Prevention of bleeding of particulate grouts using biopolymers

Construction and Building Materials 192 (2018) 202–209

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Prevention of bleeding of particulate grouts using biopolymers Hamidreza Khatami a,b,⇑, Brendan C. O’Kelly c a

School of Civil, Environmental and Mining Engineering, The University of Adelaide, South Australia 5005, Australia Formerly Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin, Ireland c Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Dublin, Ireland b

h i g h l i g h t s
Some biopolymers stabilize particulate grouts.
These biopolymers increase the grout viscosity.
These biopolymers also increase the zeta potential of the fine grout particles.
Increased viscosity and zeta potential results in reduction or inhibition of bleeding.
There is a synergy between some biopolymers.

a r t i c l e

i n f o

Article history: Received 18 May 2018 Received in revised form 8 October 2018 Accepted 15 October 2018 Available online 23 October 2018 Keywords: Biopolymer GGBS Ground improvement Particulate grout Zeta potential

a b s t r a c t The potential applicability of several biopolymers, including xanthan gum, diutan gum, acacia gum, modified starch, guar gum and a cellulosic polymer, for maintaining the solid particles of a particulate grout in suspension has been evaluated through static bleeding tests. The grout material investigated was ground granulated blast-furnace slag (GGBS) cement prepared at different water to binder ratio values ranging from 2:1 to 5:1. Among the biopolymers studied, diutan gum and xanthan gum were found to have the highest performance in bleeding prevention at very low concentrations in ranging 0.25–1.0%. A synergy was observed for xanthan gum and guar gum and for xanthan gum and cellulosic polymer combinations. This helps reduce the overall amount of biopolymer required for suspension stabilization. By measuring the zeta potential and viscosity values of the biopolymer-added grouts, the mechanisms underlying the sedimentation inhibitory action of the biopolymers are discussed. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction In a civil engineering context, the term ‘‘bleeding” refers to the separation of water from the total volume of a particulate grout or paste [11]. In this sense, bleeding is some form of sedimentation in which the solid particles present in the grout or paste tend to settle in mixing tanks, handling equipment, pipes and even when injected for ground improvement applications, especially along rock or concrete cracks. In addition to impeding the functionality of injection facilities due to loss of water and resulting higher grout viscosity, bleeding renders the overall geotechnical benefits of grouting practice less effective. For example, in applying the compensation grouting technique, ground settlements are counteracted by introducing a pressurized grout into the pore voids, thereby generating a controlled upheaval of the subject founda⇑ Corresponding author at: School of Civil, Environmental and Mining Engineering, The University of Adelaide, South Australia 5005, Australia. E-mail addresses: [email protected] (H. Khatami), [email protected] (B.C. O’Kelly). https://doi.org/10.1016/j.conbuildmat.2018.10.131 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

tion. In grouting for ground improvement, the stratification of water and cementitious materials within the soil matrix or rock fractures leads to non-homogeneous quality of grouting. Bleeding causes the water to dissipate from the solid particles of the grout and enter into the neighboring ground, in a similar process to water drainage in soil consolidation, thereby reducing the ground treatment volume for the compensation grouting in the example described [14]. Many researchers [27,28,30,34,36,37,43,14] have solely focused on the characterization, modelling and prediction of the bleeding/ sedimentation of cement-based pastes and/or grouts in different situations. However, less attention has been given in literature to proposing feasible solutions for mitigating against the bleeding problem. For instance, Bruere [4] examined the effectiveness of a number of paraffin-wax emulsions and bentonite suspensions on the bleeding of cement pastes and reported a reduction in their rate of bleeding. From a review of the literature and to the authors’ knowledge, no study has reported previously on the competence of

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2. Materials The GGBS material of superfine consistency (99% by mass finer than 22 lm, with a median size of 4 lm) employed in the present investigation was supplied by Ecocem Ireland Ltd., Dublin, Ireland. Fig. 1 shows the probability density and cumulative distribution of the constituent particles. The density and Blaine surface area of the GGBS material were measured as 2.89 g/cm3 and 7688 cm2/g, respectively. Diutan gum, xanthan gum and carboxymethylcellulose (CMC: branded as Finnfix 10000) were supplied by CP Kelco Company, UK. Guar gum, acacia gum and a modified starch (namely Starpol 136) were supplied by Cargill, Thew Arnott, UK and Tate and Lyle, UK, respectively. Table 1 presents the concentrations and blends of these biopolymer materials with the GGBS material for the two water-to-binder ratio (w/b) values of 2:1 and 5:1 investigated.

