Agents and Superplasticizers in. Cementitious and Model Systems: Rheology, Bleeding, and Segregation by N. Mikanovic, J. Sharman, C. Jolicoeur, K. Khayat,.
SP-262—5
Compatibility of Viscosity-Enhancing Agents and Superplasticizers in Cementitious and Model Systems: Rheology, Bleeding, and Segregation by N. Mikanovic, J. Sharman, C. Jolicoeur, K. Khayat, and M. Pagé Synopsis: Viscosity-enhancing agents (VEA) are often used to improve the cohesiveness and stability of self-consolidating and underwater concrete. However, because of various types of interactions occurring between the VEA polymers and other components of fresh cementitious systems, the beneficial effects of the VEA is found to depend on the nature of both the VEA and the other the other components, particularly the superplasticizer (SP). Hence, different VEA-SP combinations are found to yield different dose-response effects in application. To investigate the origin and consequences of VEA-SP interactions, the influence of two common VEAs on the properties of cementitious and reference (limestone) systems was investigated through rheological and stability (bleeding and segregation) measurements in the presence of two typical SPs, a naphthalene-based (PNS) and a carboxylate-based (PC) polymer. The rheology of cement and powdered limestone pastes was evaluated through the Kantro mini-slump test and from measurements of yield stress and plastic viscosity in simple shear, and dynamic moduli obtained through oscillatory measurements. The bleeding and sedimentation behaviors were monitored using a multipoint conductivity method. Corresponding rheology and stability data were also obtained on mortars incorporating the same VEA-SP admixture combinations. In these systems, the different VEA-SP couples demonstrate varying compatibility which impact on their performance.
Keywords: admixtures; bleeding; calcium carbonate; cement; paste; rheology; sedimentation; segregation; superplasticizers; viscosityenhancing agents.
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Nikola Mikanovic, PhD, is a Research Assistant in the department of civil engineering at the Université de Sherbrooke, QC, Canada. His research interests include cement chemistry and cement-admixture interactions and the consequence of these interactions on the rheology and microstructure of cementitious materials. Jeff Sharman, BSc, is a Research Associate and master’s student in the Department of Chemistry, Université de Sherbrooke. His research interests include the solution and colloid chemistry of cementitious systems and chemical admixtures for cement-based materials. Carmel Jolicoeur, PhD, is a Professor in the Department of Chemistry at the Université de Sherbrooke. His research interests include the solution and colloid chemistry of cementitious systems and chemical admixtures for cement-based materials. Kamal Khayat, PhD, is a Professor in the Department of Civil Engineering at the Université de Sherbrooke. His research interests include rheology and stability of cement-based materials, underwater concrete, self-consolidating, and self-leveling concrete. Monique Pagé, PhD, is the Executive Vice-President of Handy Chemicals Ltd., Candiac, QC, Canada. Pagé specializes in colloidal chemistry and heads the development of various types of concrete chemicals admixtures.
INTRODUCTION The development and acceptance of self-consolidating concrete (SCC) largely depends on the ability to design formulations that exhibit the following properties: low yield stress, high viscosity and cohesiveness, little or no bleeding and no segregation. These can sometimes be achieved through optimized gradation of aggregates and filler materials, typically by increasing the content of low-reactivity fines, e.g., limestone.12-3 The typical SCC behaviour can also be achieved through the use of chemical admixtures, usually water-soluble, high molecular weight polymers, such as derivatives of starch, cellulose and gums. This type of additive is often used in consumer and industrial products (e.g., gels, coatings) to increase the viscosity and stability of colloidal formulations. The viscosity-enhancing admixtures (VEA) operate through a combination of effects: polymer hydration and swelling, and polymer-polymer interactions; generally, they can be effective at low concentration, typically less than 1%. The introduction of VEA in fluid concrete mixes provides similar benefits, greatly improving the cohesiveness and stability of these concretes towards bleeding and segregation.4,5 While the mode of action of VEA in concrete may be similar to that outlined above for other formulations, the implementation of VEA in concrete must cope with particular conditions prevailing in cementitious systems (e.g., high pH, high Ca+2) and the presence of other chemical admixtures, particularly superplasticizers (SP). The performance of VEA in application should therefore depend on the intrinsic properties of the VEA polymers and on VEA-SP interactions. In practice, it has indeed been found that different VEA-SP pairs exhibit different performances, or compatibility6,7;
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the origin of these compatibility/incompatibility situations remains poorly described and understood. The present investigation was undertaken to examine the behavior of common VEA and SP combinations in the following systems: limestone pastes, cement pastes and mortars. Previous studies of admixture function in limestone pastes were shown useful to illustrate the intrinsic features of admixtures in non-reactive systems.8 Indeed, it was demonstrated in this study that the state of flocculation of the native limestone paste (pH ~ 10) was very similar to that of the cement paste and this produced similar results in fluidity and sedimentation when the appropriate w/s was used. Comparison of results derived from studies on limestone pastes and cement pastes or mortars therefore emphasizes the effects specific to cement chemistry and hydration. The present study involves combinations of two VEAs (a modified cellulose and a bacterial polysaccharide) and two SP (Polynaphthalene sulfonate (PNS) and Polyacrylate ethers, also known as Polycarboxylates (PC)), and focused on the influence of the VEA-SP pairs on the rheological properties and stability (bleeding and segregation) of the pastes and mortars. The results first provide a more accurate definition of VEA-SP compatibility in cementitious systems. Using additional information on the molecular properties of the VEAs and SPs, suggestions can be advanced on the phenomena which determine the level of VEA-SP compatibility.
