536 ACI Materials Journal/September-October 2011

Title no. 108-M57

ACI MATERIALS JOURNAL TECHNICAL PAPER

ACI Materials Journal, V. 108, No. 5, September-October 2011.MS No. M-2010-169.R2 received November 12, 2010, and reviewed under Institute

publication policies. Copyright © 2011, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the July-August 2012 ACI Materials Journal if the discussion is received by April 1, 2012.

Rheology of Fly-Ash-Based Geopolymer Concreteby Aminul Islam Laskar and Rajan Bhattacharjee

There is only limited literature available on the rheological behavior of slag-based geopolymer mortar and fly-ash-based geopolymer paste. An attempt is therefore made in this study to examine the rheological behavior of fly-ash-based geopolymer concrete with a mixture of sodium hydroxide and sodium silicate solutions. It was observed that thixotropy is present in fly-ash-based geopolymer concrete, and the degree of thixotropy depends on the molar strength of the sodium hydroxide solution and the ratio of silicate to hydroxide solutions. It was also observed that fly-ash-based geopolymer concrete can be assumed to behave in the Bingham model with a very good accuracy. Bingham parameters, such as yield stress and plastic viscosity, are affected by the molar strength of the sodium hydroxide solution and the ratio of silicate to hydroxide solutions.

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

INTRODUCTIONConventional single-point workability tests such as the

slump test, Vebe test, and compaction factor test are widely used to assess the workability of fresh concrete, both in the laboratory and at the job site. The majority of these test methods are empirical in nature and measure distance or time, which serves as an index of workability.1,2 These tests do not provide any information about the fundamental properties of fresh concrete. Single-point workability tests are therefore considered incapable of providing an adequate characterization of the workability of today’s much more advanced concrete mixtures.1,2 Several authors1-3 have therefore acknowledged the need for a more quantitative measurement of the workability of fresh concrete. Although the slump test is categorized as a single-point workability test, it is still currently the most widely used workability test because of its simplicity and because slump is very useful to check batch-to-batch variations in the materials being fed into the mixer. In concrete rheology, fresh concrete is assumed to be a concentrated suspension of aggregates in cement mortar, and fluid rheology methods are used to describe fresh concrete behavior.3-5 Rheology is defined as the science of the flow and deformation of matter. A material is said to flow if, under constant shear stress, its deformation increases with time. Rheology is a useful tool to characterize and describe various fresh concrete behaviors, such as workability loss, stability, compactibility, and flow behavior.

Over the years, a great deal of research has been carried out, and it has been established that cement concrete as a fluid most often behaves like a Bingham fluid with a good accuracy.1,2 In the Bingham model, flow is defined by two parameters: yield stress and plastic viscosity. Yield stress and plastic viscosity are considered to be the fundamental parameters of fresh concrete rheology. Yield stress gives initial resistance to flow and plastic viscosity governs the flow after it is initiated. Yield stress is the contribution of the skeleton—that is, it is a manifestation of friction

among solid particles. Plastic viscosity is the contribution of suspending liquid that results from viscous dissipation due to the movement of water in the sheared material. The Bingham model is represented by the following equation

0τ = τ + µγ�

where t is the shear stress; t0 is the yield stress; m is the plastic viscosity; and �γ is the shear rate.

Geopolymer concrete, on the other hand, is a developing field of recent research for using solid waste and by-products. Geopolymer concrete is a concrete that contains alkali-activated alumino-silicate mineral binder. Any pozzolanic compound or source of silica and alumina that is readily dissolved in the alkaline solution lends itself to geopolymerization. In geopolymer, polymerization takes place, yielding Si-O-Al bonds as follows

2 2[ (Si O ) Al O] . H On z nM w− − − −

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 alumino-silicate materials is a complex chemical process involving the dissolution of raw materials and the transportation, orientation, and polycondensation of the reaction products. The polymerization process involves a substantially fast chemical reaction under alkaline conditions on Si-Al minerals and results in a three-dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds.6-11 Water is released during the chemical reaction that occurs in the formation of geopolymer.

