Method for preparing high-performance cement mortar based on NaOH activated coal gangue

Abstract

The invention discloses a method for preparing high-performance cement mortar based on NaOH activated coal gangue, which comprises the following steps: putting preliminarily crushed coal gangue into a rod mill for crushing and screening to obtain undersize coal gangue and oversize coal gangue; calcining the treated coal gangue, and putting the calcined product into a rod mill for fine grinding to obtain an admixture; weighing the admixture and the cement in proportion, and uniformly stirring and mixing to obtain a cementing material; the preparation method comprises the following steps: pouring a cementing material and gravel into a stirrer according to a mortar ratio, uniformly stirring, dissolving an exciting agent NaOH and a water reducing agent into water, pouring into the stirrer, and uniformly stirring the mixed materials to obtain the final high-performance cement mortar. The coal gangue is calcined and ground by a rod mill to serve as the admixture to replace part of the Portland fly ash cement, and the exciting agent NaOH is added, so that the bending strength and the compressive strength of a test piece added with the exciting agent NaOH are increased, and the admixture is effectively excited and activated by the NaOH.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid waste recycling, and in particular, relates to a method for preparing high-performance cement mortar based on NaOH-activated coal gangue. Background Art

[0002] Gangue, derived from waste rock during coal mining and washing, is a major solid waste generated by the coal, power, and aluminum industries. Its long-term accumulation not only occupies significant land resources but also significantly pollutes the ecological environment. The disposal of this waste has long plagued the industry. Traditional landfill and stacking methods are not only inefficient in resource utilization, but can also cause soil, water, and air pollution, and even trigger geological disasters.

[0003] In recent years, with the rapid advancement of resource utilization technologies, the use of these solid wastes to prepare cementitious materials has gradually become a research focus. As an indispensable building material in civil engineering and infrastructure construction, cementitious materials' traditional production process consumes large amounts of limestone and clay resources and is accompanied by large amounts of carbon dioxide emissions, which puts significant pressure on the environment.

[0004] In view of this, in the existing technology, coal gangue, as a natural volcanic ash mixed material, has certain activity itself. Its activity can be effectively improved through methods such as thermal activation, mechanical activation, and chemical excitation, so that it can be better utilized as a resource in the field of building materials.

[0005] However, the existing activation method requires cumbersome process to treat coal gangue, and it is difficult to avoid the possibility of secondary pollution. To address the above problems, the present invention aims to develop a new coal gangue activation method and a method for preparing high-performance cement mortar based on the coal gangue obtained by this method. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention provides a new coal gangue activation method, which comprises the following steps: placing the preliminarily crushed coal gangue into a rod mill for crushing and screening to obtain undersize coal gangue and oversize coal gangue; calcining the treated coal gangue, and grinding the calcined product into a rod mill for use as an admixture; weighing the admixture and cement in proportion, stirring and mixing them evenly to obtain a cementitious material; first pouring the cementitious material and sand and gravel into a mixer according to the mortar ratio and stirring them evenly, then dissolving an activator NaOH and a water reducer into water, pouring the mixture into the mixer, and stirring the mixture evenly to obtain the final high-performance cement mortar; so as to solve the corresponding technical problems raised in the above background technology.

[0007] In order to achieve the above objects, the technical solution adopted by the present invention is: In a first aspect, the present invention relates to a method for preparing high-performance cement mortar based on NaOH activation of coal gangue, the method specifically comprising: Step 1: Put the preliminarily crushed gangue into the rod mill for crushing and screening to obtain undersize gangue and oversize gangue; Step 2: washing the coal gangue on the sieve with water, drying the precipitate obtained in the washing process; and mixing the dried precipitate with the coal gangue under the sieve obtained in step 1 to obtain mixed coal gangue; Step 3: preheating the undersize gangue obtained in step 1 or the mixed gangue obtained in step 2, calcining it at a certain temperature, taking it out after calcining for a certain period of time, and placing it in the air to cool it to obtain a calcined product; Step 4: placing the calcined product into a rod mill and grinding it into a fine powder to be used as an admixture; Step 5: Weigh the admixture and cement in a certain proportion, stir and mix them evenly to obtain a cementitious material; Step 6: First, pour the cementitious material and sand and gravel into the mixer according to the mortar-sand ratio and mix them evenly. Then, dissolve the activator NaOH and water reducer into the water, pour it into the mixer, and stir the mixture evenly to obtain the final high-performance cement mortar.

