The invention generally relates to cements and, more particularly, to low-clinker factor cements.
The rapid rate of infrastructure development in many parts of the world has resulted in an enormous demand for portland cement. In order to reduce its environmental impact, both in terms of CO2 emissions and energy consumption, it is desired that the clinker factor of cement be reduced. A number of strategies predominantly based on the use of high volumes of cement replacement materials such as fly ash and blast furnace slag have been practiced by the concrete industry to reduce cement use in concrete.
In recent years there has been increased interest in the use of limestone (CaCO3) powder as a partial cement replacement material. Limestone has the advantages of being abundant, inexpensive, and avoiding the environmental costs associated with portland cement. ASTM standards historically allowed up to 5% limestone (mass basis) in cement, and it has been shown that such low replacement levels can result in comparable or better properties as compared to plain cements. Recently ASTM C 595-12 has specified a Type IL cement that can include up to 15% of limestone powder (mass basis) as a cement replacement material. Unfortunately, high replacement levels by limestone can compromise properties, such as in terms of strength reduction.
It is against this background that a need arose to develop the low-clinker factor cements described herein.
One aspect of this disclosure relates to a manufacturing process of a “low cement content” product. In one embodiment, the manufacturing process includes: (1) forming a cementitious mixture by combining a cement, a carbonate source, and an aluminous source; and (2) curing the cementitious mixture to form the product, which can be cement paste, mortar, or concrete. The carbonate source is included in an amount greater than 20% by weight of solids combined in the cementitious mixture.
In another embodiment, the manufacturing process includes: (1) forming a cementitious mixture by combining (a) a cement in an amount corresponding to 30% to 80% by weight of solids in the cementitious mixture, (b) an aluminous source, and (c) a carbonate source in an amount corresponding to at least 40% of a remaining weight of solids combined with the cement; and (2) curing the cementitious mixture to form the product.
Another aspect of this disclosure relates a “low cement content” concrete. In one embodiment, the concrete is formed by: (1) forming a cementitious mixture by combining a cement, a carbonate source, and an aluminous source; and (2) curing the cementitious mixture to form the concrete. The carbonate source is included in an amount greater than 20% by weight of solids combined in the cementitious mixture.
In another embodiment, the concrete is formed by: (1) forming a cementitious mixture by combining (a) a cement in an amount corresponding to 30% to 80% by weight of solids in the cementitious mixture, (b) an aluminous source, and (c) a carbonate source in an amount corresponding to at least 40% of a remaining weight of solids combined with the cement; and (2) curing the cementitious mixture to form the concrete.
Other aspects and embodiments of this disclosure are also contemplated. The foregoing summary and the following detailed description are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.
For a better understanding of the nature and objects of some embodiments of this disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.
Embodiments of this disclosure provide systematic approaches to develop viable long-term solutions to reduce the use of cement in concrete and other cement products, such as cement pastes and mortars. In some embodiments, a motivation in this regard is to expand the use of powdered limestone and similar carbonaceous materials in concrete. This approach is deemed particularly attractive as the abundance of limestone and similar carbonaceous materials in nature propels them as desirable materials that can be used in isolation or in conjunction with other materials to achieve a high cement-reduction (or replacement) level in concretes, such as from about 16% to about 70%, from about 20% to about 70%, from about 25% to about 70%, from about 30% to about 70%, from about 35% to about 70%, from about 40 to about 70%, from about 45% to about 65%, or from about 45% to about 55% (mass basis). This approach, which emphasizes the application of multiple-material solutions, can optimize the use of natural-and-waste materials by selecting them for use in concrete based upon their constituent chemistry and availability to produce sustainable concretes with engineering properties as desired for infrastructure construction. These efforts can reduce the impact of: (1) cement production on CO2 emissions and climate change, and (2) CO2 taxation and environmental policy on the construction industry, which would impede growth in the infrastructure sector.
