High-strength artificial wall sand and preparation method and application thereof

Abstract

The present application relates to the technical field of building materials, in particular to a kind of high-strength artificial wall sand and its preparation method and application.The present application forms base sand by sodium hexametaphosphate auxiliary cleaning, gradation reconstruction treatment mechanism sand;Then use dopamine and tris to construct biomimetic intermediate layer, and induce iron-calcium-silicon mineral precursor to form organic-inorganic hybrid coating, finally by calcium hydroxide mineralization curing and silane gas phase grafting;With low crushing index, high interfacial bonding strength and excellent impermeability, effectively solve the deep engineering interface loose, high pressure leakage and brittle cracking problem.The material is especially suitable for deep oil and gas cementing, water-rich stratum water curtain, submarine tunnel lining and nuclear waste geological disposal repository and other high water pressure, high confining pressure engineering field.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a high-strength artificial wall-forming sand, its preparation method, and its application. Background Technology

[0002] With the rapid development of deep underground engineering, oil and gas drilling and production, and geological disposal of nuclear waste, extremely stringent requirements have been placed on the structural integrity and impermeability of wellbore walls and diaphragm walls. In these complex geological environments with high confining pressure and high water pressure, the wall-forming materials not only bear the mechanical function of transmitting formation pressure and maintaining borehole wall stability, but also constitute the first line of defense against the migration of underground fluids.

[0003] Traditional wall-building processes primarily rely on natural river sand or ordinary manufactured sand as aggregates to formulate the cementing system. However, with the depletion of high-quality natural sand resources, the application of manufactured sand has become an inevitable trend. Currently, the widely used ordinary manufactured sand faces severe technical bottlenecks when used for high-strength artificial wall construction: First, significant defects in particle morphology and gradation. Manufactured sand produced by traditional crushing processes often contains a large number of needle-like and flaky particles, resulting in high porosity and a dumbbell-shaped gradation with large ends and a small middle. This makes the cementing system prone to segregation and bleeding during pumping, failing to form a dense, self-supporting skeleton. Second, weak interfacial bonding performance. The surface of manufactured sand is usually adhered to electrostatically adsorbed fine clay and stone powder. This weak medium not only significantly increases the system's water demand but also forms a loose, porous interfacial transition zone (ITZ) between the aggregate and cement paste. Under high-pressure grouting or deep stratum stress, these microcracks rapidly propagate and penetrate, leading to brittle fracture of the wall-building layer.

[0004] Furthermore, and more critically, there is a lack of anti-seepage and plugging capabilities. Existing wall-forming sand lacks an active pore-sealing mechanism. Under high water pressure, fluid is very likely to seep along the capillary channels and interface gaps on the aggregate surface, triggering piping effects and ultimately leading to well wall instability or pit water inrush accidents.

[0005] Therefore, there is an urgent need to develop a high-strength artificial wall-building sand that integrates a high-strength skeleton, dense packing, and active hydrophobic sealing function to meet the urgent needs of deep engineering for wall-building materials in extreme environments. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a high-strength artificial wall-building sand, its preparation method and application, so as to solve the problems of existing wall-building sand having discrete gradation, loose interface, which leads to easy leakage and cracking under high pressure and insufficient strength.

[0007] To achieve the above objectives, the present invention provides a method for preparing high-strength artificial wall-building sand, comprising the following steps:

[0008] S1: The manufactured sand is wetted, stirred, and settled to remove impurities, and then subjected to hydrocyclone classification to separate fine powder and retain the settled sand; the obtained settled sand is dewatered, shaped, scrubbed and dried to obtain dry sand; the obtained dry sand is screened and mixed to obtain base sand with continuous gradation.

[0009] S2: The base sand is placed in an aqueous sodium hydroxide solution and heated and stirred to remove surface oil and activate the surface by hydroxylation. After the treatment is completed, the waste liquid is discarded and the sand is washed to obtain wet base sand.

[0010] S3: Tris(hydroxymethyl)aminomethane, dopamine hydrochloride, and colloidal silica are added to a deionized water system and stirred under alkaline conditions to form a co-deposition mother liquor; then moistened base sand is added for surface deposition to obtain mixed sand material;

[0011] S4: Add ferric nitrate, calcium nitrate and sodium silicate to the mixed sand in sequence, and keep stirring and adjust the pH value during the addition process so that the mineral precursors grow in situ on the hybrid layer on the surface of the sand. After the reaction is completed, the composite coated sand is obtained by solid-liquid separation, washing and drying.

[0012] S5: The composite coated sand is placed in a nitrogen atmosphere for programmed temperature pyrolysis, which carbonizes the organic components and induces the inorganic components to crystallize and strengthen. After cooling, pyrolytic sand is obtained.

[0013] S6: Spray calcium hydroxide suspension on the surface of pyrolytic sand and place it in a humid carbon dioxide atmosphere for mineralization curing; then place the cured sand in a closed steam environment containing triethoxymethylsilane for gas phase grafting reaction, and after the reaction is completed, dry it to obtain high-strength artificial wall sand.

