U.S. patent application number 09/829892 was filed with the patent office on 2001-09-27 for pressure-assisted molding and carbonation of cementitious materials.
Invention is credited to Dooley, Kerry M., Knopf, F. Carl.
Application Number | 20010023655 09/829892 |
Document ID | / |
Family ID | 26807383 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010023655 |
Kind Code |
A1 |
Knopf, F. Carl ; et
al. |
September 27, 2001 |
Pressure-assisted molding and carbonation of cementitious
materials
Abstract
A method is disclosed for rapidly carbonating large cement
structures, by forming and hardening cement in a mold under high
carbon dioxide density, such as supercritical or near-supercritical
conditions. The method is more reliable, efficient, and effective
than are post-molding treatments with high-pressure CO.sub.2.
Cements molded in the presence of high-pressure CO.sub.2 are
significantly denser than otherwise comparable cements having no
CO.sub.2 treatment, and are also significantly denser than
otherwise comparable cements treated with CO.sub.2 after hardening.
Bulk carbonation of cementitious materials produces several
beneficial effects, including reducing permeability of the cement,
increasing its compressive strength, and reducing its pH. These
effects are produced rapidly, and extend throughout the bulk of the
cement--they are not limited to a surface layer, as are prior
methods of post-hardening CO.sub.2 treatment. The method maybe used
with any cement or concrete composition, including those made with
waste products such as fly ash or cement slag. Surface carbonation
is almost instantaneous, and bulk carbonation deep into a form is
rapid. By combining molding, curing, and carbonation into a single
step, carbon dioxide is better distributed throughout the entire
specimen or form, producing a uniform product.
Inventors: |
Knopf, F. Carl; (Baton
Rouge, LA) ; Dooley, Kerry M.; (Baton Rouge,
LA) |
Correspondence
Address: |
PATENT DEPARTMENT
TAYLOR, PORTER, BROOKS & PHILLIPS, L.L.P
P.O. BOX 2471
BATON ROUGE
LA
70821
US
|
Family ID: |
26807383 |
Appl. No.: |
09/829892 |
Filed: |
April 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09829892 |
Apr 10, 2001 |
|
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09170480 |
Oct 13, 1998 |
|
|
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60109799 |
Oct 15, 1997 |
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Current U.S.
Class: |
106/792 |
Current CPC
Class: |
Y02P 40/18 20151101;
C04B 2103/007 20130101; C04B 40/0231 20130101; Y10S 264/43
20130101; C04B 16/06 20130101; C04B 28/02 20130101; C04B 40/0231
20130101 |
Class at
Publication: |
106/792 |
International
Class: |
C04B 002/00 |
Claims
We claim:
1. A process for making a carbonated cement, comprising the steps
of: (a) placing an uncured cement comprising hydroxides of calcium
into a gas-tight compartment that contains the entire uncured
cement; (b) reacting the uncured cement with carbon dioxide that is
introduced into the gas-tight compartment at a pressure of at least
about 400 psi, until at least about 50% of the hydroxides of
calcium have been converted to calcium carbonate; wherein the ratio
of the mass of introduced carbon dioxide to the mass of the uncured
cement prior to introduction of the carbon dioxide is at least
about 0.08; and (c) curing the cement to form a hardened cement
paste.
2. A process as recited in claim 1, wherein the carbon dioxide is a
supercritical fluid, or is a fluid whose carbon dioxide density
exceeds 0.46 g/cm.sup.3.
3. A process as recited in claim 1, wherein the ratio of the mass
of introduced carbon dioxide to the mass of the uncured cement
prior to introduction of the carbon dioxide is at least about
0.12.
4. A cement produced by the process of claim 1.
5. A cement as recited in claim 4, wherein the voidage of said
cement is at least 50% lower than the voidage of a comparison
cement that is produced by an otherwise identical process, except
that the comparison cement is not reacted with carbon dioxide while
curing, or is reacted only with ambient carbon dioxide while
curing.
6. A cement produced by the process of claim 2.
7. A process as recited in claim 1, wherein the uncured cement is
admixed with reinforcing polymeric fibers that are stable at the pH
of the cured cement.
