CN115140985B

CN115140985B - Boron carbide ultra-high performance concrete and preparation method and application thereof

Info

Publication number: CN115140985B
Application number: CN202210943668.4A
Authority: CN
Inventors: 吕亚军; 张景伟; 张鹏刚; 董树林; 侯泳辉; 乔敏; 仝玉萍; 姜晓波; 白卫峰; 马军涛; 朱晨辉
Original assignee: North China University of Water Resources and Electric Power
Current assignee: North China University of Water Resources and Electric Power
Priority date: 2022-08-08
Filing date: 2022-08-08
Publication date: 2023-04-11
Anticipated expiration: 2042-08-08
Also published as: CN115140985A

Abstract

The invention discloses a boron carbide ultra-high performance concrete and a preparation method and application thereof, belonging to the technical field of ultra-high performance concrete.A boron carbide part with the same volume is used for replacing cement in the ultra-high performance concrete to obtain the boron carbide ultra-high performance concrete; the substitution rate of the substitution with the same volume is less than or equal to 10 percent and is not 0. According to the invention, boron carbide is adopted to replace part of cement to prepare the ultra-high performance concrete, so that the shielding effect is improved, and the excellent mechanical property of the ultra-high performance concrete is maintained. Along with the replacement rate of boron carbide, the compressive strength of the concrete is gradually reduced when the concrete is increased, but the concrete still has better fluidity, and the pores are concentrated in harmless pores below 20 nanometers; with the increase of the content of boron carbide, the slow neutron shielding capability of the concrete is obviously enhanced.

Description

Boron carbide ultra-high performance concrete and preparation method and application thereof

Technical Field

The invention belongs to the technical field of ultra-high performance concrete, and particularly relates to boron carbide ultra-high performance concrete and a preparation method and application thereof.

Background

With the rapid development and progress of science and technology, nuclear technology has been widely applied to the fields of industry, agriculture, medical treatment, environmental protection, public safety and the like, and remarkable economic and social benefits are obtained, but nuclear radiation generated in the application process of the nuclear technology can seriously threaten human health, ecological environment, public safety and the like. Neutron radiation in nuclear radiation mainly consists of neutral particle flow, has the characteristics of extremely strong penetrability, easy damage to atomic structures and the like, is often generated in the industries such as medical care, nuclear power plants, aerospace and the like, can greatly harm the bodies of doctors, patients, nuclear power plant managers and aircraft crews working in the environment for a long time, even can generate various diseases such as organ failure, central nerve damage, malignant tumors and the like, and can bring unpredictable influence on some precise instruments. Therefore, an efficient radiation protection shielding measure is urgently needed to be found, and the method has great significance for reducing the damage of neutron radiation to human health, valuable equipment and the like. At present, scholars at home and abroad carry out relevant research on material performance and functions (strength, radiation protection effect and the like) of radiation protection materials such as lead, concrete, steel plates, glass and the like, and obtain certain research results. Among them, the concrete material is considered as one of the neutron radiation shielding materials with the most potential for popularization and application because of its advantages of abundant raw materials, low cost, good plasticity, strong radiation shielding effect, etc.

For neutron shielding, the main principle is to utilize the interaction of neutrons and the atomic nuclei of specific elements to slow down the propagation speed of fast neutrons so as to reduce the energy of the fast neutrons until the fast neutrons are absorbed. In the preparation process of the radiation-proof concrete, the key to improving the neutron radiation shielding is to select materials (boron elements and the like) rich in specific elements as fine aggregates or coarse aggregates. Based on the prior research, it is not difficult to discover that the element B is used for the radiation-proof material due to the high absorption cross section, wherein ¹⁰ The average cross section of the thermal neutrons of B is as high as 3840B, which is more than 20 times of that of graphite and more than 500 times of that of traditional concrete, so that the boron-containing compound can be added into the concrete to counteract the neutronsHas good shielding effect.

At present, boron-containing materials such as boron nitride, boron carbide and the like gradually enter the public vision, and most of composite materials containing boride have certain radiation protection capability and are widely applied to important fields such as protection, nuclear industry, medical treatment and the like. Some scholars at home and abroad carry out related research on the radiation protection effect of the boride-containing composite material, and obtain some achievements. B is ₃ FS (boron-containing blast furnace slag) is used as a shielding filler for preventing neutron radiation damage due to its high boron content, and is excellent in neutron radiation shielding effect; dong et al for B ₃ FS made the study and found B ₃ FS is excellent in neutron shielding radiation, and is a good shielding filler for preventing neutron radiation damage; the study by Morallo et al found that the neutron attenuation coefficient of the concrete test block increased with increasing borosilicate glass (BSGP) content in place of cement. Demir and other analyses show that the boron-magnesium stone has higher boron content than other boron minerals (such as borax and ulexite), and the prepared composite material can exert better neutron shielding capability; the researches of Sathisha and the like show that the novel composite material prepared by using ferroboron has good absorption effect on X-ray, gamma ray and neutron radiation; abdullah et al teach the incorporation of B into concrete ₄ The conclusion that the C powder can significantly improve the neutron shielding performance of concrete is concluded, and DiJulio et al found that polyethylene and B are used ₄ The neutron shielding effect of the concrete prepared by the method C reaches 40%. In conclusion, the proper amount of boron compound is doped into the concrete, so that the neutron radiation shielding capability of the concrete material can be effectively improved.

However, it has been found that the incorporation of boron compounds in concrete in appropriate amounts has some adverse effects on the hydration of the cement, resulting in a decrease in the mechanical and mechanical properties of the concrete. For example, glinicki et al found that replacement of a portion of the sand with borax and synthetic boron carbide adversely affected cement hydration and mechanical properties. Gambarini et al concluded that the use of colemanite, ulexite and borax to replace some of the sand would have a negative impact on cement hydration, setting time, compressive strength; han and Zayed et al indicate that dissolved boric acid in boron compounds lowers the pH, delays cement hydration, and reduces the durability of the cement. Based on the research results, it is not easy to find that the proper amount of boron compound is not beneficial to the development of the mechanical property of concrete, can hinder the application of the boron carbide-containing concrete in the aspect of neutron radiation protection, and a method for improving the mechanical property of the boron carbide-containing neutron radiation protection shielding concrete needs to be found.