[0014] Preferably, sodium hexametaphosphate is added as a dispersant during the impurity removal process in step S1; the cutting particle size of the cyclone classification is preferably 70-80 μm.

[0015] Preferably, the gradation scheme of the base sand in step S1 is as follows: 10% of the particle size is 150-300μm, 20% is 300-600μm, 25% is 600-1180μm, 25% is 1180-2360μm, and 20% is 2360-4750μm.

[0016] Preferably, the ratio of the amount of manufactured sand and sodium hexametaphosphate crystals in step S1 is 100-120 kg: 270-330 g.

[0017] Preferably, the manufactured sand in step S1 is an aggregate product system of Anhui Conch Cement Co., Ltd., with limestone as the lithology, fineness modulus of 2.8-3.2 (medium-coarse sand), mud content (by mass) of 3.0%-5.0%, and stone powder content (by mass) of 8%-12%.

[0018] Preferably, the processing temperature in step S2 is 35-45℃ and the processing time is 12-18 min.

[0019] Preferably, the weight ratio of the base sand and sodium hydroxide in step S2 is 100kg:225-275g.

[0020] Preferably, the flow rate of carbon dioxide introduced in step S3 is 1.5-2.5 L / min.

[0021] Preferably, the temperature of the deposition reaction in step S3 is 20-30°C.

[0022] Preferably, the weight ratio of the moist base sand, tris(hydroxymethyl)aminomethane, dopamine hydrochloride and colloidal silica in step S3 is 100 kg: 33-39 g: 135-165 g: 1350-1650 g.

[0023] Preferably, in step S4, the weight ratio of the mixed sand, ferric nitrate, calcium nitrate, and sodium silicate is 125-130 kg: 45-55 g: 270-330 g: 180-220 g, and the ferric nitrate, calcium nitrate, and sodium silicate are added in the form of an aqueous solution.

[0024] Preferably, the ferric nitrate solution in step S4 is formed by dissolving ferric nitrate nonahydrate in deionized water.

[0025] Preferably, the calcium nitrate solution in step S4 is formed by dissolving calcium nitrate tetrahydrate in deionized water.

[0026] Preferably, the sodium silicate solution in step S4 is formed by dissolving sodium silicate in deionized water.

[0027] Preferably, in step S4, the pH of the system is adjusted to 8-9 before adding the calcium nitrate solution.

[0028] Preferably, the specific process of the programmed temperature rise pyrolysis in step S5 is to first raise the temperature to 140-160℃ at 4-6℃ / min and hold it for 25-35min, and then raise the temperature to 480-520℃ at 4-6℃ / min and hold it for 110-130min.

[0029] Preferably, the weight ratio of pyrolytic sand, calcium hydroxide and triethoxymethylsilane in step S6 is 100kg:90-110g:180-220g.

[0030] Preferably, the temperature of the gas-phase grafting reaction in step S6 is 75-85°C.

[0031] Furthermore, the present invention provides a high-strength artificial wall-building sand.

[0032] Furthermore, the present invention also provides an application of high-strength artificial wall-forming sand for use in extreme engineering fields of high water pressure and high confining pressure, such as deep oil and gas cementing, deep foundation pits in water-rich formations, and underground continuous walls.

[0033] The beneficial effects of this invention are:

[0034] This invention constructs a dense, dual impermeable barrier, significantly improving sealing reliability under high water pressure: through precise gradation reconstruction and shaping technology, needle-like particles are eliminated, and the micro-gradation filling effect blocks interconnected pores; simultaneously, through in-situ mineralization and carbonization curing technology, a dense mineral crystal layer is induced to grow on the surface of the sand particles, and silanization treatment is used to give it gradient hydrophobic properties. This structure not only physically seals the microcracks on the aggregate surface, but also chemically transforms the hydrophilic interface into a hydrophobic barrier layer, effectively inhibiting capillary penetration of high-pressure water and solving the problem of easy leakage in deep engineering wall layers.

[0035] This invention strengthens and toughens the interface transition zone, significantly improving early and long-term mechanical strength. By innovatively introducing dopamine and tris(hydroxymethyl)aminomethane to construct molecular bridges, highly active anchoring points are formed on the surface of inert sand particles. Subsequently, the stepwise addition of precursors induces the directional crystallization growth of iron-calcium-silicon mineral phases. This chemical bonding completely changes the traditional physical bonding mode between aggregates and cementitious materials, transforming the weak interface transition zone into a high-strength chemical bonding zone, effectively transferring stress, significantly improving the compressive and flexural strength of the wall layer, and preventing brittle cracking.

[0036] This invention endows aggregates with excellent crushing resistance and volume stability. Through a unique synergistic technology of pyrolytic carbon skeleton reinforcement and mineralization sealing, the carbonaceous skeleton formed by the organic precursor during pyrolysis fills and supports the micropores inside the sand grains, while the dense outer mineralized shell provides additional confining pressure protection. This core-shell structure significantly reduces the crushing index, enabling the wall-building sand to maintain its skeleton integrity even under high ground stress in deep formations, ensuring the long-term structural stability of the wellbore or wall.

