Adsorption properties of polycarboxylate ether-based superplasticizer on cement particles and their resultant dispersion

Abstract

The cement dispersion performance of a polycarboxylate (PCE)-based superplasticizer is highly related to their adsorption behaviors as a function of time. This study evaluated effects of PCEs on rheological properties of cementitious materials. First, characteristics of PCEs were characterized via permeation chromatography (GPC) and Fourier-transform infrared spectrometry (FT-IR). The adsorption behavior of single and blended PCEs on cementitious composites was identified using total organic carbon analyzer (TOC). Based on the measurement of PCE adsorption, the changes of rheological properties of cementitious materials as well as the number of dispersed cement particles were characterized using a rheometer and laser spectroscopy, respectively. The experimental results support the systematic mechanism of PCE adsorption, cement dispersion, and the decrease in viscosity of cementitious materials.

Graphical abstract

Keywords

adsorption / polycarboxylate / superplasticizer / rheology / dispersion

1 Introduction

Various types of chemical admixtures have been developed to modify and improve the properties, such as workability, setting time, mechanical performance, and freezing and thawing resistance, of cementitious materials [1,2]. Superplasticizers are one of the most important ingredients and have contributed to the manufacture of various types of concrete, such as high-strength concrete and self-consolidating concrete, involving decrease of the water requirements of concrete mixtures [3–6]. Reduction of the amount of mixing water content in a concrete mix improves its durability and decreases the water-to-cementitious material ratio (w/cm).

Among various types of superplasticizers, polycarboxylate (PCE)-based superplasticizer has shown excellent water reducing performance, achieved by its cement-adsorption mechanism. PCEs consist of a long-charged backbone (carboxylic group) with polyethylene oxide (PEO) graft chains. When PCEs are added into cementitious mixes, the polymers rapidly adsorb onto the surface of hydrating cement particles due to attractions between anionic PCE backbones and cationic surfaces of cement particles [7,8]. The hydrophilic PEO side chains extend into the pore solution and generate steric repulsion to prevent flocculation of cement particles. Subsequently, the water in the flocculated cement particles is released by the dispersion of the cement particles, which results in a better workability of concrete mixtures [9]. It is known that depending on the backbone charge density and the thickness of the adsorbed layer, the capability of adsorption and dispersion of PCEs can be varied. For example, PCEs having low PEO graft density can have a strong initial dispersing effect due to the rapid adsorption of PCE polymers [10]. In contrast, PCEs having high PEO graft density show slow adsorption and are then useful for flowability retention [11]. Therefore, various types of PCEs can be developed and applied to precisely control concrete workability.

The investigation of adsorption characteristics of PCEs is helpful for understanding the relationship between PCE structures and their effect on the workability of concrete mixtures. Liu et al. [12] investigated changes in the viscosity of cement paste as a function of superplasticizer dosage. The addition of PCE in a low w/cm cement paste (0.24 to 0.32) reportedly increases the packing density of cement grains and develops water film among them effectively. Yoon and Kim [13] analyzed the relationship between the dosage of superplasticizer and changes of workability of cementitious mixtures. It was reported that the workability of cement paste was highly increased if PCE adsorption reached a plateau value, where the surface of cement particles was saturated with PCE polymers, which is known as critical dosage or saturation point [13,14]. Ma et al. [15] investigated the effect of blended PCE and aliphatic (–CH, –CH₂, and –CH₃) superplasticizer on rheological properties and stability of cement paste. They identified that the mixing ratio of two superplasticizers can be controlled appropriately to maintain and improve the rheological properties of cementitious composites. In addition, a combination of PCE and aliphatic superplasticizer is beneficial for enhancing bleeding and segregation resistance. The addition of multiple PCEs having different adsorption rate (e.g., water reducing and consistency maintaining PCEs) would show a competitive adsorption, which can be described as shown in Fig.1. The adsorption rate of water reducing PCEs is higher than that of consistency maintaining PCEs.

