Method for producing dry-mixed mortar by adding recycled coal ash

Abstract

The invention relates to the technical field of building materials, and discloses a production method of dry-mixed mortar added with recycled coal ash, the mortar is prepared from ordinary Portland cement, a modified coal ash precursor, fine aggregate, hydroxypropyl methyl cellulose ether and redispersible latex powder; the modified coal ash precursor is obtained by mixing coal ash, organic acid, light calcined magnesia and polyphosphate according to a specific proportion and performing mechanical activation treatment in an anhydrous environment; according to the method, a surface micro-reaction area is constructed through the mechanochemical effect, phosphate depolymerization and magnesium curing reaction are excited by acid, and a magnesium phosphate cementation phase is generated in situ on the surface of the fly ash; the structure is independent of a cement hydration system, the early strength, the water resistance and the interface bonding force of the mortar are improved, drying shrinkage is compensated through the micro-expansion effect, and the problems that high-content fly ash mortar is low in early strength and prone to cracking are effectively solved.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a method for producing dry-mixed mortar with the addition of recycled coal ash. Background Technology

[0002] Fly ash, as a major solid waste emitted by coal-fired power plants, plays a vital role in environmental protection and the sustainable development of building materials through its resource utilization. Adding an appropriate amount of fly ash to dry-mix mortar production can not only reduce cement usage and save production costs, but also improve the mortar's long-term durability to some extent. However, fly ash particles are mainly composed of dense aluminosilicate glass, and their chemical properties are relatively stable under normal temperature and pressure, exhibiting a potential activity hysteresis characteristic.

[0003] In existing dry-mix mortar formulations, fly ash is typically used only as a physical filler or micro-aggregate. When the proportion of fly ash replacing cement is high, its extremely low early hydration activity prevents it from contributing sufficient bonding strength in the early stages of mortar hardening, leading to a significant decrease in early mortar strength and impacting construction progress and demolding efficiency. Furthermore, the surface of fly ash particles is mostly smooth and dense, exhibiting chemical inertness and weak interfacial bonding with cement hydration products (gels). This can easily lead to structural defects in the interfacial transition zone, further reducing the tensile bond strength of the mortar.

[0004] On the other hand, cement-based dry-mixed mortars commonly suffer from volume shrinkage during the hardening process due to moisture evaporation and chemical shrinkage, which can easily lead to hollowing and cracking in the plaster or masonry layers. Although existing technologies often use the addition of expanding agents (such as calcium oxide or magnesium oxide) to compensate for shrinkage, commercially available magnesium oxide expanding agents are mostly high-temperature calcined products with low reactivity. They often only begin to hydrate and expand in the later stages of mortar hardening. This delayed expansion not only fails to compensate for early plastic shrinkage but may also damage the already formed microstructure, resulting in poor volume stability.

[0005] Current activation technologies for fly ash mostly focus on strong alkali activation (such as using sodium hydroxide or water glass). However, this can easily lead to efflorescence on the mortar surface, and the highly alkaline environment negatively impacts the performance of certain organic admixtures, while also increasing safety hazards during construction. While simple physical grinding can increase the specific surface area of ​​fly ash, it cannot fundamentally change its surface chemical inertness. Therefore, developing a technical solution that can simultaneously achieve surface chemical activation, early strength enhancement, and volume shrinkage compensation of fly ash while maintaining the stability of dry-mixed mortar in its dry powder state is a pressing technical challenge that needs to be addressed to realize the high-proportion, high-value utilization of fly ash in dry-mixed mortar. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for producing dry-mixed mortar by adding recycled fly ash, which solves the problems of low early activity, decreased early strength due to excessive addition, easy shrinkage cracking, and weak interfacial bonding between fly ash and cement matrix in existing fly ash dry-mixed mortar.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for producing dry-mixed mortar with recycled fly ash, comprising the following steps: S1. Preparation of modified fly ash precursor: Weigh fly ash, organic acid, lightly calcined magnesium oxide and polyphosphate according to the ratio, mix them, perform mechanical activation treatment and sieve; S2. Dry mixing: Add ordinary silicate cement, modified fly ash precursor obtained in step S1, and fine aggregate to the mixer, and add hydroxypropyl methylcellulose ether and redispersible latex powder to the mixer. S3. Start the mixer to mix and stir until the dry mortar material is macroscopically uniform, then discharge and package it. The dry-mixed mortar is made from the following raw materials in parts by weight: 150-250 parts of ordinary silicate cement; 80-150 parts of modified fly ash precursor; 660-700 parts of fine aggregate; 0.5-1.5 parts of hydroxypropyl methylcellulose ether; and 2.5-5.0 parts of redispersible latex powder.

[0008] By adopting the above technical solution, this invention utilizes mechanochemical action to pre-construct a surface micro-reaction zone, solving the technical problems of low early-stage activity, limited dosage, and easy cracking of traditional fly ash in dry-mixed mortar. Specifically, the modified fly ash precursor anchors the reactive components to the surface of fly ash particles in an anhydrous environment. When water is added to the mortar and stirred, the organic acids on the precursor surface dissolve rapidly, establishing a localized slightly acidic environment that triggers the hydrolysis of polyphosphate and corrodes the glassy surface of the fly ash. The dissociated phosphate ions react rapidly with the magnesium ions released from lightly calcined magnesium oxide, generating a magnesium phosphate cement phase in situ on the surface of the fly ash particles. This cement phase is independent of the silicate cement hydration system and rapidly forms a structural framework in the early stage (within 12-24 hours), improving the early strength of the mortar. At the same time, the in-situ grown crystals improve the interfacial bonding between fly ash and the matrix, enhancing the adhesion strength of the mortar.

[0009] Preferably, in step S1, the modified fly ash precursor comprises, by mass percentage, the following components: 85.0%–93.0% fly ash; 0.5%–1.5% organic acid; 5.0%–10.0% lightly calcined magnesium oxide; and 1.5%–3.5% polyphosphate. In step S1, the mechanical activation treatment is carried out in a closed ball mill, controlling the milling environment to be an anhydrous environment with a relative humidity of less than 40%, controlling the material temperature below 60°C during grinding, and the grinding activation time to be 15–30 minutes.

