Lime-mud-based magnesium oxychloride cement modified by citric acid and preparation method thereof

Abstract

The application discloses a kind of citric acid modified red mud-based magnesium oxychloride cement and preparation method thereof, belong to building material technical field, its technical scheme main point is that the red mud-based magnesium oxychloride cement is composed of the following components: light-burned magnesium oxide, magnesium chloride hexahydrate, water, red mud and citric acid, after citric acid is added in the red mud-based magnesium oxychloride cement in the application, the compressive strength and water resistance of the red mud-based magnesium oxychloride cement can be effectively improved, and at the same time, a large amount of red mud can be added in the magnesium oxychloride cement, to solve the influence of red mud solid waste on the environment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building materials, and particularly relates to a citric acid modified red mud based oxychloride magnesium cement and a preparation method thereof. BACKGROUND

[0002] Oxychloride magnesium cement is a kind of air hardening cementitious material with early strength, wear resistance, heat insulation, fire resistance and other advantages, which is mainly generated by the reaction of light magnesium oxide powder and magnesium chloride solution, and the main hydration products are 3Mg(OH)2·MgCl2·8H2O (3-phase) and 5Mg(OH)2·MgCl2·8H2O (5-phase). It can be used as raw materials for concrete, wall materials, fireproof boards, decorative materials and the like in building materials. Due to the concentrated hydration heat release of the oxychloride magnesium cement, the magnesium cement building material is prone to warping and deformation, which affects the quality. The main hydration products 3-phase and 5-phase of the oxychloride magnesium cement will become brucite after meeting water, a large number of pores are generated, the strength is reduced, and the hydration heat release and water resistance are poor, which limits the application of the oxychloride magnesium cement in building materials.

[0003] Red mud is the main alkaline industrial solid waste generated in the production of alumina, and 1.2-1.5 tons of red mud solid waste are generated per ton of alumina. China is a big country in alumina production, and the cumulative storage of red mud in China has exceeded 100 million tons. The high alkalinity and heavy metals of red mud cause soil and water resource pollution and deteriorate the ecological environment. Therefore, the comprehensive recovery and utilization of red mud has become a problem to be solved. At present, the comprehensive utilization and recovery of red mud at home and abroad mainly focuses on the extraction of heavy metals, building materials, environmental protection materials, agricultural growth and the like. Among them, the production of building materials is one of the most effective ways to provide resource utilization of red mud. The application of red mud in building materials mainly focuses on cement production, concrete blocks, decorative materials and roadbed materials and the like.

[0004] For the recovery of red mud, the existing Chinese patent CN107986739A discloses a red mud slag building material, wherein the red mud slag raw material includes 20-95wt% of red mud slag, 0-3.5wt% of fly ash, 4.1-55wt% of light burned magnesium oxide, 0.75-16.9wt% of magnesium chloride, 0.05-0.5wt% of defoaming agent, 0-1.5wt% of water reducing agent and 0.1-2.6wt% of composite modifier, and the composite modifier is composed of phosphoric acid and / or phosphate, sulfate and oxalic acid. As can be seen from the specific embodiments, when the addition amount of red mud slag is more than 80%, the 7-day compressive strength of the building material is only 4.9-23.4MPa, and when the addition amount of red mud slag is 20%, the 7-day compressive strength of the building material is only 55.2MPa. It can be seen that although a large amount of red mud slag is added in the building material, the strength of the oxychloride magnesium cement is significantly reduced, and the building material cannot be used in the scene requiring high strength such as building wall.

[0005] The existing Chinese patent CN109081672A discloses a red mud and chromium slag composite material and a preparation method thereof, wherein the composite material comprises red mud 20-70wt%, chromium slag 20-70wt%, light-burned magnesium oxide 8.1-40wt%, magnesium chloride 1.7-12.9wt%, defoaming agent 0.05-0.5wt%, water reducing agent 0-1.5wt%, and modifier 0.15-2.6wt%, the modifier being one of phosphoric acid and / or phosphate, citric acid and / or sodium citrate, and oxalic acid. According to the specific embodiment, the addition amount of red mud is basically 2 times the weight of light-burned magnesium oxide, and the 28-day compressive strength is <30MPa. Although the red mud is effectively utilized, the compressive strength of the building material obtained finally is too low to be used in scenarios with high strength requirements.