The alkali activators typically used in geopolymer are either a mixture of sodium hydroxide and sodium silicate or a mixture of potassium hydroxide and potassium silicate.6-8 Zhang,9 Khale and Chaudhary,10 and Duxson et al.11 presented an extensive review on geopolymer technology and summarized the previous research on geopolymer.

A review of the literature reveals that research on the use of geopolymer concrete for strength and durability has been plentiful, and a great deal of research has highlighted the potential use of fly-ash-based geopolymer concrete in the concrete industry. Palacios et al.12 conducted rheological tests on slag-based geopolymer mortar and concluded that slag paste and mortar followed the Bingham model when NaOH

ACI Materials Journal/September-October 2011 537

was used as an activator. Recently, Criado et al.13 investigated the rheological behavior of fly-ash-based geopolymer paste with NaOH as an activator and observed that the Bingham model could be fitted to those pastes. A great deal of research on fly-ash-based geopolymer concrete for conventional workability tests such as slump, the compaction factor, and slump flow tests is also available. There has been little or no research thus far, however, to investigate the rheological behavior of fly-ash-based geopolymer concrete in terms of yield stress and plastic viscosity; this has yet to be reported. An attempt has therefore been made in this study to carry out an experimental investigation to examine the thixotropy and rheological behavior of fly-ash-based geopolymer concrete.

RESEARCH SIGNIFICANCEResearch on the rheological behavior of fly-ash-based

geopolymer concrete is rare in the literature and needs to be explored. Conventional workability test results are qualitative and empirical in nature and rheological parameters are considered to be the fundamental properties of fresh concrete. This study, which deals with the thixotropy and rheology of fly-ash-based geopolymer concrete, will introduce a new idea about the rheological behavior of such concrete.

EXPERIMENTAL INVESTIGATIONMixture proportions

Several 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 the molar strength of NaOH and R is the ratio of the weight of the sodium silicate solution to the weight of the sodium hydroxide solution.

Fine aggregateLocally available alluvial sand with water absorption

of 1.5%, moisture content of 0.5%, and specific gravity of 2.6 was used in this study. The sieve analysis, specific gravity, moisture content, and water absorption were determined as per IS 2386.14 The particle size distribution is shown in Table 2. The sand stockpile was stored inside the laboratory throughout the experimental investigation. The sand was not sieved and was used directly from the stockpile unless otherwise mentioned.

Coarse aggregateMaximum-sized 16 mm (0.624 in.) graded crushed stone

aggregate with water absorption of 0.9%, moisture content of 0.25%, and specific gravity of 2.6 was collected and stored in the laboratory. The physical properties were determined as per IS 2386.14 The particle size distribution is presented in Table 3. The aggregates were not sieved and were used directly from the stockpile.

Fly ashClass F fly ash collected by an electrostatic precipitator was

used in this study. The specifications of the fly ash conform to IS 3812.15 The chemical composition of the fly ash used in this study is as follows: specific gravity of 2.10, loss on ignition (LOI) of 0.8%, and SiO2 + Fe2O3 + Al2O3 = 89.8%.

It should be noted that the main requirements of ASTM C618-94a16 are a minimum altogether content of 70% of silica, alumina, and ferric oxide and a maximum LOI of 6%.

Alkaline activatorsThe most common alkaline liquids used in geopolymeriza-

tion are a mixture of sodium hydroxide and sodium silicate or a mixture of potassium hydroxide and potassium silicate. A mixture of sodium hydroxide and sodium silicate was chosen in this study as an alkali activator. Sodium-based solutions

Aminul Islam Laskar is an Associate Professor in the Department of Civil Engineering at the National Institute of Technology, Silchar, India. He received his BE in civil engineering from the National Institute of Technology; 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 at the National Institute of Technology. He received his BE in civil engineering from Jorhat Government Engineering College, Assam, India, and his MTech from the National Institute of Technology.

Table 1—Mixture designations and proportions in kg/m3 (lb/yd3)Mixture no. Fly ash Sand Coarse aggregate NaOH solution Na2O·SiO2 solution Molar strength, M R Slump, mm (in.)

