report on fly ash as a soil additive -...

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REPORT

ON

FLY ASH AS A SOIL ADDITIVE

by

Ken Isenberger

OFFICE OF MATERIALS

HIGHWAY DIVISION

March 1981

HESEARCH SE.CTlON Office of Materials /

Iowa Dept. of Transportation

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• • • • • •• • • • •• •

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SUMMARY

The addition of a selected self-cementing, Class C fly

ash to ... blow .. sand soils improves their compacted strength greatly

as opposed to the minimal strength improvement when fly ash is

mixed with loess soil. By varying the percentage of fly ash

added, the resulting blow sand-fly ash mixture can function as

a low strength stabilized material or as a higher strength sub-

base. Low strength stabilized material can also be obtained by

mixing loess soils with a selected Class C fly ash.

The development of the higher strength values required for

subbase materials is very dependent upon compaction delay time

and moisture condition of the material. Results at this time

indicate that, when compaction delays are involved, excess

moisture in the material has the greatest positive effect in

achieving minimum strengths. Other added retarding agents, such

as borax and gypsum, have less effect .

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TABLE OF CONTENTS

SUMMARY

TABLE OF CONTENTS

PURPOSE AND SCOPE

MATERIALS

Fly Ash

Soils

Retarders

LABORATORY PROCEDURES

Soil-Fly Ash Mixtures

Soil-Fly Ash-Retarder

TEST RESULTS AND INTERPRETATION

Soil-Fly Ash

High Strength

Low Strength


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1

1

2

2

2

2

5

5

5

5

15

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PURPOSE AND SCOPE

This study was initiated to investigate the effects of

adding self-cementing fly ash to base and soil materials .

Two fly ashes and s'everal soil types, representing the

range of soils giving problems when utilized as construction

materials, were chosen.

The original goal was to determine if high strength mix

tures of soil-fly ash could be produced. If so, then the

resulting pavement design could take into account the contri-

bution of the soil-fly ash mixture to the overall structural

capabilities of the pavement system. As the study continued,

a need was expressed by some for a low-strength mixture. That

is, in-situations where the nature of the soil made construction

activities and traffic difficult, some means of stabilizing the

soil was desired. This stabilization should produce a mixture I

that was strong enough to drive on, yet weak enough to be trim-

med to final grade by standard equipment.

Preliminary strength results indicating a dependence upon

the time delay before compaction were responsible for incorpor

ating a study of retarders into the program.

MATERIALS

Fly Ash

The fly ashes were obtained from Council Bluffs No. 3 and

Sioux City Port Neal No. 4 generating plants, both have self-

cementing properties.

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Soils

Initially, it was intended to obtain three different soil

types: blow sand, loess and a medium clay (A-7-5 or A-7-6).

These represent the range of problem soils encountered on high-

way construction projects and the study was designed to see if

their engineering properties could be improved. Only the loess

and blow sand were obtainable when the study began and a reasses-

. ment of the feasibility (methodology and economy) of incorporating

fly ash into medium clay soils in the field resulted in the dele-

tion of it as a test soil.

Retarders

A commercial, liquid fly ash retarder was obtained for use

in the study. Several commonly available materials were also

investigated as to their retardation potential, i.e., gypsum,

borax and calgon.

LABORATORY PROCEDURES

Soil-Fly Ash Mixtures

The fly ashes were combined with the soil materials accord

ing to the following procedure:

I. Soil Characterization

a. Determine the soil classification of the two

soils.

II. Proctor Densities/Optimum Moisture

a. Determine proctor densities and optimum

r

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f r r r [

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r r r r r F

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fly ash added.


The specimens involved in the r.:3.tarder study we:r:§J;>~epared.

the same as the previous fly ash-soil samples. Pre~~minary

trial mixes were made holding most variables constant to see

if a proposed retarder had any positive effect. The two that

did show potential benefits, borax and gypsum, were included

in an expanded study that varied the amount of retarder.

TEST RESULTS AND INTERPRETATION

.soil-Fly Ash

Using the loess and blow-sand soils, soil-fly ash specimen.s

were prepared using the variable fly ash percentages, moisture

contents, compaction times and curing times. The results of

this phase are shown in Figures 2 thru 9.

High Strength

In evaluating soil admixtures for high strengths, two

schools of thought exist as to evaluation criteria. Soil-cement

proponents use a minimum compressive strength of 300 psi with

a curing time of 7 days.

Soil lime advocates, considering the slower reaction time

of lime versus portland cement, use a 28 day curing time.

Since the two fly ashes in this study have self-cementing prop-

erties, the 7 day curing time was chosen as the evaluation

criteria for soil-fly ash mixtures.

