CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 669-674, 1985. Printed in the USA 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

INFLUENCE OF FLY ASH CHARACTERISTICS ON THE STRENGTH OF PORTLAND-FLY ASH MIXTURES

P. K. Mehta University of California

Berkeley, CA 94720

(Communicated by J. Skalny) (Received Feb. 6, 1985)

ABSTRACT Modern thermal power plants are producing large amounts of f ly ash that is generally quite suitable for use as a supplementary cemen- t i t ious material in concrete. However, for ti l is purpose the f ly ash ut i l izat ion in the United States continues to remain low, mainly on account of lack of quality control. This is because the current standards on f ly ash do not contain specifications and test methods that are able to assess adequately the performance of a f ly ash in concrete. Based on tests on I I different f ly ashes and direct determination of compressive strength of test mortars made with a fixed proportion of f ly ash by weight of the cementitious material, and a fixed ratio between water and the cementitious material, i t seems that the calcium content and particle size dis- tribution of the f ly ash are the most important parameters govern- ing the strength development rate in normally cured portland cement- f ly ash mixtures.

Introduction

Statistics compiled by the National Ash Association, Inc. show that in 1983, 52.35 million tons of f ly ash were produced in the United States and only 3.62 million tons were used for making cement and concrete products. The low rate of ut i l izat ion of f ly ash as a supplementary cementing material in the cement and concrete industries has partly been attributed to the lack of quality control. For an effective quality control on f ly ash suitable for use as a pozzolan, i t is necessary to know which of the numerous physical and chemical characteristics of the material greatly influence the behavior of portland cement-fly ash mixtures, such as the rate of strength development. There is general agreement that standard test methods currently in use, e.g., the AST~ C 311Pozzolan Act iv i ty Test, do not provide a useful measure for evaluation of relative reactivity of f ly ashes from different sources.

Experimenta!

The oxide analyses of an ASTM Type I portland cement and I I f ly ashes produced under normal-load conditions by modern thermal power plants located in different parts of the United States are reported in Table I. Fly Ash Nos. I-7 were produced from the combustion of bituminous coals, and are of low

669

670 Vol. 15, No. 4 P.K. Mehta

calcium content. Fly Ash Nos. 8-11 were produced from e i the r subbituminous or l i g n i t e coals, and are of high calcium content.

The mineralogical compositions inc lud ing the glass content of the f l y ashes were determined by X-ray d i f f r a c t i o n analys is . Par t i c le size charac- t e r i s t i c s were evaluated by several methods such as sieve ana lys is , X-ray sedimentation technique, Blaine Ai r Permeabi l i ty Method, and s ing le -po in t n i t rogen adsorpt ion.

A modified ASTM C 109 tes t was used to assess the strength con t r ibu t ion potent ia l of a f l y ash to the blended mixtures contain ing port land cement and the f l y ash. According to th i s tes t , a constant r a t i o between water to the cementit ious material (por t land cement + f l y ash) was used to make 2-inch mortar cubes from blended cements contain ing 20% f l y ash by weight. In the mortar, the ra t ios of standard sand to the cementit ious mater ial and of water to the cementit ious mater ial was 2.75 and 0.485, respect ive ly . In order to determine the e f fec t of accelerated curing on s t rength, the mortar cubes were cured both at the normal temperature (73°F), and at a high temperature ( I IO°F).

TABLE 1

Oxide Analyses of Fly Ashes and Port land Cement, %

Fly Ash SiO 2 AI203 Fe203 CaO MgO SO 3 Na20 K20 19ni t ion No. Loss

1 55.1 21. 2 53.4 22 3 50.9 25 4 57.6 29 5 52.2 27 6 50.9 28 7 46.2 31 8 38.4 13 9 39.5 19

I0 36.0 19 I I 50.5 17

Port land 21.0 4 Cement

1 5.2 6.7 1.6 0.5 1.73 1.24 0 6.3 6.8 2.0 0.5 2.86 0.67 3 8.4 2.4 1.0 0.3 0.28 2.83 0 5.2 0.3 1 . I 0.2 0.30 2.90 4 9.2 4.4 1.0 0.45 0.12 0.68 9 5.4 1.4 0.9 0.4 0.32 2.54 3 8.5 1.8 0.7 0.5 0.25 1.99 0 20.6 14.6 1.4 3.3 0.40 2.(14 5 5.7 24.7 3.4 1.8 1.56 0.21 8 5.0 27.2 4.9 3.15 0.42 1.72 2 5.9 15.8 3.1 1.0 0.49 0.82 6 3.0 64.1 2.4 2.7 0.40 0.20

06 05 21 09 35 29 45 l 6 0.9 0.4 0.4 1.5

( i i i )

Results and Discussion

The mineralogical analyses of the f ly ashes revealed the following:

( i) Crystalline si l ica in the form of quartz was found to be present in all the f ly ashes. The quartz content as estimated by quantitative XRD ranged from about 2% in Fly Ash No. 2 to about I0% in Fly Ash Nos. l , 4, 8, 9, lO, and I I .

