Solid waste-based self-stress concrete filled steel tube and preparation method thereof

Abstract

The invention belongs to the technical field of building materials, and particularly relates to solid-waste-based self-stress concrete-filled steel tube and a preparation method thereof. The solid waste-based self-stress concrete filled steel tube is prepared from the following raw materials in percentage by mass: 60% of mineral powder, 30% of steel slag powder, 10% of fly ash, 6%-12% of an expanding agent, 4% of an alkali activator, 15% of steel slag fine aggregate, 20% of steel slag coarse aggregate, 15% of sand, 20% of gravel and the balance of water, totaling 100%. The steel slag is used for replacing natural sand and gravel and is supplemented with the expanding agent to form a self-stress concrete-filled steel tube material system, and the material system has the advantages of being environmentally friendly, high in self-stress, high in bearing capacity and the like. The material system also solves the problems that pores are generated between the concrete and the steel pipe due to self-shrinkage of the concrete, voids are formed, and the overall working performance is influenced.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to a solid waste-based self-stressing steel pipe concrete and its preparation method. Background Technology

[0002] Concrete-filled steel tube (CFST) structures, characterized by strong confinement, good integrity, and superior load-bearing capacity, have been widely used in high-rise buildings, bridge foundations, and municipal engineering projects. The core of the design involves concrete filling a closed steel tube, which provides circumferential constraint while bearing structural loads, forming a coordinated stress system. However, a key issue with CFST structures lies in the interface between the concrete and the steel tube. In practical applications, ordinary concrete often experiences moisture loss and drying shrinkage during hardening, leading to voids and microcracks within the steel tube. This results in insufficient interfacial bonding, reduced early load-bearing capacity, and even compromised long-term service performance. The airtightness and confined space of the steel tube make effective external curing of the internal concrete difficult, hindering timely replenishment of evaporating moisture and further exacerbating drying shrinkage. Furthermore, the steel tube cannot actively apply external prestress to the concrete, leaving the interface in a state of passive stress. Therefore, how to construct self-stress within the confined space of the steel tube and enhance the bond between the concrete and the steel tube has become a pressing issue in CFST design.

[0003] In industrial solid waste production, the total amount of bulk solid waste such as mineral powder, fly ash, and steel slag exceeds 1 billion tons. Among them, the annual output of steel slag exceeds 120 million tons, fly ash exceeds 500 million tons, and blast furnace slag exceeds 300 million tons, making it a significant issue for industrial solid waste treatment in my country. Mineral powder mainly originates from the fine grinding of blast furnace slag during steelmaking and has potential pozzolanic reactivity, making it an unavoidable byproduct of metallurgical processes. Fly ash is a fine-particle byproduct obtained from flue gas capture during coal-fired power generation. Steel slag originates from the cooling and solidification of molten metal slag in the steelmaking process. The continuous accumulation of these industrial solid wastes poses numerous environmental hazards. On the one hand, large quantities of slag and steel slag are left in the open year-round, occupying land resources and impacting ecosystems. On the other hand, fly ash contains a certain amount of heavy metals and soluble salts, which, if not properly treated, can easily cause water and soil pollution. Furthermore, steel slag, due to its complex chemical composition and unstable cementing properties, will cause volume instability if used directly without treatment, posing significant safety risks.

[0004] How to efficiently integrate bulk solid wastes such as mineral powder, fly ash, and steel slag into steel-concrete composite systems is related to the harmlessness, reduction, and resource utilization of solid waste resources. By constructing a cementitious and aggregate system with industrial by-products as the main raw materials, the engineering performance of steel-concrete composite systems can be achieved while reducing the dependence on cement and natural aggregates, thus providing raw material guarantees for the large-scale promotion of green building materials systems.

[0005] However, the hydration rate and expansion process of existing solid waste-based self-stressing steel-tube concrete materials under alkali / sulfate activation are highly sensitive to temperature, admixture dosage, and activator dosage, resulting in the difficulty in stabilizing and controlling the self-stress when solid waste-based cementitious systems are applied to steel-tube concrete. Summary of the Invention

[0006] To address the problem that the hydration rate and expansion process of existing solid waste-based self-stressing steel-tube concrete materials under alkali / sulfate activation are highly sensitive to temperature, admixture dosage, and activator dosage, leading to unstable and uncontrollable self-stress when solid waste-based cementitious systems are applied to steel-tube concrete, this invention provides a solid waste-based self-stressing steel-tube concrete and its preparation method. This invention uses mineral powder, steel slag powder, and fly ash as the main cementitious materials, supplemented with an alkali activator, and uses steel slag instead of natural sand and crushed stone, supplemented with an expansion agent to form a self-stressing steel-tube concrete material system. This material system possesses the characteristics of being green and environmentally friendly, having strong self-stress, and high load-bearing capacity. To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0007] This invention provides a solid waste-based self-stressing steel pipe concrete, made from the following raw materials by weight percentage: 5.20%~5.56% mineral powder, 2.60%~2.78% steel slag powder, 0.86%~0.92% fly ash, 0.60%~1.18% expansion agent, 2.02%~2.02% alkali activator, 7.38%~14.78% steel slag fine aggregate, 10.14%~20.28% steel slag coarse aggregate, 7.38%~14.78% sand, 10.14%~20.28% crushed stone, with the balance being water, totaling 100%.

