Toward the viscosity reducing of cement paste: Optimization of the molecular weight of polycarboxylate superplasticizers

Construction and Building Materials 242 (2020) 117984

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Toward the viscosity reducing of cement paste: Optimization of the molecular weight of polycarboxylate superplasticizers Qianqian Zhang a,⇑, Xin Shu a, Xiaohan Yu b, Yong Yang a, Qianping Ran a,b,⇑ a b

State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co., Ltd., Nanjing, Jiangsu 210008, China School of Materials Science and Engineering, Southeast University, Nanjing 210096, China

h i g h l i g h t s
Underlying mechanism of PCEs on rheology of cement pastes was uncovered.
Viscosity decreased with the increase of molecular weight of PCE.
‘‘Bridge effect” causes more adsorption needed to achieve the same yield stress.
Adsorption layer thickness correlates with packing density and viscosity.

a r t i c l e

i n f o

Article history: Received 6 November 2019 Received in revised form 26 December 2019 Accepted 28 December 2019

Keywords: Rheology Residual viscosity Polycarboxylate superplasticizer Cement paste Packing density Adsorption

a b s t r a c t The influence of polycarboxylate superplasticizers (PCEs) on rheology was investigated for viscosity reducing of cement pastes. Five types of PCEs of varied molecular weight were employed. For pastes with same w/c and consistent spread value, yield stress was almost the same, while residual viscosity decreased with the increase of molecular weight of PCE. It was suggested that it was probably the ‘‘bridging effect” causing more adsorption amount required to achieve similar yield stress for PCE of larger molecular weight. The large adsorption layer thickness was beneficial for improving packing density, which enlarged the average particle separating distance, and finally resulted in a significant decrease in residual viscosity of cement pastes. Considering that packing density improvement was pronounced for viscosity reducing of cement paste, development of PCE with high surface coverage of the long side chain should be the necessary and sufficient condition for the lowering of apparent viscosity of cement paste. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Modern concrete are often designed for high strength and they typically have a low water-to-cement ratio (w/c) [1]. However, dramatical increase in the viscosity of concrete due to the low w/c is unfavourable to construction. It brings great challenges to polycarboxylate superplasticizer (PCE), which has the advantages of excellent water-reducing efficiency, and high designability in molecular structure and has become one of the indispensable components of concrete. PCEs reduce attractive interparticle force and release water entrapped in the agglomerated cement particles mainly through steric hindrance effect, which is generated by adsorbed PCE molecules on the surface of particles [2,3]. For PCEs, the relationship between adsorption and dispersion has been a hot research topic and significant achievement has been made. It is ⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Zhang), [email protected] (Q. Ran). https://doi.org/10.1016/j.conbuildmat.2019.117984 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

generally believed that the dispersing ability of PCEs comes from the non-adsorbing side chains [4–6], the amount and length of which determine the steric hindrance [7–9]. Furthermore, the adsorption affinity of the PCEs depends strongly on the amount of carboxylic groups in the linear backbone. In addition to dispersion, it is important to reveal effect of molecular structure on rheological properties so as to improve the performance of PCEs. It has been widely accepted that the dispersion of PCEs is one of the key factors affecting fluidity of cement paste [5], which is inversely proportional to its yield stress [10]. In addition, yield stress of cement paste could be predicted based on the dispersion efficiency of PCEs [11]. Cement paste containing PCEs usually behaviors as a shear-thickening fluid [12,13]. Some research in literature showed that the apparent viscosity, which is the ratio of instantaneous shear stress to shear rate, was reduced when PCEs were added [14,15]. However, it can be known from the definition of apparent viscosity that apparent viscosity contains the contribution of yield stress, so the measured decrease in appar-

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Q. Zhang et al. / Construction and Building Materials 242 (2020) 117984

ent viscosity might have been resulted from a decrease in yield stress [16]. At present, most studies have not distinguished the difference between effects of molecular structure on yield stress and viscosity [5,17,18]. The viscosity of pastes with the same yield stress might be significantly different. Malhotra [19] and Banfill [20] found that for concrete with PCEs of different molecular structures, the same slump values could be obtained, but the viscosity was obviously different. Liu [21] reported that for pastes at a low w/c, the viscosity might increase with the increase of PCEs dosage. It seems that the underlying working mechanism of PCEs on yield stress is different from that on viscosity. The presence of PCE results in more complex interactions on rheology of cement paste, and the viscosity mechanism of the same fluidity still remained unclear. The key parameter to enhance the viscosity-reducing capacity of PCE is also a question. This paper aims at improving the understanding of the underlying mechanism concerning the influence of PCEs on rheological properties for viscosity reducing of cement pastes. To facilitate comparison, five PCE samples with only obvious difference in molecular weight were employed. The flow spread, yield stress and residual viscosity of the pastes with three levels of w/c (0.40, 0.30 and 0.20) were measured. The adsorption behavior of PCEs, packing of solid particles, and the interstitial solution viscosity was also considered for discussing the underlying mechanism.