Many studies have also focused on the effect of superplasticizers on the workability, stability (e.g., bleeding), and hardness of cementitious materials [16–19]. However, further investigation is needed for in-depth understanding of the adsorption kinetics of single and blended PCEs and their influences on rheological properties of cementitious materials as a function of time. The aim of this study is to characterize the relationship of the adsorption of single and blended PCEs with rheological properties of cementitious mixtures. Three different PCEs (two as water reducers, one as a consistency maintaining type) and two blended PCEs (combination of water reducers, and water reducer and consistency maintaining type) were tested. The molecular weight and structure of PCEs were identified by gel permeation chromatography (GPC) and Fourier-Transform Infrared Spectrometry (FT-IR). Their unique adsorption characteristics as a function of their dosages and storage time were investigated based on the results of a total organic carbon (TOC) analyzer. The performances of these superplasticizers were evaluated based on the measurement of rheological properties, and the cement dispersion were observed by optical-back reflectance measurement (ORM).

2 Experimental program

2.1 Cement

Cement used in this study was Type I ordinary Portland cement, which complied with ASTM C150. Its specific gravity and Blaine number were 3.14 and 335 m²/kg, respectively. The chemical composition of cement was characterized by X-ray fluorescence (XRF). Tab.1 reports the oxide composition of the cement.

2.2 Superplasticizers

In this study, two vinyl ether-based PCE (S1 and S3) and one methoxy-PEO PCE (S2) were used. Chemical structures of methoxy-PEO PCE and viny ether-based PCE are provided in Fig.2. Numbers of m and n in Fig.2 can be changed depending on the target performance of PCEs. The densities of these PCEs can be categorized into two groups: water reducer (S1 and S2) and consistency-maintaining agent (S3). The water reducers have a relatively long carboxylic backbone and short PEO graft chain compared to the consistency-maintaining agent. Blended polymers of M1 and M2 were prepared by combining S1 and S2, and S1 and S3 at a mass ratio of 5:5, respectively. The solid content of each sample was 50%. Using these five different PCEs, the adsorption kinetics and its effect on rheological properties of cement pastes were identified. Their molecular weights were then the combination of S1, S2, and S3, showing a highly-increased polydispersity (PDI).

The GPC analysis of PCEs was conducted using Waters e2695 (through Korea Research Institute of Chemical Technology). For the GPC test, injection volume and used columns were 100 µL PCE with eluent (water), and ultrahydrogel 120, 250, 500, and 1000 µL. A flow rate during the test was 0.6 mL/min. The average molecular weight and PDI of PCEs were identified as provided in Tab.2. If a PCE is a uniform polymer (also known as monodisperse polymer), PDI should be equal to one. A high PDI means the PCE sample contains molecules of different mass. Chemical bonding of PCEs were characterized using FT-IR at room temperature. The IR adsorption spectra of each superplasticizer indicates a specific bonding of atoms. The applied spectral range was 400 to 4000 cm^–1 with a 2 cm^–1 resolution, as shown in Fig.3. The peak at 2880 cm^–1 was attributed to C–H stretching vibration adsorption. Peaks at 1468 cm^–1 represented C–H bending. The peak at 1723 cm^–1 was attributed to carbonyl (–C=O–O–). The stretching vibration adsorption peak at 1110 cm^–1 was due to ether bonds (–C–O–C–) [13,17,20].

2.3 Sample preparation

Cement paste samples incorporating various dosages of the PCEs were prepared for the rheological test and TOC analysis. The w/cm of cement paste was fixed at 0.4. It was important to produce a stable sample when evaluating the PCE adsorption and the resultant cement dispersion. No bleeding occurred in the samples incorporating PCE, and the neat cement paste by w/cm = 0.40 showed sufficient fluidity to achieve stable measurements. The solid-content dosage of each PCE was controlled at 0.02%, 0.05%, and 0.08% of cement mass. The samples were then labeled by the PCE information. For example, S1-02 indicates the cement paste sample produced by incorporating 0.02% of S1.

