Method for improving strength of high-alkali-content Portland cement clinker

Abstract

The invention discloses a method for improving the strength of high-alkali-content Portland cement clinker, and belongs to the technical field of Portland cement clinker production, when the alkali content R2O in the Portland cement clinker is larger than or equal to 0.6%, the strength is improved by regulating and controlling the content of the following chemical components: the sulfur-alkali ratio S / R in the Portland cement clinker is controlled to be larger than or equal to 0.8; the content of F ions in the Portland cement clinker is controlled to be 0.05-0.15%; and the content of P2O5 in the Portland cement clinker is controlled to be 0.1-0.3%, so that the mineral composition and sintering behavior of the high-alkali clinker can be effectively improved, and the mechanical property of the high-alkali clinker is improved.

Description

Technical Field

[0001] This invention relates to a method for improving the strength of silicate cement clinker with high alkali content. Specifically, it relates to a method for solving the problem of low strength of silicate cement clinker and cement in the later stage due to high alkali content, and belongs to the field of silicate cement clinker production technology. Background Technology

[0002] In the production of silicate cement clinker, alkalis (mainly K₂O and Na₂O) are unavoidable due to their introduction into raw materials and fuels. These alkali elements not only affect the clinker's firing process and hydration behavior but also significantly impact the final performance of cement and concrete. Due to limitations in regional resources, some cement companies use raw materials with high alkali content, which is difficult to control effectively through conventional batching adjustments. For example, one company's limestone raw material had a K₂O content consistently higher than 1.0%, and combined with the alkali contribution from other raw materials, the average R₂O content in its clinker reached 1.14%, and the average K₂O content was as high as 1.54%. Under these circumstances, the average 3-day compressive strength of the clinker was 31.4 MPa, while the 28-day strength was only 47.2 MPa, significantly lower than the industry average. To verify the direct impact of alkali content on strength, the company attempted to purchase low-alkali limestone for production, controlling the alkali content of the clinker to below 0.6%. The results showed that the average strength of the clinker after 3 days was 29.0 MPa, and the strength after 28 days increased to 59.8 MPa, demonstrating a significant improvement in strength performance. This proves that alkali content has a key impact on the later strength of clinker.

[0003] However, research revealed that under similar alkali content conditions, some similar enterprises still achieved a clinker strength exceeding 56.5 MPa after 28 days. This indicates that even with limited raw material conditions and difficulty in reducing alkali content, it is still possible to improve the mechanical properties of high-alkali clinker through appropriate processes and technologies. Therefore, researching and developing a method to effectively improve the strength of high-alkali clinker is of great significance for solving practical production problems under raw material constraints.

[0004] Chinese patent CN116354626A discloses a method for producing cement clinker from waste high-silica and high-alkali limestone. This method is mainly applicable to cement clinker produced from high-silica and high-alkali limestone, sandstone, tailings, fly ash, and desulfurized gypsum as raw materials, yielding clinker with a CaO content of 44%–49%, SiO2 content of 8%–12%, and R2O content of 0.7%–1.2%. By controlling the saturation ratio KH to be 0.88–0.90, the silicon content SM to be 2.75–2.85, the aluminum content IM to be 1.40–1.60, and the sulfur-alkali ratio to be controlled at 0.8, the method solves the problems of difficult grinding and poor calcinability of low-grade high-silica and high-alkali limestone, achieving effective utilization of this type of waste resource. The strength properties of the prepared clinker meet the expected requirements.

[0005] However, this method mainly relies on the control of traditional ratio values ​​and sulfur-alkali ratio. Although it can promote the formation of sulfates from some alkali, reducing the adverse effects on the main minerals, it fails to fundamentally solve the problems of fly sand and poor burnability caused by excessively high liquid phase viscosity in high-alkali clinker during firing. In addition, this method does not consider the negative effects that excessive alkali sulfate formation may cause, such as abnormal coagulation, increased water demand, and limited strength development in the later stages. Therefore, the improvement in clinker strength by this patented method is relatively limited, especially when dealing with higher alkali content or more complex raw material systems, its applicability and efficiency enhancement capabilities are significantly insufficient. Summary of the Invention

[0006] This invention aims to solve the problem of low strength of silicate cement clinker in the later stage due to high alkali content (R2O≥0.6%) in raw materials in the prior art. It provides a method to improve the strength of high alkali content cement clinker. This method can effectively improve the mineral composition and firing behavior of high alkali clinker by controlling the sulfur-alkali ratio, F ion content and P2O5 content in clinker, thereby improving its mechanical properties.

