A high-strength low-carbon composite cementitious material based on gradient pouring and a preparation method thereof

Abstract

The application discloses a kind of high-strength low-carbon composite cementitious material based on gradient pouring and a preparation method thereof.The composite cementitious material includes alkali-activated base cementitious material layer and cement base cementitious material layer from inside to outside, and the alkali-activated base cementitious material layer and the cement base cementitious material layer are combined into a whole by gradient pouring.The raw materials of the alkali-activated base cementitious material layer include granulated blast furnace slag, coal gangue powder, alkali activator, aggregate and water.The raw materials of the cement base cementitious material layer include Portland cement, aggregate and water.The alkali-activated base cementitious material with granulated blast furnace slag and coal gangue powder as solid raw materials and the cement base cementitious material with Portland cement as solid raw material are combined into a whole by gradient pouring to obtain the composite cementitious material, which substantially improves the strength performance of the composite cementitious material while retaining the advantages of early strength and low carbon of alkali-activated slag, and also significantly improves the carbonation resistance.

Description

Technical Field

[0001] This invention belongs to the field of engineering materials technology, and in particular relates to a high-strength, low-carbon composite cementitious material based on gradient casting and its preparation method. Background Technology

[0002] While concrete (PC) boasts advantages such as high compressive strength and excellent durability, its production process suffers from high energy consumption and carbon emissions. Alkali-activated slag (AAS), using industrial solid waste as its main raw material, forms aluminosilicate gel at room temperature through alkali activation, reducing carbon emissions by 60-80% compared to PC. Furthermore, AAS exhibits excellent early strength and resistance to chemical attack, making it a highly promising green cementitious material for the 21st century. However, AAS still faces significant challenges in its widespread application. Strong alkali activators (such as NaOH and water glass) are expensive, limiting its economic feasibility; simultaneously, the high-alkali environment poses construction safety risks, easily causing burns to operators; most importantly, AAS has poor resistance to carbonization and volume stability, large drying shrinkage, and significantly inferior long-term durability compared to PC. Relying solely on PC or AAS makes it difficult to simultaneously meet the demands for low carbon emissions and high performance; therefore, a combination of both is considered a promising technological approach. By leveraging AAS to achieve early strength, low carbon emissions, and solid waste resource utilization, while optimizing the alkaline environment through PC improves carbonization resistance and shrinkage, complementary and synergistic performance enhancement can be achieved. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a high-strength, low-carbon composite cementitious material based on gradient casting and its preparation method. The composite cementitious material is obtained by combining an alkali-activated cementitious material made from granulated blast furnace slag and coal gangue powder as solid raw materials with a cement-based cementitious material made from silicate cement as solid raw materials through gradient casting. While retaining the early strength and low-carbon advantages of alkali-activated slag, the strength performance of the composite cementitious material is substantially improved, and its resistance to carbonation is also significantly enhanced.

[0004] The present invention proposes a high-strength, low-carbon composite cementitious material based on gradient casting, comprising an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out, wherein the alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole through gradient casting.

[0005] The raw materials for the alkali-activated cementitious material layer include granulated blast furnace slag, coal gangue powder, alkali activator, aggregate, and water; the raw materials for the cement-based cementitious material layer include silicate cement, aggregate, and water.

[0006] In this invention, in order to alleviate the problem of high energy consumption and high emissions in cement, the low-carbon potential of alkali slag is utilized to solve the key scientific problem of incompatibility between the two in compounding. A composite cementitious material suitable for gradient casting is proposed. The influence of material composition design and compounding method on the competitive mechanism of polycondensation and hydration reaction is studied, and the multi-scale coupling structure-property regulation mechanism is revealed.

[0007] Preferably, the mass ratio of silicate cement, aggregate, and water is 1:(2-4):(0.4-0.6), more preferably 1:3:0.5; the mass ratio of granulated blast furnace slag, coal gangue powder, alkali activator, river sand, and water is 1:(0.9-1.1):(0.5-0.7):(5-7):(0.4-0.6), more preferably 1:1:0.6:6:0.5;

[0008] Preferably, by mass percentage, the alkali-activated cementitious material layer accounts for 50-95 wt% of the total amount of the composite cementitious material, and the cement-based cementitious material layer accounts for 5-50 wt% of the total amount of the composite cementitious material.

[0009] Preferably, the silicate cement is PO 42.5, 42.5R, 52.5 or 52.5R ordinary silicate cement with a specific surface area ≥400m² / kg;

[0010] Its chemical composition by mass percentage is: CaO 60-65%, SiO2 20-25%, Al2O3 5-10%, Fe2O3 1-5%, MgO 0-1%, Na2O 0-1%, K2O 0-1%.

[0011] Preferably, the aggregate is river sand with a fineness modulus of 1.6-2.2.

[0012] Preferably, the granulated blast furnace slag is S75, S95 or S105 grade slag powder with a specific surface area ≥400m² / kg;

[0013] Its chemical composition by mass percentage is: CaO 35-40%, SiO2 30-35%, Al2O3 15-20%, Fe2O3 1-5%, MgO 1-5%, Na2O 0-1%, K2O 0-1%.

[0014] Preferably, the specific surface area of ​​the coal gangue powder is ≥400m² / kg;

[0015] Its chemical composition by mass percentage is: SiO2 65-70%, Al2O3 20-25%, Fe2O3 1-5%, CaO 1-5%, K2O 1-5%, MgO 0-1%, Na2O 0-1%.

[0016] Preferably, the coal gangue powder is activated coal gangue powder, which is obtained by grinding coal gangue powder with a grinding aid;

[0017] The grinding aid is obtained by reacting polyvinylpyrrolidone with ethylenediamine via an addition reaction, followed by a phosphonation reaction with phosphorous acid and formaldehyde.

[0018] The structure of the grinding aid in this invention is shown below:

[0019]

[0020] Using the polyvinylpyrrolidone composite organophosphate compound shown in the above structural diagram as a grinding aid has two advantages: firstly, it can adsorb onto the hydroxyl groups on the surface of coal gangue powder, preventing coal gangue powder agglomeration and maintaining its high specific surface area, while also contributing to the improvement of the cementitious activity of the coal gangue powder; secondly, the N, O, and P coordinating atoms contained in its structural formula can affect the Ca... 2+ Complexation is formed, driving the rapid construction of CSH gel and further improving the mechanical strength of the resulting composite material.

[0021] Preferably, the alkaline activator comprises sodium hydroxide and liquid sodium silicate;

[0022] The mass ratio of sodium hydroxide to liquid sodium silicate is 1:(4-6), preferably 1:5;

[0023] The liquid sodium silicate has the following chemical composition by mass percentage: Na2O 5-10%, SiO2 25-30%, H2O 60-65%.

[0024] This invention also proposes a method for preparing the above-mentioned high-strength, low-carbon composite cementitious material based on gradient casting, comprising:

[0025] After mixing the raw materials in the alkali-activated cementitious material layer and casting for initial setting, the raw materials in the cement-based cementitious material layer are added and cast for solidification to obtain the high-strength, low-carbon composite cementitious material.

