Coal bottom ash aggregate screening evaluation method and application in foam concrete preparation

Abstract

The present application relates to a kind of screening evaluation method of coal bottom ash aggregate and application in the preparation of foam concrete, belong to building material and solid waste resource utilization technical field.Method includes: detecting the alkali content and loss on ignition of coal bottom ash aggregate, screening aggregate with alkali below threshold and loss on ignition below threshold;d 60 Value is detected, calculates uneven coefficient and curvature coefficient, evaluates its particle size distribution index;Determine its saturated water absorption, evaluate its physical property index;Determine its content of pozzolanic activity substance, evaluate its chemical activity index;According to three indexes, judge its application range.The present application sets up veto screening checkpoint, eliminates fatal risk (AAR problem and bubble breaking) from the source, then two physical function indexes and chemical index are coupled and evaluated, predict its performance in fresh stage and hardening stage, solve the problem of immediate process stability.Only need to detect 5 core indexes to complete evaluation, greatly shorten the evaluation period.

Description

Technical Field

[0001] This invention belongs to the field of building materials and solid waste resource utilization technology, specifically involving a screening and evaluation method for coal bottom ash aggregate and its application in the preparation of foamed concrete. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Coal ash, a major solid waste generated by coal-fired power plants, is considered a potential lightweight aggregate for foamed concrete due to its lightweight and porous characteristics. However, the composition and physical properties of coal ash are greatly affected by factors such as coal source, boiler type, and combustion conditions, resulting in significant quality fluctuations. Key indicators such as particle size distribution, saturated water absorption, unburned carbon content, and alkali / reactive substance content exhibit significant differences, demonstrating variability. This strong variability leads to significant uncertainties in its application in foamed concrete: direct use without scientific evaluation and screening can easily cause a series of problems in the prepared foamed concrete, including poor workability, large strength dispersion, abnormal thermal conductivity, increased shrinkage, and uneven density, thus restricting its large-scale and high-value resource utilization.

[0004] In existing technologies, when using coal ash to replace natural aggregates in foamed concrete, the focus is usually on the impact of a single index (such as particle size) on a certain aspect of the foamed concrete's performance (such as strength). However, the comprehensive selection principles and standards for coal ash are not addressed. Due to the inherent variability of coal ash, this improvement in mechanical properties exhibits significant dispersion and fluctuation, making it difficult to guarantee the stability of the resulting foamed concrete's properties. Summary of the Invention

[0005] To address the current technological shortcomings, the present invention aims to provide a screening and evaluation method for coal ash aggregate and its application in the preparation of foamed concrete. This method establishes a correlation and screening criteria between key aggregate characteristics and core performance properties of foamed concrete (such as workability, strength, thermal insulation, density uniformity, and shrinkage performance). By comprehensively considering physical and chemical indicators and through a programmed rejection screening and key indicator combination evaluation, it can quickly determine whether a specific batch of coal ash is suitable for producing foamed concrete with the expected performance requirements, thus ensuring the stability of foamed concrete product quality from the source of raw materials.

[0006] To achieve the above objectives, the technical solution of the present invention includes the following aspects.

[0007] Firstly, a method for screening and evaluating coal bottom ash aggregate includes the following steps: S1. Detect the alkaline content and loss on ignition of coal bottom ash aggregate, and screen aggregates with alkaline content and loss on ignition below the threshold to enter the subsequent evaluation steps. S2, Detect its d 60 The values ​​are used to calculate the coefficient of uniformity and the coefficient of curvature, and to evaluate the particle size distribution index. S3. Determine its saturated water absorption rate and evaluate its physical properties. S4. Determine the content of its volcanic ash active substances, and evaluate its chemical activity index based on the content of volcanic ash active substances and the content of alkaline substances measured in S1. S5. Determine its applicable scope based on particle size distribution index, physical property index, and chemical activity index.

[0008] Secondly, the above-mentioned screening and evaluation methods for coal-fired bottom ash aggregates are applied in the preparation of foamed concrete.