[0006] Therefore, in the prior art, after adding red mud (the mixing amount is 50%) into the magnesium oxychloride cement, the water resistance of the magnesium oxychloride cement is improved, but the strength is significantly reduced, and with the increase of the addition amount of red mud, the strength of the magnesium oxychloride cement will be reduced more, which limits the use of the magnesium oxychloride cement in high-strength application scenarios. SUMMARY

[0007] In order to solve the problems in the prior art, the present application provides a citric acid modified red mud based magnesium oxychloride cement and a preparation method thereof. By reasonably controlling the addition amount of each raw material, after adding red mud and citric acid into the magnesium oxychloride cement, the influence of the red mud solid waste on the environment is solved, and at the same time, when the addition amount of red mud reaches 60%, the 28-day compressive strength of the magnesium oxychloride cement can still be maintained at 60MPa, meeting the needs of high-strength use scenarios.

[0008] The first aspect of the present application is to provide a citric acid modified red mud based magnesium oxychloride cement, which adopts the following technical scheme:

[0009] The citric acid modified red mud based magnesium oxychloride cement is composed of light-burned magnesium oxide, magnesium chloride hexahydrate, water, red mud, and citric acid; the molar ratio of active magnesium oxide in the light-burned magnesium oxide to the magnesium chloride hexahydrate is (6-8):1, the addition amount of the red mud is 20-60% of the weight of the light-burned magnesium oxide, and the addition amount of the citric acid is 0.5-1.5% of the weight of the light-burned magnesium oxide.

[0010] Preferably, the content of active magnesium oxide in the light-burned magnesium oxide is 60%.

[0011] Preferably, the water-cement ratio of the red mud based magnesium oxychloride cement is 0.27.

[0012] Preferably, the addition amount of the citric acid is 1.0-1.5% of the weight of the light-burned magnesium oxide.

[0013] Preferably, the red mud has a particle size of less than 200 mesh.

[0014] Preferably, the red mud is dried at 100-110°C for 24h, ground for 40-45min and sieved.

[0015] The second aspect of the present application provides a preparation method of the citric acid modified red mud based magnesia-chloride cement as described above, comprising the following steps:

[0016] S1, stirring and mixing the magnesium chloride hexahydrate, citric acid and water to obtain an aqueous solution;

[0017] S2, mixing the light-burned magnesia and red mud powder to obtain a mixed powder;

[0018] S3, adding the aqueous solution obtained in step S1 into the mixed powder and stirring to obtain the citric acid modified red mud based magnesia-chloride cement.

[0019] With the increase of the content of red mud in the magnesia-chloride cement, the 28-day compressive strength of the magnesia-chloride cement is also significantly reduced. By using the above technical solution, the present application can reduce the amplitude of the decrease of the compressive strength of the magnesia-chloride cement by adding citric acid to the red mud based magnesia-chloride cement. Compared with single-doped red mud, the addition of citric acid can alleviate the strength loss caused by the addition of a large amount of red mud. Moreover, when the addition amount of red mud is 60% of the weight of the light-burned magnesia and the addition amount of citric acid is 1.5% of the weight of the light-burned magnesia, the 28-day compressive strength of the red mud based magnesia-chloride cement can still reach 60MPa, which is better than that of most ordinary Portland cement. It can be seen that the addition of citric acid in the present application can alleviate the loss of the compressive strength of the magnesia-chloride cement caused by the addition of a large amount of red mud. However, when the addition amount of citric acid exceeds 1.5%, the alleviating effect is not obvious. Therefore, when the addition amount of citric acid is controlled within the range of 1.5% of the weight of the light-burned magnesia, the red mud based magnesia-chloride cement can have good mechanical properties and meet the strength requirements of building materials.

