Monitoring setting of geopolymers

Advances in Civil EngineeringMaterials

Prannoy Suraneni,1 Sravanthi Puligilla,2 Eric H. Kim,3 Xu Chen,2

Leslie J. Struble,4 and Paramita Mondal5

DOI: 10.1520/ACEM20130100

Monitoring Setting ofGeopolymers

VOL. 3 / NO. 1 / MONTH 2014

Prannoy Suraneni,1 Sravanthi Puligilla,2 Eric H. Kim,3 Xu Chen,2

1Leslie J. Struble,4 and Paramita Mondal5

Monitoring Setting of Geopolymers

Reference

Suraneni, Prannoy, Puligilla, Sravanthi, Kim, Eric H., Chen, Xu, Struble, Leslie J., and Mondal,

Paramita, “Monitoring Setting of Geopolymers,” Advances in Civil Engineering Materials,

Vol. 3, No. 1, 2014, pp. 1–16, doi:10.1520/ACEM20130100. ISSN 2165-39845

2ABSTRACT

3The setting behavior of geopolymer pastes as a function of time was studied

4using two methods: penetration resistance and ultrasonic shear wave

5reflection. Several starting materials were included—metakaolin, Class C and

6Class F fly ashes, and slag—and chemical parameters known to affect set were

7varied. The geopolymers showed a wide range of set times compared to a

8reference Portland cement paste: some were much more rapid, some were

9similar, and some were much slower. Although most geopolymers formed gels

10that were both solid and had measurable strength, some, initially, formed a

11soft gel that had no measurable strength. Therefore, to fully characterize

12setting behavior, it is necessary to use both types of tests. It was seen that

13setting behavior was sensitive to chemical parameters, with setting delayed

14somewhat with increasing silica/alumina and increasing water/alkali, and

15accelerated substantially with calcium hydroxide substitution.

Keywords

16geopolymer, setting, ultrasonic wave reflection, penetration resistance, metakaolin, fly ash,

17slag

18Introduction

19The term geopolymers refers to a broad class of aluminosilicate polymer materials20obtained by the reaction of an aluminosilicate starting material (the powder known21as the precursor) with an alkaline solution. Metakaolin, fly ash, slag, and other

Manuscript received August 27,

2013; accepted for publication

January 2, 2014; published online

xx xx xxxx.

1

Research Assistant, Institute for

Building Materials, Dept. of Civil,

Environmental and Geomatic

Engineering, ETH Zurich,

Switzerland (Corresponding

author).

2

Graduate Research Assistant, Civil

and Environmental Engineering,

Univ. of Illinois at Urbana-

Champaign, Urbana, IL 61801,

United States of America.

3

Quality Engineer, ExxonMobil

Development Company, Houston,

TX 77210, United States of

America.

4

Professor, Civil and Environmental

Engineering, Univ. of Illinois at

Urbana-Champaign, Urbana, IL

61801, United States of America.

5

Assistant Professor, Civil and

Environmental Engineering, Univ.

of Illinois at Urbana-Champaign,

Urbana, IL 61801, United States of

America.

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Advances in Civil Engineering Materials

doi:10.1520/ACEM20130100 / Vol. 3 / No. 1 / Month 2014 / available online at www.astm.org

R

Sticky Note

This is a preprint of an article published in ASTM Advances in Civil Engineering Materials, Vol. 3, No. 1, located at the following ASTM URL: http://www.astm.org/DIGITAL_LIBRARY/JOURNALS/ACEM/PAGES/ACEM20130100.htmThis version is not for distribution.

PROOF COPY [ACEM20130100]

