Slump improvement mechanism of alkalies in PNS superplasticized cement pastes

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1 Materials and Structures/Matériaux et Constructions, Vol. 33, July 2000, pp Slump improvement mechanism of alkalies in PNS superplasticized cement pastes B.-G. Kim, S. P. Jiang and P.-C. Aïtcin Department of Civil Engineering, University of Sherbrooke, Sherbrooke, QC, Canada, J1K 2R1 SCIENTIFIC REPORTS Paper received: June 25, 1999; Paper accepted: November 5, 1999 A B S T R A C T Some cement/superplasticizer combinations which result in rapid slump loss can be improved by the addition of alkali sulfates when using polynaphthalene or polymelamine sulfonates (PNS or PMS). It seems that for a given cement there exists an optimal content of soluble alkalies with sulfonated based superplasticizers. The major effect of the alkalies is to reduce the adsorption of PNS or PMS molecules by providing the necessary SO 4-2 ions that are necessary to neutralize the active sites of the interstitial phase of the cement. The second effect of the addition of alkalies is to increase the ph of the interstitial solution of the mortar or of the concrete which seems to favor the non-adsorption of the superplasticizer molecules on the C 3 A. R É S U M É Certaines combinaisons ciment/superplastifiant qui présentent des pertes d affaissement très rapides peuvent être corrigées par des additions d alcalins dans le cas où l on utilise des polynaphthalènes ou polymélamines sulfonates (PNS ou PMS). Il semblerait qu il existe d ailleurs une teneur optimale en alcalis solubles dans le cas où l on utilise un PNS ou un PMS. L effet principal des alcalis serait de réduire l adsorption des molécules de PNS ou PMS en fournissant les ions SO 4-2 nécessaires à la neutralisation des sites actifs de la phase intersticielle du ciment. Un second effet de l introduction de ces alcalis est d augmenter aussi le ph de la solution intersticielle du mortier ou du béton frais qui favorise aussi la non-adsorption des molécules de superplastifiant sur le C 3 A. 1. INTRODUCTION Nowadays, the construction industry mostly uses commercial superplasticizers based on polynaphthalene sulfonate (PNS) or polymelamine sulfonate (PMS). The addition of these superplasticizers to Portland cement concrete usually greatly increases the fluidity of fresh concrete. However, the dispersing effect of these superplasticizers can be of short duration and can result in a rapid slump loss depending on the particular combination of cement and superplasticizer used. Recently, a practical method, consisting in an addition of alkali sulfate, has been found efficient to solve slump loss problems in some superplasticized high-performance concretes [1, 2]. It has also been seen that there seems to be an optimum soluble alkali content in cements, i.e Na 2 O eq. in terms of slump and slump loss of cement pastes [3]. The effect of alkali sulfate on the fluidity of cement pastes has been explained in the literature [4-8] advocating that alkali sulfate inhibits the adsorption of PNS superplasticizer on the interstitial phases by rapidly producing sulfate ions, and then effectively reducing the positive zeta potential of C 3 A and C 4 AF by competition of sulfate ions and the sulfonate group of PNS superplasticizer [4-8]. In addition, the role of free PNS remaining in solution and its contribution to fluidizing of cement pastes has been explained [8]. On the other hand, some interesting results have been found in the literature by Luke et al. [9] showing that the amount of PNS superplasticizer adsorbed by a C 3 A/Gypsum system is significantly reduced in an alkaline solution (0.25 M KOH, M NaOH). At 5 minutes, the amount of PNS superplasticizer adsorbed is reduced to 50% of that measured on a pure water system. However, the effect of alkaline solution on the fluidity of cement pastes was not investigated in the above study. The aim of this study is to elucidate how alkalies influence the PNS adsorption and the fluidity of superplasticized cement pastes. Furthermore, this paper will discuss the slump improvement following the addition of alkalies in superplasticized cement pastes /00 RILEM 363

