Desulfurized gypsum-fly ash based high durability road base inorganic cementing material and preparation method thereof

Abstract

This invention relates to the field of road materials technology, specifically to a desulfurized gypsum-fly ash-based high-durability road base inorganic cementitious material and its preparation method. The material comprises, by weight, 6200-7100 parts modified fly ash, 200-500 parts silicate cement, 800-1200 parts calcium hydroxide, and 1500-2500 parts flue gas desulfurization gypsum. The modified fly ash is pre-activated by a composite activation solution, then sequentially treated with calcium hydroxide, sodium hexametaphosphate, and calcium nitrate tetrahydrate to construct calcium-phosphorus reaction sites. A sodium silicate solution is then added dropwise to form a hydrated calcium silicate nucleation coating. Finally, sodium tripolyphosphate is added, followed by aging and drying to obtain the final product. This invention effectively improves the surface activity and nucleation efficiency of fly ash, giving the material the advantages of high early strength, strong water resistance, and good freeze-thaw stability, making it particularly suitable for high-humidity areas and heavy-load road bases.

Description

Technical Field

[0001] This invention relates to the field of road materials technology, and in particular to a desulfurized gypsum-fly ash based high-durability road base inorganic cementing material and its preparation method. Background Technology

[0002] In the field of road base materials, the resource utilization of desulfurized gypsum and fly ash as industrial solid waste has become an important research direction. However, the stable glassy phase structure and strong surface inertness of fly ash result in a slow dissolution rate in hydration systems and limited early participation in reactions. Existing technologies typically employ physical mixing or simple alkali activation methods to attempt to enhance the activity of fly ash, but these methods are insufficient to effectively disrupt the inert layer on its glassy surface and cannot construct continuous and controllable reaction sites on the particle surface. Consequently, the early hydration product generation in the cementing system is insufficient, the interfacial transition zone structure is loose, and the initial bearing capacity of the compacted base layer is low, making it difficult to meet the requirements of heavy traffic for early opening.

[0003] Furthermore, even if early strength is improved by increasing cement content or introducing additional hydration product nuclei, problems such as concentrated hydration heat and increased shrinkage stress are easily caused. In humid or alternating wet and dry environments, the base material is prone to softening and peeling. Especially in harsh conditions such as high humidity areas or heavy-load roads in port areas, the water stability and freeze-thaw resistance of existing materials are often insufficient, resulting in significant strength fluctuations and affecting long-term service safety. In addition, if chemical activators are used to forcibly accelerate the reaction, although strength can be improved in the short term, it may lead to uneven distribution of hydration products and coarsening of the pore structure, which in turn weakens durability.

[0004] Existing modification technologies mostly focus on macroscopic ratio adjustments or single activation methods, failing to address the root cause by controlling the surface characteristics of fly ash particles to achieve a progressive deepening of directional activation, nucleation, and stabilization. The fundamental contradiction lies in the mismatch between the surface inertness of fly ash and the required hydration reaction rate, as well as the difficulty in synergizing early strength with long-term stability. Therefore, there is an urgent need for a technical approach that can fundamentally alter the surface reaction characteristics of fly ash and maintain its high durability under complex environments. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose an inorganic cementing material for high-durability road base courses based on desulfurized gypsum and fly ash, and its preparation method, so as to solve the problem that existing desulfurized gypsum-fly ash based road base courses suffer from slow dissolution of the glass phase of fly ash and strong surface inertness, which leads to limited early hydration reaction, insufficient early load-bearing capacity after compaction, and significant softening upon contact with water, making it difficult to meet the synergistic requirements of early strength and high durability for heavy-duty traffic base courses.

[0006] To achieve the above objectives, the present invention provides a desulfurized gypsum-fly ash based high-durability road base inorganic cementing material, comprising, by weight: 6200-7100 parts modified fly ash, 200-500 parts silicate cement, 800-1200 parts calcium hydroxide and 1500-2500 parts flue gas desulfurization gypsum. The modified fly ash is prepared by the following steps: the fly ash is pre-activated and filtered and washed using a composite activation solution containing anhydrous sodium sulfate and sodium hydroxide; the pre-activated fly ash wet filter cake is dispersed in water and then calcium hydroxide slurry, sodium hexametaphosphate solution and calcium nitrate tetrahydrate solution are added in sequence to construct calcium-phosphorus reaction sites; then sodium silicate solution dilution is added dropwise to the obtained fly ash slurry to construct a hydrated calcium silicate nucleation coating in situ, and sodium tripolyphosphate solution is added after the sodium silicate solution dilution is added. After static aging, filtration, washing, drying, grinding and sieving, the modified fly ash is obtained.

[0007] Preferably, the composite activation solution is prepared by mixing 7600-8200 parts of deionized water, 1000-1200 parts of anhydrous sodium sulfate and 40-80 parts of sodium hydroxide per 10000 parts of fly ash.

[0008] Preferably, the pre-activation is performed by adding fly ash in three portions at 65-75°C and stirring for 30-50 minutes.

[0009] Preferably, per 10,000 parts of fly ash, the calcium hydroxide slurry is obtained by mixing 1,500-2,000 parts of deionized water with 150-250 parts of calcium hydroxide; the sodium hexametaphosphate solution is obtained by mixing 12-20 parts of sodium hexametaphosphate with 250-350 parts of deionized water; and the calcium nitrate tetrahydrate solution is obtained by mixing 80-140 parts of calcium nitrate tetrahydrate with 150-250 parts of deionized water.