[0009] The beneficial effects of this invention are as follows: 1. This invention establishes a veto screening checkpoint: it pioneered the use of two chemical risk indicators, "alkali content" and "loss on ignition," for a one-vote veto, eliminating fatal risks (AAR issues and foam breakage) at the source and resolving long-term performance concerns. This invention establishes a functional evaluation system: it innovatively couples the evaluation of two physical functional indicators, "particle size distribution" and "saturated water absorption rate," with the chemical indicator "active substance content," rather than simply adding them together. Based on the coupling relationship of the three indicators and their comprehensive impact on the performance of foamed concrete, its performance in the fresh mixing and hardening stages is predicted, solving the problem of immediate process stability. Only five core indicators (alkaline substances / volcanic ash active substances / unburned carbon content, particle size distribution, and saturated water absorption rate) need to be tested to complete the evaluation, greatly shortening the evaluation cycle.

[0010] 2. This invention provides a graded application guide: the final "suitability level" output transforms complex material properties into clear production decision instructions, achieving a leap from "unable to judge" to "precise application." It solves the technical challenges of poor workability, large fluctuations in mechanical properties, high drying shrinkage, poor uniformity, and high thermal conductivity in foamed concrete caused by significant fluctuations in the quality of coal ash. The evaluation results directly correspond to the actual production application level, providing precise material selection criteria for producing precast foamed concrete products and cast-in-place foamed concrete walls with different performance requirements. This ensures that industrially produced foamed concrete products possess stable low thermal conductivity, high compressive strength, and good low shrinkage, providing reliable technical support for the high-value and large-scale resource utilization of bulk solid waste, coal ash. Attached Figure Description

[0011] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0012] Figure 1 This is a flowchart illustrating a screening and evaluation method for coal bottom ash aggregate in Embodiment 1 of the present invention. Detailed Implementation

[0013] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0014] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0015] One or more embodiments of the present invention provide a method for screening and evaluating coal bottom ash aggregate, comprising the following steps: S1. Detect the alkaline content and loss on ignition of coal bottom ash aggregate, and screen aggregates with alkaline content and loss on ignition below the threshold to enter the subsequent evaluation steps. S2. Detection of d in coal-fired bottom ash aggregate. 60 The values ​​are used to calculate the coefficient of uniformity and the coefficient of curvature, and to evaluate the particle size distribution index. S3. Determine the saturated water absorption rate of coal bottom ash aggregate and evaluate its physical property indicators; S4. Determine the content of pozzolanic active substances in coal bottom ash aggregate, and evaluate its chemical activity index based on the content of pozzolanic active substances and the content of alkaline substances measured in S1. S5. Determine the applicable scope of coal bottom ash aggregate based on particle size distribution index, physical property index and chemical activity index.

[0016] In the above process, a rejection screening is first conducted, testing the alkaline content and loss on ignition (LOI) to avoid the risk of alkali-aggregate reaction (AAR) and foam breakage, which seriously affect the quality of foamed concrete. Only when the alkaline content and LOI both meet their respective conditions is the rejection screening passed and the process proceeds to the next step of key indicator evaluation. In the key indicator evaluation, three types of key indicators are mainly tested: particle size distribution, chemical activity, and physical properties. A grading standard is established considering the comprehensive impact of the three indicators on the performance of foamed concrete, and then the applicability is determined, including four applicability levels: optimal combination, recommended use, consideration for use, or use with caution, to guide the application of coal bottom ash aggregate in foamed concrete.

[0017] Optionally, in S1, the level of alkaline substances in the aggregate indicates the risk of alkali-aggregate reaction (AAR) when applied to foamed concrete. Especially when the content of active substances is also high, if it is higher than this value, it means that the risk of alkali-aggregate reaction of the aggregate is extremely high, which will lead to cracking and failure, and is therefore rejected. The level of loss on ignition (unburned carbon content) mainly describes the carbon content present in coal bottom ash aggregate. During the mixing stage of slurry and foam in the preparation of foamed concrete, these carbons will adsorb a large amount of foam, causing foam to rupture and ultimately leading to the failure of foamed concrete products. Therefore, its content needs to be limited. The threshold for alkaline substance content, expressed as Na2O equivalent, is 2.0%; the threshold for loss on ignition is 5.0%, which is a threshold obtained through a large number of experiments.

