An ultra-early-strength shotcrete and a preparation method thereof

Abstract

This invention discloses an ultra-early strength shotcrete and its preparation method. By mass, it comprises the following components: 400 parts of cementitious material, which is a mixture of cement and composite admixtures; 340 parts of cement, which is ordinary Portland cement of P·O 42.5 grade or higher; 60 parts of composite admixture, which includes active mineral admixtures and additives; the active mineral admixtures include bauxite, limestone, and gypsum; the additives include tartaric acid, sodium gluconate, and polycarboxylate powder; 600 parts of aggregate, which is graded sand with a fineness modulus of 2.2; 24 parts of accelerator; and 160 parts of water, with a water-cement ratio of 0.4. This invention uses conventional P·O42.5 cement, controls the water-cement ratio at 0.4, which is easy to mix and spray. Furthermore, by using a bauxite, limestone, and gypsum to form a composite admixture, the mortar can achieve a high support strength within 3 hours after spraying. At the same time, while ensuring that the 24-hour strength meets the support requirements, it significantly reduces material costs and has good construction adaptability.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to an ultra-early strength sprayed mortar and its preparation method. Background Technology

[0002] Shotcrete is widely used in the initial support of tunnels, slopes, culverts, etc. Shotcrete is the main material. After shotcrete is applied to the wall, it needs to harden and set quickly in order to achieve the support effect. Therefore, shotcrete has early strength requirements. The early strength requirement for shotcrete is a compressive strength of not less than 10 MPa after 24 hours.

[0003] In existing technologies, traditional shotcrete is typically formulated with ordinary Portland cement, sand, aggregate, water, and admixtures. Its early strength improvement mainly relies on the introduction of admixtures or mineral admixtures, commonly including nano-early-strength agents, early-strength admixtures, or modified mineral admixtures. Adding nano-early-strength agents or early-strength admixtures to concrete can achieve 8-hour and 24-hour compressive strengths of 10-12 MPa and 19-22 MPa, respectively. Adding modified mineral admixtures to concrete can increase 3-day and 7-day compressive strengths to over 30 MPa and 44 MPa, respectively. However, its early strength improvement is slow, especially since there are no specific requirements for the 3-hour compressive strength of shotcrete.

[0004] In addition, UHPC shotcrete has higher early strength performance compared to traditional shotcrete, with a 24-hour compressive strength of over 40 MPa, indicating sufficient early strength. However, UHPC shotcrete has a water-cement ratio of less than 0.23 and uses 52.5 grade cement, resulting in higher cost and poorer adaptability compared to traditional shotcrete. Summary of the Invention

[0005] The purpose of this invention is to provide an ultra-early strength shotcrete and its preparation method to solve the problems mentioned in the background art.

[0006] To solve the above-mentioned technical problems, the present invention provides an ultra-early strength sprayed mortar, which comprises the following components by mass:

[0007] The cementitious material comprises 400 parts, which is a mixture of cement and composite admixtures. The cement comprises 340 parts, which is ordinary Portland cement of P·O 42.5 grade or higher. The composite admixture comprises 60 parts, which includes active mineral admixtures and additives. The active mineral admixtures include bauxite, limestone, and gypsum; the additives include tartaric acid, sodium gluconate, and polycarboxylate powder.

[0008] 600 parts of aggregate, wherein the aggregate is graded sand with a fineness modulus of 2.2;

[0009] 24 parts of quick-setting agent;

[0010] 160 parts water, with a water-to-gel ratio of 0.4.

[0011] Furthermore, the mass ratio of the bauxite, the limestone, and the gypsum is 5:4.4:4.7.

[0012] Furthermore, the tartaric acid accounts for 1% of the mass of the active mineral admixture, the sodium gluconate accounts for 0.2% of the mass of the active mineral admixture, and the polycarboxylate powder accounts for 1.5% of the mass of the active mineral admixture.

[0013] Furthermore, the accelerator is an iron-labeled alkali-free accelerator.

