A nuclear engineering concrete admixture, its preparation method and application

Abstract

This invention relates to the field of building materials, specifically to a concrete admixture, its preparation method, and its application. The concrete admixture comprises the following components by weight percentage: 15%-30% light-burned MgO; 15%-30% 2MgO·B₂O₃; 15%-40% 3MgO·B₂O₃; 20%-26% CaCO₃; 1%-2% CaO; the remainder being unavoidable impurities. This invention, by using a mixture of light-burned MgO, 2MgO·B₂O₃, 3MgO·B₂O₃, CaCO₃, and CaO in the above weight percentages, can improve the shrinkage, toughening, crack resistance, and radiation protection properties of concrete in which it is applied.

Description

Technical Field

[0001] This invention relates to the field of building materials, specifically to a nuclear engineering concrete admixture, its preparation method, and its application. Background Technology

[0002] Concrete quality is an important prerequisite for ensuring the safe and stable operation of the project. Most large nuclear engineering buildings, such as reactor buildings, fuel storage pools, and conventional island buildings, use large-volume concrete raft foundations. Raft foundations not only play an overall load-bearing and seismic role, but are also important parts for shielding the penetration and leakage of radioactive materials. However, since some nuclear engineering projects are located in corrosive environments, the main structure of nuclear engineering projects must not only prevent internal nuclear leakage, but also resist the erosion of external corrosive media. Therefore, the requirements for crack control are very strict.

[0003] Cracks in concrete are mainly caused by the decrease in temperature and drying shrinkage of concrete. In particular, the long-term slow loss of water in concrete causes continuous drying shrinkage. Once the accumulated shrinkage value exceeds the limit deformation of concrete, cracks will occur. These late-stage drying shrinkage cracks in concrete generally occur during the service period after nuclear engineering is put into use, which can seriously affect the safety of nuclear engineering.

[0004] The prior art discloses a multifunctional admixture composition for high-strength concrete, which reduces concrete shrinkage by using an expansion component. However, this method has certain limitations in terms of actual crack resistance. High-strength concrete is brittle and easily leads to yield fracture. In particular, nuclear engineering concrete requires both crack resistance and radiation protection, and the compatibility between crack-resistant materials and radiation-resistant materials is difficult to solve. Summary of the Invention

[0005] Therefore, the primary technical problem to be solved by the present invention is to overcome the defect of low toughness in high-strength concrete in the prior art, which leads to easy fracture of concrete, thereby providing a concrete admixture and its preparation method and application, which can improve the toughness of concrete and avoid concrete fracture and cracking.

[0006] Another technical problem to be solved by the present invention is to overcome the defect in the prior art that the setting time of concrete with radiation effect is too long, resulting in a decrease in strength, thereby providing a concrete admixture that can reduce the impact of concrete setting on concrete strength.

[0007] On one hand, the present invention provides a concrete admixture comprising the following components by weight percentage:

[0008] Lightly calcined MgO: 15%-30%;

[0009] 2MgO·B₂O₃ 15%-40%;

[0010] 3MgO·B₂O₃ 20%-40%;

[0011] CaCO3 20%-26%;

[0012] CaO 1%-2%;

[0013] The rest are unavoidable impurities.

[0014] Preferably, by weight percentage, the lightly calcined MgO is 23.2%, 2MgO·B2O3 is 15.6%, 3MgO·B2O3 is 39.1%, CaCO3 is 20.1%, and CaO is 1.9%.

[0015] The lightly calcined MgO includes medium-low activity MgO and ultra-low activity MgO.

[0016] The reaction time of the medium-low activity MgO is 100-300s; and / or, the reaction time of the ultra-low activity MgO is 350-600s.

[0017] On the other hand, the present invention provides a method for preparing concrete admixtures, comprising the following steps:

[0018] S1, after drying and dehydration, magnesite and carnallite are mixed with dimethylformamide to obtain a mixed raw material. Boric acid is added to the mixed raw material, stirred, and heated to obtain a preheated material.

[0019] S2, the preheated material is heated at a first heating temperature for a first time, at a second heating temperature for a second time, and at a third heating temperature for a third time to obtain a cooked material;

[0020] S3. After cooling the clinker obtained in step S2, mix it with limestone and crush it to obtain concrete admixture.

