WO1993010972A1 - Lightweight composites - Google Patents

Abstract

A lightweight composite includes aggregate particles (14), a high density phase which surrounds the aggregate particles (14) or forms a thin exterior shell of an aggregate particle (14) and a low density matrix phase (16) which occupies interstitial space between the aggregate particles (14) to produce a cell (24) or sandwich beam microstructure. A lightweight cement composite and a method for producing such a lightweight cement composite are provided as are structural panels including a lightweight composite layer characterized by a sandwich beam microstructure. The low density matrix phase (16) can be cellular concrete characterized by a void (10) surrounded by a more dense material (12), which can be cement.

Description

LIGHTWEIGHT COMPOSITES

Background of the invention i_s Field of the Invention

The invention relates to lightweight composites for structural applications, particularly low density cellular composites characterized by cell walls with a sandwich beam microstructure. 2. Description of the Prior Art

Prefabricated structural panels often include a lightweight core. Presently, such panels are constructed as sandwich panels including a foam core. Conventional cement foam materials with densities less than 20 pounds per cubic foot do not have sufficient strength and ductility to be building panel cores. Sandwich panels having a low density polymer foam core, while lightweight, have performance limitations as structural members such as roof or wall panels because of stiffness constraints. For example, when such polymer foam core sandwich panels are used as structural roof panels, creep of the polymer foam core limits panel performance and economy. In regions where snow loads are significant, panel spans are limited to 10 feet or less due to concerns over creep and the resulting increase in panel deflection over time. In warm regions, concern that a roof may become sufficiently warm to cause a decrease in foam stiffness limits the widespread application of foam core structural panels.

Core combustibility in polymer foam core panels also requires that they be covered with a layer of fire resistive material such as gypsum board, before they can be used as structural components.

According to current construction practice, residential foundations are typically made in one of two ways. For houses with basements, foundations are made from poured-in-place concrete. Houses with crawl spaces or shallow foundations often use concrete masonry units (CMU's) for the vertical elements in addition to cast-in- place footings. Both cast-in-place concrete and CMU construction are labor intensive and highly sensitive to weather conditions. Neither provide a basement or foundation that is well insulated, and an additional layer of polymer foam insulation must typically be applied to the exterior face of the foundation wall to provide adequate thermal insulation.

There is a need for a lightweight composite material which can be used to produce economical, prefabricated panels having the appropriate stiffness, insulating and fire resistive characteristics to be suitable for use in structural applications such as roof, wall and foundation panels.

-i-i_ι_ι τγ of the Invention The invention provides lightweight composites for use in structural applications. According to one aspect of the invention, a low density composite including at least two aggregate phase particles, further includes three phases, the aggregate particles, a high density phase associated with the aggregate particles which surrounds low overall density aggregate particles or is the outer wall or shell of hollow aggregate particles, and a low density matrix phase which occupies the interstitial space between the aggregate particles to produce a sandwich beam microstructure. The low density matrix phase can be cellular concrete or foam.

Aggregate particles are characterized by an overall low density and can be an organic material such as pre- expanded polystyrene, recycled reground plastic foam, and hollow polymeric spheres or cubes; an inorganic material such as hollow glass spheres composed of silicate and borosilicate glass and thin-walled hollow or low density cellular ceramic or steel spheres or cubes. Aggregate phase particles can be selected to have a particular geometry and can have cubic, spherical or other regular geometries. The high density phase can include cement as well as fiber and other additives such as silica fume, sand, cement paste and superplasticizers. Superplasticizers are additives which reduce the amount of water needed for mixing with the cement and other additives.

The high density phase is further characterized by a high stiffness and a density in the range of from about 85 to about 115 pounds per cubic foot and is selected so that it has a density and stiffness, i.e., Young's Modulus, at least five times greater than the density and stiffness of the matrix low density phase. The aggregate particle density, which can be in the range of from about 1 to about 10 pounds per cubic foot, is selected so that it is at most one-sixth that of the low density matrix phase, which is characterized by a density in the range of from about 0.5 to about 30 pounds per cubic foot. For cement foams, the cellular matrix density is typically in a range of from about 15 to about 30 pounds per cubic foot. Polymer foams can have densities of less than 1 pound per cubic foot.

The lightweight composite further includes a first aggregate particle and a second aggregate particle each of which is covered with or has a wall of the high density phase. The particles are separated by the matrix phase which occupies the interstitial space between the aggregate phase particles to create the desired .andwich beam microstructure. The thickness of the high density phase coating or wall is in the range of from about 20 icrons to about 100 microns and the thickness of the low density matrix phase occupying the interstitial space between the aggregate phase particles is typically in a range of from about 100 microns to about 500 microns. In another aspect of the invention, a lightweight cement composite includes lightweight aggregate particles dispersed in a matrix phase of cellular concrete. The lightweight cement composite can further include fiber, present in a concentration in a range of from about 0.3% to about 1.0% by weight of cement, which can be polymer fiber such as polyester and polypropylene fiber; inorganic fiber such as glass fiber, including silicate and borosilicate fiber, E-glass and other glass fiber coated with alkali resistant coatings. The fiber can have a strand-like geometry and be in a range of from about 0.25 to about 1.5 inches long.

The lightweight aggregate particles can be present in the composite in a quantity in the range of from about 40 to about 55% volume fraction of the composite. The lightweight aggregate particles can have a density in the range of from about 1 to about 10 pounds per cubic foot and consist of particles including a hollow or cellular lightweight interior surrounded by a shell consisting of the high, density phase material. The aggregate phase material which makes up the particle wall or shell has a density and stiffness at least ive times that of the low density matrix phase and is 20-100 μ thick.

The lightweight aggregate phase particles can have spherical, cubic or other regular geometries and range in size from about 0.5mm in diameter to about 6mm in diameter, or other relevant dimension depending on particle geometry. The aggregate phase particles can be characterized by a size distribution in the range of from about 3.0 to about 6.0mm based on a relevant particle dimension.

The lightweight aggregate particles can be polymer particles such as expanded polystyrene foam, recycled reground plastic foam and hollow polymeric spheres, cubes and other regular shapes; inorganic particles such as hollow glass spheres made from silicate or borosilicate glass and hollow or low density cellular ceramic or steel spheres, cubes or other regular shapes with thin exterior walls.

The lightweight aggregate particles are coated with a high density phase material. The low density, matrix phase can be a cellular concrete including a cement into which a preformed foam, which exhibits chemical compatibility with the cement, has been introduced. The cement can be a high early strength cement such as Portland Type III, Normal Type I Portland cement or any type of cement compatible with the preformed foam. The preformed foam can be one of several types of foaming agent suitable for the making of preformed foam cements including: saponified wood resin stabilized with animal glue; sodium compounds of aliphatic and aromatic sulfates, such as sodium lauryl, cetyl, and oleyl sulfates; sodium naphthalene isopropyl sulfate; sulfates of petroleum derivatives; complex organic compounds including keratin compounds and saponin; and inorganic compounds stabilized with organics. The foam can also be produced in situ. The composite can further include other additives such as silica fume, superplasticizers, sand, and polymer additives such as epoxy and latex.