[0007] This invention is achieved through the following technical solution: a method for improving the strength of high-alkali content silicate cement clinker, wherein when the alkali content R2O in the silicate cement clinker is ≥0.6%, the strength is improved by adjusting the content of the following chemical components: (1) Control the sulfur-alkali ratio (S / R) in silicate cement clinker to be ≥0.8; (2) Control the content of F ions in silicate cement clinker to 0.05-0.15%; (3) Control the P2O5 content in silicate cement clinker to 0.1-0.3%.

[0008] When the alkali content R2O in silicate cement clinker is less than 1.0%, its sulfur-alkali ratio S / R should be controlled at 0.8 to 1; when the alkali content R2O in silicate cement clinker is greater than or equal to 1.0%, its sulfur-alkali ratio S / R should be controlled at ≥1.

[0009] When the alkali content R2O in silicate cement clinker is less than 1.0%, the P2O5 content should be controlled between 0.1% and 0.15%; when the alkali content R2O in silicate cement clinker is greater than or equal to 1.0%, the P2O5 content should be controlled between 0.15% and 0.3%.

[0010] When adjusting the sulfur-alkali ratio in silicate cement clinker, it can be achieved through at least one of the following methods: (I) High-sulfur coal is used as fuel in the silicate cement clinker calcination process; (II) Incorporating industrial solid waste gypsum into the raw material preparation process.

[0011] When adjusting the F ion content in silicate cement clinker, it is achieved by incorporating fluorine-containing raw materials during the raw meal preparation process. The fluorine-containing raw materials are selected from at least one of fluorite, fluorite waste residue, fluorine-containing calcium mud, phosphate rock flotation tailings, or phosphate slag.

[0012] When adjusting the P2O5 content in silicate cement clinker, it is achieved by incorporating phosphorus-containing raw materials during the raw meal preparation process. The phosphorus-containing raw materials are selected from at least one of phosphate rock flotation tailings, phosphate slag, or raw phosphate rock ore.

[0013] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) By synergistically controlling the content of three specific chemical components in clinker (sulfur-alkali ratio, F ion and P2O5 content), this invention can effectively suppress the adverse effects of alkali on the mineral structure of clinker, promote the formation of C3S, and significantly improve the strength of clinker, especially the 28-day compressive strength of clinker with high alkali content.

[0014] (2) The present invention uses an optimized composition to adjust the viscosity of the liquid phase and the formation temperature, improve the scalability of the material, reduce the rapid solidification and quick solidification phenomena, reduce the amount of water used for the standard consistency of clinker, and improve the ventilation effect and thermal stability of the kiln.

[0015] (3) The present invention can improve performance and reduce production costs by using high-sulfur coal during clinker calcination or by using industrial solid waste gypsum, fluorite slag, phosphorus slag, phosphate rock flotation tailings and other waste materials as raw materials during raw material batching. This is in line with the concept of green manufacturing and resource recycling.

[0016] (4) Based on the alkali content in the clinker (R2O<1.0% or ≥1.0%), the present invention sets different control ranges for the sulfur-alkali ratio S / R and P2O5 content, which has clear adaptability and operability and is easy to promote and apply in existing production lines.

[0017] (5) Based on multiple mechanisms such as alkali damage inhibition, mineral stabilization and firing promotion, this invention solves the problem of low strength of high alkali clinker from the source of composition design and has good repeatability and stability. Attached Figure Description

[0018] Figure 1 This is the XRD quantitative analysis spectrum of the pre-clinker in Example 2 of the present invention.

[0019] Figure 2 This is the XRD quantitative analysis spectrum of the clinker after regulation in Example 2 of the present invention.

[0020] Note: The X-ray diffraction analysis of clinker mineral phases involved in this invention was performed in accordance with GB / T40407-2021, "Method for X-ray Diffraction Analysis of Mineral Phases in Silicate Cement Clinker". Some differences exist between the quantitative XRD analysis results and the results calculated using the Bouger formula method, which is normal. Detailed Implementation

[0021] The invention's objective, technical solution, and beneficial effects will be further explained in detail below.

[0022] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the claimed invention. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0023] This invention relates to a method for improving the strength of silicate cement clinker with high alkali content (R2O≥0.6%). This method involves systematically controlling the sulfur-alkali ratio (S / R) and F ion concentration (F2O) in the silicate cement clinker. - The content of three key chemical components, namely P2O5, is effectively optimized to improve the mineral composition and calcination behavior of clinker, thereby improving its mechanical properties.