[0014] A method for preparing ultra-early strength shotcrete as described in any one of the following steps:

[0015] S1. Preparation of composite admixture: Weigh bauxite, limestone and gypsum according to the proportion, calcine the weighed bauxite, limestone and gypsum together in a high-temperature furnace, and continuously grind the calcined and cooled minerals through a ball mill to obtain active mineral admixture. Then add tartaric acid, sodium gluconate and polycarboxylate powder to the ground active mineral admixture powder and mix thoroughly to prepare the composite admixture.

[0016] S2. Preparation of ultra-early strength shotcrete: Weigh cement, aggregate, water and the composite admixture prepared in step S1 according to the proportion, mix them evenly and then add the quick-setting agent.

[0017] Furthermore, in step S1, the temperature of the high-temperature furnace is 1350℃, and the calcination time is 150min.

[0018] Furthermore, in step S1, the calcined and cooled mineral is continuously ground in a ball mill until the particle size distribution meets the following requirements:

[0019] The proportion of 10nm~100nm is greater than 5%, the proportion of 0.1μm~1μm is greater than 20%, and the proportion of 1μm~10μm is greater than 65%.

[0020] Furthermore, the active mineral admixture powder contains ≥20% alumina, ≥45% calcium oxide, and ≥30% sulfur trioxide.

[0021] Furthermore, in step S2, cement, aggregate, water, and the composite admixture prepared in step S1 are weighed according to the proportion, mixed evenly, and then sprayed using a mortar wet spraying machine. During the spraying construction, an accelerator is added to the nozzle of the mortar wet spraying machine to carry out the wet spraying operation.

[0022] The beneficial effects of this invention are as follows: This invention uses conventional P·O42.5 cement, controls the water-cement ratio at 0.4, which is easy to mix and spray, and forms a composite admixture by using inexpensive industrial by-products or natural minerals (bauxite, limestone, gypsum), that is, introduces a bauxite-gypsum system. By utilizing the rapid formation mechanism of ettringite, the mortar can achieve a high support strength within 3 hours after spraying. At the same time, while ensuring that the 24-hour strength meets or even exceeds the support requirements, it significantly reduces material costs and has good construction adaptability. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are merely one embodiment of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0024] To make the objectives, technical solutions and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments.

[0025] In the following description, references to "an embodiment," "an embodiment," "an example," "example," etc., indicate that the described embodiment or example may include a particular feature, structure, characteristic, property, element, or limitation, but not every embodiment or example necessarily includes that particular feature, structure, characteristic, property, element, or limitation. Furthermore, the repeated use of the phrase "an embodiment according to this application," while possibly referring to the same embodiment, does not necessarily refer to the same embodiment.

[0026] This invention discloses an ultra-early strength sprayed mortar, which comprises the following components by weight:

[0027] The cementitious material comprises 400 parts, which is a mixture of cement and composite admixtures. The cement consists of 340 parts ordinary Portland cement of grade P·O42.5 or higher, and the composite admixtures consist of 60 parts (accounting for 15% of the total cementitious material). The composite admixtures include active mineral admixtures and additives. The active mineral admixtures include bauxite, limestone, and gypsum. The additives include tartaric acid, sodium gluconate, and polycarboxylate powder.

[0028] Bauxite, rich in active alumina, reacts rapidly with calcium hydroxide generated during cement hydration in the presence of gypsum to form ettringite. The formation rate of ettringite is extremely fast, and the crystals quickly form an early framework during growth, significantly improving the structural strength of the mortar within 3 hours. Limestone not only acts as a filler, but its fine particles also react with aluminates in the cement clinker to form aluminocarbonates, further stabilizing ettringite and preventing strength reduction due to later transformation. Tartaric acid and sodium gluconate in the admixtures are retarders. During the mixing stage before spraying, they adsorb onto the surface of cement particles, inhibiting the initial intense hydration of C3A (tricalcium aluminate) and C3S (tricalcium silicate), ensuring the mortar maintains its fluidity during pipeline transportation.

[0029] Tartaric acid and sodium gluconate in the admixtures are retarders. During the mixing stage before spraying, they adsorb onto the surface of cement particles, inhibiting the initial intense hydration of C3A (tricalcium aluminate) and C3S (tricalcium silicate), ensuring the mortar maintains its fluidity during pipeline transportation. Polycarboxylate powder, as a high-performance water-reducing agent, has carboxylate ions in its molecular structure that adsorb onto the surface of cement, bauxite, limestone, and other powder particles. Through the combined effects of electrostatic repulsion and steric hindrance, it releases the water trapped within the cement flocculation structure.