The high density phase covered aggregate or thin walled aggregate particles and interstitial cellular concrete low density matrix material form a sandwich beam microstructure within the composite. For cement composite foams, this sandwich beam microstructure results in mechanical properties for the composite which make it suitable for use as a core material for a load- bearing structural panel. At a density in the range of from about 11 to about 14 pounds per cubic foot, the material is characterized by a compressive strength in a range of from about 40psi to about 60psi, a Young's modulus in the range of from about 18,00Opsi to about 25,000psi and a modulus of rupture in the range of from about 35psi to about 55psi.

Another aspect of this invention provides a method for making a lightweight composite including steps of forming a slurry including a cement and water, adding lightweight aggregate to the slurry and mixing to form a thoroughly intermixed intermediate mixture of the lightweight aggregate particles and slurry, introducing a foaming agent into this thoroughly mixed intermediate mixture to produce a foamed mixture, molding the foamed mixture into a desired shaped article and finally curing the shaped article to form the composite.

The slurry can further include fiber and other additives including silica fume, a superplasticizer, sand and polymer additives such as epoxy and latex.

The constituents of the slurry can be mixed together in stages including dry-mixing the cement and fiber. Additives such as silica fume can be dry-mixed with the cement and fiber or they can be pre ixed with water and then combined with the dry-mixed cement and fiber. Water can be added to the dry mixed cement, fiber and other additives to form a slurry.

The lightweight aggregate particles are thoroughly intermixed with the slurry so that the lightweight aggregate particles become coated with the cement slurry which hardens to form the high density phase. The foaming agent is thoroughly intermixed with the slurry and slurry-coated lightweight aggregate particles while care is taken to preserve the foam bubbles. A shaped article can be molded from the already described mixture and cured in an atmosphere having greater than 75% relative humidity at room temperature.

In another aspect of the invention, a structural panel including a first facing layer, an inner low density matrix composite layer characterized by a sandwich beam microstructure, a means for bonding the first facing layer to a first side of the inner low density matrix composite layer, a second facing layer and a means for bonding the second facing layer to a second side of the inner low density matrix composite layer is provided. The inner low density matrix composite layer can include lightweight aggregate particles dispersed in a matrix of cellular concrete.

The inner low density layer can be bonded to the first and second facing layers which can be structural materials such as cement, steel, wood, plywood, oriented strand board, waferboard and fiber reinforced cement boards, and non-structural materials, such as gypsum wallboard, depending upon the end use of the structural panel. The means for bonding can include adhesives, such as epoxy, silicone or latex; cement hydration when bonding a cement face to a cement core and mechanical fasteners.

Depending upon whether the structural panel is to be used as a wall, roof or foundation panel, the relative thicknesses of the first facing layer, second facing layer and low density matrix composite layer can be adjusted. Both the first and second facing layers are of equal thickness, a thickness less than the low density matrix composite layer core thickness. For a wall panel, the low density matrix composite layer core is from about 4 to about 12 times thicker than a facing layer. For a roof panel, the low density matrix composite layer core is from about 8 to about 16 times the thickness of a facing layer. For a foundation panel, the low density matrix composite layer core is about 8 to about 16 times the thickness of a facing layer. Additionally, mechanical fasteners and/or grouted joints can be provided for fastening one structural panel to another structural panel.

An object of this invention is to provide a low density matrix composite including at least two aggregate particles, a high density phase which surrounds said aggregate particles or forms the exterior walls of the particles and a low density matrix phase occupying the interstitial space between said aggregate particles to produce a sandwich beam microstructure.

Another object of this invention is to provide a lightweight cement composite comprising lightweight aggregate particles dispersed in a matrix phase of cellular concrete.

Another object of the present invention is the provision of a method for making a lightweight composite. Yet another object of the invention is to provide structural panels including a low density matrix composite layer.

Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon reading the description of preferred embodiments which follows.

Brief Description of the Drawings FIG. 1 is a schematic illustration of a low density foam matrix phase/lightweight aggregate particle^* composite microstructure. FIG. 2 is a schematic expanded view of a low density matrix phase occupying interstitial space between two lightweight aggregate particles.

FIG. 3 is a scanning electron microscope (SEM) micrograph of a cement foam composite of the invention.

FIG. 4 is a scanning electron microscope (SEM) micrograph showing a cement foam cell wall between two expanded polystyrene sphere lightweight aggregate particles in a composite of the invention. FIG. 5 is a scanning electron microscope (SEM) micrograph showing a high density cement phase coating around an expanded polystyrene sphere lightweight aggregate particle.

FIG. 6 is a scanning electron microscope (SEM) micrograph also showing a high density phase cement coating surrounding an expanded polystyrene sphere lightweight aggregate particle.

FIG. 7 is a graph showing compressive strength of lightweight cement composites with and without EPS spheres as a function of density.

FIG. 8 is a graph of compressive strength of a lightweight cement composite as a function of percent volume fraction EPS spheres.

FIG. 9 is a graph of Young's Modulus of lightweight cement composites with and without EPS spheres as a function of density.

FIG. 10 is a graph of Young's Modulus of a lightweight cement composite as a function of percent volume fraction EPS spheres. FIG. 11 is a graph showing Modulus of Rupture with and without EPS spheres of lightweight cement composites as a function of density. FIG. 12 is a graph of Modulus of Rupture of a lightweight cement composite as a function of percent volume fraction EPS spheres.

FIG. 13 is a schematic illustration of a batch process for casting structural panel faces and cores.

FIG. 14 is a cross section of a lightweight cement composite structural panel core.

FIG. 15 is a schematic illustration of a step in a process for bonding full density cement faces to a lightweight foam composite core to produce a structural panel including a lightweight foam composite core.

FIG. 16 is a schematic illustration of a step in a process for bonding full density cement faces to a lightweight foam composite core to produce a structural panel including a lightweight foam composite core.

FIG. 17 is a schematic illustration of a continuous production process for a structural panel.

FIG. 18 is a schematic illustration of simplified sandwich cell geometry. Description of the Preferred Embodiment

This invention provides a low density matrix composite characterized by a sandwich beam microstructure, a lightweight cement composite comprising lightweight aggregate particles dispersed in a matrix phase of cellular concrete, a method for making a lightweight cement composite and a structural panel including an inner low density matrix composite layer characterized by a sandwich beam microstructure.