8. A process as recited in claim 7, wherein said fibers comprise a
polyamide, a polyolefin, a polyamide blend, or a polyolefin
blend.
9. A process as recited in claim 8, wherein said fibers comprise a
nylon.
10. A process as recited in claim 8, wherein said fibers comprise
polypropylene.
11. A cured cement at least 2 mm thick, wherein all interior
portions of said cement that are at least 1 mm from the nearest
surface of said cement comprise interlocking calcium carbonate
crystals at least 10 .mu.m in diameter.
12. A cement as recited in claim 11, wherein the pH of said cement
is below about 8.
13. A cement as recited in claim 12, wherein said cement is
reinforced with polymeric fibers that are stable at the pH of said
cement.
14. A cement as recited in claim 13, wherein said fibers comprise a
polyamide, a polyolefin, a polyamide blend, or a polyolefin
blend.
15. A cement as recited in claim 13, wherein said fibers comprise a
nylon.
16. A cement as recited in claim 13, wherein said fibers comprise
polypropylene.
Description
[0001] This invention pertains to carbonation of cementitious
materials, particularly to carbonation of cements using
supercritical or high density carbon dioxide.
[0002] 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.degree. 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."
[0003] Hardened or cured cements have sometimes been reacted with
high pressure or supercritical CO.sub.2 to improve their
properties. Supercritical and near-critical CO.sub.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.sub.2 at the pore entrance, a dispersed water phase associated
with the pore walls, and possibly free water at the CO.sub.2/water
interface. The high CO.sub.2 pressure increases the solubility of
CO.sub.2 in the dispersed aqueous phase. A concentration gradient
of CO.sub.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).sub.2,
but also other calcareous hydrated cement components, e.g., calcium
silicate hydrate.)
[0004] Densification Reactions
[0005] 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.sup.- and I.sup.- 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.sup.- and I.sup.- 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 .about.13 to a more
neutral value of .about.8, allowing the reinforcement of the cement
with polymer fibers such as certain polyamides (e.g., nylons) that
are incompatible with normal cements.
[0006] Carbonation of cement is a complex process. All
calcium-bearing phases are susceptible to carbonation. For calcium
hydroxide (portlandite) the reaction is
Ca(OH).sub.2+CO.sub.2.fwdarw.CaCO.sub.3+H.sub.2O
[0007] The calcium carbonate may crystallize in one of several
forms, including calcite, aragonite and vaterite. Calcite is the
most stable and common form.
[0008] In this reaction, calcium hydroxide (Ca(OH).sub.2) is
assumed first to dissolve in water, after which it reacts with
CO.sub.2. Following reaction, the calcium carbonate (CaCO.sub.3)
precipitates. Atmospheric concentrations of CO.sub.2
(.about.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.sub.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.
[0009] 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.
[0010] Supercritical Fluids in Cementitious Materials
[0011] 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.sub.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.sub.2, with mostly water but some
CO.sub.2, adsorbed onto the walls of the concrete pores. At high
pressures solubility of CO.sub.2 in water increases.
[0012] 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.
[0013] J. Bukowski et al., "Reactivity and Strength Development of
CO.sub.2 Activated Non-Hydraulic Calcium Silicates, Cement and
Concrete Research, vol. 9, pp. 57-68 (1979) discloses treating
non-hydraulic calcium silicates with CO.sub.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.
[0014] 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.
[0015] 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.sub.2.
[0016] 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.
[0017] U.S. Pat. No. 5,051,217 discloses a continuous stamping and
pressing process for curing and carbonating cementitious materials.
CO.sub.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.sub.2 to the mass of the
uncured cement was relatively low, apparently always under 0.002
(extrapolating from data given in the specification).
[0018] F. Knopf et al., "Densification and pH Reduction in Cement
Mixtures Using Supercritical CO.sub.2," Abstract of paper to be
presented at 1997 annual meeting of the American Institute of
Chemical Engineers, available on the Internet in July 1997 at
[0019]
http://www1.che.ufl.edu/meeting/1997/annualsession/100/h/index.html
[0020] discloses some of the inventors' own work, work that is
disclosed in greater detail in the present specification.
