Calcium hexaaluminate repairing material as well as preparation method and application thereof

Abstract

The invention discloses a calcium hexaluminate repairing material, a preparation method thereof and application of the calcium hexaluminate repairing material in repairing of a water beam or a stand column of a steel rolling heating furnace. The calcium hexaluminate repairing material comprises a base material, an additive and water, each 100 parts by mass of the base material comprises the following raw materials: 70-80 parts by mass of calcium hexaluminate, 5-15 parts by mass of activated alumina micro powder, 10-20 parts by mass of calcium aluminate cement and 4-9 parts by mass of aluminum dihydrogen phosphate; the additive comprises a water reducing agent accounting for 0.1-0.2% of the mass of the base material; and the mass of the water is 10-20% of the mass of the base material. The calcium hexaluminate repairing material provided by the invention solves the problems of pouring construction, long repairing time and the like after the existing heating furnace water beam is damaged, has the advantages of high strength and low linear change rate, can reduce the consumption of refractory materials, greatly shortens the repair cycle, meets the requirement of fast-paced production of steel mills, and has wide application prospects. The method is of great significance in improving the production efficiency and reducing the energy consumption and carbon emission of the heating furnace.

Description

Technical Field

[0001] The present invention relates to the technical field of refractory materials for heating furnaces, and in particular to a calcium hexaaluminate repair material and a preparation method and application thereof. Background Art

[0002] The continuous development of the iron and steel metallurgical industry is also an important stage of high-quality green development. The heating furnace is an important equipment for heating the billet before the finished product is rolled, and it is a key equipment in the steel rolling process of steel enterprises. Under the high temperature environment of normal production in the heating furnace, the refractory materials used to wrap the water beams and columns of the heating furnace will crack and fall off, which will lead to the failure of the thermal insulation lining, the intrusion of high-temperature flue gas, and even the direct exposure of the water-cooled steel pipes to the high-temperature environment, thereby significantly increasing the heat loss of the water cooling, reducing the thermal efficiency of the heating furnace, increasing energy consumption, and also bringing production safety issues. It is often necessary to remove the water beam refractory materials near the damaged part after shutting down the furnace, and then re-form and cast and cure the mold. The process is complex and time-consuming. Therefore, using repair materials to repair damaged and peeling water beam column refractory materials is of great significance for shortening the maintenance cycle of hot rolling heating furnaces, reducing production costs, and improving production efficiency.

[0003] Publication No. CN117342877A discloses a porous calcium hexaaluminate thermally insulating castable for heating furnace water beams and columns. The castable contains 63-90 parts of calcium hexaaluminate, 3-10 parts of calcined alumina powder, 3-9 parts of activated alumina fine powder, and 4-18 parts of calcium aluminate cement, with the total weight of these raw materials being 100 parts. Additives include a water reducer, a mineralizer, an air entraining agent, and a foam stabilizer. This castable utilizes the inherent low thermal conductivity of calcium hexaaluminate and its interwoven hexagonal platelet structure to form micropores. The air entraining agent is used to create pores that further reduce the castable's thermal conductivity. This overcomes the problems of existing heavy alumina-silica refractory castables for heating furnace water beams and columns, such as high thermal conductivity, poor thermal insulation, and insufficient corrosion resistance. The castable improves its thermal insulation and heat preservation properties, enhances its high-temperature mechanical strength, increases its service life, and reduces water-cooling heat loss. However, this material needs to be supported by formwork, poured, cured and baked in strict accordance with the construction process of castables to ensure the high-temperature performance of the material. It cannot be directly used to repair small-scale damaged parts of water beam columns.

[0004] Publication number CN119490346A discloses a ladle slag line repair material. Its components and percentages by mass are: 60-85% magnesia, 5-15% used magnesia-carbon brick powder, 2-11% iron-phosphorus powder, 0.3-2% metallic aluminum powder, 2-6% ultrafine graphite powder, 2-8% pig iron particles, 1-10% kaolin fine powder, and 1-5% spherical SiO2 micropowder. The material also includes 2-20% waste engine oil, 0-16% aluminum dihydrogen phosphate, and 0.2-0.6% amino-terminated polyisobutylene. This ladle slag line repair material exhibits excellent high-temperature resistance and adhesion properties, effectively improving adhesion to the ladle slag line and extending its service life. However, this hot, self-flowing magnesia repair material has low thermal shock resistance and is not suitable for use in rolling mill heating furnace water beams.

