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Description  |
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FIELD OF THE INVENTION
This invention relates to the field of fly ash and is concerned more
particularly with a new synthetic Class C fly ash and the use of the same
as a substantial replacement for portland cement in general purpose
concrete construction including read-mix concrete.
BACKGROUND OF THE INVENTION
This invention is concerned with the utilization of two industrial
by-products; namely Class F fly ash and cement kiln dust (hereinafter
CKD). When finely divided or pulverized coal is combusted at high
temperatures, for example, in boilers for the steam generation of
electricity, the ash consisting of the uncombustible residue plus a small
amount of residual combustible matter, is made up of two fractions, a
bottom ash recovered from the furnace or boiler in the form of a slag-like
material and a fly ash which remains suspended in the flue gases from the
combustion until separated therefrom by known separatory techniques, such
as electrostatic precipitation. This fly ash is an extremely finely
divided material generally in the form of spherical bead-like particles,
with at least 70% by weight passing a 200 mesh sieve and has a generally
glassy state, resulting from fusion or sintering during combustion. As
recognized in the American Society of Testing Materials (ASTM)
specification designation C618-85 entitled "Fly Ash and Raw or Calcined
Natural Pozzolan for Use as a Mineral Admixture in Portland Cement
Concrete", fly ash is subdivided into two distinct classifications;
namely, Class F and Class C. The definitions of these two classes are as
follows:
"Class F--Fly ash normally produced from burning anthracite or bituminous
coal that meets the applicable requirements for this class as given
herein. This class fly ash has pozzolanic properties.
Class C--Fly Ash normally produced from lignite or subbituminous coal that
meets the applicable requirements for this class as given herein. This
class of fly ash, in addition to having pozzolanic properties, also has
some cementitious properties. Some Class C flyashes may contain lime
contents higher than 10%."
The latter reference to "pozzolanic properties" refers to the capability of
certain mixtures which are not in themselves cementitious of undergoing a
cementitious reaction when mixed with lime in the presence of water. Class
C fly ash possesses direct cementitious properties as well as pozzolanic
properties. ASTM C618-85 is also applicable to natural pozzolanic
materials which are separately classified as Class N but are not pertinent
here.
As the above quotation indicates, the type of coal to be combusted
generally determines which class fly ash results, and the type of coal in
turn is often dependent on its geographic origin. Thus, Class C fly ash
frequently results from coals mined in the Midwest; whereas Class F fly
ash often comes from coals mined in the Appalachian region. The ASTM
specification imposes certain "chemical requirements" upon the respective
fly ash classifications thereof which are set forth below for the relevant
Classes F and C, including footnotes:
TABLE I-A
______________________________________
CHEMICAL REQUIREMENTS
Mineral
Admixture Class
F C
______________________________________
Silicon dioxide (SiO.sub.2) plus aluminum
70.0 50.0
oxide (Al.sub.2 O.sub.3) plus iron oxide
(Fe.sub.2 O.sub.3), min, %
Sulfur trioxide (SO.sub.3), max, %
5.0 5.0
Moisture content, max. %
3.0 3.0
Loss on ignition, max, %
.sup. 6.0.sup.1
6.0
______________________________________
.sup.1 The use of Class F pozzolan containing up to 12.0% loss on ignitio
may be approved by the user of either acceptable performance records or
laboratory test results that are made available.
TABLE I-B
______________________________________
SUPPLEMENTAY
OPTIONAL CHEMICAL REQUIREMENT
Mineral
Admixture Class
F C
______________________________________
Available alkalies, as Na.sub.2 O, max, %.sup.2
1.50 1.50
______________________________________
Note:
This optional requirement applies only when specifically requested.
.sup.2 Applicable only when specifically required by the purchaser for
mineral admixture to be used in concrete containing reactive aggregate an
cement to meet a limitation on content of alkalies.
