Synthesis of dispersing agents from starch – Influence on rheological properties and early age hydration of OPC

Construction and Building Materials 240 (2020) 117913

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Synthesis of dispersing agents from starch – Influence on rheological properties and early age hydration of OPC Stephan Partschefeld ⇑, Andrea Osburg Bauhaus-Universität Weimar, F.A. Finger Institute for Building Materials Science (FIB), Chair of Building Chemistry and Polymer Materials, Coudraystraße 11A, 99423 Weimar, Germany

h i g h l i g h t s  The Influence of starch-based superplasticizers on rheological behaviour of OPC.  The Influence of starch-based superplasticizers on early age hydration of OPC.  The Investigations show that thickening admixtures can be produced from starch.  The Investigations show that liquefying admixtures can be produced from starch.  One key parameter is the molecular weight of the starch admixtures.  Another key parameter is the amount of anionic charges incorporated.  The early age hydration was strongly modified by the starch superplasticizers.  The dormant period is strongly prolonged by the starch superplasticizers.

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Article history: Received 20 June 2019 Received in revised form 26 November 2019 Accepted 19 December 2019

Keywords: Superplasticizer Polysaccharides Starch Cement hydration Rheological behaviour Adsorption experiments Conductivity measurements

a b s t r a c t This study deals with the influence and effectivity of synthesized starch-based superplasticizers on rheological behaviour of cement pastes and on cement hydration. By varying molecular parameters, like molecular weight and the amount of introduced anionic charges, key parameters which induce thickening or liquefying behaviour in cement pastes were identified. Cement hydration was monitored by isothermal calorimetry, ultrasonic measurements and conductivity experiments. The investigations were supplemented by adsorption measurements by the phenol-sulphuric acid method. Results clearly show that low molecular weights and anionic charges in the molecular structure are necessary to create superplasticizers from starch. Also, the amount of anionic charges implemented in the molecular structure by chemical modification is a key parameter for a high dispersing performance. Furthermore, the starchbased superplasticizers delay the hydration of cement pastes, because of adsorption on cement surface, especially on the first hydration products. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The construction industry has already made a major change in recent decades towards innovative technologies and materials. This has been determined by both the demand for increasing productivity and by ecological aspects. With a worldwide consumption of about 4 Gt/a, cement is the most widely used binding agent [1]. Especially the organic additives for building materials are of enormous importance to regulate and improve the fresh and hardened material properties of mortars and concretes [2]. The vast majorities of currently used additives are produced syn-

⇑ Corresponding author. https://doi.org/10.1016/j.conbuildmat.2019.117913 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

thetically and are derived from energy-intensive petrochemicals. Superplasticizers are the most frequently used admixtures in concrete. In addition to polycondensate-based superplasticizers, which are less effective especially at low water-cement ratios, polycarboxylate ethers are the most effective superplasticizers on the market [3]. Due to the increasing demand for cement, especially in growing industrial countries, the demand for highly effective superplasticizers will also increase substantially in the coming years. For this reason, it is necessary to research admixtures based on renewable resources with lower energy-demand compared to petrochemical products as well as alternative binder materials. Polysaccharides represent such a renewable material, which can be adapted to the respective application by chemical modification [4]. Especially the cellulose and starch ethers played an important

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S. Partschefeld, A. Osburg / Construction and Building Materials 240 (2020) 117913

role as water retention agents and regulators of the rheological properties in the building material application as additives [5]. The interaction of modified polysaccharides as additives to cementitious systems is well known due to the wealth of scientific work [6–10]. However, in the chemical modification of starch and cellulose, the main focus was on increasing the consistency and water reduction of mortars and concretes [6,11,12]. The field of application of starch as an additive can be significantly extended by creating superplasticizer properties through suitable molecule design, thereby making concrete as a mass-produced building material accessible for admixtures derived from renewable raw materials. Vieira et al. investigated the possibility of producing starch derivatives which cause a dispersing effect in the cement paste. It was found that low molecular weights (degraded starches) are necessary to obtain dispersing properties. It is also necessary to incorporate anionic groups into degraded starches [13]. Crepy et al. also carried out investigations with starch-based polymers with viscosity-reducing properties in cementitious systems. The effect of starch acetate, starch maleate, starch succinate and starch sulphopropylate and butylate in cement mortars was investigated [14]. The authors showed that by introducing sulfopropyle or sulfobutyle side chains into starch polymers, self-leveling compounds can be generated that can replace conventional plasticizers. Shenghua et al. studied the influence and performance of synthesized poly-carboxymethyl-ß-cyclodextrin in cement pastes by measuring fresh state properties like slump, setting time, adsorption and zeta potential [15]. The present study deals with the investigation of the influence of synthesized starch-based superplasticizers on the rheological properties of ordinary portland cement paste. Molecular parameters influencing the dispersing properties should be identified. Furthermore, the influence on the early hydration of Portland cement was investigated.