[0018] Optionally, in S2, if d 60 If the aggregate gradation value, uniformity coefficient, and curvature coefficient are all within the set range, the evaluation is A; if only one is outside the set range, the evaluation is B; if at least two are outside the set range, the evaluation is C. These three indicators are the core parameters for evaluating concrete aggregate gradation. 60 The value is used to evaluate the aggregate size, the non-uniformity coefficient is used to evaluate the distribution of the size, and the curvature coefficient represents the continuity of the distribution. These ranges ensure the continuity and rationality of the aggregate gradation. Exceeding the range will lead to insufficient support of the slurry skeleton or excessive compaction, affecting workability and strength.

[0019] Optional, d 60 The setting range is 350~550μm, the setting range of the non-uniformity coefficient is >5, and the setting range of the curvature coefficient is 1~3; it has been determined through a large number of basic and repetitive tests that it can obtain excellent foamed concrete performance.

[0020] Optionally, in S3, if the saturated water absorption rate is within the set range, the evaluation is A; if it is higher than the set range, the evaluation is B; if it is lower than the set range, the evaluation is C. Coal bottom ash, as a porous material, has high water absorption and can continuously supply water to the matrix during the performance development stage of foamed concrete, thus exerting an internal curing effect. However, a higher water absorption rate does not necessarily mean better overall performance: if it is too high, the adsorbed water in the pores of the coal bottom ash will be thrown out during mixing, and after the prepared foamed concrete is poured into the mold, the water in the pores of the coal bottom ash aggregate will also seep out under gravity, leading to bleeding, settling, and segregation during the setting stage of the foamed concrete; if it is too low, the pores of the aggregate "store" less water, resulting in insufficient water supply to the matrix (or no water supply at all) during the performance development stage of the foamed concrete, which leads to the inability to improve shrinkage.

[0021] Optionally, the saturated water absorption rate can be set within the range of 50-65%. The saturated water absorption rate of coal bottom ash materials spans a wide range of approximately 10-85%. Both exceeding and falling below this set range are detrimental to the performance development of foamed concrete.

[0022] Optionally, in S4, if both the content of active volcanic ash and the content of alkaline substances are within the set range, the evaluation is A; if only one is within the set range, the evaluation is B; and if neither is within the set range, the evaluation is C. This is because both substances exist within a certain reasonable range. When both are within this reasonable range, it indicates that the aggregate has good overall performance, with advantages such as low shrinkage, high strength, and low cracking risk. However, when one substance is outside this range, it will have a certain adverse effect on performance. For example, when it is "high alkali (alkaline substance content), normal (volcanic ash) activity," "low alkali, normal activity," "normal alkalinity, high activity," or "normal alkalinity, low activity," the strength improvement is slight. If neither is within the range, the performance difference is significant. For example, "high alkali, high activity" indicates a relatively higher risk for alkali aggregates. The "alkaline substance content" here is the same data result as the "alkaline substance content" in step S1. The "preset low alkali threshold" here is equivalent to further defining the content range to judge the specific technical effect it can produce in foamed concrete, under the premise of meeting the rejection criteria.

[0023] Optionally, the content of volcanic ash active material can be set within a range of 50-70%, and the content of alkaline material can be set within a range of 0.4-1%. The content of volcanic ash active material determines its potential to improve the performance of foamed concrete, but the realization of this potential requires an alkaline environment. An appropriate content of volcanic ash active material can generate a suitable amount of secondary hydration products, which can be used to improve properties such as strength. When the content is lower than the set range (e.g., 0-0.4%), the amount of hydration products is small, and the performance improvement is weak. When the content is higher than the set range (e.g., 1-2%), the amount of hydration products is excessive, and defective substances are also generated, which will increase the risk of expansion and cracking of foamed concrete. The set range of alkaline material content here is lower than the alkaline material threshold in S1. The threshold in S1 is used to screen whether coal bottom ash aggregate can be used in foamed concrete, and the set range in S4 is used to evaluate the effectiveness of screening coal bottom ash aggregate when applied to foamed concrete.

