Method for preparing low-carbon cement based on construction waste

Abstract

The invention discloses a method for preparing low-carbon cement based on construction wastes, which comprises the following steps: carrying out deep hydrothermal reaction on the construction wastes by using an autoclave under high-temperature and high-pressure conditions, so that silicon-aluminum minerals in the construction wastes are subjected to more thorough hydrothermal rearrangement to generate more stable hydrated silicate and aluminosilicate; the reaction activity of the clinker can be improved at the subsequent lower calcining temperature, so that the utilization rate and the high activity of complex components such as the building wastes in the cement preparation process are realized. The low-carbon cement disclosed by the invention is relatively high in later compressive strength and breaking strength, the strength grade of the low-carbon cement is 32.5, and the low-carbon cement meets building requirements and can be widely applied to the fields of buildings, roads and the like. According to the invention, closed cycle of solid waste-material is realized through waste recycling, and double innovation of material performance and ecological benefit is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of cement preparation, and particularly to a method for preparing low-carbon cement based on construction waste. Background Art

[0002] In the traditional cement production process, the clinker calcination temperature is usually above 1400 °C, which not only consumes a large amount of energy but also releases a large amount of carbon dioxide (CO2), causing serious environmental impacts. Therefore, developing new cements that are low-carbon, low-energy-consuming, and resource-utilizing construction waste has become an important direction for solving the disposal of construction waste and carbon emission reduction in the cement industry.

[0003] Currently, some studies have attempted to use industrial wastes such as coal gangue, fly ash, and steel slag as alternative raw materials to reduce the clinker calcination temperature and carbon emissions. For example, the invention patent CN112456829A discloses a process for producing cement using solid waste, which uses slag and coal gangue as raw materials to prepare cement clinker, and controls the ratio of modified polyphosphoric acid potassium to cement clinker during cement production to prevent the cement from hardening too quickly. However, the calcination temperature in this method is 1300 °C to 1450 °C, with high energy consumption and high costs. The invention patent CN 109485278A discloses a cementitious material using coal gangue as a raw material and its preparation method. After mechanically-microwave composite activation of coal gangue, it is uniformly mixed with quicklime, and through chemical strengthening, hydrothermal synthesis, and low-temperature calcination methods, and ground with an ultra-fine cement mill to obtain a new cementitious material. The main components of coal gangue in this method are SiO2, Al2O3, as well as a certain amount of Fe2O3 and carbonaceous substances, which have high potential activity. Under the condition of a lower calcination temperature, it can easily form hydrated products with cementitious properties, but it is difficult to be applied to construction waste with complex components, etc. Moreover, the calcium aluminate component in the low-carbon cementitious material prepared from coal gangue is relatively high, and the setting time is fast, affecting the workability.

[0004] With the rapid development of urbanization and infrastructure construction, a large amount of construction waste is continuously generated, including waste concrete, waste bricks, waste glass, etc. If these construction wastes are not reasonably treated and reused, they will not only occupy a large amount of land resources but also cause environmental pollution. The traditional treatment methods for construction waste are mainly landfill or simple crushing and reuse, which have problems such as resource waste and low treatment efficiency. Moreover, the composition of construction waste is complex, with a relatively high CaO content, different ratios of SiO2 and Al2O3, and some components have already hydrated, resulting in low or uneven potential activity. When directly applied to the conventional cement process, its activity is low, the utilization rate is low, and additional conditioning components are required to produce cementitious properties. Summary of the Invention

[0005] Aiming at the deficiencies of the above-mentioned existing technologies, the technical problem to be solved by the present invention is: how to provide a method for preparing low-carbon cement based on construction waste, so as to solve the problems existing in the existing processes, such as high calcination temperature, high cost, waste of resources, environmental pollution caused by construction waste, complex composition, low potential activity, resulting in low utilization rate in cement preparation, etc.

