Composite early strength agent for promoting cement hydration in low-temperature environment and preparation method thereof

Abstract

The invention belongs to the technical field of building materials, and discloses a composite early strength agent for promoting cement hydration in a low-temperature environment and a preparation method thereof. The composite early strength agent is composed of a catalytic ionic liquid, a low-temperature eutectic phase change regulating agent, alkali metal nitrate, alkali metal formate and a nanoscale seed crystal inducer, cement hydration is remarkably accelerated at the temperature of 0-5 DEG C through the synergistic effect of four mechanisms of Lewis acid catalysis, phase change temperature regulation, ion regulation and seed crystal induction, the early strength is improved, and the early strength is improved. And chloride ions and sulfate ions are not contained, so that the durability of steel bars and the long-term performance of concrete are guaranteed. By constructing a catalysis-temperature regulation-ion-seed crystal composite early strength system, the internal contradiction among low-temperature early strength, steel bar compatibility, thermal stability and construction performance in the prior art is solved, and reliable technical support is provided for infrastructure construction in alpine regions.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, and relates to a composite early strength agent that promotes cement hydration under low temperature conditions and its preparation method. Background Technology

[0002] Accelerators, as a type of functional concrete admixture, are widely used to accelerate the hydration process of cement mineral phases (such as C3S and C3A), thereby improving the early mechanical properties of concrete. Traditional accelerators mainly include chlorides (such as CaCl2), sulfates (such as Na2SO4), and inorganic compounds such as nitrates and nitrites. Among these, chlorides have long held a dominant position due to their highly efficient setting-accelerating effect. These substances provide high concentrations of Cl... - Ions disrupt the double electric layer structure on the surface of cement particles, promote the rapid dissolution of the aluminate phase, and generate intermediate products such as Friedel salt, effectively shortening the induction period and significantly improving the 1-day and 3-day compressive strength.

[0003] Although chloride-based early-strength agents have excellent early-strength effects, the chloride ions they introduce can easily penetrate the concrete protective layer, accumulate on the surface of the reinforcing steel, and destroy the passivation film, inducing pitting corrosion and stress corrosion cracking, which seriously threatens the long-term service safety of the structure. Therefore, their use has been strictly restricted or prohibited in prestressed concrete, marine engineering, and important infrastructure.

[0004] While sulfate-based early-strength agents avoid the risk of chloride ion corrosion, they suffer from poor thermal stability and are prone to decomposition and failure at high temperatures. More importantly, their solubility decreases significantly at low temperatures, and the ion migration rate slows down, resulting in a substantial reduction in their coagulation-promoting efficiency, making it difficult to meet the stable early-strength performance requirements in frigid environments.

[0005] In recent years, researchers have attempted to introduce organic early-strength components or composite systems to alleviate the aforementioned contradictions. However, most solutions are still limited to physical adsorption or simple compounding, lacking the ability to directionally regulate the cement hydration pathway. Especially under low-temperature conditions (such as 0-5℃), the viscosity of the aqueous phase increases, the ion diffusion coefficient decreases, and the nucleation energy barrier rises. Traditional early-strength agents cannot effectively reduce the hydration activation energy or reconstruct the interfacial reaction microenvironment, resulting in a sharp decline in their setting efficiency.

[0006] If the early-strength component itself lacks sufficient thermal stability, it may decompose under the exothermic hydration of concrete or fluctuations in external ambient temperature. This not only results in the loss of its function but may also introduce air bubbles or byproducts, deteriorating the microstructure. Therefore, the existing technology system exhibits a significant trade-off between performance dimensions in terms of high-efficiency setting, steel reinforcement compatibility, low-temperature adaptability, and thermal stability. The fundamental problem lies in the lack of a novel functional component that combines catalytic activity, chemical inertness, and environmental responsiveness, capable of precisely intervening in the cement hydration process at the reaction kinetics level, rather than relying solely on the non-specific stimulation of high-concentration ions. Summary of the Invention

[0007] To achieve the above-mentioned objectives, this invention provides a composite early-strength agent that promotes cement hydration under low-temperature conditions and its preparation method. The composite early-strength agent, by introducing an ionic liquid component with Lewis acid catalytic activity, a low-temperature eutectic phase transformation regulating component, and a chloride- and sulfate-free alkali metal salt synergistic system, constructs a multi-scale reaction microenvironment regulation mechanism. Under low-temperature conditions of 0-5℃, it significantly reduces the activation energy of cement hydration reaction, accelerates the dissolution-nucleation-growth kinetics of C3S and C3A, and simultaneously avoids the introduction of harmful components such as chloride and sulfate ions, ensuring the durability of steel reinforcement and the long-term stability of concrete performance.