22aluminosilicates have been used as precursor materials in the formation of geopoly-23mers. The alkaline solution is usually a mixture of alkali silicate and alkali hydroxide.24Geopolymers are being promoted as a sustainable alternative to conventional Port-25land cements because less CO2 is generated during their manufacture. They can26show very high compressive strengths and their early age strength often exceeds that27of cement-based materials.28To probe the geopolymer reaction, it is important to have a method to monitor29their setting, but no method has been studied in detail. Despite the differences30between the structural basis for setting of Portland cement and geopolymer, it seems31reasonable to explore whether the setting tests used with Portland cement can be32applied to geopolymers. That is the objective of the study reported here. If the tests33are seen to provide useful measures, it seems reasonable to apply the same setting34criteria to geopolymers, especially if the geopolymer is considered for use as the35binding constituent in structural concrete.36Penetration resistance is commonly used for determining set times of cementi-37tious mixtures. Monitoring concrete setting is critical, because set times indicate38when the material is no longer workable and starts to gain strength and stiffness.39The Vicat test [1] is used for cement pastes and the Proctor test [2] is used for mor-40tar extracted from concrete. The methods are simple and inexpensive, but they are41time consuming and allow only discrete measurements. The Vicat test uses only one42needle and gives little information on the progress of stiffening, whereas the Proctor43test uses different-sized needles and monitors the evolution of stiffening, so the44Proctor test was selected for this study. Some studies have used the Vicat test to45measure geopolymer stiffening [3,4], but only one used the Proctor test, and that46was a study by our group [5].47Because penetration testing has some drawbacks, a new method, ultrasonic48wave reflection (UWR), has been studied in our laboratory for this purpose. In this49method, the intensity of the wave reflected at the interface of a buffer and sample is50monitored as a function of time. Because shear waves (S-waves) propagate in solids51but not in fluids, there is a marked increase in the intensity of the propagated wave,52and, thereby, decrease in the intensity of the reflected wave as setting takes place.53Studies of cementitious mixtures showed good agreement between set times meas-54ured using Proctor penetration resistance and set times measured using UWR with a55low impedance polystyrene buffer [6–8].56In the present study, the same two methods, penetration resistance and UWR,57were used to study geopolymer set. Some of the results in this paper were included58in a paper on Class F fly ash-slag geopolymer setting [5]; but the focus of that paper59was not on methods for monitoring setting of geopolymers in general.60Several different kinds of geopolymers were included in this study. Each is the61subject of separate ongoing research that is not addressed here. A combination of62penetration resistance and UWR was used to monitor the setting of geopolymers as63a function of time. The geopolymers showed setting behaviors ranging from very64rapid to very slow. Setting behavior was found to be sensitive to the presence of65calcium, the water/alkali ratio, and the silica/alumina ratio. Therefore, by using both66penetration resistance and UWR, mix proportions that influence setting and mix-67tures showing problematic setting can be identified. Parts of this work were included68in two graduate theses [8,9].

J_ID: ACEM DOI: 10.1520/ACEM20130100 Date: 12-January-14 Stage: Page: 2 Total Pages: 17


SURANENI ET AL. ON MONITORING SETTING 2



69Background

70GEOPOLYMERS

71The molecular structure of the geopolymer is distinctly different from the structure

72of hydrated Portland cement. Geopolymer gel, the principal binding constituent in

73geopolymers, has a network structure composed of SiO�44 or AlO�54 tetrahedra

74linked by sharing all four tetrahedral corners [10]. Alkali cations and water mole-

75cules may be present in cavities in the network. Although usually amorphous, geo-

76polymer gel is broadly similar to zeolite minerals. Calcium silicate hydrate (C-S-H),

77the principal binding constituent in Portland cement, has a layered structure com-

78posed of sheets of CaO�106 octahedra to which are linked chains (quite short in some

79cases) of SiO�44 tetrahedra [11]. Water molecules and cations (mainly Ca2þ) may be

80present between the layers. Although poorly crystalline, C-S-H is broadly similar to

81the mineral tobermorite.

82The term “setting” in cementitious mixtures refers to the development of rigid-83ity (i.e., transition to solid) in the initially fluid material before there is significant de-84velopment of compressive strength [12]. Geopolymer pastes also show a transition85from a fluid to a solid state, but this process is not well studied. Monitoring geopoly-86mer setting is critical for its use in engineering applications.87The microstructural changes associated with setting of geopolymer are also dis-88tinctly different from those of Portland cement. When alkali solution is added to the89precursor to produce a geopolymer, the precursor dissolves (at least in part) to form90a solution containing aluminate and silicate ions, probably as monomers. These then91polymerize, initially forming a solution of low molecular weight oligomers and92finally transforming to the solid aluminosilicate gel. With fly ash geopolymers, the93precursor is clearly not fully dissolved at the time of initial set, but metakaolin dis-94solves more rapidly and it is not clear how much solid precursor remains during set-95ting. When water is added to Portland cement, the cement particles may flocculate,96but the resulting suspension is nonetheless fluid. As the cement reacts with the97water, calcium silicates dissolve and C-S-H precipitates on the surface of the cement98particles. The C-S-H grows outward from the surface of the particles and serves to99bind cement particles together and produce set, even though the degree of hydration100is quite low. In both materials, setting involves gelation—that is, the transformation101of a suspension to a continuous, three-dimensional solid skeleton (as described in102general [13])—in the geopolymer by linking of aluminate and silicate ions and in the103Portland cement by binding of cement particles by precipitated C-S-H. It should be104noted that both the geopolymer gel molecular structure and the Portland cement105microstructure continue to evolve over time after set. The difference is mainly one of106scale (cement particles being much larger than aluminosilicate polymers) and bond107strength (the bonds between C-S-H-coated cement particles presumably being108weaker than the bonds between aluminosilicate tetrahedra).109Chemical factors have been reported to affect setting behavior of geopolymers.110Setting time increases with increasing silica/alumina ratio [3]. At a lower ratio the111polymerization is more likely to occur between aluminate and silicate species,112whereas at a higher ratio polymerization is more likely among silicate species.113Because the rate of condensation among silicate species is slower than that between114aluminate and silicate species, setting is delayed with a higher ratio. Calcium, which






115is found in some precursor materials, shortens setting time. Various hypotheses116have been proposed to explain the effect of calcium, including heterogeneous nuclea-117tion on additional surfaces provided by precipitated calcium hydroxide [14] and118C-S-H [15,16].

