A method for modifying a calcium oxide expander
By subjecting the calcium oxide expansive agent to a secondary expansion treatment with expandable graphite, the problem of uneven early and late shrinkage of concrete caused by the rapid reaction rate of the calcium oxide expansive agent is solved. This achieves delayed expansion effect and improved stability of concrete structure, and is suitable for concrete of different strength grades.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- JIANGSU SOBUTE NEW MATERIALS CO LTD
- Filing Date
- 2022-12-31
- Publication Date
- 2026-05-29
AI Technical Summary
Calcium oxide expansive agents react quickly, have a large early expansion effect but a small late expansion effect, which leads to uneven shrinkage of concrete in the early and late stages, making it prone to cracking. Existing modification methods are complex or have unstable effects.
Expandable graphite is used for secondary expansion treatment. By controlling the calcination temperature and time, calcium oxide is encapsulated in the porous graphite structure, which slows down the expansion reaction time. Mineral admixtures are used to optimize the internal pore structure of concrete.
It effectively prolongs the expansion time of calcium oxide expansive agent, reduces water absorption failure, optimizes the pore structure of concrete, and improves the later shrinkage compensation effect. It is suitable for concrete of different strength grades, and the process is simple and easy to promote.
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Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of concrete admixtures for building materials, and specifically discloses a method for modifying calcium oxide expanding agent. Background Technology
[0002] Concrete, a mixture of cementitious materials, sand, and aggregate, is the most widely used building material in the world. In practical engineering, due to hydration reactions and the influence of the surrounding environment, concrete undergoes drying shrinkage, autogenous shrinkage, and temperature-induced shrinkage, making it prone to cracking under the constraints of various structures. This severely affects the durability and service life of concrete. Expansion agents are key materials for controlling concrete cracking. To solve the problem of concrete cracking, a certain amount of expansion agent needs to be added to the concrete material to compensate for the shrinkage, improve the structural stability of the concrete, and control the cracks within a harmless range.
[0003] Calcium oxide expansive agents are widely used due to their minimal impact on concrete workability and mechanical properties, as well as their ability to promote admixture hydration. Calcium oxide expands upon contact with water to form calcium hydroxide, thus mitigating early concrete shrinkage. However, calcium oxide expansive agents have certain drawbacks: they react rapidly with water, are prone to moisture absorption and inactivation, and while their expansion effect is significant in the early stages of concrete's plasticity, it diminishes as the concrete solidifies and shrinks, at which point its expansion effect is largely exhausted. Furthermore, the early addition of the expansive agent generates substantial heat. Since concrete is a poor conductor of heat, this can lead to uneven heat distribution within the concrete initially. Under the influence of fluctuating ambient temperatures, this excessive temperature difference between the interior and exterior can easily cause cracking.
[0004] Currently, there are many studies on modifying calcium oxide expanding agents in order to change their hydration process and delay expansion.
[0005] For example, a modified calcium oxide-based expansive agent for cement concrete and its preparation method, wherein the modified calcium oxide-based expansive agent for cement concrete has a specific surface area of 100 m². 2 / kg~600m 2 The product, weighing approximately 1 kg, is composed of long-chain fatty acids and calcium oxide-based expansive agents. The long-chain fatty acids coat the surface of the calcium oxide-based expansive agent, and the mass ratio of calcium oxide-based expansive agent to long-chain fatty acids is 90:10 to 999:1. The resulting modified calcium oxide-based expansive agent for cement concrete exhibits improved moisture resistance, reduced hydration during the plastic stage of cement concrete, and increased expansion after hardening, thus enhancing the crack resistance of concrete. However, this method simply involves grinding and mixing hydrophobic materials and calcium oxide, resulting in unstable expansion effects of the produced expansive agent.
[0006] Another example is a modified calcium oxide-based cement concrete expansive agent, which is composed of phenolic resin, modified rosin resin, and calcium oxide-based expansive clinker. The phenolic resin and modified rosin resin work together to coat the surface of the calcium oxide-based expansive clinker, wherein the phenolic resin and modified rosin resin account for 0.01% to 10% of the total mass of the modified calcium oxide-based cement concrete expansive agent. By modifying the calcium oxide-based expansive clinker with a specific ratio of phenolic resin and modified rosin resin, the hydration process of the crack-resistant expansive agent is effectively regulated. The modified rosin resin is water-soluble, and its combination with the phenolic resin not only ensures an early expansion rate but also allows for the slow release of the expansive agent in the later stages, significantly improving the durability of concrete. However, this invention has a complex preparation process and requires a high level of production expertise.