[0008] This invention uses mineral powder, steel slag powder, and fly ash as the main cementing materials, combined with an alkali activator to improve the reactivity and hardening stability of the solid waste system. Simultaneously, it replaces natural sand and crushed stone with fine and / or coarse steel slag aggregate (replacement rate can reach 0-100%), achieving resource utilization of solid waste aggregates. Based on this, by controlling the dosage of an expansive agent, the core concrete undergoes restricted expansion in the early hardening stage. This expansion, under the constraint of the steel pipe, is transformed into a continuous compressive force on the inner wall of the steel pipe, thereby forming a stable pre-stress within the component. This self-stress can offset the risk of interfacial micro-cracks and local voids caused by the self-shrinkage and hardening deformation of the solid waste-based concrete, improving the adhesion and bonding reliability of the steel pipe-core concrete interface, thus ensuring the joint working performance and load-bearing stability of the component. Therefore, this invention ensures that while increasing the solid waste dosage and replacement rate, it achieves controllable introduction of self-stress and effective improvement of interfacial performance.

[0009] This invention significantly reduces the sensitivity of the hydration reaction to dosage and activator concentration by narrowing the proportions of mineral powder, steel slag powder, and fly ash to 5.20%~5.56%, 2.60%~2.78%, and 0.86%~0.92%, respectively, and fixing the alkali activator at 2.02%. It introduces 0.60%~1.18% of an expansion agent to the solid waste-based cementitious material, forming a stable expansion source with strong temperature adaptability. A multi-level physical constraint system is constructed using 7.38%~14.78% fine steel slag aggregate, 10.14%~20.28% coarse steel slag aggregate, and sand and gravel to synergistically regulate the expansion process, thereby precisely controlling the self-stress development rate, peak value, and stabilization period. This solves the problem in existing solid waste-based self-stressed steel-tube concrete materials where the hydration rate and expansion process under alkali / sulfate activation are highly sensitive to temperature, dosage, and activator concentration, leading to unstable and controllable self-stress when the solid waste-based cementitious system is applied to steel-tube concrete.

[0010] Furthermore, the cementitious material is composed of mineral powder, steel slag powder, and fly ash. The mass ratio of mineral powder, steel slag powder, and fly ash is 5~7:2~4:1.

[0011] Furthermore, the mass ratio of mineral powder, steel slag powder, and fly ash is 6:3:1.

[0012] Furthermore, based on the total mass of the main cementitious materials (internal admixture method), the amount of expanding agent is 6% to 12%.

[0013] Furthermore, the amount of expansion agent is 9%.

[0014] Furthermore, steel slag fine aggregate and sand together form fine aggregate; steel slag aggregate and crushed stone together form coarse aggregate. Both fine and coarse aggregates can be used as aggregates.

[0015] Furthermore, the alkali activator is a compound of sodium hydroxide and water glass. The initial water glass has a SiO2 content of 28.3%, a Na2O content of 9.4%, a water content of 62.3%, and an initial modulus of 3.0. The initial sodium hydroxide solid contains 99% NaOH. Sodium hydroxide is added to the water glass in batches under stirring conditions to adjust the modulus of the compounded water glass to approximately 1.0. The compounded water glass has a SiO2 content of 28.3%, a Na2O content of 29.245%, and a water content of 42.46%. Based on the total mass of the main cementing materials, the effective alkali (Na2O alkali equivalent) of the alkali activator is 5%~7%, and the water glass modulus is 0.95~1.1.

[0016] Furthermore, based on the total mass of the cementitious materials, the water-cement ratio is 0.35~0.45. The water-cement ratio is the ratio of the mass of the mixing water to the total mass of the cementitious materials. The mass of the mixing water (mixing water volume) is the sum of the added water and the water content in the alkali activator (solution), meaning the water content in the alkali activator is included in the mixing water volume for calculating the water-cement ratio.

[0017] Furthermore, the mineral powder is S95 grade mineral powder, with a 7-day activity index ≥75%, a 28-day activity index ≥95%, and a specific surface area of ​​400 m². 2 / kg~450m 2 / kg; The mineral powder, steel slag powder, and fly ash are compounded and formed into an alkali-activated cementitious system under the action of the above-mentioned alkali activator. After curing at 20±2℃ and RH≥95% for 28 days, the hydration products are quantified by XRD-Rietveld method, with a scanning angle of 5°~80° and a scanning speed of 2° / min. The mass fraction of hydrated calcium silicate cement (C-S-H) obtained is 19.7%, which is 22.3% higher than that of the mineral powder system under the same conditions, compared with 15.3% of hydrated calcium silicate cement (C-S-H). This is beneficial to improving the interfacial bonding performance between hardened steel pipe concrete and aggregate.