[0005] Publication number CN119320280A discloses a plastic material for repairing medium-frequency furnace linings and its preparation method. The plastic material comprises the following components in percentages: 20%-30% alumina, 35%-45% mullite, 5%-15% kyanite, 5%-10% plasticizer, 3%-7% glucose, 2%-5% silicon nitride, and 10-15% modified aluminum polyphosphate. The modified aluminum polyphosphate is prepared by reacting aluminum polyphosphate, silicon micropowder, polyvinyl alcohol, and water at 180-220°C and cooling to room temperature to obtain the modified aluminum polyphosphate. This plastic material for repairing medium-frequency furnace linings has low production costs and exhibits excellent bonding, high-temperature resistance, and thermal shock resistance, resolving issues such as poor bonding, short service life, and limited storage time associated with existing plastic materials. However, the linear change of the plastic repair material at 1100℃ is large, which is quite different from the linear change of the original water beam column refractory material. If it is used to repair damage on the water beam of a steel rolling heating furnace, cracks will be generated with the matrix under high temperature, resulting in damage to the new refractory material. Therefore, it cannot be used to repair the refractory material of the water beam of a steel rolling heating furnace. Summary of the Invention

[0006] In order to overcome the shortcomings of the above-mentioned technology, the purpose of the present invention is to provide a calcium hexaaluminate repair material and its preparation method and application, which can solve the problems of pouring construction and long repair time after the water beams and columns of the existing heating furnace are damaged, and has the advantages of high strength, low linear change rate and high plasticity.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: A calcium hexaaluminate repair material is special in that it includes a base material, additives and water; every 100 parts by mass of the base material includes the following raw materials: 70-80 parts by mass of calcium hexaaluminate, 5-15 parts by mass of activated alumina fine powder, 10-20 parts by mass of calcium aluminate cement, and 4-9 parts by mass of aluminum dihydrogen phosphate; the additive includes a water reducer accounting for 0.1-0.2% of the mass of the base material; the mass of the water is 10-20% of the mass of the base material.

[0008] As a preferred solution, the water reducer is a polycarboxylic acid water reducer.

[0009] Furthermore, the water reducer includes a general-purpose water reducer, a retarding-setting water reducer and an accelerating-setting water reducer.

[0010] Furthermore, the mass percentages of the general-purpose water-reducing agent, the retarding-setting water-reducing agent and the accelerating-setting water-reducing agent in the water-reducing agent are 60-80%, 15-30% and 5-10% respectively.

[0011] As a preferred embodiment, the calcium hexaaluminate is pre-synthesized calcium hexaaluminate with a purity greater than 99%; the particle size distribution of the calcium hexaaluminate is as follows: 30-45 wt% of calcium hexaaluminate with a particle size of 1 mm < ≤ 3 mm, 24-38 wt% of calcium hexaaluminate with a particle size of 0.074 mm ≤ ≤ 1 mm, and 20-45 wt% of calcium hexaaluminate with a particle size of less than 0.074 mm, and the sum of the weight percentages of the above components is 100%.

[0012] As a preferred embodiment, the Al2O3 content of the activated alumina powder is greater than 99%, and the particle size D50 is 1-2.5 μm; the Al2O3 content of the calcium aluminate cement is greater than 63.5%, the CaO content is less than 35.0%, and the SiO2 content is less than 0.8%; the aluminum dihydrogen phosphate is in powder form with a purity of greater than 99%.

[0013] As a preferred embodiment, every 100 parts by mass of the substrate includes the following raw materials: 73 parts by mass of calcium hexaaluminate, 7 parts by mass of activated alumina powder, 12 parts by mass of calcium aluminate cement, and 8 parts by mass of aluminum dihydrogen phosphate; the additives include 0.09% by mass of a general-purpose water reducer, 0.045% by mass of a retarding-setting water reducer, and 0.015% by mass of a accelerating-setting water reducer; the mass of the water is 14.1% by mass of the substrate.

[0014] As a preferred solution, the calcium hexaaluminate repair material has a linear change of ≤0.25% after firing under the test conditions of 1300°C×3h, a flexural strength of ≥9MPa at room temperature, and a flexural bonding strength of ≥2MPa.

