Low-carbon early strength admixture for shotcrete and preparation method thereof

Abstract

This invention discloses an early-strength admixture for low-carbon shotcrete and its preparation method. The early-strength admixture, by weight, consists of 30-80 parts quicklime, 10-50 parts calcium salt, and 0-60 parts silica fume. The preparation method involves carbonizing and grinding the above raw materials in an environment containing carbon dioxide gas. This invention, by supplementing the calcium source externally, alleviates the competition for calcium ions by early hydration products caused by aluminum sulfate-based accelerators, promoting C-S-H gel formation. Simultaneously, the highly active calcium carbonate generated by carbonization and grinding provides nucleation sites for hydration products, further accelerating early hydration. This admixture can significantly improve the early compressive strength of shotcrete (reaching over 12 MPa at 8 hours) and has a low-carbon effect by sequestering carbon dioxide, making it particularly suitable for applications requiring rapid support, such as tunnel engineering.

Description

Technical Field

[0001] This invention relates to the field of shotcrete materials technology, and more particularly to the field of shotcrete technology with early high strength requirements. Background Technology

[0002] In underground or geotechnical engineering fields such as tunnels, mines, slopes, and foundation pits, shotcrete is a crucial support method. The surrounding rock or soil in these engineering environments often has poor stability. After excavation, stress redistributes rapidly, leading to a series of risks such as deformation, loosening, and even collapse, seriously threatening project safety and construction progress. Given these risks, timely support is paramount. Shotcrete needs to reach sufficient strength within a few hours of excavation to quickly provide reliable structural support to the surrounding rock, effectively limiting deformation, preventing accidents such as rockfalls and collapses, and ensuring the safety of construction personnel and equipment. To meet this urgent need, accelerators are widely used in shotcrete, accelerating the hydration process of ordinary Portland cement and promoting rapid concrete setting.

[0003] Based on the difference in alkali content, accelerators can be divided into two main categories: alkali-containing accelerators and alkali-free accelerators. Compared with alkali-containing accelerators, alkali-free accelerators have gained wider application in engineering practice due to their advantages such as less damage to the later strength of concrete and lower impact on worker health. The main components of alkali-free accelerators include aluminum sulfate (AS), alkanolamines, and acids, among which AS is the core accelerator component. Numerous studies have shown that when AS is added to a cement system, it consumes the calcium in the pore solution of the cement paste. 2+ This process causes a large amount of ettringite (AFt) to precipitate in the liquid phase, which strongly promotes the rapid setting of cement and the rapid development of early strength. However, the effect of relying solely on the formation of AFt on improving early strength is relatively limited and cannot fully meet the stringent requirements of early support (such as the requirement of 10 MPa compressive strength at 8 hours in Chinese railway standards).

[0004] Existing technologies, such as CN 120247497 A, use plasma-enhanced mechanochemical treatment of silica fume and slag to obtain early-strength admixtures, which improve the 3-day and 7-day strength of shotcrete, but do not mention the 8-hour strength. Patent CN119874247 A uses limestone, montmorillonite, granite, bauxite, magnesium sulfate, etc., to prepare early-strength admixtures, but the improvement in the 8-hour strength of shotcrete is limited. CN 118930117 A uses early-strength admixtures prepared with calcium nitrate, nucleation accelerators, sodium gluconate, sodium carbonate, etc., to significantly improve the 8-hour strength of shotcrete, but the sodium salt content in the materials is detrimental to the later-stage durability of the shotcrete. Summary of the Invention

[0005] To address the problems mentioned above, this invention provides an early-strength admixture for low-carbon shotcrete, the technical solution of which is as follows:

[0006] An early-strength admixture for low-carbon shotcrete, by weight, comprises the following components:

[0007] 30-80 parts of slaked lime

[0008] 10-50 parts calcium salt,

[0009] 0-60 parts silica fume.

[0010] Preferably, the slaked lime contains more than 95% calcium hydroxide and has a particle size range of 30-65 μm.

[0011] Preferably, the calcium salt includes at least one of calcium formate and calcium nitrate, with a particle size range of 20-70 μm.

[0012] Preferably, the silica fume contains more than 95% SiO2 and has a particle size range of 0.1-1μm.

[0013] Preferably, the early strength admixture is processed by carbonization grinding technology, wherein carbonization grinding refers to introducing a gas containing carbon dioxide during the grinding process.

