Note: Descriptions are shown in the official language in which they were submitted.
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SYNTHETIC AGGREGATE AND
METHOD OF MANUFACTURING SAME
Background of the Invention
[0001] Concrete is a composite construction material composed primar-
ily of aggregate, cement and water. There are many formulations that have
varied properties. The aggregate is generally coarse gravel or crushed rocks
such as limestone, or granite, along with a fine aggregate such as sand. The
cement, commonly portland cement, and other cementitious materials such as
fly ash and slag cement, serve as a binder for the aggregate. Various chemi-
cal admixtures may also added to achieve varied properties. Water is then
mixed with this dry composite which enables it to be shaped (typically
poured) and then solidified and hardened into rock-hard strength through a
chemical process known as hydration. The water reacts with the cement
which bonds the other components together, eventually creating a robust
stone-like material. Concrete has relatively high compressive strength, but
much lower tensile strength. For this reason is usually reinforced with
materials that are strong in tension (often steel).
[0002] Concrete is widely used for making architectural structures,
foundations, brick/block walls, pavements, bridges/overpasses, motor-
ways/roads, runways, parking structures, dams, pools/reservoirs, pipes,
footings for gates, fences and poles and even boats.
[0003] Construction aggregate, or simply "aggregate", is a broad
category of coarse particulate material used in construction, including sand,
gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates.
Aggregates are a component of composite materials such as concrete and
asphalt concrete; the aggregate serves as reinforcement to add strength to the
overall composite material. Due to the relatively high hydraulic conductivity
value as compared to most soils, aggregates are widely used in drainage
applications such as foundation and French drains, septic drain fields, retain-
ing wall drains, and road side edge drains. Aggregates are also used as base
material under foundations, roads, and railroads. In other words, aggregates
are used as a stable foundation or road/rail base with predictable, uniform
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properties (e.g. to help prevent differential settling under the road or build-
ing), or as a low-cost extender that binds with more expensive cement or
asphalt to form concrete.
[0004] Pozzolans are commonly used as an addition (the technical term
is "cement extender") to concrete mixtures to increase the long-term strength
and other material properties, and in some cases reduce the material cost of,
concrete. A pozzolan is a material which, when combined with calcium
hydroxide, exhibits cementitious properties. Pozzolans are primarily vitreous
siliceous materials which react with calcium hydroxide to form calcium
silicates; other cementitious materials may also be formed depending on the
constituents of the pozzolan.
[0005] The pozzolanic reaction may be slower than the rest of the
reactions that occur during cement hydration, and thus the short-term strength
of concrete made with pozzolans may not be as high as concrete made with
purely cementitious materials; conversely, highly reactive pozzolans, such as
silica fume and high reactivity metakaolin can produce "high early strength"
concrete that increase the rate at which concrete gains strength.
[0006] Many pozzolans available for use in construction today were
previously seen as waste products, often ending up in landfills. Use of
pozzolans can permit a decrease in the use of Portland cement when produc-
ing concrete; this is more environmentally friendly than limiting cementitious
materials to Portland cement.
[0007] One common pozzolan used in modern concrete is fly ash. Fly
ash is one of the residues generated in combustion, and comprises the fine
particles that rise with the flue gases. In an industrial context, fly ash
usually
refers to ash produced during combustion of coal. Fly ash is generally
captured by electrostatic precipitators or other particle filtration equipment
before the flue gases reach the chimneys of coal-fired power plants, and
together with bottom ash removed from the bottom of the furnace is in this
case jointly known as coal ash. Depending upon the source and makeup of
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the coal being burned, the components of fly ash vary considerably, but all
fly ash includes substantial amounts of silicon dioxide (Si02) (both amor-
phous and crystalline) and calcium oxide (CaO), both being endemic ingredi-
ents in many coal-bearing rock strata.
[0008] Owing to its pozzolanic properties, fly ash is used as a replace-
ment for some of the cement content of concrete. It can replace up to 30%
by mass of Portland cement, and can add to the concrete's final strength and
increase its chemical resistance and durability. Recently concrete mix design
for partial cement replacement with High Volume Fly Ash (50 % cement
replacement) has been developed.
