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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a novel rice hull ash of ultra high
fineness and other particular properties, as described fully hereinbelow,
aqueous slurries of said ash and to their use as an admixture for
hydraulic cement to enhance the properties of hydraulic cement
compositions and the resultant set products made therewith.
Rice hull materials, such as the shell, hull or husk of the rice grain, are
generally a discarded waste product from rice production. The rice hull
materials are viewed as having little, if any, commercial value in their
raw material form. It is known, however, that rice hull materials
typically contain a substantial amount, typically about 16 to 20 percent,
of silica (SiO.sub.2) and when burned yield an ash which is rich in
amorphous silica. This ash, referred to herein and in the appended claims,
as "rice hull ash" or "RHA", has been used as a pozzolanic additive in
hydraulic cement compositions. The RHA is viewed as a filler material
capable of replacing or extending the more expensive ingredients of the
composition, such as portland cement and the like. (See U.S. Pat. No.
4,105,459 to P. K. Mehta.) The RHA material has also been proposed as a
filler or extender in other applications, such as in elastomeric and
plastimeric compositions (see U.S. Pat. No. 3,951,907 to P. K. Mehta). Its
use in these general manners have value in countries where rice is a major
crop and where the other components which are replaced or extended are
expensive and/or not readily available.
The rice hull material is commonly converted into RHA by uncontrolled
combustion methods in which the material is used as the fuel source. More
recently, U.S. Pat. No. 3,959,007 disclosed a process in which a higher
amount of energy is obtained when burning rice hull material under
controlled combustion while still obtaining a RHA useful as a cement
pozzolan or as an extender in other applications. The ash obtained from
conventional modes of combustion may be further processed by dry grinding
the ash to a powder form. The resultant ground material is a fluffy powder
of low bulk density having a BET surface area of about 10 sq. m./g. or
greater and, typically, a Blaine specific surface area of lower than 1
sq.m./gm. (The higher the Blaine value the smaller the particle.) Although
the BET specific surface area of RHA preparations increases as the
combustion temperature is reduced, and can be in excess of 200 sq.m./g.,
the Blaine specific surface areas of RHA's are usually much lower than
this, typically less than 1 sq.m./g. This difference is because most of
the specific surface measured by the BET technique is internal to the
particle while the internal surfaces of a particle are not measured by the
Blaine technique. The Blaine specific surface area measurements are
greatly affected by particle size changes and are more indicative of
particle size. Thus, the RHA material presently obtained and used is a
somewhat coarse particle, having a low bulk density which makes the RHA
hard to handle and deliver in desired amounts into a mix.
In the case where RHA is viewed predominantly as an extender-filler, the
particle size of the RHA is not deemed to be critical as long as it is not
so large as to disrupt the matrix to which it is added. In the case of its
use as a pozzolan in cement compositions, it is known that small increases
in the strength of the resultant hardened cement composition can be
achieved by the use of smaller particle size RHA. However, this
relationship tends to plateau as the particle size reaches a Blaine
surface area of 1 to 1.5 sq. m/g. Therefore, there has been no incentive
to attempt to develop a means to further reduce the RHA particle size and
achieve a resultant product.
In summary, rice hull ash (RHA) is generally of high silica content (at
least about 85% SiO.sub.2), but its usefulness as an additive in hydraulic
cement mixes has heretofore been limited by the difficulty in obtaining it
and handling it in a controlled, convenient and efficient manner. It has
been demonstrated that the pozzolanic reactivity of RHA may be enhanced by
burning the rice hulls at relatively low temperatures in
specially-designed furnaces (U.S. Pat. No. 3,959,007). The strength of
hardened hydraulic cements can be enhanced to small extents by using
smaller particle RHA but this relationship plateaus. Because of this as
well as the mechanical restrictions in dry grinding and the required need
of specialized combustion techniques, there has been no desire to produce
a RHA of very high Blaine value. Additional barriers to the need to form
and use a high-Blaine RHA are the difficulty believed associated with
handling the ultrafine dry powder, its presumed poor flow properties and
with its dust hazards.
