Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1998-10-13
2001-07-24
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S738000, C264SDIG004
Reexamination Certificate
active
06264736
ABSTRACT:
This invention pertains to carbonation of cementitious materials, particularly to carbonation of cements using supercritical or high density carbon dioxide.
Above a compound's “critical point,” a critical pressure and temperature characteristic of that compound, the familiar transition between gas and liquid disappears, and the compound is said to be a “supercritical fluid.” Supercritical fluids (SCFs) have properties of both gasses and liquids, in addition to unique supercritical properties. A supercritical fluid is compressible like a gas, but typically has a density more like that of a liquid. Supercritical fluids have been used, for example, as solvents and as reaction media. The critical pressure and temperature for carbon dioxide are 1071 psi and 31.3° C. The viscosity and molecular diffusivity of a supercritical fluid are typically intermediate between the corresponding values for the liquid and the gas. Compounds below, but near, the critical temperature and pressure are sometimes termed “near-critical.”
Hardened or cured cements have sometimes been reacted with high pressure or supercritical CO
2
to improve their properties. Supercritical and near-critical CO
2
increase the mobility of water that is already present in the cement matrix, water bound as hydrates and adsorbed on pore walls. A pore in the cement may initially contain supercritical or near-critical CO
2
at the pore entrance, a dispersed water phase associated with the pore walls, and possibly free water at the CO
2
/water interface. The high CO
2
pressure increases the solubility of CO
2
in the dispersed aqueous phase. A concentration gradient of CO
2
is thus produced in the concrete pores. Carbon dioxide may then react with various cement components, particularly hydroxides of calcium. (As used in the specification and claims, the term “hydroxides of calcium” includes not only Ca(OH)
2
, but also other calcareous hydrated cement components, e.g., calcium silicate hydrate.)
Densification Reactions
Carbonation reduces the permeability of cement, typically by 3 to 6 orders of magnitude. This reduction in permeability has been attributed to precipitation of carbonates in the micropores and macropores of the cement. For example, in cement grout carbonation shifts a bimodal pore distribution (pores around 2-10 nm in diameter and pores around 10-900 nm) to a unimodal distribution (pores around 2-10 nm in diameter only). Reduced permeability and smaller pore diameters slow rates of diffusion in carbonated cements. For example, Cl
−
and I
−
diffusion coefficients have been reported to be 2 to 3 orders of magnitude lower in carbonated cement than in noncarbonated cement, as have carbon-14 migration rates. (Lower Cl
−
and I
−
diffusion rates indicate greater resistance to salt intrusion. Salt intrusion is undesirable, as it can lead to fracturing or cracking.) Curing cement grout with carbon dioxide increases the strength and dimensional stability of a cement. The pH of cement in fully carbonated zones is lowered from a basic ~13 to a more neutral value of ~8, allowing the reinforcement of the cement with polymer fibers such as certain polyamides (e.g., nylons) that are incompatible with normal cements.
Carbonation of cement is a complex process. All calcium-bearing phases are susceptible to carbonation. For calcium hydroxide (portlandite) the reaction is
Ca(OH)
2
+CO
2
&rlarr2;CaCO
3
+H
2
O
The calcium carbonate may crystallize in one of several forms, including calcite, aragonite and vaterite. Calcite is the most stable and common form.
In this reaction, calcium hydroxide (Ca(OH)
2
) is assumed first to dissolve in water, after which it reacts with CO
2
. Following reaction, the calcium carbonate (CaCO
3
) precipitates. Atmospheric concentrations of CO
2
(~0.04%), do not react appreciably with completely dry concrete. Conversely, if the concrete pores are filled with water, carbonation at low pressure essentially stops before bulk carbonation of a thick cement form can occur, because the solubility and diffusivity of CO
2
in water are low under such conditions. However, bulk carbonation of cement can occur at atmospheric pressure and ambient temperatures after years of exposure to atmospheric carbon dioxide.
High pressure conditions have previously been used to carbonate the surface layers of hardened cements. However, problems resulting from bulk carbonation of hardened cements have been reported. For example, the volume changes associated with conversion of calcium hydroxide to calcium carbonate have been reported to cause microcracking and shrinkage, at least under certain conditions.
Supercritical Fluids in Cementitious Materials
Supercritical and near-critical fluids confined in narrow pores have properties that are often quite different from those of a bulk gas. Because supercritical fluids are highly compressible, a surface or wall potential can produce a strong, temperature-dependent preferential adsorption, which might not occur at all at lower fluid densities. For example, a water layer on the solid surfaces is believed to be necessary to initiate carbonation reactions. Water is, in turn, a product of carbonation. At lower pressures water can completely fill the pores and thereby limit or even prevent carbonation; in such cases the sample must be dried for carbonation to resume. However, saturation and supersaturation of water in a CO
2
-rich phase is possible at high pressure, because phase separation in the concrete pores is slower than the carbonation reaction. Also, at high pressures carbon dioxide may adsorb onto the solid surfaces, along with water. The pore environment may eventually consist of a fluid phase of water and dissolved CO
2
, with mostly water but some CO
2
, adsorbed onto the walls of the concrete pores. At high pressures solubility of CO
2
in water increases.
E. Reardon et al., “High Pressure Carbonation of Cementitious Grout,”
Cement and Concrete Research,
vol. 19, pp. 385-399 (1989) discloses treating a solid, hardened, cementitious grout with carbon dioxide gas at pressures up to 800 psi, and notes that this process can sometimes cause physical damage to specimens, including fracturing due to dehydration and shrinkage.
J. Bukowski et al., “Reactivity and Strength Development of CO
2
Activated Non-Hydraulic Calcium Silicates,
Cement and Concrete Research,
vol. 9, pp. 57-68 (1979) discloses treating non-hydraulic calcium silicates with CO
2
up to 815 psi, and notes that both the extent of the carbonation reaction and the compressive strength of the carbonated materials increased with treatment pressure.
U.S. Pat. No. 4,117,060 discloses a method for the manufacture of concrete, in which a mixture of a cement, an aggregate, a polymer, and water were compressed in a mold, and exposed to carbon dioxide gas in the mold prior to compression, so that the carbon dioxide reacts with the other ingredients to provide a hardened product.
U.S. Pat. No. 4,427,610 discloses a molding process for cementitious materials, wherein the molded but uncured object is conveyed to a curing chamber and exposed to ultracold CO
2
.
U.S. Pat. No. 5,518,540 discloses treating a cured cement with dense-phase gaseous or supercritical carbon dioxide. The patent also mentions using supercritical carbon dioxide as a solvent to infuse certain materials into a hardened cement paste. See also U.S. Pat. No. 5,650,562.
U.S. Pat. No. 5,051,217 discloses a continuous stamping and pressing process for curing and carbonating cementitious materials. CO
2
was admitted at low pressures, and could later be compressed to higher pressures in one segment of the apparatus, a segment through which an afterhardening cement mixture passed continuously. The apparatus was said to be quasi-gas-tight. Only a portion of the uncured form was subjected to high pressure at any given time. The ratio of the mass of CO
2
to the mass of the uncured cement was relatively low, apparently always under 0.002 (extrapolating from data given in the specification).
F. Knopf et al., “Densificatio
Dooley Kerry M.
Knopf F. Carl
Board of Supervisors of Louisiana State University and Agricultu
Marcantoni Paul
Runnels John H.
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