[0015] The present invention also discloses a method for preparing the calcium hexaaluminate repair material, which is special in that it comprises the following steps: The base material and additive raw materials are weighed according to the formula and premixed, and then placed in a blender for dry mixing for 1 to 3 minutes to obtain a mixture; After the dry mixing is completed, water is added to the mixture within 1 minute, and the mixture is continuously stirred for 3 to 6 minutes. After the stirring is completed, the calcium hexaaluminate repair material is obtained.

[0016] The present invention also discloses the application of the above-mentioned calcium hexaaluminate repair material in repairing the water beam or column of a steel rolling heating furnace. The special feature of the application is that the calcium hexaaluminate repair material is applied to the repaired part of the water beam or column within 25 minutes after preparation and cured for at least 24 hours.

[0017] As a preferred solution, the water is deionized water, and the water temperature is 10-25°C.

[0018] The present invention utilizes a calcium hexaaluminate material, primarily composed of a calcium hexaaluminate phase. Calcium hexaaluminate exhibits a high melting point, excellent high-temperature stability, low thermal expansion coefficient and thermal conductivity, good chemical stability, and resistance to alkaline corrosion, making it a highly effective refractory material. The addition of presynthesized calcium hexaaluminate reduces the incomplete conversion of CA and CA2 phases during the high-temperature synthesis of calcium hexaaluminate, thereby minimizing the impact on material properties. Activated alumina micropowder, acting as an aluminum source, forms an excellent bonding system with calcium aluminate cement and calcium hexaaluminate. After calcination at 1300°C, the hydration products of the calcium aluminate cement gradually transform into CA, CA2, and CA6 phases. These newly generated substances fill pores and enhance the strength of the patch. The optimized combination of three water-reducing agents significantly reduces the water requirement of the patch, mitigates porosity caused by water evaporation, and adjusts the patch's pore structure. The use of phosphates generates aluminum phosphate gel, which encapsulates aggregate particles to form a three-dimensional network structure, enhancing the patch's strength and bonding strength.

[0019] Compared with the prior art, the present invention has the following beneficial effects: The calcium hexaaluminate repair material provided by the present invention solves the problems of pouring construction and long repair time after the water beam of the existing heating furnace is damaged, and has the advantages of high strength and low linear change rate.

[0020] The present invention avoids the problem of a single water reducer's insufficient performance in improving the construction and high-temperature performance of the repair material by adding three water reducers in an optimized ratio to the calcium hexaaluminate repair material. The addition of a universal water reducer reduces the amount of water added to the system, increases fluidity, and improves medium-temperature strength. The addition of a retarding water reducer delays the peak exothermicity of calcium dialuminate hydration, reduces the generation of microcracks, and improves mechanical properties. The addition of an accelerating water reducer accelerates the hydration of the calcium monoaluminate phase to form a dense structure, improving high-temperature volume stability. The optimized combination of the three water reducers significantly improves the compressive strength and flexural bond strength of the calcium hexaaluminate repair material while maintaining the post-firing linear changes of the calcium hexaaluminate repair material. The compressive strength and flexural bond strength of the calcium hexaaluminate repair material containing the water reducer provided by the present invention are significantly higher than those of the calcium hexaaluminate repair material without the addition of a water reducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1This is a SEM photograph of the fracture surface of the repair material in Example 1 after treatment at 1300°C. DETAILED DESCRIPTION

[0022] In order to better explain the present invention, the main contents of the present invention are further illustrated below with reference to the accompanying drawings and specific embodiments, but the contents of the present invention are not limited to the following embodiments.

[0023] The present invention discloses a calcium hexaaluminate patching material for wrapping a water beam of a steel rolling heating furnace, comprising a base material, additives, and water. The base material comprises the following raw materials per 100 parts by mass: 70-80 parts by mass of calcium hexaaluminate, 5-15 parts by mass of activated alumina fine powder, 10-20 parts by mass of calcium aluminate cement, and 4-9 parts by mass of aluminum dihydrogen phosphate. The additive comprises a water reducer accounting for 0.1-0.2% by mass of the base material. The water is deionized water, accounting for 10-20% of the base material's mass.