As used herein, a lightweight composite or low density matrix composite refers to a composite having a density in the range of from about 2 to about 14 pounds per cubic foot. Also as used herein, a low density matrix phase refers to a phase with a density in the range of from about 0.5 to about 30 pounds per cubic foot and a high density phase refers to a phase which has a density in the range of from about 85 to about 115 pounds per cubic foot.

In the low density matrix composite, a low density matrix phase 16 can be cellular concrete which consists of cement combined with a foam which is either introduced into a cement slurry as a preformed foam or is formed in situ by a foaming agent, included in the cement slurry, which reacts to form bubbles, thus entraining air during cement hydration. A cellular concrete is characterized by voids, wherein air was initially entrained in the foam, surrounded by cement as shown schematically in FIG. 1 where a void 10 is surrounded by more dense material 12, which can be cement, to form a cell which is the basis for the term cellular concrete. The low density matrix phase can also be a foam such as polyurethane, expanded polystyrene and polyethylene. The densities of these foams would be in a range of from about 1 to about 10 pounds per cubic foot. If such foams are used as a matrix material instead of cellular concrete, the lightweight aggregate particles can include hollow glass spheres and spheres with a hard shell.

Void 10 is surrounded by hardened, dense material 12 of a composition corresponding to that of the foam or cement product used. In order to produce a lightweight composite material having appropriate stiffness and other mechanical properties for structural applications, the low density matrix phase is selected from materials having a density in the range of from about o.5 to about 30 pounds per cubic foot. For cement foams, the low density cellular matrix phase 16 density is typically in a range of from about 15 to about 30 pounds per cubic foot. Polymer foams can have densities of less than l pound per cubic foot. Aggregate phase particles 14, which can be organic material such as expanded polystyrene, recycled reground plastic foam; hollow polymeric spheres, cubes or other regularly shaped particles; inorganic material including hollow glass spheres such as silicate glass and borosilicate glass; and other hollow or low density cellular ceramic and steel spheres, cubes or regularly shaped particles with thin exterior walls, are embedded in low density matrix phase 16 to create the desired sandwich beam microstructure. An important consideration in selecting aggregate particles is the ratio of particle wall thickness to overall particle dimensions. For a sphere, the ratio is in the range of from about .02 to about .001. Preferred spherical aggregate particles have bulk densities less than 5 pounds per cubic foot.

While aggregate particles 14 are shown as spherical in FIG. 1, aggregate particles can have other, including cubic, geometries. Cubic or spherical geometry aggregate particles are preferred because they have isotropic outer surfaces which pack so that a sandwich beam microstructure is formed when the interstitial low density matrix phase is confined between spheres. Cubic geometry aggregate particles are particularly preferred because the surfaces which they present to interstitial low density matrix phase 16 are isotropic and planar and thus capable of readily creating the sandwich beam microstructure. The low density aggregate particles can consist of an aggregate phase material shell 20 surrounding an interior void or air-filled space 18 as shown in FIG. 2 or can be solid or cellular particles formed from a low density material. The density of the aggregate particles can be in the range of from about 1 to about 10 pounds per cubic foot, preferably in the range of from about 1 to about 3 pounds per cubic foot. Although FIG. 1 shows aggregate particles of uniform size, aggregate particles 14 need not necessarily all be identical in size and, instead, can display a size distribution which may in fact enhance their efficacy. The particle size can be in the range of from about 0.5mm to about 6mm. For cubic aggregate phase particles, identical sizes are preferred, while for other regular shapes, a particle size distribution is preferred.

A cell 24 denoted by lines in FIG. 1 is defined by a cube drawn around a sphere which includes sandwich beam microstructural features formed with neighboring spheres.

As shown most clearly in the expanded, schematic view of FIG. 2, aggregate particles 14, in this embodiment, consist of a hollow interior space or void 18 surrounded by aggregate phase material shell 20 coated with a high density phase layer 22, such as cement which can contain fiber and other additives including silica fume, superplasticizers and polymer additives such as epoxy, latex and the like. Between high density layers 22 is interstitial cellular matrix phase 16, in this embodiment, cellular concrete. The sandwich beam microstructure is thus formed by "sandwiching" low density cellular matrix phase material between high density layers 22 which is analogous to a conventional macroscopic sandwich beam which typically consists of two, thin, stiff skins such as steel or wood separated by a low density core of polymer foam or honeycomb material.

It is clear from FIGS. 1 and 2 that in order to obtain the desired sandwich beam microstructure, neighboring spheres must enclose an intervening layer of low density matrix phase material between them. The desired sandwich beam microstructure can not be created between touching spheres. Thus, lightweight aggregate particle concentration must be selected to provide an optimum concentration of aggregate phase particles which are in close proximity but not touching, so that they bound low density matrix phase material. It is undesirable to have fully packed aggregate particles since the strength of the material would then be dominated by the strength of the low density aggregate phase material. The sandwich beam microstructure is optimized for a lightweight aggregate particle concentration in the range of from about 40 to about 50% volume fraction.

Sandwich beam microstructure further requires selecting a material for the high density layer 22 which is at least five times denser than that of the material making up low density matrix phase 16. These density criteria insure that the sandwich beam microstructural features function in the same manner as a conventional macroscopic sandwich beam.

The sandwich beam microstructure extends over a scale of from about 0.5mm to about 6mm at a scale of a single cell, 0.1mm to 1mm at a scale of a single cell wall beam between aggregate particles and greater than 6mm at the scale of many cells with the high density phase layer 22 having a thickness in the range of from about 20 microns to about 100 microns surrounding aggregate phase particles having a particle size in the range of from about 0.5mm to about 6mm in diameter and characterized by a particle size distribution having a breadth in the range of from about 0.5mm to about 6mm. The lightweight cement composite provided includes lightweight aggregate particles, present in quantities in a range of from about 40 to about 50% volume fraction of the composite and having the density, geometry, size and other characteristics a.lready discussed, dispersed in a matrix phase of cellular concrete. The lightweight cement composite can further include fiber which can be a polymer fiber, such as a polyester and a polypropylene fiber and an inorganic fiber such as a glass fiber. Typical glass fibers used in a cement matrix include silicate and borosilicate glass fibers, standard E-glass and alkali resistant glass fibers.

Fibers generally have a strand-like geometry, are in the range of from about 0.25 to about 1.5 inches in length and are present in concentrations in the range of from about 0.3% by weight of cement to about 1.0% by weight of cement. Fiber length and fiber concentration are kept within these ranges to avoid problems in mixing the resulting cement slurry with the foam. To make a cellular concrete of low density, a large volume of low density, preferably 3 pounds per cubic foot, preformed foam is mixed with the cement slurry. In a cement slurry with a fiber concentration significantly above 0.1% by weight of cement, the fiber tends to separate out from the slurry and clump at the bottom of the mixing vessel when foam and slurry are combined. Longer fibers also are more likely to clump together than are shorter fibers.