The ASTM physical requirements for both fly ash classes are virtually the
same and are reproduced below exclusive of cautionary footnotes:
TABLE I-C
______________________________________
PHYSICAL REQUIREMENTS
Mineral
Admixture Class
F C
______________________________________
Fineness
Amount retained when wet-sieved
34 34
on No. 325 (45 .mu.m) sieve, max %
Pozzolanic activity index
With portland cement, at
75 75
28 days, min. percent of
control
With lime, at 7 days min,
800(5500)
. . .
psi (kPa)
Water requirement, max,
105 105
percent of control
Soundness
Autoclave expansion or contraction,
0.8 0.8
max %
Uniformity requirements
The specific gravity and fineness
of individual samples shall not vary
from the average established by
the ten preceding tests or by all
preceding tests if the number is
less than ten, by more than:
Specific gravity, max variation
5 5
from average, %
Percent retained on No. 325
5 5
(45 .mu.m), max variation,
percentage points from average
______________________________________
CKD, on the other hand, is a by-product of the production of portland
cement clinkers by the high temperature furnacing of appropriate raw
materials typically mixtures of limestone and clay or a low grade
limestone already containing a sufficient quantity of argillaceous
materials often with added quantities of lime to adjust the final
composition. The resultant clinkers are pulverized by grinding to a high
degree of fineness and these particles upon admixture with sufficient
water undergo a cementitious reaction and produce the solid product
generally referred to as concrete, which exhibits high compressive
strength and is thus highly useful in the construction of a great variety
of building or supporting structures. Generally, rotary furnaces are used
for producing portland cement clinkers and a certain quantity of finely
divided dust is produced as a by-product which is carried off in the flue
gases from such furnaces. The dust content can range from about 5% of the
clinkers output in so-called wet process plants up to as high as 15% in
dry process plants. The suspended dust is removed by various separating
techniques and remains as a by-product of the cement making operation.
Part of the CKD can be returned to the furnace as recycled raw material,
but it is not readily reincorporated into clinker formation and, in
addition, tends to excessively elevate the alkalinity of the ultimate
portland cement.
The quantities of these two by-product materials which are produced
annually are enormous and are likely only to increase in the future. As
petroleum oil as the fuel for the generation of electricity is reduced
because of conservation efforts and unfavorable economics vs. the more
readily available coal and as political considerations increasingly
preclude the construction of new nuclear power electrical generating
facilities, or even the operation of already completed units of this type,
greater reliance will necessarily fall on coal as the fuel for generating
electricity. As of 1979, the amount of CKD was estimated as accumulating
at a rate of 4-12 million tons per year in the United States alone;
whereas the amount of Class F fly ash that is available is estimated to be
about ten times what can be readily utilized. Obviously, there is an
urgent growing need to find effective ways of employing these unavoidably
industrial by-products since otherwise they will collect at a staggering
rate and create crucial concerns over their adverse environmental effect.
Various proposals have already been made for utilizing both fly ash and
CKD. According to the text The Chemistry of Cement and Concrete by Lea,
Chemical Publishing Company, Inc., 1971 edition, at page 421 et seq., fly
ash, i.e., Class F type, from boilers was first reported to be potentially
useful as a partial replacement for portland cement in concrete
construction about 50 years ago, and its utilization for that purpose has
since become increasingly widespread. It is generally accepted that the
proportion of portland cement replaced by the usual fly ash should not
exceed about 20% to avoid significant reduction in the compressive
strength of the resultant concrete, although some more cautions
jurisdictions may impose lower limits, e.g., the 15% maximum authorized by
the Virginia Department of Highways and Transportation (VDHT). As
described by Lea at page 437, the substitution of the fly ash tends to
retard the early rate of hardening of the concrete so that the concrete
shows up to a 30% lower strength after seven days testing and up to 25%
lower strength after 28 days of testing, but in time the strength levels
equalize at replacement levels up to 20%. Increasing the substitution
quantity up to 30% gives more drastic reduction in the early compression
values plus an ultimate reduction of at least about 15% after one year.