[0018] Furthermore, steel slag coarse aggregate and steel slag fine aggregate are used to partially or completely replace natural coarse and fine aggregates, with a replacement rate of 25% to 75% (by mass), preferably 50%. In the concrete mix design of this invention, steel slag coarse aggregate and steel slag fine aggregate adopt a composite system of "steel slag aggregate + natural aggregate": wherein the steel slag coarse aggregate replaces natural crushed stone, and the steel slag fine aggregate replaces natural river sand; the remaining coarse and fine aggregates are still natural crushed stone and natural river sand, respectively. Through this composite gradation design, a multi-grade gradation structure of concrete is constructed, and interface performance optimization is achieved.

[0019] Furthermore, the steel slag powder is obtained from steelmaking converter steel slag through magnetic separation to remove iron and ball milling, with a specific surface area ≥400m². 2 / kg, with a free calcium oxide content of 3%~5% and a free magnesium oxide content of 2%~4%; the free calcium oxide and free magnesium oxide in the steel slag powder can produce a synergistic expansion effect with the expansion agent. Under the constraint of the steel pipe, the expansion stress contributed by it accounts for 30%~40% of the total self-stress of the concrete. At the same time, cracking caused by excessive expansion is avoided by controlling the free calcium and magnesium content.

[0020] Furthermore, the fly ash is Class I fly ash, with a sieve residue of ≤12% on a 45μm square mesh sieve, a water requirement ratio of ≤95%, and a loss on ignition of ≤5%. The fly ash has a spherical particle morphology. According to GB / T50080-2016 method at 20±2℃ and w / b=0.4, the spread of the control group without fly ash is 480±10mm, while that with 10% fly ash (based on the total mass of cementitious materials) is 550±10mm, an increase of 12.7% compared to the control group. It can also fill the micropores of the cementitious system during the hardening process. The apparent porosity of the specimen was determined according to ASTM C642. After the specimen was dried at 110℃, vacuum saturated with water, and surface dried, the standard penetration porosity of the specimen without fly ash was 13.5%, while that with 10% fly ash was 11.9%, reducing the 28-day porosity of the concrete by 13.4%.

[0021] Furthermore, the expansive agent is an HCSA ettringite-type expansive agent from Ganglu Cement Plant. Its raw material composition, by mass fraction, includes 60%~80% calcium sulfoaluminate cement clinker and 15%~40% calcium sulfate components. A small amount of conventional inert fillers and grinding aids can be adjusted according to the heat release and expansion rate. Its 28-day restricted expansion rate is ≥0.025%, and its 3-day restricted expansion rate is ≥0.015% in an alkaline environment with pH ≥12. When the HCSA ettringite-type expansive agent works synergistically with the free calcium oxide and free magnesium oxide components in the steel slag powder, it can enable the early self-stress of concrete under steel pipe constraint to reach 60%~70% of the total self-stress of concrete. At the same time, by controlling the expansion rate, it avoids interface damage caused by asynchronous development of concrete strength.

[0022] Furthermore, the steel slag fine aggregate is made from steel slag through crushing and screening, with a fineness modulus of 2.3~3.0, crushing index ≤10%, water absorption rate ≤3%, crushing value ≤10%, maximum particle size ≤4.75mm, and has undergone at least 28 days of aging treatment; when the steel slag fine aggregate is mixed with sand at a mass ratio of 50:50, the porosity of the mixed fine aggregate is ≤35%, which is 5%~8% lower than that of single natural sand, and can significantly improve the compactness of concrete and the strength of the interface transition zone with cementitious paste.

[0023] Furthermore, the sand is natural river sand with a fineness modulus of 2.6~2.9, a mud content ≤1.5%, and a mud lump content ≤0.5%. After being blended with steel slag fine aggregate in a suitable mass ratio, the mixture should conform to the national standard "Sand for Construction" (GB / T14684~2022) on standard sieves of 4.75, 2.36, 1.18, 0.60, 0.30, and 0.15 mm. The fineness modulus of the blended fine aggregate should be controlled between 2.6 and 2.9, and it should simultaneously meet the requirement of an apparent density ≥2500 kg / m³. 3The performance limitation is a water absorption rate of ≤3%. Fine aggregates obtained according to the above rules possess both continuous gradation and high apparent density, which can balance the fluidity of the mixture and the skeleton support without significantly increasing water consumption, thus benefiting the early self-stress and strength development under steel pipe constraints.

[0024] Furthermore, the steel slag coarse aggregate is obtained by water quenching, crushing, and screening of steel slag, with a particle size range of 5~20mm, a crushing index ≤12%, a needle-like and flaky particle content ≤8%, and an apparent density ≥3000kg / m³. 3 The water absorption rate is ≤2.5%; when the steel slag coarse aggregate and crushed stone are mixed at a mass ratio of 50:50, the porosity of the mixed coarse aggregate is ≤40%, and the steel slag aggregate undergoes a micro-expansion of 0.01%~0.02% within 30 days under alkaline curing environment, which further fills the aggregate gaps and enhances the interfacial interlocking force with the steel pipe.

