A method for calcining a concrete expansive agent using a new dry process kiln

Abstract

The application discloses a method for calcining a concrete expanding agent by using a new dry-process kiln, which comprises the following steps: crushing and storing limestone, bauxite and hard gypsum respectively; taking the crushed limestone, bauxite and hard gypsum according to the mass percentage of 60-90wt%, 1-6wt% and 8-35wt% respectively, and then mixing and grinding to obtain a powdered material, i.e. raw material; pre-decomposing the mixed raw material by a preheater, and then feeding the pre-decomposed raw material into a rotary kiln for calcination; cooling the calcined raw material to obtain clinker of the concrete expanding agent; and grinding the clinker to obtain the concrete expanding agent. The method fully utilizes the advantages of the new dry-process kiln, so that the material is heated more uniformly, the calcination is more sufficient, the clinker of the expanding agent is uniform in size, the free calcium oxide content and the activity are stable, the structure is more compact, the activity of the free calcium oxide and the anhydrous calcium sulphoaluminate is more stable, and the comprehensive effect on the mechanical property of cement mortar and the limited expansion rate is better.

Description

Technical Field

[0001] This invention relates to the field of concrete expansion agent preparation technology, and specifically to a method for calcining concrete expansion agents using a novel dry kiln process. Background Technology

[0002] Shrinkage of concrete is an inherent phenomenon that easily occurs during the hydration and hardening process of cement, and drying shrinkage is one of the main causes of concrete cracking. Adding concrete expansive agents to compensate for concrete shrinkage is a common method. Commonly used expansive agents can be classified according to their basic composition and expansion source into sulfoaluminate-based expansive agents, lime-based expansive agents, magnesium oxide-based expansive agents, and composite expansive agents. Due to the increased mineral admixtures in cement or concrete, resulting in "calcium deficiency" in concrete, CSA expansive agents with CaO combined with ettringite as the expansion source have become a major research focus.

[0003] CSA expander is typically produced using tunnel kilns, wet process kilns, vertical kilns, and hollow kilns. Tunnel kilns require significant infrastructure investment and their thermal regime cannot be frequently adjusted. Vertical kilns have a large feed volume and limited temperature control methods, often resulting in clinker with significantly different activity and homogeneity compared to the required minerals. Hollow kilns have low carbonate decomposition rates, high heat consumption, and substantial heat loss, with a thermal efficiency of only around 20%. Wet process kilns have high heat consumption and low output per unit volume. Wet rotary kilns are highly adaptable to heterogeneous raw materials, producing uniform raw meal composition, stable processes, high-quality clinker with high strength, and less dust and fly ash during grinding. However, wet process production requires evaporating 30%–40% of the slurry moisture, consuming a large amount of heat, resulting in higher energy consumption than vertical and dry rotary kilns. Due to its high energy consumption and water usage, wet process production is considered a restricted and phased-out kiln type. The reactivity of anhydrous calcium sulfoaluminate and free calcium oxide in expansive agents is a key factor affecting their performance. Therefore, the calcination activity of limestone directly or indirectly determines the expansive performance; over- or under-calcined limestone will affect the expansive performance of the agent. Meanwhile, modern concrete often exhibits rapid early strength development, high brittleness, and poor stress relaxation ability, which can lead to a mismatch between the agent's and concrete's strength development, resulting in insufficient shrinkage compensation and inability to effectively prevent concrete cracking.

[0004] Patent CN103496867B discloses a calcium sulfoaluminate or calcium sulfoaluminate-calcium oxide concrete expansive agent, its preparation method, and its application. The raw material formula intentionally replaces at least 20 wt% limestone with quicklime to enable industrial production in a rotary kiln, confirming that increasing the carbonate content is beneficial for the preparation of the expansive agent. However, due to the high reactivity of quicklime, it readily reacts with air during storage to generate large amounts of calcium hydroxide and calcium carbonate, essentially not different from directly calcined limestone. Furthermore, quicklime, as a raw material, requires crushing and grinding, increasing energy consumption. The process also involves the continuous shedding of new surfaces, easily leading to reaction between the quicklime and air.