The limited substitution of fly ash for portland cement in concrete
formulations has other effects beyond compressive strength changes, both
positive and negative. The fly ash tends to increase the workability of
the cement mix and is recognized as desirably reducing the reactivity of
the portland cement with so-called reactive aggregates. On the other hand,
fly ash contains a minor content of uncombusted carbon which acts to
absorb air entrained in the concrete. Because entrained air increases the
resistance of the hardened concrete to freezing, such reduction is
undesirable but can be compensated for by the inclusion as an additive of
so-called air-entraining agents.
Utilization of fly ash for up to 20% of cement in concrete mixes at best
consumes only a fraction of the available quantities of this material, and
efforts have been made to increase its use. Dodson et al in U.S. Pat. No.
4,210,457, while recognizing this accepted limit, proposed the
substitution of larger amounts of fly ash, and preferably more, of the
portland cement with certain selected natural fly ashes having a combined
content of silica, alumina and ferric oxide content, less than 80% by
weight, and a calcium content exceeding 10%, based on five samples of such
ashes, varying from about 58-72% combined with a calcium oxide range of
about 18-30%. Six other ash samples which were not suitable at the high
levels of 50% or more were shown to vary in the combined oxide content
from about 87-92% and in calcium oxide content from about 4 to about 8%.
Evaluating these values against the ASTM C618-85, one observes that the
acceptable fly ashes came under the Class C specifications, while the
unacceptable ashes fell in the Class F specification. Thus, this patent in
effect establishes that natural Class C fly ashes are suitable for
substantially higher levels of replacement for portland cement in concrete
mixes than are Class F fly ashes, and this capacity is now generally
recognized, with Class C fly ashes being generally permitted up to about a
50% replacement level while maintaining the desirable physical properties
of the concrete especially compressive strength.
In U.S. Pat. No. 4,240,952, Hulbert et al while also acknowledging the
generally recognized permissible limit of (Class F) fly ash replacement
for portland cement of 20%, proposed replacement of at least 50% up to
80%, provided the mix contained as additives about 2% of gypsum and about
3% of calcium chloride by weight of the fly ash. The fly ash described for
this purpose, however, was a natural Class C fly ash analyzing about 28%
calcium oxide and a combined silica, alumina and ferric oxide content of
about 63%. With up to 80% of this fly ash and the specified additives,
compressive strengths comparable to straight portland cement were said to
be generally achievable. In one example using 140 pounds portland cement
and 560 pounds of fly ash (80-20 ratio) with conventional amounts of
coarse and fine aggregate, and water and including the requisite
additives, compressive strengths tested at 3180 psi for 7 days, 4200 psi
for 14 days and about 5000 psi at 28 days.
Obviously, the above patents cannot contribute to a solution to the problem
with Class F fly ash. In U.S. Pat. Nos. 4,018,617 and 4,101,332, Nicholson
proposed the use of mixtures of fly ash (apparently Class F in type),
cement kiln dust and aggregate for creating a stabilized base supporting
surface replacing conventional gravel- or asphalt-aggregate-stabilized
bases in road construction wherein the useful ranges were fly ash 6-24%,
CKD 4-16% and aggregate 60-90%, with 8% CKD, 12% fly ash and 80% aggregate
preferred. Compressive strength values for such measures as revealed in
the examples varied rather erratically and generally exhibited only small
increases in compression strength over the 7-28 day test period. Among the
better results were for the preferred mixture wherein the values increased
from about 1100 psi at 7 days to 1400 psi at 28 days. The addition of a
small amount of calcium chloride added about a 200 psi increment to these
values. On the other hand, the addition of 3% of lime stack dust recovered
from a lime kiln significantly reduced the results to about 700 psi at 7
days to 900-1300 psi at 28 days. Elimination of the aggregate reduced the
strength to a fraction of the values otherwise, a mixture of 12% CKD and
88% fly ash alone showing strength values of only about 190-260 psi over
the 28 day test period. Similarly, the choice of a finely divided
aggregate such as fill sand resulted in about the same fractional level of
strength values in the range of about 140-230 psi. A combination of finely
divided and coarse aggregate in approximately equal amounts reduced the
compressive strength values by about 1/2 with virtually no change over the
test period, giving values ranging from about 650-750 psi, except where 1%
of Type 1 portland cement was included which restored the strength values
to about their general level, except at the initial 7 day period where the
strength values were about 800-900 psi increase at 28 days to about
1200-1600 psi. Curiously, the best strength results were attained when
11.6% fly ash was combined with 3.4% lime with the balance crushed
aggregate, the CKD being omitted entirely, for which the strength values
while starting at a lower level of about 850-950 at 7 days increased to
about 1700 psi at 28 days.
