M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 8 8 – 9 5

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Impact of welan gum on tricalciumaluminate–gypsum hydration

Lei Ma, Qinglin Zhao⁎, Chukang Yao, Mingkai ZhouState Key Laboratory of Silicate Materials for Architectures, School of Materials Science and Engineering, Wuhan University of Technology,Wuhan 430070, P.R. China

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +86 27 8766 999E-mail addresses: malei198713@163.com

1044-5803/$ – see front matter © 2011 Elseviedoi:10.1016/j.matchar.2011.12.002

A B S T R A C T

Article history:Received 7 August 2011Received in revised form30 November 2011Accepted 1 December 2011

The retarding effect of welan gum on tricalcium aluminate–gypsum hydration, as a partialsystem of ordinary Portland cement (OPC) hydration, was investigated with severalmethods. The tricalcium aluminate–gypsum hydration behavior in the presence orabsence of welan gum was researched by field emission gun scanning electronmicroscopy, X-ray diffraction and zeta potential analysis. Meanwhile, we studied the sur-face electrochemical properties and adsorption characteristics of welan gum by utilizing azeta potential analyzer and UV–VIS absorption spectrophotometer. By adding welan gum,the morphology change of ettringite and retardation of hydration stages in tricalcium alu-minate–gypsum system was observed. Moreover, we detected the adsorption behaviorand zeta potential inversion of tricalcium aluminate and ettringite, as well as a rapid de-crease in the zeta potential of tricalcium aluminate–gypsum system. The reduction on nu-cleation rate of ettringite and hydration activity of C3A was also demonstrated. Thus,through the adsorption effect, welan gum induces a retarding behavior in tricalcium alumi-nate–gypsum hydration.

© 2011 Elsevier Inc. All rights reserved.

Keywords:Welan gumTricalcium aluminateEttringiteZeta potentialHydration

1. Introduction

Welan gum is a water-soluble polysaccharide commonly in-troduced into cementitious materials to induce a substantialincrease in water retention capacity, and to adjust workabili-ty. It is produced by bacteria from the Alcaligenes species inan aerobic, submerged fermentation. Welan gum is low-costand its tolerance for a high concentration of calcium ionsmakes it ideal for applications involving concrete, mortars, ce-ment grouts and high temperature drilling [1]. The repeat unitof welan gum shows a single sugar side-chain containing ei-ther L-rhamnose or L-mannose substituted on C3 of every(1→4)-linked glucose unit [2]. The long-chain molecules ofwelan gum can adhere to the periphery of water molecules,thus they adsorb and fix part of the mixing water which in-creases the yield value and plastic viscosity [3]. By the sameprinciple, welan gum is highly effective in controlling bleeding

3; fax: +86 27 8764 1294.(L. Ma), zhaoqinglin@whu

r Inc. All rights reserved.

in cement-based materials. Previous studies highlighted thatin aqueous solution, the side chain can fold back on themain chain to form hydrogen bonds with the carboxylategroups and a double helix is produced [4]. As a side effect,welan gum may also lead to a problematic retardation of ce-ment hydration.

Tricalcium aluminate (Ca3Al2O6, C3A), one of the most re-active components of Portland clinker, has an important ef-fect on the early hydration and rheological behavior ofcement slurry. The presence of gypsum strongly affects thehydration behavior of C3A and leads to the formation of ettrin-gite (Ca6[Al(OH)6]2·(SO4)3·26H2O), which armours unhydratedaluminate and slows down its hydration. The reactivity ofthe C3A–gypsum system plays a vital role in controlling andforecasting the workability of cement-based materials. Theinfluence mechanism of many additives on the C3A–gypsumsystem's hydration has been extensively investigated [5–11].

t.edu.cn (Q.L. Zhao).

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Therefore, to better understand the retarding effect ofwelan gum on OPC hydration, it is necessary to study the in-fluence mechanism of welan gum on C3A–gypsum system.For this purpose, the ettringite, one of the most crucial hydra-tion products, was synthesized and the zeta potential as wellas the adsorption amount of welan gum on C3A and ettringitewas measured. Hydration of the C3A–gypsum system wascharacterized in detail by zeta-potential analysis, UV–VISspectrophotometry, powder X-ray diffraction (XRD) and witha field emission gun scanning electron microscope (FEG-SEM).