SUMMARY
The present invention is directed to RHA of ultra high fineness as more
specifically defined hereinbelow. The present invention is further
directed to cement admixture compositions containing said RHA of this
invention which provides for a means of easily handling and metering the
ultra high fineness RHA and to improved cement compositions containing
said admixture.
The present invention has been found to be an effective means of causing
the resultant cement structures to exhibit substantial inhibition to
permeation of materials which adversely affect the durability of the
resultant structure, such as chloride ions and the like and thus form a
structure of high strength and low corrosion potential.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a rice hull ash product of ultra fine
particle size, to stable and easily handle aqueous suspensions containing
said rice hull ash and to hydraulic cement compositions to which the
present rice hull ash has been added.
In order to provide a clear description of the present invention, the
following terms are used in this description and in the appended claims
with the meaning given below.
The term "hydraulic cement" refers to a dry powder which sets and hardens
to a solid mass when mixed with water. Among the cements included under
this definition are plaster of paris, high alumina cements, lime-pozzolan
cements, blastfurnace slag cements, portland cements, and blended cements
based principally on portland cement. The term "hydraulic cement
composition" is taken to refer to a mixture of an hydraulic cement with
water, and also, if desired, with aggregate and admixtures. The term
"aggregate" refers to an essentially chemically inert filler, such as a
sand or gravel or crushed rock, whereas the term "admixture" refers to
materials which when added to a cement composition in small amounts
imparts a large influence on the physical and/or chemical properties of
the uncured composition and/or upon the cured composition. Hydraulic
cement compositions include a cement paste when there is no aggregate, a
mortar when the aggregate is a sand, and a concrete when the aggregate
also contains coarse particles such as gravel or crushed rock.
A "pozzolan" is an inorganic material which consists principally of
chemically reactive compounds of silicon and aluminum in their oxide
forms, and which is capable of reacting with lime (calcium hydroxide,
Ca(OH).sub.2), to form a hardened mass of calcium silicate hydrates and
calcium aluminate hydrates. A common application is to use pozzolans as
additives to enhance the economy or modify the properties of mixed based
primarily on portland cements. In such cases, the pozzolans react with the
lime evolved by the normal reaction between cement and water. In some
cases the pozzolan is interground or interblended with the dry solids
during the manufacture of the portland cement, while in other cases,
pozzolans are added as an admixture during the preparation of portland
cement compositions, such as concretes.
The terms "rice hull ash" and "RHA" refer to the ash obtained from the
combustion of rice hull materials, that is from the shell, hull and/or
husk of the rice grain. Rice hull material is generally viewed as a waste
product of little or no commercial value ("Rice: Chemistry and Technology"
2nd Ed. Chapter 19, by B. O. Juliano, 1985).
The terms "rice hull ash product," "RHA product," "RHA-LT product" and
"RHA-PF product" refer to the product of the present invention, as fully
described below, which is formed from conventional RHA. The present
invention is directed to a new rice hull ash product, aqueous suspensions
of this product and to its use as a hydraulic cement admixture. The rice
hull ash used to form the RHA product of the present invention must be an
amorphous silica formed by the combustion of rice hull. The combustion can
be accomplished by various modes, such as by the controlled slow
combustion of rice hulls at low to moderate temperatures, preferably below
800.degree. C. and more commonly between about 500.degree. and 600.degree.
C. Such a combustion process is described in U.S. Pat. No. 3,959,007, the
teaching of which is incorporated herein by reference, or by other known
low temperature processes. The rice hull ash produced by such low
temperature modes of combustion is herein referred to as "RHA-LT." When
the combustion period is maintained for an extended period the RHA-LT may
be light gray or off-white in color while short combustion periods tend to
yield a darker gray to black material. Material formed under such low
temperature combustion has high internal porosity as shown by its high BET
specific surface area values which are typically in the range of about 20
to 200, more typically about 100 to 200 sq.m./g. and greater. Higher
values are obtained by the lower combustion temperature. Alternately, the
rice hull ash useful in forming the present RHA product can be formed by
combustion of rice hull material in a conventional pulverized fuel burner.
The rice hulls, chopped to an average particle size of from about 0.1 to
about 1 mm, are blown into a flame along with the combustion air. The
combustion, although reaching temperatures well in excess of 1000.degree.