[0030] The aggregate consists of 600 parts, using graded sand with a fineness modulus of 2.2; it falls into the category of medium-fine sand. This type of medium sand has good gradation continuity, which can maximize the reduction of porosity between aggregates. Under the action of the accelerator, the slurry can quickly fill the gaps between sand particles, forming a dense packing structure, avoiding weak areas of strength caused by aggregate settling or segregation.

[0031] 24 parts of quick-setting agent (6% of the amount of cementitious material); the free aluminum ions in the quick-setting agent can forcefully activate tricalcium aluminate to react violently with gypsum and bauxite, promoting the formation of a network structure of calcite and CSH (hydrated calcium silicate) gel in a very short time (within minutes).

[0032] The water content is 160 parts, with a water-cement ratio of 0.4. Polycarboxylate powder is responsible for dispersing cement particles in the initial stage, reducing viscosity, thereby enabling the mortar to achieve sufficient spreadability and fluidity to meet the pumping requirements of spraying equipment. In other words, this invention achieves high density of cement stone structure at a conventional water-cement ratio of 0.4 through the micro-aggregate filling effect of the composite admixture and the optimization of hydration products.

[0033] This invention achieves early-strength, high-strength sprayed mortar by adding active admixtures to conventional mortar. The active admixtures can be activated by accelerators and rapidly accelerate cement hydration.

[0034] Tartaric acid and sodium gluconate in the composite admixture inhibit the activation of cement hydration by the active admixture and the cement's own hydration, ensuring that the mortar's fluidity remains intact over a long period. After adding the accelerator, the accelerator can rapidly activate the inhibited active admixture, quickly initiating continuous cement hydration and rapidly increasing the cement's compressive strength, thus achieving the goal of early high strength mortar.

[0035] This invention utilizes conventional P·O42.5 cement, controlling the water-cement ratio at 0.4, which facilitates mixing and spraying. Furthermore, it incorporates a composite admixture formed from inexpensive industrial byproducts or natural minerals (bauxite, limestone, gypsum), introducing a bauxite-gypsum system. By leveraging the rapid formation mechanism of ettringite, the mortar achieves high support strength within 3 hours of spraying. Simultaneously, while ensuring 24-hour strength meets or even exceeds support requirements, it significantly reduces material costs and exhibits good construction adaptability.

[0036] In addition, this invention uses the same cement and sand as shotcrete, combined with composite admixtures, to prepare ultra-early strength shotcrete with performance superior to early high strength shotcrete.

[0037] In one embodiment, the mass ratio of bauxite, limestone, and gypsum is 5:4.4:4.7. This ensures sufficient sulfate ions to react with the aluminum phase to generate a large amount of ettringite, while avoiding the risk of delayed ettringite formation due to excessive gypsum.

[0038] In one embodiment, tartaric acid accounts for 1% of the mass of the active mineral admixture, sodium gluconate accounts for 0.2% of the mass of the active mineral admixture, and polycarboxylate powder accounts for 1.5% of the mass of the active mineral admixture.

[0039] For alumina-rich systems (bauxite and gypsum), 1% tartaric acid is used as the main retarder, effectively solving the problem of easy flash setting of the aluminate phase and ensuring the reliability of shotcrete construction. Sodium gluconate at only 0.2% avoids excessive inhibition of the silicate phase, ensuring a 24-hour strength of 20-30 MPa, meeting or even exceeding the specifications for initial tunnel support. 1.5% polycarboxylate powder combined with a 0.4 water-cement ratio gives the mortar good pumpability and spray cohesiveness even with low water consumption, superior to traditional shotcrete.

[0040] In one embodiment, the accelerator is an iron-based alkali-free accelerator.

