Effect of Polycarboxylate–Ether Admixtures on Calcium Aluminate

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Effect of Polycarboxylate−Ether Admixtures on Calcium Aluminate Cement Pastes. Part 1: Compatibility Studies María del Mar Alonso,† Marta Palacios,†,‡ and Francisca Puertas*,† †

Eduardo Torroja Institute for Construction Sciences (IETcc-CSIC), Spanish National Research Council (CSIC), Madrid 28033, Spain ‡ Institute of Building Materials, ETH Zurich, CH-8093 Zurich, Switzerland S Supporting Information *

ABSTRACT: In calcium aluminate cements (CAC), it is recommended to work with low water/cement ratios (for example using superplasticizers admixtures) to avoid the negative effects of metastable hexagonal hydrate conversion to stable cubic ones. The present work studies the compatibility between CAC and three polycarboxylate ether (PCE) admixtures with different structures. The C/E ratio of PCE admixtures was less important to the adsorption values in CAC suspensions than those of Portland-based binder (OPC). The more positive zeta potential in the CAC pastes may be the reason why these admixtures (regardless of their C/E ratio) adsorb onto cement particles with a different conformation than observed in OPC pastes. The presence of PCE admixtures decreases yield stress of CAC pastes at a lower dosage than that needed for OPC pastes. However, after 15 min CAC pastes with PCE admixtures lose their fluidity, an indication of the scant compatibility between these materials.

1. INTRODUCTION The chemistry and mineralogy of calcium aluminate cement (CAC) translate into certain differences over the Portlandbased binder (OPC). Indeed, with high early age strength (with 1-day values comparable to 28-day values in OPC) and high resistance to aggressive chemical and physical agents, CACs are particularly apt for aggressive environments. Hence their use in industrial flooring, where high strength and abrasion resistance are imperative, or in refractory concretes.1 The predominance of monocalcium aluminate (CA) phase in CAC, where it accounts for over 60% of the total composition by weight, is the main source of the chemical and mineralogical differences between it and OPC. Other mineralogical phases are also often present in CAC, however, including C3A, C12A7, CA2, C2S, and C2AS. The CA−water reaction yields hydration products that vary with temperature and relative humidity. The calcium aluminates (CAHx) that form at low temperatures (T < 15 °C) are essentially hexagonal, mostly in the form of CAH10, and metastable at ambient temperature. C2AH8 and AH3 form at temperatures from 15 to 70 °C, while C3AH6 and AH3 prevail at temperatures of over 70 °C.2 Over time, these hexagonal hydrates tend to evolve into stable cubic hydrates (C3AH6), slowly at under 25 °C and more quickly with rising temperatures. Attendant upon this conversion is a decline in initial mechanical strength, due essentially to the greater porosity resulting from the evaporation of the water released in the process. This change from a hexagonal to a cubic configuration is favored when the temperature and relative humidity rise.3,4 Strength reduction can be prevented by using water/cement ratios of under 0.4, although the substantial loss of concrete workability at such low ratios calls for the use of superplasticizers admixtures. These admixtures, anionic surfactants, adsorb onto cement particles, inducing electrostatic and/or steric repulsion or both and releasing the water trapped between the flocs. Their use © 2013 American Chemical Society

lowers the water demand for a given workability or improves workability for a given liquid/solid ratio.5−7 Despite the obvious benefits of the use of superplasticisers in cement systems, in practice their application occasionally generates anomalous or undesirable effects, such as paste segregation, low initial or early loss of workability, or excessive setting retardation, to name a few. Such symptoms are indicative of admixture-cement incompatibility. The superplasticisers most commonly used with OPC cements and concretes are based either on modified lignosulphonates, melamine, and naphthalene derivatives or polycarboxylate ether (PCE) synthetic admixtures. The highest performance is attained with the latter, which reduce water demand by up to 40%, inducing the highest increase of concrete mechanical properties and durability.8 The utility of some of these admixtures, however, particularly the melamine and naphthalene derivatives, used in calcium aluminate cements and concretes, is limited because the fluidity induced declines quickly while the hydration reactions are unduly retarded.4,9−11 Monosi et al.12,13 studied the behavior of a sodium tripolyphosphate (TPP) and a carboxylate- and ester-based (CAE) acrylic polymer in CAC−silica fume blends. These authors showed that while fluidity improved substantially with both admixtures, the effect disappeared quickly with TPP, whereas the loss was more gradual with the CAE admixture. Fryda et al.14 compared the behavior of a naphthalene-based and a PCE admixture in a CAC mortars, reporting that both raised fluidity, although the desired effect was attained with a lower dosage of PCE than of the naphthalene admixture. Ng and Plank15,16 studied the effect of PCEs admixtures on fluidity Received: Revised: Accepted: Published: 17323