[0010] Preferably, per 10,000 parts of fly ash, the sodium silicate solution dilution is obtained by mixing 450-750 parts of sodium silicate solution with 300-500 parts of deionized water; the sodium tripolyphosphate solution is obtained by mixing 6-12 parts of sodium tripolyphosphate with 150-250 parts of deionized water.

[0011] Preferably, the sodium silicate solution contains 10%-12% Na2O and 26%-28% SiO2 by mass percentage.

[0012] Preferably, the static aging is carried out at 38-45℃ for 1.5-3 hours.

[0013] Preferably, the modified fly ash is ground and passed through a 250μm-350μm sieve.

[0014] Preferably, the fly ash used to prepare the modified fly ash meets the requirements of Class II fly ash specified in GB / T1596.

[0015] Preferably, the flue gas desulfurization gypsum meets the requirements for flue gas desulfurization gypsum for building materials as specified in GB / T37785.

[0016] Preferably, the silicate cement is ordinary silicate cement P·O42.5.

[0017] Furthermore, the present invention also provides a method for preparing a desulfurized gypsum-fly ash-based high-durability road base inorganic cementing material, comprising the following steps: mixing the modified fly ash, silicate cement, calcium hydroxide and flue gas desulfurization gypsum to obtain the desulfurized gypsum-fly ash-based high-durability road base inorganic cementing material.

[0018] The beneficial effects of this invention are: This invention pretreats fly ash with a composite activation solution under moderate heating and stirring conditions, significantly promoting the depolymerization of its glassy network structure and the dissolution of active silica-alumina components, providing abundant precursors for subsequent hydration reactions. This step transforms the fly ash surface from an inert to a reactive state, laying the foundation for subsequent functionalization modifications.

[0019] Calcium hydroxide slurry and sodium hexametaphosphate are sequentially introduced onto the surface of fly ash to form stable calcium-phosphorus anchoring points. Then, by adding calcium nitrate tetrahydrate, an intermediate phase of calcium phosphate is generated in situ at these anchoring points. This intermediate phase not only provides a high-density calcium source but also serves as a heterogeneous nucleation substrate, enabling the subsequent sodium silicate solution to uniformly construct a nanoscale hydrated calcium silicate nucleation coating on the fly ash particle surface. This achieves spatial binding between the nucleation function and the particle surface, effectively preventing the agglomeration or migration loss of the nucleating agent.

[0020] Sodium silicate solution is added slowly dropwise to ensure controllable nucleation of hydrated calcium silicate and a uniform and dense coating distribution. This structure is not easily damaged during mixing and compaction, and in humid environments, it can continuously promote the development of hydration products towards low porosity and high density, thereby improving the early strength and water resistance of the base material.

[0021] Sodium tripolyphosphate was introduced after the nucleation coating was formed. Its electrochemical regulation suppressed homogeneous precipitation in the solution, reduced free salt residue, and optimized the connectivity of the pore structure. This timing arrangement avoided premature complexation interference of calcium ions by polyphosphate, and instead achieved a synergistic benefit of coating homogenization, free salt control, and early strength enhancement.

[0022] The resulting cementing material forms a continuous cementing network in the hardened body with fly ash particles as the core and hydration products as the shell. The interface transition zone is strengthened and the pores are refined, so that it can maintain a high strength retention rate and volume stability under freeze-thaw cycles and alternating wet and dry environments. It is especially suitable for the long-term service requirements of the base course of heavy-duty roads in high-humidity areas, municipal roads and port areas. Detailed Implementation

[0023] 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.