[0008] The composite early-strength agent of this invention is composed of the following components by mass percentage: 2%-8% catalytic ionic liquid, 15%-30% low-temperature eutectic phase change regulator, 40%-60% alkali metal nitrate, 10%-20% alkali metal formate, and 3%-7% nano-scale seed crystal inducer. After physical mixing or solution blending, the components are dried and granulated to form a homogeneous powder product, which can be directly incorporated into concrete mixtures at a dosage of 0.8%-2.5% of the total mass of the cementitious materials.

[0009] The catalytic ionic liquid is one or a combination of two of 1-butyl-3-methylimidazolium tetrafluoroborate or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. This ionic liquid remains liquid at low temperatures; its cationic portion adsorbs onto the surface of cement particles via electrostatic interactions, weakening the stability of the silicon-oxygen tetrahedral network structure; the anionic portion contains BF4... - or TfO - It possesses Lewis acid properties and can react with Ca. 2+ The formation of weakly coordinated complexes lowers the interfacial energy barrier required for Ca(OH)2 nucleation during the initial stage of C3S hydration. At 0°C, the viscosity of the ionic liquid is below 80 mPa·s, and its conductivity is above 1.2 mS / cm, ensuring sufficient migration capacity and interfacial activity in low-temperature slurries. The thermal decomposition temperature of the ionic liquid is above 300°C, maintaining chemical structural stability within the hydration temperature rise inside concrete (typically below 70°C) and within the range of external environmental temperature fluctuations, without producing gaseous byproducts.

[0010] The low-temperature eutectic phase change regulator is composed of ethylene glycol and urea in a mass ratio of 1:1.2 to 1:1.8. This binary system forms a eutectic mixture in the range of -8 to -3℃, with a latent heat of phase change of 180-220 J / g. When the ambient temperature drops below 0℃, the eutectic system undergoes a solid-liquid phase change, releasing latent heat and locally raising the temperature of the cement paste micro-region by 1-3℃, effectively mitigating the inhibitory effect of low temperature on the hydration reaction rate. Simultaneously, the hydroxyl groups in the ethylene glycol molecules and the amino groups in the urea molecules can form a hydrogen bond network with the silanol groups on the surface of the cement hydration product CSH gel, enhancing the aggregation stability of early hydration products and inhibiting the initiation of microcracks. The eutectic system is completely liquefied above 20℃, facilitating uniform mixing with other components; during the drying and granulation process, it is encapsulated in a porous diatomaceous earth carrier using a spray drying process to prevent moisture absorption and agglomeration.

[0011] The alkali metal nitrate is sodium nitrate or potassium nitrate, with a particle size D50 of 10-30 μm. This component completely dissociates into Na in water. + / K + With NO3 - NO3 - It is not oxidizing or corrosive, and does not react electrochemically with steel reinforcement. Na + / K + It can partially replace the interlayer Ca in CSH gels 2+ This reduces interlayer spacing, promotes silicon chain condensation, and thus accelerates the densification of the gel phase. At -5℃, the solubility of sodium nitrate is 73 g / 100 g water, and potassium nitrate is 46 g / 100 g water, both significantly higher than sodium sulfate (4.76 g / 100 g water), ensuring an effective supply of ions.

[0012] The alkali metal formate is sodium formate or potassium formate, with a particle size D50 of 15-35 μm. - ) can be with Al 3+ Formation of a soluble complex [Al(HCOO)] n ] 3-n + (n=1 to 3), delaying the instantaneous hydration of C3A and preventing flash condensation; at the same time, the complex gradually dissociates when the pH rises above 12.5, releasing Al. 3+ It participates in the orderly formation of ettringite, achieving slow-release and stabilization regulation of the hydration process. The addition of formate reduces the initial fluidity loss rate of the slurry by 15%-25%, improving workability and construction.