[0005] The pre-decomposition furnace in the new dry-process kiln optimizes the combustion, heat exchange, and carbonate decomposition processes, improving carbonate decomposition efficiency. Simultaneously, the generated free calcium oxide reacts better with Al and S elements in the kiln to form highly reactive anhydrous calcium sulfoaluminate, reducing the formation of ineffective mineral C2AS. Material heating is uniform, temperature control is precise, and production continuity is good. However, the narrow sintering temperature range of the expander, its large liquid phase content, and the difficulty in controlling the firing process can easily lead to material blockage during production, making continuous industrial production in the new dry-process kiln challenging. Summary of the Invention

[0006] The purpose of this invention is to provide a method for calcining concrete expansion agents using a novel dry kiln, in order to solve the problem that CSA expansion agent clinker cannot be smoothly and continuously produced on a novel dry kiln; at the same time, it solves the technical problems of existing concrete expansion agents having poor mineral activity, long hydration time, poor overall effect on the mechanical properties and expansion rate of cement mortar, easy damage to the structure of concrete, and affecting the durability of concrete.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for calcining concrete expansion agents using a novel dry kiln process includes the following steps:

[0009] S1. Crush limestone, bauxite, and anhydrite separately;

[0010] S2. Weigh the crushed limestone, bauxite, and anhydrite in mass percentages of 60-90 wt%, 1-6 wt%, and 8-35 wt%, respectively, then mix and grind them to obtain powdered raw material.

[0011] S3. After the mixed raw materials are pre-decomposed in a preheater, they are calcined in a rotary kiln and cooled to obtain expansion agent clinker. The clinker is then ground to obtain the concrete expansion agent.

[0012] Furthermore, the calcination temperature of the rotary kiln in the new dry process kiln is 1300-1500℃, and the calcination time is 20-40 minutes. That is, the entire calcination process, from the time the powdered material enters the preheater and pre-decomposition furnace of the new dry process kiln to the time it exits the kiln and enters the grate cooler, is maintained within 20-40 minutes.

[0013] Furthermore, the decomposition furnace temperature of the novel dry process kiln is 840–860°C.

[0014] The temperature of the decomposition furnace is the same as that of the pre-decomposition furnace, and the normal process temperature of the decomposition furnace is 880–900℃. Because the raw materials contain many flux minerals and have a large liquid phase, using the normal process temperature easily leads to the problem of melt crust formation. Conversely, if the decomposition furnace temperature is controlled too low, the decomposition rate of carbonates will be low, resulting in less free calcium oxide formation, which is detrimental to the formation of clinker minerals in the rotary kiln. Therefore, this invention selects a temperature of 840–860℃, optimizes the combustion, heat exchange, and carbonate decomposition process, controls the carbonate decomposition efficiency, and solves problems such as material blockage and kiln ring formation during production.

[0015] Furthermore, the concrete expansion agent has a specific surface area of ​​200–300 m². 2 / kg.

[0016] Furthermore, the powdered material is a powder with a sieve residue fineness of ≤22% obtained by uniformly passing through a 0.08mm square hole sieve.

[0017] Furthermore, the limestone contains ≥52wt% CaO and ≤2.5wt% SiO2.

[0018] Furthermore, the content of Al2O3 in the bauxite is ≥60wt%.

[0019] Furthermore, the SO3 content in the anhydrite is ≥37wt%.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. This invention requires no additional equipment modification costs. It utilizes a new type of dry-process kiln in an existing cement plant to produce a concrete expansion agent through calcination, and all raw materials used are conventional cement clinker raw materials. This invention fully leverages the advantages of the pre-decomposition furnace in the new dry-process kiln. By reducing the temperature of the decomposition furnace and controlling it at 840-860℃, it optimizes the combustion, heat exchange, and carbonate decomposition processes, controls the carbonate decomposition efficiency, and solves problems such as material blockage and kiln ring formation during production, enabling the continuous industrial production of expansion agent clinker.

[0022] 2. This invention fully utilizes the advantages of the new dry kiln, making the material heat more evenly and calcined more fully. The resulting expansion agent clinker is uniform in size, has stable free calcium oxide content and activity, and a denser structure. The activity of free calcium oxide and anhydrous calcium sulfoaluminate is more stable, resulting in a better overall effect on the mechanical properties of cement mortar and the limitation of expansion rate. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0024] Example 1

[0025] As a preferred embodiment of the present invention, this embodiment discloses a method for calcining concrete expansion agents using a novel dry kiln method, comprising the following steps:

[0026] S1. Crush limestone, bauxite, and anhydrite separately;

[0027] S2. Weigh the crushed limestone, bauxite, and anhydrite in the following weight percentages: 82wt%, 2wt%, and 16wt%, respectively. Mix and grind them evenly, then pass them through a 0.08mm square hole sieve to obtain raw material with a sieve residue fineness of ≤22%.