2. Materials and Methods

2.1. Starting Materials

Ettringite was prepared by adding 7 g aluminum sulfate hy-drate (Al2[SO4]3·18H2O, AR grade) and 5 g of portlandite (Ca[OH]2) to 50 ml deionized water at 60 °C and stirring for30 min. The solid filtrate was dried at 50 °C and analyzed byXRD, which indicated that ettringite was the main constitu-ent. Traces of gypsum were removed by rinsing with deio-nized water following Barbarulo's method [12], to below theXRD detection limit. The surface area of ettringite determinedby BET method was 22 m2/g.

The C3A from China Building Materials Academy (Beijing,China) was produced from iron-free AR-grade chemicals. Itssurface area determined by BET method equalled 0.57 m2/g.The welan gum came from Hebei Xinhe Biochemical Co., Ltd(Hebei, China). Its performance index was as follows: the ap-pearance was off-white powder; PH (of 1% solution) was 7.0–10.0; and the particle size was no less than 95% through100 mesh. The gypsum (CaSO4·2H2O, AR-grade chemicals)was produced by Sinopharm Chemical Reagent Co., Ltd(Shanghai, China).

2.2. Hydration Experiments of C3A–gypsum System

Table 1 shows the mixture proportions. Groups A1 and A2 be-long to hydration experiments of the C3A–gypsum system. Inthe C3A–gypsum system, the higher the amount of gypsum,the longer the duration of the hydration period. For groupsA1 and A2, to observe the hydration state of C3A before andafter the gypsumwas depleted, a mass ratio of C3A to gypsumof 2:1 was determined, which corresponded to a mole ratio of

Table 1 –Mix proportion of experiment.

Group C3A (g) Gypsum (g) Ettringite (g) Welan (g)

A1 2 1 – –A2 2 1 – 0.015B1 3 – – –B2 3 – – –B3 3 – – 0.015B4 – – 3 –B5 – – 3 –B6 – – 3 0.015

a Saturated Ca(OH)2 solution in 25 °C.b Zeta potential.

1.27:1. In group A1, C3A and gypsumwere mixed and added tothe saturated calcium hydroxide solution with a liquid to solidratio equal to 3, which was closed to hydration in cementpaste because the solution was promptly saturated with calci-um hydroxide. The high L/S (liquid/solid=3) ratio was used inorder to keep the mixture homogeneous. The mixture wassealed in beaker and stirred with amagnetic stirrer. A fewmil-liliters of sample were taken out several times and dividedinto two parts for measurement below. Both the reaction pro-cess and the following measurement were conducted at roomtemperature.

Additionally, for group A2 with welan gum (0.5% by weightof solid, the ratio is suitable to observe the distinction be-tween A1 and A2), the additive was joined in saturated calci-um hydroxide solution prior to the hydration and the rest ofthe procedure was the same as for group A1.

2.2.1. Characterization by FEG-SEM and XRDSome of the sample was washed with alcohol. It was thendried at 50 °C and ground into powder for analysis with theFEG-SEM and powder XRD. Selected powder was sputteredwith gold and investigated in the FEG-SEM (JEOL JSM-6700F,at accelerating voltage of 5 kV, emission current of 10–20 μA,vacuum pressure <10−5 Torr, working distance of 8–8.3 mm).XRD measurements were performed using a PANalyticalX'Pert PW3050/60 diffractometer (CuKα anode, λ=1.54178 Å,40 kV, 40 mA) in the range of 5–80° 2θ with a step of 0.02°/s.

2.2.2. Zeta Potential AnalysisAnother part of sample was diluted with deionized water upto 500 times to determine the zeta potential. The zeta poten-tial was measured by Zeta-Meter System 3.0+ (Zeta-MeterInc, USA), which works on the basis of the electrophoresismethod.