[0021] Preferably, the first heating temperature is 700-800℃, and the first heating time is 1.5-2 hours. The second heating temperature is 1000-1100℃, and the second heating time is 0.5-1 hour. The third heating temperature is 1250-1300℃, and the third heating time is 0.2-0.4 hours.

[0022] Preferably, the roasting method is gas-phase suspension roasting.

[0023] Preferably, the heating rate of the first heating temperature is 10-15℃ / min. The heating rate of the second heating temperature is 20-25℃ / min. The heating rate of the third heating temperature is 25-30℃ / min.

[0024] The magnesite contains ≥40% MgO, ≤1.5% CaO, and ≤1.5% SiO2.

[0025] The boric acid is industrial boric acid with a content ≥99.5%, w / w.

[0026] The carnallite has a KCl content of 20-30%, a NaCl content of 10-15%, and a MgCl2 content of 25-40%.

[0027] In step S1, magnesite comprises 20-40 parts by weight, carnallite comprises 5-10 parts by weight, and boric acid comprises 20-40 parts by weight.

[0028] Preferably, the particle size of the magnesite and carnallite is 1-3 mm.

[0029] The drying temperature for magnesite and carnallite is 150-200℃, and the drying time is 4-8 hours.

[0030] The stirring steps after adding boric acid are as follows: first, stir at 80-120℃ and a stirring rate of 1.5-3 r / min for 3-5 hours, then raise the temperature to 150-200℃ and continue stirring for 1-2 hours.

[0031] The heating temperature in step S1 is 300-350℃.

[0032] In step S3, the weight of CaCO3 is 15-30 parts.

[0033] Preferably, the cooling temperature of the clinker is below 60°C.

[0034] The particle size of the cooled clinker after being mixed and crushed with limestone is 300-600 mesh.

[0035] The concrete admixture provided by this invention, or the concrete admixture prepared by the above preparation method, can be applied to the preparation of nuclear engineering concrete. The preparation method is to mix the concrete admixture with coarse aggregate and fine aggregate for 30s-60s, and then add cement, fly ash, water and admixtures and mix for 90s-120s to obtain nuclear engineering concrete. The amount of the concrete admixture is 10-20% of the amount of concrete cementitious material, w / w.

[0036] The technical solution of this invention has the following advantages:

[0037] 1. The concrete admixture provided by this invention comprises the following components by weight percentage: 15%-30% lightly calcined MgO; 15%-40% 2MgO·B₂O₃; 20%-40% 3MgO·B₂O₃; 10%-26% CaCO₃; 1%-2% CaO; the remainder being unavoidable impurities. This invention utilizes finely ground CaCO₃ to fill and compact the concrete, improving its pore structure. Simultaneously, it utilizes 2MgO·B₂O₃ whiskers, which possess high strength and high modulus mechanical properties, to uniformly disperse in the concrete, acting as a skeleton. Due to the fibrous structure of 2MgO·B₂O₃, it can develop a directional structure in the concrete without generating anisotropy. Under stress, it produces a certain deformation, eliminating interfacial stress concentration and residual stress, thus relaxing the stress. Therefore, it can inhibit concrete crack propagation and increase concrete toughness. Furthermore, 2MgO·B₂O₃ can increase the fracture surface energy of concrete, reducing its brittleness and increasing its toughness. This invention uses 2MgO·B2O3 and 3MgO·B2O3 as materials for absorbing and shielding neutrons. This not only avoids problems such as prolonged concrete setting time and unstable radiation shielding performance caused by excessive addition of boron components, but also improves the absorption and shielding effect. In particular, the concrete admixture of this invention contains 3MgO·B2O3, which has good hydration stability and will not cause crystal structure damage during the hydration process of cementitious components. Therefore, it will not release borate ions and has little impact on the setting time and strength of concrete.

[0038] 2. The concrete admixture provided by the present invention, by weight percentage, contains 23.2% lightly calcined MgO, 15.6% 2MgO·B2O3, 39.1% 3MgO·B2O3, 20.1% CaCO3, and 1.9% CaO. The concrete admixture prepared with the above-mentioned contents of each raw material can improve the strength and toughness of concrete, and at the same time, improve the shielding performance against neutrons.