[0008] Preferably, the material-to-rod ratio of the rod mill in step 1 is 25:1, a 1 mm standard sieve is used for screening, and the grinding time is 15-180 min, preferably 120 min.

[0009] Preferably, the sieve ratio of the undersize gangue to the oversize gangue is 0.26-0.49.

[0010] Preferably, the calcination temperature is 650° C. and the calcination time is 30-240 min, preferably the calcination temperature is 650° C. and the calcination time is 120 min.

[0011] Preferably, the size of the admixture in step 4 is ground to ≤0.105 mm.

[0012] Preferably, the mass ratio of admixture to cement in the cementitious material is 1:3; The mass ratio of water to cementitious material, i.e. the water-binder ratio is 0.44; The mass ratio of water reducer to cementitious material is 1:100; The mass ratio of sand and gravel to the total mass of cementitious materials and water reducer is 1:2; The mass ratio of the activator NaOH to the gelling material is 2-10:100, preferably 8:100.

[0013] Preferably, the cement is fly ash silicate cement.

[0014] Preferably, the water reducer is a polycarboxylic acid water reducer.

[0015] Preferably, the particle size of the sand and gravel is ≤2 mm.

[0016] In a second aspect, the present invention relates to a high-performance cement mortar method, which uses the above-mentioned method for preparing high-performance cement mortar based on NaOH activation of coal gangue to prepare cement mortar.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. In the technical solution of the present invention, the hardness difference of coal gangue is utilized to achieve selective crushing of coal gangue. The raw coal gangue is added to a jaw crusher for crushing and then sorted by a rod mill. The fragile material under the screen and the coarse aggregate on the screen are obtained by screening. The coarse aggregate is washed with water, and the precipitate obtained in the washing process is filtered, dried, and mixed with the fragile material under the screen. This achieves mechanical activation of the coal gangue, refines the coal gangue particles, increases the specific surface area, and improves the reaction rate. Specifically, by rod-grinding the gangue, the undersize gangue can be calcined and used as an admixture to replace part of the cement for industrial use. This approach is not only low-cost, but also high-quality and has broad development prospects. This provides more directions for the use of gangue, which is of great significance to its development and promotion.

[0018] 2. In the technical solution of the present invention, the sorted coal gangue is activated by a thermal-chemical composite activation method, and the mixed coal gangue powder material is calcined at a temperature of 650°C. The changes in its internal mineral composition are analyzed by XRD and XRF tests to obtain the optimal calcination conditions. Different amounts of NaOH are added to the product after calcining the coal gangue, and the product is ground into an admixture and mixed with ordinary Portland cement, water, polycarboxylic acid water reducer, sand and gravel to obtain a high-performance cement mortar material. The obtained high-performance cement mortar material has the characteristics of good fluidity, no segregation, good mechanical properties, good durability, etc. The consumption of coal gangue in the process is huge, which effectively reduces the pollution of coal gangue to the environment.

[0019] 3. The advantage of the present invention is that the addition of NaOH improves the flexural and compressive strengths of the mortar material to a certain extent. As the curing age increases, the flexural and compressive strengths continue to increase, that is, an alkali-activated cementitious material substrate and green high-performance mortar are provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the preparation method of the present invention; Figure 2 is the XRD pattern of the coal gangue in the present invention; Figure 3 The XRD patterns of the above-sieve and below-sieve coal gangue after sorting in the present invention are shown in FIG. Figure 4 The XRD patterns of undersize gangue at different calcination temperatures in the present invention; Figure 5 is the flexural strength of high-performance cement mortar at different NaOH dosages in the embodiment of the present invention; Figure 6 is the compressive strength of high-performance cement mortar at different NaOH dosages in the embodiment of the present invention; Figure 7 1 is a test curve of the flexural strength of cement mortar in the comparative example of the present invention; Figure 8 It is the compressive strength test curve of cement mortar in the comparative example of the present invention. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0022] Unless otherwise specified, the raw materials used in the examples of the present invention are all commercially available or available to those skilled in the art; unless otherwise specified, the methods used in the examples of the present invention are all methods known to those skilled in the art.

