Han hemp-magnesium oxychloride cement composite material and preparation method thereof

Abstract

A hemp-magnesium oxychloride cement composite material and a preparation method thereof, which relate to a cement composite material and a preparation method thereof. The present application aims at solving the problem that the prior art cannot reduce the damage of internal stress concentration to the mechanical strength of MOC while fully exerting the water resistance improvement effect of phosphonic acid modifier on MOC. The hemp-magnesium oxychloride cement composite material is prepared from magnesium chloride hexahydrate, magnesium oxide, diphosphoric acid and hemp straw. The method comprises the following steps: 1, uniformly mixing magnesium chloride hexahydrate, distilled water and a solution containing diphosphoric acid to obtain a mixed system; 2, dry mixing hemp straw and magnesium oxide powder, and then adding the mixture into the mixed system to stir and dissolve; and 3, solidifying. The present application is used for hemp-magnesium oxychloride cement composite material and preparation thereof.

Description

Technical Field

[0001] This invention relates to a cement composite material and its preparation method. Background Technology

[0002] With the deepening of the concept of green development, reducing carbon emissions in the cement industry has become a key task for the green and low-carbon transformation of my country's construction industry. Traditional silicate cement has high strength and good durability, and has been widely used in the construction of various building structures. However, the problems of heavy weight and excessive energy consumption in the production of silicate cement have not yet been effectively solved. Therefore, developing new low-carbon cement products to replace traditional silicate cement is an effective way to achieve low-carbon and efficient production in the cement industry and improve energy efficiency.

[0003] Bio-based building materials mostly use biomass resources such as bamboo, straw, and wood as building materials, which helps reduce the use of non-renewable resources and lower production energy consumption. Hemp straw, an agricultural waste product resulting from the comprehensive utilization of hemp plants, accounts for 65% to 70% of the total yield of hemp plants (by weight). It is lightweight, porous, and has heat and sound insulation properties, but is often landfilled or incinerated due to its low added value. In the 1990s, researchers successfully produced a new type of sustainable building material—hempcrete, also known as hemp concrete—using hemp, lime, and water. It has excellent fire-resistant, mildew-resistant, lightweight, and heat-insulating properties and is widely used in non-load-bearing structures such as enclosure walls, interior partitions, and floor slabs. Compared with silicate concrete, hemp concrete has lower mechanical strength, with a density of 1000 kg / m³. 3 At that time, the compressive strength was only 2 MPa, limiting its application range. To improve the strength of hemp concrete, researchers used fly ash, kaolin, slag, and other mineral admixtures in combination with a lime matrix, but without significant improvement. In recent years, magnesium-based cementitious materials have attracted widespread attention due to their extremely high strength, excellent fire resistance, and good biocompatibility. Using magnesium-based cementitious materials to prepare hemp-magnesium composite materials holds promise for solving the problem of low mechanical strength in hemp concrete.

[0004] Magnesium oxychloride cement (MOC) is mainly composed of magnesium oxide (MgO) and magnesium chloride solution (MgCl2) reacted together. It is a typical magnesium-based cementitious material with excellent characteristics such as rapid hardening, high strength, fire resistance, and thermal insulation. Its raw materials are widely available and sustainable. Magnesium oxide is mainly produced by calcining magnesite at approximately 800℃, far lower than the calcination temperature of silicate clinker; magnesium chloride (MgCl2) can be extracted from salt lakes, making it abundant and inexpensive. Therefore, comprehensively utilizing MgO and MgCl2 to produce MOC-based building materials, replacing traditional acid salt products, helps achieve resource recycling and reduces carbon emissions and energy consumption. However, MOC products generally suffer from poor water resistance, easy moisture absorption and efflorescence, and high brittleness. Their five-phase crystals (the main crystalline phase of cement) gradually decompose upon contact with water, ultimately leading to a loss of mechanical strength, severely restricting their application in the construction field. To address this issue, researchers have proposed various strategies to improve the water resistance of MOC products, the most common being the use of phosphonic acid or soluble phosphonates. However, phosphonate ions react with Mg... 2+ Excessive crystallization stress in the formed magnesium phosphonate chelates can lead to the propagation of microcracks, thus adversely affecting the mechanical strength of MOCs. Therefore, how to fully utilize the water resistance improvement effect of phosphonate modifiers on MOCs while mitigating the damage to the mechanical strength of MOCs caused by internal stress concentration remains a challenge. Summary of the Invention

[0005] The present invention aims to address the problem that existing technologies cannot fully utilize the water resistance improvement effect of phosphonic acid modifiers on MOC while reducing the damage to the mechanical strength of MOC caused by internal stress concentration, and thus provides a hemp-magnesium oxychloride cement composite material and its preparation method.