[0028] S3. After the mixed raw materials are pre-decomposed in a preheater, they are calcined in a rotary kiln at a temperature of 1450℃ for 20 minutes. The decomposition furnace temperature of the new dry kiln is 850℃. After cooling, the expansion agent clinker is obtained, and the clinker is then ground to obtain concrete expansion agent.

[0029] In this embodiment, the specific surface area of ​​the concrete expansion agent is 250m². 2 / kg.

[0030] In this embodiment, the limestone contains 53 wt% CaO and 1.3 wt% SiO2.

[0031] In this embodiment, the Al2O3 content in the bauxite is 65.1 wt%.

[0032] In this embodiment, the SO3 content in the anhydrite is 38.3 wt%.

[0033] Example 2

[0034] As a preferred embodiment of the present invention, this embodiment discloses a method for calcining concrete expansion agents using a novel dry kiln method, comprising the following steps:

[0035] S1. Crush limestone, bauxite, and anhydrite separately;

[0036] S2. Weigh the crushed limestone, bauxite, and anhydrite in the following weight percentages: 68wt%, 2wt%, and 30wt%, respectively. Mix and grind them evenly, then pass them through a 0.08mm square hole sieve to obtain raw material with a sieve residue fineness of ≤22%.

[0037] S3. After the mixed raw materials are pre-decomposed in a preheater, they are calcined in a rotary kiln at a temperature of 1450℃ for 30 minutes. The decomposition furnace temperature of the new dry kiln is 860℃. After cooling, the expansion agent clinker is obtained, and the clinker is then ground to obtain concrete expansion agent.

[0038] In this embodiment, the specific surface area of ​​the concrete expansion agent is 250m². 2 / kg.

[0039] In this embodiment, the limestone contains 53 wt% CaO and 1.3 wt% SiO2.

[0040] In this embodiment, the Al2O3 content in the bauxite is 65.1 wt%.

[0041] In this embodiment, the SO3 content in the anhydrite is 38.3 wt%.

[0042] Example 3

[0043] As a preferred embodiment of the present invention, this embodiment discloses a method for calcining concrete expansion agents using a novel dry kiln method, comprising the following steps:

[0044] S1. Crush limestone, bauxite, and anhydrite separately;

[0045] S2. Weigh the crushed limestone, bauxite, and anhydrite in the following weight percentages: 60wt%, 5wt%, and 35wt%, respectively. Mix and grind them evenly, then pass them through a 0.08mm square hole sieve to obtain raw material with a sieve residue fineness of ≤22%.

[0046] S3. After the mixed raw materials are pre-decomposed in a preheater, they are calcined in a rotary kiln at a temperature of 1350℃ for 40 minutes. The decomposition furnace temperature of the new dry kiln is 840℃. After cooling, the expansion agent clinker is obtained, and the clinker is then ground to obtain concrete expansion agent.

[0047] In this embodiment, the specific surface area of ​​the concrete expansion agent is 250m². 2 / kg.

[0048] In this embodiment, the limestone contains 53 wt% CaO and 1.3 wt% SiO2.

[0049] In this embodiment, the Al2O3 content in the bauxite is 65.1 wt%.

[0050] In this embodiment, the SO3 content in the anhydrite is 38.3 wt%.

[0051] Example 4

[0052] As a preferred embodiment of the present invention, this embodiment discloses a method for calcining concrete expansion agents using a novel dry kiln method, comprising the following steps:

[0053] S1. Crush limestone, bauxite, and anhydrite separately;

[0054] S2. Weigh the crushed limestone, bauxite, and anhydrite in the following weight percentages: 90wt%, 2wt%, and 8wt%, respectively. Mix and grind them evenly, then pass them through a 0.08mm square hole sieve to obtain raw material with a sieve residue fineness of ≤22%.

[0055] S3. After the mixed raw materials are pre-decomposed in a preheater, they are calcined in a rotary kiln at a temperature of 1500℃ for 20 minutes. The decomposition furnace temperature of the new dry kiln is 855℃. After cooling, the expansion agent clinker is obtained, and the clinker is then ground to obtain concrete expansion agent.

[0056] In this embodiment, the specific surface area of ​​the concrete expansion agent is 250m². 2 / kg.

[0057] In this embodiment, the limestone contains 53 wt% CaO and 1.3 wt% SiO2.