[0021] We have discovered that a superior method to rapidly
carbonate large cement forms or structures is to shape and harden
the cement in a mold under high carbon dioxide pressure, at
supercritical, near-supercritical, or high CO.sub.2 density
conditions. In other words, contrary to previous teachings,
supercritical, near-supercritical, or high density CO.sub.2 is
reacted with cement while the cement is still in an uncured state.
The novel carbonation method is more reliable, efficient, and
effective than are post-molding treatments with high-pressure
CO.sub.2, or treatments using low temperature, low pressure
CO.sub.2. The novel method is more effective and reliable than
methods that admit relatively small amounts of CO.sub.2 to a mold
at relatively low pressure, and then compress the uncured mixture.
The novel method is more effective in penetrating voids with
CO.sub.2, and is therefore more efficient in converting hydroxides
of calcium to CaCO.sub.3. Cements molded in the presence of
high-pressure CO.sub.2 are significantly denser than otherwise
comparable cements having no CO.sub.2 treatment, and are also
significantly denser than otherwise comparable cements treated with
CO.sub.2 after hardening.
[0022] The novel bulk carbonation of cementitious materials
produces several beneficial effects, including reducing
permeability of the cement, increasing its compressive strength,
and reducing its pH. These effects are produced rapidly, and extend
throughout the bulk of the cement--they are not limited to a
surface layer, as are prior methods of post-hardening CO.sub.2
treatment. The novel method may be used with any cement or concrete
composition, including those made with waste products such as fly
ash or cement slag. Surface carbonation is almost instantaneous,
and bulk carbonation is rapid even with forms several centimeters
thick, tens of centimeters thick, or thicker. By combining molding,
curing, and carbonation into a single step, carbon dioxide is
better distributed throughout the entire specimen or form,
producing a uniform carbonated cement product. In particular, it is
believed that this is the first cured cement in which all interior
portions of the cement that are at least 1 mm from the nearest
surface of the cement comprise interlocking calcium carbonate
crystals that are at least 10 .mu.m in diameter.
[0023] Bulk carbonation of cement with supercritical CO.sub.2 in
our laboratory has produced a dense layer of interlocking calcium
carbonate (calcite) crystals in minutes. The crystals are an order
of magnitude larger in diameter (.about.10 .mu.m) than has been
previously reported for calcite crystals in the interior of
cements. The novel process produces concretes with improved
durability and higher compressive strengths.
[0024] Uses for concretes based on the novel, bulk-carbonated
cements are numerous. The higher compressive strength allows the
use of thinner blocks and less material for a given strength
requirement. For example, the stronger concrete may be used to make
lighter weight, fire-resistant structural panels or roofing tiles.
Cement roofing is rapidly gaining acceptance. These roofs last
essentially for the lifetime of the home, have a Class A fire
rating, and can be cast into any desired appearance. Costs should
be competitive with those for shorter-lived asphalt roofing
materials.
[0025] Low-cost reinforcing fibers may be used in bulk carbonated
cements due to the near-neutral pH of these materials. Many
potential reinforcing fibers are incompatible with the higher pH
found in most cements, e.g. the pH .about.13 of conventional
Portland cements. For example, it has been estimated that 3-4
billion pounds of carpet fiber per year are land-filled in the
United States. Recycled carpet polymers could instead be used to
reinforce these cement structures of near-neutral pH, transforming
old carpets from a waste product into a useful resource.
[0026] Carbonated cementitious materials can also be used for
building artificial reefs. Near-neutral pH's are necessary for the
growth of most marine organisms.
[0027] Carbonation and polymer reinforcement produce concretes with
greater resistance to chemical attack, a property that is useful,
for example, in the petroleum, mining, metallurgical, and chemical
industries. Bulk-carbonated cements have essentially no die-swell
or warpage, an advantage in the ceramics industry.
[0028] Preparation of Carbonated and Molded Samples
[0029] Comparison samples using previously cured cements were
prepared in an existing SCF continuous treatment system. Liquid
CO.sub.2 was compressed by a positive displacement diaphragm
compressor (American Lewa model ELM-1) to 1500 psi. The compressed
CO.sub.2 was stored in surge tanks to dampen pressure fluctuations.