[0025] Furthermore, the coarse aggregate is a mixture of crushed stone and steel slag: the crushed stone is preferably limestone crushed stone, with a particle size range of 5~20mm, a crushing index ≤10%, a mud content ≤1%, and an apparent density ≥2600kgm³. 3 After being blended with steel slag coarse aggregate in a certain mass ratio, the particle composition of the blended coarse aggregate on standard sieves of 26.5, 19.0, 16.0, 13.2, 9.5, and 4.75 mm conforms to the national standard "Construction Pebbles and Crushed Stone" (GB / T14685-2002), and ensures that the ratio of the maximum particle size to the minimum particle size is ≥4. The blended coarse aggregate should also simultaneously meet the following requirements: apparent density ≥2600 kg / m³. 3 The compound coarse aggregate obtained according to the above rules can form a continuous and stable stress skeleton in the mixture, taking into account both fluidity and load-bearing requirements under the constraint of steel pipe.

[0026] This invention also provides a method for preparing solid waste-based self-stressing steel pipe concrete, comprising the following steps: first, mixing fine steel slag aggregate, sand, coarse steel slag aggregate, and crushed stone; then adding mineral powder, steel slag powder, fly ash, and an expansion agent and mixing them; then adding an alkali activator and water in sequence, stirring evenly, and finally pouring the mixture into the inside of a steel pipe. After vibration, solid waste-based self-stressing steel pipe concrete is finally obtained.

[0027] In the aforementioned steel-concrete composite material system, mineral powder, as a potential active cementitious material, can gradually generate hydrated calcium silicate gel under alkaline activation conditions, participating in the later-stage strength development. Steel slag powder, rich in free calcium oxide and magnesium oxide, possesses a certain volume expansion capacity and can produce a synergistic expansion effect with the expansive agent, inducing volume expansion in the concrete under the confinement of the steel pipe, which is then converted into effective self-stress. Fly ash, with its spherical particle morphology, helps improve the fluidity of the paste, fills the interfacial micropores, and enhances the compactness and durability of the concrete. The expansive agent reacts rapidly in the alkaline system, releasing expansion stress and effectively forming circumferential inward pressure under the closed confinement of the steel pipe, realizing the early-stage self-stress construction of the component. The coarse and fine aggregates of steel slag exhibit slight expansibility under certain humidity and alkaline conditions, and can participate in volume deformation regulation together with the expansive agent, forming internal self-stress within the steel pipe, further enhancing the interfacial bonding and synergistic deformation capacity between the core concrete and the steel pipe. The synergistic design of the above components in cementation, aggregate, and expansion effects enables the concrete system of this invention to possess good self-stress induction capacity and axial compressive bearing capacity while ensuring workability and volume stability.

[0028] The steel-concrete composite system provided in this invention exhibits excellent workability and strong expansion stability. It can induce effective self-stress under the constraint of the steel tube, and the components demonstrate superior axial compressive performance, meeting engineering application requirements. Using the concrete material system provided by this invention, axial compressive testing with steel tubes has verified that, without external prestressing, a self-stress level of 1.13 MPa to 3.63 MPa can be achieved. The peak bearing capacity of the steel-concrete composite component is increased by 6% to 12%, demonstrating good overall toughness and stability.

[0029] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention provides a solid waste-based self-stressing steel-tube concrete, using mineral powder, steel slag powder, and fly ash as the main cementing materials, and incorporating an alkali activator to improve the reactivity and setting and hardening stability of the solid waste system. Simultaneously, it replaces natural sand and crushed stone with fine and / or coarse steel slag aggregate (replacement rate can reach 0-100%), realizing the resource utilization of solid waste aggregate. Based on this, by controlling the dosage of an expansive agent, the core concrete undergoes restricted expansion in the early hardening stage. This expansion, under the constraint of the steel tube, is transformed into a continuous compressive force on the inner wall of the steel tube, thereby forming a stable pre-stress within the component. This self-stress can offset the risk of interfacial micro-cracks and local voids caused by the self-shrinkage and hardening deformation of the solid waste-based concrete, improving the adhesion and bonding reliability of the steel tube-core concrete interface, thus ensuring the joint working performance and load-bearing stability of the component. Therefore, this invention ensures that while increasing the solid waste dosage and replacement rate, it achieves controllable introduction of self-stress and effective improvement of interfacial performance.