[0009] Silica fume, is another commonly used pozzolanic material, also
known as microsilica, is an amorphous (non-crystalline) polymorph of silicon
dioxide, silica. It is an ultrafine powder collected as a by-product of the
silicon and ferro-silicon alloy production and consists of spherical particles
with an average particle diameter of 150 nm. Because of its extreme fineness
and high silica content, silica fume is a very effective pozzolanic material.
[0010] Silica fume is added to concrete to improve its properties, in
particular its compressive strength, bond strength, and abrasion resistance.
These improvements stem from both the mechanical improvements resulting
from addition of a very fine powder to the cement paste mix as well as from
the pozzolanic reactions between the silica fume and free calcium hydroxide
in the paste.
[0011] Polystyrene is an aromatic polymer made from the monomer
styrene, a liquid hydrocarbon that is manufactured from petroleum by the
chemical industry. Polystyrene is one of the most widely used plastics, the
scale being several billion kilograms per year. Polystyrene can either be a
thermoset or a thermoplastic. A thermoplastic polystyrene is in a solid
(glassy) state at room temperature, but flows if heated above its glass transi-
tion temperature of about 100 C (for molding or extrusion), and becomes
solid again when cooled. Pure solid polystyrene is a colorless, hard plastic
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with limited flexibility. It can be cast into molds with fine detail. Polysty-
rene can be transparent or can be made to take on various colors.
[0012] Polystyrene can be recycled, and has the number "6" as its
recycling symbol. The increasing oil prices have increased the value of
polystyrene for recycling. No known microorganism has yet been shown to
biodegrade polystyrene, and it is often abundant as a form of pollution in the
outdoor environment, particularly along shores and waterways especially in
its low density cellular form.
[0013] Expanded polystyrene (EPS) is a rigid and tough, closed-cell
foam. It is usually white and made of pre-expanded polystyrene beads.
Familiar uses include molded sheets for building insulation and packing
material ("peanuts") for cushioning fragile items inside boxes. Sheets are
commonly packaged as rigid panels which are also known as "bead-board".
[0014] The foregoing and other objectives, features, and advantages
of
the invention will be more readily understood upon consideration of the
following detailed description of the invention taken in conjunction with the
accompanying drawings.
Brief Description of the Several Drawings
[0015] FIG. 1 depicts a flow chart indicating the preferred process
for
manufacturing a synthetic aggregate;
[0016] FIG. 2 depicts an abstract cross-sectional view of non-
spherical
EPS particles, shredded, ground, texturized, or otherwise reduced according
to the methods described herein, suspended in a cementitious mixture.
[0017] FIG. 3 depicts a flow chart indicating the preferred process
for
manufacturing an alternative synthetic aggregate.
Detailed Description of Preferred Embodiments
[0018] In the following detailed description, numerous specific
details
are set forth in order to provide a thorough understanding of various embodi-
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ments. However, those skilled in the art will understand that the present
invention may be practiced without these specific details, that the present
invention is not limited to the described embodiments, and that the present
invention may be practiced in a variety of alternate embodiments. In other
instances, well known methods, procedures, components, and systems have
not been described in detail. One of skill in the art will appreciate various
modifications to the process, to the ingredients, and to the proportions of
ingredients that are possible. One skilled in the art will further appreciate
that the following steps can be scaled to allow larger or smaller quantities
of
product to be made. For example, the following description primarily
teaches using expanded polystyrene (EPS) content as a primary ingredient in
the aggregate. However, one of ordinary skill in the art will recognize that
any suitable form of styrene polymers can be used in place of the EPS
content with similarly beneficial results. In particular, manufactured, ex-
panded or partially expanded polystyrene beads or other prepared pieces of
suitable size may be used without departing from the scope of the aggregate
described herein. Similarly either virgin or recycled EPS can be used as the
raw EPS material.