[0023] Coal gangue refers to the solid waste generated during coal mining, washing, and processing. It is also a viable resource, possessing a dual nature. It has become one of the largest sources of accumulated and discharged industrial solid waste in my country, accounting for approximately 15% of China's total solid waste.

[0024] Gangue is often stored in open-air piles, resulting in the formation of numerous gangue mountains. As the number of gangue mountains continues to increase, significant land resources have been encroached upon, resulting in not only a waste of resources but also significant ecological impacts. The harm caused by gangue mountains to the surrounding ecological environment stems primarily from the following factors: First, they occupy a large amount of fertile farmland, preventing the land from realizing its true value and damaging the soil's organic nutrients. Second, they pollute the surrounding air. Gangue contains a certain amount of combustible materials such as residual coal, carbonaceous mudstone, broken wood, and pyrite minerals, which pose a potential risk of spontaneous combustion in hot, dry summers. Years of exposure to wind and sun can also lead to its weathering and decomposition, producing large amounts of dust that form airborne smog, seriously impacting the health of residents in mining areas. Third, they pollute land and groundwater resources.

[0025] There are some heavy metal elements in coal gangue. The sulfides in coal gangue form acidic solutions under the scouring and leaching of atmospheric precipitation. During the infiltration and migration process, mercury, chromium, arsenic, copper, cadmium, lead, etc. are dissolved and penetrate into the soil along with groundwater, thereby polluting the soil and water resources. The higher the sulfide content in coal gangue, the easier it is for harmful elements to dissolve; fourthly, since the accumulation of coal gangue is relatively loose, its firmness is not high, and it is very easy to cause geological disasters such as collapse of gangue mountains, landslides, and mudslides in rainy seasons.

[0026] As a natural volcanic ash mixed material, coal gangue itself has certain activity. Its activity can be effectively improved through thermal activation, mechanical activation, chemical excitation and other methods, so that it can be better utilized as a resource in the field of building materials.

[0027] Secondly, with the rapid growth of the construction industry, the shortage of many building materials is becoming increasingly serious. According to statistics, traditional cement accounts for approximately 15% of global energy consumption. Under immense energy pressure, there is an urgent need to find alternative materials to alleviate this pressure. To address this, the idea of ​​using coal gangue to prepare cement mortar is proposed, exploring ways to achieve large-scale, harmless, and sustainable coal gangue consumption while effectively alleviating the enormous energy pressure of traditional cement. Finally, existing uses for coal gangue primarily include power generation, fertilizer production, road construction materials, sintered bricks, and architectural ceramics, making large-scale production difficult. Furthermore, these methods still require cumbersome processing steps, unavoidable with the potential for secondary pollution. Therefore, the use of coal gangue to prepare cement mortar effectively increases gangue utilization and offers a simple, harmless, and sustainable green approach.

[0028] Based on this, the present invention proposes a new method for activating coal gangue, and uses the coal gangue obtained based on this method to prepare high-performance cement mortar.

[0029] 1. Raw materials The raw materials mainly include calcined coal gangue, cement, water, sand and gravel, release agent, water reducing agent, and sodium hydroxide. The specific types of materials are shown in Table 1 below:

[0030] 2. Activation scheme of coal gangue: The activity of coal gangue refers to the ability of its soluble components, such as SiO2 and Al2O3, to react with lime at room temperature to form a gelling hydration product after adding water. Activation pathways for coal gangue include thermal activation, mechanical activation, chemical activation, and combined activation.

[0031] (1) Mechanical activation: The purpose of mechanical activation is to refine the gangue particles, increase the specific surface area, and improve the reaction rate. Selective crushing is achieved through autogenous grinding and sorting of the gangue, so that the hard gangue (mainly containing calcium) maintains its original particle size, while the softer part (mainly containing clay minerals) is crushed into fine particles. Sieving can be used to separate the gangues of different hardness.

