Planilla de barrido del agua, limpieza y secado o inhibición del agua residual

e.g. in CPS 3,964 µeq/g), owed to the carboxylate and sulfonate groups present in the graft chains. Such high anionic charge promotes adsorption of the graft copolymer onto positively charged surfaces of cement. The ATBS-acrylic acid copolymer exhibits an even higher anionic charge while BNS possesses a much lower anionic charge than the brown coal based graft copolymer and the ATBS-acrylic acid copolymer.

Table 2 – Molar masses, polydispersity index (PDI) and polymer radii of the synthesized polymers and of BNS as reference superplasticizer sample.

Polymer Mw [g/mol] Mn [g/mol] PDI Rg(z) [nm] Rh(z) [nm]

Brown coal-ATBS-AA 443,300 216,100 2.0 37.2 22.1 ATBS-AA copolymer 183,300 93,460 2.0 23.9 14.5

BNS 140,000* - - - -

* = batch measurement

Fig. 2 – SEC spectrum of the brown coal-ATBS-AA graft

copolymer.

Table 3 – Anionic charge amounts of the polymer samples in DI water, CPS and 0.1 M NaOH solution.

Polymer DI water [µeq/g] CPS [µeq/g] 0.1 M NaOH [µeq/g]

Brown coal-ATBS-AA 3,347 3,964 4,765 ATBS-AA copolymer 4,241 3,974 4,855

BNS 3,643 2,713 3,989

For the chemical structure of the graft copolymer, the model as follows was developed: Humic acid which presents the main component in the alkali extract possesses numerous functional groups (i. e. phenolic and carboxylate groups) in its structure which provide perfect docking sites for the grafting reaction. After addition of an initiator (e.g. sodium peroxodisulfate), macroradicals can form through abstraction of hydrogen from these func- tional groups. Consequently, ATBS and acrylic acid monomers can be grafted onto these free radical sites. Simultaneously, side chain propagation can continue. As a result, a graft copolymer is formed that is composed of a humic acid backbone and grafted ATBS-co- acrylic acid side chains. A structural model of the graft copolymer is proposed in Fig. 3. Note that for humic acid, only model structures exist16 _{due to complexity of composi-}

tion and abundant natural variants. Generally, humic acid contains a number of condensed aromatic rings, and thus attains a relatively stiff, linear conformation. In contrast to this, the ATBS-co-acrylic acid graft chains exhibit high conformational flexibility and are coiled in solution. These differences in solution conformation were confirmed by the Burchard parameters (ratio of Rg(z)/Rh(z)). There, a value of 1.7 was obtained for the graft copolymer

which represents a linear, stretched random coil.17 Cement dispersion

The dispersing performance of the brown coal based graft copolymer was determined using a mini slump test. Here, the dosages were established to reach a paste flow of 26 ± 0.5 cm. The water-to-cement ratio of the neat cement paste without superplasticizer was set to produce a slump flow of 18 ± 0.5 cm (w/c ratio of 0.455). This water-to-cement

value was applied for all measurements. For the brown coal-ATBS-acrylic acid graft copo- lymer a dosage of 0.21% bwoc was required to reach the desired slump flow of 26 ± 0.5 cm. Thus, the novel graft copolymer was even more effective than a commercial BNS reference sample (dosage 0.30% bwoc). Additional mini slump testing carried out for the ATBS-acrylic acid copolymer revealed that this polymer was slightly better (dosage 0.19% bwoc) than the graft copolymer itself. The results suggest that the dispersing effect mainly originates from the ATBS-co-acrylic acid graft chains. This was confirmed further via mini slump tests, evidencing that the alkaline brown coal extract does not disperse cement.

Furthermore, the time dependent slump loss behavior of a cement paste prepared at a water-to-cement ratio of 0.455 containing the polymers at dosages required for a slump flow of 26 ± 0.5 cm was investigated. The results of these measurements are displayed in

Fig. 4. Starting from an initial slump flow of 26 ± 0.5 cm, fluidity was monitored every 15

min for a period of 120 min. According to Fig. 4, the dispersing performance of the brown coal-ATBS-acrylic acid graft copolymer quickly decreases in the first 30 min (slump flow 21.4 cm [8.43 in.]). Afterwards, the decrease becomes very slow. In contrast, the ATBS- acrylic acid copolymer exhibits better slump retention and behaves more similar than the commercial BNS superplasticizer.