2.3. Zeta Potential and Surface Adsorption Experiments ofC3A and Ettringite

From B1 to B6 in Table 1, the C3A and ettringite were added todeionized water or saturated calcium hydroxide solutionaccording a liquid/solid ratio of 3. After 20 min of magneticstirring, a 1 mL mixing sample was extracted and dilutedwith deionized water up to 500 times.

If the mixing sample contained welan gum (0.5% by weightof solid, to keep the same ratio with hydration experiment of

Limewater (g) a DI water (g) L/S Analysis method

9 – 3 FEG-SEM, XRD, ZPb

9 – 3 FEG-SEM, XRD, ZP– 9 3 ZP9 – 3 ZP9 – 3 ZP, UV–VIS– 9 3 ZP9 – 3 ZP9 – 3 ZP, UV–VIS

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C3A–gypsum system), the remaining mixing sample was fil-tered using a millipore membrane of 0.22 μm and the welangum in the filtrate was measured with a phenol-sulfuric acidmethod [13], which determined the absorbance at 490 nm bymeans of a UV–VIS spectrophotometer (LAB UV-2800, Shang-hai, China). The amount of polysaccharide adsorbed on themineral phase was calculated from the differences in the ini-tial well-known welan gum concentration and the final con-centration after adsorption. The indoor temperature wasidentical with that during previous hydration experiment.

3. Results

3.1. FEG-SEM Images of C3A–gypsum System

Themorphology of the blank C3A–gypsum system at 24 h (a, b,c) and 48 h (d, e, f) is exhibited in Fig. 1. As shown in Fig. 1a, alarge quantity of hydrates formed around C3A particles at 24 hwhile a certain amount of gypsum carrying hydration prod-ucts can be observed in the system. Based on the XRD datain Fig. 3 and their morphology in Fig. 1b and c, the hydratescan be identified as ettringite. It can be seen that the surface

Fig. 1 – SEM images of hydration stages of C3A–gypsu

of C3A is armoured by randomly oriented rod-like ettringitecrystals in Fig. 1b and that the crystal size is about 100–300 nm in length in Fig. 1c. The solid gypsum disappears at48 h in Fig. 1d, e, f. Comparing with 24 h, a certain degree ofgrowth of ettringite can be observed at 48 h and the ettringitecrystals range from 200 to 400 nm in length in Fig. 1f. Further-more, the ettringite morphology in Fig. 1e and f appears to bewider than that in Fig. 1b and c, in the blank system.

The images of the system with welan gum are shown inFig. 2. At 24 h (a, b, c), it is similar to the blank system in thatthe C3A particles are wrapped by ettringite while the gypsumhas a clean surface as seen clearly in Fig. 2a. In view of thephase composition in Fig. 4, the gel-like hydrates shelteringthe C3A surface should be ettringite in Fig. 2b [7], the detailsof which are shown in Fig. 2c. At 48 h (d, e, f), Fig. 2d indicatesthat in contrast to the blank system at 48 h, the gypsum canbe observed clearly in the additive system. Fig. 2e shows thatthe ettringite was deposited on the C3A surface as an agglom-erate where the individual crystal can hardly be distin-guished, but the surface of the C3A particle still cannot becompletely covered. The ettringite morphology shown inFig. 2f seems like spheres or short rods on the surface of agel-like phase.

m system, at 24 h (a, b, c) and 48 h (d, e, f), L/S=3.

Fig. 2 – SEM images of hydration stages of C3A–gypsum system with 0.5% welan gum, at 24 h (a, b, c) and 48 h (d, e, f), L/S=3.

Fig. 3 – X-ray diffraction patterns of C3A–gypsum system,L/S=3 (E: ettringite, G: gypsum, A: C3A, M: AFm, H: C4AH13).

