Patent Application
20160326054
The present invention generally relates to cementitious compositions. The invention particularly relates to cementitious compositions containing internal curing agents and mineral admixtures.
Cementitious compositions for forming high-performance concrete (HPC) and ultra-high-performance concrete (UHPC) commonly include various additives intended to improve various properties of the compositions and the concrete they produce. Such additives may include any ingredients other than Portland cement, water, and aggregate that are added to the composition before or during mixing. These additives are referred to herein as admixtures.
Portland-pozzolan blended cement typically contains 20-25 wt. % of a pozzolan (also referred to herein as a pozzolanic material), resulting in significant energy and cost savings as less Portland cement can be used. A pozzolan is defined by ASTM C125 (ASTM: American Society for Testing Materials) as a siliceous or siliceous/aluminous material that will chemically react with calcium hydroxide (Ca(OH)2) and/or calcium cations (Ca′) in the presence of water (referred to herein as pozzolanic reaction or reactions) at ordinary temperatures to form compounds with cementitious properties (including C—S—H which provides long-term strength and durability to concrete). During setting, pozzolanic reactions result in improved resistance to thermal cracking due to a relatively low heat of hydration (roughly half that of pure Portland cement). Additionally, the pozzolanic reaction products of very fine (sub-micron) siliceous materials are more efficient at filling capillary spaces within the cement paste without the formation of large expansive pressures and phases within in the concrete (i.e., typical of alkali-silica reactions (ASR), which can lead to cracking and decreased concrete durability), resulting in a refined pore size distribution (i.e., replacing macropores with microporous material) thus improving the strength, durability, and impermeability of the hardened concrete.
Common pozzolans include silica fume and rice husk ash which are both industrial waste products that consist of pure amorphous silica in the form of particles. Pozzolanic particles and cementitious mixtures are well described in U.S. Pat. No. 7,442,248 issued to Timmons on Oct. 28, 2008. The contents of this patent are incorporated herein by reference in their entirety. Pozzolanic particles employed in cementitious applications are typically in the size range of 50-150 nanometers. Pozzolanic particles smaller in size, for example in the range 10-50 nanometers, have higher surface area and hence increased reactivity. Smaller particles are also more likely to react completely and form C—S—H without forming expansive phases typical of ASR. However, particles in this smaller size range are difficult to handle due to the hazardous nature of these particles if inhaled. Additionally, these smaller particles typically decrease the workability of the cementitious mixture, an undesirable result which in turn requires greater pumping pressures and increases the difficulty of mixture placement.
In an effort to lower their water-to-binder ratios, cementitious mixtures used to produce HPCs and UHPCs may include internal curing agents. These agents, typically wet porous aggregate or swollen superabsorbent polymer (SAP) hydrogels, provide a continuous supply of water during curing, thus counteracting self-desiccation and reducing or potentially eliminating autogenous shrinkage and cracking of the cement and achieving a corresponding increase in compressive strength and durability.
SAP hydrogel particles may swell to as much as 100 times their original weight in the presence of water as shown in image (a) of
In view of the above, there is an ongoing desire for admixtures capable of modifying or improving the properties and characteristics of cementitious mixtures and concrete formed therewith.
The present invention provides cementitious mixtures, compositions for use in cementitious mixtures, and methods of producing cementitious mixtures wherein the compositions are suitable for modifying or improving the properties of the cementitious mixtures and concrete formed therewith. In particular, the compositions include polymers having particles of at least one pozzolanic material incorporated directly into a macromolecular network structure of the polymers.
According to one aspect of the invention, a composition for use as a curing agent in a cementitious mixture includes a superabsorbent polymer (SAP) hydrogel having a macromolecular network structure, and at least one pozzolanic material that is chemically incorporated into the macromolecular network structure of the SAP hydrogel.
According to another aspect of the invention, a cementitious mixture is provided that includes an internal curing agent comprising a superabsorbent polymer (SAP) hydrogel having a macromolecular network structure, and at least one pozzolanic material that is chemically incorporated into the macromolecular network structure of the SAP hydrogel.