( i i ) Crystalline aluminosilicate in the form of mullite, 3 A1203.2 SiO 2, was present in all the low-calcium f ly ash specimens except Fly Ash No. 2. No mullite was detected in the high-calcium f ly ashes. The mullite content ranged from about 5% for Fly Ash Nos. l , 3, 5, 6, and 7 to about I0% for Fly Ash No. 4.

Crystalline C3A, a highly reactive compound, was detected in all the

Vol. 15, No. 4 671 FLY ASH, Ca, PARTICLE SIZE DISTRIBUTION, MORTAR STRENGTH

(iv)

(v)

(vi)

shown in Table 2.

high-calcium f l y ashes usedcin th is invest iga t ion . The CRA content was estimated to be about 2~ for Fly Ash No. I I , and 5-8~% for Fly Ash Nos. 8, 9, and I0.

Crysta l l ine anhydrite (CS) was detected in a l l the high-calcium f l y ashes; however, c rys ta l l i ne CaO was detected only in Fly Ash Nos. 8 and I0. The c rys ta l l i ne CaO or free lime content in Fly Ash Nos. 8 and I0 was found to be 3.8% and 0.4%, respect ively, by the ASTM method C 114.

Magnetite (Fe~O 4) was present in Fly Ash No. 8, and periclase (crys- t a l l i n e MgO) ~ was detected in Fly Ash Nos. 9 and I0.

From the quant i ta t ive mineralogical analyses data a rough estimate of the noncrystal l ine matter or glass content was made by dif ference. I t seems that Fly Ash Nos. 2 and 5 contained ~ver 90% glass, Nos. I , 3, 6, and 7 contained 80-90% glass, and Nos. 4, 8, 9, I0, and I I con- tained 75-80% glass. The XRD diffused band, due to the aluminosi l icate glass, showed a maxima at 23-24 ° 2e (Cu ~ ) in the low-calcium (<7% CaO) f l y ashes, 26 ° 2@ in Fly Ash Nos. 8 and I I (14-16% CaO), and 32 ° 2e in Fly Ash Nos. 9 and I0 (24-28% CaO).

Diamond and Lopez-Flores ( I ) , and the author (2) have reported that th is s h i f t in the posi t ion of the diffused-band s ign i f i es a change in the composition of the glass. Whereas Diamond (3) speculates that the glass present in f l y ashes with more than 20% CaO is a calcium alumi- nate glass of 12 CaO.7 AI203 composition, the author believes (2) that , s imi lar to the glass in blastfurnace slags which contain 35-40% CaO, the glass in high-calcium f l y ashes is s t i l l an aluminosi l icate glass with considerable calcium subst i tu t ion . In any case, the glass in the high-calcium f l y ashes is more react ive than the glass in low- calcium f l y ashes.

Selected data from the par t i c le size analyses and surface area tests are I t is obvious from the data the re lat ionship between sur-

TABLE 2

Surface Area and Par t ic le Size D is t r ibu t ion

Fly Ash No. Surface Area, cm2/g Par t ic le Size

Blaine N 2 Adsorption >45~m* <lO~m**

1 3620 10300 15 48 2 3190 6000 I I 48 3 2900 8800 21 38 4 3120 6800 16 42 5 3510 II000 17 41 6 3770 18300 14 49 7 2370 8500 24 29 8 3920 18900 15 36 9 3770 26700 15 44

