A circulating fluidized bed fly ash-based cementitious material, preparation method and formulation design method

Abstract

The present invention belongs to the field of utilization of industrial solid waste, and specifically relates to a circulating fluidized bed fly ash-based cementitious material, a preparation method and a formulation design method. The circulating fluidized bed fly ash-based cementitious material comprises 30-55 parts by weight of red mud, 0-40 parts by weight of ordinary fly ash, 15-65 parts by weight of circulating fluidized bed fly ash, 0-10 parts by weight of a calcium-containing solid alkali activator, and 10-25 parts by weight of water; the total number of parts of red mud, ordinary fly ash, circulating fluidized bed fly ash and calcium-containing solid alkali activator is 100 parts. The present invention adopts the Box-Behnken response surface design method, selects three factors, namely the red mud content, the ordinary fly ash content and the calcium-containing solid alkali activator ash content, as the corresponding variables, takes the 7-day and 28-day unconfined compressive strengths of the circulating fluidized bed fly ash-based cementitious specimen as the response values, performs fitting regression and variance analysis on the corresponding variables, obtains an optimized regression equation, and thus obtains the optimal formulation of the circulating fluidized bed fly ash-based cementitious material.

Description

Technical Field

[0001] The present invention belongs to the technical field of utilization of industrial solid waste, and particularly relates to a circulating fluidized bed fly ash-based cementitious material, a preparation method and a formulation design method. Background Art

[0002] Red mud is a waste generated during the processing of bauxite. It has strong alkalinity and rich metal oxide content, causing great harm to the environment. Currently, the annual global new increase in red mud is nearly 200 million tons. The reuse of red mud mainly focuses on the extraction of elements such as iron, high-aluminum calcium, alumina, and alkali, and its utilization rate in road cementitious materials is less than 5%.

[0003] Circulating fluidized bed fly ash is a by-product of circulating fluidized bed boilers in power plants. Currently, the annual accumulation is about 120 million tons. Compared with ordinary fly ash, the application of in-furnace desulfurization technology makes the chemical components of fluidized bed fly ash contain more toxic heavy metal substances and unstable chemical components, making it difficult to be used in road base cementitious materials.

[0004] The invention patent with the application number CN202210246183.X uses circulating fluidized bed fly ash, microsilica, diatomite and cement to prepare a circulating fluidized bed fly ash-based low-carbon cement. Although it increases the dosage of fluidized bed fly ash, it cannot completely replace cement clinker, and the economic cost is relatively high. The invention patent with the application number CN202311581300.9 proposes a quaternary cementitious material mortar based on red mud-fly ash-electric slag-fly ash, but the experimental design does not consider the interaction between raw materials. Therefore, the strength still needs to be improved.

[0005] In summary, there is an urgent need to develop a cementitious material and a preparation method with low cost, good working performance and full consideration of the synergistic effect of solid waste materials. Summary of the Invention

[0006] The purpose of the present invention is to provide a circulating fluidized bed fly ash-based cementitious material, a preparation method and a formulation design method. Through formula optimization, a high-strength cementitious material mainly composed of industrial solid waste is obtained, which can reduce economic costs and simultaneously realize the synergistic utilization of industrial solid waste.

[0007] To achieve the above purpose, the present invention provides a circulating fluidized bed fly ash-based cementitious material, which includes 30-55 parts by weight of red mud, 0-40 parts by weight of ordinary fly ash, 15-65 parts by weight of circulating fluidized bed fly ash, 0-10 parts by weight of calcium-containing solid alkali activator, and 10-25 parts by weight of water; wherein, the total number of parts of the red mud, ordinary fly ash, circulating fluidized bed fly ash and calcium-containing solid alkali activator is 100 parts.

[0008] Further, by weight, it includes 30 - 55 parts of red mud, 0 - 20 parts of ordinary fly ash, 25 - 65 parts of circulating fluidized bed fly ash, 1 - 10 parts of calcium-containing solid alkali activator, and 10 - 25 parts of water. The mass ratio of ordinary fly ash to circulating fluidized bed fly ash is preferably (0 - 1.5):1, more preferably (0 - 1):1, and at the same time, the dosage of the calcium-containing solid alkali activator is not 0.

[0009] Further, by weight, it includes 40 - 55 parts of red mud, 0 parts of ordinary fly ash, 35 - 57.5 parts of circulating fluidized bed fly ash, 2.5 - 10 parts of calcium-containing solid alkali activator, and 15 - 19 parts of water.

[0010] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 47 parts of red mud, 0 parts of ordinary fly ash, 43 parts of circulating fluidized bed fly ash, 10 parts of calcium-containing solid alkali activator, and 18 parts of water.