[0037] This invention thoroughly removes harmful mud and electrostatically adsorbed fine powder adhering to the surface through deep cleaning and cyclone classification assisted by sodium hexametaphosphate, exposing a clean active surface. This not only eliminates the hidden dangers caused by mud swelling due to water absorption, but also significantly reduces the water demand of the cementing system, improves the fluidity and anti-segregation performance of the slurry, and meets the construction requirements of long-distance pumping and high-pressure injection.

[0038] In summary, the high-strength artificial wall-forming sand prepared by this invention breaks through the technical bottleneck of traditional manufactured sand in deep and complex geological environments. It integrates three core functions: high-strength skeleton support, dense gradation filling, and active hydrophobic sealing. It not only solves the fundamental defects of weak interface bonding and high pore connectivity, but also endows underground engineering structures with excellent impermeability and sealing performance and long-term mechanical stability. It is particularly suitable for major infrastructure construction fields with extreme requirements for zero leakage and high durability, such as deep oil and gas well cementing, deep foundation pit water-stop curtains in water-rich strata, submarine tunnel lining, and nuclear waste geological disposal sites. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0040] The sources or properties of the raw materials used in the embodiments and comparative examples of this invention are as follows:

[0041] Manufactured sand: aggregate product system of Anhui Conch Cement Co., Ltd., lithology is limestone, fineness modulus is 2.8-3.2 (medium-coarse sand), mud content (by mass) is 4%, stone powder content (by mass) is 10%; Sodium hexametaphosphate crystals: Sigma-Aldrich, product number 305553; Colloidal silica: Sigma-Aldrich, product number 420794, mass fraction 30wt%; Sodium silicate: Shanghai Maclean Biochemical Technology Co., Ltd., product number S871946.

[0042] Example 1: A method for preparing high-strength artificial wall-building sand, the specific steps of which are as follows:

[0043] (1) Take 100 kg of manufactured sand and add it to a forced mixing container. Add 80 kg of tap water and stir for 2 min to wet the entire sand. Then add 270 g of sodium hexametaphosphate crystals and continue stirring for 8 min. Let it stand for 1 min and then pour off the supernatant water. Repeat the above steps twice to remove the attached mud and weakly flocculated fine powder to obtain wet sand. Then transfer the wet sand to a hydrocyclone classifier and add 120 kg of tap water to form slurry. Set the hydrocyclone classifier to cut the particle size to 70 μm and separate continuously. Discard the overflow fine powder slurry, retain the sediment and rinse it with tap water. Dewater it through a vibrating dewatering screen until there is no apparent free water. Then The sand was shaped once in a vertical shaft impact shaping machine, then washed with 10 kg of tap water for 2 minutes in a scrubbing machine and immediately dehydrated and spread evenly. It was then dried at 105℃ to constant weight to obtain 100 kg of dry sand. The dry sand was then sieved and graded, and 10 kg of sand with a particle size of 150-300 μm, 20 kg of sand with a particle size of 300-600 μm, 25 kg of sand with a particle size of 600-1180 μm, 25 kg of sand with a particle size of 1180-2360 μm, and 20 kg of sand with a particle size of 2360-4750 μm were weighed and mixed for 5 minutes to obtain 100 kg of base sand with continuous gradation.

[0044] (2) Add 100 kg of base sand to the reactor, add 22.5 kg of deionized water and 225 g of sodium hydroxide, keep the temperature at 35°C and stir for 12 min, discard the alkaline solution, add 54 kg of deionized water and stir for 1 min, then discard the washing liquid to obtain wet base sand.

[0045] (3) Take another container, add 27 kg of deionized water and 33 g of tris(hydroxymethyl)aminomethane and stir until completely dissolved. Then add 135 g of dopamine hydrochloride and stir for 5 min. Then add 1350 g of colloidal silica and stir for 8 min. Adjust the pH of the mother liquor to 8 to form a co-deposition mother liquor. Then add 100 kg of moist base sand, control the system temperature at 20 °C and use a paddle stirrer to stir at 180 rpm for 25 min to obtain mixed sand.

[0046] (4) Take another 45g of ferric nitrate nonahydrate and dissolve it in 450g of deionized water to form ferric nitrate solution; dissolve 270g of calcium nitrate tetrahydrate in 2700g of deionized water to form calcium nitrate solution; dissolve 180g of sodium silicate in 3600g of deionized water to form sodium silicate solution; then add the ferric nitrate solution to 125kg of mixed sand in 1min, continue stirring at 180rpm for 8min, add sodium hydroxide to adjust the pH to 8, then add the calcium nitrate solution dropwise to the system in 8min and continue stirring for 8min, finally add the sodium silicate solution dropwise to the system in 12min and continue stirring for 25min; after the reaction is completed, filter the obtained product through a sieve and add deionized water to stir and wash 3 times to remove unreacted salts, dry at 105℃ to constant weight to obtain composite coated sand;

[0047] (5) Place 100 kg of composite coated sand in a tube furnace, introduce nitrogen as a protective gas and purge at room temperature for 25 min, then start heating, raise the temperature to 140℃ at 4℃ / min and hold for 25 min, raise the temperature to 480℃ at 4℃ / min and hold for 110 min, and then cool naturally to room temperature under nitrogen protection to obtain pyrolytic sand.