[0032] (2) Thermal activation: The purpose of thermal activation is to use high temperatures to induce intense thermal motion within the microstructure of gangue, removing bound water from the minerals. After calcination, the ultrafine kaolin in gangue undergoes dehydration, destroying its layered structure and forming amorphous aluminum silicate, metakaolin. Consequently, the calcined gangue contains significant amounts of activated silicon oxide and aluminum oxide. Because metakaolin exhibits an irregular molecular arrangement and a thermodynamically metastable state, it exhibits gelling properties when activated by chemical agents.

[0033] (3) Chemical activation: Chemical activation of coal gangue involves adding an activator to enhance its pozzolanic activity. The activator has two functions: first, it provides a highly polar environment, breaking the Si-O and Al-O bonds on the gangue surface, causing structural disintegration; second, it participates in the reaction to form a gelling substance. The main factors influencing the effectiveness of chemical activation are the type and dosage of the activator.

[0034] (4) Composite activation: Composite activation refers to the use of two or more activation methods to activate simultaneously. Generally speaking, the effect of composite activation is better than that of a single activation method.

[0035] The present invention realizes the activation of the coal gangue by adopting a method of performing thermal-chemical composite activation after the coal gangue is sorted.

[0036] First, we explored the technical conditions for gangue sorting, utilizing its hardness differences to achieve selective crushing. The raw gangue was crushed to -6mm in a jaw crusher and then sorted in a rod mill. Screening separated the brittle material below the sieve and the coarse aggregate above the sieve. The coarse aggregate was washed, and the resulting sediment was filtered and dried.

[0037] Secondly, the optimal calcination conditions for thermal activation of coal gangue were explored. The dust was washed together with the friable coal gangue powder material under the sieve, or only the friable coal gangue powder under the sieve was calcined at 650°C. The changes in the internal mineral composition were analyzed through XRD and XRF tests to obtain the optimal calcination conditions.

[0038] Finally, the effects of varying amounts of the activator NaOH on the mechanical properties of high-performance cement mortar made from coal gangue were investigated. Based on the above experiments, the calcined coal gangue product was ground and used as an admixture. By adding varying amounts of NaOH, the mixture was evenly mixed with fly ash, Portland cement, water, a polycarboxylic acid-based high-efficiency water reducer, sand, and gravel to produce high-performance cement mortar. The resulting high-performance cement mortar exhibited excellent fluidity, no segregation, excellent mechanical properties, and durability. The process consumed a significant amount of coal gangue, effectively reducing its environmental pollution.

[0039] Specifically, high-performance cement mortar is prepared with coal gangue as the main raw material, and the coal gangue is crushed and sorted in a jaw crusher to obtain fragile powdered coal gangue and hard and unbreakable block coal gangue; the block coal gangue is washed with water; the precipitate obtained in the washing process is dried, and together with the powdered coal gangue, or only the powdered coal gangue is preheated, it is calcined at a temperature of 650°C to obtain a calcined product, and the calcined product is placed in a rod mill for grinding as an admixture; the admixture and cement are weighed in proportion, and stirred and mixed to obtain a cementitious material; the cementitious material and sand and gravel are first poured into a mixer according to the mortar ratio and stirred evenly, and then the activator NaOH and the water reducer are dissolved in water, poured into the mixer, and the mixture is stirred evenly to obtain the final high-performance cement mortar. The flow chart of the specific preparation method is as follows Figure 1 The influence of the amount of activator NaOH on the mechanical properties of cement mortar was studied by conducting cement mortar strength tests after curing for 3d, 7d, and 28d.

[0040] 3. Physical and chemical properties of coal gangue: Gangue analysis (uncrushed and unscreened) The properties of coal gangue are clarified by measuring the full industrial analysis, XRD, XRF, sulfur content and calorific value of coal gangue.

[0041] (1) Full industrial testing and sulfur content testing The gangue was naturally dried and crushed before being tested for full industrial test and sulfur content test. The results of full industrial test and sulfur content test are shown in Table 2:

[0042] As shown in Table 2, the average ash content of coal gangue is 84.01%, the average sulfur content is 0.63%, the average moisture content is 2.88%, and the average volatile content is 88.53%.