MATERIALS AND METHODS
Materials A common Type-10 (ASTM Type I) portland cement was used and its composition and properties are presented in Table 1. A calcium carbonate (CaCO3) powder in the form of calcite provided by a local supplier was also used as an inert reference material. The specific surface area and mean particle diameter of the calcium carbonate and cement are presented in Table 2. The specific surfaces of the solids are comparable, although particles of calcium carbonate are somewhat finer. The sand used was a standard Ottawa sand, meeting the requirements of ASTM C778. Two commercially available superplasticizers were used for this study: a sodium polynaphtalenesulfonate (PNS) and a polycarboxylate (PC) partially esterified with intermediate molecular weight polyethylene glycol (Mw ~ 2000). Both superplasticizers were supplied by Handy Chemicals Ltd, as concentrated aqueous solutions (~ 40 wt%). Two viscosity-enhancing agents (VEA), commonly used in the concrete industry, were selected: welan gum, a polysaccharide supplied by CP Kelco, and a hydroxypropylmethylcellulose (HPMC) supplied by Dow Chemicals. Both products are supplied in a powder-form and were dissolved in the mix water prior to mixing with cement. Methods Stability of the pastes and mortars—The time evolution of the homogeneity of CaCO3 pastes, cement pastes and mortars were monitored through a method developed earlier by the authors.9,10,11-12 This methodology allows direct observation of water permeation and surface bleeding, water channeling, sedimentation and segregation of solids from electrical conductivity measurements at different heights of the sample and as a function of time. Since the electrical conductivity of particle suspensions depends
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on liquid/solid composition, the variance in the conductivity values measured at different depths at any given time is a direct measure of the heterogeneity of the sample at that instant. Such measurements can thus be used to define a stability index (SI) representing the homogeneity of the sample at a given time: SI = 1-(σ/λavg), where σ and λavg are respectively the standard deviation and average electrical conductivities measured over the height of a sample at that time. This parameter characterizes the overall homogeneity of the sample and may be evaluated as function of time. Conductivity versus time curves obtained in CaCO3 and cement pastes can be used to calculate the bleeding and sedimentation curves. The bleeding curve describes the vertical displacement of an interface between the clear supernatant solution and the rest of the sedimenting column (bleeding front); the sedimentation curve illustrates the progression of the interface between the sedimenting column and the compacted sediment (sedimentation front) with time. Figure 1 shows bleeding and sedimentation curves for a calcium carbonate paste (L/S=0.5) containing no chemical admixtures. From these curves, the following parameters, which provide quantitative information on the kinetics of solid/liquid separation can be obtained: • Bleeding volume—total volume of bleeding water accumulated at the top of the cell, at the end of the sedimentation experiment (mL) • Equilibrium stability index (SI)—value of the stability index at the end of a sedimentation experiment. • ν r0—initial bleeding rate: initial slope of the bleeding curve • ν r1—accelerated bleeding rate observed after an initial period of constant bleeding rate; this was shown earlier to signal channel bleeding; • ν s0—initial sedimentation rate: initial slope of the sedimentation curve • ν s1—accelerated rate of sedimentation after an initial constant rate period. More details on the instrumentation, optimization of the experimental parameters and data analysis procedures may be found elsewhere.9-12 Rheology—All rheological measurements in pastes were performed with a ViscoAnalyser rheometer (ATS RheoSystems) using cup and bob geometry at constant temperature [25 °C (77 °F)], while mortar measurements were realized with a ConTech mortar rheometer. All pastes and mortars investigated in this study showed typical Bingham behavior and yield stress (τo) and plastic viscosity (µ) were estimated from their flow curves (30 to 5 s-1 in 6 steps). Dynamic (oscillatory) measurements were carried out only in pastes, which were pre-sheared for 30 s at 200 Pa (2.9 x 10-2 psi) and then allowed to rest for 5 minutes before the beginning of the rheological measurement. Shear sweeps between 0.1 and 10 Pa (1.5 x 10-5 psi and 1.5 x 10-3 psi) were