A full-solid-waste self-hardening, self-curing and micro-expanding road base material and a preparation method thereof

Abstract

The application discloses a kind of full solid waste self-hardening, self-curing, micro-expansion road base material and preparation method thereof, belong to solid waste resource utilization field.The road base material is by circulating fluidized bed fly ash, circulating fluidized bed slag, after pre-grinding, pre-saturation and other modification treatments, with waste slag, red mud, water is mixed according to certain proportion, is prepared after mixing, compaction and curing preparation.Compared with prior art, all solid waste is used as raw material in the application, and has the performance characteristics of self-hardening, self-curing, micro-expansion, high frost resistance, good water stability.Not only the preparation cost of road base material is reduced, and realizes the large amount of use of CFB ash slag, and has important significance to high-grade highway construction and ecological environment protection.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization, specifically relating to a self-hardening, self-curing, and micro-expansion road base material made entirely of solid waste and its preparation method. Background Technology

[0002] With my country's power industry maintaining a long-standing "coal-based" energy structure, circulating fluidized bed (CFB) boilers have been widely adopted in domestic thermal power plants due to their ability to efficiently utilize coal gangue and low-quality coal while simultaneously desulfurizing. The total annual emissions of CFB fly ash and CFB slag exceed 150 million tons, with particularly large stockpiles in major coal-producing areas such as Shanxi, Inner Mongolia, and Ningxia. Compared to pulverized coal boiler fly ash, CFB fly ash, due to its lower calcination temperature and the use of in-furnace calcium-enrichment desulfurization technology, exhibits characteristics such as highly porous surface, high content of free calcium oxide (f-CaO) and anhydrite (Ⅱ-CaSO4), and high loss on ignition, limiting its use in cement and concrete. Furthermore, CFB slag contains abundant porosity, with a water absorption rate exceeding 20% ​​and a lower crushing value than ordinary crushed stone, making it unsuitable for use as coarse aggregate. Currently, CFB ash (CFB fly ash and CFB slag) is still mainly disposed of through landfill and stockpiling, which not only occupies land but also poses environmental risks such as dust and alkali pollution. There is an urgent need for high-volume, high-value-added utilization methods. However, because CFB ash contains abundant active aluminum-silicon resources and anhydrite phase, it has the function of self-hardening upon contact with water.

[0003] The road base course is a crucial functional layer in highway structures that bears and dissipates traffic loads. Traditional cement-stabilized crushed stone and lime-fly ash crushed stone base courses consume large amounts of natural sand, cement, and lime annually, causing ecological damage and increasing carbon emission pressure. Chinese patents CN111548098B and CN111574121A disclose methods for preparing road base course mixtures using solid waste materials such as coal gasification waste residue, slag, and fly ash, but these still require 3%–10% cement as an alkaline activator. Chinese patent CN116177943A uses a system of iron tailings + steel slag powder + water slag + lime to achieve a solid waste-based road base course material, but the lime content needs to be 2%–5%, still relying on traditional alkaline activators. Furthermore, existing road base course materials require at least 7 days of geotextile covering followed by water curing after paving and compaction, a cumbersome process with high labor costs. Existing research has shown that CFB ash possesses potential self-hardening properties: f-CaO hydrates to form Ca(OH)2, which, under sulfate activation, forms ettringite (AFt) and CSH gel, giving the system self-hardening and micro-expansion characteristics. Furthermore, CFB slag has a porous structure, allowing it to pre-adsorb a significant amount of moisture, which is then slowly released after compaction of the road base material, achieving self-curing. Therefore, how to achieve "zero-cement" self-hardening, self-curing, and micro-expansion road base materials without introducing cement or lime, utilizing only the chemical activity of CFB ash and supplemented with industrial solid waste such as red mud, has become a current technological bottleneck.