[0020] The fine particles of red mud fill the internal voids of magnesium oxychloride cement and react with magnesium ions and chloride ions to form amorphous gelling substances, which will increase the compactness of the internal part of magnesium oxychloride cement, optimize the pore structure, improve the density, and thus improve the water resistance of magnesium oxychloride cement. When the amount of red mud added is relatively small, citric acid will increase the porosity of red mud-based magnesium oxychloride cement. The reason is that citric acid reacts with metal ions in the red mud to produce a large number of pores. When the amount of red mud added exceeds 40%, a large number of red mud particles that have not participated in the reaction fill the pore structure, increasing the density of magnesium oxychloride cement, thereby improving water resistance. In addition, the retarding effect of citric acid will make the hydration reaction in the system more complete, reduce macropores and optimize the pore structure (when the amount of red mud added is 60%, after adding 1.0-1.5% citric acid, although it makes the chlorine The porosity of magnesium oxychloride cement is greater than that without citric acid, but the proportion of harmless pores and less harmful pores is greater than that when only 60% red mud is added, and the harmful pores are less than that when only 60% red mud is added. Therefore, the addition of citric acid will reduce the proportion of large pores and harmful pores, making the internal structure more uniform. Therefore, the combination of large amounts of red mud and citric acid not only increases the utilization of solid waste materials, but also reduces the reduction in the compressive strength of magnesium oxychloride cement and improves its water resistance, effectively improving the performance of magnesium oxychloride cement in construction materials.

[0021] In addition, the present application found through XRD patterns that there were obvious five-phase diffraction peaks in the magnesium oxychloride cement specimens after 28 days of curing, indicating that the main crystalline phase of magnesium oxychloride cement was five phases. As the red mud content increased, the five-phase diffraction peak in the magnesium oxychloride cement gradually weakened, and at the same time, the diffraction peak of magnesium hydroxide decreased and gradually weakened. This may be due to the obstruction of red mud particles, which reduced the amount of five phases generated, and the fine red mud particles were adsorbed on the surface of larger magnesium oxide particles, thereby inhibiting the formation of five phases and magnesium hydroxide; when citric acid was added, the five-phase diffraction peak of magnesium oxychloride cement was enhanced, and the diffraction peak of magnesium hydroxide was reduced and weakened. This shows that after the addition of citric acid, citric acid can promote the formation of five phases in red mud-based magnesium oxychloride cement, and citric acid reacts with magnesium ions to reduce the precipitation of magnesium hydroxide.

[0022] After the test block containing citric acid and red mud is soaked for 14 days, the XRD pattern shows that the diffraction peak of 5 phase in the red mud-based magnesia oxychloride cement with added citric acid is stronger than that without added citric acid, which indicates that the appropriate use of red mud and citric acid can inhibit the hydrolysis of 5 phase in the magnesia oxychloride cement. The reason is that the complexation of citric acid can slow down the hydration rate of magnesium oxide, refine the 5·1·8 phase crystals, and improve the structural density. The slender needle-like 5·1·8 phase is intertwined with a large amount of cementitious material to form a dense network structure, which strengthens the bonding between the crystals. The bonding structure protects the 5·1·8 phase whisker structure from hydrolysis, and finally significantly improves the water resistance and mechanical properties of the magnesia oxychloride cement. Moreover, C5H7O5COO- provided by citric acid is beneficial to the generation of 5 phase, and C5H7O5COO- and MgOH + combined after the hydrolysis of MgO, forms an organic magnesium·complex layer attached to the surface of the 5 phase crystals, thereby effectively improving the water stability of the magnesia oxychloride cement.

[0023] In summary, the present application has the following beneficial effects: after adding citric acid to the red mud-based magnesia oxychloride cement, the compressive strength of the magnesia oxychloride cement with single-doped red mud is significantly improved. The combination of red mud content and citric acid content optimizes the pore structure of the magnesia oxychloride cement, enhances the compactness, and citric acid can promote the generation of 5 phase and prevent the hydrolysis of 5 phase in water. Moreover, red mud significantly improves the pH of the system, promotes the dissolution of silicon dioxide, diiron trioxide and aluminum oxide, and the complex produced by the reaction of red mud and citric acid fills the pores, reduces large pores, increases small pores, optimizes the void structure, and makes the internal microstructure of the magnesia oxychloride cement more compact, thereby effectively improving the water resistance of the magnesia oxychloride cement. DETAILED DESCRIPTION

[0024] The present application will be further described in detail below in conjunction with examples. All reagents not specified by the manufacturer are conventional reagent products that can be obtained through commercial purchase.