Fig. 1. Chemical structure of the PCEs.

Table 2 Structural characteristic of synthesized PCEs. Samples

Mw (g/mol)

PDI

PCE1 PCE2 PCE3 PCE4 PCE5

21,263 29,582 40,398 49,006 63,571

1.354 1.546 1.650 1.714 1.823

Note: Mw is weight-average molecular weight and PDI is polydispersity index.

2. Experimental program

2.3. Experimental procedures

2.1. Materials

2.3.1. Fluidity measurement Fluidity of a fresh cement paste was characterized by spread diameter through a mini-cone test (top diameter: 36 mm, bottom diameter: 60 mm and height: 60 mm). Immediately after mixing, the cement paste was cast into the cone on a glass plate, and then the cone was vertically lifted. The flow spread value of the tested paste was determined by the average of two perpendicularly crossing diameters of the spread paste (the maximum diameter of the spread sample and the maximum width perpendicular to that diameter).

Ordinary Portland cement PO 42.5 was used in the experiments, and its chemical and mineral compositions are listed in Table 1. The specific gravity and Blaine specific surface of the cement were 3.06 and 397 m2/kg, respectively. Five PCE samples were synthesized (Fig. 1). Two monomers-methylallyl polyethylene glycol (MPEG, the weight-average molecular weight Mw was 2400, the polydispersity index PDI was 1.06) as long side chains and acrylic acid (AA) as adsorption groups were successfully copolymerized by aqueous free radical polymerization to produce model PCEs, named PCE1, PCE2, PCE3, PCE4 and PCE5. The feeding ratio of MPEG to AA and side chain length were kept constant, while the only variable of the molecular structure was molecular weight. The PDI of some PCEs was larger than 1.5, possibly due to the chain transfer reaction or chain termination reaction (Table 2).

2.2. Mixing procedure Three w/c (0.40, 0.30, and 0.20) were employed in this study. 4–5 dosages (by mass of cement) of each PCE, which were chosen to achieve appropriate fluidity for the reproducibility of results of flow behavior of paste, were tested at each w/c. Interactions between the early hydration products and superplasctizer is one of the important factors affecting rheology of cement pastes [22,23]. Therefore, a delay addition method proposed by Hot [16] was used to alleviate the interactions. Hot confirmed that the delay addition method, in which the PCE was added 20 min later than the mixing water (at this time, most tricalcium aluminates had already nucleated) was helpful to obtain repeatable test results such as rheological and adsorption measurement. For a given mix, cement and 90% mixing water was placed into a mixer and mixed at a low speed for 1 min. Then, the PCE together with the remaining 10% mixing water was added after resting for 19 min and the cement paste was remixed at the low speed for 2 min, and then at a high speed for another 2 min. All the mixes were repeated three times. Besides, the influence of early hydration on rheological properties of cement paste is extremely complicated. In order to eliminate the effects of hydration on rheological properties, all the tests (e.g. fluidity, rheology and adsorption) were carried out at the same time point, i.e., immediately after mixing.

2.3.2. Rheological measurement Rheological properties of cement paste were carried out using a coaxial cylinder rheometer (Brookfield R/S SST2000). After mixing, the cement paste was immediately poured into the measuring cup of rheometer and equilibrated for 30 s before testing. A commonly used rheological test program including pre-shearing (constant rate of 100 s1 for 1 min), resting (1 min) and a hysteresis loop test (the shear rate was increased from 0 to 100 s1 over 1 min and then immediately decelerated back to 0 over an additional 1 min) was adopted in this study. The rheological parameters of the tested cement pastes were derived from the linearly decreasing curve of the hysteresis loop. In addition, viscosity of simulated cement paste interstitial solutions with various dosages of PCEs was also tested by the rheometer through a measuring procedure, in which the solution were sheared at an linearly increased rate from 0 to 200 s1 within 60 s (step 1) and then the shear rate was kept constant for 60 s (step 2), before decelerated back to 0 within another 60 s (step 3). The viscosity solution was characterized by the average apparent viscosity obtained from step 2. The simulated interstitial solution was composed of 1.72 g/L CaSO42H2O, 6.959 g/L Na2SO4, 4.757 g/L K2SO4 and 7.12 g/L KOH [24]. 2.3.3. Adsorption measurement A commercial Total Organic Carbon (TOC) apparatus was used to quantify the adsorption behavior of PCEs. Solution sample for adsorption test was taken from the same cement paste as that used for fluidity and rheology test. To obtain the interstitial fluid, the cement paste was centrifuged at a speed of 10000 rpm for 5 min immediately after mixing. The filtrate was separated and acidified with 1 mol/L HCl to remove inorganic carbon (carbonates), then diluted by 20 times.

