Metakaolin-based calcium phosphate chemically bonded mortar, its preparation method and application

Abstract

The application belongs to the technical field of brick and stone cultural relics protection, and particularly discloses a metakaolin-based calcium phosphate chemical bonding mortar, a preparation method and application thereof. The mortar is composed of the following raw materials in percentage by mass: 11.0-14.0% of high-activity coarse metakaolin, 3.0-5.0% of high-activity superfine metakaolin, 13.0-17.0% of calcium dihydrogen phosphate-hydrate powder, 52.0-56.0% of mullite sand, and the balance of mixing water. Compared with the prior art, the repair material can have compatibility, safety and certain weather resistance with the brick and stone cultural relics, the molar ratio of phosphorus and aluminum of the reactant is controlled, the particle size grading of the metakaolin is adjusted, the proportion of the product crystal and amorphous structure is regulated, and then the metakaolin-based calcium phosphate chemical bonding material with a strength gradient is prepared. The setting time of the prepared material is appropriate, the shrinkage rate is low, and there is no soluble salt, so the material has high applicability for the repair and reinforcement of the brick and stone cultural relics.

Description

Technical Field

[0001] This invention relates to the field of protection technology for brick and stone cultural relics such as grottoes and ancient buildings, specifically to a metakaolin-based calcium phosphate chemically bonded mortar and its preparation method and application. Background Technology

[0002] Brick and stone cultural relics have undergone natural erosion for hundreds, thousands, or even tens of thousands of years, and currently exhibit varying degrees of weathering damage, such as flaking and detachment. To ensure the integrity of the relics and preserve their information for as long as possible, conservationists need to use repair and reinforcement materials to treat these damages, reattach detached parts, and seal and repair cracks to prevent further expansion and ensure the safety of the relics.

[0003] Repair and reinforcement materials are mainly divided into organic and inorganic materials. Organic materials have low compatibility with the physicochemical properties of inorganic materials such as bricks and stones, and are prone to aging and discoloration. They often fail due to interfacial cracking caused by asynchronous expansion and contraction with the stone matrix. Inorganic materials include cement, air-hardening lime, and hydraulic lime. Silicate cement has high mechanical strength and is a mature material, initially used for the restoration of stone artifacts. However, its high salt content easily causes white bloom on the surface, affecting the artifact's appearance. Air-hardening and hydraulic lime materials have low reinforcement strength and cannot fully meet the needs of brick and stone artifact restoration and reinforcement. Currently, there is an urgent need for a new type of brick and stone artifact restoration and reinforcement material that is compatible with the physicochemical properties of stone, while also meeting requirements such as excellent mechanical properties, no secondary damage, and high durability.

[0004] Phosphate chemically bonded materials have made new progress in industrial research as adhesives and refractory materials, possessing both the superior compatibility of inorganic materials and the good and adjustable strength properties of organic materials. However, current technologies have not assessed the safety of these materials in the application to masonry artifacts, nor have they considered their weather resistance in the complex and variable environments of these artifacts, such as freeze-thaw cycles, wet-dry conditions, and hot-dry environments. Therefore, by screening suitable phosphate chemically bonded material raw materials for the restoration and reinforcement of masonry artifacts, and developing a new type of metakaolin-based calcium phosphate chemically bonded mortar that simultaneously meets the requirements of excellent mechanical properties, compatible physicochemical properties, no derivative damage, and high durability, this research is of great value and significance for the protection of masonry artifacts in my country, especially immovable masonry artifacts in complex open-air environments. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics, its preparation method, and its application.

[0006] To achieve the above objectives, the specific technical solution adopted by the present invention is as follows:

[0007] The first aspect of the present invention is to provide a metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics, the mortar being composed of the following raw materials in the indicated mass percentages: 11.0%–14.0% highly active coarse metakaolin, 3.0%–5.0% highly active ultrafine metakaolin, 13.0%–17.0% calcium dihydrogen phosphate hydrate powder, 52.0%–56.0% mullite sand, with the balance being mixing water.

[0008] Preferably, the highly active coarse metakaolin and highly active ultrafine metakaolin are calcined coal-based metakaolin with an activity of 40% or more (activity is calculated as the ratio of active alumina content to the total mass of metakaolin) and an Al2O3 / SiO2 ratio of 0.85 or more.