[0011] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 42.5 parts of red mud, 0 parts of ordinary fly ash, 47.5 parts of circulating fluidized bed fly ash, 10 parts of calcium-containing solid alkali activator, and 18 parts of water.

[0012] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 42.5 parts of red mud, 0 parts of ordinary fly ash, 57.5 parts of circulating fluidized bed fly ash, 0 parts of calcium-containing solid alkali activator, and 18 parts of water.

[0013] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 30 parts of red mud, 0 parts of ordinary fly ash, 65 parts of circulating fluidized bed fly ash, 5 parts of calcium-containing solid alkali activator, and 18 parts of water.

[0014] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 55 parts of red mud, 0 parts of ordinary fly ash, 40 parts of circulating fluidized bed fly ash, 5 parts of calcium-containing solid alkali activator, and 16 parts of water.

[0015] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 30 parts of red mud, 20 parts of ordinary fly ash, 40 parts of circulating fluidized bed fly ash, 10 parts of calcium-containing solid alkali activator, and 15 parts of water.

[0016] In some preferred embodiments, the circulating fluidized bed fly ash-based cementitious material by weight includes 55 parts of red mud, 20 parts of ordinary fly ash, 15 parts of circulating fluidized bed fly ash, 10 parts of calcium-containing solid alkali activator, and 18 parts of water.

[0017] The present invention uses red mud, ordinary fly ash, circulating fluidized bed fly ash and slaked lime as raw materials. Through formula optimization, a cementitious material with excellent compressive strength is obtained, while the utilization rate of solid waste is improved and the economic cost is significantly reduced.

[0018] Furthermore, the composition of the ordinary fly ash by mass fraction includes SiO₂ 46.6%, Al₂O₃ 33.2%, CaO 4.2%, MgO 1.5%, Fe₂O₃ 3.2%, K₂O 0.52%, SO₃ 1.28% and TiO₂ 1.15%; the composition of the circulating fluidized bed fly ash by mass fraction includes SiO₂ 32.79%, Al₂O₃ 21.8%, CaO 24.22%, MgO 0.57%, Fe₂O₃ 5.52%, K₂O 1.10%, SO₃ 11.61% and TiO₂ 1.02%; compared with ordinary fly ash, the relatively more free CaO in the circulating fluidized bed fly ash will be better activated in the pozzolanic reaction to generate Ca(OH)₂, enabling more silicon-aluminum components to be activated and participate in the hydration reaction.

[0019] The composition of the red mud by mass fraction includes SiO₂ 22.11%, Al₂O₃ 22.20%, CaO 21.62%, Fe₂O₃ 12.91%, Na₂O 11.39%, SO₃ 0.79% and TiO₂ 5.42%.

[0020] Furthermore, the calcium-containing solid base activator is slaked lime, with the main component being Ca(OH)₂, the content ≥ 95%, in powder form, and the fineness ≤ 200 mesh.

[0021] The present invention also provides a preparation method for the circulating fluidized bed fly ash-based cementitious material as described in any one of the above, comprising the following steps:

[0022] (1) Activation of raw materials: Place the red mud, ordinary fly ash and circulating fluidized bed fly ash in a blast drying oven and dry to constant weight. After ball milling, pass through a 0.075 mm sieve to obtain fine particle materials with a fineness ≤ 75 μm for standby; place the calcium-containing solid base activator in a blast drying oven and dry to constant weight, then pass through a 200 mesh sieve for standby;

[0023] (2) Mixing of the cementitious material: Weigh the red mud, ordinary fly ash, circulating fluidized bed fly ash and calcium-containing solid base activator prepared in step (1) according to the formula, mix evenly for 2 - 3 minutes, then add water and mix evenly again for about 2 - 3 minutes.

[0024] Further, it also includes specimen forming and curing. The gelling material mixed in step (2) is cured to obtain the solidified gelling material. Preferably, it includes: adding the mixed gelling material into a cylindrical mold with a size of Φ50×50mm in three times successively, statically pressing to form, demolding with a demolding machine, and then placing it with a film covering in a curing box at 20°C and 95% RH for curing until the corresponding age to obtain the solidified gelling material.

[0025] Further, the red mud, ordinary fly ash, and circulating fluidized bed fly ash are placed in a blast drying oven at 55 - 65°C for drying; the calcium-containing solid base activator is placed in a blast drying oven at 25 - 35°C for drying.

[0026] The present invention also provides a method for designing the formula of a circulating fluidized bed fly ash-based gelling material based on the response surface method, including the following steps:

[0027] (1) According to the total number of parts of red mud, ordinary fly ash, circulating fluidized bed fly ash, and calcium-containing solid base activator being 100 parts, taking the mass parts of red mud, ordinary fly ash, and calcium-containing solid base activator as influencing factors, multiple single-factor tests are carried out on the circulating fluidized bed fly ash. According to the above-mentioned preparation method, the modified gelling material cured to the corresponding age is obtained. Taking the 7-day unconfined compressive strength of the modified gelling material as the judgment basis, the influence of each factor on the modified gelling material is obtained based on the single-factor test.

