Note: Descriptions are shown in the official language in which they were submitted.
lX81506
TITLE OF THE INVENTION
LIGHTWEIGHT AGGREGATE HAVING HIGH RESISTANCE TO WATER
ABSORPTION AND PROCESS FOR PREPARATION THEREOF
BACKGROUND OF THE INVENTION
The present invention relates to a lightweight
aggregate which is inexpensive and excellent in both
resistance to freezing and thawing and resistance to water
absorption. The present invention also pertains to a
process for preparation of such lightweight aggregate.
Conventional lightweight aggregates generally involve a
high water absorption, and the flash water absorption
percentage thereof is known to be about 30% of the 24-hour
water absorption percentage. For this reason, when such
conventional aggregate in an air-dry state is used, the
slump of concrete is degraded during mixing or conveying of
the concrete.
To overcome such disadvantage, it is conventional
practice to subject a lightweight aggregate to a prewetting
treatment, i.e., a pretreatment in which the lightweight
aggregate is watered so as to absorb water appropriately,
before it is mixed into concrete for an actual use, thereby
preventing any reduction in consistency.
However, the prewetting processing operation
disadvantageously complicates both mixing of concrete and
moisture control, and it is not a sufficiently reliable
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treatment in view of the concrete conveying system by pump
and the prevention against frost damage.
To cope with the above-described problems, various
types of processing method have already been proposed for
the purpose of lowering the water absorption of lightweight
aggregates (i.e., improving the resistance to water
absorption thereof). For e~amples, the following methods
have heretofore been known: (A) a method wherein a
lightweight aggregate is rolled over a slope, and while
doing so, it is subjected to curing application with a
viscous material or a covering material; (B) a method
wherein the surface of a lightweight aggregate is subjected
to a covering treatment using a liquid solution type
petrolic resin; and (C) a method wherein the surface of a
lightweight aggregate is subjected to a covering treatment
using straight asphalt.
The above-described conventional methods suffer,
however, from the following problems. Namely, with a
preventive technique to water absorption such as the method
(A), it is impossible to provide a lightweight aggregate
having high resistance to water absorption that can be used
in the concrete conveying system by pump wherein concrete
which has not yet solidified is placed. In other words,
there has been provided no lightweight aggregate which
satisfies the following conditions, i.e., target values
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which are to be reached:
(a) To ensure a water absorption percentage of 4% or less
under a pressure of 40kg/cm2.
(b) To possess a high resistance to freezing and thawing,
i.e., DF value 80 or more at 300 cycles.
When a petrolic resin material is employed as a coating
material as in the case of the prior art method (B), the
following disadvantages may be experienced.
First, resin materials involve the problem that, as
hardening of a resin material progresses, shrinkage occurs,
and this leads to generation of pinholes. It is necessary
in order to prevent the generation of pinholes to laminate a
multiplicity of resin layers, which means that this prior
art method is impractical from the costwise point of view.
It has been found that a thermal hardening resin
particularly shrinks through hardening reaction and cooling
taking place subsequently, resulting in a considerable
internal stress.
The internal stress in the resin leads to not only
lowering in the strength of the system but also generation
of microcracks and pinholes.
Secondly, as will also be clear from the porous
structure of lightweight aggregates, it is necessary, in
order to satisfactorily impregnate a coating material into
micron order pores (capillary tubes), to employ a coating
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material having a relatively low coefficient of viscosity
and conduct operations such as pressing impregnation with a
relatively large pressure difference, which fact involves
increased costs, disadvantageously.
Thirdly, there is the problem of interfacial peeling
caused due to the difference in terms of thermal expansion
coefficient between a lightweight aggregate and a coating
material. In regard to the water absorption performance
under pressure in the concrete conveying system by pump, the
existence of any space at the interface between the
lightweight aggregate and the coating material is
particularly fatal.
Fourthly, a resinous coating material takes a certain
period of time to terminate hardening reaction, and this
involves the problem that the tack of the coating material
bonded to the surface of the aggregate remains undesirably.