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3.2. XRD Investigation of C3A–gypsum System

The results in Fig. 3 exhibit the hydration process of the blankC3A–gypsum system. As can be seen, at 6 h, apart from thepeaks of C3A (JCPDS 38-1429) and gypsum (JCPDS 06–0046),the characteristic peak of ettringite (Ca6[Al(OH)6]2·(SO4)3·26H2-

O, JCPDS 72-0646) at 2θ=9.07° (d=9.74 Å) emerges. The ettrin-gite peak intensity increases over time. At 48 h, the traces ofgypsum disappear, which is confirmed by FEG-SEM images(Fig. 1d, e, f). The ettringite peaks still exist at 72 h, but thepeak intensity becomes weaker than in previous stages.Meanwhile, the main phase in the system is AFm (calciummonosulfoaluminate, Ca4[Al(OH)6]2·(SO4)·6H2O, JCPDS 50-1607) and its characteristic peak is situated at 2θ=10.08°(d=8.86 Å). The peak intensity of AFm increases after 72 h.Moreover, at 120 h, ettringite, C3A and gypsum wereexhausted in the hydration process. It seems that the AFmphase dominates the system, but a new peak also appears.The phase (2θ=10.90°, d=8.08 Å) identified by XRD phase anal-ysis is C4AH13 (JCPDS 33-0255) [14,15].

The phase transformation of C3A–gypsum system contain-ing welan gum is shown in Fig. 4. At 6 h, 24 h and 48 h, the

peaks of ettringite can be observed clearly, but the relative in-tensity is weaker than for the blank system. The peaks of gyp-sum exist in the system from 6 to 72 h. Obviously, the amountof gypsum was more than that in the blank system in everystage. At 120 h, the phases of AFm and C4AH13 are detected

Fig. 4 – X-ray diffraction patterns of C3A–gypsum systemwith 0.5%welan gum, L/S=3 (E: ettringite, G: gypsum, A: C3A,M: AFm, H: C4AH13).

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in XRD. It is worth mentioning that the traces of C3A can beobserved at 120 h.

3.3. Zeta Potential and Adsorbed Amount of C3A andEttringite

Fig. 5 presents the effect of saturated calcium hydroxide solu-tion and welan gum on the zeta potential of ettringite and C3A(left), as well as the adsorption of welan gum on bothminerals(right). From the results shown in Fig. 5 (left), the zeta poten-tials of C3A and ettringite in deionized water are positive,about 23.29 mV and 5.51 mV, respectively. In a saturated calci-um hydroxide solution, the zeta potential values of 33.47 mVand 10.56 mV were measured for C3A and ettringite, respec-tively, higher than those in deionized water. When 0.5%welan gum existed in saturated calcium hydroxide solution,the electrostatic potentials of both phases were inverted tohighly negative values, about −28.34 mV and −24.96 mV,respectively.

The content of welan gum adsorbed in saturated calciumhydroxide solution was quantified per mass of mineralphase. It can be seen from Fig. 5 (right) that the adsorptionamount for C3A and ettringite are 2.42 mg/g and 1.36 mg/g re-spectively, which account for 48.4% and 27.3% of the quantityintroduced initially. The adsorption amount on C3A is about 2times higher than that for ettringite.

Fig. 5 – Zeta potential of C3A and ettringite in deionized water oradsorbed amount of welan gum on C3A and ettringite in saturatright).

3.4. The Variation of Zeta Potential of C3A–gypsumSystem with and without Welan Gum

The zeta potential variation of the C3A–gypsum system isshown in Fig. 6. The zeta potential of the blank sample is neg-ative and the value is about −18.8 mV at the 1 h point. From 1to 60 h, due to the consumption of SO4

2− and gradually increas-ing amount of ettringite, the zeta potentials of blank sampletend to be positive. The isoelectric point is situated at 43 h.

With a zeta potential of −32.63 mV, the system containingwelan gum has a higher negative zeta potential value than theblank system at 1 h. At 5 h, the value rises to −35.06 mV. From5 h to the end, zeta potential values decrease from −35.06 mVto −6.15 mV.