For the neutron radiation protection shielding concrete, the neutron radiation protection shielding concrete is mostly used in special structures (such as a nuclear power station and the like) with extremely high requirements on safety, and has the characteristics of high strength, good shielding effect and the like, but the strength of the neutron radiation protection shielding concrete prepared by scholars at home and abroad at present is generally in a lower level, once the neutron radiation protection shielding concrete is damaged by explosion impact or serious disasters such as earthquake, tornado and the like, the whole structure may have potential safety hazards, the radiation shielding effect is greatly attenuated, and further the life safety of personnel is threatened. Therefore, the prepared high-strength neutron shielding concrete has wider application scenes and more important engineering significance.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to provide the boron carbide ultra-high performance concrete and the preparation method and the application thereof.

In order to achieve the purpose, the invention provides the following technical scheme:

the boron carbide ultrahigh-performance concrete is prepared by replacing cement in the ultrahigh-performance concrete with equal volume of boron carbide parts; the substitution rate of the equal-volume substitution is less than or equal to 10 percent and is not 0;

the boron carbide needs to be subjected to alkaline treatment before use, and the treatment method comprises the following steps: dissolving boron carbide in water, adding calcium hydroxide until the pH value is 10.7 after the boron carbide is dissolved, drying and filtering to obtain the boron carbide after alkaline treatment.

In order to improve the influence of boron carbide on the strength of concrete, based on the principle that calcium hydroxide powder and impurities in industrial boron carbide powder generate boric acid when meeting water, the reaction between the boric acid and an interface transition region is effectively solved, the compressive strength of the prepared concrete is further increased, the requirement of the ultra-high performance concrete is met, and the application range of the concrete is wider. The aim is to ensure the feasibility of using industrial boron carbide as an additive for the preparation of ultra-high performance concrete.

Further, the ultra-high performance concrete (UHPC) comprises the following raw materials in parts by weight: 639-800 parts of cement, 185 parts of fly ash, 148 parts of silica fume, 289-767 parts of fine aggregate, 40 parts of water reducing agent and 198 parts of water.

Further, the cement is P.II 52.5 portland cement.

Further, the fly ash is first-grade fly ash.

Furthermore, the fine aggregate is natural river sand with the diameter of 0-2.56mm.

Further, the water reducing agent is a polycarboxylic acid high-efficiency water reducing agent, the water reducing rate is 30%, and the solid content is 30%.

The invention also provides a preparation method of the boron carbide ultrahigh-performance concrete, which comprises the following steps:

weighing raw materials according to the mass, mixing cement, silica fume, fly ash, natural river sand and boron carbide, stirring for 2min at the rotating speed of 140 +/-5 r/min, then adding a water reducing agent and 70% of water by volume, continuously stirring for 2min at the rotating speed of 140 +/-5 r/min, finally adding the rest water, and stirring for 2min at the rotating speed of 280 +/-10 r/min to obtain the boron carbide ultrahigh-performance concrete.

The invention also provides application of the boron carbide ultrahigh-performance concrete as a neutron shielding radiation-proof material.

Further, the neutron shielding radiation protection material is a slow neutron shielding radiation protection material.

The term "slow neutrons" in the present invention refers to low energy neutrons having a thermal energy (0.025 ev) level, which are not of sufficient energy to cause ionization and excitation by themselves. Such neutrons are present in nuclear reactors, however, and indirectly cause several chemical changes when materials are irradiated in these reactors.

Compared with the prior art, the invention has the beneficial effects that:

according to the invention, the boron carbide is used for replacing part of cement to prepare the ultra-high performance concrete, so that the shielding effect is improved, and the ultra-high performance concrete has good mechanical properties. Along with the increase of the replacement rate of boron carbide, the compressive strength of the concrete can be gradually reduced, but the concrete still has better fluidity, and the pores are concentrated in harmless pores below 20 nanometers; with the increase of the content of boron carbide, the slow neutron shielding capability of the concrete is obviously enhanced.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.

FIG. 1 is a digital image (left) and a scanning electron microscope image (right) of boron carbide;

FIG. 2 is a schematic illustration of boron carbide alkaline treatment;

FIG. 3 shows the amount of calcium hydroxide powder added and the pH value of the solution;

FIG. 4 shows the main components of the raw materials of the present invention;

FIG. 5 is a graph of the optimization of the target for each raw material and particle size distribution according to the present invention;

FIG. 6 is a flow chart of the preparation of boron carbide ultra-high performance concrete of example 1;

FIG. 7 shows the fluidity change law of the boron carbide ultra-high performance concrete prepared by different boron carbide replacement rates in example 1;

FIG. 8 is a graph showing the set times of ultra-high performance concretes of boron carbide prepared according to different boron carbide replacement ratios in example 1;

FIG. 9 is the wet bulk density of boron carbide ultra high performance concrete prepared at different boron carbide substitution rates of example 1;

FIG. 10 is a graph showing the compressive strength of ultra-high performance concrete of boron carbide prepared according to different boron carbide replacement ratios in example 1;

FIG. 11 is a graph showing pore size distributions of ultra-high performance boron carbide concrete prepared according to different boron carbide replacement ratios in example 1; the left graph is the cumulative pore distribution of different particle sizes; the right graph shows the porosity of different particle sizes;

FIG. 12 is a histogram of pore size distribution of ultra-high performance concrete of boron carbide prepared at different boron carbide replacement rates in example 1;

FIG. 13 is a TG and DTG curve of boron carbide ultra-high performance concrete prepared at different boron carbide replacement rates in example 1;