[0048] (6) Disperse 90g of calcium hydroxide in 2700g of deionized water and stir for 4min to form a calcium hydroxide suspension. Spread 100kg of pyrolytic sand and spray the calcium hydroxide suspension in 3 times while stirring. After the spraying is completed, seal the sand and let it stand for 50min. Then transfer it to a closed carbonization box. Place deionized water at the bottom of the box to maintain the relative humidity inside the box. Introduce 540L of carbon dioxide at a flow rate of 4.5L / min and react for 110min. After the reaction is completed, take it out and dry it under hot air at 55℃ for 50min. After drying, transfer the product to a closed silanization box and mix it evenly. Place 180g of triethoxymethylsilane in an open glass dish in the box, heat it to 75℃ and maintain it for 110min so that the silane diffuses into the sand layer in the vapor phase. After the reaction is completed, take out the sand and dry it at 105℃ for 110min to obtain the finished product, which is high-strength artificial wall sand.

[0049] Example 2: A method for preparing high-strength artificial wall-building sand, the specific steps of which are as follows:

[0050] (1) Take 110 kg of manufactured sand and add it to a forced mixing container. Add 80 kg of tap water and stir for 2 min to wet the entire sand. Then add 300 g of sodium hexametaphosphate crystals and continue stirring for 10 min. Let it stand for 2 min and then pour off the supernatant water. Repeat the above steps twice to remove the attached mud and weakly flocculated fine powder to obtain wet sand. Then transfer the wet sand to a hydrocyclone classifier and add 120 kg of tap water to form slurry. Set the hydrocyclone classifier to cut the particle size to 75 μm and separate continuously. Discard the overflow fine powder slurry, retain the sediment and rinse it with tap water. Dewater it through a vibrating dewatering screen until there is no apparent free water. Then The sand was shaped once in a vertical shaft impact shaping machine, then washed with 10 kg of tap water for 3 minutes in a scrubbing machine and immediately dehydrated and spread evenly. It was then dried at 105℃ to constant weight to obtain 100 kg of dry sand. The dry sand was then sieved and graded, and 10 kg of sand with a particle size of 150-300 μm, 20 kg of sand with a particle size of 300-600 μm, 25 kg of sand with a particle size of 600-1180 μm, 25 kg of sand with a particle size of 1180-2360 μm, and 20 kg of sand with a particle size of 2360-4750 μm were weighed and mixed for 5 minutes to obtain 100 kg of base sand with continuous gradation.

[0051] (2) Add 100kg of base sand to the reactor, add 25kg of deionized water and 250g of sodium hydroxide, keep the temperature at 40℃ and stir for 15min, discard the alkaline solution, add 60kg of deionized water and stir for 2min, discard the washing liquid to obtain wet base sand.

[0052] (3) Take another container, add 30kg of deionized water and 36g of tris(hydroxymethyl)aminomethane and stir until completely dissolved. Then add 150g of dopamine hydrochloride and stir for 5 minutes. Then add 1500g of colloidal silica and stir for 10 minutes. Adjust the pH of the mother liquor to 9 to form a co-deposition mother liquor. Then add 100kg of moist base sand, control the system temperature to 25℃ and use a paddle mixer to stir at 200rpm for 30 minutes to obtain mixed sand.

[0053] (4) Take another 50g of ferric nitrate nonahydrate and dissolve it in 500g of deionized water to form ferric nitrate solution; dissolve 300g of calcium nitrate tetrahydrate in 3000g of deionized water to form calcium nitrate solution; dissolve 200g of sodium silicate in 4000g of deionized water to form sodium silicate solution; then add the ferric nitrate solution to 127kg of mixed sand in 1min, continue stirring at 200rpm for 10min, then add sodium hydroxide to adjust the pH to 9, then add the calcium nitrate solution dropwise to the system in 10min and continue stirring for 10min, finally add the sodium silicate solution dropwise to the system in 15min and continue stirring for 30min; after the reaction is completed, filter the obtained product through a sieve and add deionized water to stir and wash 3 times to remove unreacted salts, and dry at 105℃ to constant weight to obtain composite coated sand;

[0054] (5) Place 100 kg of composite coated sand in a tube furnace, introduce nitrogen as a protective gas and purge at room temperature for 30 min, then start heating, raise the temperature to 150℃ at 5℃ / min and hold for 30 min, raise the temperature to 500℃ at 5℃ / min and hold for 120 min, and then cool naturally to room temperature under nitrogen protection to obtain pyrolytic sand.