All cement paste samples were mixed by the following procedure: 1) the exact dosage of superplasticizer was added into mixing water; 2) cement powder was poured into PCE-concentrated water; and 3) each batch, more than 3.0 L, was mixed for 8 min using a planetary mixer. Each cement paste batch was used for the rheometer test equipped with a laser analyzer and for the analysis of PCE adsorption using TOC measurements. To evaluate the PCE performance over time, these tests were repeated at one-hour intervals for two hours. In between the tests, the samples were stored in sealed containers and re-mixed for 3 min prior to the tests. The details of each test are provided in Section 2.4.

2.4 Analysis

2.4.1 PCE adsorption using a TOC analyzer

The amount of PCE adsorbed on the cement particles was evaluated by the TOC analyzer (Shimadzu TOC-VCSH/CSN). The analyzer can measure the amount of inorganic and total carbon content in the pore solution of a cement paste sample. The organic carbon content can be calculated by subtracting inorganic carbon content from the TOC content.

The procedure for the measurements was as follows: 1) pore solution from a freshly mixed cement paste was obtained by using a centrifuge for 10 min at 3000 r/min; and 2) using a syringe filter having a pore size of 0.45 μm, the solution was filtered to remove the remaining cement particles. For normalization, the amount of superplasticizer adsorbed on the cement particles was calculated using the following Eq. (1):

(1)\begin{array}{r}\text{Adsorbed amount of}PCE(\text{mg}/\text{g})=TOC(\text{mg}/\text{L})\frac{V_{\text{water}}(\mathrm{L})}{M_{\text{cement}}(\mathrm{g})},\\ \end{array}

where TOC is the result of the remaining carbon in the extracted water, which is equal to the amount of unadsorbed polymer; V_water is the volume of water; and M_cement is the mass of cement.

Since the amount of TOC in the cement due to the cement grinding agents is not negligible, this amount of TOC needs to be taken into account in the measurements [21]. Hence, the TOC analyzer was calibrated with a neat cement paste. The calibration sample did not incorporate any PCE, and the TOC in its pore solution was assumedly attributed to the cement grinding agents. The difference between the TOC results of a specific PCE concentrated solution and the pore solution of the cement paste incorporating it presents the amount of its adsorption on cement grains.

2.4.2 Rheometer-ORM test

The effect of different PCEs on rheological properties of cement pastes was evaluated using the rheometer (HAAKE MARS III, Thermo Fisher Scientific Inc.), which was coupled with ORM (Sequip GmbH) as shown in Fig.4. For the rheological test, a parallel plate geometry having a diameter of 35 mm and gap of 1 mm was adopted. The apparent viscosity of cement paste was measured at 0, 1, and 2 h after the mixing. During the test, the shear rate was fixed at 100 s^–1 to prevent agglomeration of cement particles.

The ORM recorded the number of cement particles and agglomerates, which is beneficial for observing the flocculation phenomenon [22]. The particles and agglomerates to be tested were 0.5 to 100 µm in size. Fig.4 shows the configuration of the simultaneous measurement. A sapphire glass window on the tip of the probe was leveled with the surrounding bottom plate. The central axis of the laser probe was located at 8.6 mm distance from that of the torque bar (rotational axis). The focal point was within the 8.5 mm-diameter circular motion along the central axis of the laser probe, and it also moved in a direction normal to the sapphire glass window (bottom plate) within a range of 40 to 125 µm.

3 Experimental results and discussion

3.1 Adsorption of PCEs

The PCE adsorption on cement particles was characterized for a period of 2 h. Fig.5 shows the adsorption results of S1–S3, M1, and M2 in a cement paste system. Water reducers, S1 and S2, showed a rapid adsorption rate. A large amount of these water reducers was adsorbed at 0 h (after mixing), which could enhance the early-age workability of cement pastes. The rapid adsorption was due to a highly-charged backbone with a short PEO chain [11]. In the case of M1, a blended water reducer made by combining S1 and S2, showed similar adsorption rate and quantity. After 1 h, the PCE adsorption of S1, S2, and M1 polymers increased by 0.05%. After 2 h, the PCE adsorption increased by 0.08%. The plateau level of PCE adsorption was slightly higher than 0.08% of cement mass. At the plateau value, significant bleeding of the sample occurred. Excess PCE beyond the limit of its adsorption on cement caused segregation of cement paste. Therefore, the measurements were made before its plateau.