[0013] The nanoscale seed inducer is a composite powder of nano-silica and nano-calcium carbonate, with a mass ratio of 1:1 to 2:1, an average particle size of 20-50 nm, and a specific surface area of ​​80-120 m². 2 / g. The surface of nano-SiO2 is rich in silanol groups, which rapidly react with Ca in an alkaline environment.2+ The reaction generates CSH gel nuclei, providing numerous heterogeneous nucleation sites; nano-CaCO3 acts as carbonate crystal nuclei, promoting the directional precipitation of single-carbon calcium sulfoaluminate. The synergistic effect of these two processes shortens the C3S hydration induction period by 30%-50%, significantly improving the 12-hour and 24-hour compressive strength.

[0014] In a preferred embodiment of the present invention, the composite early-strength agent comprises: 5% 1-butyl-3-methylimidazolium tetrafluoroborate, 22% ethylene glycol-urea eutectic phase change regulator (mass ratio 1:1.5), 50% sodium nitrate, 18% sodium formate, and 5% nano-SiO2 / CaCO3 composite seed crystals (mass ratio 3:2). Under curing conditions at -3℃, this formula shortens the initial setting time of ordinary silicate cement paste to less than 180 min and the final setting time to less than 270 min; the 1-day compressive strength of concrete specimens reaches 12.5 MPa, and the 3-day compressive strength reaches 28.3 MPa, representing increases of 210% and 185% respectively compared to the control group; there is no shrinkage in the 28-day compressive strength, and the electrical flux is less than 800 A·s, indicating a significant reduction in chloride ion permeability.

[0015] In another preferred embodiment of the present invention, the composite early-strength agent is prepared by solution blending-spray drying. The specific steps are as follows: First, a catalytic ionic liquid and deionized water are mixed at a mass ratio of 1:3 to form a transparent solution; then, a low-temperature eutectic phase change regulator is added and stirred at 40°C until completely dissolved; next, alkali metal nitrate and alkali metal formate are added sequentially and stirred for 30 minutes until homogeneous; finally, a nanoscale seed crystal inducer is introduced using a high-speed shear dispersion method at 3000 rpm for 15 minutes to form a stable suspension; this suspension is sprayed into a 180°C hot air drying tower via a centrifugal atomizer, with the inlet air temperature controlled at 175-185°C and the outlet air temperature at 85-95°C. The resulting powder is sieved to obtain spherical particles with a particle size of 100-300 μm. This process ensures uniform microscopic distribution of each component and avoids migration and precipitation of the ionic liquid during storage.

[0016] The process for adding the composite early-strength agent to concrete is as follows: 10 seconds before adding the mixing water, the dry powder of the early-strength agent of this invention is added to the mixer and premixed with the aggregate for 15 seconds; then cement, mineral admixtures, and mixing water are added, with a total mixing time of more than 90 seconds. This order of addition ensures that the early-strength agent preferentially wets and adsorbs onto the surface of cement particles, maximizing its interfacial catalytic effect.

[0017] The mechanism of action of the composite early-strength agent described in this invention is manifested in multi-scale synergy: at the molecular scale, the Lewis acid anion of the ionic liquid reacts with Ca... 2+Coordination lowers the activation energy of C3S hydration; at the mesoscale, the eutectic phase transition regulator releases latent heat and forms a hydrogen bond network, optimizing the thermodynamic and kinetic conditions of the micro-region; at the macroscale, alkali metal salts provide highly mobile cations and functional anions, regulating the hydration pathway; at the nanoscale, seed inducers provide heterogeneous nucleation cores, accelerating the early formation of the gel phase and the crystalline phase. These four mechanisms form a positive coupling at low temperatures, breaking through the traditional single-action mode of early-strength agents that relies on high concentrations of harmful ions.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention solves the inherent contradictions between low-temperature early strength, steel reinforcement compatibility, thermal stability and construction performance in existing technologies by constructing a four-in-one composite early strength system integrating catalysis, temperature regulation, ionization and seeding, providing a reliable solution for infrastructure construction in high-altitude and cold regions. Detailed Implementation