[0024] Optionally, in S5, the applicable scope is determined based on the number of A, B, and C in the evaluation of particle size distribution index, physical property index, and chemical activity index, and then processed to improve the index with low evaluation.

[0025] Optionally, if all three indicators—particle size distribution, physical properties, and chemical activity—are rated A, then it is deemed suitable for directly preparing high-strength, high-stability, and low-shrinkage foamed concrete. If at least one of the three indicators is rated B and no C is rated C, then it is deemed suitable for producing foamed concrete with conventional performance requirements, and the foamed concrete formulation needs to be adjusted according to the lowest rated indicator. If there is only one C among the three indicators, then its use is deemed high-risk. If there is more than one C among the three indicators, then it is deemed unsuitable for preparing foamed concrete. This is because: coal ash, due to its inherent instability (affected by factors such as the degree of coal combustion), results in inconsistent quality of the prepared foamed concrete products, and some raw materials may even fail to produce products that meet national basic standards. The above technical solution first classifies the three core key factors of aggregates into different levels, and separately classifies the performance parameters that may be encountered in each part. Then, the individual aggregate parameter levels are correlated with the performance of foamed concrete: water absorption rate (potential for internal curing effect) corresponds to the shrinkage performance of foamed concrete; particle size distribution corresponds to the workability of fresh foamed concrete; alkaline substances correspond to the cracking risk of foamed concrete; and the content of active substances corresponds to the mechanical properties of foamed concrete. Finally, the individual aggregate parameters within the three parts are arranged and combined to output the suitability level of coal bottom ash in foamed concrete. In addition, during this arrangement and combination process, technicians can also predict the performance development of foamed concrete products based on the individual parameter combinations of each part.

[0026] One or more embodiments of the present invention provide the application of the above-mentioned screening and evaluation method for coal bottom ash aggregate in the preparation of foamed concrete.

[0027] Example 1 A method for screening and evaluating coal bottom ash aggregate includes the following steps: S1. Rejection Screening: The alkaline content of the coal bottom ash aggregate to be screened is detected by X-ray diffraction (XRF) and expressed as Na2O equivalent: i.e., "alkaline content = Na2O + 0.658K2O". If the alkaline content is higher than 2.0%, it is judged to have failed the rejection screening. The loss on ignition (LOI) of the coal bottom ash aggregate to be screened is detected at a temperature of 950℃ (operation method refers to GB / T 176-2017). If the LOI is higher than 5.0%, it is judged to have failed the rejection screening. The coal bottom ash aggregate that passes the rejection screening is then evaluated in subsequent steps.

[0028] S2. Evaluation of particle size distribution index in key indicator evaluation: Detection of d-particle size distribution of coal bottom ash aggregate. 60 Calculate the non-uniformity coefficient Cu and curvature coefficient Cc based on the value of d; take d 60 The setting range is 350 ~ 550 μm, the setting range for Cu is > 5, and the setting range for Cc is 1 ~ 3; if d 60 If the values ​​of Cu and Cc are all within the set range, the particle size distribution index is evaluated as A; if only one of them is outside the set range, the particle size distribution index is evaluated as B; if at least two of them are outside the set range, the particle size distribution index is evaluated as C. Where, d 60 The value refers to the cumulative mass of particles smaller than this particle size value accounting for 60% of the total mass; d 10 This refers to the cumulative mass of particles smaller than this particle size value accounting for 10% of the total mass; d 30 This means that the cumulative mass of particles smaller than this particle size value accounts for 30% of the total mass.

[0029] Non-uniformity coefficient Cu = d 60 / d 10 The curvature coefficient Cc = d 30 2 / (d 10 ·d 60 It is used to comprehensively judge the continuity and quality of aggregate gradation.