[0019] Exemplary synthetic aggregates for use with cement, stucco,
mortar or gypsum-based mixtures and exemplary methods for making such
synthetic aggregates are described herein. The aggregates described herein
can be blended to dry mix stucco, mortar, or concrete or can be directly
added to wet-mix stucco, mortar, or concrete. These aggregates advanta-
geously increase fire resistance, flexile strength, impact resistance over
conventional aggregates. The higher air-containing content of the EPS
material decreases weight and adds valuable insulative and sound-absorbing
properties to the final product (e.g., stucco, mortar, or concrete). Finely
ground and coated EPS also provides a cosmetic benefit in that raw or large
EPS beads or fragments are not visibly exposed on the mixture surface, and
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which can further adversely affect finish appearance and desirability by
creating pits, bumps, lumps other surface irregularities.
[0020] Referring to FIG. 1, in an exemplary
embodiment of a method
of preparing the present aggregate, the components of which are described in
detail below, raw EPS material 4 processed 6 whereby the raw EPS material
4 is finely crushed, shredded, texturized, or otherwise reduced into prefera-
bly non-spherical EPS particles 32 and placed in a wetting station 40. A
wetting agent 42 is introduced to the EPS particles 32. The wetting agent is
preferably water mixed with a thickening agent such as acrylic. The EPS
particles are then thoroughly combined with the wetting agent using a mixing
device, such as a paddle mixer, or other blending method, such that the EPS
content is substantially and uniformly coated with the wetting agent. After
the EPS content is coated with the wetting agent, coating agents 46 are added
in a coating step 44. The coating agents may include fly ash or silica fume or
some combination thereof. In a particular embodiment, the coating agent
consists of equal parts by volume of fly ash and silica fume, combined with
polyvinyl acetate ("PVA") fibers. Other pozzolanic materials may also serve
as effective additives. In an exemplary embodiment of the aggregate, the
proportion of ingredients are:Ingredient
Volume %
by Volume
Ground EPS Content
4 gallons 77.1%
Wetting Agent 0.125
gallons 2.4%
Fly ash 0.5 gallons
9.6%
Silica Fume 0.5 gallons
9.6 %
PVA Fibers 0.0625 gallons
1.2%
[0021] Finally the wetted, coated EPS particles
are dried 48. This
process creates a lightweight aggregate 52 for aerating cement, stucco and
other cementitious materials. The use of finely ground EPS particles maxi-
mizes the encapsulated air in the aggregate by reducing the interstitial
volume
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surrounding fully formed foam beads or scrap fragments. Larger beads used
as a filler in cement and cementitious products are typically able to decrease
cement-based product weights up to a limit of approximately 20%, while
embodiments of the aggregate described herein can decrease the same product
weights by up to 80% or more, for example as may be preferable in the case
of certain cement stucco applications, floating concrete, geo-filled slurries,
or
other products or applications that require ultra-lightweight materials.
[0022] For certain specialized uses, such as stucco, a small amount of a
surfactant may also be added to the mix at this point. After adding the
coating agents and any other dry ingredients, the aggregate is again mixed
thoroughly with mechanical paddle mixer until coating ingredients and fibers
are evenly distributed in mixture.
[0023] The fly ash, silica fume and PVA fiber ingredients should be
thoroughly blended with the EPS and other ingredients using a mechanical
mixer or similar mechanism to ensure uniform and homogenous mixing of all
ingredients in the matrix.
[0024] After the finely shredded EPS particles are wetted with the
wetting agent and the fly ash, silica fume and PVA fibers have been thor-
oughly blended with the finely shredded EPS particles, the removal of excess
moisture during the drying step 48 ensures that any potential future storage
problems which might be caused by excess moisture are minimized while the
aggregate is stored in a bulk or bagged state. The mixture can be dried using
conventional methods (e.g., circulating air) or can be allowed to air dry
through natural convection. The aggregate is preferably dried until it does
not bind together when squeezed.
[0025] Finely ground EPS particles combined with appropriate wetting
and coating materials is used to incorporate a concentrated air-infused
material into the matrix. EPS beads contain approximately 80% air by
volume and, therefore, the finer the grind, the greater the concentration of
air
that can be added to any given mixture. It has been determined that EPS
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particles sufficiently reduced in size to pass through a 3/16 inch mesh are
advantageously suitable for use in the aggregates described herein. Once the
finely ground, or otherwise reduced, EPS material is properly wetted and
surface-coated to inhibit static and add a stabilizing film of dead weight,
then
dried, the resulting additive can be quickly and uniformly integrated into the
applicable stucco or concrete mixture. Using a preferred method of process-
ing the raw EPS material 4, the size of the finely ground EPS particles 32
may range from 20 to 120 mils (0.508 to 3.048 millimeters).