[0039] 3. The concrete admixture provided by this invention includes lightly calcined MgO, comprising medium-low activity MgO and ultra-low activity MgO. In the internal environment of concrete at 40℃~60℃, medium-low activity MgO exhibits a moderate reaction rate, compensating for mid-term and late-term shrinkage of the concrete. Ultra-low activity MgO reacts very slowly, compensating for long-term shrinkage of the concrete. This invention utilizes lightly calcined MgO with different activities to compensate for early, mid-term, late-term, and long-term shrinkage of concrete under different temperature conditions, achieving continuous and stable micro-expansion in the concrete to resist volume shrinkage and prevent cracking.

[0040] 4. The method for preparing concrete admixtures provided by this invention includes mixing dried and dehydrated magnesite and carnallite with dimethylformamide to obtain a mixed raw meal; adding boric acid to the mixed raw meal, stirring, and heating to obtain a preheated material; calcining the preheated material by gas suspension at a first heating temperature for a first time, at a second heating temperature for a second time, and at a third heating temperature for a third time to obtain clinker; cooling the clinker obtained in the above steps, mixing it with limestone, and then crushing it to obtain concrete admixtures. This invention uses DMF as a solvent to complex and replace water molecules in MgCl2·6H2O in carnallite, resulting in the complete dissolution and dispersion of magnesia-carnallite crystals to form a MgCl2 complex. When adding boric acid, stirring at room temperature dehydrates the boric acid to generate metaboric acid; after heating and stirring, the metaboric acid generates pyroboric acid, which can combine with K and Na ions in carnallite to form pyroborates and form precursors for magnesium borate whiskers with the MgCl2 complex. Unreacted DMF is removed by heating the mixture after the stirring reaction. Subsequently, the precursor was converted into magnesium borate whiskers by roasting, and magnesite was converted into magnesium oxide with different activities, which in turn formed 2MgO·B2O3 and 3MgO·B2O3 crystals at different roasting temperatures.

[0041] Boron, with its large thermal neutron absorption cross section and low maximum energy for capturing gamma radiation, exhibits significant effectiveness in absorbing and shielding neutron rays, the most harmful component in nuclear reactions. Therefore, boron-containing cement has become one of the ultimate protective materials in the nuclear industry. Existing conventional methods involve incorporating borax, boric acid, and other boron-rich materials into concrete. This leads to uneven distribution of boron in the cement and unstable radiation shielding performance. Furthermore, these methods typically involve high levels of boron, prolonging cement setting time and affecting concrete strength development. This invention introduces magnesium, which readily combines with boron, forming a stable mineral phase, MgO-B₂O₃, in the clinker system. Mg in the MgO-B₂O₃ system is not released during subsequent hydration, resulting in a stable crystal structure. 3MgO·B₂O₃ exhibits excellent neutron absorption and shielding effects, with the neutron shielding effect gradually increasing as the 3MgO·B₂O₃ content in concrete increases. Meanwhile, 3MgO·B2O3 does not cause crystal structure damage during the hydration process of cementitious components. This mineral has good hydration stability and is not easily damaged, so it will not release borate ions. At the same time, it will not consume Ca in the concrete cementitious system, and can maintain the effective content of Ca element in the concrete cementitious system, and has little impact on the setting time and strength of concrete.

[0042] 5. The method for preparing concrete admixtures provided by the present invention uses a gas-phase suspension method to calcine the preheated material. It utilizes the large heat and mass transfer area between the gas and solid phases, resulting in a fast heat transfer rate. This reduces the temperature difference between the atmosphere and the preheated material, preventing over-burning and improving calcination efficiency. At the same time, the gas-phase calcination method offers flexible temperature control, enabling temperature adjustment in a short time and avoiding changes in component content due to insufficient preheated material reaction caused by a slow temperature control rate.

[0043] Compared to ordinary electric kilns, which cannot flexibly control the atmosphere to change the content of various components in clinker firing, gas phase suspension kilns have a larger upper limit of space (the flow of flames makes the temperature relatively uniform in a large space) and controllable firing atmosphere (oxidizing, neutral, weakly reducing, strong reducing), which makes the content of various components in the prepared concrete admixtures controllable.

