Jet repairing mortar for repairing acid-corrosion-resistant structures and preparation method of jet repairing mortar

Abstract

The spraying repair mortar for repairing the acid-corrosion-resistant structures comprises the following components in parts by mass: 100 parts of a cementing material which comprises sulphoaluminate cement, silica fume and biomass charcoal powder; 85-160 parts of aggregate comprising tailing sand, and the tailing sand comprises three particle size segments; 0.1 to 1.0 part of reinforcing fiber; the chemical additive is prepared from the following components in parts by weight: 0.5 to 5.0 parts of redispersible polymer powder, 0.05 to 0.5 part of cellulose ether water retention and thickening component, 0.02 to 0.5 part of retarder, 0.1 to 1.5 parts of polycarboxylic acid type water reducing agent and 0.02 to 0.3 part of defoaming agent on the basis of 100 parts of the cementing material; and mixing water, wherein the mass ratio of the mixing water to the cementing material is 0.30-0.50. The invention further provides a preparation method of the spraying repair mortar for repairing the acid-corrosion-resistant structures. Early strength and bonding are both considered, and functions of the biomass charcoal are synergistic.

Description

Technical Field

[0001] This invention belongs to the technical field of solid waste resource utilization and repair materials, and specifically relates to a multi-source solid waste-based sulfoaluminate spray repair mortar for repairing structures in acidic corrosive environments and its preparation method. The structures include, but are not limited to, municipal drainage / sewage inspection wells, sewage pipelines and their ancillary structures, sewage pumping stations, septic tanks, culverts, etc. Background Technology

[0002] Municipal drainage and sewage structures are often exposed to high humidity, nutrient-rich, and low-flow-rate sewage environments during their service life, and are frequently subjected to acidic media and microbial activity; municipal drainage / sewage inspection wells are a typical example. Under the influence of these acidic composite media, the lining material is prone to calcium component dissolution, weakening of the aggregate-mortar interface, and crack propagation, leading to cracking, spalling, and leakage, thus reducing structural durability and operational safety.

[0003] Existing trenchless spray repair methods mostly employ sulfoaluminate cement-based spray mortar, with spray workability controlled by components such as water-reducing, thickening / water-retaining, defoaming, and retarding agents (e.g., CN120441270A). These methods typically emphasize early strength and construction stability, but no specific systematic design has been found to address the performance challenges posed by the acidic corrosion risk within municipal sewage structures (typically manholes, often dominated by sulfuric acid and accompanied by organic acids).

[0004] To improve the utilization rate of solid waste, there are existing all-solid-waste shotcrete solutions (such as CN116354682B), which achieve sprayability and certain mechanical properties through the gradation of solid waste-based cementitious materials and solid waste aggregates. However, these solutions are mostly developed from the perspectives of resource utilization and workability, and there is no evidence of dedicated optimization for strength retention and failure mechanisms under strong acid / bioacid conditions, starting from the synergistic effect of early load-bearing and interfacial bonding in the sulfoaluminate system.

[0005] In addition, there are technologies that improve the acid resistance of the lining by using composite cementitious materials and introducing acid-resistant components / aggregates (e.g., CN119219387A). However, such solutions often fail to take into account the need for the co-utilization of multi-source solid waste and the replacement of natural sand with tailings sand, and still do not systematically design for the interfacial bonding stability and mechanical properties under microbially induced strong acid environments.

[0006] Existing technologies generally face a technical bottleneck in repairing structures in acidic corrosive environments (with drainage / sewage inspection wells as a typical example): traditional solutions based on sulfoaluminate cement, while meeting the early strength requirements for emergency repairs, suffer from insufficient structural stability and durability degradation under the combined effects of acidic media and microbial activity. Existing all-solid-waste spraying solutions, while improving resource utilization, are often limited by the gradation fluctuations and porous characteristics of tailings sand and other solid wastes, making it difficult to balance the stability of vertical spraying construction with interfacial bonding strength. Currently, there is a lack of a spraying mortar system that can synergistically achieve "high early strength emergency repairs," "maintaining acid resistance," and "high-proportion tailings resource utilization."

[0007] Meanwhile, addressing the issue of mortar pore structure and interface transition zone deterioration under acidic media, leading to a decline in adhesion and mechanical retention, biochar, as a porous carbon-based material, is considered to have potential for pore structure regulation and interface effects in cement-based systems. Furthermore, as it originates from biomass carbon resources, its incorporation and solidification into building materials also possess potential for carbon resource utilization and carbon sequestration, potentially contributing to climate change mitigation and environmental benefits. However, in applications involving rapid-hardening sprayed repair mortar, the introduction of biochar typically requires coordinated matching with the hydration process of the cementitious system, the admixture dispersion system, rheological and thixotropic properties, and the setting window. Otherwise, engineering risks such as uneven dispersion, rheological fluctuations, or disturbances to the setting window may occur. Existing technologies still lack systematic design and controllable introduction pathways addressing these coupling relationships.