Table 1 Chemical and mineral composition of cement. Chemical composition (wt%)

Mineral composition (wt%)

CaO

SiO2

Al2O3

Fe2O3

SO3

MgO

K2O

LOI

C3A

C2S

C3S

C4AF

60.35

19.89

5.04

3.19

2.58

1.77

0.82

3.06

3.23

14.42

55.70

11.22

Q. Zhang et al. / Construction and Building Materials 242 (2020) 117984 The diluted filtrate was used for organic carbon analysis. In addition, for the specific adsorption amount of PCE, the organic carbon content of the reference PCE solution containing only PCE and water, and the cement paste without PCE were also considered. 2.3.4. Particle packing density measurement Up to now, there has been no standard test method for packing density of fine powders. Some existing testing methods can be broadly divided into two categories: direct methods and indirect methods. The direct method usually refers to the dry packing method. Due to the van der Waals and electrostatic force between particles, the interparticle flocculation during dry packing is very serious, which leads to the low measured packing density [25]. Most indirect methods determine the packing density of powder materials by measuring the minimum amount of water required for powder materials to form a paste [26,27]. This kind of wet packing (minimum water requirement) is considered to be more reliable than the dry packing method because it could effectively reflect the effect of mixing process and superplasticizer. In this paper, the test method for packing density proposed by DeLarrard [28] was adopted. In this method, the air content of paste was neglected, and the interspaces between cement particles were assumed to be filled with water only. The packing density of cementitious materials could be calculated as follows:

/¼

qw m c qw m c þ q c m w

ð1Þ

where qc is the mean specific gravity of cementitious materials, qw is the specific gravity of water, mw is the mass of minimum water requirement, and mc is the mass of cementitious materials. During the measurement of mw, the delay addition method was also employed to alleviate the interactions between PCE and early cement hydrations. For a given cement paste mix, a certain amount of cement together with 90% of the designed water was mixed at a low speed for 1 min using a Hobart mixer. The PCE together with the remaining 10% designed water was added into the mixture after resting for 19 min. Then, the mixture was remixed at the low speed for 1 min and at a high speed for 6 min. The amount of water content was adjusted and the above steps was repeated to find the minimum water requirement (mw), which just could change the mixture from a humid powders to a thick paste.

3. Experimental results 3.1. Theoretical model A comb- like polymer is generally described as a molecule consisting of n repeating structural units. Each structural unit has N monomers in the backbone and one side chain. Each side chain contains P monomers [29]. The adsorption conformation (adsorption layer thickness) is an important parameter for its dispersibility [30]. PCEs adsorb on the surface of the cement particles by the adsorbing backbone, and disperse them mainly through the Fig. 1 steric hindrance between surfaces caused by the polymer’s side chains. The conformation of comb-like PCE adsorbed on the surface of cement particles can be characterized by a theoretical model developed by Flatt [7]. This model shows the polymer adsorbed on the surface of the cement particles, behaves as a chain in a hemisphere, which is equivalent to the blobs in solution. The radius of one adsorbed blob could be computed from the polymer molecular parameters:

 pffiffiffi 1=5 ap RAC ¼ 2 2ð1  2vÞ ap P7=10 N1=10 aN

ð2Þ

where v is the Flory parameters of the side chains, aN is the backbone monomer size, aP is the side-chain monomer size. At present, it is generally accepted that the polymer shows single layer adsorption on the surface of cement particles, and Rac could be considered as the adsorption layer thickness at full surface coverage. For the five PCE samples discussed here (consistent N, same P, with only vaired n values), the adsorption layer thickness at full surface coverage should be consistent with each other, as the calculated results listed in Table 3. Generally, flow spread of paste is used to evaluate the dispersibility of PCE on cement particles, and it is basically negatively correlated with yield stress are [10]. Therefore, it is considered that

3

the factors that affect the dispersion capacity must affect the yield stress. The solid particles in a paste could form a network structure, which would deform and fail before flow occurs. And, strength of this rigid particle’s network structure determines the yield stress [31]. The colloidal and contact interactions between cement particles in a cement paste, lead to the formation of rigid particles’ network structure. So, yield stress of cement paste is determined by the volume fraction and nature of cement particles. Based on the assumption that yield stress is mainly determined by the noncontact interaction between particles and the number of interacting particles through solid volume fraction and particle size, a theoretical analysis model called YODEL has recently been developed [11,32]. The YODEL can be expressed as:

s0 ffi m

A0 a 2

d H

 2

  /2 /  /perc /max ð/max  /Þ

ð3Þ

where m is a pre-factor, which depends on the particle size distribution and shape, A0 is the non retarded Hamaker constant, a* is the average radius of curvature of contact points among particles, d is the particle average diameter, H is the minimum separation distance, which is the surface to surface separation distance at contact points determined by the surface coverage and conformation of polymer after adsorption, /, /perc and /max are the solid volume fraction, percolation threshold and maximum packing of the powders, respectively. In a recent review [30], the calculation method for surface to surface separation distance H was also proposed. It is suggested that there are two states for the surface of particles in the presence of PCEs. One is the surface of particles is fully covered by the adsorbed PCEs and the other is the incompletely covered. At full surface coverage, H is considered as two times the adsorption layer thickness (equivalent to two times the radius of one adsorbed blob, H = 2  Rac). At the incomplete surface coverage, H is determined by the surface coverage ratio h of adsorbed PCE on the particles surface:

1 H

2

¼

h2 H2p

þ 8

hð1  hÞ ð1  hÞ2 2 þ H20 Hp þ H0

ð4Þ

where Hp is the surface to surface separation distance at full surface coverage equal to two times the thickness of the adsorbed polymer layer, H0 is the surface to surface dispersion distance without any PCEs (at zero surface coverage). The surface coverage ratio h could be calculated according to the model described as following:

h ¼ mads  NA 

S MW

ð5Þ

where mads is the adsorption amount, NA is the Avogadro number, S is the occupied area by one polymer molecule and Mw is molar mass. The occupied area by one polymer molecule S could been calculated based on the model proposed by Flatt [7], and the results have been shown in Table 3. For the five PCE samples discussed here, the values of S/Mw are almost the same. PCE of large molecular weight contains large amount of adsorption groups, indicating high adsorption affinity. At the same dosage, high adsorption amount leads to a high value of H, which leads to low yield stress. In addition, when the adsorption amounts of the five PCEs are the same, it is suggested that the values of H should be similar, and so the yield stress. 3.2. Flow spread of cement paste Flow spread of cement paste was tested through a mini-cone, and the specific experimental process was carried out as reference [21]. Fig. 2 shows the flow spread of cement pastes with PCEs of

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Table 3 P, N, n, Rac and S are calculated according to the theoretical structure of PCE and Flatt’s model [7]. Samples

N

P

n

Rac (nm)

S (nm2)

S/Mw (nm2g/mol)

PCE1 PCE2 PCE3 PCE4 PCE5

5.60 5.60 5.54 5.50 5.49

122.2 122.2 122.2 122.2 122.2

7.8 10.8 14.8 18.0 23.3

5.32 5.32 5.32 5.33 5.33

150.80 209.80 286.05 346.63 449.48

0.0071 0.0071 0.0071 0.0071 0.0071

Fig. 2. Flow spread values of the pastes: a) w/c = 0.4, b) w/c = 0.3, c) w/c = 0.2.

different molecular weights. It is clear that all the PCEs improved the flow spread of cement paste, as expected, and the flow spread increased rapidly with PCEs dosage. At fixed dosage, spread values of cement pastes first increased with the increasing molecular weight (from 21,300 g/mol to 29,300 g/mol), and then gradually decreased, while the difference between PCEs was more significant at high dosage. Among the PCE samples, PCE2 of 29,300 g/mol is the most effect superplasticizer. The dosage of PCEs required to achieve the same fluidity as listed in Table 4 also confirmed again

this conclusion. The results seem quite different from the theoretical model, which indicates PCE of high molecular weight (large adsorption amount and therefore large H) should be more effective. 3.3. Rheological properties of cement paste Generally, cement paste is a shear thinning suspension. However, this shear thinning phenomenon is usually less pronounced

Table 4 Dosage of PCEs for given flow spread. Samples

PCE1 PCE2 PCE3 PCE4 PCE5

w/c = 0.40

w/c = 0.30

w/c = 0.20

175 ± 5 mm

200 ± 5 mm

225 ± 5 mm

175 ± 5 mm

200 ± 5 mm

225 ± 5 mm

225 ± 5 mm

0.027 0.027 0.027 0.027 0.033

0.035 0.033 0.033 0.035 0.040

0.040 0.040 0.040 0.047 0.047

0.065 0.063 0.065 0.07 0.077

0.07 0.065 0.07 0.075 0.08

0.075 0.07 0.075 0.08 0.085

0.157 0.152 0.159 0.160 0.170

Q. Zhang et al. / Construction and Building Materials 242 (2020) 117984

and even shear thickening could be found when a PCE is added into the paste. Therefore, the Herschel-Bulkley’s equation was used to fit the shear stress-rate curves (down curves) in this study. The model is written as:

s ¼ s0 þ kc_ n

ð6Þ

where s is the shear stress, s0 is the yield stress, k is the consistency, c_ is shear rate, and n is the flow behavior index. The mixture is shear-thinning for n < 1 and shear-thickening for n > 1. Apparent viscosity is defined as the ratio of shear stress to shear rate, which is the result of a complex interaction of forces, including hydrodynamic force, colloidal attractive interactions and contact forces between cement particles. In general, it can be divided into two parts. One comes from the yield stress which is the energy used to break down the network of interactions between cement particles, and the other is the energy dissipation during the flow which could be called residual viscosity. The residual viscosity is written as:

gres ¼

s  s0 s0 ¼g c_ c_

ð7Þ

where gres, g are the residual viscosity and apparent viscosity, respectively. In most cases, the variation of rheological parameters of cement paste is consistent with the flow spread. High flow spread always indicated low yield stress and viscosity. Fig. 3 shows the rheological properties of cement paste with same spread value. For the pastes with same w/c and spread value (Fig. 3a), the yield stress was almost the same and not affected by molecular weight of PCEs