[0030] This invention significantly reduces the sensitivity of the hydration reaction to dosage and activator concentration by narrowing the proportions of mineral powder, steel slag powder, and fly ash to 5.20%~5.56%, 2.60%~2.78%, and 0.86%~0.92%, respectively, and fixing the alkali activator at 2.02%. It introduces 0.60%~1.18% of an expansion agent to the solid waste-based cementitious material, forming a stable expansion source with strong temperature adaptability. A multi-level physical constraint system is constructed using 7.38%~14.78% fine steel slag aggregate, 10.14%~20.28% coarse steel slag aggregate, and sand and gravel to synergistically regulate the expansion process, thereby precisely controlling the self-stress development rate, peak value, and stabilization period. This solves the problem in existing solid waste-based self-stressed steel-tube concrete materials where the hydration rate and expansion process under alkali / sulfate activation are highly sensitive to temperature, dosage, and activator concentration, leading to unstable and controllable self-stress when the solid waste-based cementitious system is applied to steel-tube concrete.

[0031] 2. Regarding the efficient utilization of solid waste, this invention breaks through the limitations of traditional low-proportion blending of solid waste. By completely replacing cement with mineral powder, steel slag powder, and fly ash, and by partially or completely replacing natural sand and crushed stone with steel slag coarse and fine aggregates (with a replacement rate of up to 100%), it achieves high-proportion synergistic utilization of bulk industrial solid waste. This not only solves the problem of long-term land occupation and environmental pollution caused by the stockpiling of slag, steel slag, and fly ash, but also transforms solid waste into high-value building materials, significantly enhancing its economic value and providing a practical and feasible technical solution for the harmless and resource-based treatment of solid waste.

[0032] In terms of self-stress regulation and interface optimization, this invention innovatively designs an "expansive-steel slag synergistic expansion system." Through the synergistic effect of the ettringite-type expansive agent and the free calcium oxide and magnesium oxide in the steel slag powder, self-stress is precisely induced under the constraint of the steel pipe, effectively solving the problem of voids and insufficient bonding strength between the steel pipe and concrete interface caused by drying shrinkage in ordinary concrete. At the same time, the micro-expansion characteristics of steel slag aggregate under alkaline curing environment further fill the aggregate gaps, enhance the interfacial interlocking force with the steel pipe, and increase the interfacial bonding strength to over 6.5 MPa, ensuring the synergistic stress-bearing performance of the component.

[0033] In terms of mechanical properties and construction adaptability, after alkali activation, the cementitious materials, mineral powder, steel slag powder, and fly ash form an active complementary relationship, increasing the amount of hydrated calcium silicate gel generated at 28 days by 20%~30%, significantly enhancing later-stage strength. The optimized gradation of steel slag and natural aggregate reduces the porosity by 5%~8%, improving the compactness of concrete. In addition, the peak bearing capacity of steel-concrete composite members is increased by 6%~12% compared to traditional systems, with a 7-day compressive strength exceeding 30MPa and a 28-day compressive strength exceeding 45MPa, while also meeting the slump and spread requirements for construction, facilitating pouring and vibration.

[0034] In terms of green environmental protection and sustainable development, this invention significantly reduces the amount of cement and natural aggregates used, reduces carbon emissions in the production process, and aligns with the "dual carbon" goal; through innovative processes, it achieves high-value utilization of solid waste, promotes the green transformation of the building materials system, conforms to the concepts of energy conservation, environmental protection and sustainable development, and has significant social, environmental and economic benefits. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the stress on the steel pipe and the core concrete in this invention. Detailed Implementation

[0036] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific 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 scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods, and the materials and reagents used in the following embodiments are commercially available unless otherwise specified.

[0037] The mineral powder is S95 grade, with a 7-day activity index ≥75%, a 28-day activity index ≥95%, and a specific surface area of ​​400~450 m². 2 / kg. Expanding agent dosage is 6%. Specific surface area of ​​steel slag powder ≥400m². 2 The steel slag powder contains 3%–5% free calcium oxide and 2%–4% free magnesium oxide. The fly ash is Grade I, with a 45μm square-hole sieve residue ≤12%, water requirement ≤95%, and loss on ignition ≤5%. The expanding agent is an ettringite-type expanding agent, and the alkali activator is prepared by mixing sodium hydroxide and water glass. The fineness modulus of the steel slag fine aggregate is 2.3–3.0, crushing index ≤10%, and water absorption ≤3%. The sand is natural river sand with a fineness modulus of 2.6–2.9, mud content ≤1.5%, and mud lump content ≤0.5%. The crushed stone is limestone crushed stone with a particle size of 5–20 mm, crushing index ≤10%, mud content ≤1%, and apparent density ≥2600 kg / m³. 3 The coarse aggregate of steel slag has a particle size of 5~20mm, a crushing index of ≤12%, a needle-like and flaky particle content of ≤8%, and a water absorption rate of ≤2.5%.

[0038] Example 1 A type of solid waste-based self-stressing steel-tube concrete is made from the following raw materials: 273 kg of mineral powder, 136.5 kg of steel slag powder, 45.5 kg of fly ash, 45 kg of expansive agent, 375 kg of fine steel slag aggregate, 375 kg of natural river sand, 515 kg of coarse steel slag aggregate, 515 kg of crushed stone, 156.44 kg of water, and 102.58 kg of alkali activator. That is, the replacement rate of both fine and coarse steel slag aggregate is 50%, and the expansive agent dosage is 9%.

