Accelerated curing of cement-based materials

A CO.sub.2 pre-curing period is used prior to accelerated (steam or high-pressure steam) curing of cement and concrete products in order to: (1) prepare the products to withstand the high temperature and vapor pressure in the accelerated curing environment without microcracking and damage; and (2) incorporate the advantages of carbonation reactions in terms of dimensional stability, chemical stability, increased strength and hardness, and improved abrasion resistance into cement and concrete products without substantially modifying the conventional procedures of accelerated curing. Depending on the moisture content of the product, the invention may accomplish CO.sub.2 pre-curing by first drying the product (e.g. at slightly elevated temperature) and then expose it to a carbon dioxide-rich environment. Vigorous reactions of cement paste in the presence of carbon dioxide provide the products with enhanced strength, integrity and chemical and dimensional stability in a relatively short time period. Subsequent accelerated curing, even at reduced time periods (with less energy and cost consumptions) would produce higher performance characteristics than achievable with the conventional pre-setting period followed by accelerated curing of cement and concrete products.

BACKGROUND--FIELD OF INVENTION 
This invention relates to cement, concrete and concrete-like products, 
especially those subjected to accelerated curing for rapid strength gain 
and dimensional stabilization. 
BACKGROUND--DESCRIPTION OF PRIOR ART 
The slow rate of setting and hardening of hydraulic cements reduce 
productivity in cement and concrete products plants. To overcome this 
problem, accelerated curing processes have been adopted by the cement and 
concrete products industry where methods such as steam and high-pressure 
steam curing have been used to increase the rate of strength gain and 
dimensional stabilization in cement-based materials. These accelerated 
curing processes subject the product to an environment with elevated 
temperature and (possibly) pressure. A pre-set period is required to 
prevent damage to the young product in such environments due to stresses 
associated with thermal expansion and vapor pressure. The pre-set period 
also allows the occurrence of chemical reactions which are needed for 
eventual chemical and dimensional stability of the system. Our invention 
utilizes CO.sub.2 curing within this pre-set period in order to uniquely 
prepare cement- and concrete-based products for conventional accelerated 
curing processes, and also to bring about improvements in the final 
product after curing. 
CO.sub.2 curing has been the subject of several patents which have used the 
process as a replacement for conventional accelerated curing processes 
such as steam or accelerated steam curing. The prior art does not, 
however, cover complementary and subsequent use of CO.sub.2 curing and 
conventional accelerated curing processes. 
U.S. Pat. No. 109,669 to James L. Rowland (Nov. 29, 1870), U.S. Pat. No. 
128,980 to James L. Rowland (Jul. 16, 1972), U.S. Pat. No. 2,496,895 to 
Ronald W. Staley (Feb. 7, 1950), U.S. Pat. No. 3,149,986 to Nissan 
Zelmanoff (Sep. 22, 1964), U.S. Pat. No. 3,468,993 to Knud Georg Bierlich 
(Sep. 23, 1969), U.S. Pat. No. 3,492,385 to Branko Simunic (Jan. 27, 
1970), U.S. Pat. No. 4,093,690 to John A. Murray (Jun. 6, 1978), U.S. Pat. 
No. 4,266,921 to John A. Murray (May 12, 1981), U.S. Pat. No. 4,362,679 to 
Roman Malinowski (Dec. 7, 1982), U.S. Pat. No. 4,427,610 to John A. Murray 
(Jan. 24, 1984), U.S. Pat. No. 4,436,498 to John A. Murray (Mar. 13, 
1984), U.S. Pat. No. 5,257,464 to Francisco Trevino-Gonzales (Nov. 2, 
1993), German lay-open print 1,915,563 (1974), German open-to-public print 
2,008,247 (1971), United Kingdom patent 781,328 (1957), United Kingdom 
patent 1,460,284 (1976), and Swedish patent 89,121 to K. P. Billner (1935) 
cover various methods of accelerated curing of concrete with carbon 
dioxide. These inventions use various concrete mixtures which are 
relatively dry or are dewatered to a relatively low moisture content, and 
are then subjected to carbon dioxide gas at atmoshpheric or elevated 
pressaures in environments with different temperatures and relative 
humidities. They present carbonation curing as an alternative to, and not 
a complement for, the conventional accelerated curing methods of steam and 
high-pressure steam curing. They rely on carbonation to reach substantial 
stages of curing. 
