Adsorption of comb type superplasticizers on calcium carbonate in alkaline medium in presence of electrolytes and rheological properties

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1 Indian Journal of Chemical Technology Vol. 15, July 2008, pp Adsorption of comb type superplasticizers on calcium carbonate in alkaline medium in presence of electrolytes and rheological properties Sarita Rai & N S Gajbhiye* Department of Chemistry, Indian Institute of Technology, Kanpur , India nsg@iitk.ac.in Received 1 October 2007; revised 8 May 2008 Adsorption behaviour of polycarboxylate type superplasticizers (PC) on CaCO 3 suspension in a basic medium has been studied in the presence of NaCl, Na 2 SO 4 and CaCl 2 having same ionic strength. The amount of PC adsorbed was found out by determining the amount of PC present in supernatant solution with the help of total organic analyzer. It is found that the adsorption decreased when the POE chain length decreased. The adsorption was increased in presence of CaCl 2 while NaCl and Na 2 SO 4 decreased the adsorption. The adsorption is not a competitive process but depends on ionic strength. Rheological measurements of suspensions have also shown that lower the POE chain length, higher the viscosity of the CaCO 3 suspension. Keywords: Polycarboxylate, Superplasticizer, Adsorption, Cement Superplasticizers are essential ingradients for the production of high performance concrete which includes high strength and high fluidity concrete. In recent years a new class of superplasticizers often referred to as polycarboxylate based comb polymers, has emerged as having the unique ability to improve the quality and economics of concrete products 1-4. The polycarboxylate (PC) type superplasticizers (SP) contain polycarboxylic acid backbone with attached polyoxyalkylene groups. PC based superplasticizers have gained much more acceptance in cement industry due to their higher efficiency in reducing water /cement ratio, and preventing slump loss in comparison with the traditional superplasticizers based on naphthalene sulfonate and melamine sulfonate condensates. Further, polycarboxylate based superplasticizer is a safer material for health because it does not contain formaldehyde, so its application is expected to expand in future. An exhaustive literature survey on concrete admixtures, with particular emphasis to PC superplasticizers and their functions have recently been publsihed 5,6. Polycarboxylate type superplasticizers with a polyethylene oxide (PEO) grafted chains is one of the materials used to make self compacting concrete (SCC) because of its superior dispersing ability for cement, especially at low water/cement ratio. A large number of variations in these polymers are possible by varying the length of the backbone, the content of ionic groups, and the length, the amount and the type of side chains. These structural variations of the polymer have great influence on the performance of the PC. Sodium salts of formaldehyde condensates of β-naphthalene sulphate disperse cement particles by electrostatic repulsion that results from the adsorption of superplasticizer on cement surfaces. Polycarboxylate type superplasticizers with graft chains disperse cement agglomerates by steric hindrance 1,7,8. By varying the polymer structure, properties like adsorption, electrostatic and steric effects of the superplasticizers can be optimized. When the same level of superplasticizers is used, the surrounding environment of the concrete or ingredients significantly affect the rheological properties of the concrete as an initial slump or slump loss 9,10. Such problems were known as an incompatibility phenomenon between the cement and the superplasticizer Different studies have shown that the amount of alkaline sulphate significantly affects the adsorption of superplasticizers on the surface of cement particles 15,16. Portland cement consists of four mineral phases namely Ca 3 SiO 5, Ca 2 SiO 4, Ca 3 Al 2 O 6 and Ca 4 Al 2 Fe 2 O 10, which are abbreviated as C 3 S, C 2 S, C 3 A and C 4 AF. In addition to these phases there is a small amount of gypsum also. For these cement