[0028] (2) Based on the test results of the single factor, taking the mass parts of red mud, ordinary fly ash, and calcium-containing solid base activator as influencing factors, and taking the 7-day and 28-day unconfined compressive strengths of the modified gelling material as response values (when conducting the experiment, 3 specimens for each of the 7-day and 28-day ages are prepared), the Box - Benhnken design method in DesignExpert13 software is used to design the test plan for the three-factor and three-level of the circulating fluidized bed fly ash-based gelling material.

[0029] (3) Based on the data in step (2), multiple regression analysis and model optimization are carried out to determine the optimal ratio of the circulating fluidized bed fly ash-based gelling material under the target unconfined compressive strength.

[0030] The present invention adopts the Box - Behnken response surface design method, selects three factors of the red mud content, fly ash content, and hydrated lime content as the corresponding variables, takes the 7-day and 28-day unconfined compressive strengths of the circulating fluidized bed fly ash-based gelling specimen as response values, conducts fitting regression and variance analysis on the corresponding variables, obtains the optimized regression equation, and thus obtains the optimal formula of the circulating fluidized bed fly ash-based gelling material. Experimental verification shows that the error between the actual compressive strength of the obtained formula and the predicted value is within 5%. Therefore, the formula design has high accuracy and is convenient for popularization and application.

[0031] Further, in step (1), the mass parts of red mud, ordinary fly ash, and calcium-containing solid base activator are 30 - 55 parts, 0 - 40 parts, and 0 - 10 parts respectively;

[0032] In step (3), the formula with the maximum unconfined compressive strength at 7d and 28d is: 47 parts of red mud, 10 parts of calcium-containing solid base activator, 0 parts of ordinary fly ash, 43 parts of circulating fluidized bed fly ash, and 18 parts of water. Under these conditions, three parallel tests were conducted, and the error between the measured mean value and the predicted value did not exceed 3%, indicating that the model established by the response surface method is effective and feasible.

[0033] Further, in step (3), the regression equations for the unconfined compressive strength at 7d and 28d after model optimization are as follows:

[0034] Regression equation for 7d compressive strength:

[0035] Y1 = 15.46 - 0.82A - 3.91B + 2.90C - 2.47AB + 1.14BC - 1.42A2 - 1.69B2 (1)

[0036] Regression equation for 28d compressive strength:

[0037] Y2 = 23.14 - 1.26A - 6.08B + 4.47C - 2.47AB - 2.74A2 - 2.40B2 - 2.81C2 (2)

[0038] In the formula, Y1 is the 7d compressive strength, Y2 is the 28d compressive strength, A is the percentage content of red mud, B is the percentage content of fly ash, C is the percentage content of calcium-containing solid base activator, and here the percentage content refers to the percentage relative to the total amount of red mud, ordinary fly ash, circulating fluidized bed fly ash, and calcium-containing solid base activator.

[0039] Specifically, taking slaked lime as an example, it includes the following steps:

[0040] (2.1) Raw material activation: Take red mud, ordinary fly ash, and circulating fluidized bed fly ash and dry them to constant weight in a 60°C forced-air drying oven, then pass them through a 0.075mm sieve after ball milling to obtain fine particle materials with a fineness ≤ 75μm; place slaked lime in a 30°C forced-air drying oven and dry it to constant weight, then pass it through a 200-mesh sieve;

[0041] (2.2) Taking the contents of red mud, ordinary fly ash, and hydrated lime in step (1) as influencing factors and the unconfined compressive strengths at 7 days and 28 days of the circulating fluidized bed fly ash-based modified cementitious specimens as response values, a test scheme with three factors and three levels is designed; that is, taking the contents of red mud, ordinary fly ash, and hydrated lime as three influencing factors, and designing three dosage levels for the content of each factor. For example, the content of red mud is designed as 30 parts, 42.5 parts, and 55 parts; the content of ordinary fly ash is designed as 0 parts, 20 parts, and 40 parts; the content of hydrated lime is designed as 0 parts, 5 parts, and 10 parts.

[0042] (2.3) Cementitious material mixing: Weigh the dosages of each material accurately according to the mass fractions designed by the response surface, mix at room temperature for 2 - 3 minutes, and then, based on the results determined by the optimum moisture content test, add the corresponding mass of water and mix evenly again with a small spatula for about 3 minutes.

[0043] (2.4) Specimen molding and curing: Add the mixed cementitious material into a cylindrical mold with a size of Φ50×50mm in three batches in sequence, and use the static pressure molding method to prepare specimens with a size of Φ50×50mm. Then, cover with a film and place them in a curing box at 20℃ and 95% RH.