typically run at a constant frequency of 1 Hz and the values of elastic and loss modulus (respectively G’ and G”) in the linear viscoelastic region, as well as the value of phase angle, were determined. Frequency sweeps were also performed at frequencies ranging from 0.01-10 Hz; these experiments permit the determination of rheological parameters such as the relaxation time (inverse value of the frequency where the phase shift reaches 45˚, i.e. the loss and storage moduli are equal) and the zero-shear viscosity (low frequency extrapolation of the complex viscosity). The rheological behavior of the pastes and mortars was also characterized through ‘slump-flow’ tests typically used in studies of cement-based materials.
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Mixing procedure—The paste and mortar samples used in stability measurements were prepared using a Hobart mixer according to ASTM C305. The required amount of water or electrolyte solution (10 g/L NaCl solution was used in limestone pastes measurements to provide the ionic strength typically found in cement pastes) and both superplasticizers and viscosity-enhancing admixtures were introduced in a mixing bowl prior to mixing with solids. The solid phase was then gently introduced into the mixer and this moment is time zero on bleeding and sedimentation curves. The total mixing time was four minutes. Thereafter, the sample was transferred into the conductivity cell and conductivity measurements started usually 5-6 minutes after initial contact between the solid and liquid phases. The rest of the sample was used to perform air entrainment and rheological measurements described above. The mixing protocol and the sequence of measurements are schematically described in Figure 2.
RESULTS
Effect of VEAs on paste stability The effect of the type and dosage of VEA on the bleeding/sedimentation behaviour and on rheology was studied in CaCO3 and cement pastes containing two different superplasticizers. A liquid to solid ratio (L/S) of 0.5 was selected for the CaCO3 paste to obtain a stable paste without any chemical admixture; the dosage of PNS and PAE type superplasticizers (0.05 and 0.03 wt.%) was than selected to get relatively unstable reference pastes, having a volume of bleed water representing 10-15% of the total paste volume. A relatively high L/S of 0.65 for cement pastes was chosen to obtain similar bleeding volumes when a typical dosages of PNS and PC superplasticizer were employed (0.7 and 0.1 wt.%, respectively). The data illustrating the effect of two VEAs (HPMC and welan gum) on the stability of these pastes are collected in Table 3, while the parameters describing their rheological properties are presented in Table 4. The selected dosage range of welan gum and HPMC in paste experiments is typical for applications in concrete technology (0.01-0.1% by mass of cement). Judging from the data in Table 3, both VEA appear to be more effective in improving overall stability in cement pastes than in CaCO3 pastes. Examination of the stability index values and cumulative bleeding volumes collected over CaCO3 pastes shows that the cellulosebased VEA provides little or no added stability to the limestone paste. Welan gum, however, lowers bleeding volume and increases the equilibrium stability index, but its effect is smaller than expected regardless of the type of SP used (less than 50% of reduction of bleeding volume and an increase of 0.15 SI units (15%) when 0.03% of welan was used). On the other hand, in cement pastes, the overall stability increases and the total volume of bleeding water decreases significantly with increasing VEA dosage. The difference in the performance of both VEAs in a non-hydrating paste of mineral particles versus cement paste will be discussed below from a mechanistic point of view. A better understanding of the effect of VEAs on the bleeding and segregation processes can be achieved by analyzing the kinetic parameters presented in Table 3. In reference CaCO3 pastes containing only superplasticizers, the bleeding and sedimentation curves are simple: bleeding and sedimentation occur at constant rates until the system reaches a pseudo-equilibrium state. In the presence of welan gum, the shape of these