[0007] Therefore, how to modify calcium oxide-based expansion agents to delay their expansion effect in a simple way remains a direction worthy of further research. Summary of the Invention
[0008] To address the problems in the prior art, the inventors of this invention, based on long-term research on concrete expansion agents, proposed a modification method for calcium oxide-based expansion agents. This method utilizes the expandability of expandable graphite and rationally controls the expansion process of expandable graphite to reasonably coat calcium oxide, thus solving the problem of premature release of expansion effect and failure to compensate for later shrinkage of concrete caused by the rapid reaction of calcium oxide expansion agent in a simple way.
[0009] The present invention specifically adopts the following technical solution:
[0010] A method for modifying a calcium oxide expanding agent, comprising the following steps:
[0011] S1. Take 10% to 25% of the first expandable graphite and calcine it at 300℃ to 400℃ for 10 to 30 minutes to obtain porous expanded graphite.
[0012] S2. Mix expanded graphite with 30%–50% calcium oxide and 10%–30% mineral admixtures to obtain a mixture.
[0013] S3. Mix the mixture with 20% to 40% of the second expandable graphite, calcine at 250°C to 350°C for 30 to 120 minutes, and quench in air to obtain the modified calcium oxide expander.
[0014] The above mass percentages are calculated based on the total mass of the first expandable graphite, calcium oxide, mineral admixtures, and the second expandable graphite, which is 100%.
[0015] Preferably, in step S1 above, the first portion of expandable graphite has a particle size of 40-60 mesh, while in step S3 above, the second portion of expandable graphite has a particle size of 150-170 mesh.
[0016] The aforementioned mineral admixtures can be conventional mineral admixtures in this field, such as fly ash and silica fume.
[0017] After obtaining the modified calcium oxide expander through quenching, it is preferable to grind it to a specific surface area of 200 m². 2 / kg~450m 2 / kg of powder.
[0018] In the above-described modification method of the present invention, the first expandable graphite expands during calcination and fully expands at 400°C to obtain a porous structure, which can serve as a carrier for subsequent calcium oxide and mineral admixture particles. The second expandable graphite obtained by mixing also fills the pores inside the porous expanded graphite. During the second expansion process, this part of the expandable graphite particles expands and fills the interior of the expanded graphite, and seals the calcium oxide and mineral admixture particles inside the network structure.
[0019] Thus, the modified calcium oxide expansive agent obtained through the modification treatment, when incorporated into concrete, effectively delays the action time of the modified expansive agent due to the expansion reaction that occurs only when free water comes into contact with the internal expansion clinker, caused by the mesh-like expanded graphite surrounding the calcium oxide. This also reduces the possibility of calcium oxide absorbing water and becoming ineffective. Furthermore, the mineral admixtures inside the mesh-like expanded graphite effectively optimize the internal pore structure of the concrete, thereby improving its strength.
[0020] The beneficial effects of this invention are as follows:
[0021] 1) Sintering effective expanded clinker—calcium oxide—into the network structure of expanded graphite extends the contact distance with free water in concrete, effectively delaying the time for the modified calcium oxide expansion agent to exert its expansion effect, which is beneficial for the later shrinkage compensation of concrete.
[0022] 2) This invention uses mineral admixtures as one of the main raw materials for modification, providing an effective way to treat solid waste, which is of great significance for saving resources and protecting the environment.
[0023] 3) The product of this invention is highly adaptable and applicable to concrete of different strength grades.
[0024] 4) The modification method of the calcium oxide expansive agent for cement concrete provided by the present invention has readily available raw materials, a simple modification process, and is easy to promote. Detailed Implementation
[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.
[0026] Example 1
[0027] First, weigh 50-mesh expandable graphite, calcine it at 350°C for 15 minutes, remove it and cool it to room temperature in air to obtain porous expanded graphite.
[0028] Then, the calcium oxide powder, fly ash, expanded graphite and 160 mesh expandable graphite are mixed in a mass ratio of 3:1:2:4 to obtain a mixture.
