110-m46

ACI Materials Journal/September-October 2013 513

Title no. 110-M46

ACI MATERIALS JOURNAL TECHNICAL PAPER

ACI Materials Journal, V. 110, No. 5, September-October 2013.MS No. M-2011-389.R2 received July 17, 2012, and reviewed under Institute

publication policies. Copyright 2013, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including authors closure, if any, will be published in the July-August 2014 ACI Materials Journal if the discussion is received by April 1, 2014.

Effect of Plasticizer and Superplasticizer on Rheology of Fly-Ash-Based Geopolymer Concreteby Aminul Islam Laskar and Rajan Bhattacharjee

where t is the shear stress; t0 is the yield stress constant; m is the plastic viscosity; and g is shear strain rate.

Geopolymer concrete is a promising field of research because it uses industrial waste and by-products. Fly-ash-based geopolymer concrete contains alkali-activated fly ash as binder. Fly ash is readily dissolved in the alka-line solution and lends itself to geopolymerization. In geopolymer concrete, polymerization takes place yielding Si-O-Al bonds as follows

Mn[(SiO2)zAlO]n wH2O

where Mn is the alkaline element; the symbol indicates the presence of a bond; z is 1, 2, or 3; and n is the degree of polymerization.6-11 The end product is an amorphous polymer. The alkali activation of fly ash and other mineral admixtures is a complex chemical process involving disso-lution of raw materials, transportation or orientation, and polycondensation of the reaction products. Fly-ash-based geopolymer concrete has shown its superior dura-bility over ordinary portland-cement concrete. The alkali activators normally used in geopolymer concrete are either a mixture of sodium hydroxide and sodium silicate or a mixture of potassium hydroxide and potassium silicate.6-8,12 Zhang,9 Khale and Chaudhary,10 and Duxson et al.11 presented a review on geopolymer and summarized previous research on geopolymer.

There has been extensive research on geopolymer concrete for strength and durability in recent years and several researchers highlighted the potential use of fly-ash-based geopolymer concrete in the concrete industry. Published literature on fly-ash-based geopolymer concrete for conven-tional single-point workability tests is also available. A review of the literature shows that no attempt has been made so far to investigate the effect of mixture parameters and superplasticizer dosage on rheological parameters of fly-ash-based geopolymer concrete. However, there are few reports available on rheological behavior of geopolymer paste and geopolymer mortar. Palacios et al.13 conducted rheological tests for slag-based geopolymer mortar and concluded that slag paste and mortar followed the Bingham Model when NaOH was used as an activator. Criado et al.14 studied rheo-logical behavior of fly-ash-based geopolymer paste with NaOH as an activator and observed that the Bingham Model could be fitted to those pastes. It is only recently that Laskar

An attempt has been made in this investigation to study the varia-tion of yield stress, plastic viscosity, and slump of fly-ash-based geopolymer concrete with the variation of lignin-based plasticizer dosage and polycarboxylic-ether-based superplasticizer dosage. It has been observed that a critical value of molar strength of sodium hydroxide exists that is equal to 4 M. Beyond this critical molar strength, superplasticizer and plasticizer have an adverse effect on yield stress, plastic viscosity, and slump of fly-ash-based geopolymer concrete. Below the critical molar strength of sodium hydroxide, there is a decrease in both the rheological parameters and an increase in slump. Lignin-based first-generation plasticizer shows better performance in terms of workability over third-generation superplasticizer below the critical value of molar strength. It was also observed that there is a good correlation between the rheolog-ical parameters and slump for fly-ash-based geopolymer concrete incorporating plasticizer and superplasticizer.

Keywords: geopolymer concrete; plastic viscosity; rheology; superplasti-cizer; thixotropy; yield stress.

INTRODUCTIONSingle-point workability tests such as the slump test, Vebe

test, and compaction factor test are widely used test methods to assess the workability of fresh concrete. The majority of these test methods are empirical in nature and they measure distance or time that serves as an index of workability. These tests do not provide any information about funda-mental properties of fresh concrete rheology. Tests such as the slump, compacting factor, and Vebe test are empirical in nature and these tests cannot characterize workability of todays advanced concrete mixtures.1,2 Researchers3-5 treat fresh concrete as a concentrated suspension of aggregates in cement mortar; therefore, fluid rheology methods are used to describe fresh concrete behavior. Rheology is defined as the science of flow and deformation of matter. Fluid rheology is a useful tool that is used by researchers to characterize and describe various fresh concrete properties such as workability loss, stability, compactibility, and pumpability. Rheological parameters are used to understand the interac-tions among the ingredients in concrete.