[0044] 6. The method for preparing concrete admixtures provided by the present invention, wherein the first heating temperature is 700-800℃ and the first time is 1.5-2h; and / or, the second heating temperature is 1000-1100℃ and the second time is 0.5-1h; and / or, the third heating temperature is 1250-1300℃ and the third time is 0.2-0.4h. The present invention uses a gas-phase suspension method to maintain the preheated material at a first heating temperature of 700-800℃ for 1.5-2h. During this process, magnesite begins to decompose to form highly active magnesium oxide, and the precursor generates 2MgO·B2O3. NaCl and KCl in carnallite act as composite fluxes, which are more conducive to whisker growth. The second heating temperature is maintained at 100-1100℃ for 0.5-1h. During this process, medium-to-low activity magnesium oxide is generated, and simultaneously, magnesium oxide reacts with unreacted borate to generate 2MgO·B2O3 crystals. Finally, when the third heating temperature is maintained at 1250-1300℃ for 0.2-0.4 hours, this process can generate a small amount of ultra-low activity magnesium oxide, and some 2MgO·B₂O₃ crystals will transform into 3MgO·B₂O₃ crystals. This invention, by gradually increasing the heating temperature, enables the generation of lightly calcined magnesium oxide, 2MgO·B₂O₃ crystals, and 3MgO·B₂O₃ crystals with different activities during the calcination process. Simultaneously, the flexibility of temperature adjustment using the gas-phase suspension method effectively controls the duration of the temperature-changing process, avoiding instability in the content of various components due to prolonged temperature changes.

[0045] 7. The concrete admixture provided by this invention is applied to the preparation of concrete for nuclear engineering, enabling the concrete used in nuclear engineering to simultaneously possess the properties of compensating for shrinkage, toughening and crack resistance, and radiation protection. Attached Figure Description

[0046] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0047] Figure 1 This is a process flow diagram of a specific example of the preparation method of concrete admixtures in Embodiment 1 of the present invention. Detailed Implementation

[0048] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0049] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0050] Example 1

[0051] This embodiment provides a method for preparing concrete admixtures, with specific steps and parameters as follows:

[0052] Please see Figure 1 As shown, 4 tons of magnesite and 0.5 tons of carnallite were mixed and crushed to an average particle size of 3 mm. The crushed raw material was then transported to a dryer for primary flow drying and dehydration at a controlled temperature of 180℃ for 6 hours. The magnesite used contained 48.3% MgO, 1.2% CaO, and 1.0% SiO2. The carnallite contained 27% KCl, 13% NaCl, and 32% MgCl2. The contents of each component are mass fractions.

[0053] After drying, the mixed raw materials are naturally cooled to 100°C. While stirring the mixed raw materials, industrial-grade dimethylformamide (DMF) is sprayed into the dryer to ensure that the surface of the mixed raw materials is in full contact with the DMF.

[0054] After stopping the spraying of DMF, 3 tons of industrial boric acid were added to the dryer. The mixture was stirred slowly at 1.5 r / min for 3.5 hours at 80°C. The temperature was then raised to 180°C and stirred at 1.5 r / min for another 2 hours. The boric acid content of the industrial boric acid was 99.5% w / w.

[0055] After stirring is stopped, the mixture in the dryer is conveyed to the preheating cyclone to heat the mixture to 320°C to obtain the preheated material.

[0056] The preheated material is fed into the main furnace for gas suspension roasting. The temperature is increased to 750℃ at 12℃ / min and held for 2 hours, then increased to 1050℃ at 20℃ / min and held for 0.8 hours, and finally increased to 1250℃ at 25℃ / min and held for 0.3 hours to obtain clinker.

[0057] After firing, the clinker is sent to a flow cooling device to cool the clinker temperature to 150°C at a rate of 5°C / min before being sent to the silo.

[0058] After the clinker is naturally cooled to below 60°C in the silo, it is mixed with 2.5 tons of crushed limestone, crushed into small particles, and then fed into a mill to be ground to an average particle size of 300 mesh, thus obtaining concrete admixture.