According to another aspect of the invention, a method includes providing a polymer composition that is end-functionalized with a chemically reactive group, incorporating a chemically reactive functional group on surfaces of a quantity of particles of a pozzolanic material to form surface-functionalized pozzolanic particles, chemically reacting the surface-functionalized pozzolanic particles with the end-functionalized polymer composition to form a plurality of polymer-grafted pozzolanic particles, incorporating a cross-linking agent into the plurality of polymer-grafted particles to form a composite hydrogel with a macromolecular structure comprising the particles of the pozzolanic material chemically incorporated therein, and incorporating the composite hydrogel into a cementitious mixture.
According to another aspect of the invention, a method includes providing a fluid medium having a quantity of particles of a pozzolanic material therein, incorporating a composition on surfaces of the particles of the pozzolanic material to form surface-functionalized pozzolanic particles, chemically reacting the surface-functionalized pozzolanic particles with at least one monomer composition and at least one chain-transfer agent to form a plurality of polymer-grafted particles, incorporating a cross-linking agent into the plurality of polymer-grafted pozzolanic particles to form a composite hydrogel with a macromolecular structure comprising the particles of the pozzolanic material chemically incorporated therein, and incorporating the composite hydrogel into a cementitious mixture.
Technical effects of the compositions, cementitious mixtures, and methods described above preferably include the capability of incorporating pozzolanic materials into cementitious mixtures with reduced handling risks, while modifying or improving the beneficial effects of both the pozzolanic materials and the superabsorbent polymers within the cementitious mixture relative to separate additions of the two materials.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
Composite hydrogel-based internal curing agents and cementitious mixtures comprising such curing agents are described herein. The curing agents include SAP hydrogels having pozzolanic particles chemically incorporated directly into their polymer macromolecular network structure. Such curing agents have been observed to exhibit increased reactivity and impart greater strength and durability to cured cement relative to conventional internal curing agents, and yet are believed to be less hazardous in preparation and use.
Certain aspects of the invention are described herein in reference to poly(acrylamide(AM)-acrylic acid(AA)) hydrogels; however, it should be understood that the invention is believed to be applicable to any SAP hydrogel. Further, during synthesis of poly(acrylamide(AM)-acrylic acid(AA)) hydrogels, the acrylic acid monomer may be neutralized by adding a base (such as NaOH) to the reaction solution, which converts the acrylic acid to sodium acrylate. As such, the terms poly(acrylamide(AM)-acrylic acid(AA)) and poly(sodium acrylate(PANa)-acrylamide(AM)) are used interchangeably herein.
As previously stated, SAP hydrogels are able to provide a continuous supply of water during curing of cementitious mixtures, thus counteracting self-desiccation and reducing or eliminating autogenous shrinkage and cracking of a cement and achieving a corresponding increase in compressive strength and durability. However, SAP hydrogel particles may leave behind voids in the hardened cement as shown in
It has been observed that portlandite (a naturally occurring form of calcium hydroxide (Ca(OH)2)) may form in the voids remaining from dehydrated SAP hydrogel particles within a few hours of setting, as shown in
The hydrogel-based internal curing agents described herein are intended to promote the cement curing process not only by providing water to fuel the curing reaction, but also by facilitating beneficial pozzolanic reactions to convert calcium hydroxide (portlandite) to calcium silicate hydrates (C—S—H; the main product of the hydration of Portland cement) within the cementitious mixtures that further refines the microstructure and improves the strength and durability of the hardened HPC. To accomplish this goal, the composite hydrogel-based internal curing agents contain pozzolanic materials chemically incorporated into their polymer macromolecular network structures. This is achieved by chemically (covalently) attaching (“grafting”) polymer molecules to the surfaces of pozzolanic particles and subsequently cross-linking the polymer molecules to neighboring polymer molecules, forming a three-dimensional superabsorbent polymer-pozzolan composite hydrogel networks referred to herein as “SAPP” hydrogels.
