PROCESS FOR PRODUCING IMPROVED BUILDING MATERIAL AND PRODUCT THEREOF

Abstract: A process for producing a paste of improved quality. In one aspect, the process comprises allowing hydraulic cement and water to mix sufficiently to reach early age equilibrium, exposing the mixing water and cementitious material to a carbon source selected from the group consisting of CO2, CO, carbonate salts, and mixtures thereof, and then allowing the resulting mixture to again reach early age equilibrium.

Description: PROCESS FOR PRODUCING IMPROVED BUILDING MATERIAL AND PRODUCT THEREOF BACKGROUND 1. Field of the Invention The present invention relates generally to the production of building materials formed from hydraulic cements, clays, and other materials. More particularly, the present invention relates to a process .for improving the quality of cementitious products. 2. Description of Related Art In the construction and building industry, concrete is generally defined as a mixture of two components — aggregates and cement paste. Mortar is defined as a mixture of sand and cement paste. Aggregates include without limitation rock, fibers, steel balls, and other noncementitious materials. The cement paste, which typically includes cementitious material and water, binds the aggregates into a rocklike mass ( ■• concrete") as the paste hardens. In this regard, the technology of concrete is discussed in S.H. Kosmatka, "Design and Control of Concrete Mixtures," 13th Edition, Portland Cement Association, 1988; see, also S. Mindess and J. Francis Young, "Concrete," Prentice-Hall, Inc. (1981). The cement paste hardens because of a chemical hydration reaction between hydraulic cement and water. Examples of typical hydraulic cements known in the art include: the broad family of Portland cements (including ordinary Portland cement without gypsum) , calcium aluminate cements (including calcium aluminate cements without set regulators, e.g. , gypsum) , plasters, silicate cements (including β dicalcium silicates, tricalcium silicates, and mixtures thereof, silicoaluminate cements, pozzolanic cements, etc.) , gypsum cements, phosphate cements, and magnesium oxychloride cements. Other cementitious compositions include fly ash, silica fume, and slag cement. The term hydration as used herein is intended to describe the chemical reactions that take place between the hydraulic cement and water. The chemistry of hydration is extremely complex and can only be approximated by studying the hydration of pure cement compounds. For simplicity in* describing cement hydration, it is often assumed that the hydration of each compound takes place independently of the others that are present in the cement mixture. In reality, cement hydration involves complex interrelated reactions of the each compound in the cement mixture. With respect to Portland cement, the principal cement components are dicalcium silicate (2CaO»Si0 2 , often abbreviated "C 2 S") and tricalcium silicate (3CaO-Si0 2 , often abbreviated "C 3 S") . Portland cement generally contains smaller amounts of tricalcium aluminate (3CaO-Al 2 0 3 , often abbreviated "C 3 A") and tetracalcium aluminum ferrite (4CaO-Al 2 0 3 -Fe 2 0 3 , often abbreviated "C t KF" ) . Cement hydration begins as soon as water is added to the hydraulic cement. On first contact with water, CaO and Si0 2 dissolve from the surface of each C 3 S grain, and the concentration of calcium and hydroxide ions rapidly increases. The pH rises to over 12 very rapidly. The rate of this hydrolysis slows down quickly but continues throughout a dormant period if gypsum or other set regulating chemicals are present. After several hours under normal conditions, the hydration products, calcium hydroxide ("CaOH") and Calcium- Silicate-Hydrate ("C-S-H") , start to form rapidly, and the reaction again proceeds rapidly. Dicalcium silicate hydrates in a similar manner, but is much slower because it is a less reactive compound than C 3 S. For additional information about the hydration reactions, reference is made to F.M. Lea, Chemistry of Cement and Concrete. 3rd edition, pp. 177-310 (1970). The term "setting" is defined as the onset of rigidity or stiffness in fresh concrete or cement paste. Setting is to be distinguished from "hardening," which describes the development of useful and measurable strength. Both setting and hardening are gradual changes which are affected by the continuing cement hydration process. Setting is a transitional period between the states of true fluidity and true rigidity. Standardized penetration tests known in the art are often used to determine setting. The term "initial set," which indicates that the cement paste is beginning to stiffen, occurs approximately 2 to 4 hours after mixing, about the time the hydration reaction proceeds with renewed vigor at the end of the dormant period. Initial set is marked by the beginning of a rapid temperature rise of the cement paste, which reaches a maximum rate at "final set." Final set, which occurs approximately 4 to 8 hours after mixing, indicates an initial hardening of- the cement to the point at which it can sustain some load. After final set, the reaction rate again slows down until it reaches a steady state within approximately 12 to 24 hours after mixing. As mentioned above, when a hydraulic cement is first mixed with water, there is a rapid dissolution of the cement components (CaO, Si0 2 , and A1 Z 0 3 , etc.). After this initial rapid reaction period, the rate of chemical change is very slow and remains slow until the beginning of initial set. The period of slow reaction or dormant period will hereinafter be referred to as "early age equilibrium." It will be appreciated that the exact characteristics of true chemical equilibrium are not fully satisfied by this definition, but the reaction rate is sufficiently slow to be easily distinguished from the initial reaction rates and the subsequent reaction rates. Furthermore, this "early age equilibrium" period is of great significance in the processing and handling' of cementitious materials (see with respect to early age equilibrium reaction rate constants, etc.), E.M. Gartner and H.M. Jennings, "Thermodynamics of Calcium Silicate Hydrates and Their Solutions", J. Am. Ceram. Soc. , 70[10] 473-49 (1987) ; and H.M. Jennings, "Aqueous Solubility Relationships for Two Types of Calcium Silicate Hydrate", J.Am. Ceram. Soc, 69[8] 614-18 (1986). The quality of. concrete depends to a great extent on the quality of the cement paste used in its formation. For example, the quality of the cement paste influences characteristics of the concrete such as its rheology, compressive, and flexural strength, and its ability to resist weathering and cracking. One limitation of hardened cementitious products, such as concrete, is weathering or cracking caused by carbonation. Carbonation occurs when carbon dioxide present in the air penetrates and reacts with the cementitious product to form carbonates. This "post- hydration" carbonation reaction often produces shrinkage which, in turn, leads to cracking. Generally, carbon dioxide penetration in dense, high quality cementitious products is very shallow; therefore, only the surface of these products is affected. However, carbonation is significantly increased in cementitious products having a high water-cement ratio and/or.a porous paste. To improve the quality of cementitious products, it has long been known in the cement and concrete industry to intentionally carbonate hydrating cementitious materials during the curing process. For example, after mixing and processing is completed and after the cementitious product is poured into a mold, it is believed that deliberately exposing the cementitious product to carbon dioxide induces irreversible shrinkage. in those cases where carbon dioxide exposure is intended, the processed cementitious product is poured into a mold, and the mold placed in an autoclave having a carbon dioxide enriched atmosphere. Initially, the carbon dioxide only reaches the surface of the cementitious product. After several days of exposure, well after final set, and depending upon the thickness of the product, the carbon dioxide may penetrate throughout the product. Thus, using such techniques, future shrinkage of the resulting "pre- shrunk" product due to exposure to the atmosphere may be reduced. See, e.g. , G.J. Verbeck, Carbonation of Hydrated Portland Cement. ASTM Special Technical Publication No. 205 (1958) . It has been recognized that the presence of carbonates, such as KC0 3 , CaC0 3 , and NaC0 3 , in the initial concrete mix may retard the cure rate when in small quantities (for example, 0.2% by weight) and may accelerate the cure rate when in larger quantities (for example, 1%-10% by weight) . See, Egide M.M.G. Niel, "Supplementary Paper 11-117 The Influence of Alkali-Carbonate on the Hydration of Cement," 4th Tokyo International Symposium on Cement at pages 472- 486 (1968) . To produce such changes in curing rates, it is known to directly add cement to a solution of sodium carbonate dissolved in water and thereafter begin mixing or to add the carbonate salt upon the onset of mixing. Unfortunately, the effect on the rate of cure using carbonate additives at the onset of mixing is unpredictable. Furthermore, carbonate additives have been shown to have an adverse affect on the compressive strength of the final product (typically 12 hours after mixing) . It is also know that tap water typically contains from about 0.0017% to 0.0042% by volume C0 2 , and that air typically contains from about 0.033% to 1.0% by volume C0 2 . BRIEF SUMMARY OF THE INVENTION The present invention is directed to a process for producing a cement building material of improved strength, handling characteristics and overall performance. One aspect of the process of the present invention includes mixing water and hydraulic cement in the presence of a carbon source selected from the group consisting of C0 2 , CO, carbonate salts, and mixtures thereof after the mixed cement and water have reached early age equilibrium and then subsequently allowing the resulting mixtur to again reach early age equilibrium, with these steps all occurring prior to initial set of the hydrated cement material. Carbonate salts are those which under the mixing conditions release C0 2 , C0 3 " , or C0 3 2' ions into the liquid phase of the mixing cement and water. In another aspect of the present invention, the process includes exposing mixing water and hydraulic cement to a carbon source selected from the group consisting of C0 2 , CO, and mixtures thereof prior to the initial set of the hydrated cement material. Such processes result in the cement/water mixture having additional components of carbon having oxidation states greater than zero. The term "carbon source" as used herein, is intended to mean an amount of carbon in excess of that present in the constituents prior to processing, including cement, water, any admixtures, and the atmosphere contiguous with the mixing cement, water, and any admixtures. A comparison between the compressive strength of cured conventional paste and cured paste produced by the process of the present invention shows the cured paste produced by the process of the present invention to be significantly stronger. In addition, it is believed that the tensile strength, bonding strength, toughness, freeze thaw resistance, and Young's Modulus are also improved. It is further believed that cement pastes prepared according to the processes of the present invention have a superior aggregate-bonding capability and reduced permeability. Building materials incorporating the paste also show a superior resistance to the deleterious affects of natural carbonation. Still further, the new cement based materials produced according to the present invention have many advantages over comparable wood, plastic, metal, and ceramic materials now being used in certain applications. For instance, cementitious materials within the scope of the present invention are low cost, environmentally stable, environmentally compatible, water resistant, fireproof, and formed at low temperatures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional schematic side view of one embodiment of the HEM Mixer. Figure 2 is a cross-sectional schematic view of one embodiment of the HEM Mixer showing certain dimensions. Figure 3 is a top view of the upper set of downthrust blades of one embodiment of the HEM Mixer. Figure 4 is a partial cross-sectional end view of an upper downthrust blade of one embodiment of the HEM Mixer. Figure 5 is a partial cross-sectional end view of an upper upthrust blade of one embodiment of the HEM Mixer. Figure 6 is a top view of the lower downthrust blades of one embodiment of the HEM Mixer. Figure 7 is a partial cross-sectional end view of a lower downthrust blade of one embodiment of the HEM Mixer. Figure 8 is a cross-sectional top view of the lower downthrust blades of one embodiment of the HEM Mixer. DETAILED DESCRIPTION OF THE INVENTION The following description is of the best presently contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken* in a limiting sense. The scope of the invention is best determined by reference to the appended claims. In one aspect of the process of the present invention, the process includes producing a paste by (1) allowing hydraulic cement and water to reach early age equilibrium, (2) exposing the mixing water and hydraulic cement to a carbon source, and (3) allowing the cement, water and carbon source to again reach early age equilibrium, with all of these steps (1) through (3) occurring prior to the initial set, and with the carbon source being selected from the group consisting of C0 2 , CO, carbonate salts, and mixtures thereof. In another aspect of the present invention, the process includes exposing mixing water and hydraulic cement to a carbon source selected from the group consisting of C0 2 , CO, and mixtures