[0008] Therefore, there is an urgent need to propose a repair mortar system and its preparation method suitable for trenchless spraying repair of structures in acidic corrosive environments. This system should be able to balance spraying stability, early load-bearing capacity, and interfacial bonding performance under controllable construction windows, while also improving performance retention under acidic media. Simultaneously, it should achieve efficient synergistic utilization of tailings sand and multi-source industrial solid waste while maintaining cost control. Furthermore, this system can introduce functional components (such as biochar) under the constraints of defined formulation boundaries and controlled introduction processes (e.g., using pre-dispersion / stepwise addition and shear homogenization pathways) to leverage their potential regulatory role on pore structure and interfacial stability without significantly disturbing the coagulation and spraying rheological stability. Summary of the Invention

[0009] To overcome the shortcomings of existing technologies, this invention provides a sprayed repair mortar for acid-resistant structures and its preparation method. The sulfoaluminate rapid-hardening system incorporates multi-source solid waste and biomass char powder as cementing components, and the tailings sand aggregate is segmented and the coarse-fine ratio is constrained. At the same time, a retarder solution is added in two stages and a mixing path of shear homogenization is adopted, thereby achieving comprehensive optimization in terms of sprayed construction stability, early load-bearing capacity and interfacial bonding, and performance retention after the action of acid media.

[0010] The technical solution adopted by this invention to solve its technical problem is: A sprayed repair mortar for repairing acid-corrosion resistant structures, comprising, by weight: 100 parts of cementitious material, wherein the cementitious material includes sulfoaluminate cement, silica fume and biochar powder; The aggregate consists of 85 to 160 parts, wherein the aggregate includes tailings sand, and the tailings sand comprises three particle size ranges. Reinforcing fiber 0.1 to 1.0 parts; The chemical additives, based on the 100 parts of the gelling material, include 0.5 to 5.0 parts of redispersible polymer powder, 0.05 to 0.5 parts of cellulose ether water-retaining and thickening components, 0.02 to 0.5 parts of retarder, 0.1 to 1.5 parts of polycarboxylate superplasticizer, and 0.02 to 0.3 parts of defoamer; And mixing water, wherein the mass ratio of the mixing water to the cementitious material is 0.30 to 0.50.

[0011] Furthermore, the cementing material also includes fly ash and hemihydrate desulfurization gypsum; and, by mass, the cementing material is composed of 65-85 parts of sulfoaluminate cement, 2-15 parts of fly ash, 5-10 parts of hemihydrate desulfurization gypsum, 5-12 parts of silica fume, and 0.5-3 parts of biochar powder, and the sum of the mass parts of each component is 100 parts.

[0012] Preferably, the hemihydrate desulfurized gypsum is obtained by drying and dehydrating dihydrate industrial desulfurized gypsum.

[0013] The tailings sand accounts for 70-100% of the total mass of the aggregate.

[0014] The tailings sand comprises at least three particle size ranges: 26-40 mesh, 40-70 mesh, and 70-110 mesh. With a total tailings sand volume of 100 parts, the 26-40 mesh tailings sand accounts for 20-30 parts, the 40-70 mesh tailings sand accounts for 40-55 parts, and the 70-110 mesh tailings sand is the balance. The sum of the mass parts of the above three particle size ranges is 100 parts.

[0015] Preferably, the tailings sand with a particle size range of 26-40 mesh and 40-70 mesh together account for more than 50%.

[0016] The reinforcing fiber comprises polypropylene fiber, and the polypropylene fiber is a short, easily dispersible fiber with a length of 5-7 mm. This solution can improve toughness and crack resistance, while taking into account fiber dispersibility, spray molding stability, and mechanical toughening effect. Under the premise of the technical route of this invention, other alkali-resistant fibers can also be selected for reinforcement.

[0017] The redispersible polymer powder is VAE latex powder, and the cellulose ether water-retaining and thickening component is hydroxypropyl methylcellulose (HPMC).

[0018] Preferably, the retarder is boric acid.

[0019] A method for preparing sprayed repair mortar for acid corrosion resistant structures, employing a solid-phase stepwise dry mixing—two-stage addition of retarder solution—shear homogenization and stirring process, including the following steps: S1. Mix the cementitious materials, aggregates and reinforcing fibers to obtain the first mixture; S2. Add the remaining chemical additives, excluding the retarder, to the first mixture and stir to obtain a dry mixture; wherein, the remaining chemical additives include at least redispersible polymer powder, cellulose ether water-retaining and thickening components, polycarboxylate superplasticizers, and defoamers; S3. The retarder and mixing water are premixed to form a retarder solution. The retarder solution is added to the dry mix in two parts. First, the dry mix is ​​stirred at low speed to fully wet the dry mix and form a uniform mortar mixture. Then, the stirring intensity is increased to perform shear homogenization stirring to obtain the sprayed repair mortar.

[0020] In step S3, the amount of the retarder solution added for the first time is 60% to 80% of the total amount, and the amount added for the second time is 20% to 40%; the low-speed stirring speed is 200 to 300 rpm, the shear homogenization stirring speed is 400 to 600 rpm; the low-speed stirring time is 45 to 60 s, and the shear homogenization stirring time is 60 to 90 s.