The combination of fly ash and lime stack dust incidentally mentioned in
the later patent was explored further by Nicholson in U.S. Pat. No.
4,038,095 which governs mixtures of about 10-14% fly ash, about 5-15% lime
stack dust with the balance aggregate in the range of 71-85%. Somewhat
inexpicably, the compressive results reported here for such mixtures do
not reach the high level specified in the first two patents, the strength
values specified being only about 1000 psi with the more general levels
well below that depending on particular proportions.
In U.S. Pat. No. 4,268,316, Wills discloses the use of mixtures of kiln
dust and fly ash as a replacement for ground limestome and gypsum for
forming a mortar or masonry cement, using proportions of about 25-55%
portland cement, about 25-65% CKD and 10-25% fly ash. When these mortar
formulations were mixed with damp sand in the proportions of about one
part cement mixture to 2.5-3 parts sand, compression strengths comparable
to those of standard masonry cement composed of 55% cement clinkers, 40%
limestone and 5% gypsum were shown for mixtures containing 50% cement,
25-40% CKD and 15-25% fly ash. Inexplicably, in one example, when the
cement content was increased to 55% with 35% CKD and 10% fly ash, the
compressive strengths dropped by about 30-40% at both the 7 day and 28 day
ages to levels inferior to the standard material. As the cement content
was decreased, with corresponding increases in the CKD, the compressive
strength values dropped drastically. On the other hand, in another similar
example mixtures containing 55% cement, 35% CKD and 10% ash proved
superior, particularly at the 28 day age, in compressive strength mixtures
containing 50% cement, 35% fly ash and 15% CKD as well as other standard
masonry cements containing 50% cement, 47% limestone and 3% gypsum.
Indeed, strength values dropped about 40% for the mixtures with a 5%
reduction in cement and a corresponding 5% increase in the fly ash to
values definitely inferior to the standard cements. Similar variations
were shown under laboratory test conditions for comparable 50/35/15
mixtures dependent on the source of the fly ash while under actual
construction conditions for the same mixtures, compressive strength values
were reduced by about 50% for both the conventional masonry cement
containing 55% portland cement and comparable mixtures within the patented
concept. The fly ash was preferably Class F with Class C materials being
less desirable.
In U.S. Pat. No. 4,407,677 Wills went on to teach that in the manufacture
of concrete products such as blocks or bricks, the fly ash usually
employed in combination with portland cement therein could be replaced in
its entirety by CKD with modest improvement in early compressive strength
values for such products. Thus, at one day and two day tests, compressive
strength values were shown of about 500-800 psi, but were said to increase
to about 1200 psi after 28 days. The mixes disclosed here contained
0.4-0.9 parts cement, about 0.1-0.6 parts CKD and 10-12 parts aggregate
combining both fine and coarse materials, such as expanded shale and
natural sand in a weight ratio of 80/20. Masonry cements generally develop
at least about 95% of their strength properties at 28 days age so that
additional aging of the patent products would not be expected to result in
any significant increase in their compressive strength values.
In a different vein, an improved highly activated fly ash is obtained by
Minnick in U.S. Pat. No. 3,643,115 by injecting lime together with
bituminous coal into the combustion boiler to give a synthetic fly ash
developing early strength as high as five times that obtained
conventionally. The improved highly active fly ash can be mixed in
proportions of 80-90 parts with 5-87 parts aggregate and 5-30 parts water.