thereof prior to initial set of the hydrated cement material. It should be appreciated that in addition to hydraulic cements, other cementitious materials which can be added and processed according to the present invention include fly ash, silica fume, and slag cement. After the water and cementitious material have been exposed to the carbon source for a predetermined time sufficient to allow the pH to drop, the resulting mixture may be allowed to gel as discussed further below. Thereafter the gelled paste can be further processed. It has been observed that for increased strength to be obtained, the hydraulic cement and water often must continue to mix after the carbon source is added and at least until the mixture reaches early age equilibrium. If such mixing is stopped too early, significant increases in strength often do not occur, although other properties are improved such as reduced bleeding and shrinkage. Typically, it takes about 30 seconds to 5 minutes of mixing for hydraulic cement and water to reach early age equilibrium. This time period may change depending upon the vigor of the mixing: vigorous mixing will cause the mixture to reach early age equilibrium faster. Likewise, after the mixture has been exposed to the carbon source, it takes from about a few seconds to about a few minutes to again reach early age equilibrium, depending upon the vigor of the mixing. In one preferred embodiment, the hydraulic cement is Portland cement including one or more of the following compounds: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium alu inoferrite. The Portland cement is in- a powdered form and when mixed with water forms a cementitious paste. The water to cement ratio can range rom 0.1 to about 20, with 0.2 to 10 preferred, and with 0.3 to 3.0 more preferred. The amount of "carbon source" exposed to the mixing cement and water should exceed the amount of carbon normally present in the paste as a result of conventional mixing. For example, in a conventional mortar-type 2 cubic feet capacity drum mixer rotated at 15-20 rpm, with a 0.40 water to cement ratio, the amount of carbon in the paste after 5 minutes of mixing was observed to be about 0.058% by weight; whereas, the present invention caused the concentration of carbon in the paste to exceed 0.058% by weight. In this regard, the present invention includes combining cement and water in the initial mixing process with C0 2 in a quantity greater than that present in the cement, water, any admixtures, and atmosphere contiguous with the mixing cement and water. To obtain improved strength, it is preferred that the carbon source such as C0 2 be added to mixing cement and water after the mixture reaches early age equilibrium. On the other hand, if the carbon source such as C0 2 is added initially, other improved characteristics of the paste are obtained including stability, gelling, and reduced bleeding, among others. * Carbon dioxide -is the currently preferred carbon source exposed to the mixing water and hydraulic cement. Other preferred carbon sources include, without limitation, carbon dioxide enriched air and carbon monoxide enriched air in such amounts as to increase the amount of carbon in the paste above that amount present in the cement before mixing. Carbonate salts are another preferred carbon source if the mixing water and cement are first allowed to reach early age equilibrium before the addition of the carbon source, as in the first aspect of the present invention. Carbonate salts are intended to mean those compounds which, when combined with the hydraulic cement, water, and any admixtures, are capable of releasing C0 2 , C0 3 ~ , or C0 3 z" into the liquid phase of the mixture. Examples of some currently preferred carbonate salts include, without limitation, KC0 3 , CaC0 3 , and NaC0 3 . (For convenience, the carbon source selected from the group consisting of C0 2 , CO, carbonate salts, and mixtures thereof shall be referred to as "Group I Carbon Sources." Group I Carbon Sources without carbonate salts shall be referred to as "Group II Carbon Sources.") In one embodiment of the present invention, water, cement, and air containing carbon dioxide are mixed together by high shear agitation. Carbon dioxide from the air becomes entrained in the cement/water mixture during agitation. In this embodiment, the amount of carbon dioxide exposed to the mixture is dependent upon the length of time and velocity of mixing or agitation. Additional carbon dioxide from a carbon dioxide source (e.g. , gas tank), can be flowed into the high shear mixer. A high shear mixing apparatus capable of entraining carbon dioxide from the air in the cement/water mixture is described in United States Patent Nos. 4,944,595 and 4,946,504 which are incorporated herein by reference. The high shear mixer described in these patents is hereafter referred to as the "HEM Mixer". The HEM Mixer is also believed to facilitate formation of the paste of the present invention by inducing a substantially bimodal size distribution of cement particles in the presence of a Group I Carbon Source. The HEM Mixer not only deflocculates the hydraulic cement particles, but it also breaks down large cement particles into smaller particles so that a new subset of particles is created. These two effects are believed to provide more uniform hydration of the cement particles which results in superior physical characteristics of the final hydrated product. The HEM Mixer is unique from other high shear mixing processes in that it not only mixes cement and water and disperses cement particles, but the HEM Mixer also mixes gas therein. Each speed (rpm) of the HEM Mixer has a characteristic volume flow pattern. The addition or subtraction of cement and/or water does not change the dimension of the overall mixing envelope containing liquid, solid, and gas (the "volume flow pattern") , but merely increases or decreases the volume of gas being mixed with the solid and liquid components. It has also been observed that the HEM Mixer raises the pH of the liquid phase of the mixing cement and water over the pH of conventional mixing of the cement and water at the same time interval. The principal reason for the increase in pH is that the HEM Mixer causes the liquid phase to become more saturated with calcium and hydroxyl ions. It is also believed that other high shear mixers cause this same effect. In such cases of increased calcium and hydroxyl concentration, reactive silicas can be added to facilitate the production of a paste having an increased compressive strength. Reactive silicas are known in the art and include without limitation fly ash, silica fume, slag cement, and glass. When the liquid phase of the cement/water mixture has an increased concentration of calcium and hydroxyl ions, reactive silicas may be added having a weight percent in the range from about 0.25% to about 100%, preferably about 5% to about 60%, and most preferably about 10% to about 40%. Other mixing