[0021] The technical concept of this invention is to adopt a synergistic cementing system of "multi-source solid waste-biochar-sulfoaluminate", combined with segmented graded tailings sand aggregate, and introduce reinforcing fibers and chemical additives to achieve synergistic control of spraying workability and setting time, thereby improving structural stability and performance retention under acidic media.

[0022] The present invention has the following functions and features: (1) Under the conditions of spraying construction on the facade / top plate, the sprayed repair mortar material has thixotropy, low rebound and a certain single forming thickness; (2) It has high early strength and reliable bonding in emergency repair scenarios; (3) In a composite acidic environment with sulfuric acid as the main component and accompanied by organic acids, the repaired structure has good strength retention and durability; (4) While meeting the above performance requirements, it increases the proportion of engineering utilization of multi-source solid waste and tailings sand and takes into account cost control.

[0023] This invention helps to solve the application problem of tailings sand replacing natural sand in the field of shotcrete materials. Existing solid waste resource utilization technologies in shotcrete often only involve the replacement of a small amount of admixtures, which limits the types and amounts of solid waste. Furthermore, due to the fluctuation of particle shape and gradation of bulk solid wastes such as tailings sand, the stability of mixing, pumping and spraying is easily reduced, resulting in engineering difficulties in achieving "full replacement of natural sand with tailings sand" without sacrificing workability and mechanical properties.

[0024] This invention constructs a synergistic cementing system of "multi-source solid waste – biochar – sulfoaluminate". The aggregate includes tailings sand and the tailings sand is designed with segmented gradation. Through a mixing process of solid phase step-by-step dry mixing, two-stage addition of retarder solution and shear homogenization stirring, the coagulation process and spray molding process of the fast-hardening system are controlled, thereby achieving early strength, good spraying performance, maintenance of acid environment performance and efficient utilization of solid waste.

[0025] Based on a synergistic cementitious system, tailings sand segmented gradation constraints, and a mixing process involving the two-stage addition of retarder solution and shear homogenization stirring, this invention forms a mortar system suitable for trenchless shotcrete repair of structures in acidic corrosive environments, achieving a comprehensive balance in shotcrete construction stability, early mechanical strength, and performance retention under acidic media.

[0026] From an application scenario perspective, the inner walls of municipal sewage structures with a high risk of acid corrosion (typically municipal drainage / sewage inspection wells) are usually situated in a complex acidic environment dominated by sulfuric acid and accompanied by organic acids. This invention improves the interfacial bonding stability and strength retention of the material under acidic media by introducing a synergistic cementing system and the functionality of biochar, combined with tailings sand gradation to regulate the pore structure and interfacial transition zone.

[0027] Compared with existing shotcrete used for the repair of structures in acidic corrosive environments, existing materials are prone to rebound and insufficient forming stability under vertical / roof spraying conditions. Moreover, in emergency repair scenarios, it is often difficult to simultaneously meet the requirements of early load-bearing capacity, interfacial bonding, and performance retention under the action of acidic media. To address the above contradictory needs, this invention focuses on (1) segmenting and grading tailings sand and constraining the coarse-fine ratio to ensure that it can still be sprayed stably even when replacing natural sand in a high proportion or even in its entirety; (2) using a pre-dissolved and step-by-step addition of a retarder, combined with a shear-homogenized mixing path, to stabilize and control the setting process and sprayable window of the fast-hardening system; and (3) introducing functional components such as biochar to regulate the pore structure and interfacial transition zone, thereby achieving comprehensive optimization between spraying construction stability, early mechanical strength and interfacial bonding, and performance retention under the action of acidic media.

[0028] The beneficial effects of this invention are mainly reflected in: (1) Balancing early strength and adhesion: This invention utilizes the synergistic effect of the sulfoaluminate cementitious system, fine active components, polymers, and fibers to enable the sprayed repair mortar to maintain high interfacial adhesion reliability and crack resistance while meeting the early strength requirements for rapid repair. Preferably, in some embodiments, the 1-day compressive strength and 28-day tensile bond strength can reach a high level. For example, the 1-day compressive strength of Example 1 is 26.4 MPa, and the 28-day tensile bond strength is 3.5 MPa (see Table 2).

[0029] (2) Synergistic Function of Biochar: Introducing biochar powder into the synergistic cementing system of this invention can utilize its porous structure and surface characteristics to regulate the pore structure and interface transition zone, thereby helping to improve bonding stability and performance retention after exposure to acidic media. Preferably, in some embodiments, compared with the control sample without biochar, the combined performance of bonding and acid resistance is better (for example, the trend shown in Table 2).

[0030] (3) Sprayability under high proportion (or full) tailings sand replacement: The present invention performs segmented gradation and coarse-fine ratio constraints on tailings sand, so that it can still obtain stable flow-thixotropic balance and spray molding stability when replacing natural sand in a high proportion or even as the only fine aggregate, thereby reducing rebound, increasing single-pass molding thickness and reducing the risk of sagging. Preferably, in some embodiments, the spray rebound rate and the maximum single-pass spray thickness can be within the engineering acceptable range (e.g., the range shown in Table 2).