[0044] (2.5) Repeat the above steps (3) and (4) to make 3 specimens each for the 7-day and 28-day ages.

[0045] (2.6) Unconfined compressive strength measurement: After each specimen is cured to the specified age, use a universal testing machine with a measurement accuracy of ±1% and a loading rate set at 1mm / min to measure the strength, record the maximum pressure P (N) when the specimen fails, and calculate the corresponding compressive strength R C (MPa).

[0046] (3) Take the mean value of the compressive strengths of each specimen at each age in each group as the response result, and conduct regression fitting, variance analysis, and model optimization for the 7-day and 28-day ages respectively. Finally, determine the optimal ratio of the solid waste co-modified circulating fluidized bed fly ash-based cementitious material.

[0047] The present invention uses the unconfined compressive strengths at 7 days and 28 days as the indexes to evaluate the performance of the circulating fluidized bed fly ash-based modified cementitious material. The optimized model obtained in this way can accurately obtain the formula of the circulating fluidized bed fly ash-based cementitious material with excellent unconfined compressive strength, and can also obtain the corresponding formula of the circulating fluidized bed fly ash-based cementitious material according to the target required unconfined compressive strength. The design method is flexible, providing a fast and effective way for the design and application of such cementitious materials.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] (1) The fluidized bed fly ash and ordinary fly ash adopted in the present invention provide rich active Si and Al substances for the reaction system. The incorporation of red mud and calcium-containing solid base activator creates a good alkaline environment for the reaction system, increasing the raw materials participating in the hydration reaction and accelerating the formation of various hydraulic components, thereby effectively improving the compressive strength of the cementitious material at different ages.

[0050] (2) Due to the difference in furnace temperature, ordinary fly ash and fluidized bed fly ash have significant differences in fineness, phase composition, and chemical components. Through the compound admixture of the two in the present invention, it is found that when the mass fractions of red mud and slaked lime remain unchanged and the mass fraction of ordinary fly ash is 0, the strength performance of the obtained circulating fluidized bed fly ash-based cementitious material is better.

[0051] (3) The present invention uses the response surface Box-Behnken design method to optimize the compressive strength of the circulating fluidized bed fly ash-based cementitious material, and obtains the optimal ratio of the solid waste synergistic modified cementitious material as follows: the mass fractions of red mud, ordinary fly ash, and slaked lime are 47 parts, 0 part, and 10 parts in sequence; under this condition, three groups of parallel tests are carried out, and the error between the measured mean value and the predicted value does not exceed 3%, indicating that the model established by the response surface method is effective and feasible.

[0052] (4) The preparation method of the present invention is simple, closer to the actual technical requirements, and the formula design method is flexible, providing a fast and effective way for the design and application of such cementitious materials, and contributing to the subsequent research on other properties of the circulating fluidized bed fly ash-based cementitious material. Description of the Drawings

[0053] Figure 1 It is the response surface diagram and contour diagram of the influence of the interaction between red mud and ordinary fly ash on the circulating fluidized bed fly ash-based cementitious material at the 7-day age;

[0054] Figure 2 It is the response surface diagram and contour diagram of the influence of the interaction between ordinary fly ash and slaked lime on the circulating fluidized bed fly ash-based cementitious material at the 7-day age;

[0055] Figure 3 It is the response surface diagram and contour diagram of the influence of the interaction between red mud and ordinary fly ash on the circulating fluidized bed fly ash-based cementitious material at the 28-day age;

[0056] Figure 4 It is the scatter diagram of the actual value and the predicted value at the 7-day age;

[0057] Figure 5 It is the scatter diagram of the actual value and the residual at the 7-day age;

[0058] Figure 6 It is the scatter diagram of the actual value and the predicted value at the 28-day age;

[0059] Figure 7 It is the scatter plot of the actual value and the residual at the age of 28 days;

[0060] Figure 8 From top to bottom are the diagrams of the circulating fluidized bed fly ash-based cementitious specimens and the specimens after unconfined compressive strength test in turn. Specific implementation manners

[0061] The following combines the embodiments of the present invention and is described in detail in conjunction with the accompanying drawings.

[0062] In the following embodiments, the compositions of red mud, ordinary fly ash (fly ash) and circulating fluidized bed fly ash (CFB ash) are shown in Table 1. It can be seen that the most intuitive difference between ordinary fly ash and CFB ash is the large difference in the contents of CaO and SO3.

[0063] Table 1 Main component compositions of red mud, fly ash and CFB ash

[0064]

[0065]

[0066] The following simultaneously describes in detail the circulating fluidized bed fly ash-based cementitious material and the formula design method provided by the present invention through the experimental design of optimizing the circulating fluidized bed fly ash-based cementitious material by response surface.