[0043] (2) Heat generation test The coal gangue was ground to -0.2 mm, and the dry basis high calorific value of the coal gangue was measured by CT5000A automatic calorimeter, which was 0.03 MJ / kg.

[0044] (3) XRF testing The chemical composition of the gangue was analyzed using an X-ray fluorescence spectrometer. The analysis results are shown in Table 3:

[0045] As shown in Table 3, the chemical composition of coal gangue includes SiO2 content of 44.4% and Al2O3 content of 19.7%, which belongs to clay rock type coal gangue.

[0046] (4) XRD test The gangue was ground to -0.074 mm and measured by X-ray diffractometer with a scanning angle of 10° to 80°. The analysis spectrum was as follows: Figure 2 Shown: By Figure 2 Analysis shows that the mineral composition of coal gangue is mainly quartz and kaolinite, with a small amount of pyrite.

[0047] 4. Coal gangue sorting technology (mechanical activation): Through full industrial analysis, XRD, XRF, sulfur content and calorific value determination, the properties of undersize coal gangue are clarified.

[0048] (1) Analysis of the mass ratio between undersize and oversize At a material-to-rod ratio of 25:1, gangue was dry-ground using a three-roller, four-drum rod mill. Screening was performed using a 1mm standard sieve, and the mass of the above-screen and below-screen products was weighed. The relationship between the above-screen and above-screen mass ratios and grinding time was investigated for grinding times of 15, 30, 60, 120, and 180 minutes. The test results are shown in Table 4.

[0049] When the material-rod ratio is 25:1, the under-sieve to over-sieve mass ratio shows an overall increasing trend with the increase of grinding time; although the under-sieve to over-sieve mass ratio is the largest at 180 min, it is not much different from the under-sieve to over-sieve mass ratio at the sorting time of 120 min. Considering energy consumption and cost, the optimal grinding time is selected as 120 min.

[0050] (2) Full industrial testing and sulfur content testing The full industrial test and sulfur content test were carried out on the coal gangue under the sieve after grinding for 120 minutes. The results of the full industrial analysis and sulfur content analysis are shown in Table 5:

[0051] It can be seen from Table 5 that when the grinding time is 120 min, the average ash content of the undersize gangue is 81.32%, which is 2.69% lower than that of the gangue.

[0052] (3) Heat generation test The undersize gangue was ground to -0.2 mm, and the dry basis high calorific value of the undersize gangue was measured by CT5000A automatic calorimeter, which was 2.29 MJ / kg. Compared with the gangue, the calorific value increased by 2.26 MJ / kg, indicating that the sorting effect was significant.

[0053] (4) XRF testing The chemical composition of the oversize and undersize gangue after grinding for 120 minutes was analyzed by X-ray fluorescence spectrometer. The analysis results are shown in Table 6:

[0054] Table 6 shows that the chemical composition of the oversize and undersize gangue after sorting is essentially the same as that of the gangue, differing only in the component contents. The oversize gangue has increased SiO2 and Al2O3 contents, while the Fe2O3 content has decreased. The undersize gangue has decreased SiO2 and Al2O3 contents, while the Fe2O3 content has increased. This indicates that the oversize gangue is a hard material rich in quartz, while the undersize gangue is a brittle material rich in kaolinite and pyrite.

[0055] (5) XRD test The coal gangue on the sieve and the coal gangue under the sieve were ground to -0.074 mm and measured by X-ray diffractometer with a scanning angle of 10° to 80°. The analysis spectrum is as follows: Figure 3 As shown by Figure 3 It can be seen that compared with the gangue, the XRD patterns of the undersize gangue and oversize gangue ground for 120 min show that the quartz characteristic peak of the oversize gangue increases, while the quartz characteristic peak of the undersize gangue decreases.

[0056] 5. Thermal activation technology of coal gangue: Thermal activation involves activating gangue through high-temperature calcination, increasing its volcanic ash activity. This process simultaneously stimulates volcanic ash activity and removes organic matter, which may have been introduced during the mining process and is detrimental to its gelling properties. Thermal activation of gangue is performed in an SX410 muffle furnace. The specific experimental steps are as follows: ① Weigh 400g of mechanically activated gangue powder and evenly place it in an ash dish; ② Set the muffle furnace temperature and calcine the gangue powder at 650°C. Once the temperature reaches the set value, place the ash dish in the muffle furnace, calcine for 2 hours, remove it, allow it to cool in air, and then place it in a sealed bag with a label for later use.

