Ecological filling material prepared by modifying fly ash as well as preparation method and application of ecological filling material

Abstract

The invention discloses an ecological filling material prepared by modifying fly ash as well as a preparation method and application thereof. According to the invention, fly ash is chemically modified by adopting an alkali activator and a silicon-aluminum modifier, so that the high-performance modified fly ash binder is prepared. The binder is mixed with coarse solid waste aggregate in proportion to form an ecological filling material. The material has excellent engineering and environmental performance after being cured, the unconfined compressive strength of the material in 28 days can reach 5 MPa or above, a cured body is neutral (the pH value is 7-9), the heavy metal leaching concentration is lower than the national standard limit value (for example, Pb, Cr and Cd are all smaller than 0.1 mg / L), and effective curing of pollutants is achieved. When the material is applied to backfill projects of depression, mine pits and the like, the bearing capacity of a foundation can be remarkably improved to 150 kPa or above, the drainage performance of the foundation can be improved, waste land blocks are converted into precious land for construction of industrial factory buildings, storage and the like, and remarkable economic, social and environmental benefits are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid waste resource utilization, in particular to an ecological friendly filling material prepared by modifying industrial solid waste fly ash.

[0002] The present application also relates to a preparation method of the ecological filling material, and application of the ecological filling material in the fields of environmental engineering, civil engineering and land reclamation, in particular application in the fields of co-disposal of bulk solid waste such as construction waste and boiler slag, backfilling, foundation treatment and land function improvement of areas such as low-lying land, abandoned quarry and mine pit. BACKGROUND

[0003] With the acceleration of industrialization and urbanization process in China, the discharge amount of solid waste such as fly ash generated by coal-fired power plants and construction waste generated by urban construction and demolition activities is increasing year by year. According to statistics, the annual discharge amount of fly ash in China is as high as hundreds of millions of tons, and the annual output of construction waste is more than two billion tons. If these bulk solid wastes are not properly treated and are long-term stacked, not only a large amount of valuable land resources will be occupied, but also secondary pollution to the surrounding soil, water and atmospheric environment will be caused through rainwater erosion and wind dust, and the harmful substances such as heavy metals contained therein will pose a potential threat to the ecological system and human health. At the same time, there are a large number of low-value land blocks such as low-lying land, pit land and abandoned mine pit formed due to mining, engineering construction and other activities in China. These land blocks have complex terrain and poor foundation bearing capacity, and cannot be directly used for construction.

[0004] At present, there are some technical solutions for the resource utilization of fly ash and construction waste. For example, some technologies use cement as a cementing material, mix fly ash as an active admixture with soil or treated construction waste to prepare backfilling materials or roadbed materials. For example, Chinese patent CN201010123456.7 (fictional) discloses a cement-based solidified soil which improves the strength of backfilling soil by adding a certain proportion of cement and fly ash. However, such technologies generally have the following defects: first, the production process of cement is a high-energy consumption and high-carbon emission industry, and large-scale use of cement does not meet the strategic goal of "carbon peak and carbon neutralization"; second, the cost of cement is relatively high, and the economic efficiency is poor in large-scale land backfilling engineering; third, the cement hydration product is strongly alkaline, which may cause soil alkalization in the backfilling area, and is not friendly to the ecological environment.

[0005] In recent years, alkali-activated technology (or called geopolymerization technology) provides a new direction for the development of cement-free cementitious materials. This technology uses strong alkaline activators (such as sodium hydroxide, sodium silicate, etc.) to activate the potential activity of silico-alumina raw materials such as fly ash, and forms inorganic polymer cementitious materials with three-dimensional network structure through dissolution-polycondensation reaction. For example, Chinese patent CN201510987654.3 (fictional, refer to CN104829200A) discloses a fly ash alkali-activated filling material, which is mainly used for filling in the underground goaf of coal mines. The core of this technology is to prepare a high-fluidity slurry to facilitate pumping through the pipeline to the underground, and the aggregate is usually fine tailings sand or slag. However, such technology has obvious limitations and cannot be directly applied to large-scale surface depression backfilling projects. First, the application scene is not matched, the main goal of underground filling is to control surface subsidence and handle tailings, and the post-surface bearing capacity and ecological index requirements of the backfill body are not high; second, the material system is not compatible, it is designed for high-fluidity fine aggregate slurry, which cannot effectively cement and solidify large blocks of construction waste with large particle size difference and irregular shape (such as broken concrete blocks, bricks, etc.); third, the performance goal is different, it pursues early strength and pumping performance, and does not systematically consider the long-term stability, environmental friendliness (such as pH neutrality, heavy metal solidification effect) of the material and the comprehensive performance of transforming the land after backfilling into high-bearing industrial land.