[0014] This invention also discloses a method for preparing the above-mentioned low-carbon shotcrete early-strength admixture, characterized by comprising the following steps:

[0015] (1) Weigh out quicklime, calcium salt and silica fume according to the weight ratio;

[0016] (2) Place the weighed raw materials into a carbonization grinding equipment and grind them under conditions of passing carbon dioxide gas;

[0017] Preferably, the carbonization grinding equipment is a ball mill that passes carbon dioxide gas, and the concentration of the carbon dioxide gas is not less than 25%.

[0018] Preferably, the temperature of the grinding process is controlled at 20-40℃.

[0019] The present invention also discloses a shotcrete, characterized in that it comprises cement and the above-mentioned early-strength admixture, wherein the amount of early-strength admixture is 5%-15% of the cement mass.

[0020] Beneficial effects

[0021] (1) Precise design of the material system: To address the calcium ion competition dilemma caused by aluminum sulfate-based accelerators, a unique ternary composite system was constructed, consisting of 'highly active quicklime' as the core calcium source and carbonization carrier, 'functional calcium salts' as a rapid synergist, and 'ultrafine silica fume' as a post-construction structural optimizer. This system works synergistically from three dimensions: rapid calcium ion supply (quicklime, calcium salts), accelerated hydration reaction (calcium salts, carbonization products), and sustained microstructural densification (silica fume). None of these components can be omitted. Discarding or replacing any component will compromise the integrity and early strength effect of the system.

[0022] (2) Scientific definition of the proportion range: Through extensive experimental verification, the specific proportion range of each component is the key window for achieving the above-mentioned synergistic effect. Quicklime (30-80 parts) ensures the basic calcium effect and carbonization potential; calcium salt (10-50 parts) provides key early functional synergy under controllable cost; the wide range of silica fume (0-60 parts) gives the product excellent flexibility to cope with different engineering priorities (pure early strength vs. early strength and high durability). This proportion range is the technical guarantee that this scheme can simultaneously meet ultra-high early strength (8h ≥12MPa) and good later strength (28d ≥35MPa).

[0023] (3) Activation Innovation in Preparation Process: The 'carbonization grinding' mechanochemical process is adopted, which is not a simple mixing and refining, but rather an 'activation pre-processing' of quicklime during the preparation stage. This process converts some Ca(OH)2 into highly active nano-calcium carbonate crystal nuclei and optimizes particle dispersibility. These 'pre-formed nuclei' can immediately play a role during concrete hydration, significantly shortening the hydration induction period, thereby producing an early strength enhancement effect far exceeding that of ordinary mechanical mixing. This process cleverly combines material performance enhancement with carbon dioxide sequestration, reflecting the innovative thinking of green manufacturing. Attached Figure Description

[0024] Figure 1 Bar chart showing the influence of admixture composition and preparation process on the compressive strength of shotcrete;

[0025] Figure 2 Bar chart showing the effect of admixture dosage on the compressive strength of shotcrete. Detailed Implementation

[0026] Currently, aluminum sulfate-based alkali-free accelerators are mainly used in shotcrete to achieve rapid early-stage compressive strength development. The principle is that the aluminum sulfate in the aluminum sulfate-based alkali-free accelerator reacts with the calcium in the cement paste. 2+ The reaction generates AFt, enabling rapid early strength development. However, the formation of AFt simultaneously consumes a large amount of Ca in the cement paste pore solution. 2+This reduces the saturation of CSH in the cement paste, thereby inhibiting the formation of CSH precipitates. Ultimately, this makes it difficult for shotcrete using aluminum sulfate-based alkali-free accelerators to meet the requirements for ultra-early strength in its early stages. Therefore, this invention proposes a method to enhance the early strength of shotcrete by adding exogenous calcium-containing admixtures, the principle of which is based on Ca... 2+ Regulation, through external supplementation of Ca 2+ This promotes CSH gel formation. Simultaneously, it guides the formation of AFt at exogenous calcium sources, reducing its accumulation on the surface of cement particles and mitigating its impact on ion dissolution and diffusion during cement hydration, thus promoting hydration. This achieves the goal of improving the early compressive strength of shotcrete.