OBJECTS AND ADVANTAGES 
Accordingly, several objects and advantages of my invention are to 
incorporate the CO.sub.2 curing process into the pre-set period of the 
conventional accelerated curing methods of cement and concrete products in 
order to make them more capable of withstanding the elevated temperature 
and pressure of the accelerated curing prcesses, to incorporate the 
advantages of CO.sub.2 curing in terms of improved dimensional stability, 
increased impermeability and strength, and enhanced chemical stability, 
longevity, abrasion resistance, efflorescence resistance and hardness, and 
enhanced compatibility of the cement-based matrix with inclusions such as 
glass and wood fibers into concrete and cement products, and to reduce the 
time and energy required for accelerated curing of concrete and cement 
products. 
Still further objects and advantages will become apparent from a 
consideration of the ensuing description and accompanying drawings.

SUMMARY 
A method of accelerated curing of cement and concrete products where a 
CO.sub.2 pre-curing period precedes the conventional accelerated curing 
processes such as steam and high-pressure steam curing. The CO.sub.2 
pre-curing step involves the adjustment of the moisture content of the 
product as necessary, for example through drying, followed by exposure of 
the product to a carbon dioxide-rich environment at different temperatures 
and relative humidities. Exposure to carbon dioxide causes vigorous 
reactions with cement paste which produce desirable levels of strength and 
integrity for the product to satisfactorily withstand the high 
temperatures and pressures involved in conventional accelerated curing 
procedures without microcracking and damage, and to provide the product 
with the advantages of carbonation reactions in terms of improved 
dimensional stability, chemical stability and mechanical properties. The 
process also presents the potential to reduce the time and energy 
consumption in accelerated curing of cement and concrete products, and 
provides a value-added application for industrial emissions which are 
typically rich in carbon dioxide. 
PREFERRED EMBODIMENT--DESCRIPTION 
As shown in FIG. 1, the invention can be used in pre-carbonation of cement 
or concrete products prior to high-pressure steam curing in an autoclave. 
Following production (by molding, slurry-dewatering, or other techniques) 
in the production unit 1, the product 2 is subjected to a drying 
environment 3 where heat, air circulation and/or low humidity of the 
environment are used to reduce the moisture content of the product to 
levels which favor the diffusion and reaction of carbon dioxide. 
Subsequently, the products with adjusted moisture content enter the 
pre-carbonation environment where the moisture content may be maintained 
at a desirable level usng a solution 5, through which carbon dioxide or 
carbon dioxide-rich gases (e.g. flue gas) 6 are introduced into the 
environment. The pre-carbonated products are then moved into an autoclave 
7 for high-pressure steam curing. The variables of high-pressure steam 
curing may be adjusted because of the pre-carbonation of the products. For 
example, the temperature can be raised more rapidly and the duration of 
high-pressure steam curing may be reduced. 
In FIG. 2, concrete products (masonry units, pipes, etc.) 1 are subjected 
to drying (FIG. 2a), carbonation (FIG. 2b) and steam curing (FIG. 2c) 
within the same chamber 2. Drying (FIG. 2a) is accomplished by the 
applicaiton of heat 3 in the chamber (possibly with air circulation); some 
low-moisture-content products may not need any drying. Subsequently, 
carbon dioxide or carbon dioxide-rich gases (e.g. flue gas) 4 are 
introduced into the camber for a sufficient time period to accomplish 
pre-carbonation. The chamber is then loaded with steam 5 at an appropriate 
temperature for steam curing of the concrete products. The steam curing 
variables (rate of temperature rise, duration, etc.) can be adjusted (e.g. 
duration can be reduced) due to pre-carbonation. 
As shown in FIG. 3, in applications similar to that in FIG. 1, the concrete 
products (e.g. cement or fibrous cement panel) 2 can be dried through the 
application of vacuum 3. Subsequently, the product would be pre-carbonated 
4 and loaded into autoclave 5 for high-pressure steam curing. 