[0041] The "Iron Standard" refers to compliance with the requirements of the Chinese railway industry standard Q / CR 807—2020, "Alkali-Free Accelerator for Shotcrete." This standard sets strict requirements for core indicators such as alkali content (≤1.0%), setting time (initial setting ≤3 min, final setting ≤8 min), early strength (6-hour compressive strength ≥1.0 MPa, 24-hour compressive strength ≥10.0 MPa), and 28-day strength retention rate (≥90%). This means that the accelerator is alkali-free, sets quickly, and has low strength loss characteristics.

[0042] Alkali-free means that the alkali metal content in the accelerator is extremely low (usually ≤1.0%). Compared with traditional alkaline accelerators (alkali content can reach 15%~25%), it does not introduce a large amount of free alkali, thus avoiding the threat to the later durability of concrete (such as alkali-aggregate reaction), while significantly reducing the corrosiveness to the skin of construction workers and the corrosiveness to equipment.

[0043] Iron Standard alkali-free quick-setting agent introduces almost no free alkali and does not damage the hydration products of cement, achieving the ideal effect of preparing non-setting and quick-setting on the wall, meeting the requirements of spraying construction.

[0044] A method for preparing ultra-early strength shotcrete as described in any of the above embodiments, comprising the following steps:

[0045] S1. Preparation of composite admixture: Weigh bauxite, limestone and gypsum according to the proportion, calcine the weighed bauxite, limestone and gypsum together in a high-temperature furnace, and continuously grind the calcined and cooled minerals through a ball mill to obtain active mineral admixture. Then add tartaric acid, sodium gluconate and polycarboxylate powder to the ground active mineral admixture powder and mix thoroughly to prepare the composite admixture.

[0046] Calcination can pre-convert these raw materials into mineral phases with higher thermodynamic activity. Furthermore, the calcined products react faster and more thoroughly when they come into contact with water. The calcination process also removes carbon dioxide from limestone and water of crystallization from gypsum, thus avoiding abnormal volume fluctuations during later hydration.

[0047] The mechanical force during ball milling causes defects and lattice distortion in mineral crystals, further increasing surface energy and reactivity.

[0048] S2. Preparation of ultra-early strength mortar: Weigh cement, aggregate, water and the composite admixture prepared in step S1 according to the proportion, mix them evenly and then add the quick-setting agent.

[0049] In one embodiment, in step S1, the temperature of the high-temperature furnace is 1350°C and the calcination time is 150 min.

[0050] Conventional low-temperature calcination (800~1000℃) mainly achieves limestone decomposition, gypsum dehydration, and a small amount of surface activation. The products are still mainly in amorphous or microcrystalline form, and the reactivity depends on the alkaline activation during the later hydration stage. In contrast, the high-temperature calcination at 1350℃ in this scheme can pre-synthesize highly active minerals, converting bauxite, limestone, and gypsum into calcium sulfoaluminate C4A3S and calcium aluminate C through solid-state reactions. 12 A7. These two minerals are the core components of sulfoaluminate cement. They hydrate extremely rapidly upon contact with water, without relying on the Ca(OH)2 activation provided by ordinary silicate cement. That is, the alkaline environment provided by ordinary silicate cement is no longer a necessary condition for the reaction of the active minerals. The reaction does not require external alkali, is extremely fast, and is accompanied by significant early strength growth. A 150-minute heat treatment time ensures complete decomposition of limestone and full participation of free calcium oxide in the reaction.

[0051] In one embodiment, in step S1, the calcined and cooled mineral is continuously ground in a ball mill until the particle size distribution meets the following requirements:

[0052] The proportion of 10nm~100nm is greater than 5%, the proportion of 0.1μm~1μm is greater than 20%, and the proportion of 1μm~10μm is greater than 65%.

[0053] 10nm~100nm particles, accounting for >5%, provide extremely high surface energy and nucleation sites, triggering ultra-early hydration. These nanoparticles (especially 10~50nm) have an extremely high surface atomic ratio, resulting in exponentially increasing chemical activity. This significantly reduces the nucleation barrier of ettringite (AFt) and CSH gel, allowing hydration products to be generated in large quantities within the first few minutes after spraying. 0.1μm~1μm particles, accounting for >20%, fill the capillary pores between micron-sized particles, increasing initial packing density and reducing water demand. They can be used in conjunction with polycarboxylate superplasticizers to avoid loss of flowability due to increased fine particles. 1μm~10μm particles, accounting for >65%, serve as the main body of the active mineral admixture, providing sufficient raw materials for the hydration reaction. Particles in this size range have a suitable specific surface area, ensuring rapid penetration of the hydration medium (water, ions) without causing a dramatic increase in water demand due to excessive fineness.