1

100

0.9

Density distribution

80

0.8

70

0.7

60

0.6

50

0.5 0.4

40 30

Cumulative distribution

0.3

20

0.2

10

0.1

Density distribution

90 Cumulative distribution (%)

biopolymer materials in reducing the bleeding of particulate grouts. Few benefits of using these materials in a geotechnical context (for example, in improving the mechanical properties of sand) have mainly been investigated in the past [22,7,33,1,6]. In addition, there are some investigations reporting on the effectiveness of biopolymer additives in reducing the hydraulic conductivity of treated soils [42,5,8]. The present investigation, however, focuses on the ability of some biopolymers for suspension stabilization. A geotechnical engineer can beneficially utilize this characteristic for several purposes, perhaps most notably being the stabilization of particulate grouts for ground improvement applications. It has been an ongoing challenge for geotechnical engineers to find an additive that keeps practically all solid particles in a homogenous suspension for a reasonable length of time demanded by the grouting treatment application [18]). In the present investigation, a number of biopolymer materials including xanthan gum, diutan gum, acacia gum, modified starch, guar gum, and carboxymethylcellulose (CMC) were investigated for this purpose. Different concentrations of these biopolymers were mixed with ground granulated blast-furnace slag cement (GGBS) at two different water-to-binder ratio values. The synergistic properties of these biopolymers were also investigated for this purpose. Here, synergism refers to the phenomenon of two biopolymer influences operating in conjunction to produce an effect (e.g. change in viscosity or zeta potential) that is greater than the sum of the effects each biopolymer is capable of contributing to the system individually [35]. The results of the bleeding tests performed over a 3-h period indicated that some of the biopolymers investigated effectively reduced the degree of water separation for the prepared grout mixtures. In addition to environmental benefits of using an industrial waste material for grouting, the GGBS used in this study had much finer grading in comparison with normal Portland cement, thereby providing easier and more effective infiltration of the prepared GGBS-based grouts into the pore void spaces of the soil material under treatment. Note that GGBS can be activated by sole application of some alkalis, such as sodium hydroxide; that is, there is no need for being blended with Portland cement material. To understand the mechanism(s) by which the biopolymers stabilized the grout suspensions for the duration the bleeding tests, the zeta potential of dilute solutions of neat GGBS and biopolymer–GGBS grouts were first measured by means of the Laser Doppler Velocimetry (LDV) technique. Then, the significant changes in the rheological properties of the biopolymer–GGBS grouts were studied. By combining these two aspects of the stabilization of the GGBS based grout suspensions, the mechanism of bleeding prevention with biopolymers is illustrated.

0

0 0.5

1

2

4 Particle size (μm)

8

16

Fig. 1. Particle size analysis of the superfine GGBS investigated.

Table 1 Biopolymer concentrations and combinations investigated. Biopolymer

Concentration (percentage by mass of water)

Diutan gum (DG) Carboxymethylcellulose (CMC) Xanthan gum (XG) Guar gum (GG) Acacia gum (AG) Starch Xanthan gum and CMC Xanthan gum and guar gum Guar gum and CMC

0.25% 0.25% (for 2:1 w/b)–1.0% (for 5:1 w/b) 0.25% (for 2:1 w/b)–1.0% (for 5:1 w/b) 0.25% (for 2:1 w/b)–1.0% (for 5:1 w/b) 1.0% and 2.0% (for 2:1 w/b) 0.5% (for 2:1 w/b) 0.25% XG and 0.25% CMC (for 5:1 w/b) 0.25% XG and 0.25% GG (for 5:1 w/b) 0.25% GG and 0.25% CMC (for 5:1 w/b)