[0027] This invention further enhances the performance of exogenous calcium-containing admixtures through carbonation and grinding technology. The principle is as follows: carbonation and grinding convert calcium sources such as calcium hydroxide in the admixture into metastable, highly reactive calcium carbonate (CC), providing nucleation sites for hydration products such as AFt, calcium monosulfoaluminate (AFm), and CSH during the early hydration of cement, thus promoting early cement hydration. Furthermore, the highly reactive CC can react with C3A in the cement to form Mc and Hc, which helps refine the pore structure. Ultimately, this enhances the performance of exogenous calcium-containing admixtures while achieving a certain degree of carbon fixation.

[0028] Based on the above principles, this invention provides an early-strength admixture for low-carbon shotcrete and its preparation method, which achieves a shotcrete strength of 12 MPa in 8 hours while solidifying a portion of carbon dioxide.

[0029] The core objective of this invention is to provide highly active, rapidly released calcium ions (Ca) in the very early stage (8 hours). 2+ This approach aims to resolve the competition between accelerators and cement hydration for calcium ions, while also considering later-stage microstructure optimization and low-carbon effects. The selection of these three materials is based on a deep understanding of the early-stage hydration chemistry of shotcrete, forming a complementary and synergistic "calcium ion regulation and enhancement system."

[0030] This invention uses only three materials: quicklime, calcium salts, and silica fume. The reasons for this selection are as follows:

[0031] Quicklime (main calcium source and carbonization carrier):

[0032] Reason for selection: Quicklime (Ca(OH)2) is the primary calcium source in this invention. Compared to limestone powder (CaCO3, low activity) and ordinary silicate cement (where the rate of calcium ion release during hydration is inhibited by competition from the aluminum phase), quicklime has extremely high chemical activity, dissolves rapidly in an aqueous environment, and directly and quickly replenishes a large amount of Ca into the pore solution of the slurry. 2+This directly meets the need for rapid calcium ion generation in the early stages of AFt and CSH gels.

[0033] Synergistic Effects and Irreplaceability: Its high activity makes it an ideal material for carbonization grinding. Introducing CO2 during the grinding process can partially convert it into metastable, highly active nano-sized calcium carbonate (CC). This CC is not only a calcium source itself, but more importantly, it provides a vast number of nucleation sites for hydration products such as AFt and CSH, greatly accelerating the precipitation and growth of these products. This is one of the key mechanisms for achieving ultra-early strength. Other inert or low-activity calcium sources (such as ordinary limestone) cannot achieve this "in-situ activation" effect.

[0034] Calcium salts (functional calcium supplements and strength modifiers):

[0035] Reason for selection: Relying solely on quicklime results in a relatively singular release pattern of calcium ions. This invention introduces specific calcium salts (such as calcium formate and calcium nitrate) as auxiliary calcium sources, a carefully designed approach.

[0036] Calcium formate: Not only is it a calcium source, but its formate ions are also well-known early strength agents. They can accelerate the hydration of tricalcium silicate (C3S) and further promote the early formation of CSH, thus creating a time-dependent synergistic effect with the calcium-supplementing effect of quicklime.

[0037] Calcium nitrate: While providing calcium ions, nitrate ions can significantly lower the freezing point of the solution and improve early strength, which is especially important for construction in low-temperature or water-rich tunnel environments.

[0038] Synergistic Effects and Irreplaceability: The dissolution kinetics of calcium salts differ from those of slaked lime. Calcium salts provide an immediate and rapid burst of calcium ions in the early stages of hydration, forming a "fast-slow combination" calcium supply curve with the slightly slower but more sustained release of calcium ions from slaked lime, ensuring sufficient calcium ion concentration throughout the early hydration process. Using only slaked lime may not be sufficient to handle the most intense calcium competition in the early stages of hydration; using only calcium salts is costly, may introduce harmful ions (such as chlorides), and lacks the carbonization activation and later alkalinity maintenance functions of slaked lime. The combination of the two achieves a calcium supplementation effect greater than the sum of its parts (1+1>2).

[0039] Silica fume (microstructure optimizer and post-construction strength guarantee):

[0040] Reason for selection: Silica fume (SiO2>95%) is an extremely fine spherical glass. In this invention, its core function is not to provide early strength, but to perform "back-end optimization".

[0041] Synergistic effects and irreplaceability:

[0042] Physical filling: Its ultra-fine particle size (0.1-1μm) can fill the gaps between quicklime, calcium salt particles and cement particles, making the initial packing of the slurry more compact.