[0010] By adopting the above technical solution and strictly controlling the anhydrous sealed environment and grinding temperature, the thermal decomposition of organic acids or premature reactions between components are prevented, ensuring that chemical potential energy is stored in the precursor. The mechanical activation process not only refines the particle size of the material and increases the specific surface area, but more importantly, it introduces lattice distortion energy and achieves tight coating of fly ash particles by acid, magnesium, and phosphorus components at the micron scale. This synergistic physical and chemical pretreatment ensures that the components coexist stably in a dry state, and can immediately undergo chemical reactions in sequence upon contact with water.

[0011] Preferably, the organic acid is oxalic acid dihydrate or L-tartaric acid; the polyphosphate is sodium tripolyphosphate or sodium hexametaphosphate. The activity value of the lightly calcined magnesium oxide is 30-60 seconds, and the average particle size D50 is ≤20μm; the fly ash is Class II F fly ash that has been pre-dried to a moisture content ≤0.5wt%.

[0012] By employing the above technical solution, the activity value and particle size of lightly calcined magnesium oxide are limited, ensuring that its dissolution rate is highly matched with the hydrolysis rate of polyphosphate, thereby achieving precise control of the hydration process. The reaction mechanism of this invention proceeds in the following steps: Step 1 (Acidic Activation and Depolymerization): After water is added to the mortar, the solid-phase organic acids preferentially dissolve, forming a transient weakly acidic microenvironment around the precursor particles. This environment promotes the hydrolysis and breakdown of the long-chain structure of polyphosphate into highly active orthophosphate groups, while simultaneously protonally eroding the fly ash vitreous network and exposing active sites.

[0013] Step 2 (In-situ Growth of the Cemented Phase): Simultaneously with acid activation, the lightly calcined magnesium oxide releases magnesium ions, which react with the dissociated phosphate ions in an exothermic acid-base reaction to generate hydrated magnesium phosphate gel and a crystalline phase. This product grows in situ using fly ash particles as nucleation sites, rapidly connecting to form an early-strength framework.

[0014] Step 3 (Volume Compensation and Densification): As the reaction proceeds, the pH value of the system gradually increases. Some of the magnesium oxide that did not participate in the acid-base reaction slowly hydrates to form magnesium hydroxide in the later stage of hardening. This process is accompanied by a small amount of solid phase volume expansion, which offsets the drying shrinkage caused by the hydration of ordinary silicate cement and water evaporation, prevents mortar cracking, and improves the volume stability and durability of the mortar.

[0015] Preferably, in step S2, the mixer is a twin-shaft paddle zero-gravity mixer; the hydroxypropyl methylcellulose ether and redispersible latex powder need to be premixed before being added to the mixer. In step S3, the mixing and stirring process parameters are controlled as follows: spindle speed 45-60 r / min, blade speed 1400-1500 r / min, and stirring time 180-300 seconds. In step S2, the fine aggregate is natural river sand that has been dried to a moisture content ≤0.5 wt%; the viscosity of the hydroxypropyl methylcellulose ether is 40000-100000 mPa·s.

[0016] By employing the above technical solutions, specific mixing equipment and process parameters ensure the uniform dispersion of trace additives in a large quantity of base materials. The critical scattering velocity generated by the twin-shaft paddles, combined with the high-speed shearing of the flying blades, breaks up agglomerates of cellulose ethers and adhesive powders, avoiding performance defects caused by localized enrichment. The premixing process further guarantees the uniform distribution of lightweight components. Strict control of the moisture content of fine aggregates is to prevent precursors from becoming damp and deactivated, ensuring the quality stability of the finished mortar during storage.

[0017] This invention provides a method for producing dry-mixed mortar by adding recycled coal ash. It has the following beneficial effects: 1. This invention modifies fly ash in situ using solid organic acids, lightly calcined magnesium oxide, and polyphosphates. In the initial stage of mortar water addition, the organic acids on the precursor surface dissolve to establish a local weakly acidic environment, promoting the hydrolysis of polyphosphates and corroding the fly ash surface. The dissociated phosphate ions react rapidly with magnesium ions to generate magnesium phosphate hydration products. This reaction is independent of the cement hydration process and quickly forms a structural skeleton in the early stage, solving the problem of low early strength of high-volume fly ash mortar.

[0018] 2. This invention uses mechanical activation treatment in an anhydrous environment to make the reactive components adhere tightly to the surface of the refined fly ash particles. This treatment promotes the in-situ growth of the subsequently generated magnesium phosphate cement phase on the fly ash matrix, improves the structure of the interface transition zone between fly ash and cement paste, and thus improves the tensile bond strength and surface wear resistance of the mortar.

[0019] 3. This invention limits the activity value and particle size of lightly calcined magnesium oxide to match its reaction rate with the gelation process of the system. During the hardening process, some magnesium oxide participates in the formation of magnesium phosphate cement phase, while the other part hydrates to generate magnesium hydroxide, resulting in a small amount of solid phase volume expansion. This expansion effect offsets the drying shrinkage caused by the hydration of silicate cement and the evaporation of water, giving the mortar good volume stability and reducing the risk of cracking. Detailed Implementation

[0020] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The specifications of the raw materials used in this invention are as follows: Ordinary Portland cement: Commercially available P.O42.5 grade; Fly ash: Class II, F type fly ash, pre-dried to a moisture content ≤0.5wt% before use; Lightly calcined magnesium oxide: activity value 30-60 seconds, average particle size D50≤20μm; Fine aggregate: natural river sand, dried to a moisture content of ≤0.5wt%.

[0022] Preparation Examples 1-5: Preparation Example 1: This preparation example provides a modified coal ash precursor, which, by mass percentage, consists of the following components: 90.0% fly ash, 1.0% oxalic acid dihydrate, 6.0% lightly calcined magnesium oxide, and 3.0% sodium tripolyphosphate.

[0023] Its preparation method includes the following steps: Weigh out each ingredient according to the proportions and put them into a V-type mixer for premixing for 5 minutes; The premixed materials are fed into a closed ball mill and mechanically activated in an anhydrous environment with a relative humidity of less than 40%. The grinding time is 20 minutes, and the material temperature is controlled to be below 50°C during the grinding process. The material is discharged and sieved through an 80-mesh screen to obtain the modified fly ash precursor.

[0024] Preparation Example 2: This preparation example provides a modified coal ash precursor, which, by mass percentage, consists of the following components: 93.0% fly ash, 0.5% oxalic acid dihydrate, 5.0% lightly calcined magnesium oxide, and 1.5% sodium tripolyphosphate.