[0030] S3. Evaluation of physical property indicators in key indicator evaluation: Measure the saturated water absorption rate of coal bottom ash aggregate. If it is within the range of 50-65%, its physical property indicator is evaluated as A. If it is higher than this range, its physical property indicator is evaluated as B. If it is lower than this range, its physical property indicator is evaluated as C. S4. Evaluation of Chemical Activity Indicators in Key Indicator Evaluation: The content of pozzolanic active substances in coal bottom ash aggregate is determined by XRF (X-ray fluorescence spectrometry), specifically the sum of Al2O3 and SiO2 content, with a set range of 50-70% for the sum of Al2O3 and SiO2 content, and a set range of 0.4-1% for alkaline substance content. If both the pozzolanic active substance content and the alkaline substance content measured in S1 are within the set range, the chemical activity index is evaluated as A. If only one is within the set range, the chemical activity index is evaluated as B. If neither is within the set range, the chemical activity index is evaluated as C.

[0031] S5. Applicability Level Determination: Based on particle size distribution index, physical property index and chemical activity index, without considering the order of the three, only the quantity of A, B and C is considered to determine the applicability range of coal bottom ash aggregate. If all three key indicators are rated A, the rating is "optimal combination," suitable for preparing high-strength, high-stability, and low-shrinkage high-quality foamed concrete.

[0032] If at least one of the three key indicators is B and no C is included, such as AAB, ABB, BAA, or BBB, its rating is "Recommended for Use". It is suitable for preparing foamed concrete with conventional performance requirements and requires slight adjustments to the formula (according to actual production needs).

[0033] If there is only one "C" among the three key indicators, such as ABC, ACB, BAC, or CBA, the level is judged as "consider using". The risk of preparing foamed concrete with conventional performance requirements is high, and it can only be considered for use after extensive test verification and major adjustments to the formula.

[0034] If there are more than one "C" among the three key indicators, such as ACC, BCC, or CCA, the rating is "use with caution" and it is generally not recommended for use in foamed concrete.

[0035] The bottom ash of coal from multiple random regions and multiple batches (including samples 1 to 9) was screened and evaluated according to the above steps, and the results are shown in Table 1.

[0036] Table 1. Screening and evaluation results of different types of coal bottom ash aggregate

[0037] Example 2 Samples 1 to 9 from Example 1 were used to prepare foamed concrete products (design target dry density: 900 kg / m³) under the guidance of the same theoretical mix proportions and design methods, referring to JGJ-T 341 "Technical Specification for Application of Foamed Concrete". 3 The workability (including flowability, 24h bleeding rate and sedimentation rate) was tested according to relevant standards, and the results are shown in Table 2; the test results of mechanical properties are shown in Table 3; the test results of 28d dry bulk density, thermal conductivity and drying shrinkage rate are shown in Table 4; and the density uniformity is shown in Table 5.

[0038] Table 2. Workability Test Results of Freshly Mixed Foamed Concrete with Different Coal-Fired Bottom Ash Aggregate Selection

[0039] Table 3. Test results of compressive strength of foamed concrete with different types of coal-fired bottom ash aggregate.

[0040] Table 4. Test results of dry density, thermal conductivity and drying shrinkage of foamed concrete with different types of coal-fired bottom ash aggregate.

[0041] Table 5. Test results of density uniformity of foamed concrete with different types of coal-fired bottom ash aggregate.

[0042] Note 5: 1. The density uniformity test sample is a cylindrical specimen with a diameter of 50 mm and a height of 1000 mm. The density at the top surface of the specimen is taken as the baseline "1". The density at the middle part (500 mm from the top) and the bottom are compared with the density value at the top surface to obtain the data in this table. The density uniformity is described by the relative density ratio at the three points of each specimen.

[0043] 2. The greater the difference between the bottom, middle, and top, the worse the uniformity of the sample; conversely, the smaller the difference, the more uniform the sample.