5

as expected. Yet, for pastes with the same spread value at different w/c, the lower the w/c was, the greater the yield stress was. The relationship between yield stress and fluidity of paste or concrete has been extensively studied, and some mathematical formula between yield stress and flow spread has been proposed [10,33,34]. Results from Roussel N shows that the bulk weight of paste is the key factor [35]. For paste with the same w/c, the bulk weight was considered to be uniform, thus yield stress of pastes with similar fluidity was almost the same. However, the bulk weight was significantly affected by the w/c of paste. The bulk weight was about 1950 kg/m3 for cement paste with w/c of 0.40, but increased to 2320 kg /m3 when w/c decreased 0.20. Therefore, yield stress of cement paste with the same fluidity increased gradually with the decreasing w/c. Fig. 3b shows the fluid behavior index n of cement pastes with same spread value. At fixed w/c, the fluid index n increased with the increasing molecular weight of PCE and spread value. As the w/c decreased from 0.40 to 0.30, the shear-thickening behavior was enhanced, while with the w/c further decreased to 0.20, the shear-thickening behavior of paste became less pronounced. Fig. 3c and d show the residual viscosity of cement paste at given shear rates (20 s1 and 80 s1). At both shear rates (20 and 80 s1), residual viscosity of pastes with w/c of 0.40 and 0.30 decreased when molecular weights of PCEs and dosage of PCEs (spread value) increased. Yet, residual viscosity of pastes at 80 s1 was higher than that at low shear rate due to shear thickening behavior. The rheological behavior of pastes with a w/c of 0.20 was similar to that of pastes with a w/c of 0.40. Moreover, as expected, an increase in solid volume fraction (decreased w/c) led to an increase in residual viscosity.

Fig. 3. Effect of molecular weight of PCEs on the rheology of cement paste: a) yield stress, b) fluid index n, c) residual viscosity at 20 s1, d) residual viscosity at 80 s1.

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Q. Zhang et al. / Construction and Building Materials 242 (2020) 117984

Fig. 4 shows the adsorption behavior of different PCE samples. For paste of the same w/c and consistent fluidity, PCE with a larger molecular weight showed a larger adsorption amount, compared to that with a smaller molecular weight. When the spread value of pastes was 225 mm, the adsorption amount of PCE5 in the cement pastes with w/c of 0.40, 0.30 and 0.20 was 26.3%, 36.0% and 37.5% higher than that of PCE1, respectively. In addition, as the w/c and PCE dosage varied, the adsorption amount and efficiency of each PCE were slightly different, and the differences in adsorption behavior due to various molecular weights gradually increased in the pastes with decreased w/c and increased PCE dosage. It is also interesting to find that, although the delay addition procedure was applied, to achieve the same flow spread, the required adsorption amount is higher for PCE of large molecular weight, especially for paste of large flow spread at low w/c. During the early stage of mixing procedure, a large amount of PCE molecules was believed to be consumed by C3A hydration to form the orgnomineral phase [36]. In the delay addition procedure, most of the PCE adsorbed could be considered as attaching on the surface of cement particles or hydration products.

measurement. Then viscosity of the simulated interstitial solutions (prepared with synthetic solution) was measured, as shown in Fig. 5a. For cement paste with a w/c of 0.4, the dosage of PCEs was 0.027%–0.047% to satisfy the given spread values. As molecular weight increased, the content of PCEs remaining in the interstitial solution decreased due to the increase of adsorption efficiency, and then viscosity of interstitial solution decreased. As the w/c increased to 0.3, the required dosage of PCEs was 0.063%–0.085%. More non-adsorbed PCEs remained in the interstitial solution, resulting in an increased viscosity. Although the adsorption efficiency of PCEs increased with molecular weight, viscosity of PCEs solution with the same PCEs concentration increased as well, as shown in Fig. 5b. Therefore, the viscosity of interstitial fluid decreased first and then increased with the increased molecular weight of PCEs. For the pastes with a w/c of 0.2, the effect of molecular weight on viscosity of interstitial solution was similar to that of pastes with a w/c of 0.3. Compared with the pastes with a w/c of 0.3, the concentration of non-adsorbed PCEs in the pastes with a w/c of 0.20 increased 4–6 times due to the increased dosage of PCEs. The large increase in the concentration of non-adsorbed PCEs resulted in a significant increase in the viscosity of interstitial solution.