[0026] The exemplary embodiment may advantageously utilize EPS
material obtained from post-consumer or manufacturing waste products,
whereby foam pieces of various sizes are received from store outlets and
other commercial sources. Although Type 1 EPS (1 lb/cu. ft.) and Type 2
EPS (2 lb/cu. ft.) are preferred and are described below, other polystyrene
materials, including other densities of EPS material, or other materials based
on styrene polymer variants, such as cups, trays, containers, and egg cartons
may be also be used. Other recycled products such as pulverized rubber tire
fragments and/or other miniaturized plastic components, as well as organic
byproducts such as rice hulls or wood chips may also be incorporated into the
mix design.
[0027] Referring again to FIG. 1, in an exemplary method of preparing
the EPS material for use in the aggregates described herein, during the EPS
processing step 6, the raw EPS material 4 is sorted 8 and any relatively large
foam pieces are broken into smaller sizes no larger than approximately 6" in
the longest dimension 12. During this process, the raw EPS pieces are
inspected for foam of the wrong type or color and for any loose, foreign
objects, such as screws, tape, paper, and other foreign objects incorporated
into foam packaging and any such offending material is removed. High and
low density foam chunks are also mixed into rough proportion such that, after
processing, the EPS particles 32 have an approximate final density of 1.5
pounds per cubic foot. The reduced foam is then put through a pre-break
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milling process 16, whereby the foam is ground, crushed, cut or otherwise
reduced to pieces no more than roughly 2 inches across 20. The reduced
foam is then put through a final milling step 24 whereby the EPS material
further processed, texturized, shredded, granulated, beaded, or otherwise
reduced until the pieces are substantially non-spherical and small enough to
be filtered through a 3/16 inch screen 28.
[0028] Referring to FIG. 2, grinding, shredding texturizing or otherwise
reducing the EPS content to this size advantageously allows the introduction
of higher volume of EPS content into a cementious mixture than possible
using conventional EPS-containing aggregates. This provides lighter,
smoother and more easily transportable, mixable, trowelable and/or pump-
able mixtures and these properties combine to cause much less physical strain
on work-persons, machinery and equipment. Additionally, reducing the EPS
in the manner described above creates irregularly shaped materials that are
more easily mixed and resist settling or rising in the mixture (either wet or
dry) under vibration. The increased air-containing particulate density also
retards the mixture drying process, which helps create better hydration and
increases mixture "pot life", and extends applicator "working time." In the
case of the utilization of recycled EPS waste material in the mixture, the
higher the concentration, the greater the "green" environmental benefit.
[0029] Referring again to FIG. 1, pre-coating the EPS particles 32 with
a wetting agent 42 including a thickening agent 43 permanently encapsulates
the EPS particles with a thin layer of the thickening agent. This layer helps
to ensure a uniform bonding of the EPS particles 32 with the later applied
coating materials 46 and further allows for a greater percentage of aeration
of
the bulk mixture than the conventional method of adding EPS content directly
to cement products.
[0030] The wetting agent 42 allows the EPS-coating additives, described
below, to adhere to the surface of the EPS particles 32. The introduction of
the moisture in this step also helps weigh down the EPS particles 32, making
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it easier to uniformly mix with the coating agent(s) 46 in a later step 46.
Wetting and later coating the EPS particles also reduces static electricity,
which results in a smoother, more readily integrated mixing process. The
addition of a thickening agent, such as acrylic, increases the wetting agent's
stickiness, which allows for a stronger adhesion of the coating agent 46 to
the
surface of the EPS particles 32.
[0031] In certain embodiments of the aggregate disclosed herein, PVA
is used as part of the wetting agent, preferably in a ratio of 3 parts water
to 1
part PVA. In other embodiments, such as for use in the creation of stucco
products, the addition of a small amount of a surfactant may be added to the
wetting agent to improve the viscosity and body of the mix and to help
homogenize the mix. The surfactant emulsifies with the water when water is
added at the time of mixing of the product. Addition of the surfactant
improves the mixability and workability of the mixture. The surfactant
improves water resistance, flame and fire resistance and compressive
strength. It also reduces the surface PH of stucco materials. In the propor-
tions described in Table 1, the addition of 0.17 oz of surfactant has proven
effective.