FIG. 14 is a heat flow curve of ultra high performance boron carbide concrete prepared at different boron carbide replacement rates of example 1; the left graph is an accumulated released heat curve, and the right graph is a thermal power curve;

FIG. 15 is a graph showing the compressive strength of ultra-high performance concrete prepared with boron carbide according to different boron carbide replacement ratios in example 1;

FIG. 16 is a graph showing the effect of porosity on compressive strength of ultra-high performance concrete of boron carbide prepared at different boron carbide replacement ratios in example 1;

FIG. 17 is a neutron test shielding model of boron carbide ultra-high performance concrete of example 1;

FIG. 18 is a graph showing the effect of different boron carbide substitution rates on neutron shielding performance of boron carbide ultra-high performance concrete prepared in example 1;

FIG. 19 is a neutron spectrum of a common element;

FIG. 20 is a plot of half-value layer as a function of neutron energy;

fig. 21 is a plot of neutron energy versus neutron macroscopic cross-section.

Detailed Description

Reference will now be made in detail to various exemplary embodiments of the invention, the detailed description should not be construed as limiting the invention but as a more detailed description of certain aspects, features and embodiments of the invention.

It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Further, for numerical ranges in this disclosure, it is understood that each intervening value, between the upper and lower limit of that range, is also specifically disclosed. Every smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in a stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference herein for the purpose of disclosing and describing the methods and/or materials associated with the documents. In case of conflict with any incorporated document, the present specification will control.

It will be apparent to those skilled in the art that various modifications and variations can be made in the specific embodiments of the present disclosure without departing from the scope or spirit of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification. The specification and examples are exemplary only.

As used herein, the terms "comprising," "including," "having," "containing," and the like are open-ended terms that mean including, but not limited to.

The boron carbide ultrahigh-performance concrete is prepared by replacing cement in the ultrahigh-performance concrete with equal volume of boron carbide parts; the substitution rate of the substitution with the same volume is less than or equal to 10 percent and is not 0. Preferably 7.5% or less, more preferably 5% or less.

In some preferred embodiments, the boron carbide needs to be subjected to alkaline treatment before use, and the treatment method comprises the following steps: dissolving boron carbide in water, adding calcium hydroxide after the boron carbide is dissolved until the pH value is 10.7, drying and filtering to obtain the boron carbide after alkaline treatment.

In some preferred embodiments, the ultra-high performance concrete comprises the following raw materials in parts by weight: 639-800 parts of cement, 185 parts of fly ash, 148 parts of silica fume, 289-767 parts of fine aggregate, 40 parts of water reducing agent and 198 parts of water.

In some preferred embodiments, the cement is PIII 52.5 portland cement.

In some preferred embodiments, the fly ash is a primary fly ash.

In some preferred embodiments, the fine aggregate is natural river sand with a diameter of 0-2.56mm.

In some preferred embodiments, the water reducing agent is a polycarboxylic acid type high-efficiency water reducing agent, the water reducing rate is 30%, and the solid content is 30%.

weighing raw materials according to mass, mixing cement, silica fume, fly ash, natural river sand and boron carbide, stirring for 2min at the rotating speed of 140 +/-5 r/min, then adding a water reducing agent and 70% of water by volume, continuously stirring for 2min at the rotating speed of 140 +/-5 r/min, finally adding the rest water, and stirring for 2min at the rotating speed of 280 +/-10 r/min to obtain the boron carbide ultrahigh-performance concrete.

The "parts" in the present invention are all parts by mass unless otherwise specified.

The raw materials used in the invention are as follows:

the cement is P.II 52.5 silicate cement produced by Henan Yongan cement Co., ltd; the fly ash is first-grade fly ash provided by a Rongchang environmental protection material factory; the silica fume is produced by Loyang Yumin micro silica powder Limited company; the calcium hydroxide powder is produced by Shandong Longhui chemical industry Co., ltd; boron carbide is produced by Henan Sicheng grinding science and technology Limited, and the form is shown in figure 1; the water reducing agent is a polycarboxylic acid high-efficiency water reducing agent produced by Jiangsu Subert company, the water reducing rate is 30 percent, and the solid content is 30 percent.

The boron carbide used in the invention is industrial grade boron carbide, the grain size of the boron carbide is 0.1-70 microns, and the industrial grade boron carbide contains a small amount of boron oxide. The composition is shown in table 1.

TABLE 1 main chemical composition of boron carbide

TABLE 2 chemical composition of cement, silica fume and fly ash (%)

	Na ₂ O	MgO	Al ₂ O ₃	SiO ₂	P ₂ O ₅	SO ₃	K ₂ O	CaO	Fe ₂ O ₃	LOI
											Cement	0.07	1.73	4.24	18.25	0.08	3.25	0.87	65.03	3.38	3.10
Silica fume	0.25	0.37	0.22	94.85	0.13	0.79	0.64	0.32	0.18	2.25
											Fly ash	0.28	0.36	38.85	46.82	0.07	0.62	0.84	7.8	2.85	1.51

Example 1

1. Alkaline treatment of boron carbide:

in order to better obtain the doping amount of calcium hydroxide and ensure the precision of a simulation result of a shielding experiment, boron carbide (57 g) with the doping amount of 10% is dissolved in 198g of water in the test process, and after the boron carbide is completely dissolved, a pH value of the solution is 6.1 by using a pH-B-3 pen type pH meter (Shanghai Sanxin) test; then adding 0.5g of calcium hydroxide powder each time, fully stirring and testing the pH value until the pH value of the solution is stable and unchanged, and determining that the effect is optimal when the pH value of the solution is 10.7, wherein the doping amount of the calcium hydroxide is determined to be 2g, thus obtaining the boron carbide solution after alkaline treatment. And (3) placing the boron carbide solution subjected to the alkaline treatment in a drying box for heating, setting the highest temperature of the drying box to be 150 ℃, continuing for 24 hours, then closing the drying box, reducing the temperature to room temperature, and filtering to obtain the boron carbide subjected to the alkaline treatment. The schematic diagram of the boron carbide alkaline treatment and the change of the solution pH value are shown in FIGS. 2 and 3 respectively.