3.5. Viscosity of interstitial solution

3.6. Packing density of particles

A large amount of unabsorbed PCE remained in the interstitial solutions of the cement pastes, and the PCE concentration in the interstitial fluid was calculated according to the adsorption

Fig. 6 shows the packing density of the cement particles based on the minimum water requirement method. It is clear that the packing density of particles in pastes with higher flow spread

3.4. Adsorption behavior

Fig. 4. Adsorption behavior of PCEs with various molecular weights on cement: a) adsorption amount, b) adsorption efficiency.

Fig. 5. Solutions viscosity of: a) simulated interstitial solutions; b) effect of PCEs concentration.

Q. Zhang et al. / Construction and Building Materials 242 (2020) 117984

Fig. 6. Effect of molecular weight of PCEs on packing density.

and lower w/c (higher PCE dosage) was obviously higher. The flocculation structures formed in pastes was a key factor affecting the packing density of particles. Particles could more easily get closely packed at lower flocculation state [37]. Due to the interparticles forces such as van der Waals and static electric force, cement particles spontaneously coagulated after mixing with water. PCEs were helpful for deflocculation through electrostatic repulsion effect or steric hindrance effect, thereby improve particle packing. With the increase of PCE dosage, more PCE molecules adsorbed on the surface of cement particles which further increased the dispersing degree of particles, and thus the packing density could be improved. However, for pastes of the same w/c and spread value, packing density appeared to increase gradually with the increased molecular weight of PCEs. For pastes with w/c of 0.2 and spread value of 225 mm, packing density with PCE5 was 1.5% higher than that with PCE1. This indicated different flocculation structures in the pastes. It seemed that PCE of large molecular weight could induce smaller and more uniform flocculation structure, and also result in a denser packing, which might be derived from the high adsorption amount and probably large adsorption layer thickness.

4. Discussion

7

authors reasoned that Amax should also be consistent with each other for the PCE samples discussed here. Another interesting result was the flow spread of cement paste at relatively higher dosage at w/c of 0.2. It could be noted from Fig. 2c that the flow spread approached a ‘‘plateau” after PCE dosage of 2 mg/g cement, especially for PCE5 of the highest molecular weight. Yet, the ‘‘plateau” of PCE5 was much lower than that of the other PCE samples. In our former report [38], the ‘‘plateau” only appeared until further addition of PCE could not increase the surface coverage (surface ‘‘saturation”). This was quite different from expectation, since the adsorption amount of PCE5 was always the largest, yet the largest adsorption layer thickness was considered to be consistent with the other samples. The assumptions of the authors’ analysis were therefore taken into account again: (1) The adsorption affinity gradient toward surfaces of cement particles, for the PCE samples only contained carboxylic groups and of consistent grafting density, was considered to be comparable or similar with each other, due to the same adsorption mechanism. (2) Multi-layer adsorption was neglected here, due to PCE of high steric hindrance (long side chains of 2400 g/mol and relatively low grafting density). Indeed only few reports showed this type of adsorption [39]. (3) Some other interactions occurred in the presence of polymer molecules enhanced the strength of rigid particle network (e.g. large polymer molecule contacted with different particles, the so-called ‘‘bridging effect”, as always observed for cement suspension with thickening agent [40]). PCE did not attach onto single particle but multi particles obviously could increase the yield stress, which could not be distinguished with each other by TOC method. Ye [41] observed by focused beam reflectance that at low PCE dosage, bigger and more agglomerates were found compared with control sample without PCE due to the bridging effect. Kashani [42] suggested that the bridging effect was much more pronounced for long backbone chain or large polymer molecules. Recently Kong [43] reported the yield stress increase due to further adding of PCE molecules of low grafting density (1/22, molar ratio of macromonomer unit to AA unit). This type of interaction was possibly related to static charge interaction, which depended on the charge density and amount of polymer molecule. ‘‘Bridging effect” should be more effective for PCE of large molecular weight (Fig. 7). The balance effect of steric hindrance (PCE adsorption on single particle) and ‘‘bridging effect” (PCE attached to multi particles) finally deter-

4.1. Yield stress and dispersing performance of PCE The aforementioned results showed that, at fixed dosage, paste incorporating PCE of moderate molecular weight exhibited the lowest yield stress, indicating the best dispersing performance of this type of PCE. The yield stress was determined by the strength of rigid particle network of the cement paste, which was closely related to the particle packing and the particle separation distance (YODEL, [32]), while the distance was depended on the adsorption conformation. The adsorption of PCE would increase the particle separation distance and improve the packing density, which could weaken the attractive interparticle force (e.g. Van der Waals interaction). For the PCE samples with only varied molecular weight, the adsorption layer thickness of these PCE samples at full coverage was considered to be consistent with each other. Yet, higher adsorption amount for PCE of higher molecular weight was required to obtain the given flow spread. Herein the delay addition procedure minimized the effect of C3A hydration. Based on Flatt’s report, the maximum adsorption amount of PCE (designated as Amax, adsorption at full surface coverage) for a certain system with fixed specific surface area, was inversely proportional to the surface area occupied by single PCE molecule, which was just determined by grafting density [3]. The

Fig. 7. Schematic diagram of effect of PCE of different molecular weight (top: low molecular weight, bottom: high molecular weight).