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curves changes; after an initial period of bleeding and sedimentation at a constant rate, acceleration in the bleeding and sedimentation processes occur, and the rates later slowly decrease to zero (pseudo-equilibrium state). As described elsewhere,13 an increase of the bleeding rate which occurs several hours into the phase separation process is an indication of the onset of channel bleeding, i.e., liquid migration through preferential, least-resistance vertical paths (channels). It is to be noted that the bleeding and sedimentation curves show related features, i.e., the onset of accelerated bleeding and sedimentation happens at the same time on both curves; this illustrates that the two phenomena are interrelated and are occurring concomitantly. The authors have shown that the onset of channel bleeding is correlated with a change in the settling mechanism of solid particles in a paste.12 It is interesting to note that accelerated bleeding and sedimentation do not occur in all CaCO3 pastes containing welan gum, only in those having moderate slump spreads of ~110-120 mm (~4.3-4.7 in.)in diameter (Table 4) and, preferentially, containing PNS. This is in agreement with an observation made by Loh et al.13 that channeling occurs only in particle dispersions which are neither too dense, nor too fluid; the latter probably explains the absence of channeling phenomena in CaCO3 pastes containing the cellulose-based VEA. In cement pastes, the interactions involving the hydrating cement particles rapidly “freezes” the bleeding and sedimentation processes after 1-2 hours, prior to the formation of channels which would normally occur after this period in dispersions of non-reactive minerals. Both of the VEAs examined tend to decrease the initial (ν r0, ν s0 ) and accelerated (ν r1, ν s1 ) bleeding and sedimentation rates in CaCO3 pastes, even when these admixtures do not appear to improve the overall paste stability, as is the case with HPMC (Table 3). The addition of VEAs to the cement pastes results in more significant stabilization than in limestone pastes. With certain SP-VEA pairs, however, abnormal behavior is occasionally observed. With low VEA dosages there is only a slight decrease and, in certain cases, even an increase in the initial bleeding and sedimentation rates; this has been observed with PC/welan gum and with PNS/HPMC. As the VEA dosage is further increased, the bleeding and sedimentation rates are significantly reduced and become comparable to those for the PNS/welan gum and PC/HPMC combinations. Effect of VEAs on paste rheology The effect of VEAs on the rheological properties of CaCO3 and cement pastes depends on the nature of the solid particles and on SP/VEA combinations, as evidenced in Table 4. In CaCO 3 pastes, the addition of welan gum reduces slump spread and increases both yield stress (τo) and plastic viscosity (µ); however, the addition of the cellulose-based VEA shows either no effect or even makes limestone pastes more fluid, as is the case for pastes containing PC. Surprisingly, both dynamic moduli (G’ and G”) decrease as the VEA concentration increases, while the decrease in phase angle and the increase in characteristic relaxation time (tc) suggest that the addition of VEA results in pastes with a more solid-like (elastic) behavior. When added to cement pastes, both VEAs demonstrate a more important effect on the rheology as compared to limestone pastes. All flow parameters (slump spread, τo,
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µ) indicate that the VEAs reduce the fluidity of cement pastes with increasing dosage regardless the SP/VEA combination. The dynamic rheological results, however, do not show any specific trends and are not very conclusive. Results in mortars The effect of VEA on the stability and rheology of mortars was studied on cement equivalent mortars (CEM) corresponding to a typical self-consolidating concrete (SCC) having a cement content of 460 kg/m3. Again, the water-cement ratio (w/c) was fixed at 0.65 to obtain unstable mortars prone to bleeding and sedimentation and segregation when typical dosages of superplasticizers were employed (0.6% and 0.135% relative to the weight of cement for PNS and PC, respectively). In addition, the VEA dosage range was typical for SCC applications (0.01-0.1 %). The results on mortar testing are collected in Table 5. Generally, the presence of VEAs improves somewhat the overall stability (stability index), increasing both yield stress and plastic viscosity and reducing slump spread of the CEM; mortars containing PC superplasticizer appear to be relatively stable (and more stable than those with PNS), so the effect of both VEA on stability index values is limited in these mortars. However, some unexpected results were obtained for certain SP/VEA combinations and will be discussed below in the light of SP/VEA (in)compatibility.