[0057] (1) Loss on ignition analysis Loss on ignition, also known as loss on ignition, refers to the loss of material after the crystallization water discharged during the firing process, the CO2 decomposed from carbonates, the SO2 decomposed from sulfates, and the organic impurities are removed.

[0058] Take 100g of undersize gangue and calcine it at 650℃ for 30, 60, 90, 120, 180 and 240min. The results are shown in Table 7.

[0059] As holding time increases, the LOI increases rapidly at first and then slowly. When the holding time exceeds 120 minutes, the LOI slowly increases, likely due to sample inhomogeneity. From an energy consumption perspective, 120 minutes is the optimal holding time.

[0060] (2) XRF test of undersize gangue at different holding times. The test results are shown in Table 8:

[0061] It can be seen from Table 8 that with the increase of holding time, SiO2 and Al2O3 increased, and Fe2O3 decreased compared with the under-sieve gangue.

[0062] (3) XRD test of undersize gangue at different holding times 100g of undersize gangue was calcined at 650°C for holding times of 30, 60, 90, 120, 180, and 240 minutes to examine the changes in its mineral composition. The calcined product was ground to -0.074mm and measured using an X-ray diffractometer at a scanning angle of 10° to 80°.

[0063] The XRD test patterns of undersize gangue and undersize gangue at different holding times are as follows: Figure 4 As shown: Depend on Figure 4 As can be seen, the characteristic peaks of kaolinite and pyrite decrease with increasing holding time, indicating that their mineral structures begin to change. At 120 minutes, the characteristic peaks of kaolinite and pyrite in the gangue essentially cease to decrease. Because kaolinite is an aluminosilicate mineral, it can be dehydrated by calcination to form metastable metakaolinite. Pyrite decomposes into ferric oxide at high temperatures.

[0064] The chemical changes that occur in this process are: ; ;;

[0065] In summary, the XRF and XRD of the undersize gangue were compared with those of the undersize gangue kept warm for different times, and the loss on ignition data were analyzed. Considering the cost and energy consumption, this experiment selected a calcination temperature of 650℃ and a holding time of 120min as the best experimental conditions.

[0066] 6. Test analysis: A method for preparing high-performance cement mortar based on NaOH activation of coal gangue, the method specifically comprising: Step 1: Put the preliminarily crushed gangue into the rod mill for crushing and screening to obtain undersize gangue and oversize gangue; Step 2: washing the coal gangue on the sieve with water, drying the precipitate obtained in the washing process; and mixing the dried precipitate with the coal gangue under the sieve obtained in step 1 to obtain mixed coal gangue; Step 3: preheating the undersize gangue obtained in step 1 or the mixed gangue obtained in step 2, calcining it at a certain temperature, taking it out after calcining for a certain period of time, and placing it in the air to cool it to obtain a calcined product; Step 4: placing the calcined product into a rod mill and grinding it into a fine powder to be used as an admixture; Step 5: Weigh the admixture and cement in a certain proportion, stir and mix them evenly to obtain a cementitious material; Step 6: First, pour the cementitious material and sand and gravel into the mixer according to the mortar-sand ratio and mix them evenly. Then, dissolve the activator NaOH and water reducer in water, pour them into the mixer, and stir the mixture evenly to obtain the final high-performance cement mortar. The rod mill material to rod ratio in step 1 is 25:1, the sieving adopts 1mm standard sieve, and the grinding time is 120min.

[0067] The calcination temperature is 650° C. and the calcination time is 120 minutes.

[0068] The size of the admixture in step 4 is ground to -0.105 mm.

[0069] The mass ratio of admixture to cement in cementitious materials is 1:3; The mass ratio of water to cementitious material, i.e. the water-binder ratio is 0.44; The mass ratio of water reducer to cementitious material is 1:100; The mass ratio of sand and gravel to the total mass of cementitious materials and water reducer is 1:2; The cement is fly ash silicate cement.

[0070] The water reducer is a polycarboxylic acid water reducer.