[0024] Example 1: The fly ash used in this embodiment was purchased from Rizhao Iron & Steel Holding Group Co., Ltd. (a building powder). Its loss on ignition, water requirement ratio, and fineness met the requirements for Grade II fly ash as specified in GB / T1596. The flue gas desulfurization gypsum was purchased from Rizhao Iron & Steel Holding Group Co., Ltd. (labeled W5). Its calcium sulfate dihydrate content, moisture content, and harmful ion index met the requirements for flue gas desulfurization gypsum for building materials as specified in GB / T37785. The silicate cement used was Anhui Conch Cement Co., Ltd.'s Conch brand ordinary silicate cement P·O42.5. The sodium silicate solution (water glass) used was Sigma-Aldrich product number 338443 (approximately 11% Na2O and 27% SiO2 by mass fraction, with a SiO2 to Na2O mass ratio of approximately 5:2). Step S1: Weigh 7800g of deionized water, 1100g of anhydrous sodium sulfate, and 60g of sodium hydroxide and add them to a stirring container. Stir at 600r / min for 10min at 70℃ to obtain a composite activation solution. While maintaining the solution temperature at 70℃, add 10000g of fly ash in three portions and continue stirring at 600r / min for 40min to form a uniform slurry. Immediately afterward, vacuum filter to obtain a pre-activated fly ash filter cake, and rinse twice with deionized water to obtain a pre-activated fly ash wet filter cake. Step S2: Take the pre-activated fly ash wet filter cake obtained in step S1 and add 2000g of deionized water. Stir at 600r / min for 5min at 30℃. Separately, take 1800g of deionized water and 200g of calcium hydroxide to premix and prepare calcium hydroxide slurry. Add the calcium hydroxide slurry to the fly ash dispersion slurry all at once and continue stirring at 600r / min for 20min. Then, dissolve 15g of sodium hexametaphosphate in 300g of deionized water, add it slowly and stir for 10min. Then, dissolve 100g of calcium nitrate tetrahydrate in 200g of deionized water, add it dropwise and continue stirring for 10min to obtain fly ash slurry with calcium-phosphorus reaction sites enhanced. Step S3: Heat the fly ash slurry obtained in Step S2 to 35℃ and maintain a stirring speed of 800 r / min; premix and dilute 600 g of sodium silicate solution with 400 g of deionized water, and then add the sodium silicate solution dilution evenly over 20 min by dropwise addition. After the dropwise addition is completed, continue stirring for 30 min; then dissolve 8 g of sodium tripolyphosphate in 200 g of deionized water, add it while continuing to stir and stir for 10 min; then let the slurry stand and age at 40℃ for 2 h; after aging, vacuum filter to obtain modified fly ash wet filter cake, and rinse once with deionized water; place the wet filter cake in a 65℃ forced-air drying oven to dry to constant weight, cool, grind and pass through a 300 μm sieve to obtain modified fly ash; Step S4: Weigh 6700g of modified fly ash, 300g of silicate cement, 1000g of calcium hydroxide, and 2000g of flue gas desulfurization gypsum and add them to a dry mixer in sequence. First, dry mix 6700g of modified fly ash and 300g of ordinary silicate cement for 2 minutes, then add 1000g of calcium hydroxide and dry mix for 2 minutes, and finally add 2000g of flue gas desulfurization gypsum and dry mix for 3 minutes to obtain a desulfurization gypsum-fly ash based high-durability road base inorganic cementitious material. Step S5: Weigh 10,000g of continuously graded crushed stone (particle size 0-25mm), 500g of desulfurized gypsum-fly ash based high-durability road base inorganic binder, and 700g of deionized water; first, dry mix 10,000g of continuously graded crushed stone and 500g of inorganic binder powder for 60s, then add 700g of deionized water in two batches and wet mix for 120s to obtain a uniform mixture; fill the mixture into a cylindrical mold with a diameter of 100mm, form it in two layers and compact it, controlling the compaction degree to be above 98% of the maximum dry density of the standard compaction; after demolding, cure it in an environment of 20℃ and relative humidity not less than 95% for 28 days.

[0025] Example 2: The raw materials used in this embodiment are the same as those in Example 1. Compared with Example 1, this embodiment makes the following optimizations to the core formula and key process parameters: Step S1: Weigh 8000g of deionized water, 1000g of anhydrous sodium sulfate, and 80g of sodium hydroxide and add them to a stirring container. Stir at 600r / min for 10min at 75℃ to obtain a composite activation solution. While maintaining the solution temperature at 75℃, add 10000g of fly ash in three portions and continue stirring at 600r / min for 45min to form a uniform slurry. Immediately vacuum filter the slurry to obtain a pre-activated fly ash filter cake, and rinse it twice with deionized water to obtain a pre-activated fly ash wet filter cake. Step S2: Take the pre-activated fly ash wet filter cake obtained in step S1 and add 2200g of deionized water. Stir at 600r / min for 5min at 35℃. Separately, take 1800g of deionized water and 220g of calcium hydroxide to premix and prepare calcium hydroxide slurry. Add the calcium hydroxide slurry to the fly ash dispersion slurry at once and continue stirring at 600r / min for 25min. Then, dissolve 20g of sodium hexametaphosphate in 350g of deionized water, add it slowly and stir for 12min. Then, dissolve 120g of calcium nitrate tetrahydrate in 250g of deionized water, add it dropwise and continue stirring for 12min to obtain fly ash slurry with calcium-phosphorus reaction sites enhanced. Step S3: Heat the fly ash slurry obtained in step S2 to 38℃ and maintain a stirring speed of 850 r / min; premix and dilute 650 g of sodium silicate solution with 350 g of deionized water, and then add the sodium silicate solution dilution solution evenly over 25 min by dropwise addition. After the dropwise addition is completed, continue stirring for 35 min; then dissolve 10 g of sodium tripolyphosphate in 250 g of deionized water, add it while continuing to stir and stir for 10 min; then let the slurry stand at 45℃ for 2 h for aging; after aging, vacuum filter to obtain modified fly ash wet filter cake, and rinse once with deionized water; place the wet filter cake in a 65℃ forced-air drying oven to dry to constant weight, cool, grind and pass through a 300 μm sieve to obtain modified fly ash; Step S4: Weigh 6400g of modified fly ash, 200g of silicate cement, 900g of calcium hydroxide, and 2500g of flue gas desulfurization gypsum and add them to a dry mixer in sequence. First, dry mix 6400g of modified fly ash and 200g of ordinary silicate cement for 2 minutes, then add 900g of calcium hydroxide and dry mix for 2 minutes, and finally add 2500g of flue gas desulfurization gypsum and dry mix for 3 minutes to obtain a desulfurization gypsum-fly ash based high-durability road base inorganic cementitious material. Step S5: Weigh 10,000g of continuously graded crushed stone (particle size 0-25mm), 450g of desulfurized gypsum-fly ash based high-durability road base inorganic binder, and 680g of deionized water; first, dry mix 10,000g of continuously graded crushed stone and 450g of inorganic binder powder for 60s, then add 680g of deionized water in two portions and wet mix for 120s each to obtain a uniform mixture; fill the mixture into a 100mm diameter cylindrical mold, form it in two layers, and compact it, controlling the compaction degree to be above 98% of the maximum dry density of the standard compaction; after demolding, cure it in an environment of 20℃ and relative humidity not less than 95% for 28 days. The remaining conditions are the same as in Example 1.