I0 4010 16500 12 50 I I 3000 I I I 00 15 44

*residue on No. 325 mesh sieve **by X-ray sedimentation analysis

672 Vol. 15, No. 4 P.K. Mehta

face areas and pa r t i c l e size d i s t r i b u t i o n s obtained by the d i f f e ren t charac- t e r i za t i on techniques is ra ther poor. Surface areas obtained by ni t rogen ad- sorpt ion tend to be much higher than by the Blaine Ai r Permeabi l i ty Method, espec ia l l y in the case of high-calcium f l y ashes. This is probably because in the former the presence of very small pa r t i c l es of calcium and a l ka l i su l fa tes covering the spherical grains of f l y ash made a large con t r ibu t ion to the sur- face area. Comparison of typ ica l scanning e lectron micrographs of a high- calcium f l y ash (Fig. IA) and a low-calcium f l y ash (Fig. IB) c lea r l y showed that the spherical pa r t i c l es of the l a t t e r f l y ash were r e l a t i v e l y free from surface deposits. I t appears that from the standpoint of pozzolanic a c t i v i t y the surface area determined by the Blaine method would be more meaningful than

FIG. 1

Typical Scanning Electron

Micrographs of a High-

Calcium Fly Ash (A), and

a Low-Calcium Fly Ash (B)

the ni t rogen adsorpt ion method. The pa r t i c l e size analys is by wet seiv ing revealed that except fo r two low-calcium f l y ashes, v iz . No. 3 and 7, a l l others contained well under I0 um, as determined by the X-ray sedimentation ana lys is , were also useful as another index of pozzolanic a c t i v i t y . Except fo r Fly Ash No. 7, a l l f l y ashes contained approximately 40 to 50% par t i c les <I0 ~m; in fac t , Fly Ash Nos. I , 2, 6, 9, I0 and I I contained close to 45-50% par t i c les <I0 um.

Vol. 15, No. 4 673 FLY ASH, Ca, PARTICLE SIZE DISTRIBUTION, MORTAR STRENGTH

In Table 3 a comparison of the pozzolanic a c t i v i t y index data (ASTM C 311) is shown with the modified ASTM C 109 mortars cured e i ther at 23°C (73°F) or at 43°C (I IO:F) for 7, 28, and 90 days. The data c lear ly show that the poz- zolanic a c t i v i t y index does not provide a helpful c r i t e r i on for assessment of the re la t i ve rates of r e a c t i v i t y of the various f l y ashes. For instance, among the low-calcium f l y ashes for normal curing temperature the rates of strength development for the mortars made with Fly Ash Nos. 1 and 6 were con- siderably higher than for those containing Fly Ash Nos. 2 and 3, yet the pozzolanic a c t i v i t y indices of al l the four f l y ashes were s imi lar (80-83). Since the modified ASTM C 109 mortar test used in th is invest igat ion closely simulates the f i e l d pract ice, the resul ts of th is tes t are believed to provide a better index of the re la t i ve r e a c t i v i t y of a f l y ash than the pozzolanic a c t i v i t y test data from the ASTM Method C 311.

I f the 90-day strengths of normally cured mortar are assumed to be indica- t i ve of the ult imate strength contr ibut ion potent ial of a f l y ash in a portland cement-fly ash mixture, i t seems from the data in Table 3 that accelerated curing at 43°C (IIO°F) for 28 days w i l l be adequate for assessment of the rela- t i ve r e a c t i v i t y of a f l y ash because there is a good corre lat ion between the 90-day normal-cured and the 28-day accelerated-cured strengths. Some low- calcium f l y ashes seem to benef i t more from accelerated curing than others. For instance, the 28-day strength of 43°C specimens containing Fly Ash No. 4 or 5 were substant ia l ly higher than the 90-day strength of the normally-cured specimens, while the reverse was the case for Fly Ash No. I . Accelerated curino strength at 7-days was generally lower than the 28-day normal strength, however, i t appears that a sa t is fac to ry 7-day accelerated test can be developed by ra is ing the curing temperature to 50 or 55°C.

in regard to factors governing the strength contr ibut ion rate of a f l y ash to a portland cement-fly ash mixture, i t appears that in addit ion to the calcium content the par t i c le size d i s t r i bu t i on is most important. The calcium

TABLE 3

Modified ASTM C 109 Compressive Strengths and Pozzolanic Ac t iv i ty Index

Cement only

Ash 1

2

ASTM CI09 (20% Fly Ash + 80% Cement)