[0031] (4) Stable and controllable setting and construction window: The present invention adopts a process path of pre-dissolving and adding the retarder in stages, followed by low-speed wetting and shear homogenization, which can reduce the risk of uncontrolled setting caused by local enrichment of the retarder component, improve the uniformity of mixing and the controllability of the spraying construction window, and maintain good construction stability even when the ratio fluctuates or the amount of retarder is adjusted. Preferably, in some embodiments, the fluctuations in initial / final setting time, rebound rate and single-shot forming thickness are small (as shown in Table 2, for example).

[0032] (5) Synergistic utilization and comprehensive benefits of multi-source solid waste: This invention uses multi-source solid waste such as fly ash, desulfurization gypsum, silica fume, tailings sand and biochar powder as synergistic cementing and structural regulation components. Under the premise of ensuring the spraying workability, early strength and acid resistance, it realizes the resource utilization of high-volume solid waste, while taking into account material costs and environmental benefits. Among them, the introduction of biochar also has certain carbon resource utilization and carbon sequestration potential, which can bring about climate change mitigation and environmental benefits. Attached Figure Description

[0033] Figure 1 This is a flowchart of the preparation method of sprayed repair mortar for repairing acid-corrosion resistant structures. Detailed Implementation

[0034] The present invention will now be further described with reference to the accompanying drawings.

[0035] Reference Figure 1 A sprayable repair mortar for acid-resistant structures is suitable for repairing the inner walls of structures with a high risk of acidic media exposure (typically drainage / sewage inspection wells). It meets the requirements of spraying construction for forming stability, low rebound and workable time window, while also taking into account early load-bearing capacity, interfacial bonding and acid resistance.

[0036] The sprayed repair mortar for acid-resistant corrosion-resistant structures in this embodiment comprises, by weight: 100 parts of cementitious material, which includes sulfoaluminate cement, silica fume, and biochar powder; 85-160 parts of aggregate, which includes tailings sand and the tailings sand contains three particle size ranges; 0.1-1.0 parts of reinforcing fiber; chemical additives, based on the 100 parts of cementitious material, including 0.5-5.0 parts of redispersible polymer powder, 0.05-0.5 parts of cellulose ether water-retaining and thickening component, 0.02-0.5 parts of retarder, 0.1-1.5 parts of polycarboxylate superplasticizer, and 0.02-0.3 parts of defoamer; and mixing water, wherein the mass ratio of mixing water to cementitious material is 0.30-0.50.

[0037] Furthermore, the cementing material further includes fly ash and hemihydrate desulfurization gypsum; and, by weight, the cementing material consists of 65-85 parts of sulfoaluminate cement, 2-15 parts of fly ash, 5-10 parts of hemihydrate desulfurization gypsum, 5-12 parts of silica fume, and 0.5-3 parts of biochar powder, with the sum of the weight parts of all components being 100 parts. Preferably, the hemihydrate desulfurization gypsum is obtained by drying and dehydrating dihydrate industrial desulfurization gypsum.

[0038] The tailings sand in the aggregate accounts for 70-100% of the total aggregate by mass. The tailings sand comprises at least three particle size ranges: 26-40 mesh, 40-70 mesh, and 70-110 mesh. Based on a total tailings sand volume of 100 parts, 20-30 parts are 26-40 mesh, 40-55 parts are 40-70 mesh, and the remainder is 70-110 mesh, with the sum of the mass portions of the three particle size ranges being 100 parts. Preferably, the combined proportion of tailings sand in the 26-40 mesh and 40-70 mesh particle size ranges is greater than 50%.

[0039] The reinforcing fiber comprises polypropylene fiber, and the polypropylene fiber is a short, easily dispersible fiber with a length of 5-7 mm. This solution can improve toughness and crack resistance, while taking into account fiber dispersibility, spray molding stability, and mechanical toughening effect. Under the premise of the technical route of this invention, other alkali-resistant fibers can also be selected for reinforcement.

[0040] The redispersible polymer powder is VAE latex powder, and the cellulose ether water-retaining and thickening component is hydroxypropyl methylcellulose (HPMC). Preferably, the retarder is boric acid.

[0041] The initial setting time of the sprayed repair mortar is 60–80 min, and the final setting time is 75–95 min. When tested according to current national standards or equivalent standards (e.g., flexural / compressive strength according to GB / T 17671 or its current version, tensile bond strength according to JGJ / T 70 or its current version), its mechanical properties meet the following requirements: a) 1-day compressive strength not less than 22 MPa; b) 28-day compressive strength not less than 38 MPa; c) 1-day flexural strength not less than 4.0 MPa; d) 28-day flexural strength not less than 5.5 MPa; e) 28-day tensile bond strength (wet curing) not less than 2.5 MPa. When tested according to current industry standards or equivalent standards (e.g., rebound rate according to relevant standards for sprayed concrete / sprayed mortar or their current versions), the spraying performance meets the following requirements: single spray thickness not less than 14 mm, and rebound rate not higher than 7%. After being corroded with 5% sulfuric acid solution for 24 hours, no blistering, peeling, or cracking was observed on the surface. After being soaked in 5% sulfuric acid solution for 24 hours, the 28-day flexural strength retention rate was not less than 100%, and the 28-day compressive strength retention rate was not less than 96%.

