TECHBRIEF Evaluation of High-Volume

Fly Ash Mixtures (Paste and 

Mortar Components) Using a 

Dynamic Shear Rheometer and

an Isothermal Calorimeter

FHWA Publication No.: FHWA-HRT-12-062

NTIS Accession No. of the report covered in this TechBrief: 

PB2012-112546

FHWA Contact: Ahmad Ardani, HRDI-10, (202) 493-3422, 

ahmad.ardani@dot.gov

This document is a technical summary of the unpublished Federal 

Highway Administration (FHWA) report, Evaluation of High-Volume 

Fly Ash (HVFA) Mixtures (Paste and Mortar Components) Using a 

Dynamic Shear Rheometer (DSR) and an Isothermal Calorimeter, 

available through the National Technical Information Service (NTIS).(1)

Objective

The primary objective of this study was to develop a rationale for 

using a dynamic shear rheometer and an isothermal calorimeter as 

practical, quick scanning tools for the following purposes:

• Predict and assess early-age behavior of concrete mixtures 

containing different types and levels of cement and fly ash.

• Identify incompatible blends.

• Verify performance.

Introduction

Many transportation experts in State transportation departments, 

the concrete industry, and academia are exploring ways to make concrete more sustainable and environmentally friendly. Supplementary 

cementitious materials (SCMs) such as fly ash, slag cement, and 

natural pozzolans have been used by many transportation agencies 

to achieve sustainability through the following features:

• Improved concrete performance and durability.

• Improved rheological properties (workability, finishability). 

• Increased use of byproducts.

• Reduced carbon footprint associated with concrete production.

• Reduced overall cost of concrete.

Although the use of SCMs has increased steadily over the last two 

decades because of the benefits they afford, their use in highway 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research/
2

applications still poses many unanswered questions. There is no sound, systematic protocol that 

can be used to routinely evaluate and proportion 

SCMs into concrete mixtures while ensuring that 

performance and durability are not compromised. 

Chemical, mineralogical, and granulometric characteristics of fly ash can vary from one source to 

another and within the same source. This variability 

in fly ash could have a profound impact on fresh and 

hardened properties of concrete.

Many transportation agencies have been using fly ash 

in their concrete pavement mixtures, with replacement levels ranging from 10 to 30 percent (typically 

20 percent of the total cementitious material); however, these specifications are often based on empirical 

estimates that lack sound engineering evaluation. In 

an attempt to reduce the carbon footprint associated 

with cement production, reduce its adverse impact 

on the environment, and ultimately improve concrete 

performance, many transportation departments have 

expressed interest in using higher dosages of fly ash 

in concrete infrastructure. 

While high-volume fly ash concrete can be proportioned to produce durable concrete, its use is 

not without problems. Some issues include slow 

strength gain at early ages, delayed setting, and 

reduced bleeding that results in extended curing 

time requirements and eventually slows down concrete paving during construction. 

Experimental Investigation

In the study, a total of 12 mortar mixtures and 

14  paste mixtures were prepared. Two different 

Type I portland cements (low alkali and high alkali) 

and two different fly ashes (Class F and Class C) 

were used at three replacement levels (20, 40, and 

60 percent). Table 1 shows a summary of the paste 

and mortar mixtures. A water-cementitious materials 

(w/cm) ratio of 0.40 was used for all the mixtures. 

Mortar mixtures were mixed following ASTM C305, 

except for the mixer requirements.(2) Flow tests 

(ASTM C1437), modified unit weight using the base 

of the rollameter, setting time (ASTM C403), and 

compressive strength (ASTM C109) at the ages of 3, 

7, 28, 56, 91, and 119 days were carried out.(3–5) Three 

cubes were tested at each age.

Paste mixtures were prepared according to ASTM 

C1738.(6) All materials were kept at 73 ±5 °F (23 ±3 °C) 

for at least 1 day before mixing the paste.

Isothermal calorimetry was performed at 77 °F (25 °C) 

for 72 h, following ASTM C1679.(7) Four replicates per 

mixture were run, with masses ranging from 0.157 

to 0.169 oz (4.44 to 4.78 g). The heat of hydration 

was measured via a commercial eight-channel heat 

conduction isothermal calorimeter. 