[0055] (6) Disperse 100g of calcium hydroxide in 3000g of deionized water and stir for 5min to form a calcium hydroxide suspension. Spread 100kg of pyrolytic sand and spray the calcium hydroxide suspension in 3 times while stirring. After the spraying is completed, seal the sand and let it stand for 60min. Then transfer it to a closed carbonization box. Place deionized water at the bottom of the box to maintain the relative humidity inside the box. Pass 600L of carbon dioxide at a flow rate of 5L / min and react for 120min. After the reaction is completed, take it out and dry it under hot air at 60℃ for 60min. After drying, transfer the product to a closed silanization box and mix it evenly. Place 200g of triethoxymethylsilane in an open glass dish in the box, heat it to 80℃ and maintain it for 120min so that the silane diffuses into the sand layer in the vapor phase. After the reaction is completed, take out the sand and dry it at 105℃ for 110-130min to obtain the finished product, which is high-strength artificial wall sand.

[0056] Example 3: A method for preparing high-strength artificial wall-building sand, the specific steps of which are as follows:

[0057] (1) Take 120 kg of manufactured sand and add it to a forced mixing container. Add 80 kg of tap water and stir for 2 min to wet the entire sand. Then add 330 g of sodium hexametaphosphate crystals and continue stirring for 12 min. Let it stand for 3 min and then pour off the supernatant water. Repeat the above steps twice to remove the attached mud and weakly flocculated fine powder to obtain wet sand. Then transfer the wet sand to a hydrocyclone classifier and add 120 kg of tap water to form slurry. Set the hydrocyclone classifier to cut the particle size to 80 μm and separate continuously. Discard the overflow fine powder slurry, retain the sediment and rinse it with tap water. Dewater it through a vibrating dewatering screen until there is no apparent free water. Then The sand was shaped once in a vertical shaft impact shaping machine, then washed with 10 kg of tap water for 4 minutes in a scrubbing machine and immediately dehydrated and spread evenly. It was then dried at 105℃ to constant weight to obtain 100 kg of dry sand. The dry sand was then sieved and graded, and 10 kg of sand with a particle size of 150-300 μm, 20 kg of sand with a particle size of 300-600 μm, 25 kg of sand with a particle size of 600-1180 μm, 25 kg of sand with a particle size of 1180-2360 μm, and 20 kg of sand with a particle size of 2360-4750 μm were weighed and mixed for 5 minutes to obtain 100 kg of base sand with continuous gradation.

[0058] (2) Add 100 kg of base sand to the reactor, add 27.5 kg of deionized water and 275 g of sodium hydroxide, keep the temperature at 45 °C and stir for 18 min, discard the alkaline solution, add 66 kg of deionized water and stir for 3 min, discard the washing liquid to obtain wet base sand.

[0059] (3) Take another container, add 33kg of deionized water and 39g of tris(hydroxymethyl)aminomethane and stir until completely dissolved. Then add 165g of dopamine hydrochloride and stir for 5 minutes. Then add 1650g of colloidal silica and stir for 12 minutes. Adjust the pH of the mother liquor to 9 to form a co-deposition mother liquor. Then add 100kg of moist base sand, control the system temperature to 30℃ and use a paddle stirrer to stir at 220rpm for 35 minutes to obtain mixed sand.

[0060] (4) Take another 55g of ferric nitrate nonahydrate and dissolve it in 550g of deionized water to form ferric nitrate solution; dissolve 330g of calcium nitrate tetrahydrate in 3300g of deionized water to form calcium nitrate solution; dissolve 220g of sodium silicate in 4400g of deionized water to form sodium silicate solution; then add the ferric nitrate solution to 130kg of mixed sand in 1min, continue stirring at 220rpm for 12min, then add sodium hydroxide to adjust the pH to 9, then add the calcium nitrate solution dropwise to the system in 12min and continue stirring for 12min, finally add the sodium silicate solution dropwise to the system in 18min and continue stirring for 35min; after the reaction is completed, filter the obtained product through a sieve and add deionized water to stir and wash 3 times to remove unreacted salts, and dry at 105℃ to constant weight to obtain composite coated sand;

[0061] (5) Place 100 kg of composite coated sand in a tube furnace, introduce nitrogen as a protective gas and purge at room temperature for 35 min, then start heating, raise the temperature to 160℃ at 6℃ / min and hold for 35 min, then raise the temperature to 520℃ at 6℃ / min and hold for 130 min, and then cool naturally to room temperature under nitrogen protection to obtain pyrolytic sand.

