THE WORKABILITY AND STRENGTH DEVELOPMENT OF GROUND FLY ASH-CONCRETES

M. Akram Tahir*, UET, Lahore, Pakistan Ammara Nusrat, UET, Lahore, Pakistan

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31st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 – 17 August 2006, Singapore

THE WORKABILITY AND STRENGTH DEVELOPMENT OF GROUND FLY ASH-CONCRETES

M. Akram Tahir*, UET, Lahore, Pakistan Ammara Nusrat, UET, Lahore, Pakistan

Abstract The strength development of fly ash-concretes is governed by reactivity of fly ash. It has been noted from the vast literature survey that reactivity of fly ash can be increased by increasing its specific surface area. The specific surface area can be increased in two ways; beneficiation by air classification and by mechanical processing or grinding. However, grinding breaks the spherecity of ash particles and workability is affected drastically due to irregular broken particles. This study is conducted on a low calcium fly ash where specific surface area was increased by grinding the ash. The concrete mixtures were prepared using fly ash-binder ratios of 0.20, 0.35 and 0.50 with water-binder ratios of 0.40, 0.50, 0.60 and 0.70. The ash was ground to three different grades of fineness by varying the grinding time from 4 to 12 hours. The workability of fresh concrete was noted by measuring the compacting factors. The strength of hardened concretes for one of the mixtures was determined by crushing 100 mm cubes after 7, 28, 90 and 180 days of standard moist curing. It was noted that workability in general decreased by using ground ash, however, increase in workability was observed with ground fly ash concretes in case of two higher water-binder ratios.

Keywords: Fineness, fly ash, grinding, specific surface area, strength development, workability.

1. Introduction

Like many other pozzolanic materials fly ash when added to concrete improves workability of concrete by making the mixture more plastic and by decreasing the segregation and bleeding. It is noticed that workability is generally enhanced with finer fly ashes. However, when specific surface area is increased by grinding the ash the workability of ash decreases due to irregular particles as a result of mechanical processing.

Ravina [1] found out that there is a linear relationship between the specific surface of fly ash and its pozzolanic activity index while the relation between the amounts retained on sieve #325 was linear only over a limited middle range. It was concluded that air permeability method ash subject to upper maximum carbon content, is a better than sieve analysis to find out the fineness of fly ash. Hughes and Al-Ani [2] presented the results of an investigation in which it was found that neither the standard air permeability nor the residue on sieve #325 provided good indication of the complete size distribution of a fly ash. The air permeability provided a value, which did not necessarily vary with the specific surface, and the sieve #325 merely considered the division at one boundary only. Ukita et al. [3] after classifying conventional fly ash by air separation produced three classes of classified fly ash

with the maximum particle diameters of 20, 10, and 5 m. It was observed that whether the classified fly ash was used alone or in combination with ground granulated blast furnace slag, it improved the workability and strength of concrete with lower water binder ratio.

Tanaka et al. [4] performed experimental investigations in which sorted fly ash was used as cement replacement of 10 and 30 percent by weight of the cement. It was observed that with 10% lesser water binder ratio, workability of the concrete increased with an increase in amount of fly ash. Cornelissen et al. [5] made mortars and concretes from mechanically processed fly ash particle sizes of which were between 1 to 5 m. It was noted that very fluid mixtures could be produced with excellent strength and durability properties, by using ground fly ashes. Ground fly ashes also resulted in higher strength in comparison with air-classified fly ash incorporated in concrete having similar workability. Monzo et al. [6] presented the results of a study on the influence of ground fly ashes on the workability and strength of mortars. The grinding of fly ashes produced enormous affect on their nature. The fineness was increased, mean diameter decreased and shape became more angular. It was noticed that the workability of mortars was reduced with increase in both grinding time and replacement levels.

Wecharatana et al. [7] conducted an experimental study on the characteristics of processed fly ash and found out that the processed fly ashes perform equally or better than cement when mixed as cement replacement in concrete. It was noticed that the addition of processed fly ash as cement replacement not only exhibited early strength development of fly ash concrete but also enhanced its durability. Hassan and Cabrera [8] studied the use of air-classified fly ash to produce high performance concrete. The investigation was carried out using two fly ashes obtained from air classifier. High performance concrete with fly ash-binder ratio of 0.30 exhibited excellent strength and durability properties. Tahir and Pichai [9] presented a model for predicting strength development of fly ash concretes based on chemical composition and fineness of binder materials. It was noted that pozzolanic strength improved with increase in fineness of binder materials.

2. Experimental Program

A low calcium or class F ash was employed for the present study. An ordinary Portland cement with the crushed limestone as coarse aggregate and the coarse river sand as fine aggregate was used to make concretes. The chemical composition and physical properties of the two binder materials are reported in Table 1. The specific gravity of the coarse aggregate was 2.72 and that of fine aggregate was 2.54.