119SETTING

120In a previous study in our laboratory [17], the Proctor test, a standard test for setting121of concrete, was adapted for use with Portland cement pastes. Penetration resistance122values are initially very low and increase in an exponential manner. Initial and final123set times in paste were assigned to penetration resistance values of 2MPa and12414MPa, respectively.125Considerable research has been done in our laboratory on the use of UWR to126measure set times of Portland cement mixtures [6–8,18]. When a normal ultrasonic127wave encounters a boundary between two materials, the wave is in part transmitted128and in part reflected. The relative amount of wave energy reflected depends on the129relative acoustic impedances of the two materials (in this case, the hydrating paste130and a buffer) according to

R ¼Zp � Zb

Zp þ Zb

��

��

(1)

131where:132Zp and Zb are the acoustic impedances of the paste and buffer, respectively.133To detect small changes in the paste impedance, a buffer must be chosen whose134acoustic impedance is close to that of the paste; and most of our studies have utilized135a very low-impedance polystyrene for this purpose. When Zp and Zb become equal,136R becomes zero and changes direction, called an inversion.137In cement paste stiffening [7], the R(t) curve is seen to behave as follows.138Because S-waves do not pass through the paste while it is fluid, R is initially equal to1391. There is a small drop in the first 20min that is attributed to reflocculation of the140cement particles after the disruption as a result of pouring the paste into the test141container. Thereafter, R remains roughly constant during the induction period and142then decreases fairly rapidly because of hydration. Initial set was assigned empiri-143cally to the time R reaches a value of 0.83. There is an inflection point in the R(t)144response indicating a change in the rate of reaction, and final set was assigned145empirically to this inflection point. Using UWR, we were able to clearly identify146initial and final set for various cementitious mixtures, and set times from both UWR147and penetration resistance were associated in a roughly linear manner with a moder-148ately strong correlation. For initial set, with both UWR and penetration resistance,149the coefficient of variation was around 5 %.150In applying these setting methods to geopolymer mixtures, it is necessary to151consider not only whether the method provides a reasonable test for setting but also152whether the criteria developed for Portland cement mixtures may sensibly be applied153to geopolymers. If the geopolymer is being explored as a binder in a structural con-154crete, then the set criteria for both initial and final set using penetration resistance155seem quite reasonable. It is shown here that the set criterion for initial set using156UWR is applicable to geopolymers but that the criterion gives somewhat different






157initial set times than found using penetration resistance. It is unlikely that the set cri-158terion for final set using UWR is even applicable to geopolymers, given that the159inversion in R(t) reflects hydration kinetics in the Portland cement, and it is shown160here that this inversion was not observed in the geopolymer mixtures.

161Experimental Procedures

162MATERIALS

163Four aluminosilicate precursors were used to make geopolymer pastes. Their chemi-164cal compositions are presented in Table 1. The metakaolin (MK), a dehydroxylated165form of kaolinite obtained by heating to a temperature of approximately 800�C, is166widely used in geopolymer research. The MK used in this study was essentially167amorphous (as evidenced by only sharp x-ray diffraction peaks attributed to anatase)168and does not have any calcium, and therefore the geopolymerization reaction is169expected to be relatively simple. The ground granulated blast furnace slag (hereafter170referred to as slag, SL), is entirely amorphous (as evidenced by the lack of any sharp171x-ray diffraction peak) but contains appreciable calcium. The Class F fly ash (FFA)172is low in calcium but contains several crystalline components (quartz, mullite, and173hematite were detected using x-ray diffraction), so the resulting aluminum and174silicon levels in solution are difficult to predict. The Class C fly ash (CFA) is high in175calcium and also contains crystalline components (which are gypsum, quartz, anhy-176drite, periclase, lime, and C3A), so its reaction is expected to be especially complex.177The metakaolin, fly ash, and slag geopolymers were designed based on parame-178ters suggested by Davidovits [19] and Davidovits and Sawyer [20]. The designs179included different silica/alumina ratios and calcium contents to provide different set-180ting behaviors, based on relationships noted in the background discussion. They also181included different water/alkali ratios. Chemical moduli–molar ratios of SiO2 to182Al2O3, Na2O or K2O to SiO2, H2O to Na2O or K2O, and CaO to SiO2þAl2O3,183denoted as S/A, M/S, H/M (where M is the alkali), and C/(SþA) are listed in184Table 2. They were controlled by varying the amounts of alkali hydroxide and alkali185silicate in the solutions and the amount of solution mixed with the precursor. The186MK and CFA geopolymers were made with reagent grade sodium hydroxide (in the187form of pellets) and industrial grade sodium silicate (in the form of water glass), and188the FFA and SL geopolymers were made with reagent grade potassium hydroxide189(in the form of pellets) and potassium silicate (in solution form). The use of different190alkalis reflects the fact that each precursor was the focus of a separate and distinct

TABLE 1

Chemical composition of the precursor materials, given as weight percentage.a

MK CFA FFA SL

Na2O 0.23 2.15 0.81 0.26

K2O 0.19 0.32 2.13 0.48

SiO2 53 30.83 60.17 35.7

Al2O3 43.8 18.61 21.91 11.21

CaO 0.02 28.56 1.81 39.4

aProvided by the manufacturer.






