Method for improving high temperature resistance of engineering cement-based composite material

Abstract

The invention discloses a method for improving high temperature resistance of an engineering cement-based composite material, and belongs to the technical field of cement-based composite materials. The coral sand powder is used for replacing quartz powder in the engineering cement-based composite material at the volume replacement rate of 0-50%, a proper amount of the coral sand powder is used for replacing the quartz powder in the engineering cement-based composite material, the binding force of an interface transition zone (ITZ) is improved due to the introduction of the coral sand powder, the comprehensive performance of the engineering cement-based composite material is effectively improved, and the engineering cement-based composite material is suitable for being used as an engineering cement-based composite material. Particularly, the material shows relatively good mechanical properties in a high-temperature environment. Moreover, when the replacement rate of the coral sand powder is 25%, the fluidity of the engineering cement-based composite material is good, the residual compressive strength and the axial tensile property are most excellent, and the compressive strength after exposure at 400 DEG C is 60.6 MPa.

Description

Technical Field

[0001] The invention belongs to the technical field of cement-based composite materials, and in particular relates to a method for improving the high-temperature resistance of engineering cement-based composite materials. Background Art

[0002] In recent years, with the accelerated development of oceanic island and reef construction, traditional cement-based materials, due to their inherent brittleness and limited ductility, have become unable to meet the long-term service requirements in harsh marine environments. In this context, engineered cement-based composites (ECCs) have become a current research hotspot, demonstrating significant advantages in complex load-environment coupling scenarios due to their unique strain-hardening behavior and superior deformation capacity (ultimate tensile strain typically exceeding 3%).

[0003] However, using ECC for island construction requires large quantities of raw materials, which are expensive to transport from the mainland to offshore locations and are also subject to seasonal fluctuations. In tropical and subtropical waters, coral reef ecosystems form unique layered calcium carbonate aggregates through calcareous deposition. After crushing and grading, coral aggregate can be used as an alternative to quartz aggregate in island construction. Coral aggregate is typically characterized by irregular shapes, rough surfaces, complex particle shapes, high porosity, and low strength and stiffness.

[0004] In actual engineering, building materials are often exposed to extreme temperature environments such as fire and high temperature climate, resulting in significant deterioration of mechanical properties. In addition, island strategic facilities may be threatened by sudden events such as explosions, and it is necessary to focus on the impact of high temperature, high strain rate and their coupling on structural safety. Since ECC has excellent crack control capabilities, it usually plays a role in the form of reinforcement or superposition in the tensile zone of the component. When the building structure faces a fire, the tensile zone of the component is the first to be affected by extreme high temperature. However, the performance response of coral aggregate to ECC under extreme high temperature is not yet clear. Its micro-activity may improve the interface effect between the matrix and the aggregate, but the decomposition of calcium carbonate components at high temperature will also cause significant volume changes. This coupling effect may change the high temperature damage mode of ECC, thereby affecting the high temperature resistance of ECC. Therefore, how to use coral aggregate to improve the high temperature resistance of engineering cement-based composites has become a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention proposes a method for improving the high temperature resistance of engineering cement-based composite materials.

[0006] To achieve the above objectives, the present invention provides the following technical solutions:

[0007] The present invention provides a method for improving the high-temperature resistance of an engineering cement-based composite material. The method comprises replacing quartz powder with an average particle size of 100-150 μm in the engineering cement-based composite material with coral sand powder with a median particle size of 150-250 μm at a volume replacement rate of 0-50%. The particle size ratio of the coral sand powder to the quartz powder is (1.2-2.0):1, the fineness modulus of the coral sand powder is 1.9-2.1, and the value of the volume replacement rate is not 0%.

[0008] Furthermore, the specific surface area of the coral sand powder is 450-550m 2 / kg, the moisture content of the dried coral sand powder is less than 0.5%; the density of the coral sand powder is 2.78g / cm 3 The present invention uses coral sand powder having a larger particle size than quartz powder because the larger particles of coral sand powder decompose relatively slowly at high temperatures, thereby delaying the volume shrinkage and structural degradation caused by high temperatures.

[0009] Furthermore, the coral sand powder is obtained by jaw crushing and ball milling coral sand. The main component of the coral sand powder is CaCO3, which is mainly composed of aragonite and a small amount of calcite.

[0010] Furthermore, the quartz powder has a particle size of 100-150 μm accounting for more than 90%; the silicon dioxide content of the quartz powder is ≥98.5%; the density of the quartz powder is 2.65 g / cm 3 .