2 Materials and Structures/Matériaux et Constructions, Vol. 33, July MATERIALS AND TESTS 2.1 Portland cement The chemical/phase compositions and fineness of the cements used in this study are presented in Table 1. Cements E and F are CSA Type 10 (ASTM Type I with a 5% max. limestone filler content), while Cements L and N are ASTM Type I. These four cements present a wide range of C 3 A composition, their C 3 A contents vary from 6% for Cement F up to 11% for Cement N, according to Bogue composition. The form of C 3 A is only cubic for Cements E, F, and N while a mixture form of cubic and orthorhombic for Cement L has been found by XRD analysis. The Na 2 O equivalent contents of these cements vary from 0.31% for Cement N to 0.92% for Cement E. 2.2 Superplasticizer A pure sodium salt of polynaphthalene sulfonate (PNS) superplasticizer was used in this investigation. It is an aqueous solution having 41% solids. Its ph (10% solution) is 7.85; its sulfate content is 1.08%; its viscosity is 65 cps (22 C); and its specific gravity is Paste mixing procedure The cement pastes were prepared at a constant water/cement ratio (W/C) of Superplasticizer was added to the mixing water, then the solution was added to the cement. The cement pastes were mixed manually for 1.5 minutes, then with a high-speed hand-held mixer for 2.5 minutes in order to obtain a well-dispersed paste. The mixing procedure was carried out under controlled temperature of 25 ± 1 C. The superplasticizer dosage is expressed as dry weight (weight of PNS superplasticizer solids relative to the weight of cement): 0.6% for Cement E, F, and L; 1.0% for Cement N. 2.4 Mini slump The mini-slump test [10] was used to assess the effect of the different PNS superplasticizers on cement-paste fluidity. The procedure involves transferring the cement paste in a minicone, then lifting the cone smoothly and quickly. The area of the paste spread on a polyethylene plate is measured and expressed in cm Filtering system A pressure filtering system was applied to extract the interstitial solution from cement pastes. Nitrogen gas pressure of 345 kpa (50 psi) and a membrane of 0.45 µm diameter were applied in the filtering process. Samples were taken at different times (2, 5, 15, and 30 minutes). Table 1 Chemical and phase composition of the cements Chemical Cement Composition (%) E F L N SiO Al 2 O Fe 2 O CaO MgO SO K 2 O Na 2 O Na 2 O eq Na 2 O soluble eq.* C 3 S C 2 S C 3 A C 4 AF Blaine (m 2 /kg) *Measured by ICP at W/C=0.35 at 30 minutes after mixing. 2.6 Measurement of ph After the filtering process, the ph was directly measured by putting the sensor of an Orion ph meter in the filtrate. The ph meter was calibrated by using two buffer solutions at ph 10 and Adsorption measurement PNS adsorption on the surface of cement particles was evaluated by measuring the amount of PNS in the solution extracted from the fresh cement pastes, expressed as a percentage (mass of PNS present in the extracted solution relative to the mass of added PNS superplasticizer). The original filtered solution was diluted with deionized water to obtain adequate PNS concentration. The PNS concentration in solution was measured at the peak absorbance wavelength between 276 and 294 with a Hewlett-Packard diode array UVvisible spectrometer. 2.8 Measurement of sulfate ion concentration in solution The total ionic sulfur (from the sulfate ions in cement and the sulfonate ions of the PNS) concentration in solution in fresh cement pastes was measured using the ICP (Inductivity Coupled Plasma) spectrometry. The measurements were carried out 30 minutes after mixing. The original filtrate solution obtained by the system described above was diluted to 1:150 with 5% HCl. The sulfate ion concentration from the cement is calculated by subtracting the ionic sulfur from the PNS superplasticizer. 364

3 Kim, Jiang, Aïtcin 3. RESULTS 3.1 Effect of alkalies addition on mini slump It is clearly seen in Fig. 1 that cements E and F are compatible with the PNS superplasticizer while cements L and N are incompatible when considering minislump measurements and as shown in another paper by the Marsh cone test [2]. These results show that the cement pastes made with cement E and F including a given dosage of PNS superplasticizer have a high initial mini-slump value and maintain this high value for 90 minutes. On the contrary, in the case of cement L, the initial slump is maintained for 15 minutes but the slump rapidly decreases thereafter. The cement paste made with cement N has a relatively low slump even at 2 minutes although the dosage of superplasticizer is higher than that of the three other cements. In order to investigate the effect of an addition of alkali hydroxide on the fluidity of cement pastes made with cements N and L, mini slump tests were done on grouts where the amount of NaOH was increased and the results were compared with the addition of alkali sulfate and hemihydrate, as presented in Figs. 2a and 2b. The comparison shows that sodium hydroxide is as effective to reduce the slump loss as sodium sulfate. Moreover, a relatively small amount of sodium hydroxide influences the fluidity of cement pastes more than sodium sulfate, although