阅读说明：本技术 一种碱式硫酸镁水泥中高效固定氯离子的方法 (Method for efficiently fixing chloride ions in basic magnesium sulfate cement ) 是由 方莉 贾真真 雷帅帅 程芳琴 雷晓东 孔祥贵 于 2021-09-08 设计创作，主要内容包括：本发明公开了一种碱式硫酸镁水泥中高效固定氯离子的方法,属于建筑材料技术领域。针对采用盐湖卤水沉淀法得到的氧化镁制备碱式硫酸镁水泥会引入氯离子的问题,本发明采用煅烧水滑石和偏高岭土作为混合固氯剂加入到含有氯离子的碱式硫酸镁水泥浆体中,搅拌均匀后浇筑到模具中,在空气中养护得到碱式硫酸镁水泥。本发明提供的固定氯离子方法,其中,煅烧水滑石通过吸附氯离子结构重建,偏高岭土通过参与水化反应生成新的胶凝相对氯离子产生静电吸附,二者协同固氯,在碱式硫酸镁水泥中的固氯率可达72％,且对碱式硫酸镁水泥中的5·1·7相晶体生长及其力学性能几乎没有影响,可实现长期、稳定、高效固氯。(The invention discloses a method for efficiently fixing chloride ions in basic magnesium sulfate cement, and belongs to the technical field of building materials. Aiming at the problem that chloride ions are introduced when basic magnesium sulfate cement is prepared by magnesium oxide obtained by a salt lake brine precipitation method, calcined hydrotalcite and metakaolin are used as a mixed chlorine fixing agent to be added into basic magnesium sulfate cement slurry containing chloride ions, the basic magnesium sulfate cement slurry is poured into a mould after being uniformly stirred, and the basic magnesium sulfate cement is obtained through maintenance in the air. According to the method for fixing chloride ions, the calcined hydrotalcite is reconstructed by absorbing a chloride ion structure, metakaolin participates in hydration reaction to generate new gel which generates electrostatic adsorption relative to chloride ions, the metakaolin and the chloride ions cooperate to fix chloride, the chloride fixing rate in basic magnesium sulfate cement can reach 72%, the growth of 5.1.7 phase crystals in the basic magnesium sulfate cement and the mechanical properties of the crystals are hardly influenced, and long-term, stable and efficient chloride fixing can be realized.)

1. A method for efficiently fixing chloride ions in basic magnesium sulfate cement is characterized by comprising the following steps: the method comprises the following steps:

step 1, adding magnesium chloride, magnesium sulfate heptahydrate and an additive into water to prepare a magnesium sulfate solution containing chloride ions and the additive;

step 2, placing the hydrotalcite in a muffle furnace, calcining at a certain temperature to remove anions and water to obtain calcined hydrotalcite, and then uniformly mixing with metakaolin to obtain a mixed chlorine fixing agent;

step 3, adding magnesium oxide and a mixed chlorine fixing agent into the magnesium sulfate solution containing chloride ions and the admixture obtained in the step 1, and stirring at a high speed to obtain basic magnesium sulfate cement paste;

and 4, pouring the cement paste obtained in the step 3 into a conventional mold, curing in air at room temperature until demolding, and continuing curing until the testing age to obtain the basic magnesium sulfate cement.

2. The method for efficiently fixing chloride ions in basic magnesium sulfate cement according to claim 1, wherein the method comprises the following steps: the molar ratio of the magnesium oxide to the magnesium sulfate heptahydrate is 5:1, and the addition amount of the additive is 1% of the mass of the magnesium oxide.

3. The method for efficiently fixing chloride ions in basic magnesium sulfate cement according to claim 2, characterized in that: the content of chloride ions in the magnesium sulfate solution containing chloride ions and the additive in the step 1 is 0.5-2% of the mass of magnesium oxide.

4. The method for efficiently fixing chloride ions in basic magnesium sulfate cement according to claim 3, wherein the method comprises the following steps: the mass ratio of the calcined hydrotalcite to the metakaolin in the step 2 is 1: 1-10.

5. The method for efficiently fixing chloride ions in basic magnesium sulfate cement according to claim 4, wherein the method comprises the following steps: in the step 2, the calcining temperature is 450-600 ℃, and the calcining time is 2-5 h.