In consequence, there is a risk of particles of the
aggregate bonding to each other in the shape of a millet and
rice cake. For this reason, it is extremely difficult, with
the existing technology, to effect coating of a lightweight
aggregate for each individual particle thereof.
The upper limit cost of coating which is allowed for a
preventive technique to water absorption is generally needed
to be 50~ or less of the total cost of the lightweight
aggregate employed from the economical point of view. If,
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for example, a multiplicity of coating layers of a petrolic
resin are provided on the surface of a lightweight aggregate
in a plurality of steps, the cost of the lightweight
aggregate is raised by 200 to 300% because of the cost of
the resin and the process costs.
In addition, general resinous coating materials involve
excessively high costs as materials which are used for
prevention of water absorption of aggregates, i.e., even
relatively inexpensive ones are 200 to 300 yens per
killogram.
On the other hand, it may also be possible to employ
various kinds of polymer emulsion as coating materials.
However, polymer emulsions suffer from the following
disadvantages:
(a) The dynamic strength of polymer emulsions is lower than
those of general thermal plastic resins and thermal
hardening resins.
(b) It is difficult to form a relatively thick covering
layer.
In addition, since water (generally about 50%)
contained in a polymer emulsion is stored for a long time in
the aggregate by the covering layer formed on the surface of
the aggregate, the water may have a bad influence on a
freezing and thawing test.
A preventive technique to water absorption using
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straight asphalt such as the method (C) also has the problem
that it is not possible to attain prevention of water
absorption which is satisfactorily effective under a
pressure of 40kg/cm2 in the concrete conveying system by
pump.
SUMMARY OF THE INVENTION
In view of the above-described problems of the prior
art, it is a primary object of the present invention to
provide a lightweight aggregate having excellent resistance
to water absorption.
The basic technique to provide the lightweight
aggregate according to the present invention features
provision of a "waterproof plug" of an appropriate material
in each of the open pores in the outer surface layer of a
lightweight porous aggregate among a multiplicity of open
pores which are continuously present in the aggregate, and
adoption of so-called sol-to-gel conversion technique
(conversion of a sol into a gel) for forming the "waterproof
plug".
More specifically, the present invention provides: (1)
a lightweight aggregate having improved resistance to water
absorption wherein a gel material is filled in at least open
pores in the surface layer of a lightweight porous aggregate
among a multiplicity of open pores therein; (2) a process
for preparation of a lightweight aggregate having high
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resistance to water absorption wherein a raw material
solution for gel formation is injected in at least open
pores in the surface layer of a lightweight porous aggregate
among a multiplicity of open pores therein, and the raw
material solution is then allowed to gel, thereby forming a
gel material in at least the open pores in said surface
layer so as to fill them with the gel material; and (3) a
process for preparation of a lightweight aggregate having
high resistance to water absorption wherein a heated
lightweight porous aggregate is submerged in a raw material
solution for gel formation so as to fill the material
solution in at least open pores in the surface layer of the
aggregate among a multiplicity of open pores therein through
suction by the reduced pressure effect caused by cooling of
the aggregate, and the material solution filled in the open
pores is then converted to a gel, thereby forming a gel
material in said open pores so as to fill them with the gel
material.
BRIEF DESCRIPTION OF T~E DRAwIN~s-
Fig. 1 is a flow sheet schematically showing thepreparation process according to the present invention; and
Fig. 2 is a graph showing the pore system distribution
of a marketed lightweight aggregate (A).
DETAILED DESCRIPTION OF THE INv-E-N-TIoN
The present inventor accomplished this invention after
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the following exhaustive study of the porous structure and
water absorption percentage of lightweight aggregates.
The present inventor first noticed the following facts.
Table l below shows the water absorption percentage of
an ordinary lightweight aggregate (A) produced by firing
ground shale.
Table l
Water absorption percentage of lightweight aggregate (A)
_ . . _
specific 24-hour boil saturation
gravity absorption absorption coefficient
~ percentage percentage
Ordinary product 1.25 5~ 10% 0.5
. .
Ultra-lightweight 0.8 12% 50% 0.24
product ~
By the examination of the pore system distribution and
characteristics of expansion and shrinkage of the
lightweight aggregates, the following results have been
obtained.