[0062] (6) Disperse 110g of calcium hydroxide in 3300g of deionized water and stir for 6min to form a calcium hydroxide suspension. Spread 100kg of pyrolytic sand and spray the calcium hydroxide suspension in 3 times while stirring. After spraying, seal the sand and let it stand for 70min. Then transfer it to a closed carbonization box. Place deionized water at the bottom of the box to maintain the relative humidity inside the box. Introduce 660L of carbon dioxide at a flow rate of 5.5L / min and react for 130min. After the reaction is completed, take it out and dry it under hot air at 65℃ for 70min. After drying, transfer the product to a closed silanization box and mix it evenly. Place 220g of triethoxymethylsilane in an open glass dish in the box, heat it to 85℃ and maintain it for 130min so that the silane diffuses into the sand layer in the vapor phase. After the reaction is completed, take out the sand and dry it at 105℃ for 130min to obtain the finished product, which is high-strength artificial wall sand.

[0063] Comparative Example 1: The difference from Example 2 is that sodium hexametaphosphate crystals are not added in step (1), while the remaining steps are the same as in Example 2.

[0064] Comparative Example 2: The difference from Example 2 is that the shaping and wiping operations are omitted in step (1), while the remaining steps are the same as in Example 2.

[0065] Comparative Example 3: The difference from Example 2 is that in step (1), no screening, grading and mixing operations are performed. Instead, dehydrated and dried single-size sand is used directly as the base sand. The remaining steps are the same as in Example 2.

[0066] Comparative Example 4: The difference from Example 2 is that in step (3), only colloidal silica is added for single deposition, and tris(hydroxymethyl)aminomethane and dopamine hydrochloride are not added. The remaining steps are the same as in Example 2.

[0067] Comparative Example 5: The difference from Example 2 is that in step (4), ferric nitrate, calcium nitrate, and sodium silicate are mixed and added at the same time, while the other steps are the same as in Example 2.

[0068] Comparative Example 6: The difference from Example 2 is that in step (4), sodium silicate solution is added first, followed by calcium nitrate solution, while the remaining steps are the same as in Example 2.

[0069] Comparative Example 7: The difference from Example 2 is that only triethoxymethylsilane liquid is used for gas-phase grafting in step (6), while the other steps are the same as in Example 2.

[0070] Performance testing

[0071] All sand samples from Examples 1-3 and Comparative Examples 1-7 were dried to constant weight in an oven at 105°C before testing, and then cooled to room temperature in a desiccator; each sample was subjected to at least 3 parallel tests, and the results were taken as the arithmetic mean.

[0072] Particle crushing strength: The crushing index of sand in Appendix C of GB / T 14685-2022 "Construction Gravel and Crushed Stone" was tested. 500g of finished sand from Examples 1-3 and Comparative Examples 1-7 were dried at 105℃ to constant weight and cooled to room temperature. The sand was then passed through a 2.36mm sieve, and sand particles with a diameter of 1.18-2.36mm were collected as test samples. 150g of the sample was loaded into a steel mold with an inner diameter of 77mm in three portions (after each loading, it was tamped 30 times with a tamping rod). The mold was placed in a press and pressurized to 400kN at a rate of 200N / s and held for 5min. The pressure was then released, and particles that passed through a 0.6mm sieve were removed. The crushing index value was calculated.

[0073] Gradation stability: According to Clause 6.3 of GB / T 14684-2022 "Construction Gravel and Crushed Stone", 500g of finished sand from Examples 1-3 and Comparative Examples 1-7 were added to 1.5L of tap water and stirred for 10min (300rpm). After standing for 5min, the middle slurry was taken and passed through a 75μm wet sieve. The material on the sieve was dried and weighed, and the stone powder content after wet sieving was calculated.

[0074] Mortar strength and interfacial microhardness testing: Tests were conducted according to GB / T 17671-2021 "Test Method for Strength of Cement Mortar".

[0075] Mortar strength: The finished sand from Examples 1-3 and Comparative Examples 1-7 was mixed with PO 42.5 cement at a mass ratio of 3:1 and a water-cement ratio of 0.5. After mixing in a planetary mixer, the mixture was compacted in a 40mm×40mm×160mm triple mold and cured under standard conditions for 3 days and 28 days. The compressive strength was then determined using a flexural and compressive strength testing machine.

[0076] Interface hardness: 28-day-old test blocks were selected, cut, inlaid with epoxy resin, and polished to make smooth sheets. A Vickers microhardness tester was used to apply a load of 0.49 N and hold the load for 10 s. Starting from the edge of the sand grains, a test point was selected every 10 μm towards the cement stone. The microhardness values ​​within a 50 μm range were continuously tested, and the average value was taken to characterize the interface strength.

[0077] High-pressure impermeability test: The test was conducted according to Chapter 6 (Water permeability test - stepwise pressure method) of GB / T 50082-2024 "Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete". C30 concrete was prepared by replacing ordinary sand with finished sand from Examples 1-3 and Comparative Examples 1-7. Frustum specimens with a top diameter of 175 mm, a bottom diameter of 185 mm, and a height of 150 mm were made, with 6 specimens per group. After standard curing for 28 days, a sealant was applied and the specimens were placed in an HS-4 type impermeability tester. The initial water pressure was set to 0.1 MPa, and the water pressure was increased by 0.1 MPa every 8 hours until water seepage was observed on the end face of 3 out of 6 specimens. The test was then stopped, and the pressure value H was recorded. The impermeability grade P = 10H-1 was calculated.