[0006] A hemp-magnesium oxychloride cement composite material is prepared from magnesium chloride hexahydrate, magnesium oxide, diphosphoric acid and hemp straw.

[0007] A method for preparing a hemp-magnesium oxychloride cement composite material, comprising the following steps:

[0008] 1. Add magnesium chloride hexahydrate to distilled water and stir until well mixed. Then add a solution containing diphosphoric acid dropwise and continue stirring until well mixed to obtain a mixed system.

[0009] 2. Dry mix hemp straw with magnesium oxide powder, then add it to the mixing system and stir to dissolve, to obtain MOC slurry;

[0010] 3. Place the MOC slurry in a mold for pre-curing, then demold and continue curing until the hydration reaction is complete, thus completing the preparation method of hemp-magnesium oxychloride cement composite material.

[0011] The beneficial effects of this invention are:

[0012] The hemp-magnesium oxychloride cement composite material prepared in this invention possesses lightweight, high strength, high toughness, and excellent water resistance. Hemp straw acts as a supporting framework, working with magnesium oxychloride cement to transfer stress and inhibit microcrack propagation, thus positively impacting toughness and water resistance. However, XRD and FT-IR results show that the addition of hemp straw does not inhibit the decomposition of the five-phase crystals. (The text then mentions diphosphoric acid and Mg...) 2+ The magnesium phosphonate complex generated through the complexation reaction plays a crucial protective role for the five-phase crystals, thereby improving the softening coefficient of the hemp-magnesium oxychloride cement composite. However, the formation of this chelate generates excessively high internal crystallization stress, leading to the propagation of microcracks within the system and impairing its mechanical strength. In this invention, an "organic-inorganic" hybrid system is constructed in MOC through the synergistic effect of hemp straw and diphosphoric acid, simultaneously improving the mechanical properties and water resistance of the composite material. The hemp-magnesium oxychloride cement composite exhibits a maximum compressive strength of 48.21 MPa, demonstrating excellent mechanical properties. Simultaneously, the water resistance of the hemp-magnesium oxychloride cement composite is improved, with a softening coefficient reaching a maximum of 0.81. The production process of the hemp-magnesium oxychloride cement composite is simple and easy to implement, with widely available and low-cost raw materials, making it suitable for industrial production. Combining hemp straw, an agricultural waste, with MOC achieves resource recycling and endows MOC with lightweight and high-toughness characteristics, providing a sustainable alternative for the building materials industry. Attached Figure Description

[0013] Figure 1 The graph shows a comparison of the compressive strength, softening coefficient, water content-water absorption rate, and flexural strength of the hemp-magnesium oxychloride cement composite materials prepared in Examples 1 to 4 and the comparative experiment. a represents the compressive strength of the hemp-magnesium oxychloride cement composite material before and after soaking in water. 28d represents 28 days of curing in step 3 without soaking, and 7d represents 7 days of soaking in water after 28 days of curing in step 3. b represents the softening coefficient, c represents the water content-water absorption rate, and d represents the flexural strength.

[0014] Figure 2 The infrared spectrum, X-ray diffraction (XRD) crystal phase diagram, and corresponding crystal phase content diagram of the hemp-magnesium oxychloride cement composite material prepared in Example 4 and the comparative experiment are shown in Figure a. Infrared spectrum of M10 and M10H2 before and after soaking in water for 7 days; Figure b. Crystal phase diagram of M10 and M10H2 before and after soaking in water for 7 days; Figure c. Crystal phase content diagram of M10 and M10H2 before and after soaking in water for 7 days.