[0026] Preferably, the initial setting time is 10-30 minutes.

[0027] The beneficial effects of this invention are:

[0028] This invention takes "gradient casting of alkali-activated slag-based cementitious materials-cement-based cementitious materials" as its starting point, elucidating the kinetic interaction mechanism of the condensation reaction of alkali-activated slag-based cementitious materials and the hydration reaction of cement-based cementitious materials in the composite, with the key ion (OH-) - Ca 2+This study investigates migration and competition behaviors, the evolution of microstructures in the interface region, and the compatibility of reaction products. It establishes a multi-scale "cooperative" structure-property relationship encompassing reaction competition, interface structure, and macroscopic performance. The study focuses on the controllable construction of composite materials. By synergistically regulating the alkaline environment of the system through cement-based cementitious materials, it substantially improves the system's resistance to carbonization and drying shrinkage while retaining the early strength and low-carbon advantages of alkali-activated slag-based cementitious materials. It reveals the reaction pathways and the essence of interfacial interactions in composite materials, developing low-carbon, environmentally friendly, structurally stable, and performance-balanced composite materials. The study also explores their application potential in scenarios requiring early strength, durability, and green performance, such as rapid repair engineering, corrosion-resistant linings, and low-carbon precast components. This provides a solution with both theoretical depth and practical feasibility for the development of high-performance low-carbon cementitious materials. Attached Figure Description

[0029] Figure 1 The infrared spectra of the composite cementitious materials described in the embodiments and comparative examples of the present invention are as follows: (a) is the infrared spectrum of the composite cementitious material described in Comparative Example 1; (b) is the infrared spectrum of the composite cementitious material described in Comparative Example 2; (c) is the infrared spectrum of the composite cementitious material described in Comparative Example 4; (d) is the infrared spectrum of the cement-based cementitious material layer in the composite cementitious material described in Example 1; (e) is the infrared spectrum of the alkali-activated cementitious material layer in the composite cementitious material described in Example 1; and (f) is the infrared spectrum of the interface transition zone of the composite cementitious material described in Example 1.

[0030] Figure 2 This is a BSE image (CBS) of the composite cementitious material described in Comparative Example 4 of the present invention after 28 days of curing.

[0031] Figure 3 This is a BSE image (ETD) of the composite cementitious material described in Comparative Example 4 of the present invention after 28 days of curing.

[0032] Figure 4 This is a BSE image (CBS) of the cementitious material layer after 28 days of curing of the composite cementitious material described in Example 1 of the present invention.

[0033] Figure 5 This is a BSE image (ETD) of the cementitious material layer after 28 days of curing of the composite cementitious material described in Example 1 of the present invention.

[0034] Figure 6 This is a BSE image (ETD) of the alkali-activated cementitious material layer after 28 days of curing of the composite cementitious material described in Example 1 of the present invention.

[0035] Figure 7 This is a BSE image (CBS) of the alkali-activated cementitious material layer after 28 days of curing of the composite cementitious material described in Example 1 of the present invention.

[0036] Figure 8 This is a BSE image (ETD) of the interface transition zone of the composite cementitious material described in Example 1 of the present invention after 28 days of curing.

[0037] Figure 9 This is a BSE (CBS) image of the interface transition zone of the composite cementitious material described in Example 1 of the present invention after 28 days of curing.

[0038] Figure 10 This is a schematic diagram of the composite cementitious material described in the comparative example of the present invention;

[0039] Figure 11 This is a schematic diagram of the composite cementitious material described in an embodiment of the present invention;

[0040] Figure 12 This is the infrared spectrum of the grinding aid described in Example 4 of the present invention. Detailed Implementation

[0041] The present invention will now be described in detail through specific embodiments. However, these embodiments are clearly provided for illustrative purposes and are not intended to limit the scope of the present invention.

[0042] In the following examples and comparative examples, the silicate cement (PC, pH 12.68) used was ordinary PO 42.5 silicate cement; the alkali-activated slag (AAS, pH 14.47) used was granulated blast furnace slag (GBFS) and coal gangue powder (CG), with GBFS being S95 grade and CG prepared by a combined crushing-ball milling process; the alkali activator used was analytical grade NaOH (purity ≥99.5%) and industrial grade liquid sodium silicate (YG, Na2O content 9.82wt%, SiO2 content 25.84wt%, H2O content 64.15wt%); and the fine aggregate used was river sand (fineness modulus 1.8); the chemical composition and specific surface area (m²) of PC, GBFS, and CG were described below. 2 ( / kg) is shown in Table 1 below:

[0043] Table 1. Chemical composition percentage of raw materials (PC, GBFS, and CG)

[0044]

[0045] As shown in Table 1 above, the main components of PC and GBFS are CaO, SiO2 and Al2O3, while the main components of CG are SiO2 and Al2O3.

[0046] Example 1

[0047] A high-strength, low-carbon composite cementitious material based on gradient casting includes an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out. The alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole through gradient casting.

[0048] The raw materials for the alkali-activated cementitious material layer include 180g of granulated blast furnace slag, 180g of coal gangue powder, 115.2g of alkali activator, 1080g of river sand, and 100g of water; the alkali activator includes 19.2g of sodium hydroxide and 96g of industrial-grade liquid sodium silicate; the raw materials for the cement-based cementitious material layer include 90g of silicate cement, 270g of river sand, and 45g of water.

[0049] The above-mentioned preparation method of high-strength low-carbon composite cementitious material based on gradient casting includes:

[0050] (1) Add sodium hydroxide and industrial-grade liquid sodium silicate, which are the raw materials in the alkali-activated cementitious material layer, to water to make an alkali activator solution. Then add granulated blast furnace slag and coal gangue powder and stir at low speed for 30s. Then add river sand and stir at high speed for 60s to obtain alkali-activated cementitious material. Add silicate cement, which is the raw material in the cement-based cementitious material layer, to water and stir at low speed for 30s. Then add river sand and stir at high speed for 90s to obtain cement-based cementitious material.

[0051] (2) The alkali-activated cementitious material was poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After 15 minutes, it was initially set. Then, cement-based cementitious material was poured in and compacted by vibration. The resulting specimen was demolded 24 hours after molding and transferred to a curing box for curing at a temperature of 20±1℃ and a humidity of 95%. The high-strength low-carbon composite cementitious material based on gradient casting, abbreviated as LP2G8, was obtained.

[0052] Example 2

[0053] A high-strength, low-carbon composite cementitious material based on gradient casting includes an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out. The alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole through gradient casting.

[0054] The raw materials for the alkali-activated cementitious material layer include 135g of granulated blast furnace slag, 135g of coal gangue powder, 86.4g of alkali activator, 810g of river sand, and 75g of water; the alkali activator includes 14.4g of sodium hydroxide and 72g of industrial-grade liquid sodium silicate; the raw materials for the cement-based cementitious material include 180g of silicate cement, 540g of river sand, and 90g of water.