[0039] The above-mentioned method for preparing solid waste-based self-stressing steel-tube concrete includes the following steps: First, fine aggregate of steel slag, sand, coarse aggregate of steel slag, and crushed stone are poured into a concrete mixer and mixed evenly. Then, mineral powder, steel slag powder, fly ash, and expansion agent are added and mixed evenly. Next, water glass and water are added in sequence and stirred evenly to obtain a mixture. Half of the mixture is poured into the interior of a Q235 steel pipe and fully vibrated. The pouring and vibration process is repeated twice to obtain solid waste-based self-stressing steel pipe concrete.

[0040] Example 2 A type of solid waste-based self-stressing steel-tube concrete is made from the following raw materials: 264 kg of mineral powder, 132 kg of steel slag powder, 44 kg of fly ash, 60 kg of expansive agent, 375 kg of fine steel slag aggregate, 375 kg of natural river sand, 515 kg of coarse steel slag aggregate, 515 kg of crushed stone, 156.44 kg of water, and 102.58 kg of alkali activator. Specifically, the replacement rate of both fine and coarse steel slag aggregate is 50%, and the expansive agent dosage is 12%.

[0041] The preparation method of solid waste-based self-stressing steel tube concrete is the same as in Example 1.

[0042] Example 3 A type of solid waste-based self-stressing steel-tube concrete is made from the following raw materials: 282 kg of mineral powder, 141 kg of steel slag powder, 47 kg of fly ash, 30 kg of expansive agent, 375 kg of fine steel slag aggregate, 375 kg of natural river sand, 515 kg of coarse steel slag aggregate, 515 kg of crushed stone, 156.44 kg of water, and 102.58 kg of alkali activator. Specifically, the replacement rate of both fine and coarse steel slag aggregate is 50%, and the expansive agent dosage is 6%.

[0043] Example 4 A type of solid waste-based self-stressing steel-tube concrete is made from the following raw materials: 273 kg of mineral powder, 136.5 kg of steel slag powder, 45.5 kg of fly ash, 45 kg of expansive agent, 187.5 kg of fine steel slag aggregate, 562.5 kg of natural river sand, 257.5 kg of coarse steel slag aggregate, 772.5 kg of crushed stone, 156.44 kg of water, and 102.58 kg of alkali activator. That is, the replacement rate of both fine and coarse steel slag aggregate is 25%, and the expansive agent dosage is 9%.

[0044] The preparation method of solid waste-based self-stressing steel tube concrete is the same as in Example 1.

[0045] Example 5 A type of solid waste-based self-stressing steel-tube concrete is made from the following raw materials: 273 kg of mineral powder, 136.5 kg of steel slag powder, 45.5 kg of fly ash, 45 kg of expansive agent, 562.5 kg of fine steel slag aggregate, 187.5 kg of natural river sand, 772.5 kg of coarse steel slag aggregate, 257.5 kg of crushed stone, 156.44 kg of water, and 102.58 kg of alkali activator. That is, the replacement rate of both fine and coarse steel slag aggregate is 75%, and the expansive agent dosage is 9%.

[0046] Comparative Example 1 A type of ordinary steel-concrete composite is made from the following raw materials: 500 kg of cement, 375 kg of natural river sand, 1030 kg of crushed stone, and 200 kg of water. It does not contain mineral powder, steel slag powder, expansion agent, alkali activator, or steel slag aggregate, i.e., it is a solid waste-free core concrete.

[0047] The above-mentioned method for preparing ordinary steel-tube concrete includes the following steps: First, pour natural river sand and gravel into a concrete mixer and mix them evenly. Then add cement and water and mix evenly. Take half of the mixture and pour it into the inside of a Q235 steel pipe and vibrate it thoroughly. Repeat the pouring and vibration process twice to obtain ordinary steel pipe concrete.

[0048] Comparative Example 2 A solid waste-based steel pipe concrete is made from the following raw materials: 300 kg of mineral powder, 150 kg of steel slag powder, 50 kg of fly ash, 750 kg of natural river sand, 1030 kg of crushed stone, 156.44 kg of water, and 102.58 kg of alkali activator; that is, the steel slag content is 0% and the natural aggregate content is 100%.

[0049] The preparation method of solid waste-based steel pipe concrete is the same as in Example 1.

[0050] Comparative Example 3 A solid waste-based steel pipe concrete is made from the following raw materials: 300 kg of mineral powder, 150 kg of steel slag powder, 50 kg of fly ash, 750 kg of fine steel slag aggregate, 1030 kg of coarse steel slag aggregate, 156.44 kg of water, and 102.58 kg of alkali activator; that is, the steel slag content is 100% and the natural aggregate content is 0%.

[0051] The preparation method of solid waste-based steel pipe concrete is the same as in Example 1.

[0052] Slump, spread, compressive strength and self-stress were tested on the concrete of Examples 1 to 6 and Comparative Examples 1 to 3, respectively.