[0011] Furthermore, the volume replacement rate is negatively correlated with the median particle size of the coral sand powder, specifically:

[0012] When the volume replacement rate is 15-50%, coral sand powder with a median particle size of 150-200 μm is used to replace quartz powder;

[0013] When the volume replacement rate is 0-15%, coral sand powder with a median particle size of 200-250 μm is used to replace quartz powder, and the value of the volume replacement rate is not 0.

[0014] The present invention provides a high-temperature resistant engineering cement-based composite material based on coral sand powder, which is composed of the following raw materials per unit volume mass: cement (PC) 600kg / m 3 、Fly ash (FA) 600kg / m 3 , quartz powder (QP) 216-432kg / m 3 , Coral sand powder (CSP) 0-226kg / m 3 , Polyethylene fiber (PE) 19.4kg / m 3 , water reducing agent (HRWR) 3.6kg / m 3、Defoaming agent (DF) 1.8kg / m 3 and seawater (ASW) 348kg / m 3 ; and the amount of the coral sand powder is not 0.

[0015] Furthermore, the seawater is artificial seawater; the components of the artificial seawater are: NaCl 24.53g / L, MgCl2·6H2O 5.2g / L, Na2SO4 4.09g / L, CaCl2 1.16g / L, KC1 0.695g / L, NaHCO3 0.201g / L, KBr 0.101g / L and H3BO3 0.027g / L.

[0016] Furthermore, the polyethylene fiber has a length of 12 mm, a diameter of 24 μm, and an elastic modulus greater than 116 GPa.

[0017] Furthermore, the cement is P.II 52.5R Portland cement.

[0018] Furthermore, the fly ash is Class F fly ash.

[0019] The main oxides in cement and fly ash are CaO, SiO2, Al2O3 and Fe2O3, and the total mass of CaO, SiO2, Al2O3 and Fe2O3 accounts for about 90%.

[0020] Furthermore, the water reducer is a polycarboxylic acid-based high-efficiency water reducer; the solid content of the polycarboxylic acid-based high-efficiency water reducer is 49-51%.

[0021] Furthermore, the defoaming agent is a polyether-modified silicon defoaming agent.

[0022] The present invention also provides a method for preparing the high-temperature resistant engineering cement-based composite material based on coral sand powder according to the above technical solution, comprising the following steps:

[0023] Cement, fly ash, quartz powder, coral sand powder and a defoaming agent are stirred and mixed to obtain a dry material mixture; a mixture of seawater and a water reducer is added to the dry material mixture and stirred to obtain a slurry; polyethylene fiber is added to the slurry to obtain the high-temperature resistant engineering cement-based composite material based on coral sand powder.

[0024] Furthermore, the preparation method of the high-temperature resistant engineering cement-based composite material based on coral sand powder comprises the following steps:

[0025] (1) Cement, fly ash, quartz powder, coral sand powder and defoamer were added to a mixer and stirred at a speed of 75 r / min for 3 min to obtain a dry material mixture;

[0026] (2) adding a mixture of seawater and a water reducer to the dry material mixture within 1 minute, and then stirring at a speed of 135 rpm for 1-3 minutes to obtain a slurry;

[0027] (3) slowly adding polyethylene fiber to the slurry at a rotation speed of 75 r / min, completing the addition of the polyethylene fiber within 3 minutes, and then stirring at a rotation speed of 135 r / min for 2-3 minutes to ensure that the fibers are evenly distributed, thereby obtaining the high-temperature resistant engineering cement-based composite material based on coral sand powder.

[0028] Compared with the prior art, the present invention has the following advantages and technical effects:

[0029] The present invention uses coral sand powder to partially replace quartz powder in engineering cement-based composite materials, and the fluidity of the engineering cement-based composite materials is significantly improved. In particular, when the coral sand powder replacement rate is 25%, the material exhibits optimal fluidity. At this time, the residual compressive strength and axial tensile properties of the material are also the best. After exposure to 400°C, the compressive strength is 60.6 MPa, with only a loss of 15.3%. This shows that the present invention effectively improves the mechanical properties of the cement-based composite materials by introducing an appropriate amount of coral sand powder.