Table 1. Physical properties and chemical composition of the cement and fly ash

Binder Density SSA† LOI ‡ Weight of Oxide, %

kg/m3 m2/kg % SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3

OPC 3150 350 1.32 21.1 4.82 3.17 64.7 1.62 0.80 0.15 2.15

Ash 2665 295 3.32 38.4 20.5 17.7 8.7 2.98 2.13 2.12 2.54

Note: † SSA denotes the Blaine specific surface area and ‡ LOI is Loss on Ignition.

The concretes were cast using binder content equal to 350 kg/m3 of concrete. Three fly ash-binder ratio of 0.20, 0.35 and 0.50 were combined with four water-binder ratios of 0.40, 0.50, 0.60 and 0.70 to cast concrete. Four grades of ash fineness namely "Natural, 4 hour, 8 hour and 12 hour" were used where hours denoted the grinding time. The specific surface area of natural ash was 295 m2/kg where as those of ground for 4 hour, 8 hour and 12-hour had Blaine specific surface areas equal to 445, 610 and 765 m2/kg respectively. The mixture proportions of the basic twelve concretes are given in Table 2. Each mixture was repeated four times with four fineness grades of fly ash. The compacting factors were measured for all the forty-eight mixtures. The plots of compacting factors are shown on Figs.1 and 2. Twelve 100mm cubes were cast from each of the sixteen concrete mixtures in which fly ash-binder ratio was 0.20. Three cubes each were tested for crushing strength after 7, 28, 90, 180 days of standard moist curing. The compressive strength developments of the sixteen mixtures are reported in Figs. 3 and 4.

Table 2. Mixture proportions in kg/m3 of concretes

Mixture ID Cement Fly Ash Water Coarse Agg Sand F/(C+F) W/(C+F)

F20W40

280 70

140 1280 645

0.20

0.40

F20W50 175 1240 600 0.50

F20W60 210 1210 550 0.60

F20W70 245 1180 500 0.70

F35W40

228 122

140 1280 645

0.35

0.40

F35W50 175 1240 600 0.50

F35W60 210 1210 550 0.60

F35W70 245 1180 500 0.70

F50W40

175 175

140 1280 645

0.50

0.40

F50W50 175 1240 595 0.50

F50W60 210 1210 540 0.60

F50W70 245 1170 495 0.70

Note: Each mixture was repeated with 4 fineness grades of ash; Natural, 4-hour, 8-hour and 12-hour.

3. Discussion of The Results

The compacting factor (CF) vs. water-binder ratio has been plotted in Fig. 1 for four fineness grades of fly ash. In first part of Fig. 1 the curves are plotted for fly-ash binder ratio of 0.20. It may be noted that workability increased almost in a linear fashion with increase in water-binder ratio for natural fly ash. The workability was decreased for water-binder ratios of 0.40 and 0.50 for concretes incorporating fly ash that had been ground for four hours.

The workability was further decreased as the grinding time of fly ash has been increased to 8 and 12 hours. It may be noticed that decrease in workability from natural ash-concrete to 4-hour ash-concrete was more pronounced than the corresponding decreases from 4-hour ash-concrete to 8-hour ash-concrete and from 8-hour ash-concrete to 12-hour ash-concrete.

It is interesting to note that workability increased dramatically for the water-binder ratios of 0.60 and 0.70 for ground fly ash-concretes. It seemed as if water-binder ratio of 0.54 was "magical" water-binder ratio for the concrete mixtures having fly ash-binder ratio of 0.20.

The trend is more or less same for concretes mixtures cast with fly ash-binder ratio of 0.35, as may be noted from 2nd part of the Fig. 1. However, the decrease in workability was more pronounced in case of 4-hour ash-concrete compared to natural concrete and the difference was much more as compared to the other wise similar concrete with fly ash-binder ratio of 0.20. The magical water-binder ratio for this class of concrete mixtures is 0.56

It is not surprising at all to note that ground ash-concretes have shown remarkably decreased workability in comparison to natural ash-concretes

0.7

0.8

0.9

1

0.4 0.5 0.6 0.7

Water-Binder Ratio

CF

Natural 4 Hour8 Hour 12 Hour

Fly Ash-Binder Ratio=0.20

0.7

0.8

0.9

1

0.4 0.5 0.6 0.7

Water-Binder Ratio

CF

Fly Ash-Binder Ratio=0.35

0.7

0.8

0.9

1

0.4 0.5 0.6 0.7

Water-Binder Ratio

CF

Fly Ash-Binder Ratio=0.50

Fig. 1. Compacting factors vs. W/(C+F)

even in case of mixtures with fly ash-binder ratio of 0.50. The magical water-binder ratio in this case equaled to 0.58 beyond which the concretes with fly ashes ground for higher times have shown higher workability.