[0054] In one embodiment, the alumina content in the active mineral admixture powder is ≥20%, ensuring sufficient aluminum source to react with sulfates under the triggering of the accelerator, generating a sufficient number of ettringite crystals to form an early strength framework. The calcium oxide content is ≥45%, and the high CaO content provides a strongly alkaline environment (pH>12.5), accelerating the hydration of cement and active minerals. Simultaneously, CaO is an essential element for the formation of ettringite and CSH gel. Sufficient calcium source ensures that hydration products can fully fill pores, improving density. The sulfur trioxide content is ≥30%, meaning that a large amount of soluble sulfate exists in the system, which reacts with Al in the accelerator in the early stages. 3+In synergy with the Al2O3 system, it rapidly generates ettringite, during which continuous dissolution provides SO4. 2- This reaction with the later C3A hydration prevents structural damage caused by direct C3A hydration.

[0055] In one embodiment, in step S2, cement, aggregate, water, and the composite admixture prepared in step S1 are weighed according to a certain ratio, mixed evenly, and then sprayed using a wet mortar spraying machine. During the spraying operation, an accelerator is added to the nozzle of the wet mortar spraying machine for wet spraying. That is, the accelerator is atomized under the action of compressed air and fully mixed with the high-speed sprayed mortar flow. It takes effect rapidly during the spraying process (about 0.1~0.3 seconds) and after impacting the rock surface, achieving catalytic setting.

[0056] Embodiments of the present invention:

[0057] The material specifications and mixing ratios for each cubic meter of ultra-early strength shotcrete are as follows:

[0058] The cementitious material comprises 400 parts, including 340 parts of cement (using Ya-Tung P·O 42.5 grade ordinary Portland cement) and 60 parts of composite admixture, which includes active mineral admixtures and additives. The active mineral admixtures include bauxite, limestone, and gypsum in a mass ratio of 5:4.4:4.7. The additives include tartaric acid, sodium gluconate, and polycarboxylate powder. Tartaric acid accounts for 1% of the mass of the active mineral admixtures, sodium gluconate accounts for 0.2% of the mass of the active mineral admixtures, and polycarboxylate powder accounts for 1.5% of the mass of the active mineral admixtures.

[0059] 600 parts of aggregate, using graded sand with a fineness modulus of 2.2;

[0060] 24 parts of quick-setting agent, using iron standard alkali-free quick-setting agent;

[0061] 160 parts water, with a water-to-gel ratio of 0.4.

[0062] The preparation method is as follows:

[0063] S1. Preparation of composite admixtures:

[0064] (1) Bauxite, limestone, and gypsum were calcined together in a high-temperature furnace at 1350℃ for 150 minutes in a mass ratio of 5:4.4:4.7. After cooling, the minerals were continuously ground by a ball mill until they were powdered with a particle size of 10nm~100nm >5%, 0.1μm~1μm >20%, and 1μm~10μm >65%. The powder had an alumina content ≥20%, a calcium oxide content ≥45%, and a sulfur trioxide content ≥30%. This yielded the active mineral admixture.

[0065] (2) Add tartaric acid (1% of the active mineral admixture), sodium gluconate (0.2% of the active mineral admixture), and polycarboxylate powder (1.5% of the active mineral admixture) to the ground active mineral admixture powder, and mix thoroughly. The composite admixture is then prepared.

[0066] S2. Preparation of ultra-early strength mortar:

[0067] Weigh the materials according to the proportions and mix them evenly. Test the initial mortar flowability, the mortar flowability after 0.5 hours, and the mortar flowability after 1 hour.

[0068] S3. Then, a wet mortar spraying machine is used for spraying. During the spraying construction, 6% of the amount of cementitious material added by Iron Standard alkali-free quick-setting agent is added to the spray gun nozzle of the wet mortar spraying machine and mixed with the mortar. The wet spraying operation is carried out and sprayed into the standard large slab test mold to test the compressive strength at 3h, 8h, 24h, 7d and 28d.