[0004] On the other hand, red mud is a highly alkaline waste residue emitted during the Bayer process for alumina production, with a national stockpile exceeding 400 million tons. Rich in Fe2O3, Al2O3, and alkaline components, it can be used as an alkali activator or filler. Waste soil is widely available and inexpensive, serving to regulate gradation and fill pores. However, the high water content and alkalinity of red mud can lead to poor material volume stability, while waste soil, due to its large compositional fluctuations and high water content, is difficult to utilize directly. Therefore, how to synergistically utilize CFB ash, red mud, and waste soil, and through pre-grinding, pre-saturation, and hydrophobic surface modification, to achieve self-hardening, self-curing, crack resistance, freeze resistance, and water resistance in materials, has become a key technical problem urgently needing to be solved in the intersection of solid waste resource utilization and road engineering. Summary of the Invention

[0005] The purpose of this invention is to address the technical bottleneck of using existing multi-element solid wastes such as CFB ash, red mud, and waste soil directly as road base materials. By fully utilizing the composition and structural characteristics of CFB ash and red mud, this invention provides a method for preparing a self-hardening, self-curing, and micro-expanding road base material from all solid wastes.

[0006] This invention provides a self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste, comprising the following raw materials by weight:

[0007] Circulating fluidized bed fly ash (CFB fly ash): 10~40 parts

[0008] Circulating fluidized bed slag (CFB slag): 20-50 parts

[0009] Waste soil: 5-25 parts

[0010] Red mud: 0~15 parts.

[0011] Furthermore, the circulating fluidized bed fly ash is desulfurization and dust removal ash emitted from power plants that use coal gangue and low-quality coal as fuel and circulating fluidized bed boilers as burners, with a 28-day activity index ≥80%; the circulating fluidized bed slag is bottom ash emitted from circulating fluidized bed boilers, with a particle size of 0.1~20mm and an apparent particle density of 1.0~2.4g / cm³. 3 The saturated surface dry water absorption rate is 10~30%.

[0012] Furthermore, the circulating fluidized bed fly ash and circulating fluidized bed slag are composed of the following chemical components by mass percentage: Al2O3: 20~35%, SiO2: 35~50%, Fe2O3: 4~7%, CaO: 5~18%, SO3: 4~12%, and loss on ignition: 2~10%.

[0013] Furthermore, the red mud is a waste product discharged during the Bayer process for extracting alumina, and is composed of the following chemical components by mass percentage: Al2O3: 10~20%, SiO2: 5~20%, Fe2O3: 30~50%, CaO: 5~15%, Na2O: 2~10%, Ti2O: 2~10%.

[0014] Furthermore, the road base material has an unconfined compressive strength of 2~5MPa at 7 days, an unconfined compressive strength of 4~8MPa at 28 days, an expansion rate of 0~10 mm / m at 28 days, and a residual compressive strength ratio of ≥90% after 5 freeze-thaw cycles.

[0015] A method for preparing a self-hardening, self-curing, and micro-expansion road base material made entirely of solid waste includes the following steps:

[0016] S1 Place the circulating fluidized bed fly ash in a steel ball mill, add 0.1% of the mass of the circulating fluidized bed fly ash as a hydrophobic activating grinding aid, and mill for 10~30 minutes before use;

[0017] S2 uses a screening machine to remove particles larger than 10mm from the circulating fluidized bed slag, and adds water at 10% of the slag mass after removing particles larger than 10mm. The mixture is then stirred evenly, sealed, and aged for 6-12 hours before use.

[0018] S3 crushes naturally air-dried waste soil and red mud into particles with a size of <2mm and tests their moisture content;

[0019] S4 uses a mixer to mix water, circulating fluidized bed fly ash pretreated in step S1, waste slag and red mud pretreated in step S3 for 2-5 minutes, then adds circulating fluidized bed slag pretreated in step S2 and mixes evenly for 1-3 minutes.

[0020] S5 uses a hydraulic press to compress the mixture obtained from S4 into shape, with a compaction degree of 92%~94%, and wraps it with geotextile. It is then cured at room temperature for 7~28 days to produce a self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste.

[0021] Furthermore, the hydrophobic activating grinding aid in step S1 is any one or a combination of calcium stearate, oleic acid, γ-aminopropyltriethoxysilane, and stearamide.

[0022] Furthermore, the particle size of the circulating fluidized bed fly ash particles after ball milling in step S1 meets the requirement that the sieve residue is ≤5% after passing through an 80μm square hole sieve, and ≤20% after passing through a 45μm square hole sieve.

[0023] Furthermore, the amount of water added in step S4 is calculated by adding 5% to the optimum moisture content obtained from the heavy compaction test, and then subtracting the amount of water introduced from the circulating fluidized bed slag, waste slag, and red mud. The specific calculation formula is as follows:

[0024] Water addition = (optimal moisture content + 5%) × total dry material mass - circulating fluidized bed slag mass × 10% - waste slag content × waste slag moisture content - red mud content × red mud moisture content.

