Cement-based composite material crack self-repairing method cooperating with utilization of solid waste and carbon dioxide

Abstract

The invention discloses a cement-based composite material crack self-repairing method cooperating with utilization of solid waste and carbon dioxide, and belongs to the technical field of building materials. Solid waste is used as a raw material, and after being sealed and soaked, the solid waste is properly pretreated to prepare the self-repairing agent with a certain particle size; the self-repairing agent is added into an existing cement-based material system, short fibers are added in a matched mode, and the cement-based composite material is prepared. The short fibers are added to limit the crack width of the cement-based composite material, after the cement-based composite material cracks, the embedded self-repairing agent reacts with carbon dioxide to achieve crack self-repairing, and the whole process is simple, economical and environmentally friendly. The problem that in the prior art, the self-repairing capacity is weakened due to the fact that CaO is insufficient is solved, a technical path is provided for high-value utilization of industrial solid waste, and the problem of recycling of a large amount of solid waste is solved.

Description

Technical Field

[0001] The invention relates to the technical field of building materials, and in particular to a method for self-repairing cracks in cement-based composite materials by coordinated utilization of solid waste and carbon dioxide. Background Art

[0002] Cement is an indispensable raw material in modern infrastructure construction. In 2023, the global demand for cement reached 4 billion tons, and China's demand for cement accounted for 52% of the global total demand. With the further expansion of the cement industry, the carbon emission problem brought about by its production process has become increasingly prominent. There are two main aspects of carbon emissions in the production process of cement, namely energy carbon emissions and chemical carbon emissions; among them, energy carbon emissions are caused by the energy consumption of the calcination process, which relies on various types of fossil fuels to maintain a calcination temperature of 1450°C; and chemical carbon emissions are generated by the high-temperature decomposition of limestone, the raw material of cement, which provides sufficient CaO components for cement clinker. With the development of global green energy transformation, energy carbon emissions in the cement production process have been gradually reduced, and how to reduce chemical carbon emissions has become one of the bottlenecks for the cement industry to achieve a dual-carbon strategy.

[0003] In view of the carbon emission characteristics of cement clinker, there are currently two main technical paths for developing low-carbon cement and its products (such as concrete): first, reducing the dependence of cement production on limestone calcination, either through chemical excitation (such as patent CN118084367A, through the combined excitation of three activators: calcium sulfate, sodium hydroxide, and sodium silicate) to form a calcination-free cementitious system, or developing low-calcium clinker (such as high-belite cement) to reduce dependence on limestone raw materials. Second, composite cement is formed by using auxiliary cementitious materials and mineral admixtures (such as patent CN116903334A, using calcined clay and limestone powder to replace silicate cement in large quantities to ensure environmental benefits; patent CN117105595A, using calcined clay and lightly burned dolomite powder to replace silicate cement in large quantities to ensure environmental benefits; patent CN118239734A, using a reasonable combination of metakaolin, limestone powder and cement to reduce carbon emissions, energy consumption and costs) to reduce the clinker content in cement, thereby reducing the carbon emissions per unit mass of cement. Therefore, by reducing the CaO content in the cementitious component, chemical carbon emissions can be effectively reduced, and it is expected to achieve carbon neutrality from the source of production.

[0004] Crack self-healing is a spontaneous phenomenon of cement-based materials. Generally speaking, after the cracking of cement-based materials occurs, the hydration products near the crack surface and the incompletely hydrated cement clinker are directly exposed to the air, and under certain humidity conditions, a series of dissolution-precipitation reactions occur. The generated mineral precipitation will fill the crack space, forming a self-healing phenomenon. When the cement hydration age is short, the further hydration of the cement clinker has a more significant effect on filling the cracks. As the age increases, the remaining clinker content decreases, and the carbonization reaction between the hydration products near the crack surface (such as hydrated calcium silicate, calcium sulfonate, calcium hydroxide, etc.) and carbon dioxide in the air gradually dominates the later crack self-healing. This self-healing phenomenon inherent in cement-based materials can effectively repair fine cracks and improve the material's impermeability and service life.