[0024] Specifically, the present invention defines the following control ranges for chemical components: (1) Sulfur-alkali ratio S / R ≥ 0.8 By rationally controlling the sulfur-alkali ratio, it is possible to effectively promote the preferential combination of alkali components with sulfur to form alkali sulfates, reducing the solid dissolution and damage of silicate and aluminate minerals by alkali, thereby mitigating their negative impact on the clinker mineral structure and early hydration. This not only helps improve the stability of the firing process but also creates favorable conditions for the synergistic effect of subsequent fluorine and phosphorus elements. To further enhance the targeting and applicability of the control, this invention is refined according to different levels of clinker alkali content, as follows: When the alkali content R2O < 1.0%, the sulfur-alkali ratio S / R should be controlled between 0.8 and 1, and the P2O5 content should be controlled between 0.1 and 0.15%. When the alkali content R2O≥1.0%, the sulfur-alkali ratio S / R≥1, and the P2O5 content is controlled between 0.15 and 0.3%.

[0025] (2) The F ion content is controlled between 0.05% and 0.15%. By rationally controlling the range of F ion content, the viscosity of the liquid phase during clinker firing can be reduced, promoting liquid phase flow and mass transfer processes, and improving burnability. At the same time, the regulation of F ion content can also reduce the formation temperature of tricalcium silicate (C3S), promote the crystallization and development of C3S, thereby optimizing the clinker mineral phase composition and laying a structural foundation for strength improvement.

[0026] (3) The P2O5 content is controlled at 0.1-0.3%.

[0027] Appropriate control of P2O5 content can effectively inhibit the premature precipitation of alkali sulfates during the calcination process, delay cement setting, and improve the standard consistency water requirement of cement. Simultaneously, P2O5 can induce C3S lattice distortion, promoting its activity and facilitating the sustained development of cement strength in the later stages. Furthermore, the synergistic effect of P2O5 and F further optimizes the properties of the calcined liquid phase and the occurrence state of alkali.

[0028] Therefore, this invention achieves systematic optimization of the performance of high-alkali silicate cement clinker in multiple aspects by synergistically regulating the sulfur-alkali ratio, F ion content, and P2O5 content. Specifically, it increases the 28-day compressive strength of the clinker by 8–12 MPa, with continued strength growth in the later stages; effectively reduces liquid phase viscosity, reduces sand flying, improves burnability, and makes the firing process more stable; optimizes cement workability, reduces the water required for standard cement consistency (by approximately 3–5%), and allows for reasonable control of setting time, avoiding problems such as rapid setting and quick setting.

[0029] In the specific implementation process, the following methods can be used to regulate each component: To regulate the sulfur-alkali ratio, high-sulfur coal can be used as fuel during clinker burning, or industrial solid waste gypsum can be added during the raw material preparation process. To regulate the F ion content, fluorite, fluorite waste residue, fluorine-containing calcium mud, phosphate rock flotation tailings, or phosphate slag can be added during the raw material preparation process for regulation. To regulate P2O5 content, phosphate rock flotation tailings, phosphate slag, or raw phosphate rock ore can be added during the raw material preparation process.

[0030] Among them, the sulfur content (St) of high-sulfur coal supplied to the plant is greater than 2.5%; Industrial solid waste gypsum includes desulfurization gypsum, phosphogypsum, and fluorogypsum. Desulfurization gypsum is a byproduct of flue gas desulfurization processes in coal-fired power plants and steel mills, with its main component being calcium sulfate dihydrate (CaSO4·2H2O) and a purity typically ≥90%. It is formed by the reaction of limestone slurry with sulfur dioxide, and after dehydration, its moisture content is approximately 10%–12%. Phosphogypsum is a byproduct of wet-process phosphoric acid production, with its main component being calcium sulfate dihydrate (CaSO4·2H2O), containing small amounts of phosphorus, fluorine, and other impurities. It is acidic and has a high moisture content; its storage can pollute soil and water and release harmful substances. Fluoropypsum is an industrial byproduct of hydrofluoric acid production, with its main component being anhydrous calcium sulfate (CaSO4), with a content of 80%–95%. Approximately 3.6–4 tons of fluorogypsum are produced for every ton of hydrofluoric acid produced. It is strongly acidic (low pH) and contains small amounts of calcium fluoride and unreacted sulfuric acid.

[0031] Fluorite (fluorite ore) is an important non-metallic mineral, mainly composed of calcium fluoride (CaF2), and is widely used in metallurgy, glass, ceramics, chemical industry and other fields. Fluorite waste is the waste residue left over from fluorite mining and beneficiation. Its main component is calcium fluoride (CaF2), and it may contain impurities such as rare earth elements, iron, aluminum, and silicon.