[0025] The heavy compaction test involves first preparing samples according to the raw material ratio of the road base material, then mechanically compacting the samples to rearrange the particles, reduce air content, and increase density. While maintaining a relatively constant moisture content, this process achieves increased sample strength and reduced permeability. A moisture content-dry density curve is plotted during the test, with the point corresponding to the maximum dry density representing the optimum moisture content. The total mass of the dry material refers to the sum of the masses of the circulating fluidized bed fly ash after pretreatment in step S1, the circulating fluidized bed slag after pretreatment in step S2, and the waste soil and red mud after pretreatment in step S3.

[0026] The key technical problems solved by this invention include:

[0027] (1) CFB fly ash particles have small particle size and a porous structure, resulting in a large specific surface area and high water absorption. When used directly to prepare road base materials, it will significantly increase the optimum moisture content of the road base materials, affecting the compaction degree and subsequent water resistance of the materials. Therefore, this invention uses mechanical grinding combined with a hydrophobic activating grinding aid to modify CFB fly ash, thereby effectively reducing the water requirement of CFB fly ash and improving its activity;

[0028] (2) CFB slag particles have a loose and porous structure and a high crushing value, making it difficult to directly replace crushed stone in the preparation of road base materials. Therefore, in this invention, waste soil, red mud and other mud-like materials are combined with slag to construct a "suspended dense" road base structure, which reduces the damage to CFB slag during the compaction process;

[0029] (3) Road base materials require continuous watering and maintenance during application, resulting in high labor costs and significant water consumption. This invention fully utilizes the characteristics of CFB slag particles being loose, porous, and having a high water absorption rate. By pre-saturating the slag with water, CFB slag is used as a "water source" for endogenous curing, achieving self-curing of road base materials without affecting compaction.

[0030] The beneficial effects of this invention are as follows:

[0031] (1) This invention does not use traditional cementitious materials such as cement, lime, and gypsum. Instead, it achieves self-hardening of the material by means of minerals such as active aluminum silicon and calcium sulfate contained in CFB ash, which are catalyzed and activated by alkaline substances provided by red mud. The material preparation cost is low and it is green and low-carbon.

[0032] (2) This invention overcomes the shortcomings of traditional road base materials that require long-term humidification and maintenance, and makes full use of the loose and porous structure of CFB slag particles. By pre-saturating with water, CFB slag is used as the "water source" for endogenous maintenance. Without affecting the compaction and molding, only a film covering is needed to achieve self-maintenance of the road base material.

[0033] (3) The solid waste self-hardening, self-curing, and micro-expansion road base material obtained by the present invention has high strength, excellent water resistance and crack resistance. During the curing process, the material can generate ettringite crystals with micro-expansion properties, which not only makes up for the shrinkage during the service process of the material, but also the ettringite crystals have excellent impermeability, which can significantly improve the water resistance and frost resistance of the road base material. Attached Figure Description

[0034] Figure 1 Microscopic morphology of CFB fly ash.

[0035] Figure 2 This is a microscopic morphology image of CFB slag.

[0036] Figure 3The image shows the microstructure of the self-hardening, self-curing, and micro-expanding road base material prepared from solid waste in Example 1. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply.

[0038] Example 1 A self-hardening, self-curing, and micro-expanding road base material made entirely of solid waste, comprising the following raw materials by weight: 35 parts CFB fly ash, 30 parts CFB slag, 15 parts waste soil, and 10 parts red mud.

[0039] The 28-day activity index of CFB fly ash was 88%; the particle size of CFB slag was 0.1~12mm, and the apparent density of the particles was 1.5~2.4 g / cm³. 3 The saturated surface dry water absorption rate is 18%. The chemical composition of the CFB fly ash, CFB slag, and red mud used is shown in Appendix Table 1.

[0040]

[0041] Depend on Figure 1 It can be seen that CFB fly ash is composed of fine particles, with a particle size of approximately 1 μm, possessing a large specific surface area, which is the basis for its good activity. Figure 2 It can be seen that the loose and porous surface of CFB slag particles is the main reason for their high water absorption rate.