[0029] Next, the above mixture is calcined at 300°C for 60 minutes, then removed and cooled to room temperature in air. The resulting mixture is then ground in an electromagnetic vibratory mill for 1 minute. After grinding, the modified calcium oxide expansion agent for cement concrete is obtained.
[0030] Example 2
[0031] First, weigh 50-mesh expandable graphite, calcine it at 300°C for 10 minutes, remove it and cool it to room temperature in air to obtain porous expanded graphite.
[0032] Then, the calcium oxide powder, fly ash, expanded graphite and 160 mesh expandable graphite are mixed in a mass ratio of 5:2:1:2 to obtain a mixture.
[0033] Next, the above mixture is calcined at 250°C for 120 minutes, then removed and cooled to room temperature in air. The resulting mixture is then ground in an electromagnetic vibratory mill for 1 minute. After grinding, the modified calcium oxide expansion agent for cement concrete is obtained.
[0034] Example 3
[0035] First, weigh 50-mesh expandable graphite, calcine it at 400℃ for 30 minutes, remove it and cool it to room temperature in air to obtain porous expanded graphite.
[0036] Then, the calcium oxide powder, fly ash, expanded graphite and 160 mesh expandable graphite are mixed in a mass ratio of 4:3:1:2 to obtain a mixture.
[0037] Next, the above mixture is calcined at 350°C for 120 minutes, then removed and cooled to room temperature in air. The resulting mixture is then ground in an electromagnetic vibratory mill for 1 minute. After grinding, the modified calcium oxide expansion agent for cement concrete is obtained.
[0038] Example 4
[0039] First, weigh 50-mesh expandable graphite, calcine it at 300℃ for 30 minutes, remove it and cool it to room temperature in air to obtain porous expanded graphite.
[0040] Then, the calcium oxide powder, fly ash, expanded graphite and 160 mesh expandable graphite are mixed in a mass ratio of 4:3:1:2 to obtain a mixture.
[0041] Next, the above mixture is calcined at 250°C for 30 minutes, then removed and cooled to room temperature in air. The resulting mixture is then ground in an electromagnetic vibratory mill for 1 minute. After grinding, the modified calcium oxide expansion agent for cement concrete is obtained.
[0042] To demonstrate the necessity of each process parameter in the modification method of the calcium oxide expanding agent of the present invention, the following comparative experiment was conducted.
[0043] Comparative Example 1
[0044] The purpose of this comparative example is to demonstrate the influence of the number of expansion cycles on the final modification effect in the modification method.
[0045] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 1 and Example 1 is that after mixing calcium oxide powder, fly ash, expanded graphite, and 160-mesh expandable graphite to obtain a mixture, it is not subjected to secondary calcination and expansion, but is directly ground; the rest is as described in Example 1 to obtain the first comparative expanding agent.
[0046] Comparative Example 2
[0047] The purpose of this comparative example is to demonstrate the effect of the relative amount of expandable graphite used in the two calcination expansions on the modification effect in the modification method.
[0048] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 2 and Example 1 is that calcium oxide powder, fly ash, expanded graphite, and 160-mesh expandable graphite are mixed in a mass ratio of 3:1:4:2, and the mixture is then subjected to secondary calcination and expansion. The rest is the same as described in Example 1 to obtain the second comparative expanding agent.
[0049] Comparative Example 3
[0050] The purpose of this comparative example is to demonstrate the influence of the first calcination expansion time on the modification effect in the modification method.
[0051] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 3 and Example 1 is that 50-mesh expandable graphite was weighed, calcined at 350°C for 40 minutes, and then cooled to room temperature in air to obtain porous expanded graphite; the rest is the same as described in Example 1 to obtain the third comparative expanding agent.
[0052] Comparative Example 4
[0053] The purpose of this comparative example is to demonstrate the influence of the first calcination expansion time on the modification effect in the modification method.
[0054] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 4 and Example 1 is that 50-mesh expandable graphite was weighed, calcined at 350°C for 5 minutes, and then cooled to room temperature in air to obtain porous expanded graphite; the rest is the same as described in Example 1 to obtain the fourth comparative expanding agent.