[0025] Its preparation method includes the following steps: Weigh out each ingredient according to the proportions and put them into a V-type mixer for premixing for 3 minutes; The premixed materials are fed into a closed ball mill and mechanically activated in an anhydrous environment with a relative humidity of less than 40%. The grinding time is 15 minutes, and the material temperature is controlled to be below 50°C during the grinding process. The material is discharged and sieved through an 80-mesh screen to obtain the modified fly ash precursor.

[0026] Preparation Example 3: This preparation example provides a modified fly ash precursor, which is composed of the following components by mass percentage: 85.0% fly ash, 1.5% oxalic acid dihydrate, 10.0% lightly calcined magnesium oxide, and 3.5% sodium tripolyphosphate.

[0027] Its preparation method includes the following steps: Weigh out each ingredient according to the proportions and put them into a V-type mixer for premixing for 5 minutes; The premixed materials are fed into a closed ball mill and mechanically activated in an anhydrous environment with a relative humidity of less than 40%. The grinding time is 30 minutes, and the material temperature is controlled to be below 60°C during the grinding process. The material is discharged and sieved through an 80-mesh screen to obtain the modified fly ash precursor.

[0028] Preparation Example 4: This preparation example provides a modified coal ash precursor. Except for replacing the oxalic acid dihydrate in the raw material with L-tartaric acid in equal amounts, the types and proportions of the other components are exactly the same as in Preparation Example 1.

[0029] Its preparation method is exactly the same as that of Preparation Example 1.

[0030] Preparation Example 5: This preparation example provides a modified coal ash precursor. Except for replacing sodium tripolyphosphate in the raw material with sodium hexametaphosphate in equal amounts, the types and proportions of the other components are exactly the same as in Preparation Example 1.

[0031] Its preparation method is exactly the same as that of Preparation Example 1.

[0032] Examples 1-5: Example

[0033] This embodiment provides a dry-mixed mortar with recycled coal ash added, the raw material formula of which is as follows by weight: 200 parts of ordinary silicate cement, 120 parts of the modified fly ash precursor obtained in Preparation Example 1, 675 parts of fine aggregate, 1.0 part of hydroxypropyl methylcellulose ether, and 4.0 parts of redispersible latex powder.

[0034] Its preparation method includes the following steps: Ordinary silicate cement, modified fly ash precursor, and fine aggregate are added to a twin-shaft paddle zero-gravity mixer; Hydroxypropyl methylcellulose ether and redispersible latex powder are premixed and then fed into the mixer through the additive feeding port; Start the mixer, control the spindle speed to 50 r / min and the blade speed to 1450 r / min, and mix for 240 seconds until the material is macroscopically uniform; The material is discharged and sealed in packaging to obtain the finished dry-mixed mortar. Example

[0035] This embodiment provides a dry-mixed mortar with recycled coal ash added, the raw material formula of which is as follows by weight: 250 parts of ordinary silicate cement, 80 parts of the modified fly ash precursor obtained in Preparation Example 2, 667 parts of fine aggregate, 0.5 parts of hydroxypropyl methylcellulose ether, and 2.5 parts of redispersible latex powder.

[0036] Its preparation method includes the following steps: Ordinary silicate cement, modified fly ash precursor, and fine aggregate are added to a twin-shaft paddle zero-gravity mixer; Hydroxypropyl methylcellulose ether and redispersible latex powder are premixed and then fed into the mixer through the additive feeding port; Start the mixer, control the spindle speed to 45 r / min and the blade speed to 1400 r / min, and mix for 180 seconds until the material is macroscopically uniform; The material is discharged and sealed in packaging to obtain the finished dry-mixed mortar. Example

[0037] This embodiment provides a dry-mixed mortar with recycled coal ash added, the raw material formula of which is as follows by weight: 150 parts of ordinary silicate cement, 150 parts of the modified fly ash precursor obtained in Preparation Example 3, 693.5 parts of fine aggregate, 1.5 parts of hydroxypropyl methylcellulose ether, and 5.0 parts of redispersible latex powder.

[0038] Its preparation method includes the following steps: Ordinary silicate cement, modified fly ash precursor, and fine aggregate are added to a twin-shaft paddle zero-gravity mixer; Hydroxypropyl methylcellulose ether and redispersible latex powder are premixed and then fed into the mixer through the additive feeding port; Start the mixer, control the spindle speed to 60 r / min and the blade speed to 1500 r / min, and mix for 300 seconds until the material is macroscopically uniform; The material is discharged and sealed in packaging to obtain the finished dry-mixed mortar. Example

[0039] This embodiment provides a dry-mixed mortar with recycled coal ash added, the raw material formula of which is as follows by weight: 200 parts of ordinary silicate cement, 120 parts of the modified fly ash precursor (containing L-tartaric acid) obtained in Preparation Example 4, 675 parts of fine aggregate, 1.0 part of hydroxypropyl methylcellulose ether, and 4.0 parts of redispersible latex powder.

[0040] Its preparation method is exactly the same as that in Example 1. Example

[0041] This embodiment provides a dry-mixed mortar with added recycled coal ash. The raw material formula is as follows by weight: 200 parts of ordinary silicate cement, 120 parts of the modified coal ash precursor (containing sodium hexametaphosphate) obtained in Preparation Example 5, 675 parts of fine aggregate, 1.0 part of hydroxypropyl methylcellulose ether, and 4.0 parts of redispersible latex powder.

[0042] Its preparation method is exactly the same as that in Example 1.

[0043] Comparative Examples 1-5: Comparative Example 1: Compared with Example 1, the difference is that the modified fly ash precursor used was prepared without the addition of oxalic acid dihydrate, lightly calcined magnesium oxide and sodium tripolyphosphate, and was only 100% fly ash. However, the precursor still underwent the same ball milling process as in Example 1; the remaining preparation steps and parameters were the same as in Example 1.

[0044] Comparative Example 2: Compared with Example 1, the difference is that the pre-ball milling mechanical activation step of the modified fly ash precursor was omitted. Specifically, the fly ash, oxalic acid dihydrate, lightly calcined magnesium oxide, and sodium tripolyphosphate in the formulation of Example 1 were directly added to a twin-shaft paddle zero-gravity mixer in their original powder state along with ordinary silicate cement, fine aggregate, and admixtures; all other proportions and mixing parameters were the same as in Example 1.

[0045] Comparative Example 3: Compared with Example 1, the difference is that when preparing the modified fly ash precursor, the raw material does not contain component A (oxalic acid dihydrate), and the missing mass fraction is made up by fly ash; otherwise, it is the same as Example 1.