[0005] Since the main healing product of cement-based materials during their service life is calcium carbonate, when the CaO content is insufficient (especially in the above-mentioned low-calcium cementitious system), its self-healing process will be inhibited. In order to improve its self-healing ability, the existing technologies include implanting microcapsules containing cementitious components in concrete (such as patents CN117623732, CN108558260, CN103396652), implanting microorganisms that can mineralize and generate calcium carbonate (such as patents CN112142409, CN112919847, CN113264729), etc. Although these methods improve the self-healing ability of the material to a certain extent, they increase the material cost and process complexity, and increase other risks in practical applications, such as material defects introduced by microcapsules, environmental problems caused by microorganisms, etc. In addition, the long-term stability of its self-healing ability remains to be verified. Therefore, it is very necessary to establish an economical and convenient self-repair method for low-calcium cement-based materials. Summary of the invention

[0006] In view of the deficiencies of the above-mentioned prior art, the present invention provides a method for self-repairing cracks in cement-based composite materials by synergistically utilizing solid waste and carbon dioxide for low-calcium cement-based composite materials. After cracking occurs in the cement-based composite material, its self-repairing ability of cracks is restored or enhanced through natural carbonization in the air or artificial accelerated carbonization, thereby solving the problem of weakened self-repairing ability caused by insufficient CaO component.

[0007] To achieve the above purpose, the specific technical solutions of the present invention are as follows:

[0008] A method for self-repairing cracks in cement-based composite materials by synergistically utilizing solid waste and carbon dioxide, wherein a self-repairing agent is added to a cement-based material system, and short fibers are added in combination to prepare a cement-based composite material; when cracks occur in the cement-based composite material, the self-repairing agent reacts with carbon dioxide to achieve self-repair of the cracks;

[0009] The preparation method of the self-healing agent comprises the following steps:

[0010] S1. Soak the solid waste naturally in a sealed state;

[0011] S2, filtering, vacuum drying;

[0012] S3. Pre-treating the solid waste to obtain a self-repairing agent with a certain particle size, and sealing and storing it for future use; the pre-treatment includes but is not limited to one or more of physical grinding, heat treatment, and chemical weathering.

[0013] The present invention uses alkaline solid waste as raw material, and uses it as a self-repairing agent after appropriate pretreatment for its chemical components with low hydration activity and high carbonization activity, and is matched with an appropriate amount of short fibers for use in cement-based composite materials; during the natural use of cement-based composite materials, the added short fibers can constrain the width of the cracks, thereby reducing the difficulty of self-repair of the cracks in the later stage; and the self-repairing agent embedded in the cement-based composite material can react with carbon dioxide after the cement-based composite material cracks to achieve the repair process of its own cracks. The whole process is simple, economical and environmentally friendly, and the self-repairing process does not require the addition of other chemicals. The self-repairing agent has a sufficient content of calcium silicate mineral components, such as γ-C2S, CS, and C3S2; the present invention is based on the specific chemical components and mineral components in the solid waste, and regulates the content of CaO or SiO2 through different pretreatment methods, and completes crystallization through heat treatment, so as to increase the content of calcium silicate mineral components in the self-repairing agent. When the cement-based composite material has not cracked, the self-healing agent does not participate in the hydration reaction, but acts as an admixture, forming an effective bond with the cementitious material and playing a microscopic filling role when the particle size is small enough; since the calcium silicate mineral component in the self-healing agent is inert in the cement-based composite material, its potential cementing ability is retained, and since the density of the self-healing agent is comparable to that of traditional admixtures, it will not introduce physical defects and affect the mechanical properties of the material; after the cement-based composite material cracks, the self-healing agent encounters carbon dioxide (in the air or provided artificially), and the chemical products formed by the reaction of the two can fill the cracks, achieving a self-healing effect, and can also further increase the density of the cement matrix, improving its strength and impermeability.

[0014] Preferably, the physical grinding is: slowly ball milling the solid waste at room temperature for at least 1 hour, or mechanically impact milling the solid waste at room temperature for at least 30 seconds; the heat treatment is: treating the solid waste at 800-1100°C for 1-2 hours and cooling naturally; the chemical weathering is: adding lime, silica fume or kaolin to the ground solid waste, mixing evenly, treating at 800-1100°C for 1-2 hours, and cooling naturally.

