Polymer-cement composites and methods of making same

Details Polymer-cement composites and methods of making same Polymer-cement composites and methods of making same

2000-03-17

2003-05-27

Medley, Margaret (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C106S696000, C106S724000, C106S737000, C428S446000, C428S500000, C428S688000

Reexamination Certificate

active

06569923

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to polymer-cement composites, and more particularly to polymer-cement composites having both cementitious and polymer bonding and products made from the cured polymer-cement composites.
2. Description of the Related Art
Portland cement comprises, essentially, a heterogeneous mixture of calcium silicate and calcium aluminate phases that hydrate simultaneously. The calcium silicate phases make up about 75% by weight of the cement and are responsible for most of the strength development. The products of hydration are calcium-silicate-hydride (C-S-H), the cementitious binding phase, and calcium hydroxide. The C-S-H is present as a continuous, poorly crystallized, rigid gel phase, and the calcium hydroxide forms large, equiaxed crystals predominantly in large pores and capillaries. The presence of calcium hydroxide in the large pores and capillaries tends to make the cement susceptible to acid and sulfate attack. Calcium hydroxide can be leached to the surface where it carbonates to form discoloring deposits (efflorescence). The leaching increases the porosity, making the material more susceptible to infiltration and attack. Also, the presence of relatively weak calcium hydroxide crystals in the pores prevents filling of the pores with stronger C-S-H, causing a reduction in the attainable strength.
Cementitious products formed with binding phases from only cement and water typically have low strengths and are brittle, i.e., have low flexibility. A commonly used way to increase strength, by reducing porosity in cements, mortars, and concretes, is to reduce the water content, commonly reported as the water-to-cement ratio (w/c). Lowering the batch w/c ratio has a tendency to reduce the cured porosity by reducing the open pore space vacated by evaporation of excess water.
The addition of a colloidal suspension of polymer solids in water, commonly referred to as latex, to the batch improves workability and usually allows a reduction in the w/c ratio. Tile improvement in workability is attributed to the spherical latex particles (that act like microscopic ball bearings) and to the surfactants that are typically added to help stabilize the suspension. Thus, adequate plasticity, or flow, is attained for lower water contents. Cured product containing latex must be dried to form a continuous polymer film that coats the open surfaces of the solid particles, cementitious matrix, pores and capillaries. This continuous coating of dried latex increases the strength, flexibility, wear resistance, impact resistance, and chemical resistance relative to cement. Latex additions to a batch also improve the adhesion or bonding to other materials.
However, prior art compositions typically have used high latex additions (a volume fraction of latex solids to cement (ls/c) between 0.4 and 0.7 or higher). This resulted in very long cement curing times and a detrimental level of water susceptibility (permeability). There is, therefore, a need in the art for an improved latex-cement or polymer-cement composition having normal or accelerated setting times, and low permeability. In addition to the foregoing, cement and latex-cement are not very flexible. It would additionally be advantageous to be able to adjust such characteristics as strength, flexibility and durability in a polymer-cement composite.
In addition to the foregoing, the methods that can be employed to form known cement or latex-cement compositions are limited due to the high viscosity of the green (uncured) body. There is, therefore, a need in the art for an improved polymer-cement composition wherein the viscosity of the uncured batch can be adjusted to accommodate almost any forming method.
It is an object of this invention to provide a polymer-cement composite wherein unique combinations of strength, flexibility and durability, can be effected by both composition and curing procedures.
It is a further object of the invention to provide polymer-cement composite which can be made by most conventional forming methods.
It is another object of the invention to provide a polymer-cement composite such that products can be formed from the composite without the use of water-soluble polymers, thereby greatly reducing the susceptibility of the products to water-based attack or degradation.
It is still a further object of the invention to provide a polymer-cement composite for forming products wherein the flexibility of the products can be adjusted to facilitate installation methods, unlike rigid or brittle construction materials.
SUMMARY OF THE INVENTION
The foregoing and other objects, features and advantages are achieved by this invention which is a polymer-cement composite in which the physical properties of the composite are determined by the combined effects of two distinct binding phases, cementitious and polymer (latex). The composite of the present invention basically comprises an inert, inorganic filler material, such as sand, latex, cement, reactive silica, and water. In preferred embodiments, the reactive silica is pozzolanic. Conventional additives, such as pigments and admixtures, are optional components. In preferred embodiments, all solid material components have particle sizes less than 300 microns.
In particularly preferred embodiments, the composite comprises, by weight percent, about 40% to 50% inert, inorganic filler material; about 12% to 23% latex; about 20% to 25% cement; and about 7% to 13% reactive silica.
The term “pozzolanic” refers to materials which contain high amounts of silica (SiO
2
) that are of sufficient reactivity to react at room temperature, in the presence of water, with calcia (CaO) or calcium hydroxide (Ca(OH)
2
) in the cement to form C-S-H. Calcium hydroxide is produced, for example, by hydrating portland cement. Pozzolan additions in hydrating calcium aluminate cements typically react to form stratlingite (hydrated gehlenite, a calcium aluminate silicate hydrate), resulting in better strength retention with time than in products not containing pozzoians.
The addition of a sufficient quantity of pozzolanic material to the batch significantly reduces porosity and permeability in the cured product, and increases long term strength. Pozzolanic reactions are slower than those of the cement components, but they react with the calcium hydroxide and deposit C-S-H into the large pores and capillaries. This can result in filling of the open capillaries and large pores, greatly reducing permeability. Filling of large pores with strong reaction product instead of relatively weak calcium hydroxide results in increased strength of the product. Reduction in the amount of calcium hydroxide that can be leached to the surface reduces the tendency to effloresce. The setting time of the composite of the present invention is normal or accelerated.
As used herein, the term “sand” means essentially inert, inorganic filler materials having particle sizes ranging from about 50 to 300 microns. These fillers include, but are not limited to, materials such as silica sand, ground nepheline syenite, ground sandstone, ground limestone, ground dolomite, coarse fly ash, and ground basalt. Lightweight, fine aggregate materials such as fly ash, perlite, and vermiculite, may be used in applications where product densities must be minimized. In preferred embodiments, the inorganic filler is silica sand.
The term “latex” means a colloidal suspension of polymer solids in water. A latex typically contains about 50 percent by weight of spherical polymer particles ranging in size from about 0.01 micron to 1 micron in diameter. The preferred latexes are those most commonly used in latex-modified concretes. These include well-known elastomeric (rubber-like), thermoplastic polymers. In specific preferred embodiments, the polymer may be, but is not limited to, polyacrylate, styrene-butadiene, or styrene-acrylate. Of course, other latex polymers, known and used by those of ordinary skill in the art, such as the alkali-swellable latexes described in U.S. Pat. Nos. 4,861

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