[0032] Fluorine-containing calcium sludge usually refers to fluorine-containing calcium sludge, which is a solid waste generated during the treatment of fluorine-containing industrial wastewater through chemical precipitation (such as the addition of lime). Its main component is calcium fluoride (CaF2), and its appearance is often muddy or gelatinous.

[0033] Phosphate ore flotation tailings refer to the tailings remaining after high-grade phosphate ore has been extracted through flotation. Their composition typically includes approximately 5%–7.5% fluorapatite, approximately 2%–3.8% P₂O₅, approximately 0.2%–0.3% F, and the remaining main minerals are dolomite and a small amount of quartz. The chemical composition of a certain phosphate ore flotation tailings is shown in Table 1 below.

[0034] Table 1

[0035] Phosphorus slag is an industrial waste residue discharged during the electric furnace process for producing yellow phosphorus. Its main components are silicon dioxide (SiO2) and calcium oxide (CaO), accounting for over 80%, and it also contains small amounts of alumina (Al2O3), iron oxide (Fe2O3), phosphorus pentoxide (P2O5), and fluorides. Its structure is glassy and possesses certain potential activity, making it suitable for use as a cement admixture or ingredient. Its chemical composition analysis is shown in Table 2 below.

[0036] Table 2

[0037] Phosphate ore refers to phosphate rock mined from mines. For example, the Mabian phosphate mine in Sichuan contains approximately 46%–70% fluoroapatite, about 20%–28% P2O5, and 1.8%–2.5% F. The remaining main minerals are dolomite and quartz. The Wengfu phosphate mine in Guizhou has an average P2O5 content as high as 34.2% and an average F content of 3.04%.

[0038] The specific implementation of the present invention will be described below with reference to the embodiments. Of course, the scope of protection of the present invention is not limited to the following embodiments.

[0039] Example 1: Before adjustment, the raw material ratio was limestone:red sand:aluminum waste rock = 82.9:7.3:9.8. After mixing and grinding, the raw materials were calcined in a rotary kiln to produce silicate cement clinker. The controlled ratio and mineral composition were: KH = 0.92, n = 2.5, p = 1.4, C3S = 59.6%, C2S = 15.0%, C3A = 7.8%, C4AF = 9.9%. The clinker alkali content R2O = 0.84% ​​(of which, K2O content is 1.12%, Na2O content is 0.11%). Cement was prepared according to GB / T21372 "Silica Cement Clinker", with clinker:dihydrate gypsum = 95:5 grinding, controlling the specific surface area to be 352.5 m². 2 / kg. The mortar strength was tested according to GB / T17671. The 3-day compressive strength was 30.0 MPa, and the 28-day compressive strength was 49.6 MPa.

[0040] After adjustment, the composition of the raw materials was modified based on the above raw materials, and the raw meal ratio was adjusted to limestone:red sand:alumina waste rock:anhydrite:phosphorus slag = 80.6:6.8:8.2:1.6:2.4, wherein the sulfur-alkali ratio (S / R) was controlled at 0.86, the P2O5 content at 0.12%, and the F ion content at 0.1%. KH = 0.92, n = 2.5, p = 1.4 were maintained; the mineral composition was C3S = 62.5%, C2S = 16.0%, C3A = 8.8%, C4AF = 9.6%. Cement was prepared according to the requirements of GB / T21372 "Silicate Cement Clinker", and the clinker:dihydrate gypsum was ground at a ratio of 95:5, controlling the specific surface area to be 349.2 m². 2 / kg. Mortar strength was tested according to GB / T17671. The 3-day compressive strength was 34.1 MPa, and the 28-day compressive strength was 58.4 MPa. The 28-day strength increased by 8.8 MPa compared to before adjustment.

[0041] Example 2: Before adjustment, the raw material ratio was limestone:red sand:aluminum waste rock = 82.3:7.9:9.8. After mixing and grinding, the raw materials were calcined in a rotary kiln to produce silicate cement clinker (process same as in Example 1). The control rate values ​​and mineral composition were: KH = 0.91, n = 2.5, p = 1.6, C3S = 57.29%, C2S = 15.9%, C3A = 8.%, C4AF = 10.3% (see...). Figure 1The clinker alkali content R2O = 1.12% (of which K2O content is 1.38% and Na2O content is 0.21%). Cement was prepared according to the requirements of GB / T21372 "Silicate Cement Clinker", and ground at a ratio of clinker:dihydrate gypsum = 95:5, controlling the specific surface area to be 353.4 m2 / kg. Mortar strength was tested according to GB / T17671, with a 3-day compressive strength of 29.1 MPa and a 28-day compressive strength of 50.1 MPa.