[0006] To solve the above technical problems, the present invention adopts the following technical solutions: A method for preparing low-carbon cement based on construction waste, comprising the following steps:

[0007] S1: Crush and screen the construction waste to remove wood blocks, metals and other impurities that cannot be utilized, grind it to a particle size of less than 50 μm, the purpose is to increase the specific surface area for subsequent hydrothermal reaction, and then soak it in an alkali solution. After solid-liquid separation to remove the filtrate, a construction waste slurry is obtained;

[0008] S2: Add an activator, a hydrothermal auxiliary powder and water to the slurry in step S1, stir evenly to obtain a reaction solution, and then place the reaction solution in an autoclave for reaction. The reaction temperature is 180°C to 250°C. After the reaction ends, the product is dried at a low temperature to obtain a low-carbon cement precursor; in this way, under the conditions of high temperature and high pressure, the construction waste is subjected to a deep hydrothermal reaction, so that the silico-aluminate minerals in it undergo a more thorough hydrothermal rearrangement, generating more stable hydrated silicates and aluminosilicates, and improving the reaction activity of subsequent clinker calcination.

[0009] S3: Calcinate the precursor obtained in step S2 at 850°C to 1000°C to obtain clinker;

[0010] S4: Mix the clinker obtained in step S3 with mineral admixtures, and then grind the mixture to fine powder, namely the low-carbon cement.

[0011] In this way, during the alkali pretreatment of construction waste, the silico-aluminate minerals are activated, releasing soluble SiO2 and Al2O3. In the hydrothermal synthesis stage, under the conditions of high temperature and high pressure hydrothermal, the dissolved SiO2, Al2O3, Ca 2+ react again to form highly active calcium silicate C5S6H5, C6S6H and aluminosilicate phase C-A-S-H. Finally, during the low-temperature calcination process, these hydrothermal phases dehydrate and recombine to form β-C2S, CaSiO3, C4A3S - , and low-carbon cementitious phases. Therefore, through efficient alkali pretreatment + high-temperature and high-pressure hydrothermal reaction, the silico-aluminate mineral components in construction waste are deeply activated and hydrothermally rearranged, generating a precursor with high hydration activity, thereby reducing the clinker calcination temperature, reducing energy consumption and CO2 emissions.

[0012] Preferably, the construction waste in step S1 may be selected from one or more of waste concrete, waste bricks and waste glass.

[0013] Preferably, the alkaline solution in step S1 is Na2CO3 or / and NaOH, and the concentration of the alkaline solution is 1-4 mol / L. Specifically, when the alkaline solution is NaOH, its concentration is 2-4 mol / L; when the alkaline solution is Na2CO3, its concentration is 1-3 mol / L; the soaking time is 4-7 h. In this way, the silicon-aluminum components in the construction waste are pre-activated.

[0014] Preferably, the hydrothermal auxiliary powder in step S2 may be selected from one or more of red mud, fluorogypsum and phosphogypsum. In this way, the hydrothermal auxiliary not only serves as a conditioning component for adjusting the calcium-silicon-aluminum components of the low-carbon cement, but also promotes the hydrothermal reaction of aluminosilicates to form products with cementitious activity (such as tobermorite and xonotlite). And the particle size of the hydrothermal auxiliary powder is below 50 μm to increase the specific surface area for subsequent hydrothermal reactions; the activator is NaOH or a mixture of NaOH and Na2CO3. Using NaOH alone is suitable for high-efficiency activation and is suitable for treating waste brick powder and glass powder with high silicon and aluminum contents, which can quickly dissolve silicon-aluminum minerals and improve activity; the combination of NaOH + Na2CO3 is suitable for mild activation and is suitable for materials with a relatively high calcium content (such as waste concrete) to avoid excessive dissolution of minerals caused by too high a pH value and reduce the instability caused by too high alkalinity; Na2CO3 cannot be used alone because the alkalinity of sodium carbonate itself is weak and it cannot sufficiently activate silicon-aluminum minerals and is difficult to effectively promote hydrothermal reactions. In summary, it is more reasonable to keep the molar ratio of NaOH and Na2CO3 between 1:0 and 1:2. The dosage of the hydrothermal auxiliary is 10%-30% of the actual mass of the construction waste in the slurry.