[0030] The present invention adopts an appropriate amount of coral sand powder to replace the quartz powder in the engineering cement-based composite material. The introduction of coral sand powder helps to improve the bonding strength of the interface transition zone (ITZ), effectively improving the comprehensive performance of the engineering cement-based composite material, especially showing better mechanical properties under high temperature environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0032] Figure 1 The following are SEM images of coral sand powder in Examples and Comparative Examples of the present invention;

[0033] Figure 2 Schematic diagrams of the structures of prismatic specimen (a), cubic specimen (b), and dumbbell-shaped specimen (c);

[0034] Figure 3 The effect of different CSP replacement rates on the fluidity of SCECC paste;

[0035] Figure 4 The water absorption rate of SCECC with different CSP replacement rates after exposure to 200℃ for 48h changes with time the changing trend of

[0036] Figure 5Figure 1 shows the actual image of the heating device (a), the heating curve of the residual mechanical properties test (b), and the heating curve of the high-temperature spalling test (c).

[0037] Figure 6 The spalling degree of SCECC with different CSP replacement rates at 0 and 14 days, where (a) and (e) are R-0, (b) and (f) are R-25, (c) and (g) are R-50, (d) and (h) are R-100, (a)-(d) are 0 days, and (e)-(f) are 14 days;

[0038] Figure 7 is the mass loss of SCECC with different CSP replacement rates;

[0039] Figure 8 The changes in the residual compressive strength of SCECC with different CSP replacement rates after exposure to different temperatures, where (a) is the compressive strength and (b) is the compressive strength loss rate;

[0040] Figure 9 is the axial tensile failure mode of SCECC with different CSP replacement ratios;

[0041] Figure 10 The average crack width (a) and crack density (b) of SCECC with different CSP replacement rates at different temperatures;

[0042] Figure 11 Axial tensile stress-strain curves of SCECC with different CSP replacement rates at 20℃ and 200℃;

[0043] Figure 12 Ultimate tensile strength (a) and ultimate tensile strain (b) of SCECC with different CSP replacement rates at different temperatures;

[0044] Figure 13 EDS images of the ITZ between the aggregate and the matrix in SCECC with different CSP replacement rates under a scanning electron microscope at room temperature. DETAILED DESCRIPTION

[0045] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0046] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] Unless otherwise specified, the raw materials in the examples of the present invention were purchased from commercial sources.

[0048] The SEM images of the coral sand powder in the embodiments of the present invention and the comparative examples are shown in FIG. Figure 1 .

[0049] The physical and mechanical parameters of the polyethylene fibers in the examples of the present invention and the comparative examples are shown in Table 1.

[0050] Table 1 Physical and mechanical parameters of polyethylene fibers

[0051]

[0052] The components of the artificial seawater in the examples of the present invention and the comparative examples are shown in Table 2.

[0053] Table 2 Composition of artificial seawater

[0054] Element NaCl <![CDATA[MgCl2·6H2O]]> <![CDATA[Na2SO4]]> <![CDATA[CaCl2]]> KC1 <![CDATA[NaHCO3]]> KBr <![CDATA[H3BO3]]> Concentration (g / L) 24.53 5.2 4.09 1.16 0.695 0.201 0.101 0.027

[0055] In the examples and comparative examples of the present invention, the fly ash is Class F fly ash; the water reducer is a polycarboxylic acid-based high-efficiency water reducer with a solid content of 49%; and the defoamer is a polyether-modified silicon defoamer.

[0056] Example 1-2 and Comparative Example 1-2

[0057] A method for improving the high temperature resistance of an engineering cement-based composite material, comprising replacing quartz powder with an average particle size of 100-150 μm in the engineering cement-based composite material with coral sand powder with a median particle size of 150-200 μm at volume replacement rates of 0, 25%, 50%, and 100%, respectively. The particle size ratio of the coral sand powder to the quartz powder is (1.2-2.0):1, the fineness modulus of the coral sand powder is 1.9-2.1, and the specific surface area of the coral sand powder is 450-550 m 2 / kg, the moisture content of the dried coral sand powder is less than 0.5%; the particles with a particle size of 100-150μm account for more than 90% of the quartz powder, and the silicon dioxide content of the quartz powder is ≥98.5%.

[0058] A high-temperature resistant engineering cement-based composite material based on coral sand powder is composed of the following raw materials: cement (PC), fly ash (FA), quartz powder (QP), coral sand powder (CSP), polyethylene fiber (PE), water reducing agent (HRWR), defoaming agent (DF) and seawater (ASW);

[0059] The raw material ratios of Example 1-2 and Comparative Example 1-2 are shown in Table 3.