[0059] Take 69 kg of the concrete admixture prepared above, 665 kg of river sand with a fineness of 2.8, and 1130 kg of crushed stone with a particle size of 5-25 mm, and put them into a mixer and dry mix for 30 seconds. Then, add 350 kg of PO42.5 cement, 31 kg of Grade I fly ash, 165 kg of water, and 4.6 kg of polycarboxylate superplasticizer into the mixer and mix for 90 seconds to obtain a 1m³ concrete mix. 3 Concrete for nuclear engineering.

[0060] Example 2

[0061] This embodiment provides a method for preparing concrete admixtures. The specific steps and parameters are the same as in Embodiment 1, except that the amount of magnesite is 3 tons, carnallite is 0.8 tons, industrial boric acid is 3.5 tons, and limestone is 2.7 tons.

[0062] Take 39 kg of the concrete admixture prepared above, 665 kg of river sand with a fineness of 2.8, and 1130 kg of crushed stone with a particle size of 5-25 mm, and put them into a mixer to dry mix for 60 seconds. Then, add 360 kg of PO42.5 cement, 31 kg of Grade I fly ash, 165 kg of water, and 4.6 kg of polycarboxylate superplasticizer into the mixer and mix for 120 seconds to obtain a 1m³ concrete mix. 3 Concrete for nuclear engineering.

[0063] Example 3

[0064] This embodiment provides a method for preparing concrete admixtures. The specific steps and parameters are the same as in Embodiment 1, except that the amount of magnesite is 2 tons, carnallite is 1 ton, industrial boric acid is 4 tons, and limestone is 3 tons.

[0065] Take 77 kg of the concrete admixture prepared above, 665 kg of river sand with a fineness of 2.8, and 1130 kg of crushed stone with a particle size of 5-25 mm and put them into the mixer and dry mix for 60 seconds. Then, add 350 kg of PO42.5 cement, 31 kg of Grade I fly ash, 165 kg of water and 4.6 kg of polycarboxylate superplasticizer into the mixer and mix for 120 seconds to obtain concrete for nuclear engineering.

[0066] Example 4

[0067] Four tons of magnesite and 0.5 tons of carnallite were mixed and crushed to an average particle size of 3 mm. The crushed raw material was then transported to a dryer for primary flow drying and dehydration at a controlled temperature of 180℃ for 6 hours. The magnesite used contained 48.3% MgO, 1.2% CaO, and 1.0% SiO2, while the carnallite contained 27% KCl, 13% NaCl, and 32% MgCl2.

[0068] After drying, the mixed raw materials are naturally cooled to 100°C. While stirring the mixed raw materials, industrial-grade dimethylformamide (DMF) is sprayed into the dryer to ensure that the surface of the mixed raw materials is in full contact with the DMF.

[0069] After stopping the spraying of DMF, 3 tons of industrial boric acid were added to the dryer. The mixture was slowly stirred at 3 r / min at 120°C for 3.5 hours, then heated to 180°C and stirred for another 2 hours. The content of industrial boric acid was 99.5% w / w.

[0070] After stirring is stopped, the mixture in the dryer is conveyed to the preheating cyclone to heat the mixture to 320°C to obtain the preheated material.

[0071] The preheated material is fed into the main furnace for gaseous suspension roasting. The temperature is increased to 750℃ at 12℃ / min and held for 2 hours, then increased to 1100℃ at 20℃ / min and held for 0.5 hours, and finally increased to 1300℃ at 25℃ / min and held for 0.4 hours to obtain clinker.

[0072] After firing, the clinker is sent to a flow cooling device to cool the clinker temperature to 150°C at a rate of 5°C / min before being sent to the silo.

[0073] After the clinker is naturally cooled to below 60°C in the silo, it is mixed with 2.5 tons of crushed limestone, crushed into small particles, and then fed into a mill to be ground to an average particle size of 300 mesh, thus obtaining concrete admixture.

[0074] 1m of concrete was prepared according to the method for preparing concrete in Example 1. 3 Concrete for nuclear engineering.

[0075] Example 5

[0076] Four tons of magnesite and 0.5 tons of carnallite were mixed and crushed to an average particle size of 2 mm. The crushed raw material was then transported to a dryer for primary flow drying and dehydration at a controlled temperature of 150℃ for 8 hours. The magnesite used contained 46.2% MgO, 0.9% CaO, and 1.2% SiO2, while the carnallite contained 25% KCl, 14.8% NaCl, and 38.5% MgCl2.