[0026] Example 3: The raw materials used in this embodiment are the same as those in Example 1. Compared with Example 1, this embodiment makes the following optimizations to the core formula and key process parameters: Step S1: Weigh 7600g of deionized water, 1200g of anhydrous sodium sulfate, and 50g of sodium hydroxide and add them to a stirring container. Stir at 500r / min for 8 minutes at 65℃ to obtain a composite activation solution. While maintaining the solution temperature at 65℃, add 10000g of fly ash in three portions and continue stirring at 500r / min for 30 minutes to form a uniform slurry. Immediately afterward, vacuum filter to obtain a pre-activated fly ash filter cake, and rinse twice with deionized water to obtain a pre-activated fly ash wet filter cake. Step S2: Take the pre-activated fly ash wet filter cake obtained in step S1 and add 1800g of deionized water. Stir at 600r / min for 5min at 25℃. Separately, take 1500g of deionized water and 150g of calcium hydroxide to premix and prepare calcium hydroxide slurry. Add the calcium hydroxide slurry to the fly ash dispersion slurry at once and continue stirring at 600r / min for 20min. Then, dissolve 12g of sodium hexametaphosphate in 250g of deionized water, add it slowly and stir for 10min. Then, dissolve 80g of calcium nitrate tetrahydrate in 150g of deionized water, add it dropwise and continue stirring for 10min to obtain fly ash slurry with calcium-phosphorus reaction sites enhanced. Step S3: Heat the fly ash slurry obtained in Step S2 to 30℃ and maintain a stirring speed of 700 r / min; premix and dilute 450 g of sodium silicate solution with 300 g of deionized water, and then uniformly add the sodium silicate solution dilution over 15 min by dropwise addition. After the dropwise addition is completed, continue stirring for 20 min; then dissolve 6 g of sodium tripolyphosphate in 150 g of deionized water, add it while continuing to stir and stir for 10 min; then let the slurry stand and age at 38℃ for 1.5 h; after aging, vacuum filter to obtain modified fly ash wet filter cake, and rinse once with deionized water; place the wet filter cake in a 60℃ forced-air drying oven to dry to constant weight, cool, grind and pass through a 350 μm sieve to obtain modified fly ash; Step S4: Weigh 7000g of modified fly ash, 300g of silicate cement, 800g of calcium hydroxide, and 1900g of flue gas desulfurization gypsum and add them to a dry mixer in sequence. First, dry mix 7000g of modified fly ash and 300g of ordinary silicate cement for 2 minutes, then add 800g of calcium hydroxide and dry mix for 2 minutes, and finally add 1900g of flue gas desulfurization gypsum and dry mix for 3 minutes to obtain a desulfurization gypsum-fly ash based high-durability road base inorganic cementitious material. Step S5: Weigh 10,000g of continuously graded crushed stone (particle size 0-25mm), 600g of desulfurized gypsum-fly ash based high-durability road base inorganic binder, and 750g of deionized water; first, dry mix 10,000g of continuously graded crushed stone and 600g of inorganic binder powder for 60s, then add 750g of deionized water in two portions and wet mix for 120s each to obtain a uniform mixture; fill the mixture into a 100mm diameter cylindrical mold, form it in two layers, and compact it, controlling the compaction degree to be above 98% of the standard compacted maximum dry density; after demolding, cure it for 28 days in an environment of 20℃ and relative humidity not less than 95%. The remaining conditions are the same as in Example 1.

[0027] Example 4: The raw materials used in this embodiment are the same as those in Example 1. Compared with Example 1, this embodiment makes the following optimizations to the core formula and key process parameters: Step S1: Weigh 8200g of deionized water, 1100g of anhydrous sodium sulfate, and 40g of sodium hydroxide and add them to a stirring container. Stir at 700r / min for 12min at 70℃ to obtain a composite activation solution. While maintaining the solution temperature at 70℃, add 10000g of fly ash in three portions and continue stirring at 700r / min for 50min to form a uniform slurry. Immediately afterward, vacuum filter to obtain a pre-activated fly ash filter cake, and rinse twice with deionized water to obtain a pre-activated fly ash wet filter cake. Step S2: Take the pre-activated fly ash wet filter cake obtained in step S1 and add 2400g of deionized water. Stir at 600r / min for 5min at 30℃. Separately, take 2000g of deionized water and 250g of calcium hydroxide to premix and prepare calcium hydroxide slurry. Add the calcium hydroxide slurry to the fly ash dispersion slurry all at once and continue stirring at 600r / min for 30min. Then, dissolve 18g of sodium hexametaphosphate in 300g of deionized water, add it slowly and stir for 15min. Then, dissolve 140g of calcium nitrate tetrahydrate in 200g of deionized water, add it dropwise and continue stirring for 15min to obtain fly ash slurry with calcium-phosphorus reaction sites enhanced. Step S3: Heat the fly ash slurry obtained in Step S2 to 40℃ and maintain a stirring speed of 900 r / min; premix and dilute 750 g of sodium silicate solution with 500 g of deionized water, and then uniformly add the sodium silicate solution dilution solution dropwise over 30 min. After the dropwise addition is completed, continue stirring for 40 min; then dissolve 8 g of sodium tripolyphosphate in 200 g of deionized water, add it while continuing to stir and stir for 10 min; then let the slurry stand and age at 40℃ for 3 h; after aging, vacuum filter to obtain modified fly ash wet filter cake, and rinse once with deionized water; place the wet filter cake in a 70℃ forced-air drying oven to dry to constant weight, cool, grind and pass through a 250 μm sieve to obtain modified fly ash; Step S4: Weigh 6200g of modified fly ash, 500g of silicate cement, 1200g of calcium hydroxide, and 2100g of flue gas desulfurization gypsum and add them to a dry mixer in sequence. First, dry mix 6200g of modified fly ash and 500g of ordinary silicate cement for 2 minutes, then add 1200g of calcium hydroxide and dry mix for 2 minutes, and finally add 2100g of flue gas desulfurization gypsum and dry mix for 3 minutes to obtain a desulfurization gypsum-fly ash based high-durability road base inorganic cementitious material. Step S5: Weigh 10,000g of continuously graded crushed stone (particle size 0-25mm), 400g of desulfurized gypsum-fly ash based high-durability road base inorganic binder, and 650g of deionized water; first, dry mix 10,000g of continuously graded crushed stone and 400g of inorganic binder powder for 60s, then add 650g of deionized water in two portions and wet mix for 120s each to obtain a uniform mixture; fill the mixture into a 100mm diameter cylindrical mold, form it in two layers, and compact it, controlling the compaction degree to be above 98% of the maximum dry density of the standard compaction; after demolding, cure it for 28 days in an environment of 20℃ and relative humidity not less than 95%. The remaining conditions are the same as in Example 1.