[0044] It can be seen that: Samples 1 and 2, which were judged as AAA, have balanced and excellent key indicators (d 60The homogeneity, saturated water absorption, alkali and active content, and loss on ignition are all within the optimal range. The prepared foamed concrete exhibits the best comprehensive performance: moderate fluidity (155~161 mm), extremely low bleeding rate (0.8~1.2%), and small settling rate (0.51~0.77%), indicating good slurry homogeneity, excellent foam stability, and the strongest process adaptability. Mechanical properties meet and are excellent: 28-day compressive strength reaches 5.8 MPa and 5.5 MPa respectively, significantly exceeding the national standard requirement of A09 grade (≥3.5 MPa), and the strength development is stable. Thermal insulation and shrinkage performance are excellent: dry density is approximately 900 kg / m³. 3 It exhibits low thermal conductivity (0.1615~0.1651 W / (m•K)), far exceeding the national standard requirements for A09 grade products (≤0.24 W / (m•K)). The 28-day drying shrinkage is low (863.29~905.62 μm / m), indicating good volume stability and low risk of cracking. The hardened body has a uniform structure: in the density uniformity test, the relative density ratios of the top, middle, and bottom of the specimen showed small changes (Example 1: 1.000 / 1.038 / 1.057; Example 2: 1.000 / 1.026 / 1.043), proving that no significant segregation occurred during the molding process, resulting in good structural uniformity and excellent performance.

[0045] Samples 3, 4, and 5, whose results include at least one B but not C, have minor defects in some indicators (such as slightly higher alkali content, d...). 60 While the overall performance is still relatively good (though the saturated water absorption rate may be slightly low or high), the fresh mix performance is generally controllable: the fluidity (130~158 mm) and bleeding rate (1.1~5.1%) basically meet the process requirements, but the bleeding rate of some groups (such as sample 5) is slightly higher, requiring optimization by slightly adjusting the amount of water-reducing agent or foam stabilizer during production; the mechanical properties meet the requirements: the 28-day compressive strength is between 4.15-4.3 MPa, which meets the strength requirements of A09 grade, but is slightly lower than the "optimal combination" grade; the thermal insulation and shrinkage performance are good: the thermal conductivity (0.1731~0.1769 W / (m•K)) is still better than the national standard. The drying shrinkage value (806.34~879.25 μm / m) is within the acceptable range; it can be applied to conventional scenarios: after slight adjustments to the formula, this grade of aggregate can be stably used to produce conventional foamed concrete products with good performance requirements, but targeted fine adjustments to the formula are required.

[0046] The judgment result includes a C sample 6 with particle size distribution (d) 60The aggregate (590μm, Cu=9.8, Cc=0.6) and alkali content (1.6%) have significant defects: poor fresh-mixed stability: although the fluidity is acceptable (160 mm), the bleeding rate is high (3.8%), indicating that the foam stability is affected; substandard mechanical properties: the 28-day compressive strength is only 3.2 MPa, failing to meet the minimum requirement of 3.5 MPa for A09 grade, indicating insufficient strength development ability; poor volume stability: the drying shrinkage value increases to 1055.64 μm / m, significantly higher than the preferred group; limited application range: this grade of aggregate can only be considered for occasions with lower mechanical performance requirements, such as foundation pit backfilling, and requires extensive test verification and major formula adjustments. As an aggregate added to foamed concrete, its performance has significant defects, limiting its use.

[0047] Of the samples 7 and 8 that failed the rejection screening, although sample 7 had high chemical activity, good gradation, and abnormally high 3-day strength (6.26 MPa, due to high alkali activation in the early stage), its 7-day and 28-day strengths showed severe shrinkage (the 28-day strength decreased by about 12% compared to the 7-day strength), and its drying shrinkage was extremely large (1088.51 μm / m), indicating a high risk of AAR (Aging Anxiety Reduction). Sample 8, due to its high loss on ignition (8.2%), directly damaged the stability of the fresh mix (the bleeding rate was as high as 5.4%), resulting in a high bulk density after hardening (1095.26 kg / m³). 3 This is due to the large amount of activated carbon in the coal bottom ash breaking down after mixing, resulting in poor thermal conductivity (0.1932 W / (m•K)) and poor overall performance. As an aggregate added to foamed concrete, its performance defects are significant, proving that excellent single index cannot guarantee overall performance. The comprehensive evaluation method of this invention is necessary and efficient.

[0048] Sample 9, which had multiple "C" marks in the judgment results, had an excessively low aggregate saturated water absorption rate (30%) and extremely poor gradation, resulting in poor workability of the freshly mixed slurry (flowability 109 mm, bleeding rate 6.2%) and a high bulk density after hardening (1058.44 kg / m³). 3 Its low strength (3.4 MPa) and poor overall performance indicate that it can only be considered for non-structural applications where performance is not required, such as outdoor maintenance components, and its use is extremely risky.