[0015] Preferably, the particle size of the repair agent is adjusted according to its mineral composition and the cement-based composite material system, and the particle size of the repair agent is 10 µm~4.75 mm.

[0016] Preferably, the self-healing agent is used to replace one or more of the following materials in the existing cement-based material system: a. The self-healing agent is used to replace 10% to 100% of fine aggregate; b. The self-healing agent is used to replace 10% to 100% of inert fillers; c. The self-healing agent is used to replace 10% to 50% of auxiliary cementitious materials.

[0017] Preferably, the source of the carbon dioxide includes one or more of air, industrial waste gas, and artificial synthetic gas, and the minimum concentration thereof is 150 ppm.

[0018] Preferably, after cracking occurs in the cement-based composite material, the self-repairing of the cracks is achieved by the reaction of the self-repairing agent with carbon dioxide in an outdoor rain and air exposure environment; or, the self-repair of the cracks is achieved by artificially accelerated carbonization; the specific operation of artificially accelerated carbonization is: using carbon dioxide gas with a concentration of 5-10% or more, at a relative humidity of 20%-70%, and a moisture content of the cement-based composite material of 30%-70% under saturated state, and reacting for at least 30 minutes.

[0019] Preferably, the amount of short fibers added is 0.5% to 2.0% of the total volume of the cement-based material system.

[0020] Preferably, the short fibers are selected from one or more of steel fibers, polyethylene fibers, polyvinyl alcohol fibers, polypropylene, and carbon fibers.

[0021] Preferably, the CaO content in the solid waste is 20~30 wt.%, and the SiO2 content is 10~20 wt.%; the source of the solid waste includes but is not limited to one or more of converter slag, electric furnace slag, open-hearth slag, and waste incineration bottom ash.

[0022] Preferably, the CaO content of the cement-based material system is 0-10 wt.%, and the cement includes at least one of ordinary Portland cement, high-belite Portland cement, Portland composite cement, alkali-activated cementitious material, recycled micro-powder low-carbon cement, aluminate cement, and sulphoaluminate cement.

[0023] Preferably, the cement-based material system includes one or more of ordinary concrete, mortar, grouting material, fiber concrete, fiber-reinforced cement-based composite materials, and ultra-high performance concrete.

[0024] Preferably, the soaking temperature is at least 23° C., and the soaking time is 24 hours to 14 days.

[0025] Preferably, the solvent used for soaking is tap water or deionized water.

[0026] Preferably, the liquid-to-solid ratio of the soaking is (6-10):1.

[0027] Preferably, the vacuum drying temperature is 60-105° C., and the vacuum drying time is 2-48 hours.

[0028] The present invention removes or reduces the influence of heavy metals or soluble salts in solid waste by soaking treatment in a sealed state, and then prepares a self-repairing agent of a certain particle size by appropriate pretreatment, and uses it in the existing cement-based material system to replace natural aggregates or fillers to prepare cement-based composite materials, saving the amount of traditional natural aggregates or fillers, and promoting the resource utilization of solid waste. After the obtained cement-based composite material cracks, the crack self-repair can be achieved in the air, or under artificial acceleration conditions. The crack self-repair process requires the participation of water and carbon dioxide. In the air, natural rainwater and carbon dioxide in the atmosphere can provide a self-repairing environment; under artificial acceleration conditions, carbon dioxide is used to achieve crack self-repair at appropriate humidity; under artificial acceleration conditions, the cement-based composite material needs to be in a semi-dry state, and its moisture content is 30-70% of the saturated state. The carbon dioxide used can be industrial flue gas or artificially synthesized carbon dioxide gas, and the latter can be used for artificial repair of modular prefabricated components.