Rheological properties were tested following the procedure suggested by Ferraris and Obla at ages of 8, 30, 

50, 70, and 90 min after the cement contacted water, 

except for mixture LA20F, which was tested only at 8, 

30, and 50 min.(8) The tests were carried out at a controlled temperature of 77 ±0.4 °F (25 ±0.2 °C). 

A parallel plate rheometer was used to determine 

yield stress and plastic viscosity. In order to avoid 

slippage, 1.4-inch (35-mm) serrated plates were 

used. A 0.02-inch (0.4-mm) gap was selected to 

represent the median distance between aggregates 

in concrete.(9) The shear rates selected ranged from 3 

to 50 s-1, following the Ferraris and Obla procedure.(8)

Results and Discussion

Fresh Properties

Table 2 shows the fresh property test results for 

mortar. Class C fly ash mixtures exhibited higher 

flow for all replacement levels and for both cements. 

The flow of mixtures containing Class C fly ash also 

increased with increasing fly ash content. This trend 

was not observed in Class F fly ash mixtures. 

Table 1. Mixtures in the experimental program.

Mixes

Fly Ash 

(percent)

Cement 

Type

Fly Ash 

Class

LA1 0 Low alkali None

LA20F 20 Low alkali Class F

LA40F 40 Low alkali Class F

LA60F 60 Low alkali Class F

LA20C 20 Low alkali Class C

LA40C 40 Low alkali Class C

LA60C 60 Low alkali Class C

HA1 0 High alkali None

HA20F 20 High alkali Class F

HA40F 40 High alkali Class F

HA60F 60 High alkali Class F

HA20C 20 High alkali Class C

HA40C 40 High alkali Class C

HA60C 60 High alkali Class C

1Only paste mixtures prepared.
3

For the same replacement level, mixtures containing Class C fly ash and high alkali cement presented 

a higher flow than the correspondent mixtures 

with low alkali cement. Again, this trend was not 

observed in Class F fly ash mixtures.

As expected, as the fly ash content increased, the 

initial and final setting times also increased, but 

this trend was even more pronounced in Class C fly 

ash mixtures.

Compressive Strength

Figure 1 and figure 2 show the strength development over time. As expected, compressive strength 

decreased with an increase of fly ash content, and 

the decrease was more pronounced at early ages. 

Nevertheless, the compressive strengths achieved 

were quite high for replacements of 20 and 40 percent, reaching at least 3,000 psi (21,000 kPa), even at 

3 days. There was little strength increase from 91 to 

119 days.

As shown in figure 1, mixtures containing low alkali 

cement with Class C fly ash yielded higher strengths, 

especially at longer ages, with the exception of 

mixtures containing only 20 percent fly ash. In mixtures containing high alkali cement, there was no 

significant difference in strength between mixtures 

with Class F fly ash and Class C fly ash at longer ages, 

as shown in figure 2.

Calorimetry

A typical heat profile from isothermal calorimetry 

shows three peaks. An initial peak occurs immediately 

after mixing the water with the cementitious materials 

due to the rapid dissolution of C3A and initial 

formation of ettringite (AFt) phases. In the current 

experiment, this peak is not shown because the mixtures were prepared externally prior to insertion into 

the calorimeter. The second peak is related to the 

hydration of C3S, and the third peak, also called the 

“sulfate depletion peak,” corresponds to the reaction 

of C3A. It has been suggested that the third peak 

relates to the renewed formation of ettringite.(10)

The heat flow over time is presented in figure 3 and 

figure 4 for low and high alkali cement, respectively. 

In the figures, LA is a mixture containing only low 

alkali cement and HA is a mixture containing only 

Table 2. Fresh properties of mortars.

Mixes

Flow 

(percent)

Unit 

weight 

(lb/ft3)

Initial 

setting 

(min)

Final 

setting 

(min)

LA20F 94.5 139 214 311

LA40F 95.6 135 225 345

LA60F 88.1 132 232 363

LA20C 108.0 138 312 416

LA40C 125.7 137 423 562

LA60C 137.5 139 514 680

HA20F 99.5 136 205 302

HA40F 95.9 134 223 338

HA60F 84.8 131 247 421

HA20C 123.0 138 299 413

HA40C 138.0 138 422 567

HA60C 147.4 140 653 875

1 lb/ft3 = 16.02 kg/m3

4000

6000

8000

10000

12000

Compressive strength (psi)

0

2000

0 20 40 60 80 100 120 140

Age (days)

LA20F LA40F LA60F LA20C LA40C LA60C

Figure 1. Compressive strength development of mortar mixtures containing low alkali cement.