The injected lime combines with the sulfur dioxide released during
combustion of the coal, and additional sulfur may be needed if the coal
has insufficient sulfur, giving a fly ash having a considerably increased
sulfate content as well as calcium oxide and magnesium contents.
OBJECTS OF THE INVENTION
The object of the present invention is a new Class F fly ash-CKD blend in
proportions within the range of about 40-60 fly ash and 60-40 CKD giving
in the blend a combined content of silica, alumina and ferric oxide of at
least about 50% but below about 70% with a calcium oxide content of at
least 10% and thus fulfills the salient specification requirements of ASTM
C618-85 for a Class C fly ash.
Another object is a synthetic Class C fly ash blend which can be used in
lieu of natural Class C fly ashes in the art, especially in combination
with portland cement for making concrete.
A further object is a concrete mix of the general or all purpose variety in
which in excess of about 25% and preferably in excess of about 30% up to
about 50% by weight of the portland cement used therein is replaced by a
synthetic Class C fly ash blended from Class F fly ash and cement kiln
dust.
SUMMARY OF THE INVENTION
It has now been discovered that regular or common Class F fly ash having a
combined silica, alumina and ferric oxide content of at least about 70%
and preferably at least about 80% by weight, as well as a calcium oxide
content below about 10%, when blended homogeneously with cement kiln dust
(CKD) in the proportions of 60-40/40-60% by weight yields a blend having a
combined silica, alumina and ferric oxide content of at least 50% but
below about 70% with a calcium oxide content above about 10% and
preferably above about 20% and satisfies the essential specification
requirements for a Class C fly ash of ASTM 1618-85 and thus constitutes a
new synthetic Class C fly ash product. This product has been found to be
comparable with natural Class C fly ash in its important properties and
especially for purposes of combination with portland cement for producing
concrete. In particular, it effectively replaces the same large
proportions of portland cement up to about 50% by weight as is generally
approved for natural Class C fly ash in contrast to the substantially
lower replacement levels allowed for Class F fly ash. By replacing more
than about 25% and preferably more than about 30% of the portland cement
up to the 50% limit, substantial economic savings can be achieved because
of the favorable pricing structure for the waste products Class F fy ash
and CKD compared to the valuable commodity portland cement. For general or
all purpose cement mixes, including so-called ready mixes as prepared and
delivered in cement mixer trucks as well as bagged or bulk mixes for
general or all purpose concrete construction, which would normally contain
about 400-700 lbs. portland cement, about 1600-2000 of a coarse aggregate
and a sufficient quantity of a fine generally sand-like aggregate plus
minor amounts of any optional ingredients as to yield one cubic yard of
concrete when admixed in water equal in amount to about 40-70% weight of
the portland cement, the new synthetic class C fly ash blend of this
invention can replace more than 25% and as much as 50% by weight of the
portland cement up to a limit of about 200 lbs wt, while effectively
achieving substantially the same structural properties, especially
compressive strength in the resultant concrete structure as achieved with
100% portland cement.
DETAILED DESCRIPTION OF THE INVENTION
Any standard or common Class F fly ash obtained from boilers and like
furnaces used for the combustion of pulverized coal, particularly of a
bituminous or anthracite type, and especially from coal-fired,
steam-generating plants of electrical utilities, is suitable for use as
the Class F fly ash component of this invention. Such fly ash should have
a combined silica, alumina and ferric oxide content of at least about 70%
and preferably 80% or higher by weight and a calcium oxide content below
about 10%, usually about 6% by weight or less. A specific fly ash found to
give particularly good results in the invention is so-called "Carbo" fly
ash obtained from the Clinch River Power Plant of the American Electric
Power Service Corporation at Carbo, Va. This specific fly ash is available
from Ash Management Corp., a subsidiary of JTM Industries of Marietta, Ga.
An analysis of this preferred Carbo fly ash appears in the following Table
II which sets forth the chemical composition and certain physical
properties of fly ash samples taken periodically from this plant over a
nine month period.