[0042] The key to this invention lies in the synergistic design of the following three technologies: (1) a synergistic cementing system of “multi-source solid waste – biochar – sulfoaluminate”, which is used to balance early strength and structural stability under acidic conditions; (2) the aggregate includes tailings sand and is designed with segmented gradation, preferably using all tailings sand as the only aggregate, and controlling spray stability and rebound by gradation and coarse-fine ratio; (3) a wet mixing process in which retarder solution (e.g., boric acid) is pre-dissolved and added in steps, which is used to stabilize and control the coagulation and sprayable window of the fast-hardening system. In this invention, tailings sand can be used in a high proportion or even entirely as aggregate. Through segmented particle size distribution and coarse-to-fine ratio constraints, it forms a more rational particle packing structure with the cementitious matrix, thereby reducing porosity and improving the continuity of the interfacial transition zone. Combined with the bridging and crack pinning effect of polypropylene short fibers, this enhances the toughness and crack resistance of the sprayed repair layer. Furthermore, the mixing process employing solid-phase stepwise dry mixing, two-stage addition of the retarder solution, and shear-homogenizing stirring helps suppress powder agglomeration and fiber clumping, resulting in a more uniform mixture and thus improving the stability and consistency of the spraying operation.

[0043] In this invention, the sulfoaluminate cement is a rapid-hardening sulfoaluminate cement with a strength grade of not less than 42.5; the fly ash can be high-calcium fly ash; the hemihydrate desulfurized gypsum can be a commercially available product or obtained by grinding dihydrate desulfurized gypsum after baking at 140-160℃ for 30-50 minutes; the silica fume can be industrial silica fume. Biochar powder can be obtained from agricultural and forestry biomass such as bamboo under isolated or anaerobic conditions through pyrolysis / carbonization, followed by cooling, crushing, and grinding; alternatively, biochar powder with similar functions can be used.

[0044] The tailings aggregate is preferably obtained from metal mine tailings through crushing and screening, such as iron ore tailings or copper ore tailings; its particle size distribution preferably satisfies the following: the tailings aggregate contains at least three particle size ranges: 26-40 mesh, 40-70 mesh, and 70-110 mesh; based on a total tailings aggregate of 100 parts, the 26-40 mesh tailings aggregate is 20-30 parts, the 40-70 mesh tailings aggregate is 40-55 parts, and the 70-110 mesh tailings aggregate is the balance, and the sum of the mass parts of the above three particle size ranges is 100 parts. The tailings aggregate is mainly composed of SiO2 (e.g., about 86 wt%), and contains a certain amount of Al2O3, CaO, K2O, Na2O, etc.; its mineral phase, in addition to quartz, may contain a small amount of calcite, potassium feldspar, etc. (see Table 1, Table 1 is only an example and does not constitute a limitation). Therefore, compared with standard quartz sand, this tailings sand is not limited to "inert filling" in the synergistic cementing system of the present invention. Its contribution to the interfacial zone and acid resistance is reflected by the comparison results of the examples and comparative examples (see Table 2).

[0045] Without departing from the overall concept of this invention, the above-mentioned chemical additives can be equivalently replaced: the redispersible polymer powder can be VAE-based, acrylate-based, styrene-acrylic, or other redispersible latex powders; the cellulose ether-based water-retaining and thickening component can be HPMC, HEMC, MC, etc.; the retarder can be boric acid, borates, or other retarding components that can delay the setting of sulfoaluminate cement systems; the polycarboxylate superplasticizer can be a polycarboxylate high-efficiency superplasticizer with different functional group structures; the defoamer can be a polyether-based, organosilicon-based, or mineral oil-based defoaming component. In addition to polypropylene fibers, reinforcing fibers can also be PVA fibers, basalt fibers, glass fibers, etc., to improve crack resistance and toughness.

[0046] The typical chemical compositions of the raw materials used in this invention, such as fly ash, hemihydrate desulfurization gypsum, silica fume, biochar powder, and tailings sand, are shown in Table 1. The data in Table 1 are for illustrative purposes only and do not constitute a limitation of this invention.

[0047] Table 1 shows the main chemical composition (wt%) of each raw material.

[0048] like Figure 1 As shown, a method for preparing sprayable repair mortar for acid-corrosion-resistant structures involves a solid-phase stepwise dry mixing process. A retarder is premixed with mixing water to form a retarder solution, which is then added in two stages. First, the mixture is stirred at low speed to fully wet the materials and form a homogeneous mixture. Then, the stirring intensity is increased to perform shear homogenization mixing to obtain the sprayable repair mortar. Finally, spraying is applied. This process helps improve the dispersion uniformity and spraying stability of the mixture.