Some embodiments of this disclosure provide strategies to engineer sustainable concretes with a reduced cement content while maintaining properties (e.g., compressive strength at both early and later ages) comparable to those of traditional “pure-cement” concretes. In developing sustainable concretes including large quantities (on mass or volume basis) of cement replacement materials, strategies can involve relating the constituent chemistry and physical properties of the components to the rate of chemical reactions and the resultant liquid and solid phase assemblages. Also, the strategies can develop an understanding of the pore structure and its relation to the macroscopic engineering properties. In some embodiments, high replacement levels of cement can be achieved through the synergy and interaction of large quantities of limestone (or other carbonate-rich materials) when used individually or in combination with at least one supplementary cementing material (SCM). Specifically, limestone can be rendered a reactive component of a cementitious mixture, by manipulation of the overall cement (binder) chemistry. This allows for the use of limestone, which is otherwise chemically inert, as a reactive part of the binder. The approach in some embodiments is based on altering the cement chemistry to promote the formation of certain binder phases, which can provide strength and structure to the overall cementitious mixture. Some embodiments can be implemented by blending or inter-grinding cement (e.g., in the form of powder or clinker) and limestone, along with a SCM. The SCM can be a chemical activator to promote reactions with limestone (carbonate).
In some embodiments, proportioning of sustainable concretes involves blending or otherwise incorporating limestone powder to cement. This strategy is particularly attractive as: (1) the wide-spread and abundant availability of limestone in the earth's crust allows for the incorporation of an ecologically inert material to replace a part of the cement in concrete, and (2) quarried limestone involves little processing other than crushing and powdering before use in concrete. This is a significant improvement compared to cement production because raw limestone powder utilization reduces CO2 emissions associated with the decarbonation of the limestone in the cement kiln, and the energy involved for grinding quarried limestone is significantly lower than that involved to heat the cement kiln to about 1450° C.
Limestone powder can serve as a physical filler in cementitious mixtures. The increase in the effective water-to-cement ratio (dilution) facilitated by the use of limestone powder can result in enhanced early-age cement hydration (filler-effect). However, in addition to filler-effects, limestone addition can also induce chemical effects, which can be attributed to carbonate (CO32−) anion-substitutions in the monosulfoaluminate (SO42−AFm) phase to produce carboaluminate structures (CO32−AFm). According to some embodiments, AFm can refer to one or more members of a family of hydrated calcium aluminate hydrate phases (aluminate-ferrite-monosubstituent phases). Its crystalline layer structure can be derived from that of portlandite, Ca(OH)2, but with about one third of the Ca2+ ions replaced by a trivalent ion, nominally Al3+ or Fe3+. The resulting charge imbalance gives the layers a positive charge, which is compensated by intercalated anions; the remaining interlayer space is filled with H2O. In some embodiments, its general formula can be represented as [Ca2(Al,Fe)(OH)6].X.xH2O, where X represents a monovalent ion or 0.5 of a divalent interlayer anion, and x represents the number of water molecules. While the SO42−AFm (Ca4Al2(OH)12(SO4).6H2O, monosulfoaluminate) is the form most commonly encountered in cement-based mixtures, anion-substitutions of the sulfate by CO32− can result in the formation of alternate AFm phases such as calcium monocarboaluminate (CO32− AFm; Ca4Al2(OH)12(CO3).5H2O) and calcium hemicarboaluminate (AFm including OH− and CO32− in about 2:1 molar ratio; Ca4Al2(OH)13(CO3)0.5.5.5H2O). The carbonate (CO32−) anion-substitutions can also enhance the quantity of ettringite (AFt; aluminate-ferrite-trisubstituted; trigonal crystalline compound that can be represented as (Ca6Al2(OH)12(SO4)3.26H2O)) formed due to the release of SO42− species from the monosulfoaluminate phase, and can also increase the total solid volume of the reaction products formed due to increased (either, or both, hemi and mono) carboaluminate phase formation. This response can be related to the chemistry and mass-content of the reactive (e.g., cement, limestone, and chemical activator) components. Leveraging this chemistry and promoting the activation of carbonate-rich materials using suitable chemical activators can pave the way for the better utilization of abundant natural materials such as limestone (powder) as more than a filler in concrete.