The pressure was controlled by a Tescom regulator (model 44-1124)
to within .+-.5 psi. Pressure was monitored by a Heise digital
pressure gauge (model 710A). The specimen (10 mm by 10 mm by 40 mm)
was held in a tube immersed in a Plexiglas 25.degree. C. constant
temperature bath. The CO.sub.2 flow rate was .about.0.8 g/s, and
the run time was 1 hour.
[0030] A prototype device was constructed to evaluate the novel
one-step method for molding, curing, and supercritical (or
near-critical or high density) CO.sub.2 treatment. Specimens were
treated in a simple cylindrical mold operated by a piston, which
was sealed on its outer surface by O-rings. CO.sub.2 gas (at
.about.700 psi) was introduced below the piston. The pressure above
the piston was rapidly increased using water as a driver fluid. The
increased pressure initiated the molding process. As the piston
moved rapidly toward the sample, the gas pressure above the sample
rose to equalize. But simultaneously the CO.sub.2 reacted with the
cement, tending to lower the pressure. A 2000 psi water pressure
was applied to the piston, and the samples were generally molded
for .about.3 hours, although shorter or longer times can be used.
The molded specimens in the prototype embodiment were cylindrical,
39 mm diameter by 13 mm height. The prototype unit allowed various
modes of CO.sub.2 addition to be studied, without the complexities
inherent in filling the mold with uncured cements under pressure.
However, the scope of the invention is not limited by the manner
used to fill the mold. The amount of CO.sub.2 added to the cement
matrix could be readily controlled by adjusting the initial height
of the piston above the cement.
[0031] Characterization of Chemical and Physical Properties of
Cements
[0032] The porosities of conventionally cast samples (i.e.,
conventionally molded without high pressure CO.sub.2) and samples
produced by the novel process were determined indirectly by
measuring surface areas at a fixed initial composition. Higher
surface areas are often associated with void-filling and therefore
with decreased pore volumes, when small pores are created from
larger pores without significant pore closure. The amount of
nitrogen or other inert gas adsorbed (in determining surface area)
includes contributions from capillary condensation in small pores.
However, as voids are completely filled surface areas decrease
significantly. A discussion of physical adsorption mechanisms in
porous materials can be found in standard works on this subject,
for example, D. M. Ruthven, Principles of Adsorption and Adsorption
Processes (1984).
[0033] Thus an increase in surface area upon carbonation indicates
a small reduction in voidage, while a decrease in surface area
indicates almost complete closure of voids in the specimen,
accompanied by densification. Surface areas were estimated using
the one-point BET method at 30% relative saturation, using a
Micromeritics 2700 Pulse Chemisorption apparatus. Water was first
removed under vacuum at 1 torr for 24 h at ambient temperature,
then under flowing N.sub.2/He for at least 2 h. The surface areas
of selected samples were checked by the full BET N.sub.2 adsorption
method using an Omnitherm (model Omnisorp 360) adsorption
apparatus. The pore volume was determined in water by displacement
(Archimedes' principle). All specimens used in density and porosity
measurements were dried under vacuum at 1 torr at ambient
temperature prior to measurement.
[0034] A Scintag PAD-V automated X-ray Powder Diffractometer was
used to identify crystalline phases. Specimens were step-scanned
from 3-60.degree. 2.theta., at a 0.02.degree. step size, 3
second/step. A Perkin-Elmer thermogravimetric analyzer was used to
quantify weight losses from water evolution (from hydrates),
hydroxide (e.g., Ca(OH).sub.2) to oxide (e.g., CaO) conversions,
and carbonate (e.g., CaCO.sub.3) to oxide (e.g., CaO) conversions.
The carrier gas was helium at 1 atm. The temperature program was
200-700.degree. C., 5.degree. C./min, hold at 700.degree. C.
[0035] Results, Post-Treated Samples
[0036] The "post-treatments" (i.e., carbonations of previously cast
samples) used near-critical CO.sub.2 (1500 psi and 25.degree. C.).
