Brown coal – The used brown coal was mined in the Lusatia brown coal mining district near Cottbus, Germany.
Chemicals – 2-Acrylamdio-2-tert.butyl sulfonic acid (ATBS), acrylic acid (AA), sodium hydroxide (NaOH), sodium peroxodisulfate (Na2S2O8), sodium pyrosulfite (Na2S2O5) and
EDTA were used as per obtained.
Industrial superplasticizer sample – A spray dried powder BNS superplasticizer was used as an industrial grade reference sample.
Cement – The cement used for this study was an ordinary Portland cement CEM I 52.5 N. Its phase composition as determined by X-ray diffraction and subsequent Rietveld refine- ment is illustrated in Table 1. Its specific surface area was 3,583 cm2/g (Blaine method)
and its particle size (d50 value) was 11.5 µm [45.3 · 10-5 in.] (laser granulometer). The
density as obtained by helium pycnometry was 3.15 g/cm3. Extraction of brown coal
The individual steps performed in the extraction process are schematically presented in
Fig. 1. First, chunks of brown coal were coarsely crushed with a hammer and the residue
was sieved to < 250 µm [9.84 · 10-3 in.] to obtain a brown coal powder exhibiting a narrow Table 1 – Phase composition of the CEM I 52.5 N sample determined by Q-XRD using Rietveld refinement.
Phase wt.% C3S, m 54.14 C2S, m 26.63 C3A, c 3.28 C3A, o 4.26 C4AF, o 2.45
Free Lime (Franke) 0.10
Periclase (MgO) 0.03 Anhydrite 2.64 Hemihydrate* 1.21 Dihydrate* 0.02 Calcite 3.61 Quartz 1.16 Arcanite (K2SO4) 0.46 * determined by thermogravimetry
Fig. 1 – Scheme for extraction of alkali soluble components
from brown coal.
particle size distribution and a high specific surface area. For extraction of the alkali soluble components, 70 g [2.47 oz.] of the brown coal powder were mixed with 700 mL [23.7 fl. oz.] of 0.5 M NaOH placed in a 1 L [33.8 fl. oz.] round-bottom flask.9 The mixture was
refluxed for three hours at 90 °C [194 °F] under constant stirring. Next, the dark solution was cooled to ambient temperature and centrifuged two times for 10 minutes at 8,500 rpm. The supernatant was separated from the residue and freeze dried, yielding 21.7 g [0.765 oz.] of a black solid (theor. yield 31%). It is noteworthy that many coal producers offer such extracts in liquid or powder form designated as “caustic lignite”.
Synthesis of brown coal-ATBS-acrylic acid graft copolymer
The novel brown coal-ATBS-acrylic acid graft copolymer was synthesized by aqueous free radical copolymerization using sodium peroxodisulfate as initiator. Here, 2-acryl- amido-2-tert.butyl sulfonic acid (ATBS) and acrylic acid were grafted onto the extracted alkali soluble components (i.e. humic and fulvic acid) of brown coal. The molar ratio between ATBS and acrylic acid was 1: 0.15 and the weight ratio between the brown coal and the grafted monomers was 20: 80 (wt/wt). In a five necked, 1 L [33.8 fl. oz.] round- bottom flask equipped with stirrer, thermometer, reflux condenser and inlet for N2 gas,
13.2 g [0.466 oz.] of the dry alkali soluble components were dissolved in 206 mL [6.97 fl. oz.] DI water. Thereafter, 8.8 g [0.31 oz.] NaOH pellets were added to adjust a pH of 12 and then cooled to 18 °C [64 °F]. Next, 50.0 g [1.76 oz.] ATBS (241 mmol, 1.0 eq) were dissolved stepwise and the temperature was kept constantly under 25 °C [77 °F] to avoid homopolymerization of ATBS. 2.60 g [0.092 oz.] acrylic acid (36.1 mmol, 0.15 eq), 600 mg [0.021 oz.] EDTA and 1.00 g [0.035 oz.] of an organo-modified polysiloxane defoamer were added and the mixture was purged with N2 for 1 h at room temperature. After heating
to 50 °C [122 °F] the first portion of sodium peroxodisulfate initiator was added (8 g [0.282 oz.]) and the reaction mixture was stirred for 50 minutes at this temperature. The second part of initiator was added (8 g [0.282 oz.]) and the polymerization continued for additional 70 minutes at 50 °C [122 °F]. Then the temperature was increased to 60 °C [140 °F] and kept there for 1 h. Finally, the reaction flask was heated to 80 °C [176 °F], 3.60 g [0.127 oz.] of sodium pyrosulfite were added to quench remaining radicals and stirring continued for 1 h at 80 °C [176 °F]. The solution was cooled to room temperature to obtain a viscous, dark brownish polymer solution with a solid content of 29 wt.% and a pH of 2.5. The polymer solution was used without any further purification.
Synthesis of ATBS-acrylic acid copolymer
An ATBS-acrylic acid copolymer was synthesized according to the procedure described above except that no alkali soluble components were present. Here, a pale yellowish, 27.6 wt.% aqueous solution with low viscosity and a pH of 2.5 was obtained. This polymer was used for comparison.