Typically, portland cements are constituted to a sulfate-to-alumina (SO3/Al2O3, SA, molar mass-basis) ratio ranging between about 0.5 and about 0.9. The SA ratio indicates the quantity/balance of (mono- and tri-) sulfoaluminate phases that can be produced in a cementitious mixture. Some embodiments provide the reduction of portland cement content in concretes, such as on the order of about 40-70% (mass basis) by: (1) the addition of particle-size classified limestone powders at high levels, such as ranging between about 20% and about 50% or about 25% and 50% (mass basis), and (2) altering the SA ratio of the binder by controlled additions of one or more aluminous-containing (or alumina (Al2O3)-containing) materials, such as calcined or non-calcined clays (e.g., Kaolin group, such as kaolinite, dickite, halloysite, and nacrite; calcined or dehydroxylated clays of Kaolin group, such as metakaolin; Smectite group, such as montmorillonite, nontronite, and saponite; Illite group, such as illite and clay-micas; Chlorite group; and other clays such as sepiolite and attapulgite), alternate cements (e.g., high-alumina cements or calcium aluminate cements), steel and aluminum slags, fly ash (Class F or other fly ash conforming to ASTM C 618), non-standard fly ashes such as CFBC fly ash (e.g., with high aluminate content and potentially incompatible with ASTM C 618), municipal waste incineration ash, aluminum dross, chemical agents (e.g., calcined aluminas and inorganic salts), and other hydratable aluminous (or alumina) sources. In some embodiments, suitable aluminous (or alumina) sources include those having an alumina content of at least about 30% by weight, such as at least about 35% by weight, at least about 40% by weight, or at least about 45% by weight, and up to about 95% by weight or more. The use of aluminous (or alumina) sources can amplify carboaluminate phase formation and, thus, aid in the development of sustainable concretes with significantly reduced cement contents. In place of, or in combination with, limestone, other carbonate sources can be used, such as other similar carbonaceous materials including but not restricted to magnesium carbonates, dolomite, high magnesium limestone, their variants and derivatives (either of natural or synthetic origin), and other carbonate-rich materials. In some embodiments, suitable carbonate (or calcium carbonate) sources include those having a calcium carbonate content of at least about 35% by weight, such as at least about 40% by weight, at least about 50% by weight, at least about 60% by weight, at least about 70% by weight, or at least about 80% by weight, and up to about 97% by weight or more. In some embodiments, suitable carbonate (or calcium carbonate) sources include those in powder form and having a median particle size in the range of about 0.1 μm to about 100 μm, such as from about 0.1 μm to about 20 μm, from about 0.1 μm to about 15 μm, from about 0.1 μm to about 10 μm, from about 0.1 μm to about 7 μm, from about 0.1 μm to about 5 μm, from about 0.1 μm to about 3 μm, or from about 0.5 μm to about 3 μm.
Aspects of the some embodiments can be further understood using a phase-description diagram developed using a thermodynamic modeling package (GEMS-PSI) as shown in
b considers the role of alumina content in a cementitious mixture that initially includes 40% of limestone powder and 60% of cement by mass (water-to-binder mass ratio of 0.50). Here, the cement content is systematically reduced by replacement with hydratable alumina (SA ranges between about 0.56 at about 0% alumina and about 0.07 at about 15% alumina replacement (mass basis)). At high pH levels, the alumina provides readily-soluble aluminate ions (Al(OH)4) in (aqueous) solution, which in the presence of sufficient calcium and carbonate species promotes the formation of monocarboaluminate phase. The role of the alumina content is revealed as the increased volume of monocarboaluminate formed (with increasing alumina replacement) promotes an increased solid hydrate volume as compared to mixtures which include alumina intrinsic solely to the cement (0% replacement in
Other than aspects related to phase assemblage information, the phase diagrams can also provide insights related to the durability response of the mixture. As an example, consider the portlandite-excess and deficient regions shown in
According to some embodiments, manufacturing of a “low cement content” concrete is carried out by incorporating at least one carbonate source (e.g., limestone) and at least one aluminous source (e.g., metakaolin) into a cementitious mixture including clinker (e.g., as a powder) and water. Desired amounts of either, or both, the carbonate source and the aluminous source can be added into a mixing water used to prepare the concrete. Either, or both, the carbonate source and the aluminous source can be added directly into a cement, or a suitably optimized cement clinker by addition or replacement as a powder. Examples of cements include portland cement, including ASTM C150 compliant ordinary portland cements (OPCs) such as Type I OPC, Type Ia OPC, Type II OPC, Type II(MH) OPC, Type IIa OPC, Type II(MH)a OPC, Type III OPC, Type IIIa OPC, Type IV OPC, and Type V OPC, as well as blends or combinations of two or more of such OPCs, such as Type I/II OPC, Type II/V OPC, and so forth. Other examples of cements include energetically modified cements, portland cement blends, and non-portland hydraulic cements including calcium aluminate/sulfoaluminate cements amongst others.