[0060] The preparation method of high-temperature resistant engineering cement-based composite material based on coral sand powder comprises the following specific steps:

[0061] (1) Cement, fly ash, quartz powder, coral sand powder and defoamer were added to a mixer and stirred at a speed of 75 r / min for 3 min to obtain a dry material mixture;

[0062] (2) adding a mixture of seawater and a water reducer to the dry material mixture within 1 minute, and then stirring at a speed of 135 rpm for 3 minutes to obtain a slurry;

[0063] (3) Polyethylene fibers were slowly added to the slurry at a rotation speed of 75 r / min, and the addition of polyethylene fibers was completed within 3 min. The slurry was then stirred at a rotation speed of 135 r / min for 3 min to ensure uniform distribution of the fibers, thereby obtaining a high-temperature resistant engineering cement-based composite material based on coral sand powder.

[0064] Table 3 Raw material ratios of Examples 1-2 and Comparative Examples 1-2 (kg / m 3 )

[0065]

[0066] Note: Each group of materials is named RX, where X represents the volume replacement rate of CSP to QP.

[0067] Performance testing:

[0068] 1. Liquidity test

[0069] According to ASTM C1437-2013, the fluidity of fresh slurry was tested using the platform method: a custom-made conical mold (top Φ70mm, bottom Φ100mm, height 50mm) was vibrated in a standard manner, and the maximum diameter of the slurry in the orthogonal direction was measured. The average of the two values was taken as the fluidity index. The results are shown in the table. Figure 3 .

[0070] Figure 3 The effect of different CSP replacement rates on the fluidity of SCECC paste. Figure 3 It can be seen that, unlike the traditional coral aggregate whose fluidity continues to decrease after being added to engineering cement-based composites, the fluidity of SCECC shows a trend of first increasing and then decreasing as the CSP replacement rate increases. Figure 3 As shown in Figure 2, when the CSP replacement rate increases from 0 vol% to 25 vol% and 50 vol%, the fluidity of R-25 and R-50 increases to 84% and 81%, respectively. However, when the CSP replacement rate further increases to 100 vol%, the fluidity of R-100 decreases to 75%. 2+Strong ion adsorption imbues its surface with positive charges, causing electrostatic repulsion between CSP particles and other particles in the slurry. Therefore, adding a small amount of CSP can improve the dispersion of aggregate particles in the slurry. Compared with QP, CSP has a more irregular shape and a rougher surface. Adding a large amount of CSP will increase frictional resistance between particles and enhance the adsorption of free water. These factors have a negative impact on the fluidity of SCECC.

[0071] 2. Water absorption test

[0072] Use 40mm×40mm×160mm prism specimen (see Figure 2 Water absorption was tested as described in part (a) of the test. Before testing, the specimens were heated to 200°C at a rate of 1°C / min and held in an oven for 48 hours to avoid adverse thermal stress. After heating, the specimens were allowed to cool naturally. The top surface was sealed with plastic film and the sides with epoxy resin to ensure that only the bottom surface was exposed to water. The test lasted 3 hours, with mass changes recorded every 15 minutes for the first 2 hours and every 30 minutes for the last hour.

[0073] The water absorption calculation formula is shown in formula (1):

[0074]

[0075] Among them, m t is the mass of the sample changing with time, g; a is the exposed area of the sample, mm 2 ; d is the density of water, g / mm 3 ;The unit of water absorption I is mm.

[0076] The adsorption coefficient K is expressed by I and time The linear fitting is obtained as shown in formula (2):

[0077]

[0078] Where c is the fitting constant.

[0079] Figure 4 The water absorption rate of SCECC with different CSP replacement rates after exposure to 200℃ for 48h changes with time The changing trend of Figure 4 It can be seen that the adsorption capacity of the specimen for water gradually weakens over time. In addition, the slope of the curve (adsorption coefficient K) gradually decreases with the increase of CSP replacement rate, which means that the adsorption capacity of the specimen gradually decreases. Specifically, the adsorption coefficient of the R-0 group is The permeability of SCECC is 18%, 37%, and 97% higher than that of R-25, R-50, and R-100, respectively. This indicates that the permeability of SCECC decreases significantly with increasing CSP replacement rate, a phenomenon likely related to the rough surface characteristics of CSP. Because PE fibers persist after melting, CSP forms a stronger bond with the melt than the smoother QP, causing it to adhere to the pores, resulting in a reduction in effective pore size and permeation paths, thereby reducing permeability.