It has been established that cement concrete as a fluid can be assumed to behave like a Bingham fluid with a good accuracy.1,2 In the Bingham Model, flow is described by two parameters: yield stress and plastic viscosity. Yield stress and plastic viscosity are considered to be fundamental parameters of fresh concrete rheology. Yield stress gives initial resistance to flow and plastic viscosity governs the flow after it is initiated. The Bingham Model is represented by the following equation

0 = + (1)

514 ACI Materials Journal/September-October 2013

in this study. The specifications of the fly ash conform to the Indian Standard Code of Practice IS 3812.17 The chemical composition of fly ash is presented in Table 3.

Alkaline activatorsIn geopolymer concrete, either a mixed solution of sodium

hydroxide and sodium silicate or a mixed solution of potas-sium hydroxide and potassium silicate is commonly used. A mixed solution of sodium hydroxide and sodium silicate was chosen in this study as alkali activators. Sodium-based solutions were chosen because they were less expensive and because it was reported that NaOH possesses a greater capacity to liberate silicate and aluminate monomers.11 It is also reported that sodium cations have better zeolitization capabilities in geopolymer forming systems.11 The commercial-grade sodium hydroxide in pellets (purity = 97%; specific gravity = 2.13) and sodium silicate solution (Na2O = 18.2%; SiO2 = 36.7%; water = 45.1%; specific gravity = 1.53) were used to prepare the solution. The mass of NaOH pellets in a solution varied according to molar strength M and the mass of silicate solution was taken according to the ratio R.

Water-reducing admixturesTo study the effect of water-reducing admixtures (WRAs)

on rheological parameters, two types of chemical admix-turesnamely, first-generation lignin-based water reducer (lignin) and third-generation polycarboxylic-ether-based high-range water reducer (PC) were used. The water reducers dosage used in this study is the weight of water reducer expressed as a percentage of fly ash content.

MixingConcrete was mixed in a tilting mixer (laboratory type).

The weight of the materials was taken with a digital weighing balance. The mixing sequence was as follows:

1. Requisite quantity of sodium hydroxide and sodium silicate solution was prepared 24 hours in advance.

2. Solutions of sodium hydroxide and sodium silicate solution was mixed on the day of casting.

3. Coarse aggregate, fine aggregate, and fly ash was mixed for 2 minutes in a tilting mixer.

4. Alkaline solutions and WRA were added during mixing and mixed for two more minutes.

5. Mixing was stopped and the concrete mixture was poured.

Mixing and testing of geopolymer concrete mixtures were performed at approximately 20 to 22C (68 to 72F).

Rheological measurementsThe rheological measurements were performed with a

rate-controlled concrete rheometer developed at the National Institute of Technology, Silchar, India. The details of the rheom-eter, such as conceptual design, actual design, calibration, and validation, are published elsewhere.18 Figure 1 shows the schematic diagram of the rheometer.

The prepared concrete was transferred to the cylindrical container with a trowel from the same height every time.

Aminul Islam Laskar is a Professor in the Department of Civil Engineering, National Institute of Technology, Silchar, India. He received his BE (Civil) from the National Institute of Technology, Silchar; his MTech from the Indian Institute of Technology, New Delhi, India; and his PhD from the Indian Institute of Technology, Guwahati, India. His research interests include high-performance concrete and geopolymer concrete.

Rajan Bhattacharjee is a Research Scholar in the Department of Civil Engineering, National Institute of Technology, Silchar. He received his BE (Civil) from Jorhat Government Engineering College, Assam, India; and his MTech from the National Institute of Technology, Silchar.

and Bhattacharjee15 reported that fly-ash-based geopolymer concrete behaves like a Bingham fluid with good accuracy. An attempt has therefore been made in this paper to study the effect of several mixture parameters as well as plasticizer and superplasticizer dosage on the rheological parameters of fly-ash-based geopolymer concrete.