[0049] The preparation method of the sprayed repair mortar for repairing acid-corrosion-resistant structures includes the following steps: S1. Solid phase stepwise dry mixing (Phase I): Weigh the cementitious materials (sulfoaluminate cement, fly ash, hemihydrate desulfurized gypsum, silica fume, biochar powder), tailings sand aggregate, and polypropylene fiber according to the proportion. Put the above solid materials into the mixer and carry out the first low-speed mixing at 200-300 rpm for 2 minutes. This stage is the key period of physical dispersion. By utilizing the dry shear and friction between the tailings sand aggregate particles, the polypropylene fiber bundles are effectively dispersed, avoiding the clumping of fibers in the wet mixing stage. S2, Chemical Additive Dispersion (Phase II): Add the remaining chemical additives (e.g., VAE latex powder, HPMC, PCE and defoamer) except for the retarder to the mixture obtained in S1, and continue to stir at a low speed of 200-300 rpm for 1 minute. This short-term low-speed mixing aims to ensure the uniform dispersion of trace chemical functional components in the gel matrix and reduce the agglomeration and local enrichment / stratification of lightweight ultrafine powders (such as silica fume and latex powder) during the dry mixing process, while reducing the risk of dust dispersion. S3. First addition of retarder solution (low-speed wetting and mixing): The retarder (e.g., boric acid) of the specified amount is pre-dissolved in the mixing water to prepare a retarder solution. First, add 60%–80% of the total retarder solution (preferably 70%) and stir at a low speed of 200–300 rpm for 45–60 seconds. At this stage, the mortar mixture is in a plastic state. Low-speed mixing initially wets the powder particles and causes them to agglomerate into nuclei, preventing fine powder from flying away. At the same time, the higher solid-liquid ratio helps the polycarboxylate superplasticizer (PCE) to preferentially adsorb onto the surface of cement particles, initially exerting its steric hindrance effect. The second addition of the retarder solution (shear homogenization): Add the remaining 20%–40% of the total retarder solution (preferably 30%), and quickly increase the stirring speed to 400–600 rpm for 60–90 seconds of high-speed shear mixing. This high shear force breaks down the flocculated structure of cement particles, releases trapped water, and accelerates the unfolding and dissolution of HPMC molecular chains, resulting in a stable thixotropic fluid in the mortar mixture. This meets the dual requirements of fluidity and anti-sagging properties for shotcrete applications. In this specification, "shear homogenization mixing" refers to achieving a uniformly dispersed mortar mixture without significant agglomeration by increasing the stirring speed and shear intensity; "thixotropy" refers to the rheological properties of mortar where viscosity decreases under shear and partially recovers after shearing stops, which can be characterized by indicators such as the change in fluidity over time.

[0050] S4. Spraying Construction: One hour before the spraying operation, the repair surface of the structure to be repaired is moistened with water (keeping it moist but without standing water). For structures to be repaired, such as drainage / sewage inspection wells, the prepared spray repair mortar is loaded into the spraying equipment and sprayed onto the repair surface to form a dense repair layer, thus completing the repair construction.

[0051] The preparation process, which involves solid-phase stepwise dry mixing, two-stage addition of the retarder solution, and shear homogenization stirring, helps to fully disperse and uniformly mix the components in the mortar system. This results in a more uniform distribution of the retarder in the system, thereby more stably controlling the setting time and sprayable application window of the fast-setting cementitious system. It also reduces the risk of clogging of the spraying equipment due to excessively rapid setting and the risk of increased sagging and rebound due to excessively slow setting.

[0052] To verify the effectiveness and controllability of the technical solution of the present invention, Examples 1-3 and Comparative Examples 1-2 were set up. Unless otherwise stated, the raw materials used in each example and comparative example are all commercially available industrial-grade products, the mortars are all prepared according to the above preparation method, and the samples are cured to the specified age according to relevant standards before testing.

[0053] Example 1 By weight, the cementitious material comprises 100 parts, including: 76 parts sulfoaluminate cement, 5 parts fly ash, 8 parts hemihydrate desulfurized gypsum, 10 parts silica fume, and 1 part biochar powder; the aggregate is 120 parts whole tailings sand; the admixtures and functional components are: 2.0 parts VAE latex powder, 0.1 parts hydroxypropyl methylcellulose, 0.25 parts boric acid, 0.4 parts polycarboxylate-based high-performance water-reducing agent, 0.1 parts defoamer, 0.4 parts polypropylene fiber, and 40 parts mixing water (the mass ratio of mixing water to cementitious material is 0.40). The sprayed repair mortar is prepared according to the above method.

[0054] Example 2 Based on Example 1, the composition of the cementitious materials was adjusted (by mass): 67 parts sulfoaluminate cement, 15 parts fly ash, 10 parts hemihydrate desulfurized gypsum, 6 parts silica fume, and 2 parts biochar powder; the remaining components and preparation methods were the same as in Example 1.