In contrast, S3 consistency maintaining PCE showed a slow and small adsorption. These characteristics were attributed to short carboxylic backbone and long PEO graft chain as described in Fig.1. Even though the adsorption rate of S3 at 0 h was slower than those of the others, the amount of adsorbed S3 increased greatly with time. In the case of M2, which is a combination of S1 and S3, its adsorption characteristics were in between those of water reducers and the consistency maintaining PCE.

3.2 Rheological properties

To identify the effect of the PCE samples on the rheological properties of cementitious mixtures, their apparent viscosities were measured using the rheometer. Fig.6 shows the changes in the rheological properties of cement pastes depending on the types and dosages of the PCEs. The age of each paste sample was 0 to 2 h. In the case of the water reducers of S1, S2, and M1, the initial apparent viscosities of cement pastes at 0 h highly decreased with a higher dosage. A large number of polymers were rapidly adsorbed on the cement particles, resulting in the improvement of apparent viscosities. In particular, M1 was the most effective water reducer for decreasing the initial apparent viscosity, which was attributed to synergetic adsorption of polymer S1 and S2. After 1 and 2 h, their apparent viscosities increased even though the additional adsorptions of S1, S2, and M1 were observed as shown in Fig.6(a)–6(c). The formation of organo-mineral phases on the surface of cement particles dissipates the effect of the PCE having short PEO graft chains [19]. Consequently, the dispersion of cement particles caused by adsorbed polymers becomes ineffective and the workability of cement paste becomes poor over time.

On the contrary, S3 showed relatively higher initial apparent viscosity than the water-reducing types, S1 and S2. The advantage of S3 was keeping the consistency of the sample, or even decreasing the apparent viscosity after 2 h. Sample M2, which is a combination of S1 and S3, showed low viscosities at 0 and 2 h. For example, the apparent viscosities of M2-08 at 0 and 2 h were 0.15 and 0.20 Pa·s, respectively. The viscosities were comparable with those of S1-08 (0.08 Pa·s at 0 h and 0.21 Pa·s at 2 h) and S3-08 (0.47 Pa·s at 0 h and 0.35 Pa·s at 2 h). A combination of the water-reducing type and consistency-maintaining type improves not only early-age, but also long-term workability of cementitious materials.

A linear relationship between the PCE adsorption and the apparent viscosities of cement paste was identified, as shown in Fig.7. Regardless of the type, or its polymeric structure, of PCEs, the apparent viscosity linearly decreased with higher adsorption of PCE. A high PCE adsorption can improve particle packing and inter-particle distance, which are related to colloidal attractive interaction and hydrodynamic effect [16,23]. It corresponds to the strong linear correlation (R² = 0.95 at 0 h and 0.92 at 2 h) between apparent viscosity and adsorbed amount of PCEs seen in Fig.7. Thereby, the adsorption rate of PCE governs the changes in the viscosity. Analysis of number of cement particles which will be discussed in next section can support the linear relationship between apparent viscosity and adsorbed amount of PCEs.

3.3 Dispersion of cement particles

The number of dispersed cement particles was counted using the ORM, which evaluated the effect of each PCE sample on cement dispersion. It should be noted that the control mixture is a neat cement paste produced by w/cm = 0.4 without incorporating a PCE. When the dosages of S1 and S2, the water reducers, increased, the relative number of dispersed cement particles increased, as can be seen in Fig.8(a) and 8(b). The relative number of cement paste incorporating 0.08%S2 was slightly higher than that of the sample containing 0.08%S1. However, they did not differ greatly in adsorption or rheological properties. Cement dispersion exceeding a certain degree did not contribute to a decrease in viscosity.