2 350 INDIAN J. CHEM. TECHNOL., JULY 2008 component minerals, superplasticizers are not expected to adsorb uniformly. Ohta and Uomoto 17 have reported the compatibility problem between certain cements and comb-type superplasticizers, which can adversely affect concrete workability. Due to complexities of the system it is difficult to clarify the adsorption and dispersion behaviour of comb type polymers during the hydration of cement in the presence and absence of certain ions found in the pore solution. Therefore use of model system (hydraulically unreactive) powders will be essential for better understanding of the superplasticizers chemistry. The study of adsorption of non-ionic hydro soluble polymer [poly(ethylene oxide)] on CaCO 3 as a model system and the rheological behaviour of concentrated aqueous dispersions as a function of the dispersant molecular structure will be interesting and from the studies structure property relationships could be drawn 18. Sakai et al. 19 studied the influence of molecular structure of comb-type superplasticizers and inorganic electrolytes on the dispersion mechanisms of limestone powder and proposed a mechanism. Geffroy et al. 20 studied the adsorption of polyacrylates on the calcite in water at ph 9. They found that some macromolecules of intermediate molar mass are adsorbed first and build up an electrostatic barrier that prevents the adsorption of other macromolecules. However, due to increase of ionic strength when this electrostatic barrier is reduced to certain optimum value, the high molar mass macromolecules is no longer rejected. Thus, adsorption of macromolecules is related to ionic strength. Comparet et al., 21 studied the adsorption of comb-type superplasticizers onto carbonate in basic medium in the presence of sodium sulphate and found that decrease in adsorption is mainly due to ionic strengths and not due to ion adsorption. In the present communication CaCO 3 suspension is chosen as a model system and the ph of the medium is kept at 12.6 (as in the case of Portland cement). Three polycarboxylate type superplasticizers namely, chryso-1, chryso-2 and chryso-3 having same backbone length and esterification level but different POE chain length have been used for adsorption studies in the presence of different electrolytes of same ionic strength. Rheological properties have been studied and mechanism of adsorption has been proposed. Experimental Procedure Materials The CaCO 3 powder (SOCAL-31) was purchased from Solvay, France. The average diameter of the powdered particle was 70 nm and was aggregated into flocks of large size and small density. Three comb- type polymers synthesized by esterification reaction between polymethacrylic acid and methoxy terminal of polyethylene oxide chains were procured from Chryso (France). The polymers are abbreviated as Chryso-1, Chryso-2 and Chryso-3. All the polymers have same backbone length and esterification level but they differed in the length of POE chain. The lengths of POE chain for Chryso-1, Chryso-2, and Chryso-3 were 2000, 1100 and 750g/mol respectively. The chemical structure of the main component of the superplasticizer is given in Fig. 1. The ph of all the polymers in solution was 7.0 indicating that all are neutral solutions. Adsorption studies Adsorption studies of superplasticizers on calcite suspension were made at 25 C with a liquid/solid ratio of 5.0 in the presence of Na 2 SO 4, CaCl 2 and NaCl of same ionic strength. The suspensions of CaCO 3 in NaOH solution of ph 12.6 were prepared in a rotary bath using a magnetic stirrer. After 3 days of mixing the suspensions were centrifuged for 10 min at 9000 rpm. The amount of superplasticizers in the supernatant solution was determined with Total Organic Carbon analyzer (TOC-5000A Shimadzu). The amount of superplasticizers added minus that left in the solution gave the percentage of adsorption on the surface of calcite. Fig. 1 Structure of the superplasticizers

3 RAI & GAJBHIYE: ADSORPTION OF SUPERPLASTICIZERS ON CALCIUM CARBONATE 351 Rheological measurement The suspensions were mixed in a blender at liquid/ solid ratio of 0.7 for 5 min and kept in a water bath for 1 h at 25 0 C. Measurements of viscosity as a function of shear rate at constant level of polymer dosage were made. All the experiments were done at 25 C by using Couette analogy with a Rheometer RSI Orchestrator. The couette geometry consisted of two rotating concentric cylinders with the fluid contained in the gap. Results and Discussion In the presence of superplasticizers, dispersions occurred, the dispersion mechanisms being mainly linked to the adsorbed superplasticizers. Superplasticizers that are adsorbed on the cement can induce ionic and steric repulsive forces, leading to better dispersion. Therefore the adsorption behaviour of superplasticizers on calcite was studied and correlated with the flow properties. The adsorption isotherms of all the three PC s, chryso-1, chryso-2 and chryso-3, on