[0055] The preparation method of the high-strength low-carbon composite cementitious material based on gradient casting is as described in Example 1, abbreviated as LP4G6.

[0056] Example 3

[0057] A high-strength, low-carbon composite cementitious material based on gradient casting includes an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out. The alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole through gradient casting.

[0058] The raw materials for the alkali-activated cementitious material layer include 113g of granulated blast furnace slag, 112g of coal gangue powder, 72g of alkali activator, 675g of river sand, and 63g of water; the alkali activator includes 12g of sodium hydroxide and 60g of industrial-grade liquid sodium silicate; the raw materials for the cement-based cementitious material include 225g of silicate cement, 675g of river sand, and 113g of water.

[0059] The preparation method of the high-strength low-carbon composite cementitious material based on gradient casting is as described in Example 1, abbreviated as LP5G5.

[0060] Example 4

[0061] A high-strength, low-carbon composite cementitious material based on gradient casting includes an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out. The alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole through gradient casting.

[0062] The raw materials for the alkali-activated cementitious material layer include 90g of granulated blast furnace slag, 90g of coal gangue powder, 57.6g of alkali activator, 540g of river sand, and 50g of water; the alkali activator includes 9.6g of sodium hydroxide and 48g of industrial-grade liquid sodium silicate; the raw materials for the cement-based cementitious material layer include 270g of silicate cement, 810g of river sand, and 135g of water.

[0063] The preparation method of the high-strength low-carbon composite cementitious material based on gradient casting is as described in Example 1, abbreviated as LP6G4.

[0064] Example 5

[0065] A high-strength, low-carbon composite cementitious material based on gradient casting includes an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out. The alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole through gradient casting.

[0066] The raw materials for the alkali-activated cementitious material layer include 45g of granulated blast furnace slag, 45g of coal gangue powder, 28.8g of alkali activator, 270g of river sand, and 25g of water; the alkali activator includes 4.8g of sodium hydroxide and 24g of industrial-grade liquid sodium silicate; the raw materials for the cement-based cementitious material layer include 360g of silicate cement, 1080g of river sand, and 180g of water.

[0067] The preparation method of the high-strength low-carbon composite cementitious material based on gradient casting is as described in Example 1, abbreviated as LP8G2.

[0068] Example 6

[0069] A high-strength, low-carbon composite cementitious material based on gradient casting, abbreviated as L`P2G8, has the same raw materials and preparation method as in Example 1, except that the coal gangue powder is activated coal gangue powder, which is prepared by the following method: 180g of coal gangue powder and 10g of grinding aid are added to a ball mill, and then tungsten carbide grinding balls are added. The mixture is ball-milled for 12 hours at a speed of 350 rpm to obtain the activated coal gangue powder.

[0070] The grinding aid is prepared by the following method: 10g of polyvinylpyrrolidone (PVP K30) is dissolved completely in anhydrous ethanol, then 1g of formic acid is added and stirred until well mixed. 0.5g of ethylenediamine is then added, and the mixture is heated to 60℃ and stirred for 3 hours. After removing the anhydrous ethanol, the resulting product, along with 2g of phosphorous acid and 3g of formaldehyde, is added to a 20wt% hydrochloric acid solution. The mixture is heated to 60℃ and stirred for 6 hours. After filtration, washing with water, and drying, the grinding aid is obtained. Its infrared spectrum is as follows: Figure 12 As shown.

[0071] Example 7

[0072] A high-strength, low-carbon composite cementitious material based on gradient casting, abbreviated as L``P2G8, has the same raw materials and preparation method as in Example 1, except that the coal gangue powder is activated coal gangue powder, which is prepared by the following method: 180g of coal gangue powder and 10g of grinding aid are added to a ball mill, and then tungsten carbide grinding balls are added. The mixture is ball-milled at 350rpm for 12h to obtain the activated coal gangue powder; the grinding aid is polyvinylpyrrolidone (PVP K30).

[0073] Comparative Example 1

[0074] A composite cementitious material includes a cement-based cementitious material layer; the raw materials of the cement-based cementitious material layer include 450g of silicate cement, 1350g of river sand and 225g of water; its pH is 12.68.

[0075] The preparation method of the above-mentioned composite cementitious material includes:

[0076] (1) Add the raw material silicate cement in the cement-based cementitious material layer to water and stir at low speed for 30s, then add river sand and stir at high speed for 90s to obtain cement-based cementitious material.

[0077] (2) The cement-based cementitious material is injected into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After the specimen is molded for 24 hours, it is demolded and transferred to a curing box for curing at a temperature of 20±1℃ and a humidity of 95%. The composite cementitious material, abbreviated as P10, can be obtained.

[0078] Comparative Example 2

[0079] A composite cementitious material, comprising an alkali-activated cementitious material layer;

[0080] The raw materials for the alkali-activated cementitious material layer include 225g of granulated blast furnace slag, 225g of coal gangue powder, 144g of alkali activator, 1350g of river sand, and 125g of water; the alkali activator includes 24g of sodium hydroxide and 120g of industrial-grade liquid sodium silicate; its pH is 14.47.

[0081] The preparation method of the above-mentioned composite cementitious material includes:

[0082] (1) Add sodium hydroxide and industrial-grade liquid sodium silicate, which are raw materials in the alkali-activated slag-based cementitious material layer, to water to make an alkali activator solution. Then add granulated blast furnace slag and coal gangue powder and stir at low speed for 30s. Then add river sand and stir at high speed for 60s to obtain alkali-activated cementitious material.

[0083] (2) The alkali-activated cementitious material is injected into a 40mm×40mm×160mm triple mold and compacted on a vibration table. After the specimen is molded for 24 hours, it is demolded and transferred to a curing box for curing at a temperature of 20±1℃ and a humidity of 95%. The composite cementitious material, abbreviated as G10, can be obtained.

[0084] Comparative Example 3

[0085] A composite cementitious material, the raw materials of which are as described in Example 1.

[0086] The preparation method of the above-mentioned composite cementitious material includes:

[0087] Sodium hydroxide and industrial-grade liquid sodium silicate were added to all water to prepare an alkali activator solution. Then, silicate cement, coal gangue powder and granulated blast furnace slag were dry-mixed and pretreated. The resulting mixture was stirred at low speed with the alkali activator solution for 30 seconds, and then all river sand was added and stirred at high speed for 60 seconds. The mixture was then poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After the specimens were molded for 24 hours, they were demolded and transferred to a curing chamber for curing at a temperature of 20±1℃ and a humidity of 95%. The resulting composite cementitious material, abbreviated as MP2G8, was obtained.

[0088] Comparative Example 4

[0089] A composite cementitious material, the raw materials of which are as described in Example 2.