[0055] Example 3 Based on Example 1, the composition of the cementitious materials was adjusted (by mass): 82 parts sulfoaluminate cement, 2 parts fly ash, 6 parts hemihydrate desulfurized gypsum, 9.5 parts silica fume, and 0.5 parts biochar powder; the remaining components and preparation methods were the same as in Example 1.

[0056] Comparative Example 1 This study investigated the effect of biochar powder on performance. Compared to Example 1, only 1 part of biochar powder was removed, and the silica fume was increased from 10 parts to 11 parts; the remaining components and preparation methods were the same as in Example 1.

[0057] Comparative Example 2 This study investigated the role of tailings sand aggregate. Compared to Example 1, the only difference was that the tailings sand was replaced with standard quartz sand of equal mass and the same gradation; the remaining components and preparation methods were the same as in Example 1.

[0058] Performance testing: Setting time shall be tested according to GB / T 1346; flowability shall be tested according to GB / T 2419; compressive strength and flexural strength shall be tested according to GB / T 17671; tensile bond strength (wet curing) shall be tested according to JGJ / T 70; spray rebound rate and maximum single spray thickness shall be tested according to JGJ / T 372. If any of the above standards are revised, the current version or equivalent standard shall prevail.

[0059] Acid resistance test: Samples prepared and cured for 28 days were immersed in a 5% sulfuric acid solution for 24 hours. After removal, the apparent changes were observed, and the mass loss rate, compressive strength, and flexural strength before and after corrosion were tested. The strength retention rate was calculated (strength retention rate = strength after corrosion / strength before corrosion × 100%). The test results of each example and comparative example are shown in Table 2.

[0060] Table 2 shows a comparison of the performance test results of sprayed repair mortar;

[0061] As shown in Table 2, the embodiments of the present invention exhibit good performance in terms of setting time, fluidity, mechanical properties, spray application performance, and acid resistance. The initial setting time of Examples 1-3 is 63-76 min, the final setting time is 75-93 min, and the fluidity is 188-203 mm. Simultaneously, the 1-day compressive strength of Examples 1-3 is 22.5-28.2 MPa, the 28-day compressive strength is 38.6-49.2 MPa, the 28-day tensile bond strength is 2.8-3.5 MPa, the spray rebound rate is 5.5%-6.7%, and the maximum single spray thickness is 14-16 mm.

[0062] Setting time is primarily used to characterize the operational window for spraying. Combined with the mixing process of this invention—"pre-dissolving and adding the retarder (e.g., boric acid) in batches, followed by low-speed wet mixing and shear homogenization"—the setting process in this embodiment remains within a window suitable for spraying repair work, which is beneficial for continuous on-site construction and operability control.

[0063] (1) Comparison of Example 1 and Comparative Example 1 (Example 1 introduced biochar powder). The overall proportions of the cementitious materials in both examples are similar, but Comparative Example 1 replaced biochar with silica fume. Table 2 shows that the 28-day tensile bond strength of Example 1 is 3.5 MPa, which is higher than 2.6 MPa of Comparative Example 1; the spray rebound rate is 5.5%, which is lower than 6.1% of Comparative Example 1; and the maximum single spray thickness is 16 mm, which is higher than 13 mm of Comparative Example 1. After soaking in 5% sulfuric acid solution for 24 h, the mass loss rate of Example 1 is 0.09%, which is lower than 0.17% of Comparative Example 1, and the 28-day flexural strength retention rate is 101.5%, which is significantly higher than 82.5% of Comparative Example 1. This indicates that in the system of the present invention, biochar powder, as a functional component, helps to improve the interfacial bonding performance and enhance the mechanical retention ability after acid resistance, while also playing a positive role in spray anti-sagging and rebound control.

[0064] (2) Comparison of Example 1 and Comparative Example 2 (the aggregate in Example 1 was replaced with whole tailings sand instead of standard sand). The cementitious material system and admixture system of both were the same, except that the aggregate in Example 1 was replaced with segmented graded whole tailings sand instead of standard quartz sand. Table 2 shows that the 28-day compressive strength of Example 1 was 46.2 MPa, which was higher than 41.0 MPa of Comparative Example 2; the spray rebound rate was 5.5% (5.9% for Comparative Example 2), and the maximum single spray thickness was 16 mm (14 mm for Comparative Example 2). After soaking in 5% sulfuric acid solution for 24 h, the mass loss rate of Example 1 was 0.09%, which was lower than 0.19% of Comparative Example 2, and the 28-day flexural strength retention rate was 101.5%, which was significantly higher than 76.7% of Comparative Example 2. This demonstrates that, under the combined effect of the synergistic cementitious system and tailings sand gradation of this invention, the whole tailings sand is not a simple substitute material. It can achieve resource utilization while obtaining better comprehensive performance than the standard sand system, especially in terms of acid resistance and sprayable workability.