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mined the dispersion of PCE and then the yield stress of cement paste. 4.2. Apparent viscosity of cement paste Residual viscosity is actually the apparent viscosity without the contribution of yield stress. Therefore, for cement paste of fixed flow spread, the residual viscosity depends primarily on the relative solid volume fraction (///max), average separating distance between particles, viscosity of interstitial solution, which were the important factors affecting hydrodynamic force [16,44]. The true volume fraction of solid particle in paste could be reduced due to the decrease of solid concentration and increase of packing density of solid particles, thereby increasing the average separating distance between particles. Additionally, from dimensional point of view, microscopic shear rate exists between the rigid cement particles. The average microscopic shear rate increases with the increasing macroscopic shear rate applied to the paste and the ratio of particle diameter to the average separating distance between particles [11]. When a certain constant macroscopic shear rate is applied, increasing particle dispersion distance could reduce the microscopic shear rate and reduce the hydrodynamic force. Similarly, reducing the viscosity of interstitial solution could decrease viscous resistance during shearing, and then reduce the residual viscosity. As PCE is added into a paste, the adsorbed PCE on the surface of cement particle increases the particle separation distance (Eq. (3)), while the remained PCE enhanced the interstitial solution viscosity. Noteworthy, the flocculation state at different PCE dosages and shear rate was different in cement paste. Due to different amount of entrapped water, the true volume fraction of ‘‘solid particle” was always higher than the mixing proportion. PCE adsorption could break the flocs formed by particles, reduce the true volume fraction, and improve the particle packing. At high PCE dosages, although the adsorption amount increased, even larger amount of PCE remained in the continuous phase (with concomitant decrease in adsorption ratio). For the PCE samples discussed herein, the intrinsic viscosity of PCE of high molecular weight is higher than that of low molecular weight. For pastes with the same flow spread (yield stress), the interstitial solution viscosity (Fig. 5) appeared to be lowest for PCE of moderate molecular weight. Both PCE1 and PCE5 induced high interstitial solution viscosity. Therefore, it is reasoned by the authors that the improvement of packing density was the main reason for the low apparent viscosity of cement paste under test conditions. The viscosity-reducing effect of improved packing density should be much higher than the fluctuation of viscosity of the continuous phase. For pastes with w/c of 0.2 and spread value of 225 mm, packing density with PCE5 was 1.5% higher than that with PCE1, but residual viscosity at 80 s1 decreased by more than 20%. This was also consistent with the result that the apparent viscosity (residual viscosity) always gradually decreased with the increased molecular weight (Fig. 2). The viscosity reducing effect became more obvious at low w/c. It is really interesting to propose the following working mechanism for PCE of varied molecular weight (Fig. 6): The adsorption of PCE of high molecular weight was always stronger than that of low molecular weight, including the ‘‘bridging effect”. Yet, to achieve the same yield stress (flow spread), even higher surface coverage (adsorption amount) for PCE of high molecular weight was required to increase the particle separation distance and reduce the interparticle attractive force. The concomitant effect was the low flocculation state and high packing density, which was much more effective to reduce viscosity (residual viscosity) of cement paste against the opposite contribution from the increased viscosity of interstitial solution. It is probably the bridging effect, which