[0069] Comparative Example 1:

[0070] The material specifications and formulation examples for each cubic meter of comparative example 1 are as follows:

[0071] 480 parts of ordinary Portland cement of P·O 42.5 grade were selected;

[0072] 846 parts of fine aggregate, using medium sand of zone II with a fineness modulus of 2.6;

[0073] 846 parts of coarse aggregate, using 5-10mm cavitary crushed stone;

[0074] 48 parts of nano-CSH early strength agent, accounting for 10% of the total cement content;

[0075] 3.26 parts of retarded polycarboxylate superplasticizer, accounting for 0.68% of the total cement content;

[0076] 48 parts of alkali-free quick-setting agent, accounting for 10% of the total cement content;

[0077] 178 parts water, water-to-gel ratio 0.37.

[0078] The preparation method is as follows:

[0079] S1. Accurately weigh the cement, crushed stone, medium sand, nano CSH early strength agent, water-reducing agent, and water according to the specified proportions. Put the cement, crushed stone, and medium sand into a twin-shaft forced mixer for dry mixing to ensure that the coarse and fine aggregates are fully and evenly mixed with the cement.

[0080] S2. Add all the mixing water, nano-CSH early-strength agent, and polycarboxylate superplasticizer to the dry materials, and continue forced mixing to prepare a homogeneous concrete mixture. After mixing, test the initial flowability, the flowability of the mixture at 0.5 hours, and the flowability of the mixture at 1 hour.

[0081] S3. Then, a wet concrete spraying machine is used to mix the mixture with 10% of the total cement content of alkali-free quick-setting agent at the nozzle and spray it into a standard large slab mold to test the compressive strength at 3h, 8h, 24h, 7d and 28d.

[0082] Comparative Example 2:

[0083] The material specifications and mixing ratios for each cubic meter of comparative example 2 are as follows:

[0084] 404 parts of ordinary Portland cement of P·O 42.5 grade were selected;

[0085] 780 parts of sand, using medium sand with a fineness modulus of 2.6;

[0086] 846 portions of crushed stone, using 5-10mm crushed stone;

[0087] 4.3 parts of water-reducing agent, using polycarboxylate water-reducing agent;

[0088] 26 parts of admixture, which is a modified admixture obtained by plasma-enhanced mechanochemical treatment of silica fume and slag;

[0089] The alkali-free quick-setting agent is 30.1 parts, and its dosage accounts for 7% of the total amount of cementitious materials (the sum of cement and admixtures).

[0090] 194 parts water, water-to-gel ratio 0.44.

[0091] Preparation method:

[0092] S1. Preparation of the admixture: Silica fume and slag are accurately weighed at a mass ratio of 1:4. Then, silica fume, slag, and 0.3% (by mass of silica fume) of ethylene glycol dispersant are added together to a plasma ball mill. Zirconium beads are used as the milling media, with a ball-to-material ratio of 6:1. The rotation speed is controlled at 950 r / min and the time at 50 min. Simultaneously, the plasma system is turned on, and the discharge current is controlled at 230 mA, the discharge voltage at 4.8 kV, and the discharge frequency at 9.5 kHz. The treatment atmosphere is air. Finally, a composite admixture of nano-silica fume coated with slag is obtained.

[0093] S2. Preparation of Shotcrete: Weigh and blend the raw material components according to the proportions to obtain a mixture. Then, use a wet shotcrete machine to mix the mixture with an alkali-free quick-setting agent. Measure the initial flowability, 0.5-hour flowability, and 1-hour flowability of the shotcrete. Shotcrete is sprayed into standard large-slab molds, and the compressive strength is tested at 3 hours, 8 hours, 24 hours, 7 days, and 28 days.