[0090] The preparation method of the above-mentioned composite cementitious material includes:

[0091] Sodium hydroxide and industrial-grade liquid sodium silicate were added to all water to prepare an alkali activator solution. Then, silicate cement, coal gangue powder and granulated blast furnace slag were dry-mixed and pretreated. The resulting mixture was stirred at low speed with the alkali activator solution for 30 seconds, and then all river sand was added and stirred at high speed for 60 seconds. The mixture was then poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After the resulting specimen was molded for 24 hours, it was demolded and transferred to a curing chamber for curing at a temperature of 20±1℃ and a humidity of 95%. The resulting composite cementitious material, abbreviated as MP4G6, was obtained.

[0092] Comparative Example 5

[0093] A composite cementitious material, the raw materials of which are as described in Example 3.

[0094] The preparation method of the above-mentioned composite cementitious material includes:

[0095] Sodium hydroxide and industrial-grade liquid sodium silicate were added to all water to prepare an alkali activator solution. Then, silicate cement, coal gangue powder and granulated blast furnace slag were dry-mixed and pretreated. The resulting mixture was stirred at low speed with the alkali activator solution for 30 seconds, and then all river sand was added and stirred at high speed for 60 seconds. The mixture was then poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After the resulting specimen was molded for 24 hours, it was demolded and transferred to a curing chamber for curing at a temperature of 20±1℃ and a humidity of 95%. The resulting composite cementitious material, abbreviated as MP5G5, was obtained.

[0096] Comparative Example 6

[0097] A composite cementitious material, the raw materials of which are as described in Example 4.

[0098] The preparation method of the above-mentioned composite cementitious material includes:

[0099] Sodium hydroxide and industrial-grade liquid sodium silicate were added to all water to prepare an alkali activator solution. Then, silicate cement, coal gangue powder and granulated blast furnace slag were dry-mixed and pretreated. The resulting mixture was stirred at low speed with the alkali activator solution for 30 seconds, and then all river sand was added and stirred at high speed for 60 seconds. The mixture was then poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After the specimens were molded for 24 hours, they were demolded and transferred to a curing chamber for curing at a temperature of 20±1℃ and a humidity of 95%. The resulting composite cementitious material, abbreviated as MP6G4, was obtained.

[0100] Comparative Example 7

[0101] A composite cementitious material, the raw materials of which are as described in Example 5.

[0102] The preparation method of the above-mentioned composite cementitious material includes:

[0103] Sodium hydroxide and industrial-grade liquid sodium silicate were added to all water to prepare an alkali activator solution. Then, silicate cement, coal gangue powder and granulated blast furnace slag were dry-mixed and pretreated. The resulting mixture was stirred at low speed with the alkali activator solution for 30 seconds, and then all river sand was added and stirred at high speed for 60 seconds. The mixture was then poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After the specimens were molded for 24 hours, they were demolded and transferred to a curing chamber for curing at a temperature of 20±1℃ and a humidity of 95%. The resulting composite cementitious material, abbreviated as MP8G2, was obtained.

[0104] Comparative Example 8

[0105] A composite cementitious material based on gradient casting includes a cement-based cementitious material layer and an alkali-activated cementitious material layer from the inside out, wherein the cement-based cementitious material layer and the alkali-activated cementitious material layer are bonded together as a whole through gradient casting.

[0106] The raw materials for the cement-based cementitious material layer include 90g of silicate cement, 270g of river sand, and 45g of water; the raw materials for the alkali-activated cementitious material layer include 180g of granulated blast furnace slag, 180g of coal gangue powder, 115.2g of alkali activator, 1080g of river sand, and 100g of water; the alkali activator includes 19.2g of sodium hydroxide and 96g of industrial-grade liquid sodium silicate.

[0107] The above-mentioned method for preparing composite cementitious materials based on gradient casting includes:

[0108] (1) Add silicate cement, the raw material in the cement-based cementitious material layer, to water and stir at low speed for 30s, then add river sand and stir at high speed for 90s to obtain cement-based cementitious material; add sodium hydroxide and industrial-grade liquid sodium silicate, the raw materials in the alkali-activated cementitious material layer, to water to prepare alkali activator solution, then add granulated blast furnace slag and coal gangue powder and stir at low speed for 30s, then add river sand and stir at high speed for 60s to obtain alkali-activated cementitious material;

[0109] (2) The cement-based cementitious material was poured into a 40mm×40mm×160mm triple mold and compacted on a vibrating table. After 15 minutes, it was initially set. Then, alkali-activated cementitious material was injected and compacted by vibration. The resulting specimen was demolded 24 hours after molding and transferred to a curing box for curing at a temperature of 20±1℃ and a humidity of 95%. The high-strength low-carbon composite cementitious material based on gradient casting, abbreviated as LG8P2, was obtained.

[0110] The compressive strength of the composite cementitious materials described in the above embodiments and comparative examples was tested, specifically according to GB / T17671-2021. The compressive strength at 1d, 3d, 28d, and 60d was tested using a DYE-300S mortar press, and the results are shown in Table 2 below:

[0111] Table 2 shows the compressive strength of the composite cementitious materials described in the examples and comparative examples at different ages.

[0112]

[0113] As shown in Table 2 above, the compressive strength of P10 (pH=12.68) reached 19.6 MPa and 35.2 MPa at 1d and 3d respectively, entered the stable period at 57.7 MPa at 28d, and increased slightly to 62.4 MPa at 60d; the compressive strength of G10 (pH=14.47) was 22.5 MPa and 40.8 MPa at 1d and 3d respectively, showing significant early strength characteristics, reaching 59.6 MPa at 28d (3.3% higher than P10), and continued to increase to 65.3 MPa at 60d. The later strength growth rate (9.6% increase from 28 to 60d) was higher than that of P10.

[0114] The compressive strength of MP8G2 obtained by mixed casting significantly degraded, decreasing to 9.9 MPa and 15.2 MPa at 1 day and 3 days, respectively, and only 24.7 MPa and 26.3 MPa at 28 days and 60 days, respectively, a decrease of 50%-70% compared to P10 and G10. Although the AAS content in MP8G2 was only 20%, the pH of the system decreased from the optimal AAS environment (14.47) to 13.71. The low alkalinity environment led to insufficient dissociation of active substances in GBFS / CG, reducing the polymerization efficiency and hindering the formation of polymerization products. However, the optimal pH for PC hydration is 12.5-13.0, and the pH of MP8G2 is lower than that of PC. 13.71 The alkaline environment leads to the deharmonicity of the hydration reaction, which not only hinders the PC hydration reaction but also inhibits the formation and development of CSH, resulting in a sharp drop in compressive strength. As the AAS content increases, the alkaline environment of MP6G4 to MP2G8 gradually improves, becoming more suitable for the polycondensation reaction, which promotes the continuous formation of high-strength C-(A)-SH gel. Although PC hydration is increasingly inhibited, the PC content decreases, and the dominant role is gradually replaced by AAS. This phase transition process restores the strength of the mixture, but does not alleviate the incompatibility of the two-phase reaction, which is manifested in the fact that the development of compressive strength always lags behind P10 or G10.

