A modified low-shrinkage cement paste with MGO-CNT and its preparation process

Abstract

This invention relates to the field of building materials technology, and in particular to an MGO-CNT modified low-shrinkage cement paste and its preparation process. The raw materials for this MGO-CNT modified low-shrinkage cement paste include cement, MGO, and MWCNT; the sum of the mass of the MGO and MWCNT accounts for 0.05-0.3% of the mass of the cement. By incorporating MGO and MWCNT into the cement paste, this invention can enhance the mechanical properties of the cement paste, improve its early mechanical properties, and also inhibit cement paste shrinkage to a certain extent, reducing the shrinkage rate.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to an MGO-CNT modified low-shrinkage cement paste and its preparation process. Background Technology

[0002] Cement, when mixed with water, can harden in air or water, and it can firmly bind materials such as sand and stone together. Therefore, cement is widely used in construction engineering as a cementing material, for example, in concrete, adhesives, and ultra-high performance concrete. However, during the hardening process of cement-based materials, the volume of the cement paste shrinks; this phenomenon is called cement shrinkage. Cement shrinkage induces tensile stress within the material, leading to cracks. Cracks reduce the durability of cement-based materials, so controlling cement paste shrinkage is crucial.

[0003] To improve the shrinkage resistance of cement paste, the main current technology involves adding expansive agents, such as magnesium oxide expansive agent (MEA) and shrinkage reducing agent (SRA). MEA compensates for shrinkage stress through its expansion stress, thereby reducing the shrinkage of cement-based materials. The main function of SRA is to reduce the autogenous shrinkage of cement-based materials by lowering the surface tension in the pore solution of the cement matrix. However, both MEA and SRA can lead to a certain degree of reduction in early mechanical properties. Summary of the Invention

[0004] Based on the above, this invention provides an MGO-CNT modified low-shrinkage cement paste (hereinafter referred to as modified cement paste) and its preparation process. This invention can improve the mechanical properties of cement paste while reducing its shrinkage rate.

[0005] To achieve the above objectives, the present invention provides the following solution:

[0006] One of the technical solutions of the present invention is an MGO-CNT modified low-shrinkage cement paste, the raw materials of which include cement, MGO and MWCNT; the sum of the mass of the MGO and MWCNT accounts for 0.05-0.3% of the mass of the cement.

[0007] The second technical solution of the present invention is a method for preparing the above-mentioned MGO-CNT modified low-shrinkage cement paste, comprising the following steps:

[0008] The water-reducing agent was added to a portion of the water, followed by the addition of MGO (graphene oxide) and MWCNT (multi-walled carbon nanotubes) and the mixture was dispersed evenly to obtain an MGO-CNT dispersion.

[0009] Add the MGO-CNT dispersion to the cement and stir. Add the remaining water and continue stirring to obtain the MGO-CNT modified low-shrinkage cement paste.

[0010] The third technical solution of the present invention is a method for reducing the shrinkage value of cement paste by adding MGO and MWCNT to cement.

[0011] The present invention discloses the following technical effects:

[0012] This invention enhances the mechanical properties of cement paste and improves its early mechanical properties by incorporating MGO and MWCNT into it. It also inhibits the shrinkage of cement paste and reduces the shrinkage rate to a certain extent. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 The shrinkage value of the cement paste prepared in Examples 1-6 and Comparative Example 1 of this invention is the 28-day shrinkage value. Detailed Implementation

[0015] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0016] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0017] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0018] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0019] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0020] The first aspect of the present invention provides an MGO-CNT modified low-shrinkage cement paste, the raw materials of which include cement, MGO and MWCNT; the sum of the mass of the MGO and MWCNT accounts for 0.05-0.3% of the mass of the cement.