[0015] Figure 3 The images are SEM-EDS images of the hemp-magnesium oxychloride cement composite material prepared in Example 4 and the comparative experiment. a is M10, b is M10 after soaking in water for 7 days, c is M10H2, and d is M10H2 after soaking in water for 7 days.

[0016] Figure 4 The above are XPS comparison images of the hemp-magnesium oxychloride cement composite material prepared in Example 4 and the comparative experiment. a is the Mg 2p peak of M10, b is the Mg 2p peak of M10H2, c is the O 1s peak of M10, and d is the O 1s peak of M10H2. Detailed Implementation

[0017] Specific implementation method one: This implementation method is a hemp-magnesium oxychloride cement composite material, which is prepared from magnesium chloride hexahydrate, magnesium oxide, diphosphoric acid and hemp straw.

[0018] The addition of hemp straw leads to a decrease in compressive strength (dry) because the straw replaces part of the cement without contributing strength itself. However, it also introduces lightweight aggregate, reducing the cement's self-weight and resulting in a lightweight composite material. While the addition of hemp straw can improve water resistance to some extent (increased softening coefficient), it does not prevent the decomposition of the five-phase crystals; therefore, suitable modifiers are still needed to improve the composite material's water resistance.

[0019] This embodiment utilizes an "organic-inorganic" hybrid strategy, introducing the environmentally friendly metal chelating agent bisphosphonic acid to further improve the water resistance of HMOC, successfully preparing a lightweight, high-strength, and water-resistant hemp-magnesium oxychloride cement composite material. Diphosphonic acid, as an organic phosphonic acid metal chelating agent, can chelate with magnesium ions in the five-phase crystal to form magnesium phosphate chelates that protect the five-phase crystal and improve water resistance; it can also form hydrogen bonds with hydroxyl groups on hemp straw, optimizing the interfacial bonding between cement and straw and improving the compressive strength (dry) of the composite material. Simultaneously, the hemp straw acts as a stress dissipation unit, dispersing the internal crystallization stress generated during the formation of the magnesium phosphonate chelates. Through the synergistic effect of hemp straw and bisphosphonic acid, an "organic-inorganic" hybrid system was successfully constructed in the MOC system, achieving both improved water resistance and suppression of internal microcracks generated by the bisphosphonic acid chelation reaction, thus improving compressive strength (dry). This results in simultaneous improvement of the mechanical properties and water resistance of the hemp-magnesium oxychloride cement composite material, enhancing its application potential in the construction field.

[0020] This embodiment introduces bisphosphonic acid, whose phosphonic acid groups can react with Mg in phase 5. 2+ Chelation forms a stable complex that is not easily hydrolyzed. If diphosphate is added to MOC alone, phosphonate and Mg... 2+ The magnesium phosphonate salts formed can generate internal expansion stress, which severely impairs mechanical strength. However, the addition of hemp straw can disperse internal stress and improve the toughness of MOC composites.

[0021] The beneficial effects of this embodiment are:

[0022] The hemp-magnesium oxychloride cement composite material prepared in this embodiment possesses lightweight, high strength, high toughness, and excellent water resistance. Hemp straw acts as a supporting framework, working with magnesium oxychloride cement to transfer stress and inhibit microcrack propagation, thus positively impacting toughness and water resistance. However, XRD and FT-IR results show that the addition of hemp straw does not inhibit the decomposition of the five-phase crystals. (The text then mentions diphosphoric acid and Mg...) 2+ The magnesium phosphonate complex generated through the complexation reaction plays a crucial protective role for the five-phase crystals, thereby improving the softening coefficient of the hemp-magnesium oxychloride cement composite. However, the formation of this chelate generates excessively high internal crystallization stress, leading to the propagation of microcracks within the system and impairing its mechanical strength. In this embodiment, an "organic-inorganic" hybrid system is constructed in MOC through the synergistic effect of hemp straw and diphosphoric acid, simultaneously improving the mechanical properties and water resistance of the composite material. The hemp-magnesium oxychloride cement composite exhibits a maximum compressive strength of 48.21 MPa, demonstrating excellent mechanical properties. Simultaneously, the water resistance of the hemp-magnesium oxychloride cement composite is improved, with a softening coefficient reaching a maximum of 0.81. The production process of the hemp-magnesium oxychloride cement composite is simple and easy to implement, with widely available and low-cost raw materials, making it suitable for industrial production. Combining hemp straw, an agricultural waste, with MOC achieves resource recycling and endows MOC with lightweight and high-toughness characteristics, providing a sustainable alternative for the building materials industry.