[0115] The 1-day and 3-day compressive strengths of LP8G2 obtained by gradient casting reached 20.1 MPa and 31.2 MPa, respectively, rising to 55.2 MPa at 28 days (only 8.3% lower than P10), and stabilizing at 60.8 MPa at 60 days (the difference narrowed to 5.1%). This strength growth characteristic contrasts sharply with that of mixed casting, revealing the effectiveness of the separation strategy. AAS achieves temporal isolation through rapid setting (initial setting time <15 min), ensuring that the lower AAS has already stabilized and initially set when the upper PC layer is cast, completely blocking the high concentration of OH in the AAS. -Interpenetration and migration with low-alkaline solutions of PC ensure the chemical integrity of the reaction products in both phases. When the proportion of AAS increases to 80%, the early strength of LP2G8 reaches 27.5 MPa (1 day), which is 40.3% and 22.2% higher than P10 and G10, respectively. The strength at 60 days increases to 68.8 MPa, which is 10.3% and 5.4% higher than P10 and G10, respectively. The calcium hydroxide (CH) formed by PC hydration is considered a mechanically weak phase. However, in the PC-AAS complex, CH acts as a Ca 2+ The supplementary source releases Ca 2+ Under the influence of concentration gradient, CH diffuses into the AAM region, which not only promotes the continuous dissolution of CH in PC, but also participates in the alkali activation reaction, promoting the formation of high-strength, low-calcium-silicon ratio CSH or CASH gels. Therefore, the superior compressive strength of the PC-AAS composite stems from the establishment of an integrated system characterized by complementary chemical reactions, dynamic ion migration and synergistic product formation. CH has undergone a functional transformation from a "mechanical weakness" to a "reactive resource", which promotes the synergistic coupling of the two cementation reactions.

[0116] The 1-day and 3-day compressive strengths of L`P2G8 obtained by gradient casting reached 30.1 MPa and 50.3 MPa, respectively. After 28 days, the strength increased to 72.3 MPa and stabilized at 77.4 MPa after 60 days, which is a significant improvement over the compressive strength of LP2G8. However, the improvement in compressive strength of L``P2G8 relative to LP2G8 is not obvious, which illustrates the strength growth characteristics of the grinding aid described in Example 6.

[0117] Both LG8P2 and LP2G8 are produced by gradient casting, but the compressive strength of LG8P2 is much lower than that of LP2G8 due to different casting sequences.

[0118] The carbonation resistance of the composite cementitious materials described in the above embodiments and comparative examples was tested, specifically according to GB / T 50082-2009, at carbonation ages of 7d, 14d, and 28d. The results are shown in Table 3 below:

[0119] Table 3 shows the carbonation depth of the composite cementitious materials described in the examples and comparative examples at different ages.

[0120]

[0121] As shown in Table 3 above, the composite cementitious material described in the examples also has excellent carbon resistance.

[0122] Figure 1 The infrared spectra of the composite cementitious materials described in the examples and comparative examples are shown below. (Refer to...) Figure 1 From ab, we know that P10 is at 1109cm. -1 It exhibits Si-O-Si vibrational peaks, combined with 865 cm⁻¹ -1Characteristic peaks, all belonging to the silicon-oxygen tetrahedral features of CSH gel, and high intensity confirms the abundant formation of CSH gel; 608 cm⁻¹ -1 / 665cm -1 The double peaks correspond to Si-O-Al bond vibrations, indicating the formation of aluminum-oxygen tetrahedral CASH gels, and their peak intensity reflects high CASH gel abundance; the gel-bound water formation characteristics are observed through 1618 cm⁻¹. -1 / 1679cm -1 H-OH vibration at 3411 cm -1 / 3539cm -1 The -OH vibration at the point of origin is corroborated, with the latter pointing to Ca(OH)₂ crystals; the above confirms that hydration products such as C-(A)-SH are synergistically generated in P10, supplemented by byproducts such as Ca(OH)₂, revealing an excellent hydration reaction process. Although G10 is at 889 cm⁻¹... -1 / 1265cm -1 Si-O-Si vibrations were still detected, but the peak position shifted significantly to the right compared to the characteristic peak of P10, and the peak intensity decreased, attributed to a decrease in the degree of polymerization of silicon-oxygen tetrahedra, indicating a reduction in CSH gel formation; however, G10 showed a peak intensity at 522 cm⁻¹. -1 / 662cm -1 Exhibits significant Si-O-Al vibration, 662 cm⁻¹ -1 The vibration intensity was significantly higher than that of P10, which strongly confirms that a large amount of CASH gel was formed under the dominance of the condensation reaction; this indicates that the strength development mechanism of G10 is a CASH-dominated structural support mode.

[0123] Figure 1 The spectral characteristics of MP6G4 c reveal that both hydration and condensation reactions are systematically suppressed; 1278 cm⁻¹ -1 Although the Si-O-Si vibrational peaks of the silicon-oxygen network were observed, their intensity was significantly lower than that of P10 and G10, indicating that the PC hydration reaction was strongly interfered with, resulting in insufficient CSH gel formation; at the same time, 518 cm⁻¹ -1 Only a single, severely weakened Si-O-Al vibrational peak was observed at this point, a stark contrast to the broadened, strong peak of G10. This confirms that the AAS aluminosilicate condensation reaction in MP6G4 was severely distorted, and the aluminum-oxygen tetrahedra failed to effectively integrate into the gel network, resulting in a sharp decrease in the amount of key CASH gel formed. Furthermore, at 3316 cm⁻¹... -1 The intensity of the -OH peak was also significantly lower than that of P10 and G10, indicating that the hydration of PC was hindered, resulting in insufficient formation of the Ca(OH)2 crystalline phase; the HOH vibration peaks of the gel interlayer and adsorbed water (1618-1679 cm⁻¹) -1The absence of [a specific element] corroborates the severe stagnation of the hydration and condensation processes in the system. These phenomena demonstrate that the mixed casting process leads to an inhibitory effect of the liquid-phase chemical environment on both types of gelation reactions; AAS introduces a high concentration of OH [a specific element]. - Na + The active silica-alumina monomers altered the ion balance and pH evolution pathway of the system. The high alkalinity and ionic composition of AAS disrupted the Ca in PC. 2+ The dissolution and precipitation equilibrium forms a dense silicate (aluminate) coating layer, which blocks the ion migration and ordered precipitation processes essential for CSH gel nucleation and growth within PC particles, thus hindering its nucleation kinetics. Simultaneously, PC hydration releases a large amount of Ca. 2+ It invades the AAS reaction domain in the reverse direction and strongly competes for binding with [SiO4]. 4- and [Al(OH)4] - Monomers form inert ion pairs that inhibit polymerization, altering the local pH and coordination environment of aluminum species. This significantly slows down the polymerization kinetics of aluminosilicate oligomer formation, recombination, and three-dimensional network extension. Therefore, the mixed casting environment simultaneously stifles both PC hydration (characterized by ion precipitation) and AAS reactions (characterized by monomer polymerization), ultimately leading to a severe weakening of the formation of the C-(A)-S-critical gel phase. This manifests macroscopically as deterioration of mechanical properties, revealing the severe incompatibility between PC and AAS under non-optimized composite conditions.