The CO.sub.2 density at these conditions was 0.83 g/cm.sup.3, well
above the density at the critical pressure and temperature (0.46
g/cm.sup.3). Table 1 summarizes X-ray diffraction (XRD) results for
five different concrete mixes. The samples for the XRD measurements
were taken from the surfaces of the specimens. For each mix both a
control sample (no carbonation) and a test sample (carbonated) were
measured. The reported weights of the additives were normalized to
the initial weight of concrete. For all samples, a weight ratio of
0.603 water to 1.0 cement (ASTM Type III) was used in the initial
mix. The five mixes represent typical fast set concretes, some of
which included one or more of the following additives: glass
fibers, Kevlar fibers, calcite, lime, and a plasticizer.
1TABLE 1 XRD Phase Characterization of Carbonated Specimens,
Continuous Flow Treatment Ratio of XRD peak heights,
portlandite/calcite Additives Control 1 2.9 none Test 1 0.029
Control 2 3.9 0.021 lime Test 2 <0.035 0.007 calcite Control 3
4.1 0.021 calcite Test 3 0.08 0.022 WRDA 19 plasticizer Control 4
2.6 0.105 lime Test 4 0.09 0.021 calcite Control 5 3.6 0.105 lime
0.021 calcite 0.022 WRDA 19 plasticizer Test 5 <0.035 0.007
E-glass fiber 0.010 Kevlar 49 fiber
[0037] The portlandite peak reported in Table 1 occurred at
18.1.degree. 2.theta., and the calcite peak at 29.5.degree.. The
reported ratios of portlandite to calcite are not strictly
quantitative, because detailed calibrations of peak height versus
the weight of a given phase were not made, and also because careful
microtome sectioning procedures were not used. Nevertheless, the
five control samples showed a reasonably consistent ratio range,
2.6-4.1.
[0038] As compared to the controls, the test samples showed a
significant increase in calcite (CaCO.sub.3) peak heights, and a
corresponding decrease in portlandite (Ca(OH).sub.2) peak heights.
The relative ratio of P/C (portlandite/calcite) for the control and
test samples (i.e., (P/C).sub.control/(P/C).sub.test) ranged from a
low of 29 for sample 4 to a high of 111 for sample 2. Despite the
semi-quantitative nature of these initial XRD measurements, it is
still clear that carbonation caused a 1-2 order of magnitude change
in the ratio of portlandite to calcite in samples taken from the
surface. These experiments show that the presence of typical cement
additives did not hinder the carbonation process substantially.
[0039] Scanning electron microscope (SEM) photomicrographs showed
qualitatively similar appearances for control and test samples at
magnification 33.times.: individual, rounded sand grains coated
with the cement. At higher magnifications, 650.times. and
3700.times., significant differences in the crystalline structures
became apparent. Before carbonation, the cement comprised primarily
calcium silicate hydrate, calcium hydroxide, and ettringite. The
carbonated cement, by contrast, showed large calcium carbonate
crystals (average diameter 10 .mu.m), with partially developed
crystal faces. The average grain size was an order of magnitude
greater than that previously reported for carbonated cements. The
calcium carbonate crystals formed interlocking grains, suggesting
that permeability of the cement was thereby reduced. Also, adhesion
between the carbonated layer and the noncarbonated layer, as well
as adhesion between the carbonated layer and aggregate, both
appeared to be good.
[0040] Derivative thermogravimetric analysis (TGA) of a Portland
cement mortar before and after carbonation was used to estimate
content of calcium carbonate and hydroxides of calcium. The complex
chemical nature of a typical cement precludes exact quantitation by
TGA, so the TGA results are considered to provide relative
comparisons only. A large increase in calcium carbonate content
following carbonation was evident, as was a proportional decrease
in the content of hydroxides of calcium. The content of ettringite
and other stable hydrates appeared to be unaffected by the
carbonation.
[0041] The SEM micrographs suggested that surface carbonation was
extensive. Derivative thermogravimetry, on the other hand,
indicated that about half of the hydroxides of calcium did not
undergo any change. This discrepancy is explained by the fact that
the SEM probed only the top few micrometers of the surface, while
the thermal analysis was representative of the top several
millimeters of the sample. Thus the deeper one probed into the
sample, the lower the degree of carbonation for the post-treated
samples. As shown below, the results were quite different for
samples produced by the novel supercritical molding treatment.