Namely, it has been confirmed that the total amount of
pores in each lightweight aggregate is substantially in
inverse proportion to the size of the apparent specific
gravity thereof. As to the relationship between the 24-hour
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absorption percentage and the total amount of pores, a
lightweight aggregate which has a relatively high absorption
percentage does not always have a relatively large total
amount of pores. For example, in pumice, about 50% of the
total amount of pores are filled with absorbed water,
whereas, in the above-described lightweight aggregate, only
8% of the total amount of pores are filled with absorbed
water.
Fig. 2 shows results of examination of pore system
distribution of the above-described lightweight aggregate,
the amount of pores having a diameter of 750A or less
accounts for 50% of the total amount of pores.
The present inventor examined the relationship between
the porous structure and water absorption percentage of the
above-described lightweight aggregate and reached the
conclusion that it is preferable to provide a "waterproof
plug" of an appropriate material in each of the open pores
which are contiguous with the outer surface of the
lightweight aggregate (which pores have pore diameters
around 7.5 to 50~m according to the results shown in
Fig. 2), and to adopt so-called sol-to-gel conversion
technique (conversion from a sol into a gel) for forming the
"waterproof plugn.
The present inventor is convinced that it is
unnecessary to apply coating in such a manner that the
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covering layer extends to the surface of the skeletal
portion of the lightweight aggregate in addition to the open
pores, i.e., over the whole surface of the aggregate.
On the basis of the above-described technical idea, the
present invention has been accomplished.
Thus, the present invention provides: (1) a
lightweight aggregate having improved resistance to water
absorption wherein a gel material is filled in at least open
pores in the surface layer of a lightweight porous aggregate
among a multiplicity of open pores therein; (2) a process
for preparation of a lightweight aggregate having high
resistance to water absorption wherein a raw material
solution for gel formation is injected in at least open
pores in the surface layer of a lightweight porous aggregate
among a multiplicity of open pores therein, and the raw
material solution is then allowed to gel, thereby forming a
gel material in at least the open pores in said surface
layer so as to fill them with the gel material; and (3) a
process for preparation of a lightweight aggregate having
high resistance to water absorption wherein a heated
lightweight porous aggregate is submerged in a raw material
solution for gel formation so as to fill the material
solution in at least open pores in the surface layer of the
aggregate among a multiplicity of open pores therein through
suction by the reduced pressure effect caused by cooling of
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the aggregate, and the material solution filled in the open
pores is then converted to a gel, thereby forming a gel
material in said open pores so as to fill them with the gel
material.
It is impossible to satisfactorily fill a raw material
solution for gel formation in micron order pores in a
lightweight aggregate simply by carrying out, for example,
a conventional submerging technique as derived from the
Hagen-Poiseuille's method.
Examples of metbods which may be employed to solve the
above-described problem include one in which a raw material
solution for gel formation is pressurized and another in
which a raw material solution for gel formation is brought
into contact with a lightweight aggregate which is placed in
a reduced pressure atmosphere produced by vacuum suction.
A method which is particularly recommended in the
present invention is such that a heated lightweight porous
aggregate is submerged in a colloid solution ~a raw material
solution for gel formation) of fine particles prepared by
dispersing them into a medium such as water or oil, thereby
filling the material solutation in open pores in the
aggregate. More specifically, in this method the
temperature of the heated lightweight aggregate lowers when
it is submerged in the raw material solution for gel
formation, resulting in a reduction in pressure within the
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open pores in the aggregate, and thus allowing the material
solution to be readily filled in the open pores. In this
method, it suffices to set ~T in the formula PV=nRT at about
100C. Thus, it is possible to readily fill a preventive
agent against water absorption in open pores within a
lightweight porous aggregate.
In actual industrial production, the method according
to the present invention can be practiced simply by adding
an auxiliary apparatus to a conventional lightweight
aggregate production line, and in this case production costs
are considerably low. Although such production line
generally needs a step of cooling a fired lightweight
aggregate, it is possible, according to the method of the
present invention, to replace the cooling step by the step
of submerging the aggregate in a raw material solution for
gel formation.