[0029] The specific method of the present invention for pre-treating solid waste to prepare a self-repairing agent is selected according to the specific components of the solid waste. For example, for solid waste with coarser particles, a ball mill or impact mill is used to make the solid waste into a self-repairing agent with a finer particle size, and the reaction activity is increased by increasing the specific surface area; for solid waste with a low content of calcium silicate, a high-temperature treatment or a method of mixing and calcining with other materials is selected to make the obtained self-repairing agent have more calcium silicate content to participate in the subsequent self-repairing reaction. In addition, the particle size and dosage of the self-repairing agent depend on the specific type and mix ratio of the cement-based material system, and can be used to replace sand or micropowder with similar particle size. In some application scenarios, free calcium oxide in solid waste needs to be removed in advance to avoid endangering the volume stability of the material; the self-repairing agent of the present invention should be stored in a confined space, isolated from the air, and mixed evenly with other components in the cement-based material system when used, without changing the mixing process or process.

[0030] The present invention also provides a cement-based composite material involved in the crack self-repairing method.

[0031] Compared with the prior art, the present invention is beneficial in that:

[0032] 1. Most low-carbon cements have a low calcium content, resulting in weak self-repairing ability in the later stage. The present invention provides a method for improving the self-repairing ability of low-calcium cement-based composite materials. A self-repairing agent is combined with an appropriate amount of short fibers and used in the cement-based composite materials. The added short fibers are used to constrain the width of cracks; the self-repairing agent serves as a supplementary calcium source and reacts with carbon dioxide to achieve self-repair of cracks in the cement-based composite materials. The present invention promotes the improvement of the self-repairing ability of low-calcium cement-based composite materials, solves the problem of insufficient calcium source in the low-calcium cementitious material system, and also alleviates the mining pressure and transportation cost of natural raw materials.

[0033] 2. The present invention utilizes the characteristics of "high carbonization activity and low hydration activity" of some mineral components in solid waste and makes them into self-healing agents. These mineral components react with carbon dioxide to achieve self-repair of cracks. The repair product is similar to ordinary cement-based composite materials and has good compatibility with the crack surface. The repair process is also a mineralization process of carbon dioxide, which promotes carbon sequestration in building materials and the synergistic high-value utilization of carbon dioxide and solid waste.

[0034] 3. After the self-healing agent of the present invention reacts with carbon dioxide, the chemical products formed inside the cement-based composite material can not only fill the cracks to achieve the effect of crack healing, but also further increase the density of the cement matrix, improve its strength and impermeability, so as to enhance the safety and durability of infrastructure.

[0035] 4. Compared with microcapsule self-repairing materials, the method of the present invention does not change the physical structure of cement and concrete. The self-repairing agent used as an admixture has the same physical and mechanical properties as traditional admixtures to ensure good dispersibility and compatibility in concrete. The self-repairing agent used belongs to the same calcium silicate system as traditional cementitious materials, and there are no problems such as increasing material defects, reducing strength, and long-term aging of self-repairing products. Compared with microbial mineralization self-repairing, the method of the present invention has a wider range of applicability and no pollution risk.

[0036] 5. The method of the present invention uses solid waste as the raw material of the self-healing agent, and utilizes the self-healing agent to react with carbon dioxide (air or artificially provided) to achieve self-repair of cracks. It has the advantages of low cost and simple process. The preparation and application of the self-healing agent can rely on existing equipment to complete the entire process, which is conducive to industrial promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 The tensile stress-strain curve of the cement-based composite material prepared in Example 1 and the crack repaired thereof;

[0038] Figure 2The tensile stress-strain curves of the cement-based composite material prepared in Example 1 at different damage degrees after crack repair for 7 days; wherein, Figure 2 A is the tensile stress-strain curve of the cement-based composite material stretched to 1% strain, 7 days after the crack was repaired; Figure 2 B is the tensile stress-strain curve of the cement-based composite material stretched to a strain of 1.5% 7 days after the crack was repaired; Figure 2 C is the tensile stress-strain curve of the cement-based composite material stretched to a strain of 2% 7 days after the crack was repaired;

[0039] Figure 3 The degree of crack repair characterized by the UPV test method;

[0040] Figure 4 This is the crack repair morphology under microscope observation in Test Example 1;

[0041] Figure 5 This is the crack repair morphology under microscope observation in Test Example 4. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] The present invention provides a method for self-repairing cracks in cement-based composite materials by using solid waste in combination with carbon dioxide. A self-repairing agent is added to a cement-based material system, and short fibers are added in combination to prepare a cement-based composite material. When cracks occur in the cement-based composite material, the self-repairing agent reacts with carbon dioxide to achieve self-repair of the cracks.