[0023] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the diphosphonic acid is one or a combination of several of the following: aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphonic acid. Everything else is the same as in Specific Implementation Method One.

[0024] Specific Implementation Method 3: This implementation method provides a preparation method for hemp-magnesium oxychloride cement composite materials, which is carried out according to the following steps:

[0025] 1. Add magnesium chloride hexahydrate to distilled water and stir until well mixed. Then add a solution containing diphosphoric acid dropwise and continue stirring until well mixed to obtain a mixed system.

[0026] 2. Dry mix hemp straw with magnesium oxide powder, then add it to the mixing system and stir to dissolve, to obtain MOC slurry;

[0027] 3. Place the MOC slurry in a mold for pre-curing, then demold and continue curing until the hydration reaction is complete, thus completing the preparation method of hemp-magnesium oxychloride cement composite material.

[0028] In this specific implementation method, steps one and two can be combined, that is, magnesium chloride hexahydrate, distilled water, a solution containing diphosphoric acid, hemp straw and magnesium oxide powder are directly mixed to obtain MOC slurry.

[0029] Specific Implementation Method Four: This implementation method differs from Specific Implementation Method Three in that: the molar ratio of magnesium chloride hexahydrate to distilled water in step one is 1:(8~12); the mass percentage of the solution containing diphosphoric acid in step one is ≥70%. Everything else is the same as in Specific Implementation Method Three.

[0030] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method Three or Four in that the diphosphonic acid mentioned in step one is one or a combination of several of the following: aminotrimethylene phosphate, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphonic acid. Everything else is the same as in Specific Implementation Method Three or Four.

[0031] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods Three to Five in that the particle size of the hemp straw mentioned in step two is 5mm to 10mm. Everything else is the same as in Specific Implementation Methods Three to Five.

[0032] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods Three to Six in that the mass of hemp straw in the MOC slurry described in step two is 5% to 20% of the mass of magnesium oxide powder. Everything else is the same as in Specific Implementation Methods Three to Six.

[0033] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods Three to Seven in that the molar ratio of magnesium chloride hexahydrate to magnesium oxide powder in the MOC slurry described in step two is 1:(7~11). Everything else is the same as in Specific Implementation Methods Three to Seven.

[0034] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods Three to Eight in that the mass of diphosphoric acid in the MOC slurry described in step two is 0.5% to 2% of the mass of magnesium oxide powder. Everything else is the same as in Specific Implementation Methods Three to Eight.

[0035] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods Three to Nine in that: the pre-curing described in step three is specifically carried out in a constant temperature and humidity environment of 20℃~25℃ and 50%~70% for 24h~48h; the continued curing until the hydration reaction is complete described in step three is specifically carried out in a constant temperature and humidity environment of 20℃~25℃ and 50%~70% for 20 days~28 days until the hydration reaction is complete. Everything else is the same as in Specific Implementation Methods Three to Nine.

[0036] The beneficial effects of the present invention are verified using the following embodiments:

[0037] Example 1:

[0038] A method for preparing a hemp-magnesium oxychloride cement composite material, comprising the following steps:

[0039] 1. Under the condition of stirring speed of 400 r / min, magnesium chloride hexahydrate is added to distilled water and stirred until homogeneous. Then, under the condition of stirring speed of 400 r / min and dropping speed of 5 mL / min to 10 mL / min, a solution containing hydroxyethylidene diphosphonic acid is added dropwise and stirred until homogeneous to obtain a mixed system.