TABLE II
__________________________________________________________________________
ANALYSIS OF TYPICAL CLASS F FLY ASH
Sample
1 2 3 4 5 6 7 8 9 10
__________________________________________________________________________
CHEMICAL ANALYSIS
Silica 50.0
50.7
51.2
53.0
53.0
49.4
50.6
50.9
56.3
54.5
Aluminum Oxide 29.8
28.5
25.5
26.1
24.7
25.4
25.2
24.6
23.1
24.8
Iron Oxide 6.8
7.0
7.6
7.8
7.4
8.4
8.2
8.0
7.7
7.3
Combined 1, 2 & 3 86.6
86.2
84.3
86.9
85.1
83.2
84.0
83.5
87.1
86.6
Titanium Dioxide 1.6
1.6
1.6
1.7
1.7
1.5
1.5
1.5
1.5
1.7
Calcium Oxide 6.1
6.3
6.6
4.1
5.7
7.0
9.1
6.8
5.8
5.6
Magnesium Oxide 1.7
1.7
1.8
1.6
1.6
1.8
2.0
1.8
1.4
1.6
Sodium Oxide 0.8
0.6
0.6
0.6
0.7
0.6
0.7
0.6
0.6
0.6
Potassium Oxide 2.5
2.8
2.6
2.9
2.4
2.8
2.5
2.9
2.7
2.6
Sulfur Trioxide 0.9
0.8
0.7
0.5
0.6
0.9
1.0
0.9
0.9
0.8
Phosphorous Pentoxide
0.3
0.2
0.2
0.2
0.2
0.2
0.2
0.3
0.3
0.2
Other Constituents
1.2
1.0
1.1
0.7
0.7
0.6
0.6
0.7
0.7
1.0
Available Alkalies, %
.65
.78
.77
.76
.66
.78
.76
.70
.78
.83
Average Combined 1, 2 & 3 = 85.3;
Av. CaO = 6.21
PHYSICAL TESTS
Moisture Content, %
.02
.14
.05
.03
.02
.18
.05
.01
.12
Neg Ignition Loss 1.07
1.27
1.01
0.94
0.60
0.76
.62
.82
.75
Retained on No. 325, Wet-sieved, %
19.99
21.68
20.39
20.58
19.34
17.66
19.13
22.53
20.80
Specific Gravity 2.27
2.24
2.22
2.25 2.26
2.26
2.27
2.26
__________________________________________________________________________
Identification of other common Class F fly ashes can be found throughout
the literature. For example, U.S. Pat. No. 4,210,457 mentioned previously,
provides an analysis in Table I thereof for six samples of Class F fly
ashes, identified F-K from which the content of constituents thereof
expressed as a weight percent range and an average value are summarized in
the following Table III.
TABLE III
______________________________________
VARIOUS CLASS F FLY ASHES FROM USP 4,210,457
Main Components, % wt.
Range Average
______________________________________
SiO.sub.2 43.3-56.5
49.3
Al.sub.2 O.sub.3 18.5-31.0
24.65
Fe.sub.2 O.sub.3 5.6-29.9
14.2
CaO 4.3-7.7 5.95
SiO.sub.2 + Al.sub.2 O.sub.3 + Fe.sub.2 O.sub.3
83.9-91.7
88.1
______________________________________
In addition, the Lea text indentified above gives in Table 71 at page 422
an analysis for four Class F fly ashes, two from the U.S.A. and two from
Great Britain. Similarly, analysis for representative fly ashes, both
Class F and Class C, from Texas sources appear at pages 175-178 in
Research Report 450-1, project 3-9-85-450 by Center for Transportation
Research, Bureau of Engineering Research, the University of Texas at
Austin, entitled "Effectiveness of Fly Ash Replacement in the Reduction of
Damage Due to Alkali-Aggregate Reaction in Concrete" by Farbiarz and
Carrasquillo, May 1986. In this report, the Class F fly ashes are
identified as "Type A"; while the Class C fly ashes are identified as
"Type B" according to whether the fly ashes contain a high content of
combined silica, alumina and ferric oxide with low calcium oxide or vice
versa. One sample described in this report illustrates the unreliability
of a classification of fly ashes along geological lines, i.e., whether
derived from bituminous and anthracite coal versus subbituminous and
lignite coal, that sample being considered a Type B (Class C) ash despite
its derivation from sublignite and a normal classification by its supplier
as Class F.