[0046] Comparative Example 4: Compared with Example 1, the difference is that when preparing the modified fly ash precursor, the raw material does not contain component C (sodium tripolyphosphate), and the missing mass fraction is made up by fly ash; otherwise, it is the same as Example 1.

[0047] Comparative Example 5: Compared with Example 1, the difference is that when preparing the modified coal ash precursor, component B (lightly calcined magnesium oxide) in the raw material is replaced with an equal mass of darkly calcined magnesium oxide (activity value > 1200 seconds); all other aspects are the same as in Example 1.

[0048] Test Examples 1-5: Test Example 1: Comparative Test of Physical and Mechanical Properties and Volumetric Stability Experimental method description: This test aims to compare the comprehensive performance of the dry-mixed mortars obtained in Example 1 and Comparative Example 1 to verify the actual effectiveness of the multi-component solid-phase in-situ modification technology. The experiment was conducted according to the "Standard for Test Methods of Basic Performance of Building Mortar" (JGJ / T70-2009), and the specific test items and procedures are as follows: Sample preparation: Accurately weigh each component according to the proportions set in Example 1 and Comparative Example 1. The amount of water added was determined according to the consistency test in the "Standard for Test Methods of Basic Performance of Building Mortar", and the mortar consistency was controlled within the range of 90mm ± 5mm to ensure consistent workability. The well-mixed mortar was poured into a 70.7mm × 70.7mm × 70.7mm steel triple mold, compacted using a vibrating table, and the surface was smoothed.

[0049] Curing conditions: The specimens were cured in a standard curing room with a temperature of 20±2℃ and a relative humidity of over 90%. After 24 hours, the specimens were demolded and cured under the same conditions until the specified age.

[0050] Setting time determination: The initial and final setting times of the mortar were determined using a penetration resistance meter.

[0051] Compressive strength test: The compressive strength of the specimens was determined using a pressure testing machine at 1 day (1d), 3 days (3d), and 28 days (28d) of age, with the loading rate controlled between 0.25 and 1.5 kN / s. The result was the arithmetic mean of a set of 3 specimens.

[0052] Tensile bond strength test: A concrete slab was used as the substrate, and specimens with a molding size of 40mm×40mm were formed. After curing for 14 days, the tensile bond strength was determined using a pull-out tester.

[0053] Drying shrinkage test: Prepare prism specimens of 25mm×25mm×280mm and measure the natural drying shrinkage rate at 28 days.

[0054] Experimental results: The physical and mechanical performance test results of Example 1 and Comparative Example 1 are shown in Table 1.

[0055] Table 1. Mortar Performance Test Data Recording Table for Example 1 and Comparative Example 1 Test Project unit Example 1 Comparative Example 1 Consistency Water Quantity % 16.4 15.8 Initial setting time min 265 230 Final freezing time min 382 345 Compressive strength (1d) MPa 5.8 2.4 Compressive strength (3d) MPa 12.7 6.1 Compressive strength (28d) MPa 23.4 16.8 Tensile bond strength (14d) MPa 1.12 0.58 28-day drying shrinkage % 0.035 0.118 Surface condition (28d) - Smooth and free of exudation A slight white precipitation was observed. Note: The “-” in the table indicates that the test item has no unit of measurement.

[0056] Results Analysis and Conclusions: Based on the data in Table 1 and the reaction mechanism of this invention, the experimental results are analyzed as follows: Early intensity difference analysis: Data shows that the 1-day compressive strength (5.8 MPa) of Example 1 is 2.4 times that of Comparative Example 1 (2.4 MPa), and the 3-day strength is 2.08 times that of Comparative Example 1. Although Comparative Example 1 underwent the same ball milling treatment, increasing the specific surface area of ​​fly ash, it is essentially a physical filler. In the early stages of hydration, fly ash only exhibits a micro-aggregate effect, and the pozzolanic reaction is extremely slow, failing to contribute to early strength. In contrast, Example 1 introduced organic acids, magnesium oxide, and polyphosphates. In the initial stage of contact with water, the organic acids establish a localized low pH environment on the fly ash surface, promoting the dissolution of the vitreous phase and the hydrolysis of polyphosphates. Subsequently, the released phosphate ions react with magnesium ions and dissolved aluminum ions, rapidly generating magnesium phosphate and phosphoaluminate cementitious phases. These products exhibit rapid hardening and early strength characteristics, forming an effective cementitious skeleton on the surface and between fly ash particles before cement hydration is fully completed, thereby improving the early physical and mechanical properties of the mortar.

[0057] Mechanism for improving interfacial bonding and adhesion strength: The tensile bond strength of Example 1 (1.12 MPa) was higher than that of Comparative Example 1 (0.58 MPa). In Comparative Example 1, the fly ash particles had smooth surfaces and were chemically inert, relying mainly on van der Waals forces and physical interlocking with the cement hydration products (CSH gel), resulting in structural defects in the interfacial transition zone. In Example 1, however, the reactive components were embedded in the fly ash surface through ball milling pretreatment. The phosphate crystalline phase, which grew in situ during hydration, was directly anchored to the fly ash matrix and extended outward to form chemical bonds with the cement paste. This in-situ growth mode increased the surface roughness and chemical activity of the fly ash, improved the microstructural density of the interfacial transition zone, and thus enhanced the macroscopic bond strength.

[0058] Compensation mechanism for volume stability: The drying shrinkage data show that the shrinkage deformation of Example 1 (0.035%) is much smaller than that of Comparative Example 1 (0.118%). The ordinary cement-fly ash system (Comparative Example 1) experiences significant volume shrinkage during hardening due to moisture evaporation and chemical shrinkage, which easily leads to cracking. In Example 1, the lightly calcined magnesium oxide introduced undergoes a slight solid-phase volume expansion during hydration to magnesium hydroxide and participation in the magnesium phosphate reaction. This expansion effect effectively offsets the drying shrinkage stress of the matrix during the structure formation period. Simultaneously, the resulting magnesium phosphate cementitious phase exhibits low drying shrinkage characteristics, further enhancing the volume stability of the matrix.