A decrease in the number of dispersed cement particles was observed in the paste samples incorporating S1 and S2. These results correspond with the decrease in their apparent viscosities, as illustrated in Fig.8(a) and 8(b). A more intensive result was observed in the cement paste incorporating M1. That was attributed to the competitive adsorption between two different adsorbing polymers. The cement particles were more saturated by the polymers, which resulted in a significant decrease in the apparent viscosity.

In the case of S3, the consistency-maintaining type, there was no big difference in the relative numbers at 0 h even with high dosages. The number of dispersed cement particles increased at 1 and 2 h, and the adsorption concurrently occurred as reported in the previous section. As a result, the viscosity decreased. In the case of M2, which is a combination of S1 and S3, the cement dispersion showed both water-reducing and consistency-maintaining characteristics. The increase in the relative number of particles at 0 h was observed in a range of 0.02% to 0.08%.

4 Conclusions

This research investigated the effect of water reducing, consistency maintaining, and blended PCEs (vinyl ether-based methoxy-PEO PCEs) on rheological properties and dispersion of cement particles. It verified that a rapid adsorption of water reducing PCE improved early-age rheological properties. On the contrary, the initial adsorption rate of a consistency-maintaining type PCE was slow, but a significant increase in its adsorption was observed at 1 and 2 h later. Because of its delayed adsorption for a period of 2 h, the apparent viscosity was maintained. Comparing each PCE, the blend of two water-reducing types showed a high initial adsorption resulting in a low apparent viscosity. The use of PCE blend, water-reducing and consistency-maintaining types, can effectively improve early and long-term workability of cementitious materials.

The linear relationship between the PCE adsorption and the viscosity of cement pastes was identified regardless of the type of PCE. The dispersion of cement particles was concurrently observed with the PCE adsorption. This result would be caused by the improvement of apparent viscosity with an increase of PCE adsorption in cement paste by improving cement packing as well as inter-particle distance. Therefore, it was clearly demonstrated that PCE adsorption, the cement dispersion, and finally the decrease in viscosity, are systematically controlled by the polymeric structure of PCE.

References

[1] Plank J, Sakai E, Miao C W, Yu C, Hong J X. Chemical admixtures—Chemistry, applications and their impact on concrete microstructure and durability. Cement and Concrete Research, 2015, 78 : 81–99

[2] Houst Y F, Bowen P, Perche F, Kauppi A, Borget P, Galmiche L, Le Meins J F, Lafuma F, Flatt R J, Schober I, Banfill P F G, Swift D S, Myrvold B O, Petersen B G, Reknes K. Design and function of novel superplasticizers for more durable high performance concrete (superplast project). Cement and Concrete Research, 2008, 38( 10): 1197–1209

[3] Mardani-Aghabaglou A, Tuyan M, Yilmaz G, Ariöz Ö, Ramyar K. Effect of different types of superplasticizer on fresh, rheological and strength properties of self-consolidating concrete. Construction & Building Materials, 2013, 47 : 1020–1025

[4] Zeyad A M, Almalki A. Influence of mixing time and superplasticizer dosage on self-consolidating concrete properties. Journal of Materials Research and Technology, 2020, 9( 3): 6101–6115

[5] Li P P, Yu Q L, Brouwers H J H. Effect of PCE-type superplasticizer on early-age behaviour of ultra-high performance concrete (UHPC). Construction & Building Materials, 2017, 153 : 740–750

[6] Dubey R, Kumar P. An experimental study for optimization of high range water reducing superplasticizer in self compacting concrete. Frontiers of Structural and Civil Engineering, 2013, 7( 1): 62–71

[7] ZhangKPanLLiJLinC. What is the mechanism of the fiber effect on the rheological behavior of cement paste with polycarboxylate superplasticizer? Construction & Building Materials, 2021, 281: 122542