RESEARCH SIGNIFICANCERheological behavior of fly-ash-based geopolymer

concrete incorporating plasticizers and superplasticizers has never been reported and needs to be explored. This study reveals the effect of plasticizer/superplasticizer after interacting with alkaline solutions on yield stress and plastic viscosity of fly-ash-based geopolymer concrete. The outcome of this investigation is new and will be very useful for a better understanding of the behavior of the material during the production stage.

EXPERIMENTAL INVESTIGATIONMixture proportions

Two geopolymer mixtures were prepared with different mixture proportions with Class F fly ash, sand, and coarse aggregates. The mixture proportions and designations are listed in Table 1. In Table 1, M represents molar strength of NaOH, and R is the ratio of the weight of sodium silicate solution to the weight of the sodium hydroxide solution.

Fine aggregateLocally available river sand (water absorption = 1.5%;

moisture content = 0.5%; specific gravity = 2.6) was used in this work. Sieve analysis, specific gravity, moisture content, and water absorption were determined as per Indian Stan-dard Code IS 2386.16 The particle size distribution is shown in Table 2.

Coarse aggregateGraded crushed stone aggregate (water absorption = 0.9%;

moisture content = 0.25%; specific gravity = 2.6) of 16 mm (0.624 in.) maximum size was collected and stored in the laboratory. The physical properties were determined as per the code stated previously. The particle size distribution is presented in Table 2. Aggregates were not sieved and were used as received directly from the stockpile.

Fly ashClass F fly ash (specific gravity = 2.10) collected by elec-

trostatic precipitator, obtained from Farakka, India, was used

Table 1Mixture designation and mixture proportions, kg/m3 (lb/yd3)Mixture No. Fly ash Sand Coarse aggregate NaOH solution Na2OSiO2 solution Molar strength R Slump, mm (in.)

1 546 (906) 598 (993) 864 (1434) 300.2 (498) 68.4 (113.5) 2.5 M 0.23 200 (8)2 546 (906) 598 (993) 864 (1434) 328.5 (498) 39.4 (113.5) 6.3 M 0.12 140 (5.6)


The rheological test was carried out after exactly 15 minutes from the instant of the addition of the alkaline solution. Every time, a new batch of concrete with the desired composition for a particular mixture was prepared. The mixing sequence and the time at which the rheological test was performed were identical for each batch and for all mixtures.

Over the years, it was established by many workers that the down-curve of the torque-speed plot follows the Bingham model in the case of cement-based materials such as cement paste, cement mortar, and cement concrete. Recently, Laskar and Bhattacharjee15 observed that thixotropy is also present in fly-ash-based geopolymer concrete. Thixotropy will lead to erroneous test results in terms of yield stress and plastic viscosity if it is not properly taken care of. Mewis19 used the term thixotropy to describe an isothermal gel-sol transi-tion due to mechanical agitation. Barnes et al.20 has given a comprehensive review of the topic and described thixotropy as a decrease of the apparent viscosity under constant shear stress or shear rate, followed by a gradual recovery when stress or shear rate is removed. Thixotropy is governed by a combination of reversible coagulation, dispersion, and then recoagulation of cement particles.21 Researchers suggested several approaches to investigate and measure thixotropy. The simplest approach mentioned by Mewis19 is to measure torque under a linear increase and then a decrease in rota-tional frequency. The hysteresis loop, if obtained, indi-cates the presence of thixotropy, although the loops do not provide a good basis for quantitative treatment. For cement-based materials, a stepwise increasing shear rate sequence

followed by a stepwise decreasing shear rate sequence is used to investigate the presence of thixotropy, if any, for concrete mixtures.22 In this study, a stepwise increasing shear rate sequence and then decreasing shear rate sequence was used for every mixture for rheological measurements. The down-curve was used to estimate Bingham param-eters. To minimize the danger for particle migration, the maximum rotational frequency was taken as 40 rpm in this study for all the observations.