[0059] in conclusion: Experimental results confirm that while simple mechanical activation (Comparative Example 1) improves the physical dispersibility of fly ash, it cannot overcome its inherent defects of low early activity and weak interfacial bonding. Test results in Example 1 show that, compared to Comparative Example 1, this invention, through the synergistic modification of organic acids, magnesium oxide, and polyphosphates, can generate a magnesium phosphate-based cementitious phase with high early strength and volume stability on the fly ash surface, thereby improving the early strength, adhesion, and volume stability of dry-mixed mortar.

[0060] Test Example 2: The Influence of Mechanical Activation Process on Mortar Hardening Process and Strength Development Experimental method description: This test case primarily examines the crucial role of the pre-ball milling mechanical activation process in the final properties of the materials. The experimental subjects are Example 2 and Comparative Example 2.

[0061] The experimental procedure strictly followed the "Standard for Test Methods of Basic Performance of Building Mortar" (JGJ / T70-2009).

[0062] First, the consistency was measured, and the amount of water added was adjusted to keep the consistency of both mortars at 90mm±5mm in order to eliminate the interference of rheological differences on the test results.

[0063] A 70.7 mm cube specimen was molded for compressive strength testing, and a 40 mm × 40 mm × 160 mm prism specimen was molded for flexural strength testing.

[0064] The specimens were cured in a standard curing room (20±2℃, RH≥90%).

[0065] The monitoring frequency of condensation time was increased in order to capture differences in reaction kinetics.

[0066] Mechanical strength data were measured at 1 day, 7 days and 28 days.

[0067] Experimental results: The performance test data of Example 2 and Comparative Example 2 are shown in Table 2.

[0068] Table 2 Comparative data on the effects of mechanical activation process on mortar performance.

[0069] Test metrics unit Example 2 Comparative Example 2 Actual water consumption g 245 238 Mortar wet density <![CDATA[kg / m 3 ]]> 1985 2010 Initial setting time h:min 4:15 11:20 Final freezing time h:min 6:05 18:45 Compressive strength (1d) MPa 4.9 0.8 Compressive strength (7d) MPa 15.3 8.2 Compressive strength (28d) MPa 21.6 14.5 Flexural strength (28d) MPa 4.2 2.8 Appearance of the test block (at demolding) - Well hardened The surface is soft and some corners are chipped. Note: The “-” in the table indicates that the test item has no unit of measurement.

[0070] Results Analysis and Conclusions: Based on the data in Table 2, the mechanism of action of the mechanical activation process in the system is analyzed as follows: Condensation time and reaction kinetics analysis The data shows that the setting time of Example 2 is within the normal range (initial setting time is about 4 hours), while Comparative Example 2 shows a slow setting phenomenon, with the initial setting time extended to more than 11 hours and the final setting time approaching 19 hours.

[0071] This difference directly confirms that the pre-ball milling process alters the chemical reaction pathway.

[0072] In Example 2, oxalic acid, magnesium oxide, and polyphosphate were ball-milled and adhered tightly to the surface of coal ash. After the addition of water, a high-concentration reaction zone was rapidly formed in the micron-scale diffusion layer, where acid-base neutralization and magnesium-phosphorus cementation reactions preferentially occurred, promoting slurry coagulation.

[0073] In Comparative Example 2, the components were randomly dispersed. Organic acids and polyphosphates dissolved directly in the mixing water and rapidly diffused throughout the cement paste system. Due to the lack of local spatial confinement, the dissolved organic acids underwent a large-scale neutralization reaction with the calcium hydroxide produced during cement hydration, interfering with the normal hydration process of silicate cement. At the same time, polyphosphates, as a strong retarder, adsorbed onto the surface of cement particles without being consumed by magnesium oxide, further hindering the hydration reaction and leading to a prolonged setting time.

[0074] Differences in the mechanisms of early strength development: Example 2 has a 1-day compressive strength of 4.9 MPa, which is sufficient for demolding; while Comparative Example 2 has a compressive strength of only 0.8 MPa, and breaks during demolding.

[0075] This indicates that the expected magnesium phosphate cementitious material was not effectively generated in Comparative Example 2. In the direct physical mixing system, the large amount of calcium ions (Ca) in the cement paste... 2+ Calcium ions have a strong binding affinity to dissolved phosphate ions, rapidly forming a poorly soluble and non-cementing calcium phosphate precipitate. Calcium ions exhibit a clear competitive advantage in kinetics, consuming the phosphate ions that would otherwise bind with magnesium ions (Mg²⁺). 2+ The magnesium oxide and polyphosphate were combined. In Example 2, mechanical activation was used to achieve a submicron-level close contact between magnesium oxide and polyphosphate in space. Upon contact with water, the concentration of magnesium ions and phosphate ions in the local area was extremely high. According to the principle of chemical reaction kinetics, the magnesium-phosphorus reaction occurred preferentially in the local area, preferentially generating magnesium phosphate hydrate with cementing strength, thereby avoiding the interference of calcium ions.

[0076] As a result of the contribution to the final mechanical properties, the compressive strength of Example 2 was approximately 49% higher than that of Comparative Example 2 at 28 days of age.

[0077] In Comparative Example 2, due to the lack of pretreatment, fly ash only served as an inert filler, and due to the side effects of the retarder, the cement stone structure was loose.

[0078] Example 2 utilizes mechanochemical processes to achieve not only in-situ growth of the magnesium-phosphorus cementing phase but also enhances the chemical activity of the fly ash surface through lattice distortion energy introduced during the ball milling process, enabling it to better participate in the pozzolanic reaction in the later stages. This dual enhancement mechanism (magnesium-phosphorus cementation and activity activation) ensures the mortar's superior long-term mechanical properties.

[0079] in conclusion: This test case fully demonstrates that the anhydrous mechanical activation preparation of precursors in this invention is not a simple physical mixing process, but a key technology for constructing a specific chemical reaction microenvironment. This process successfully solves the problem of competitive reaction control in multi-component gelling systems, ensuring the formation of the magnesium-phosphorus cementing phase and avoiding severe retardation and early strength loss caused by simple component mixing.

[0080] Test Example 3: The regulatory effect of solid-phase acid sources on reaction initiation and early intensity Experimental method description: This test case focuses on the chemical initiation function of component A (crystalline organic dicarboxylic acid) in the modified system. The comparison objects are Example 3 (a high-dosage formulation containing oxalic acid dihydrate) and Comparative Example 3 (without oxalic acid dihydrate, otherwise the same as Example 3).