[0065] (3) Overall results and acid resistance of Examples 1-3. Table 2 shows that after soaking in 5% sulfuric acid solution for 24 h, Examples 1-3 showed no blistering, peeling, or cracking on the surface; the mass loss rate was 0.09%-0.21%, the flexural strength retention rate after 28 days was 101.5%-113.8%, and the compressive strength retention rate after 28 days was 96.4%-99.4%, demonstrating good acid resistance and durability. This effect is related to the sulfoaluminate-based multi-source solid waste cementing system (with low free Ca(OH)2 content) used in this invention, the densification brought about by the tailings sand gradation, and the synergistic regulation of pore structure and interface zone by biochar, which can reduce acid penetration and erosion channels and improve structural stability.

[0066] The embodiments described in this specification are merely examples of implementations of the inventive concept and are for illustrative purposes only. The scope of protection of this invention should not be considered limited to the specific forms described in these embodiments; rather, it extends to equivalent technical means conceived by those skilled in the art based on the inventive concept.

Claims

1. A sprayed repair mortar for repairing acid-corrosion-resistant structures, characterized in that, Included by weight parts: 100 parts of cementitious material, wherein the cementitious material includes sulfoaluminate cement, silica fume and biochar powder; The aggregate consists of 85 to 160 parts, wherein the aggregate includes tailings sand, and the tailings sand comprises three particle size ranges. Reinforcing fiber 0.1 to 1.0 parts; The chemical additives, based on the 100 parts of the gelling material, include 0.5 to 5.0 parts of redispersible polymer powder, 0.05 to 0.5 parts of cellulose ether water-retaining and thickening components, 0.02 to 0.5 parts of retarder, 0.1 to 1.5 parts of polycarboxylate superplasticizer, and 0.02 to 0.3 parts of defoamer; And mixing water, wherein the mass ratio of the mixing water to the cementitious material is 0.30 to 0.

50.

2. The sprayed repair mortar for repairing acid-corrosion-resistant structures as described in claim 1, characterized in that, The cementing material further includes fly ash and hemihydrate desulfurization gypsum; and, by mass, the cementing material is composed of 65-85 parts of sulfoaluminate cement, 2-15 parts of fly ash, 5-10 parts of hemihydrate desulfurization gypsum, 5-12 parts of silica fume, and 0.5-3 parts of biochar powder, and the sum of the mass parts of each component is 100 parts.

3. The sprayed repair mortar for repairing acid-corrosion-resistant structures as described in claim 2, characterized in that, The hemihydrate desulfurization gypsum is obtained by drying and dehydrating dihydrate industrial desulfurization gypsum.

4. A sprayed repair mortar for repairing acid-corrosion-resistant structures as described in any one of claims 1 to 3, characterized in that, The tailings sand accounts for 70-100% of the total mass of the aggregate.

5. The sprayed repair mortar for repairing acid-corrosion-resistant structures as described in claim 4, characterized in that, The tailings sand comprises at least three particle size ranges: 26-40 mesh, 40-70 mesh, and 70-110 mesh. With a total tailings sand volume of 100 parts, the 26-40 mesh tailings sand accounts for 20-30 parts, the 40-70 mesh tailings sand accounts for 40-55 parts, and the 70-110 mesh tailings sand is the balance. The sum of the mass parts of the above three particle size ranges is 100 parts.

6. The sprayed repair mortar for repairing acid-corrosion-resistant structures as described in claim 5, characterized in that, Of the tailings, the tailings with a particle size of 26-40 mesh and 40-70 mesh account for more than 50% in total.

7. A sprayed repair mortar for repairing acid-corrosion-resistant structures as described in any one of claims 1 to 3, characterized in that, The reinforcing fiber comprises polypropylene fiber, and the polypropylene fiber is a short, easily dispersible fiber with a length of 5 to 7 mm.

8. A sprayed repair mortar for repairing acid-corrosion-resistant structures as described in any one of claims 1 to 3, characterized in that, The redispersible polymer powder is VAE latex powder, the cellulose ether water-retaining and thickening component is hydroxypropyl methylcellulose (HPMC), and the retarder is boric acid.

9. A method for preparing sprayed repair mortar for repairing acid-corrosion-resistant structures as described in claim 1, characterized in that, The method includes the following steps: S1. Mix the cementitious materials, aggregates and reinforcing fibers to obtain the first mixture; S2. Add the remaining chemical additives, excluding the retarder, to the first mixture and stir to obtain a dry mixture; wherein, the remaining chemical additives include at least redispersible polymer powder, cellulose ether water-retaining and thickening components, polycarboxylate superplasticizers, and defoamers; S3. The retarder and mixing water are premixed to form a retarder solution. The retarder solution is added to the dry mix in two parts. First, the dry mix is ​​stirred at low speed to fully wet the dry mix and form a uniform mortar mixture. Then, the stirring intensity is increased to perform shear homogenization stirring to obtain the sprayed repair mortar.

10. The preparation method according to claim 9, characterized in that, In step S3, the amount of the retarder solution added for the first time is 60% to 80% of the total amount, and the amount added for the second time is 20% to 40%; the low-speed stirring speed is 200 to 300 rpm, and the shear homogenization stirring speed is 400 to 600 rpm; the low-speed stirring time is 45 to 60 s, and the shear homogenization stirring time is 60 to 90 s.

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