191study, and the effect of different alkalis on the setting is not studied here. The S/A192was varied from 2.85 to 6.00, the M/S was fixed at 0.25, and the H/M was either 15193or 20. The resulting water/solid weight ratio was 0.3–0.4 for CFA and SL geopoly-194mers and 0.6–0.8 for MK and FFA geopolymers. The alkali hydroxide contents var-195ied from about 92 to 156 g for 2000 g of the mixture and the alkali silicate contents196varied from 20 to 825 g for 2000 g of the mixture; lower alkali hydroxide and silicate197contents were used for CFA and slag and higher contents for FFA and MK. The198large range in S/A values and alkali contents was because of the wide compositional199variation in the precursor materials. In this study, only the bulk ratios were200considered.

201MIXING

202Geopolymer pastes were made by mixing the precursor and a solution. The solution203was made by mixing alkali silicate and alkali hydroxide with the required amount of204deionized water. The solution in some cases generated significant heat, so it was pre-205pared about 3 h prior to mixing and allowed to cool to ambient temperature. Solu-206tion and precursor were mixed in a paddle mixer, first placing precursor in the207mixer and then adding solution. These were mixed at the lowest speed for 150 s, the208mixer was stopped for 60 s, while paste was scraped off the container and the paddle,209and then they were mixed at the intermediate speed for 150 s. The mixes were not210tested for their workability, as the UWR detects changes in reflected wave intensity211during fluid to solid transformation and will work regardless of the initial workabil-212ity. However, it was noted visually that the mixes possessed moderately fluid work-213ability until they set.

214PENETRATION RESISTANCE

215The standard penetration resistance procedure [2], modified to work on pastes [17],216was used to evaluate the penetration resistance of the mixes. Around 2 kg of paste217was poured into a plastic container (150mm by 150mm by 100mm). Once in the218container, the specimen was covered with a moist towel to prevent evaporation of219water. Penetration was controlled using a screw-driven load frame with a 100-kN-220load cell. Penetration measurements were made at time intervals chosen according

TABLE 2

Geopolymer chemical moduli.a

Mix No. Precursor S/A (molar) H/M (molar) Water/solids (weight) C/(SþA) (molar)

1 MK 3.00 15 0.59 0.00

2 MK 3.20 15 0.60 0.00

3 MK 3.00 20 0.79 0.00

4 CFA 2.85 15 0.33 0.73

5 CFA 3.00 15 0.34 0.70

6 CFA 3.20 15 0.35 0.67

7 CFA 3.00 20 0.45 0.70

8 FFA 6.00 15 0.62 0.02

9 Slag 6.00 15 0.38 0.91

10 MKþCH 3.00 15 0.59 0.61

aAll mixes had M/S molar ratio 0.25.






221to the discretion of the operator. Measurements were more frequent for pastes that222showed more rapid stiffening. Six needles with standard cross sections of 645 mm2,223323 mm2, 161 mm2, 65 mm2, 32 mm2, and 16 mm2 were used, larger needles for224initial measurements and smaller ones for later measurements, and changed as225required to maintain the penetration depth at about 25mm without exceeding the226capacity of the load cell. The measured load was divided by area of the needle to cal-227culate penetration resistance.

228ULTRASONICWAVE REFLECTION

229The UWR testing followed the procedure developed in our laboratory [6]. After230mixing, around 150ml of paste were poured into a container, 50mm by 60mm by23150mm made using high impact polystyrene. A contact type 2.25-MHz S-wave trans-232ducer was attached to the bottom of the container using phenyl salicylate as cou-233plant. The transducer was connected to a pulser/receiver unit, which was connected234to a computer equipped with a digitizer. Time domain signals were obtained and235converted to frequency domain by use of a fast Fourier transform. The reflection236coefficient R was calculated from the frequency data using a compensating proce-237dure [5] based on the ratio of the reflection amplitudes of cement paste and air.

238Results and Discussion

239Figures 1 and 2 show the setting behavior of geopolymers made with different pre-240cursors. The S/A values differed (MK and CFA geopolymers had S/A 3.00 and FFA241and SL geopolymers had S/A 6.00), but other values (H/M and M/S) were the same242for all geopolymers.243The penetration resistance results (Fig. 1) indicate that the MK and CFA geopol-244ymer behaviors were broadly similar to that of the Portland cement paste (also245shown), first almost zero penetration resistance and then a rapidly increasing resist-246ance. The SL geopolymer, on the other hand, showed only a very short region with

FIG. 1

Setting behavior of

geopolymers made with

different precursors (MK and

CFA mixes S/A 3.00 and FFA

and SL mixes S/A 6.00),

showing penetration resistance

results, with Portland cement

paste w/c 0.40 shown for

comparison.