[0081] The experiment consisted of two parts: monitoring the pH evolution of the slurry and testing the mechanical properties of the mortar. Precursor pH evolution test: 5.0 g of the modified coal ash precursor powder prepared in Example 3 and Comparative Example 3 were weighed and quickly added to 50 ml of deionized water (liquid-solid ratio 10:1). The mixture was then placed on a magnetic stirrer and stirred at 300 r / min. The pH change of the slurry was continuously monitored from 0 to 30 minutes after contact with water using a high-precision pH meter, and data were recorded every 5 minutes to characterize the acidity and alkalinity of the microenvironment.

[0082] Mortar physical property testing: Following GB / T17671-2021 standard, two formulations were prepared into standard prism specimens of 40mm×40mm×160mm. Since this test focuses on reaction initiation characteristics, the very early compressive strength at 12 hours and 24 hours, and the compressive strength at 28 days were specifically tested. Setting time was also measured to determine whether the hydration reaction had started normally.

[0083] Experimental results: The precursor pH evolution and mortar performance test data of Example 3 and Comparative Example 3 are recorded in Table 3.

[0084] Table 3. Effects of acid source deficiency on pH environment and mechanical properties of the system. Test Project Time Nodes / Indicators Example 3 Comparative Example 3 Pulp pH monitoring Stir for 1 minute 4.25 9.85 Stir for 5 minutes 4.82 10.42 Stir for 15 minutes 6.15 11.2 Stir for 30 minutes 8.4 11.85 Mortar performance Initial setting time (min) 215 580 Compressive strength (12h) (MPa) 2.1 0.0 (Not demolded) Compressive strength (24h) (MPa) 5.4 1.2 Compressive strength (28d) (MPa) 24.8 13.5 Specimen cross-sectional condition (28 days) The structure is dense, and the fracture surface penetrates the aggregate. Loose structure, dusty fracture surface Results Analysis and Conclusions: Based on the data in Table 3 and the principles of microscopic reaction kinetics, the analysis is as follows: Microenvironment pH regulation mechanism: pH monitoring data directly reflects the differences in the chemical environment at the initial stage of the reaction. In Example 3, the slurry maintained a weakly acidic environment (pH 4.25–6.15) for the first 15 minutes after contact with water. This was due to the rapid dissolution of the ball-milled, micron-sized oxalic acid crystals, releasing hydrogen ions. This acidic window period is the core start-up condition of this technical solution: on the one hand, the lower pH value promotes the hydrolysis of sodium tripolyphosphate, causing it to break down from its long-chain structure into highly reactive orthophosphate ions; on the other hand, the acidic environment causes proton erosion on the surface of the fly ash vitreous, inducing the depolymerization of the aluminosilicate network.

[0085] In contrast, Comparative Example 3 exhibited strong alkalinity (pH>9) upon contact with water, and the pH rapidly increased to above 11 as magnesium oxide hydrated. Under alkaline conditions, sodium tripolyphosphate hydrolyzed extremely slowly, and the fly ash surface remained passivated, preventing the establishment of the expected chemical reaction pathways.

[0086] Chemodependence of early strength establishment: Strength data indicate that the absence of organic acids leads to a near loss of early strength. Example 3 achieved a strength of 2.1 MPa at 12 hours and 5.4 MPa at 24 hours, indicating that the magnesium-phosphorus bonding reaction occurred rapidly and provided an early framework.

[0087] Comparative Example 3 failed to demold after 12 hours, and its strength after 24 hours was only 1.2 MPa. This confirms that, in the absence of acid activation, magnesium oxide and unhydrolyzed polyphosphate are difficult to react directly at room temperature. At this point, magnesium oxide either acts as an inert filler or undergoes slow hydration to form magnesium hydroxide, which is a loose, layered crystal with no binding capacity. The strength of Comparative Example 3 in the later stage (28 days) mainly comes from the normal hydration of the cement components; the fly ash did not contribute additional strength through chemical modification, and therefore it was lower than that of Example 3.

[0088] Reaction timing control: In Example 3, the pH value rose back to 8.40 after 30 minutes, indicating that the acidic component had been consumed and the system gradually became alkaline, which aligns with the alkaline requirements of cement hydration. This dynamic pH evolution, from acid to alkali, ensured the occurrence of the modification reaction while avoiding the damage to the stability of cement hydration products (CSH) caused by prolonged acidity. Comparative Example 3 remained highly alkaline throughout, lacking the crucial acid initiation step, leading to the failure of the magnesium phosphate cementing system.

[0089] in conclusion: This test case demonstrates the crucial role of solid organic acids in the system. By comparing pH changes and strength data, it is evident that the acidic environment created by the dissolution of organic acids promotes the hydrolysis of polyphosphates and the dissolution of magnesium oxide, thereby initiating subsequent chemical reactions. As shown in Comparative Example 3, without organic acids, the system remains in a highly alkaline environment, failing to form an effective cementing phase, resulting in extremely low early strength.

[0090] Test Example 4: Effect of polyphosphate components on cemented phase construction and water resistance Experimental method description: This test case aims to verify the core role of component C (polyphosphate) in forming a chemically cemented framework in the modified system. The comparison objects are Example 4 (a complete system containing L-tartaric acid and sodium tripolyphosphate) and Comparative Example 4 (lacking sodium tripolyphosphate, containing only L-tartaric acid and lightly calcined magnesium oxide).

[0091] The experiment focused on the water resistance strength retention rate (softening coefficient) and surface abrasion resistance of hardened mortar to characterize the chemical stability and bonding strength of the cemented products.

[0092] Water resistance strength (softening coefficient) test: Two sets of mortar specimens were prepared according to JGJ / T70-2009 standard. One set was cured for 28 days under standard conditions (dry strength was measured); the other set was cured for 21 days, then immersed in water at 20±2℃ for 7 days, and after removing and wiping off the surface moisture, the wet strength was measured. The softening coefficient was calculated as: wet strength / dry strength.

[0093] Surface abrasion resistance test: Refer to the abrasion resistance test method in JC / T985-2005 "Cement-based self-leveling mortar for flooring", use a JM-V type abrasion tester, load 500g, grind for 200 revolutions, and measure the wear amount on the surface of the specimen.

[0094] Experimental results: The water resistance and abrasion resistance test data of Example 4 and Comparative Example 4 are shown in Table 4.