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of CAC and OPC pastes. They concluded that more anionic PCEs disperse CAC less PCEs with low ionic charge, this effect of PCEs superplasticizers on CAC fluidity being opposite to that found for OPC pastes. However, PCE admixture behavior in CAC systems has been scantly studied, and the results reported in the literature to date are contradictory. While the factors affecting OPC−PCE compatibility are now well established,17−20 very few studies address CAC−PCE compatibility from the standpoint of paste, mortar, and concrete rheology. The present study was designed to explore and ascertain the interaction and compatibility between calcium aluminate cement and three PCE admixtures having different molecular structures, based on adsorption and rheological analyses.

Table 2. Chemical and Physical Characterization of Admixtures solids content (%) (UNE-EN 480-8) rotational viscosity (mPa·s) carboxylic groups/ester groups (C/E) Mw (D) Mn (D) PDI (polydispersity index) radius of gyration (calculated according to ref 23) (nm) Na content (ppm)

Table 1. Cements’ Mineralogical Composition (wt %)a OPC

a

62.5 8.3 4.0 11.9

(±0.2) (±0.5) (±0.2) (±0.2)

3.7 5.4 1.3 0.6 1.7

(±0.2) (±0.1) (±0.1) (±0.1) (±0.1)

0.5 (±0.1)

CAC

3.6 74.2 1.9 4.0

PC2

PC3

39.67 865.02 0.70 123000 55900 2.20 6.21

39.74 918.08 0.40 189000 78700 2.40 7.26

13200

8375

5625

(a) Determination of Superplasticizer Adsorption Isotherms on Cement Pastes. A 20 g portion of cement was mixed with 40 g of a solution containing a PCE admixture and stirred for 30 min at 25 °C. The liquid phase extracted by centrifugation was analyzed for total organic carbon content (TOC) on a Shimadzu TOC-VCSH/CSN TOC analyzer. The difference between the amount of PCE initially added and the amount detected in the liquid phase in terms of TOC was defined as the measure of admixture consumed. Flatt and Houst18 concluded that the total admixture present in cement pastes was either adsorbed onto cement particles or consumed in the formation of an organo-mineral phase. Consequently, given that adsorption isotherms do not distinguished between these two forms, the most accurate term for this fraction of superplasticizer is “consumed” admixture. (b) Effect of PCEs on Cement Suspension Zeta Potential. The effect of the different dosages of superplasticizer on the zeta potential of cement suspensions was analyzed with a Colloidal Dynamics Acoustosizer IIs. In these measurements, 30 g of binder were placed in 160 g of water (solid fraction in the suspension = 0.16) and magnetically stirred for 15 min. The zeta potential of the suspensions was determined in a measuring cell after 5-min ultrasound treatment. An automatic titrator was then used to add dosages of PCE admixture ranging from 0 to 7 mg polymer/g cement to these suspensions. The zeta potential was calculated by subtracting the background signal attributed to the pore solution from the readings. In this way the PCE-cement colloidal-chemical interaction in the cement suspensions was studied. (c) Effect of PCEs on OPC and CAC Paste Rheology. The liquid/cement ratio used in the rheological tests was 0.4 for the OPC pastes and 0.35 for the CAC pastes. At these values, the initial consistencies of the two pastes were similar. The rheology tests conducted included flow curves using a rotational viscosimeter and minislump, for which dosages of 0, 0.2, 0.4, and 1.2 mg polymer/g cement were added to the pastes. • Plastic viscosity and yield stress were determined on a Haake Rheowin Pro RV1 rotational viscometer fitted with a serrated cylindrical rotor. A 100 g portion of cement was added to the amount of water required to reach the respective liter per second ratio and mixed for 3 min with a blade stirrer at 600 rpm. The rheological test consisted of 1 min of preshear at 100 s−1 followed by a ramp-down to 10 s−1, a return to a shear rate of 100 s−1 for 2.5 min, and finally reduced over the same period of time to 10 s−1. • Minislump test: the polycarboxylate admixtures were added to the cement with the water. These pastes were