PREFERRED EMBODIMENT--OPERATION 
The operation of the system incorporating our pre-carbonation procedure is 
further described here with reference to FIGS. 1, 2 and 3. In order to 
accomplish pre-carbonation prior to accelerated (e.g. steam or 
high-pressure steam) curing of cement and concrete products, the 
relatively wet products have to be partially dried; some direr products 
may not require this adjustment of moisture content, and some very dry 
products may actually need the introduction of some additional moisture. 
In FIGS. 1 and 2, drying is accomplished at elevated temperature (in the 
drying environment 3 of FIG. 1 and through the application of heat 3 into 
the chamber 2 in FIG. 2). In FIG. 3, vacuum-dewatering 3 is used to 
accomplish drying. Alternatively, one could store the products in 
environments with relatively low relative humidity, with or without the 
introduction of heat, in order to adjust their moisture content. In either 
case, particularly when heat is used for the purpose of drying, 
exccessively high temperatures can be damaging to the product. The reason 
for the adjustment of the product moisture content is that high moisture 
contents mitigate thorough penetration of the carbon dioxide gas into the 
product and thus prevent effective carbonation. Low moisture contents also 
do not favor the process of carbonation where the carbon dioxide gas 
should be dissolved in water to react with cement. Once the moisute 
content is adjusted, the products are exposed to carbon dioxide or carbon 
dioxide-rich gases in order to achieve pre-carbonation. In FIG. 1, the 
carbon dioxide gas 6 is introduced through a solution 5 which adjusts the 
moisture content of the pre-carbonation environment and the relative 
humidity of the gas. Again, very low or very high moisture contents and 
relative humidities may slow down the rate of carbonation. In FIG. 2, 
pre-carbonation is accomplished through the introductin of the carbon 
dioxide gas 4 (FIG. 2b) into the same chamber 2 which was used for drying 
of products (FIG. 2a) and will subsequently be used for steam curing (FIG. 
2c). The relative humidity of the environment and the gas may have to be 
adjusted for optimum pre-carbonation. The duration of the pre-carbonation 
step depends on a number of factors, including the concentration of 
CO.sub.2, the thickness and composition of the product, the temperature 
and relative humidity of the pre-carbonation environment, the 
characterisitcs of the follow-on accelerated curing process, and the 
targeted properties of the product after accelerated curing. During the 
pre-carbonation process, CO.sub.2 dissolves in the moisute within the 
pores in the product, and initiates a vigorous reaction with cement, which 
takes place at a substantially higher rate than conventional hydration of 
cement and rapidly yields dimensionally and chemically stable hydration 
and carbonation products which provide high levels of strength and 
stability. After the precarbonation process, the products are subjected to 
accelerated curing, which could be, for example, high-pressure steam 
curing in an autoclave 7 (FIG. 1) or steam curing in a chamber 2 (FIG. 
2c). The pre-carbonation process provides the product with sufficient 
strength and stability to withstand the elevated temperature of such 
accelerated curing processes without suffering much damage and 
microcracking. Furthermore, the chemical changes associated with 
pre-carbonation favor the development of an improved structure within 
cement-based materials. One may enhance the effects of pre-carbonation 
through the adjustment of the composition of the product. For example, 
moisture content may be recduced, and lime, ground limestone, or other 
products promoting carbonation and bonding with carbonation products may 
be included in the composition of the cement-based product. 
CONCLUSIONS, RAMIFICATIONS, AND SCOPE 
Accordingly, it can be seen that we have developed a pre-curing 
(pre-carbonation) method which efficiently prepares cement and concrete 
products for accelerated (steam and high-pressure steam) curing, and 
enhances the end product performance characteristics. Our new method 
combines, in a complementary manner, the CO.sub.2 curing with the 
conventional methods of accelerated curing of cement and concrete 
products. The method makes limited changes in the conventional methods of 
accelerated curing of cement and concrete products, and can thus be 
conveniently into the production scheme of the existing cement and 
concrete prducts plants. 