73°F IIO°F

I ' 7 days 28 days 90 days 7 days 28 daysd9Odays

4615 5850 7000 5050 5850 7040

3465 5525 7425 4975

3710 4950 6675 5225

3640 4765 6425 4300

4925 6900 5105

4940 6910

4 3650

5 351o

6 3685 5600 7225

7 3335 4365 6100

8 4085 5700

9 4225 ! 6115

lO 4125 I 6150

11 I 4o15 ! 5600

7240

7400

6975

7125

Pozzolanic Activity (ASTM C311)

lO0°F, 28 days psi index

5800

5670

5475

4100

5300

5850

5275

5125

6750

6885

6210

7625

7425

7690

6025

7415

7585

6835

6935

7630

759O

6300

8120

7890

8540

6260

7350

8410

8100

8090

4665 81

4840 83

4775 82

4490 77

5275 91

4640 80

4175 72

4970 86

5000 86

4625 80

5415 93

674 Vol. 15, No. ,~ P.K. Mehta

content controls the mineralogy, i .e . the composition of the c rys ta l l i ne minerals (quartz and mul l i te vs C~A and CS) and the glass present (high-calcium vs low-calcium glass). For low-c~Icium f l y ashes from modern furnaces which usually produce low-carbon (<5% carbon) and high-glass (80-90% glass) f l y ash, i t seems that in general the r e a c t i v i t y was d i rec t l y proportional to the amount of par t ic les less than I0 ~m, and inversely proportional to the amount of par- t i c les greater than 45 ~m. For instance, Fly Ash Nos. I , 6, 9, I0, and I I which contained 45-50% par t ic les <I0 ~m gave s imi lar 28-day strength on normal curing in the blended cements when compared to the control portland cement. On the other hand, Fly Ash Nos. 3 and 7 which contained more than 20% par t ic les >45 ~m showed the slowest rate of strength development at normal temperature. For high-calcium f l y ashes such as Nos. 8-11, however, due to the high react i - v i t y of both the c rys ta l l i ne and the glassy phases the rates of r eac t i v i t y were less affected by the par t i c le size d i s t r i bu t i on .

Conclusions

I . Under normal operating condit ions, i t appears that modern thermal power plants are capable of producing f l y ash that is generally low in carbon (<5%), high in glass (>75%), and has f ine par t ic le size d i s t r i bu t i on (>40% under I0 ~m, and <20% above 45 ~m). This f l y ash is well suited for use as a cement supplement in the cement and concrete indust r ies.

2. Except for the calcium content, var ia t ions in the other chemical const i - tuents of f l y ash appeared to have no ef fect on i t s r e a c t i v i t y . Superior r e a c t i v i t y of the high-calcium f l y ashes, compared to the low-calcium ones, was probably due to both the presence of react ive c rys ta l l i ne compounds such as C3A and a more act ive calcium aluminosi l icate glass.

3. For f l y ashes with normal carbon and glass contents, i t seems that in addit ion to the calcium content, the par t ic le size d i s t r i bu t i on is an important parameter governing the r e a c t i v i t y of a f l y ash. For low- calcium f l y ashes the r e a c t i v i t y was found to be d i rec t l y proportional to the amount of par t ic les <I0 um, and inversely proportional to par t ic les >45 um. High calcium f l y ashes seem to be r e l a t i v e l y less sensi t ive to par t i c le size d i s t r i bu t i on .

4. As expected, the pozzolan a c t i v i t y index (ASTM Method C 311) did not pro- vide a useful yardst ick for grading the re la t i ve r e a c t i v i t y of f l y ashes from d i f fe ren t sources. In th is regard, more useful information was obtained from normally-cured test mortars containing 20% f l y ash by weight of cement, and 0.485 ra t io between water to cement plus f l y ash. The data from an accelerated test (43°C curing temperature for 7 days) generally related well with the data from the 28-day normally cured mortars.

AcknowledBement

Financial support from the E lec t r ic Power Research I n s t i t u t e (EPRI) is grate- f u l l y acknowledged. Suh Chen carried out most of the experimental work.

References

I . S. Diamond and F. Lopez-Flores, "On the D is t r ibu t ion Between Physical and Chemical Character ist ics Between L i gn i t i c and Bituminous Fly Ashes," Pro- ceedings, Symp. on Effects of Fly Ash Incorporation in Cement and Concrete, Materials Research Society, Editor S. Diamond, 34-44 (1981).

2. P. K. Mehta, "Pozzolanic and Cementitious Byproducts as Mineral Admixtures for Concrete - A Cr i t i ca l Review," Prodeedings, F i rs t Internat ional Con- ference on the Use of Fly Ash, S i l i ca Fume, Slag, and Other Mineral By- products in Concrete, ACI SP-79, Editor V. M. Malhotra, 1-46 (1983).

3. S. Diamond, "On the Glass Present in Low-Calcium and in High-Calcium Fly- ashes," Cem. Conc. Res. 13, 459-464 (1983).