[0015] Preferably, the liquid-solid ratio in the reaction solution in step S2 is 2.5:1-3.5:1, and the pH value of the reaction solution is 12-13.

[0016] Preferably, the reaction pressure in the autoclave in step S2 is 1.5-3 MPa, and the reaction time is 4-12 h.

[0017] Preferably, the dosage of the mineral admixture in step S4 is 10%-40% of the mass of the clinker; the mineral admixture may be selected from one or more of fly ash, steel slag, volcanic ash and silica fume. In this way, by adjusting the ratio between the hydrothermal auxiliary and the mineral admixture, low-carbon cements with different properties can be customized according to specific engineering requirements.

[0018] Preferably, the specific surface area of the fine powder in step S4 is 300-400 m 2 / kg.

[0019] Another object of the present invention is to provide low-carbon cement prepared by the above method.

[0020] Another object of the present invention is to provide the application of the above low-carbon cement in the fields of construction and roads.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The method provided by the present invention converts construction waste into low-carbon active cementitious materials through multi-stage synergistic effects. First, the physical properties of the waste are optimized by crushing, screening and ball milling to enhance the reaction interface activity; subsequently, the chemical stability of inert components (such as vitreous or clay minerals) is destroyed by alkali pretreatment to release silicon-aluminum active substances; in the hydrothermal reaction, silicon-aluminum precursors and calcium sources are directionally formed into cementitious phases such as C-S-H gel and calcium aluminate hydrate under high temperature and high pressure; finally, through medium-temperature calcination at 850-1000 °C, the intermediate products are recrystallized into low-temperature active minerals such as β-C2S and C3A. This method is applicable to a wider range of construction waste types, including waste concrete, waste bricks, waste glass, etc., and further optimizes the mineral composition through hydrothermal additives (such as red mud, phosphogypsum, fluorogypsum) to achieve more efficient resource utilization. The present invention realizes the closed-loop cycle of "solid waste - material" through waste resource utilization, combining both material performance and ecological benefits.

[0023] 2. The present invention uses an autoclave to carry out deep hydrothermal reaction on construction waste under high temperature and high pressure conditions, so that the silicon-aluminum minerals in it undergo more thorough hydrothermal rearrangement, generating more stable hydrated silicates and aluminosilicates, improving the reaction activity of subsequent clinker calcination, and thus realizing the utilization rate and high activity of complex components such as construction waste in the cement preparation process. The low-carbon cement prepared by the present invention has relatively high later compressive strength and flexural strength, and its strength grade is 32.5, meeting the construction requirements and can be widely used in the fields of construction and roads.

[0024] 3. The present invention uses construction waste such as waste concrete, waste bricks, waste glass, etc. as the main raw materials, reduces the dependence on natural resources (such as limestone), realizes the high-value utilization of construction waste, and reduces the environmental problems brought by landfilling. Combining industrial by-products such as red mud and phosphogypsum as hydrothermal additives further promotes the comprehensive utilization of solid waste, meeting the requirements of green and sustainable development. Compared with the clinker sintering method of traditional cement production, the low-carbon cement of the present invention also reduces the calcination temperature by about 450-600 °C on the premise of ensuring its durability and mechanical properties, avoiding the high-temperature, high-energy-consuming and large amount of carbon dioxide emission process, and reducing the production cost. The process of the present invention is simple, with low implementation difficulty and flexible production. This method can be compatible with existing cement production equipment and is easy to realize large-scale industrial application. Detailed implementation mode

[0025] The present invention will be further described in detail below in conjunction with embodiments. It should be understood that the following embodiments are only for illustrative explanation of the present invention and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention. Unless otherwise specified, the raw materials and reagents used in the following embodiments are commercially available products or can be prepared by known methods. The "parts by weight" in the following embodiments are weighed according to the same standard, that is, the mass of each part by weight is the same.