In addition to the rheological test, a slump test was also carried out as per Indian Standard Code of Practice IS 7320.23 A slump cone was filled with concrete in three layers. Each layer was tamped 25 times with a standard 16 mm (0.624 in.) diameter steel rod and the top was struck off by means of a screeding and rolling motion of a tamping rod. Immediately after filling, the cone was lifted vertically and the decrease in the height of the center of the concrete was measured as slump.

EXPERIMENTAL RESULTS AND DISCUSSIONThe dosage of plasticizer/superplasticizer affecting yield

strength and plastic viscosity of fly-ash-based geopolymer concrete, the effectiveness of plasticizer/superplasticizer at different molar strengths, and the correlation between slump and rheological parameters have been studied and presented in the subsequent paragraphs.

Figure 2 presents the variation of slump with the variation of WRA dosage for Mixture 1. It can be observed that there is an appreciable increase in slump incorporating both the admixturesnamely, lignin and PC. The admixtures seem to serve their plasticizing effect by dispersing the particles apart, which in turn improves workability by releasing water trapped in the flocs. It may also be observed that the improve-ment of workability in this study is greater with lignin-based water reducer (first-generation plasticizer) than with PC-based admixture (third-generation superplasticizer). At a lignin dosage of 1.5% and above, there was segregation of the mixture; the particles constituting the paste separated, forming a dense lower phase and a creamy upper phase. The variation of workability in this study contradicts the works reported by Criado et al.,14 Bakharev et al.,24 and Douglas and Brandstetr.25 It should be mentioned that Criado et al.14 observed improvement in flow value for fly-ash-based

Table 2Sieve analysis of aggregates

Sieve size, mm (in.)Percent by mass passing

Fine aggregate Coarse aggregate16 (0.624) 10012.5 (0.50) 42.2010 (0.393) 31.806.3 (0.246) 25.04.75 (0.185) 98.2 0.92.36 (0.0937) 96.5 1.70 (0.0663) 94.6 1.18 (0.0469) 91.2 0.60 (0.0234) 66.3 0.30 (0.0117) 20.3 0.15 (0.0059) 1.6

Table 3Chemical composition of fly ashParticulars Composition, %

SiO2 57

Al2O3 27.1

Fe2O3 5.4

CaO 6.1

MgO 2.0

K2O 0.6

SO3 1.4

Loss on ignition 0.8

Fig. 1Schematic diagram of rheometer used in this study.


mortar. Palacios and Puertas26 studied the chemical stability of different types of superplasticizers (melamines, naphtha-lenes, vinyl copolymers, and polycarboxylates) in highly alkaline media and concluded that all (except the naphthalene-based admixture) in an NaOH environment were chemically unstable at pH > 13. At such high values, polycarboxylate-based admixtures underwent alkaline hydrolysis that altered their struc-ture and consequently their dispersing and fluidizing properties.

The variations of yield stress and plastic viscosity of Mixture 1 with the WRA dosages are presented in Fig. 3 and 4, respectively. It can be observed that both the rheological parameters decrease with the increase in WRA dosage, and the decrease is greater in mixtures with lignin. The admix-tures were able to make the mixtures more fluid; therefore, less yield stress and plastic viscosity were obtained compared to the control mixture. The admixtures served their purpose by dispersing the particles apart; subsequently, smaller forces (compared to the control mixture) were required to initiate flow and, hence, the mixture had lower yield stress. Criado et al.14 observed similar variation of yield stress and plastic viscosity of fly-ash-based geopolymer mortar incor-porating PC- and lignin-based admixtures.

To study the effect of WRAs on the thixotropy of geopolymer concrete, the up- and down-curves of mixtures are plotted as shown in Fig. 5. It can be observed that up- and down-curves of all the mixtures, including the control mixture (Mixture 1), form thixotropic loops. It is also can be observed from Fig. 5 that the area under the loop (although this is not a very good method to estimate thixotropy), and hence the degree of thixotropy, continuously decreases with the increase in PC dosage. The degree of thixotropy is the least for the mixture with lignin. The admixtures in this study are able to prevent the formation of flocs to an extent, depending on the dosage of the admixtures. The reason for the least degree of thixotropy with lignin is unknown at the moment and needs further study.