weakened the dispersing performance of PCE of high molecular weight, yet obviously enhanced the viscosity-reducing effect at consistent flow spread. In the present test conditions, the shear-thickening behavior of pastes, which was mostly contributed by dosage of PCEs and high volume fraction of solids, was very obvious [45]. The side chain of the polymer absorbed on the surface of cement particles could enter the interstitial solution and be in a disordered state. In such a state, hydroclusters could form easily especially under a high shear rate [12,46]. Additionally, as higher dosage of the PCEs was added, more un-adsorbed polymers remained in the interstitial solution and then the polymers were more easily to become entangled, which contributes to an enhancement of the shear-thickening behavior. Therefore, the shear-thickening behavior of the pastes with larger molecular weight was more pronounced. Furthermore, the increase of solid concentration due to the decrease of w/c could reduce the particle spacing and then the repulsive forces between the particles. As a result, the particles are prone to collide with each other and appear disorder dispersion, which results in a more pronounced shear-thickening behavior of pastes with lower w/c. However, for paste with a w/c of 0.2, the yield stress was higher, and more agglomerations would appear. It is possible that these agglomerations moved in an orderly manner during the shear process. As a result, the shear-thickening behavior of paste was less pronounced compared with the paste with w/c of 0.3. However, the shear-thickening behavior of cement paste is complicated, further investigation is necessary to find out the key parameters that dominant this rheological behavior. 4.3. Key parameter for the apparent viscosity Under test conditions, the cement paste of the lowest apparent viscosity was prepared by the PCE of the highest molecular weight. The fundamental beginning is the high adsorption amount, and therefore the large particle separation distance and high packing density (high amount of water originally entrapped being released). The key parameter for cement paste is the packing density-the most effective parameter for the microstructure of a suspension (e.g. cement paste) which could be easily measured. For a certain type of cement, high packing density indicated low flocculation degree, therefore low yield stress and apparent viscosity. The question is which parameter of PCE is the key parameter for the apparent viscosity. Herein it seemed answer was the adsorption amount (Fig. 8). Pourchet [47] also suggested that it was the adsorption amount which was more closely related with the dispersing performance. Indeed, the adsorption layer thickness, which determined the steric hindrance effect, might be another answer. Yet, the average adsorption layer thickness depended on the surface coverage and also RAC of PCE based on Flatt’s model [3]. It could also be supposed that, comb-like PCE molecule attached on the surface seemed like polymer brush. Each repeat unit (‘‘blob” in Flatt’s model) was a ‘‘comb tooth”. Based on the model for surface-tethered chains [48], the average adsorption layer thickness only was determined by the length and density of side chain. The ‘‘generalized” density (‘‘grafting density” of side chain on the particle surface) depended on both the PCE surface coverage and the PCE grafting density within single molecule. The side chain length, grafting density, and RAC for the PCE samples discussed here were considered as constant. Therefore, it seemed that the adsorption amount was the key parameter. It should also be noted that, no preferential adsorption was compared or discussed in this paper. The complicated binder system of the cementitious material contained particle surface of different properties (e.g. type and amount of surface charge). Even in a cement paste without any mineral admixture (e.g. silica fume, fly

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indeed low true volume fraction of ‘‘solid” particle, which finally accounted for the low residual viscosity (apparent viscosity). Generally, the most effective strategy to reduce the viscosity of cement paste should be the improving of packing density, as indicated by [44], the deflocculation process. To develop PCE of high viscosity-reducing capacity, the researchers should try to improve the surface coverage of the long side chain toward all the particle surfaces in cement paste. This would be reported in a later research. However, the influence of apparent viscosity on the workability of cement mortar, or concrete (in the presence of aggregates), the situation might be different, since aggregates might get locally concentrated and greatly enhanced the macroscopic shear viscosity. Fig. 8. Evolution of the residual viscosity as a function of the adsorption amount of PCEs.

ash), the surface property of different mineral phases or hydration products should be different. To achieve high packing density (low flocculation degree), superplasticizers should attach uniformly, completely onto all the particle surfaces. In this regard, the authors reasoned that, the most effective parameter for the apparent viscosity should be the surface coverage, more exactly, the surface coverage of the attached long side chains. High surface coverage of the long side chain should be the necessary and sufficient condition for the lowering of apparent viscosity of cement paste. The above discussed condition is for most of the commonlyused cementitious material of proper flowability, in which the PCE dosage is moderate (except for ultra-high performance concrete). The ‘‘plateau” of the adsorption amount has not been achieved yet. Further addition of PCE could effectively improve the packing density. For cementitious material of extremely low water to binder ratio, most of the PCE remained in the solution phase, the influence of solution viscosity became more pronounced, as reported by Liu [21] and our former report [38].

CRediT authorship contribution statement Qianqian Zhang: Methodology, Investigation, Writing - original draft. Xin Shu: Investigation, Validation, Formal analysis, Visualization, Software. Xiaohan Yu: Writing - review & editing. Yong Yang: Resources, Writing - review & editing, Supervision. Qianping Ran: Conceptualization, Resources, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the funding by the National Key Research and Development Program (Grant NO. 2018YFC0705400) and National Natural Science Foundation of China (Grant NO. 51578269).

5. Concluding remarks The influence of five types of PCEs with various molecular weights on the rheology was investigated for viscosity reducing of cement pastes in this study. Based on the experimental results, the following conclusions can be drawn: 1) PCE of moderate molecular weight exhibited the best dispersing performance (the capacity to reduce the yield stress of cement paste). ”Bridging effect”, which indicated one PCE molecule attached with multi-particles, probably enhanced the interparticle interaction, might account for the lower dispersing performance of PCE of high molecular weight. 2) For pastes with same w/c and spread value, yield stress was almost the same. Although the adsorption efficiency of the PCEs enhanced with the increase of molecular weight, more adsorption amount was needed to achieve the same yield stress due to the ‘‘bridge effect”. 3) Under test conditions, residual viscosity of the cement pastes decreased with the increased molecular weight and dosage of PCEs, while shear thickening behavior was much pronounced and residual viscosity increased with shear rate. 4) At constant flow spread, due to the high surface coverage of PCE of high molecular weight, the adsorption layer thickness and the particle separation distance was high. Although the viscosity of the interstitial solutions was high, the highly deflocculating effect resulted in high packing density and

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