Although the description above contains many specificities, these should 
not be construed as limiting the scope of the invention but as merely 
providing illustrations of some of the presently preferred embodiments of 
this invention. Various other embodiments and ramifications are possible 
within it's scope. For example, the pre-carbonation my not have to be 
preceded by a drying period if the product is produced at a desirable 
moisture content. The terms cement and concrete are used here in a general 
sense. All calcareus and other hydraulic binders capable of recating with 
CO.sub.2 can constitute the binding material in cement and concrete 
products processed according to this invention. Exposure to the carbon 
dioxide gas for the purpose of pre-carbonation in this invention may take 
place under pressure and at different temperatures. Alternatively, vacuum 
may be applied to create a sub-pressure which is maintained to facilitate 
the dffusion of the carbon dioxide gas. The carbon dioxide gas may also be 
replaced with a solusion of CO.sub.2 or carbonic acid which is used during 
mixing or applied to cement or concrete products after molding for the 
purpose of pre-carbonation. The invention is applicable to cement and 
concrete products of different geometries and sizes, processed through 
diverse production techniques, including those utilizing relatively dry or 
wet mixes, those compacting the material through vibration or under 
pressure, those using vacuum-dewatering, injection molding, extrusion, 
conventional molding, and all other methods of making cement and concrete 
products. The accelerated curing methods following pre carbonation also 
can be any applicable method other than steam and high-pressure steam 
curing, including immersion in hot water, and other methods. The invention 
may also be used in application to cement or concrete products with 
inclusions such as glass or wood fibers where carbonation reactions make 
the cement-based matrix more compatible with such fibers and inclusions. 
Thus the scope of the invention should be determined by the appended claims 
and their legal equivalents, rather than by the examples given. 
EXAMPLE 1 
A normal-weight concrete masonry mix was prepared with a coarse aggregate 
(pea gravel)-ro-cement ratio (by weight) of 2.1, fine aggregate (concrete 
river sand)-to-cement ratio of 3.57, and water-to-cement ratio of 0.5. The 
mix was consolidated through vibration under slight pressure (as is 
customary in the production of concrete masonry units) into 50-mm cubes. 
The control pre-set condition consisted of two hours of storage at 30 
degrees centigrade and 65% relative humidity. For the purpose of CO.sub.2 
pre-curing, this pre-set period was replaced with one hour of drying at 30 
degrees centigrade and 50% relative humidity, followed by one hour of 
exposure to an exhaust gas with approximately 20% CO.sub.2 concentration 
at 35 degrees centigrade and 60% relative humidity. In both cases, the 
products, subsequent to the pre-set period or CO.sub.2 pre-curing, were 
subjected to 4 hours of steam curing at 50 degrees centigrade and 100% 
relative humidity. The cubes were then stored in laboratory air and 
subjected to compression tesing at the age of 24 hours. With CO.sub.2 
pre-curing the compressive strength was more than 55 MPa, while with 
conventional pre-setting, the compressive strength was of the order of 40 
MPa. 
EXAMPLE 2 
Cellulose fiber reinforced cement composites were prepared in the form of 
10-mm panels with 8% fiber mass fracion and silica sand-cement ratio of 
0.75 through a slurry-dewatering manufacturing procedure which simulated 
the Hatscheck process. The conventional pre-set period in this case 
consisted of storage in an oven at 50 degrees centigrade for 2 hours. For 
the purpose of CO.sub.2 pre-curing, this pre-set period was replaced with 
1 hour of oven storage at 50 degrees centigrade followed by 1 hour of 
exposure to an environment with 20% CO.sub.2 concentration and 90% 
relative humidity. All panels were subsequently subjected to high-pressure 
steam curing at 150 degrees centigrade for seven hours. The panels were 
subjected to flexure test and moisture movement (as relative humidity 
increased from 30% to 90%) test at the age of 14 days. Conventional 
pre-setting produced a flexural strength of 8 MPa and a moisture movement 
of 0.07%, while CO.sub.2 pre-curing produced a flexural strength of 11 MPa 
and a moisture movement of 0.04%.