these three starches. The starch derivative S2-B was not degraded in molecular weight, only anionic charges were implemented. The specific charge of the synthesized admixtures was measured by particle charge detector with titration unit. Ordinary portland cement (OPC) CEM I 52.5 milke-classic Ò from HeidelbergCement AG was used in this study. The designation follows the German standard DIN EN 197-1. This cement is typically used for investigations in building chemistry in Germany. The chemical composition and mineralogical phases, which are determined by an X-ray diffractometer Seifert XRD 3003 TT with Euler cradle and X-Y table and Rietveld analysis, are listed in (Table 2).

2. Experimental

The influence of the starch superplasticizers on the rheological behaviour of OPC pastes was determined with a rotation viscometer (Rheotec, Brookeflied DV III-ultra with SC-4 29 spindle) and compared with the dispersing performance of commercially available polycarboxylate ether (PCE) (Melflux 1641FÒ, BASF) and a melamine based polycondensate (PC) (Ronasil F20Ò RCF, Chemie + Faser GmbH). The deionized water-cement ratio (w/c) of the OPC pastes was set to 0.35. The determined flow curves and viscosity curves are the result of mean value determinations from six measured values at each rotation speed. The influence of the starch superplasticizers on early hydration was determined by calorimetric studies with an isothermal calorimeter (mc- calÒ, C3-Prozesstechnik) at 20 °C. Mixing of OPC pastes took place outside the calorimeter, as described by Sauvat et al. [19] and Ramachandran [20]. Each calorimetric experiment was performed three times and a mean heat flow curve was determined. In

2.1. Materials Three basic starches S1 (manioc starch from Ingredion Germany GmbH), S2 (wheat starch from Carl Roth GmbH & Co. KG) and S3 (potato waste starch of food manufacturer Ablig Feinfrost GmbH) were chemically modified and used as potential superplasticizers. The properties of the basic starches, before and after chemical modification are given in (Table 1). The basic starches were characterized by size exclusion chromatography (SEC). This method is commonly used to investigate the molecular size and weight distribution of polymeric products like starch derivatives [16]. The method is based on the separation of different molecule sizes by the residence time in the pores of the adsorbent. Four starch superplasticizers (S1-A; S1-B; S2-A; S3-A) were produced on the basis of

2.2. Synthesis of the starch superplasticizers The synthesis of superplasticizers from starch takes place in a two-stage chemical modification process. First, the molecular weight of the starch was reduced by acid hydrolysis [17]. Therefor the starch was suspended in deionized water (starch/waterdeionized = 1/2) and mixed with 20 ml of 20 M hydrochloric acid. By varying the stirring time from 8 h to 24 h different molecular weights were achieved. The molecular degradation was achieved by addition of 100 ml of 10 wt% sodium hydroxide solution. The second part of the chemical modification was the introduction of anionic charges by adding 25 wt% sodium-vinyl-sulfonate (NaVs) solution (starch/NaVs = 1/3,25) at alkaline conditions and temperature of 60 to 80 °C [18]. By varying the stirring time from 24 h to 72 h different amounts of anionic charges were implemented in the molecular structure of the starches. (Fig. 1) shows the reaction pattern for the synthesis of the starch superplasticizers. 2.3. Methods of investigation

Table 1 Properties of the starches before and after chemical modification.

a b

Admixture

Molecular weight Mn/Mw (Da)a

Degree of polymerization DPn/ DPw

Specific charge in H2O [C/g]b

Specific charge in 0.02 M (CaOH)2 [C/g]b

S1 S2 S3

1.03*105/5.3*105 7.13*105/2.1*106 6.32*105/1.25*106

63/327 438/1296 387/766

– – –

– – –

S1-A S1-B S2-A S2-B S3-A

Properties of the starches after chemical modification analysis 1.03*105/5.3*105 1.03*105/5.3*105 5.04*105/1.5*106 7.13*105/2.1*106 2.09*105/9.3*105

63/327 63/327 309/933 438/1296 256/723

11.2 1.2 9.8 9.8 6.9

10.4 0.9 9.5 9.5 6.4

Determined by size exclusion chromatography with a pu-980 pump and separating columns NOVEMA3000 und NOVEMA300, DMSO/LiBr-solvent at 65 °C. Determined by particle charge detector MütekÒ PCD-04 with titration unit, Solvent Poly-DADMAC.