2. Elemental composition calculation in concrete

The element composition in the test piece is an important factor influencing the neutron shielding capability, and researches have found that the hydrogen element and the boron element respectively have remarkable shielding capability for fast neutrons and slow neutrons. The hydration reaction process of the concrete is shown in formulas (1) to (4):

2(3CaO·SiO ₂ )+6H ₂ O→3CaO·2SiO ₂ ·3H ₂ O+3Ca(OH) ₂ (1)

2(2CaO·SiO ₂ )+4H ₂ O→3CaO·2SiO ₂ ·3H ₂ O+Ca(OH) ₂ (2)

Al ₂ O ₃ ·3CaO+2H ₂ O·3CaSO ₄ +26H ₂ O→Al ₂ O ₃ ·3CaO·3CaSO ₄ ·32H ₂ O (3)

Al ₂ O ₃ ·3CaO·3CaSO ₄ ·32H ₂ O+2(3CaO·Al ₂ O ₃ )+4H ₂ O→3(Al ₂ O ₃ ·3CaO·CaSO ₄ ·12H ₂ O) (4)

the content of hydrogen in the cement after hydration can be calculated by the formulas (1) to (4), and the bound water content required by the hydration reaction is further obtained. Considering that cement cannot be completely hydrated in the concrete hydration reaction, the application coefficient of cement hydration is 0.95 according to the minimum clinker content in the cement given by the composition, specification and compliance standard of cement-common cement (BSEN 197-1.

According to the test results, the hydrated P.II 52.5 cement contained 50-60wt% of CSH (3 CaO.2 SiO) ₂ ·3H ₂ O)、20-25wt％CH(Ca(OH) ₂ ) And 15-20wt% of AFm (3 CaO. Al) ₂ O ₃ ·CaSO ₄ )·12H ₂ O); the lowest hydrogen content and the most unfavorable shielding effect in terms of elemental composition were determined, and then CSH: CH: the mass ratio of AFm is 60:25:15. based on this, the contents of the main elements after hydration can be calculated as shown in table 3.

TABLE 3 chemical composition of hydrated Cement

When the boron carbide replacement rate is 2.5%, the hydration cement in the radiation shielding concrete is: river sand: fly ash: silicon powder: the mass ratio of boron carbide is 32.08; calculating according to the steps of fig. 4, and finally determining each element Ca of the radiation shielding concrete: o: h: s: al: si: b: the mass ratio of other elements is 12.85:50.54:0.74:0.29:2.28:32.27:0.52:0.51. finally, the composition and the ratio of the main elements in the concrete test block are calculated in the above manner, as shown in fig. 4.

3. Design of mix proportion

The dense particle packing structure is a key factor in the preparation of UHPC. As shown in fig. 5, the modified a & a model was used until the particle size distribution curve of the boron carbide concrete was closest to the target curve, reaching the tightest packing structure. Adopting untreated boron carbide with the volume fraction of 0%, 2.5%, 5%, 7.5% and 10% and alkaline treated boron carbide with the volume fraction of 10% to replace the cement content in the UHPC by the same volume to prepare 6 types of boron carbide ultra-high performance concrete with different boron carbide doping amounts, determining a target curve according to a mixing ratio scheme shown in table 4 (shown in a formula 5), and adjusting each ratio of a mixture in the boron carbide UHPC.

Wherein D-particle size (μm), P (D) -percentage of particles having a particle size smaller than D, D _max Maximum particle size (. Mu.m), D _min Minimum particle size (. Mu.m), q-distribution modulus, of 0.23.

TABLE 4 boron carbide concrete mix ratio (g/m) ³ )

Note: b represents boron carbide; c represents calcium hydroxide alkaline treatment; 0. 2.5, 5, 7.5 and 10 show that the replacement rate of boron carbide for cement is 0, 2.5, 5, 7.5 and 10%, and the replacement is carried out by using the same volume.

4. Preparation method

Weighing raw materials by mass, mixing cement, silica fume, fly ash, natural river sand and boron carbide, stirring for 2min at the rotating speed of 140 +/-5 r/min, then adding a water reducing agent and 70% of water by volume, continuously stirring for 2min at the rotating speed of 140 +/-5 r/min, finally adding the rest water, and stirring for 2min at the rotating speed of 280 +/-10 r/min to obtain the boron carbide ultrahigh-performance concrete (see figure 6).

The element contents in the boron carbide ultra-high performance concrete test blocks prepared by partially replacing cement with different boron carbide contents (0, 2.5%, 5%, 7.5%, 10%) are shown in table 5.

TABLE 5 elemental mass percentages (%)

5. Performance test

1) Flow properties

According to the method for measuring fluidity of cement mortar (GB/T2419-2005), the fluidity of the prepared boron carbide ultrahigh-performance concrete is tested by a jumping-table method. And (3) putting the mixture into a truncated cone metal circular mould in two layers, tamping, lifting the circular mould, starting a jump table, measuring two diameters in the mutually perpendicular direction by using a caliper after finishing 25 times of jumping, wherein the average value of the two diameters is the fluidity of the prepared boron carbide ultrahigh-performance concrete (see a result in fig. 7).

As can be seen in FIG. 7, the fluidity of the UHPC blend gradually decreased as the boron carbide replacement rate increased. The flow of the UHPC blend after incorporation of the different replacement rates of 2.5%, 5%, 7.5%, 10% boron carbide decreased by 2.38%, 5.44%, 8.16%, 10.88% and 11.56%, respectively, compared to the maximum flow of 294mm for the reference group (B0). When the doping amount of the boron carbide is 10%, the fluidity of the UHPC mixture containing the alkali-treated boron carbide is slightly changed compared with that of B10, and the fluidity is only reduced by 2mm, so that the influence of whether the fluidity of the UHPC mixture containing the boron carbide is influenced by the alkali treatment of the boron carbide is very weak. It is noteworthy that the incorporation of a certain amount of boron carbide significantly reduces the fluidity of the UHPC mix, mainly due to the fact that boron carbide is smaller in particle size than replacement cement and has a larger specific surface area, and fine particles of boron carbide adsorb free water in the mortar more easily, making the mix more viscous, resulting in a reduced fluidity.