[0071] The particle size of the sand and gravel is -2mm.

[0072] The cement mortar specimens required for the test of the present invention were prepared according to the mix ratio; the amount of activator NaOH in the comparative example was 0% (the mass ratio of activator NaOH to cementitious material was 0:100), the amount of activator NaOH in Example 1 was 2% (the mass ratio of activator NaOH to cementitious material was 2:100), the amount of activator NaOH in Example 2 was 4% (the mass ratio of activator NaOH to cementitious material was 4:100), the amount of activator NaOH in Example 3 was 6% (the mass ratio of activator NaOH to cementitious material was 6:100), the amount of activator NaOH in Example 4 was 8% (the mass ratio of activator NaOH to cementitious material was 8:100), and the amount of activator NaOH in Example 5 was 10% (the mass ratio of activator NaOH to cementitious material was 10:100).

[0073] The specific process is as follows: Chemical activation of coal gangue involves the introduction of a small amount of activator, which participates in and accelerates the secondary reaction between the gangue and cement hydration products. This primarily disrupts the relatively stable Si-O network structure. Under the action of alkali, the covalent Si-O-Si and Al-O-Al bonds within the structure break, forming ions that enter the solution. [SiO4]4- and [AlO4]5- combine to form a three-dimensional polyaluminate structure. However, the chemical activation process alone is relatively slow, so to enhance the activation effect, it is generally combined with thermal activation.

[0074] The strength of mortar specimens was determined in accordance with GB / T17617-1999 "Test method for strength of cement mortar (ISO method)". The size of the triple test mold used in the experimental test mold was 40mm×40mm×160mm. A one-factor six-level experiment was conducted, with two groups for each level and one specimen in each group. After standard curing for 3d, 7d, and 28d, the WAW-600C microcomputer-controlled electro-hydraulic servo universal testing machine was used to first test the flexural strength of the specimens and then measure the compressive strength.

[0075] (1) Preparation of cement mortar from coal gangue activated by NaOH separation 1) Before the experiment, wipe the triple test mold clean, apply the release agent evenly on the contact surface between the mortar and the test mold, and assemble them tightly.

[0076] 2) Evenly mix the stone sand and cementitious materials according to the mortar-sand ratio. About 3130g of material is required for each six samples.

[0077] 3) First, mix the stone sand and cementitious materials evenly, then dissolve the activator and water reducer in water, pour them into the mixer, and start the machine to mix the materials evenly.

[0078] 4) After the mixing is completed, use a shovel to directly load the material from the mixer into the mold that has been brushed with the release agent. Shake the mold to form the cement mortar sample, and use a shovel to smooth the surface of the sample and seal it with plastic wrap.

[0079] 5) Cure the sample in air for 24 hours to allow the body to solidify and harden.

[0080] 6) After 24 hours, demould the specimen and seal it with plastic wrap for curing.

[0081] 7) Place the specimen in a medium-pressure test bench and cure at room temperature. After the set curing time, remove it and test its flexural and compressive strength.

[0082] (2) Mechanical properties evaluation of specimens with NaOH content The chemical reagent NaOH was selected as the experimental alkali activator, and the effect of the addition of NaOH at 0%, 2%, 4%, 6%, 8%, and 10% on the mechanical properties of the molded mortar specimens was explored. The flexural strength of the specimens was measured as shown in Table 9 and Figure 5 As shown: Flexural strength: Flexural strength of the specimen / MPa, as shown in Table 9:

[0083] It can be seen that when the water-binder ratio is 0.44 and the NaOH content is 8%, the activation effect of the activator is the best, and the flexural strengths are 11MPa, 26MPa, and 28MPa, respectively, which are 8MPa, 12MPa, and 4MPa higher than those without adding NaOH, among which the flexural strength is increased by as much as 2.7 times.

[0084] The compressive strength of the specimens is shown in Table 10 and Figure 6 As shown: Compressive strength: compressive strength of the specimen / MPa, as shown in Table 10:

[0085] As shown in Table 3.9, the compressive strength of the specimens decreases with increasing NaOH content. The activation effect is optimal when the water-binder ratio is 0.44 and the NaOH content is 8% at 3d, 7d, and 28d. The compressive strengths are 18 MPa, 29 MPa, and 33 MPa, respectively, representing increases of 12 MPa, 14 MPa, and 8 MPa compared to those without NaOH, with the maximum increase in compressive strength reaching 2 times.