247zero penetration resistance and then a very rapidly increasing resistance. The FFA248geopolymer did not develop any significant penetration resistance in the time period249tested (1 day). Initial set times (using the criteria reported previously for cement250paste [17]) ranged from around 30min for the SL geopolymer to around 175min251and 205min for the CFA and MK geopolymers to something more than 1 day for252the FFA geopolymer.253In the UWR tests, the geopolymer pastes showed substantial shrinkage, some-254times leading to loss of contact between the paste and the buffer, at which time the255UWR response abruptly became that of air (R¼ 1). This change is not presented to256avoid confusion. Results are also not shown after inversion. Finally, it was not possi-257ble to determine final set for the geopolymers using the inflection criterion proposed258previously for cement paste [6], as the geopolymers tested here did not show the259inflection.260The UWR results (Fig. 2) indicate that the geopolymer behaviors were broadly261similar to those observed using penetration resistance and just described. They all262lacked the initial decrease (within the first 20min of mixing) in R observed with263cement paste (also shown in the figure) and attributed to reflocculation. Otherwise264the behavior of the MK geopolymer was similar to that of the Portland cement paste,265a plateau in R followed by a substantial reduction starting at around 200min. The266FFA geopolymer did not show any appreciable decrease in R throughout the test,267indicating a complete lack of stiffening. The CFA geopolymer and the SL geopoly-268mer both showed an immediate decrease in R, almost linear with time, indicating269immediate and progressive stiffening. Initial set times (using the criteria reported270previously for Portland cement paste) ranged from around 20min for the SL geopol-271ymer to around 70min for the CFA geopolymer to around 265min for the MK geo-272polymer to something more than 1 day for the FFA geopolymer.273Thus, the behaviors of all geopolymers except one were broadly similar by the274two methods. The SL geopolymer showed immediate and rapid stiffening, the MK275geopolymer showed a period of no stiffening lasting for around 200min followed by

FIG. 2

Setting behavior of

geopolymers made with

different precursors (MK and

CFA mixes S/A 3.00 and FFA

and SL mixes S/A 6.00),

showing UWR results, with

Portland cement paste w/c

0.40 shown for comparison.






276progressively more rapid stiffening, and the FFA geopolymer showed no stiffening277for at least 1 day using both methods. However, the behavior of the CFA geopolymer278was quite different; penetration resistance showed an initial period with no stiffening279followed by progressive stiffening, whereas UWR showed progressive stiffening right280from the start of the test. Because the behavior of the CFA geopolymer using UWR281was so different from that of the other geopolymers, the test was repeated several282times on fresh samples; but results were always the same, that stiffening began im-283mediately and was more rapid using UWR than using penetration resistance. In the284results shown here, the mixture was seen to set at around 70min using UWR,285whereas penetration resistance was almost zero at this time; and the mixture set at286around 175min using penetration resistance, whereas R at this time was around 0.3,287a very low value indicating that the paste was quite solid. The low penetration resist-288ance at the time when UWR showed initial set suggests that a soft gel had formed,289solid but with very little strength; therefore, reducing S-wave reflection but not290increasing penetration resistance. This gel subsequently gained strength (as shown291by the increasing penetration resistance). For the CFA geopolymer, it appears that292set time and the nature of the material at set can only be understood using both293methods.294Results for MK geopolymers with different S/A values are shown in Fig. 3. Mixes295were made with three values of S/A, but tests could not be performed on the mix296with S/A 2.85 because it was too stiff to mix properly (the mixer stopped frequently).297Using both methods, the other two pastes showed a plateau followed by stiffening.298The mix with S/A 3.20 showed much later set than the mix with S/A 3.00 (initial set299using penetration resistance was around 435min compared to around 205min).300Higher values of S/A were not tested as the set times were expected to be extremely301long (following the trend of S/A 3.20). Thus, small increases in S/A produced large302delays in setting.303Results for CFA mixtures with different S/A values are shown in Fig. 4. With304S/A 2.85 stiffening began immediately and increased rapidly, showing that this mix305was forming a hard solid almost instantaneously. The mixture had an initial set time

FIG. 3

Setting behavior of MK

geopolymers with different S/A

values, showing penetration

resistance and UWR results.

Penetration resistance values

start from zero and

progressively increase, whereas

S-wave R values start from one

and progressively decrease.






306using penetration resistance of around 5min, an extremely rapid set. With S/A 3.00307and 3.20, stiffening began later and increased more slowly. On the whole, the trend308was that set time increased with increasing S/A ratio. All three mixes began to stiffen309quickly without an initial plateau when tested using UWR, although the two higher310S/A mixes showed a plateau (shorter in duration than for MK mixes) using penetra-311tion resistance. Therefore, it can be concluded that the mixes with S/A 3.00 and 3.20312initially formed a soft gel (which later hardened), whereas the mix with S/A 2.85313formed a hard gel.314Although the S/A was seen to be important in determining set time, its specific315effect differed between the two geopolymer precursors, as shown in Fig. 5. Set times316(initial and final set using penetration resistance and initial set using UWR) broadly317increased with increasing S/A. However, the effect was much stronger in the MK318mixtures than in the CFA mixtures. Also, the CFA S/A 3.20 mixture did not follow

FIG. 5

Set time as a function of S/A

ratio, showing both initial and

final set using both ultrasonic

wave reflection and penetration

resistance.