2) Initial setting and final setting time

In order to investigate the influence of different proportions of boron carbide addition on the setting time of concrete and the performance of daily work, the setting time of the boron carbide ultra-high performance concrete was determined using a vicat instrument according to the method for testing water consumption, setting time and stability at standard consistency of cement (GB/T1346-2011) [52] (see fig. 8 for results).

As can be seen from fig. 8, the initial setting time of the sample is significantly shortened as the replacement ratio of the industrial-grade boron carbide increases, and when the replacement ratio is 2.5, 5, 7.5, 10%, the initial setting time of the sample is advanced by 10.14%, 21.73%, 27.54%, 31.88%, respectively (relative to the replacement ratio of 0), the initial setting time of the alkali-treated boron carbide (BC 10) is 235min. This is mainly because boron carbide powder has a smaller particle size and a larger specific surface area than cement, and as boron carbide powder increases, the amount of free water in the mixture for hydration decreases, and in early hydration, soluble substances in cement dissolve in water and are more likely to reach a saturated state, and more Ca (OH) is generated in a short time ₂ The crystal and the ettringite crystal, each crystal particle is continuously connected into a net, and the net structure is continuously strengthened, so that the setting time of the cement paste is continuously advanced.

As the replacement rate of boron carbide increases, the difference between the initial setting time and the final setting time increases, mainly due to two reasons: 1) The industrial-grade boron carbide contains boron oxide, and can generate metaboric acid and boric acid when dissolved in water, and acidic substances can be used as a cement retarder to delay the setting time of cement; 2) Boric acid is readily available with OH ^- An insoluble precipitate is formed, and calcium diborate hexahydrate (CaO. B) is formed ₂ O ₄ ·6H ₂ O). During the reaction, hydrogen ions (H) are released ⁺ ) Consuming hydroxyl anion (OH) ^- ) The time required for the cement to hydrate to saturation is delayed, resulting in an increase in the initial set and final set time differential. However, in the boron carbide sample (BC 10) after the alkaline treatment, the boric acid and the metaboric acid are greatly reduced, and the initial setting and final setting time difference is not influenced too much.

B ₂ O ₃ +3H ₂ O＝2B(OH) ₃ (6)

2B(OH) ₃ +Ca ²⁺ +7OH ^- +H ₂ O＝CaO·B ₂ O ₄ ·6H ₂ O (8)

3) Determination of Wet bulk Density

In order to research the compactness of the mixture under different mixing proportions, the wet bulk density method is adopted for characterization, and the wet bulk density method can be calculated according to the formulas (9) and (10),

wherein the content of the first and second substances,

is the bulk density, V is the mold volume to be filled with the sample, taking 220ml.

Wherein Mmax-the maximum mass of the various mixed materials after mixing, X-different gelled materials,

the set of cements used, pw, ps, px are the densities of water, sand and cement, respectively, and RW, RS, RX are the volume ratios of water, sand and cement to the total solids content of the mixture, respectively (see figure 9 for results). />

As can be seen from FIG. 9, the wet bulk density of the blend showed a tendency to increase with increasing boron carbide replacement rate, and generally speaking, the wet bulk densities of boron carbide did not differ much when the replacement rates were 0, 2.5, 5, 7.5, 10%, respectively, and the wet bulk densities were 0.769, 0.773, 0.785, 0.799, 0.806g/cm, respectively ³ When the boron carbide substitution rate after the alkali treatment was 10%, the wet bulk density was 0.809g/cm ³ . The reason is that the boron carbide particles with the smaller particle size are finer, and the replacement rate is increased, so that the gap in the mixture can be filled more favorably, and the internal compact structure can be formed more favorably.

4) Microstructure

The microscopic morphology of the boron carbide ultra-high performance concrete was tested using a field emission scanning electron microscope (Hitachi S4800). During the test, the accelerating voltage is 15kV, a cured 28d test block is selected for testing, and the test block is dried in an oven at 50 ℃ for 2h before testing (see the result in FIG. 10).

As can be seen from fig. 10, for the reference group (B0), the hydration process of the material is relatively sufficient, and the cement-based material is relatively dense; when the non-alkalized boron carbide is doped into the material, the hydration process of the concrete is adversely affected, the microstructure of the cement-based material gradually becomes loose with the increase of the doping amount, the existence of a large number of pores can be seen, and the phenomenon becomes more obvious with the increase of the replacement rate, as shown in fig. 10 (b, c, d and e). When the alkalization treatment boron carbide (BC 10) is added, hydration products are stacked mutually, the structure is compact, the pores are obviously reduced, and excessive cracks and needle-shaped hydration products are not generated. The results show that after the industrial grade boron carbide is subjected to alkaline treatment, the generation of boric acid can be effectively reduced, and the influence of the boric acid on concrete hydration is reduced, which is the reason why the BC10 test block can keep good mechanical properties.

5) Pore structure

The pore structure of the ultra-high performance boron carbide concrete test block cured for 28 days was tested by mercury porosimeter (mike autopore ev 9600) with a maximum pressure of 421MPa and a contact angle of 130 ° (see fig. 11, 12 and table 6 for results).