S. Partschefeld, A. Osburg / Construction and Building Materials 240 (2020) 117913 Table 2 Chemical and mineralogical composition of the used OPC [wt%]. CEM I 52.5 N milke-classic Chemical composition SiO2 Al2O3 Fe2O3 CaO MgO TiO2 K2O Na2O MnO SO3

23.4 4.3 1.3 64.0 0.8 0.18 0.75 0.23 0.04 2.6

Mineralogical composition by XRD and Rietveld analysis 49.1 C3S C2S 30.0 C3A 6.3 C4AF 4.4 Bassanite 3.4 Anhydrite 2.0 Arcanite 0.9 Calcite 3.6 Periclase 0.3

addition, the setting points and hardening times of OPC pastes mixed with starch superplasticizers were determined by ultrasonic measurements. An ultrasonic measuring device (ultratest IP-8Ò of the company ultratestÒ) was used for this purpose, which works according to the ultrasonic transmission method. In combination with Vicat tests according to DIN EN 196-3 (vicatronic MA-E044), the solidification start of the used cement paste was determined at 190 min, which corresponds to an ultrasonic velocity of approximately 1000 m/s. The end of solidification was determined at 240 min, which corresponds to an ultrasonic velocity of approxi-

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mately 1200 m/s. The investigation of the hydration kinetics of OPC by electrical conductivity experiments enables the identification of nucleation, growth and precipitation processes of hydrates like C-S-H or portlandite [21]. By the use of highly concentrated lime solutions (0.02 M Ca(OH)2) it is possible to track the hydration kinetic at high liquid to solid ratios of 20, which are comparable to pastes. The time of portlandite precipitation corresponds to a dropin conductivity and is a benchmark for the delay in hydration by addition of admixtures [22,23]. In order to avoid carbonation influences, the suspensions were exposed to nitrogen gas during the measurements. Adsorption experiments using the phenolsulphuric acid method were performed to clarify the mechanism of action [24,25]. The used OPC was brought into contact with the starch superplasticizers and a highly concentrated lime solution (0.02 M Ca(OH)2) (liquid/solid-ratio of 20), stirred for 2 h and then centrifuged at 2215 G (EppendorfÒ Centrifuge 5804 R). The supernatant was mixed with 5 wt% phenol solution and 96 wt% sulphuric acid and the resulting yellow colour was measured with a spectral photometer (Schott InstrumentsÒ, UvLine 9100) at a wavelength of 490 nm. The investigations were performed in kinetic mode, so that each solution was measured six times and the mean value was calculated. The concentration of the starch superplasticizers in the supernatant was determined by prior calibration with various concentrated glucose solutions. By comparing the concentration with and without the mineral phase, the total amount of adsorbed starch superplasticizer was calculated [26,27]. 3. Results and discussion The investigations initially focused on the influence on the rheological behaviour of the OPC paste as a result of the addition of the synthesized starch superplasticizers and also the PC and PCE superplasticizers (Figs. 2 and 3). The yield stress was defined as the lowest adjustable speed of the rotation viscometer used at

Fig. 1. Chemical modification steps to create superplasticizers from starch [18].

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Fig. 2. Influence of the synthesized starch-derivatives S2-A and S2-B on the flow curves of CEM I 52.5N at w/c-ratio of 0.35 in comparison to commercially available superplasticizers (PC and PCE).

Fig. 3. Influence of the synthesized starch superplasticizers S1-A; S1-B and S3-A on the flow curves of CEM I 52.5N at w/c-ratio of 0.35 in comparison to commercially available superplasticizers (PC and PCE) and CEM I 52.5N at w/c-ratio of 0.60.