[0042] Results, Molded Specimens; and Comparisons to Post-Treated
Specimens
[0043] The details of the treatments and initial compositions used
in the molding experiments are given in Tables 2 and 3. All initial
cure times were 3 hours. All comparison samples were prepared in
the molding device with 2000 psi water pressure on the driver side.
Some comparison samples were set with air only (i.e., with no more
than ambient levels of CO.sub.2.) Some comparison samples were set
in air for three hours initially, and the partially cured materials
were then contacted with CO.sub.2 for an additional two hours.
2TABLE 2 Composition and Treatments for Molded Portland Cement (PC)
and Fly Ash Samples Mass of Mass of 5 M NaOH Fiber Type and Mass,
PC or Solution, as a as a Percentage of Sample Number Fly Ash
Percentage of Mass Mass of PC or and Description (g) of PC or Fly
Ash Fly Ash 3A-PC, set in air 50 32 polypropylene, 1.4 3B-PC, set
with 50 32 polypropylene, 1.4 CO.sub.2 2A-fly ash, set 25 40 none
in air 2B-fly ash, set 25 40 none with CO.sub.2 11-fly ash, set 25
40 none with water P = 2000 psi, then CO.sub.2 5-fly ash, set 25 45
polypropylene, 1.6 with CO.sub.2 15-fly ash, set in 25 40
polypropylene, 1.6 air then CO.sub.2 16-fly ash, set 25 44 nylon,
1.6 with CO.sub.2, foamed.sup.1 17-fly ash, set 25 44
polypropylene, 1.6 with CO.sub.2, foamed.sup.1 .sup.1foamed with
aqueous solution comprising 73% 5 M NaOH and 27% aqueous (30 wt%)
H.sub.2O.sub.2
[0044]
3TABLE 3 Composition and Treatments for Molded Cement Slag Samples
Mass of Mass of 5 M NaOH Fiber Type and Cement Solution, as a Mass
as a Sample Number Slag Percentage of Mass Percentage of and
Description (g) of Slag Mass of Slag 4-set with CO.sub.2 25 44
polypropylene, 1.4 6-set in air, foamed.sup.1 25 44 0 8-set in air
25 40 polypropylene, 1.6 9-set with CO.sub.2 30 43 polypropylene,
4.3 10-set with CO.sub.2 25 45 0 foamed.sup.2 12-set in air,
CO.sub.2 25 45 0 post-setting .sup.1foamed with aqueous solution
comprising 55% 5 M NaOH and 45% aqueous (30 wt %) H.sub.2O.sub.2
.sup.2foamed with aqueous solution comprising 76% 5 M NaOH and 24%
aqueous (30 wt %) H.sub.2O.sub.2
[0045] For the fly ash and cement slag specimens, a 5 M NaOH
solution was used to reduce curing times, following the method of
U.S. Pat. No. 5,435,843. In some experiments, H.sub.2O.sub.2 was
used as a foaming agent to see whether it would affect contact
between the CO.sub.2 and the cements. The Portland cement used was
Type I. The fly ash was Class C. The cement slag was standard
pig-iron blast furnace slag.
[0046] After demolding, sectioned samples were tested for increases
in carbonate content by TGA. The reactions used to estimate
Ca(OH).sub.2 and CaCO.sub.3 content were as follows:
[0047] Hydrates.fwdarw.Silicates , Carbonates (T<300.degree.
C.)
[0048] MgCO.sub.3.fwdarw.MgO+CO.sub.2 [MW=44] (T
.about.300-350.degree. C.)
[0049] Ca(OH).sub.2 [MW=74.1].fwdarw.CaO+H.sub.2O [MW=18]
(350.degree. C.<T<450.degree. C.)
[0050] CaCO.sub.3 [MW=100.1].fwdarw.CaO+CO.sub.2 [MW=44]
(T>600.degree. C.)