[0042] The preparation method and process parameters of a self-hardening, self-curing, and micro-expansion road base material made entirely of solid waste are as follows:

[0043] S1. CFB fly ash is placed in a steel ball mill, and calcium stearate of 0.1% of the mass of CFB fly ash is added. After ball milling for 15 minutes, it is ready for use. The CFB fly ash after ball milling has a sieve residue of 2.5% after passing through an 80μm square hole sieve and a sieve residue of 11.0% after passing through a 45μm square hole sieve.

[0044] S2 uses a screening machine to remove particles larger than 10mm from the CFB slag, and adds water at 10% of the mass of the removed CFB slag, mixes it evenly, and ages it in a sealed container for 12 hours before use.

[0045] S3 crushed naturally air-dried waste soil and red mud into particles with a size of <2mm, and tested their moisture contents to be 2.1% and 3.2%, respectively.

[0046] S4 uses a mixer to mix the CFB fly ash pretreated by S1, the waste slag and red mud pretreated by S3 with water in a certain proportion for 2 minutes, and then adds the CFB slag pretreated by S2 and mixes it evenly for 2 minutes.

[0047] S5 uses a hydraulic press to compress the mixture in S4 into a 94% compaction degree, and then wraps it with geotextile. It is cured at room temperature for 7 days and 28 days respectively to produce a self-hardening, self-curing, and micro-expansion road base material made from solid waste.

[0048] The self-hardening, self-curing, and micro-expansion road base material prepared after sealed curing is shown in Appendix Table 2. The physical properties of this material are shown in Appendix Table 2, and its microstructure is shown in Appendix Table 3. Figure 3 As shown.

[0049]

[0050] Note: Expansion rate - a positive value indicates expansion, and a negative value indicates contraction.

[0051] Example 2 uses the same raw materials as Example 1, but with the following proportions: 25 parts CFB fly ash, 40 parts CFB slag, 20 parts waste soil, and 5 parts red mud. The preparation process and parameters are the same as in Example 1. The physical properties of the road base material obtained after curing to a certain age are shown in Appendix Table 2.

[0052] Example 3 uses the same raw material composition as Example 1, but with the following proportions: 40 parts CFB fly ash, 50 parts CFB slag, and 10 parts waste soil. The preparation method and process parameters are the same as in Example 1. The physical properties of the road base material obtained after curing to a certain age are shown in Appendix Table 2.

[0053] Example 4 uses the same raw material composition and proportions as Example 1, and the preparation steps are the same as in Example 1. However, in the process parameters, the hydrophobic activating grinding aid is a mixture of γ-aminopropyltriethoxysilane and stearamide in a 1:1 mass ratio. After curing for a certain period, the physical properties of the resulting road base material are shown in Appendix Table 2.

[0054] Example 5 uses the same raw material composition and proportions as Example 1, and the preparation steps are the same as in Example 1, but the compaction degree is 92%. The physical properties of the road base material after curing to a certain age are shown in Appendix Table 2.

[0055] Comparative Example 1 differs from Example 1 in that an equal amount of Grade II fly ash replaces the CFB fly ash in the raw materials. The chemical composition of the Grade II fly ash is shown in Appendix Table 1. The preparation method and process parameters of the material are the same as in Example 1. The physical properties of the road base material after sealed curing to a certain age are shown in Appendix Table 2.

[0056] Since Class II fly ash produced by pulverized coal furnace combustion contains almost no CaSO4, when it replaces CFB fly ash, the hydration of the prepared road base material does not produce hydrated products such as ettringite. Not only is the early strength of the material low and it does not have rapid hardening characteristics, but the material also lacks self-expansion characteristics. After 28 days of curing, it exhibits a shrinkage rate of 1.6 mm / m, which has a significant impact on the crack resistance of the road.

[0057] Comparative Example 2 differs from Example 1 in that, in step S2 of the preparation method, the sieved CFB slag is not pre-wetted with water. The physical properties of the road base material cured in a sealed environment for a certain period are shown in Appendix Table 2.

[0058] Because CFB slag has a strong water absorption capacity, failure to pre-wet it with water will lead to insufficient hydration during the curing stage due to water shortage, affecting strength development. Simultaneously, the lack of water limits the formation of the hydration product ettringite, resulting in a smaller expansion rate of the sample compared to Example 1. Furthermore, due to the internal water deficiency in the CFB slag of Comparative Example 2, insufficient hydration products were not formed internally, leading to a higher water absorption rate in the later stages of material formation and reduced freeze-thaw resistance; therefore, the freeze-thaw cycle strength was significantly lower than that of Example 1.