[0026] The main chemical compositions of the waste concrete powder (calcium silicate source) used in the following embodiments are: 25-40% SiO2, 30-45% CaO, 5-15% Al2O3, 2-10% Fe2O3, 1-5% MgO, 1-4% K2O, Na2O, and the loss on ignition is 15-25%; the main chemical compositions of the waste brick powder (silica-aluminum source) are: 45-60% SiO2, 20-30% Al2O3, 5-10% CaO, 3-8% Fe2O3, 1-5% MgO, 0.5-2% K2O, Na2O, and the loss on ignition is 10-20%; the main chemical compositions of the waste glass powder (calcium source) are: 70-80% SiO2, 10-15% Na2O, 1-5% Al2O3, 1-3% Fe2O3, 5-10% CaO, 0.5-2% MgO, and the loss on ignition is 2-5%.

[0027] I. A method for preparing low-carbon cement based on construction waste

[0028] Example 1: The following steps are adopted in this example:

[0029] S1: Weigh by weight: 35 parts of waste concrete powder, 25 parts of waste brick powder, and 10 parts of waste glass powder, then crush, screen, and grind them into fine powder with a particle size below 50 μm. Then soak the fine powder in a 2 mol / L NaOH solution for 5 h, and after solid-liquid separation to remove the filtrate, a construction waste slurry is obtained.

[0030] S2: Weigh by weight: 8 parts of red mud, 5 parts of fluorogypsum, and 3 parts of desulfurized gypsum. Then grind the weighed hydrothermal auxiliary materials to a particle size below 50 μm, and add them to the slurry in step S1, add water to keep the liquid-solid ratio between 3:1, stir evenly, and add Na2CO3 or NaOH to adjust the pH value of the system to 12 to obtain a reaction solution.

[0031] S3: Place the reaction solution obtained in step S2 in an autoclave at a pressure of 2 MPa and react for 8 h, with a reaction temperature of 200 °C. After the reaction, naturally dry the product or dry it in an oven at 50 °C - 80 °C to obtain a low-carbon cement precursor.

[0032] S4: Calcinate the low-carbon cement precursor obtained in step S3 at 850 °C to obtain clinker, and then weigh by parts by weight: 14 parts of mineral admixture (fly ash: steel slag = 1:1), mix it evenly with the clinker, and then grind the mixture to a fine powder with a particle size of a specific surface area of 300 - 400 m 2 / kg, namely the low-carbon cement.

[0033] Example 2: This example adopts the following steps:

[0034] S1: Weigh by parts by weight: 29 parts of waste concrete powder, 21 parts of waste brick powder, and 8 parts of waste glass powder. Then crush, screen, and grind them into a fine powder with a particle size below 50 μm. Then soak the fine powder in a NaOH solution with a concentration of 2 mol / L for 5 h. After solid-liquid separation to remove the filtrate, a building waste powder slurry is obtained.

[0035] S2: Weigh by parts by weight: 6 parts of red mud, 4 parts of fluorogypsum, and 2 parts of desulfurized gypsum. Then grind the weighed hydrothermal auxiliary materials to a particle size below 50 μm, and add them to the powder slurry in step S1. Add water to keep the liquid-solid ratio between 3:1, stir evenly, and add Na2CO3 or NaOH to adjust the pH value of the system to between 12 to obtain a reaction solution.

[0036] S3: Place the reaction solution obtained in step S2 in an autoclave at a pressure of 2 MPa and react for 8 h. The reaction temperature is 200 °C. After the reaction, naturally dry the product or dry it in an oven at 50 °C - 80 °C to obtain a low-carbon cement precursor.

[0037] S4: Calcinate the low-carbon cement precursor obtained in step S3 at 900 °C to obtain clinker, and then weigh by parts by weight: 30 parts of mineral admixture (fly ash: steel slag = 1:1), mix it evenly with the clinker, and then grind the mixture to a fine powder with a particle size of a specific surface area of 300 - 400 m 2 / kg, namely the low-carbon cement.

[0038] Comparative Example 1: In this comparative example, the liquid-solid ratio is 4:1, and other steps are the same as in Example 1.

[0039] Comparative Example 2: In this comparative example, the calcination temperature is 1100 °C, and other steps are the same as in Example 1.

[0040] Comparative Example 3: In this comparative example, the pressure in the hydrothermal reaction is normal pressure, and other steps are the same as in Example 1.