[0009] Preferably, the highly active coarse metakaolin is calcined coal-based metakaolin with an average particle size of 12.0–14.0 μm, and the highly active ultrafine metakaolin is calcined coal-based metakaolin with an average particle size of 3.0–3.5 μm.

[0010] Preferably, the calcium dihydrogen phosphate hydrate powder is an analytical grade reagent with a purity of 92% or higher.

[0011] A second aspect of the present invention is to provide a method for preparing the above-mentioned metakaolin-based calcium phosphate chemically bonded mortar, specifically implemented according to the following steps:

[0012] Step 1: Weigh the required raw materials according to the proportions;

[0013] Step 2: Stir the high-activity coarse metakaolin, high-activity ultrafine metakaolin and calcium dihydrogen phosphate hydrate powder at low speed for 30 seconds. After mixing evenly, pause the process, add mixing water, and stir at low speed for 60 seconds.

[0014] Step 3: After the powder and mixing water are evenly mixed, pause the process, add mullite sand, mix at low speed for 60 seconds, pause for 30 seconds, and continue mixing at high speed for 60 seconds to obtain the mortar.

[0015] A third aspect of the present invention is to provide the application of the above-mentioned metakaolin-based calcium phosphate chemically bonded mortar, which is used for sealing cracks, repairing missing parts, and bonding and reinforcing fragments of brick and stone cultural relics.

[0016] Preferably, the ambient temperature for the application of this mortar should be between 0 and 50°C, and the relative humidity should be between 60 and 95%.

[0017] The present invention has the following beneficial effects:

[0018] 1. This invention uses a multi-scale compounding reaction of highly active metakaolin and calcium dihydrogen phosphate-hydrate to obtain a metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics. It can achieve high mechanical strength while obtaining a series of materials with adjustable strength within a certain range by adjusting the proportion.

[0019] 2. The highly active metakaolin calcination process used in this invention is mature, has high purity, and is economically readily available. The calcium dihydrogen phosphate hydrate powder is a chemically pure reagent with a purity greater than 92%, widely available, and stable in properties. The stability of the raw material properties and source ensures the stability of the properties of the cured product.

[0020] 3. The metakaolin-based calcium phosphate chemically bonded mortar provided by this invention for the restoration and reinforcement of brick and stone cultural relics exhibits high compatibility with the physical and chemical properties of the stone, without causing derivative damage, and is safe and friendly to brick and stone cultural relics. The incorporation of mullite aggregate results in a material with low shrinkage and volume stability, possessing strong resistance to freeze-thaw cycles and wet-dry cycles, enabling it to cope with many adverse environmental factors in the complex preservation environment of outdoor brick and stone cultural relics.

[0021] 4. The method for preparing metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics provided by the present invention is simple. The resulting mortar has good fluidity, no exothermic phenomenon when water is added and stirred, high safety performance, moderate setting time, strong plasticity, and is easy for operators to use for the protection and restoration of brick and stone cultural relics. It has excellent working performance. Attached Figure Description

[0022] Figure 1 This is a SEM image of the metakaolin-based calcium phosphate chemically bonded material in this invention.

[0023] Figure 2 This is a comparison of optical photographs of the metakaolin-based calcium phosphate chemically bonded material and rock material used in this invention. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0025] Example 1

[0026] A metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics is composed of the following raw materials in the following mass percentages: 13.1% highly active coarse metakaolin, 3.2% highly active ultrafine metakaolin, 13.1% calcium dihydrogen phosphate hydrate powder, 55.4% mullite sand, and the balance being mixing water.

[0027] The specific ingredients for the above raw materials are as follows:

[0028] The highly active coarse metakaolin is calcined coal-based metakaolin with an activity of 40% and an Al2O3 / SiO2 ratio of 0.85, and an average particle size of 12.0–14.0 μm.

[0029] The highly active ultrafine metakaolin is calcined coal-based metakaolin with an activity of 40% and an Al2O3 / SiO2 ratio of 0.85, and an average particle size of 3.0–3.5 μm.

[0030] The calcium dihydrogen phosphate hydrate powder is an analytical grade reagent with a purity of 92% or higher.