FIG. 4

Setting behavior of CFA

geopolymers with different S/A

values, showing penetration

resistance and UWR results.

Penetration resistance values

start from zero and









319this trend for initial set, and initial set by both methods was slightly lower than the320value for CFA S/A 3.00. It is unclear why the results with the CFA S/A 3.20 mixture321do not conform to the general trend. The strong effect of S/A on MK geopolymer set322times was observed previously, although conditions were somewhat different (simi-323lar S/A ratios), however, testing was performed with a Vicat needle at 40�C) [3,21].324Similar to results presented here, the effect of S/A on the set times of CFA geopoly-325mers was not as strong as on the MK geopolymers over similar S/A ratios [3,21,22].326However, the reported effect of the S/A ratio on set times of CFA geopolymers seems327to be complex (broadly, set times increased with increasing S/A ratio until S/A of328about 3.50, and subsequently decreased) [22].329Tests were also done on MK and CFA geopolymers with varying H/M values.330Increasing the H/M ratio was accomplished by increasing the water/solids ratio in331the geopolymer. Results are shown in Fig. 6. Both MK geopolymers showed an initial332plateau followed by rapid stiffening. Set was delayed with the higher H/M (initial set333using penetration resistance increased from around 205min to around 250min).334Both CFA geopolymers again showed different behavior with the two tests, stiffening335after a plateau using penetration resistance and stiffening immediately using UWR.336But set was also delayed when H/M was increased (initial set using penetration re-337sistance increased from around 175min to around 285min), although there was338almost no difference with UWR. It is not clear from these experiments what mecha-339nism is responsible for the delay in set with increasing H/M. As noted, in these mix-340tures increasing H/M coincided with increasing water/solids (because M/S was kept341constant). Results may indicate that higher water content delays gelation because a342critical alumina and/or silica concentration is required for the gel to form. Alterna-343tively, the difference in the delay between the two methods, seen here only for the344CFA geopolymer, may indicate that increasing H/M does not affect gel formation345but does affect gel strength. To the knowledge of the authors, the effect of H/M on346the set times of geopolymers has not been directly studied to date, although FTIR347evidence shows that increasing H/M decreases the rate of the geopolymerization348reaction (which, in principle, should increase the set time) [23].

FIG. 6

Setting behavior of MK and

CFA geopolymers with

different H/M values (all with S/

A 3.00). Penetration resistance

values start from zero and









349As discussed previously, the presence of calcium has been seen to shorten geo-350polymer set times. This effect probably explains why the CFA and the SL geopoly-351mers, both of which contain calcium, showed more rapid setting. To test this effect352further, calcium hydroxide (CH) was added to an MK geopolymer with S/A 3.00,353replacing 30 % by weight of the metakaolin. Results are shown in Fig. 7. The geopol-354ymer with calcium hydroxide replacement stiffened much more rapidly (using pene-355tration resistance, initial set was reduced from around 205min to around 30min).356Furthermore, the geopolymer with calcium began stiffening immediately and357showed no initial plateau. However, the results using the two methods agreed for358both mixtures, so calcium is not implicated in the lack of agreement seen here with359CFA geopolymers. Thus, the behavior of the MK geopolymer was completely360changed by the substitution of a large amount of calcium. In fact, the behavior of the361MK geopolymer with calcium hydroxide replacement was very similar to that of the362SL geopolymer (immediate and rapid stiffening, with initial set times of about36315min from UWR and about 30min from penetration resistance), although it did364not have as much calcium (Table 2). These results agree with previously published365results; calcium hydroxide and calcium oxide have been shown to significantly accel-366erate setting of geopolymer mixes [14].367The initial set times using UWR and penetration resistance correlated well on368all except the CFA mixtures, which initially formed a soft gel. It can be seen in Fig. 8

369that the set times using these two methods correlated well (points were fitted by a370single line with a high correlation coefficient) for many of the geopolymer mixtures371and for the Portland cement paste, although the set times were not equal (as evi-372denced by the divergence between the fitted line and the unity line). In earlier work,373on set time of Portland cement paste [5], we showed better agreement between the374two methods (the points could be fitted by the unity line, although with not nearly375this high a correlation coefficient), and here the Portland cement results fell close to376the unity line. More importantly, the CFA mixtures that initially formed a soft gel377did not fall on the same line as the geopolymers with other precursors. Thus, the378correspondence in set times between the two methods was quite different for CFA

FIG. 7

Setting behavior of MK

geopolymers, plain and with

calcium hydroxide

replacement. Penetration

resistance values start from

zero and progressively

increase, whereas S-wave R

values start from one and

progressively decrease.