[0044] The preparation method of the self-healing agent comprises the following steps:

[0045] S1. Soak the solid waste naturally in a sealed state;

[0046] S2, filtering, vacuum drying;

[0047] S3. Pre-treating the solid waste to obtain a self-repairing agent with a certain particle size, and sealing and storing it for future use; the pre-treatment includes but is not limited to one or more of physical grinding, heat treatment, and chemical weathering.

[0048] In some examples, the self-healing agent is used to replace one or more of the following materials in an existing cement-based material system: a. The self-healing agent is used to replace 10% to 100% of fine aggregate; b. The self-healing agent is used to replace 10% to 100% of inert fillers; c. The self-healing agent is used to replace 10% to 50% of auxiliary cementitious materials.

[0049] In some examples, the source of the carbon dioxide includes one or more of air, industrial waste gas, and artificial synthetic gas, and the concentration of the carbon dioxide is as low as 150 ppm.

[0050] In some examples, after cracking occurs in the cement-based composite material, the self-repair of the cracks is achieved through the reaction of the self-healing agent with carbon dioxide in an outdoor rain and air exposure environment; or, the self-repair of the cracks is achieved through artificially accelerated carbonization; the specific operation of artificially accelerated carbonization is: using carbon dioxide gas with a concentration of 5% to 10% or more, at a relative humidity of 20% to 70%, and the moisture content of the cement-based composite material is 30% to 70% of the saturated state, and reacting for at least 30 minutes.

[0051] In some examples, the amount of short fibers added is 0.5% to 2.0% of the total volume of the cement-based material system.

[0052] In some examples, the short fibers are selected from one or more of steel fibers, polyethylene fibers, polyvinyl alcohol fibers, polypropylene, and carbon fibers.

[0053] In some examples, the CaO content in the solid waste is 20~30 wt.%, and the SiO2 content is 10~20wt.%; the source of the solid waste includes but is not limited to one or more of converter slag, electric furnace slag, open-hearth slag, and waste incineration bottom slag.

[0054] In some examples, the CaO content of the cement-based material system is 0~10 wt.%, and the cement includes at least one of ordinary Portland cement, high-belite Portland cement, Portland composite cement, alkali-activated cementitious material, recycled micro-powder low-carbon cement, aluminate cement, and sulphoaluminate cement.

[0055] In some examples, the cement-based material system includes one or more of ordinary concrete, mortar, grouting material, fiber concrete, fiber reinforced cement-based composite materials, and ultra-high performance concrete.

[0056] In some examples, the soaking temperature is as low as 23° C. and the soaking time is 24 hours to 14 days.

[0057] In some examples, the solvent used for the soaking is tap water or deionized water.

[0058] In some examples, the liquid-to-solid ratio of the soaking is (6-10):1.

[0059] In some examples, the vacuum drying temperature is 60-105° C., and the vacuum drying time is 2-48 hours.

[0060] Example 1

[0061] A cement-based composite material for crack self-repairing is prepared by using steel slag, and the preparation method is as follows:

[0062] (1) Select industrial waste steel slag and soak it naturally in tap water at 25 °C for 24 hours in a sealed state, with a liquid-to-solid ratio of 6:1;

[0063] (2) Filter and vacuum dry at 60 °C for 48 hours;

[0064] (3) Screening and slow ball milling at room temperature for 3 hours until the particle size is less than 200 µm to obtain a self-healing agent, which is then sealed and stored for later use.

