Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2002-10-15
2004-08-31
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S705000, C106SDIG001
Reexamination Certificate
active
06783585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the process and the products obtained by a partial removal of carbon from coal ash which is used in the production of concrete. The carbon removed, through a thermal process of Partial Carbon Burn-Out (PCBO) can no longer reduce the activity of air entraining agents, which increase the stability, the number and the size of air voids in the concrete. The presence of appropriate air voids improves the freeze-thaw resistance of concrete.
2. Description of the Prior Art
Concrete consists of a cement binder, a pozzolanic material such as fly ash which will react with a mineral alkali, an aggregate material and sufficient water to cause the cement to set and bind the components into a hardened and durable material. A pozzolanic material reacts with calcium hydroxide, a by-product of Portland cement hydration to form compounds having the cementitious properties.
Unburned carbon in fly ash has a broad particle size distribution ranging from coarse char (>75 &mgr;m) to very fine soot sized (=40 nm) amorphous carbon particles that has a very high surface area (Gao, Y. M., H. S. Shim, R. H. Hurt, E. M. Suuberg, N. Yang, “Effects of Carbon on Air Entrainment in Fly Ash Concrete: Role of Soot and Carbon Black”, Energy & Fuels 11, 457, 1997). The very fine unburned carbon in fly ash has properties similar to that of activated carbon and as such has an affinity for molecules that have hydrophobic moieties, such as air entraining agents.
The durability of concrete to freeze-thaw cycles is dependent on its level of air entrainment. When water freezes its volume increases 9%. If concrete is fully saturated, all air voids are filled with water, the tensile stresses generated by the freezing water are sufficient to cause cracking and deterioration. Concrete has excellent strength in compression but its tensile strength is approximately 7%-10% of the compressive strength. When concrete begins to freeze the expanding ice forces water into the unfrozen regions of the cement binder—this movement of water creates large hydraulic pressures and generates tensile stress.
Many factors affect the durability of concrete to cycles of freezing and thawing: the cement binder content; amount and type of pozzolan; water to cement ratio; quality of the aggregates; and, the presence of air voids with an optimum spacing and size distribution.
Air is naturally entrapped in concrete through the folding and shearing action of mixing the cement paste. The entrapped air voids are large and not stable in concrete unless air entraining agents are used. Air entraining agents are surface active agents or surfactants. These agents reduce the surface tension of water, which tends to promote the fragmentation of large air voids into smaller ones, and to stabilize air voids by collecting at the water interface and forming a film.
The unburned carbon residue present in fly ash has a high adsorptive capacity for air entraining agents. The time dependent adsorption of air entraining agents causes a loss of entrained air during mixing and placement, and ultimately affects durability of the concrete by degrading the air void system.
LOI (Loss On Ignition) is a measure of the residual combustible material, primarily carbon in the coal ashes. There are several processes in commercial use that aim to reduce the LOI of moderate to high LOI fly ashes significantly to a level below 3% by weight. These include methods employing electrostatic separation or by carbon combustion. It should be noted that carbon makes up most of the measured LOI (to within about 10%) but more particularly it is the adsorption capacity of fly ash for air entraining agents and not the LOI, that is the ultimate criteria for whether the fly ash is commercially useful.
U.S. Pat. No. 5,399,194 describes a thermal treatment for fly ash in a fluid bed between 800 and 1300° F., or (426 and 700° C.) while U.S. Pat. No. 5,160,539 describes the use of a fluid bed to reduce the LOI in a temperature range from 1300 to 1800° F. or 700 to 982° C. and is clearly designed to eliminate as much carbon as possible.
SUMMARY OF THE INVENTION
One object of the invention is to provide a process for producing a coal ash having a low adsorption affinity for air entraining agents in concrete. This process comprising a partial combustion of a carbon residue of the feed coal ash wherein a fine carbon fraction of said carbon residue that is responsible for adsorption of air entraining agents in concrete is combusted, while leaving a non-combusted carbon fraction with a lower adsorption capacity for air entraining agents; and a recovery of the coal ash product.
It is another object of this invention to provide a method for inhibiting adsorption characteristics of coal ash bearing a carbon residue. The method comprising combusting an adsorbing fraction of said carbon residue responsible for adsorption of air entraining agents in concrete, while leaving non-adsorbing carbon of said carbon residue un-combusted in said coal ash.
Another object of the invention is a coal ash product which has low levels of a fine carbon fraction responsible for adsorbing air entraining agents in concrete. The fly ash product for use in concrete, is produced by a partial combustion of a carbon residue of a feed coal ash wherein the fine carbon fraction of said carbon residue responsible for adsorption of air entraining agents in concrete is combusted, while leaving a non-combusted carbon fraction of non-air entrainment agent absorbing carbon in the coal ash product.
Yet another object of the invention is a concrete product. The concrete product having a cement binder, an aggregate, and an air entraining agent characterized in that it includes, a coal ash product which has low levels of a fine carbon fraction responsible for adsorbing air entraining agents in concrete produced by, a partial combustion of a carbon residue of a coal ash wherein the fine carbon fraction of said carbon residue responsible for adsorption of air entraining agents in concrete is combusted, while leaving a non-combusted carbon fraction of non-air entrainment agent absorbing carbon in the coal ash product, which are all mixed together with water to produce said concrete product.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
When concrete contains sufficient cement (>225 kg/m
3
) and the water to cement ratio is less than 0.55:1, the concrete will be durable providing that an air entraining agent is used to generate an air void system consisting of very fine bubbles (50 to 200 &mgr;m). The average distance from any point in the cement paste or the spacing factor, must not be more than 200 &mgr;m from the nearest air void in order to protect the cement paste. The air voids in air entrained concrete relieve the pressure (i.e. tensile stresses) by accommodating the hydraulic pressure produced by the expansion of water at lower temperatures and prevent damage to the concrete. Air entrained concrete that has adequate strength and is appropriately saturated with air will withstand hundreds of freezing and thawing cycles without dilating or losing strength.
Air entraining agents promote the adhesion of air voids to the surface of hydrating cement grains which further stabilizes the air void in the cement paste. In general, as the dosage of the air entraining agent increases, the air content increases up to a maximum value, after which additional increases in air entraining agent do not increase air content significantly. The dosage rate of air entraining agent required to achieve a target air content is dependent on the type of air entraining agent, concrete temperature, concrete materials, such as sand, cement, fly ash, and mixture proportions.
The adsorption of air entraining agents by the carbon contained in coal ashes, reduces their concentration in solution, which in turn causes the destabilization of air voids. Under such conditions air voids coalesce into larger voids, with some rising to the surface and being lost. The remaining fewer and coarser
Oates David Bridson
Zacarias Philip Souza
Bachman & LaPointe P.C.
Koslow C. Melissa
Lafarge Canada Inc.
Manlove Shalie
LandOfFree
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