[0053] The schematic diagram of the calculation process for the self-stress test is shown below. Figure 1 As shown. The specific testing method is as follows: Two pairs of strain gauges are symmetrically arranged on the outer wall of the middle part of the steel pipe to measure the circumferential and longitudinal strains, respectively. The strain gauges are resistance strain gauges with a resistance value of 350±0.5Ω, a strain limit of 2%, and a sensitivity coefficient of 2.0±1%. Before bonding, the surface of the steel pipe is ground and cleaned to ensure firm bonding and signal stability. After the pouring is completed, the strain gauges are connected to the TDS-630 static strain acquisition system to start recording data. Data is continuously collected for 28 days to measure the circumferential and longitudinal strains of the outer wall of the steel pipe, thereby indirectly obtaining the self-stress value generated by the concrete.

[0054] The slump test method shall be determined according to GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures", as follows: The experiment was conducted at 20℃±2℃ using an Abrams conical cylinder (approximately 300mm high, with an upper inner diameter of approximately 100mm and a lower inner diameter of approximately 200mm) and a matching tamping rod and base plate. The apparatus was cleaned and moistened before the experiment. Immediately after mixing the mixture, a sample was taken and thoroughly mixed. The conical cylinder was then placed on a horizontal base plate and firmly secured. The mixture was filled in three layers, and after each layer was filled, it was tamped 25 times with the tamping rod, which should penetrate the current layer and slightly extend into the next. After filling, the cylinder opening was leveled. Make the top surface of the mixture flush with the opening of the cylinder, and then lift the cone cylinder vertically and smoothly within about 5 to 10 seconds, avoiding twisting, tilting and impact. After lifting the cylinder, read the slump within 30±5 seconds. The slump value is the height difference (mm) between the height of the cone cylinder and the highest point of the sample after collapse. Obtain the 0h slump using the above method. At the same time, take the same batch of mixture, seal it and let it stand, and keep it at 20±2℃ for 2 hours. Stir it lightly for 12 seconds, and then measure the 2h slump using the same method.

[0055] Spread (mm) test method: After lifting the cone, the mixture is naturally spread out and allowed to stand still. Measure two perpendicular diameters and take the average value.

[0056] The compressive strength test method shall be performed in accordance with GB / T50081-2019 "Standard for Test Methods of Mechanical Properties of Ordinary Concrete". 150mm×150mm×150mm cube specimens (or equivalent dimensions allowed by the standard) shall be used. After mixing, the specimens shall be demolded 24±2 hours later and cured in a standard curing room at 20±℃ and relative humidity ≥95% until the specified age. A calibrated pressure testing machine shall be used for loading, with the loading rate controlled according to the standard requirements, and the maximum load shall be recorded. P max And the area under pressure. For each age period n=3, the average value is taken as the 7d / 28d compressive strength (MPa) of the group; outliers are handled in accordance with the standard clauses.

[0057] The formula for calculating compressive strength is as follows: ; In the above formula, f c This refers to the axial compressive strength of concrete. A The area under pressure.

[0058] The results are shown in Table 1.

[0059] Table 1. Concrete test data of Examples 1-5 and Comparative Examples 1-3 As shown in Table 1, in Comparative Example 1, without the addition of solid waste materials and expansion agents, the 28-day core concrete strength is higher than that of Example 5. However, because it does not have an expansion effect, it cannot interact with the steel pipe to form self-stress. The ultimate bearing capacity of the steel pipe specimen filled with the core concrete of this material is 745kN, which is 17.9% lower than that of the preferred Example 1.

[0060] Comparative Example 2, without the addition of steel slag aggregate and using only natural sand and gravel, has a slightly higher 28-day core concrete strength than some of the embodiments, but its self-stress generated solely by the expansion agent is lower, and its ultimate bearing capacity is lower than that of Preferred Example 1, decreasing by 14.7%.

[0061] In Comparative Example 3, without adding natural aggregate, steel slag coarse aggregate and steel slag fine aggregate were used to replace natural crushed stone and sand. Although the self-stress value was as high as 2.75 MPa, the 28-day core concrete strength was much lower than that of the example. Due to the increased steel slag content, excessive expansion led to more internal pores and microcracks, which offset the advantage of self-stress.

[0062] The self-stress of steel-concrete composite tubes provided by this invention is quantitatively verified by placing resistance strain gauges longitudinally and transversely on the outer wall of the steel tube to collect minute strain changes generated in the steel tube during concrete expansion. Through elasticity, the measured circumferential strain of the steel tube is used to estimate the radial stress exerted by the core concrete on the inner wall of the steel tube, indirectly obtaining the self-stress value generated by the concrete. This method is simple to operate, highly sensitive, and suitable for the quantitative evaluation and engineering application of the self-stress effect of the system of this invention.