apparatuses capable of agitation to allow dispersion of the cement particles in water may also be utilized in connection with the present invention including a Waring blender, a Bosch electronic mixer, a food processor, a Hobart mixer, a drum mixer, GKN Colcrete mixer, a paddle mixer, or hand mixing. When using the foregoing mixers, the amount of C0 2 carbon source originating from air which is exposed to the cement/water mixture may not be sufficient for purposes of the present invention since such mixers have a lower surface area contact when compared to the HEM Mixer. Accordingly, as discussed below, an external carbon source such as carbon dioxide or carbon monoxide gas can be used to increase the amount of carbon source in connection with these other mixers. It has also been observed that another aspect of the process of the present invention is contacting a gaseous carbon source during mixing of the cementitious material and water such that the carbon source thoroughly contacts the liquid phase of the cement/water mixture prior to initial set. Preferably, the carbon source is selected from the group consisting of C0 2 , CO, and mixtures thereof, although C0 2 is the most preferred choice. In addition, preferably a high shear mixer causes the mixing, with the HEM Mixer most preferred. It is also within the scope of the present invention to bubble carbon dioxide-containing air through the cement/water mixture during mixing. Alternatively, the cement/water mixture may be exposed to carbon dioxide gas by allowing the carbon dioxide to flow over the mixing hydraulic cement and water such that the atmosphere contiguous with the mixing cement and water is enriched in carbon dioxide. In this embodiment, any apparatus capable of mixing water and cement may be utilized, although the HEM Mixer is preferred. In the embodiment where the contiguous atmosphere is enriched in carbon dioxide, the atmosphere should contain at least 1% by volume carbon dioxide, preferably from about 5% to about 100% by volume carbon dioxide, and more preferably from about 75% to about 100% carbon dioxide. When reference is made herein to exposing the mixing water and cementitious material to a C0 2 carbon source, such reference is meant to include an atmosphere of C0 2 ranging from about 5% by volume to about 100% by volume, unless otherwise stated. The same is intended with respect to CO and C0 2 /CO mixtures. In yet another embodiment within the scope of the present invention, the water is carbonated by C0 2 , CO, or mixtures thereof before the cementitious material is added. The water may be carbonated by agitating water which is exposed to carbon dioxide-containing air, such that the carbon dioxide is entrained in . the water. A similar result may be obtained by bubbling carbon dioxide-containing air, carbon dioxide, or carbon monoxide through the water. It is believed that the introduction of a carbon source such as carbon dioxide or a weak acid including carbonic acid, sulfuric acid, hydrochloric acid, and acetic acid into the mixing water and cement prior to initial set reduces the alkalinity of the cement and water mixture, and induces the formation of an early lattice-work matrix or gel. The formation of an early lattice-work matrix or gel increases cement paste stability, reduces bleeding, and provides other improved properties. The gel is induced by lowering the pH of the mixture by, for example, at least 0.05 pH units and preferably at least 0.1 pH units. Such matrix is observable because the paste of the present invention gels or stiffens much faster than conventional paste not exposed to an elevated amount of carbon dioxide or a weak acid. The gelling or stiffening occurs within the first few minutes of combining cement and water in the presence of the carbon source or weak acid, and after the combined mixture is allowed to remain stationary. It has been observed that if (1) the cement and water are combined until early age equilibrium is reached, and (2) the carbon source is then exposed to the mixing cement and water, and (3) the cement and water are then allowed to remain stationary, then gelation will occur. On the other hand, if the cement and water is not allowed to remain stationary after being exposed to the carbon source and early age equilibrium is allowed to recur, then if the cement and water mixture is thereafter allowed to remain stationary, gelation will not occur. It should be appreciated that by mixing the paste with CO 2 for an extended period of time, the mixture becomes very stable and thick. This is considered a different state of matter than the "set." The stiffness of the paste can be controlled by regulating the amount of Group I and II Carbon Sources exposed to the mixing cement and water. Thus more carbon dioxide tends to increase the stiffness. On the other hand, upon further mixing of the gelled paste, the workability of the paste is increased. A concrete or mortar having an increased compressive strength may be produced as follows: A cementitious paste is formed by exposing a mixture of water and hydraulic cement to a carbon source according to one of the embodiments described above. The cementitious paste is then combined with aggregate or sand. The resulting paste- aggregate mixture is allowed to cure to thereby form a hardened concrete having an increased compressive strength. Testing and comparative analysis according to ASTM standards have been conducted to determine the effect of mixing, water to cement ratio, and carbon dioxide addition on various hardened cement paste properties such as compressive strength, carbon content, viscosity, and shrinkage. Tables 1-4 summarize the results of these tests. Table 1 The Effect Of Mixing On Hardened Cement Paste Properties Strength * High Energy Mixer Table 2 HDBART* 0.40 110 300 4420 6120 0.08 45 *Control Table 4 1 Air atmosphere contiguous with mixing cement. Carbon dioxide saturated stmosphere. It has also been determined that the paste cure may also be regulated by use of Group II Carbon Sources or preferably carbon dioxide exposure. Specifically, as the volume of carbon dioxide mixed with water and cementitious material prior to hydration increases, the rate of cure and hydration can be accelerated. Thus, the cure rate of a predetermined paste may be regulated by exposing a predetermined volume of carbon dioxide to a predetermined water/cement mixture to form a paste. Any of the methods for mixing the water, cementitious material and carbon sources described above may be utilized. The paste is then allowed to cure by hydration. The rate of setting of concrete may also be regulated by mixing aggregate with cementitious paste produced according to this method. The above disclosure generally describes the present invention. A more complete understanding may be obtained by reference to the following specific examples. EXAMPLE 1 Fifty pounds of Portland cement and 20 pounds of water (0.40 water to cement ratio) were mixed