379mixes (except the mix with S/A 2.85) than for the other precursors in this study and380for Portland cement. This unexpected difference seems to possibly reflect that the381Class C fly ash precursor formed initially soft gels, which subsequently hardened, as382opposed to the metakaolin and slag precursors, which immediately formed a hard383gel, especially at higher H/M and higher S/A. The existence of multiple gels with dif-384ferent mechanical properties in geopolymerization is known [24,25]; however, it is385unclear why this behavior depends on the precursor, and we are investigating the386topic further.387It appears that the two methods are different in a way not evidenced in our pre-388vious study with Portland cement. Based on our understanding of setting in the two389systems (as described in the background sections), it is reasonable to conclude that390the ultrasonic behavior is more sensitive to formation of geopolymer gel and the391penetration resistance is more sensitive to strength of the gel. With most geopolymer392mixtures, initial set time can be determined using either a penetration test or an ul-393trasonic test; but with some mixtures it is necessary to use both tests to fully charac-394terize setting.395As noted throughout this section, many but not all mixtures showed an initial pe-396riod with no stiffening, similar to the induction period observed typically and shown397here for Portland cement paste. This behavior was seen with geopolymers made using398MK and FFA (although the latter observation is not especially helpful given that the399mixture did not set at all during the test period) but not with geopolymers made using400SL or CFA or MK with Ca(OH)2 replacement. Interestingly, this behavior was seen401more commonly using penetration resistance than using UWR, perhaps again indi-402cating that the UWR is more sensitive to gelation than to gel strength.

403Conclusions

404The following conclusions were drawn from the study:

4051. 406Either S-wave UWR or penetration resistance can be used to monitor the set-407ting behavior of geopolymers. To fully characterize setting of geopolymers, it408is necessary to measure using both methods.

FIG. 8

Comparison of initial set times

using UWR and penetration

resistance. Best fit and unity

lines are shown.






4092. 410Geopolymers show a wider range of setting behavior compared to Portland411cement, some mixtures set much more rapidly, some set in the same time412period as Portland cement, and some mixtures set much more slowly. Geopol-413ymers made with metakaolin showed an initial period during which no stiff-414ening was observed, much as the induction period in Portland cement paste,415but geopolymers made with slag or Class C fly ash did not show this behavior,416especially when tested using UWR. The behavior of most of the tested Class C417fly ash geopolymers was different between UWR and penetration resistance,418indicating that they formed a soft gel.4193. 420Geopolymer composition plays an important role in determining setting421behavior. Increasing the H2O/(Na,K)2O delayed setting somewhat. Increasing422the SiO2/Al2O3 delayed setting somewhat, especially in the metakaolin geopol-423ymers. Replacement of a portion of the metakaolin with Ca(OH)2 substan-424tially accelerated setting.

425ACKNOWLEDGMENTS

426The writers are grateful to Prof. John Popovics at the University of Illinois for use of427his laboratory facilities for ultrasonic testing, and to Dr. Chul-woo Chung at428Pukyong National University and Dr. Givanildo Azeredo at Universidade Federal da429Paraıba for helpful discussions about the method. Some of the work reported here is430supported by NSF (DMR 1008102) and some is supported by the Department of431Civil Engineering at the University of Illinois. The metakaolin used in this study was432supplied by BASF, the Class C fly ash by Boral, and the Class F fly ash and slag by433Lafarge.

434References

435[1] ASTM C191: Test Method for Time of Setting of Hydraulic Cement by Vicat436Needle Annual Book of ASTM Standards, ASTM International, West Consho-437hocken, PA, 2008.438[2] ASTM C403: Test Method for Time of Setting of Concrete Mixtures by Pene-439tration Resistance, Annual Book of ASTM Standards, ASTM International,440West Conshohocken, PA, 2008.441[3] De Silva, P., Sagoe-Crenstil, K., and Sirivivatnanon, V., “Kinetics of Geopoly-442merization: Role of Al2O3 and SiO2,” Cement Concrete Res., Vol. 37, No. 4,4432007, pp. 512–518.444[4] Cheng, T. W. and Chiu, J. P., “Fire-Resistant Geopolymer Produced by445Granulated Blast Furnace Slag,” Miner. Eng., Vol. 16, No. 3, 2003, pp.446205–210.447[5] Puligilla, S. and Mondal, P., “Role of Slag in Microstructural Development and448Hardening of Fly Ash-Slag Geopolymer,” Cement Concrete Res., Vol. 43, 2013,449pp. 70–80.450[6] Chung, C.-W., 2010, “Ultrasonic Wave Reflection Measurements on Stiffening451and Setting of Cement Paste,” Ph.D. dissertation, University of Illinois at452Urbana-Champaign, Champaign, IL, 197 pp.