C., occurs over a very short time period. Materials formed in this manner
have BET surface area values of about 20 to 50 sq.m./g. and are identified
herein as "RHA-PF." The use of a pulverized fuel burner as the combustion
means is a simple method of manufacturing the needed and useful RHA,
because the technology for pulverized fuel combustion is well established
and is also well suited to the use of the combustion heat for steam or
electricity generation.
The RHA found useful in forming the RHA-product of the present invention
must have a silica (SiO.sub.2) content of at least 80% by weight and
preferably at least 85% and most preferably at least 90% by weight. It
should consist primarily of amorphous silica as determined by x-ray
diffractometry. The formation of RHA under high temperature for extended
periods results in crystallization of the silica dto cristobalite or
quartz. Such RHA is not useful. The carbon content should be less than 10%
and more preferably less than 6% by weight. Therefore, an amorphous silica
RHA having at least 80% by wt. SiO.sub.2 and less than 10% by wt. carbon
is suitable but the preferred RHA would contain at least 85% by wt.
SiO.sub.2 and less than 6% by wt. carbon. The BET specific surface area
should be at least 20 sq.m./g. and normally will range from 20 sq.m./g. to
200 sq.m./g. Higher values are useful.
The particle size of RHA which is ground by conventional means of dry
processes normally has a Blaine specific surface area of less than 1
sq.m/gm. with the value rising as high as about 1.5 sq.m./g. By excessive
dry grinding a Blaine specific surface area value of about 2 sq.m./g. may
be achievable. Values in excess of about 2 sq. m./g. are not achievable by
conventional means of dry grinding. The Blaine specific surface areas of
RHA's are usually much lower than the BET values for the same material
because most of the specific surface measured by the BET technique is
internal to the particle, in the form of fine pores, which are not
measured by the Blaine technique. Thus, the BET specific surface area is
not significantly increased by further grinding RHA preparations, whereas
the Blaine specific surface area is very amenable to increase by grinding
up to the limitations discussed above.
The RHA-product of the present invention is required to have all of the
physical characteristics described above for RHA, that is the
characteristics of morphology, carbon and silica content, and BET surface
area except that its particle size must be significantly smaller than
previously taught. It has been unexpectedly found that by processing such
RHA by the manner described below one can achieve a product of ultra high
fineness, that is the particle diameter is ultra small. The ultra-small
dimensions of the particles of the present RHA-product is of values that
can not be accurately measured by conventional Blaine surface area
measurements. An appropriate means of measuring the present ultra-fine
particle size is by a laser-light scattering analyzer which gives a
statistical analysis of the volume-median particle diameter (D.sub.50).
The present RHA-product must have a volume-median particle diameter
(D.sub.50) of up to 4 micrometers, preferably up to about 3 micrometers
and most preferably up to about 2.5 micrometers. Because of the systematic
differences among the various analytical techniques for determining size
with the presently required ultra-high fineness product, the laser-light
scattering analysis is the basis used here. Specifically, a Leeds and
Northrup "Microtrac" laser-light scattering analyzer or its equivalent was
used. The Blaine air permeability method of specific surface area
determination, which is commonly used in the measurement of the fineness
of cements and even of microsilicas, is not recommended herein because it
is an imprecise means of analyzing materials of ultra-small particle size
especially if the particles are also very non-spherical and internally
porous, as is the case with RHA. For example, a sample of presently formed
RHA, which was determined to have a D.sub.50 of 3.7 micrometers by a
laser-light scattering analyzer, was also found to have a Blaine air
permeability specific surface area of 4.5 sq.m./g. Using this Blaine value
and assuming that all of the particles were uniformly-sized spheres, a
calculated diameter of only about 0.6 micrometers would be obtained.
Knowing that the laser technique directly measures particle diameter, one
readily observes that the Blaine technique is inappropriate for the
present ultra fine product.
The process required to achieve the RHA-product of ultra-high fineness is
to grind RHA by wet grinding technique. The liquid medium should be water.