[0051] Hydroxylated Silicas, Aluminas.fwdarw.SiO.sub.2,
Al.sub.2O.sub.3+H.sub.2O (T<650.degree. C.)
[0052] Other Carbonates.fwdarw.Oxides+CO.sub.2 (T>500.degree.
C.)
[0053] In most instances the MgCO.sub.3 peak could not be resolved
from the Ca(OH).sub.2 peak. Also, the final dehydrations of the
surfaces of other hydroxides such as SiO.sub.2 take place at
temperatures that overlap CaCO.sub.3 decomposition. The TGA results
should therefore be viewed as estimates of the amounts of
Ca(OH).sub.2 and CaCO.sub.3 in these materials. The TGA results are
nevertheless useful in relative comparisons of carbonated versus
non-carbonated (but otherwise identical) materials.
[0054] Standard samples were used to calibrate appropriate
temperature ranges for the dehydration and decarbonation reactions
in the TGA analysis. Each standard was a homogeneous physical
mixture, containing {fraction (2/3)} mold specimen 3A (Portland
cement, set in air), and {fraction (1/3)} of the additive being
tested. These components were ground to a powder with a mortar and
pestle. The additives used in separate samples were as follows:
CaCO.sub.3, which produced a high-temperature reaction;
Ca(OH).sub.2, which produced a range of multiple dehydrations from
.about.350-450.degree. C.; Al(OH).sub.3, for which bulk dehydration
occurred at low temperatures, in the hydrate-loss region; and
Na.sub.2SiO.sub.3, which produced a peak at .about.570-640.degree.
C., an evolution of water from silicate surfaces that can affect
quantitation of the carbonate peak--however, the relatively small
size of this peak suggests that rough quantitation of CaCO.sub.3 by
TGA is still possible. Tables 4 and 5 give the TGA results for the
molded samples. In the Tables, the designations "M" and "T" refer
to samples that were removed from the middle of the specimen and
the top surface of the specimen, respectively.
4TABLE 4 TGA Results, Fly Ash Samples % Water Loss % Hydroxide as %
Carbonate as Sample from Hydrates Ca(OH).sub.2 CaCO.sub.3 2A-fly
ash, set in 2.8 9.6 6.5 air 2B-fly ash, set with 1.6 3.3 13.7
CO.sub.2 11T 2.3 6.0 5.9 11M 2.7 5.2 6.0 5M 0.89 3.8 10.7 5T 0.74
4.0 11.0 15-fly ash, set in 0.68 13.3 15.1 air, CO.sub.2
post-setting 16-fly ash, set with 0.55 3.7 14.5 CO.sub.2,
foamed
[0055]
5TABLE 5 TGA Results, Cement and Cement Slag Samples % Water Loss %
Hydroxide as % Carbonate as Sample from Hydrates Ca(OH).sub.2
CaCO.sub.3 3A, set in air 1.5 16.0 5.1 3B, set with CO.sub.2 1.3
13.4 7.0 4, slag, set with 0.86 18.5 7.7 CO.sub.2 6, slag, set in
air, 0.45 14.4 1.1 foamed 8, slag, set in air 1.8 14.7 3.8 9-slag,
set with 1.1 16.7 5.8 CO.sub.2 10-slag, set with 0.56 12.6 2.3
CO.sub.2 foamed 12-slag, set in air, 1.2 12.4 1.6 CO.sub.2
post-setting
[0056] Note in Table 4 a general increase in measured CaCO.sub.3
content for all the CO.sub.2-molded samples as compared to the
non-carbonated samples. When CO.sub.2 was not used directly in the
molding process, but was instead applied as a post-cure treatment
in the mold, the measured carbonate content sometimes increased
(sample 15), and sometimes did not (sample 11). In Table 5 the
carbonate content increased where CO.sub.2 was used in the molding,
except for one of the H.sub.2O.sub.2-foamed samples. However, the
carbonate content did not increase when CO.sub.2 was used to treat
an already-hardened cement slag (sample 12). These experiments show
that the high-pressure CO.sub.2 molding process is more reliable
and effective than is a post-molding treatment with high pressure
CO.sub.2.