[0028] Example 5: The raw materials used in this embodiment are the same as those in Example 1. Compared with Example 1, this embodiment makes the following optimizations to the core formula and key process parameters: Step S1: Weigh 7800g of deionized water, 1150g of anhydrous sodium sulfate, and 70g of sodium hydroxide and add them to a stirring container. Stir at 650r / min for 10min at 72℃ to obtain a composite activation solution. While maintaining the solution temperature at 72℃, add 10000g of fly ash in three portions and continue stirring at 650r / min for 40min to form a uniform slurry. Immediately afterward, vacuum filter to obtain a pre-activated fly ash filter cake, and rinse twice with deionized water to obtain a pre-activated fly ash wet filter cake. Step S2: Take the pre-activated fly ash wet filter cake obtained in step S1 and add 2000g of deionized water. Stir at 600r / min for 5min at 32℃. Separately, take 1800g of deionized water and 200g of calcium hydroxide to premix and prepare calcium hydroxide slurry. Add the calcium hydroxide slurry to the fly ash dispersion slurry all at once and continue stirring at 600r / min for 20min. Then, dissolve 15g of sodium hexametaphosphate in 300g of deionized water, add it slowly and stir for 10min. Then, dissolve 100g of calcium nitrate tetrahydrate in 200g of deionized water, add it dropwise and continue stirring for 10min to obtain fly ash slurry with calcium-phosphorus reaction sites enhanced. Step S3: Heat the fly ash slurry obtained in Step S2 to 35℃ and maintain a stirring speed of 800 r / min; premix and dilute 600 g of sodium silicate solution with 400 g of deionized water, and then add the sodium silicate solution dilution evenly over 20 min by dropwise addition. After the dropwise addition is completed, continue stirring for 30 min; then dissolve 12 g of sodium tripolyphosphate in 200 g of deionized water, add it while continuing to stir and stir for 10 min; then let the slurry stand at 40℃ for 2.5 h for aging; after aging, vacuum filter to obtain modified fly ash wet filter cake, and rinse once with deionized water; place the wet filter cake in a 68℃ forced-air drying oven to dry to constant weight, cool, grind and pass through a 300 μm sieve to obtain modified fly ash; Step S4: Weigh 7100g of modified fly ash, 300g of silicate cement, 1100g of calcium hydroxide, and 1500g of flue gas desulfurization gypsum and add them to a dry mixer in sequence. First, dry mix 7100g of modified fly ash and 300g of ordinary silicate cement for 2 minutes, then add 1100g of calcium hydroxide and dry mix for 2 minutes, and finally add 1500g of flue gas desulfurization gypsum and dry mix for 3 minutes to obtain a desulfurization gypsum-fly ash based high-durability road base inorganic cementitious material. Step S5: Weigh 10,000g of continuously graded crushed stone (particle size 0-25mm), 550g of desulfurized gypsum-fly ash based high-durability road base inorganic binder, and 720g of deionized water; first, dry mix 10,000g of continuously graded crushed stone and 550g of inorganic binder powder for 60s, then add 720g of deionized water in two portions and wet mix for 120s each to obtain a uniform mixture; fill the mixture into a 100mm diameter cylindrical mold, form it in two layers, and compact it, controlling the compaction degree to be above 98% of the standard compacted maximum dry density; after demolding, cure it for 28 days in an environment of 20℃ and relative humidity not less than 95%. The remaining conditions are the same as in Example 1.

[0029] Comparative Example 1: The difference from Example 1 is that steps S1, S2, and S3 are omitted in Comparative Example 1. Fly ash is used directly instead of modified fly ash in step S4. Specifically, in step S4, 6700g of fly ash, 300g of silicate cement, 1000g of calcium hydroxide, and 2000g of flue gas desulfurization gypsum are weighed and added sequentially to a dry mixer, and the inorganic binder material is prepared according to the dry mixing sequence and time of Example 1. All other conditions are the same as in Example 1.