[0049] The above data demonstrates that the coupled evaluation method of this invention yielded unexpected results. For example, Sample 7 (high alkali): although its gradation and water absorption were good, the excessive alkali content led to severe AAR problems and subsequent strength shrinkage. This proves that a good single indicator does not necessarily mean overall usability. Sample 8 (high loss on ignition): even if other indicators were qualified, the high carbon content directly damaged foam stability, leading to bleeding and sedimentation, demonstrating the criticality of the veto indicator. Sample 4 (judgment result: BAB) and Sample 1 (judgment result: AAA) represent the difference between "recommended use" and "best combination," respectively. The data (such as shrinkage value and uniformity) show a clear performance gradient, proving that the grading system of this invention can accurately predict performance differences.

[0050] In addition, the d of aggregate in sample 4 (BAB) 60 The value (320 μm) is slightly lower than the lower limit of the preferred range (350 μm) set in this invention, and the foamed concrete prepared from it is slightly inferior to sample 1 (AAA grade) in terms of uniformity (see Table 5) and strength. Meanwhile, the d value of the aggregate in sample 9 (CCB grade) is... 60 The value (280 μm) further decreased, far exceeding the reasonable range, leading to a sharp deterioration in its fresh-mixed hardening properties (bleeding rate 6.2%, strength 3.4 MPa); this proves that the parameter range set in this invention (such as d) is within acceptable limits. 60 The range of 350~550μm is not something that can be easily obtained by those skilled in the art through a limited number of experiments. It is a critical range that has been verified by a large number of experiments and distinguishes between the "usable" and "unusable" properties of materials, achieving unexpected technical effects.

[0051] The experimental data above fully demonstrate that the screening and evaluation method provided by this invention can accurately and reliably predict the performance of coal bottom ash in foamed concrete: "optimal combination" aggregates can produce high-performance products; "recommended" aggregates require minor adjustments and can be used in conventional products; "consider use" aggregates have obvious performance defects and their use is limited; while aggregates that fail screening or are "used with caution" have poor performance and high risk, and are not recommended for use. This method provides key technical support for the precise selection of solid waste raw materials and the assurance of product quality in industrial applications.

[0052] Existing technologies have disclosed solutions such as using coal ash as coarse and fine aggregates to replace part of the natural sand, aiming to improve matrix density and compressive strength through gradation optimization; or using coal ash of a specific particle size (8.6~593 μm, average particle size 170 μm) as aggregate in foamed concrete, aiming to improve mechanical properties and thermal insulation; or using a much higher amount of coal ash than the sum of cement, calcined stone powder, and fly ash powder, resulting in a reduction in bulk density and a significant increase in strength. However, all of these solutions lack a systematic evaluation of the comprehensive performance of the aggregate itself, and there is a technical bias in the previous evaluation system: that is, overemphasizing the chemical composition of coal ash (such as the total content of SiO2 and Al2O3), believing that its pozzolanic activity is the dominant factor affecting concrete performance, while generally ignoring the fundamental influence of its physical properties (such as pore structure, water absorption behavior, and particle size distribution). This bias has led to two one-sided approaches: One approach is "chemical determinism": relying excessively on chemical analysis reports such as XRF, assuming that base coats with similar chemical compositions will necessarily have similar performance. However, in practice, it has been found that base coats with similar chemical compositions can have drastically different application effects. Another approach is the "single-index method": attempting to stabilize performance by controlling a single index (such as fineness modulus or particle size), but the effect is not significant and cannot solve the complex problems caused by the coupling variation of multiple indexes in base coats.

[0053] The root of the aforementioned technical bias lies in the failure of those skilled in the art to recognize that, for foamed concrete, a porous material highly dependent on a paste-bubble stabilization system, the physical properties of aggregates (such as water absorption and gradation) are the immediate dominant factors controlling the stability of the fresh mixing process (workability, bleeding, and settling), while their chemical properties (such as alkali content) are potential risk factors affecting long-term durability (alkali-aggregate reaction, strength reduction). Viewing physical and chemical indicators in isolation is the fundamental reason why the application challenges of base mortar cannot be solved.

