more than those of sodium lignosulfonate by approximately 100%. GSL has a stronger water reducing ability than FDN in mortar and concrete. The concrete with GSL has little slump loss, moderate air content, and strong compressive strength. In addition, GSL can be used to control the release of hydration heat in mass concrete. (4) GSL with higher molecular weight has stronger dispersive performance due to the thicker adsorbed film from an increase steric hindrance in spite of a lower adsorption amount.

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ASSOCIATED CONTENT

S Supporting Information *

Tables and graphs showing the spread of the cement paste, water reducibility in mortar, and hydration time. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*X. Qiu. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial supports of the China Excellent Young Scientist Fund (20925622), the National Science and Technology Support Plan Projects of China (2011BAE06B06-3), and International S&T Cooperation Program of China (2013DFA41670).

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REFERENCES

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