Diutan gum has only been recently developed by CP Kelco Company and hence there is not much information openly available about its chemistry. However, it is established that diutan gum is a linear polysaccharide with side branches containing one or two monosaccharides [32]. Carboxylate groups are attached to the backbone of the diutan gum’s structure, thereby providing an anionic charge. Diutan gum shows pseudoplastic properties and can be used as a fluid rheology modifier. Pseudoplasticity is characterized by an immediate change in viscosity with the exertion or withdrawal of shear stress [13]. In other words, diutan gum solution readily flows when agitated or pressurized, but returns to a gelatinous state almost instantaneously after removal of the shear stress, thereby stabilizing components within the fluid. Xanthan gum is a polysaccharide secreted by the bacterium Xanthomonas campestris in a fermentation process [2]. Xanthan gum has a molecular formula of (C35H49O29)n, with each molecule consisting of approximately 7000 pentamers, and its chemical structure possesses a main-chain pentasaccharide repeat of linear b-1,4-D-glucose [40]. The side chains of xanthan gum contain alternate glucose-monomers-with-mannose and glucuronic acid with pyruvic acid [41]. Xanthan gum is widely used in the petroleum industry as an effective viscosifier. It remains stable in the presence of acids, alkalis and salt and also at high temperatures, providing a very high low-shear pseudoplastic viscosity and shear-thinning character [20]. Carboxymethylcellulose (CMC), with a chemical formula of C8H15NaO8, is a derivative of cellulose formed by its reaction with alkali and monochloroacetic acid. Cellulose itself is a plant-based polysaccharide (as distinct from diutan and xanthan gums which are microbe-based polysaccharides) principally sourced from tree pulps. The chemical structure of cellulose consists of a linear chain of several hundreds to many thousands of glycosidic linkages [25]. During industrial manufacturing, carboxymethyl groups are attached to some of the hydroxyl groups of the glucose units of the cellulose backbone. CMC dissolves in cold water and is mainly

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used for controlling viscosity without gelling (at typical concentrations, CMC does not gel even in the presence of calcium ions). CMC’s control of viscosity allows for its use as a thickener, emulsion stabilizer and suspending agent. Hydrophobic lowersubstituted CMCs are thixotropic whereas extended higher substituted CMCs are pseudoplastic [16,19]. CMC is synthesized with different molecular weights of the cellulose backbone structure, offering a variety of functional properties. The CMC material considered in the present investigation is branded as Finnfix 10,000 and had very high viscosity ( 10,000 cP) in cold water and at 1% concentration. Guar gum is a polysaccharide extracted from the seeds of the leguminous shrub Cyamopsis tetragonoloba. The powdered seed is composed of 75–85% polysaccharide (the actual biopolymeric hydrocolloid), 5–6% protein, 8–14% moisture, as well as some other constituents [3]. A high-molecular-weight polysaccharide known as galactomannan, which is based on a mannan backbone with galactose side groups, constructs the chemical structure of guar gum [29]. The ratio of galactose to mannan varies modestly depending on the source of the leguminous seeds. Guar gum hydrates fairly rapidly in cold water to produce highly viscous pseudoplastic solutions of generally greater low-shear viscosity when compared with other hydrocolloids. High concentrations (1%) are very thixotropic but lower concentrations (0.3%) are far less so [9]. This gum has found many different applications from explosives, mining and oil industries to pharmaceutical, cosmetics, textile and nanoparticle industries. Acacia gum is a highly water-soluble natural polymer secreted by the Acacia tree. It is mainly composed of glycoproteins and polysaccharides with a large molecular weight, estimated to be in the range of 2  105 to 6  105 Da [21]. Acacia gum mainly consists of a backbone of 1,3-linked b-D-galactopyranosyl units and side chains of two or five 1,3-linked b-D-galactopyranosyl units joined to the main chain by 1,6-linkages [31]. Similar to guar gum, the chemical composition of acacia gum varies with the origin of the plant and its growth conditions. Acacia gum’s principal uses include surfactant and viscosifier industries and pharmacy. Like most natural products, acacia gum is subject to chemical variability that sharply affects its functional properties. The exact chemical structure of the gum is complex and has not yet been fully explained in the literature. Natural starch is either non-ionic or slightly anionic, depending on the material’s source. Starch is a white, tasteless, odorless powder that does not dissolve in cold water or ethanol. It consists of linear and helical amylose, along with branched amylopectin, which consists of glucose monomers. The particular starch considered in the present investigation was Starpol 136, which is derived from waxy corn starch and modified chemically to increase water solubility at low temperatures, resistance to shear breakdown, water retention, and its viscosity ([44]). In the waxy corn starch, the linear amylose component present in natural or normal starch does not exist [12]. Starpol 136 is generally used in adhesives, oil drilling fluids, tile mortar and grouts.