The variation of slump with the variation of plasticizer/superplasticizer dosage for Mixture 2 is presented in Fig. 6. It may be observed that there is a tremendous decrease in slump after the addition of plasticizer/superplasticizer, which is just opposite to that shown in Fig. 2. As before, lignin-based plasticizer shows better performance compared to third-generation PC superplasticizer. The probable reason for the decrease in slump may be a higher molar concentration of NaOH solution compared to Mixture 1, although the exact reason is to be investigated further. The

geopolymer mortar incorporating PC but observed a decrease in flow value using lignin-based chemical admixture. Bakharev et al.24 and Douglas and Brandstetr25 concluded that presence of a naphthalene-based admixture (second-generation superplasticizer) did not improve alkaline system workability. Criado at el.14 also observed that the addition of lignin resulted in segregation of fly-ash-based geopolymer

Fig. 2Variation of slump with water-reducer dosage (Mixture 1).

Fig. 3Variation of yield stress with water-reducer dosage (Mixture 1).

Fig. 4Variation of plastic viscosity with water-reducer dosage (Mixture 1).

Fig. 5Effect of superplasticizer/plasticizer type and dosage on thixotropy (Mixture 1).


slump values at different water-reducer dosage were very low after the addition of chemical admixtures, and the rheo-logical tests were not conducted for Mixture 2.

To investigate the probable effect of molar strength of NaOH solution on the effectiveness of plasticizer/super-plasticizer, Mixture 2 was chosen as the reference mixture. A slump test was carried out for mixtures by changing the molar strength of NaOH for Mixture 2. The results are shown in Fig. 7. It may be observed that at 4 M concen-tration, the effect of the addition of superplasticizer/plas-ticizer is negligible. Mixtures containing NaOH solutions with molar strength above 4 M show a decrease in slump, whereas there is an increase in workability for all mixtures with chemical admixtures at molar strengths less than 4 M. It may also be observed from Fig. 7 that the performance of lignin-based plasticizer is still better compared to third-generation superplasticizer at all molar strengths of NaOH solution, except 1.5 M.

The behavior of fly-ash-based geopolymer concretes using lignin-based plasticizer and PC-based superplasti-cizer are contradictory to the OPC system incorporating these admixtures. In the OPC system, PC is much more effective compared to lignin-based admixture. It is to be remembered that these admixtures are proven suitable for the OPC system and established theories are available for their mode of action. The hydration reaction mechanism of OPC and the alkali activation reaction mechanism are completely different; the hydration product of OPC and reaction product of fly-ash-based geopolymer concrete are also different. Water reducers, including PC, are designed to form complexes with the dissolved Ca2+

formed during early phases of OPC hydration.27 In the case of a fly ash system, there are no dissolved Ca2+ ions. Therefore, the mode of action by which slump increases or decreases is difficult to explain within the context of this study and needs further investigation. For the same reason, it is not clear why lignin imparts better workability than PC admixtures.

The segregation of fly-ash-based geopolymer concrete after the addition of lignin also needs further study. In a study of activated slag systems, Palacios and Puertas26,28 reported changes in infrared spectra of a naphthalene-based admix-ture when stored in a mixed solution of sodium hydroxide and sodium silicate. The changes in spectra were attributed to certain changes in sulfonates responsible for a plasticizing effect. Lignin-based admixture might have experienced similar alterations in this study, and concrete containing lignin suffered from segregation.

Correlation between slump and rheological parameters such as yield stress and plastic viscosity determined by the present rheometer has been studied. Immediately after the rheological test for each sample, concrete was transferred to the mixer. Leftover concrete in the cylindrical container was cleaned manually so that all the mortar is taken out. Concrete was mixed again for 2 minutes and transferred for subsequent testing. A slump test was performed after 30 minutes from the addition of water. It should be noted that that similar procedure was adopted by Wallevik.21 Moreover, it has already been demonstrated that there is no significant change in the workability of fly-ash-based geopolymer concrete for a longer period because there is hardly any geopolymeriza-tion at a room temperature of approximately 20C (68F).