2. EXPERIMENTAL SECTION 2.1. Materials. The following materials were used in the present study: • A calcium aluminate cement (CAC) and, as the reference or control material, European standard EN 197-1:2011classified CEM I 42.5R (OPC) Portland cement. • Three poly(acrylic acid) derivatives, here labeled PC1, PC2, and PC3, as the PCE admixtures. Table 1 lists cements’ mineralogical composition, found by Rietveld analysis of the X-ray diffraction patterns and expressed in values normalized to 100% of the crystalline phases.

C3S C2S C3A C4AF CA C12A7 FeO CaCO3 CaSO4·2H2O CaSO4·1/2H2O CaSO4 alkaline sulfates Ca20Al26Mg3Si3O68 CaTiO3 other

PC1 39.94 432.86 1.20 61000 35000 1.74 4.96

(±0.1) (±0.2) (±0.1) (±0.1)

5.3 (±0.4) 9.6 (±0.1) 1.4 (±0.2)

Parentheses = standard deviation.

The analytical techniques used to characterize the admixtures included FTIR, FT-Raman, 1H and 13C NMR-MAS, GPC, and rotational viscosimetry.21,22 The major physical−chemical characteristics of the three PCEs are given in Table 2, along with their molecular weights. On the basis of the information provided by the manufacturers and the physical−chemical and structural characterization conducted, PC1, PC2, and PC3 were found to have the same side chain lengths (PEO chains = 5500 D) but different backbone chain lengths (PC1 > PC2 > PC3). The C/E ratios likewise differed, with C/E (carboxylic/ester groups) values of 1.20, 0.70, and 0.40, respectively, and a concomitant decrease in charge density from PC1 to PC3. 2.2. PCE Admixture Compatibility with OPC and CAC. The following tests were carried out: 17324

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Figure 1. Adsorption isotherms of the three PCEs on OPC and CAC cement pastes.

again PC1 and PC2 have a higher affinity than PC3. If we compare both cements, results confirm the lower affinity of CAC for PC1 and PC2 with respect to the OPC cement, while PC3 has a slightly higher affinity. 3.2. Effect of Polycarboxylate-Based Superplasticisers on Cement Suspension Zeta Potential. The zeta potential values obtained for CAC and OPC suspensions in the absence of chemical admixtures were positive in both cements and perceptibly higher in CAC, with values of +1.1 ± 0.4 for OPC and +6.7 ± 0.3 for CAC. Figure 2 shows the variation in the zeta potential values for OPC and CAC pastes after inclusion of the three PCE-based admixtures. Note that in all cases, the values were close to 0 mV, confirming that steric repulsion was the prevalent superplasticising mechanism, while the electrostatic contribution was negligible in the PCE admixtures used in this study.25 As a rule, the OPC zeta potential was unaltered at dosages of over 0.5 mg of polymer per gram of cement. In the CAC pastes, the zeta potential flattened at much higher dosages (6 mg of polymer per gram of cement). The zeta potential values were low and not observed to vary with the PCE admixture used in either type of cement. 3.3. Admixture Effect on OPC and CAC Paste Rheology. The effect of PCE on OPC and CAC paste yield stress and plastic viscosity is showed in Figure 3. In Table 4, PCE dosages and percentage of yield stress reduction induced in the two cements are compared to the values for a paste with no admixtures. Since dosages of 1.2 mg polymer/g of cement induced segregation in the CAC pastes, these values are not shown on the graph. In these cases, trials were run with additional dosages of 0.2 mg polymer per gram of CAC. Further to the findings for the OPC pastes (see Figure 3), admixtures PC1 and PC2 lowered plastic viscosity (by 26 and 48%, respectively) more intensely with rising dosages of admixture. Viscosity was modified much more substantially by these two admixtures than by PC3, which induced no change in this parameter. The plastic viscosity of CAC paste was not modified by dosages of 0.2 mg of polymer/g of cement. At dosages of 0.4 mg polymer/g of cement, however, viscosity declined significantly, by 29% in the presence of PC2 and even more steeply with PC3 (37%). The yield stress values for the OPC pastes showed that paste fluidity was affected similarly by admixtures PC1 and PC2 and much more intensely than by PC3 (see Table 4 and Figure 3). Dosages of 0.2 mg PC1 or PC2/g of cement failed to lower the