http://www.astm.org/Standards/C191


http://dx.doi.org/10.1016/j.cemconres.2007.01.003

http://dx.doi.org/10.1016/S0892-6875(03)00008-6



453[7] Chung, C.-W., Suraneni, P., Popovics, J. S., and Struble, L. J., “Setting Time454Measurement Using Ultrasonic Wave Reflection,” ACI Mater. J., Vol. 109, No.4551, 2011, pp. 109–118.456[8] Suraneni, P., 2011, “Ultrasonic Wave Reflection Measurements on Self-457Compacting Pastes and Concretes,” M.S. thesis, University of Illinois at458Urbana-Champaign, Champaign, IL, 119 pp.459[9] Puligilla, S., 2011, “Understanding the Role of Slag on Geopolymer Hardening460and Microstructural Development,” M.S. thesis, University of Illinois at461Urbana-Champaign, Champaign, IL, 72 pp.462[10] Fernendez-Jimenez, A. and Palomo, A., “Nanostructure/Microstructure of Fly463Ash Geopolymers,” Geopolymers, J. L. Provis and J. S. J. van Deventer, Eds.,464CRC Press, Boca Raton, 2009, Chap. 6, 91 pp.465[11] Taylor, H. F. W., Cement Chemistry, 2nd ed., Thomas Telford, London, 1997,466Chap. 5.467[12] ASTM C125: Standard Terminology Relating to Concrete and Concrete Aggre-468gates, Annual Book of ASTM Standards, ASTM International, West Consho-469hocken, PA, 2011.470[13] Brinker, C. J. and Scherer, G. W., Sol-Gel Science, Academic, San Diego, 1990,471Chap. 1.472[14] Lee, W. K. W. and van Deventer, J. S. J., “The Effect of Ionic Contaminants on473the Early-Age Properties of Alkali-Activated Fly Ash-Based Cements,” Cement474Concrete Res., Vol. 32, No. 4, 2002, pp. 577–584.475[15] Yip, C. K., Lukey, G. C., Provis, J. L., and van Deventer, J. S. J., “Effect of Cal-476cium Silicate Sources on Geopolymerization,” Cement Concrete Res., Vol. 38,477No. 4, 2007, pp. 554–564.478[16] Lloyd, R. R., Provis, J. L., and van Deventer, J. S. J., “Microscopy and Micro-479analysis of Inorganic Polymer Cements. 2: The Binder Gel,” J. Mater. Sci., Vol.48044, No. 2, 2009, pp. 620–631.481[17] Chung, C.-W., Mroczek, M., Park, I. Y., and Struble, L. J., “On the Evaluation482of Setting Time of Cement Paste Based on ASTM C403 Penetration Resistance483Test,” J. Test. Eval., Vol. 38, No. 5, 2010, pp. 61–68.484[18] Chung, C.-W., Suraneni, P., Popovics, J. S., Struble, L. J., and Weiss, J. W.,485“Application of Ultrasonic P-Wave Reflection to Measure Development of Early-486Age Cement-Paste Properties,”Mater. Struct., Vol. 46, No. 6, 2013, pp. 987–997.487[19] Davidovits, J., Geopolymer Chemistry and Applications, 2nd ed., Institut Geo-488polymere, Saint-Quentin, France, 2008, Chap. 9.489[20] J. Davidovits and J. L. Sawyer, “Early High-Strength Mineral Polymer,” 1985,490U.S. Patent No. 4,509,985.491[21] De Silva, P. and Sagoe-Crenstil, K., “The Effect of Al2O3 and SiO2 on Setting492and Hardening of Na2O-Al2O3-SiO2-H2O Geopolymer Systems,” J. Austr.493Ceram. Soc., Vol. 44, No. 1, 2008, pp. 39–46.494[22] Chindaprasirt, P., De Silva, P., Sagoe-Crenstil, K., and Hanjitsuwan, S., “Effect of495SiO2 and Al2O3 on the Setting and Hardening of High Calcium Fly Ash-Based496Geopolymer Systems,” J. Mater. Sci., Vol. 47, No. 12, 2012, pp. 4876–4883.497[23] Rees, C. A., 2007, “Mechanisms and Kinetics of Gel Formation in Geopol-498ymers,” Ph.D. dissertation, University of Melbourne, Melbourne, Australia,499176 pp.






http://dx.doi.org/10.1016/S0008-8846(01)00724-4

http://dx.doi.org/10.1016/S0008-8846(01)00724-4


http://dx.doi.org/10.1007/s10853-008-3078-z


http://dx.doi.org/10.1617/s11527-012-9948-5

http://dx.doi.org/10.1007/s10853-012-6353-y


500[24] Duxson, P., Fernandez-Jimenez, A., Provis, J. L., Lukey, G. C., Palomo, A., and501van Deventer, J. S. J., “Geopolymer Technology: The Current State of the Art,”502J. Mater. Sci., Vol. 42, No. 9, 2007, pp. 2917–2933.503[25] Fernandez-Jimenez, A., Palomo, A., Sobrados, I., and Sanz, J., “The Role Played504by the Reactive Alumina Content in the Alkaline Activation of Fly Ashes,”505Micropor. Mesopor. Mater., Vol. 91, Nos. 1–3, 2006, pp. 111–119.





http://dx.doi.org/10.1007/s10853-006-0637-z

http://dx.doi.org/10.1016/j.micromeso.2005.11.015

Monitoring setting of geopolymers

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