[0094] Comparative Example 3:

[0095] The material specifications and mixing ratios for each cubic meter of material in Proportion 3 are as follows:

[0096] 700 parts of ordinary Portland cement of P·O 52.5 grade were selected;

[0097] 150 parts fly ash;

[0098] 150 parts silica fume;

[0099] 4 parts water-reducing agent;

[0100] 1000 parts of quartz sand;

[0101] 0.8 parts of defoamer (total mass of cementitious material 700+150+150=1000 parts, 1000×0.1%=1 part, actually take 0.8 parts);

[0102] Early strength agent (0.85 parts each of nano calcium carbonate and nano aluminum oxide, totaling 1.7 parts, 1000 × 0.2% = 2 parts, actually taking 1.7 parts);

[0103] 60 parts of quick-setting agent (6% of the total mass of cementitious materials);

[0104] 170 parts water, water-to-gel ratio 0.17.

[0105] Preparation method:

[0106] S1. Carefully pour the weighed ordinary 52.5 silicate cement, fly ash, silica fume, and early-strength agent into a forced mixer in sequence. Start the mixer, set the mixing speed to 350 r / min, and mix for 2 minutes. During the mixing process, observe that the materials gradually mix evenly, the color tends to be uniform, and there is no obvious powder agglomeration, thus obtaining the first-grade mixture.

[0107] S2. Keep the mixer running and add 1000 parts of quartz sand evenly to the primary mixture at a slow speed. Increase the mixing speed to 450 r / min and mix for 2.5 minutes. During this time, you can see through the observation window that the quartz sand is evenly dispersed in the mixture without obvious particle accumulation, indicating that the secondary mixture has been successfully prepared.

[0108] S3. In a clean plastic bucket, slowly pour 4 parts of water-reducing agent into 85 parts of water while continuously stirring with a stirring rod until the water-reducing agent is completely dissolved. Slowly pour the prepared water-reducing agent solution into the secondary mixture, maintaining a stirring speed of 350 rpm for 1 minute to allow the water-reducing agent to initially mix evenly with the secondary mixture. Next, slowly pour in the remaining 85 parts of water, adjust the stirring speed to 400 rpm, and continue stirring for 2 minutes. At this point, the mixture has good flowability and a uniform texture, yielding the tertiary mixture.

[0109] S4. Add 0.8 parts of defoamer to the grade III mixture and stir for 30 seconds to disperse it evenly, thus obtaining the grade IV mixture. After stirring, test the initial flowability, 0.5-hour flowability, and 1-hour flowability of the grade IV mixture. Then, using a wet concrete spraying machine, mix the grade IV mixture with 6% of the total mass of cementitious material (alkali-free quick-setting agent) at the nozzle and spray it into a standard large slab mold to test the compressive strength at 3 hours, 8 hours, 24 hours, 7 days, and 28 days.

[0110] The specific performance test data is shown in the table below:

[0111]

[0112] As can be seen from the comparison of this embodiment with Comparative Example 1, Comparative Example 2, and Comparative Example 3 in the table above:

[0113] This embodiment exhibits no collapse within 1 hour, outperforming all comparative examples. It can meet complex working conditions such as long-distance pumping, multi-face rotation construction, and temporary equipment failure waiting, avoiding pipe blockage, waste, and rework problems caused by excessively rapid loss of fluidity. Simultaneously, the fluidity is stable, and the slurry exhibits uniform cohesion during spraying, preventing the phenomenon of excessively thin flow in the early stages and excessively dry clogging of the nozzle in the later stages, significantly improving the continuity and stability of the spraying operation.

[0114] In this embodiment, after adding the accelerator, the compressive strength is ≥10MPa at 3 hours, ≥20MPa at 8 hours, and ≥30MPa at 24 hours. The strength increases linearly without sudden increases or stagnation, allowing for accurate prediction of support strength at each time point and ensuring construction safety. Furthermore, the strength develops steadily in the later stages, reaching 68.5MPa at 28 days, higher than Comparative Examples 1 and 2, indicating better durability and extending the service life of the support structure while reducing later maintenance costs. Although lower than Comparative Example 3, it avoids sacrificing workability and increasing costs by pursuing excessively high strength.

[0115] This embodiment uses only general-purpose engineering raw materials (ordinary Portland cement of P·O 42.5 grade and above, ordinary graded sand, and iron standard alkali-free accelerator), ensuring a stable supply chain, low procurement costs, and eliminating the need for customized production. Furthermore, accelerators are among the most expensive admixtures in shotcrete; the 6% dosage in this embodiment is the lowest, effectively reducing costs and preventing alkali-aggregate reaction.