[0025] Light-burned magnesium oxide is produced by Liaoning Yingkou Huiteng Refractory Materials Co., Ltd.;

[0026] Bayer red mud is from Shandong Aluminum Co., Ltd.;

[0027] Magnesium chloride hexahydrate is produced by Shanghai Minhang Shenlong Light Chemical Co., Ltd.;

[0028] Citric acid is of analytical purity. Example 1

[0029] A preparation method of citric acid modified red mud-based magnesia oxychloride cement, comprising the following steps:

[0030] Ingredients: 10 kg of light-burned magnesia, the active magnesium oxide content in the light-burned magnesia is 60%, the molar ratio of active magnesium oxide to magnesium chloride hexahydrate is 7:1, the addition amount of magnesium chloride hexahydrate is 4.35 kg, the addition amount of red mud is 20% of the weight of light-burned magnesia, that is, the amount of red mud is 2 kg, the addition amount of citric acid is 0.5% of the weight of light-burned magnesia, that is, the amount of citric acid is 0.05 kg, and the water-cement ratio is 0.27;

[0031] S1, dry the red mud at 100℃ for 24h, grind for 40min, and sieve after passing through a 200 mesh sieve for standby;

[0032] S2, mix magnesium chloride hexahydrate, citric acid and water to prepare an aqueous solution;

[0033] S3, mix light-burned magnesia and red mud powder to obtain a mixed powder;

[0034] S4, add the aqueous solution obtained in step S2 to the mixed powder, first stir at low speed for 60s, stand for 90s, then stir at high speed for 60s, pour the stirred slurry into a mold, vibrate on a cement vibration table for 60s, use a scraper to remove excess slurry to make the surface smooth, demold after curing for 24h, and test the performance under the conditions of temperature 20±2℃ and humidity 60±5% for 7 days, 28 days and 42 days respectively. Example 2

[0035] A preparation method of citric acid modified red mud based magnesium oxychloride cement, comprising the following steps:

[0036] Ingredients: 10 kg of light-burned magnesia, the active magnesium oxide content in the light-burned magnesia is 60%, the molar ratio of active magnesium oxide to magnesium chloride hexahydrate is 7:1, the addition amount of magnesium chloride hexahydrate is 4.35 kg, the addition amount of red mud is 40% of the weight of light-burned magnesia, that is, the amount of red mud is 4 kg, the addition amount of citric acid is 0.5% of the weight of light-burned magnesia, that is, the amount of citric acid is 0.05 kg, and the water-cement ratio is 0.27;

[0037] S1, dry the red mud at 110℃ for 24h, grind for 45min, and sieve after passing through a 200 mesh sieve for standby;

[0038] S2, mix magnesium chloride hexahydrate, citric acid and water to prepare an aqueous solution;

[0039] S3, mix light-burned magnesia and red mud powder to obtain a mixed powder;

[0040] S4, the aqueous solution obtained in step S2 is added to the mixed powder, first stirred at low speed for 60s, stand for 90s, then stirred at high speed for 60s, the stirred slurry is poured into a mold, vibrated on a cement vibrating table for 60s, the excess slurry is scraped off with a scraper to make the surface smooth, demolded after curing for 24h, cured under the conditions of temperature 20±2℃ and humidity 60±5% for 7d, 28d and 42d respectively, and the performance is tested. Example 3

[0041] A preparation method of a citric acid modified red mud based magnesia-chloride cement, comprising the following steps:

[0042] Ingredients: 10kg of light burned magnesia, the active magnesium oxide content in the light burned magnesia is 60%, the molar ratio of active magnesium oxide to magnesium chloride hexahydrate is 7:1, the addition amount of magnesium chloride hexahydrate is 4.35kg, the addition amount of red mud is 60% of the weight of light burned magnesia, i.e. the amount of red mud is 6kg, the addition amount of citric acid is 0.5% of the weight of light burned magnesia, i.e. the amount of citric acid is 0.05kg, and the water-cement ratio is 0.27;

[0043] S1, dry the red mud at 100-110℃ for 24h, grind for 40-45min, sieve and pass through a 200 mesh sieve for standby use;

[0044] S2, mix magnesium chloride hexahydrate, citric acid and water to obtain an aqueous solution;

[0045] S3, mix the light burned magnesia and red mud powder to obtain a mixed powder;

[0046] S4, add the aqueous solution obtained in step S2 to the mixed powder, first stirred at low speed for 60s, stand for 90s, then stirred at high speed for 60s, the stirred slurry is poured into a mold, vibrated on a cement vibrating table for 60s, the excess slurry is scraped off with a scraper to make the surface smooth, demolded after curing for 24h, cured under the conditions of temperature 20±2℃ and humidity 60±5% for 7d, 28d and 42d respectively, and the performance is tested. Example 4

[0047] A preparation method of a citric acid modified red mud based magnesia-chloride cement, which is different from example 3 in that the addition amount of citric acid is 1.0% of the weight of light burned magnesia, i.e. the addition amount of citric acid is 0.1kg, and the others are the same as example 3. Example 5

[0048] A preparation method of a citric acid modified red mud based magnesia-chloride cement, which is different from example 3 in that the addition amount of citric acid is 1.5% of the weight of light burned magnesia, i.e. the addition amount of citric acid is 0.15kg, and the others are the same as example 3. Example 6

[0049] A preparation method of a citric acid modified red mud based magnesia-chloride cement, which is different from example 5 in that the addition amount of citric acid is 2.0% of the weight of light burned magnesia, i.e. the addition amount of citric acid is 0.2 kg, and the others are the same as example 5. Example 7

[0050] A preparation method of a citric acid modified red mud based magnesia-chloride cement, which is different from example 3 in that the total amount of light burned magnesia is 10 kg, and the molar ratio of active magnesia to magnesium chloride hexahydrate in light burned magnesia is 6:1, and the others are the same as example 3. Example 8

[0051] A preparation method of a citric acid modified red mud based magnesia-chloride cement, which is different from example 3 in that the total amount of light burned magnesia is 10 kg, and the molar ratio of active magnesia to magnesium chloride hexahydrate in light burned magnesia is 8:1, and the others are the same as example 3. Comparative example 1

[0052] A preparation method of a red mud based magnesia-chloride cement, which is different from example 3 in that no citric acid is added, and the others are the same as example 3.

[0053] Performance detection

[0054] The compressive strength and water resistance of the magnesia-chloride cement obtained in the above examples and comparative examples were detected, and the detection results are shown in Table 1.

[0055] The compressive strength test was carried out according to the relevant provisions in GB / T 17671-2021, and a universal testing machine was used for testing, and the compressive strength test was carried out under the condition of loading rate of 2400 N / s.

[0056] Water resistance test: after curing for 28 days, the test block was immersed in water, and the residual compressive strength was tested after soaking for 7 days and 14 days, and the water resistance was indicated by the softening coefficient, which was the ratio of the residual compressive strength to the compressive strength at 28 days.

[0057] Table 1: Magnesia-chloride cement detection results

[0058]

[0059] The control example is a magnesia-chloride cement without adding red mud and citric acid.

[0060] In the examples 1-3 of the present application, when the addition amount of citric acid is 0.5%, the compressive strengths of the magnesia oxychloride cement at 3 days, 14 days, 28 days and 42 days all show a gradually decreasing trend with the increase of the content of red mud, and compared with the control example, the decrease amplitude of the compressive strength of the magnesia oxychloride cement significantly increases with the increase of the content of red mud, but the water resistance coefficients of the examples 1-3 are all much higher than that of the control example, thus it can be illustrated that the addition of red mud and citric acid increases the compactness of the magnesia oxychloride cement, optimizes the pore structure and improves the density, thereby improving the water resistance of the magnesia oxychloride cement.