DISCUSSION
Mechanistic aspects The VEAs increase the cohesiveness of cement-based materials through a combination of several physico-chemical phenomena which have been the object of considerable investigation.14-17 It is believed that these admixtures increase the apparent viscosity of the mixing water and that of the fresh cementitious material through a combination of the mechanisms illustrated in Figure 3 and described as follows:5,18 1. Water retention—The long-chain hydrophilic VEA molecules adsorb and immobilize free water molecules; in doing so, their apparent volume increases by swelling, which increases the viscosity of mix water. 2. Polymer-polymer interaction and entanglement—Adjacent VEA polymer chains can develop attractive forces, resulting in the formation of a gel-like network, thus further blocking the motion of water, and increasing the viscosity of the whole system; at higher concentrations, the VEA polymer molecules can entangle, resulting in further increase in the apparent viscosity. 3. Polymer-particle interaction—Adsorption of polymer onto particles leads to an increased particle size and an increase in drag; furthermore, higher polymer concentrations lead to particle-particle bridging producing a relatively rigid network.16,17 The first two of these phenomena may be observed in bulk solutions in the absence of solids; the third effect will be observable in slurries or pastes in which particles can undergo significant interaction with the VEA. The particle bridging effect results in the formation of large flocs in which free water may be entrapped. Some recent studies showed that this effect opposes the dispersion function of superplasticizers used in highly flowable cementitious materials; Yammamuro et al suggest, therefore, the use
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of non-adsorptive VEAs.16 As emphasized earlier, both VEAs improve stability while reducing the fluidity of cement pastes (increase yield stress and plastic viscosity). In a simplified approach, the properties of a fresh cementitious system may be related to a sum of contributions originating from “physical” effects due to the dimensional, electrostatic, steric, and other features of the system, and “chemical effects” resulting from cement hydration. Given that CaCO3 exhibits surface and colloidal properties similar to that of cement particles at early stages of hydration,19 the lack of performance of both VEAs in the model paste suggests that a principal contribution to the mode of action of these molecules is related to inherent differences between limestone and cement; this may be due to differences in solution composition or due to cement hydration and/or to specific interactions. Under equilibrium conditions, the zeta potential of CaCO3 particles is near zero, as is the case of portland cement particles. Contrary to cement, the limestone suspension is only slightly alkaline (pH ~10) and the concentration of Ca2+ ions is an order of the magnitude lower than in a typical cement paste solution [the saturation concentration of the limestone, hemihydrate, gypsum and portlandite are respectively 0.5, 45, 15, and 23 mmol/L in pure water at 25°C (77°F)]. The difference in performance of both VEAs in model and cement pastes may originate from the fact that the conformation of the subject polymers is sensitive to the solution composition. However, our rheological measurements and data published elsewhere20 show that pH and calcium concentration has very little effect on the viscosity of welan gum solutions. This behaviour is related to weak association of ordered chain segments and formation of a 3D network typical for welan gum. On the other hand, the viscosity of solutions containing cellulose-based VEA, such as methyl cellulose (MC) or hydroxyethyl cellulose (HEC), may be somewhat reduced at very low or high Ph.20 These observations may be explained by the fact that that below pH=3, acid-catalyzed hydrolysis becomes significant, and above pH = 11, oxidative degradation takes place.21 However, in our preliminary measurements, we observed no significant changes in the viscosity of HPMC solutions when lime or NaOH (pH = 12.5) solutions were employed instead of water. Therefore, we believe that solution composition has only a small effect on the lack of performance of these VEAs in the limestone paste. In light of the preceding discussion, it would appear that an adsorption mechanism plays the most significant role in the mode of action of VEAs. Some preliminary results on adsorption of these molecules in 10% CaCO3 and cement slurries demonstrate that adsorption is, effectively, more pronounced on cement particles than on CaCO3. In fact, the adsorption of both VEA on limestone particles