[0021] The reason why the sum of the mass of MgO and MWCNT is limited to 0.05-0.3% of the cement mass in this invention is that if the addition amount of MgO and MWCNT is less than 0.05%, their role in enhancing and improving cement performance may not be fully realized; if the addition amount is too high, MgO and MWCNT may form agglomerates, leading to decreased material dispersibility and reduced material performance, while also increasing preparation costs. In order to minimize the impact on the performance of cement itself while ensuring the improvement of the mechanical properties and microstructure of cement paste, the above-mentioned addition amount limited by this invention can balance material performance and cost to a certain extent, while also taking into account the synergistic effects and interactions of the materials.

[0022] In a preferred embodiment of the present invention, the raw materials also include water and a water-reducing agent.

[0023] In a preferred embodiment of the present invention, the water-cement ratio is 0.35-0.5.

[0024] In a preferred embodiment of the present invention, the amount of water-reducing agent added accounts for 0.1-1.5% of the mass of the cement.

[0025] A second aspect of the present invention provides a method for preparing the above-mentioned MGO-CNT modified low-shrinkage cement paste, comprising the following steps:

[0026] Add the water-reducing agent to a portion of the water, then add MGO and MWCNT and disperse evenly to obtain an MGO-CNT dispersion;

[0027] Add the MGO-CNT dispersion to the cement and stir. Add the remaining water and continue stirring to obtain the MGO-CNT modified low-shrinkage cement paste.

[0028] In the step of adding water-reducing agent to a portion of water, followed by adding MGO and MWCNT and dispersing them evenly to obtain an MGO-CNT dispersion, the amount of water used accounts for 15% of the mass of the cement; the method of even dispersion is water-tight ultrasonic dispersion.

[0029] In the step of adding the MGO-CNT dispersion to the cement and stirring, then adding all the remaining water and continuing to stir to obtain the MGO-CNT modified low-shrinkage cement paste, before adding the MGO-CNT dispersion to the cement, the cement is first stirred at low speed (rotation: 140±5 r / min, revolution: 62±5 r / min) for 30 seconds using a cement paste mixer. After adding the MGO-CNT dispersion, stirring is continued at low speed (rotation: 140±5 r / min, revolution: 62±5 r / min) for 30 seconds. During the stirring process, all the remaining water is added. Then, stirring is carried out at high speed (rotation: 285±10 r / min, revolution: 125±10 r / min) for 30 seconds. After that, stirring is stopped for 90 seconds. Finally, stirring is carried out at high speed (rotation: 285±10 r / min, revolution: 125±10 r / min) for 60 seconds to obtain the MGO-CNT modified low-shrinkage cement paste.

[0030] The purpose of stopping mixing for 90 seconds is to allow the mixture to settle and fully mix. During this 90-second break, the particles in the cement paste have a chance to rearrange and settle, which helps to further mix and achieve uniform dispersion. Furthermore, stopping mixing allows air bubbles in the paste to gradually rise and dissipate, thereby reducing its porosity and improving the density and strength of the cement.

[0031] The purpose of starting with low speed and then increasing to high speed is to achieve thorough mixing and dissolution. During the low-speed mixing stage, the cement powder and water / dispersion mix to form a preliminary paste. Low-speed mixing helps prevent cement powder from splashing and generating excessive air bubbles. Furthermore, low-speed mixing helps to wet and evenly disperse the cement particles, allowing for better subsequent high-speed mixing.

[0032] During the high-speed mixing stage, the agitator rotates at a high speed to further mix and evenly disperse cement particles. High-speed mixing helps accelerate the interaction and reaction rate between cement particles, improving the uniformity and stability of the cement paste. Simultaneously, high-speed mixing also removes air and bubbles, making the cement paste more refined and robust. In summary, by employing a mixing process that transitions from low speed to high speed, it is possible to ensure that the cement paste is mixed evenly and dissolved thoroughly, while eliminating excess air bubbles, thereby improving the quality and performance of the cement.

[0033] Mixing cement paste at a uniform speed may lead to uneven mixing and unstable paste quality. This is because low-speed and high-speed mixing serve different purposes during the mixing process; only by combining both can cement powder and water be effectively mixed together to achieve thorough mixing and uniform dispersion.