[0095] Table 4. Data on the impact of polyphosphate deficiency on the water resistance and abrasion resistance of mortar. Test metrics unit Example 4 Comparative Example 4 28-day air curing compressive strength MPa 22.4 14.2 28-day saturated compressive strength MPa 20.8 7.9 Softening coefficient - 0.93 0.56 Surface wear (200 revolutions) g 0.45 1.82 Appearance characteristics after immersion in water - Surface integrity The surface is powdery and has a white leaching substance. 1d compressive strength MPa 5.2 0.9 Note: The “-” in the table indicates that the test item has no unit of measurement.

[0096] Results Analysis and Conclusions: Based on the data in Table 4 and the chemical reaction mechanism, the analysis is as follows: The essential differences between cemented phases: Strength and abrasion resistance data show that Comparative Example 4, lacking polyphosphate, exhibits severely degraded performance. In Example 4, the hydrogen ions dissociated from L-tartaric acid promote the dissolution of magnesium oxide, releasing magnesium ions (Mg... 2+It rapidly reacts with sodium tripolyphosphate to produce hydrogen phosphate (HPO4). 2- ) and phosphate (PO4) 3- A chemical reaction occurs, generating a continuous cementitious phase mainly composed of struvite (MgNH4PO4·6H2O) or amorphous magnesium phosphate (Mg-PO4-H2O). This phase exhibits ceramic-like hardness and brittleness, along with extremely high binding strength, tightly binding the fly ash particles.

[0097] In Comparative Example 4, due to the lack of phosphate source, the reaction of L-tartaric acid with magnesium oxide mainly produces magnesium tartrate or locally forms magnesium hydroxide (Mg(OH)2). Magnesium hydroxide mainly exists as loose, flaky crystals, lacking cementing ability, and organic magnesium salts generally have low strength. Therefore, Comparative Example 4 exhibits macroscopically low strength and poor surface abrasion resistance (the amount of wear is 4 times that of Example 4), indicating that an effective inorganic cementing framework has not been formed internally.

[0098] Factors determining water resistance: The softening coefficient data revealed differences in the solubility characteristics of the modified products. The softening coefficient of Example 4 was as high as 0.93, indicating that the generated magnesium phosphate cement phase has excellent water resistance and insolubility, and is structurally stable in an aqueous environment.

[0099] Comparative Example 4 showed a softening coefficient of only 0.56, with a significant decrease in strength after immersion in water. This is because, in the absence of phosphates, the organic magnesium salts generated in the system are mostly water-soluble or slightly soluble, dissolving during immersion. Simultaneously, the magnesium hydroxide, a simple magnesium oxide hydration product, lacks chemical bonding with the fly ash surface, leading to the failure of physical bonding upon water penetration into the interface. Furthermore, unconsumed L-tartaric acid, being a small-molecule organic acid, can easily cause osmotic pressure changes upon contact with water, damaging the cement stone structure.

[0100] Analysis of the limitations of the acid-magnesium system: The experimental results of Comparative Example 4 show that, even with the addition of organic acid and magnesium oxide, the strength and water resistance of the specimens were poor in the absence of polyphosphate. This indicates that simple organic acid magnesium salts or magnesium hydroxide precipitates cannot provide sufficient bonding strength and water resistance.

[0101] in conclusion: This test example demonstrates that polyphosphates are key components in forming a high-strength, water-resistant cementitious phase. The magnesium phosphate hydrate generated in Example 4 exhibits excellent water resistance and bonding strength, acting as a skeletal support on the surface of fly ash particles; while Comparative Example 4, lacking phosphate groups, could not generate this type of insoluble gel, resulting in a decrease in mortar strength after immersion in water.

[0102] Test Example 5: Effect of magnesium oxide activity on reaction compatibility and volume stability Experimental Method Description This test case compares Example 5 (system containing sodium hexametaphosphate and lightly calcined magnesium oxide) with Comparative Example 5 (system containing sodium tripolyphosphate and recalcined magnesium oxide). Although the types of phosphates are slightly different, the core purpose of this test is to examine the active state of magnesium oxide and the kinetic matching relationship of the acid-phosphorus system, as well as its decisive influence on the volume stability and stability of the material.

[0103] The experiments were mainly conducted in accordance with GB / T23439-2017 "Concrete Expansion Agent" and GB / T1346-2011 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement".

[0104] Restricted expansion rate test: Prismatic specimens with dimensions of 40mm×40mm×160mm were fabricated, with a longitudinal reinforcing cage (reinforcement ratio of 0.76%) pre-embedded in the center of the mold. After molding, the specimens were covered with plastic wrap and demolded after 24 hours, and the initial length was measured immediately. The specimens were then immersed in water at 20±2℃ for curing, and the length changes were measured at 1d, 3d, 14d, and 28d to calculate the restricted expansion rate.

[0105] Autoclaving stability test (Raychalcogenide clamp method and strength method): To accelerate the detection of delayed hydration hazards of inactive magnesium oxide, 28-day-old hardened specimens were placed in an autoclave and treated at a constant temperature of 2.0 MPa steam pressure (corresponding to a temperature of approximately 215.7℃) for 3 hours. The appearance integrity of the specimens after treatment was observed, and the compressive strength loss rate before and after autoclaving was measured.

[0106] XRD phase analysis: Hardened slurry samples with a curing age of 1 day were taken, hydration was terminated with anhydrous ethanol, and after grinding and sieving, X-ray diffraction analysis was performed to qualitatively compare the intensity of the characteristic peak of MgO and whether magnesium ammonium phosphate (Struvite) or similar cementing phases were formed.

[0107] Experimental results: The volume stability and microstructure analysis data of Example 5 and Comparative Example 5 are shown in Table 5.

[0108] Table 5. Data on the effect of magnesium oxide activity on reaction matching and volume stability. Test Project Age / Condition Example 5 Comparative Example 5 Limiting expansion rate (%) 1d 0.006 -0.015 3d 0.012 -0.028 14d 0.018 -0.046 28d 0.021 -0.062 Pressure stability Appearance description The edges and corners are intact and there are no cracks. Surface cracked, bubbly Strength loss rate 0.032 0.385 XRD features (1d) MgO (2θ=42.9°) Peak intensity extremely weak Peak intensity is sharp (unconsumed) Mg-P cemented phase There is a clear diffuse peak Not detected Mechanical properties 1-day compressive strength (MPa) 5.8 1.9 28-day compressive strength (MPa) 23.5 16.1 In the table, "+" represents expansion and "-" represents contraction in the limit expansion rate.