6. The method for efficiently fixing chloride ions in basic magnesium sulfate cement according to claim 5, wherein the method comprises the following steps: the addition amount of the mixed chlorine fixing agent in the step 3 is 5% of the mass of the magnesium oxide.

7. The method for efficiently fixing chloride ions in basic magnesium sulfate cement according to claim 6, wherein the method comprises the following steps: the mass ratio of the water to the total amount of magnesium oxide and magnesium sulfate was 0.85.

8. Basic magnesium sulfate cement prepared by the method for efficiently fixing chloride ions in the basic magnesium sulfate cement according to claims 1 to 7, wherein: the basic magnesium sulfate cement is composed of magnesium oxide, magnesium sulfate heptahydrate, a chlorine fixing agent and an additive, wherein the molar ratio of the magnesium oxide to the magnesium sulfate heptahydrate is 5:1, the mass ratio of water to the total amount of the magnesium oxide and the magnesium sulfate is 0.85, the addition amount of the additive is 1% of the mass of the magnesium oxide, and the synergistic chlorine ion fixing capacity of the mixed chlorine fixing agent is up to 72%.

Technical Field

The invention belongs to the technical field of building materials, and relates to a method for efficiently fixing chloride ions in basic magnesium sulfate cement.

Background

The basic magnesium sulfate cement is a modified magnesium oxysulfate cement prepared from active magnesium oxide and magnesium sulfate aqueous solution in the presence of weak acids such as phosphoric acid, citric acid, malic acid, etc., and is prepared from needle bar-shaped basic magnesium sulfate crystals (5Mg (OH)₂·MgSO₄·7H₂O, 5, 1 and 7 phases for short) are mutually staggered, and has the advantages of light weight, high strength, fire resistance, wear resistance, environmental protection and the like. At present, the magnesium oxide used as the raw material for preparing basic magnesium sulfate cement is mainly light-burned magnesium oxide, wherein the impurity content is high, the active magnesium oxide content is low, and the long-term stability of the cement is easy to be poor. The magnesium oxide prepared by precipitation from salt lake brine or seawater has the advantages of high purity, high active magnesium oxide content and the like, and is a suitable raw material for preparing high-performance basic magnesium sulfate cement. However, residual chloride ions are inevitably contained in magnesium oxide obtained from brine or seawater, and thus, moisture absorption and halogen reversion may occur, which may deteriorate the corrosion resistance and durability of basic magnesium sulfate cement. Therefore, the method has important practical significance for realizing the efficient fixation of residual chloride ions in the basic magnesium sulfate cement.

At present, the fixation of free chloride ions in cement is mainly a method for improving the compactness of the cement by adding mineral admixtures, additives and the like. Patent CN106746851B discloses a method for improving the fixing ability of chloride ions in hardened cement paste, in which the curing rate is improved from 60.34% to 81.15% after glass powder and triethanolamine are added into concrete with the mass ratio of fly ash to cement being 3:7 as chloride ion adsorbents. Patent CN108129051A discloses a concrete admixture for curing chloride ions with high efficiency for a long time, which uses calcined bentonite containing aluminum silicon phase components and kaolin to obtain concrete admixture with Fischer-Tropsch salt reaction activityAccording to the hole wall, different modification components are added to regulate the electrostatic adsorption of the hole wall to chloride ions, the physical adsorption capacity and stability of the C-S-H gel to the chloride ions are enhanced, the total curing amount of the chloride ions of 360d is about 47%, but the additive is complex in component, needs to be prepared in advance, and consumes long time. However, basic magnesium sulfate cement belongs to a weakly alkaline system, and it is difficult to activate the pozzolanic activity of aluminosilicate such as fly ash, and chlorine ion fixation is achieved by forming fischer-tropsch salts. Hydrotalcite (LDH) is a layered metal hydroxide and has recently received much attention in the immobilization of chloride ions due to its properties such as ion exchange, thermal stability and structure reconstruction. Patent CN108298845B discloses an anti-halogen-returning magnesium oxychloride cement and a preparation method thereof, wherein ultrathin NO modified by a self-made surfactant is used₃ ^-The type hydrotalcite nanosheet is added into magnesium oxychloride cement as an anti-halogen back-halogen agent, and forms chloride ion intercalated hydrotalcite through ion exchange, so that free chloride ions are effectively captured. Patent CN104276777B discloses a chloride ion curing agent, which utilizes Ca-Al-LDH and its water reducing agent intercalation product (Ca-Al-SP-LDH) to react with chloride in magnesium oxychloride cement to generate Feillite salt to fix chloride ions, while Ca-Al-Si-LDH and Mg-Al-LDH can also fix chloride ions through ion exchange.