[0067] According to the total number of parts of red mud, ordinary fly ash, circulating fluidized bed fly ash and calcium-containing solid base activator being 100 parts, multiple single-factor tests are carried out on the circulating fluidized bed fly ash with the mass parts of red mud, ordinary fly ash and calcium-containing solid base activator as influencing factors. According to the preparation method of the cementitious material, the modified cementitious material cured for 7 days is obtained. Taking the 7-day unconfined compressive strength of the modified cementitious material as the judgment basis, the influence of each factor on the modified cementitious material is obtained according to the single-factor test; furthermore, the preferred ranges of the mass parts of red mud, ordinary fly ash and calcium-containing solid base activator are obtained as 30 - 55 parts, 0 - 40 parts and 0 - 10 parts, and a three-factor three-level experimental scheme design is carried out within this range.

[0068] Example 1

[0069] (1) Activation of raw materials: Place red mud, ordinary fly ash and circulating fluidized bed fly ash in a 60°C blast drying oven and dry to constant weight. After ball milling, pass through a 0.075 mm sieve to obtain fine particle materials with a fineness ≤ 75 μm for standby; place slaked lime in a 30°C blast drying oven and dry to constant weight, and pass through a 200-mesh sieve for standby.

[0070] (2) Mixing of cementitious materials: According to the response surface design scheme, the total number of dry materials is 100 parts. Weigh successively 30 parts of red mud dry materials with a particle size ≤ 75μm that have been dried in a 60°C drying oven, 0 parts of ordinary fly ash dry materials, 65 parts of circulating fluidized bed fly ash dry materials, and 5 parts of hydrated lime that has been dried to constant weight in a 30°C drying oven and has a fineness lower than 200 mesh. Use a small spatula to uniformly mix the dry materials for 2 - 3 minutes, then add 18 parts of water, and use the small spatula to uniformly mix for about 3 minutes again.

[0071] (3) Specimen molding and curing: Add the mixed cementitious materials into a cylindrical mold with a size of Φ50×50mm in three times successively. After static pressure molding, use a demolding machine to demold, and then place it with a film covering in a curing box at 20°C and 95% RH for curing to the corresponding age.

[0072] (4) Repeat the above steps (2) - (3) to make 3 specimens each for the 7d and 28d ages.

[0073] (5) After curing to the corresponding age, use a universal testing machine with a measurement accuracy of ±1% and a loading rate set at 1mm / min to measure the strength, record the maximum pressure P (N) when the specimen fails, and calculate the corresponding unconfined compressive strength R C (MPa).

[0074] Unconfined compressive strength R C It is calculated according to the following formula in units of Newton per square millimeter (MPa):

[0075] In the formula, P is the maximum pressure (N) when the specimen fails, and A is the cross-sectional area of the specimen (mm 2 );

[0076] D is the diameter of the specimen (mm).

[0077] As Figure 8 shown, it can be seen that the circulating fluidized bed fly ash-based cementitious specimens have good formability, and the specimens rupture after the unconfined compressive strength test.

[0078] Example 2

[0079] The preparation method is the same as that of Example 1, except that the number of parts of red mud is 55, the number of parts of ordinary fly ash is 0, the number of parts of hydrated lime is 5, the number of parts of fluidized bed fly ash is 40, and the number of parts of water is 16.

[0080] Example 3

[0081] The preparation method is the same as that of Example 1, except that the number of parts of red mud is 30, the number of parts of ordinary fly ash is 40, the number of parts of hydrated lime is 5, the number of parts of fluidized bed fly ash is 25, and the number of parts of water is 16.

[0082] Example 4

[0083] The preparation method is the same as that of Example 1, except that the amount of red mud is 30 parts, the amount of ordinary fly ash is 20 parts, the amount of slaked lime is 0 part, the amount of fluidized bed fly ash is 50 parts, and the amount of water is 19 parts.

[0084] Example 5

[0085] The preparation method is the same as that of Example 1, except that the amount of red mud is 30 parts, the amount of ordinary fly ash is 40 parts, the amount of slaked lime is 5 parts, the amount of fluidized bed fly ash is 25 parts, and the amount of water is 17 parts.

[0086] Example 6

[0087] The preparation method is the same as that of Example 1, except that the amount of red mud is 55 parts, the amount of ordinary fly ash is 20 parts, the amount of slaked lime is 0 part, the amount of fluidized bed fly ash is 25 parts, and the amount of water is 18 parts.

[0088] Example 7

[0089] The preparation method is the same as that of Example 1, except that the amount of red mud is 30 parts, the amount of ordinary fly ash is 20 parts, the amount of slaked lime is 10 parts, the amount of fluidized bed fly ash is 40 parts, and the amount of water is 15 parts.