1 417 (703) 640 (1078) 956 (1611) 212.5 (358) 173.4 (292) 11.4 0.82 Flowing concrete

2 573 (966) 582 (981) 907 (1528) 239 (403) 107 (180) 6 0.45 180 (7.02)

3 561 (945) 570 (960) 888 (1496) 258.4 (435) 155 (261) 10.5 0.60 Flowing concrete

4 561 (945) 570 (960) 888 (1496) 258.4 (435) 155 (261) 17 0.60 180 (7.02)

5 561 (945) 570 (960) 888 (1496) 258.4 (435) 155 (261) 14 0.60 Flowing concrete

6 413 (696) 633 (1067) 946 (1594) 233 (393) 106.4 (179) 10 0.45 150 (6)

7 423 (714) 648 (1095) 968 (1636) 168 (284) 168 (284) 16 1.0 210 (8.4)

8 417 (703) 640 (1078) 956 (1611) 212.5 (358) 173.4 (292) 6 0.82 240 (9.36)

9 413 (696) 633 (1067) 945 (1594) 214.4 (362) 171.4 (290) 3 0.80 70 (2.8)

10 438 (730) 671 (1131) 1002 (1688) 215 (362) 72.7 (122.5) 6 0.34 185 (7.22)

11 440 (743.6) 675 (1141) 1007 (1702) 220 (372) 32 (55) 7 0.15 165 (6.6)

12 444 (748) 681 (1147) 1017 (1714) 184.4 (311) 74 (124.7) 3 0.40 180 (7.02)

13 413 (696) 633 (1067) 945 (1594) 202.9 (343) 202.9 (343) 6 1.0 Flowing concrete

14 450 (758) 690 (1163) 1035 (1744) 174 (293) 75 (126) 5 0.43 65 (2.54)

15 450 (758) 690 (1163) 1035 (1744) 174 (293) 75 (126) 1 0.43 25 (1)

16 417 (703) 640 (1078) 956 (1611) 212.5 (358) 173.4 (292) 20 0.82 185 (7.22)

17 423 (714) 648 (1095) 968 (1636) 230.5 (389.5) 105.4 (178) 16 0.46 40 (1.6)

18 556 (940) 565 (955) 881 (1489) 220.7 (417) 156.2 (264) 3 0.71 Flowing concrete

538 ACI Materials Journal/September-October 2011

were chosen because they are cheaper and because it has been reported that NaOH possesses a greater capacity to liberate silicate and aluminate monomers.11 It has also been reported that sodium cations have better zeolitization capabilities in geopolymer-forming systems.11 The commercial grade sodium hydroxide in pallets (purity of 97% and specific gravity of 2.13) and sodium silicate solution (Na2O = 18.2%, SiO2 = 36.7%, water of 45.1%, and specific gravity of 1.53) were used to prepare the solution. The mass of NaOH pallets in a solution varied according to molar strength M and the mass of silicate solution was taken according to the ratio R.

MixingThe concrete was mixed in a tilting mixer (laboratory

type). The weight of the materials was measured with a

digital weighing balance. The requisite quantities of the sodium hydroxide and sodium silicate solutions were prepared 24 hours in advance. The coarse aggregate, fine aggregate, and fly ash were mixed in the mixer for 2 minutes and then the alkaline solutions were added. The mixture was then mixed for 2 minutes. The mixing sequence was identical for each and every mixture throughout the investigation.

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 rheometer, such as the conceptual design, actual design, calibration, and validation, are published elsewhere.17 Figure 1 presents the schematic diagram of the rheometer.

The prepared concrete was transferred to the cylindrical container with a trowel from the same height each time. The rheological test was carried out exactly 15 minutes after the addition of the alkaline solution. Each 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 the mixtures. Rheological testing was carried out in the laboratory at room temperature (20°C [68°F]) with the relative humidity varying from 70 to 80%.

ThixotropyAn important phenomenon that affects the rheological

measurements of cement-based materials is thixotropy. Thixotropy will lead to erroneous test results in terms of yield stress and plastic viscosity if it is not properly addressed. Mewis18 used the term thixotropy to describe an isothermal gel-sol transition due to mechanical agitation. Barnes et al.19 gave a comprehensive review of this topic and described thixotropy as a decrease of the apparent viscosity under constant shear stress or shear rate, followed by a gradual recovery when the stress or shear rate is removed. The thixotropic sample experiences a reversible, time-dependent decrease in viscosity and depicts a hysteresis loop when the torque is measured under a linear increase and then a decrease in rotational frequency.20 Over the years, it has been established by many researchers1-3 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.