Disclosure of Invention

Aiming at the problem of fixing chloride ions in a basic magnesium sulfate cement alkalescent system, the invention provides a method for efficiently fixing chloride ions in basic magnesium sulfate cement.

In order to achieve the purpose, the invention adopts the following technical scheme:

a method for efficiently fixing chloride ions in basic magnesium sulfate cement comprises the following steps:

step 1, adding magnesium chloride, magnesium sulfate heptahydrate and an additive into water to prepare a magnesium sulfate solution containing chloride ions and the additive;

step 2, placing the hydrotalcite in a muffle furnace, calcining at a certain temperature to remove anions and water to obtain calcined hydrotalcite, and then uniformly mixing with metakaolin to obtain a mixed chlorine fixing agent;

step 3, adding magnesium oxide and a mixed chlorine fixing agent into the magnesium sulfate solution containing chloride ions and the admixture obtained in the step 1, and stirring at a high speed to obtain basic magnesium sulfate cement paste;

and 4, pouring the cement paste obtained in the step 3 into a conventional mold, curing in air at room temperature until demolding, and continuing curing until the testing age to obtain the basic magnesium sulfate cement.

Further, the molar ratio of the magnesium oxide to the magnesium sulfate heptahydrate is 5:1, and the addition amount of the additive is 1% of the mass of the magnesium oxide.

Further, the content of chloride ions in the magnesium sulfate solution containing chloride ions and the additive in the step 1 is 0.5-2% of the mass of magnesium oxide.

Further, the mass ratio of the calcined hydrotalcite to the metakaolin in the step 2 is 1: 1-10.

Further, the calcining temperature in the step 2 is 450-600 ℃, and the calcining time is 2-5 h.

Furthermore, the addition amount of the mixed chlorine-fixing agent in the step 3 is 5% of the mass of the magnesium oxide.

The mixed chlorine fixing agent in the step 2 is a mixture of metakaolin and calcined hydrotalcite.

Further, the mass ratio of the water to the total amount of magnesium oxide and magnesium sulfate was 0.85.

The basic magnesium sulfate cement is composed of magnesium oxide, magnesium sulfate heptahydrate, a chlorine fixing agent and an additive, wherein the molar ratio of the magnesium oxide to the magnesium sulfate heptahydrate is 5:1, the mass ratio (water-cement ratio) of water to the total amount of the magnesium oxide and the magnesium sulfate is 0.85, the addition amount of the additive is 1% of the mass of the magnesium oxide, the synergistic chlorine ion fixing capacity of the mixed chlorine fixing agent is 72%, and the chlorine fixing agent has no obvious influence on the hydration reaction of the basic magnesium sulfate cement.

Compared with the prior art, the invention has the following advantages:

(1) according to the invention, the calcined hydrotalcite is added into the basic magnesium sulfate cement, so that chloride ions are absorbed between laminates of the hydrotalcite, and the fixation of the chloride ions is realized through the structural reconstruction of the hydrotalcite.

(2) The invention selects metakaolin containing a large amount of active silicon-aluminum components, can participate in the hydration reaction of the alkalescent system of the basic magnesium sulfate cement to generate a new aluminum-magnesium silicate gel phase without strong alkali excitation, and the positively charged hydrated ions on the surface of the new aluminum-magnesium silicate gel phase can realize the adsorption and fixation of chloride ions through electrostatic attraction.