CaCO 3 in strongly basic medium (ph=12.6) in presence and absence of different electrolytes are shown in Figs 2-4. The concentrations of the electrolytes used were: Na 2 SO 4, 50 mmol/l; CaCl 2, 50 mmol/l and NaCl, 130 mmol/l (130 mmol is kept in order to keep the same ionic strength as that of 50 mmol Na 2 SO 4 or CaCl 2 ). The ionic strengths of the electrolyte solutions were , and mol/kg for NaCl, Na 2 SO 4 and CaCl 2 respectively indicating that the ionic strengths of all the three electrolytes are the same. The shape of the adsorption isotherms is almost similar. The only differences are that the amount of superplasticizers adsorbed is different under different conditions. The adsorption increased gradually with the increase of equilibrated concentration of PC and then approached to a constant value. The adsorption isotherms showed a typical Langmuir curve. The maximum amounts of PC adsorbed are given in Table 1. The results given in Table 1 show that the adsorption of PC on CaCO 3 surface in the absence of any electrolyte (reference) follows the sequence: Chryso-1 > chryso-2 > chryso-3 In the presence of NaCl and Na 2 SO 4 of same ionic strength the maximum amount of PC adsorbed is almost the same and lower as compared to the reference. However, in the presence of CaCl 2 the maximum amount of PC adsorbed is higher than the reference. It appears that higher the POE chain length i.e. higher the molecular weight of the polymer, higher the extent of adsorption on the surface of CaCO 3. Fig. 2 Adsorption isotherm of Chryso-1 on calcium carbonate, l/s=5 ph= 12.6 (NaOH) in presence of Na 2 SO 4,CaCl 2 and NaCl Fig. 3 Adsorption isotherm of Chryso-2 on calcium carbonate, l/s=5 ph=12.6 (NaOH) in presence of Na 2 SO 4, CaCl 2 and NaCl Fig. 4 Adsorption isotherm of Chryso-3 on calcium carbonate, l/s=5 ph=12.6 (NaOH) in presence of Na 2 SO 4, CaCl 2 and NaCl Table 1 Maximum amount of PC adsorbed on CaCO 3 in presence of electrolytes Electrolyte Amount of PC adsorbed (mg/g of CaCO 3 ) Ionic strength of electrolytes (mol/kg) Chryso-1 Chryso-2 Chryso-3 0 (reference) NaCl Na 2 SO CaCl

4 352 INDIAN J. CHEM. TECHNOL., JULY 2008 Understanding and quantifying the effect of PC even in the non-hydraulic system like the one under consideration, is a complex task. The CaCO 3 crystal structure 22 is given as A calcite crystal can be cleaved along three planes making an angle of with each other. The common phase is the plane with crystallographic ind/*exes (1014), corresponding to a rhombohedral surface. The adsorption may occur preferentially at a particular plane. It is reported that citrates adsorb on the surface of CaSO 4. 2H 2 o preferentially on a particular crystal plane 23, block the growth in the corresponding direction. Electrolytes may affect the electrostatic repulsive forces on the conformation (distribution of tails, loops and trains) of the superplasticizers on the surface of CaCO 3 particles. Different electrolytes may affect this conformation in different ways. Depending on the change in conformation, extent of adsorption will depend 24. Perhaps CaCl 2 changes the conformation in such a way that the adsorption is increased whereas the changes in conformation in the presence of NaCl and Na 2 SO 4 decrease the adsorption. For same ionic strength of NaCl and Na 2 SO 4 the change in conformation may be the same resulting into same amount of adsorption. From the above it appears that the adsorption is not a competitive process. Electrolytes at the surface of CaCO 3 particle modify the superplasticizers conformations. If the conformation is compatible at a particular surface the adsorption is increased. However if the conformation is not compatible to the surface, the adsorption is decreased. Further, rheology allows us to study macroscopic structure of colloidal systems by measuring the macroscopic flow behaviour of the system under the shear stress. The study of flow behaviour of the concentrated suspensions has always been of great interest in rheology since concentrated suspensions exhibit numerous types of peculiar rheological characteristics such as thixotropy, dilatancy and viscoelasticity etc. The steady shear viscosities of suspensions containing superplasticizers having different PEO chain length are shown in Figs 5-7 Fig. 5 Shear rate dependence of apparent viscosity for calcite suspensions in a 3 wt% chryso-1. Fig. 6 Shear rate dependence of apparent viscosity for calcite suspensions in 3 wt% chryso-2. Fig. 7 Shear rate dependence of apparent viscosity for calcite suspensions in 3 wt% chryso-3.