In some embodiments, at least one carbonate source (e.g., limestone) is incorporated in an amount wcarbonate greater than about 15% by weight of all solids (e.g., cement+carbonate source(s)+aluminous source(s)) (dry mass basis) combined with water to form a cementitious mixture, such as at least or greater than about 20% by weight, at least about 23% by weight, at least about 25% by weight, at least about 27% by weight, at least about 30% by weight, at least about 33% by weight, or at least about about 35% by weight, and up to about 40% by weight, up to about 45% by weight, or more. For example, wcarbonate can be in the range of 15%<wcarbonate≦45%, 20%<wcarbonate≦45%, 20%<wcarbonate≦40%, 20%<wcarbonate≦35%, or 20%<wcarbonate≦30%. In some embodiments, two or more different carbonate sources are incorporated in a combined amount wcarbonate,combined greater than about 15% by weight of all solids (e.g., cement+carbonate source(s)+aluminous source(s)) (dry mass basis) combined with water to form a cementitious mixture, such as at least or greater than about 20% by weight, at least about 23% by weight, at least about 25% by weight, at least about 27% by weight, at least about 30% by weight, at least about 33% by weight, or at least about about 35% by weight, and up to about 40% by weight, up to about 45% by weight, or more. For example, wcarbonate,combined can be in the range of 15%<wcarbonate,combined≦45%, 20%<wcarbonate,combined≦45%, 20%<wcarbonate,combined≦40%, 20%<wcarbonate,combined≦35%, Or 20%<wcarbonate,combined≦30%.
In some embodiments, at least one aluminous source (e.g., metakaolin) is incorporated in an amount waluminous greater than about 1% by weight of all solids (e.g., cement+carbonate source(s)+aluminous source(s)) (dry mass basis) combined with water to form a cementitious mixture, such as at least or greater than about 3% by weight, at least about 5% by weight, at least about 7% by weight, at least about 10% by weight, at least about 13% by weight, at least about 15% by weight, or at least about about 17% by weight, and up to about 20% by weight, up to about 25% by weight, or more. For example, waluminous can be in the range of 1%<waluminous≦25%, 3%<waluminous≦25%, 5%<waluminous≦25%, 7%<waluminous≦25%, 10%<waluminous≦25%, or 10%<waluminous≦20%. In some embodiments, two or more different aluminous sources are incorporated in a combined amount waluminous,combined greater than about 1% by weight of all solids (e.g., cement+carbonate source(s)+aluminous source(s)) (dry mass basis) combined with water to form a cementitious mixture, such as at least or greater than about 3% by weight, at least about 5% by weight, at least about 7% by weight, at least about 10% by weight, at least about 13% by weight, at least about 15% by weight, or at least about about 17% by weight, and up to about 20% by weight, up to about 25% by weight, or more. For example, waluminous,combined can be in the range of 1%<waluminous,combined≦25%, 3%<waluminous,combined≦25%, 5%<waluminous,combined≦25%, 7%<waluminous,combined≦25%, 10%<waluminous,combined≦25%, or 10%<waluminous≦20%.
In some embodiments, a cement is incorporated in an amount wcement corresponding to about 30% to about 84% by weight of all solids (e.g., cement+carbonate source(s)+aluminous source(s)) (dry mass basis) combined with water to form a cementitious mixture, such as from about 30% to about 80%, about 30% to about 75%, about 30% to about 70%, about 30% to about 65%, about 30% to about 60%, about 35% to about 60%, about 45% to about 60%, about 35% to about 55%, or about 45% to about 55%. In some such embodiments, at least one carbonate source (e.g., limestone) is incorporated in an amount corresponding to at least about 40% of a remaining weight of solids (e.g., carbonate source(s)+aluminous source(s)) combined with the cement, such as at least about 45%, at least about 50%, at least about 55%, at least about 60%, or at least about 65%, and up to about 70%, up to about 75%, or up to about 80% or more. In some such embodiments, at least one carbonate source and at least one aluminous source are combined in a mass or weight ratio in a range of about 2:3 to about 6:1, such as from about 2:3 to about 5:1, from about 2:3 to about 4:1, from about 9:11 to about 3:1, from about 1:1 to about 7:3, from about 11:9 to about 7:3, from about 3:2 to about 7:3, or from about 13:7 to about 7:3.