[0041] Comparative Example 4: In this comparative example, no alkali solution soaking pretreatment is carried out, and other steps are the same as in Example 1.

[0042] Comparative Example 5: No mineral admixture was added in this comparative example, and other steps were the same as those in Example 1.

[0043] II. Performance Verification

[0044] 1. The low-carbon cements prepared in Examples 1-2 and Comparative Examples 1-5 were respectively tested for compressive strength, flexural strength, initial setting time and final setting time, and the results are shown in Table 1.

[0045] Table 1

[0046]

[0047] As can be seen from Table 1, compared with the comparative examples, the low-carbon cement prepared by the present invention can significantly improve the compressive strength and flexural strength of the cement, and its strength grade is 32.5, meeting the building requirements. And experiments found that: too high liquid-solid ratio is not conducive to the final strength. Too high calcination temperature has little improvement on the cement strength. In addition, the hydrothermal reaction under high temperature and high pressure and strong alkaline (pH≥12) conditions is more conducive to the formation of active phases. Moreover, alkali pretreatment can improve the reactivity of silicon-aluminum components, enhance the formation of hydration products, and improve the cement performance; when there is no alkali pretreatment, the initial setting time of the cement is shortened but the strength is reduced. Appropriate amount of mineral admixtures (fly ash, steel slag) can enhance the cement performance, improve the strength and durability.

[0048] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing low-carbon cement from construction waste, characterized in that, It includes the following steps: S1: Crush, screen, and grind construction waste to a particle size below 50 μm, then soak it in an alkali solution. After solid-liquid separation to remove the filtrate, a construction waste slurry is obtained; S2: Add an activator, a hydrothermal auxiliary powder, and water to the slurry obtained in step S1, stir evenly to obtain a reaction solution, and then place the reaction solution in an autoclave for reaction. The reaction temperature is 180°C to 250°C. After the reaction ends, the product is dried at a low temperature to obtain a low-carbon cement precursor; S3: Calcine the precursor obtained in step S2 at 850°C to 1000°C to obtain clinker; S4: Mix the clinker obtained in step S3 with a mineral admixture, and then grind the mixture to fine powder, namely the low-carbon cement.

2. The method for preparing low-carbon cement based on construction waste according to claim 1, characterized in that, The construction waste in step S1 can be selected from one or more of waste concrete, waste bricks, and waste glass.

3. The method for preparing low-carbon cement based on construction waste according to claim 1, characterized in that, The alkali solution in step S1 is Na2CO3 or / and NaOH, and the concentration of the alkali solution is 1 to 4 mol / L; the soaking time is 4 to 7 h.

4. The method for preparing low-carbon cement based on construction waste according to claim 1, characterized in that, The hydrothermal auxiliary powder in step S2 can be selected from one or more of red mud, fluorogypsum, and phosphogypsum, and the particle size is below 50 μm; the activator is NaOH or a mixture of NaOH and Na2CO3; the dosage of the hydrothermal auxiliary powder is 10% to 30% of the mass of the construction waste in the slurry.

5. The method for preparing low-carbon cement based on construction waste according to claim 1, wherein, In the reaction solution in step S2, the liquid-solid ratio is 2.5:1 to 3.5:1, and the pH value of the reaction solution is 12 to 13.

6. The method for preparing low-carbon cement based on construction waste according to claim 1, wherein In step S2, the reaction pressure in the autoclave is 1.5 to 3 MPa, and the reaction time is 4 to 12 h.

7. The method for preparing low-carbon cement based on construction waste according to claim 1, wherein, In step S4, the dosage of the mineral admixture is 10% to 40% of the mass of the clinker; the mineral admixture can be selected from one or more of fly ash, steel slag, volcanic ash, and silica fume.

8. The method for preparing low-carbon cement based on construction waste according to claim 1, characterized in that, The specific surface area of the fine powder described in step S4 is 300-400 m 2 / kg.

9. The low-carbon cement prepared by the method according to any one of claims 1 to 8.

10. An application of the low-carbon cement according to claim 9 in the fields of construction and roads.

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