[0030] Comparative Example 2: The difference from Example 1 is that sodium hexametaphosphate is not added in step S2 of Comparative Example 2. Instead, after adding the calcium hydroxide slurry and stirring at 600 r / min for 20 min, 100 g of calcium nitrate tetrahydrate is directly dissolved in 200 g of deionized water and added dropwise while stirring for 10 min. The remaining conditions are the same as in Example 1.

[0031] Comparative Example 3: The difference from Example 1 is that in Comparative Example 3, calcium nitrate tetrahydrate is not added in step S2. That is, after 15g of sodium hexametaphosphate is pre-dissolved in 300g of deionized water and slowly added while stirring for 10 minutes, the calcium nitrate tetrahydrate solution is not added, and the process proceeds directly to step S3. The remaining conditions are the same as in Example 1.

[0032] Comparative Example 4: The difference from Example 1 is that in Comparative Example 4, the order of adding sodium hexametaphosphate and calcium nitrate tetrahydrate in step S2 is reversed. Specifically, after adding the calcium hydroxide slurry and stirring at 600 rpm for 20 minutes, 100 g of calcium nitrate tetrahydrate, pre-dissolved in 200 g of deionized water, is added dropwise while stirring for 10 minutes. Then, 15 g of sodium hexametaphosphate, pre-dissolved in 300 g of deionized water, is slowly added while stirring for 10 minutes. All other conditions are the same as in Example 1.

[0033] Comparative Example 5: The difference from Example 1 is that in Comparative Example 5, the sodium silicate solution dilution in step S3 is not added dropwise, but rather 600g of sodium silicate solution is premixed and diluted with 400g of deionized water and then added all at once within 1 minute, followed by stirring for 30 minutes. All other conditions are the same as in Example 1.

[0034] Comparative Example 6: The difference from Example 1 is that the timing of adding sodium tripolyphosphate in step S3 of Comparative Example 6 is changed. Specifically, 8g of sodium tripolyphosphate is dissolved in 200g of deionized water, then added and stirred for 10 minutes at 35°C and 800 rpm. Next, 600g of sodium silicate solution is premixed and diluted with 400g of deionized water and added dropwise over 20 minutes, followed by stirring for another 30 minutes after the addition is complete. All other conditions are the same as in Example 1.

[0035] Comparative Example 7: The difference from Example 1 is that in Comparative Example 7, sodium tripolyphosphate was not added in step S3. That is, after adding the sodium silicate solution dilution and stirring for 30 minutes, no more sodium tripolyphosphate solution was added. The mixture was directly allowed to stand at 40°C for 2 hours and then the modified fly ash was prepared according to the subsequent steps of Example 1. All other conditions were the same as in Example 1.

[0036] Comparative Example 8: The difference from Example 1 is that in Comparative Example 8, sodium silicate solution was not added in step S3. Instead, the fly ash slurry obtained in step S2 was heated to 35°C and stirred at a speed of 800 r / min, and then stirred for 50 min while maintaining the same stirring conditions. Subsequently, 8 g of sodium tripolyphosphate was dissolved in 200 g of deionized water and added, and stirred for 10 min. After that, the slurry was allowed to stand and age at 40°C for 2 h, and the modified fly ash was prepared according to the subsequent steps of Example 1. The remaining conditions were the same as in Example 1.

[0037] Performance testing: Sample preparation: Desulfurized gypsum-fly ash based high-durability road base inorganic binder powder was prepared according to the methods of Examples 1-5 and Comparative Examples 1-8, and continuous graded crushed stone mixture specimens were prepared according to their respective steps S5. To ensure fair comparison, the continuous graded crushed stone (particle size 0-25mm), deionized water, mixing equipment, 100mm diameter cylindrical molds, compaction methods, demolding timing, and curing environment were kept consistent. Unconfined compressive strength, immersion unconfined compressive strength, and freeze-thaw strength tests were all conducted using 100mm diameter and 100mm height cylindrical specimens, with 6 specimens prepared for each age group. Drying shrinkage tests were conducted using 100mm×100mm×400mm prismatic specimens, with 3 specimens prepared for each group of samples.

[0038] Unconfined compressive strength: Unconfined compressive strength was tested according to JTG 3441-2024 "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering". Cylindrical specimens with a diameter of 100 mm and a height of 100 mm were cured for 7 days and 28 days respectively. The diameter and height of the specimens were measured and the average value was taken. The specimens were placed at the center of the upper and lower pressure plates of a compression testing machine, and displacement-controlled loading was used at a loading rate of 1 mm / min. The failure load P (N) was recorded. The unconfined compressive strength Rc (MPa) was calculated according to Rc=P / A, where A is the area of ​​the specimen under pressure, A=π×(D / 2). 2 D is the measured average diameter (mm); 6 specimens were tested for each group of samples at each age and the average value was calculated, as shown in Table 1.

[0039] Unconfined compressive strength under immersion and its ratio to immersion strength: The unconfined compressive strength under immersion was tested according to JTG 3441-2024 "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering". Cylindrical specimens with a diameter of 100 mm and a height of 100 mm were first cured at 20℃ and a relative humidity of not less than 95% for 6 days. Then, the specimens were completely immersed in still water at 20℃ for 24 hours. After removal, the surface water was quickly wiped off with a damp cloth, and the unconfined compressive strength test was completed within 10 minutes. The loading rate and strength calculation were the same as for the unconfined compressive strength test, yielding the unconfined compressive strength under immersion, R. w (MPa); the dry unconfined compressive strength at the same age is R d Water immersion strength compared to Kw =R w / R d Six specimens were tested in each group of samples and the average value was calculated, as shown in Table 1.