[0040] 2. Under the condition of stirring speed of 140 r / min, hemp straw and magnesium oxide powder are dry mixed for 5 min, and then added to the mixing system under the condition of stirring speed of 140 r / min and stirred to dissolve, so as to obtain MOC slurry;

[0041] 3. Place the MOC slurry in a mold and pre-cur it for 24 hours in a constant temperature and humidity environment of 25℃ and 60% RH. Then demold it and continue to cure it for 28 days in a constant temperature and humidity environment of 25℃ and 60% RH until the hydration reaction is complete, to obtain hemp-magnesium oxychloride cement composite material.

[0042] The molar ratio of magnesium chloride hexahydrate to distilled water mentioned in step one is 1:10.

[0043] The solution containing hydroxyethylidene diphosphonic acid (HEDP) in step one has a mass percentage of 70%, and the solvent is water.

[0044] The particle size of the hemp straw mentioned in step two is 5mm~10mm.

[0045] In step two, the mass of hemp straw in the MOC slurry is 10% of the mass of magnesium oxide powder.

[0046] In step two, the molar ratio of magnesium chloride hexahydrate to magnesium oxide powder in the MOC slurry is 1:9.

[0047] The mass of hydroxyethylidene diphosphonic acid in the MOC slurry described in step two is 0.5% of the mass of magnesium oxide powder.

[0048] The magnesium chloride hexahydrate (MgCl2·6H2O) mentioned in step one is industrial grade with a content ≥64%; the magnesium oxide powder (MgO) mentioned in step two is industrial grade with a content ≥85%; the hemp straw mentioned in step two is provided by Sunwu County, Heilongjiang Province.

[0049] Example 2: This example differs from Example 1 in that the mass of hydroxyethylidene diphosphonic acid in the MOC slurry in step 2 is 1% of the mass of magnesium oxide powder. Everything else is the same as in Example 1.

[0050] Example 3: This example differs from Example 1 in that the mass of hydroxyethylidene diphosphonic acid in the MOC slurry described in step 2 is 1.5% of the mass of magnesium oxide powder. Everything else is the same as in Example 1.

[0051] Example 4: This example differs from Example 1 in that the mass of hydroxyethylidene diphosphonic acid in the MOC slurry described in step 2 is 2% of the mass of magnesium oxide powder. Everything else is the same as in Example 1.

[0052] Comparative Experiment: This comparative experiment differs from Example 1 in that the addition of the solution containing hydroxyethylidene diphosphonic acid is omitted in step one. Everything else is the same as in Example 1.

[0053] The cement composites prepared according to the amount of hydroxyethylidene diphosphate added are named as follows: the hemp-magnesium oxychloride cement composite prepared in the comparative experiment is M10 (addition amount 0g), the hemp-magnesium oxychloride cement composite prepared in Example 1 is M10H0.5 (addition amount 0.5%), the hemp-magnesium oxychloride cement composite prepared in Example 2 is M10H1 (addition amount 1%), the hemp-magnesium oxychloride cement composite prepared in Example 3 is M10H1.5 (addition amount 1.5%), and the hemp-magnesium oxychloride cement composite prepared in Example 4 is M10H2 (addition amount 2%). The abbreviation for hydroxyethylidene diphosphate is HEDP; the abbreviation for magnesium oxide is MgO; and the abbreviation for magnesium chloride hexahydrate is MgCl2·6H2O.

[0054] The following tests were conducted on the hemp-magnesium oxychloride cement composite material:

[0055] (1) Compressive strength:

[0056] Place the specimen on the universal testing machine and adjust its position so that its axis is aligned with the center of the press plate. Gradually apply pressure at a loading rate of 5 mm / min until the specimen fails. Take at least 3 specimens from each group of samples as test samples. Each group of compressive strength test requires at least 3 specimens. Take the arithmetic mean of the test results of the 3 specimens as the flexural strength value of that group. If the deviation of a single value from the average value exceeds 5%, the test must be repeated.