[0065] (4) Weigh the raw materials of cement-based composite materials: 165 parts of cement, 50 parts of kaolin, 25 parts of calcium carbonate powder, 660 parts of fly ash, 348 parts of self-healing agent, 240 parts of water, 5 parts of water reducing agent, and 20 parts of short fibers;

[0066] (5) Place the weighed cement, metakaolin, calcium carbonate powder, fly ash, and self-healing agent in a mortar mixer and stir for 3 minutes to obtain a mixed dry material;

[0067] (6) Pour the weighed water and water reducing agent into the mortar mixer and stir for 3 minutes to allow them to fully react with the mixed dry materials to form a slurry;

[0068] (7) Add the weighed short fibers into the slurry three times and stir at high speed for 5 minutes to evenly disperse the short fibers;

[0069] (8) The uniformly stirred slurry was molded and allowed to stand at 25 °C for 24 hours before demolding. The slurry was then cured for 28 days (temperature was controlled at 20 ± 2 °C and humidity was ≥ 95%) to obtain a self-healing cement-based composite material.

[0070] The cement used in this embodiment is P·O 42.5 ordinary Portland cement; the fly ash is Class F first-class fly ash; the water reducing agent is a polycarboxylic acid-based high-efficiency water reducing agent; and the short fiber is polyvinyl alcohol (PVA) fiber.

[0071] The measured compressive strength of the cement-based composite material prepared in Example 1 is 44.3 MPa, the tensile strength is 3.6 MPa, the tensile initial crack strength is 2.6 MPa, and the uniaxial tensile strain is 3.8%; the uniaxial tensile stress-strain curve of the cement-based composite material is as follows: Figure 1As shown in curve 1.

[0072] The following is a crack self-repair test on the cement-based composite material prepared in Implementation 1. The cement-based composite material is stretched by a uniaxial tensile instrument, and cracks of different widths are pre-fabricated in the tensile region of the cement-based composite material to simulate material crack damage caused by load during use.

[0073] Test Example 1

[0074] The cement-based composite material was stretched to a strain of 1% and then unloaded. In order to accelerate the self-repair process of the cracks, a high concentration of carbon dioxide gas was introduced into the damaged cement-based composite material to cause a dissolution-precipitation carbonization reaction at the cracks. The repair morphology of the material cracks was observed using an optical microscope. The specific method is as follows:

[0075] (1) The cement-based composite material prepared in Implementation 1 is stretched to a strain of 1% and then unloaded;

[0076] (2) Use an optical microscope to photograph the crack morphology of cement-based composites;

[0077] (3) Dry the cement-based composite material at 23 °C in a ventilated place for 2 hours until the water loss in the cement-based composite material reaches 40 ± 5%. At this time, the cement-based composite material is in a semi-dry state;

[0078] (4) In order to accelerate the observation of the repair process of carbon dioxide on the material sample, the semi-dry cement-based composite material was placed in a sealed pressure container, and 99.9% carbon dioxide gas was introduced into the container to maintain the gas pressure in the container at 0.1 MPa;

[0079] (5) After 7 days of reaction, the repair morphology of the cracks in the cement-based composite material was observed using an optical microscope;

[0080] (6) The repaired cement-based composite material is stretched to fracture using a uniaxial tensile instrument to obtain a stress-strain curve. Figure 1 Curve 2 and Figure 2 As shown in A; the crack distribution state and crack repair results of cement-based composite materials are shown in Figure 4 shown.

[0081] Test Example 2

[0082] The cement-based composite material was stretched to a strain of 1.5% and then unloaded to obtain the crack distribution state at this strain level, and then repaired under accelerated carbonization conditions; the evaluation method of the self-repair process and crack repair morphology of the cement-based composite material was the same as that of Test Example 1. The stress-strain curve of the repaired cement-based composite material is shown in Figure 2 As shown in B.

[0083] Test Example 3

[0084] The cement-based composite material was stretched to a strain of 2% and then unloaded to obtain the crack distribution state at this strain level, and then repaired under accelerated carbonization conditions. The evaluation method of the self-repair process and crack repair morphology of the cement-based composite material was the same as that of Test Example 1. The stress-strain curve of the cement-based composite material after crack repair is shown in Figure 2 As shown in C.