[0116] This embodiment achieves superior performance by synergistically designing the cementitious material system (ordinary silicate cement and high-temperature calcined composite admixture), admixtures (tartaric acid, sodium gluconate, and polycarboxylate powder), particle size distribution (nano to micron level), and the use of ferroalloy-free accelerator. This results in zero flowability loss at 1 hour, and strengths of 12.5 MPa at 3 hours, 23.2 MPa at 8 hours, and 33.5 MPa at 24 hours with a 6% accelerator dosage. Compared to traditional early-strength shotcrete (Comparative Examples 1 and 2), this embodiment exhibits higher early strength, less accelerator consumption, and better flowability retention. Compared to ultra-high-strength materials (Comparative Example 3), this embodiment demonstrates greater construction adaptability, lower cost, and more prominent engineering practicality. Therefore, this embodiment represents an ideal ultra-early-strength shotcrete solution that meets the requirements of modern high-performance tunnel support systems.

[0117] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An ultra-early strength shotcrete, characterized in that: Based on parts by mass, it includes the following components: The cementitious material comprises 400 parts, which is a mixture of cement and composite admixtures. The cement comprises 340 parts, which is ordinary Portland cement of P·O 42.5 grade or higher. The composite admixture comprises 60 parts, which includes active mineral admixtures and additives. The active mineral admixtures include bauxite, limestone, and gypsum; the additives include tartaric acid, sodium gluconate, and polycarboxylate powder. 600 parts of aggregate, wherein the aggregate is graded sand with a fineness modulus of 2.2; 24 parts of quick-setting agent; 160 parts water, with a water-to-gel ratio of 0.

4.

2. The ultra-early strength shotcrete according to claim 1, characterized in that: The mass ratio of the bauxite, the limestone, and the gypsum is 5:4.4:4.

7.

3. The ultra-early strength shotcrete according to claim 1, characterized in that: The tartaric acid accounts for 1% of the mass of the active mineral admixture, the sodium gluconate accounts for 0.2% of the mass of the active mineral admixture, and the polycarboxylate powder accounts for 1.5% of the mass of the active mineral admixture.

4. The ultra-early strength shotcrete according to claim 3, characterized in that: The quick-setting agent is a ferric standard alkali-free quick-setting agent.

5. A method for preparing ultra-early strength shotcrete as described in any one of claims 1-4, characterized in that: Includes the following steps: S1. Preparation of composite admixture: Weigh bauxite, limestone and gypsum according to the proportion, calcine the weighed bauxite, limestone and gypsum together in a high-temperature furnace, and continuously grind the calcined and cooled minerals through a ball mill to obtain active mineral admixture. Then add tartaric acid, sodium gluconate and polycarboxylate powder to the ground active mineral admixture powder and mix thoroughly to prepare the composite admixture. S2. Preparation of ultra-early strength shotcrete: Weigh cement, aggregate, water and the composite admixture prepared in step S1 according to the proportion, mix them evenly and then add the quick-setting agent.

6. The ultra-early strength shotcrete and its preparation method according to claim 5, characterized in that: In step S1, the temperature of the high-temperature furnace is 1350℃ and the calcination time is 150min.

7. The ultra-early strength shotcrete and its preparation method according to claim 5, characterized in that: In step S1, the calcined and cooled mineral is continuously ground in a ball mill until the particle size distribution meets the following requirements: The proportion of 10nm~100nm is greater than 5%, the proportion of 0.1μm~1μm is greater than 20%, and the proportion of 1μm~10μm is greater than 65%.

8. The ultra-early strength shotcrete and its preparation method according to claim 5, characterized in that: The active mineral admixture powder contains ≥20% alumina, ≥45% calcium oxide, and ≥30% sulfur trioxide.

9. The ultra-early strength shotcrete and its preparation method according to claim 5, characterized in that: In step S2, cement, aggregate, water, and the composite admixture prepared in step S1 are weighed according to the proportion, mixed evenly, and then sprayed using a mortar wet spraying machine. During the spraying operation, an accelerator is added to the nozzle of the mortar wet spraying machine for wet spraying.

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