[0059] Comparative Example 3 differs from Example 1 in that an equal amount of 5-10mm crushed stone replaces the CFB slag in the raw materials. Other preparation methods and process parameters are the same as in Example 1. The physical properties of the road base material after natural curing to a certain age are shown in Appendix Table 2.

[0060] Because pre-saturated CFB slag particles were not added in this case, there was insufficient water to participate in the hydration reaction of CFB fly ash during the curing stage. As a result of insufficient hydration, the strength of the samples at various ages was low.

Claims

1. A self-hardening, self-curing, and micro-expansion road base material made entirely of solid waste, characterized in that, It was prepared by the following method: S1 Place the circulating fluidized bed fly ash in a steel ball mill, add 0.1% of the mass of the circulating fluidized bed fly ash as a hydrophobic activating grinding aid, and mill for 10~30 minutes before use; S2 uses a screening machine to remove particles larger than 10mm from the circulating fluidized bed slag, and adds water at 10% of the slag mass after removing particles larger than 10mm. The mixture is then stirred evenly, sealed, and aged for 6-12 hours before use. S3 crushes naturally air-dried waste soil and red mud into particles with a size of <2mm and tests their moisture content; In step S4, a mixer is used to mix water, pretreated circulating fluidized bed fly ash from step S1, pretreated waste slag and red mud from step S3, and other materials for 2-5 minutes. Then, pretreated circulating fluidized bed slag from step S2 is added and mixed evenly for 1-3 minutes. The amount of water added is the optimum moisture content obtained from the heavy compaction test plus 5%, minus the water introduced by the circulating fluidized bed slag, waste slag, and red mud. The specific calculation formula is as follows: Water addition = (optimal moisture content + 5%) × total dry material mass - circulating fluidized bed slag mass × 10% - waste slag content × waste slag moisture content - red mud content × red mud moisture content; S5 uses a hydraulic press to compress the mixture obtained from S4 into shape, with a compaction degree of 92%~94%, and wraps it with geotextile. It is then cured at room temperature for 7~28 days to obtain a self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste. The raw material usage, by weight, is as follows: Circulating fluidized bed fly ash: 10-40 parts Circulating fluidized bed slag: 20-50 parts Waste soil: 5-25 parts Red mud: 5-15 parts.

2. The self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste as described in claim 1, characterized in that: The circulating fluidized bed fly ash is desulfurization and dust removal ash emitted from power plants that use coal gangue and low-quality coal as fuel and circulating fluidized bed boilers as burners, with a 28-day activity index ≥80%; the circulating fluidized bed slag is bottom ash emitted from circulating fluidized bed boilers, with a particle size of 0.1~20mm and an apparent particle density of 1.0~2.4g / cm³. 3 The saturated surface dry water absorption rate is 10~30%.

3. A self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste, as described in claim 1 or 2, is characterized in that... The circulating fluidized bed fly ash and circulating fluidized bed slag are composed of the following chemical components by mass percentage: Al2O3: 20~35%, SiO2: 35~50%, Fe2O3: 4~7%, CaO: 5~18%, SO3: 4~12%, and loss on ignition: 2~10%.

4. The self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste according to claim 1, characterized in that, The red mud is a waste product discharged during the Bayer process for extracting alumina, and it consists of the following chemical components by mass percentage: Al2O3: 10~20%, SiO2: 5~20%, Fe2O3: 30~50%, CaO: 5~15%, Na2O: 2~10%, Ti2O: 2~10%.

5. The self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste according to claim 1, characterized in that: The road base material has an unconfined compressive strength of 2~5MPa at 7 days, an unconfined compressive strength of 4~8MPa at 28 days, an expansion rate of 0~10 mm / m at 28 days, and a residual compressive strength ratio of ≥90% after 5 freeze-thaw cycles.

6. The self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste according to claim 1, characterized in that: The hydrophobic activating grinding aid in step S1 is any one or a combination of calcium stearate, oleic acid, γ-aminopropyltriethoxysilane, and stearamide.

7. The self-hardening, self-curing, and micro-expansion road base material made entirely from solid waste according to claim 1, characterized in that: The particle size of the fly ash particles after ball milling in step S1 meets the requirement that the residue after passing through an 80μm square hole sieve is ≤5%, and the residue after passing through a 45μm square hole sieve is ≤20%.

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