1 rpm, which corresponds to a shear rate of 0.25 s 1. The S2-B sample, which has the highest average molecular weight, was the only sample that caused a thickening effect in the OPC paste, although the same amount of anionic charges are implemented as in the sample S2-A. The yield stress of the cement paste including the sample S2-B increased to 51.5 N/m2, which corresponds to a factor of approximately 7 compared to OPC paste without admixtures. In contrast, the sample S2-A degraded to DPw < 1000 shows a very high liquefying effect and a low yield stress of 0.42 N/m2, which corresponds to a reducing factor of approximately 18 compared to the OPC paste without admixtures. The samples S1-A, S1-B and S3-A, also reduced the yield stress to a level of lower than 1 N/m2 and showed superplasticizer effects in the rheological behaviour of the OPC paste. (Table 3) shows the measured yield stresses of the OPC pastes with varying types of admixture. The

investigations confirm the research results of Vieira et al. who found that besides ionic groups like carboxyl and sulphate derivatives, the molecular weight is a key parameter to design dispersing Table 3 Influence of the synthesized starch additives on the yield stress in OPC pastes in comparison to PC and PCE. Type of additive

Yield stress [N/m2] in OPC paste w/c = 0.35

S-1A S-1B S-2A S-2B S3-A PC PCE without admixtures

0.42 0.50 0.42 51.50 0.50 4.33 0.83 7.5

S. Partschefeld, A. Osburg / Construction and Building Materials 240 (2020) 117913

materials based on polysaccharides [13]. In contrast it was found that the degree of polymerisation should be in the range of DPw > 1000 to produce thickening properties in OPC pastes and DPw < 1000 to create superplasticizers. The OPC paste without admixture shows the typical structure viscous flow properties of a Bingham fluid described by other authors in the literature [30–32]. The flow curves increase with increasing shear rate. Only the paste with the sample S2-B shows a drop-in shear stress by increasing the rotation speed. Furthermore, it can be seen that with increasing rotation speed, the shear stress of the cement paste with the sample S2-A is at a comparable level as the specimen with PCE superplasticizer. In addition, the dispersing performance of the synthesized starch superplasticizer is much higher than that of PC, because of their decrease in effectiveness at low w/c < 0.4 [33]. The comparison of the flow curves with starch superplasticizers S-1A and S1-B indicates that the amount of charges introduced into the molecular structure influences the effectiveness of the dispersing performance. The higher the amount of sulphuric acid groups, the higher the dispersing effect in OPC pastes. The dispersing performances of the OPC pastes with samples S1-A and S3-A are almost identical, although the molecular weight of S3-A is higher by a factor of two. The amount of anionic charges in the molecular structure however is nearly the same. Only the flow curve of OPC paste including sample S1-B with a lower charge quantity decreases slightly compared to the other starch superplasticizers. The amount of anionic charges in the molecular structure of the starch superplasticizers is another key parameter for high dispersing performance. The flow curves of the OPC pastes with starch superplasticizer S-1A and the PCE superplasticizer at a w/c of 0.35 and 0.5 wt% addition are comparable to the OPC paste without admixtures at a w/c of 0.6. This results in a water reduction potential of approximately 42 wt%. The viscosity curves of the OPC pastes show a uniform course, although there are differences in the order of magnitude of the viscosities (Figs. 4 and 5). The viscosity decreases with increasing shear rate and is on the highest level at slow shear rates, which is also described by other authors [34,35]. The OPC pastes with the starch superplasticizers and the sample including the PCE reach equilibrium at higher shear rates >5 s 1. The viscosity of OPC paste with the sample S2-B, which has not