1 psi = 6.89 kPa
4

high alkali cement. The substitution of cement by fly 

ash caused a dilution effect due to the fact that fly 

ashes are normally inert during the first few hours. As 

a consequence, the maximum heat flow decreased 

with increasing fly ash content, and in some cases, 

there was retardation in the heat flow. For the same 

mass replacement, Class C fly ash mixtures yielded 

higher degrees of retardation than Class F fly ash 

mixtures; although the volume of Class C fly ash for 

the same mass was slightly lower due to its higher 

specific gravity. Similar behavior was observed by 

Bentz when using fly ashes from the same sources.(11)

384

214

395

225 424

232

489

312 643

423

744

514

403

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Heat Flow (W/g)

514

0

0 200 400 600 800 1000

Time (min)

LA LA20F LA40F LA60F LA20C LA40C LA60C

Figure 3. Heat flow of mixtures containing low alkali cement obtained through isothermal calorimetry.

453

205

460

223

506

247

968

653

593

299

422

427

768

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

Heat Flow (W/g)

0

0 200 400 600 800 1000

Time (min)

HA HA20F HA40F HA60F HA20C HA40C HA60C

Figure 4. Heat flow of mixtures containing high alkali cement obtained through isothermal calorimetry.

4000

6000

8000

10000

12000

Compressive strength (psi)

0

2000

0 20 40 60 80 100 120 140

Age (days)

HA20F HA40F HA60F HA20C HA40C HA60C

Figure 2. Compressive strength development of mortar mixtures containing high alkali cement.

1 psi = 6.89 kPa
5

A small fourth peak can be observed in mixtures containing Class C fly ash. This peak increases with the 

increase in fly ash content and occurs between 22 and 

23 h. This peak has been attributed to the hydration 

of C4AF as well as the conversion of AFt to the AFm 

phase.(12) However, in the present study, this peak 

was found to increase with the increase of Class  C 

fly ash. Consequently, it was concluded that either 

the fly ash promotes the hydration of the cement and 

serves as a nucleation site for the cement hydration 

(and more specific to the hydration of C3A), or the 

pozzolanic reaction of the fly ash could manifest itself 

in the fourth hydration peak.(12) This peak appears 

slightly bigger with mixtures containing low alkali 

cement, which has a lower C3A content and a higher 

C4AF content than the high alkali cement.

The curves for the high alkali cement mixtures shifted 

to the right, indicating a delay compared to the low 

alkali cement mixtures. The delay on the maximum 

heat flow varied from 24 min for plain mixtures to 

223 min for mixtures containing 60 percent Class C 

fly ash. The difference between low alkali and high 

alkali cement mixtures containing Class F fly ash was 

less pronounced, ranging from 69 to 83 min.

In each curve shown in figure 3 and figure 4, with the 

exception of the mixtures containing only cement, two 

markers are shown. The first represents the initial set 

time of the respective mortar mixture, and the second represents the time of the maximum heat flow. In 

figure 5, these two markers are plotted against each 

other, correlating the time of maximum heat flow of 

pastes and the initial setting time of the mortars containing the same proportions of cementitious materials 

and the same w/cm ratio. There is a very good correlation (R2

 = 0.93), indicating that calorimetry measurements can be used to predict the initial setting time. A 

similar correlation was obtained between final setting 

time and time of the maximum heat flow (R2 = 0.92). 

This shows that isothermal calorimetry can be used 

as a surrogate test for setting time (ASTM  C403), a 

laborious test to identify incompatibilities.(4) It is important to emphasize that the linear regression in figure 5 

remains to be validated for different w/cm ratios and 

different cements and fly ashes.

In a study on incompatibility of combinations of concrete materials, Taylor et al. suggested a test protocol 

where a combination of materials would be considered incompatible when the time of maximum heat 

flow is delayed by more than 60 min.(13) According to 

the criteria presented by Taylor et al., only mixtures 

LA20F, LA40F, LA60F, HA20F, and HA40F would be 

considered compatible.(13) All the mixtures containing 

Class C fly ash would be considered incompatible.