mixed for 3 min in a laboratory mixer and placed in 190 mm × 81 mm × 572 mm truncated mold. The mold was subsequently removed, and the samples were subsequently shaken 10 times on a flow table. The demolded samples and the arithmetic mean of four measurements of the resulting diameter, taken in different directions, is given as the final value. The test was conducted at 5, 15, 30, and 60 min after the pastes were mixed.24

3. RESULTS 3.1. Determination of Superplasticizer Adsorption Isotherms on Cement Suspensions. The isotherms for superplasticiser adsorption on the OPC and CAC in suspension are reproduced in Figure 1. Here the amount of polymer added per gram of solid was plotted against the amount consumed and the resulting curve was fitted to an exponential equation. The plateau values and the slope of the linear range of the adsorption isotherm for each curve are listed in Table 3. Table 3. Adsorption Data (“Plateau Values”) of Three Superplasticizers on Two Cement Pastes PC1 PC2 PC3

plateau value (mg PCE/g cement) slope plateau value (mg PCE/g cement) slope plateau value (mg PCE/g cement) slope

OPC

CAC

1.83 0.91 1.45 0.98 0.61 0.65

1.55 0.88 1.54 0.82 1.23 0.70

Figure 1 shows that in OPC pastes, total polymer adsorption was greatest in PC1 followed by PC2 and smallest in PC3. The C/E ratio of the admixtures followed the same pattern (see Table 2), confirming that the higher the charge density of a polymer, the higher is the adsorption.25 The differences in superplasticiser adsorption on the CAC cement pastes were smaller. Moreover, for PC1 the value was about 15% lower in CAC than in OPC; for PC2, adsorption was essentially the same in the two cement pastes; and for admixture PC3, adsorption was 101% higher in CAC than in the Portland pastes. Slope of the linear range of the adsorption isotherm is related to affinity between the polymers and the two cements. In the case of the OPC cement, PC1 and PC2 have a similar slope while PC3 shows an affinity 30% lower. In the case of CAC, 17325

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Figure 2. OPC and CAC paste zeta potential values in the presence of PCEs.

practically constant throughout the 60-min trial in all the OPC pastes and at all admixture dosages. In the CAC pastes, however, at dosages of 0.4 mg of polymer/g of cement, all the admixtures, including PC3, induced a considerable rise (64−88%) in fluidity in the first 5− 15 min, although this effect disappeared very quickly. In stark contrast to the findings for OPC, the most significant increases in fluidity (88%) were attained with PC3.

4. DISCUSSION The obtained results, allow us to assess the compatibility between CAC pastes and the three structurally different PCE admixtures. The discussion was based on admixture adsorption on CAC pastes and the effect of the superplasticizers on cement paste fluidity and rheological behavior, compared to OPC results. The adsorption values for each admixture and dosage are plotted in Figure 5 against the normalized yield stress value for each admixture at that dosage. This normalized yield stress depicts the reduction of this rheological parameter with respect to pastes with no admixtures. At the two dosages studied, the differences in both adsorption values and fluidizing effect induced in the OPC pastes between admixtures PC1 and PC2 (left side in Figure 5) were smaller with respect to the differences induced by PC3. Admixture PC3, with a low C/E ratio, exhibited lower adsorption values, lower affinity for OPC, and a lesser effect on yield stress, an indication that side chain density conditioned admixture adsorption and the resulting dispersion in the OPC pastes.25 In the CAC pastes (right side in Figure 5), the three PCEs show higher fluidizing properties than in OPC pastes. In addition, the three admixtures behaved similarly at each of the two dosages used, with no substantial differences in adsorption or fluidization. In these pastes, admixture PC3 was the most effective at both dosages. This finding was confirmed by minislump test (see Figure 4), in which this admixture induced a steep rise in fluidity (88%). These results show that the C/E ratio does not appear to be as relevant in CAC as in OPC pastes These results concurred partially with the findings reported by Ng and Plank,15 whose data showed that, contrary to what was observed in OPC systems, the admixture with the lowest ionic charge induced the greatest rises in fluidity in CAC pastes. These authors attributed that behavior to the intercalation of the PCE admixtures in-between the layers of the calcium

Figure 3. Plastic viscosity versus yield stress variations in OPC and CAC pastes containing admixtures.