[0024] The calcium hexaaluminate is pre-synthesized calcium hexaaluminate with a purity of >99%; the particle size distribution of the calcium hexaaluminate is as follows: 30-45 wt% of calcium hexaaluminate with a particle size of 1 mm < ≤ 3 mm, 24-38 wt% of calcium hexaaluminate with a particle size of 0.074 mm ≤ ≤ 1 mm, and 20-45 wt% of calcium hexaaluminate with a particle size of <0.074 mm, and the sum of the weight percentages of the above components is 100%.

[0025] The preparation method of the calcium hexaaluminate repair material comprises the following steps: The calcium hexaaluminate, activated alumina powder, calcium aluminate cement, aluminum dihydrogen phosphate and water reducer were weighed according to the formula and premixed, and then placed in a mixer for dry mixing for 2 minutes; After the dry mixing is completed, deionized water with a water temperature of 10~25℃ is gradually added within 1 minute, and the mixture is continuously stirred for 4 minutes. After the stirring is completed, calcium hexaaluminate repair material is obtained, and the repair of the damaged part of the water beam column is completed within 25 minutes.

[0026] Examples 1 to 5 and Comparative Examples 1 to 4 The present invention is further described below by using specific examples 1 to 5 and comparative examples 1 to 4.

[0027] The components and their contents in Examples 1-5 (S1-S5) and Comparative Examples 1-4 (D1-D4) are shown in Table 1. The weight percentage of each raw material added in Table 1 refers to the weight percentage per 100 parts of the base material, and the percentage percentage is the percentage calculated by comparing the weight of the additive or water to the weight of the base material. In Table 1: A: calcium hexaaluminate, parts by mass; purity of calcium hexaaluminate > 99%; A1: calcium hexaaluminate with particle size 1 mm < ≤ 3 mm, parts by mass; A2: calcium hexaaluminate with particle size 0.074 mm ≤ ≤ 1 mm, parts by mass; A3: calcium hexaaluminate with particle size < 0.074 mm, parts by mass.

[0028] B: Activated alumina powder, parts by mass; the Al2O3 content of the activated alumina powder is greater than 99%, and the particle size D50 is 2 μm.

[0029] C: calcium aluminate cement, parts by mass; in calcium aluminate cement, Al2O3 content>63.5%, CaO content<35.0%, SiO2 content<0.8%.

[0030] D: Aluminum dihydrogen phosphate, parts by mass; Aluminum dihydrogen phosphate is in powder form with a purity of >99%.

[0031] E: Water reducer, %; E1: General-purpose water reducer; E2: Retarding water reducer; E3: Accelerating water reducer. Water reducers were purchased from Hubei Siman New Materials Co., Ltd.: WSM-R1 for general-purpose water reducer, WSM-R2 for retarding water reducer, and WSM-R3 for accelerating water reducer.

[0032] F: Deionized water, %.

[0033] Among them, the difference between Comparative Examples 1 to 4 and Example 1 is that: no water reducer is added in Comparative Example 1; only a general-purpose water reducer is added in Comparative Example 2; only a general-purpose water reducer and a retarding water reducer are added in Comparative Example 3; only a general-purpose water reducer and an accelerating water reducer are added in Comparative Example 4; due to the differences in the type selection and proportion of the water reducers in Comparative Examples 1 to 4, more deionized water needs to be added during the preparation process compared to Example S1.

[0034] Table 1: Ingredients and their contents in Examples and Comparative Examples The raw materials in each example and comparative example were premixed, dry-blended, added with deionized water, and continuously stirred according to the above-described preparation method. The mixture was then poured into a 25 mm × 25 mm × 150 mm mold and compacted to form the mold. The molded samples were cured in an environment at a temperature of 25°C and a humidity of 80% for 24 hours, then dried at 110°C for 24 hours. The dried strip samples were then heat-treated at 1300°C for 3 hours to obtain the calcium hexaaluminate patching material samples of the present invention. The calcium hexaaluminate patching material samples prepared in the above examples and comparative examples were subjected to various performance tests: the compressive strength of the patching material was tested according to GB / T 5072-2023; the linear change of the sample after high-temperature treatment was measured according to GB / T 5988-2022, the test method for permanent linear change of refractory materials upon heating.

[0035] The test results of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 2. The SEM photograph of the fracture surface of the repair material in Example 1 after treatment at 1300°C is shown in Table 2. Figure 1 shown.