Once formed, a cementitious mixture is cured (e.g., water-cured) to promote hydration reactions to form a resulting “low cement content” concrete. In some embodiments, curing includes reacting at least one carbonate source (e.g., limestone) to form one or more binder phases, and an extent of the carbonate source that is reacted (as determined based on a mass fraction (dry mass basis) of the carbonate source in the concrete after curing relative to a mass fraction (dry mass basis) of the carbonate source in the cementitious mixture before curing) is at least about 1%, such as at least about 3%, at least about 5%, at least about 7%, at least about 10%, at least about 13%, at least about 15%, at least about 17%, at least about 20%, at least about 23%, at least about 25%, at least about 27%, at least about 30%, at least about 40%, or at least about 50%, and up to about 80%, up to about 90%, or more. In some embodiments, the concrete includes one or more AFm phases, including a monocarboaluminate phase in an amount of at least about 1% by weight (dry mass basis), such as at least about 2% by weight, at least about 3% by weight, at least about 5% by weight, at least about 7% by weight, or at least about 10% by weight, and up to about 15% by weight or more. In some such embodiments, the concrete also includes stratlingite phase in an amount of at least about 0.1% by weight (dry mass basis), such as at least about 0.2% by weight, at least about 0.3% by weight, at least about 0.5% by weight, at least about 0.7% by weight, or at least about 1% by weight, and up to about 1.5% by weight or more. In some such embodiments, any portlandite phase is included in the concrete in an amount no greater than about 20% by weight (dry mass basis), such as no greater than about 17% by weight, no greater than about 15% by weight, no greater than about 13% by weight, no greater than about 10% by weight, no greater than about 7% by weight, or no greater than about 5% by weight, and down to about 1% by weight or less.
Surprisingly, and despite the high replacement level of cement by limestone, a resulting “low cement content” concrete is a high strength material, with a compressive strength of at least about 15 MPa, such as at least about 20 MPa, at least about 25 MPa, at least about 30 MPa, at least about 35 MPa, at least about 40 MPa, at least about 45 MPa, at least about 50 MPa, at least about 55 MPa, at least about 60 MPa, or at least about 65 MPa, and up to about 70 MPa, up to about 80 MPa, or more. In some embodiments, a resulting “low cement content” concrete has a compressive strength that is at least about 50% of a compressive strength of a reference (pure or 100% cement) concrete, such as at least about 55%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, or at least about 80%, and up to about 85%, up to about 90%, or more. In some such embodiments, a porosity (e.g., a ratio of a volume of pores to a total volume) of the “low cement content” concrete is no greater than about 25%, such as no greater than about 23%, no greater than about 20%, no greater than about 18%, no greater than about 15%, or no greater than about 12%, and down to about 10%, down to about 8%, or less. The above-stated values of the porosity and the compressive strength can correspond to 1-day values, 7-day values, 14-day values, 28-day values, 56-day values, 90-day values, or values after longer periods of time.
The following examples describe specific aspects of some embodiments of this disclosure to illustrate and provide a description for those of ordinary skill in the art. The examples should not be construed as limiting this disclosure, as the examples merely provide specific methodology useful in understanding and practicing some embodiments of this disclosure.
This example describes the influence of limestone particle size and the type of (partial) cement replacement material on hydration and the mechanical properties of cement pastes. Limestone powders having median particle sizes of about 0.7, about 3, and about 15 μm, at ordinary portland cement (OPC) replacement levels between about 0% and about 20% (volume basis), and two other replacement materials of differing reactivity (i.e., Class F fly ash or metakaolin) at replacement levels between about 0% and about 10% (volume basis), are used to proportion ternary binder formulations. Fine limestone accelerates early-age hydration, resulting in comparable or better 1-day compressive strengths, and increased calcium hydroxide (CH) contents as compared to pure cement pastes. The incorporation of metakaolin in conjunction with limestone powder alters the heat release (e.g., kinetic) response significantly. A ternary blend of this nature, with about 20% total cement replacement, demonstrates the highest 1-day strength and lowest CH content. Thermal analysis reveals distinct peaks corresponding to the formation of the carboaluminate phases after 28 days in the limestone-metakaolin modified pastes, whereas the incorporation of similar levels of fly ash does not change the response markedly. It is shown that the synergistic effects of limestone and metakaolin incorporation results in improved properties at early ages, while maintaining later age properties similar to that of traditional OPC systems.
There has been increased interest in the use of limestone powder as a cement replacement material. The use of an additional, reactive cement replacement material with appropriate chemical characteristics can facilitate the use of higher levels of limestone powder with little or no attendant property loss. Towards this goal, this example provides a systematic investigation of the effects of limestone fineness on the behavior of binders including supplementary cementing materials (SCMs). Specifically, this example develops a more detailed understanding of the influence of limestone fineness and additions on the behavior of ternary binder systems (cement+limestone+SCM), including SCMs of differing chemical reactivity. As such, this example focuses on clarifying the role of limestone fineness and the type of SCM (metakaolin or a Class F fly ash) on early and later-age behavior to understand the possibility to proportion ternary binder formulations that display properties similar to traditional OPC systems.