TABLE 6 particle size distribution results in boron carbide ultra high performance concrete

As can be seen from fig. 11 and 12, for the boron carbide ultra-high performance concrete without alkali treatment, the porosity of the sample tends to decrease first and then increase with the increase of the replacement rate, the porosities of the B0, B2.5, B5, B7.5 and B10 samples respectively reach 12.54%, 10.51%, 10.57%, 12.30% and 13.55%, and the porosity of the boron carbide ultra-high performance concrete with alkali treatment is 10.24%, which is decreased by 18.34% compared with the control group B0 (see table 6). When the replacement rate is 2.5% and 5%, the boron carbide with smaller grain size fills the pores to form a tighter packing structure, and the total porosity is reduced; when the replacement rate is 7.5% and 10%, the generated borax hinders the hydration process, so that higher porosity is generated; when the boron carbide (BC 10) after alkaline treatment is added, the adverse effect generated by borax is eliminated, the compactness of the material is improved, and the porosity of the sample is reduced.

6) Thermogravimetric analysis

Thermogravimetric analysis was performed using a thermogravimetric analyzer (STA 449F5, tolz, germany). The specific test steps are as follows: firstly, soaking boron carbide ultra-high performance concrete samples in isopropanol for 7 days, then drying samples to be tested at 100 ℃ for 2 hours, crushing and grinding the samples, filtering and collecting powder with the particle size of less than 75 microns, heating the powder at 900 ℃ from room temperature (22 ℃), wherein the heating rate is 10 ℃/min, and using helium as a protective gas (see the result in FIG. 13).

As can be seen from the graph of fig. 13 (TG), the sample mass decreased at a constant rate as the temperature increased, which may be due to decomposition of certain components in the sample after heating. The mass loss of the sample increases with the increase of the substitution rate of boron carbide, because the boric acid reacts with the calcium hydroxide to generate bound water, and then the bound water is decomposed at high temperature.

From the graph of FIG. 13 (DTG), three peak curves are seen, the first peak of the mass change rate occurring at 60-250 ℃ corresponding to the decomposition of calcium silicate hydrate gel and ettringite, and it is noted that the mass change rate tends to increase with the increase of the boron carbide substitution rate, mainly because more CaO.B.is generated due to the increase of the boron carbide substitution rate ₂ O ₄ ·6H ₂ O, free water generated after heating and evaporated, causing an increase in the rate of change of mass; the second mass loss peak is at 380-500 deg.C, corresponding to decomposition of portlandite, and the rate of mass change tends to decrease as the rate of boron carbide substitution increases, mainly due to the formation of B (OH) by addition of boron carbide ₃ And Ca (OH) ₂ The reaction takes place, consuming Ca (OH) from the sample ₂ (ii) a The third mass loss peak is 600-750At this temperature, corresponds to the decomposition of the carbonic acid phase.

7) Heat of hydration

The hydration heat of the cements with different proportions of boron carbide incorporated in the cements was determined using a hydration heat tester model I-CAL4000/8000 for 72 hours, according to the Standard practice for measuring the hydration kinetics of mixtures of Hydraulic cements by isothermal calorimetry (ASTMc 1679), and in order to avoid mutual interference, a single heat sink was used for each sample (see results in FIG. 14).

As shown in fig. 14 (left), the total heat release of the non-alkali treated boron carbide doped samples tended to decrease with the increase of the boron carbide, and the total heat release of the B0, B2.5, B5, B7.5 and B10 samples were 0.258, 0.246, 0.236, 0.215 and 0.2102J, respectively, because the cement content decreased due to the increase of the cement replacement on the one hand, and the dilution effect generated by the boron carbide doping is stronger than the nucleation effect on the other hand, and the cement hydration and the total hydration heat release are limited by the boron carbide doped with the non-alkali treatment. The total exotherm for the alkalized boron carbide ultra-high performance concrete was 0.2262J for the same 10% boron carbide replacement. Compared with the total heat release of the boron carbide ultrahigh-performance concrete prepared after the replacement of the non-alkaline treated boron carbide, the total heat release is slightly increased.

It can be seen from fig. 14 (right) that the peak of the heat release rate is advanced with the increase of boron carbide, and the time for the heat release rate of B0, B2.5, B5, B7.5, B10, BC10 samples to reach the peak is 50, 48, 40, 39, 36, 34h respectively, because the industrial grade boron carbide has smaller particle size, and the fine particles accelerate the early hydration process of UHPC due to the nucleation effect. This phenomenon was verified in the initial setting and final setting experiments.

8) Mechanical Properties

Uniaxial compressive strength samples with the specification of 40mm multiplied by 160mm are prepared, and are placed in a standard curing box with the temperature (20 +/-1) DEG C and the humidity of 95% for curing for 7d and 28d after being demoulded. According to the Cement mortar Strength test method (ISO method) (GB/T17671-1999), an electrohydraulic servo pressure tester is used for testing the compressive strength at a loading rate of 2.4kN/s, and the average value of the compressive strength of 3 test blocks in each group is taken as a test value (see the result in FIG. 15).

As can be seen from FIG. 15, after the 7d and 28d curing, the compressive strength of the test block showed a tendency of rising first and then falling, the compressive strength of the cured 7d test block, B0, B2.5, B5, B7.5 and B10 test block were 85.2, 93.13, 94.6, 74 and 67.33MPa, respectively, and the compressive strength of the cured 28d test block was 103.5, 109.8, 110.3, 92.8 and 87.8MPa, respectively. When the substitution rates were 2.5% and 5%, the compressive strength increased due to the lower porosity of the test piece, which is the lowest porosity from the porosity test section. When the replacement rate is B7.5 and B10, the compressive strength is obviously reduced, mainly because the excessive addition of the industrial-grade boron carbide generates boric acid and other acidic substances by the contained boron oxide, and the reaction is generated with the generated calcium hydroxide, so that the hydration of cement is hindered, and the mechanical property of the sample is reduced. When the boron carbide (BC 10) subjected to alkaline treatment is used for replacement, the compressive strength of 7d and the compressive strength of 28d respectively reach 89.6Mpa and 112.1Mpa, and the strength reaches the highest, because the adverse effect of boric acid on the mechanical property of the test block is eliminated after the alkaline treatment, in addition, the maintenance of the concrete test block is more facilitated under an alkaline environment.