Sulfate tolerance of graft copolymer

The presence of alkali sulfates (i.e. K2SO4, Na2SO4) in cement can have a significant

impact on the dispersing performance of superplasticizers. This phenomenon (the so-called “sulfate effect”) has been observed mainly for polycarboxylates. In literature, two mecha- nisms are discussed for the negative impact of sulfate on PCEs: competitive adsorption between sulfate ions and PCEs18 _{and shrinkage of PCE molecules.}19 _{To investigate whether}

the brown coal based graft copolymer is also affected by sulfate ions, mini slump tests at increasing additions of sodium sulfate were carried out. The graft copolymer was applied at a dosage of 0.21% bwoc (slump flow 26 ± 0.5 cm). The results are illustrated in Fig. 5.

Fig. 4 – Time dependent development of the slump flow

of a cement slurry (w/c = 0.455) containing 0.21% bwoc of brown coal-ATBS-AA graft copolymer, 0.19% bwoc of ATBS-AA copolymer and 0.3% bwoc of BNS respectively.

There, it was observed that the graft copolymer is not much affected by the presence of different concentrations of sulfate, thus confirming high sulfate tolerance for the grafted product. In contrast, the ATBS-acrylic acid copolymer shows strong sensitivity to sulfate. At increasing Na2SO4 additions, its initial slump flow of 26.3 cm [10.4 in.] quickly drops

to 20.9 cm [8.22 in.]. Furthermore, a physical blend of 20 wt.% brown coal extract and 80 wt.% of the ATBS-acrylic acid copolymer (as present in the graft copolymer) was tested. It was found that this physical mixture exhibits the same poor sulfate tolerance than the ATBS-acrylic acid copolymer, thus providing further evidence that in fact a chemical reac- tion had occurred between the brown coal substrate and the monomers. A potential expla- nation for the different behaviors of the brown coal-ATBS-acrylic acid graft copolymer and the ATBS-acrylic acid copolymer are the different molar masses of the polymers. The graft copolymer possesses a higher Mw than the copolymer, so its adsorption is mainly driven

by entropic effects (desorption of a huge amount of ions and water molecules adsorbed on the surface of cement), thus producing a high Gibbs energy of adsorption.20 _Whereas

sulfate ions, whose energy of adsorption results from an enthalpic contribution only and is comparatively low cannot displace already adsorbed polymers from the surface of cement.

Adsorbed layer thickness

Additional experiments were carried out to prove successful grafting of ATBS and acrylic acid monomers onto the alkaline extract of brown coal. For this purpose, the adsorbed layer thicknesses of the graft copolymer, the ATBS-acrylic acid copolymer and the brown coal extract were measured and compared. Monodisperse, spherical cationic polysty- rene nanoparticles were taken as adsorbent and layer thicknesses were determined using dynamic light scattering. This method allows facile determination of the adsorbed layer thickness of negatively charged polyelectrolytes at high pH conditions such as in cement pore solution.14 _{Layer thicknesses were measured as a function of polymer concentration}

until the point of saturated adsorption was reached. The results are displayed in Fig. 6. The alkaline extract of brown coal reaches the point of saturated adsorption at an adsorbed layer thickness of ~ 2.5 nm [9.8 · 10-8 _{in.] only whereas the brown coal-ATBS-acrylic acid graft}

Fig. 5 – Effect of different Na2SO4 dosages on the slump

flow of cement pastes (w/c = 0.455) holding different polymer samples.

polymer exhibits a substantially higher layer thickness of ~ 6.4 nm [2.5 · 10-7 _{in.]. Contrary}

to this, the ATBS-acrylic acid copolymer produces an adsorbed layer thickness of ~ 1.7 nm [6.7 · 10-8 _{in.] only. These values signify that grafting of the monomers onto the brown coal}

substrate has indeed occurred, and that the graft product possesses pendants of ATBS-co- acrylic acid. However, BNS shows a very low adsorbed layer thickness of only ~ 0.3 nm.

Effect on cement hydration

The influence of the superplasticizers on cement hydration was tested by means of isothermal heat flow calorimetry. The heat evolution from cement hydration was monitored for cement pastes prepared at a water-to-cement ratio of 0.455 holding 0.25% bwoc of the brown coal-ATBS-acrylic acid graft copolymer, of ATBS-acrylic acid copolymer and of BNS respectively. The results are illustrated in Fig. 7. It was observed that the brown coal- ATBS-acrylic acid graft copolymer causes very minor retardation, apparently caused by the ATBS-co-acrylic acid pendant groups.