The water should contain or have introduced therein at least one low or
high range water reducing agent used in cement formulations to achieve a
resultant stable slurry. The wet grinding can be accomplished by various
means, such as a ball mill, tube mill, sand mill, or any type of stirred
or vibrated media mill, operated either in batch or continuous mode. Use
of stirred media mills, such as an Attritor Mill has been found to be
particularly effective for ultrafine grinding of RHA to produce slurries
of the subject invention. The wet grinding has been found to achieve the
desired ultra fine particle sizes and also provides low operating costs,
reduced dust emissions and reduced noise levels. Dry grinding does not
achieve the present RHA-product of ultra-high fineness.
Although the resultant RHA-product may be dried and used in its powder
form, it is preferable to maintain and use the RHA-product in aqueous
suspension. The suspension shall contain the RHA-product in from about 20
to 80%, preferably from about 30 to 70% and most preferably from about 40
to 60% by weight. The RHA-product suspension must also contain small
amounts of conventional water reducing agent cement admixture material,
such as naphthalene sulfonate formaldehyde condensate, melamine sulfonate
formaldehyde condensate, lignin sulfonates, polyacrylic acid and its
alkali and alkaline earth metal salts as well as copolymers of the
polyacrylate and the like. The term "polyacrylate" shall include polymers
of acrylic acid, methacrylic acid as well as C.sub.1 -C.sub.3 alkyl esters
thereof which are water soluble by having a sufficient amount of metal
salt therein. The copolymer may be mixtures of the polyacrylate monomeric
units indicated above or with other olefinic monomeric units including
ethylene, hydroxyalkyl acrylates and the like. The water reducing agent
must be present in at least 0.01% by weight of solids contained in the
suspension, with normal range of from 0.01 to 10%, preferably from 0.1 to
5%. The most preferred suspension contains a mixture of a naphthalene
sulfonate formaldehyde condensate and a polyacrylate. It has been
unexpectedly found that the present RHA-product provides a stable,
non-setting and non-settling suspension even when used in high
concentrations, such as greater than 40% by weight.
The solid RHA-product of the present invention must contain substantially
uniformly mixed therewith a water reducing agent, as described above, in
amounts given above for the solid, powder RHA-product composition.
In addition, other conventional cement admixture materials may be combined
in the RHA-product suspension without causing detrimental effects such as
solidifying, gelling or settling of the suspension. Other types of
admixtures which have been found to be desirable to be incorporated into
the present RHA-product suspension includes viscosifiers, such as
polyhydroxyalkyl celluloses, polyvinyl alcohol, polyethylene oxide and the
like; wetting agents such as vinsol resins, sulfonated organic compounds,
polyethoxylated alkyl phenols and the like; and set retarders such as
sugar derivatives, polycarboxylic acids, hydroxycarboxylic acids,
phosphates, phosphonic acids, borates and the like; accelerators, such as
chlorides, sulfates, formates and nitrates of alkali and alkaline earth
metals, especially of calcium, sodium or potassium. Another class of
accelerator which is very suitable for this application is the
hydroxyalkylated amines, such as triethanolamine, and corrosion inhibitors
such as alkali and alkaline earth metal nitrites.
The subject slurry can be used directly as a hydraulic cement admixture.
The slurry of RHA-product is stable and storable and is a means of readily
transporting and metering the desired amount of RHA-product without
incurring the handling and health (breathing) hazards associated with dry
powders of rice hull ash and other conventional pozzolans. In addition,
because the present RHA-product is capable of being contained in the
slurry in high concentrations, the water content of the slurry does not
have an adverse effect on the water to cement ratio and on the physical
properties of the resultant cement composition.
Another embodiment of the present invention relates to a particularly
useful and unexpected observation that stable and pourable suspensions can
be formed of a combination of highly desired materials. Specifically, when
the subject RHA-product suspension, as described above, is formed from RHA
described above as RHA-PF, one can include into the suspension high
concentrations, such as up to about 10% by weight, and even up to about
15% by weight of the total suspension, of calcium nitrite. Calcium nitrite
is a known agent which effectively inhibits corrosion of metal pieces
(such as rebars and the like) contained in cement compositions when used
in concentrations ranging from at least 0.5 to 10 percent and preferably
of at least about 2 percent based on the portland cement contained
therein. The present suspension permits the addition of the desired
RHA-product and of sufficient calcium nitrite by a single application.