[0057] (2) Flexural strength:

[0058] The MOC flexural strength test was conducted according to GB / T 17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)". The specimen was placed on two supports of the universal testing machine, with the side (40mm × 160mm) of the specimen as the compression surface, and a span of 100mm. The specimen axis was ensured to coincide with the center line of the testing machine supports, and the indenter was positioned between the two support points. A uniform load was applied at a rate of (50±10) N / s until the specimen fractured. The flexural strength was calculated using the following formula:

[0059] ;

[0060] Where: f t : Flexural strength (MPa), accurate to 0.1MPa; F: Maximum failure load (N); L: Support span (100mm); b: Specimen cross-sectional width (40mm); h: Specimen cross-sectional height (40mm); Each flexural strength test requires at least 3 specimens. The arithmetic mean of the test results of the 3 specimens is taken as the flexural strength value of the group. If the deviation of a single value from the average value exceeds 5%, the test must be repeated.

[0061] (3) Softening coefficient:

[0062] The softening coefficient is an important indicator for measuring the water resistance of a material, referring to the ratio of the compressive strength of the material in a water-saturated state to its compressive strength in a dry state. Another set of identical specimens were immersed in water for 7 days to ensure they were water-saturated. After 7 days, the specimens were removed, the surface moisture was wiped off with a damp cloth, and their compressive strength was immediately tested according to the test method in (1). The average value was taken as P1. Substituting into the formula... The result should be rounded to two decimal places. The closer the softening coefficient is to 1, the less the material's strength decreases after absorbing water, and the better its water resistance.

[0063] (4) Moisture content and water absorption rate:

[0064] Moisture content testing was conducted according to GB / T 4111-201. First, a cube with a side length of approximately 10 mm was cut from the sample, ensuring no obvious defects. Immediately after cutting, the mass under natural conditions was measured using a balance with an accuracy of 0.01 g and recorded as m1. The sample was then placed in an oven at 105±5℃ to dry until the difference between two consecutive weighings did not exceed 0.01 g. The dried sample was then removed and placed in a desiccator to cool to room temperature, and the dried mass m0 was measured. Finally, the moisture content was determined according to the formula... Calculate the results, round them to two decimal places, and take the average of at least three parallel specimens in each group.

[0065] For the water absorption test, take specimens of the same specification, dry them at 105±5℃ to constant weight, and weigh them. Then, completely immerse the dried specimens in distilled water for 48 hours until they reach water saturation (the difference between two weighings ≤0.01g). After saturation, remove the specimens, quickly wipe off the surface moisture with a damp cloth, and immediately weigh them at the saturated state (m2). Finally, calculate the water absorption rate using the formula... Calculations should also be performed, retaining two decimal places, and the average value of at least 3 parallel specimens in each group should be taken.

[0066] Figure 1 The graph shows a comparison of the compressive strength, softening coefficient, water content-water absorption rate, and flexural strength of the hemp-magnesium oxychloride cement composites prepared in Examples 1 to 4 and the comparative experiment. Figure a shows the compressive strength of the hemp-magnesium oxychloride cement composite before and after soaking in water. 28d represents 28 days of curing in step three (without soaking), and 7d represents 7 days of soaking in water after 28 days of curing in step three. Figure b shows the softening coefficient, c shows the water content-water absorption rate, and d shows the flexural strength. As shown in the graph, with the increase of HEDP content, the flexural strength of the HEDP-modified hemp-magnesium oxychloride cement continuously decreases, but the trend is relatively slow. The flexural strength of M0H0.5 is 15.8 MPa, only 13.6% lower than that of M10. The compressive strength first increases and then decreases, reaching a maximum of 48.21 MPa without soaking, exhibiting the best mechanical properties, an improvement of 17.27% compared to M10. This is attributed to the chelation and hydrogen bonding between HEDP and MOC and hemp straw, respectively, which enhances the interfacial bonding force and facilitates effective stress transfer during deformation and fracture of the composite material, thereby improving its mechanical properties. With increasing HEDP content, the softening coefficient of HEDP-modified hemp-magnesium oxychloride cement first increases and then decreases, reaching a maximum of 0.81, a 24.6% increase compared to M10. Simultaneously, the moisture content and water absorption of the composite material also continuously decrease; the water absorption of M10H2 is 11.4%, a 21.4% decrease compared to M10, demonstrating superior water resistance.