Similarly, any common cement kiln dust (CKD) which is produced as a
by-product during the industrial production of portland cement would in
principle be suitable for purposes of this invention. One specific CKD,
obtained as a matter of convenience, from the Tarmac Lone Star Cement
Company cement plant at Roanoke, Va., has been found entirely useful in
the invention, and an analysis of this CKD is set forth in the following
Table IV.
TABLE IV
______________________________________
ANALYSIS OF TYPICAL CEMENT KILN DUST (CKD)
______________________________________
Chemical Analysis, %
Insoluble Residue
SiO.sub.2 16.80
Al.sub.2 O.sub.3 5.12
Fe.sub.2 O.sub.3 2.38
CaO 45.89
MgO 2.22
SO.sub.3 4.93
K.sub.2 O 2.65
Na.sub.2 O 40
Total Alkali 2.14
Combine 1, 2 & 3 24.3
Loss on Ignition 19.60
______________________________________
Particle Size
Sieve # % Passing
______________________________________
20 98.6
50 96.5
100 93.8
200 85.4
325 74.6
______________________________________
Composition for various samples of CKD can be found in the patent
literature wherein, for example, nine different samples have been analyzed
by Nicholson in U.S. Pat. No. 4,018,617 mentioned previously. A summary of
the range and average of the weight percent values for contents of the
main components of these nine samples as taken from this patent appears
below as Table V.
TABLE V
______________________________________
VARIOUS CKD'S FROM USP 4,018,617
Main Components, % wt
Range
Ingredient Low High Average
______________________________________
SiO.sub.2 6.0 28.5 16.5
Al.sub.2 O.sub.3
3.2 9.6 4.35
Fe.sub.2 O.sub.3
0.8 5.9 2.66
CaO 16.0 65.0 47.6
Combined 1, 2 & 3
10.0 44.0 23.5
______________________________________
In preparing the new synthetic Class C fly ash blends of the invention,
40-60% by weight of a standard Class F fly ash is uniformly and
homogeneously admixed with 60-40% by weight of CKD. Usually, blends within
these limits will yield a blended synthetic product having a combined
silica, alumina and ferric oxide content of at least about 50% but below
the level of at least about 70% as is characteristic by ASTM standards as
a Class F fly ash. However, as the analyses for Class F fly ashes and
CKD's set forth above establishes, both Class F fly ashes and CKD's are
subject to wide variation in composition. Consequently, if certain CKD's
having uncharacteristically high contents of silica, alumina and ferric
oxide, i.e., exceeding about 20%, were to be combined with a Class F fly
ash having a combined content of the same components at the upper end of
the range recognized therefor, say exceeding 90%, then the possibility
exists of the ultimate synthetic Class C blended product having a combined
silica, alumina and ferric oxide content failing to satisfy the ASTM
standards for a Class C fly ash, i.e., in excess of 70% by weight. In that
event, the upper limit of the range for the fly ash might need to be
adjusted from 60% to perhaps about 55% or so in order to stay within the
ASTM limit.
Normally, the content of calcium oxide will be sufficiently high in any
ordinary CKD as to give a calcium oxide content exceeding the minimum 10%
by weight limit when blended with any of the generally available Class F
fly ashes.
An analysis of a typical synthetic Class C fly ash blended product of the
invention containing 42% Class F Carbo fly ash and 58% CKD from the Lone
Star Roanoke cement plant is set forth below as Table VI.