[0085] Test Example 4

[0086] This test verifies the self-healing ability of cement-based composite materials under natural conditions. The cement-based composite materials are stretched to a strain of 1% and then unloaded. A dry-wet cycle is used, with one cycle every 4 days, to simulate the natural environment conditions for repair. At the end of each cycle, a Pundit Lab ultrasonic detector is used to measure the ultrasonic pulse velocity (UPV) passing through the material to evaluate the degree of crack repair in the cement-based composite material. The test results are shown in the figure below. Figure 3 The crack morphology was photographed using an optical microscope before and after the crack repair to observe the crack repair effect, crack distribution and crack repair results. Figure 5 shown.

[0087] Comparative Example 1

[0088] In this comparative example, the self-repairing cement-based composite material is prepared using steel slag. The preparation method is basically the same as that of Example 1, except that the self-repairing agent prepared in step (3) is not sealed and stored, but is used after natural weathering for 3 months.

[0089] The cement-based composite material prepared in Comparative Example 1 was stretched to a strain of 1% and then unloaded. A dry-wet cycle was used, with one cycle every 4 days, to simulate the natural environment for self-repair. At the end of each cycle, a Pundit Lab ultrasonic detector was used to measure the speed of ultrasonic pulses passing through the material to evaluate the degree of crack repair in the cement-based composite material. The test results are shown in Figure 2. Figure 3 As shown (weathering). The crack morphology was photographed using an optical microscope before and after the crack repair to observe the crack repair effect.

[0090] Figure 1 The tensile stress-strain curve of the cement-based composite material prepared in Example 1 and the crack repaired thereof; Figure 1It can be clearly seen that the initial crack strength and final tensile strength of curve 2 (stress-strain curve of the cement-based composite material after the cracks are repaired after the cement-based composite material is stretched to 1% strain and then unloaded) are greater than those of curve 1, and the strain hardening behavior is more obvious, and the final ductility is also improved. This shows that the self-repairing process of the cement-based composite material of the present invention not only fills the cracks, but also promotes the connection of the matrix of the crack surface, so that the initial crack strength value of the cement-based composite material after the cracks are repaired has been significantly improved. In addition, since carbon dioxide will further stimulate the potential activity of steel slag, the matrix in the entire cement-based composite material will be more compact and more tightly bonded to the internal fibers, so that the final tensile strength is significantly increased and the ductility is also improved to a certain extent.

[0091] Figure 2 The uniaxial tensile stress-strain curves of cement-based composites after 7 days of accelerated repair at different strain levels. Figure 2 It can be seen that the cement-based composite materials all show a significant increase in initial crack strength after crack repair. Since the filling effect of the crack will not increase the initial crack strength of the material, the above phenomenon shows that the gel-like products generated by the carbonization of the calcium silicate component in the steel slag have a bonding effect on the crack. Under this effect, the cement-based composite materials after crack repair will not first crack at the original crack when stretched, achieving the effect of repairing the original crack damage of the material.

[0092] Figure 3 The crack repair condition is characterized using the UPV test method. Figure 3 It can be seen that the cement-based composite material prepared in Example 1 has a good crack repairing effect under natural conditions of dry-wet cycles. In addition, the weathering of steel slag caused by unsealed storage will cause a significant decrease in the crack repairing effect.

[0093] Figure 4 , Figure 5 They are the crack repair forms of test examples 1 and 4 respectively. Figure 4 It can be seen that the low-calcium cement-based composite material that uses steel slag to achieve crack self-repair can completely repair cracks <50μm and partially repair cracks <100μm under the repair conditions of accelerated carbonization in 7 days, significantly reducing the crack width. Figure 5 It can be seen that under the conditions of simulating the actual use of cement-based composite materials, that is, repeated cycles of rainfall and drying, appropriately extending the repair cycle can also achieve the repair effect of accelerated carbonization.