[0078] The test results are shown in Table 1.

[0079] Table 1 Performance Test Results

[0080] Crushing index (%) Stone powder content on wet sieve (%) 3D compressive strength (MPa) 28-day compressive strength (MPa) 28d interfacial hardness (HV) Permeability grade (P) Example 1 8.5 4.2 28.7 45.3 68.3 P10 Example 2 7.8 3.9 30.5 48.7 72.6 P10 Example 3 8.9 4.5 27.9 44.6 66.8 P10 Comparative Example 1 15.7 8.3 20.3 35.1 45.2 P6 Comparative Example 2 13.4 6.7 23.6 38.9 52.7 P8 Comparative Example 3 12.1 7.2 22.1 37.5 48.9 P7 Comparative Example 4 11.2 5.1 24.8 42.8 51.3 P8 Comparative Example 5 14.3 6.5 22.9 40.1 49.8 P7 Comparative Example 6 18.5 5.8 21.5 36.2 47.1 P6 Comparative Example 7 10.5 4.8 26.3 43.5 58.4 P5

[0081] Data Analysis: As can be seen from the data in Examples 1-3 in Table 1, the high-strength artificial wall-building sand prepared by this invention exhibits excellent anti-crushing properties, low dust content, high early and later strength, good interfacial bonding ability, and high impermeability. It demonstrates outstanding comprehensive advantages in all key performance indicators, reflecting the significant effectiveness of the combined process of particle size distribution reconstruction, in-situ surface mineralization, and pyrolysis carbonaceous skeleton reinforcement. This improvement in comprehensive performance may be attributed to the multi-step synergistic optimization process in this invention: Sodium hexametaphosphate dispersion washing and cyclone classification effectively remove mud and fine powder, reducing water demand; shaping and scrubbing operations improve particle morphology, reducing needle-like and flaky particles; particle size reconstruction optimizes particle packing density; polydopamine / colloidal silica co-deposition and subsequent mineralization coating form a strong and tough interfacial layer; gradient pyrolysis and mineralization curing further seal pores and enhance hydrophobicity. The combination of these technical features enables sand particles to form a dense interfacial transition zone in the cementing system, thereby maintaining structural integrity and impermeability stability under high pressure. This successfully solves the leakage problem of well casing or foundation pit wall layers under high water pressure, achieving a dual breakthrough in mechanical strength and durability.

[0082] As can be seen from the data in Example 2 and Comparative Example 1 in Table 1, the sodium hexametaphosphate-assisted deep cleaning process is a prerequisite for ensuring the interfacial bonding performance of the wall-building sand. Its absence will lead to a comprehensive decline in the material's impermeability and mechanical properties. It is speculated that the sodium hexametaphosphate dispersion washing step effectively removes the mud coating on the surface of the sand particles, exposing the active hydroxyl groups, providing a good foundation for the subsequent coating reaction. At the same time, it reduces the increase in water demand and interfacial weakening caused by mud content, thereby significantly improving the mechanical properties and impermeability of the wall-building sand.

[0083] As can be seen from the data in Example 2 and Comparative Example 2 in Table 1, the shaping and scrubbing processes have a decisive impact on improving particle geometry and reducing packing porosity, directly affecting the overall density and compressive stability of the wall layer. This is presumably because unshaped manufactured sand contains a large number of needle-like and flaky particles. These irregular particles are prone to stress concentration and fracture under pressure, thus compromising the stability of the aggregate skeleton. Furthermore, the bridging effect of these needle-like and flaky particles leads to a large number of ineffective pores in the packing system that cannot be filled by the slurry. This not only reduces the density of the concrete but also provides a convenient channel for high-pressure water penetration.

[0084] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, constructing a continuous gradation system is a key means to block water seepage in artificial wall construction. Single-size particle packing cannot meet the technical requirements of a high-permeability wall layer. Through precise sieving and recombination, the micro-gradation filling effect of particles of different sizes is utilized—large particles construct the skeleton, medium particles fill the voids, and fine powder further densifies the micropores—forming a dense-suspended or skeleton-dense structure. This physically dense packing, combined with the hydrophobic mineralization layer on the surface, forms a dual defense of physical filling and chemical barrier, thereby achieving a qualitative leap in permeability resistance while ensuring high strength.

[0085] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, the biomimetic intermediate layer constructed from dopamine and tris(hydroxymethyl)aminomethane plays an irreplaceable molecular bridging role in the organic-inorganic composite system. Its absence will lead to coating layer peeling and interface failure. It is speculated that this is because the use of colloidal silica as a single deposition method, lacking the bridging effect of polydopamine, results in insufficient adhesion between the coating layer and the sand grain surface, a weak interfacial transition zone, and affects the overall performance.