In the lightweight composite, the cellular concrete occupies interstitial space between the aggregate particles. The cellular concrete includes a cement and a foam which can be preformed or formed in situ. The cement can be a high early strength cement such as a Portland Type III cement, a Normal Portland Type I cement, or any type of cement chemically compatible with the preformed foam so that foaming action is not inhibited and entrained air is not eliminated. For example, rapid hardening high gypsum content cements are to be avoided because they cause the foam to loose its aeration. The preformed foam can include an inorganic foam; stabilized organic foam; hydrolyzed protein based foam, including sodium lauryl, cetyl, and oleyl sulfates; sodium naphthalene isopropyl sulphate; sulfates of petroleum derivatives; complex organic compounds further including keratin compounds, and saponin; and inorganic compounds stabilized with organics such as the commercial foam concentrate products Elastizell™ and EMG™ both made by Elastizell Corporation of America, Ann Arbor, MI. Alternatively, the foam can be formed in situ by the reaction of lime in the cement slurry with finely powdered metals such as aluminum and zinc. The powdered metals react with lime in the slurry during cement hydration to form bubbles resulting in desired air entrainment. The volume of entrained air needed is in the range of about 75-85% of the total volume of the foam. In addition to powdered metals, other in situ foaming agents are foam concentrate, added undiluted, to the cement slurry and then agitated vigorously with the slurry. Foams can be based on metals (e.g., aluminum, nickel and copper) or ceramics (e.g., Sic and glass).

The lightweight cement composite can also contain other additives including silica fume, such as Force

10,000™, manufactured by W.R. Grace Company, Cambridge, Massachusetts; superplasticizers and polymer additives such as epoxy and latex.

The very small particles of silica making up silica fume can fill voids between hydrated cement grains reducing the permeability of cements, and may result in some increase in compressive strength. When low density foams are used, silica fume can increase foam cohesiveness during setting, thus resulting in higher composite strength. Superplasticizers reduce the amount of water required in the slurry to maintain a flowable, workable slurry consistency. The lower water/cement ratio required to produce this workable slurry consistency produces a cement paste characterized by higher stiffness and strength. Polymer additives such as latex and epoxy can increase cement tensile strength, toughness and ductility.

This lightweight cement composite is characterized by a sandwich beam microstructure which contributes to its superior mechanical properties including a compressive strength in a range of from about 45psi to about 60psi, a Young's modulus in the range of from about 18,000 psi to about 25,000 psi, a modulus of rupture in the range of from about 35 psi to about 55 psi, and a density between 11 and 14 pounds per cubic foot to make the lightweight composite suitable for use as a core material in a load-bearing structural member.

A method for making a lightweight composite includes forming a slurry including cement and water, mixing lightweight aggregate particles into the slurry so that the aggregate becomes completely coated with the slurry mixture, introducing a foaming agent into the aggregate/slurry mixture, molding the foamed mixture into a shaped article and curing the shaped article. The characteristics of the cement, lightweight aggregate particles and foaming agent have already been described. The slurry can further include other additives such as fiber, silica fume, superplasticizer and polymer additives such as epoxy and latex which have also already been described in greater detail. The slurry can be prepared by dry mixing cement and fiber with silica fume and a superplasticizer and adding water to the silica fume and superplasticizer solution to form a cementitious . slurry as a final step. Alternatively, dry mixed cement and fiber can be wet by combining them with premixed water, silica fume and superplasticizer to form a slurry.

The slurry produced according to either of these two methods can then be combined and thoroughly blended with a preformed foam while care is taken to retain foam aeration. For in-situ foaming methods, the foam concentrate would be added to the water/silica fume/superplastizer solution and then mixed vigorously with the cement to create a foamy slurry. It is desirable that the cement be fully hydrated. Sufficient water is used in forming the slurry to produce a water/cement ratio in the slurry of approximately 0.6. Since the superplasticizer functions as a surfactant, its concentration is kept below approximately 0.5% by weight of cement to avoid disruption of the foam.

This method for making a lightweight composite can be used for preparation of large or small composite batches and can be scaled up or down using mixing apparatus appropriate to the mass of composite being prepared without adversely impacting properties of the resulting composite. The composite slurry can be handled using conventional techniques including pumping.

A structural panel is provided which includes facing layers which can be made of a structural building material such as cement, wood, plywood, oriented strand board, waferboard and fiber reinforced cement board or a non-structural material such as gypsum wallboard. The facing layers can be bonded to opposing faces of an inner low density matrix composite layer having characteristics already described using adhesives such as epoxy, silicone and latex; cement hydration when bonding a cement face to a cement core and mechanical fasteners such as embedded clips, bolts and couplings. Materials for the facing layers are selected according to the panel's end use as a wall panel, roof panel or foundation panel. Foundation panels can be constructed with cement board on both faces bonded to a cement foam composite core. Roof panels can have cement, steel or wood faces bonded to the cement foam composite core. Wall panels could use cement, wood or steel faces bonded to the core, as well.

The ratio of the relative thicknesses of the first facing layer with respect to the inner lightweight composite foam layer and to the second facing layer can also be adjusted depending upon the intended panel use. For foundation panels, cement board faces, in a range of from about 1/2" to about 3/4" thick can be bonded to cores in a range of from about 6" to about 8" thick. Wall panels can be made from wood and/or cement faces in a range of from about 1/2" to about 3/4" thick bonded to a core in a range of from about 2" to about 6" thick. Roof panels can have in a range of from about 1/2" to about 3/4" thick faces of cement or wood bonded to a core in a range of from about 6" to about 12" thick. Steel faces, less than 1/16" thick can also be used for faces on wall and roof panels, with cores from in a range of from about 2" to about 12" thick. Face and core thicknesses are determined by a combination of structural and thermal insulation requirements, and can depend on factors such as the roof span, loading conditions and thermal insulation requirements.

The structural panels can be provided with means for fastening individual panels together with the particular fastening means selected according to panel use as a wall, roof or foundation panel. Foundation panels with cement skins can be grouted together using a shear key type joint or fastened using mechanical fasteners such as embedded clips, bolts and couplings. Wall panels with cement faces can be fastened in the same manner as foundation panels. Wall panels with wooden faces can be fastened using a spline system similar to that used in current residential sandwich panel construction. Panels with steel faces can use connections well known in the art and currently used for joining steel-faced sandwich building panels.