5 RAI & GAJBHIYE: ADSORPTION OF SUPERPLASTICIZERS ON CALCIUM CARBONATE 353 which represent the viscosity as a function of shear rate at constant level of superplasticizers dosage i.e. 3% by weight at l/s =1.0. As can be observed, all samples exhibit shear thinning behaviour. The system of this type becomes thinner on stirring because the structure is broken down and becomes thicker again on standing because the structure is re-established. The shear thinning characteristic is called pseudo plasticity. Figures 5, 6 and 7 represent plots of viscosity at shear rate levels of 2, 5,10 and 20 s -1. The viscosity in the case of chryso 3 is not stable at shear rate of 2 s -1 and it appears that it is periodic. It is clear from the data that chryso 1 is more fluid than chryso 2 and chryso 3. Increase in the viscosity of calcite suspensions with the adsorbed POE molecules can be explained with changes in the concentration of the free polymer chains in the suspending media. When the adsorption isotherm is reached up to platue, most of the added polymer will stay in the solution increasing the concentration of the free polymer chains in the suspending media. This causes an increase in the magnitude of the depletion forces in the system which is accompanied by an increase in the viscosity of the dispersion. The apparent viscosity decreases with the shear rate (Fig. 8). The results showed that lower the PEO chain, higher the viscosity. It can be observed from Fig. 9 that there is a significant increase in the viscosity of suspension as the Na 2 SO 4 concentration is increased. The adsorption isotherm indicates that the adsorption density of the superplasticizers is lower in the presence of salt. It may be concluded that over the range of salt concentration at which the suspension is stable, the tendency of superplasticizers molecules to migrate on the surface of the particles decreases due to compression of the double layers with increasing salt concentration and change in conformation. This causes a decrease in the adsorption density of the superplasticizers onto the surface of the particles and as a result viscosity is increased. However PC containing graft chains with longer chain length are effective for preventing the fluidity decrease when SO 2 4 is added. Ohta and Uomoto 17 on the other hand, have reported that fluidity of concrete can decrease with one type of superplasticizers if the sulphate ion concentration is high. Fig. 8 Apparent viscosity as the function of shear rate. Fig. 9 Viscosity increases in presence of Na 2 SO 4 for chryso-1 Conclusions Based on the above results, the following conclusions can be drawn: (i) The amount of adsorption is highest in the presence of chryso-1, because molecular weight is highest. (ii) It is observed that the addition of inorganic electrolytes containing anions which form relatively insoluble calcium salts significantly decrease the adsorption on limestone powder suspensions and also the fluidity. Decrease in adsorption in the presence of NaCl and Na 2 SO 4 is the same since the ionic strength is the same. On the other hand CaCl 2 increased the adsorption. Both increase and decrease in adsorption in the presence of electrolyte is due to conformational changes in the polymer structure of superplasticizers.