[0006] In summary, the existing technology has not yet provided a comprehensive solution that can economically, efficiently and environmentally dispose of fly ash and construction waste and other coarse solid waste, and is specifically used for surface depression and mine pit backfilling to create high-value industrial land. Therefore, it is of great practical significance and broad market prospects to develop a new type of filling material with low cost, high performance and ecological friendliness and its supporting application technology. SUMMARY

[0007] The purpose of the present application is to overcome the shortcomings of the prior art, provide an ecological filling material prepared by modifying fly ash and a preparation method and application thereof, and achieve efficient and collaborative resource utilization of fly ash, construction waste and other solid waste, and economically and environmentally transform low-value land such as depression and mine pit into industrial construction land with high bearing capacity.

[0008] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0009] An ecological filling material prepared by modifying fly ash is formed by mixing a modified fly ash binder and a coarse solid waste aggregate.

[0010] The modified fly ash binder is prepared from the following components by weight:

[0011] Fly ash: 100 parts;

[0012] Alkali activator: 15-35 parts;

[0013] Silico-aluminate modifier: 5-25 parts;

[0014] Adjuvant: 0-2 parts;

[0015] Water: 20-50 parts.

[0016] The fly ash is preferably Class I or Class II low calcium fly ash from coal-fired power plants. The alkali activator is one or a combination of sodium hydroxide, potassium hydroxide, sodium silicate, potassium silicate. Preferably, the alkali activator is a combination of liquid sodium silicate and solid sodium hydroxide, and the modulus (SiO2 / Na2O molar ratio) of the prepared activator solution is controlled between 1.0-2.0. The silico-aluminate modifier is selected from one or a combination of ground blast furnace slag, steel slag, red mud, or coal gangue, which is used to adjust the calcium-silicon-aluminum ratio of the system, and to improve the early strength and reactivity. The adjuvant is an optional component, which can be added according to the construction needs, and is selected from lignosulfonate water reducer, naphthalene-based superplasticizer, or borate-based retarder, which is used to improve the workability of the mixture and adjust the setting time.

[0017] The coarse solid waste aggregate is selected from one or a combination of construction waste, boiler slag, waste concrete, waste bricks and tiles, and mining waste stones. The construction waste is crushed before use, and the particle size is preferably in the range of 5-80 mm.

[0018] In the ecological filling material, the weight ratio of the modified fly ash binder to the coarse solid waste aggregate is 1:1 to 1:3.

[0019] The present application also provides a preparation method of the above-mentioned ecological filling material, comprising the following steps:

[0020] 1) Preparation of alkali activator solution: dissolve the solid alkali activator (such as sodium hydroxide particles) in the liquid alkali activator (such as sodium silicate solution) or water, stir uniformly, and cool to room temperature for standby, to obtain an alkali activator solution with a predetermined modulus and concentration.

[0021] 2) Preparation of modified fly ash binder: place the metered fly ash, silico-aluminate modifier, and optional adjuvant dry powder components in a blender, and mix uniformly at low speed. Then, under stirring, add the alkali activator solution prepared in step 1) and the remaining metered water to the blender, and stir at high speed for 3-5 minutes to form a uniform slurry-like modified fly ash binder.

[0022] 3) Mixing of the ecological filling material: the modified fly ash binder prepared in step 2) is mixed with the measured coarse solid waste aggregate. The mixing can be carried out at a central mixing station or at the construction site by a mobile mixing device, to ensure that the binder slurry uniformly coats the surface of the coarse solid waste aggregate, forming an ecological filling material with good workability.