[0043] Volcanic ash reaction: Under the activation of Ca(OH)2 generated during early hydration, silica fume undergoes a volcanic ash reaction in the later stage (after 1 day), generating additional CSH gel, which continuously densifies the microstructure of the slurry, ensuring and improving the later strength. This is a "responsive" enhancement after the consumption of quicklime.

[0044] Stabilizing effect: The presence of silica fume can improve the cohesiveness of the slurry, reduce jet rebound, and help stabilize the network of hydration products formed in the early stages.

[0045] The profound meaning of the "0 parts" design: The technical solution allows for zero parts of silica fume, which precisely reflects the clear layering of the invention's concept. This is particularly relevant when the project only pursues early strength (8h) and not 28-day strength.

[0046] When performance requirements are generally low, some later-stage performance can be sacrificed, relying entirely on the calcium chemistry system of quicklime and calcium salts. This provides the formulation with great flexibility and engineering specificity.

[0047] These three materials are not randomly combined: quicklime serves as the active calcium source and carbonization core, calcium salts act as a functional early-strength synergist, and silica fume optimizes and stabilizes the microstructure in later stages. They respectively target the three key nodes of early hydration: "explosive calcium demand," "accelerated reaction," and "structural durability," forming a logically closed-loop solution. Adding other materials (such as slag and fly ash) results in insufficient early-stage activity; removing any one of them will disrupt the integrity of this synergistic system, leading to a significant decrease in early-strength effect, later-stage stability, or low-carbon benefits.

[0048] All raw materials used in this invention were purchased, and their sources are detailed in Table 1.

[0049] cement Beijing Jinyu Co., Ltd. slaked lime Nantong Shengrui Environmental Protection Technology Co., Ltd. silica ash Wuhan Huashen Intelligent Technology Co., Ltd. Calcium formate Guangzhou Daxiao Chemical Co., Ltd. fine aggregate Manufactured sand Calcium nitrate Guangzhou Daxiao Chemical Co., Ltd. coarse aggregate Manufactured sand Water reducing agent Subote New Materials Co., Ltd. quick-setting agent Subote New Materials Co., Ltd.

[0050] This invention discloses an early-strength admixture for low-carbon shotcrete, which is composed of the following components by weight: 30-80 parts of slaked lime: Lower limit (30 parts): This is the minimum effective amount required to alter the calcium ion chemical environment of the slurry and produce a significant carbonation enhancement effect. Below this value, the external calcium source effect and the amount of active CC generated by carbonation are insufficient, failing to effectively counteract the calcium consumption by the accelerator, and the early strength enhancement effect is not significant (as shown in the comparative example).

[0051] Upper limit (80 parts): Quicklime itself has no cementitious properties. Excessive amounts will dilute the cement, resulting in excessively alkaline paste, which may affect the effectiveness of water-reducing agents. Furthermore, excessive Ca(OH)2, if not fully carbonized or reacted, may become a potential durability hazard. 80 parts is the critical point to ensure a balance between early strength and the final performance of the concrete structure.

[0052] 10-50 parts calcium salt,

[0053] Lower limit (10 samples): This is the minimum threshold amount required to provide measurable early strength enhancement and functional ionic effects (such as formate catalysis and nitrate antifreeze). Below this value, its auxiliary rapid-release calcium and early strength functions cannot be effectively demonstrated;

[0054] Maximum (50 parts): Calcium salts (especially calcium nitrate) are expensive. Excessive calcium salt content will significantly increase costs and may lead to premature hydration, difficulty in controlling setting time, or potential risks to long-term durability from introduced acid radicals (although this invention prefers chlorine-free, low-hazard salts). 50 parts is the optimal balance between economics and technical effectiveness.

[0055] 0-60 parts silica fume:

[0056] The wide range (0-60) reflects the formulation's engineering adaptability:

[0057] 0 parts: For applications requiring extreme early strength and minimum cost, relying entirely on calcium chemistry systems.

[0058] 10-30 parts (preferred): The range with the best overall performance. Within this range, the filling effect of silica fume and the pozzolanic reaction can perfectly compensate for the slight increase in porosity that may be caused by the quicklime-calcium salt system in the later stage, achieving both high early strength and high later strength.

[0059] Upper limit (60 parts): Silica fume requires a large amount of water. Excessive silica fume content will lead to poor workability, requiring more water-reducing agent, increasing costs, and excessively fine particles may require more water for lubrication, which is detrimental to early strength development. 60 parts is the technical upper limit for maintaining a balance between sprayable concrete and strengthening effect.