TABLE VI
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ANALYSIS OF TYPICAL SYNTHETIC CLASS C
BLENDED FLY ASH CONTAINING 42% CLASS F
FLY ASH AND 58% CKD
Constituents wt. %
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SiO.sub.2 31.24
Al.sub.2 O.sub.3 16.78
Fe.sub.2 O.sub.3 4.65
CaO 30.29
MgO 2.56
SO.sub.3 1.37
Total Loss on Ignition
13.00
Carbon Dioxide 8.25
Moisture 0.45
Effective Loss on Ignition
4.75
(Total Loss - CO.sub.2)
Combined 1, 2 & 3 52.67
Carbon (c) 0.42
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As is known, Class F fly ashes generally contain a certain amount of
elemental carbon resulting from incomplete combustion of the pulverized
coal and because of the undesirable effects of the carbon content thereof
on the air entrainment capability of concrete containing the same, as
stated before, the amount of such carbon is ordinarily restricted. An
upper limit of 6% is common for many jurisdictions; although others such
as the VDHT permit only 2.5% carbon. On the other hand, certain countries,
such as Canada, allow up to 12% carbon. Because in the practice of the
invention, the Class F fly ash is admixed with about 40-60% by weight of
CKD, the effective amount of carbon contained in the new synthetic Class C
blended fly ash product is proportionately reduced from the amount in the
starting Class F fly ash whereby higher fly ash contents in the latter are
tolerable. Thus, the carbon content of the present synthetic blends is
well within even the lower permissible limits and often is negligibly
small.
As will be established later, within the limits the blends of the invention
specified above, such blends exhibit substantially comparable properties
for use in general purpose cement construction, especially compressive
strength. This being the case, economic considerations may be an important
factor in selecting a specific mix within such ranges. Under present
market conditions, and dependent upon transportation distances from the
available sources of the two components, CKD can be purchased somewhat
more cheaply than can a standard Class F fly ash. For example, fly ash
might be purchased at a cost of $20.00 per ton including transportation
expense of about $7.00 per ton; whereas CKD can be purchased for about
$9.00 per ton including about $4.00 transportation expense. Where the
relative expense significantly favors one of the products, such as the
CKD, it is economically advantageous to utilize substantially the maximum
amount of the cheaper constituent. Based on this rationale, a 42% Class F
fly ash/58% CKD blend is deemed preferable.
As seen from the above analysis of a typical sample of CKD, such sample had
a relatively high loss on ignition in the order of 20%. This high loss is
due to the evolution during ignition of carbon dioxide from the carbonate
compounds present in this material. Consequently, when this CKD is blended
with Class F fly ash to form the inventive blends, such blends tend to
exhibit a loss on ignition that is nominally higher than the limit set by
the ASTM standard. However, the purpose of the ASTM standard was to
provide an indication of the content of uncombusted carbon present in
either class of fly ash because of the possible deleterious effects of
such carbon for reasons already explained. Since natural fly ashes, both
Class F and Class C, are residues of combustion, any carbonates present in
the originally finely pulverized coals would already have escaped during
the combustion process. Consequently, the materials for which the ASTM
standards were directly devised were not subject to any loss on ignition
due to the evolution of carbon dioxide and such standards, therefore, are
not directly applicable to materials which are subject to carbon dioxide
loss on ignition. As the analysis shows, after correction for the loss of
carbon dioxide, the effective ignition loss of the synthetic blends of the
invention is within ASTM standards and thus the inventive products
essentially comply with such standards. Also, some CKD's have a quite low
loss on ignition and blends using these would in any case fit the standard
limit.
The choice of aggregate material for concrete mixes using the present
blends will pose no problem to the person skilled in the design of such
mixes. The coarse aggregate should have a minimum size of about 3/8 inch
and can vary in size from that minimum up to one inch or larger,
preferably in gradations between these limits. Crushed limestone, gravel
and the like are desirable coarse aggregates, and the material selected in
any case should exhibit a considerable hardness and durability inasmuch as
crumbly, friable aggregates tend to significantly reduce the strength of
the ultimate concrete. The finely divided aggregate is smaller than 3/8
inch in size and again is preferably graduated in much finer sizes down to
200 sieve size or so. Ground limestone, sand and the like are common
useful fine aggregates.
Several different types of Portland cement are available and all are in
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