[0090] Example 8

[0091] The preparation method is the same as that of Example 1, except that the amount of red mud is 55 parts, the amount of ordinary fly ash is 20 parts, the amount of slaked lime is 10 parts, the amount of fluidized bed fly ash is 15 parts, and the amount of water is 18 parts.

[0092] Example 9

[0093] The preparation method is the same as that of Example 1, except that the amount of red mud is 42.5 parts, the amount of ordinary fly ash is 0 part, the amount of slaked lime is 0 part, the amount of fluidized bed fly ash is 57.5 parts, and the amount of water is 18 parts.

[0094] Example 10

[0095] The preparation method is the same as that of Example 1, except that the amount of red mud is 42.5 parts, the amount of ordinary fly ash is 40 parts, the amount of slaked lime is 0 part, the amount of fluidized bed fly ash is 17.5 parts, and the amount of water is 18 parts.

[0096] Example 11

[0097] The preparation method is the same as that of Example 1, except that the amount of red mud is 42.5 parts, the amount of ordinary fly ash is 0 part, the amount of hydrated lime is 10 parts, the amount of fluidized bed fly ash is 47.5 parts, and the amount of water is 17 parts.

[0098] Example 12

[0099] The preparation method is the same as that of Example 1, except that the amount of red mud is 42.5 parts, the amount of ordinary fly ash is 40 parts, the amount of hydrated lime is 10 parts, the amount of fluidized bed fly ash is 7.5 parts, and the amount of water is 17 parts.

[0100] Example 13

[0101] The preparation method is the same as that of Example 1, except that the amount of red mud is 42.5 parts, the amount of ordinary fly ash is 20 parts, the amount of hydrated lime is 5 parts, the amount of fluidized bed fly ash is 32.5 parts, and the amount of water is 18 parts.

[0102] Experimental design and result analysis of response surface optimization of circulating fluidized bed fly ash-based cementitious materials:

[0103] 1. Selection of factors and levels and response surface design

[0104] Based on the single-factor experiments, a response surface experiment design was carried out. Three factors, namely the red mud content (A), fly ash content (B), and hydrated lime content (C), were selected as the corresponding variables, and the 7-day and 28-day unconfined compressive strengths of the circulating fluidized bed fly ash (CFB)-based modified cementitious specimens were used as the response values. The Box-Behnken response surface design method was adopted to design a three-factor and three-level experimental scheme. The experimental factors and levels are set as shown in Table 2, and the response surface experimental design and results are shown in Table 3.

[0105] Table 2 Setting of experimental factors and levels

[0106]

[0107] According to the design of Design-Expert-13 software, a total of 17 groups of optimization experiments were designed for the circulating fluidized bed fly ash-based modified cementitious materials, including 5 groups of central point repeated experiments (Examples 13 - 17). The experimental design and response value results are shown in Table 3.

[0108] Table 3 Response surface experimental design and results

[0109]

[0110] As can be seen from Table 3, within the range of the circulating fluidized bed fly ash-based cementitious material formula defined in the present invention, the unconfined compressive strengths at 7 days and 28 days are relatively excellent. In particular, the unconfined compressive strength is even better when the ordinary fly ash is 0 - 20 parts, the circulating fluidized bed fly ash is 25 - 65 parts, and the calcium-containing solid base activator is 1 - 10 parts.

[0111] 2. Model Fitting and Variance Analysis

[0112] Analyze the response value results in Design-Expert 13 software. After comparing the fitting indexes such as variance and mean square deviation of different models, it is found that the fitting result of the quadratic model is the best. The statistical results of the unconfined compressive strength model are shown in Table 4, the model verification results are shown in Table 5, and the variance analysis results are shown in Table 6.

[0113] Table 4 Statistical Results of Compressive Strength Model

[0114]

[0115] Note: Response models are constructed with the compressive strength values at 7 days and 28 days, denoted as Y1 and Y2 respectively.

[0116] Table 5 Verification of Each Response Model

[0117]

[0118] Table 6 Variance Analysis of Response Regression Model

[0119]

[0120]

[0121] As can be seen from Table 4, the P values of both compressive strength models are less than 0.001, indicating that the significance of each model is good; the P values of the lack-of-fit terms are all greater than 0.05, representing a relatively high degree of model fitting. The regression equation can be used to summarize and analyze the actual test results. As can be seen from Table 5, the determination coefficients R 2 of the goodness of fit of both models are greater than 0.99, and the differences from the predicted R 2 are all less than 0.1, indicating that the quadratic model has a consistent fitting trend for the compressive strengths at different ages and has a relatively high credibility; the adjusted determination coefficients R 2 of the two models for the modified degrees of freedomThe values are 0.9832 and 0.9804 in sequence, both above 0.95, indicating that in multiple regression analysis, the fitting effects of each model are more accurate; the signal-to-noise ratios of each model are all greater than 10, indicating that the models have good effects in predicting the set response values. On the other hand, the coefficient of variation (C.V.%) of the two models, 3.62% and 4.42%, are both less than 10%, indicating that the regression equations have a high degree of fitting for each data, with small overall differences and high credibility.