It would be just arbitrary to note that workability for any type of concrete mixture was increased with increase in water-binder ratio. This phenomenon was just in line with general norms of workability of concrete mixtures. As may be depicted from Fig. 1, the increase of workability with increase in water-binder ratio was not surprising at all. For a given binder content when water-binder ratio was increased the free water increased also which directly affected the workability in a positive manner. The decrease of workability with increased grinding time of ash is also easily justifiable. The more the grinding time, the more was the specific surface of the ash and more wear and tear resulted in higher irregularity in shape of ash particles.

The lower workability of concrete mixtures as compared to other wise similar composition natural ash concrete mixtures with water-binder ratios equal to 0.40 and 0.50 may easily be explained by the foregoing broken irregular particle theory. However, it was quite intriguing that 4-hour ground ash when incorporated in concrete with water-binder ratio of 0.60 or 0.70 resulted in higher workability as compared to natural ash concrete.

This was even more intriguing when 12-hour ash-concretes gave still higher workability than 8-hour ash concretes, which were in turn more workable than 4-hour ash concretes for the high water-binder ratios. The so-called magical water-binder ratios of 0.54, 0.56 and 0.58 have deep influence on workability for three fly ash-binder ratios equal to 0.20, 0.35 and 0.50 respectively. This phenomenon will be discussed later on.

The compacting factor vs. fly ash binder ratio plots of Fig. 2 reveal more information about workability yet in another style. It may be noticed that for water-binder ratio of 0.40, the natural ash-concretes showed higher workability as the fly ash-binder ratio was increased. However, for ground fly ashes the workability decreased with an increase in fly ash-binder ratio. This was not surprising at all. For a given amount of free water in a mixture when ash particles to the same degree of irregularity were increased through increase in ash content, the workability was expected to decrease.

The strength development of concretes with fly ash-binder ratio of 0.20 has been reported in Fig. 3. The strength development of 4 concrete mixtures for water-binder ratio of 0.40 can be seen in first part of Fig. 3. The strength of natural fly ash concrete was the lowest at all ages. There was increase in strength with the increase in grinding time of incorporated fly ash. The more was the grinding time, the higher was the specific surface area and hence more was the strength at all ages. The difference of strength was about 5 MPa at the age of 28 days and this difference has been maintained up to the observed age of 180 days. It may be noticed that the rate of strength development from 28 day to 180 day was almost same for the four concrete mixtures with

0.75

0.8

0.85

0.9

0.2 0.3 0.4 0.5

Fly Ash-Binder Ratio

CF

Water-binder Ratio=0.50

0.75

0.8

0.85

0.2 0.3 0.4 0.5

Fly Ash-Binder Ratio

CF

Water-binder Ratio=0.40

0.85

0.9

0.95

1

0.2 0.3 0.4 0.5

Fly Ash-Binder Ratio

CF

Water-binder Ratio=0.60

0.9

1

0.2 0.3 0.4 0.5

Fly Ash-Binder Ratio

CF

Natural 4 Hour8 Hour 12 Hour

Water-binder Ratio=0.70

Fig. 2. Compacting Factor vs. F/(C+F)

identical fly ash-binder and water-binder ratios despite the difference of fineness. However, the finer fly ash concrete has developed strength more rapidly from 7 to 28 days as can be seen from the initial part of the strength-age curves.

The second part of Fig. 3 depicts the strength development of 4 concrete mixtures with fly ash-binder ratio of 0.20 and water-binder ratio of 0.50. It may however, be noted that strengths at all ages have been remarkably reduced as compared to the identical concrete mixtures of water-binder ratio of 0.40. This was direct consequence of the water-binder ratio law; i.e., an increase in water-binder ratio causes decrease in strength. It is interesting to note that rate of strength development from 7 to 180 day was slower as compared to the otherwise identical concrete mixtures with water-binder ratio of 0.40. The difference of strengths of concretes incorporating the fly ash with two extreme fineness grades was again approximately 5 MPa at 28 days.

The strength development of concrete mixtures with water-binder ratio of 0.60 has been shown on 3rd part of the Fig. 3. The trends are almost same as of the concretes with two water-binder ratios of 0.40 and 0.50. The strength at all ages has dropped quite sharply. The rate of strength development has further been slowed as compared to the concretes with water-binder ratio of 0.50. The difference of strength at 28 days between natural ash-concrete and 12-hour ground ash-concrete is of the order of 3 MPa.

A sharp change may be noticed in strength development of 4 concretes with water-binder ratio of 0.70. The strength of natural ash concrete was merely 2 MPa less than the concrete incorporating the finest grade of fly ash. It may be noted that the strength of concretes with different grades of fly ash was almost identical.