is very low, i.e. less than 1 mg/g of solid, close to the limits of the measurement method. Within these limits, it was found that welan adsorbs 3-4 times more than HPMC onto limestone. The higher adsorption of welan relative to HPMC could explain the fact that welan is more effective than HPMC as a VEA in limestone pastes. On the other hand, relative to CaCO3 VEA adsorption on cement particles is approximately 200% and 500% higher for welan and HPMC, respectively. Again compared to CaCO3 adsorption PC and PNS8 on cement particles can be 100% and 300-400% higher. The substantially higher adsorption of both VEAs (and SPs) on cement versus CaCO3 particles is likely to be the result of cement hydration. Regardless of the exact nature of the observation, the difference
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in binding capacity of the subject VEAs offers a reasonable explanation for their relative performances in cement and limestone pastes. Rheology versus stability in pastes and mortars From the foregoing discussion on the mode of action, one may expect that the performance of the subject VEAs is correlated with their ability to alter the rheology of pastes and mortars. The relation between the stability parameter (equilibrium stability index) and mini-slump spread for CaCO3 pastes is shown in Figure 4(a), while a similar relation for cement pastes is presented in Figure 4(b). The mini-slump was chosen over other rheological parameters to follow the effect of VEAs on the paste rheology, because of its practical relevance and wide accessibility. It is widely accepted that the slump spread and some oscillatory (dynamic) rheological properties are well correlated with the yield stress of cement-based materials,22 so the following comments are general. There is a linear correlation between the fluidity (mini-slump values) and stability of CaCO3 pastes containing the VEAs at varying dosages (Fig. 4(a)); this correlation does not appear to depend on the type of superplasticizer used, but depends on the nature of the VEA. Both VEAs appear to be ineffective in the limestone paste, i.e. a large decrease in fluidity results in only a very minor increase in stability; it is to be noted, however, that welan gum performs better than HPMC in limestone pastes. The linear correlation between the fluidity and stability of cement pastes (Fig. 4(b)) depends on the nature of the VEA, but also on the type of superplasticizer. Figure 4(b) represents a performance chart for the different VEA/SP combinations used in this study and the following conclusions about their (in)compatibility can be advanced. In view of the opposing effects of VEA and SP on the rheology and on overall stability, incompatibility may occur with certain combinations of these admixtures when they yield unpredictable cohesion (little or no stability improvement) and/or flowability behaviors (abnormal decrease in fluidity when the VEA is added). In the case of a compatible SP/VEA combination, a small slump spread (fluidity) reduction with an increase of the VEA dosage should be observed, while the overall stability should be significantly improved. The data presented in Fig. 4(b) suggest that PNS/welan gum and especially PC/HPMC can be considered as compatible SP/VEA combinations. On the other hand, the presence of the VEA lowers slump spread significantly (increase τo and µ), while stability is only slightly improved for incompatible SP/VEA combinations, i.e. PNS/HPMC and PC/welan gum. It is well known that the cellulose-based VEAs are incompatible with PNS-type superplasticizers; on the other hand, there does not appear to be any references indicating a compatibility issue involving welan gum. The paste observations on (in)compatibility between the SP and VEA used in this study are further validated in concrete equivalent mortars, where the compatibility issues are even more evident than in pastes. A close examination of the data in Table 5 shows that a correlation between the stability index and the bleeding volume, which can be readily observed in pastes, is inexistent in mortars. This is due to the electrode arrangement on the probe used for mortar measurements, which does not monitor the changes at the top and bottom of the cell.9 Therefore, for discussion purposes, the bleeding volume will be used as the stability parameter.