Figure 6 shows the relation between the cumulative 

heat for the first 72 h of paste hydration and the 3-day 

compressive strength of mortar cubes made with the 

same cementitious proportions and w/cm ratios. It is 

important to emphasize that the linear regression in 

figure 6 needs to be validated for different w/cm ratios 

and different cements and fly ashes. Nevertheless, isothermal calorimetry appears to be a reliable screening 

tool for selecting mixture proportions. 

Rheology

Figure 7 and figure 8 present the yield stress of mixtures at 8, 30, 50, 70, and 90 min, and figure 9 and 

figure 10 present the plastic viscosity.

For mixtures containing Class F fly ash, both the 

plastic viscosity and the yield stress increased with 

increasing fly ash content, with the exception of 

mixture LA60F at 8 min. For mixtures containing 

Class C fly ash, this trend was not observed, mainly 

y = 26.198x - 1543.7

R2 = 0.9118

2000

3000

4000

5000

6000

ompressive strength (psi)

0

1000

0 50 100 150 200 250 300

3-day co

Cumulative heat (J/g)

Figure 6. Relation between cumulative heat for 

the first 72 h of hydration of pastes and 3-day 

compressive strength of respective mortars.

1 psi = 6.89 kPa

y = 0.7629x - 103.37

R2 = 0.9253

200

300

400

500

600

700

ting time of mortar (min)

0

100

0 200 400 600 800 1000 1200

Initial sett

Time of maximum heat flow (min)

Figure 5. Relation between time of maximum heat 

flow of pastes and initial setting time of respective 

mortar.
6

because both plastic viscosity and yield stress were 

very low and differences between mixtures were 

small and within the variability of the test. 

When comparing mixtures using Class F fly ash with 

those made with Class C fly ash, the Class F fly ash 

mixtures yielded higher plastic viscosities and much 

higher yield stresses at all levels of cement replacement. It should be noted that the Class F fly ash used 

in this study was much coarser than the Class  C 

fly ash. Figure 7 through figure 10 also show that 

the yield stress and plastic viscosity of the mixtures 

that presented considerable setting delays (mixes 

LA20C, LA40C, LA60C, HA20C, HA40C, and HA60C) 

did not change considerably over time.

Figure 11 shows the relationship between the flow 

of mortars and the yield stress of pastes. The graph 

shows higher flows with lower yield stresses, but no 

good correlation was found. Nevertheless, it appears 

that yield stress measurement may be a better tool 

to differentiate mixtures exhibiting low flows (below 

100 percent), whereas the flow test may be a more 

appropriate technique for differentiating mixtures 

with low yield stresses (below 0.0029 psi (20 Pa)). 

Conclusions

From the results presented, the following conclusions 

can be made:

• For the materials used in this study, Class F fly 

ash did not significantly affect setting time, even at 

60 percent cement replacement. The compressive 

strength of mixtures with up to 40 percent replacement was found to be satisfactory at 3 days.

• For the materials used in this study, Class C 

fly ash significantly affected setting time, even 

at 20 percent cement replacement. Mixtures 

containing high alkali cement were affected to a 

greater level. The 3-day compressive strength of 

mixtures with up to 40 percent replacement was 

found to be satisfactory.

• For the materials used in this study, mixtures 

containing Class C fly ash gave rise to higher flow, 

lower yield stress, and lower plastic viscosity.

20

40

60

80

100

120

Yield Stress (Pa)

0

0 20 40 60 80 100

Time (min)

LA LA20F LA40F LA60F LA20C LA40C LA60C

Figure 7. Yield stress over time of mixtures containing low alkali cement.

1 Pa = 1.45 × 10-4 psi

20

40

60

80

100

120

140

Yield Stress (Pa)

0

0 20 40 60 80 100

Time (min)

HA HA20F HA40F HA60F HA20C HA40C HA60C

Figure 8. Yield stress over time of mixtures containing high alkali cement. 

1 Pa = 1.45 × 10-4 psi
7

• Isothermal calorimetry was confirmed to be a 

good screening tool to detect incompatibilities 

(when related to delayed setting time) and can be 

used for setting time and compressive strength 

prediction at early ages.

• The rheological methodology used in this study 

can give an indication of setting delay, since 

neither yield stress nor plastic viscosity of the 

mixtures exhibiting setting delays changed 

significantly over time.

References

1. Tanesi, J., Ardani, A., Meininger, R., and 

Nicolaescu, M. (2012). Evaluation of HighVolume Fly Ash (HVFA) Mixtures (Paste and 

Mortar Components) Using a Dynamic Shear 

Rheometer (DSR) and Isothermal Calorimeter, 

Report No. PB2012-112546, National Technical 

Information Service, Springfield, VA. 