The variation of yield stress and plastic viscosity of Mixture 1 with various plasticizer/superplasticizer dosages with the variation of slump are shown in Fig. 8. It can be

observed that there is a good correlation between slump and yield stress, and slump and plastic viscosity. As slump increases, yield stress and plastic viscosity of concrete with plasticizer/superplasticizer decreases. It should be mentioned in this connection that Wallevik,21 Laskar,29 and other

Fig. 6Variation of slump with water-reducer dosage (Mixture 2) at 6.3 M.

Fig. 7Effectiveness of plasticizer/superplasticizer with variation of molar strength of NaOH solution (Mixture 2).

Fig. 8Correlation between slump and rheological param-eters (Mixture 1). (Note: 1 Pa = 0.0201 lb/ft2; 1 Pa.s = 0.0201 lb.s/ft2.)


9. Zhang, Y. S., Research on Structure Formation Mechanism and Prop-erties of High-Performance Geopolymer Concrete, PhD thesis, Southeast University, Nanjing, China, 2003.

10. Khale, D., and Chaudhary, R., Mechanism of Geopolymerisation and Factors Influencing Its Development: A Review, Journal of Materials Science, V. 42, 2007, pp. 729-746.

11. Duxson, P.; Fernandez-Jimenez, A.; Provis, J. L.; Palomo, A.; and van Deventer, J. S. J., Geopolymer Technology: The Current State of the Art, Journal of Materials Science, V. 42, 2007, pp. 2917-2933.

12. Bondar, D., Alkali Activation of Iranian Natural Pozzolans for Producing Geopolymer Cement and Concrete, PhD dissertation, Univer-sity of Sheffield, Sheffield, UK, 2009.

13. Palacios, M.; Banfill, P. F. G.; and Puertas, F., Rheology and Setting of Alkali-Activated Slag Pastes and Mortars: Effect of Organic Admix-tures, ACI Materials Journal, V. 105, No. 2, Mar.-Apr. 2008, pp. 140-148.

14. Criado, M.; Palomo, A.; and Fernendez-Jimenez, A., Alkali Acti-vated Fly Ash: Effect of Admixtures on Paste Rheology, Rheologica Acta, V. 48, 2009, pp. 447-455.

15. Laskar, A. I., and Bhattacharjee, R., Rheology of Fly Ash Based Geopolymer Concrete, ACI Materials Journal, V. 108, No. 5, Sept.-Oct. 2011, pp. 536-542.

16. IS 2386, Methods for Tests for Aggregates for Concrete, Bureau of Indian Standards, Indian Standard Code of Practice, New Delhi, India, 1999.

17. IS 3812, Specification for Fly Ash for Use as Pozzolana and Admixture, Bureau of Indian Standards, Indian Standard Code of Practaice, New Delhi, India. 1999.

18. Laskar, A. I., and Talukdar, S., Design of a New Rheometer for Concrete, Journal of ASTM International, V. 5, No. 1, Jan. 2008, Paper ID JAI101096.

19. Mewis, J., ThixotropyA General Review, Journal of Non-Newtonian Fluid Mechanics, V. 6, No. 1, 1979, pp. 1-20.

20. Barnes, H. A.; Hutton, J. F.; and Walters, K., An Introduction to Rheology, Rheology Series: 3, Elsevier, Amsterdam, 1989.

21. Wallevik, J. E., Rheology of Particle Suspensions, PhD thesis, The Norwegian University of Science and Technology, Trondheim, Norway, 2003.

22. Lapasin, R.; Papo, A.; and Rajgelj, S., The Phenomenological Description of the Thixotropic Behaviour of Fresh Cement Pastes, Rheo-logica Acta, V. 22, 1983, pp. 410-416.

23. IS 7320, Specification for Concrete Slump Test, Bureau of Indian Standards, Indian Standard Code of Practice, New Delhi, India, 1999.

24. Bakharev, T.; Sanjayan, J. G.; and Cheng, Y. B., Effect of Admix-tures on Properties of Alkali-Activated Slag Concrete, Cement and Concrete Research, V. 30, 2000, pp. 1367-1374.

25. Douglas, E., and Brandstetr, J., A Preliminary Study on the Alkali Activation of Ground Granulated Blast-Furnace Slag, Cement and Concrete Research, V. 20, 1990, pp. 746-756.