Example I A lightweight cement composite characterized by a sandwich beam microstructure was prepared by dry mixing in a standard bakery type mixing machine five pounds of Portland Type III cement and 0.017 pounds polyester fiber until the mixture had the appearance of greenish-flour. Next, 2.625 pounds of water at 120°F ± 5°P, were pre¬ mixed with 0.73 pounds silica fume solution such as (Force 10,000™ made by W.R. Grace and Co., Cambridge Massachusetts and 0.025 pounds superplasticizer ZIP™, Elastizell Corp. of America, Ann Arbor, MI, a surfactant which changes the surface tension to provide a more workable consistency for the cement while reducing the water needed to produce a slurry of workable consistency. The water/cement ratio of the slurry used in this embodiment of the invention was 0.6. The final water cement ratio for the cement foam composite was 0.8. The cement foam composite has a higher water content than the slurry because it includes water incorporated into the cement from the aqueous preformed foam during hydration. Expanded polystyrene (EPS) spheres were intermixed with the slurry until they were completely coated. The slurry coating hardens to form the high density phase of the composite.

The cement slurry and slurry-coated EPS spheres were then transferred to a mechanical mixer operated at approximately 60 RPM and a preformed foam was injected. The EPS spheres must be completely wetted with the full density cement slurry before introduction of the foam. A mechanical mixer was used for this stage of the mixing process and it was operated at a slow speed to preserve air entrained within the preformed foam. After introduction of the foam, the combined cement slurry, EPS spheres and foam mixture was further mixed for at least one minute to completely incorporate the foam within the slurry containing the EPS spheres but for not longer than eight minutes because mixing beyond eight minutes can crush the foam. The foamed cement slurry was mixed for one minute mechanically, for one minute by hand, and for another minute with the mechanical mixer. The foamwas prepared from a liquid concentrate, (EMG™, Elastizell Corporation of America, Ann Arbor, MI) in a proportion of one part foam concentrate to thirty parts water to form an aqueous based foam solution. The strength of the resulting foam is determined by the concentration of foam concentrate used. The foam solution concentration can be in the range of from about 1 part foam concentrate to about 20 parts water by volume to about 1 part foam concentrate to about 40 parts water by volume. The aqueous foam solution was placed in a thirty gallon boiler tank connected to compressed air tanks operated at 100 psi and air was blown into the foam solution at a rate of thirty cubic feet per minute to produce a foam having the appearance of shaving cream, but very stiff and able to maintain a percentage of entrained air in the range of from about 75% to about 85% of the total volume of the foam within the foam bubbles. The foamed mixture was poured into styrofoam molds to produce three inch by six inch cylinders. The cylinders began to harden in about two hours, becoming warmer as hydration began to occur. After curing overnight, the cylinders were hard to the touch and it was possible to tap on a cylinder surface; however, they were not near their full strength. The cylinders were placed in a curing chamber having a relative humidity in the range of from about 80% to about 95% at room temperature for 7 days. Samples were then removed from the styrofoam molds and returned to the curing chamber for six days, air dried on the thirteenth day and tested on the fourteenth day after they were originally poured. The cylinders exhibited standard cement hydration behavior with the use of Portland Type III cement enabling attainment of high early strength, approximately 80% of the material's ultimate strength, during the fourteen day cure. It is assumed for such material, that after 28 days, full strength is reached. Beams of the lightweight composite material were also successfully molded and cured.

The microstructure of the resulting lightweight composite was examined using scanning electron microscopy. FIG. 3 is a scanning electron microscope

(SEM) micrograph of a composite having 50% volume fraction EPS spheres 30 surrounded by a nominal 20 pounds per cubic foot density cement foam matrix 32 to form a 12 pounds per cubic foot lightweight cement composite 35. Dark areas 34 are large air bubbles resulting from air entrained in the cement foam.

FIG. 4 is a micrograph showing the sandwich beam microstructure depicted schematically in the expanded view of FIG. 2. Cement foam matrix material 32 intervenes between EPS spheres 30 forming the sandwich beam microstructure.

FIG. 5 is an SEM micrograph showing a 19.3 micron thick high density cement coating 36 which surrounds EPS void 30 which is filled with polymethylmethacrylate (PMMA) . To prepare the sample shown in FIG. 5, the EPS was removed from a composite specimen and the specimen was impregnated with PMMA. (FIG. 3 was taken of a saw- cut specimen wherein the EPS was left in place.) Dark spots 34 correspond to voids left behind by foam in matrix material 32 and are also referred to as cement foam cells.

FIG. 6 shows a 54.2 micron thick high density cement coating 36 surrounding an EPS sphere 30. Cement foam cells 34 are formed in matrix material 32.

FIGS. 5 and 6 show a lightweight composite having a cement foam matrix material 32 characterized by a density in the range of from about 18 to about 20 pounds per cubic foot wherein the EPS spheres are embedded. The EPS spheres themselves have a thin coating 36 of full density cement in the range of from about 20 to about 100 microns thick. When the EPS spheres are present in the composite at high volume fractions between about 40% to about 50% volume fraction, a sandwich beam microstructure results as shown in FIG. 4 and schematically in FIG. 2. Here, the full density cement coating on the EPS spheres acts as a stiff material and the matrix material acts as a low density core and together they form a sandwich beam microstructure. This sandwich beam microstructure gives improved stiffness and strength per unit weight by comparison with conventional cement foam composites lacking the sandwich beam microstructure.

The molded cylinders and beams were tested to determine their mechanical properties. Cylinders were subjected to a compressive test. Data obtained from these tests is plotted in FIGS. 7-12. Results are compared to the mechanical properties of an equivalent density non-composite material without EPS spheres in FIGS. 7, 9 and 11. Cylinders were loaded in uniaxial compression. The maximum compressive load was used to calculate compressive strength. Young's Modulus was determined by measuring cylinder displacement vs. load. A three point bending test was used to determine the modulus of rupture.

Composite strength and stiffness were monitored as a function of percent volume fraction of EPS spheres for lightweight foam composites and as a function of composite density for lightweight foam composites both with and without EPS spheres. Results are shown in FIGS. 7-12.

FIG. 7 shows compressive strength as a function of overall lightweight foam composite density for composites including EPS spheres shown by points with error bars, and for composites without EPS spheres shown by triangles and having a cellular matrix phase material density in the range of from about 18.3 to about 21.9 pounds per cubic foot. It is clear from FIG. 7, that a lightweight foam composite containing EPS spheres exhibits a greater compressive strength than a lightweight foam composite of equivalent overall density, but not containing EPS spheres. This effect is particularly marked at densities less than 14 pounds per cubic foot. FIG. 8 shows the dependence of compressive strength on the percent volume fraction of EPS spheres for a lightweight foam composite including a cellular matrix phase characterized by a density in the range of from about 18.3 to about 21.9 pounds per cubic foot. These data indicate that up to 30% volume fraction EPS sphere addition, strength decreases relative to the strength of the cellular concrete without EPS spheres which has a compressive strength in the range of from about 50. to about 60 psi. However, as the percent volume fraction of EPS spheres increases from about 40% volume fraction to about 50% volume fraction, the compressive strength of the lightweight cement composite increases with respect to that of pure cellular concrete. The compressive strength of a lightweight cement composite containing above 55% volume fraction EPS spheres declines, since at this concentration of EPS spheres, the compressive strength of the composite is dominated by the compressive strength of the EPS spheres which are characterized by a compressive strength of approximately 30 psi.