[0122] The regression equations of the compressive strength of each model can be obtained from the equation coefficients in Table 6. According to the principle of statistical analysis, the significance index α is set to 0.1. If the P-value of an influencing factor is greater than the significance index α, it is determined to be insignificant; if the P-value is less than the significance index α, it is determined to be significant. Thus, the factors significantly affecting the unconfined compressive strength of the multi-component cementitious materials can be further analyzed after compounding CFB ash. In the Y1 model, the P-values of AC and C2 both exceed α, so these two items are removed from the regression equation; in the Y2 model, the P-values of AC and BC are both higher than α, so these two items are removed from the regression equation. The regression equations Y1 and Y2 of the optimized response models are shown in Equations (1) and (2).

[0123] 7-day compressive strength:

[0124] Y1 = 15.46 - 0.82A - 3.91B + 2.90C - 2.47AB + 1.14BC - 1.42A2 - 1.69B2 (1)

[0125] 28-day compressive strength:

[0126] Y2 = 23.14 - 1.26A - 6.08B + 4.47C - 2.47AB - 2.74A2 - 2.40B2 - 2.81C2 (2)

[0127] 3. Response surface plot analysis

[0128] According to the regression equations of the optimized response models, in the Y1 model, the P-values of AB and BC are both less than α, indicating that the interaction effects of red mud, ordinary fly ash, and hydrated lime have a significant impact on the 7-day compressive strength of the circulating fluidized bed fly ash-based cementitious materials; in the Y2 model, the P-value of AB is lower than the significance index α, indicating that the interaction effect of red mud and ordinary fly ash has a significant impact on the 28-day compressive strength of the modified cementitious materials. By analyzing the three-dimensional response surface plots, the influence of each influencing factor on the response target value can be significantly reflected.

[0129] From Figure 1It can be seen that the response surface is significantly concave and convex and the contour lines are densely distributed, indicating that the interaction between the two factors A and B has a significant impact on the 7-day compressive strength. When the mass fraction of the influencing factor B is 0, as A increases from 30 to 55, the compressive strength gradually increases. When B is 40 and A increases from 30 to 55, the compressive strength rapidly decreases. The reason is that when the content of ordinary fly ash in the reaction system is low, circulating fluidized bed fly ash is the main source of silicon-aluminum components. As a stimulating material, with the increase of its content, the activity of more silicon-aluminum components is stimulated, and a variety of hydration products such as C-S-H and ettringite are generated, improving the early strength. Excessive amount of red mud in the reaction system leads to a high-alkali environment, and the hydration products C-(A)-S-H and AFt are rapidly generated and wrap the unreacted silicon-aluminum raw materials, delaying the entry of water, reducing the pozzolanic reaction in the system, and decreasing the strength.

[0130] It can be seen from Figure 2 that the 7-day compressive strength shows a trend of increasing linearly with the increase of the slaked lime content and gradually decreasing with the increase of the ordinary fly ash content. When the mass fractions of ordinary fly ash and slaked lime are 0 part and 10 parts respectively, the compressive strength reaches the maximum. When the mass fraction of the influencing factor B is 40 parts and the content of C increases from 0 part to 10 parts, the strength of the cementitious material increases by 123.4%. This is because the increase in the mass fraction of slaked lime increases the alkalinity value of the reaction system and the content of Ca 2+ , accelerating the breakage of Si-O and Al-O bonds in ordinary fly ash and circulating fluidized bed fly ash, generating a large number of gel components, filling the voids inside the system, and increasing the 7-day compressive strength. When the mass fraction of C is 10 parts and the content of B increases from 0 part to 40 parts, the strength of the gel decreases by 30.4%. This indicates that with the increase of the mixing ratio of ordinary fly ash, the proportion of activated silicon-aluminum materials is reduced, and the content of generated hydration products is reduced, resulting in lower hydraulicity of the cementitious system and reducing the compressive strength of the solid waste cementitious material.

[0131] Figure 3 It reflects the influence of the interaction between red mud and ordinary fly ash on the compressive strength at the 28-day age. It can be seen that when the content of red mud is within the designed range, the compressive strength of the gel decreases with the increase of the ordinary fly ash content, and the change trend is similar to that at the 7-day age. When the mass fraction of ordinary fly ash increases from 0 part to 40 parts, the trend of the strength of the solid waste aggregate cementitious material changing with the red mud content is similar to that at the 7-day age, and the maximum strength values are both obtained when the mass fraction of ordinary fly ash is 0. Compared with the cementitious material at the 7-day age, due to the significantly increased curing time, the alkali activation reaction and hydration reaction occurring inside the 28-day cementitious material are more complete, and the strength value shown is higher.