Characterization of polymers
Size exclusion chromatography – Molecular weights (Mw and Mn) and polymer radii
(Rh(z) and Rg(z)) of all synthesized polymers were determined by size exclusion chroma-
tography. The instrument is equipped with a RI detector and an 18 angle dynamic light scattering detector. Polymer solutions exhibiting a concentration of 2 g/L were prepared
for the SEC analysis. The polymers were separated on a precolumn and two columns using 0.2 M NaNO3 solution (adjusted with NaOH to pH 9) as an eluent at a flow rate of 1.0 mL/
min. The value of dn/dc used to calculate Mw and Mn was 0.218 mL/g (value for lignin).15
Anionic charge amount of the polymers – The anionic charge of the polymers was deter- mined via polyelectrolyte titration using a particle charge detector. 0.001 M cationic polyd- iallyl dimethyl ammoniumchloride (polyDADMAC) solution was employed as titrator. Polymer solutions with a concentration of 0.1 g/L were prepared in DI water, in 0.1 M NaOH and in cement pore solution (CPS). Cement pore solution was freshly prepared by vacuum filtration of neat cement slurries using a water-to-cement ratio of 0.455. In a typical experiment, 10 mL [0.34 fl. oz.] of the polymer solution were pipetted into a PTFE cylinder with an oscillating PTFE piston in the center. The dissolved polymers can adsorb via Van der Waals forces on the surface of the cylinder and the piston. Because of the oscil- lating movement of the piston, counter ions are removed from the immobilized polymers and a streaming current results which can be measured by two platinum electrodes located within the PTFE cylinder. The polyDADMAC solution was titrated until the isoelectronic point was reached. For every polymer sample the measurement was repeated three times and the values were averaged. From the consumption of polyDADMAC the amount of negative charge per gram of polymer was calculated.
Heat flow calorimetry – To investigate the influence of the synthesized polymers on cement hydration, isothermal heat flow calorimetric measurements were carried out. There, 4 g [0.141 oz.] cement were filled into 20 mL [0.68 fl. oz.] glass ampoules and mixed with the respective amount of aqueous polymer solution to obtain a water-to-cement ratio of 0.455. The ampoules were sealed, homogenized for 1 min in a wobbler and then placed into the isothermal conduction calorimeter. Data logging was continued until heat evolu- tion from the hydration reaction subsided completely.
Performance of the synthesized polymers
Mini slump test – The dispersing effectiveness of the synthesized polymers was assessed utilizing a mini slump test following in principle DIN EN 1015, but with some modifica- tions. At first, the water-to-cement ratio required to reach a slump flow of 18 ± 0.5 cm [7.1 ± 0.2 in.] was established for the cement paste without polymers. At this specific water- to-cement ratio, the dosage of the polymers was determined to attain a spread flow of 26 ± 0.5 cm [10.2 ± 0.2 in.]. In a typical experiment, the superplasticizer was dissolved in the required amount of mixing water placed in a porcelain cup. The amount of water contained in the polymer solution was subtracted from the amount of mixing water. 300 g [10.6 oz.] of cement were added to the mixing water over a period of 1 min, then rested for 1 min and subsequently were stirred manually for 2 min with a spoon. Immediately after the end of stirring, the cement slurry was poured into a Vicat cone (height 40 mm [1.57 in.], top diameter 70 mm [2.76 in.], bottom diameter 80 mm [3.15 in.]) placed on a glass plate, filled to the brim and the cone was lifted vertically. The resulting paste spread was measured twice, the second measurement being perpendicular to the first one and averaged to obtain the slump flow value.
Time dependent mini slump test – Development of dispersing performance over time was investigated via time dependent mini slump testing. Here, 400 g [14.1 oz.] cement were mixed with the required amount of water and polymer to achieve an initial slump flow
of 26 ± 0.5 cm. The procedure was the same as described above. After each measurement the cement paste was transferred back into the porcelain cup and covered with a wet towel to avoid desiccation. Prior to each measurement the cement paste was vigorously stirred for two minutes. Measurements were conducted every 15 minutes over a total period of 120 minutes.
Adsorbed layer thickness
The adsorbed layer thickness of the polymers was captured by dynamic light scattering. Here, monodisperse polystyrene nanoparticles exhibiting an average particle size of 75.5 ± 0.5 nm [2.97 · 10-6 in.] were utilized as adsorbent. Starting from a 150 mg/L stock solu-
tion of each polymer in 0.1 M NaOH, different concentrations were prepared by dilution (diluent 0.1 M NaOH). Prior to the measurements, the solutions were filtered through a 0.2 µm [7.9 · 10-6 in.] filter to remove undesired dust particles that can disturb measurements
because of their high scattering intensity. Next, 50 µL [1.7 · 10-3 fl. oz.] of a suspension
of cationic polystyrene nanoparticles (for preparation see14) were added and sonicated for
5 min. The polymer solution was filled into a glass cuvette and then placed in the instru- ment. Every measurement was repeated 150 times per sample and the average value was calculated. Each test consisted of a 10 s light scattering run taken at a temperature of 25 °C [77 °F]. Measurements were continued at increasing polymer concentrations until a stable, final value was reached that was regarded as the point of saturated adsorption. The adsorbed layer thickness was calculated using equation 1.
adsorbed layer thickness nm = dads nm - dpolystyrene [nm]2 (1)
where dads represents the particle size of the polystyrene nanoparticle holding adsorbed
polymer, and dpolystyrene represents the particle size of the native polystyrene particle. Zeta potential measurement
Zeta potential measurements were performed at room temperature on an electro acoustic spectrometer. This instrument measures a vibration current induced by an acoustic wave which causes the aqueous phase to move relative to the cement particles. From that, a potential difference results which can be measured and designated as zeta potential. Imme- diately after mixing, the freshly prepared cement slurries holding the respective dosage of the polymer samples required for a slump flow of 26 ± 0.5 cm were filled into the cup of the instrument and measured under continuous stirring for a total period of 30 min.
EXPERIMENTAL RESULTS AND DISCUSSION