by a conventional mortar-type 1.2 cubic feet capacity drum mixer with the shaft revolving at 25 rpm. ' The paste was mixed for 15 minutes, allowed to cure, and the compressive strength according to ASTM standards was measured as 6143 p.s.i at 7 days. As a comparison, and according to the present invention, carbon dioxide was allowed to flow for 15 minutes over the mixing cement and water by holding a tube of flowing carbon dioxide inside the drum mixer and over the mixing paste, so that the atmosphere contiguous with the mixing cement and water was greater than 5% by volume C0 2 . The paste was allowed to cure, and the compressive strength according to ASTM standards was measured as 7480 p.s.i. at 7 days. EXAMPLE 2 The same as Example 1 except the HEM Mixer of Embodiment 2 described below was used at a shaft rpm of 500. The ASTM compressive strength was 8422 p.s.i. at 7 days. In addition to enriching the mixing paste with a carbon source prior to initial set, the resulting paste can be subjected to a localized stress which also is believed to increase the quality of the cured paste. By the term "localized stress," it is meant subjecting the paste to a localized force sufficient to deform or disrupt the paste in a local region of the paste. Methods of inducing such localized stresses include without limitation subjecting the paste to additional mixing with a spoon, or cutting the paste wit a sharp object such as a blade, including metal blades, passing the paste through a screen, adding aggregate to the paste, or extruding the paste. By so doing, further improvements in strength and/or toughness have been achieved. EXAMPLE 3 The paste of Example 2 was subjected to localized stressing by multiple sharp blade cutting through the paste, then cast and cured according to ASTM procedures, The compressive strength was 9190 p.s.i. at 7 days. One way of characterizing the cured paste of the present invention is to determine the carbon content of the paste at an age less than 1 hour after mixing the cement and water. Freshly manufactured hydraulic cement typically has a carbon content of less than about 0.01% by weight, usually in the form of trace organic carbon. The paste of the present invention at the 1 hour age has an increased carbon content over that present in the total volume of unprocessed cement and water to be processed of at least 1%, preferably at least 10% and optimally greater than 100%. It should be appreciated that an important aspect of the present invention is that the carbon source exposure occur before the initial set. Typically, for cement ratios in the range from about 0.30 to about 0.50 and with carbon dioxide as the carbon source, the exposure should occur within about ten (10) minutes after mixing begins. Preferably the mixing and carbon source exposure occurs as fast as possible, optimally within less than 2 minutes, after mixing begins. In one aspect of the present invention, it is desirable that the mixed paste rest or sit after mixing, preferably for about 30 seconds to about 1 hour. Preferably, the resulting paste is then subjected to localized stress before rapid curing begins. In another aspect of the present invention, a carbonate salt, such as NaCo 3 , KC0 3 , CaC0 3 , or other carbonate salts, can be used to increase the short and long-term strength of paste and concrete. To do so, and like the other Group I Carbon Sources, one combines and mixes a hydraulic cement and water for a sufficient amount of time for the calcium,. silica, and alumina of cement to go into the early age equilibrium. After the early age equilibrium is reached, the salt can be added in an amount ranging from about 0.001% to about 25% by weight of cement and preferably from about 0.25 to -about 0.5%. Then the material is further mixed at least until the early age equilibrium is reached again. If the material is not further mixed, but rather is left stationary, the mixture will gel. After allowing the paste to gel, localized stressing can then be applied to increase the toughness of the final cement product. Upon additional mixing, followed by stationary rest, the added Group I and Group II Carbon Sources cause the pH of the mixture to decrease (typically by about 0.1 pH units or more) , thereby causing gelation. Thus, by decreasing the pH, together with letting the mixture to remain stationary, gelation can be induced which improves certain paste properties. For example, gelation provides a paste having decreased bleeding and reduced shrinkage upon curing. The pH can also be decreased to induce gelation by adding sulfuric acid, acetic acid, carbonic acid, hydrochloric acid, other weak acids, carbonate salts, C0 2 and CO or mixtures thereof. THE HEM MIXER The HEM Mixer employs a hollow generally cylindrical housing. Disposed in the radial center of the housing is a rotatable shaft, having its upper end coupled to a shaft- rotating mechanism. The HEM Mixer housing includes a downthrust generating component and an upthrust-generating component. These components act in cooperation with a directional control means to form turbulent liquid (cement and water) mass flow patterns which move in several opposing directions relative to each other within the hollow housing. A preferred embodiment of the inner walls of the housing of the HEM Mixer includes an upper cylindrical portion adjacent to a middle conical portion which is adjacent to a lower conical portion. The downthrust-generating component within the housing includes a single upper and a lower set of downthrust blades. The upthrust-generating component includes an upper and a lower set of upthrust blades. The single upper set of substantially horizontal downthrust blades, which rotate within the cylindrical portion of the housing, are disposed in spaced relation relative to each other and are affixed to the drive shaft. The lower set of downthrust blades, which rotate within the middle conical portion of the housing, includes inclined blades disposed in spaced relation relative to each other. The lower set of downthrust blades have a trailing edge and a leading edge. The upper upthrust blades, which rotate within the cylindrical portion of the housing, are substantially horizontal and are disposed in spaced relation relative to each other and are coupled to the outer ends of the upper set of downthrust blades. The lower set of upper upthrust blades includes substantially vertical blades disposed in spaced relation relative to each other and affixed to the lower portion of the drive shaft. Each of these lower upthrust blades " has a leading edge and a trailing edge. The housing also includes several paired upper and lower vertically disposed baffles extending inwardly from the inner wall of the housing. The lower end of the upper baffle of a baffle pair and the upper end of the lower baffle are spaced from each other, and the upper upthrust blade is rotatable within the baffle space. FIGS. 1-8 show one embodiment of the HEM Mixer. The