[0132] 4. Response Optimization and Experimental Verification

[0133] Red mud, ordinary fly ash, and hydrated lime have a combined effect on the strength of the circulating fluidized bed fly ash-based modified cementitious material. When the ratios of the three factors are optimal, the synergistic effect of the multi-solid waste is the strongest, and the strength performance is more excellent. From Figures 4 to 7 It can be seen that the experimental values and predicted values at each age are distributed near the fitting curve, very close and with good consistency, indicating that after removing the insignificantly influencing factors, each response surface analysis model has good fitting accuracy and significance.

[0134] After optimization, the optimal dosages of the three factors A, B, and C are 47 parts, 0 part, and 10 parts in sequence. A verification test is carried out on the optimal ratio design scheme, and the actual values, predicted values, and the errors between the two are summarized in Table 8. Analyzing Table 8, it can be known that the experimental values of the compressive strength at 7d and 28d under the optimal ratio are 18.97 MPa and 28.26 MPa in sequence, indicating that the multi-cementitious material in the present invention can be used as a bonding material for road bases, so as to efficiently and high-quality utilize solid waste materials and reduce the environmental protection cost; at the same time, the errors between the measured values and the predicted values are all lower than 3%, indicating that the optimized model is more accurate and has higher credibility.

[0135] Table 7 Response Optimization Criteria

[0136]

[0137]

[0138] Table 8 Experimental Values and Predicted Values of the Optimal Ratio

[0139]

[0140] In summary, the present invention optimizes the formula of the circulating fluidized bed fly ash-based cementitious material through single-factor experiments and the response surface method, and finally obtains the optimal ratio of the solid waste-modified cementitious material as follows: 47 parts of red mud, 10 parts of calcium-containing solid base activator (hydrated lime), 0 part of fly ash, 43 parts of circulating fluidized bed fly ash, and 18 parts of water. The established regression model has high credibility and accuracy.

[0141] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these changes and modifications.

Claims

1. A circulating fluidized bed fly ash-based cementitious material, characterized in that: The components include 47 parts of red mud, 10 parts of calcium-containing solid alkali activator, 43 parts of circulating fluidized bed fly ash, and 18 parts of water by weight; The composition of the circulating fluidized bed fly ash includes SiO2 32.79%, Al2O3 21.8%, CaO24.22%, MgO 0.57%, Fe2O3 5.52%, K2O 1.10%, SO3 11.61% and TiO2 1.02% by mass fraction; The components of the red mud include SiO2 22.11%, Al2O3 22.20%, CaO 21.62%, Fe2O312.91%, Na2O 11.39%, SO3 0.79% and TiO2 5.42% by mass fraction; The calcium-containing solid alkali activator is slaked lime.

2. The circulating fluidized bed fly ash-based cementitious material according to claim 1, characterized in that: The main component of the slaked lime is Ca(OH)2, with a content of ≥95%, and is in powder form with a fineness of ≤200 meshes.

3. A method for preparing a circulating fluidized bed fly ash-based cementitious material according to any one of claims 1 to 2, characterized in that: The following steps are involved: (1) Activation of raw materials: Red mud and circulating fluidized bed fly ash are placed in a blast drying oven and dried to constant weight. After ball milling, they are sieved through a 0.075 mm sieve to obtain microparticles with a fineness of ≤75 μm for use; Place the calcium-containing solid base activator in a blast drying oven and dry it to constant weight, then pass it through a 200-mesh sieve for later use; (2) Mixing cementitious materials: Weigh the red mud, circulating fluidized bed fly ash and calcium-containing solid alkali activator prepared in step (1) according to the formula, mix them evenly for 2-3 minutes, then add water and mix them evenly again for 2-3 minutes.

4. The preparation method according to claim 3, characterized in that: The method also includes curing the cementitious material mixed in step (2) to obtain a solidified cementitious material.

5. The preparation method according to claim 4, characterized in that: The step of curing the mixed cementitious material in step (2) to obtain a solidified cementitious material comprises the following steps: adding the mixed cementitious material into a cylindrical mold of Φ50 × 50 mm in three times in sequence, demolding the material using a demoulding machine after static pressure molding, and then coating the material and placing the material in a curing box at 20° C. and 95% RH for curing to a corresponding age to obtain a solidified cementitious material.

6. The preparation method according to any one of claims 3 to 5, characterized in that: The red mud and circulating fluidized bed fly ash are placed in a blast drying oven at 55-65°C for drying; the calcium-containing solid alkali activator is placed in a blast drying oven at 25-35°C for drying.

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