The following is a description of the raw material
solution for gel formation employed in the present
invention.
As a raw material solution for gel formation, it is
possible to employ any material which is able to gel alone
with some stimulation such as a temperature change in
cooling or heating, e.g., agar-agar, gelatin and egg white,
or any material which is converted to a gel through a
reaction with another substance (e.g., acids, alkalis,
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alcohol or salts), e.g., sodium silicate and casein.
In the case of employing a raw material solution for
gel formation which gels through a reaction with another
substance, a lightweight aggregate which has already been
injected with the raw material solution may be brought into
contact with the second substance, or conversely, the raw
material solution may be brought into contact with the
lightweight aggregate injected with the second substance.
It should be noted that, in place of the injection of a
raw material solution for gel formation, an ultra fine
particle powder may be injected by means of spraying or
deposition.
Fig. 1 i5 a flow sheet which schematically shows
processing steps of the process according to the present
invention.
The present invention, which employs a gel material as
a waterproof material for a lightweight aggregate, provides
the following advantages:
(1) Since the gel material is a flabby jellylike substance
such as solidified agar-agar, when such gel material is
filled in the space surrounded by rigid porous walls of the
skeletal portion of a lightweight aggregate, the gel
material is able to change its shape in response to any
external pressure applied thereto in the direction in which
it is compressed. Therefore, any gap generated between the
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porous walls of the aggregate and the gel can readily be
filled up with the gel which is deformed by the external
pressure. In consequence, even when an external pressure
such as water pressure is applied to the lightweight
aggregate, intrusion of water into the aggregate is
prevented, advantageously.
(2) The gel material is not readily frozen even when the
outside temperature is below freezing point, and is able to
absorb or lessen any increase in pressure caused by the
volume expansion of the water retained in the skeletal
aggregate at low temperature. Thus, the gel has excellent
low-temperature characteristics, and it is therefore
possible to obtain a high resistance to freezing and
thawing.
t3) Since a hydrosol or a non-aqueous sol as a raw material
solution for gel formation is capable of readily permeating
into capillary tubes in the aggregate skeleton, it is
possible to extremely readily carry out the step of
injecting the raw material solution into open pores in the
aggregate. A sol is a colloidal dispersion of a substance
in the form of particles which are larger than atoms or low-
molecular weight molecules although this cannot be confirmed
by a microscope. Therefore, impregnation is also
considerably readily effected.
(4) Gel materials generally involve low costs, and it is
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possible, according to the present invention, to select a
common inexpensive material, e.g., gelatin, glue and arabic
rubber. When such gel material is formed into a gel using
an oil (vegetable oil, mineral oil, useless oil, etc.) as a
solvent, pores in a lightweight aggregate are injected with
a gel which is lightweight and has water repellent
properties, which means that the lightweight aggregate is
greatly improved in the resistance to freezing and thawing.
In such case, the oily gel material applied to the surface
of the aggregate is readily removed and therefore
constitutes no obstacle to bonding between the aggregate and
a cement paste.
(5) The water permeability of the gel material filled in
pores favorably decreases as the pressure applied by the
concrete conveying system by pump increases by virtue of its
compressibility.
(6) Unlike a lightweight aggregate subjected to the
conventional resin coating method, the lightweight aggregate
subjected gelation treatment involves no risk of aggregate
particles bonding to each other in the shape of a millet and
rice cake.
The present invention will be described hereinunder in
more details by way of examples. It should be noted that the
following examples are only illustrative and the present
invention is not limited thereby.
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First o~ all, water was added to sodium silicate to
prepare an aqueous sodium silicate solution having a
specific gravity of 1.15.
A lightweight aggregate silton* heated to
200C was submerged in the aqueous sodium silicate solution
maintained at room temperature. The aggregate was submerged
in the aqueous solution for 10 minutes while being vibrated.
During the submerging step, the bath temperature of the
aqueous sodium silicate solution was raised by the
introduction of the heated aggregate, but the bath
temperature of the solution was adjusted so as not to rise
above 50C by circulating cooling water.