[0034] If a uniform stirring speed is used, the following problems may occur:

[0035] Cement particles cannot be fully wetted and dispersed, resulting in uneven paste, which in turn affects the quality and strength of concrete;

[0036] Failure to remove air bubbles in time will result in excessive pores in the cement paste, which in turn reduces the density and durability of the concrete.

[0037] Excessive force and wear on the agitator may shorten its lifespan or even cause it to malfunction.

[0038] A third aspect of the present invention provides a method for reducing the shrinkage value of cement paste by adding MGO and MWCNT to cement.

[0039] In a preferred embodiment of the present invention, the amount of MGO and MWCNT added accounts for 0.05% of the mass of the cement.

[0040] In a preferred embodiment of the present invention, the mass ratio of MGO to MWCNT is (0-5):1, and MGO is not 0.

[0041] Unless otherwise specified, all raw materials used in the embodiments of this invention can be obtained through commercial channels.

[0042] The MGO powder used in the embodiments and comparative examples of this invention has the following characteristics: average thickness of approximately 6 nm, single-layer thickness of 0.6-1.2 nm, number of layers of 6-10, sheet size of 10-50 μm, purity >95%, and specific surface area of ​​100-300 m². 2 / g; MWCNT: Diameter 10-15nm, length 5-15nm, purity >95%, specific surface area >200m² 2 / g.

[0043] The cement used in the embodiments and comparative examples of this invention is PⅡ52.5R silicate cement.

[0044] The water-reducing agent used in the embodiments and comparative examples of this invention is a polycarboxylate high-efficiency water-reducing agent with a water reduction rate of ≥25%.

[0045] The preparation method of modified cement paste in the following embodiments of the present invention comprises the following steps:

[0046] Step 1, Preparation of MGO-CNT dispersion:

[0047] 1) Dissolve the water-reducing agent in water at 15% of the cement mass to obtain water containing the water-reducing agent; then add MGO powder and MWCNT powder to the water containing the water-reducing agent to obtain a mixed solution.

[0048] 2) Place the above mixed solution in an ultrasonic cleaner (the water level in the ultrasonic cleaner is higher than the liquid level of the mixed solution in the reagent bottle), and sonicate in water for 30 minutes, with an ultrasonic frequency of 40 Hz and a power of 375 W, to finally obtain a uniformly dispersed MGO-CNT dispersion.

[0049] Step 2, Preparation of MGO-CNT modified low-shrinkage cement paste:

[0050] 1) Weigh the cement, stir it at low speed (rotation: 140±5r / min, revolution: 62±5r / min) for 30s in a cement paste mixer, and then add the MGO-CNT dispersion prepared in step 1.

[0051] 2) Continue stirring at low speed (rotation: 140±5 r / min, revolution: 62±5 r / min) for 30 s. During stirring, add all the remaining water after preparing the MGO-CNT dispersion at a water-cement ratio of 0.35. Then stir at high speed (rotation: 285±10 r / min, revolution: 125±10 r / min) for 30 s, and then stop stirring for 90 s.

[0052] 3) Finally, stir at high speed (rotation: 285±10 r / min, revolution: 125±10 r / min) for 60 seconds to obtain MGO-CNT modified low-shrinkage cement paste (abbreviated as modified cement paste); pour the modified cement paste into a 25×25×280mm... 3 After vibrating the mold, cover it with plastic wrap and cure for 24 hours before removing it from the mold. Finally, place it in a curing box for curing (curing temperature 20±2℃, relative humidity ≥90%).

[0053] Example 1

[0054] A modified cement paste is composed of cement, water, MgO, MWCNT and water-reducing agent in a mass ratio of 500:175:0:0.25:0.5.

[0055] Example 2

[0056] A modified cement paste is composed of cement, water, MgO, MWCNT and water-reducing agent in a mass ratio of 500:175:0.05:0.2:0.5.

[0057] Example 3

[0058] A modified cement paste is composed of cement, water, MgO, MWCNT and water-reducing agent in a mass ratio of 500:175:0.1:0.15:0.5.