[0067] Figure 2 The infrared spectrum, X-ray diffraction (XRD) crystal phase diagram, and corresponding crystal phase content diagram of the hemp-magnesium oxychloride cement composite material prepared in Example 4 and the comparative experiment are shown in Figure a. Figure a shows the infrared spectrum of M10 and M10H2 before and after soaking in water for 7 days; Figure b shows the crystal phase diagram of M10 and M10H2 before and after soaking in water for 7 days; Figure c shows the crystal phase content diagram of M10 and M10H2 before and after soaking in water for 7 days. As can be seen from the figures, at 3200 cm⁻¹… -1 ~3500cm -1 The broad peak at 3610 cm⁻¹ corresponds to the asymmetric stretching vibration peak of OH in the water of crystallization in the 5-phase crystal; -1 3650cm -1 and 3690cm -1The absorption peak at 1150 cm⁻¹ corresponds to the stretching vibration of Mg-OH in a 5-phase crystal structure. Cl-O shows an absorption peak at 1150 cm⁻¹. -1 The absorption peak intensities of stretching vibrations at the same location are almost identical, indicating similar internal crystal content. After soaking in water for 7 days, compared with M10H2, M10 showed a higher absorption peak intensity at 3690 cm⁻¹. -1 The Mg-OH stretching vibration peak belonging to Mg(OH)₂ becomes stronger. In the 5-phase structure, the OH peak is located at 3200–3500 cm⁻¹. -1 and 1600cm -1 The characteristic peaks of asymmetric stretching and bending vibrations at 1150 cm⁻¹ decrease and disappear, respectively. Cl-O at 1150 cm⁻¹... -1 The tensile vibration absorption peak at the point of absorption weakened or even disappeared, indicating that the internal crystal structure of the hemp-MgO cement composite material was destroyed, and the five-phase crystal structure was hydrolyzed into Mg(OH)2. The infrared spectrum of M10H2 was basically consistent with that of the unsoaked sample, indicating that hemp straw and HEDP played a crucial protective role in the MOC crystal structure.

[0068] The crystal phase composition and content changes of hemp-magnesium oxychloride cement composite were further analyzed by XRD. The internal crystal contents of M10 were 68% (5 phases), 19% (MgCO3), and 13% (MgO). The 5 phase content of M10H2 was 71%, with a slight decrease in magnesium oxide and magnesium carbonate contents. After soaking in water, the hemp-magnesium oxychloride cement composite showed characteristic peaks of Mg(OH)2 (001), (011), (012), and (110) crystal planes at 2θ = 18.4°, 37.9°, 50.6°, and 58.6°. After soaking in water, the contents of 5 phases, Mg(OH)2, MgCO3, and MgO in M10 cement were 38%, 53%, 6%, and 3%, respectively. After adding HEDP, the 5 phase crystal content of M10H2 after soaking in water was 69%, and the amount of conversion to Mg(OH)2 decreased. In this system, the "fibrous skeleton" constructed from hemp straw can disperse the stress generated during cement hydrolysis, thus improving the stability of the cement structure. Secondly, the phosphonic acid groups of HEDP react with Mg... 2+ The resulting dense chelate resists the invasion of external moisture.

[0069] Figure 3The images show SEM-EDS images of the hemp-magnesium oxychloride cement composite material prepared in Example 4 and the comparative experiment. a) is M10, b) is M10 after soaking in water for 7 days, c) is M10H2, and d) is M10H2 after soaking in water for 7 days. As can be seen from the images, after adding hemp straw, M10 exhibits a large number of gel-like structures. Mapping point scan analysis revealed that the main elements are Mg, O, and Cl, suggesting it is a gel-like 5-phase crystal. After soaking M10 in water for 7 days, a clear crack appeared between the hemp straw and the MOC matrix. This is because most of the 5-phase crystals transformed into Mg(OH)2, and the loose magnesium hydroxide caused volume expansion, resulting in internal stress and the extension of microcracks at the interface. Mapping point scan analysis revealed that the main elements are Mg and O, indicating that although the addition of hemp straw increases the softening coefficient, it does not prevent the decomposition of the 5-phase crystals. Therefore, it is still necessary to find suitable modifiers to improve the water resistance of the M10 composite material. The simultaneous addition of hemp straw and HEDP to the system resulted in significant changes in its internal microstructure. A smooth, dense, and continuous gel-like crystal structure emerged in the matrix. These gel-like substances adhered to the hemp straw, protecting the crystals from external moisture damage. Mapping point scan analysis revealed that the main elements were Mg, O, and Cl, with the five-phase structure stably existing within the matrix. After soaking in water for 7 days, numerous short needle-like rod-shaped five-phase crystal structures remained at the cross-section of M10H2. Mapping point scan analysis again revealed that these structures were primarily composed of Mg, O, and Cl, indicating that these dense structures played a crucial protective role for the five-phase crystals.