HEM Mixer generally indicated as 10 includes a generally cylindrical hollow housing or enclosure generally indicated as 12 having a feed inlet generally indicated as 14a formed in the upper portion of the housing to supply dry hydraulic cement to the interior of the hollow housing 12, a feed inlet 14b for supplying water, a discharge outlet generally indicated as 16 formed in the lower portion of the housing to discharge the resulting paste and a longitudinally disposed rotatable shaft 28. As shown in FIG. 1, the inner surface of the housing 12 includes a substantially cylindrical upper portion 66, a conical middle portion 68 and a conical lower portion 70. The housing 12 is configured to operatively house a thrust-generating assembly and a directional control assembly to cooperatively form liquid mass flow patterns moving in various directions relative to each other within the hollow housing 12. The thrust generating assembly includes a downthrust- generating component and an upthrust-generating component. As shown in FIG. 1, the downthrust generating component includes a single upper set and a lower set of blades generally indicated as 18 and 20, respectively. The upthrust-generating component includes an upper and a lower set of blades generally indicated as 22 and 24, respectively. As best shown in FIGS. 1 and 3, the single upper set of downthrust blades 18 includes at least two and preferably six substantially horizontal blades 26 coupled to drive shaft 28 in spaced relation relative to each other by an upper collar 30. The outer end of each of the substantially horizontal upper downthrust blades 26 is interconnected by an upper annular support ring 32. As best shown in FIGS. 1, 3, and 5, the upper set of upthrust blades 22 includes at least six and preferably twelve substantially horizontal blades each indicated as 34 affixed in spaced relation relative to each other on the upper annular support ring 32. As shown in FIG. 5, each of the plurality of substantially horizontal upper upthrust blades 34 includes a leading and trailing edge indicated as 40 and 42, respectively. The smallest distance between the leading edge 40 of the upper baffle 62 is "Gl." The smallest distance between the trailing edge 42 of the upper upthrust blade and the upper edge 64a of the lower baffle 64 is "G2," and "G4" is the smallest distance between the outermost end of the upper upthrust blade 34a and the cylindrical inner wall portion of the housing 66a. As shown in FIGS. 1, 3, and 4, each of the plurality of substantially horizontal upper downthrust blades 26 is substantially pie-shaped in configuration, with each having a leading edge and trailing edge indicated as 36 and 38, respectively. As shown in FIGS. 1, 3, and 6, the lower set of downthrust blades 20 includes at least two and preferably six inclined blades each indicated as 44, with each including a leading edge 46 and a trailing edge 48 and having a configuration similar to that of the substantially horizontal upper downthrust blades 26. The lower portion of each inclined intermediate downthrust blade 44 is attached to the drive shaft 28 by an intermediate collar 50, and the upper portions are affixed to an intermediate support ring 52 such that the intermediate downthrust blades 44 form a substantially conical configuration relative to the drive shaft 28. As described below, the conical plane of the intermediate downthrust blades 44 is substantially parallel to lower portion 70 of the housing 12. To direct the vertical movement of the mixing paste, upper and lower directional control means are provided. As shown in FIG. 1, the upper directional control means preferably includes a plurality, preferably 2-12, and more preferably 8, vertically disposed flat baffles each indicated as 62 extending radially inwardly from the housing 12. The lower directional control means includes a plurality, preferably 2-12, and more preferably 8 vertically disposed flat baffles each indicated as 64 extending radially inwardly from the housing 12. With further reference to FIG. 2, "G5" is the smallest distance between the trailing edge 58 of the lower upthrust blade and the lower free edge 76 of the opposing lower battle. "G3" is the smallest horizontal distance between the trailing edge 48 of the lower downthrust blade and the outer edge of the opposing lower baffle 64. "G6" is the smallest distance between the leading edge 60 of the lower upthrust blade and the lower conical inner wall portion of the housing 70a. As shown in FIGS. 1 and 7, the lower set of upthrust blades 24 includes at least two and preferably six flat substantially vertical blades each indicated as 54 coupled to the drive shaft 28 by lower collar 56. As shown in FIG. 1, the outer portion of each flat substantially vertical lower upthrust blade 54 includes an upper and lower edge indicated as 58 and 60, respectively, inclined relative to each other. The lower edge is preferably parallel to the lower conical wall portion 70 of the housing. It should also be appreciated that the lower set of upthrust blades includes the upper and lower inclined edges 58 and 60. This configuration in the present invention further adds to the increased turbulence and multiplicity of the mechanical and liquid shear zones which further impact upon the ability of the HEM Mixer. As shown in FIG. 1, the lower inclined portion 70, the inner edges 72 of the lower portion 74 of the lower baffles 64, the conical plane formed by the lower downthrust blades 44 and the lower edges 60 of that flat substantially parallel relative to each other. Similarly, the lower edges 76 of the lower portion 74 of the lower baffles 64 are substantially parallel to the upper edges 58 of the flat substantially vertical lower upthrust blades 54. Also with reference to FIG. 2, three radii Rl, R2, and R3, three heights HI, H2, and H3, and two vertical distances H2a and H3a can be defined. Rl is the radius of the cylindrical portion of the inner wall of the housing, R2 is the smallest radius of the middle conical portion of the inner wall of the housing within the plane containing the upper edge 54a of the lower upthrust blade, and R3 is the smallest radius of the lower conical portion of the inner wall of the housing along the plane containing the lower edge 54b of the lower upthrust blade. HI is the vertical distance, along the shaft, between the horizontal plane containing the leading edge of the upper upthrust blade and lower end of the cylindrical portion 66 of the housing, H2 is the distance, along the shaft 28, between the horizontal plane containing the lower end of the middle conical portion 68 of the housing, and H3 is the distance, along the shaft 28, between the horizontal plane containing the lower end of the middle portion of the housing 68 and the horizontal plane containing the lower end of the lower conical portion 70 of the housing. It should be appreciated that the middle and lower conical portions can be modified and changed