[0042] After adjustment, the composition of the raw materials was modified based on the above-mentioned raw materials, and the raw meal ratio was adjusted to limestone:red sand:alumina waste rock:desulfurized gypsum:phosphate rock:fluorine-containing calcium mud = 80.9:6.8:8.5:2.2:1:0.5, wherein the sulfur-alkali ratio (S / R) was controlled at 1.09, the F ion content at 0.12%, and the P2O5 content at 0.20%. KH = 0.91, n = 2.5, p = 1.6 were maintained; the mineral composition was C3S = 59.1%, C2S = 15.0%, C3A = 8.5%, C4AF = 9.1% (see...). Figure 2 Cement was prepared according to the requirements of GB / T21372 "Silicate Cement Clinker", and ground at a ratio of clinker:dihydrate gypsum = 95:5, controlling the specific surface area to be 350.2 m². 2 / kg. The mortar strength was tested according to GB / T17671. The 3-day compressive strength was 33.91 MPa, and the 28-day compressive strength was 58.2 MPa. The 28-day strength was 8.1 MPa higher than before the adjustment.

[0043] The above examples demonstrate that by optimizing the raw meal ratio and introducing auxiliary raw materials such as desulfurized gypsum, phosphate rock, and fluorine-containing calcium mud, the mineral composition and trace element content of clinker can be effectively controlled. Under the premise of maintaining stable basic ratio values ​​(KH, n, p values), the C3S content can be significantly increased and the C2S content can be reduced. At the same time, by controlling the sulfur-alkali ratio and the amount of F ions and P2O5 introduced into the clinker, the hydration activity of the clinker is enhanced, and the negative impact of sulfuric acid and alkali on hydration is reduced. The 28-day compressive strength is generally increased by more than 8 MPa, indicating that this control process has a stable and significant effect on improving the early and late strength of cement.

[0044] Further analysis revealed that the synergistic effect of F ions and P2O5 promotes allit (C3S) crystal development while inhibiting belite (C2S) crystal transformation, thus improving clinker density. Furthermore, the introduction of F ions lowers the liquid phase formation temperature, improves the firing regime, and enhances the reaction rate of materials within the kiln and the sintering quality of the clinker. Appropriate phosphorus, through solid dissolution in the silicate phase and the resulting crystal distortion, increases the hydration activity of the minerals. This synergistic effect not only optimizes the clinker microstructure but also enhances the mineral hydration kinetics. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for improving the strength of high-alkali content silicate cement clinker, characterized in that: When the alkali content (R₂O) in silicate cement clinker is ≥ 0.6%, the strength can be improved by adjusting the content of the following chemical components: (1) Control the sulfur-alkali ratio (S / R) in silicate cement clinker to be ≥0.8; (2) Control the content of F ions in silicate cement clinker to 0.05-0.15%; (3) Control the P2O5 content in silicate cement clinker to 0.1-0.3%.

2. The method according to claim 1, characterized in that: When the alkali content R2O in silicate cement clinker is less than 1.0%, its sulfur-alkali ratio S / R should be controlled at 0.8 to 1; when the alkali content R2O in silicate cement clinker is greater than or equal to 1.0%, its sulfur-alkali ratio S / R should be controlled at ≥1.

3. The method according to claim 1, characterized in that: When the alkali content R2O in silicate cement clinker is less than 1.0%, the P2O5 content should be controlled between 0.1% and 0.15%; when the alkali content R2O in silicate cement clinker is greater than or equal to 1.0%, the P2O5 content should be controlled between 0.15% and 0.3%.

4. The method according to claim 1, characterized in that: When adjusting the sulfur-alkali ratio in silicate cement clinker, it can be achieved through at least one of the following methods: (I) High-sulfur coal is used as fuel in the silicate cement clinker calcination process; (II) Incorporating industrial solid waste gypsum into the raw material preparation process.

5. The method according to claim 1, characterized in that: When adjusting the F ion content in silicate cement clinker, it is achieved by incorporating fluorine-containing raw materials during the raw meal preparation process. The fluorine-containing raw materials are selected from at least one of fluorite, fluorite waste residue, fluorine-containing calcium mud, phosphate rock flotation tailings, or phosphate slag.

6. The method according to claim 1, characterized in that: When adjusting the P2O5 content in silicate cement clinker, it is achieved by incorporating phosphorus-containing raw materials during the raw meal preparation process. The phosphorus-containing raw materials are selected from at least one of phosphate rock flotation tailings, phosphate slag, or phosphate rock ore.

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