[0070] Figure 4 The XPS comparison images of the hemp-magnesium oxychloride cement composite materials prepared in Example 4 and the comparative experiment are shown. a) is the Mg 2p peak of M10, b) is the Mg 2p peak of M10H2, c) is the O 1s peak of M10, and d) is the O 1s peak of M10H2. As shown in the figure, the Mg 2p spectrum of M10 exhibits characteristic peaks of Mg-OH / MgO, Mg-Cl, and MgCO3 at 49.66 eV, 51.50 eV, and 53.30 eV, respectively. The Mg 2p peak of M10H2 shifts to 49.47 eV, 50.75 eV, and 52.41 eV, respectively. Furthermore, after peak fitting, a new peak appears at 53.87 eV for the Mg 2p of M10H2. These changes indicate that the phosphonic acid group of HEDP interacts with Mg... 2+ Chelation forms polynuclear complexes, and this coordination bond allows Mg to form polynuclear complexes. 2+ The decrease in the density of the surrounding electron cloud leads to an increase in the binding energy of the inner electrons, resulting in a higher binding energy peak fitted in the XPS spectrum. This is attributed to the interaction between HEDP and the 5-phase crystal.

Claims

1. A hemp-magnesium oxychloride cement composite material, characterized in that... It is made from magnesium chloride hexahydrate, magnesium oxide, diphosphoric acid and hemp straw.

2. The hemp-magnesium oxychloride cement composite material according to claim 1, characterized in that... The diphosphoric acid is one or a combination of several of the following: aminotrimethylene phosphate, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphoric acid.

3. The preparation method of the hemp-magnesium oxychloride cement composite material as described in claim 1, characterized in that... It is done in the following steps:

1. Add magnesium chloride hexahydrate to distilled water and stir until well mixed. Then add a solution containing diphosphoric acid dropwise and continue stirring until well mixed to obtain a mixed system.

2. Dry mix hemp straw with magnesium oxide powder, then add it to the mixing system and stir to dissolve, to obtain MOC slurry; 3. Place the MOC slurry in a mold for pre-curing, then demold and continue curing until the hydration reaction is complete, thus completing the preparation method of hemp-magnesium oxychloride cement composite material.

4. The preparation method of a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... The molar ratio of magnesium chloride hexahydrate to distilled water in step one is 1:(8~12); the mass percentage of the solution containing diphosphoric acid in step one is ≥70%.

5. The preparation method of a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... The diphosphoric acid mentioned in step one is one or a combination of several of the following: aminotrimethylene phosphate, ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphoric acid.

6. The method for preparing a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... The particle size of the hemp straw mentioned in step two is 5mm~10mm.

7. The preparation method of a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... In step two, the mass of hemp straw in the MOC slurry is 5% to 20% of the mass of magnesium oxide powder.

8. The preparation method of a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... In step two, the molar ratio of magnesium chloride hexahydrate to magnesium oxide powder in the MOC slurry is 1:(7~11).

9. The method for preparing a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... In step two, the mass of diphosphoric acid in the MOC slurry is 0.5% to 2% of the mass of magnesium oxide powder.

10. The method for preparing a hemp-magnesium oxychloride cement composite material according to claim 3, characterized in that... The pre-curing mentioned in step three specifically involves curing for 24 to 48 hours in a constant temperature and humidity environment of 20°C to 25°C and 50% to 70% humidity. The continued curing until the hydration reaction is complete, mentioned in step three, specifically involves curing for 20 to 28 days in a constant temperature and humidity environment of 20°C to 25°C and 50% to 70% humidity until the hydration reaction is complete.

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