Thixotropy is governed by a combination of reversible coagulation, dispersion, and recoagulation of cement particles.21 Researchers1-3 have suggested several approaches to investigate and measure thixotropy. The simplest approach, developed by Mewis,18 is to measure torque under a linear increase and then a decrease in rotational frequency. The hysteresis loop, if obtained, indicates the presence of thixotropy, although the loops do not provide a good basis for quantitative treatment. Such a linear increase and then linear decrease of the shear rate was adopted by Westerholm and Lagerblad22 and Sheinn et al.23 for the rheological testing of cementitious materials.

The stepwise decreasing shear rate sequence is the most commonly used method for measuring the rheological parameters of concrete such as yield stress and plastic viscosity.24 In a vane rheometer, a rotational frequency of 60 rpm is attained linearly within 2 to 3 seconds and is maintained for 25 seconds.25 The rotational frequency is then

Table 2—Sieve analysis of fine aggregateSieve size, mm (in.) Passing, %

4.75 (0.185) 98.2

2.36 (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 3—Sieve analysis of coarse aggregateSieve size, mm (in.) Passing, %

16 (0.624) 100

12.5 (0.50) 42.20

10 (0.393) 31.80

6.3 (0.246) 25.0

4.75 (0.185) 0.9

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

ACI Materials Journal/September-October 2011 539

there is an overshoot in the torque response and then a steady decrease in the torque. This type of behavior in a material is an important clue that leads the authors to conclude that the material is thixotropic in nature and not rheopeptic.27,29 The measured torque decreases slightly after 60 seconds (not shown in Fig. 4) due to segregation by gravel migration.

As mentioned previously, the area of the thixotropic loop does not provide a good basis for quantitative treatment for thixotropy. To have a qualitative idea about the effect of molar strength and the R-value on the thixotropic behavior of fly-ash-based geopolymer concrete, however, thixotropic loops for different M- and R-values were studied. The loops for different molar strengths of sodium hydroxide at constant R = 0.80 and R = 0.71 are shown in Fig. 5 and 6, respectively.

decreased in steps up to 6 rpm in another 25 seconds. In two-point tests, experiments were performed by decreasing the shear rates in steps from 60 to 6 rpm in 30 seconds.26 A similar application of the shear rate can be applied with a coaxial cylinder rheometer.27 In this study, a stepwise increasing shear rate sequence followed by a stepwise decreasing shear rate sequence was used for all the mixtures. The reason for using an increase and then a stepwise decrease in the shear rate and thus creating a hysteresis loop was mainly to investigate and understand the degree of thixotropic presence, if any, in fly-ash-based geopolymer concrete.

In addition to the application of the shear rate and the duration of the rheological test, the maximum value of the rotational frequency is also important. High rotational frequency in a rheological test introduces a large potential for particle migration of unknown magnitude. Consequently, the impeller will be surrounded by a creamy layer and the test results are erroneous and misleading. To minimize the danger for particle migration, the maximum rotational frequency was taken as 40 rpm in this study for all the observations and the total duration for the up and down curves was 50 seconds.

EXPERIMENTAL RESULTS AND DISCUSSIONInvestigation of thixotropy

The typical torque-speed relationship for two mixtures from this study is presented in Fig. 2 and 3. It can be observed that the starting torque is very high and then decreases rapidly and nonlinearly with the increase in the rotational frequency for the first few seconds. This decrease is due to a continuous thixotropic breakdown, as reported by many in the field in the case of cement-based materials. After a certain rotational frequency, the torque slowly decreases with the increase in rotational frequency. During the down curve, the torque continuously decreases as the rotational frequency decreases. The down curve in all the mixtures under experimental investigation does not follow the up curve, as shown in Fig. 2 and 3. In fact, the down curve always lies below the up curve, which indicates the presence of thixotropy in fly-ash-based geopolymer concrete. It should be noted that the torque-speed relationship of fly-ash-based geopolymer concrete mixtures resembles cement-based materials in this regard.