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been degraded in molecular weight, shows the highest dynamic viscosity at all shear rates. In comparison to cement paste without admixtures, the dynamic viscosity increases significantly by a factor of 7 at a shear rate of 0.25 s 1. In contrast, the sample S2-A shows a decrease by a factor of 180 at the same shear rates. In comparison to the investigated PC the synthesized starch superplasticizers S1-A, S1-B, S2-B and S3-A are much more efficient in liquefying the OPC paste and are comparable to the investigated PCE regarding their effectiveness. Polysaccharides such as starch and cellulose and also their derivatives are known to significantly influence the hydration of OPC and especially the hydration of the main clinker phases C3A and C3S [13,22,23,25,27,28]. The delaying effect on hydration of the synthesized starch superplasticizers was determined on the basis of the heat release curves (Fig. 6). Different periods were defined, as proposed by Taylor [29], that correspond to the initial period (t1), dormant period (t2) and main hydration period (t3). The heat released in 48 h of hydration was calculated by curve integration from t1 to t48 h (Table 4). The intensity of the initial period was not significantly changed by the addition of the starch superplasticizers. However, the dormant period was extended remarkably. The OPC paste with sample S1-A showed the highest extension of the dormant period to 14.5 h, while the OPC paste without admixtures finished the dormant period after 2.5 h. In contrast, the paste with sample S1-B with the smallest amount of anionic charges depicts the shortest extension of the dormant period to 8.5 h. The extension of the dormant period of the OPC pastes is dependent on the amount of anionic charges of the starch superplasticizer. The higher the amount of anionic charges, the longer the dormant period in following order of starch superplasticizers S1-B  S3-A  S2-A  S1-A (left: lowest extension, right: highest extension of the dormant period). This is also reflected by the values for the heat release after 24 h. The sample S1-A and S2-A showed the smallest heat release of 80 J/g and 88 J/g, which is approximately half of the heat release of pure OPC paste. After 48 h, all samples including the starch-based superplasticizers reach nearly the same heat release as pure OPC paste. Also, the main phase of hydration is influenced by the synthesized starch superplasticizers. While starch superplasticizers with low amount of anionic charges cause the smallest extension

Fig. 4. Influence of the synthesized starch superplasticizers S2-A and S2-B on the viscosity curves of CEM I 52.5N at w/c-ratio of 0.35 in comparison to commercially available superplasticizers (PC and PCE).

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Fig. 5. Influence of the synthesized starch superplasticizers S1-A, S2-B and S3-A on the viscosity curves of CEM I 52.5N at w/c-ratio of 0.35 in comparison to PCE.

Fig. 6. Influence of the starch superplasticizer (w/c ratio 0.35, by addition of 0.5 wt% superplasticizer) on the early age hydration of CEM I 52.5N by calorimetric studies. Table 4 Times and heat released in 48 h as a function of the addition of 0.5 wt% of superplasticizers. Formula

Initial period t1 [h]

Dormant period t2 [h]

Main period t3 [h]

Q24 h [J/g]

Q48 h [J/g]

CEM CEM CEM CEM CEM

0.7 0.7 0.7 0.7 0.7

2.5 ± 0.3 14.5 ± 0.6 8.5 ± 0.8 12.5 ± 0.5 11 ± 0.3

10.3 ± 0.5 26 ± 0.7 19 ± 0.9 26 ± 0.5 22 ± 0.6

177 ± 5 80 ± 3 124 ± 5 88 ± 4 116 ± 5

230 207 219 202 221

I I I I I

+ + + +

0.5 0.5 0.5 0.5

wt% wt% wt% wt%

S1-A S1-B S2-A S3-A

± ± ± ± ±

0.1 0.1 0.1 0.2 0.1

of the main hydration period to 3 h (S1-B) and 3.5 h (S3-A) in comparison to pure OPC, starch superplasticizers S1-A and S2-B show the highest extension of the main hydration period to 4 h and 6 h. The calorimetric studies indicate that the hydration of clinker

± ± ± ± ±

3 5 5 4 5

phases C3A and C3S is influenced significantly by addition of starch superplasticizers. The previously described effects of the developed superplasticizers on cement hydration are also reflected by the ultrasonic