[0019] This invention provides a composite early-strength agent that promotes cement hydration in low-temperature environments and its preparation method. The core of this method lies in a four-pronged synergistic mechanism: molecular-scale Lewis acid catalysis, mesoscopic-scale phase transition temperature regulation, macroscopic-scale ion control, and nanoscale seed induction. This mechanism significantly accelerates the hydration kinetics of silicate cement in a 0-5℃ low-temperature environment, while simultaneously mitigating the risks of steel corrosion and thermal stability defects caused by the introduction of chloride and sulfate ions. The composite early-strength agent uses a catalytic ionic liquid, a low-temperature eutectic phase transition regulator, a chloride- and sulfate-free alkali metal salt, and a nanoscale seed inducing agent as its basic building blocks. It is prepared into a homogeneous powder through a solution blending-spray drying process. When incorporated into concrete, it achieves a unified effect of rapid early strength enhancement, long-term durability assurance, and optimized construction performance.

[0020] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0021] Example 1: 5% catalytic ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate); 22% low-temperature eutectic regulator (ethylene glycol: urea = 1:1.5); 50% sodium nitrate, 18% sodium formate; 5% nanocrystal seeds (silicon dioxide: calcium carbonate = 3:2); doping amount 1.5%; Preparation process: Preparation of ionic liquid mother liquor → addition of eutectic regulator → mixing of alkali metal salts → high-speed dispersion of nanocrystal seeds → spray drying → sieving.

[0022] Example 2: 2% catalytic ionic liquid, the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1.

[0023] Example 3: 8% catalytic ionic liquid, the remaining formulation and process are the same as in Example 1; Preparation process: Same as in Example 1.

[0024] Example 4: 5% catalytic ionic liquid (1-ethyl-3-methylimidazolium trifluoromethanesulfonate), with the remaining formulation and process the same as in Example 1; Preparation process: Same as in Example 1.

[0025] Example 5: 15% low-temperature eutectic regulator, the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1.

[0026] Example 6: 30% low-temperature eutectic regulator, the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1.

[0027] Example 7: 3% nanocrystal seeds (silicon dioxide: calcium carbonate = 1:1), the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1.

[0028] Example 8: 7% nanocrystal seeds (silicon dioxide: calcium carbonate = 2:1), the rest of the formulation and process are the same as in Example 1; Preparation process: Same as in Example 1.

[0029] Comparative Example 1: Calcium chloride 5% + sodium sulfate 15% + sodium nitrate 60% + sodium formate 20%; without ionic liquid, eutectic regulator, or nanocrystal seed; doping amount 1.5%; Preparation process: Dry material mixing → crushing → sieving.

[0030] Comparative Example 2: Sodium nitrate 55% + sodium formate 20% + nanocrystal seeds 5% + filler 20%; no ionic liquid or eutectic regulator; doping amount 1.5%; Preparation process: Dry material mixing → Adding nano-crystal seeds → Crushing → Sieving.

[0031] Test method: Early strength and setting test: According to GB / T1346 standard, the initial setting / final setting time of the cement paste under curing at -3℃ was determined; the compressive strength of concrete at 1 day, 3 days and 28 days was determined.

[0032] Durability testing: Based on ASTM C1202 standard, the 28-day electrical flux was measured to assess chloride ion permeability; the corrosion current density of the reinforcing steel was tested.

[0033] Construction performance testing: Determine the initial slump and 1-hour loss rate of freshly mixed concrete; verify the compatibility of early strength agent and cement.

[0034] The test data comparisons are shown in Table 1 and Table 2.

[0035] Table 1 Comparison of Initial Setting Time, Final Setting Time, and 1-Day Compressive Strength Test Project Initial setting time (min) Final setting time (min) 1-day compressive strength (MPa) Example 1 170 250 13 Example 2 180 270 12.5 Example 3 160 240 14 Example 4 165 245 13.5 Example 5 185 275 12 Example 6 155 235 14.5 Example 7 175 255 12.8 Example 8 150 230 15 Comparative Example 1 150 220 14 Comparative Example 2 240 360 6 Table 2 Comparison of 3-day compressive strength and 28-day electrical flux Test Project 3-day compressive strength (MPa) 28-day electrical flux (A·s) Example 1 29 750 Example 2 28.3 780 Example 3 30 720 Example 4 29.5 740 Example 5 27 790 Example 6 31 710 Example 7 28 770 Example 8 32 700 Comparative Example 1 30 1800 Comparative Example 2 15 800 Example 1 shows a compressive strength of ≥12MPa at 81 days and an electrical flux of ≤790A·s at 28 days, which is far superior to the comparative example. Comparative example 1 has extremely poor durability due to chloride salt, and comparative example 2 has a weak early strength effect without synergistic system, which confirms that the core system is the key to low-temperature high-efficiency early strength.