[0086] As can be seen from the data in Example 2 and Comparative Example 5 in Table 1, the stepwise dripping process of the precursor solution is key to controlling the growth kinetics of mineral crystals and forming a dense core-shell structure, while the one-pot mixing method leads to defects in the modified layer structure. It is speculated that this is because the stepwise dripping strategy ensures that each mineral layer grows densely on the basis of the previous layer, thereby constructing a truly effective protective barrier. A loose coating layer cannot provide effective confining pressure support (leading to a high crushing index) nor can it effectively seal the microcracks on the surface of sand particles (leading to poor impermeability).

[0087] As can be seen from the data in Example 2 and Comparative Example 6 in Table 1, a specific iron-calcium-silicon feeding sequence is crucial for inducing the directional crystallization of high-strength mineral phases; reversing the order will destroy the mechanical integrity of the mineralized layer. It is speculated that this is because the ordered feeding utilizes the coordination cross-linking of iron ions and the gradual adsorption of calcium ions, ultimately forming a dense crystal layer tightly bound to the matrix through the infiltration reaction of sodium silicate, thus providing effective reinforcement.

[0088] As can be seen from the data in Example 2 and Comparative Example 7 in Table 1, the carbon dioxide mineralization curing step is the decisive step in achieving the water-blocking function of the wall-forming sand. Although simple pyrolysis carbonization can improve the strength of the skeleton, it cannot solve the seepage problem. It is speculated that this is because the mineralization curing step was omitted, and only silanization treatment was used. Although some of the carbon skeleton strength was retained, the pores were not effectively sealed, resulting in a significant decrease in seepage resistance and failure to meet the wall-forming requirements under high water pressure.

[0089] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing high-strength artificial wall-building sand, characterized in that, Includes the following steps: S1: The manufactured sand is wetted, stirred and settled to remove impurities, and sodium hexametaphosphate crystals are added during the process. After being treated by cyclone classification, the settled sand is retained. The settled sand is then dehydrated, shaped, scrubbed and dried to obtain dry sand. Finally, after screening and mixing, the base sand with continuous gradation is obtained. S2: The base sand is placed in an aqueous sodium hydroxide solution and heated and stirred to obtain moistened base sand; S3: Tris(hydroxymethyl)aminomethane, dopamine hydrochloride, and colloidal silica are added to a deionized water system and stirred under alkaline conditions to form a co-deposition mother liquor; then moistened base sand is added to obtain mixed sand material; S4: Ferric nitrate, calcium nitrate and sodium silicate are added to the mixed sand in sequence, and a composite coated sand is formed after in-situ growth reaction; S5: The composite coated sand is subjected to programmed temperature pyrolysis to obtain pyrolytic sand; S6: Spray calcium hydroxide suspension onto the surface of pyrolytic sand and place it in a humid carbon dioxide atmosphere for mineralization curing; then place the cured sand in a closed vapor environment containing triethoxymethylsilane for gas-phase grafting reaction to form high-strength artificial wall-building sand.

2. The preparation method according to claim 1, characterized in that, The gradation scheme of the base sand in step S1 is as follows: 10% of the particles are 150-300μm, 20% are 300-600μm, 25% are 600-1180μm, 25% are 1180-2360μm, and 20% are 2360-4750μm.

3. The preparation method according to claim 1, characterized in that, The ratio of manufactured sand to sodium hexametaphosphate crystals in step S1 is 100-120 kg: 270-330 g.

4. The preparation method according to claim 1, characterized in that, The weight ratio of the base sand and sodium hydroxide in step S2 is 100kg:225-275g.

5. The preparation method according to claim 1, characterized in that, The weight ratio of the moist base sand, tris(hydroxymethyl)aminomethane, dopamine hydrochloride and colloidal silica in step S3 is 100kg:33-39g:135-165g:1350-1650g.

6. The preparation method according to claim 1, characterized in that, The weight ratio of the mixed sand, ferric nitrate, calcium nitrate and sodium silicate in step S4 is 125-130 kg: 45-55 g: 270-330 g: 180-220 g.

7. The preparation method according to claim 1, characterized in that, The specific process of the programmed temperature rise pyrolysis in step S5 is to first raise the temperature to 140-160℃ at 4-6℃ / min and hold it for 25-35min, and then raise the temperature to 480-520℃ at 4-6℃ / min and hold it for 110-130min.

8. The preparation method according to claim 1, characterized in that, The weight ratio of pyrolytic sand, calcium hydroxide and triethoxymethylsilane in step S6 is 100kg:90-110g:180-220g.

9. A high-strength artificial wall-building sand, characterized in that, It is prepared according to any one of claims 1-8.

10. An application of the high-strength artificial wall-building sand according to claim 9, characterized in that, It is used in extreme engineering fields with high water pressure and high confining pressure, such as deep oil and gas cementing, deep foundation pits in water-rich formations, and underground continuous walls.