[0040] Strength retention rate and mass loss rate after freeze-thaw cycles: Freeze-thaw durability tests were conducted according to JTG 3441-2024 "Test Procedures for Inorganic Binder Stabilized Materials in Highway Engineering". Cylindrical specimens with a diameter of 100 mm and a height of 100 mm were cured at 20℃ and relative humidity not less than 95% for 28 days. They were then immersed in still water at 20℃ for 24 hours to reach saturation. The specimens were then removed, the surface water was wiped off, and the initial mass m0 (g) was measured. Subsequently, freeze-thaw cycles were performed: freezing at -20℃ for 16 hours and thawing at 20℃ for 8 hours, constituting one cycle, for a total of 15 cycles. After the freeze-thaw cycle, the specimens were removed, the surface water was wiped off, and the mass m1 (g) was measured. The mass loss rate W = (m0 - m1) / m0 × 100% (%). The unconfined compressive strength R after freeze-thaw cycles was then tested. ft (MPa), and calculate the freeze-thaw strength ratio K. ft =R ft / R 28 , where R 28 The unconfined compressive strength of the samples in the same group after 28 days in the dry state is shown in Table 1.

[0041] Drying shrinkage strain (28d): Drying shrinkage tests were conducted according to JTG 3441-2024 "Test Procedures for Inorganic Binder Stabilized Materials in Highway Engineering". A 100mm×100mm×400mm prism specimen prepared for test item one was used. Two stainless steel length measuring nails (6mm diameter, 10mm exposed length) were pre-embedded in the center of the specimen's end face. After demolding, the specimen was cured at 20℃ and relative humidity not less than 95% for 7 days. At 7 days, the initial length L0 (mm) was measured using a length comparator. Subsequently, the specimen was placed in a drying environment at 20℃ and 60% relative humidity, and the length L was measured after 28 days of drying. 28 (mm), calculate the drying shrinkage strain ε 28 =(L0-L 28 ) / L0×10 6 (με), as shown in Table 1.

[0042] Table 1 Performance Test Results Performance testing: As can be seen from the data in Table 1, the desulfurized gypsum-fly ash based high-durability road base inorganic cementitious material prepared by the present invention shows an overall trend of synergistic improvement in early load-bearing capacity, later strength growth, water immersion strength retention, freeze-thaw strength retention and volume stability. Furthermore, different embodiments have achieved simultaneous optimization between strength improvement and durability stability by optimizing the composite excitation strength, the degree of surface calcium-phosphorus reaction site construction, the nucleation method of sodium silicate solution drop addition, and the timing of sodium tripolyphosphate addition. The possible reasons are as follows: the composite activating solution promotes the directional dissolution of the glassy phase of fly ash and forms a more reactive precursor; in the presence of calcium hydroxide slurry, sodium hexametaphosphate is first introduced to anchor the site, and then calcium nitrate tetrahydrate is introduced to generate calcium phosphate intermediate phase in situ at the site, so that a high-density, sustainably supplied nucleating substrate is formed on the particle surface; subsequently, sodium silicate solution is added dropwise to construct a hydrated calcium silicate nucleating coating in situ at the above site, realizing the spatial binding of the nucleating function with the surface of fly ash particles; and the addition of sodium tripolyphosphate after nucleation is beneficial to the electrical regulation and inhibition of homogeneous precipitation, reducing the risk of free salt and pore connectivity, so that it can still maintain a denser and more stable cemented structure under complex working conditions such as humidity and freeze-thaw, which is suitable for the early opening and long-term service needs of the base course of heavy-duty roads in high-humidity areas, municipal roads and port areas.

[0043] As can be seen from the data in Table 1 for Example 1 and Comparative Example 1, when steps S1 to S3 are omitted and fly ash is directly used to replace modified fly ash in the preparation of the cementitious material, the early and later strength growth, water immersion strength retention, and freeze-thaw durability of the system are systematically weakened, and the drying shrinkage strain is more likely to be high. The main reason is that the inertness of fly ash is not directionally activated, the particle surface lacks controllable calcium-phosphorus reaction sites and hydrated calcium silicate nucleation coating, and the hydration products tend to be generated disorderly in the pores, making it difficult to form a continuous, dense, and stable cementitious network on the surface of fly ash particles; at the same time, the lack of multi-step constraints of site anchoring-nucleation coating-later electrical regulation makes the interface transition zone more prone to weakness after mixing and compaction, and water softening and freeze-thaw micro-damage are more likely to accumulate.

[0044] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 2, 3, and 4, when sodium hexametaphosphate or calcium nitrate tetrahydrate is missing in step S2, or when the order of their addition is reversed, the retention of immersion strength and the retention of strength after freeze-thaw cycles both show varying degrees of decline, accompanied by an increase in mass loss rate and worsening of shrinkage control. The possible reason is that the anchoring effect of sodium hexametaphosphate and the soluble calcium source provided by calcium nitrate tetrahydrate need to be coupled in time and space to form uniform, high-density calcium-phosphorus reaction sites on the surface of fly ash particles and generate calcium phosphate intermediate phase in situ. If any step is missing or the order is disrupted, the site density and uniformity decrease, limiting the efficiency and stability of the subsequent sodium silicate solution in constructing a hydrated calcium silicate nucleation coating at the sites. This results in insufficient pore refinement and uneven interfacial bonding, making the particles more prone to softening and microcrack damage under humid and freeze-thaw conditions. This result demonstrates that the synergistic effect of introducing calcium nitrate tetrahydrate after sodium hexametaphosphate anchoring is not a simple additive effect.