3. Experimental methods 3.1. Viscosity tests Petroleum engineers have long used the Marsh Funnel (MF) device [24]) to quickly and effectively determine the relative viscosity of drilling fluids. Using this device, the MF ‘‘viscosity” property is determined as the time period in seconds required for the test liquid to pass through a particular orifice and fill a one-quart container. The quart MF time period (t is used as an empirical indicator of the relative thickness of fluids being tested; that is, the

more viscous the fluid the longer it takes to fill the one-quart container. To calibrate the MF device, tap water is poured into the funnel and its measured viscosity should be in the range of 26 ± 0.5 s. Pitt [26] proposed an equation to correlate the MF t value (given in seconds) with the effective viscosity (ge ), expressed in terms of centipoises (cP) in the centimeter-gram-second physical system, as follows:

ge ¼ qðt  25Þ

ð1Þ

where q is the density of the test fluid (g/cm ). Although initially developed for drilling muds, Eq. (1) can also be applied for the particulate grouts containing biopolymers investigated in the present research because the texture and rheological properties of these two fluids are quite similar. Drilling muds are usually comprised of small solid particles (e.g. bentonite or the cuttings from the geologic formation being drilled) and either xanthan gum or CMC materials. Further, both types of fluids exhibit nonNewtonian behavior. Tables 2 and 3 list the measured MF t, calculated effective viscosity and density values of the investigated neat GGBS and biopolymer–GGBS grout materials, respectively, prepared at different w/b ratios. In this manner, changes in performance arising from the biopolymer additives could be deduced relative to the neat GGBS grouts (controls). 3

3.2. Bleeding tests To prepare homogeneous grouts, appropriate amounts of the GGBS and biopolymer materials were separately added to tap water with a pH value of 6.5, which was used for all viscosity and bleeding tests. In preparing the biopolymer–GGBS grouts, the GGBS solutions were slowly added to the biopolymer solutions and thoroughly stirred using a mixer rotating at 500 rpm for different time periods, depending on the proficiency of the biopolymer hydration in water. This approach was taken since preliminary testing indicated that the addition of dry GGBS powder to a biopolymer solution was found to create numerous small packets of GGBS material within the solution, demanding up to a few hours

Table 2 Density and viscosity of the neat GGBS grouts investigated. Water to binder ratio

Density (g/cm3)

Quart MF time period, t (s)

ge (cP)

1.5:1 2:1 3:1 4:1 5:1

1.32 1.28 1.18 1.14 1.12

27.43 26.53 26.32 26.27 26.20

3.2 2.0 1.6 1.4 1.3

Effective viscosity,

Table 3 Density and viscosity of the biopolymer–GGBS grouts investigated. Water to binder ratio (biopolymer dosage)

Density (g/cm3)

Quart MF time period, t (s)

Effective viscosity, ge (cP)

2:1 5:1 2:1 5:1 2:1 5:1 2:1 5:1 5:1 5:1 5:1

1.21 1.10 1.25 1.11 1.25 1.09 1.25 1.10 1.11 1.11 1.10

43.00 40.25 39.01 81.90 43.87 47.85 38.10 95.83 36.60 39.74 34.66

21.8 16.8 17.5 63.2 23.6 24.9 16.4 78 13.0 16.4 10.6

(0.25% DG) (0.25% DG) (0.25% XG) (1.0% XG) (0.25% CMC) (1.0% CMC) (0.25% GG) (1% GG) (0.25% XG and 0.25% CMC) (0.25% XG and 0.25% GG) (0.25% GG and 0.25% CMC)

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2:1 w/b ratio

55

5:1 w/b ratio

50

Bleeding (%)

44 40

40

30 25 20.31 19.5

20

13.8

10 3.13 1.17

0

0.8 0

0.8

0

0

0 0.4

0 0.4

0

Fig. 2. Results of bleeding tests on biopolymer–GGBS grouts investigated.

of agitation in order to produce a grout having uniform consistency throughout. The 800 ± 20 ml volume of each grout prepared was poured into a 1 l graduated plastic cylinder of 6 cm diameter and its mass determined in order to calculate the grout density. The level of the bleed water in each grout sample was measured over a 3-h period at 10-min intervals for the first hour and hourly thereafter. The 3-h standing period considered is a reasonable amount of time within which the grout injection process is expected to reach completion and for the cementitious grout to start setting. After this 3-h period, the bleed water that had gathered at the top of each sample was carefully drained off using a pipette and the final measured volume of this water was used in calculating the ratio of bleeding (RB, as %) value according to the following equation:

RB ¼

Vw  100 Vt

ð2Þ

where Vw and Vt are the volumes of the bleed water and the grout sample at the onset of the test, respectively. Fig. 2 displays the efficiency of the different biopolymer additives in reducing the RBs of the GGBS grouts prepared at 2:1 and 5:1 w/b ratios. In this content, RB values lower than 2% can practically be considered as null bleeding, with acceptable RB values for grouting in geotechnical applications considered as lower than 5%. 4. Results of viscosity and bleeding tests Xanthan gum and guar gum hydrated in water more readily and quickly than any of the other biopolymer materials considered. In terms of the ease of hydration, diutan gum was the most stubborn of the biopolymers and sometimes the presence of gum lumps in the water necessitated leaving the specimens overnight to achieve complete hydration. The operative should also be aware that the biopolymers’ characteristic properties arise from their long chemical chains. Consequently, improper agitation and mixing could damage these structures and may be the primary factor in accounting for the failure or reduced effectiveness of biopolymers, especially since they are generally used at low concentrations. In addition to potential concerns regarding the mixing procedure, optimum pH values usually exist for which maximum biopolymer dispersion and/or hydration are achieved. Acidic pH values facilitate biopolymer dispersion in water, whereas alkaline pH values usually bring about maximum hydration. The operative is referred

to literature from the biopolymer material manufactures regarding specific pH ranges appropriate. Table 2 shows that the viscosity of the neat GGBS grouts reduced as a function of increasing w/b ratio value, although when a neat particulate grout has a low solid (high w/b) ratio, more water is expected to bleed out of the grout. Table 3 illustrates that biopolymer concentration is more influential on grout viscosity than the mass of GGBS per unit volume of water. For all grouts considered, viscosity was found to increase as a result of the addition of more biopolymer, even though the w/b ratio values of the grouts were increased considerably. For example, the MF viscosity value of the 0.25% xanthan gum–GGBS grout with 2:1 w/b ratio was approximately doubled when the concentration of the gum was increased to 1.0%, despite the fact that w/b ratio had also been increased to 5:1. From the results of the bleeding tests shown in Fig. 2, it can be pointed out that most of the biopolymers investigated had the effect of significantly reducing bleeding when compared with the neat GGBS grouts (controls). From the biopolymers investigated, diutan gum can be considered as the most effective grout stabilizer. Since the viscosity of this biopolymer increases dramatically with concentration, it should always be used at very low dosage (typically <0.25%). In other words, injection of a high dosage diutan-gum grout will not be practical for typical geotechnical engineering applications. Xanthan gum also proved to be an excellent bleeding inhibitor. However, in some instances, the added viscosity and economic cost of this material additive may be prohibitive for use in geotechnical engineering practice. Fig. 2 also reveals that xanthan gum forms striking synergistic interactions with both the guar gum and CMC materials. For the 5:1 w/b grouts, the combined effect of two biopolymers showing synergy was to reduce the material dosage required from 1.0% to 0.5%, whilst still delivering the same overall bleeding inhibitory property. Biopolymer synergy also decreased the viscosity of low-solid ratio grouts. These outcomes result in easier grout injection. No synergistic compatibility was recognized between the guar gum and CMC materials. In the case of the starch and acacia gum, further testing was dispensed with since no reduction in bleeding of the 2:1 w/b grout was perceived. Fig. 3 shows the change in ratio of bleeding with the MF viscosity for the different biopolymer–GGBS grouts and the neat GGBS grout (control) prepared at w/b ratios of 2:1 and 5:1. As can be inferred from this figure, except for the case of the CMC biopolymer, increased viscosity resulted in a reduction in bleeding. To obtain a fundamental understanding of the mechanism(s) by which some of the investigated biopolymers were capable of reducing the bleeding of the GGBS grout (or stabilizing grout suspension), the zeta potential values of grout suspensions were measured and analyzed along with the rheological investigations. 70

Ratio of bleeding (%)

60

60

60

Neat GGBS

50

CMC–GGBS

40

DG–GGBS

30

XG–GGBS 20 10 0 0

20

40

60

80

100

t (s) Fig. 3. Bleeding versus quart MF time period (t) for neat GGBS grout and different biopolymer–GGBS grouts prepared at w/b ratios of 2:1 and 5:1.

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5. Zeta potential measurements Zeta potential (mV)