[0059] Example 4

[0060] A modified cement paste is composed of cement, water, MgO, MWCNT and water-reducing agent in a mass ratio of 500:175:0.15:0.1:0.5.

[0061] Example 5

[0062] A modified cement paste is composed of cement, water, MgO, MWCNT and water-reducing agent in a mass ratio of 500:175:0.2:0.05:0.5.

[0063] Example 6

[0064] A modified cement paste is composed of cement, water, MgO, MWCNT and water-reducing agent in a mass ratio of 500:175:0.25:0:0.5.

[0065] Comparative Example 1

[0066] A cement paste is composed of cement, water and water-reducing agent in a mass ratio of 500:175:0.1.

[0067] The above-mentioned method for preparing cement paste includes the following steps:

[0068] 1) Weigh the cement and mix it in a cement paste mixer at low speed (rotation: 140±5r / min, revolution: 62±5r / min) for 30s. Then add water while continuing to mix at low speed for 30s. Then mix at high speed (rotation: 140±5r / min, revolution: 62±5r / min) for 30s. After that, stop mixing for 90s.

[0069] 2) Finally, mix at high speed (rotation: 285±10r / min, revolution: 125±10r / min) for 60s to obtain cement paste; pour the cement paste into a 25×25×280mm3 mold, vibrate it, cover it with plastic wrap and cure for 24h, then remove the mold and finally place it in a curing box for curing (curing temperature 20±2℃, relative humidity ≥90%).

[0070] Example 1 of effect verification

[0071] I. Analysis of the Influence of MGO and MWCNT Doping

[0072] 1.1 Experimental Design

[0073] The effects of MgO and MWCNT admixtures on the mechanical properties of cement paste were investigated. Specimens were 40×40×160mm in size. 3The test blocks were specifically designed for Comparative Example 1 and Examples 1-6 (raw material composition and mixing ratio are shown in Table 1).

[0074] Table 1. Experimental mix proportions

[0075]

[0076] To investigate the effect of different dosages of MgO and MWCNT on the shrinkage of cement paste, specimens were prepared using MgO:MWCNT ratios of 0:5 (Example 1, CNT), 1:4 (Example 2, 1G4C), 2:3 (Example 3, 2G3C), 3:2 (Example 4, 3G2C), 4:1 (Example 5, 4G1C), and 5:0 (Example 6, MgO). The specimens prepared in each example and comparative example were cured for 3 days and 7 days, respectively, and their flexural and compressive strengths were measured. The results are shown in Table 2.

[0077] Further research was conducted to investigate the effects of MgO and MWCNT admixtures on the shrinkage of cement paste. The specimens were 25×25×280mm in size. 3 The test blocks were specifically designed for Comparative Example 1 and Examples 1-6.

[0078] To investigate the effect of different dosages of MGO and MWCNT on the shrinkage of cement paste, specimens were prepared using MGO:MWCNT ratios of 0:5 (Example 1, CNT), 1:4 (Example 2, 1G4C), 2:3 (Example 3, 2G3C), 3:2 (Example 4, 3G2C), 4:1 (Example 5, 4G1C), and 5:0 (Example 6, MGO). The specimens prepared in each example and comparative example were cured for 28 days, and the shrinkage values ​​were measured. The results are shown in Table 3. Figure 1 (Plain in the figure represents the comparative example).

[0079] 1.2 Results and Analysis

[0080] Table 2 Results of Mechanical Property Tests

[0081]

[0082] Table 2 shows that after incorporating two nanomaterials, MGO and MWCNT, into cement paste, the compressive strength at 3 days was not effectively improved compared to Comparative Example 1. However, the addition of MGO or MWCNT monomers at 7 days (Examples 1 and 6) enhanced the compressive strength of the cement. This enhancement is attributed to the superior mechanical properties of MGO and MWCNT, as well as their ability to fill pores and bridge cracks. MGO and MWCNT monomers can fill the tiny pores in cement and bind tightly with hydration products, thereby increasing the cement's resistance to loads under stress.