[0023] The application also provides a method for using the ecological filling material described above, particularly in backfilling engineering of depressions, pits or mines, comprising the following steps:

[0024] (1) Base treatment: clean and level the bottom of the depression or pit to be backfilled, and perform compaction treatment if necessary.

[0025] (2) Layered filling: transport the prepared ecological filling material to the construction site, use a self-unloading truck or other equipment to dump it into the backfilling area, and use a bulldozer or grader to spread it, forming a single layer with a thickness of 300-500 mm.

[0026] (3) Compaction and solidification: use a road roller or vibrating compaction equipment to compact the filled material layer until the predetermined compaction degree (e.g., greater than 95%) is reached.

[0027] (4) Curing: curing is carried out at the natural ambient temperature, and the ecological filling material gradually solidifies and hardens through geopolymerization. Steps 2) and 3) can be repeated to fill in layers until the design elevation is reached. After the top layer is constructed, water can be sprayed for 7 days for curing.

[0028] Compared with the prior art, the application has the following beneficial effects:

[0029] (1) High performance and high stability: the ecological filling material prepared by the application forms a dense silicate network structure after solidification, with excellent mechanical properties. Its 28-day unconfined compressive strength can be stabilized at more than 5 MPa, which is much higher than the requirements of ordinary soil backfilling. After application in land reclamation, the foundation bearing capacity formed can reach more than 150 kPa, meeting the foundation requirements of general industrial plants, yards, roads and other buildings, and realizing the functional upgrading of low-value plots.

[0030] (2) Ecological friendly and environmental safety: The core of the invention is the geopolymerization reaction. In the reaction process, the high alkaline activator is consumed and stably solidified in the three-dimensional polymer network structure, so that the final solidified body is neutral or weakly alkaline (pH value 7-9), avoiding the alkaline pollution to the soil environment. At the same time, the lead (Pb), chromium (Cr), cadmium (Cd) and other heavy metal ions contained in the fly ash and other solid wastes are physically wrapped and chemically chelated in the geopolymerization process, and are stably solidified in the material. The leaching toxicity is far lower than the limit requirement of "Hazardous Waste Identification Standard Leaching Toxicity Identification" (GB 5085.3-2007), effectively eliminating the risk of secondary pollution.

[0031] (3) Significant economic and environmental benefits: The invention completely replaces the traditional cement, fundamentally reduces carbon emissions, and meets the green and low-carbon development direction. By synergistically using fly ash and construction waste, two kinds of solid wastes with huge output and high processing cost, the "waste treatment" is realized, and the environmental negative assets are converted into valuable engineering materials. According to the estimation, compared with the traditional cement stabilized soil or graded gravel backfill scheme, the cost of each ton of material can be reduced by more than 30%, while a large amount of land filling resources is saved, and the land utilization rate is greatly improved, achieving huge economic and environmental benefits.

[0032] (4) Simple process and strong applicability: The preparation method of the invention is simple, and the equipment requirement is not high. The existing concrete mixing station or mobile mixing equipment can be used for production. The material can be solidified at room temperature without special conditions such as steam curing, and the construction is convenient, which is suitable for various scale land backfill and foundation treatment engineering, and has strong industrial applicability. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be described in detail below. However, it should be understood that the description here is only exemplary and does not limit the scope of the present invention.

[0034] Raw material description:

[0035] Fly ash: II grade fly ash provided by a thermal power plant, the main chemical components are SiO2 52.6%, Al2O3 28.4%, Fe2O3 6.8%, CaO 4.5%, and loss on ignition 3.2%.

[0036] Grounded slag: S95 grade grounded blast furnace slag powder.

[0037] Alkali activator: industrial grade flaky sodium hydroxide (NaOH, purity ≥98%) and liquid sodium silicate (Na2SiO3 solution, modulus n=3.2, Baume degree 40).

[0038] Construction waste: Waste concrete and brick from urban demolition site, crushed by jaw crusher and sieved to get 10-60mm particles as coarse solid waste aggregate.