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For incompatible combinations, i.e. PNS/HPMC and PC/welan gum, the addition of the VEA results in an unexpected increase in bleeding volume at low VEA dosages relative to that of the corresponding reference mortars (without VEA); for these mixes, the bleeding volume remains important even at high VEA dosages. At the same time, both yield stress and plastic viscosity of these mixes increase significantly (slump spread decreases) with an increase in dosage of the VEA. Quite the opposite is observed in mortars containing compatible SP/VEA combinations: a clear increase of the overall stability, significant decrease of the bleeding volume and only a slight effect on the rheological parameters of mortars (slump, yield stress and plastic viscosity) is observed. VEA-SP compatibility (interactions) As noted above, the VEA mode of action involves different relative contributions from three potential mechanisms, illustrated in Figure 3. Interference, or competition, with any of these phenomena involving the SP can therefore influence one or more of the key properties required to achieve stable fluid cementitious systems. VEA-SP interactions, direct or indirect, can lead to an erratic response (incompatibility) with respect to one or more desirable feature, namely: low yield stress, high viscosity and cohesiveness, minimal bleeding and absence of surface settlement and segregation. VEA-SP incompatibility related to the first and second mechanisms (hydration, swelling and polymer-polymer interactions) can be examined in bulk solutions of the individual or combined admixtures. In preliminary studies, it was clearly observed that, while HPMC acts as a viscosity-enhancing agent in water (or lime solution), it exhibits no significant viscosity enhancement in the presence of PNS. With compatible VEA-SP pairs, viscosity effects are little influenced by the presence of the superplasticizer. These and related observations, to be reported elsewhere,24 indicate that the erratic behavior (incompatibility) of some VEA-SP couples can, in part, originate from solvation effects and polymer interactions occurring in solution. Such interactions, as observed, for example, with the PNS-HPMC couple, can be expected to alter the combined action of these admixtures. In practice, all polymers can participate in interactions (polymer-polymer, or polymerparticle) of various types and energies, depending on composition (hydrophobic, hydrophilic, ionic charges), molecular weight and morphology. Entropic effects related to polymer chain motions or configuration or solvent molecules will also contribute to the interactions according to the type and solution properties of the polymers. Incompatibility situations related to the third mode of action (Fig. 3) involve interactions of VEA’s and SP’s with particles of the system (cement, limestone, fine aggregate). While VEA-particle interactions can be expected to enhance the cohesiveness of the paste (reducing particle segregation and surface settlement), competition for surface-binding sites with SP molecules could reduce the stabilizing effect of the VEA. For example, PNS is known to adsorb more strongly than PC on cement and other particles, a behavior which can be attributed to the higher charge density of PNS. The presence of the latter may thus repress VEA-particle binding and bridging phenomena and their contribution to paste stabilization. Such effects most likely alter the performance of the PNS-HPMC couple discussed above, given the very low ionic charge density of HPMC relative to PNS. Qualitatively, similar mechanisms can be inferred to explain the
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PC-welan gum incompatibility. In this case, however, little or no deleterious interaction was observed in bulk solution24 suggesting that surface-binding competition plays a primary role in the incompatibility of this pair. The detailed elucidation of different types of deleterious interactions involving VEA and SP in cementitious systems will clearly require complementary physico-chemical information. Nonetheless, the results reported here provide an adequate basis to qualify compatibility issues in several systems of important practical interest.
CONCLUSIONS In practice, highly flowable cement-based materials with adequate resistance to bleeding, segregation, settlement, etc., can be obtained through adequate combinations of VEA and SP. However, as confirmed by the present results, the choice of VEA-SP admixtures can have a marked influence on the overall properties of a fresh cementitious system. Potential VEA-SP compatibility issues must therefore be identified and rationalized in order to achieve optimal benefits from the combined use of these chemical admixtures. The systematic comparison of limestone and cement pastes performed in this study provides valuable information about the mode of action of VEAs and the consequences of VEA-SP compatibility. Among the admixture combinations examined here, it is apparent from different measurement types on pastes and mortars, that optimal SP-VEA combinations are readily identified, i.e., PC-Cellulose and PNSwelan gum, whereas the opposite pairs yield sub-optimal, or erratic results.