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------------------------------------+---------+---------+---------+---------+---------+------------------------------3X OPT OPT +3Y. OPT 60 HIN 60 MIN 60 ~IN MOISTURE

TIME DELAY -3% OPT

0 MIN OPT

0 HIN ,._3r. OPT

0 MIN

TREATM£NT

0--'- -·<:·-:>'.' ·;c·-•• '-VC:'.,,;,.., .. ~"---"""- .-.~~...,,~,~"I" .•... .,_._-.-.-. "'-"'""'"°""""'"'~~c..,..,.;'>.~;<J.,~>O"'.Oi>"-'oC>'><-""'

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STRENGTH

500

450

400

350

300

~so

200

150

100

50

0

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+ I I I + I I I + I I I + I I I + I I I + I I I +

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FIGURE 3

NEAL ~4 ASH AND LOESS

COMPRESSIVE STRENGTH <PSI> AS A FUNCTION OF 1- = 16X ASH 2 = 20Y. A~H 3

28 DAY CUF\E

FL:Y(!tSH X .!>.

= rrr. ASH

TREATMENT

3 ~

2~2~"'3 ,_ ~~

3 .

t /1~ ::.---. ~ ---- 3 / . ~1'L)

I I I ------------------------------------+---------+---------+---------+---------+--·-------+----------"'."------------------

MOISTURE "" -3% OPT OPT +3% OPT . 7'3r. Of'T DPT +3% OPT . I . TIME DELAY ~ 0 MIN 0 MIN 0 MIN 60 HIN 60 MIN 60 MIN

TREATMENT

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STRENGTH

500 + I I I

450 + I I I

400 + I I I

350 +

300 +

250 +

200 +

150 +

100 +

50 +

• +

FIGURE 4

COUNCIL BLUFFS ASH AND LOESS

COMPRESSIVE STRENGTH <PSI> AS A FUNCTION OF FLYASH X t. 1 = 16X ASH 2 = 20Y. ASH 3 ~: 25X ASH

7 DAY CURE

•

! TREATME~T

; ;~~ '" j 1(1) 3 3.....- <1(2.)

,

------------------------------------+---------+---------+---------+---------+---------+-----------------------------MOISTURE

TIME DELAY -3X OPT

0 MIN OPT

0 MIN +3Y. OPT

0 MIN -3% OPT OPT +3X OPT 60 MIN 60 MIN 60 MIN

TREATMENT

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:----.~"'~~~-~---- II • • Ill • • • II II •···. 111 a ·~ · ':WI·; ·· ... · -----~···~---. -.-.- .---.--.--W.--41A·•rfl

STr\ENOTH

500 + I I I

450 +

400 + I I I

350 +

300 +

250 + I

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200 +

150 +

100. +

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FIGURE 5

COUNCIL BLUFFS ASH AND LOESS

COMPRESSIVE STRENGTH {PSI) AS A FUNCTION OF FLYASH X b t "' 16% ASH 2 = 20% ASH 3 = 25X ASH

TREATMENT

28 DAY CURE

3 3~ 2----" ' 1~~

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---------------------~--------------+---------+---------+---------+---------+---------+-----------------------------MOISTURE = -3X OPT OPT +3Y. OPT -3X OPT OPT +Jr. OPT

TIME DELAY = 0 MIN 0 MIN 0 MIN 60 MIN 60 MIN 60 HIN

TREATMENT I

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FIGURE 7

NEAL :4 AND BLOW SAND

COMPRESSIVE STRENGTH CPSI> AS A FUNCTION OF FLYASH X & TREATMENT 1 ~ 1bX ASH 2 = 20X ASH 3 = 25X ASH

29 DAY CURE

-3% OF·T OPT +31. OPT

I 1 ------------------------------------+---------+---------+---------+---------+---------+------------~----------------

MOISTURE TIME DELAY

-3r. OPT 0 MIN

OPT +3X OPT G MIN G MJ°N 60 MIN 60 HIN 60 MIN

TREATMENT

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------------------------------------+---------+---------+---------+~--------+---------+-----------------------------

MOISTURE = -3X OPT OPT .+3~ OPT -3X OPT OPT +3% OPT TIME DELAY s 0 MIN G MIN 0 MIN 60 MIN 60 MIN 60 MXN

f~eAT~~NT

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- - • _.. _.. •. -4' , _... i _... , _..; _.. j •• , ; -ti' i -tl''''i ; -t1·'''''', ·.·· -11.'P/" · ti',,, i ·~ --- -·- - -- -- - - --- ---- --- ... ·---- -,-- - --

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FIGURE 9

COUNCIL FLUFFS ASH AND BLOW SAND

COMPRESSIVE STRENGTH <PSI> AS A FUNCTION OF FLYASH X & TREATMENT 1 = 16X ASH 2 = 20Y. ASH 3 = 25X ASH

28 DAY CURE

STRENGTH

. 900 + 3 I

850 + I

BOO +

750 +

700 +

650 +

600 +

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500 + 3

450 +

400 + 2

350 +

300 +

250 +

200 +

tSO +

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100 +

SC +

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----------------------~-------------~---------+---------·---------+~--------+---------+---~-------------------------HO I STURE a -3X OPT OPT +3X OPT -3% OPT OPT +3X OPT

TIME DELAY =a 0 HIN 0 MIN 0 HIN 60_ HIN 60 HIN 60 MIN

T' ,.,., .. ,,, .

