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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
Soil-Fly Ash-Retarder
Page
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iii
1
1
2
2
2
2
5
5
5
5
15
17
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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.
- 2 -
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
f
f r r r [
f r r ' I
r r r r r F
r r !
4
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fly ash added.
Soil-Fly Ash-Retarder
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>'><-""'
·-- -· -·-· ·-· ·-·--·-- ---•-• 1111. W.····91
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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f4"Tti"'"ii'F''t'etrrr:ni T'Wizren11me'trt::T&'ffff i''W'iew= iii '"; . rtnBt :zf~·xrmr' .. ifttr . F ·rw: . I - .. ~;-.•t"Jf''}'YTtt' ,. '"·-''Y""'F'- '"Zt't""''"'-'§f-'''"CT?tr·'""'"EitJtlr§·M{-.i;.,
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
1' I !
00
L
:----.~"'~~~-~---- 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
. I I
200 +
150 +
100. +
so +
0 + I I
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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'" 2 -3____-:3 ,----·-===2 '
•
---------------------~--------------+---------+---------+---------+---------+---------+-----------------------------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
"....;_, __ -
- - • _.. _.. •. -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 +
5so + I
500 + 3
450 +
400 + 2
350 +
300 +
250 +
200 +
tSO +
.1·~. I
100 +
SC +
0 + I I I I
----------------------~-------------~---------+---------·---------+~--------+---------+---~-------------------------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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STR
500 + I I I
450 +
400 + I I I
350 +
300 +
250 +
200 +
150 +
10G +
50 +
0 +
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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500 + I I I
. 450 + I I I
400 + I I I
350 +
300 +
250 +
200 +
150 +
100 +
50 •
0 • I I
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
•
B/B\ H H~G
~~~~\
H "" fX GYPSUM
.----·'\. --A
H G A ~
~--
,,
~----------------------+---------+---------+---------+----~----+---------+---------+---------+---------+-----------------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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kif'~f'&i,;j' </·•'• .• ·> .. ,, .. .,.,·;,
$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
5Q + I I I
• +
\
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
If:: KV E
F
,/\ E
L :F
E'
F
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E
'v V\ . I \_L
L-F
L' K~K''
E.----E
F
~of'' E
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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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