Thus it alleviates on-site multiple application of admixture materials,
and provides for accurate dosage of the desired materials. The ability to
combine the present microsilica product with high concentrations of
calcium nitrite to provide a storage stable, pourable suspension is
unexpected as suspensions formed with other microsilicas, such as
condensed silica fume, with calcium nitrite harden after only a few hours
to unworkable consistency which can not be poured or metered. Thus, a
particularly novel and useful form of this invention is suspensions
containing from about 20 to 80% (preferably from 40% to 60%) RHA-PF, from
about 0.5 to 10% (preferably from 2 to 8%) Ca(NO.sub.2).sub.2, and 0.01 up
to 5% (preferably, 0.1 to 5%) by dry weight of a water reducing agent, as
described above, which is preferably selected from naphthalene sulfonate
formaldehyde or melamine sulfonate formaldehyde condensates. This has
exceptionally good properties as an economical durability enhancing
admixture for use in reinforced portland cement mortars or concretes,
since it acts both to reduce the permeability of the hardened concrete to
aggressive species such as chloride or sulfate ions, and also to inhibit
the onset of corrosion of reinforcing steel even if aggressive species,
including carbon dioxide, penetrate through to the steel.
The RHA-product admixture slurry of the present invention can be added to
conventional hydraulic cement compositions in amounts ranging from about
0.1 to 30, preferably to 20 percent solids of the slurry based on the dry
cement used in the cement composition (S/S).
The following examples are set forth to further illustrate and describe the
present invention, and are not meant to limit its scope in any way except
as defined in the claims appended hereto. All parts and percentages are by
weight unless otherwise indicated.
EXAMPLE 1
This example illustrates the production of RHA slurries by the wet-milling
process. Rice hull ash of the RHA-PF class was purchased from Agrilectric
Power Partners, Inc., of Lake Charles, La. They were waste byproducts of
an electricity generation process. The samples had bulk densities of
between 17 and 22 lb/cu.ft., silica contents of 92 to 93 wt.%, carbon
contents of 2.5 to 5.5 wt.% and a moisture content of less than 2 wt. %,
the remainder of the mass being principally in the form of calcium,
potassium and magnesium oxide compounds. The median particle diameter was
65 um, as measured by laser-light scattering technique using a Microtrac
(TM). The RHA was milled in a 3.4 gallon ceramic jar mill, with a grinding
medium consisting of 23.3 kg of steel balls (1/4 and 3/8 inch). The mill
was of 12-inch internal diameter, and was rotated on rollers at 38 rpm. It
was first charged with the steel balls plus 2250 g of an aqueous solution
containing 110 g of sodium naphthalene sulfonate formaldehyde condensate
(Daxad-19) sold by W. R. Grace & Co. Then 2750 g of RHA was added in three
increments over a period of 3 hours, after which the mill was allowed to
continue running for a further 16.5 hours. At the end of this period, the
resulting RHA product slurry was withdrawn from the mill, and analyzed for
particle size distribution using a Microtrac laser-light scattering
analyzer. The mass median particle diameter was found to be 2.6 um. The
slurry was a stable liquid with a total RHA solids content of 55% by
weight. It has a viscosity of about 1400 centipoise as measured by a
Brookfield viscometer at 60 rpm, and was pourable.
EXAMPLE 2
This example illustrates the production of a slurry RHA-LT. The RHA-LT was
produced by burning raw rice hulls in a current of air at a temperature
well below 700.degree. C., using a 2-foot diameter by 24 foot long
externally-heated steel-shelled rotary calciner. The resulting product was
off-white in color, and had a BET specific surfact area of about 150
sq.m./g, and a carbon content of less that 0.5%. This RHA-LT sample was
ground in a 5-gallon steel ball mill, with a charge of 23.3 kg of steel
balls (1/4 and 3/8 inch), operating at 54 rpm. The mill was first charged
with the steel balls plus 3200 grams of a 2% aqueous solution of sodium
naphthalene sulfonate formaldehyde (Daxad-19), and then 3200 grams of the
RHA-LT was added in small increments over a period of 1.3 hours. The mill
was then operated for a further 29.5 hours, with samples being taken at
intermediate times. These samples were subjected to particle size analysis
using the laser-light scattering analyzer (Microtrac). Results are
summarized in Table 1. The resulting slurries contained 50% RHA and 1%
Daxad-19 by weight, and were all pourable liquids. The samples were stored
and later observed as being a readily pourable liquid after brief
agitation.