[0063] This invention addresses the problems of unstable and controllable self-stress and unreliable bonding and adhesion between the steel pipe and core concrete interface when solid waste-based cementitious systems are applied to steel-concrete composites. It proposes a solid waste-based self-stressed steel-concrete composite and its preparation method. This technical solution uses mineral powder, steel slag powder, and fly ash as the main cementitious materials, combined with an alkali activator to improve the reactivity and hardening stability of the solid waste system. Simultaneously, it replaces natural sand and crushed stone with fine and / or coarse steel slag aggregate (replacement rate can reach 0-100%), realizing the resource utilization of solid waste aggregates. Based on this, by controlling the dosage of an expansive agent, the core concrete undergoes restricted expansion in the early hardening stage. Under the constraint of the steel pipe, this expansion is transformed into a continuous compressive force on the inner wall of the steel pipe, thereby forming a stable pre-stress within the component. This self-stress can offset the risk of interfacial micro-cracks and local voids caused by the self-shrinkage and hardening deformation of the solid waste-based concrete, improving the bonding and adhesion reliability of the steel pipe-core concrete interface, and thus ensuring the overall working performance and load-bearing stability of the component. Thus, this invention ensures that while increasing the amount of solid waste added and the replacement rate, it also achieves the controllable introduction of self-stress and the effective improvement of interface properties.

[0064] This invention significantly reduces the sensitivity of the hydration reaction to dosage and activator concentration by narrowing the proportions of mineral powder, steel slag powder, and fly ash to 5.20%~5.56%, 2.60%~2.78%, and 0.86%~0.92%, respectively, and fixing the alkali activator at 2.02%. It introduces 0.60%~1.18% of an expansion agent to the solid waste-based cementitious material, forming a stable expansion source with strong temperature adaptability. A multi-level physical constraint system is constructed using 7.38%~14.78% fine steel slag aggregate, 10.14%~20.28% coarse steel slag aggregate, and sand and gravel to synergistically regulate the expansion process, thereby precisely controlling the self-stress development rate, peak value, and stabilization period. This solves the problem in existing solid waste-based self-stressed steel-tube concrete materials where the hydration rate and expansion process under alkali / sulfate activation are highly sensitive to temperature, dosage, and activator concentration, leading to unstable and controllable self-stress when the solid waste-based cementitious system is applied to steel-tube concrete.

[0065] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the inventive concept of the present invention, can make other changes and modifications to these embodiments, all of which fall within the scope of the present invention.

[0066] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If such modifications and variations fall within the scope of equivalents of this invention, then this invention also intends to include these modifications and variations.

Claims

1. A solid waste-based self-stressing steel-tube concrete, characterized in that, It is made from the following raw materials by weight percentage: 5.20%~5.56% mineral powder, 2.60%~2.78% steel slag powder, 0.86%~0.92% fly ash, 0.60%~1.18% expanding agent, 2.02%~2.02% alkali activator, 7.38%~14.78% steel slag fine aggregate, 10.14%~20.28% steel slag coarse aggregate, 7.38%~14.78% sand and 10.14%~20.28% crushed stone, with the balance being water, and the total of all raw materials being 100%.

2. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The mineral powder is S95 grade, with a 7-day activity index ≥75%, a 28-day activity index ≥95%, and a specific surface area of ​​400 m². 2 / kg~450m 2 / kg, with an expansion agent dosage of 6%.

3. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The specific surface area of ​​steel slag powder is ≥400m². 2 / kg, the free calcium oxide content in steel slag powder is 3%~5% and the free magnesium oxide content is 2%~4%.

4. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The fly ash is Class I fly ash, with a 45μm square hole sieve residue of ≤12%, water requirement ratio of ≤95%, and loss on ignition of ≤5%.

5. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The expanding agent is an ettringite-type expanding agent, and the alkali activator is prepared by mixing sodium hydroxide and water glass.

6. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The fineness modulus of steel slag fine aggregate is 2.3~3.0, the crushing index is ≤10%, and the water absorption rate is ≤3%.

7. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The sand is natural river sand with a fineness modulus of 2.6-2.9, a mud content of ≤1.5%, and a mud lump content of ≤0.5%. The crushed stone is limestone crushed stone with a particle size of 5mm-20mm, a crushing index of ≤10%, a mud content of ≤1%, and an apparent density of ≥2600kg / m³. 3 .

8. The solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, The steel slag coarse aggregate has a particle size of 5mm~20mm, a crushing index of ≤12%, a needle-like and flaky particle content of ≤8%, and a water absorption rate of ≤2.5%.

9. The method for preparing solid waste-based self-stressing steel-tube concrete according to claim 1, characterized in that, Includes the following steps: Take mineral powder, steel slag powder, fly ash, expanding agent, alkali activator, fine steel slag aggregate, coarse steel slag aggregate, sand, crushed stone and water respectively, and prepare the materials according to the raw material ratio; First, steel slag fine aggregate, sand, steel slag coarse aggregate, and crushed stone are mixed. Then, mineral powder, steel slag powder, fly ash, and expansion agent are added and mixed. After that, alkali activator and water are added in sequence, and after being stirred evenly, the mixture is poured into the inside of the steel pipe and vibrated to obtain solid waste-based self-stressing steel pipe concrete.

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