[0061] Compared with the example 3, when the addition amount of red mud is 60%, the compressive strengths of the magnesia oxychloride cement at 3 days, 14 days, 28 days and 42 days obtained by the examples 4 and 5 are all improved with the increase of the content of citric acid, which can be seen that when a large amount of red mud is added, the addition of 1-1.5% of citric acid can reduce the decrease amplitude of the compressive strength of the magnesia oxychloride cement, so that the compressive strengths of the magnesia oxychloride cement obtained by the examples 4-5 are improved compared with the example 3, and the water resistance of the magnesia oxychloride cement of the examples 4-5 is also significantly improved compared with the example 3, and the reason is that when the addition amount of red mud is relatively small, the citric acid increases the porosity of the red mud-based magnesia oxychloride cement, and the reason is that the citric acid reacts with the metal ions in the red mud to generate a large number of pores, and when the addition amount of red mud exceeds 40%, a large amount of red mud particles which do not participate in the reaction are filled in the pore structure, thereby increasing the density of the magnesia oxychloride cement and improving the water resistance, and the retarding effect of the citric acid can make the hydration reaction in the system more complete, reduce the large pores and refine the pore structure, so that the structure inside is more uniform.

[0062] Compared with the example 5, when the addition amount of red mud is 60% and the addition amount of citric acid is 2%, the compressive strength of the magnesia oxychloride cement obtained by the example 6 does not further increase with the further increase of the content of citric acid compared with the compressive strength in the example 5, thus it can be illustrated that when the addition amount of citric acid exceeds 1.5%, it has no effect on the increase of the compressive strength of the red mud-based magnesia oxychloride cement.

[0063] Compared with the example 3, when only red mud is added without adding citric acid, the compressive strength of the magnesia oxychloride cement obtained by the comparative example 1 is significantly lower than that of the magnesia oxychloride cement to which citric acid is added, which illustrates that the addition of citric acid not only improves the compressive strength of the red mud-based magnesia oxychloride cement, but also improves the water resistance of the red mud-based magnesia oxychloride cement.

[0064] The examples of the specific embodiment are the preferred examples of the present application, but do not limit the protection scope of the present application, so that: any equivalent changes made according to the structure, shape, principle of the present application should be covered within the protection scope of the present application.

Claims

1. A citric acid-modified red mud-based magnesium oxychloride cement, characterized by, The red mud-based magnesium oxychloride cement is composed of light-burned magnesium oxide, magnesium chloride hexahydrate, water, red mud and citric acid; the mole ratio of active magnesium oxide in the light-burned magnesium oxide to the magnesium chloride hexahydrate is (6-8):1, the adding amount of the red mud is 20-60% of the weight of the light-burned magnesium oxide, and the adding amount of the citric acid is 1.0-1.5% of the weight of the light-burned magnesium oxide.

2. The citric acid modified red mud based magnesium oxychloride cement according to claim 1, characterized in that: The content of active magnesium oxide in the light-burned magnesium oxide is 60%.

3. The citric acid modified red mud based magnesium oxychloride cement as claimed in claim 1, wherein: The water-cement ratio of the red mud-based magnesium oxychloride cement is 0.

27.

4. The citric acid modified red mud based magnesium oxychloride cement as claimed in claim 1, wherein: The particle size of the red mud is less than 200 mesh.

5. The citric acid modified red mud based magnesium oxychloride cement according to claim 4, characterized in that: The red mud is dried at 100-110°C for 24h, ground for 40-45min and then sieved.

6. A process for the preparation of a citric acid modified red mud based magnesium oxychloride cement as claimed in any one of claims 1 to 5 characterised in that, The method comprises the following steps: S1, stirring and mixing magnesium chloride hexahydrate, citric acid and water to obtain an aqueous solution; S2, mixing light-burned magnesium oxide and red mud powder to obtain a mixed powder; S3, adding the aqueous solution obtained in step S1 into the mixed powder and stirring to obtain a citric acid-modified red mud-based magnesium oxychloride cement.

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