[0036] like Figure 1 It can be seen that after the repair material is treated at 1300°C, the material structure is dense and uniform, without obvious defects; a large number of orderly grown columnar or lamellar crystals appear, which can enhance the structural stability; it shows good high-temperature strength, thermal shock stability and corrosion resistance; it is suitable for long-term use in harsh furnace lining environments such as high temperature, high thermal shock, and high corrosion.

[0037] As shown in Table 2, Comparative Examples 1-4, which did not simultaneously use a general-purpose water-reducing agent, a retarding water-reducing agent, or an accelerating water-reducing agent, exhibited significant post-fire linear variation under the 1300°C x 3h test conditions, and their room-temperature flexural strength and flexural bond strength were significantly lower than those of the examples. The calcium hexaaluminate patching material in Comparative Example 1, which did not include a water-reducing agent, performed the worst, with a post-fire linear variation of 0.34% under the 1300°C x 3h test conditions, a room-temperature flexural strength of 5.56 MPa, and a flexural bond strength of 1.28 MPa. With the gradual addition of water-reducing agents, the post-fire linear variation of the patching material decreased somewhat, and its strength increased somewhat.

[0038] In Examples 1-5, the calcium hexaaluminate repair materials containing all three water-reducing agents exhibited a post-firing linear change of 0.18-0.25%, room-temperature flexural strength of 9.78-13.17 MPa, and flexural bond strength of 2.33-2.63 MPa under the test conditions of 1300°C for 3 hours. These results indicate a significant decrease in the post-firing linear change compared to the control materials, while significantly increasing the room-temperature flexural strength and flexural bond strength. In particular, Example 5 achieved a post-firing linear change of 0.18%, flexural strength of 13.17 MPa, and flexural bond strength of 2.63 MPa. This demonstrates that the addition of the three water-reducing agents in an optimized ratio significantly improves the strength and flexural bond strength of the calcium hexaaluminate repair materials while maintaining their post-firing linear change.

[0039] Table 2: Performance results of calcium hexaaluminate repair materials in examples and comparative examples In summary, the flexural strength and flexural bond strength of the calcium hexaaluminate patch containing a water reducer provided by the present invention are significantly higher than those of calcium hexaaluminate patching materials without a water reducer. The addition of presynthesized calcium hexaaluminate reduces the impact of material performance degradation caused by incomplete conversion of CA and CA2 phases during the high-temperature synthesis of calcium hexaaluminate. Activated alumina powder, acting as an aluminum source, forms an excellent bonding system with calcium aluminate cement and calcium hexaaluminate. After calcination at 1300°C for 3 hours, the hydration products of the calcium aluminate cement gradually transform into CA, CA2, and CA6 phases. These newly generated substances fill the pores, enhancing the strength of the patch. The optimized combination of the three water reducers significantly reduces the patching material's water requirement, reduces the porosity caused by water evaporation, adjusts the patch's pore structure, and improves the material's mechanical strength while maintaining the post-calcination linear changes of the calcium hexaaluminate patch.

[0040] Application Examples The repair materials described in Examples 1 to 5 were used to repair the water beams and columns of a heating furnace. Compared to the prior art method of re-molding and then pouring and curing, the rapid repair of damaged refractory materials can extend their service life and reduce refractory consumption. Taking Example 3 as an example, during the furnace repair process of a steel mill's rolling mill heating furnace (heating temperature 1250°C), it was found that the water beams and columns had multiple areas of refractory material blocks that were less than 20*30*60mm in size. The repair material components and proportions described in Example 3 were used to weigh the base material and additives. After dry mixing for 2 minutes, water at a temperature of 10-25°C was measured in proportion, stirred for 5 minutes, and then applied to the refractory block-like areas to the same apparent diameter as the original water beam and column. The damaged areas of the water beam and column were repaired and completed within 25 minutes. After completion, the repair was naturally cured for 24 hours and then baked and heated with the furnace. One year after the repair, during the furnace shutdown for maintenance, the appearance was intact, with no further cracking or shedding, and the refractory block continued to be used in the furnace.