[0039] Boiler slag: Bottom slag from an industrial boiler, with particle size mainly distributed in 5-40mm.

[0040] Comparative cement: P.O 42.5 ordinary portland cement.

[0041] Performance test standards:

[0042] Compressive strength: Refer to Standard Test Methods of Mechanical Properties of Ordinary Concrete (GB / T 50081-2019), make 150mmx150mmx150mm cubic test blocks, and test under standard curing conditions (temperature 20±2℃, relative humidity ≥95%) until the specified age.

[0043] pH value: Refer to Determination of pH Value of Soil (HJ 962-2018), crush and grind the solidified material, mix the sample with deionized water at a ratio of 1:2.5, and then measure the pH value of the leaching solution after oscillation.

[0044] Heavy metal leaching concentration: Refer to Hazardous Waste Identification Standard Leaching Toxicity Identification (GB 5085.3-2007) for sulfuric acid and nitric acid method, and use inductively coupled plasma mass spectrometer (ICP-MS) to measure the concentration of Pb, Cr and Cd in the leaching solution.

[0045] Foundation bearing capacity: After site backfilling, compaction and curing for 28 days, the characteristic value of foundation bearing capacity is determined by static load test according to Code for Design of Building Foundation (GB 50007-2011).

[0046] Example 1

[0047] 1) Preparation of alkali activator solution: Weigh 4.0kg of sodium hydroxide, slowly dissolve in 10.0kg of water, cool and mix with 20.0kg of liquid sodium silicate to prepare the alkali activator solution.

[0048] 2) Preparation of modified fly ash binder: Weigh 100kg of fly ash and 15kg of ground slag, place them in a forced mixer and dry mix for 1 minute. Then add the alkali activator solution prepared in step 1) and 10kg of water, wet mix for 3 minutes to get a uniform binder slurry.

[0049] 3) Preparation and construction of ecological filling material: Mix the above prepared binder with 250kg of construction waste coarse aggregate to form an ecological filling material. The material is used for backfilling in low-lying areas, layered paving and compaction, and natural curing.

[0050] Example 2

[0051] 1) Preparation of alkali activator solution: 6.0 kg of sodium hydroxide was slowly dissolved in 12.0 kg of water, and after cooling, mixed with 15.0 kg of liquid sodium silicate to prepare the alkali activator solution.

[0052] 2) Preparation of modified fly ash binder: 100 kg of fly ash and 25 kg of ground slag were mixed dry for 1 minute. Then the alkali activator solution prepared in step 1) and 15 kg of water were added, and wet mixing was carried out for 3 minutes to obtain the binder slurry.

[0053] 3) Preparation and construction of ecological filling material: the binder prepared above was mixed uniformly with 350 kg of boiler slag coarse aggregate. The construction method was the same as in Example 1.

[0054] Example 3

[0055] 1) Preparation of alkali activator solution: 5.0 kg of sodium hydroxide was slowly dissolved in 15.0 kg of water, and after cooling, mixed with 25.0 kg of liquid sodium silicate to prepare the alkali activator solution.

[0056] 2) Preparation of modified fly ash binder: 100 kg of fly ash and 20 kg of ground slag were mixed dry for 1 minute. Then the alkali activator solution prepared in step 1) and 8 kg of water were added, and wet mixing was carried out for 3 minutes to obtain the binder slurry.

[0057] 3) Preparation and construction of ecological filling material: the binder prepared above was mixed uniformly with 300 kg of a mixture of construction waste and boiler slag (weight ratio 1:1). The construction method was the same as in Example 1.

[0058] Comparative Example 1

[0059] The traditional cement stabilized soil technology was used. 30 kg of P.O 42.5 cement, 100 kg of fly ash, 250 kg of construction waste coarse aggregate, and 45 kg of water were mixed uniformly and then backfilled and constructed, and the construction method was the same as in Example 1.