The instrument used for this purpose was a Zetasizer Nano ZS device (from Malvern Instruments, UK) which is capable of measuring zeta potential using Doppler electrophoresis as the basic principle. Trial measurements obtained for initial non-diluted samples of the neat GGBS and biopolymer–GGBS grouts showed immense scatter and noise, such that these test results were deemed unrepeatable. Accordingly, dilution of biopolymeric concentrations and partitioning of the GGBS particle size range were considered in order to address these issues. A second series of trial tests was performed to identify suitable GGBS and biopolymer concentrations and an appropriate methodology for preparing the zeta potential samples. In this regard, the Zetasizer software accurately determined whether: (i) the concentration of the sample under assessment was too high or low; (ii) air bubbles were present or (iii) the sample suspension was aggregating. The particle size analysis of the GGBS material investigated (Fig. 1) indicated that, statistically, 30% by mass of its solid particles were smaller than 2 lm, which is a suitable range of particle sizes for zeta potential measurements using the Zetasizer instrument employed. Therefore, 10 g of the GGBS powder was added to 1 l of distilled water and thoroughly mixed for a 4-min period at the speed of 500 rpm. The suspension was then poured into a suction-aided filter paper set-up, with filter apertures sized to 2 lm, thereby obtaining a 3g/l suspension of superfine GGBS material comprising particles smaller than 2 lm. Different biopolymers amounting to 0.15% by mass of water were then gradually added to the vortex of the slowly agitated GGBS suspensions and the mixing process was then continued until clear liquids (indicative of the complete hydration of the biopolymers) were obtained. The magnitude of zeta potential is tremendously affected by pH, and as such, reporting zeta potential measurements without quoting the associated pH values is rather meaningless. In this series of testing, the pH values of all samples were measured and found to be in the range of pH 9.4 ± 0.2. Sufficient amounts of the biopolymer–GGBS suspensions were sampled using a double-distilled water-washed syringe and delicately injected into a folded capillary cell (type DTS1060) in order to avoid creating air bubbles within the cell. If visual inspection confirmed that there were no air bubble inclusions present, then the cell was inserted into the Zetasizer instrument and allowed to stand for a 1-min period in order to equilibrate the cell chamber temperature to a standard value of 25 °C, prior to recording the zeta potential measurements. This temperature value was set and maintained by the Peltier elements in the sample holder of the instrument. The next step was to determine how many zeta potential measurements were necessary for a given sample in order to achieve a reliable estimate of the true average value of the zeta potential for that sample. The central limit theorem (CLT) in the context of probability and statistics suggests that the minimum number of observations in an empirical study should be at least 32. However, the exact number of replicates depends on the: level of confidence chosen; acceptable margin of error; variability in the population studied and expense and time required for the data collection. In this study, the number of measurements for each sample was set to 40, meaning that the Zetasizer instrument was used to obtain 40 measurements of the zeta potential value for every test sample investigated, with the average of the data set obtained for a given sample reported as its definitive zeta potential value. The Zetasizer instrument measures electrophoretic mobility by Laser Doppler Velocimetry (LDV) and then applies Henry’s equation in order to indirectly determine zeta potential. Here, electrophoretic mobility is defined as the net velocity of the colloidal

-60 -50 -40 -30 -20 -10 0 Diutan gum Xanthan gum

CMC

Guar gum

Xanthan and guar gums

GGBS

Fig. 4. Zeta potential values for diluted biopolymer-added GGBS suspensions at 0.15% biopolymer concentration.

GGBS particles in water, which is manifested when an electric field is induced by the instrument. The relationship between electrophoretic mobility (uE) and zeta potential (f) is given by:

uE ¼

2
f f ðjaÞ 3g

ð3Þ

where e is the dielectric constant of water; a is the particle radius; j is the inverse of Debye length and g is the viscosity of the dispersion medium. The value of the f(ja) term (refers to Henry’s function) is calculated either by Smoluchowski’s approximation or Hückel’s approximation [17]. Smoluchowski assumed that an induced electric field in the vicinity of a solid particle is uniform and parallel to the particle surface [15], whereas Hückel disregarded the deformation of the applied field by the presence of the particle [17]. However, if the non-conducting particles in an aqueous system are larger than 0.2 lm and dispersed in an electrolyte containing more than 0.001 M salt, then Smoluchowski’s assumption becomes valid. Hence, this approximation was assumed for the purposes of performing the calculations in the present study, and accordingly, the value of Henry’s function was set to 1.5. Fig. 4 shows the zeta potential values determined using the procedure described above for the different biopolymers in suspended solutions of the superfine GGBS material. It can be seen that diutan gum increased the zeta potential of neat GGBS particles by nearly tenfold, with the next best performance achieved for a combination of the xanthan and guar gums, indicating that these two biopolymers are fully compatible with each other and form a synergistic mixture. This excellent synergic property allows the geoengineer to prepare a well-suspended particulate grout at lower biopolymer concentration and having lower viscosity using a biopolymer (in this case guar gum) that by itself has a high susceptibility to bleeding (see Fig. 2). 6. Discussion A number of different types of forces act on solid particles (such as the GGBS particles) suspended in a liquid medium. These include colloidal, Brownian, hydrodynamic (buoyancy and drag) and gravitational forces [38]. In static rheological experiments such as the bleeding tests performed in this investigation, the Brownian and gravitational forces are competing with each other, in that Brownian forces bring about random motion of the particles in a fluid whereas gravitational forces tend to cause the settlement of these particles. The value of the following dimensionless ratio can be used to assess which of these two forces is dominant in a particulate suspension [38]:

a4 ðqd  qw Þg kB T

ð4Þ

207

where a is the solid particle radius, qd and qw are the densities of the dispersed and continuous phases, respectively (that is, the GGBS particles and water for the present investigation), g is the gravitational acceleration, kB is Boltzmann’s constant and T is the absolute temperature in Kelvin. If the ratio value is greater than unity, then the suspension has a tendency to settle out. Inputting pertinent values for the mean GGBS particle size in Eq. (4) gives a ratio value of 72, meaning that compared with Brownian forces, gravitational forces are significant and may trigger the sedimentation of the GGBS particles if not counterbalanced. Gravitational forces can be ignored in considering the proportion of the GGBS particles finer than approximately 1.5 lm in diameter (probability analysis of the GGBS particle sizes presented in Fig. 1 indicated that approximately 20% of the constituent particles were smaller than 1.5 lm). Now, Stokes’ equation can be employed to predict the settling velocity (V) of a suspended particle with a continuous phase viscosity (gcp ), as follows:

V¼

2a2 ðqd  qw Þg 9gcp

Repulsion Energy barrier

Superimposed energy

0

van der Waals attraction Particles distance (nm) Fig. 5. Schematic representation of net potential energy according to DLVO theory [17].

ð5Þ

Eq. (5) implies that the sedimentation of solid particles is delayed if the viscosity of the continuous phase is increased. Viscosity can only slow down the rate of settling, but the addition of a ‘yield stress’ (rY ) to the rheological properties can create a permanent suspension. In the context of rheology, the rY value is commonly defined as the initial resistance to flow under an applied shear stress; that is the minimum shear stress imparted from mixing, shaking or pumping required to initiate flow. For objects suspended in media with a yield stress, it has been hypothesized that if the gravitational force acting on the object is not sufficient to overcome the yield stress then the object will remain suspended indefinitely [23]. The minimum rY value required to offset the gravitational force can be estimated by the following equation after Goodwin and Hughes [13]:

rY ¼ 0:33ðqd  qw Þa  g

Potential Energy (J)

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ð6Þ

For the largest GGBS particle of practically 22 lm size (see Fig. 1), the minimum rY value necessary to prevent sedimentation occurring due to gravitational forces is estimated as approximately 0.07 Pa. The miniature fixed-head vane shear test device used by geotechnical engineers in characterization of very soft and soft soils was manipulated for the present investigation to approximately obtain the rY values of four biopolymer–GGBS grouts. For a 0.25% diutan gum–GGBS grout, the deduced yield stress was approximately 3 Pa for a high solid ratio (i.e., 2:1 w/b), compared with practically zero for a low solid ratio (5:1 w/b). Synergic 0.25% xanthan and 0.25% guar gum additives to the GGBS grout with a low solid ratio (5:1 w/b) mobilized a rY value of 6 Pa. By comparison, 1.0% xanthan gum–GGBS grout with a w/b ratio of 5:1 mobilized a rY of 15 Pa. Since the last two biopolymer grouts with at 5:1 w/b ratio delivered much higher rY values than the required minimum of approximately 0.07 Pa necessary to overcome gravity forces, they theoretically qualify as permanent particulate grouts. To examine this theoretical expectation, the grout samples were allowed to stand, undisturbed, in their cylinder containers for a 10-day period, at the end of which no bleeding was detected, thereby confirming the theoretical calculations in practice. Brownian motion can especially maintain the ultrafine portion of the GGBS material (i.e., particles < 1.5 lm size) in suspension provided that promoted collisions of particles due to random particle movements do not lead to aggregation. This can be achieved by increasing the surface charge associated with these particles; in other words, their zeta potential. The DLVO theory (named after four authors, Derjaguin, Landau, Verwey, and Overbeek) ([39,10] states that the stability of a colloidal system is

Biopolymer GGBS particle

GGBS particle

Fig. 6. Stabilization of GGBS suspension by biopolymer adsorption on the particle surfaces.