26. Palacios, M., and Puertas, F., Stability of Superplasticiser and Shrinkage-Reducing Admixtures in High Basic Media, Materiales de Construccin, V. 54, 2004, pp. 65-86.

27. Uchikawa, H.; Sawaki, D.; and Hanehara, S., Influence of Kind and Added Timing of Organic Admixture on the Composition, Structure and Property of Fresh Cement Paste, Cement and Concrete Research, V. 25, 1995, pp. 353-364.

28. Palacios, M., and Puertas, F., Effect of Superplasticiser and Shrinkage-Reducing Admixtures on Alkali-Activated Slag Pastes and Mortars, Cement and Concrete Research, V. 35, 2005, pp. 1358-1367.

29. Laskar, A. I., Correlating Slump, Slump Flow, Vebe, and Flow Test to Rheological Properties of High-Performance Concrete, Materials Research, V. 12, No. 1, 2009, pp. 63-69.

researchers observed that for cement-based concrete, there is a good correlation between yield stress and slump, and plastic viscosity and slump. For fly-ash-based geopolymer mortar, however, Criado et al.14 reported that no correlation exists between slump and rheological parameters of fly-ash-based geopolymer mortar incorporating lignin-based plasti-cizer and PC-based superplasticizer.

CONCLUSIONSThe following conclusions may be derived from this study:1. Plasticizer and superplasticizer dosage improves work-

ability (measured by slump test) of fly-ash-based geopolymer concrete for a molar strength of a NaOH solution less than 4 M. As the dose of water reducer increases, there is a decrease in the value of rheological parameters. First-gener-ation lignin-based water reducer has been found to be more effective than third-generation PC-based superplasticizer for mixtures having a molar strength greater than 4 M. However, segregation of concrete takes place with the addition of lignin-based water reducer at 1.5% and above.

2. The degree of thixotropy, assessed by area under a torque-speed curve, decreases as WRA dosage increases and the thixotropy is the least with lignin-based WRA. This is observed for molar strength of an NaOH solution less than 4 M.

3. At a higher molar concentration of NaOH (more than 4 M), plasticizer/superplasticizer dosage has an adverse effect on slump and the rheological parameters of fly-ash-based geopolymer concrete. Lignin-based plas-ticizer still shows better performance, as measured by the slump test, than PC-based superplasticizer at higher molar concentrations.

4. The correlation between rheological parameters and slump test results has been found to be good for fly-ash-based geopolymer concrete containing plasticizer and superplasticizer.

REFERENCES1. Tattersall, G. H., Workability and Quality Control of Concrete, E&FN

Spon, London, UK, 1991, 272 pp.2. Tattersall, G. H., and Banfill, P. F. G., The Rheology of Fresh Concrete,

Pitman Publishing, Marshfield, MA, 1983, 368 pp.3. Tattersall, G. H., The Workability of Concrete, A Viewpoint Publica-

tion, Cement and Concrete Association, 1976, 138 pp.4. de Larrard, F.; Szitkar, F.; Hu, C.; and Joly, M., Design of a Rheom-

eter for Fluid Concrete, Special ConcreteWorkability and Mixing, RILEM, 1993, pp. 201-208.

5. Beaupre, B., Rheology of High Performance Shotcrete, PhD thesis, University of British Columbia, Vancouver, BC, Canada, 1994.

6. Palomo, A.; Grutzeck, M. W.; and Blanco, M. T., Alkali Activated Fly Ashes: A Cement for Future, Cement and Concrete Research, V. 29, No. 8, 1994, pp. 1323-1329.

7. Palomo, A.; Alonso, S.; Fernandez-Jimenez, A.; Sorbados, I.; and Sanz, J., Alkaline Activation of Fly Ashes: A NMR Study of the Reaction Products, Journal of the American Ceramic Society, V. 87, No. 6, 2004, pp. 1141-1145.

8. Hardjito, D.; Wallah, S. E.; Sumajouw, D. M. J.; and Rangan, B. V., On the Development of Fly Ash Based Geopolymer Concrete, ACI Materials Journal, V. 101, No. 6, Nov.-Dec. 2004, pp. 467-472.

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110-m46