[0060] The aforementioned proportion range (30-80 parts quicklime, 10-50 parts calcium salt, 0-60 parts silica fume) is not a simple list of experimental data, but rather an optimization window derived from the functional boundaries and synergistic effects of each component. This proportion range defines a multi-dimensional performance optimization space, allowing engineers to precisely fine-tune the formulation within this space based on different priorities for "8-hour strength," "28-day strength," "cost," and "durability" for specific projects. This demonstrates the creativity in the method of preparing early-strength admixtures for low-carbon shotcrete by selecting appropriate proportions.

[0061] Example 1: Preparation method of early-strength admixture for low-carbon shotcrete 1

[0062] Weigh out 20 parts of quicklime, 60 parts of calcium salt, and 20 parts of silica fume according to the mass composition, and grind them in a carbonization grinding system for 2 hours. This mixture is recorded as early strength admixture 1.

[0063] Example 2: Preparation method of early-strength admixture for low-carbon shotcrete 2

[0064] Weigh out 40 parts of quicklime, 40 parts of calcium salt, and 20 parts of silica fume according to the mass composition, and grind them in a carbonization grinding system for 2 hours. This mixture is recorded as early strength admixture 2.

[0065] Example 3: Preparation method of early-strength admixture for low-carbon shotcrete 3

[0066] Weigh out 70 parts of quicklime, 10 parts of calcium salt, and 20 parts of silica fume according to the mass composition, grind them in a carbonization grinding system for 2 hours, and record it as early strength admixture 3.

[0067] Example 4: Preparation method of early-strength admixture for low-carbon shotcrete 4

[0068] Weigh out 90 parts of quicklime, 10 parts of calcium salt, and 0 parts of silica fume according to the mass composition, grind them in a carbonization grinding system for 2 hours, and record it as early strength admixture 4.

[0069] Comparative Example 1: Cement Grinding Method 1

[0070] Weigh 100 parts of cement according to the mass composition, grind them in a ball mill for 2 hours, and record this as cement after grinding without carbonization.

[0071] Comparative Example 2: Cement Grinding Method 2

[0072] Weigh 100 parts of cement according to the mass composition, and grind them in a ball mill with a carbon dioxide concentration of 25% for 2 hours. This is recorded as the carbonized and ground cement.

[0073] Comparative Example 3: Preparation method of early-strength admixture for low-carbon shotcrete 5

[0074] Weigh out 70 parts of quicklime, 10 parts of calcium salt, and 20 parts of silica fume according to the mass composition, grind them in a ball mill for 2 hours, and record this as early strength admixture 5.

[0075] Example 5: Preparation of Shotcrete 1

[0076] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts early strength admixture, 8 parts quick-setting agent, and 1 part water-reducing agent.

[0077] (2) Mix PO cement and early-strength admixture 1 evenly to obtain a cementitious material;

[0078] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0079] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0080] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0081] (6) The spraying parameters for shotcrete are selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete is obtained.

[0082] Example 6: Preparation of Shotcrete II

[0083] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts early strength admixture, 2.8 parts quick-setting agent, and 1 part water-reducing agent.

[0084] (2) Mix PO cement and early-strength admixture 2 evenly to obtain a cementitious material;

[0085] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0086] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0087] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0088] (6) The spraying parameters for shotcrete are selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete II is obtained.

[0089] Example 7 Preparation of Shotcrete III

[0090] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts early strength admixture, 3 parts quick-setting agent, 8 parts water-reducing agent.

[0091] (2) Mix PO cement and early-strength admixture 3 evenly to obtain cementitious material;

[0092] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0093] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0094] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0095] (6) The spraying parameters for shotcrete are selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete is obtained.

[0096] Example 8: Preparation of Shotcrete IV

[0097] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts early strength admixture, 4 parts quick-setting agent, 8 parts water-reducing agent.

[0098] (2) Mix PO cement and early-strength admixture 4 evenly to obtain cementitious material;

[0099] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0100] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0101] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0102] (6) The spraying parameters for shotcrete were selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, to obtain shotcrete four. Comparative Example 4: Preparation of shotcrete five

[0103] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts cement after non-carbonation grinding, 8 parts quick-setting agent, and 1 part water-reducing agent.