TABLE 1
______________________________________
WET GRINDING OF RHA-LT SLURRIES
Sample Grinding Time, hrs.
Vol. median diameter, um
______________________________________
E2-1 7.0 4.0
E2-2 8.0 3.8
E2-3 13.5 3.3
E2-4 29.5 3.3
______________________________________
EXAMPLE 3
This example illustrates the formation of RHA product slurries using a
stirred-media mill. Samples of RHA-PF and RHA-LT as described in Examples
1 and 2 above, respectively, were milled in a 30-gallon "Attritor" stirred
media mill, as supplied by Union Process, Inc. The milling process was
started by adding an aqueous solution of the indicated water reducing
agent to the mill charge in the mill chamber, the mill charge consisting
either of 1/4 inch ceramic balls or 1/8 inch stainless steel balls. The
mill stirrer motor was then started, and unground, dry RHA was fed in
slowly at the top of the mill until the desired quantity had been added.
The mill was allowed to run until the median particle diameter of the RHA
in samples taken from the mill had reached the desired value, as
determined by laser-light scattering (Microtrac) analysis, after which the
slurry was discharged by pumping it out of the bottom of the mill chamber.
Results of six such experiments are shown in Table 2. It is clear that
both types of RHA can be ground to the desired particle size range of 4 um
or less by this technique, and that median diameters as low as 1.1 um are
achievable.
TABLE 2
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RHA Slurry Production with a Stirred Media Mill
Code Ingredients, (% by mass)
Media Median
No. RHA Type* Diam um
______________________________________
E3-1 RHA-PF (52%); DX-19 (1%)
C 4.0
E3-2 RHA-PF (52%); DX-19 (1%)
C 2.9
E3-3 RHA-LT (50%); DX-19 (0.3%);
C 3.0
DX-36 (0.3%)
E3-4 RHA-PF (48%); DX-19 (2%)
C 2.4
E3-5 RHA-LT (46%); DX-36 (0.3%)
S 2.1
E3-6 RHA-PF (49%); DX-19 (2%)
S 1.6
E3-7 RHA-PF (49%); DX-19 (1%)
S 1.1
______________________________________
Notes:
*C = 1/4 inch diameter ceramic balls
S = 1/8 inch diameter stainless steel balls
DX-19 = Daxad19, (calcium neutralized NSFC, W. R. Grace & Co.)
DX-36 = Daxad36, (sodium neutralized polyacrylic acid, W. R. Grace & Co.)
EXAMPLE 4
This example illustrates the manufacture of a hydraulic cement composition
using RHA product prescribed by the present invention. A standard
high-strength concrete mix was formulated, using a water/cement ratio of
0.35, and tested as-is or with a RHA product slurry, the actual slurry
used being slurry E3-6, as described under Example 3. Full details of the
cement compositions are given in Table 3. As indicated additional amount
of water reducing agent was introduced one minute after mixing commenced
to cause the slump of the samples to be approximately equal. D.C.
resistivities were measured on 4 inch diameter by 8 inch long concrete
cylinders, by applying 60 volts end to end, using a 3.0N sodium hydroxide
solution as the contacting medium. The method is a modification of the
Federal Highway Administration's Rapid Chloride Permeability Test
(FHWA-Report No. RD-81/119 [1981]), and gives results which can be
correlated with chloride impermeability, (i.e., a higher resistivity
indicates less permeability to chloride ions). The formed hydraulic cement
compositions having the subject RHA-product (E4-2 and E4-3) therein
exhibited multifold resistance to allowing adverse chloride ions to
permeate therein in comparison to the untreated comparative sample (E4-1).
TABLE 3
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HIGH STRENGTH CONCRETES MADE WITH MICROSILICA SLURRIES
Fresh concrete Properties
Compressive Strength
D.C. Resistivity
Fresh Concrete Mix Proportions. lb/cu.yd.