FIG. 9 shows Young's Modulus as a function of overall lightweight foam composite density for lightweight foam composites with EPS spheres and without EPS spheres, both including a cellular matrix phase material characterized by a density in the range of from about 18.3 to about 21.9 pounds per cubic foot. These data indicate that a lightweight foam composite including EPS spheres shown by circles with error bars, exhibits a greater stiffness than a lightweight foam composite not including EPS spheres, shown by triangles, having an equivalent density.

FIG. 10 shows Young's Modulus as a function of percent volume fraction EPS spheres for a lightweight foam composite including a matrix phase characterized by a density in the range of from about 18.3 to about 21.9 pounds per cubic foot and shows that Young's Modulus is initially decreased relative to the Young's Modulus of cellular concrete without EPS spheres upon addition of EPS spheres but increases up to about 50% volume fraction EPS spheres in a manner similar to the observed compressive strength behavior.

FIG. 11 shows modulus of rupture data as a function of overall lightweight foam composite density for lightweight foam composites with EPS spheres, shown by circles with error bars, and for lightweight foam composites without EPS spheres, shown by triangles, with both lightweight foam composites including a cellular matrix phase characterized by density in the range of from about 20.8 to about 23.4 pounds per cubic foot. These data indicate that a lightweight foam composite including EPS spheres exhibits a greater modulus of rupture than that of a lightweight foam composite not including EPS spheres but having an equivalent overall density.

FIG. 12 shows modulus of rupture data as a function of percent volume fraction EPS spheres for a lightweight foam composite including EPS spheres and characterized by a matrix density in the range of from about 20.8 to about 23.4 pounds per cubic foot and indicates that the modulus of rupture increases substantially monotonically with EPS sphere addition.

EXAMPLE II Structural panels having full density cement faces can be prepared using a batch method involving two separate batch processes. A first batch process is for casting the full density cement faces and a second batch process is for casting the lightweight foam composite cores. According to the first batch process, full density cement faces are cast in battery mold 40 as shown in Fig. 13 mounted on rollers 42 which allow battery mold 40 to roll beneath mixer hopper 44 so that compartments 46 can be filled with full density cement to cast full density cement faces not shown. Battery mold 40 is typically characterized by length 48 of approximately 8 feet, height 50 of approximately 4 feet and overall width 51 of approximately 4 feet divided into compartments 46 each having a width of approximately 3/4 inch when mold 40 is used for casting a full density cement face or approximately 8 inches when mold 40 is used in casting a lightweight foam composite core.

A full density cement mixture is made by combining cement, silica sand, fiber, fly ash, silica fume, water and other additives in a mixer. The mixture is poured into battery mold 40 from mixer hopper 44 to fill compartments 46 with full density cement to form full density cement faces. The faces are allowed to cure in battery mold 40 overnight and then are stripped from the mold and stacked for curing, allowing reuse of battery mold 40. For a typical process, two battery molds, each capable of casting 16 full density cement faces, are needed to produce 512 square feet of total panel output. In the second batch process, water, cement, fiber, and other additives are combined in a slurry pre-mix. The pre-mix is further mixed with EPS spheres. Finally, a pre-formed foam mixture is added to the slurry and EPS mixture. Compartments 46 of battery mold 40 are then filled with the slurry, EPS and foam mixture to cast lightweight foam composite core billets not shown. Billets are allowed to set up in the mold for twelve hours and are then stripped from the mold and cured for a period of fourteen days. In a typical process, 6 lightweight foam composite core billets, each 8 inches thick, can be poured into a single battery mold. Other mold dimensions and arrangements can readily be used in the process.

After both the full density cement faces and the lightweight composite cores are cured for fourteen days, to allow for shrinkage, the full density cement faces and lightweight foam composite cores are bound together as shown in Figs. 15 and 16. The edges of lightweight foam composite core 60 are cut with a router to produce routed grooves 62 as shown in Fig. 14. Lightweight foam composite core 60 is then mounted on stand 64 which supports panel 60 with brackets 65 which are received within routed grooves 62. Full density face panels 66 and 67 are each supported by a frame 68. Adhesive can be applied to a top surface 70 of panel 60 and to a bottom surface of panel 60 not shown using a manually or mechanically driven roller 72. The surfaces of full density cement face panels 66, 67 which are to be contacted with lightweight composite core 60 can also be coated with adhesive.

After contacting surfaces of panels 60 and 66 are coated with adhesive, lightweight foam composite core 60 and face panel 67 can be lowered as by the use of jacks, so that core lower surface 69 is in contact with upper surface 74 of panel 66 and so that core upper surface 70 is in contact with a lower surface not shown of face panel 67. Light pressure, less than 1 psi, is applied and the bonded panels are allowed to cure for approximately 30 minutes as shown in FIG. 16. Adhesives suitable for this bonding process include epoxy, silicone and latex, and are well known in the art. After the approximately 30 minute cure is complete, the bonded structural panel is removed from frames 68 and stand 64, stacked and cured overnight before shipping.

Example III Alternatively, a continuous process can be used to fabricate structural panels including a lightweight composite core. Such a continuous fabrication line is shown schematically in FIG. 17.

Full density cement for forming a full density cement face on the structural panel is mixed as already described in Example II and supplied from continuous mixer 80 to produce panel bottom face 82 which can be in the range of from about 1/2" to about 3/4" thick. Alternatively, for steel-faced structural panels, the continuous mixer for full density cement application can be replaced with rolled steel. Screed 84 acts as a leveling bar to smooth the full density cement supplied by continuous mixer 80.

A second continuous mixer 86 continuously supplies lightweight foam composite mixture 88 which is produced according to the method already described in Example II and is leveled by screed 90 to produce lightweight foam composite panel core 92.

A third continuous mixer 94 applies full density cement for a full density top face which is leveled by screed 96 to form top panel face 98 which can be in the range of from about 1/2" to about 3/4" thick.

Conveyor 100 moves sufficiently slowly to allow panel material to spend approximately 1 hour on the conveyor so that the cement can begin hardening before cutting by cutter 102 to form structural panel 104 including a lightweight foam composite core 92. Structural panel 104 is then transferred to a curing chamber on rack 106. Practical production considerations require very rapid setting cement or a very long conveyor. The lightweight foam composite core is characterized by a higher water/cement ratio than is the full density cement face material. High cement shrinkage during drying, and a shrinkage differential between the faces and core may cause cracking of the structural panel if the face and core are sequentially cast wet and uncured in contact with each other as according to the already described continuous production process. It may be desirable to reduce and equalize the water/cement ratios of the full density cement face material and the lightweight foam composite core material by selecting cement foaming agents with low water requirements and high stability. Non-water based foams, or low water usage foams, such as acrylic or latex foams, would be desirable. Also, low shrinkage cements and polymer additives such as latex can be added to the core-forming slurry.