[0124] Figure 1 The cement-based gel material layer (P2) in LP2G8 of d is at 1015 cm -1 With 1164cm -1 Significant Si-O-Si bond vibrations were observed, with a 607 / 668 cm⁻¹ frequency response. -1 Strong Si-O-Al bond vibrations confirmed that C-(A)-SH gel formation was not interfered with by the high alkalinity of AAS; 1422 cm⁻¹ -1 CO3 2- Vibration and 1618cm -1 The H-OH bond vibration reappeared, verifying the sufficiency of the hydration reaction; however, 3539 / 3611 cm -1 The weakening of the -OH vibration intensity suggests that the generated Ca(OH)2 migrates to the interface region through a dissolution-diffusion mechanism. Figure 1 The alkali-activated cementitious material layer (G8) in LP2G8 of e is 888 / 1266cm -1 The vibrational intensity of the Si-O-Si bond is significantly improved compared to G10, while 515 / 661 cm⁻¹ -1 The reduced half-width at half-maximum (WHM) of the Si-O-Al bond vibrations indicates that the spatial isolation strategy effectively avoids the inhibition of the AAS condensation reaction by the low-alkali environment of PC, promoting the formation of C-(A)-SH gel; key evidence comes from the 3223 cm⁻¹. -1The intensity of the -OH vibration at the interface increases compared to G10, which is related to the directional dissolution and transport of CH in the interfacial region. Figure 1 The interface transition zone in f's LP2G8 is 877 / 1109cm. -1 Bimodal Si-O-Si bond vibration, 612 / 667 cm⁻¹ -1 The strong Si-O-Al bond vibrations at 667 cm⁻¹ all confirm the presence of interfacial catalytic effects in this region; -1 The vibration at this location suggests an increased degree of polymerization of aluminum-oxygen tetrahedra, which is related to the migration of Ca to this location. 2+ Closely related to triggering the bridging effect; 1452cm appeared simultaneously. -1 Carbonate characteristic peaks and 1627 / 1678 cm⁻¹ -1 Combined with water vibration, and 3413 / 3536 cm -1 The intensity of the -OH vibration was increased compared to P2; P2 provided continuous Ca 2+ The active source replenishes Ca for the chemical deposition of the aluminosilicate precursor (G8). 2+ ; Migrating Ca 2+ It acts as a multivalent bonding center, and its essential difference from monovalent alkali metal ions lies in its coordination flexibility and charge strength; Ca 2+ It can react simultaneously with two or more negatively charged [AlO4]. 5- Tetrahedral coordination forms strong bridging bonds similar to ≡Si–O–Ca–O–Al≡, achieving topological adjustment at the molecular scale. This not only balances the charge but also establishes additional covalent-ionic hybrid bonds between adjacent silicon-oxygen and aluminum-oxygen chains, thus implanting high-strength "chemical anchors" into the original three-dimensional network structure (NASH); Ca 2+ The bridging effect further induced the directional evolution of network polymerization, breaking the local charge symmetry of the original gel. To obtain a more stable coordination environment, neighboring [SiO4] tetrahedra tended to connect more adjacent tetrahedra, promoting the in-situ growth of a gel phase with higher polymerization and denser density in the interface region. Ultimately, the interface was not simply composed of two independent gels (C-(A)-SH), but evolved into a C-(N)-ASH hybrid phase with gradient composition and interwoven structure. At the nanoscale, this hybrid phase exhibited a biomimetic "rigid-flexible" composite framework. The highly polymerized silicon-aluminum-oxygen framework provided rigid support, while the embedded calcium-oxygen chains and dynamic ionic bonds endowed it with excellent deformation and energy dissipation capabilities. The ultimate manifestation of this synergistic effect was the formation of a new material region at the interface with performance exceeding that of the single body. This region achieved cross-scale synergistic enhancement from microscopic chemical bonding to macroscopic mechanical properties. Gradient casting solved the AAS-PC incompatibility problem through the dual effects of physical isolation and chemical synergy, avoided alkaline environment conflicts through spatial isolation, and utilized Ca 2+Directional migration enables media reuse, while a high-density C-(N)-ASH crosslinking network is formed in the interface region. This multi-scale synergistic effect significantly improves compressive strength.

[0125] Figure 2-3 The microstructure morphology of MP6G4 was revealed, with the most significant feature being the dominance of numerous unreacted particles. These particles exhibited a high-brightness grayscale and irregular morphology, closely matching the original material, and were densely distributed within the cross-section, forming the main framework of the microstructure. This indicates that the hydration / polymerization process of the system was hindered, resulting in a low overall reaction degree. First, the particle edges were clear and sharp, and no "reaction edges" with medium to low grayscale characteristics formed by the dissolution-reprecipitation process were observed, revealing low surface reactivity of the particles. The ion concentration in the liquid phase did not reach the supersaturation required for nucleation, leading to ineffective nucleation and growth of the gel phase. Second, the gel matrix was poorly developed and discontinuous. The key binder phase, calcium (aluminum) silicate, was present in low amounts and exhibited a discrete island-like distribution, failing to form a continuous, dense three-dimensional network. This resulted in a lack of structural bonding in the system, which is the fundamental reason for the low macroscopic strength. Finally, a highly developed multi-scale pore network was observed, with numerous particles of varying sizes within the microstructure. The interconnected pores form a continuous permeation channel, and microcracks extend mostly along the boundaries of unreacted particles or weak interfaces. The low degree of reactivity restricts the formation of the microstructure, resulting in the reaction products being unable to fully fill the pore network formed by the accumulation of original particles. At the same time, the lack of sufficient gel to form a continuous skeleton in the system leads to a lack of plastic strength to resist shrinkage stress in the early stages of hardening, thereby causing local stress concentration and the initiation and propagation of microcracks. In addition, the inherent differences in physicochemical properties between different components lead to shrinkage incoordination, further amplifying the strain mismatch at the interface and exacerbating interface debonding and crack propagation. In summary, the MP6G4 microstructure exhibits low reactivity and poor structural development characteristics. The skeletonization of unreacted particles, the severe lack of gel binder phase, and the highly interconnected defect network together constitute the triple fragility of the microstructure, resulting in severely insufficient macroscopic mechanical properties, significantly deteriorated durability, and poor overall volume stability.