the mole amounts were considered. As shown in Figs. 2a and 2b, the addition of hemihydrate is less effective to increase the fluidity of cement pastes. The amount of PNS adsorbed was measured as a function of time in the absence or presence of alkalies and the results are presented in Fig. 3. First of all, in the absence of alkalies, the amount of PNS adsorbed in Cement N rapidly increases up to 70 % at 2 minutes, but after 5 minutes, the increase in the amount of PNS adsorbed is not significant. On the contrary in the case of Cement L, the amount of PNS adsorbed continously increases as a function of time in the absence of alkalies. The tendency of PNS adsorption is not changed by the addition of alkalies in both cements as shown in Fig. 3. These results seem to be related to the form of C 3 A as well as the amount of C 3 A and C 4 AF in cement. It has Fig. 1 Mini-slump of superplasticized cement pastes vs hydration time. The dosage of superplasticizer: 0.6 % for cement E, F, and L; 1.0 % for cement N. Fig. 2a The effect of additives on the mini-slump of Cement N. a) Na 2 SO 4, b) NaOH, c) CaSO 4 1/2H 2 O addition. 365

4 Materials and Structures/Matériaux et Constructions, Vol. 33, July 2000 Table 2 Solubility of additives Product Solubility (g/l) at 25 C Sodium Sulfate 279 Sodium Hydroxide 500 Hemihydrate α (Ca SO 4 1/2H 2 O) 6.20 Hemihydrate β 8.15 Futhermore, when compared to hemihydrate, alkalies such as sodium sulfate and sodium hydroxide, significantly reduce the amount of PNS adsorbed on cement particles, probably because of much higher solubility of alkalies as shown in Table Effect of SO 3 ion concentration on the amount of PNS adsorbed on cement particles As shown in Fig. 4, an inverse relationship between the SO 3 concentration and the amount of PNS adsorbed at 30 minutes was found in both cements. It has been found that sodium sulfate rapidly dissolves sulfate ions, which increase when the dosage of sodium sulfate increases. However, calcium sulfate does not decrease significantly the amount of PNS adsorbed because it does not rapidly increase the concentration of SO 4 2- ions. In the case of alkali hydroxides such as sodium hydroxide and potassium hydroxide, it has been found that they also increase the SO 4 2- concentration. It seems that the addition of alkali hydroxide increases the solubility of calcium sulfate present in Portland cement [12]. 3.3 Effect of alkalies addition on ph of cement pastes Fig. 2b The effect of additives on the mini-slump of Cement L. a) Na 2 SO 4, b) NaOH, c) CaSO 4 1/2H 2 O addition. been found by Nawa et al. [4] that the adsorption rate of C 4 AF is lower than that of C 3 A. Moreover, the cubic form of C 3 A is more reactive than the orthorhombic form [11]. The tendency of adsorption in Cement E and F is similar to that of Cement N although the magnitude of the amount of PNS adsorbed in Cement E and F is lower than in Cement N. In order to see the ph difference in compatible (cement E and F) and incompatible (cement L and N) cements, the ph was measured with or without PNS superplasticizer as shown in Fig. 5. These results show that compatible cements (cement E and F) have higher ph values than incompatible cements (cement L and N) because of their higher alkalies content [13]. The difference in ph between compatible and incompatible cements is about in the cement pastes under study. In all of these, the ph increases with time. The addition of PNS superplasticizer does not affect the ph of the cement pastes because its ph is already neutralized at The effect of alkalies on ph of cement pastes was investigated and the results presented in Fig. 6. It was found that the measured ph value depends not only on the type of addition but also on its amount. The effect of additives on the ph value is as follows: 0.8 % NaSO 4 (NS) > 0.2 % NaOH (NH)> 0.8% CaSO 4 1/2 H 2 O (CS) for both cement pastes. The relationship between the ph and the amount of PNS adsorbed at 30 minutes following the addition is presented in Fig. 7. A close inverse rela- 366