It should be noted that the loop test curves are similar for thixotropic and rheopeptic materials.27 Of course, rheopeptic materials show an increase in viscosity and hence shear stress when sheared at a constant shear rate over a period of time.28 A rheopeptic material has many features in common with a thixotropic material. Rheopeptic materials show a breakdown at high shear rates that is best presented by a loop test similar to that of a thixotropic material. A rheopeptic material rebuilds its structure faster at a low shear rate, whereas the opposite is true for a thixotropic sample.27 To be able to distinguish between thixotropic and rheopeptic material, a different test was performed on Mixture 11, where the sample was subjected to two constant rotational frequencies (40 and 10 rpm for 90 seconds). The mixture after testing at constant 10 rpm was discarded and a fresh mixture (Mixture 11) was again prepared for 40 rpm. The resulting torque plot for 10 rpm up to 35 seconds is presented in Fig. 4. Data logging for the observed torque was done at every 0.5-second interval. It can be observed that at a constant rotational frequency, the measured torque decreases with time and eventually reaches a steady value. Of course,

Fig. 2—Up and down curve of Mixture 15.

Fig. 3—Up and down curve of Mixture 13.

Fig. 4—Stress growth at constant rotational frequency (Mixture 11).

540 ACI Materials Journal/September-October 2011

in Fig. 8 and 9. It can be observed that the torque steadily decreases as the rotational frequency decreases. Regression analysis of the down curve results in a straight line in all the mixtures under investigation. Thus, torque is a linear function of rotational frequency in this case, which implies that the relationship between shear stress and shear rate follows the Bingham model. It is worthwhile to mention here that the torque-speed relationship in a concrete rheometer is of the form T = a + bN only when it is assumed that the material is a Bingham material, where a and b are constants; T is the measured torque; and N is the rotational frequency.

Mixture 1 was subjected to a two-cycle rheological test without any resting period in between the cycles, and the

It can be observed from Fig. 5 that the area of the thixotropic loop for 20 M sodium hydroxide is the lowest. There is no significant difference between the areas at 6 and 11 M molar strengths. In Fig. 6, the area of the thixotropic loop for 20 M sodium hydroxide is the lowest. No significant difference is observed between the loop areas for the mixtures at 10 and 3 M molar strength of NaOH. Thus, the degree of thixotropy (shown by the area of the thixotropic loop) is not sensitive at lower molar strengths, but the area of the loop decreases to a large extent at a relatively higher molar strength of the NaOH solution.

The effect of the R-value on the area of the thixotropic loop is presented in Fig. 7. It can be observed that the area under the loop is high at a lower R-value and the area decreases as the R-value increases. The area of the thixotropic loop increases again with a further increase in the R-value. Thus, there exists an optimum R-value at which thixotropy is at a minimum. The reason for such a variation of thixotropy with the molar strength of hydroxide and the R-value is not clear and needs to be further investigated.

Rheological parametersThe down curves of the rheological tests were considered

to investigate whether the mixtures under investigation behave like a Bingham, shear-thinning, or shear-thickening material. The typical down curves for two mixtures are shown

Fig. 5—Variation of thixotropic loops at R = 0.80.

Fig. 6—Variation of thixotropic loops at R = 0.71.

Fig. 7—Variation of thixotropic loop area with R-value at 6 M NaOH.

Fig. 8—Down curve of Mixture 9.

Fig. 9—Down curve of Mixture 7.

ACI Materials Journal/September-October 2011 541

studied. Figure 11 shows the variation of yield stress with the molar strength of NaOH. It can be observed that the yield stress increases with the increase in molar strength. NaOH is a viscous liquid, and any increase in NaOH content makes the mixture more cohesive, thereby increasing the yield stress. Moreover, an increase in the molar strength of NaOH means that there is less water available in the mixture, thereby resulting in a decrease in workability. At a higher molar concentration, a significant increase in yield stress is not observed. Plastic viscosity, on the other hand, increases continuously with the increase in molar strength (Fig. 12). The higher the molar strength of NaOH, the more viscosity introduced by the hydroxide solution. The increase in plastic viscosity, however, is also lower at higher molar concentrations.