[0094] In summary, the present invention uses industrial solid waste as raw material, and after sealing and soaking treatment, a self-repairing agent rich in calcium silicate mineral components is prepared by pre-treating it, and the self-repairing agent is added to the existing cement-based material system, and short fibers are added to prepare a cement-based composite material; after the cement-based composite material is damaged during service, the embedded self-repairing agent can react with carbon dioxide to achieve self-repair of cracks, and the whole process is simple, economical and environmentally friendly. The present invention not only solves the problem of weakened self-repairing ability caused by insufficient CaO components in the prior art, but also provides a technical path for the high-value utilization of industrial solid waste and solves the problem of recycling of bulk solid waste. In addition, after the self-repairing agent of the present invention undergoes carbonization reaction, the chemical products formed inside the cement-based composite material can not only fill microcracks to achieve the effect of crack healing, but also further increase the density of the cement matrix, improve the strength and impermeability of the cement matrix, so as to improve the safety and durability of infrastructure.

[0095] The above specific embodiments describe the implementation of the present invention in detail, but the present invention is not limited to the specific details in the above embodiments. Within the scope of the claims and technical concept of the present invention, the technical solution of the present invention can be modified and changed in many simple ways, and these simple modifications all belong to the protection scope of the present invention.

Claims

1. A method for self-repairing cracks in cement-based composite materials by synergistically utilizing solid waste and carbon dioxide. It is characterized in that A self-repairing agent is added to a cement-based material system, and short fibers are added to prepare a cement-based composite material; when cracks occur in the cement-based composite material, the self-repairing agent reacts with carbon dioxide to achieve self-repair of the cracks; The preparation method of the self-healing agent comprises the following steps: S1. Soak the solid waste naturally in a sealed state; S2, filtering, vacuum drying; S3. Pre-treating the solid waste to obtain a self-repairing agent with a certain particle size, and sealing and storing it for future use; the pre-treatment includes but is not limited to one or more of physical grinding, heat treatment, and chemical weathering.

2. A method for self-repairing cracks in cement-based composite materials by synergistically utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The physical grinding comprises: subjecting the solid waste to slow ball milling at room temperature for at least 1 hour, or subjecting the solid waste to mechanical impact milling at room temperature for at least 30 seconds; the heat treatment comprises: subjecting the solid waste to treatment at 800-1100° C. for 1-2 hours, and cooling naturally; the chemical weathering comprises: adding lime, silica fume or metakaolin to the ground solid waste, mixing evenly, subjecting the solid waste to treatment at 800-1100° C. for 1-2 hours, and cooling naturally.

3. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The particle size of the repair agent is 10 µm~4.75 mm.

4. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The self-healing agent is used to replace one or more of the following materials in the existing cement-based material system: a. The self-healing agent is used to replace 10% to 100% of fine aggregate; b. The self-healing agent is used to replace 10% to 100% of inert fillers; c. The self-healing agent is used to replace 10% to 50% of auxiliary cementitious materials.

5. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The sources of the carbon dioxide include one or more of air, industrial waste gas, and artificial synthetic gas, and the minimum concentration of the carbon dioxide is 150 ppm.

6. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that After the cement-based composite material cracks, the self-healing agent reacts with carbon dioxide in the air in an outdoor rain and air exposure environment to achieve self-repair of the cracks; alternatively, the self-repair of the cracks is achieved by artificially accelerated carbonization, wherein the artificially accelerated carbonization is specifically performed by using carbon dioxide gas with a concentration of 5 to 10% or more, at a relative humidity of 20 to 70%, and a water content of the cement-based composite material of 30 to 70% of the saturated state, and reacting for at least 30 minutes.

7. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The amount of the short fibers added is 0.5% to 2.0% of the total volume of the cement-based material system.

8. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The CaO content in the solid waste is 20-30 wt.%, SiO 2 The content is 10~20wt.%; the sources of the solid waste include but are not limited to one or more of converter slag, electric furnace slag, open-hearth slag, and waste incineration bottom slag.

9. The method for self-repairing cracks in cement-based composite materials by utilizing solid waste and carbon dioxide according to claim 1, It is characterized in that The CaO content in the cement-based material system is 0-10 wt.%, and the cement includes at least one of ordinary Portland cement, high-belite Portland cement, Portland composite cement, alkali-activated cementitious material, recycled micro-powder low-carbon cement, aluminate cement, and sulphoaluminate cement.

10. A cement-based composite material involved in the crack self-repairing method according to any one of claims 1 to 9.

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