[0109] Results Analysis and Conclusions: Based on the data in Table 5 and the principles of multiphase reaction kinetics, the analysis is as follows: Kinetic matching mechanism of reaction window period: The core mechanism of this invention lies in using organic acids to create a short acidic window during which the depolymerization of polyphosphate and the dissolution-precipitation of magnesium oxide are coupled.

[0110] The lightly calcined magnesium oxide used in Example 5 (activity value 30-60s) has a porous structure and high lattice energy, and its dissolution rate matches that of sodium hexametaphosphate. XRD data confirmed that at 1 day, the characteristic peak of MgO in Example 5 decreased, indicating that most of the MgO had participated in the reaction and transformed into the magnesium phosphate cement phase, thus contributing to the early strength of 5.8 MPa.

[0111] Conversely, the recalcined magnesium oxide used in Comparative Example 5 has a dense crystal structure and an extremely low specific surface area, making it an inert component. During the acidic window, its dissolution rate is far lower than the rate at which the acid is neutralized. Once the system's pH becomes strongly alkaline due to cement hydration, even if phosphate hydrolyzes, it cannot find sufficient Mg. 2+ The bonding process leads to the failure of the gelation reaction. XRD shows that MgO in Comparative Example 5 was hardly consumed, and the early strength was only 1.9 MPa, mainly due to the contribution of cement hydration.

[0112] Volume compensation effect and shrinkage control: The controlled expansion rate data showed that Example 5 exhibited stable micro-expansion characteristics during water curing (+0.021% at 28 days). This controlled chemical expansion stemmed from the crystal growth pressure of the hydrous magnesium phosphate phase generated during the reaction and an appropriate amount of magnesium hydroxide, which effectively counteracted the chemical shrinkage of the cement matrix and played a role in crack resistance.

[0113] Comparative Example 5 showed continuous shrinkage (-0.062% at 28 days), and its deformation behavior was no different from that of ordinary cement mortar, indicating that the recalcined magnesium failed to provide any volume compensation effect during the normal temperature curing period.

[0114] Stability risks caused by delayed hydration: The autoclaving test results revealed the potential hazards of reburned magnesium oxide. Comparative Example 5, after high-temperature and high-pressure treatment, showed surface cracking and a strength loss of nearly 40%. This is because the residual reburned magnesium oxide was forced to hydrate and form magnesium hydroxide under autoclaving conditions, a reaction accompanied by an approximately 148% increase in solid phase volume. Since the cement stone skeleton had already hardened at this point, the enormous internal crystallization pressure led to microstructural damage.

[0115] The lightly calcined magnesium in Example 5, due to its high activity, was almost completely consumed through chemical reactions in the early stage (transformed into a cemented phase or stable hydration products), thus exhibiting excellent volume stability in the later stage.

[0116] in conclusion: This test case confirms the decisive role of magnesium oxide activity in this technical solution. Only by selecting lightly calcined magnesium oxide with suitable activity (30–60 s) can dissolution and cementation be achieved simultaneously in a specific chemical microenvironment, realizing the dual technical effects of early strength enhancement and volume shrinkage compensation. Using recalcined magnesium oxide not only leads to the failure of the modification mechanism but also introduces serious volume stability problems due to delayed hydration.

Claims

1. A method for producing dry-mixed mortar with recycled coal ash, characterized in that, Includes the following steps: S1. Preparation of modified fly ash precursor: Weigh fly ash, organic acid, lightly calcined magnesium oxide and polyphosphate according to the ratio, mix them, perform mechanical activation treatment and sieve; S2. Dry mixing: Add ordinary silicate cement, modified fly ash precursor obtained in step S1, and fine aggregate to the mixer, and add hydroxypropyl methylcellulose ether and redispersible latex powder to the mixer. S3. Start the mixer to mix and stir until the dry mortar material is macroscopically uniform, then discharge and package it. The dry-mixed mortar is made from raw materials comprising the following parts by weight: 150-250 parts of ordinary Portland cement; 80-150 parts of modified fly ash precursor; Fine aggregate 660-700 parts; Hydroxypropyl methylcellulose ether, 0.5–1.5 parts; 2.5 to 5.0 parts of redispersible latex powder.

2. The method for producing dry-mixed mortar with recycled coal ash according to claim 1, characterized in that, The modified coal ash precursor consists of the following components by mass percentage: Fly ash content: 85.0%–93.0%; Organic acids 0.5%–1.5%; Lightly calcined magnesium oxide 5.0%–10.0%; Polyphosphate 1.5%–3.5%.

3. The method for producing dry-mixed mortar with recycled coal ash according to claim 2, characterized in that, In step S1, the mechanical activation treatment is carried out in a closed ball mill, the ball mill environment is controlled to be an anhydrous environment with a relative humidity of less than 40%, the material temperature is controlled to be below 60°C during the grinding process, and the grinding activation time is 15 to 30 minutes.

4. The method for producing dry-mixed mortar with recycled coal ash according to claim 2, characterized in that, The organic acid is oxalic acid dihydrate or L-tartaric acid; the polyphosphate is sodium tripolyphosphate or sodium hexametaphosphate.

5. The method for producing dry-mixed mortar with recycled coal ash according to claim 2, characterized in that, The lightly calcined magnesium oxide has an activity value of 30-60 seconds and an average particle size D50 ≤ 20 μm; the fly ash is Class II F fly ash that has been pre-dried to a moisture content ≤ 0.5 wt%.

6. The method for producing dry-mixed mortar with recycled coal ash according to claim 1, characterized in that, In step S2, the mixer is a dual-shaft blade zero-gravity mixer.

7. The method for producing dry-mixed mortar with recycled coal ash according to claim 1, characterized in that, In step S2, the hydroxypropyl methylcellulose ether and redispersible latex powder need to be premixed before being fed into the mixer.

8. The method for producing dry-mixed mortar with recycled coal ash according to claim 1, characterized in that, In step S3, the process parameters for mixing and stirring are controlled as follows: spindle speed 45-60 r / min, cutter speed 1400-1500 r / min, and stirring time 180-300 seconds.

9. The method for producing dry-mixed mortar with recycled coal ash according to claim 1, characterized in that, The fine aggregate is natural river sand that has been dried to a moisture content of ≤0.5wt%.

10. A method for producing dry-mixed mortar with recycled coal ash according to claim 1, characterized in that, The viscosity of the hydroxypropyl methylcellulose ether is 40,000 to 100,000 mPa·s.