[0104] (2) PO cement and non-carbonated cement are ground uniformly to obtain a cementitious material;

[0105] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0106] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0107] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0108] (6) The spraying parameters for shotcrete are: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete is obtained.

[0109] Comparative Example 5: Preparation of Shotcrete VI

[0110] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts carbonized and ground cement, 8 parts quick-setting agent, and 1 part water-reducing agent.

[0111] (2) Grind PO cement and carbonized powder into a uniform cementitious material;

[0112] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0113] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0114] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0115] (6) The spraying parameters for shotcrete are selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete VI is obtained.

[0116] Comparative Example 6: Preparation of Shotcrete VII

[0117] (1) Accurately weigh the following raw materials by weight: including 94 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 6 parts early strength admixture, 5 and 8 parts quick-setting agent, and 1 part water-reducing agent.

[0118] (2) Mix PO cement and early-strength admixture 5 cement evenly to obtain cementitious material;

[0119] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0120] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0121] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0122] (6) The spraying parameters for shotcrete are: spraying air pressure 1.0 MPa, spraying distance 1.2 m, to obtain shotcrete No. 7.

[0123] Example 9: Preparation of Shotcrete VIII

[0124] (1) Accurately weigh the following raw materials by weight: including 96 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 4 parts early strength admixture, 3 parts quick-setting agent, 8 parts water-reducing agent.

[0125] (2) Mix PO cement and early-strength admixture 3 evenly to obtain cementitious material;

[0126] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0127] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0128] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0129] (6) The spraying parameters for shotcrete are selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete is obtained.

[0130] Example 10: Preparation of Shotcrete 9

[0131] (1) Accurately weigh the following raw materials by weight: including 91 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 9 parts early strength admixture, 3 parts quick-setting agent, and 1 part water-reducing agent.

[0132] (2) Mix PO cement and early-strength admixture 3 evenly to obtain cementitious material;

[0133] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0134] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0135] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0136] (6) The spraying parameters for shotcrete are: spraying air pressure 1.0 MPa, spraying distance 1.2 m, to obtain shotcrete.

[0137] Example 11 Preparation of Shotcrete

[0138] (1) Accurately weigh the following raw materials by weight: including 88 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 12 parts early strength admixture, 3 parts quick-setting agent, and 1 part water-reducing agent.

[0139] (2) Mix PO cement and early-strength admixture 3 evenly to obtain cementitious material;

[0140] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0141] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0142] (5) Add the admixture solution and the quick-setting agent to the dry mix at the same time and stir at high speed until uniform. (6) Select the following spraying parameters for shotcrete: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and obtain shotcrete.

[0143] Example 12 Preparation of Shotcrete XI

[0144] (1) Accurately weigh the following raw materials by weight: including 84 parts cement, 36 parts water, 175 parts fine aggregate, 175 parts coarse aggregate, 16 parts early strength admixture, 3 parts quick-setting agent, and 1 part water-reducing agent.

[0145] (2) Mix PO cement and early-strength admixture 3 evenly to obtain cementitious material;

[0146] (3) Add coarse aggregate and fine aggregate to the cementitious material and dry mix to obtain dry mixture;

[0147] (4) Add the water-reducing agent to water to obtain an admixture solution;

[0148] (5) Add the admixture solution and the quick-setting agent to the dry mixture at the same time and stir at high speed until uniform.

[0149] (6) The spraying parameters for shotcrete are selected as follows: spraying air pressure 1.0 MPa, spraying distance 1.2 m, and shotcrete eleven is obtained.

[0150] Table 2 Statistical Table of Compressive Strength of Shotcrete

[0151] Shotcrete 9.8 21.5 37.6 Shotcrete II 12.3 23.6 36.6 Shotcrete III 12.7 24.7 38.1 Shotcrete 4 12.4 23.1 35.5 Shotcrete 5 7.8 19.6 36.0 Shotcrete 6 8.1 20.7 36.3 Shotcrete 7 11.4 21.8 37.3 Shotcrete 8 8.4 21.4 37.1 Shotcrete 9 12.7 23.5 37.5 Shotcrete 10 11.7 22.0 36.5 Shotcrete 11 10.8 20.8 34.8

[0152] in:

[0153] Examples 1-4 (Early-Strength Admixtures 1-4): Conclusion: The performance of different proportions of early-strength admixtures (quicklime: calcium salt: silica fume 20:60:20, 40:40:20, 70:10:20, 90:10:0 respectively) was demonstrated. The results showed that the proportion with a high quicklime content (70 parts) and an appropriate amount of silica fume (20 parts) (Example 3) showed the best overall effect on both early strength (12.7 MPa at 8h) and later strength (38.1 MPa at 28d). When the silica fume content was 0 (Example 4), the early strength was acceptable, but the later strength decreased slightly, indicating that silica fume has a positive effect on the development of later strength.