2. ASTM C305. (2006). Standard Practice for 

Mechanical Mixing Hydraulic Cement Pastes 

and Mortars of Plastic Consistency, ASTM 

International, West Conshohocken, PA.

3. ASTM C1437 (2007). Standard Test Method 

for Flow of Hydraulic Cement Mortar, ASTM 

International, West Conshohocken, PA.

4. ASTM C403/C403M (2008). Standard Test 

Method for Time of Setting of Concrete Mixtures 

by Penetration Resistance, ASTM International, 

West Conshohocken, PA.

0.2

0.4

0.6

0.8

1.0

1.2

Plastic Viscosity (Pa.s)

0.0

0 20 40 60 80 100

Time (min)

LA LA20F LA40F LA60F LA20C LA40C LA60C

Figure 9. Plastic viscosity over time of mixtures containing low alkali cement.

0.20

0.40

0.60

0.80

1.00

1.20

Plastic Viscosity (Pa.s)

0.00

0 20 40 60 80 100

Time (min)

HA HA20F HA40F HA60F HA20C HA40C HA60C

Figure 10. Plastic viscosity over time of mixtures containing high alkali cement. 

40

60

80

100

120

Yield stress (Pa)

0

20

80% 90% 100% 110% 120% 130% 140% 150% 160%

Flow

Figure 11. Relation between flow and yield stress.

1 Pa = 1.45 × 10-4 psi
8

5. ASTM C109/C109M (2008). Standard Test Method 

for Compressive Strength of Hydraulic Cement 

Mortars (Using 2-in. or 50-mm Cube Specimens), 

ASTM International, West Conshohocken, PA.

6. ASTM C1738 (2011). Standard Practice for HighShear Mixing of Hydraulic Cement Paste, ASTM 

International, West Conshohocken, PA.

7. ASTM C1679 (2009). Standard Practice for 

Measuring Hydration Kinetics of Hydraulic 

Cementitious Mixtures Using Isothermal 

Calorimetry, ASTM International, West 

Conshohocken, PA.

8. Ferraris, C. and Obla, K. (2001). “Influence of 

Mineral Admixtures on the Rheology of Cement 

Paste and Concrete.” Cement and Concrete 

Research, Vol. 31, No. 2, pp. 245–255.

9. Ferraris, C. (1999). “Measurement of the 

Rheological Properties of Cement Paste: A New 

Approach.” International RILEM Conference on 

the Role of Admixtures in High Performance 

Concrete, RILEM, Monterrey, Mexico. 

10. Baert, G., Van Driessche, I., Hoste, S., De Schutter, 

G., and De Belie, N. (2007) “Interaction Between 

the Pozzolanic Reaction of Fly Ash and the 

Hydration of Cement.” Proceedings of the 

12th International Congress on the Chemistry 

of Cement, International Congress on the 

Chemistry of Cement.

11. Bentz, D. (2010). “Blending Different Fineness 

Cements to Engineer the Properties of Cement 

Based Materials.” Magazine of Concrete Research, 

62(5), pp. 327–338.

12. Lagier, F. and Kurtis, K. (2007). “Influence of 

Portland Cement Composition on Early Age 

Reactions with Metakaolin.” Cement and Concrete 

Research, 37, pp. 1411–1417.

13. Taylor, P., Johansen, V., Graf, L., Kozikowski, R., 

Zemajtis, J, and Ferraris, C. (2006). Identifying 

Incompatible Combinations of Concrete Materials: 

Volume II-Test Protocol, Report No. HRT-06-080, 

Federal Highway Administration, McLean, VA.

Researchers—Jussara Tanesi, Ph.D. and Ahmad Ardani, P.E. at the FHWA Turner-Fairbank Highway Research 

Center. For additional information, contact Mr. Ardani at (202) 493-3422 or through the FHWA Office of 

Infrastructure Research and Development located at 6300 Georgetown Pike, McLean, VA, 22101-2296.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution 

is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by email to 

report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.

gov/research. The complete report is available through the National Technical Information Service, Report 

No. PB2012-112546. 

Key Words—Fly Ash, Rheology, Calorimetry, Incompatibility.

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October 2012 FHWA-HRT-12-062

HRDI-10/10-12(500)E