[0077] After drying, the mixed raw materials are naturally cooled to 100°C. While stirring the mixed raw materials, industrial-grade dimethylformamide (DMF) is sprayed into the dryer to ensure that the surface of the mixed raw materials is in full contact with the DMF.

[0078] After stopping the spraying of DMF, 2 tons of industrial boric acid were added to the dryer. The mixture was slowly stirred at 2 r / min at 100°C for 4.8 hours. Then the temperature was raised to 150°C and stirring was continued for another 1.5 hours. The boric acid content of the industrial boric acid was 99.7%, w / w.

[0079] After stirring is stopped, the mixture in the dryer is conveyed to a preheating cyclone to heat the mixture to 350°C, thus obtaining a preheated material.

[0080] The preheated material is fed into the main furnace for gas suspension roasting. The temperature is increased to 800℃ at 10℃ / min and held for 1.5h, then increased to 1000℃ at 25℃ / min and held for 1h, and finally increased to 1250℃ at 30℃ / min and held for 0.2h to obtain clinker.

[0081] After firing, the clinker is sent to a flow cooling device to cool the clinker temperature to 150°C at a rate of 5°C / min before being sent to the silo.

[0082] After the clinker is naturally cooled to below 60°C in the silo, it is mixed with 1.5 tons of crushed limestone, crushed into small particles, and then fed into a mill to be ground to an average particle size of 600 mesh, thus obtaining concrete admixture.

[0083] 1m of concrete was prepared according to the method for preparing concrete in Example 1. 3 Concrete for nuclear engineering.

[0084] Example 6

[0085] Four tons of magnesite and 0.5 tons of carnallite were mixed and crushed to an average particle size of 1 mm. The crushed raw material was then transported to a dryer for primary flow drying and dehydration at a controlled temperature of 200℃ for 4 hours. The magnesite used contained 45.6% MgO, 1.5% CaO, and 1.5% SiO2, while the carnallite contained 20.5% KCl, 10.2% NaCl, and 25.4% MgCl2.

[0086] After drying, the mixed raw materials are naturally cooled to 100°C. While stirring the mixed raw materials, industrial-grade dimethylformamide (DMF) is sprayed into the dryer to ensure that the surface of the mixed raw materials is in full contact with the DMF.

[0087] After stopping the spraying of DMF, 3 tons of industrial boric acid were added to the dryer. The mixture was slowly stirred at 2.5 r / min for 3 hours at 80°C. The temperature was then raised to 200°C and stirred for another hour. The boric acid content of the industrial boric acid was 99.7%, w / w.

[0088] After stirring is stopped, the mixture in the dryer is conveyed to a preheating cyclone to heat the mixture to 300°C, thus obtaining a preheated material.

[0089] The preheated material is fed into the main furnace for gaseous suspension roasting. The temperature is increased to 700℃ at 15℃ / min and held for 2 hours, then increased to 1050℃ at 20℃ / min and held for 0.8 hours, and finally increased to 1250℃ at 25℃ / min and held for 0.3 hours to obtain clinker.

[0090] After firing, the clinker is sent to a flow cooling device to cool the clinker temperature to 150°C at a rate of 5°C / min before being sent to the silo.

[0091] After the clinker is naturally cooled to below 60°C in the silo, it is mixed with 2.5 tons of crushed limestone, crushed into small particles, and then fed into a mill for grinding until the average particle size is 400 mesh, thus obtaining concrete admixture.

[0092] 1m of concrete was prepared according to the method for preparing concrete in Example 1. 3 Concrete for nuclear engineering.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing concrete for nuclear engineering, with the specific steps and parameters as follows:

[0095] Take 69 kg of lightly calcined MgO expanding agent, 665 kg of river sand with a fineness of 2.8, and 1130 kg of crushed stone with a particle size of 5-25 mm, and dry mix them in a mixer for 30 seconds. Then, add 350 kg of PO42.5 cement, 31 kg of Grade I fly ash, 165 kg of water, and 4.6 kg of polycarboxylate superplasticizer to the mixer and mix for 90 seconds to obtain 1 m³ of [unclear text - possibly a specific product or product]. 3 Concrete used in nuclear engineering, in which the lightly calcined MgO expanding agent is commercially available.