[0154] Examples 5-7 (Shotcrete I, II, and III using admixtures 1-3): Conclusion: The excellent effect of the optimal mix proportion (admixture in Example 3) in actual shotcrete was verified (concrete III 8-hour strength 12.7 MPa), and it was shown that quicklime is the key to providing early strength calcium source, while calcium salts (especially calcium nitrate, etc.) contribute to ensuring the continuous development of strength.

[0155] Examples 8-11 (using admixture 3 at dosages of 4%, 9%, 12%, and 16% for shotcrete 8-11): Conclusion: The optimal dosage range for the early-strength admixture was determined. Too low a dosage (4%, concrete 8) resulted in limited early-strength improvement (8.4 MPa); at around 9% (concrete 9), the early-strength effect was significant (12.7 MPa) and the later-stage strength was good; too high a dosage (16%, concrete 11) led to a decrease in later-stage strength (34.8 MPa) due to the relatively low cement content. Therefore, the preferred dosage is 5-15% of the cement mass, more preferably about 9%.

[0156] Comparative Examples 1 & 4 (non-carbonized milled cement): Conclusion: As a blank control group, it was shown that the baseline early strength (7.8 MPa) of shotcrete was lower when no early strength admixtures were added and only ordinary milled cement was used.

[0157] Comparative Examples 2 & 5 (Carbonized Grinding of Cement): Conclusion: It is shown that carbonized grinding of cement alone has a very limited effect on improving the early strength of shotcrete (8.1 MPa vs 7.8 MPa), proving that the role of carbonized grinding is mainly to activate the specific admixture components of the present invention, rather than to have a general early strength effect on the cement itself.

[0158] Comparative Examples 3 & 6 (Admixture 5 without carbonation grinding): Conclusion: By comparing the early strength admixtures with the same mix proportions that have undergone carbonation grinding (Admixture 3, Concrete III) and those that have not undergone carbonation grinding (Admixture 5, Concrete VII), the necessity and significance of the carbonation grinding process in improving the early strength performance of admixtures are directly demonstrated (8h strength: 12.7MPa vs 11.4MPa).

[0159] Comparative Example 6 (same as Comparative Example 3): Conclusion: Same as above, emphasizing the key role of carbonization grinding process.

[0160] In addition, through Figure 1 Further conclusions from the bar charts: Comparing shotcrete I, II, III, IV, V, and VI, it is evident that introducing exogenous calcium ions can effectively improve the 8-hour and 1-day compressive strength of shotcrete. Comparing shotcrete I and II, it is shown that calcium introduced by quicklime is more helpful for the early compressive strength growth, and calcium salts have a greater effect on ensuring the later strength of shotcrete. Comparing shotcrete III and IV, it is shown that silica fume can promote the development of later compressive strength in shotcrete. The mechanism is that silica fume reacts with calcium hydroxide and other products through a pozzolanic reaction, promoting the densification of the shotcrete paste. Comparing shotcrete III and VII, it is shown that carbonation grinding can improve the early effect of early-stage admixtures. The mechanism is that carbonation grinding causes calcium sources such as calcium hydroxide in the admixtures to generate metastable, highly active calcium carbonate (CC), providing nucleation sites for hydration products such as AFt, calcium monosulfoaluminate (AFm), and CSH during the early hydration process of cement, thus promoting the early hydration of cement. Furthermore, highly reactive CC can react with C3A in cement to form Mc and Hc, which helps refine the pore structure. In summary, the content of each component in early-strength admixtures should be within a certain range to influence the admixture dosage. For example... Figure 2As shown, comparing shotcrete types 3, 8, 9, 10, and 11, a lower dosage of early-strength admixture has a limited effect on improving early compressive strength, while a higher dosage results in poor later-stage compressive strength due to the reduced cement content. Therefore, there is a certain range for the admixture dosage.