The porosity is an important factor influencing the compressive strength of the concrete, the influence of the porosity of the test block on the compressive strength of the boron carbide ultrahigh-performance concrete is shown in fig. 16, and as can be seen from fig. 16, the compressive strength of the test block is in a negative correlation with the porosity, the compressive strength of the test block is reduced when the total porosity is increased, the compressive strength is increased when the total porosity is reduced, the porosity of BC10 is the lowest, and the compressive strength is kept at the highest level.

The pores of cement-based materials are classified according to the size of the pore diameter and the influence generated, the pores of the gel with the diameter less than 10nm are considered to be related to the self-shrinkage, creep and permeability of the concrete and do not have adverse effect on the strength of the concrete, the pores with the diameter of 10-100nm are called less harmful pores and have small influence on the strength of the concrete, and the pores with the diameter more than 100nm have adverse effect on the mechanical property of the cement matrix. After 28 days of curing, the boron carbide ultra-high performance concrete prepared from the boron carbide treated by alkalization has the porosity of 0-10nm and the porosity of 10-100nm of 11.81 percent and 50.7 percent, which are the highest level in each sample, and the total porosity of the sample of 10.24 percent, which is the lowest level in each sample. Therefore, after the industrial-grade boron carbide is subjected to alkalization treatment, the pore structure can be obviously improved, the total porosity is reduced, the number of harmful pores is reduced, and the number of harmless pores is increased.

Test example 1

Neutron shielding calculation method

1) Monte Carlo (MCNP) software shielding simulation method

MCNP is a simulation calculation software of particle transmission based on a Monte Carlo method, and is widely used for neutron, photon and electron radiation shielding calculation. The method is characterized in that a FORTRAN programming principle is adopted to realize the fusion of the FORTRAN programming principle and MCNP software, a neutron shielding simulation calculation model of the ultra-high performance concrete containing boron carbide, with the test block length and width of 10 x 10cm and the thickness of 1cm, 5cm, 10cm and 15cm, is respectively constructed, parameters such as cross sections of particle interaction, mass percentages of elements, density of materials and the like are input, calculation is carried out through the following formula, and the shielding effect of an object on particles is calculated and obtained. A neutron shielding schematic built in software, as shown in fig. 17. In the process of carrying out shielding simulation, the shielding test block material is assumed to be uniform, free of pores, free of shrinkage and expansion, the geometric dimension is constant, and the density of the boron carbide test block with different replacement rates can be obtained through indoor tests and is respectively 2.31, 2.35, 2.37, 2.31, 2.3 and 2.35g/cm ³ 。

An SDEF universal source card is adopted in software, an isotropic neutron point source is used as a radioactive source, and a neutron shielding process is reproduced; the number of simulated particles in the radiation-proof shield is 10 ⁷ And the errors are controlled within 0.5 percent so as to ensure the simulation precision. Considering that the F1 counter can calculate the total count on a certain surface of the detector, the energy section is counted by adopting the F1 counting card, so that the penetration amount of the neutron irradiation device without a test block can be better obtained. It is worth mentioning that the penetration rate is mainly used in the research to evaluate the shielding effect of the boron carbide ultra-high performance concrete, and the smaller the transmittance is, the better the shielding performance is.

2) Research results show that neutrons can generate attenuation to a certain degree when penetrating through a shielding layer, and the change rule of the neutron transmittance in the shielding layer accords with an exponential attenuation rule, and can be approximately described by an exponential function, namely the neutron transmittance:

wherein N-projects an over-neutron flux; n is a radical of ₀ -a raw neutron flux; N/N ₀ -neutron transmission, dimensionless; l-material thickness, cm; sigma-shaped _R Material neutron macroscopically divided cross-section, cm ^-1 ，I ₀ -neutron source intensity, R-source to measurement point distance.

Neutron macroscopic separation section (Sigma) _R (cm ^-1 ) Is an important parameter for evaluating the neutron shielding performance of the material and mainly depends on the neutron microscopic reaction section and density of the material, and the theoretical formula of the neutron macroscopic separation section is as follows:

in the formula, ρ _s Density of the mixture, g/cm ³ ；ρ _i Density of the ith element, W _i -mass fraction of the ith element.

And evaluating the efficiency of the concrete mixture for absorbing and eliminating thermalization neutrons by calculating a macroscopic absorption section. The macroscopic thermal neutron mass absorption cross section can be expressed as:

wherein (sigma) _a ) i-ith element thermal neutron microAbsorption cross section (cm) ² Atom(s); m _i -a molar mass; n is a radical of _A -avogalois constant, (atoms/mol); atomic density of N-element, (atoms/cm 3); sigma _a Macroscopic thermal neutron mass absorption cross section, (cm) ² /g)。

The initial half-absorbed thermal neutron intensity (HVL) was calculated using a formula _thn ) Desired thickness, HVL _thn Half-ply value, as follows:

1) Influence of boron carbide doping on neutron shielding performance

The neutron shielding effect under irradiation of different energies is shown in fig. 18. It can be seen that boron carbide content has a significant effect on slow neutrons, but the shielding effect on fast neutrons is not significant. Taking a test block with the thickness of 5cm and without alkaline treatment as an example, the neutron shielding effect can refer to the data in the table 7, when the neutron energy is 0.1ev and the boron carbide replacement rate is 0, 2.5, 5, 7.5 and 10 percent, the transmissivity of the test block is 0.25, 0.05, 0.007, 0.002 and 0.0004 respectively, and the boron carbide content has obvious effect on slow neutron shielding; when an energy of 100ev was used as the neutron source, the boron carbide substitution rates were 0, 2.5, 5, 7.5, and 10%, the transmittances of the test pieces were 0.33, 0.29, 0.26, 0.25, and 0.24, respectively, and the boron carbide content was significantly reduced for the neutron shielding effect.