The materials used in this example include: a commercially available Type I/II OPC conforming to ASTM C 150, a Class F fly ash and metakaolin conforming to ASTM C 618, and limestone powder conforming to ASTM C 568. Limestone powders with three different nominal median particle sizes—about 0.7 μm, about 3 μm, and about 15 μm were used. The particle size distributions of the cement, limestone, fly ash, and metakaolin are shown in
Isothermal calorimetry was carried out as per ASTM C 1702. The pastes were mixed externally as described in ASTM C305 prior to being loaded into the calorimeter. The time elapsed between the instant water was added to the powder(s) and the paste loaded into the calorimeter was about 2 min. Isothermal calorimetry was performed over a period of about 48-72 h. The powders were dry-blended using a hand mixer at low speed prior to adding water.
Compressive strengths were determined in accordance with ASTM C109 on 50 mm cubes stored in saturated limewater until the age of testing. Simultaneous thermal analysis (thermogravimetric analysis (TGA) and differential thermal analysis (DTA)) was carried out on selected pastes at ages of 1, 7, and 28 days to determine the calcium hydroxide (CH) and calcium carbonate (CC) contents. The tests were carried out in a pure nitrogen environment, at a flow rate of about 20 ml/s. A heating rate of about 10° C./min was employed, and the pastes were heated from ambient to about 950° C. The non-evaporable water content (wn) was calculated as the difference between the mass measurements at about 950° C. and about 105° C., normalized by the mass at about 950° C., and corrected for the loss on ignition of the cement powder (based on its mass fraction in the paste) and the calcium carbonate content (about 650-800° C.). This value was found to be very similar to the mass fraction of the paste remaining after heating to about 600° C. The CH contents were determined based on the mass change measured between temperatures in the DTA curve corresponding to the CH peak.
Results and Discussions
Early-Age Behavior of Binary and Ternary Cementitious Pastes:
Isothermal calorimetry was carried out on binary and ternary paste blends including several dosages of limestone powder of different particle sizes. The following sections provide insights into the influence of limestone fineness, dosage, and the synergistic effects of limestone powder and metakaolin or fly ash on the calorimetric response. The timing of the primary and secondary hydration peaks, their amplitudes, and the slopes of the acceleration and deceleration regimes are used to describe the influence of blend composition on the calorimetric response. Since a large set of mixtures is evaluated, a computer program was developed to extract these parameters from the calorimetric curves to expedite analysis. For ease of discussion, the data reported focuses on OPC replacements levels of about 10% and about 20% by limestone and about 10% by fly ash or metakaolin.
Effect of Limestone Fineness and Dosage on the Progress of Reactions:
The influence of limestone dosage for varying particle sizes on the calorimetric response is shown in
Influence of Fly Ash and Metakaolin Replacements on the Progress of Reactions:
The influence of aluminous cement replacement materials such as fly ash and metakaolin (bulk Al2O3 contents of 24% and 43% respectively, mass basis) was examined for the early age heat release response of pastes including limestone powder.
An analysis of the response of pastes including fly ash or metakaolin involves considerations of their particle sizes and reactivity. Fly ash has a median particle size similar to the coarsest limestone powder whereas the median particle size of metakaolin is much smaller (about 5 μm). When the heat release parameters for the about 10% metakaolin modified paste are compared to those of the OPC paste with about 10% of about 3 μm limestone powder, the samples are quite similar, demonstrating the influence of particle size. The peak amplitude is slightly higher for the metakaolin modified paste, but the peaks appear at virtually the same time. The normalized CH contents of the fly ash and metakaolin modified pastes are similar to that of the OPC paste at age of 1 day as observed from
Progress of Reactions in Fly Ash/Metakaolin-Containing Limestone-Containing Pastes:
The calorimetric response of ternary blends including up to about 10% of fly ash or metakaolin with different particle sizes/dosages of limestone powder are discussed in this section.