[0161] The carbonization grinding of this invention is not a simple "grinding + ventilation", but a directional mechanochemical activation and carbon fixation integrated process:

[0162] The essential difference from traditional grinding: Traditional grinding (Comparative Example 3): It only achieves material refinement and mixing, while the chemical properties of the material remain basically unchanged.

[0163] Carbonation grinding (this invention): While mechanically crushing and mixing, CO2 is introduced to react with the freshly fractured, highly active Ca(OH)2 surface. This not only fixes carbon but also "pre-formulates" the material. The effects of this carbonation grinding are as follows: Generation of "pre-formed nuclei": The in-situ generated nano-active calcium carbonate already exists before the admixture is added to the concrete. As "pre-formed hydration nuclei," they immediately provide growth sites for AFt and CSH upon contact with water, skipping the slow induction period of nucleus formation in traditional hydration. This is the fundamental reason for the sudden increase in early strength (comparative Example 3 and Comparative Example 3); Improved material dispersibility: The reacted fine calcium carbonate coats or spaces between particles, preventing the agglomeration of ultrafine powders (especially silica fume), making it easier to disperse in concrete and exert its effect; Achieving low-carbonization of the process: Industrial CO2 is encapsulated in the admixture, giving the early-strength product itself "negative carbon" or "low-carbon" properties, combining material strengthening with innovation.

[0164] In summary, the early-strength admixture prepared by the present invention through a specific ratio of quicklime / calcium salt / silica fume and combined with carbonization grinding process can most effectively synergistically improve the early (8h) and later compressive strength of shotcrete when the appropriate dosage (such as 9% of cement mass) is used. Its effect is significantly better than that of cases where the admixture is not used, the carbonization grinding process is not adopted, or the ratio is improper.

[0165] This invention proposes an early-strength admixture for low-carbon shotcrete and its preparation method, the principle of which is based on Ca 2+ Regulation, through external supplementation of Ca 2+This process promotes CSH gel formation and guides the formation of AFt at exogenous calcium sources, reducing its accumulation on the surface of cement particles and mitigating its impact on ion dissolution and diffusion during cement hydration, thus promoting hydration. This achieves the goal of improving the early compressive strength of shotcrete. Furthermore, carbonation grinding technology further enhances the performance of exogenous calcium-containing admixtures. The principle is that carbonation grinding causes calcium sources such as calcium hydroxide in the admixture to generate metastable, highly active calcium carbonate (CC), providing nucleation sites for hydration products such as AFt, calcium monosulfoaluminate (AFm), and CSH during early cement hydration, promoting early cement hydration. In addition, the highly active CC can react with C3A in the cement to form Mc and Hc, which helps refine the pore structure. Ultimately, this enhances the performance of exogenous calcium-containing admixtures and also has a certain carbon fixation effect. Through an innovative design that integrates materials, proportions, and processes, this invention provides a highly efficient and low-carbon solution that fundamentally addresses the problem of insufficient early-stage strength in shotcrete. Its technological effects far exceed the simple superposition of existing technologies, exhibiting outstanding substantive characteristics and significant progress.

[0166] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A low carbon early strength admixture for sprayed concrete, characterized in that By weight, it consists of the following components: 30-80 parts of slaked lime 10-50 parts calcium salt, 0-60 parts silica fume; The quicklime contains more than 95% calcium hydroxide and has a particle size range of 30-65 μm; the calcium salt includes at least one of calcium formate and calcium nitrate and has a particle size range of 20-70 μm; the silica fume contains more than 95% SiO2 and has a particle size range of 0.1-1 μm; the early strength admixture is processed by carbonization grinding technology, which refers to the introduction of carbon dioxide gas during the grinding process.

2. A method for preparing an early-strength admixture for low-carbon shotcrete as described in claim 1, characterized in that, Includes the following steps: (1) Weigh out quicklime, calcium salt and silica fume according to the weight ratio; (2) Place the weighed raw materials into the carbonization grinding equipment and grind them under the condition of passing carbon dioxide gas.

3. The method according to claim 2, characterized in that, The carbonization grinding equipment is a ball mill that passes carbon dioxide gas, and the concentration of the carbon dioxide gas is not less than 25%.

4. The method according to claim 3, characterized in that, The temperature during the grinding process is controlled at 20-40℃.

5. A type of shotcrete, characterized in that, It comprises cement and the early-strength admixture as described in claim 1, wherein the amount of the early-strength admixture is 5%-15% of the cement mass.

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