to a single spherical portion having the same volume. In such case H2 is defined as the smallest distance between the lowest point on the housing shaft where the inner radius is Rl and the highest point on the shaft where the inner radius is R2; and H3 is the smallest distance between the lowest point on the housing shaft where the inner radius is R2 and the highest, point on the shaft where the inner radius is R3. In general, the upper downthrust/upper upthrust blade assembly must be higher than the middle downthrust blades which in turn must be higher than the lower upthrust blades. With reference to FIGS. 1 and 2, H2a is the smallest vertical distance between the height (along the shaft) containing the horizontal plane of the uppermost portion of the lower downthrust blade and the horizontal plane containing the largest radius of the middle conical portion 68 of the housing. H3a is the smallest vertical distance between the bottom 80 of the mixer and the lowest portion of the lower upthrust blade. In order to obtain the best results from the HEM Mixer, it is necessary that the radii, height, and vertical distance dimensions be chosen to be within certain ratios. For Rl ranging from 4.0 to 48.0 inches, the ratio of Rl to HI ranges from 0.39 to 0.45, the ratio of Rl to R2 ranges from 0.80 to 0.83, the ratio of Rl to H2 ranges from 0.59 to 0.61, the ratio of Rl to R3 ranges from 0.36 to 0.41, the ratio of Rl to H3 ranges from 0.30 to 0.32, the ratio of Rl to H2a ranges from 0.002 to 1.0, and the ratio of Rl to H3a ranges from 0.001 to 1.0. The gap distances G1-G6 shown in FIG. 2 constitute six mechanically induced shear zones. For Rl ranging from 4.0 to 48.0 inches, G1-G6 should range from 0.1 to 2.0 inches. The preferred ranges for G1-G6 are Gl - 0.20 ± 0.125, G2 = 0.20 ± 0.125, G3 = 0.20 ± 0.125, G4 = 0.38 ± 0.125, G5 = 0.25 ± 0.125, and G6 = 0.50 ± 0.125 inches. With reference to FIG. 2, it should be appreciated that the angle between the outer face of the support ring 32 and the trailing edge of the upper upthrust blade ranges from 45= _ go-, and is preferably 80°. For Rl = 8.0 inches, shaft rpm can range from about 300 rpm to about 900 rpm, and preferably about 500 rpm. For Rl = 24 inches, shaft rpm can range from about 150 rpm to about 250 rpm, and preferably about 165 rpm. In these preferred cases, G1-G3 are 0.20, G4 is 0.38, G5 is 0.25, and G6 is 0.50. In addition to the dimensions discussed above, the relationship between shaft rpm and Rl should be such that the product of Rl multiplied by shaft rpm ranges from about 2,000 to about 7,000, with the preferred range being from about 4,000 to about 5,000. The stationary paste volume should range from a height of 0.5H1 + H2 + H3 as a minimum to a height of HI + H2 + H3 + (4 x Rl) as a maximum. The preferred range is between a resting volume height on the shaft of HI + H2 + H3 + 0.5R1 to as resting volume height on the shaft of HI + H2 + H3 + 2.5R1. Mix time ("MT") is proportional to volume, water to cement ratio, and rpm. The relationship is such that the higher the water to cement ratio the less critical the mix time, and the higher the volume the longer the mix time. In general, the paste produced by the generator of the present invention and the rpm and volume ranges of the present invention, in a preferred mix time being 60 to 120 seconds. Water to Portland cement ("W/C") ratio changes from 0.20 to 2.00 with a preferred range of 0.30 to 0.50. This range is using typically available hydraulic cement and typically available water, but without the effect of admixtures or other chemicals. The addition of such and other chemicals may alter the total range and/or preferred range of water to cement ratios. Lastly, it should be appreciated that the shaft is rotated by -mechanism means for rotation 78 which is apparent to those of skill in the art. HEM Embodiment 1 A cement pate generator was constructed wherein, in inches, Rl = 24.75, HI = 9.75, R2 = 19.81, H2 = 14.50, R3 = 9.00, H3 = 7.50, H2a = 0.11, H3a = 0.002, G1-G3 = 0.20, G4 = 0.38, G5 = 0.25, and G6 = 0.50. HEM Embodiment 2 A cement paste generator was constructed wherein, in inches, Rl = 8.00, HI = 3.0, R2 = 6.50, H2 = 4.90, R3 = 3.25, H3 = 2.37, H2a = 0.06, H3a = 0.07, G1-G3 = 0.20, G4 = 0.38, G5 = 0.25, and G6 = 0.50. A cement paste generator was constructed wherein, in inches, Rl = 24.00, HI = 11.75, R2 = 19.81, H3a = 0.01, GI¬ GS = 0.20, G4 = 0.38, G5 = 0.25, and G6 = 0.50. The present invention is not to be limited in scope by the examples which are intended as illustrations of one aspect of the invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims. WHAT IS CLAIMED IS:

Claims: 1. A method for preparing a cementitious material comprising the steps of,: (a) combining hydraulic cement and water to form a cement/water mixture, said mixture being allowed to reach early age equilibrium; (b) exposing the cement/water mixture to a carbon source; and thereafter (c) allowing the cement/water mixture to reach early age equilibrium again prior to initial set. 2. A method • for preparing a cementitious material as defined in claim 1, wherein the carbon source is selected from the group consisting of C0 2 , CO, C0 2 enriched air, and mixtures thereof. 3. A method for preparing a cementitious material as defined in claim 2, wherein the hydraulic cement and water are combined in an atmosphere enriched with carbon dioxide. 4. A method for preparing a cementitious material as defined in claim 3, wherein the carbon dioxide enriched atmosphere includes from about 5% to about 100% carbon dioxide by volume. 5. A method for preparing a cementitious material as defined in claim 1, wherein the carbon source includes a carbonate salt. 6. A method for preparing a cementitious material as defined in claim 1, wherein the water to cement ratio of the cement/ ater mixture is in the range from about 0.1 to about 20. 7. A method for preparing a cementitious material as defined in claim 1, wherein the water to cement ratio of the cement/water mixture is in the range from about 0.3 to about 2. 8. A method for preparing a cementitious material as defined in claim 1, wherein the hydraulic cement and water are combined to form the cement/water mixture by high shear agitation. 9. A method for preparing a cementitious material as defined in claim 1, further comprising the step of adding an aggregate to the cement/water mixture after the second occurrence of early age equilibrium. 10. A method for preparing a cementitious material as defined in claim 1, further comprising the step of adding sand to the cement/water mixture after the second occurrence of early age equilibrium. 11. A method for preparing a cementitious material comprising the steps of: (a) combining hydraulic cement and water to form a cement/water mixture, said mixture being allowed to reach early age equilibrium; (b) exposing the cement/water mixture to a carbon source; and thereafter (c) allowing the cement/water mixture to gel. 12. A method for preparing a cementitious material as defined in claim 11, further comprising the step of subjecting the gelled cement/water mixture to a localized stress. 