While it is well known that the addition of pozzolanic material to cementitious mixtures can enhance the curing reaction and result in stronger and more durable concrete at reduced cost, there are significant tradeoffs in material processing and properties which can limit the use of pozzolanic materials in HPC and UHPC. However, such shortcomings can be overcome or at least partially alleviated by chemically incorporating pozzolanic particles directly into the polymer macromolecular network structures of the SAPP hydrogels, in a manner that enables the benefits of pozzolan addition in HPC and UHPC to be realized along with the added benefits of easier material handling, improved processability, and reduced autogenous shrinkage. In particular, the benefits to cementitious mixtures by incorporation of the SAPP hydrogels therein are expected to be greater than if the two components (pozzolanic particles and SAP hydrogels) were used and added separately as discrete additives to cementitious mixtures, as they typically are currently used in the construction industry. For example, by chemically incorporating the pozzolanic particles into the polymer macromolecular network structures of SAPP hydrogels, the particles remain in the presence of water (and will thus continue to beneficially catalyze the hydration reaction) but they will be much easier and safer to handle and add to the cementitious mixtures relative to loose particulate pozzolans.
Additionally, as previously discussed, SAP hydrogels tend to attract multi-valent ions (including calcium ions) which are key reactants in the cement hydration reaction. These ions may promote the pozzolanic reaction and potentially even “fill in” the void space that would typically remain in the cement following hydrogel deswelling with cement binder, catalyzed locally by the pozzolanic particles within the hydrogel. Even if the voids are not entirely filled in, the reaction may enhance strength in the void walls. As seen in
Regardless of whether the voids are entirely filled, chemical incorporation of pozzolan particles within the SAPP hydrogels is able to decrease the heat of hydration, increase the internal relative humidity (RH), decrease the autogenous shrinkage of the mixture, and/or minimize the appearance of large pores and microcracks within the hardened cement microstructure, thereby increasing the compressive strength and durability of the concrete. The magnitude of these effects appears to be directly dependent on the pozzolan reactivity, which is related to the polymer grafting density (with higher grafting densities resulting in reduced reactivity). Thus, the grafting density of polymer molecules on the surface of the pozzolanic particle may be used to control the reactivity of the particle and thus create a tunable response for different types of concrete.
Various methods may be used to produce the SAPP (and SAPP+P, described hereinafter) hydrogels with controlled variation in pozzolan content and morphology, polymer-pozzolan grafting density, and polymer macromolecular network structure and chemical composition.
According to one nonlimiting method for producing a SAPP hydrogel, polymer-grafted pozzolanic particles are created with a “grafting to” method utilizing a pre-synthesized uncross-linked polymer composition that is end-functionalized so that it can covalently react with the surface of pozzolanic materials. A polymer composition with a desired molecular weight may be synthesized by controlled radical polymerization from free monomer, initiator, and chain transfer agent in solution. A nonlimiting example of a suitable polymer composition includes a silane- or alkyne-terminated poly(AA-AM) copolymer (molecular weight, MW, of 500-100,000 g/mol). A chemically reactive functional group (nonlimiting example, chlorosilane) is incorporated on surfaces of particles of a pozzolanic material to form surface-functionalized particles of the pozzolanic material of varying grafting density (from 0.05 to 0.5 molecules/nm2), for example, under nitrogen atmosphere at about 70° C. via sonication or reflux in a solvent. The end-functionalized polymer composition is then chemically reacted with the surface-functionalized pozzolanic particles, for example, under nitrogen at about 70° C. in a solvent, to form polymer-grafted pozzolanic particles. The grafting density of polymer on the surface of the pozzolanic particles can be varied. To form a SAPP hydrogel, a covalent cross-linking agent is added (nonlimiting example, n,n-methylenebisacrylamide, at a concentration of 0.2-10% by weight of polymer) to a plurality of the polymer-grafted pozzolanic particles to create a covalently bonded, three-dimensional superabsorbent polymer-pozzolan composite (SAPP) hydrogel with a macromolecular structure containing particles of the pozzolanic material. Weight percent of pozzolanic particles in the SAPP hydrogel may range from about 0.5 percent particles by weight of polymer (for a relatively high polymer grafting density on the pozzolanic particle surface and a relatively high density of covalent cross-links) to about 90 percent particles by weight of polymer (for a relatively low polymer grafting density on the pozzolanic particle surface and a relatively low density of covalent cross-links).