[0057] The CO.sub.2 in-situ molded specimens were also denser than
the air-molded samples, as seen in Tables 6 and 7. Because
carbonation filled pores and cracks in the cement, the dry surface
area should decrease upon significant carbonation, as seen in
Tables 6 and 7, even when polymer fibers were present. The bulk
density of the dry carbonated materials increased, as carbonates
are generally denser than hydroxides--with one exception, sample
16, which was a H.sub.2O.sub.2-foamed sample (compared to
non-foamed standard 2A). Similar results were found for cement
slags. (See Table 7.) Note that the voidages for the carbonated
samples decreased significantly as compared to samples set in air
(Tables 6 and 7). These lower voidages demonstrate that the novel
carbonated cementitious materials possess excellent barrier
properties, e.g. to ionic transport. The decreased ionic
permeabilities lend these cements to uses such as housing and
marine applications. In addition, reinforcing polymer fibers
blended with such cements would be less susceptible to degradation
by reaction with ions transported in water, especially saltwater or
wastewater.
6TABLE 6 Porosity and Density Results, Fly Ash Samples BET Voidage
(based on water Surface Bulk displacement: (sample Area, density,
volume-water displaced)/ Sample m.sup.2/g kg/m.sup.3 sample volume
2A-fly ash, set in air 8.5 1.82 0.18 2B-fly ash, set with 5.4 1.94
0.065 CO.sub.2 5-fly ash, set with 8.4 1.95 0.049 CO.sub.2 15-fly
ash, set in air, 5.7 0.049 then CO.sub.2 16-fly ash, set with 4.6
1.68 0.089 CO.sub.2, foamed 17-fly ash, set with 7.0 1.80 0.015
CO.sub.2, foamed
[0058]
7TABLE 7 Porosity and Density Results, Cement Slag Samples BET
Voidage (based on water Surface Bulk displacement: (sample Area,
density, volume-water displaced)/ Sample m.sup.2/g kg/m.sup.3
sample volume 4-set with CO.sub.2 3.8 2.00 0.14 8-set in air 5.8
1.69 0.29 9-set with CO.sub.2 0.4 1.93 0.14 10-set with CO.sub.2,
4.7 1.93 0.15 foamed
[0059] This process can be conducted at any pressure above
.about.400 psi, preferably between .about.600 psi and .about.2000
psi. Although there is no upper limit on pressure, as a practical
matter it becomes increasingly more difficult to handle fluids
above a pressure .about.5000 psi. A delivery pressure to the mold
of .about.700-800 psi is particularly convenient in many
applications, because this is the pressure at which carbon dioxide
is delivered from a tank of liquid carbon dioxide at room
temperature (i.e., this is the vapor pressure of carbon dioxide at
room temperature). Subsequent molding would increase the pressure
within the mold. The temperature should be between -56.degree. C.
(the triple point of CO.sub.2) and 200.degree. C., preferably
between 0.degree. and 50.degree. C. More specifically, the
pressure/temperature combination should be such as to produce a
CO.sub.2 density near or exceeding the critical density of
CO.sub.2, 0.46 g/cm.sup.3. For example, at 25.degree. C. the
density of CO.sub.2 in a near-critical state of 1000 psi is 0.74
g/cm.sup.3. This density easily suffices to give uniformly
carbonated products.
[0060] As used in the specification and claims, unless context
clearly indicates otherwise, the term "carbon dioxide" refers to
any liquid, gas, or supercritical fluid containing a substantial
amount of CO.sub.2, at least 20% by weight (as measured before
reaction with, or dilution into, other components). The term
"cement" or "cementitious material" refers to any calcareous
material which, when mixed with appropriate amounts of water (and,
optionally, other curing additives), can be used as a binder for
aggregates formed from materials such as sand, gravel, crushed
stone, organic polymers, and other materials. A cement may include
such aggregate or polymeric materials as blended mixtures. Examples
of cementitious materials include Portland cements, fly ash, and
cement slags such as blast furnace slag.
[0061] The complete disclosures of all references cited in this
specification are hereby incorporated by reference. In the event of
an otherwise irreconcilable conflict, however, the present
specification shall control.
* * * * *
References