13. A method for preparing a cementitious material as defined in claim 12, wherein the localized stress is caused by extruding the cement/water mixture. 14. A method for preparing a cementitious material as defined in claim 12, wherein the localized stress is caused by cutting the cement/water mixture. 15. A method for preparing a cementitious material as defined in claim 12, wherein the localized stress is caused by passing the cement/water mixture through a screen. 16. A method for preparing a cementitious material as defined in claim 12, wherein the localized stress is caused by adding aggregate to the cement/water mixture. 17. A method for preparing a cementitious material as defined in claim 11, wherein the gelation of the cement/water mixture is induced by decreasing the pH of the cement/water mixture at least 0.05 pH units. 18. A method for preparing a cementitious material as defined in claim 11, wherein the gelation of the cement/water mixture is induced by adding a weak acid to the cement/water mixture prior to initial set. 19. A method for preparing a cementitious material as defined in claim 11, wherein the carbon source is selected from the group consisting of C0 2 , CO, C0 2 enriched air, and mixtures thereof. 20. A method for preparing a cementitious material as defined in claim 11, wherein the hydraulic cement and water are combined in an atmosphere enriched with carbon dioxide. 21. A method for preparing a cementitious material as defined in claim 20, wherein the carbon dioxide enriched atmosphere includes from about 5% to about 100% carbon dioxide by volume. 22. A method for preparing a cementitious material comprising the steps of: (a) enriching a quantity of water with C0 2 ; and (b) combining hydraulic cement with said water to form a cement/water mixture. 23. A method for preparing a cementitious material as defined in claim 22, wherein the water is enriched with C0 2 by agitating the water to entrain carbon dioxide from a carbon dioxide-containing atmosphere. 24. A method for preparing a cementitious material as defined in claim 22, wherein the water is enriched with C0 2 by bubbling carbon dioxide-containing air through the water. 25. A method for preparing a cementitious material comprising the steps of: (a) mixing hydraulic cement and water under high shear conditions such that a substantially bimodal size distribution of dispersed hydraulic cement particles is created in the water; (b) exposing the resulting cement/water mixture to a .carbon dioxide enriched atmosphere; and (c) allowing the resulting cement/water mixture to remain stationary and form a gel prior to initial set of the cement/water mixture. 26. A method for regulating the rate of hydration of hydraulic cement comprising exposing mixing water and hydraulic cement to a carbon source selected from the group consisting of C0 2 , CO, and mixtures thereof prior to initial set. 27. A method for reducing the shrinkage of cured cement paste comprising exposing mixing water and hydraulic cement to a carbon source selected from the group consisting of C0 2 , Co, and mixtures thereof prior to initial set. 28. A method for reducing the bleeding of cement paste comprising exposing mixing water and hydraulic cement to a carbon source selected from the group consisting of C0 2 , CO, and mixtures thereof prior to initial set. 29. A method for preparing a cementitious material comprising the steps of: (a) mixing hydraulic cement and water under high shear conditions such that the pH of the resulting cement/water mixture is increased; and (b) adding a reactive silica to the cement/water mixture. 30. A method for preparing a cementitious material as defined in claim 29, wherein the reactive silica is selected from the group consisting of fly ash, silica fume, slag cement, glass, and mixtures thereof. 31. A method for preparing a cementitious material as defined in claim 29, wherein from about 0.25% to about 100% of reactive silica by weight of the cement is added to the cement/water mixture. 32. A method for preparing a cementitious material as defined in claim 29, wherein from about 10% to about 40% of reactive silica by weight of the cement is added to the cement/water mixture. 33. A cementitious material prepared by the process comprising the steps of: (a) combining hydraulic cement and water to form a cement/water mixture, said mixture being allowed to reach early age equilibrium; (b) exposing the cement/water mixture to a carbon source; and thereafter (c) allowing the cement/water mixture to reach early age equilibrium again prior to initial set. 34. A cementitious material prepared by the process as defined in claim 33, wherein the carbon source is selected from the group consisting of C0 2 , CO, C0 Z enriched air, and mixtures thereof. 35. A cementitious material prepared by the process as defined in claim 34, wherein the hydraulic cement and water are combined in an atmosphere enriched with carbon dioxide. 36. A cementitious material prepared by the process as defined in claim 35, wherein the carbon dioxide enriched atmosphere includes from about 5% to about 100% carbon dioxide by volume. 37. A cementitious material prepared by the process as defined in claim 33, wherein the carbon source includes a carbonate salt. 38. A cementitious material prepared by the process as defined in claim 33, wherein the water to cement ratio of the cement/water mixture is in the range from about 0.3 to about 2. 39. A cementitious material prepared by the process as defined in claim 33, wherein the hydraulic cement and water are combined to form the cement/water mixture by high shear agitation. 40. A cementitious material prepared by the process as defined in claim 33, further comprising the step of adding an aggregate to the cement/water mixture after the second occurrence of early age equilibrium. 41. A cementitious material prepared by the process as defined in claim 33, further comprising the step of adding sand to the cement/water mixture after the second occurrence of early age equilibrium. 42. A cementitious material prepared by the process comprising the steps of: (a) combining hydraulic cement and water to form a cement/water mixture, said mixture being allowed to reach early age equilibrium; (b) exposing the cement/water mixture to a carbon source; and thereafter (c) allowing the cement/water mixture to gel. 43. A cementitious material prepared by the process as defined in claim 42, further comprising the step of subjecting the gelled cement/water mixture to a localized stress.

Inventors: HODSON SIMON K, JENNINGS HAMLIN M, HODSON, SIMON, K., JENNINGS, HAMLIN, M.
Applicants: CONCRETE TECHNOLOGY CORP, CONCRETE TECHNOLOGY CORPORATION
Application year: 1990
Main classification: B28C5
IPC codes: B01F5, B01F7, B28B11, B28C5, C04B7, C04B22, C04B38, C04B40