According to another nonlimiting method for producing a SAPP hydrogel, polymer-grafted pozzolanic particles are created with a “grafting from” controlled radical polymerization method. The method includes providing a quantity of pozzolanic material in a particulate form in a volume of fluid medium. A small molecule with acceptable functionality for reactivity or chain transfer agent is incorporated on surfaces of the particles of pozzolanic material to form surface-functionalized particles of the pozzolan, for example, under nitrogen atmosphere at about 70° C. via sonication or reflux in solvent. The surface-functionalized particles are then chemically reacted with at least one monomer composition and at least one free chain-transfer agent, resulting in a plurality of polymer-grafted pozzolanic particles with controlled polymer MW of 500-100,000 g/mol and varying grafting density (from 0.05 to 0.5 molecules/nm2). A cross-linking agent (nonlimiting example, n,n-methylenebisacrylamide, at a concentration of 0.2-10% by weight of polymer) is then incorporated into the plurality of polymer-grafted particles to create a covalently bonded, three-dimensional superabsorbent polymer-pozzolan composite (SAPP) hydrogel with a macromolecular structure containing particles of the pozzolanic material. Weight percent of pozzolanic particles in the SAPP hydrogel may range from about 0.5 percent particles by weight of polymer (for a relatively high polymer grafting density on the pozzolanic particle surface and a relatively high density of covalent cross-links) to about 90 percent particles by weight of polymer (for a relatively low polymer grafting density on the pozzolanic particle surface and a relatively low density of covalent cross-links).
As a modification to the above described synthesis method, rather than reacting the cross-linking agent solely with polymer-grafted pozzolanic particles, select ratios of pre-synthesized polymer (uncross-linked, with MW ranging from 500-50,000 g/mol) may be mixed with a plurality of polymer-grafted particles and the entire mixture cross-linked with the cross-linking agent to yield a SAPP hydrogel network with additional ungrafted polymer incorporated into the polymer macromolecular network structure, referred to herein as a SAPP+P hydrogel. The weight fraction range of pozzolanic materials in SAPP+P hydrogels may be similar to the range previously specified and is ultimately dependent on the molecular weight of the grafted polymer and uncross-linked polymer.
In general, pozzolanic materials capable of use in embodiments described above can be any siliceous or siliceous and aluminous materials that has little to no hydraulic reactivity, that is, it does not form a binder in the presence of water. Non-limiting examples of pozzolanic materials for SAPP and SAPP+P hydrogels include, but not limited to, Class C fly ash, silica fume, metakaolin, rice husk ash, Class F fly ash, slag, and/or calcined shale. Various sizes of the pozzolanic particles may be used, including particle sizes commonly used in cementitious mixtures. For example, silica nanoparticles may have diameters of about 10-100 nm, silica fume particles may have diameters of about 100-1000 nm, and rice husk ash and other natural pozzolanic particles may have diameters greater than 1 μm, though pozzolanic particles of other sizes are also within the scope of the invention. The SAPP and SAPP+P hydrogels preferably comprise pozzolanic particles in an amount of about 75 to 90 percent by weight in order to both catalyze a beneficial reaction in the concrete while still comprising enough polymer to swell and act as an internal curing agent.
In addition, it should be recognized that a cementitious mixture may include other pozzolanic materials or other additives in addition to those contained in the SAPP or SAPP+P hydrogels, or may include multiple types of SAPP or SAPP+P hydrogels each containing different pozzolanic materials. Further, the cementitious mixture may include SAPP or SAPP+P hydrogels that comprise only one type of pozzolanic material, or may comprise more than one pozzolanic material chemically incorporated into its macromolecular network structure.
While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the chemical and physical composition of the SAP hydrogels could differ from that described, other materials could be incorporated into the macromolecular network structure of the SAP hydrogels, and materials and processes/methods other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.
This application claims the benefit of U.S. Provisional Application No. 62/154,183, filed Apr. 29, 2015, the contents of which are incorporated herein by reference.
This invention was made with government support under Contract No. 1454360-CMMI awarded by the National Science Foundation and Contract No. 1333468-DGE awarded by the National Science Foundation. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62154183 | Apr 2015 | US |