[0126] Figure 4-5 The microstructure morphology of P2 was revealed, exhibiting highly reactive and densified characteristics. Firstly, the high-brightness unreacted PC particles showed a significant reduction in both number density and size, confirming that most of the gelling components participated in ion release through surface dissolution. Secondly, the PC particles were surrounded by a uniform, continuous, low-to-medium grayscale interfacial reaction transition zone, providing direct evidence of a "dissolution, diffusion, and precipitation" kinetic production line, demonstrating the Ca dissolved from the particle surface. 2+ [SiO4] 4- and [AlO4] 5-After the plasma migrates in the interfacial liquid phase and reaches supersaturation, it uses the original particles as a heterogeneous nucleation substrate to epitaxially grow a C-(A)-SH type gel coating layer with a nanoscale composition gradient. This gradient interface not only achieves a smooth transition of chemical composition, but also optimizes the stress transmission from rigid particles to flexible matrix through its modulus buffering effect, effectively suppressing the initiation of microcracks.

[0127] Figure 6-7 The microstructure morphology of G8 was revealed, showing that the main phase evolved into a continuous, homogeneous, and dominant medium-to-low grayscale amorphous gel matrix. Its uniform contrast and dense morphology clearly correspond to a three-dimensional cross-linked network structure primarily composed of calcium-(aluminum)-silicate hydrates, confirming the presence of Ca dissolved in the liquid phase. 2+ [SiO4] 4- and [AlO4] 5- Plasma achieves topological recombination and chemical bonding through condensation polymerization, thereby tightly encapsulating and cementing residual unreacted particles into a structural whole. This is the core driving force for achieving nanoscale densification of the matrix. The numerous drying shrinkage cracks that appear at the same time are not indicators of structural defects, but rather microscopic morphological evidence that the AAS condensation polymerization reaction has fully progressed to an advanced stage. When the condensation polymerization reaction is rapid and complete, a high-density, low-porosity rigid gel skeleton is generated inside the system. With the consumption and evaporation of water, the drying shrinkage stress caused by capillary tension and van der Waals forces between gel particles increases significantly. In the highly densified and plastically lost gel matrix, this shrinkage stress cannot be completely released through viscous flow, thus concentrating in local weak areas (such as areas with different degrees of micro-hydration), resulting in the formation of a regular, randomly oriented drying shrinkage crack network. Therefore, the G8 microstructure exhibits a dialectical characteristic of the coexistence of a "highly continuous matrix" and "controllable drying shrinkage cracks".

[0128] Figure 8-9 The study revealed highly dynamic and structurally ordered micro-features in the P2G8 interface transition region. This region exhibits a reaction front dominated by chemical dissolution, ion migration, and gradient precipitation, with the most significant characteristic being the substantial reduction in AAS drying shrinkage cracks. The continuously shrinking size of high-brightness unreacted particles and their dissolution morphology at the edges confirm that, driven by the interfacial chemical potential gradient, not only is the GBFS silicon-aluminum network accelerated depolymerization, but the PC dissolution kinetics are also significantly enhanced. The instantaneous dissolution of PC releases a high concentration of Ca. 2+ Establishing a localized high-calcium environment significantly enhances ion activity and supersaturation, resulting in active aluminosilicate species ([Al(OH)4)) provided by the continuous dissociation of GBFS and FA. -Oligosilicate ions diffuse into this region; the two meet in the particle-solution interface diffusion layer and interact, assembling in situ through heterogeneous nucleation and sequential co-precipitation mechanisms to form a gradient gel composite layer with a continuous transition in chemical composition from high-calcium CSH to silicon-aluminum-rich (N,C)-ASH; the PC hydration-derived C-(A)-SH phase forms first, providing an early rigid framework and nucleation sites, followed by the higher-polymerization (N,C)-ASH gel phase formed by AAS, which penetrates, fills and interweaves with it due to its superior spatial connectivity, ultimately constructing an interpenetrating and interlocking composite network at the nanoscale; therefore, the continuous gradient of chemical composition and mechanical properties formed by the deep coupling of the two types of reactions, PC hydration and AAS polymerization, fundamentally optimizes the stress transfer from rigid particles to flexible matrix, and effectively improves damage tolerance through crack deflection, bridging and other mechanisms.

[0129] Figure 10 , 11 The diagrams shown are schematic diagrams of the composite cementitious materials described in the comparative examples and embodiments, respectively. (Refer to...) Figure 10 It can be seen that MP6G4 obtained by mixed casting is fundamentally incompatible with the reaction kinetics and liquid-phase ionic environment. AAS polymerization is a dissolution-condensation process driven by strong alkali (pH > 14), and its reaction initiation and rate-controlling steps lie in the depolymerization of the network structure of aluminosilicate precursors (such as GBFS, FA) and the dissolution of active aluminosilicate species. In contrast, PC hydration depends on a high-calcium and relatively mild alkaline environment, and its early hydration is mainly due to Ca. 2+ It is characterized by rapid dissolution and release of CH4 and high calcium-to-silicon ratio CSH precipitation; however, the mixing of the two creates a key "chemical environment conflict," as PC dilutes the original strong base activator concentration of AAS, while PC violently dissolves and releases a large amount of Ca2+. 2+ With OH - This rapidly alters the liquid-phase chemical equilibrium, causing the pH value to fail to reach the alkali threshold (typically >14) required for efficient activation of GBFS and FA, severely inhibiting the depolymerization kinetics of the silicon-aluminum network and hindering the AAS reaction from the initiation stage; simultaneously, Ca dissolved from the surface of PC particles... 2+The active silicate and aluminate ions released by AAS are immediately captured, preferentially forming a low calcium-to-silica ratio, aluminum-rich C-(N)-ASH type amorphous gel coating layer on the surface of PC particles. This inhibits the continuous hydration of PC, causing the classic path of PC hydration, which is inherent to achieve volume filling and strength development through the formation of high-calcium CSH and CH, to be completely changed. Ultimately, the two reaction pathways failed to synergistically form a hybrid gel transition region (such as C-(N)-ASH) with a continuous gradient and strong bonding ability. Instead, a "chemically disordered region" was formed where both reactions were incomplete and the interfacial bonding was weak. This region is composed of poorly developed, mixed-component gel phases and a large number of unreacted particles, lacking effective chemical bridges and microstructural continuity. This fundamentally explains the weak interfacial, well-developed pores, and poor overall performance observed in the microstructure of MP6G4.