[0036] Increasing the proportion of ionic liquids (Examples 2→1→3) shortens the solidification time and increases the strength; increasing the eutectic regulator (Examples 5→1→6) optimizes the low-temperature hydration kinetics; increasing the proportion of nanocrystal seeds (Examples 7→1→8) enhances the nucleation efficiency.

[0037] The example contains no corrosive ions, meeting the durability requirements of reinforced concrete; the slump loss of fresh concrete is ≤20mm, and the workability is good; it can still guarantee early strength at -3℃, making it suitable for winter construction.

[0038] Compared to traditional chloride salt early strength agents (Comparative Example 1), the durability of the example is improved by 58%, with no risk of corrosion; compared to the non-synergistic system (Comparative Example 2), the strength is improved by 100% in 1 day, solving the industry problem that traditional early strength agents cannot achieve both early strength and durability.

[0039] The composite early strength agent described in this invention achieves high-efficiency early strength and high durability at low temperatures through a four-fold synergistic mechanism, with different parameter combinations, making it suitable for concrete engineering in cold regions.

[0040] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite early-strength agent that promotes cement hydration under low-temperature conditions, characterized in that, The composite early strength agent is in powder form and consists of the following components by mass percentage: Catalytic ionic liquids: 2%-8%; Low-temperature eutectic phase change regulator 15%-30%; Alkali metal nitrates: 40%-60%; Alkali metal formate 10%-20%; Nanoscale seed inducer 3%-7%.

2. The composite early-strength agent according to claim 1, characterized in that, The catalytic ionic liquid is one or a combination of two of 1-butyl-3-methylimidazolium tetrafluoroborate or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, with a viscosity of less than 80 mPa·s, an electrical conductivity of more than 1.2 mS / cm, and a thermal decomposition temperature of more than 300℃ at 0℃.

3. The composite early-strength agent according to claim 1, characterized in that, The low-temperature eutectic phase change regulator is composed of ethylene glycol and urea in a mass ratio of 1:1.2 to 1:1.8, forming a eutectic mixture in the range of -8 to -3℃, with a latent heat of phase change of 180-220 J / g.

4. The composite early-strength agent according to claim 1, characterized in that, The alkali metal nitrate is sodium nitrate or potassium nitrate.

5. The composite early-strength agent according to claim 1, characterized in that, The alkali metal formate is sodium formate or potassium formate; the nanoscale seed inducer is a composite powder of nano-silica and nano-calcium carbonate, with a mass ratio of 1:1 to 2:

1.

6. The composite early-strength agent according to claim 2, characterized in that, The catalytic ionic liquid is 5% by mass in the composite early strength agent and is 1-butyl-3-methylimidazolium tetrafluoroborate.

7. The composite early-strength agent according to claim 3, characterized in that, The mass ratio of ethylene glycol to urea in the low-temperature eutectic phase change regulator is 1:1.5, and its mass percentage in the composite early strength agent is 22%.

8. The composite early-strength agent according to claim 4, characterized in that, The alkali metal nitrate is sodium nitrate, which accounts for 50% by mass in the composite early strength agent; the alkali metal formate is sodium formate, which accounts for 18% by mass in the composite early strength agent.

9. The composite early-strength agent according to claim 5, characterized in that, The mass ratio of nano-silica to nano-calcium carbonate in the nano-scale seed inducer is 3:2, and its mass percentage in the composite early strength agent is 5%.

10. A method for preparing the composite early-strength agent as described in any one of claims 1-9, characterized in that, Includes the following steps: A transparent mother liquor is formed by mixing a catalytic ionic liquid with deionized water at a mass ratio of 1:

3. Add the low-temperature eutectic phase transition modifier and stir until completely dissolved; Add alkali metal nitrate and alkali metal formate in sequence, and stir until well mixed; Then, a nano-sized seed inducer is introduced by high-speed shear dispersion to form a stable suspension; The suspension is sprayed into a hot air drying tower via a centrifugal atomizer, and the resulting powder is sieved to obtain spherical particles.