(3) The mixed chlorine fixing agent consisting of the metakaolin and the calcined hydrotalcite realizes synergistic and efficient chlorine fixing, the chlorine fixing rate in basic magnesium sulfate cement can reach 72 percent, 5.1.7 phase crystal growth and mechanical properties in the basic magnesium sulfate cement are hardly influenced, and long-term stable chlorine fixing can be realized.

Drawings

FIG. 1 is an XRD pattern of basic magnesium sulfate cements (28d) prepared in examples 1-4 and comparative example 1.

FIG. 2 shows the compressive strengths of basic magnesium sulfate cements prepared in examples 1-4 and comparative examples 1-3 at different curing ages.

Detailed Description

The technical solution of the present invention is further illustrated by the following specific examples. It should be understood by those skilled in the art that the specific embodiments are only for the understanding of the present invention and should not be construed as the specific limitations of the present invention.

Example 1

Step 1, adding 4.41g of magnesium chloride (0.5%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 526g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, putting the hydrotalcite into a muffle furnace, calcining for 3 hours at 500 ℃ to remove anions and water to obtain calcined hydrotalcite, and mixing 6.59g of calcined hydrotalcite (1%) with 26.36g of metakaolin (4%) to obtain a mixed chlorine-fixing agent;

step 3, adding 659g of magnesium oxide and a mixed chlorine fixing agent into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and stirring at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement slurry;

and 4, pouring the cement paste obtained in the step 3 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain the basic magnesium sulfate cement.

Grinding basic magnesium sulfate cement test blocks maintained at different ages, weighing 10mg, soaking in 100mL deionized water, shaking for 2min, standing for 24h, filtering the suspension with filter paper, taking 20mL filtrate, and measuring the content C of free chlorine ions with a potentiometric titrator_fUsing the total chloride ion content C_tFurther obtaining the chloride ion fixing capacity C_b。

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, the XRD characterization is shown in figure 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in figure 2.

Example 2

Step 1, adding 8.82g of magnesium chloride (1%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 529g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, putting the hydrotalcite into a muffle furnace, calcining for 3 hours at 500 ℃ to remove anions and water to obtain calcined hydrotalcite, and mixing 6.59g of calcined hydrotalcite (1%) with 26.36g of metakaolin (4%) to obtain a mixed chlorine-fixing agent;

step 3, adding 659g of magnesium oxide and a mixed chlorine fixing agent into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and stirring at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement slurry;

step 4, pouring the cement paste obtained in the step 3 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain basic magnesium sulfate cement;

chloride fixing capacity was tested as in example 1.

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, the XRD characterization is shown in figure 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in figure 2.

Example 3

Step 1, adding 17.64g of magnesium chloride (2%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 562g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, putting the hydrotalcite into a muffle furnace, calcining for 3 hours at 500 ℃ to remove anions and water to obtain calcined hydrotalcite, and mixing 6.59g of calcined hydrotalcite (1%) with 26.36g of metakaolin (4%) to obtain a mixed chlorine-fixing agent;

step 3, adding 659g of magnesium oxide and a mixed chlorine fixing agent into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and stirring at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement slurry;

step 4, pouring the cement paste obtained in the step 3 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain basic magnesium sulfate cement;

chloride fixing capacity was tested as in example 1.

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, the XRD characterization is shown in figure 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in figure 2.

Example 4

Step 1, adding 17.64g of magnesium chloride (2%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 562g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, putting the hydrotalcite into a muffle furnace, calcining for 3 hours at 500 ℃ to remove anions and water to obtain calcined hydrotalcite, and mixing 13.18g of calcined hydrotalcite (2%) with 19.77g of metakaolin (3%) to obtain a mixed chlorine fixing agent;

step 3, adding 659g of magnesium oxide and a mixed chlorine fixing agent into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and stirring at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement slurry;

step 4, pouring the cement paste obtained in the step 3 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain basic magnesium sulfate cement;

chloride fixing capacity was tested as in example 1.

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, the XRD characterization is shown in figure 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in figure 2.

Comparative example 1

Step 1, adding 17.64g of magnesium chloride (2%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 509g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, adding 659g of magnesium oxide into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and stirring at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement paste;

and 3, pouring the cement paste obtained in the step 2 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain the basic magnesium sulfate cement.

Chloride fixing capacity was tested as in example 1.