Thereafter, the lightweight aggregate was taken out of
the aqueous sodium silicate solution and rotated on a woven
metal sieve to remove surplus aqueous sodium silicate
solution attached to the surface of the aggregate.
Then, the lightweight aggregate was submerged in an
aqueous sodium alginate solution or 3 to 5 minutes, the
temperature of the solution being adjusted at between 10C
and 25C, and then the aggregate was taken out of the
solution and left to stand at 30C or lower so as to become
a gel.
The lightweight aggregate thus obtained and a
non-treated lightweight aggregate for comparison underwent a
pressure absorption test (40kg/cm2x lOmin). Results of the
* Trade Mark of Sumitomo Metal Mining Co., Ltd., Japan.
16
,
.
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test are shown in Table 2 below.
Table 2
Weight before Weight after Absorption
absorption of absorption of percentage
water (g) water (g) (%)
. ... _
Sample 1 309.3 311.6 0.74
. _ _ _
Sample 2 306.6 308.6 0.65
Sample 3 301.0 303.3 0.76
, .
Non-treated 299.5 350.5 17.0
As will be understood from the results of the test, the
fact that the water absorption percentage under a pressure
of 40kg/cm2 is less than 1~ means that the lightweight
aggregates according to the present invention can
satisfactorily be employed as lightweight aggregates for the
concrete conveying system by pump.
Examples of gel materials which may be employed in the
present invention are as follows.
(1) Inorganic gel materials:
silicate gel, alminium hydroxide gel, ferric hydroxide
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gel, magnesium hydroxide gel, stabilized calcium
carbonated gel
(2) Organic gel materials:
cellulose acetate gel, nitrocellulose gel (collodion),
alkohol+sodium oleic acid gel
(3) Natual organic gels:
dextran gel (cephadex), casein gel, gelatin gel, agar gel
(4) Oil gels:
vegetable gel, mineral oil gel, useless oil gel, etc.
As has been described above in detail, the present
invention offers the following great advantages:
(1) The gel material which is filled in the space surrounded
by porous walls of the skeletal portion of a lightweight
aggregate is able to change its shape in response to any
external pressure applied thereto in the direction in which
it is compressed. Therefore, any gap generated between the
porous walls of the aggregate and the gel can readily be
filled up with the gel which is deformed by the external
pressure. In consequence, even when an external pressure
such as water pressure is applied to the lightweight
aggregate, intrusion of water into the aggregate is
prevented, advantageously. Thus, the lightweight aggregate
has excellent resistance to water absorption.
(2) The gel material is not readily frozen even when the
outside temperature is below freezing point, and is able to
18
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absorb or lessen any increase in pressure caused by the
volume expansion of the water retained in the skeletal
aggregate at low temperature. Thus, the gel has excellent
low-temperature characteristics, and it is therefore
possible to obtain a lightweight aggregate having high
resistance to freezing and thawing.
(3) Since a hydrosol or a non-aqueous sol as a raw material
solution for gel formation is capable of readily permeating
into capillary tubes in the aggregate skeleton, it is
possible to extremely readily carry out the step of
injecting the raw material solution into open pores in the
aggregate, so that it is easy to produce a lightweight
aggregate having high resistance to water absorption.
(4) In the step in which a heated lightweight aggregate
immediately after firing is submerged in a raw material
solution for gel formation, the aggregate is suddenly cooled
to reduce the air pressure in the air gaps defined within
open pores in the aggregate, whereby the raw material
solution is effectively injected into the open pores in the
aggregate by means of suction. Thus, it is possible to
readily produce a lightweight aggregate having excellent
resistance to water absorption effectively utilizing heat
energy simply by adding an auxiliary apparatus to a
conventional lightweight aggregate production line.
(5) Unlike a lightweight aggregate subjected to the
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conventional resin coating method, the lightweight aggregate
subjected geletion treatment involves no risk of aggregate
particles bonding to each other in the shape of a millet and
rice cake.
(6) Since gel materials generally involve low costs, it is
possible to obtain a lightweight aggregate having high
resistance to water absorption at low costs.
Th~s, the lightweight aggregate and the process for
preparation thereof according to the present invention
are novel and provide a large number of advantages.