4. Discussion

4.1. Microscopic Morphology of C3A–gypsum System in theAbsence and Presence of Welan Gum

The size of ettringite crystals in the blank sample (Fig. 1) issmaller than that in the former study [16,17]. There are vari-ous factors that affect the nucleation and growth of ettringitein solution, such as the ratio of C3A and gypsum, concentra-tion of solution, water/solid, adding additive or not, pH andso on. During the initial hydration stage of the C3A–gypsumsystem without additive, C3A releases numerous Ca2+ and[Al(OH)4]− [18], which can be nucleation sites for ettringitecrystals in the presence of gypsum in solution. In this study,the mole ratio of C3A and gypsum is 1.27, which means thatthere is twice as much C3A as gypsum in system. Thus, atthe early hydration stage, the concentration of Ca2+ and[Al(OH)4]− reached a very high level instantaneously and alarge number of ettringite crystal nuclei formed in solution.Then, as shown in Fig. 1, numerous tiny ettringite crystalsemerged in the blank system [19].

In the system containing welan gum, Fig. 2a–c (24 h) showthat the initial formation of ettringite was influenced by thewelan gum. The study of Merlini et al. [7] indicated that atthe beginning of the hydration stage of the C3A–gypsum sys-tem, the ettringite is amorphous. The blank sample at 24 h(Fig. 1) has experienced the period of generation of gel-like

saturated Ca(OH)2 solution with and without welan gum(left),ed Ca(OH)2 solution (0.5% welan gum by weight of mineral,

Fig. 6 – Zeta potential ofC3A–gypsumsystemwith andwithoutwelan gum as a function of hydration time (in saturatedCa(OH)2 solution, 0.5% welan gum by weight of mineral).

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ettringite. Moreover, in the system containing an additive,some additive molecules can bind into the gel-like ettringiteand stabilized it, so that the period of ettringite nucleationand growth is extended [8]. Therefore, as a result of adsorption,the welan gummolecules entered the gel-like ettringite and slo-wed ettringite nucleation. At 48 h (Fig. 2d), in comparison withFig. 1d, it is obvious that the consumption of gypsum wasdelayed by welan gum. On the C3A surface, the electrostaticforce evoked by the adsorption of welan gummolecules blockedthe SO4

2− and decreased the hydration activity of adsorbed re-gions. This is the reason why ettringite does not cover the sur-face of C3A thoroughly in Fig. 2b and e. The adsorption of welangum on ettringite (Fig. 5, right) caused the variation in the sizeand morphology of ettringite [16]. In other words, the change inthe ettringite crystal (Fig. 2f) induced by welan gum was basedupon preferential adsorption on specific crystal faces [20].

4.2. The Transformation of Hydration Products in C3A–gypsum System with and without Additive

The amount of sulfate in the C3A–gypsum system plays a vitalrole in determining the hydration product. If the mole ratio ofC3A and gypsum is 1:3, it allows complete conversion of thesystem to ettringite. The reaction is

Ca3Al2O6 þ 3CaSO4d2H2O þ 26H2O——Ca6½AlðOHÞ6�2dðSO4Þ3d26H2O

ð1Þ

For full conversion of the system to AFm, a ratio of 1:1 isnecessary. From a thermodynamic point of view, AFm ismore stable than ettringite [21]. The reaction is

Ca6½AlðOHÞ6�2dðSO4Þ3d26H2O þ 2Ca3Al2O6

þ4H2O——3Ca4½AlðOHÞ6�2dðSO4Þd6H2O

ð2Þ

In this study, the mole ratio is 1.27:1, which means when allgypsumconverts toAFm, calciumhydroaluminateswill be gen-erated from the hydration of redundant C3A. The reaction is:

C3A þ CaðOHÞ2 þ 12H2O——C4AH13 ð3Þ

Pourchez et al. [6] and Minard et al. [21] did a lot of work onthe hydration of the C3A–gypsum systems. They emphasizedthat the hydration period of the system where the mole ratioof C3A and gypsum is greater than 1 can be divided into two

parts. The first part begins with the ettringite precipitationand stops due to the exhaustion of sulphate ions. The secondcorresponds to the C3A dissolution and calcium hydroalumi-nates precipitation. In Fig. 3, two stages of the hydration peri-od can be observed.