The effect of the R-value on yield stress is shown in Fig. 13. It can be observed from Fig. 13 that there is an initial decrease in the yield stress with the increase in the R-value. A further increase in the R-value beyond the minimum increases the yield stress further. The initial decrease in the yield stress is possibly due to the availability of more water with the increase in the sodium silicate solution. With more and more sodium silicate solution in the mixture, however, the cohesion of the mixture increases and dominates the effect of the additional water that is incorporated. Except for mixtures at 10 M of NaOH, the increase or decrease in the yield stress is not significant. The variation of plastic viscosity with the R-value is shown in Fig. 14. The mixtures at 3 M molar strength do not show a significant change in plastic viscosity over the range of the R-values investigated in this study. Other mixtures show a continuous increase in

plot is shown in Fig. 10. It can be observed from Fig. 10 that the down curve of the second cycle is below the down curve of the first cycle and a linear equation can be fitted to both the cycles. It is evident from Fig. 10 that particle migration took place to some extent during the second cycle due to the longer testing period. The variation of rheological parameters estimated from both the cycles, however, is less than 10%.

The down curves obtained in this study were used to estimate yield stress and plastic viscosity. Out of several parameters affecting the workability of geopolymer concrete, only the effect of molar strength M of the hydroxide and the R-value (the ratio of silicate to hydroxide, as defined previously) on yield strength and plastic viscosity were

Fig. 10—Two-cycle test on Mixture 1.

Fig. 11—Effect of molar strength on yield strength.

Fig. 12—Effect of molar strength on plastic viscosity.

Fig. 13—Effect of R-value on yield strength.

Fig. 14—Effect of R-value on plastic viscosity.

542 ACI Materials Journal/September-October 2011

plastic viscosity with the increase in the R-value. This is due to the increased viscosity of the mixture due to more and more sodium silicate solution in the mixture.

A comparison of the variation of the yield stress with respect to the molar strength and the R-value with reference to Fig. 11 and 13 shows that the yield stress is more sensitive with respect to the molar strength of the NaOH solution. Similarly, it can be observed from Fig. 12 and 14 that the plastic viscosity is sensitive to both the molar strength of the NaOH solution and the sodium silicate solution content in the mixture. It should be mentioned that the sodium silicate solution is a viscous fluid and in fact is reported to be more viscous than the NaOH solution.30

CONCLUSIONSIn this study, fly-ash-based geopolymer concrete mixtures

with slump varying from 25 mm (1 in.) to flowing concrete and molar strengths of NaOH varying from 1 to 20 M were tested with a rate-controlled concrete rheometer to study their rheological behavior. The following conclusions may be derived from this study:• Thixotropy is present in fly-ash-based geopolymer concrete.

The sample subjected to a stepwise increased and decreased shear rate sequence shows a thixotropic loop.

• The degree of thixotropy, assessed by the area of the thixotropic loop, depends on the molar strength of the NaOH solution and the ratio of the weight of the sodium silicate solution to the sodium hydroxide solution.

• The down curve of the torque-speed plot shows a linear relationship between the torque and rotational frequency and follows the Bingham equation with a very good accuracy. The rheological parameters in terms of yield stress and plastic viscosity can be estimated from the down curve of the torque-speed plot.

• Rheological parameters such as yield stress and plastic viscosity are affected by molar strength and the ratio of the weight of the sodium silicate solution to the sodium hydroxide solution.

• Yield stress is more sensitive to changes in the molar strength of the NaOH solution and is comparatively less sensitive to the quantity of the sodium silicate solution in the mixture. Plastic viscosity, on the other hand, is sensitive to both the molar strength and the quantity of the sodium silicate solution in the mixture.

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25. Koehler, E. P., and Fowler, D. W., “Development of a Portable Rheometer for Fresh Portland Cement Concrete,” ICAR Report 105-3F, ICAR, The University of Texas at Austin, Austin, TX, 2004.

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