TABLE 7 shielding parameters of test blocks of thickness 5cm at different energies

When the neutron energy is larger, the shielding effect can be effectively improved by increasing the thickness of the shielding layer, and the shielding effect is 1 multiplied by 10 ⁴ For example, the neutron source of eV is such that when the boron carbide content is in the range of 0 to 10%, and the thickness of the test piece is 1cm, 5cm, 10cm and 15cm, the neutron transmittances are 0.77 to 0.12, 0.76 to 0.09, 0.76 to 0.0.08, 0.75 to 0.08 and 0.76 to 0.07, respectively. It is worth mentioning that, under the same doping amount of boron carbide,the shielding effect of the alkali-treated boron carbide sample is not much different from that of the industrial boron carbide-containing test block, which shows that the alkali-treated boron carbide does not influence the neutron shielding effect of the sample.

The reason why the boron carbide has better shielding performance is that the boron carbide has a larger microscopic section for slow neutrons with low energy and does not have obvious advantages for the microscopic section of fast neutrons with high energy compared with other elements. As shown in FIG. 19, at 1X 10 ^-4 -1×10 ² In the energy range of the MeV neutrons, ¹⁰ the microscopic section of B is far greater than the rest of H, ca and other elements, and the neutron energy is greater than 1 × 10 ³ The microscopic sections of eV and B are very close to those of other elements, so that boron carbide can be mainly used for shielding low-energy slow neutrons, and the neutron shielding effect of the test block cannot be obviously changed by the alkalization treatment.

2) Half value layer and macroscopic section

The half-value layer versus neutron energy is shown in fig. 20, and HVL (corresponding to the thickness of the shielding layer required to eliminate half the initial neutron intensity, flux or dose) increases progressively with increasing neutron energy, corresponding to the neutron energy versus neutron macroscopic cross-section (fig. 21). And 10 ^-8 Half-value layers of B0 (1.15 cm), B2.5 (0.56 cm), B5 (0.28 cm), B7.5 (0.21 cm), B10 (0.17 cm) and BC10 (0.17 cm) under Mev energy irradiation had a neutron energy of 10 ^-2 For Mev, the half-value layers of B0 (3.28 cm), B2.5 (3.17 cm), B5 (3.12 cm), B7.5 (3.15 cm), B10 (3.12 cm) and BC10 (3.12 cm) were increased by 65%, 82%, 91%, 93%, 95% and 94%, respectively. The half value layer gradually increases with the increase of energy, and the shielding effect is better with the increase of the yield of boron carbide.

In the entire energy spectrum, sigma _R The value (macroscopic section of the neutron) decreases gradually with increasing energy. However, when the energy is less than 1keV, Σ, compared to the rest of the energy spectrum _R The value drops very quickly. This rapid drop is due to the fact that in the energy range of 0.01-1keV, absorption occurs mainly, and the neutrons, after colliding with the nuclei in this region, have the energy to form complex nuclei with mass number A +1The amount, absorption cross section, is proportional to the inverse of the energy opening. Whereas in the energy range 1-100keV elastic scattering interactions dominate the neutron interaction process. Furthermore, sigma _R And the numerical values of concrete show that sigma varies from 0.01keV to 10-2MeV due to the energy of B0, B2.5, B5, B7.5, B10 and BC10, respectively _R The values range from 0.604 to 0.072, 1.234 to 0.073, 2.491 to 0.074, 3.256 to 0.072 and 4.051 to 0.072. (Sigma) _R )B0<B2.5<B5<B7.5<The trend of B10 is determined by the chemical composition of the concrete, which can be attributed to the dependency on boron element. Under low energy, the microscopic reaction section of 11Bde is far larger than other elements, so that the larger the boron content, the larger the macroscopic section of the neutron, and the smaller the HVL. The trend for HVL is the same as for neutron transmission, Σ _R The trend of (a) is exactly opposite to that of neutron transmission, and in the whole spectrum, B10 and BC10 have the highest macroscopic cross-sectional values, have the lowest transmission and the lowest half-value layer, which also indicates that in the prepared concrete, the neutron absorption effect is the best among them.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included therein.

Claims

1. The boron carbide ultrahigh-performance concrete is characterized in that the equal volume of boron carbide parts is used for replacing cement in the ultrahigh-performance concrete to obtain the boron carbide ultrahigh-performance concrete; the substitution rate of the substitution with the same volume is less than or equal to 10 percent and is not 0; the boron carbide is industrial grade boron carbide, and the particle size of the boron carbide is 0.1-70 microns;

the boron carbide needs to be subjected to alkaline treatment before use, and the treatment method comprises the following steps: dissolving boron carbide in water, adding calcium hydroxide after the boron carbide is dissolved until the pH value is 10.7, drying and filtering to obtain boron carbide after alkaline treatment;

the ultra-high performance concrete comprises the following raw materials in parts by weight: 639-800 parts of cement, 185 parts of fly ash, 148 parts of silica fume, 289-767 parts of fine aggregate, 40 parts of water reducing agent and 198 parts of water.

2. The ultra-high performance boron carbide concrete according to claim 1, wherein the cement is p.ii 52.5 portland cement.

3. The boron carbide ultra-high performance concrete according to claim 1, wherein the fly ash is a first grade fly ash.

4. The boron carbide ultra-high performance concrete according to claim 1, wherein the fine aggregate is natural river sand with a diameter of 0-2.56mm.

5. The boron carbide ultrahigh-performance concrete of claim 1, wherein the water reducing agent is a polycarboxylic acid-based high-efficiency water reducing agent, the water reducing rate is 30%, and the solid content is 30%.

6. A method for preparing the boron carbide ultra-high performance concrete according to any one of claims 1 to 5, comprising the steps of:

7. Use of the boron carbide ultra-high performance concrete as defined in any one of claims 1 to 5 as a neutron shielding radiation protection material.

8. The use of claim 7, wherein said neutron shielding radiation protection material is a slow neutron shielding radiation protection material.