5 Kim, Jiang, Aïtcin adsorbed is progressively reduced when the ph value of the interstitial solution increases through the addition of alkalies. Fig. 3 Effect of additives on the PNS adsorption. NS:Na 2 SO 4, NH:NaOH, CS:CaSO 4 1/2H 2 O. Fig. 4 Relationship between the SO 3 content and the amount of PNS adsorbed at 30 minutes by the addition of additives. NS:Na 2 SO 4, NH:NaOH, KH:KOH, CS:CaSO 4 1/2H 2 O. Fig. 5 ph vs hydration time in four different cements. E1:water+cement, E2:water+cement+PNS and the same meaning in the other cements. tionship between the ph value and the adsorption of PNS can also be established, as shown in Fig. 7. It can be seen that for a given cement paste the amount of PNS DISCUSSION Slump improvement mechanism of alkalies in superplasticized cement pastes In a previous study by the authors [8], it was found that incompatible cements have a tendency to rapidly adsorb a high amount of PNS superplasticizer, while in the case of compatible cements, a high amount of a PNS superplasticizer remains in solution after a small initial adsorption. For a given cement, it seems that the adsorption capacity of a cement is mainly controlled by the soluble alkali content when using a PNS superplasticizer. Futhermore, it was found that the addition of alkali sulfate was a very effective method to reduce the amount of PNS adsorbed on cement particles and to increase the fluidity of cement pastes. However, the working mechanism of alkalies on reducing the amount of PNS adsorbed was not fully explained. Nawa et al. [5] have suggested the SO 4 2- /PNS competition mechanism. That is, both the polymer anion of the PNS superplasticizer and the SO 4 2- ion have negative charges, both may be adsorbed by positively charged surfaces on early C 3 A hydration products, or incorporated into such products, almost immediately after the beginning of hydration. This mechanism has been proven by experimental results [4-6] showing that the adsorption of the PNS on C 3 A was greatly reduced in the presence of Na 2 SO 4. The presence of sulfate ions, derived from either calcium sulfate or alkali sulfate, decreases the adsorption of PNS on C 3 A and C 4 AF but increases the adsorption on C 3 S [4]. In this study, it has also been found that when the concentration of sulfate ion is increased by the addition of different alkalies, the amount of PNS adsorbed decreases, as shown in Fig. 4. Moreover, alkali hydroxides such as sodium hydroxide and potassium hydroxide, as well as sodium sulfate, increase the concentration of sulfate ion by increasing the solubility of calcium sulfate present in cement. On the other hand, the influence of the ph on the adsorption affinity of PNS superplasticizer has been

6 Materials and Structures/Matériaux et Constructions, Vol. 33, July 2000 Fig. 6 Effect of additives on the ph in cement N and cement L. Fig. 7 Relationship between the ph and the amount of PNS adsorbed at 30 minutes by the addition of additives. NS:Na 2 SO 4, NH:NaOH, KH:KOH, CS:CaSO 4 1/2H 2 O. advocated in the literature [14, 15]. The adsorption of PNS on TiO 2 is strongly dependent on the ph of the suspension. For example, at ph 3, TiO 2 bears a strong positive surface charge and the adsorption is maximum. However, the surface charge of TiO 2 becomes negative when the ph is over the zero-point of zeta-potential, and the zeta-potential depends on the ph of the cement suspension [16-19]. As shown in Fig. 5, the interstitial solution of compatible cement pastes (cement E and F) have a higher ph value than the incompatible ones (cement L and N). The difference in ph is on the order of As a mater of fact, the difference in ph results from the difference in alkali content because the alkalies increase the ph as well as sulfate ion concentration in cement paste systems. In order to understand the effect of the ph on adsorption, it is necessary to study it in a pure system not including sulfate ions. Tadros et al. [18] examined, therefore, the dependence of the surface charges upon the ph using pure C 3 A with aqueous solutions not including sulfate ions. They have found that the surface charges and the zeta potential of the C 3 A is significantly dependent on the ph of the solution. Their study showed that the zeta potential of C 3 A is positive above the zero-point 368 of the zeta potential, i.e. ph = 12.2 but that it goes down very sharply when approaching the zero-point. For example, the positive values of zeta-potential of C 3 A are approximately 57, 50, 44, 34, 26, 19, 10, 0 mv at 8, 10, 11, 11.7, 11.9, 12, 12.1, 12.2 ph values, respectively. These results indicate that a small difference of ph near the zero-point of zeta-potential could significantly affect the amount of PNS adsorbed on cement particles. Experimental results obtained in this study, presented in Fig. 6, showed that the addition of alkalies rapidly increases the ph of the interstitial solution. The dependency of the amount of PNS adsorbed on ph in solution is also well proven in Fig. 7. In conclusion, the fluidizing mechanism of superplasticized cement pastes is closely related to the amount of PNS adsorbed on cement particles, especially on the positively charged aluminate phase, because the adsorption rate of PNS on pure C 3 A and C 4 AF was found to be much larger than that adsorbed on pure C 3 S [5, 20]. Moreover, the amount of PNS adsorbed on the aluminate phase is mainly determined by the alkalies content, especially soluble alkalies content [2] that rapidly increase not only the concentration of sulfate ions but also the ph value of the interstitial solution. 