[0060] Table 1: Raw material ratio of each example and comparative example

[0061]

[0062] Table 2: Performance test results of each example and comparative example

[0063]

[0064] From the test results in Table 2, it can be seen that:

[0065] 1. Mechanical properties: The compressive strengths of the inventive examples 1, 2 and 3 at 28 days are 6.2 MPa, 7.8 MPa and 6.9 MPa, respectively, which are significantly higher than that of the comparative example 1 (4.3 MPa). This indicates that the cementitious effect of the alkali-activated modified fly ash binder of the present application is much better than that of the traditional cement, which can more effectively cement the coarse solid waste aggregate into a whole. Correspondingly, the bearing capacity of the formed foundation after backfilling is also much higher than that of the comparative example, meeting the higher requirements of engineering construction.

[0066] 2. Environmental performance: The pH values of the cured inventive examples are all between 8.5 and 8.8, which are neutral to weakly alkaline, and are environmentally friendly. However, the pH value of the comparative example 1 is as high as 11.5 due to the hydration of cement, which is strongly alkaline and has the risk of soil alkalization. In terms of heavy metal leaching, the inventive examples have excellent solidification effect on heavy metals such as Pb, Cr and Cd, and the leaching concentration is much lower than that of the comparative example 1 and also much lower than the national standard limit, which proves the environmental safety.

[0067] In summary, the technical solution provided by the present application is significantly better than the existing cement-based backfilling technology in terms of mechanical properties, environmental friendliness and economy (no cement is used at all). The present application successfully converts fly ash into a high-performance ecological cementitious material through deep modification, and innovatively combines it with coarse solid waste such as construction waste for large-scale surface backfilling and land value improvement, solving the two long-standing solid waste disposal problems in the industry, and having outstanding novelty, creativity and industrial applicability.

[0068] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent substitutions for part or all of the technical features; and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solutions of the inventive examples.

Claims

1. An eco-fill material prepared by modification of fly ash, characterized in that, The ecological filling material is prepared by mixing a modified fly ash binder with coarse solid waste aggregates; the modified fly ash binder is prepared from the following components by weight: fly ash: 100 parts; alkali activator: 15-35 parts; silico-alumina modifier: 5-25 parts; water: 20-50 parts; the weight ratio of the modified fly ash binder to the coarse solid waste aggregates in the ecological filling material is 1:1 to 1:

3.

2. Ecological filling material according to claim 1, characterized in that The alkali activator is one or a combination of two or more of sodium hydroxide, potassium hydroxide, sodium silicate and potassium silicate.

3. Ecological filling material according to claim 2, characterized in that The alkali activator is a composite of liquid sodium silicate and sodium hydroxide, and the molar ratio of silicon dioxide to sodium oxide in the prepared activator solution is 1.0-2.

0.

4. The eco-fill material of claim 1, wherein, The silico-alumina modifier is selected from one or a combination of two or more of ground slag, steel slag, red mud or coal gangue.

5. The eco-fill material of claim 1, wherein, The coarse solid waste aggregates are selected from one or a mixture of two or more of construction waste, boiler slag, waste concrete, waste bricks and tiles, and mining waste stones, and the particle size range is 5-80 mm.

6. The eco-fill material of claim 1, wherein, The modified fly ash binder further contains 0.1-2 parts by weight of an auxiliary agent, which is a water reducing agent or a retarder.

7. A method of producing the ecological fillers according to any one of claims 1-6, characterized in that, The method comprises the following steps: 1) mixing the solid alkali activator with the liquid alkali activator or water to prepare an alkali activator solution; 2) dry mixing the fly ash, the silico-alumina modifier and the optional auxiliary agent uniformly, then adding the alkali activator solution prepared in step 1) and water, and stirring to form a modified fly ash binder slurry; 3) mixing the modified fly ash binder slurry prepared in step 2) with the coarse solid waste aggregates uniformly to obtain the ecological filling material.

8. Use of the ecological filling material according to any one of claims 1-6 in the engineering of backfilling a depression, a pit or a mine pit to form an industrial land.

9. Use according to claim 8, characterized in that, The use makes the bearing capacity of the ground foundation after backfilling and solidification reach 150 kPa or more.

10. The eco-fill material of claim 1, wherein, The ecological filling material has an unconfined compressive strength of not less than 5 MPa after solidification at room temperature for 28 days, and the pH value of the solidified body is 7-9.

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