S. Partschefeld, A. Osburg / Construction and Building Materials 240 (2020) 117913

measurements (Fig. 7). The setting point of the OPC paste was delayed for several hours as also found by Vieira et al. [13]. The latest setting start at 15 h showed the OPC paste with the sample S1A. The lowest delay in setting start at 10.2 h is achieved by sample S1-B with the lowest amount of anionic charges. The OPC paste including sample S3-A shows an earlier setting start (13.7 h) than the paste including sample S2-A (14 h) because of the lower amount of anionic charges in the molecule structure. It is remarkable that the time period from beginning to end of setting is not extended by the starch superplasticizers. This corresponds with the calorimetric studies and is another sign for the delay in the dormant period. Both investigations indicate that the hydration delay is due to disturbed dissolution and precipitation processes. These results are supported by the conductivity measurements (Fig. 8). The conductivity curves clearly show that the precipitation of portlandite occurs much later due to the addition of starch superplasticizers. It is also evident that the nucleation of C-S-H and ettringite is not altered significantly. However, the dissolution processes of the anhydrous phases as well as the precipitation of CSH and ettringite were delayed. This results in a smaller increase in the conductivity curve. The highest delay in the precipitation of portlandite was determined for the OPC paste with sample S1-A (8.5 h). The sample S1-B shows the shortest delay in portlandite precipitation of 7 h, because of the lowest amount of anionic charges. From the results of the conductivity investigations, it can be concluded that the mechanism of action is based on adsorption processes of the starch-based superplasticizers on the first hydration products of the cement. This hypothesis is confirmed by the adsorption experiment carried out by the phenol-sulphuric acid method (Fig. 9). The highest adsorption rate at all concentrations was calculated for sample S-1A with the lowest molecular weight and the highest amount of anionic charges. In contrast, sample S1B with the lowest amount of anionic charges shows a lower increase of the adsorption rate with concentration of superplasticizer. The samples S1-A and S-2A with approximately the same amount of anionic charges but different molecular weight show, that the smaller the molecular weight, the more superplasticizer can be adsorbed on the surfaces of the cement hydration products. This results from the smaller space requirement of sample S1-A.

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Furthermore, it becomes apparent that sample S2-A already saturates the surfaces of the cement hydration products at an addition of 0.7 wt%. This is due to the higher molecular weight and the higher space requirement on the surfaces of the cement hydration products. Furthermore, the adsorption experiments show that a low molecular weight in the range of DPw < 500 leads to high dispersing performance which was also found by Vieira et al [13]. In addition, a high anionic charge quantity leads to high adsorption rates and liquefying properties in OPC pastes. 4. Conclusions The investigations conducted show that both thickening and liquefying admixtures can be produced by chemical modification of starch, irrespective of the type of starch used. Even waste starches can be used. The key parameters are the molecular weight and the amount of anionic charges incorporated into the starch structure. In order to achieve liquefying properties in OPC pastes, DPw should be less than 1000. For thickening properties, DPw of higher than 1000 is required. Additionally, small starch molecules (DPw <500) are necessary to ensure that the starch superplasticizers are highly efficient. Furthermore, the efficiency of the dispersing effect depends on the amount of anionic charges introduced. The higher the amount of anionic charges, the stronger is the liquefying or thickening effect. The results obtained by calorimetric, ultrasonic and conductivity studies show, that the addition of a small amount of synthesized starch superplasticizers (0.5 wt%) induces a strong retardation of early cement hydration. Retarding ability is dependent on molecular structure (molecular weight, amount of anionic charges) and is as follows: S1-A < S2-A < S3-A < S1-B. Especially the dormant period of cement hydration is strongly prolonged by the starch superplasticizers, which is caused by adsorption on the surface of the first hydration products of the OPC. The main hydration period is also prolonged. The comparison of the dispersing performance of the starch superplasticizers to common superplasticizers especially PC shows, that these novel type superplasticizers also works at low w/c-ratios of <0.4. The comparison of dispersing performance with the investigated PCE shows, that dependent on the molecular structure the liquefying effects are on the same level.

Fig. 7. Development of ultrasonic velocity of OPC pastes (w/c ratio 0.35) by addition of 0.5 wt% of starch superplasticizer.

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Fig. 8. Hydration mechanism on conductivity curve of OPC CEM I 52.5N in 0.02 M Ca(OH)2-Solution (l/s = 20) and addition of starch superplasticizers of 0.5 wt%.

Fig. 9. Total amount of the adsorbed starch-based superplasticizers relative to cement determined by the phenol-sulfuric acid method.

Further investigations, especially on the influence on hydration of the single clinker phases can help to further clarify the mechanism of action. Also, the influences on pore solution and strength development are important topics for future investigations. CRediT authorship contribution statement Stephan Partschefeld: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Andrea Osburg: Project administration, Resources, Writing - review & editing.

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. References [1] R.M. Andrew, Global CO2 emissions from cement production, Earth Syst. Sci. Data 10 (1) (2018) 195–217. [2] K.H. Khayat, Viscosity-enchancing admixtures for cement-based materials – an overview, Cem. Concr. Compos. 20 (1998) 171–188. [3] A. Leemann, F. Winnefeld, The effect of viscosity modifying agents on mortar and concrete, Cem. Concr. Res. 29 (2007) 341–349.

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