– Stage 1 corresponds to the C3A hydration in the presence ofgypsum that caused the formation of ettringite. The peaksof gypsumhave almost disappeared at 48 h, so the terms be-fore 48 h belong to stage 1. The termination of stage 1 oughtto be the disappearance of sulphate ions in solution, but avery tiny amount of gypsum can be seen at 48 h. Therefore,the sulphate ions still existed at 48 h, which implies thatthe termination of stage 1 is situated between 48 h and 72 h.

– Stage 2 of hydration period started when sulphate ionswere entirely consumed and C3A began to dissolve. Theettringite and calcium hydroaluminates transformed intoAFm (72 h). When all ettringite was converted to AFm, theredundant C3A continued to dissolve and calcium hydroa-luminates (C4AH13) presented in the system individually(120 h). To summarize, stage 2 can be divided into twosteps depending on the presence or not of ettringite.

The retardation induced by welan gum in hydration ofC3A–gypsum system can be seen in Fig. 4. It is obvious thatboth hydration stages of C3A–gypsum system were delayedby welan gum.

– The time necessary for stage 1 exceeded 72 h compared tothe blank system. From 6 to 72 h, it appears that the dissolu-tion of gypsum was slowed down in the presence of welangum. From the results of Fig. 5, we may assume that thewelan gum was adsorbed on the surface of C3A, postponingthe precipitate of calcium hydroaluminates. Meanwhile,the consumption of SO4

2−was delayed aswell. Consequently,the gypsum remained in equilibration with SO4

2- in solutionfor a longer time than in the system without welan gum.

– The hydration process of stage 2 was also retarded. Theinitial point of stage 2 was situated between 72 h and120 h. As can be seen in Fig. 3, the C3A was almostexhausted and the AFm was the primary phase at 72 h.However, the data for 120 h shown in Fig. 4 indicate thata little C3A coexisted with the AFm and C4AH13. This im-plies that the sequence of steps in the hydration processin stage 2 was disordered by the presence of welan gum.Moreover, the FEG-SEM images in Fig. 2 illustrate that inthe presence of welan gum, the surface of C3A was onlywith difficulty covered completely by ettringite. This effectmight be explained by the regions that adsorbed welangum molecule on C3A surface reacting very slowly whilethe hydration of regions without additives proceeds asusual. Therefore, the hydration process of the regionswith additivemolecules on C3A did not end until the ettrin-gite was used up and C4AH13 began to precipitate.

4.3. Zeta Potential and Adsorbed Amount of C3A andEttringite

In a colloidal solution, a diffuse electric double layer formsaround the suspended particle [22]. The composition of the

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solvents and the chemical and physical properties of the sus-pended particles determine the ionic species in the doublelayer, as well as its zeta potential [20,23]. Therefore, it is im-portant to control the ionic components of the solvents to ob-tain the accurate zeta potential values. In this study, theconductivity of the deionized water is less than 15 μS/cm, sothe influence of ions in water can be ignored. In deionizedwater, the zeta potential is controlled exclusively by the sur-face property of the mineral. Both C3A and ettringite in Fig. 5(left), which are positively charged, belong to aluminatephase in Portland cement composition.

According to Zingg et al. [23], the zeta potentials of bothC3A and ettringite are positive in 0.1 mM KOH solution, butin 0.1 mM Ca(OH)2 solution, the value for ettringite is posi-tive and 4 times higher than that in the KOH solution.Elakneswaran et al. [24] indicates that cement particleshave positive zeta potential in Ca(OH)2 solution above1 mM and the value becomes higher with increasingCa(OH)2 concentration. Therefore, it can be explained thatdue to high concentration of calcium ions, the ionic compo-sition in the diffuse double layer of minerals can be changedin a saturated Ca(OH)2 solution. Thus, in a saturated Ca(OH)2solution, the calcium ions that entered into the diffuse dou-ble layer of C3A and ettringite led to the increase in positivezeta potential values.

When welan gum was added, the negatively chargedgroups in its structure, such as carboxyl and hydroxyl,resulted in strong adsorption on the surface of the aluminatephase. Consequently, welan gum dominated the ionic compo-sition between the face of particles and the slip surface in so-lution. Then the electrostatic potentials of C3A and ettringiteshifted. Additionally, Fig. 5 (right) shows the amount ofwelan gum adsorbed on C3A is more than that on ettringite,so the negative zeta potential value for C3A is higher than itis for the ettringite, too.