[0031] The above-mentioned metakaolin-based calcium phosphate chemically bonded mortar was prepared according to the following method:

[0032] Step 1: Weigh the required raw materials according to the proportions;

[0033] Step 2: Stir the high-activity coarse metakaolin, high-activity ultrafine metakaolin and calcium dihydrogen phosphate hydrate powder at low speed for 30 seconds. After mixing evenly, pause the process, add mixing water, and stir at low speed for 60 seconds.

[0034] Step 3: After the powder and mixing water are evenly mixed, pause the process, add mullite sand, mix at low speed for 60 seconds, pause for 30 seconds, and continue mixing at high speed for 60 seconds to obtain the mortar.

[0035] In the above method of this embodiment, a planetary cement mortar mixer is used for mixing. The speed of low-speed mixing is 62±5 rpm, and the speed of high-speed mixing is 125±10 rpm.

[0036] Example 2

[0037] A metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics is composed of the following raw materials in the indicated mass percentages: 11.5% highly active coarse metakaolin, 4.8% highly active ultrafine metakaolin, 13.1% calcium dihydrogen phosphate hydrate powder, 55.4% mullite sand, and the balance being mixing water.

[0038] The preparation method of this mortar is the same as in Example 1.

[0039] Example 3

[0040] A metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics is composed of the following raw materials in the following mass percentages: 13.1% highly active coarse metakaolin, 3.2% highly active ultrafine metakaolin, 16.3% calcium dihydrogen phosphate hydrate powder, 52.7% mullite sand, and the balance being mixing water.

[0041] The preparation method of this mortar is the same as in Example 1.

[0042] Example 4

[0043] A metakaolin-based calcium phosphate chemically bonded mortar for the restoration and reinforcement of brick and stone cultural relics is composed of the following raw materials in the following mass percentages: 11.5% highly active coarse metakaolin, 4.8% highly active ultrafine metakaolin, 16.3% calcium dihydrogen phosphate hydrate powder, 52.7% mullite sand, and the balance being mixing water.

[0044] The preparation method of this mortar is the same as in Example 1.

[0045] Performance testing:

[0046] The mortars from Examples 1, 2, 3, and 4 were prepared into specimens measuring 40*40*160mm and cured for 28 days. Mechanical property tests were then conducted on the specimens according to the "Test Method for Strength of Cement Mortar (ISO Method)". The coefficient of thermal expansion was determined according to Part 8, "Determination of Linear Thermal Expansion," of GB / T3810.8-2016, "Test Methods for Ceramic Bricks". The salt content was determined according to the "Electrode Method for Determination of Soil Electrical Conductivity" (HJ 802-2016). The freeze-thaw cycle strength loss rate was determined according to JGJ / T70-2009, "Standard for Test Methods of Basic Properties of Building Mortar".

[0047] The results are shown in Table 1. The embodiments obtained by the present invention have high and adjustable mechanical properties, high compatibility with general brick and stone materials in terms of thermal expansion, are not prone to cracking, and the cured products contain almost no soluble salts, which is safe for cultural relics. They also have high weather resistance and can resist the cold and freezing weather and dry and wet change environment commonly seen in open-air brick and stone cultural relics.

[0048] Table 1. Comprehensive performance test results of the mixed mortars obtained in Examples 1-4

[0049]

[0050] Figure 1 The image shows a SEM image of the metakaolin-based calcium phosphate chemically bonded material in this invention. It can be seen that the metakaolin-based calcium phosphate chemically bonded material for the restoration and reinforcement of brick and stone cultural relics prepared in this invention has a composite structure of crystalline and amorphous materials, with a tight structural connection and no loose pores.

[0051] Figure 2 The image shows a comparison of optical photographs of the metakaolin-based calcium phosphate chemically bonded material and rock material in this invention. It can be seen that the metakaolin-based calcium phosphate chemically bonded material for the restoration and reinforcement of brick and stone cultural relics prepared in this invention can be used for the surface restoration of rocks. It has a high similarity to the appearance, texture, color and particle size of rocks, which can meet the principle of restoring cultural relics without changing their original state.