For the pastes including limestone powder along with about 10% fly ash,
e shows the heat release response of pastes including about 10% limestone and about 10% metakaolin as (partial) cement replacement materials, while
The normalized 1-day CH contents of metakaolin modified pastes are found to be consistently lower than those of pastes including only limestone or limestone/fly ash as OPC replacement materials. The reaction of carbonate from limestone with Al from the OPC and metakaolin results in the formation of the mono/hemi-carboaluminate hydrates. Thermodynamic calculations indicate that the formation of the carboaluminates initiates as early as 1 day in limestone-containing systems, a point which can be confirmed by X-ray diffraction (XRD). The formation of carboaluminates (specifically, hemi-carboaluminate) consumes CH. While this action partly explains the reduced CH content observed in ternary blends including metakaolin, an additional contribution of the pozzolanic action of metakaolin at ages as early as 1 day may be another factor involved.
Compressive Strength Development:
The compressive strengths of plain, binary, and ternary cement paste blends up to 28 days of hydration are shown in
The compressive strength development of ternary blends including fly ash or metakaolin along with limestone powder is provided in
Thermal Analysis of Pastes: Influence of Limestone and Fly Ash/Metakaolin:
Analysis of Thermogravimetric (TG) and Differential Thermogravimetric (DTG) Curves:
The earlier discussions in this example indicate that fine limestone powder in combination with metakaolin results in higher heat release and strength as compared to pastes composed with fine limestone and fly ash. TG and DTG curves for the 1-day hydrated systems are provided in
The peak at about 100° C., linked to the decomposition of C—S—H and ettringite, is slightly higher for the ternary blend including metakaolin, potentially suggesting either, or both, increased amounts of C—S—H and the stabilization of ettringite in the presence of limestone. However, no monocarbonate is observed in the 1-day DTG curves, though slight (if any) formation would be expected at such early ages.
The TG and DTG curves of OPC, fly ash, and metakaolin modified pastes cured for 28 days are shown in
The TG and DTG results for a larger replacement level of cement with 3 μm limestone powder along with metakaolin or fly ash is shown in
For the limestone-fly ash ternary blends shown in
Bound Water and CH Contents:
If the total mass loss value at 600° C. (wn) is assumed indicative of a reasonable measure of the volume of reaction products, then the increased reaction product volume (even at about 20% less cement in the paste, and a reduced CH content) can be considered to be contributed by (i) accelerations in hydration facilitated by the filler effect of limestone powder, (ii) higher reactivity of metakaolin to form pozzolanic C—S—H, and (iii) the formation of carboaluminates through the reaction between the aluminates from metakaolin and carbonates from limestone powder. These effects compensate for the reduced cement content in ternary blend with metakaolin (and limestone) to provide a compressive strength similar to that of the OPC paste (
The non-evaporable water contents (wn) and the CH contents after 1, 7, and 28 days of hydration, normalized by the mass fractions of cement in the pastes are shown in
This example describes the influence of limestone fineness and the reactivity of the alumina source on the early-age heat release response, the compressive strength and hydration products formed for cement pastes including limestone powder of three different median particle sizes or a combination of limestone powder and small amounts (about 10%) of fly ash or metakaolin. Fine limestone powders (about 0.7 and about 3 μm) were found to accelerate the early-age cement hydration at all the dosages studied. The paste with about 10% of 0.7 μm limestone powder was found to have better 1-day strength and increased normalized non-evaporable water (wn) and CH contents than the OPC paste. Increasing limestone coarseness and dosage reduced the compressive strength. Cement replacement by metakaolin in binary blends resulted in a higher heat release rate while replacement by fly ash did not produce large changes in the calorimetric response.
The calorimetric response of the pastes including limestone was not considerably modified by the presence of fly ash whereas significant changes in the calorimetric response was observed when metakaolin was used in conjunction with fine limestone powder (about 0.7 and about 3 μm). The enhanced reaction kinetics in ternary blends including about 10% 0.7 μm limestone powder and about 10% metakaolin resulted in the highest 1-day compressive strength, and the 1-day normalized CH content was among the lowest of all the evaluated pastes. While CH reduction can also be partially attributed to carboaluminate formation, it was not detected in the thermal decomposition signatures of these pastes. The enhanced aluminate phase reaction also can contribute to increased incorporation of Al3+ in the C—S—H at early ages rather than forming carboaluminates.