essentially governed by the balance between van der Waals attractive forces and the electro-osmotic repulsive forces that particles experience as they approach one another in the dispersion medium. The electric repulsive energy is proportional to the second power of zeta potential. Hence, any increase in the value of the former will have a significant effect on the repulsive forces acting among particles. This theory also proposes the existence of an ’energy barrier‘; that is, the maximum value of the curve produced by the summation of the repulsion potential and van der Waals’ attraction curves, as typified by Fig. 5. In order for flocculation to occur, two approaching particles must carry sufficient kinetic energy to prevail over this energy barrier. When this condition is achieved, van der Waals attractive forces cause particles to cling together strongly and irreversibly [17]. Inevitably, particles must have adequately high repulsive potential energy in order to prevent flocculation from occurring. The biopolymers added to the GGBS grouts were adsorbed onto the surfaces of the GGBS particles, thereby increasing their zeta potential which, in turn, resulted in a remarkable increase in their repulsive potential energy. In other words, the biopolymer addition had the effect of increasing the energy barrier, inhibiting the particle surfaces from moving towards each other thereby preventing flocculation from occurring. Another desirable effect of the biopolymer addition is the provision of a steric hindrance in the suspension. Once a thick enough biopolymer coating forms around every GGBS particle, the particles are then separated by steric repulsions between biopolymeric layers (Fig. 6). 7. Conclusions Some biopolymers can effectively stabilize GGBS based grout suspensions at very low dosages (0.25–1.0%). Among the different biopolymers investigated, diutan gum, xanthan gum, CMC and guar gum were found to reduce the level of bleeding of the GGBS

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grouts to varying degrees, depending on their concentrations and w/b ratio values. In contrast, acacia gum and modified starch were found to have virtually no effect on the GGBS grouts’ bleeding characteristics. It was also observed that xanthan gum provided synergy with both guar gum and CMC, lowering the overall amount of biopolymer material required to decrease bleeding of the GGBS grout for the same w/b ratio. No significant synergistic effect occurred when CMC and guar gum were utilized together. Diutan gum and the synergistic combination of xanthan gum and guar gum showed the highest performance in terms of reducing the RB values of the GGBS grouts. It was observed that biopolymers generally contribute two principal changes to a particulate grout, depending on their chemistry and also the size, shape and density of the dispersed particles. They can alter both the zeta potential of the suspended particles and the rheological behavior of the medium of dispersion. Measurements of zeta potential for the diluted biopolymer–GGBS grouts revealed that the biopolymers were adsorbed on the surfaces of the very fine GGBS particles smaller than 2 lm, thereby raising their zeta potential. This, in turn, increases electrostatic repulsive forces acting between the GGBS particles, keeping them apart and uniformly dispersed in water for prolonged periods of time. Steric repulsion is also another consequence of the biopolymer adsorption process. Further, the viscosifying effect of the biopolymers modified the rheology of the GGBS grouts, providing a counterbalancing force against gravity, which for the coarser particles resulted in either ceasing or slowing down of their settlement velocity. In order to make an appropriate selection from the effective biopolymers identified with regard to reducing bleeding, the engineer should consider the viscosity induced by any given biopolymer as well as considering potential impacts on the final mechanical properties of the grouted ground. It is worth mentioning that while a significant increase in viscosity might seem as a limiting attribute for a biopolymer, in some practical situations, the possible contribution to the mechanical improvement of the ground could compensate for this increased viscosity. The shear-thinning property of the biopolymers should also be taken into account as this facilitates easier grout pumping and injection in practice. Conflict of interest The authors declare that there is no conflict of interest regarding this paper. Acknowledgements The first author acknowledges the Irish Research Council (IRC) for financial support under the Embark Postgraduate Research Scholarship Scheme 2010 (RS 2010/2571). This paper is honoring the memory of Mr. Martin Carney. Martin passed away in July 2018 after four decades of dedicated service as Technical Officer in the geotechnical laboratories of Trinity College Dublin. Ar dheis Dé go raibh a anam dílis. References [1] M. Ayeldeen, A. Negm, M. El-Sawwaf, M. Kitazume, Enhancing mechanical behaviors of collapsible soil using two biopolymers, J. Rock Mech. Geotech. Eng. 9 (2017) 329–339, https://doi.org/10.1016/j.jrmge.2016.11.007. [2] G.C. Barrere, C.E. Barber, M.J. Daniels, Molecular cloning of genes involved in the production of the extracellular polysaccharide xanthan by Xanthomonas campestris pv. campestris, Int. J. Biol. Macromol. 8 (1986) 372–374, https:// doi.org/10.1016/0141-8130(86)90058-9. [3] J.N. BeMiller, Hydrocolloids, in Food Science and Technology, Gluten-Free Cereal Products and Beverages, Academic Press, 2008, pp. 203–215, https://doi. org/10.1016/B978-012373739-7.50011-3.

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