The structural panels described in Examples I and II include a sandwich beam microstructure as shown schematically in FIGS. 1 and 2 and as observed in the composites of the invention and shown in SEM micrographs of FIGS. 3-6. Using the simplified geometry for a sandwich cell 10 shown in FIG. 18, a relationship can be developed to serve as a guide for design of a sandwich beammicrostructure based on t_f, thickness 113 of sandwich beam faces 112, t_c, thickness 115 of beam foam core 114, E_f, the Young's Modulus of faces 112, G_c, the shear modulus of core 114 and I, the overall length 116 of the sandwich cell. Sandwich beam microstructure will be produced when the criterion

C* 72

0.5 < 0.05 -≤ — < 2.0

is satisfied.

Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.

Claims

What is claimed is:

1. A composite comprising at least two aggregate particles, a high density phase associated with said aggregate particles and a low density matrix phase which occupies the interstitial space between said aggregate phase particles to produce a sandwich beam microstructure.

2. The composite of claim 1 wherein said low density matrix phase is a cellular concrete.

3. The composite of claim 1 wherein said low density matrix phase is a foam.

4. The composite of claim 3 wherein said foam is a polymer foam selected from the group consisting of rigid polyurethane foams, expanded polystyrene foams, polyethylene foams and polyester foams.

5. The composite of claim 3 wherein said foam is an inorganic foam selected from the group consisting of metallic foams or ceramic foams.

6. The composite of claim 1 wherein said matrix phase is characterized by a first density and a first

Young's Modulus and said high density phase is characterized by a second density and a second Young's Modulus and wherein said second density is at least five times greater than said first density and said second Young's Modulus is at least five times greater than said first Young's Modulus.

7. The composite of claim 6 wherein said aggregate particles are characterized by a third density and said third density is in the range of from about 1 to about 10 pounds per cubic foot.

8. The composite of claim 6 wherein said second density is in the range of from about 85 to about 115 pounds per cubic foot.

9. The composite of claim 1 wherein said high density phase is a coating surrounding said aggregate particles.

10. The composite of claim 9 wherein said coating has a thickness in the range of from about 20 microns to about 100 microns.

11. The composite of claim 1 wherein said high - density phase is the shell of said aggregate particles.

12. The composite of claim 1 wherein said aggregate particles are characterized by a third density and said low density matrix phase is characterized by a first density in the range of from about 6 to about 10 times said third density.

13. The composite of claim 12 wherein said first density is in the range of from about 0.5 to about 30 pounds per cubic foot.

14. The composite of claim 1 wherein said aggregate phase particles are an organic material.

15. The composite of claim 14 wherein said organic material is a polymeric material selected from the group consisting of expanded polystyrene and recycled reground plastic.

16. The composite of claim 1 wherein said aggregate particles are an inorganic material.

17. The composite of claim 16 wherein said inorganic material is an inorganic material selected from the group consisting of hollow glass, ceramic and steel particles.

18. The composite of claim 1 wherein said aggregate particles are characterized by a hollow, regular geometry and wherein said regular geometry includes spheres, cubes, hexagonal prisms, rhombic dodecahedra and tetrakaidecahedra.

19. The composite of claim 1 wherein said high density phase further comprises a cement.

20. The composite of claim 19 wherein said high density phase further comprises a fiber.

21. The composite of claim 20 wherein said high density phase further comprises other additives selected from the group consisting of silica fume, superplasticizers, and silica sand.

22. The composite of claim 1 further including a first aggregate particle and a second aggregate particle wherein said sandwich beam microstructure is formed between said first aggregate particle which is coated with said high density phase and said second aggregate particle which is coated with said high density phase.

23. The composite of claim 1 further including a first aggregate particle having a first outer shell composed of said high density phase and a second aggregate particle having a second outer shell composed of said high density phase wherein said sandwich beam microstructure is formed between said first aggregate particle and said second aggregate particle.

24. The composite of claim 1 wherein said sandwich beam microstructure at a scale of a single cell extends over a range of from about 0.5mm to about 6mm.

25. The composite of claim 1 further comprising a first aggregate particle coated with a high density phase coating and a second aggregate particle also coated with a high density phase coating wherein said first aggregate particle is separated from said second aggregate particle by interstitial, low density matrix phase material having a thickness in the range of from about 100 microns to about 500 microns.

26. The composite of claim 1 further comprising a first aggregate particle including a thin, high stiffness, high density, exterior shell and a second aggregate particle also including a thin, high stiffness, high density exterior shell wherein said first aggregate particle is separated from said second aggregate particle by a thickness of interstitial, low density matrix phase material having a thickness in the range of from about 100 microns to about 500 microns.

27. The composite of claim 1 wherein said aggregate phase is in the range of from about 40% to about 55% volume fraction with respect to the total volume of said composite.

28. A lightweight cement composite comprising lightweight aggregate particles dispersed in a matrix phase of cellular concrete.

29. The composite of claim 28 further comprising a fiber.

30. The composite of claim 29 wherein said fiber is a polymer fiber.

31. The composite of claim 30 wherein said polymer fiber is a polymer fiber selected from the group consisting of polyester, polypropylene, E-glass, or E- glass coated with alkali resistant coating fiber.

32. The composite of claim 29 wherein said fiber is an inorganic fiber.

33. The composite of claim 32 wherein said inorganic fiber is an inorganic fiber selected from the group consisting of silicate glass fiber and borosilicate glass fiber.

34. The composite of claim 29 wherein said fiber is characterized by a geometry and wherein said geometry is strand-like.

35. The composite of claim 34 wherein said fiber is in the range of from about 0.25 to about 1.5 inches in length.

36. The composite of claim 29 wherein said lightweight cement composite further includes cement and said fiber is present in a concentration in the range of from about 0.3% to about 1.0% by weight of said cement.

37. The composite of claim 28 wherein said lightweight aggregate particles are present in a quantity in the range of from about 40% to about 55% volume fraction of said composite.

38. The composite of claim 28 wherein said lightweight aggregate particles are characterized by a density and wherein said density is in the range of from about 1 to about 10 pounds per cubic foot.

39. The composite of claim 28 wherein said lightweight aggregate particles include a hollow interior and a shell of aggregate phase material.