[0055] Comparative Example 5
[0056] The purpose of this comparative example is to demonstrate the influence of the first calcination expansion temperature on the modification effect in the modification method.
[0057] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 5 and Example 1 is that 50-mesh expandable graphite was weighed, calcined at 150°C for 15 minutes, and then cooled to room temperature in air to obtain porous expanded graphite; the rest is the same as described in Example 1 to obtain the fifth comparative expanding agent.
[0058] Comparative Example 6
[0059] The purpose of this comparative example is to demonstrate the influence of the second calcination expansion temperature on the modification effect in the modification method.
[0060] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 6 and Example 1 is that the mixture was calcined at 450°C for 60 minutes; the rest is the same as described in Example 1, to obtain the sixth comparative expanding agent.
[0061] Comparative Example 7
[0062] The purpose of this comparative example is to demonstrate the influence of the second calcination expansion temperature on the modification effect in the modification method.
[0063] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 7 and Example 1 is that the mixture was calcined at 200°C for 60 minutes; the rest is the same as described in Example 1, to obtain the seventh comparative expanding agent.
[0064] Comparative Example 8
[0065] The purpose of this comparative example is to demonstrate the influence of the second calcination expansion time on the modification effect in the modification method.
[0066] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 8 and Example 1 is that the mixture was calcined at 350°C for 180 min; the rest is the same as described in Example 1, to obtain the eighth comparative expanding agent.
[0067] Comparative Example 9
[0068] The purpose of this comparative example is to demonstrate the influence of the second calcination expansion time on the modification effect in the modification method.
[0069] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 9 and Example 1 is that the mixture was calcined at 300°C for 15 minutes; the rest is the same as described in Example 1, to obtain the ninth comparative expanding agent.
[0070] Comparative Example 10
[0071] This comparative example is designed to demonstrate the effect of the relative amounts of calcium oxide powder and mineral admixtures on the modification effect in the modification method.
[0072] In this comparative example, the similarities with Example 1 will not be repeated here; only the differences from Example 1 will be described. The difference between Comparative Example 10 and Example 1 is that calcium oxide powder, fly ash, expanded graphite, and 160-mesh expandable graphite are mixed in a mass ratio of 2:5:1:2, and the mixture is then subjected to secondary calcination for expansion; the rest is the same as described in Example 1 to obtain the tenth comparative expanding agent.
[0073] The performance of the expanding agents obtained from the above embodiments and comparative examples was tested, and the basic properties of the prepared expanded cement paste and mortar are as follows:
[0074] (1) Deformation properties of expanded cement paste
[0075] The aforementioned expansion agents were added separately to the cement paste at a dosage of 6% (based on cement mass percentage, external admixture method), with a water-cement ratio of 0.30. The cement used was P.II 52.5 grade Portland cement produced by a cement company in Nanjing. After molding the specimens, they were cured under standard conditions for (24±2) hours. After demolding, their initial length (L0) was measured. Then, the specimens were placed in water at 20℃, and at the predetermined age, their expansion (L) was measured using a micrometer. t ), calculate its free expansion rate using the following formula:
[0076] Free expansion rate (%): (L) t -L0)×100 / R
[0077] In the formula: R - when the specimen length is 40mm, the effective length is 35mm.
[0078] The expansion performance was determined according to ASTM C150-86 standard, and the free expansion rate of cement paste is shown in Table 1 below.
[0079] Table 1. Expanding performance of the expanding agents in each embodiment and comparative example.
[0080]
[0081]
[0082] Compared with the free expansion rate of the slurry, the modified calcium oxide expanding agents obtained by the modification methods in each embodiment of the present invention all exhibited good delayed expansion effects. Compared with the expanding agent in Comparative Example 1 that used expandable graphite as a carrier during primary expansion, the expanding agents in each embodiment that used expandable graphite for secondary expansion all showed a more obvious delayed expansion effect. In Comparative Example 2, more expandable graphite was used during the first calcination expansion. The corresponding comparative expanding agent, because of its smaller amount of secondary-filled expandable graphite, was insufficient to encapsulate the internal calcium oxide component, resulting in earlier hydration of the calcium oxide and exhibiting poor delayed expansion.