Mechanism of dispersion

Adsorption of superplasticizers on cement particles can be tracked via zeta potential measurements. Here, the zeta potential of cement slurries holding different polymer samples was measured. Concentrations of the polymer samples were those required for a slump flow of 26 ± 0.5 cm. The neat cement paste exhibited a slightly negative zeta poten- tial of – 3.3 mV. After addition of the polymer samples, the zeta potentials of all cement slurries decreased to similar values: brown coal-ATBS-acrylic acid graft copolymer – 28.0 mV; ATBS-acrylic acid copolymer – 27.3 mV and BNS – 29.3 mV. These results signify that the brown coal-ATBS-acrylic acid graft copolymer adsorbs and also achieves disper- sion through an electrostatic repulsion effect.

Fig. 6 – Concentration-dependent adsorbed layer thicknesses

of the brown coal-ATBS-AA graft copolymer, of ATBS-AA copolymer, of the alkaline brown coal extract and BNS.

CONCLUSIONS

An alkali brown coal extract holding humic and fulvic acids was successfully used as substrate for the synthesis of a brown coal based superplasticizer. Acrylic acid and ATBS were successfully grafted onto the extracted lignite, as was confirmed via SEC analysis and measurements of the adsorbed layer thicknesses of the graft copolymer and an ATBS- acrylic acid copolymer prepared under comparable conditions. A structural model for the novel graft copolymer suggests that humic/fulvic acid backbones hold grafted chains of ATBS-acrylic acid copolymer. The synthesized brown coal-ATBS-acrylic acid graft copo- lymer presents an effective cement dispersant which is superior over industrial BNS and exhibits excellent sulfate tolerance. Its working mechanism relies on a combination of a strong electrostatic and a minor steric effect.

The study shows that caustic lignite extracts present interesting and low-cost starting materials for the development of novel superplasticizers holding unique structural motifs. Future research should focus on the replacement of the ATBS monomer by less expensive alternatives such as e.g. itaconic, methacrylic, styrene sulfonic, allyloxy hydroxy propyl sulfonic acid or esters thereof. Due to the chemical variability of brown coal and the number of natural variants, different brown coal types should be tested as well. We consider our work as an initial study which may stimulate further ideas to exploit the potential of this concept, especially in countries where more sophisticated monomers are not available.

AUTHOR BIOS

Manuel Ilg studied chemistry and received his B.Sc. and M.Sc. from Technische Univer-

sität München. He is currently a Ph.D. student at the Chair for Construction Chemistry in Garching where he works on new structural concepts for superplasticizers.

Fig. 7 – Isothermal heat flow calorimetry of cement slurries

(w/c = 0.455) holding 0.25% bwoc of brown coal-ATBS- AA graft copolymer, ATBS-AA copolymer and BNS respectively.

Johann Plank is a full Professor at the Institute of Inorganic Chemistry of Technische

Universität München, Germany. Since 2001, he holds the Chair for Construction Chemistry there. His research interests include cement chemistry, concrete admixtures, organic-inorganic composite and nano materials, concrete, dry-mix mortars and oil well cementing.

ACKNOWLEDGEMENTS

The authors would like to thank Vatenfall Europe Mining AG for providing the brown coal samples and Lubrizol for the supply of the ATBS monomer.

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Cementitious suspensions all feature common flow characteristics when their flow curve is observed. When plotted as shear stress vs shear rate a minimum in stress is observed towards low shear rates which may be related to the hydration of cement. When plotted as apparent viscosity versus shear rate a minimum often appears towards high shear rates, beyond which the suspension enters a shear-thickening regime the origin of which remains unclear.

In between these two limits of shear rate, the expected shear-thinning behaviour takes place, where apparent viscosity may be linked to a shear-rate-dependent degree of suspen- sion flocculation.

The present paper aims at shedding some light onto the origins of those features in the context of mix design and superplasticizer technology.

Keywords: pumping; civil engineering concrete; viscosity; superplasticizer;

phosphonate.

INTRODUCTION

Whereas the rheological behaviour of cementitious suspensions has often been discussed under the framework of the Bingham model,1-3 _{thorough observations at the steady-state}

and under no-slip conditions indicate more complex behaviour. It has been reported that high dosages of superplasticizer may lead to a shear-thickening behaviour4-7 _{when shear}

rate increases, which is quite well simulated by the Herschel-Bulkley model, but with no satisfactory explanation of the underlying mechanism.3,5 _{Low shear rate regimes have}

attracted less attention, but some studies have shown that the ageing nature of a suspension may lead to a noticeable change in the flow curves with the occurrence of an increasing stress branch at low strain rates.8,9 _{Between these two limits, the material features the often}

reported shear-thinning behaviour.

The present work aims at showing that cementitious compositions all feature such char- acteristics, though in various intensities and ranges, depending on the mixture proportions

SP-302-06

Influence of Superplasticizers on