[0032] After wetting 40, a coating agent of fly ash, silica fume and/or,
in some embodiments of the present aggregate, cement and/or PVA fibers is
applied to the EPS particles 32. The coating agent 46 is mixed with the EPS
particles 32 such that the coating agent 46 is uniformly distributed through
the
mass of EPS particles 32. The surface of the individual EPS particles are
thereby coated and the coating agents bond with the individual EPS particles.
[0033] The addition of fly ash to the coating agent 46 improves the
workability and flowability of the additive. Fly ash is extremely fine,
adheres well and improves coverage to small grind EPS particulate surfaces.
Fly ash acts as a water-reducer or super-plasticizer. Fly ash fines fill up
interstitial spaces in and around the fine grind EPS beads, thereby helping to
create a less permeable mixture. Fly ash is a common extender of cement.
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On vertical applications, the addition of fly ash to cement or cementitious
products provides for better adhesion, improves "slump" and helps to "hold"
the wall better. The fly ash should be added to the mixture after the moisture
is added and before the moisture begins to evaporate. Fly ash also serves to
weigh down the foam. Fly ash also allows the material to dry slower (retards
the cure rate) because it holds the moisture longer. This also lengthens
applicator workability time and helps decrease shrinkage and cracking.
Addition of the fly ash keeps the mixture sticky and pliable.
[0034] Silica fume, like fly ash, is another recycled waste product and is
derived from the production of silicon metal or ferrosilicon alloys in
electric
arc furnaces. Because of its chemical and physical properties, silica fume is
a very reactive pozzolan. Concrete containing silica fume can feature very
high strength and can be very durable. Because of its extreme fineness and
high silica content, silica fume is a very effective pozzolanic material.
Silica
fume is commonly added to portland cement concrete to improve its proper-
ties, in particular its compressive strength, bond strength, and abrasion
resistance. These improvements stem from both the mechanical improve-
ments resulting from addition of a very fine powder to the cement paste mix,
as well as from the pozzolanic reactions between the silica fume and free
calcium hydroxide in the paste.
[0035] The addition of silica fume to the coating agent 46 also reduces
the permeability of concrete to chloride ions, which protects the reinforcing
steel of concrete from corrosion. With the addition of silica fume, the slump
loss with time is directly proportional to increase in the silica fume content
due to the introduction of large surface area in the concrete mix by its
addition. Although the slump decreases, the mix remains highly cohesive.
[0036] Silica fume also reduces bleeding significantly because the free
water is consumed in wetting of the large surface area of the silica fume and
hence the free water left in the mix for bleeding also decreases. Silica fume
also helps improve hydration because it blocks the pores in the fresh concrete
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so that water within the concrete is not allowed as readily to rise or migrate
to the surface.
[0037] Silica fume's fine size and inherent stickiness also improves its
ability to both coat and bond to the wetted EPS surfaces. It also enhances
product hardness.
[0038] Although it is known to use fly ash and silica fume as ingredients
in common cement based admixtures, aggregates created by the method
described herein are unique because the reduced EPS content is first coated
with the wetting agent and the fly ash and silica fume are used to adhere to
the EPS content in order to minimize the interstitial spaces between the
individual EPS particles and enhance the bond strength between both the EPS
particles and the cement particles within the desired admixture. Once the
resultant lightweight mixture has dried and all ingredients are fully bonded,
the resultant lightweight aggregate product ultimately helps to allow greater
aeration of the intended aggregate mixture, up to 80%.
[0039] Cement may be used as an added ingredient to the coating agents
in order to catalyze the hardening of the surface coating of the coated,
reduced EPS content. This, in turn, increases the resultant compressive
strength of the resultant product. Other pozzolanic materials may also be
used as coating agents without departing from the scope of the aggregate
described herein.
[0040] In additional embodiments of the present aggregate, PVA fibers
may be added to the mixture to form a fine, interlocking mesh within the
aggregate that helps suspend, stabilize and reinforce the coated EPS and other
components in the mixture. The PVA fibers improve the tensile and com-
pressive strength of the concrete manufactured using the present aggregate.