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8. Mikanovic, N., and Jolicoeur, C., “Influence of Superplasticizers on the Rheology and Stability of Limestone and Cement Pastes,” Cement and Concrete Research, V. 38, No. 7, 2008, pp. 907-919. 9. Pavate, T. V.; Khayat, K. H.; and C. Jolicoeur, “In-Situ Conductivity Method for Monitoring Segregation, Bleeding, and Strength Development in Cement-Based Materials,” Proceedings of the 6th CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, SP-195, V. M. Malhotra, ed., American Concrete Institute, Farmington Hills, MI, 2000, pp. 535-559. 10. Jolicoeur, C.; Khayat, K. H.; Pavate, T.; and Pagé, M., “Evaluation of Effect of Chemical Admixtures and Supplementary Cementitious Materials on Stability of Cement-Based Materials using In-Situ Conductivity Method,” Proceedings of the 6th CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, SP-195, V. M. Malhotra, ed., American Concrete Institute, Farmington Hills, MI, 2000, pp. 461-483. 11. Mikanovic, N.; Jolicoeur, C.; Khayat K.; and Pagé, M., “Model Systems for Investigation of the Stability and Rheological Properties of Cement-Based Materials,” Supplementary Papers of the 8th CANMET/ACI International Conference on Recent Advances in Concrete Technology, 2006, pp. 267-304. 12. Mikanovic, N., Méthode conductométrique pour l’étude des phénomènes de ressuage et de sédimentation dans les matériaux cimentaires, PhD thesis, Université de Sherbrooke, QC, Canada, 2006. 13. Loh, C.-K.; Tan, T.-S.; Yong, K.-Y.; and Wee, T.-H., “An Experimental Study on Bleeding and Channelling of Cement Paste and Mortar,” Adv. Cem. Res., V. 10, 1998, pp. 1-16. 14. Izumi, T., “Special Underwater Concrete Admixtures,” Concrete Engineering, V. 28, No. 3, 1990, 23 pp. 15. Khayat, K. H., “Effects of Antiwashout Admixtures on Fresh Concrete Properties,” ACI Materials Journal, V. 92, 1995, pp. 164-171. 16. Yammamuro, H.; Izumi, T.; and Mizunuma, T., “Study of Non-Adsorptive Viscosity Agents Applied to Self-Compacting Concrete,” Proceedings of the Fifth CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, SP-173, V.M. Malhotra, ed., American Concrete Institute, Farmington Hills, MI, 1997, pp. 427-444. 17. Nawa, T.; Izumi, T.; and Edamatsu, Y., “State-of-the-Art Report on Materials and Design of Self-Compacting Concrete,” Proceedings of International Workshop on Self-Compacting Concrete, Japan, 1998, pp. 160-190. 18. Jolicoeur, C.; Mikanovic, N.; Simard, M.-A.; and Sharman, J., “Chemical Admixtures: Essential Components of Quality Concrete,” Indian Concrete Journal, V. 76, 2002, pp. 537-549. 19. Mikanovic, N.; Pagé, M.; and Jolicoeur, C., “Aqueous CaCO3 Dispersions as Reference Systems for Early-Age Cementitious Materials,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, V. 291, 2006, pp. 202-211. 20. Sakata, N.; Maruyama, K.; and Minami, M., ”Basic Properties and Effects of Welan Gum on Self-Consolidating Concrete,” Proceedings of the International RILEM Conference on Production Methods and Workability of Concrete, P. J. M. Bartos, D. L. Marrs, and D. J. Cleland, eds., E&FN Spon, 1996, pp. 237-253. 21. Flatt, R. J.; Larosa, D.; and Roussel, N., “Linking Yield Stress Measurements: Spread Test Versus Viskomat,” Cement and Concrete Research, V. 36, 2006, pp. 99-109. 22. Struble, L. J., and Ji, X., “Rheology,” Analytical Techniques in Concrete Science and Rheology, V. S. Ramachandran and J. J. Beaudoin, eds., Noyes Publications, Norwich, NY, 1999, pp. 333-367.
Superplasticizers and Other Chemical Admixtures Table 1—Properties of the portland cement Composition, % Chemical analysis CaO
62.9
SiO2
21.0
Al2O3
4.2
Fe2O3
3.1
MgO
2.3
SO3
2.7
Na2O eq.
0.76
Mineralogy C3S, %
52
C2S, %
21.5
C3A, %
5.7
C4AF, %
9.5
Physical properties % passing 45 µm
96
Specific surface BET, m /g
1200
Specific Blaine, m /g
345
2
2
Table 2—Physical properties of selected CaCO3 reference Material
Source
Specific surface, m2/g
Average particle size, µm
CaCO3
Omya Canada
1.2
10
Cement
St-Laurent
1.2
20-30
79
80
Mikanovic et al. Table 3—Stability of CaCO3 (L/S=0.5) and cement pastes (L/S=0.65)
Admixtures
Air, %
Bleeding, mL
SI equil.
ν r0, cm/h
ν r1, cm/h
ν s0, cm/h
ν s1, cm/h
0.05% PNS
—
71.5
0.555
0.45
—
3.4
—
+ 0.01% Welan
57
0.61
0.3
0.55
2.8
4.65
+ 0.02% Welan
50
0.65
0.18
0.23
1.3
2.15
+ 0.03% Welan
46.5
0.69
0.05
0.085
0.5
1.0
65.5
0.58
0.3
—
2.55
—
CaCO3, L/S=0.5
+ 0.01% HPMC
1.0
+ 0.025% HPMC
0.7
63.5
0.59
0.2
—
1.65
—
+ 0.05% HPMC
0.2
62.5
0.59
0.1
—
0.85
—
0.03% PC
—
83
0.50
0.45
—
2.7
—
+ 0.01% Welan
73.5
0.55
0.425
—
3.0
—
+ 0.02% Welan
50
0.585
0.33
0.63
2.4
5.8
Cement, L/S=0.65
+ 0.03% Welan
48
0.66
0.2
—
2.0
—
+ 0.01% HPMC