The interpretation of adsorption results can be classifiedinto two aspects. On the one hand, as in Fig. 5 (left), the zetapotential of C3A in saturated Ca(OH)2 solution is 33.47 mV,which is far more than ettringite, 10.56 mV. On the otherhand, the hydration of C3A generated a large amount of calci-um hydroaluminates (C2AH8-C4AH13), which was a plate-likestructure and enlarged the specific surface area of C3A parti-cles. Therefore, C3A particles can adsorb even more welangum molecules.

4.4. Influence of Welan Gum on the Zeta Potential Trend ofC3A–gypsum System

In the blank system in Fig. 6, at the beginning of hydration, thedissolution of gypsum led to SO4

2− saturation instantaneously.The negative SO4

2− was adsorbed on the surface of C3A directlyto produce the negative value [23]. Afterwards, the C3A and SO4

2−

transformed into ettringite. It can be seen from XRD data inFig. 3 that at 48 h, only ettringite and C3A phase existed in theblank system. In Fig. 6, the zeta potential value at 48 h is positive,which corresponds with XRD data.

In the system containing welan gum, the carboxylic acidgroups in welan gum possess stronger adsorption abilitythan a sulfonic acid group [25], so it can be confirmed thatthe adsorbing capability of welan gum is stronger than that

of SO42−. Moreover, there are abundant negatively charged hy-

droxyl groups in the molecular structure of welan gum to pro-duce high anionic charge density. The result of competitiveadsorption between welan gum and SO4

2− suggests that theionic composition of the Stern and diffuse layers is dominatedby welan gum, hence the higher negative zeta potential. At5 h, the ascent of the value can be explained by the delayedC3A hydration (Fig. 4) due to the effect of the adsorption ofwelan gum and the competitive adsorption, which delayedthe adsorption balance between welan gum and C3A. Inother words, from 1 h to 5 h, the welan gum did not reachthe adsorption balance on C3A. After 5 h, the zeta potentialtended toward the positive, similar to the blank system. TheXRD data presented in Fig. 4 shows that from 6 h to 72 h themain hydration product was ettringite, which covered the sur-face of C3A (Fig. 2). Therefore, with the increase of ettringite,the welan gum adsorbed on C3A surfaces can be covered byettringite and the zeta potential was influenced to shift to thepositive direction [26]. Besides, the welan gum molecule con-tains a single sugar side-chain including either L-rhamnoseor L-mannose substituted on the backbone [2]. In view of theslope of the zeta potential in the C3A–gypsum system con-taining additive, the side chain of welan gum may notprovide a steric repulsive force [27,28] among particles insolution sufficient to keep mineral particles dispersed andstable [29].

5. Conclusions

1. In the C3A–gypsum system, small ettringite crystals areformed rapidly and dispersed on the surface of C3A in asaturated Ca(OH)2 solution of liquid/solid equal to 3. Thehydration process of the system can be divided into twostages. In the system with welan gum, the adsorption ofadditive molecules slow down the rate of ettringite nucle-ation and lead to the morphological change of ettringitecrystals. In the same way, the hydration activity of C3A isreduced and the two stages of hydration period are bothdelayed.

2. In a saturated Ca(OH)2 solution, the positive zeta potentialsof ettringite and C3A are much higher than that in deio-nized water. The significant adsorption of welan gum onC3A and ettringite is obvious in this solution, which is con-firmed by highly negative zeta potential values.

3. With the consumption of SO42−, the zeta potential of C3A–

gypsum system shifts to the positive within 60 h. The iso-electric point is located at 43 h. In the presence of 0.5%welan gum, the adsorption of welan gumproduces a highlynegative zeta potential value of the system. But the forma-tion of ettringite changes the surface of C3A and results in acovering of the adsorbedmolecules, then the zeta potentialvalues tend to positive direction as well.

Acknowledgements

We gratefully acknowledge financial support from the Funda-mental Research Funds for the Central Universities (NO. 2010-VI-012).

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