[0052] Comparative Example 1

[0053] Compared to Example 1, it is largely the same, except that calcium dihydrogen phosphate hydrate is replaced with an equimolar amount of aluminum dihydrogen phosphate. Because a mild acidic phosphate reactant was not used, the mixed slurry exhibited strong exothermic activity, with the slurry temperature exceeding 50°C, posing a safety hazard to operators. It was also difficult to stir, had poor workability, and the solidified body cracked, making it unsuitable for the restoration and reinforcement of brick and stone artifacts.

[0054] Comparative Example 2

[0055] Compared to Example 1, the majority of the contents are the same, except that calcium dihydrogen phosphate hydrate is replaced with an equimolar amount of ammonium dihydrogen phosphate. Due to the lack of acidic phosphate reactants that can quickly dissolve in the slurry and react with metakaolin, the use of ammonium dihydrogen phosphate greatly prolongs the setting time, resulting in slow hardening speed, excessive fluidity, poor cementing properties, and low curing strength.

[0056] Comparative Example 3

[0057] Compared to Example 1, the same aggregate, mullite sand, was used. The highly reactive coarse metakaolin, highly reactive ultrafine metakaolin, and calcium dihydrogen phosphate hydrate powder were replaced with an equal mass of ordinary silicate cement. Because the raw materials free of harmful soluble salts were not used as required by this invention, the electrical conductivity of the solidified body was greater than 6900 μs / cm. After using it for surface repair and reinforcement of brick and stone cultural relics, soluble salt crystals caused efflorescence, severely affecting the appearance of the relics and accelerating their weathering. Therefore, it is unsuitable for the protection of brick and stone cultural relics.

[0058] Comparative Example 4

[0059] Compared to Example 1, the same aggregate, mullite sand, was used. However, the highly reactive coarse metakaolin, highly reactive ultrafine metakaolin, and calcium dihydrogen phosphate hydrate powder were replaced with an equal mass of natural hydraulic lime (NHL 5), making it a silicate system material. Because phosphate system raw materials were not used as required by this invention, the mechanical strength of the specimen increased slowly and the cured strength was low. After freeze-thaw cycles, the material cracked and failed, with a compressive strength loss rate exceeding 5%, which cannot meet the requirements for the preservation of brick and stone cultural relics under complex climatic conditions.

[0060] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A metakaolin-based calcium phosphate chemically bonded mortar, characterized in that: The raw materials are composed of the following percentages by mass: 11.0%~14.0% highly active coarse metakaolin, 3.0%~5.0% highly active ultrafine metakaolin, 13.0%~17.0% calcium dihydrogen phosphate hydrate powder, 52.0%~56.0% mullite sand, with the balance being mixing water; the highly active coarse metakaolin and highly active ultrafine metakaolin are calcined coal-based metakaolin with an activity of 40% or higher and an Al2O3 / SiO2 ratio of 0.85 or higher; the average particle size of the highly active coarse metakaolin is 12.0~14.0 μm, and the average particle size of the highly active ultrafine metakaolin is 3.0~3.5 μm.

2. The metakaolin-based calcium phosphate chemically bonded mortar as described in claim 1, characterized in that: The calcium dihydrogen phosphate hydrate powder is an analytical grade reagent with a purity of 92% or higher.

3. The preparation method of metakaolin-based calcium phosphate chemically bonded mortar as described in any one of claims 1-2, comprising the following steps: Step 1: Weigh the required raw materials according to the proportions; Step 2: Stir the high-activity coarse metakaolin, high-activity ultrafine metakaolin and calcium dihydrogen phosphate-hydrate powder at low speed for 30 seconds. After mixing evenly, pause the process, add mixing water, and stir at low speed for 60 seconds. Step 3: After the powder and mixing water are evenly mixed, pause the process, add mullite sand, mix at low speed for 60 seconds, pause for 30 seconds, and continue mixing at high speed for 60 seconds to obtain the mortar.

4. The application of the metakaolin-based calcium phosphate chemically bonded mortar as described in any one of claims 1-2, characterized in that: This mortar is used for sealing cracks, repairing missing parts, and bonding and reinforcing fragments of brick and stone cultural relics.

5. The application of the metakaolin-based calcium phosphate chemically bonded mortar as described in claim 4, characterized in that: The ambient temperature for this mortar application should be between 0 and 50°C, and the relative humidity should be between 60 and 95%.

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