[0096] Comparative Example 2

[0097] This comparative example provides a method for preparing concrete admixtures. The specific steps and parameters are the same as in Example 1. The only difference is that the preheated material is fed into the main furnace for gas suspension calcination. The temperature is increased to 750°C at 12°C / min and held for 2 hours. Then, the temperature is increased to 1050°C at 20°C / min and held for 1.1 hours to obtain clinker. The remaining steps are the same as in Example 1 to obtain concrete admixtures.

[0098] 1m of concrete was prepared according to the method for preparing concrete in Example 1. 3 Concrete for nuclear engineering.

[0099] Experimental Example 1

[0100] The main chemical composition and content of concrete admixtures prepared in Examples 1-4 and Comparative Examples 1-2 were tested using the method of "Chemical Analysis Methods for Cement" GB / T 176-2017. The test results are shown in Table 1.

[0101] Table 1. Main chemical composition and content (%) of concrete admixtures in the examples and comparative examples.

[0102] project Lightly calcined MgO <![CDATA[2MgO·B2O3]]> <![CDATA[3MgO·B2O3]]> <![CDATA[CaCO3]]> Free CaO Example 1 28.6 19.1 30.4 20.3 1.4 Example 2 21.1 29.5 25.6 22.4 1.1 Example 3 15.7 37.5 20.6 25.1 0.9 Example 4 23.2 15.6 39.1 20.1 1.9 Comparative Example 1 85.7 0 0 4.2 0.4 Comparative Example 2 31.6 46.3 0 20.6 1.0

[0103] Experiment Example 2

[0104] The compressive strength, flexural strength, axial tensile strength, and ultimate tensile strength of the concrete prepared in Examples 1-4 and Comparative Examples 1-2 were tested according to the "Standard for Test Methods of Mechanical Properties of Ordinary Concrete" GB / T50081-2016. The restricted expansion rate of the concrete prepared in Examples 1-4 and Comparative Examples 1-2 was tested according to the "Technical Specification for Application of Magnesium Oxide Expansion Agent for Concrete" T / CECS 540-2018. The mechanical properties and restricted expansion rate of the concrete are shown in Table 2.

[0105] Table 2 shows the mechanical properties and restricted expansion rate of concrete in Examples 1-4 and Comparative Examples 1-2.

[0106]

[0107] In the table, Δξ(28d-7d) is the difference between the restricted expansion rate of the concrete sample cured in water at 40℃ for 28 days and the restricted expansion rate cured for 7 days.

[0108] It can be seen that the compressive strength of Comparative Example 1 is significantly lower than that of Example 1, indicating that the compressive strength of concrete containing only lightly calcined magnesium oxide is poor. Combined with the data from Comparative Example 2, it can be seen that the poor compressive strength of the concrete prepared in Comparative Example 1 is due to the fact that the improvement in compressive strength by 2MgO·B2O3 partially offset the negative impact of magnesium oxide. Meanwhile, comparing the flexural strength and axial tensile strength in Example 1 and Comparative Example 1, it can be seen that the addition of 2MgO·B2O3 helps to improve the toughness and crack resistance of the prepared concrete. Through the component content and mechanical property data of the concrete admixtures prepared in Examples 1-3 in Tables 1 and 2, it can be seen that with the increase of 2MgO·B2O3 content, the 28-day flexural strength and axial tensile strength of the concrete continuously increase, further proving that 2MgO·B2O3 whiskers can improve the toughness and crack resistance of concrete.

[0109] Experimental Example 3

[0110] The neutron shielding performance of the concrete prepared in Examples 1-4 and Comparative Examples 1-2 was tested using a D5008 neutron measuring device. 252 Cf is a neutron-derived spontaneous fission, with an average neutron energy of 2.06 MeV, T 1 / 2 =2.34a, neutron emission rate 1.0×10 6 s -1 The BF3 proportional counter utilizes 10 B(n, α) 7 Li reaction measurement. The concrete test samples prepared in Examples 1-4 and Comparative Examples 1-2 were spherical shell structures formed from concrete, with a cavity diameter of 500 mm and a wall thickness of 100 mm. The test results are shown in Table 3.