[0054] This invention breaks through the aforementioned technical biases of "emphasizing chemistry while neglecting physics" or "isolated indicators," and for the first time proposes a screening concept of "physical-chemical dual dimensions" and "risk-functional dual assessment." In particular, it reveals a previously unrecognized coupling relationship between physical and chemical indicators: this invention unexpectedly discovers that high water absorption (physical indicator) can partially offset the shrinkage risk caused by high alkali content (chemical indicator) (see the comparison of shrinkage data between sample 3 and sample 1). Simultaneously, good gradation (physical indicator) can provide skeletal support for the slurry, inhibiting foam coarsening and sedimentation caused by unburned carbon (chemical indicator). This interaction and compensatory effect between physical and chemical indicators is previously unknown and unutilized in the field. Therefore, the three key indicators in this invention are not simply an additive sum of indicators, but rather reveal their intrinsic correlation and establish a novel, multi-level evaluation system. This solves the technical problems of poor workability, large fluctuations in mechanical properties, large drying shrinkage, poor uniformity, and high thermal conductivity in foamed concrete caused by significant fluctuations in the quality of coal ash. This method can control the quality of raw materials from the source, ensuring that industrially produced foamed concrete products have stable low thermal conductivity, high compressive strength and good low shrinkage, providing reliable technical support for the high-value and large-scale resource utilization of bulk solid waste coal ash.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the sizing of coal combustion bottom ash aggregates, characterized by, Including the following steps: S1. Detect the alkaline content and loss on ignition of coal bottom ash aggregate, and screen aggregates with alkaline content and loss on ignition below the threshold to enter the subsequent evaluation steps. S2, detecting d of coal combustion bottom ash aggregate 60 value, calculate the non-uniformity coefficient and the curvature coefficient, and evaluate the particle size distribution index thereof S3. Determine the saturated water absorption rate of coal bottom ash aggregate and evaluate its physical property indicators; S4. Determine the content of pozzolanic active substances in coal bottom ash aggregate, and evaluate its chemical activity index based on the content of pozzolanic active substances and the content of alkaline substances measured in S1. S5. Determine the applicable scope of coal bottom ash aggregate based on particle size distribution index, physical property index and chemical activity index; In S1, the threshold for alkaline substance content is 2.0%; the threshold for loss on ignition is 5.0%. In S2, if d 60 If the value, non-uniformity coefficient, and curvature coefficient are all within the set range, the rating is A; if only one is outside the set range, the rating is B; if at least two are outside the set range, the rating is C. d 60 The set range of the unevenness coefficient is >5, and the set range of the curvature coefficient is 1~3. In S4, if both the content of active volcanic ash and the content of alkaline substances are within the set range, the evaluation is A; if only one is within the set range, the evaluation is B; if neither is within the set range, the evaluation is C. The unevenness coefficient Cu = d 60 / d 10 The curvature coefficient Cc = d 30 2 / (d 10 ·d 60 ); for comprehensive judgment of the continuity and advantages and disadvantages of aggregate gradation.

2. The method of evaluating the sizing of coal combustion bottom ash aggregates of claim 1, wherein, In S3, if the saturated water absorption rate is within the set range, the rating is A; if it is higher than the set range, the rating is B; if it is lower than the set range, the rating is C.

3. The method of evaluating the sizing of coal combustion bottom ash aggregates of claim 2, wherein, The saturated water absorption rate is set within the range of 50-65%.

4. The method of evaluating the sizing of coal combustion bottom ash aggregates of claim 1, wherein, The set range for the content of active substances in volcanic ash is 50-70%, and the set range for the content of alkaline substances is 0.4-1%.

5. The screening evaluation method of coal-fired bottom ash aggregates according to claim 2 or 3, characterized in that, In S5, the scope of application is determined based on the number of A, B, and C in the evaluation of particle size distribution index, physical property index, and chemical activity index.

6. The application of a screening and evaluation method for coal bottom ash aggregate as described in any one of claims 1-5 in the preparation of foamed concrete.

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