Table 4. Yield Stress Reduction for Each Cement and Admixture Dosagea PC1

PC2

PC3

admixture dosage (mg polymer/g cement)

OPC

CAC

0.2 0.4 1.2 0.2 0.4 1.2 0.2 0.4 1.2

ndb 45% 95% ndb 47% 92% ndb 21% 25%

6% 68% ndb 0% 52% ndb 19% 63% ndb

a

Shown as percent of yield stress in a paste with no admixtures. bNot determined.

yield stress values in CAC pastes, whereas at that same dosage, PC3 induced a 19% decline. At dosages of 0.4 mg polymer/g of cement, the reduction was steeper in all cases than at the same dosage in OPC pastes (see Table 4). The minislump test results for OPC and CAC pastes are shown in Figure 4. In the OPC pastes, at 0.4 mg polymer/g of cement, admixtures PC1 and PC2 induced much greater fluidity (with rises of around 11%) than PC3 (4%) and this difference widened at 1.2 mg polymer/g of cement: 30% for PC1 and PC2 compared to 7% for PC3. Fluidity remained 17326

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Figure 4. Minislump test values for CAC and OPC pastes in the presence and absence of PCE admixtures.

Figure 5. Admixture adsorption versus normalized yield stress in OPC and CAC pastes.

presence of all the admixtures in OPC and CAC suspensions led to zeta potential values close to zero, confirming that the electrostatic contribution to cement particles dispersion was negligible and that steric repulsion was the prevalent mechanism.26 However, CAC suspensions were found to have a zeta potential of approximately +6.7 mV, a value substantially more positive than the zeta potential for OPC suspensions (+1.1 mV). This more positive zeta potential would explain the differences in admixture (regardless of the C/E ratio) adsorption conformation on CAC and OPC particles and the larger number of anchorage points in the former. When adsorbing onto CAC particles, PCEs may be regarded to be in more of a train-type than the loop-type conformation27 most likely adopted by the admixtures when adsorbing onto OPC particles. A train-type conformation would provide for more

aluminate hydrates resulting from CAC hydration, forming an organo-mineral phase. The higher the ionic charge of the admixture, the greater would be the portion so intercalated, as a result, less polymer would be available in the system, and more polymer would be needed to induce a given fluidity. In the present study, however, the findings show no significant differences between the adsorption values for admixtures with different C/E ratios and consequently different charge density. However, the PCEs used in the present study have a different molecular structure that those used in ref 15. In addition to possible amount of PCE admixture intercalating in aluminate hydrates such as AFm phases, other differences in the interaction and behavior of CAC and OPC pastes with PCE admixtures can be found: (i) Difference in the Surface Charge on CAC and OPC Particles. According to the results shown in Figure 2, the 17327

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Figure 6. PCE admixture adsorption and conformation in (a) OPC (loop conformation) and (b) calcium aluminate cements (train conformation).