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, the XRD characterization is shown in figure 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in figure 2.

Comparative example 2

Step 1, adding 17.64g of magnesium chloride (2%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 562g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, 659g of magnesium oxide and 32.95g of metakaolin (5%) are added into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and the mixture is stirred at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement slurry;

and 3, pouring the cement paste obtained in the step 2 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain the basic magnesium sulfate cement.

Chloride fixing capacity was tested as in example 1.

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in table 2.

Comparative example 3:

step 1, adding 17.64g of magnesium chloride (2%), 783g of magnesium sulfate heptahydrate and 6.59g of citric acid into 562g of water to prepare a magnesium sulfate solution containing chloride ions and citric acid;

step 2, putting the hydrotalcite into a muffle furnace, and calcining for 3 hours at 500 ℃ to remove anions and water to obtain calcined hydrotalcite;

step 3, adding 659g of magnesium oxide and 32.95g of calcined hydrotalcite (5%) into the magnesium sulfate solution containing chloride ions and citric acid obtained in the step 1, and stirring at a high speed of 800r/min for 10min to obtain basic magnesium sulfate cement slurry;

step 4, pouring the cement paste obtained in the step 3 into a prismatic steel mould with the thickness of 40mm multiplied by 40mm, carrying out air curing at the temperature of 25 ℃ for 24 hours, then demoulding, and continuing curing to a test age to obtain basic magnesium sulfate cement;

chloride fixing capacity was tested as in example 1.

The chloride ion fixation rate of the prepared basic magnesium sulfate cement is shown in table 1, and the compressive strength of the basic magnesium sulfate cement in different curing ages is shown in table 2.

TABLE 1 chloride ion fixation rate of basic magnesium sulfate cement

It can be seen from table 1 that the chloride ion fixation rate of the metakaolin is higher than that of the calcined hydrotalcite with single addition, because the calcined hydrotalcite must realize structure reconstruction by interlayer adsorption of chloride ions, the viscous cement paste may block the migration of chloride ions, resulting in reduction of the adsorption amount, and the chloride fixation rate of the mixed chloride-fixing agent consisting of metakaolin and calcined hydrotalcite is obviously higher than that of the single-component chloride-fixing agent, which indicates that the chloride-fixing ability of the metakaolin and calcined hydrotalcite is obviously enhanced when the metakaolin and the calcined hydrotalcite act synergistically. In addition, the higher the chloride ion content, the higher the chlorine fixing rate, which indicates that the chlorine fixing agent plays a more significant role at higher chloride ion content.

As can be seen from the XRD diffraction pattern in figure 1, all the test blocks have 517-phase characteristic peak, and the peak intensity has no obvious change, which indicates that the existence of a small amount of chloride ions and chlorine fixing agents has little influence on the growth of 517-phase crystals in the basic magnesium sulfate cement. Characteristic diffraction peaks of the hydrotalcite appear in the XRD patterns of examples 3 and 4, and the calcined hydrotalcite is proved to have structural reconstruction in basic magnesium sulfate cement slurry containing chloride ions.

The compressive strength of the basic magnesium sulfate cement prepared in fig. 2 according to examples 1, 2 and 3 and comparative example 1 is gradually reduced, probably because the magnesium chloride and the magnesium oxide are possibly subjected to hydration reaction to form magnesium oxychloride cement with lower strength as the content of chloride ions is increased; the compression strength of the cement obtained in the examples 3 and 4 is slightly higher than that of the cement obtained in the comparative examples 2 and 3, which shows that on one hand, the compression strength of the basic magnesium sulfate cement is adversely affected by the large amount of the doped hydrotalcite, and on the other hand, the synergistic effect of the metakaolin and the calcined hydrotalcite can fix more chloride ions, reduce the amount of the magnesium oxychloride hydration product and have better mechanical properties.

Those skilled in the art will appreciate that the invention may be practiced without these specific details. Although illustrative embodiments of the present invention have been described above to facilitate the understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, and various changes may be made apparent to those skilled in the art as long as they are within the spirit and scope of the present invention as defined and defined by the appended claims, and all matters of the invention which utilize the inventive concepts are protected.