Final after moist curing
after moist curing
Mix RHA Air
Slump
Set,
Density,
1 day,
28 days,
for 28 days,
Code
Cement
Sand
Gravel
Water
Prod.
DX-19
% in. hr lb/cu.ft.
kpsi kpsi ohm-m
__________________________________________________________________________
E4-1
744 1421
1707
260 0 10.2
2.2
6.5 8.8
153 3.6 8.2 81
E4-2
730 1395
1675
256 55 8.2 2.6
5.0 5.7
152 4.7 10.5 280
E4-3
740 1289
1696
259 111 12.0
2.5
4.5 7.5
152 4.8 12.0 530
__________________________________________________________________________
EXAMPLE 5
This example illustrates the effect of RHA type and particle size on the
resultant permeability properties of cement compositions made using the
methods of the present invention. Mortar cement compositions were mixed in
a Hobart mixer, using a Type 1 portland cement, a concrete sand at a
sand/cement ratio of 2.5, and a water/cement ratio of 0.45, including
water added with any admixtures. Rice hull ash product was added, in
slurry form, as a direct volume for volume replacement for sand, at a
total dosage rate of 10% by weight of cement. A total of 1% (solids basis,
relative to cement,) of water reducing agent (Daxed 19) was included in
each mix, some of it coming from the RHA product slurries and the rest
added directly with the mix water. Mortar cubes and cylinders were cast
and moist cured following ASTM C109 procedures, and tested for
permeability by measuring D.C. resistivity of the cylinders after 28 days.
Results are summarized in Table 4. All samples containing the RHA product
slurries exhibited low permeability as measured by electrical resistivity,
compared to the blank. Compressive strength (cubes) of the blank was 8.4
kpsi while those containing RHA-product were within the range of about
10-12 kpsi after 28 days of cure.
For comparative purposes, a class F fly ash pozzolan was used in the same
manner as above to produce a sample. The sample was formed to have about
the same air and flow properties but the D.C. resistivity exhibited very
little increase. Further, two samples were made in the same manner using
RHA-PF material having vol. median diameters of 65 and 6 micrometers,
respectively. These samples also showed very little benefit to the
composition.
TABLE 4
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EFFECT OF MICROSILICAS AND OTHER
ADDITIVES ON MORTAR PROPERTIES
Vol. median
Fresh Mortar
28-day Cured Mortar
Additive type
Diameter, um
Air, %
Flow, %
Resistivity, ohm-m
__________________________________________________________________________
None (Avg. 3)
-- 7.3 92 36
RHA-PF 4.0 2.2 116 145
RHA-PF 2.7 4.0 120 188
RHA-PF 1.8 1.7 102 219
RHA-PF 1.1 1.3 89 272
RHA-LT 3.3 4.2 64 301
RHA-LT 2.1 3.1 60 368
Class F Fly Ash*
3.0 126 41
RHA-PF* 65 4.3 50 49
RHA-PF* 6 2.4 107 78
__________________________________________________________________________
*Comparative Examples.
EXAMPLE 6
This example illustrates the ability to form aqueous slurries of
RHA-product and calcium nitrite which are useful as cement admixtures.
Samples of the formed slurry E3-4 described hereinabove were mixed with
calcium nitrite in the amounts indicated in Table 5 below. The particle
size of the RHA-product used was not altered by the required additional
mixing to provide a substantially homogeneous composition. The pH of the
samples was measured with a glass electrode immersed directly into the
suspension. The samples were shaken and their ability to flow was
observed. This visual observation is indicated in the Table 5.
TABLE 5
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RHA-PRODUCT/CALCIUM NITRITE SLURRIES
Ca(NO.sub.2).sub.2
Storage Condition Stability
Sample
% of Slurry
Days .degree.C.
pH Fluidity
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1 0 84 23 -- Stable; Fluid
2 5 0 23 8.1 Stable; Sl.
Thicker than
Sample 1
3 5 4 50 7.4 Stable; Fluid
4 5 23 23 7.4 Stable; Fluid
5 8 4 23 8.0 Stable; Thick
liquid
6 8 7 50 7.7 Stable; Thick
liquid
7 8 23 23 7.3 Stable; Fluid
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