effective adsorption, lowering the amount of admixture needed to fully cover the surface, as the above adsorption studies showed. Figure 6 depicts a possible scheme of admixture conformations, previously proposed for OPC and CAC particles. However, further studies are needed to establish the different mechanisms of absorption of PCE on OPC and CAC particles. (ii) Difference in the Ionic Strength of CAC and OPC Paste Aqueous Solutions. Previous studies have confirmed that the ionic strength of the aqueous phase of CAC cements (with lower Si4+ and SO42− ion and alkali contents) is lower than in OPC pastes.28,29 Yamada et al. and Comparet30,31 confirmed that a rise in ionic strength in the medium led to a contraction of the ether side chains, lessening the fluidizing power of the admixtures. Lower ionic strength would therefore reduce the contraction of larger ether side chains and consequently provide for greater steric repulsion. This could partially explain the greater fluidizing effect observed in CAC pastes. While all the admixtures induced greater initial fluidity in the CAC pastes, the minislump tests showed that this effect disappeared very quickly (after 15 min). That finding did not concur with previous reports, according to which the high CAC fluidity induced by PCE admixtures was maintained over time.14 This quick loss of fluidity observed could be probably due to the fast reactivity of CAC, much faster than OPC. At the low dosages of PCEs used in CAC pastes (0.2 and 0.4 mg of polymer/g of cement), initially PCE is mostly adsorbed onto CAC surface and not much amount of PCE remains in solution (according to the adsorption results). In this way, the PCEs induce an initial dispersion of the cement particles and the observed increase of fluidity. However, the quick reaction of CAC and formation of new hydration products, including AFm phase that could intercalate PCEs, may conduct to the fast agglomeration and loss of fluidity observed. This explanation merits further future study to be confirmed.

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reason that the admixtures (regardless of their C/E ratio) adsorb onto cement particles with a possible different conformation than in OPC pastes. 4. The lower ionic strength in the CAC paste aqueous phase (with lower Si4+ and SO42− and alkali contents) might reinforce the intense initial fluidization induced by the PCE admixtures in this cement. The lower the ionic strength in the medium, the weaker is the contraction of the side chains, providing for greater steric repulsion and explaining the high fluidization observed. 5. PCE admixtures improve fluidity in CAC pastes but only briefly (for around 15 min), an indication of the scant compatibility between these two materials.

ASSOCIATED CONTENT

S Supporting Information *

Chemical composition, Blaine fineness, and particle size distribution of cements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +34 91 302 04 40. Fax: +34 91 302 07 00. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded under Project BIA2010-15516. The authors wish to thank P. Rivilla, A. Gil, and F. Morales for their assistance with the tests.

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REFERENCES

(1) Scrivener, K. L.; Cabiron, J. L.; Letourneux, R. High-performance concretes from calcium aluminate cements. Cem. Concr. Res. 1999, 29, 1215. (2) Rivas-Mercury, J. M.; De Aza, A. H.; Turrillas, X.; Pena, P. Hidratación de los cementos de aluminato de calcio. Bol. Soc. Esp. Ceram. V 2003, 42, 269. (3) Vázquez, T.; Triviño, F.; Ruiz de Gauna, A. Estudio de las transformaciones del cemento aluminoso hidratado. Influencia del ́ anhidrido carbónico, temperatura, humedad y adición de caliza en polvo. In Monografiá del Instituto de Ciencias de la Construcción Eduardo Torroja CSIC; Madrid, 1976; Vol. 334. (4) Scrivener, K. L.; Capmas, A. Calcium aluminate cements. In Lea’s Chemistry of Cement and Concrete; Hewlett, P.C., Ed.; Elsevier: New York, 1998, p 713. (5) Alonso, M. M.; Palacios, M.; Puertas, F.; De la Torre, A. G.; Aranda, M. A. G. Effect of polycarboxylate admixture structure on cement paste rheology. Mater. Constr. (Madrid, Spain) 2007, 57 (286), 65. (6) Ghorab, H. Y.; Kenawi, I. M.; Abdel All, Z. G. Interaction between cements with different composition and superplasticizers. Mater. Constr. (Madrid, Spain) 2012, 62 (307), 359.

5. CONCLUSIONS The conclusions drawn from the foregoing are as follows: 1. In OPC-type cements, the C/E ratio conditions admixture adsorption on cement particles, with greater adsorption observed at higher C/E ratios and substantial differences between plateau values. In CAC suspensions, in contrast, the C/E ratio is less relevant, for similar adsorption values were recorded for the three studied admixtures. 2. The presence of PCE admixtures lowers yield stress in CAC pastes at a lower dosage that needed for OPC pastes. The decline in cement paste yield stress strongly depends on the dosage of PCE and C/E ratio. 3. CAC pastes exhibit a higher zeta potential (+6.7 ± 0.3 mV) than OPC pastes (+1.1 ± 0.4 mV). This more positive zeta potential in the CAC pastes may be the 17328

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dx.doi.org/10.1021/ie401615t | Ind. Eng. Chem. Res. 2013, 52, 17323−17329