5. CONCLUSION Alkalies can be used to increase the fluidity of a superplasticized cement paste by decreasing the amount of PNS adsorbed on aluminate phase, allowing more PNS to remain in the solution. The working mechanisms of alkali sulfate are as follows: Rapidly increasing the ph in the interstitial solution - a small increase in ph can significantly affect the amount of PNS adsorbed because at the near zero-point of zetapotential, the zeta-potential value is significantly changed by a small difference in ph. Rapidly increasing the concentration of sulfate ions - when they rapidly dissolve, the amount of PNS adsorbed is significantly reduced by being preferentially adsorbed on the positive surface, such as C 3 A and C 4 AF, and

7 Kim, Jiang, Aïtcin reducing the positive zeta-potential value. ACKNOWLEDGMENTS The authors would like to thank Kyunggi Chemicals Ind., Co. Ltd. of Korea for its financial support and A. Lemieux, Department of Chemistry, University of Sherbrooke for carrying out the ICP analysis. REFERENCES [1] Jiang, S. P., Kim, B.-G. and Aïtcin, P.-C., Some practical solutions dealing with cement and superplasticizer compatibility, Proceedings of the 4th Beijing International Symposium on Cement and Concrete, Beijing, October, 1998, Vol.1, [2] Jiang S. P., Kim B.-G. and Aïtcin P.-C., Importance of adequate soluble alkali content to ensure cement/superplasticizer compatibility, Cem. Concr. Res. 29 (1) (1999) [3] Jiang, S. P., Kim, B.-G. and Aïtcin, P.-C., A practical method to solve slump loss problem in superplasticized high-performance concrete, Cem.Concr. Aggre. (June 2000). [4] Nawa, T. and Eguchi, H., Effect of cement characteristics on the fluidity of cement paste containing an organic admixture, Proceedings of the 9th Intl. Congress on the Chemistry of Cement, New Delhi, 1992, Vol. 4, [5] Nawa, T. and Eguchi, H., Effect of sulfate on adsorption behavior of superplasticizer, 43th CAJ Proceedings of Cement and Concrete, 1989, [6] Nawa, T., Eguchi, H. and Fukaya, Y., Effect of alkali sulfate on the rheological behavior of cement paste containing a superplasticizer, Proceedings of the 3rd CANMET/ACI Intl. Conf. on Superplasticizers and Other Chemical Admixtures in Concrete, Ottawa, 1989, (ACI SP 119). [7] Yamada, K., Hanehara, S. and Honma, K., The effect of naphthalene sulfonate type and polycarboxylate type superplasticizers on the fluidity of belite-rich cement concrete, Self-Compacting Concrete Workshop, Kochi, August, [8] Kim, B.-G., Jiang, S. P. and Aïtcin, P.-C., The adsorption behavior of PNS superplasticizer and its relation to fluidity of cement paste, Cem. Concr. Res. (1999) (Submitted for publication). [9] Luke, K. and Aïtcin, P.-C., Effect of superplasticizer on ettringite formation, Advances in cementious Materials 16 (1990) (American Ceramic Society) [10] Kantro, D. L., Influence of water reducing admixtures on the properties of cement pastes - a miniature slump test, Cem. Concr. Aggre. 2 (2) (1980) [11] Regourd, M., Cristallisation et réactivité de l aluminate tricalcique dans les ciments Portland, Il Cemento. 3 (1978) [12] Bombled, J. P., Influence of sulfates on the rheological behavior of cement pastes and on their evolution Proceedings of the 7th Intl. Congress on the Chemistry of Cement, Paris, 1980, Vol. III, VI164 VI169. [13] Rechenberg, W. and Sprung, S., Composition of the solution in the hydration of cement, Cem. Concr. Res. 13 (1) (1983) [14] Nkinamubanzi, P.-C., Effet des dispersants polymériques (superplastifiants) sur les propriétés des suspensions concentrées et des pâtes de ciment, Ph.D dissertation, Université de Sherbrooke, 1993 (in French). [15] Pierre, A., Carouille, C., Lamarche, J. M., Foissy, A. and Mercier, R., Adsorption d un polycondensat d acide naphthaléne sulfonique (PNS) et de formaldéhyde sur le dioxide de titane Cem. Concr. Res. 18 (1988) [16] Nagele, E., The zeta-potential of cement. Cem. Concr. Res. 15 (1985) [17] Nagele, E., The zeta-potential of cement, Part II: Effect of phvalue, Cem. Concr. Res. 16 (1986) [18] Tadros, M. E., Jackson, W. Y. and Skalny, J., Study of the dissolution and electrokinetic behavior of tricalcium aluminate, in Colloid and Interface Science, (Kevker M, 1976). [19] Andersen, P. J., Roy, D. M. and Gaidis, J. M., The effect of adsorption of superplasticizers on the surface of cement, Cem. Concr. Res. 17 (1987) [20] Ramanchandran, V. S., Interaction of chemical admixtures in the cement-water system, Il Cemento 83 (1) (1986)