[0086] Depend on Figure 7 and Figure 8It can be seen that compared with the strength of the blank mortar and the mortar with NaOH added, the 3d, 7d, and 28d flexural and compressive strengths first increase and then decrease with the increase in NaOH content. This shows that NaOH effectively activates the calcined coal gangue, and the prepared cement mortar material has better mechanical properties.

[0087] From the experimental analysis, we can draw the following conclusions: (1) Through the separation of three-roller four-drum rod mill, according to the mass ratio of undersize to oversize, the XRD and XRF analysis of undersize gangue showed that the grinding time of 120 min was the optimal condition; (2) At T = 650 ° C, the best condition is the holding time of t = 120 min according to the loss on ignition, XRD and XRF analysis; (3) Through experimental exploration of calcination activation, it was found that the optimal process conditions for undersize gangue were T = 650 ° C and t = 120 min. The undersize gangue was calcined under these conditions and then ground to -0.105 mm using a rod mill as an admixture. The results showed that within the experimental range, when the NaOH content was 8%, the 3d, 7d, and 28d flexural strength of the specimens could reach 11 MPa, 26 MPa, and 28 MPa; the 3d, 7d, and 28d compressive strength could reach 18 MPa, 29 MPa, and 33 MPa.

[0088] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-performance cement mortar based on NaOH activation of coal gangue, characterized in that: The method is specifically as follows: Step 1: Put the preliminarily crushed gangue into the rod mill for crushing and screening to obtain undersize gangue and oversize gangue; Step 2: washing the coal gangue on the sieve with water, drying the precipitate obtained in the washing process; and mixing the dried precipitate with the coal gangue under the sieve obtained in step 1 to obtain mixed coal gangue; Step 3: preheating the undersize gangue obtained in step 1 or the mixed gangue obtained in step 2, calcining it at a certain temperature, taking it out after calcining for a certain period of time, and placing it in the air to cool it to obtain a calcined product; Step 4: placing the calcined product into a rod mill and grinding it into a fine powder to be used as an admixture; Step 5: Weigh the admixture and cement in a certain proportion, stir and mix them evenly to obtain a cementitious material; Step 6: First, pour the cementitious material and sand and gravel into the mixer according to the mortar-sand ratio and mix them evenly. Then, dissolve the activator NaOH and water reducer into the water, pour it into the mixer, and stir the mixture evenly to obtain the final high-performance cement mortar.

2. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 1, characterized in that: The rod mill in step 1 has a material-to-rod ratio of 25:1, a 1 mm standard sieve is used for screening, and the grinding time is 15-180 min, preferably 120 min.

3. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 1, characterized in that: The sieve ratio of the undersize gangue to the oversize gangue is 0.26-0.

49.

4. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 1, characterized in that: The calcination temperature is 650° C., and the calcination time is 30-240 min. Preferably, the calcination temperature is 650° C., and the calcination time is 120 min.

5. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 1, characterized in that: The size of the admixture in step 4 is ground to ≤0.105 mm.

6. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 1, characterized in that: The mass ratio of admixture to cement in cementitious materials is 1:3; The mass ratio of water to cementitious material, i.e. the water-binder ratio is 0.44; The mass ratio of water reducer to cementitious material is 1:100; The mass ratio of sand and gravel to the total mass of cementitious materials and water reducer is 1:2; The mass ratio of the activator NaOH to the gelling material is 2-10:100, preferably 8:

100.

7. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 6, characterized in that: The cement is fly ash silicate cement.

8. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 6, characterized in that: The water reducer is a polycarboxylic acid water reducer.

9. The method for preparing high-performance cement mortar based on NaOH activation of coal gangue according to claim 6, characterized in that: The particle size of the sand and gravel is ≤2mm.

10. A high performance cement mortar, characterized in that: A cement mortar material prepared by the method for preparing high-performance cement mortar based on NaOH activation of coal gangue as described in any one of claims 1 to 9.

Images