The strength and workability has been plotted vs. specific surface area of incorporated ash in view of water-binder ratios in Fig. 4. It may be noted from first part of Fig. 1 and Fig. 4 that with an increase in workability strength has decreased upto water-binder ratio of 0.58. Beyond this ratio boh the workability and strength increased simultaneously with increase in specific surface of fly ash incorporated in concrete.

Fig. 4 gives another view of relationship between workability and strength vs. water-binder ratio with increase in specific surface area of incorporated concrete. It may be noted that for W/(C+F)=0.4 and 0.5 as specific surface increases, the strength is increased and workability is decreased. However, it is quite surprising that for the two water-binder ratios of 0.6 and 0.70 both the workability and strength are increased simultaneously. This shows as if ground ash had the effect of a super plasticiser for water-binder ratios of 0.60 and 0.70 for F/(C+F)=0.20.

No strength data was collected for the concretes cast with fly ash-binder ratios of 0.35 and 0.50. Hence simultaneous increase in strength and workability for the two water-binder ratios of 0.60 and 0.70 cannot be generalized. This phenomenon needs detailed research.

20

30

40

50

60

0 30 60 90 120 150 180

Str

engt

h,M

Pa

Natural 4 Hours8 Hours 12 Hours

Age, days

15

25

35

45

0 30 60 90 120 150 180

Str

engt

h,M

Pa

Age, days

15

25

35

0 30 60 90 120 150 180

Str

engt

h,M

Pa

Age, days

10

20

30

0 30 60 90 120 150 180

Str

engt

h,M

Pa

Age, days

Water-binder ratio=0.40

Water-binder ratio=0.50

Water-binder ratio=0.60

Water-binder ratio=0.70

Fig. 3. Strength vs. Age Plots

4. Conclusions

The results of this study have shown that beneficiation of ash by grinding resulted in decreased workability for water-binder ratios of 0.40 and 0.50 for all fly ash-binder ratios. The workability decreased as grinding time was increased. However, the workability improved dramatically as water-binder ratio was increased to 0.60 and 0.70. The strength at all ages increased with decrease in water-binder ratio and increase in specific surface area. To that extent this study corroborated the results of previous studies. However, loss of workability with increase in grinding time at water-binder ratios of 0.40 and 0.50 and then sudden gain in workability with water-binder ratios of 0.60 and 0.70 is a paradox.

Fig. 4. Strength and workability vs. SSA plots .

5. References

[1] Ravina D, “Optimized Determination of PFA Fineness with Reference to Pozzolanic Activity", Cement and Concrete Research, 1980, Volume 10, No. 4, pp. 573-580.

[2] Hughes B.P., and Al-Ani M.N.A., "PFA Fineness and Its Use in Concrete", Magazine of Concrete Research, 1989, Volume 41, No. 147, pp. 99-105.

[3] Ukita K., Ishii M., Yamamoto K., Azuma K., and Kohno K, "Properties of High Strength Concrete Using Classified Fly Ash", Proceedings of 4th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Volume 1, pp. 37-52, 1992, Istanbul, Turkey.

[4] Tanaka H., Wingnarajah S., Sugimoto K., Ukida K., and Ishii M., "Characteristics of Concrete Incorporating Sorted Fly Ash", Proceedings of 4th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Supplementary Papers, pp. 971-986, 1992, Istanbul, Turkey.

[5] Cornelissen H.A.W., Hellewaard R., and Vissers J.L., "Processed Fly Ash for Performance Concrete", Proceedings of 5th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Volume 1, pp. 67-80, 1995, Milwaukee, USA.

[6] Monzo J., Paya, J., Peris-Mora E., and Borrachero M.V., "Mechanical Treatment of Fly Ashes: Strength Development and Workability of Mortars Containing Ground fly Ashes", Proceedings of 5th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Volume 1, pp. 339-354, 1995, Milwaukee, Wisconsin, USA

[7] Wecharatana M., Bumrongjareon W., and Rachakornkij M., "Characteristics of Processed Fly Ash for Concrete", Proceedings of 6th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Supplementary Papers, pp. 1-16, 1998, Bangkok, Thailand

[8] Hassan K.E., and Cabrera J.G., "The Use of Classified Fly Ash to Produce High Performance Concrete", Proceedings of 6th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Volume 1, pp. 21-38, 1998, Bangkok, Thailand.

[9] Tahir M.A., and Nimityongskul P., "A Model for Predicting Strength Development of Fly Ash Concretes" Proceedings of 6th CANMET/ACI International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete (Edited by V. M. Malhotra), Supplementary Papers, pp. 33-48, 1998, Bangkok, Thailand.

1530

45

200 400 600 800

Str

engt

h,M

Pa

0.75

0.8

0.85

0.9

0.95

1

CF

0.4 0.5 0.6 0.7

SSA, m2/kg