6 354 INDIAN J. CHEM. TECHNOL., JULY 2008 (iii) When Na 2 SO 4 is added, the fluidity of suspensions is reduced due to the inhibition of adsorption of superplasticizers and the shrinkage of polymers may occur. (iv) It is found that lower the PEO chain length, higher the viscosity of the CaCO 3 suspension. Acknowledgment Authors are thankful to Prof. Andre Nonat and Ms. Sylvie Pourchet of Universite de Bourgogne, Dijon, Cedex, France for useful discussions. S Rai is thankful to DST, New-Delhi for awarding Women s fellowship. References 1 Uchikawa H, Hanehara S & Sawaki D, Cem Concr Res, 27 (1997) Ramachandran V S, in Adv in Concrete Technology, 2 nd edn, edited by V M Malhotra (Oxford Press, Oxford, UK), 1994, Malhotra V M, Durability of Concrete, paper presented at the 5 th CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, Barcelona, 2-4 April 1997, ACI special publication SP (1997). 4 Yoshioka K, Tazawa E, Kawai K & Enohata T, Cem Concr Res, 32 (2002) Uchikawa H, Proc RILEM Int Symp, Monterrey, Mexico, 1999, Tanaka Y, Otha A & Sngiyama T, Proc RILEM Int Symp, Monterrey, Mexico, 1999, Flatt R J, Houst Y F, Bowen P & Hofmann H, Proc 6 th CANMET/ACI Int Conf on Superplasticizers and other Chemical Admixtures in Concrete, Nice, France, edited by V M Malhotra, ACI SP-195, 2000, Kinoshita M, Nawa T, Iida M & Ichoiboji H, Proc 6 th CANMET/ACI Int Conf on Superplasticizers and other Chemical Admixtures in Concrete, Nice, France, edited by V M Malhotra, ACI SP-195, 2000, Tagnit-Hamou A & Aitcin P C, World Cem Res Develop, 24(8) (1993) Hanehara S & Yamada K, Cem Concr Res, 29 (1999) Kato H, Yoshioka K & Ushiyawa M, JCA Proc Cem Concr, 51 (1997) Jiang S, Kim B G & Aitcin P C, Cem Concr Res, 29 (1999) Jolicoeur C, Otis N, Lebel A, Simard M A & Page M, Proc V th CANMET/ACI Int Conf Superplasticizers and other Chemical Admixtures, edited by V M Malhotra, ACI SP 173 V (1997). 14 Jolicoear C & Simard M A, Cem Concr Comp, 2 (1998) Yamada K, Ogawa K & Takahashi M, Proc. 2 nd Int symp Self Compacting Concrete (2001). 16 Kim B G, Jolicoeur C & Aitcin P C, Cem Concr Res, 30 (2000) Ohta A & Uomoto T, Cem Sci Concr Technol, 52 (1998) Mosquet M, Chevalier Y, Le Perchec P, Foissy A & Guicquero J P, Colloid Polym Sci, 277 (1999) Sakai E, Atarashi D, Kawakami A & Daimon M, Proc 7 th CANMET/ACI Int Conf Superplasticizers and other Chemical Admixture in Concrete, Berlin, edited by V M Malhotra, ACI SP (2003). 20 Geffroy C, Persello J, Foissy A, Lixon P, Tournilhac F & Cabane B, Colloids Surfaces A, 162 (2000) Comparet C, Nonat A, Pourchet S, Mosquet M & Maitrasse P, Proc 7 th CANMET/ACI Int Conf Superplasticizers and other Chemical Admixtures in Concrete, Berlin, ACI SP (2003). 22 Geffroy C, Foissy A, Persello J & Cabane B, J Colloid Interface Sci, 211 (1999) Tadros M E & Mayes I, J Colloid Interface Sci, 72 (1979) Sakai E, Kahng J K & Diamon M, Proc. 5 th Int Symp Cem Concr, Shanghai, 1 (2002) 195.

INFLUENCE OF TEMPERATURE, CEMENT AND PLASTICIZER TYPE ON THE RHEOLOGY OF PASTE

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