[0083] The reason why the MGO and MWCNT blends (Examples 2-5) exhibit higher compressive strength than the single-blended blends (Examples 1 and 6) is that, firstly, MGO and MWCNT can synergistically disperse in water due to electrostatic repulsion and steric hindrance, which is a prerequisite for synergistic reinforcement. Secondly, the hydroxyl groups on the surface of MWCNT are covalently linked to the carboxyl groups on the surface of MGO, or the monomer materials cross-link with each other through the π-π stacking effect. This ensures that MGO and CNT form a 3D reinforcing structure at the nanoscale. Finally, this 3D reinforcing structure, along with the free MGO and CNT monomer materials, can covalently bond with calcium hydroxide and hydrated calcium silicate in cementitious materials, achieving a strong bond.

[0084] Table 3 Results of the shrinkage test

[0085]

[0086] From Table 3 and Figure 1 As shown, the shrinkage of cement paste was reduced to varying degrees after incorporating two nanomaterials, MGO and MWCNT. Figure 1 It can be seen that as the MGO content increases, its ability to inhibit shrinkage weakens and the shrinkage value increases. When the MGO:MWCNT ratio is 1:4, the shrinkage of the cement paste is the lowest. At 28 days, the shrinkage of Example 2 is reduced by 15.38% compared with Comparative Example 1.

[0087] Molybdenum oxide (MGO) is an oxidized derivative of graphene. While retaining the stable structure and properties of graphene, it introduces a large number of oxygen-containing functional groups into the surface of the graphene sheet structure through oxidation. These functional groups have strong hydrophilicity. The incorporation of MGO can change the pore structure of cement paste, generating a large number of gelling pores. In the later stages of hydration, due to the bonding of MGO with some hydration products, it leads to increased shrinkage.

[0088] Multi-walled carbon nanotubes (MWCNTs) are less expensive than single-walled carbon nanotubes. They possess a large specific surface area and specific surface energy, allowing them to absorb significant amounts of water. The incorporation of MWCNTs maintains the relative humidity within the cement paste. The decrease in relative humidity in the pore solution leads to the formation of menisci, which in turn subjectes the pore walls to considerable stress. This improves the shrinkage resistance of the cement paste. As a high aspect ratio fiber material, MWCNTs can also restrain the propagation of microcracks in the cement paste, thus inhibiting cracking. Simultaneously, CNTs also contribute to filling voids within the paste, forming a dense microstructure that reduces shrinkage and further delays cracking.

[0089] Due to their ease of stacking, MgO monomers exhibit a neat and orderly multi-layered structure within the cement matrix. In contrast, CNTs exhibit poor dispersibility in cement, with monomers becoming entangled and agglomerated, resulting in an intertwined fibrous network distribution within the cement matrix. When MgO and MWCNTs are co-blended, MgO promotes the dispersion of MWCNTs. The ionization of MgO generates a large amount of negative charge, and the ionization of MWCNTs also generates a certain amount of negative charge. Electrostatic repulsion keeps the nanomaterials at a certain distance, achieving a good dispersion state. The intertwined linkages between MgO and MWCNT monomers form a distinct 3D microstructure at the nanoscale. This microscopic 3D structure within the cement matrix can fill internal voids in the cement, connect cracks, and reduce shrinkage.

[0090] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for reducing the shrinkage value of cement paste while improving its early mechanical properties, characterized in that, The addition of MgO and MWCNT to cement can reduce the shrinkage value of cement paste while improving its early mechanical properties. The MGO powder has the following characteristics: average thickness 6 nm, single-layer thickness 0.6-1.2 nm, number of layers 6-10, sheet size 10-50 μm, purity >95%, and specific surface area 100-300 m². 2 / g; The MWCNTs have a diameter of 10-15 nm, a length of 5-15 nm, a purity of >95%, and a specific surface area of ​​>200 m². 2 / g; The improvement in the early mechanical properties of cement paste refers to the compressive strength at 3 days and 7 days. The sum of the masses of MGO and MWCNT accounts for 0.05% of the mass of the cement. The mass ratio of MGO to MWCNT is 0.15:0.1.

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