[8] Qiu X, Peng X, Yi C, Deng Y. Effect of side chains and sulfonic groups on the performance of polycarboxylate––Type superplasticizers in concentrated cement suspensions. Journal of Dispersion Science and Technology, 2011, 32( 2): 203–212

[9] Li Y, Yang C, Zhang Y, Zheng J, Guo H, Lu M. Study on dispersion, adsorption and flow retaining behaviors of cement mortars with TPEG-type polyether kind polycarboxylate superplasticizers. Construction & Building Materials, 2014, 64 : 324–332

[10] Liu J, Ran Q, Miao C, Qiao M. Effects of grafting densities of comb-like copolymer on the dispersion properties of concentrated cement suspensions. Materials Transactions, 2012, 53( 3): 553–558

[11] Feng H, Feng Z, Wang W, Deng Z, Zheng B. Impact of polycarboxylate superplasticizers (PCEs) with novel molecular structures on fluidity, rheological behavior and adsorption properties of cement mortar. Construction & Building Materials, 2021, 292 : 123285

[12] Liu J, Wang K, Zhang Q, Han F, Sha J, Liu J. Influence of superplasticizer dosage on the viscosity of cement paste with low water-binder ratio. Construction & Building Materials, 2017, 149 : 359–366

[13] Yoon J Y, Kim J H, Evaluation on the consumption and performance of polycarboxylates in cement-based materials. Construction and Building Materials, 2018, 158: 423–431

[14] ÖzenSAltunM GMardani-AghabaglouARamyarKEffectof nonionic side chain length of polycarboxylate-ether-based high-range water-reducing admixture on properties of cementitious systems. Frontiers of Structural and Civil Engineering. 2020, 14: 1573–1582

[15] Ma B, Qi H, Tan H, Su Y, Li X, Liu X, Li C, Zhang T. Effect of aliphatic-based superplasticizer on rheological performance of cement paste plasticized by polycarboxylate superplasticizer. Construction & Building Materials, 2020, 233 : 117181

[16] Hot J, Bessaies-Bey H, Brumaud C, Duc M, Castella C, Roussel N. Adsorbing polymers and viscosity of cement pastes. Cement and Concrete Research, 2014, 63 : 12–19

[17] Cheah C B, Chow W K, Oo C W, Leow K H. The influence of type and combination of polycarboxylate ether superplasticizer on the mechanical properties and microstructure of slag-silica fume ternary blended self-consolidating concrete. Journal of Building Engineering, 2020, 31 : 101412

[18] Perrot A, Lecompte T, Khelifi H, Brumaud C, Hot J, Roussel N. Yield stress and bleeding of fresh cement pastes. Cement and Concrete Research, 2012, 42( 7): 937–944

[19] HabbabaADaiZPlankJ. Formation of organo-mineral phases at early addition of superplasticizers: The role of alkali sulfates and C3A content. Cement and Concrete Research. 2014, 59: 112–117

[20] Chen W, Deng L, Jiang Y, Li J. Synthesis of the VPEG polycarboxylate superplasticizer with controllable activity and its properties. IOP Conference Series—Earth and Environmental Science, 2021, 647( 1): 012064

[21] ZhangKPanLLiJLinCCaoYXuNPangS. How does adsorption behavior of polycarboxylate superplasticizer effect rheology and flowability of cement paste with polypropylene fiber? Cement and Concrete Composites, 2019, 95: 228–236

[22] Kim J H, Yim H J, Il Choi B, Shin T Y, Shah S P. Influence of particle dispersion on the viscosity change in highly-concentrated cement suspensions: An experimental correlation. Journal of Rheology, 2020, 64( 3): 637–642

[23] Zhang Q, Shu X, Yu X, Yang Y, Ran Q. Toward the viscosity reducing of cement paste: Optimization of the molecular weight of polycarboxylate superplasticizers. Construction & Building Materials, 2020, 242 : 117984

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