[0058] In this embodiment, the Al2O3 content in the bauxite is 65.1 wt%.

[0059] In this embodiment, the SO3 content in the anhydrite is 38.3 wt%.

[0060] Comparative Example 1

[0061] Except for the decomposition furnace temperature of the new dry kiln being 880°C, the concrete expansion agent in this comparative example was prepared in the same way as in Example 1.

[0062] Comparative Example 2

[0063] In this comparative example, the concrete expansion agent was prepared in the same manner as in Example 1, except that the limestone contained 51 wt% CaO and 3.8 wt% SiO2; the bauxite contained 45.8 wt% Al2O3; and the anhydrite contained 38.3 wt% SO3.

[0064] The concrete expansive agents calcined in Examples 1 to 4 and Comparative Examples 1 to 2 were tested for restricted expansion of molding mortar according to GB23439-2017 "Standard Method for Concrete Expansive Agents". The cement used was the reference cement specified in GB0876. The dosage of concrete expansive agent was 5% and 10 wt% of the cement content. The results are shown in Table 1 below.

[0065] Table 1 Results of the Restricted Expansion Test of Molding Mortar

[0066]

[0067] As shown in Table 1, the restricted expansion rate of the expanding agent prepared in the examples is significantly better than that in the comparative examples, indicating that the expanding agent prepared by the present invention has good expansion performance. In Comparative Example 1, due to the high temperature of the decomposition furnace, the decomposition rate of carbonates was very high when the raw materials reacted in the pre-decomposition furnace, resulting in a higher content of free calcium. Combined with the high volatilization rate of gypsum decomposition and a large liquid phase in the clinker, severe molten crusting occurred, leading to material blockage. After three days of continuous production, the kiln had to be shut down for manual ringing, affecting continuous industrial production. In Comparative Example 2, due to the increased silicon content and decreased calcium content in the limestone, the content of free calcium oxide generated when the raw materials passed through the preheater was low. After entering the rotary kiln, the raw materials formed anhydrous calcium sulfoaluminate, silicates, and other minerals with other minerals, leaving very low levels of free calcium oxide, which affected the expansion performance of the expanding agent. Furthermore, the reduced aluminum content in bauxite resulted in insufficient aluminum content for the formation of anhydrous calcium sulfoaluminate, leading to the formation of intermediate minerals with low hydration activity. The reduced content of anhydrous calcium sulfoaluminate also affected the performance of the expanding agent.

[0068] In summary, the method provided by this invention enables the continuous industrial production of expanding agents in a novel dry process kiln, and the product also has good expansion performance.

[0069] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention used to illustrate the technical solutions of the present invention, and are not intended to limit the invention, nor are they intended to limit the patent scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. That is to say, any changes or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but whose technical problems are still consistent with the present invention, should be included within the protection scope of the present invention. In addition, the direct or indirect application of the technical solutions of the present invention to other related technical fields are similarly included within the patent protection scope of the present invention.

Claims

1. A method for calcining concrete expansion agents using a novel dry kiln process, characterized in that, Includes the following steps: S1. Crush limestone, bauxite, and anhydrite separately; S2. Weigh the crushed limestone, bauxite, and anhydrite in the following weight percentages: 60-90 wt%, 1-2 wt%, and 8-35 wt%, respectively. Mix and grind them to obtain powdered raw material. The sum of the weight percentages of limestone, bauxite, and anhydrite is 100%. S3. After the mixed raw materials are passed through a preheater and a decomposition furnace, they are calcined in a rotary kiln and cooled to obtain expansion agent clinker. The clinker is then ground to obtain the concrete expansion agent. The decomposition furnace temperature of the new dry process kiln is 840~850℃; The limestone contains ≥52wt% CaO and ≤2.5wt% SiO2. The content of Al2O3 in the bauxite is ≥60wt%; The SO3 content in the anhydrite is ≥37wt%; The rotary kiln of the new dry process kiln has a calcination temperature of 1300~1500℃ and a calcination time of 20~40min.

2. The method for calcining a concrete expansion agent using a novel dry kiln according to claim 1, characterized in that, The concrete expansion agent has a specific surface area of ​​200~300m². 2 / kg.

3. The method for calcining concrete expansion agent using a novel dry kiln according to claim 1, characterized in that, The powdered raw material is a powder with a sieve residue fineness of ≤22% obtained by uniformly passing through a 0.08mm square hole sieve.

Images