40. The composite of claim 39 wherein said shell of aggregate phase material consists of material characterized by a third density and a third stiffness and said matrix phase is characterized by a first density and a first stiffness and wherein said third density and said third stiffness are at least 6 times greater than said first density and said first stiffness.

41. The composite of claim 28 wherein said lightweight aggregate particles are characterized by a regular geometry.

42. The composite of claim 41 wherein said lightweight aggregate particles are characterized by a ' regular geometry selected from the group consisting of hollow spheres and cubes.

43. The composite of claim 28 wherein said lightweight aggregate particles consist of particles characterized by a size in the range of from about 0.5mm in diameter to about 6mm in diameter.

44. The composite of claim 28 wherein said lightweight aggregate particles are polymer particles.

45. The composite of claim 44 wherein said polymer particles are polymer particles selected from the group consisting of expanded polystyrene and recycled reground plastic.

46. The composite of claim 28 wherein said lightweight aggregate particles are inorganic particles.

47. The composite of claim 46 wherein said inorganic particles are inorganic particles selected from the group consisting of hollow glass spheres; hollow ceramic spheres, cubes and regular shapes with thin exterior shells; and low density cellular spheres, cubes and regular shapes.

48. The composite of claim 28 further comprising lightweight aggregate particles and a high density phase wherein said aggregate particles are coated with said high density phase.

49. The composite of claim 28 wherein said composite further includes interstitial space and said cellular concrete occupies said interstitial space between said lightweight aggregate particles.

50. The composite of claim 28 wherein said cellular concrete further includes a cement and a foam.

51. The composite of claim 50 wherein said foam is a preformed foam.

52. The composite of claim 50 wherein said foam is an in-situ formed foam.

53. The composite of claim 50 wherein said cement is characterized by chemical compatibility with said foam.

54. The composite of claim 53 wherein said cement is a high early strength cement.

55. The composite of claim 54 wherein said high early strength cement is a high early strength cement selected from the group consisting of Portland Type III, and Normal Portland Type I cement.

56. The composite of claim 28 wherein said foam is a preformed foam selected from the group consisting of inorganic foams; stabilized organic foams; hydrolyzed protein-based foams; saponified wood resin stabilized with animal glue; sodium containing aliphatic and aromatic sulfates, such as sodium lauryl sulfate, sodium cetyl sulfate, and sodium oleyl sulfate; sodium naphthalene isopropyl sulfate; sulfates of petroleum derivatives; complex organic compounds such as keratin compounds and saponin; and inorganic compounds stabilized with organics.

57. The composite of claim 28 further comprising other additives wherein said other additives are selected from the group consisting of silica fume, superplasticizers, sand and polymer additives such as epoxy and latex.

58. The composite of claim 28 wherein said composite is characterized by a sandwich beam microstructure.

59. The composite of claim 28 wherein said composite is characterized by mechanical properties which make said composite suitable for use as a load-bearing structural composite.

60. The composite of claim 28 wherein said composite is characterized by a compressive strength in the range of from about 40psi to about 60psi, a Young's modulus in the range of from about 18,000psi to about 25,000psi, a modulus of rupture in the range of from about 35psi to about 55psi, and a density between about 11 and about 14 pounds per cubic foot.

61. A method for making a lightweight composite comprising:

(a) forming a slurry including a cement and water; (b) adding lightweight aggregate particles to said slurry of step (a) and mixing said lightweight aggregate particles and said slurry to form an intermediate mixture;

(c) introducing a foaming agent into said intermediate mixture of step (b) and mixing to produce a foamed mixture;

(d) molding said foamed mixture of step (c) into a shaped article; and

(e) curing said shaped article of step (d) to form said lightweight composite.

62. The method of claim 61 wherein said step (c) of introducing a foaming agent further comprises adding a preformed foam.

63. The method of claim 61 wherein said step (c) of introducing a foaming agent further comprises adding an in-situ foam producer.

64. The method of claim 61 wherein said slurry of step (a) is formed so that it further comprises a fiber and other additives including silica fume, superplasticizer, sand and polymer additives such as epoxy and latex.

65. The method of claim 61 wherein said slurry forming step (a) further comprises a step of dry-mixing said cement and. said fiber.

66. The method of claim 65 further including a step of adding premixed water, silica fume and a superplasticizer to said cement and said fiber to form said slurry.

67. The method of claim 61 wherein in step (b) said lightweight aggregate particles are thoroughly intermixed so that said lightweight aggregate particles become coated with said slurry.

68. The method of claim 61 wherein in step (c) said mixing is conducted so that said preformed foam is thoroughly blended with said intermediate mixture of step (b) while the foam bubbles are preserved.

69. The method of claim 61 wherein said curing step (e) further comprises a step of providing an atmosphere having greater than 75% relative humidity at room temperature for said shaped article.

70. A structural panel comprising: a first facing layer; an inner low density matrix composite layer having a first side and a second side and characterized by a sandwich beam microstructure; means for bonding said first facing panel to said first side of said inner low density matrix composite layer; a second facing layer; and means for bonding said second facing layer to a second side of said inner low density matrix composite layer.

71. The structural panel of claim 70 wherein said inner low density matrix composite layer includes lightweight aggregate particles dispersed in a matrix of cellular concrete.

72. The structural panel of claim 70 wherein said means for bonding include an adhesive and mechanical fasteners.

73. The structural panel of claim 70 wherein said means for bonding include cement hydration which occurs when a cement facing layer is bonded to a cement inner low density matrix composite layer.

74. The structural panel of claim 72 wherein said adhesive is an adhesive selected from the group consisting of epoxy, silicon and latex.

75. The structural panel of claim 70 wherein said first facing layer and said second facing layer include a structural material selected from the group consisting of cement, wood, plywood, oriented strand board, waferboard and fiber-reinforced cement board.

76. The structural panel of claim 70 wherein said first facing layer and said second facing layer include a non-structural material selected from the group consisting of gypsum wallboard.

77. The structural panel of claim 70 wherein said layers are characterized by thicknesses and said inner low density matrix composite layer has a thickness in a range of from about 4 to about 12 times the thickness of said first facing layer and said second facing layer, adapting said structural panel for use as a wall panel.

78. The structural panel of claim 70 wherein said layers are characterized by thicknesses and said inner low density matrix composite layer has a thickness in a range of from about 8 to about 16 times the thickness of said first facing layer and said second facing layer, adapting said structural panel for use as a roof panel.

79. The structural panel of claim 70 wherein said layers are characterized by thicknesses and said inner low density matrix composite layer has a thickness in a range of from about 8 to about 16 times the thickness of said first facing layer and said second facing layer, adapting said structural panel for use as a foundation panel.

80. The structural panel of claim 70 wherein said structural panel further includes fasteners for fastening said structural panels together.

81. The structural panel of claim 80 wherein said fasteners further include mechanical fasteners and grouted joints.