[0041] In view of the shrinkage cracks that occur in the original water beam insulation lining structure or material during long-term service, which have not yet reached the level of complete fall-off, the extrusion method is used to fill the cracks with the repair material, and react with it at the interface at high temperature to form a whole, thereby improving the strength and preventing further deterioration of the insulation structure.

[0042] Compared with the existing technology of removing the refractory materials of the water beam near the damaged part after the furnace is shut down, and then re-forming and pouring and curing to form, the present invention shortens the repair time of the water beam column from 5 to 6 days to less than 1 day, reduces the consumption of refractory materials by more than 70%, and can greatly shorten the maintenance cycle to meet the needs of the fast-paced production of steel mills. It is of great significance to improve production efficiency and reduce energy consumption and carbon emissions of heating furnaces.

[0043] Other parts not described belong to the prior art.

Claims

1. A calcium hexaaluminate repair material, characterized by: The invention comprises a substrate, additives and water; every 100 parts by mass of the substrate comprises the following raw materials: 70-80 parts by mass of calcium hexaaluminate, 5-15 parts by mass of activated alumina powder, 10-20 parts by mass of calcium aluminate cement, and 4-9 parts by mass of aluminum dihydrogen phosphate; the additive comprises a water reducer accounting for 0.1-0.2% by mass of the substrate; and the mass of the water is 10-20% by mass of the substrate.

2. The calcium hexaaluminate repair material according to claim 1, characterized in that: The water reducer is a polycarboxylic acid water reducer.

3. The calcium hexaaluminate repair material according to claim 1, characterized in that: The water reducer includes a general water reducer, a retarding water reducer and an accelerating water reducer.

4. The calcium hexaaluminate repair material according to claim 3, characterized in that: The mass percentages of the universal water reducer, the retarding water reducer and the accelerating water reducer in the water reducer are 60-80%, 15-30% and 5-10% respectively.

5. The calcium hexaaluminate repair material according to claim 1, characterized in that: The calcium hexaaluminate is pre-synthesized calcium hexaaluminate with a purity greater than 99%. The particle size distribution of the calcium hexaaluminate is as follows: 30-45 wt% of calcium hexaaluminate with a particle size of 1 mm < ≤ 3 mm, 24-38 wt% of calcium hexaaluminate with a particle size of 0.074 mm ≤ ≤ 1 mm, and 20-45 wt% of calcium hexaaluminate with a particle size less than 0.074 mm. The sum of the weight percentages of the above components is 100%.

6. The calcium hexaaluminate repair material according to claim 1, characterized in that: The Al2O3 content of the activated alumina micropowder is greater than 99%, and the particle size D50 is 1-2.5 μm; the Al2O3 content of the calcium aluminate cement is greater than 63.5%, the CaO content is less than 35.0%, and the SiO2 content is less than 0.8%; the aluminum dihydrogen phosphate is in powder form and has a purity of greater than 99%.

7. The calcium hexaaluminate repair material according to claim 3, characterized in that: Every 100 parts by mass of the substrate includes the following raw materials: 73 parts by mass of calcium hexaaluminate, 7 parts by mass of activated alumina powder, 12 parts by mass of calcium aluminate cement, and 8 parts by mass of aluminum dihydrogen phosphate; the additives include 0.09% by mass of a general-purpose water reducer, 0.045% by mass of a retarding-setting water reducer, and 0.015% by mass of an accelerating-setting water reducer; the mass of the water accounts for 14.1% by mass of the substrate.

8. The calcium hexaaluminate repair material according to any one of claims 1 to 7, characterized in that: The calcium hexaaluminate repair material has a linear change of ≤0.25% after firing under the test conditions of 1300° C.×3h, a flexural strength of ≥9MPa at room temperature, and a flexural bonding strength of ≥2MPa.

9. A method for preparing the calcium hexaaluminate repair material according to any one of claims 1 to 8, characterized in that: The following steps are involved: The base material and additive raw materials are weighed according to the formula and premixed, and then placed in a blender for dry mixing to obtain a mixture; After the dry mixing is completed, water is added to the mixture and stirred to obtain the calcium hexaaluminate repair material.

10. Use of the calcium hexaaluminate repair material according to any one of claims 1 to 8 in repairing water beams or columns of a steel rolling heating furnace, characterized in that: Apply the calcium hexaaluminate repair material to the repaired part of the water beam or column within 25 minutes after preparation and cure for at least 24 hours.

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