Analysis of the data obtained shows that loess-fly ash

mixtures failed to reach 300 psi regardless of moisture con-

tent, compaction delay time, .fly ash content or being allowed

to cure for 28 days (Figures 2 thru 5) •

The results of the blow sar1d-"fly ash testing (Figures 6

thru 9) indicate that the 300 psi limit can be exceeded by

utilizing a high percentage of Council Bluffs ash and a mois-

ture content that varies depending upon the compaction delay

time. The interrelationships between moisture content and

delay time were studied further in the retarder phase.

The distinction made by ASTM C618 between Class F and

Class C ashes is based on the total amount of silicon dioxide

plus aluminum oxide plus iron oxide present. Class F requires

a minimum of 70% of the above oxides and Class C requires a

minimum of 50%. The inference being that since Class C ashes

contain less of the three listed oxides they contain more

calcium oxides and therefore may be self-cementing. Although

calcium oxide in itself is not responsible for the cementing

action of a fly ash, it is an indicator of the presence and

relative abundance of cementing compounds.

It would appear that the ASTM Class F and Class C charac-

terization of an ash can be a first guide to its suitability

as a soil additive to produce high strengths. As evidence of

this, the Council Bluffs ash exhibits consistent chemical re-

sults that classify it as a Class c ash only. Neal No. 4 chem-

ical results show variability to the point that it would have

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500 + I I I

450 +

400 + I I I

350 +

300 +

250 +

200 +

150 +

10G +

50 +

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FIGURE 10

RETARDER STUDY COMPRESSIVE ~TRENGTH <PSI) AS FUNCTION OF ADDITIVE X & TREATMENT

FLOW SAND - 25X COUNCIL BLUFFS ASH - LIQUID RETARDER E a NO ADDITIV~ ~ = 4 FL. OZ LIQUID RETARDER / CWT OF FLY ASH

E/E E

Q

E

Q

Q

E--------E E/E

E

-----------------------+---------+---------+---------+---------+---------+---------+-----~---+---------·-----------------MOISTURE = -3X OPT OPT +3X OPT -3X OPT OPT +3% OPT -3X·OPT OPT +3X OPT ,

TIME DELAY = 0 MIN G MIN 0 MIN 60 MIN 60 MIN '60. MIN 120 MIN 120 MIN t2& MIN

TREATMENT

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. 450 + I I I

400 + I I I

350 +

300 +

250 +

200 +

150 +

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FIGURE 12

RETARDER STUDY COMPRESSIVE STRENGTH (PSI) AS FUNCTION OF ADDITIVE X 6 TREATMENT

BLOW SAND - i6X COUNCIL BLUFFS ASH

A = NO ADDITIVE B # 0.5X BORAX G = O.SX GYPSUM

#:~H He A~~."" A B~\~G ·B

A

D

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B/B\ H H~G

~~~~\

H "" fX GYPSUM

.----·'\. --A

H G A ~

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~----------------------+---------+---------+---------+----~----+---------+---------+---------+---------+-----------------HOISTURE "" -3X OPT OPT +3X OPT -3X OPT OPT +3% OPT -3X OPT OPT +3X OPT

TIME DELAY "" 0 MIN G MIN O MIN 60 MIN 60 MIN 60 tt-IN 120 HIN 12G HIN 120 HIN

TREATMENT

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$TR

500 + I I I

450 + I I I

400 +

350 +

300 + I I I

. 250 +

200 +

150 + I I I

100 + I I I

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FIGURE 14

RETARDER STUDY COMPRESSIVE STRENGTH (PSI> AS FUNCTION OF ADDITIVE XX TREATMENT

BLOW SAND - 25Y. COUNCIL BLUFFS ASH

£ = NO ADDITIVE f· = O.SY. BORAX k = 0.SY. GYPSUM L = 1X GYPSUM

L

K

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F

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L :F

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-----------------------+---------+---------+---~----+---------+---------+---------+---------+---------+-----------------HOISTURE • -3X OPT OPT +3X OPT -3X OPT OPT +3X OPT :3X OPT· OPT +3X OPT TIHE DELAY .,,· 0 HIN G HIN G HIN 60 MIN 60 HIN ~O MIN .120 MIN 120 HIN 120 HIN.".'.

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report on fly ash as a soil additive -...

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