The fine limestone powder (about 0.7 and about 3 μm) modified pastes at about 10% cement replacement level showed compressive strengths comparable to those of OPC pastes until 28 days. The ternary blend of metakaolin along with about 10% 0.7 μm limestone powder resulted in compressive strengths that were higher than either of the corresponding binary blends, even at a higher overall cement replacement level. Such a response was not observed in the case of fly ash. The normalized wn at 28 days for the ternary blends of 0.7 and 3 μm limestone powder and metakaolin was higher than that of the OPC paste, the binary blends, and the ternary blends including fly ash. While the normalized wn of these pastes increased with age, the normalized CH contents were found to reduce or remain unchanged with age, indicating changes in reaction products. The DTG curves for the 28 day cured ternary pastes with both 0.7 and 3 μm limestone powder confirmed this through the observation of the presence of carboaluminates. Thus, this example sets forth the role of the overall chemical compatibility of cement replacement materials, with a view towards selecting the replacement material (in terms of its physical and chemical characteristics) to produce synergistic effects and optimal OPC replacement efficiency. As such, this example advances approaches to utilize multiple material solutions based on limestone and metakaolin to proportion ternary binders, dedicated to reducing the use of OPC in concrete.
Limestone (CaCO3) can be used to partially replace OPC. Replacement by limestone can cause dilution and early age acceleration, but can also result in strength reduction. Reduction is strength is an issue, and it is proposed that this can be addressed by increasing aluminate content of cement
Materials and Mixture Proportioning:
OPC-based mixtures were prepared with a fixed water-to-solid ratio (w/s) of about 0.45 on a mass basis. The mixtures included: (1) plain OPC; (2) about 30% mass replacement of OPC by limestone; (3) about 5%-15% mass replacement of OPC by aluminous materials; and (4) about 30% mass replacement of OPC by limestone and an additional about 5%-15% replacement by aluminous materials. The aluminous materials were metakaolin or alphabond 300, which is a hydratable alumina binder available from Almatis B.V. Oxide compositions of the aluminous sources and cement used are set forth in Table 3.
Gibbs Energy Minimization (GEMS):
A geochemical modeling code was used to perform GEMS simulations, based on the principle of the minimization of the total Gibbs energy of a complex chemical system. Inputs to the stimulations included initial phase assemblage, and degree of hydration (DOH) of OPC fixed at about 83.5% at age=28 days. Outputs of the stimulations included equilibrium solid and liquid phase assemblage as function of extent of reaction.
Influence of Metakaolin:
From the simulation results, an increase in metakaolin reduces portlandite (Ca(OH)2 or CH) in the system. Metakaolin undergoes pozzolanic reaction with portlandite to form C—S—H (calcium silicate hydrate or xCaO.SiO2.yH2O). At higher replacements, portlandite is depleted, and formation of stratlingite (dicalcium aluminate monosilicate-8-hydrate or Ca2Al2SiO7.8H2O or 2CaO.Al2O3.SiO2.8H2O or C2ASH8) is initiated. Formation of hydrogarnet (tricalcium aluminate-6-hydrate or Ca3Al2(OH)12 or 3CaO.Al2O3.6H2O or C3AH6) is also noted to increase with an increase in metakaolin.
From the simulation results, addition of about 30% limestone favors the formation of CO3-AFm over SO4-AFm. Unreacted limestone can be reduced as metakaolin content increases. As more limestone reacts, formation of CO3-AFm phase is enhanced.
For the prepared mixtures, strength increases somewhat with increasing metakaolin replacement level. This strength enhancement can be attributed to the formation of more C—S—H as a result of pozzolanic reaction.
Influence of Alphabond:
From the simulation results, an increase in alphabond suppresses formation of portlandite. At higher replacements, portlandite is depleted, and formation of stratlingite is initiated. At higher replacements, formation of C3AH6 is also enhanced.
From the simulation results, addition of about 30% limestone favors the formation of CO3-AFm over SO4-AFm. Unreacted limestone can be reduced as alphabond content increases. As more limestone reacts, formation of CO3-AFm phase is enhanced.
For the prepared mixtures, strength reduces with reduction in OPC by alphabond replacement. Strength values are higher than dilution values (quartz) at 28 days, but are lower at 90 days.
Portlandite and Limestone Contents:
Portlandite contents reduce substantially proportional to OPC replacement levels for both metakaolin and alphabond. For corresponding replacement levels, alphabond causes limestone to react more as compared to metakaolin.
While the invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention as defined by the appended claims. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the invention. All such modifications are intended to be within the scope of the claims appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations is not a limitation of the invention.
This application claims the benefit of U.S. Provisional Application No. 61/692,606 filed on Aug. 23, 2012, the disclosure of which is incorporated herein by reference in its entirety.
This invention was made with Government support of Grant No. CMMI-1066583, awarded by the National Science Foundation. The Government has certain rights in this invention.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US2013/056493 | 8/23/2013 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
61692606 | Aug 2012 | US |