The workability of the concrete is also improved while brittleness is reduced.
The fibers may be treated with oil to keep the fiber from bonding to the
matrix. The oiled and asbestos-like nature of the selected ultra-high perfor-
mance fiber causes the fiber to react to stresses in the concrete as though
the
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fiber is protected by and moving within a sleeve, which allows the material to
essentially "tear" rather than snap or shatter under severe stress forces.
[0041] The addition of the PVA fibers at this point allows the fibers to
bond directly to the coating additives, which are in turn directly bonded to
the
ground EPS content material. Thus, when then resulting dried lightweight
aggregate is then added to a stucco, concrete, mortar or gypsum based
mixture, the individual fiber infused lightweight product particles stay
better
suspended within the matrix. The bonding of the fibers to the EPS-coated
content also helps hold the lightweight aggregate in uniform suspension
throughout packaging shipping, handling and dry storage conditions.
[0042] Polypropylene, nylon, fiberglass and other types of fibers may
serve as appropriate alternatives to the PVA fibers for some uses or applica-
tions.
[0043] Referring to FIG. 3, in an alternative embodiment 68 of the
present aggregate, the wetting agent disclosed above is replaced by a wetting
agent 56 including water soluble, glass forming silicate salt ("liquid glass")
such as, preferably, potassium silicate, sodium silicate or lithium silicate.
In
such embodiments, the raw EPS material 4 may be ground, texturized, or
otherwise reduced in the same manner described above to produce the EPS
particles 32 having the same characteristics. The alternative wetting agent 56
is prepared by mixing thirty pounds of, preferably liquid, potassium silicate
with one hundred pounds of water. In some embodiments, of citric acid (not
shown) and/or commercially available pigment agents, such as fly ash or iron
oxide (not shown) are also added to the wetting agent. Seven gallons of the
alternative wetting agent 56 is sufficient to coat approximately forty five
cubic feet of EPS particles 32. In a preferred method of manufacturing such
embodiments of the present aggregate, the wetting agent is applied to the EPS
particles 32 by spreading the EPS particles 32 over a relatively wide area,
then misting the wetting agent 56 over the EPS particles while agitating the
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EPS particles for approximately two minutes to encourage complete coating
60.
[0044] The alternative wetting agent 56 infuses and encapsulates the
EPS particles 32 with a smooth, glass-like, fire-retardant coating, thereby
increasing the weight of the particles, reducing the accumulation of static
charges and generally improving the overall handling characteristics of both
the aggregate and the wet concrete product made therefrom. The resulting
concrete's structural integrity and water repellency are respectively improved
due to the potassium silicate's natural acid resistance and tendency to bind
with excess calcium in the concrete. Coating EPS particles in the manner
described above has other beneficial attributes during handling of the materi-
als, including lessening of static electricity, reducing the electric spark or
flame potential, and decreasing dust. The liquid glass coating also enhances
the overall compressive strength of the resulting concrete.
[0045] The alternative aggregate 68 advantageously does not require any
additional coating agents, such as fly ash, silica fume, or PVA fibers, al-
though such agents may be used to adjust the properties of the resulting
aggregate without departing from the scope of the aggregate disclosed herein.
[0046] For example, in certain embodiments of the alternative aggregate
68, a pigment, such as black iron oxide, may be added in order to increase
the cosmetic appeal of the aggregate as well as to act as a visual aid to
ensure
uniform coating of the wetting agent on the reduced EPS material. The
pigment may also be silica fume or fly ash that can be mixed with the liquid
glass in the wetting agent 56. In certain embodiments of the alternative
aggregate 68, citric acid is added to the wetting agent in order to catalyze
the
curing of the selected silicate. The drying time of the aggregate is thereby
reduced. The citric acid also advantageously reacts with calcium in the
resulting concrete.
[0047] The terms and expressions which have been employed in the
foregoing specification are used therein as terms of description and not of
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limitation, and there is no intention in the use of such terms and expressions
of excluding equivalents of the features shown and described or portions
thereof, it being recognized that the scope of the invention is defined and
limited only by the claims which follow.