[0083] By controlling the temperature and calcination time of the first expansion of 50-mesh expandable graphite, and the temperature and calcination time of the second expansion of 160-mesh expandable graphite, it can be found that, as in Comparative Example 3, when the first calcination time exceeds 30 minutes (as in Comparative Example 3), when the second calcination temperature exceeds 350℃ (as in Comparative Example 6), and when the second calcination time exceeds 120 minutes (as in Comparative Example 8), the effect on the expansion effect of the expanding agent is not significant. However, when the first calcination expansion time is less than 10 minutes (as in Comparative Example 4), or the temperature is less than 300℃ (as in Comparative Example 5), due to the short calcination time or low calcination temperature, the 50-mesh expandable graphite does not expand completely and cannot form an effective carrier, resulting in insufficient internal encapsulation of calcium oxide, thus weakening its delayed effect. Similarly, when the calcination temperature for the second calcination expansion does not reach 250℃ (as in Comparative Example 7) or the calcination time does not reach 30min (as in Comparative Example 9), the sealing effect formed by the secondary expansion of expandable graphite is poor, thus causing calcium oxide to be unable to be effectively encapsulated, thereby affecting its delayed expansion effect.
[0084] In contrast, the expansion agent in Comparative Example 10 had a lower final expansion effect due to the addition of a larger proportion of fly ash, resulting in a controllable expansion effect.
[0085] (2) Mechanical properties of expansive mortar
[0086] The modified calcium oxide expansive agents provided in the above embodiments were respectively incorporated into cement mortar. The dosage of the expansive agent was 6% (according to the mass percentage of cement, external admixture method), the water-cement ratio was 0.5, the cement-sand ratio was 1:3, and the water-reducing agent was 0.4% of the cement content. After the specimens were molded, they were cured under standard conditions for (24±2) hours, demolded, and then cured in water at (20±2)℃ for 3 days and 28 days, after which the strength was measured.
[0087] The cement used was P·Ⅱ52·5 grade Portland cement produced by a cement company in Nanjing; the sand was medium sand with a fineness modulus of 2.79; and the water-reducing agent was a naphthalene-based high-efficiency water-reducing agent. The strength of the cement mortar was tested according to the national standard GB / T17671-1999, "Test Method for Strength of Cement Mortar".
[0088] The strength data of cement mortar are shown in Table 2 below.
[0089] Table 2. Effects of modified calcium oxide expanding agents on the compressive strength of cement mortar in each embodiment.
[0090]
[0091] As can be seen from the compressive strength data in Table 2, compared with the baseline group, the strength of the mortar mixed with the modified calcium oxide expansive agent provided in each embodiment does not change significantly. That is, the use of the modified calcium oxide expansive agent will not have an adverse effect on the mechanical properties of the mortar.
[0092] In summary, expanded graphite, as a carrier, can effectively encapsulate the calcium oxide anti-cracking agent components through a secondary expansion process. However, controlling the temperature and calcination time of both the primary and secondary expansion stages is still necessary to optimize the anti-cracking agent with a suitable delayed expansion effect. By adjusting the ratio of mineral admixtures and calcium oxide within the anti-cracking agent, the expansion performance of the expander can be made process-controllable.
[0093] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for modifying a calcium oxide expanding agent, characterized in that, Includes the following steps: S1. Take 10%~25% of the first part of expandable graphite and calcine it at 300℃~400℃ for 10 min~30 min to obtain porous expanded graphite; the particle size of the first part of expandable graphite is 40 mesh~60 mesh. S2. The expanded graphite is mixed with 30%~50% calcium oxide and 10%~30% mineral admixtures to obtain a mixture. S3. The mixture is mixed with 20% to 40% of the second expandable graphite, calcined at 250°C to 350°C for 30 to 120 minutes, and quenched in air to obtain the modified calcium oxide expander; the particle size of the second expandable graphite is 150 mesh to 170 mesh. The above mass percentages are calculated as 100% of the total mass of the first expandable graphite, calcium oxide, mineral admixtures, and the second expandable graphite.
2. The modification method according to claim 1, characterized in that, The mineral admixture is fly ash and / or silica fume.
3. The modification method according to claim 1 or 2, characterized in that, After obtaining the modified calcium oxide expander through quenching, it is ground to a specific surface area of 200 m². 2 / kg~450 m 2 / kg of powder.