[0130] Reference Figure 11 It can be seen that the compatibility of LP2G8 obtained by gradient casting lies in the multi-scale synergistic mechanism of "spatial isolation, chemical compensation and structural toughening" constructed by the layered design; PC and AAS create a controllable independent reaction field, avoiding direct conflicts between the two due to pH requirements, ion release kinetics and incompatibility of early products; at the same time, the interlayer interface evolves into a dynamic functional region of "controlled ion diffusion and reaction coupling"; P2 continuously releases Ca through hydration. 2+ With OH - Driven by the concentration gradient, it migrates directionally towards G8; this controlled migration first triggers a precise chemical compensation reaction in the interface region, namely, the reaction of dissolved ions with the active aluminum phase in AAS, and the accompanying crystallization expansion can generate beneficial micro-prestress in the early stage, effectively offsetting the chemical shrinkage and drying shrinkage tensile stress of AAS; secondly, Ca 2+By continuously introducing alterations to the ionic composition and strength of the pore solution in the interfacial region, the capillary pressure driving force leading to plastic cracking is weakened through the physicochemical effects of compressing the electric double layer and reducing the surface tension of the solution. Ultimately, through ion-interactive precipitation and co-condensation, a C-(N)-ASH hybrid gel layer with continuously varying chemical composition and mechanical modulus is grown in situ in the interfacial transition region. This gradient structure not only achieves a smooth stress transfer from the rigid PC layer to the flexible AAM layer, avoiding stress concentration caused by abrupt performance changes, but also features a "rigid-flexible" structure composed of nanoscale interpenetrating rigid CSH and flexible NASH networks. The "flexible" composite microstructure can efficiently dissipate fracture energy through mechanisms such as crack deflection, bridging, and bifurcation, greatly improving fracture toughness. Therefore, the essence of LP2G8's crack resistance is to transform the chemical activity of PC from a "destructive interference source" into a "structural reinforcing agent." Through a multi-scale coupling mechanism, from ion migration and reaction at the molecular scale, to gradient gel construction at the nanoscale, and then to expansion stress compensation and crack guidance at the microscale, it guides and transforms volume change form energy into a positive factor that enhances structural integrity. Macroscopically, it achieves fundamental suppression of cracking tendency and synergistic optimization of comprehensive performance.

[0131] The above results indicate that mixed casting leads to mutual inhibition between the two processes, resulting in hindered hydration and reduced polycondensation efficiency, with a 28-day compressive strength of only 21.4 MPa (a decrease of 66.5%). Gradient casting constructs independent reaction units, and Ca2+ is formed in the interfacial transition zone. 2+ The directional migration pathway, with an alkali slag to cement ratio of 8:2 showing the best synergistic effect, increased the 28-day strength to 62.7 MPa (an increase of 12.4%). The alkali slag absorbs the cement hydration byproduct Ca(OH)2, promoting the formation of calcium aluminosilicate gel (C-(N)-ASH) through interfacial ion exchange, thereby enhancing the interfacial structure and overall mechanical properties. This study provides a theoretical basis and technical approach for the proportioning design of low-carbon and environmentally friendly composite cementitious materials.

[0132] Mixed casting leads to alkaline environment interference between PC and AAS. The high pH of AAS inhibits the hydration kinetics of PC, resulting in a decrease in the formation of hydration products. Simultaneously, the diffusion of low-alkaline PC solution causes a decrease in the pH of the AAS system, hindering the condensation reaction. The deterioration of both results in a 50%-70% reduction in 28-day strength, exhibiting significant incompatibility. Gradient casting constructs isolated reaction units, effectively blocking the interpenetration of PC and AAS solutions. An 8:2 mass ratio of AAS to PC forms a stable pH gradient field, promoting both types of gelation reactions and facilitating their efficient execution. The 28-day strength reaches 64.4 MPa, an increase of 69.6%, verifying the performance synergy effect. C-(N)-ASH characteristic gel symbiosis is detected in the interfacial transition zone, forming a rigid-flexible multi-scale structure. The strength of calcium aluminosilicate is significantly enhanced compared to P2 and G8, further supporting the contribution of ion migration-induced evolution mechanism to interfacial strengthening.

[0133] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-strength, low-carbon composite cementitious material based on gradient casting, characterized in that, It includes an alkali-activated cementitious material layer and a cement-based cementitious material layer from the inside out, and the alkali-activated cementitious material layer and the cement-based cementitious material layer are bonded together as a whole by gradient casting; The raw materials for the alkali-activated cementitious material layer include granulated blast furnace slag, coal gangue powder, alkali activator, aggregate, and water; the raw materials for the cement-based cementitious material layer include silicate cement, aggregate, and water. The mass ratio of silicate cement, aggregate, and water is 1:(2-4):(0.4-0.6); the mass ratio of granulated blast furnace slag, coal gangue powder, alkali activator, river sand, and water is 1:(0.9-1.1):(0.5-0.7):(5-7):(0.4-0.6). By mass percentage, the alkali-activated cementitious material layer accounts for 5-95 wt% of the total amount of the composite cementitious material, and the cement-based cementitious material layer accounts for 95-5 wt% of the total amount of the composite cementitious material. The coal gangue powder is activated coal gangue powder, which is obtained by grinding coal gangue powder with a grinding aid; the grinding aid is obtained by adding polyvinylpyrrolidone and ethylenediamine, and then reacting it with phosphorous acid and formaldehyde in a phosphonation reaction. The above-mentioned method for preparing high-strength, low-carbon composite cementitious material based on gradient casting includes: mixing the raw materials in the alkali-activated cementitious material layer and casting it for initial setting, then adding the raw materials in the cement-based cementitious material layer and casting it for solidification, thereby obtaining the high-strength, low-carbon composite cementitious material.

2. The high-strength, low-carbon composite cementitious material based on gradient casting according to claim 1, characterized in that, The silicate cement is PO 42.5, 42.5R, 52.5 or 52.5R ordinary silicate cement with a specific surface area ≥400m² / kg; Its chemical composition by mass percentage is: CaO 60-65%, SiO2 20-25%, Al2O3 5-10%, Fe2O3 1-5%, MgO 0-1%, Na2O 0-1%, K2O 0-1%.

3. The high-strength, low-carbon composite cementitious material based on gradient casting according to claim 1 or 2, characterized in that, The aggregate is river sand with a fineness modulus of 1.6-2.

2.

4. The high-strength, low-carbon composite cementitious material based on gradient casting according to claim 1 or 2, characterized in that, The granulated blast furnace slag is S75, S95, or S105 grade slag powder with a specific surface area ≥400 m². 2 / kg; Its chemical composition by mass percentage is: CaO 35-40%, SiO2 30-35%, Al2O3 15-20%, Fe2O3 1-5%, MgO 1-5%, Na2O 0-1%, K2O 0-1%.

5. The high-strength, low-carbon composite cementitious material based on gradient casting according to claim 1 or 2, characterized in that, The specific surface area of ​​the coal gangue powder is ≥400m². 2 / kg; Its chemical composition by mass percentage is: SiO2 65-70%, Al2O3 20-25%, Fe2O3 1-5%, CaO 1-5%, K2O 1-5%, MgO 0-1%, Na2O 0-1%.

6. The high-strength, low-carbon composite cementitious material based on gradient casting according to claim 1 or 2, characterized in that, The alkaline activator includes sodium hydroxide and liquid sodium silicate; The mass ratio of sodium hydroxide to liquid sodium silicate is 1:(4-6). The liquid sodium silicate has the following chemical composition by mass percentage: Na2O 5-10%, Si2O 25-30%, H2O 60-65%.

7. The high-strength, low-carbon composite cementitious material based on gradient casting according to claim 1, characterized in that, The initial setting time is 10-30 minutes.

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