[0045] As can be seen from the data in Table 1 for Example 1 and Comparative Example 5, when the sodium silicate solution dilution in step S3 is added all at once instead of dripping, the material still achieves a certain strength increase, but the water immersion strength and freeze-thaw durability are more prone to fluctuation, and the mass loss rate is more unfavorable. The main reason is that a single addition causes excessively high local instantaneous concentrations, easily inducing homogeneous precipitation in the solution and heterogeneous nucleation on the particle surface in parallel. This results in uneven thickness of the nucleation coating, local agglomeration and encapsulation, forming a locally over-cemented and locally under-cemented interface structure. During compaction and service under water exposure, this non-uniform interface is more likely to form interconnected pore channels, amplifying water damage, softening, and freeze-thaw micro-damage. Therefore, the dripping process not only affects the reaction rate but also determines the spatial distribution and stability of the nucleation coating, making it a key process point for achieving high durability.

[0046] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 6 and 7, when sodium tripolyphosphate is added before the sodium silicate solution is added, or when sodium tripolyphosphate is not added at all, the retention of immersion strength, the retention of strength after freeze-thaw cycles, and the mass loss rate show unfavorable changes, and it is more difficult to simultaneously control shrinkage. The possible reason is that sodium tripolyphosphate has complexing and electrochemical regulation characteristics: if added before the formation of the nucleation coating, it easily complexes soluble calcium ions and disturbs the nucleation process at the sites, making it difficult for the hydrated calcium silicate nucleation coating to grow uniformly at the calcium-phosphorus reaction sites; if not added, it is difficult to mildly regulate ion migration and homogeneous precipitation after nucleation, leading to an increased risk of free salt residue and pore connectivity, thus making it easier for softening and micro-damage accumulation to occur in immersion and freeze-thaw environments. These results indicate that controlling the timing of adding sodium tripolyphosphate after nucleation has unexpected comprehensive benefits, demonstrating a significant synergistic effect.

[0047] 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 desulfurized gypsum-fly ash based high-durability road base inorganic binder, characterized in that, By weight, it includes: 6200-7100 parts modified fly ash, 200-500 parts silicate cement, 800-1200 parts calcium hydroxide and 1500-2500 parts flue gas desulfurization gypsum. The modified fly ash is prepared by the following steps: the fly ash is pre-activated and filtered and washed using a composite activation solution containing anhydrous sodium sulfate and sodium hydroxide; the pre-activated fly ash wet filter cake is dispersed in water and then calcium hydroxide slurry, sodium hexametaphosphate solution and calcium nitrate tetrahydrate solution are added in sequence to construct calcium-phosphorus reaction sites; then sodium silicate solution dilution is added dropwise to the obtained fly ash slurry to construct a hydrated calcium silicate nucleation coating in situ, and sodium tripolyphosphate solution is added after the sodium silicate solution dilution is added. After static aging, filtration, washing, drying, grinding and sieving, the modified fly ash is obtained.

2. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, The composite activation solution is prepared by mixing 7,600-8,200 parts of deionized water, 1,000-1,200 parts of anhydrous sodium sulfate and 40-80 parts of sodium hydroxide per 10,000 parts of fly ash.

3. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, The pre-activation process involves adding fly ash in three portions at 65-75℃ and stirring for 30-50 minutes each time.

4. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, Based on 10,000 parts of fly ash, the calcium hydroxide slurry is obtained by mixing 1,500-2,000 parts of deionized water with 150-250 parts of calcium hydroxide; the sodium hexametaphosphate solution is obtained by mixing 12-20 parts of sodium hexametaphosphate with 250-350 parts of deionized water; and the calcium nitrate tetrahydrate solution is obtained by mixing 80-140 parts of calcium nitrate tetrahydrate with 150-250 parts of deionized water.

5. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, Based on 10,000 parts of fly ash, the sodium silicate solution dilution is obtained by mixing 450-750 parts of sodium silicate solution with 300-500 parts of deionized water; the sodium tripolyphosphate solution is obtained by mixing 6-12 parts of sodium tripolyphosphate with 150-250 parts of deionized water.

6. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 5, characterized in that, The sodium silicate solution contains, by mass percentage, 10%-12% Na2O and 26%-28% SiO2.

7. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, The static aging process involves aging at 38-45℃ for 1.5-3 hours.

8. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, The modified fly ash is ground and passed through a 250μm-350μm sieve.

9. The desulfurized gypsum-fly ash based high-durability road base inorganic binder material according to claim 1, characterized in that, The fly ash used to prepare the modified fly ash meets the requirements of Class II fly ash specified in GB / T1596; the flue gas desulfurization gypsum meets the requirements of flue gas desulfurization gypsum for building materials specified in GB / T37785; and the silicate cement is ordinary silicate cement P·O42.

5.

10. A method for preparing a desulfurized gypsum-fly ash based high-durability road base inorganic cementing material according to any one of claims 1-9, characterized in that, The process includes the following steps: mixing the modified fly ash, silicate cement, calcium hydroxide, and flue gas desulfurization gypsum to obtain a desulfurization gypsum-fly ash-based high-durability road base inorganic cementing material.

Images