[0111] Table 3. Neutron shielding performance data of concrete in Examples 1-4 and Comparative Examples 1-2.

[0112]

[0113] Comparing Examples 1-4 and Comparative Examples 1-2, it can be seen that both 2MgO·B₂O₃ and 3MgO·B₂O₃ have good neutron shielding performance, while lightly calcined MgO has no effect on the neutron shielding performance of concrete. Combining Examples 1 and Comparative Example 2, it can be seen that the neutron shielding performance of 3MgO·B₂O₃ is significantly better than that of 2MgO·B₂O₃. Furthermore, the data in Table 1 shows that the neutron shielding effect of concrete gradually increases with the increase of 3MgO·B₂O₃ content.

[0114] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A concrete admixture, characterized in that, The components include the following weight percentages: Lightly calcined MgO 15%-30%; 2MgO·B₂O₃ 15%-40%; 3MgO·B₂O₃ 20%-40%; CaCO3 20%-26%; CaO 1%-2%; The remainder are unavoidable impurities; The method for preparing the concrete admixture includes the following steps: S1, after drying and dehydration, magnesite and carnallite are mixed with dimethylformamide to obtain a mixed raw material. Boric acid is added to the mixed raw material, stirred, and heated to obtain a preheated material. S2, the preheated material is heated at a first heating temperature for a first time, at a second heating temperature for a second time, and at a third heating temperature for a third time to obtain a cooked material; S3, after cooling the clinker obtained in step S2, mix it with limestone and crush it to obtain concrete admixture; The first heating temperature is 700-800℃, and the first heating time is 1.5-2h; The second heating temperature is 1000-1100℃, and the second heating time is 0.5-1h; The third heating temperature is 1250-1300℃, and the third heating time is 0.2-0.4h; The roasting method is gas-phase suspension roasting.

2. The concrete admixture according to claim 1, characterized in that, By weight percentage, the lightly calcined MgO contains 23.2%, 2MgO·B₂O₃ 15.6%, 3MgO·B₂O₃ 39.1%; CaCO₃ 20.1%; and CaO 1.9%. The lightly calcined MgO includes medium-low activity MgO and ultra-low activity MgO; The reaction time of the medium-low activity MgO is 100-300s; The reaction time of the ultra-low activity MgO is 350-600s.

3. The concrete admixture according to claim 1, characterized in that, The heating rate at the first heating temperature is 10-15℃ / min; and / or, The heating rate at the second heating temperature is 20-25℃ / min; and / or, The heating rate of the third heating temperature is 25-30℃ / min.

4. The concrete admixture according to claim 1, characterized in that, By mass fraction, the magnesite contains ≥40% MgO, ≤1.5% CaO, and ≤1.5% SiO2; and / or, The boric acid is industrial boric acid with a boric acid content ≥99.5%, w / w; and / or, By mass fraction, the carnallite contains 20-30% KCl, 10-15% NaCl, and 25-40% MgCl2.

5. The concrete admixture according to claim 4, characterized in that, In step S1, magnesite comprises 20-40 parts by weight, carnallite 5-10 parts by weight, and boric acid 20-40 parts by weight; and / or, The magnesite and carnallite have a grain size of 1-3 mm; and / or, The magnesite and carnallite are dried and dehydrated at a temperature of 150-200℃ for 4-8 hours; and / or, The stirring steps after adding boric acid are as follows: first, stir at 80-120℃ and a stirring rate of 1.5-3 r / min for 3-5 hours; then, raise the temperature to 150-200℃ and continue stirring for 1-2 hours; and / or, The heating temperature in step S1 is 300-350℃.

6. The concrete admixture according to claim 1, characterized in that, In step S3, the weight of CaCO3 is 15-30 parts; and / or, The cooling temperature of the clinker is below 60°C; and / or, The particle size of the cooled clinker after being mixed and crushed with limestone is 300-600 mesh.

7. The application of a concrete admixture according to any one of claims 1-6 in the preparation of nuclear engineering concrete, characterized in that, The concrete admixture is mixed with coarse and fine aggregates for 30-60 seconds, and then cement, fly ash, water and admixtures are added and mixed for 90-120 seconds to obtain nuclear engineering concrete. The amount of the concrete admixture is 10-20% of the amount of concrete cementitious material, w / w.

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