[0080] 3. Performance test at high temperature

[0081] 3.1 Heating system

[0082] After curing for 28 days, the specimens were dried naturally for 30 days before heating to equalize the moisture content. The high temperature exposure instrument used was a Guangshu GS-3S-III type programmable temperature controlled muffle furnace (see Figure 5 (a) in the figure), the PID temperature control system has an accuracy of ±1°C, and the thermocouple in the furnace can monitor the temperature in the furnace in real time, and its temperature uniformity is ≤±5°C. The heating process is: heating to the target temperature (200°C / 400°C / 600°C / 800°C) at a rate of 8°C / min. At the same time, in order to make the temperature inside and outside the specimen uniform, when the temperature in the furnace reaches the target temperature, the cube specimen is kept warm for 120 minutes, and the dumbbell specimen is kept warm for 30 minutes, and then naturally cooled to room temperature for subsequent mechanical properties testing. The heating curve of the residual mechanical properties test is shown in Figure 5 Part (b) of the Figure 5 Part (c) shows the heating curve used for the spalling behavior test, which is heated according to the ISO 834 fire curve.

[0083] 3.2 Peeling behavior test

[0084] Use 40mm×40mm×160mm prism specimen (see Figure 2 (a) in the ISO 834 standard fire curve (see Figure 5 The samples (part (c) in the figure) were exposed to high temperature for 1 hour, allowed to cool naturally in the furnace for 1 hour, and then removed from the furnace. The degree of surface peeling was visually assessed, and the mass loss rate before and after heating was calculated.

[0085] 3.2.1 Peeling behavior

[0086] Figure 6 Figure 2 shows the peeling behavior of SCECC under different CSP replacement rates, where (a) and (e) are R-0, (b) and (f) are R-25, (c) and (g) are R-50, (d) and (h) are R-100, (a)-(d) are 0 days, and (e)-(f) are 14 days. Figure 6As shown in sections (a)-(d) of the figure, after 1 hour of exposure to the ISO 834 fire curve (i.e., day 0), none of the test groups exhibited explosive spalling. This is significantly different from the spalling behavior of conventional concrete and PE fiber reinforced concrete subjected to elevated fire temperatures in previous studies. For example, one specimen in the R-0 group exhibited surface spalling, while the remaining specimens only developed numerous surface cracks. Each specimen in the R-100 group exhibited deeper spalling. Clearly, the extent of SCECC cracking and spalling due to high temperatures increases with increasing CSP replacement rate. Under fire exposure, the PE fibers within the specimens melted, losing their bridging function. When vapor migration was restricted, resulting in vapor pressure buildup exceeding the bearing capacity, the specimens, unable to escape immediately, experienced cracking and even spalling. Furthermore, decomposition of the CSH gel in the matrix and the CaCO₃ in the CSP led to interfacial bond failure, also causing spalling. The above mechanism explains why the spalling behavior of SCECC is aggravated by the increase in CSP substitution rate. It also shows that the steam escape mechanism has a significant effect on the spalling behavior of concrete materials. The decomposition of CSP at high temperature aggravates the spalling of SCECC.

[0087] In addition, when the CSP replacement rate is large, the specimen will experience secondary peeling after cooling for a period of time. Figure 6 As shown in sections (e)-(h), R-100 disintegrated after 14 days of exposure to natural conditions, while the R-50 group also exhibited new spalling on its surface. This phenomenon is attributed to the reaction of calcium oxide, produced by the decomposition of calcium carbonate, with water vapor in the air to form calcium hydroxide (CaO + H2O → Ca(OH)2), which expands in volume. Although SCECC with a high CSP volume replacement rate disintegrated after a period of storage, it did not experience severe explosive spalling when exposed to fire, which helped protect the steel bars from being directly exposed to flames, softening them, and ultimately causing brittle failure of the structure. Therefore, when using SCECC to construct island facilities with potential fire environments, it is not advisable to use too much CSP to replace quartz powder.

[0088] 3.2.2 Quality loss

[0089] Figure 7 is the mass loss of SCECC with different CSP replacement rates. Figure 7It can be seen that with increasing CSP replacement rates, the mass loss of SCECC gradually increases from 18.7% to 23.3%. Although this is consistent with the aforementioned spalling behavior, the degree of SCECC spalling becomes more severe with increasing CSP replacement rates, resulting in greater mass loss. However, the worsening mass decline is primarily attributed to the decomposition of calcium carbonate, the main component of CSP, into calcium oxide and carbon dioxide at 600-800°C, with the carbon dioxide disappearing as gas. Notably, the mass loss of SCECC is close to 20%, which is caused by water loss and decomposition of hydration products in the material. On the one hand, the increase in temperature drives the outward migration of water in the form of vapor. On the other hand, calcium silicate hydrate (CSH gel) begins to decompose at 560°C and is nearly complete at 800°C. In addition, PE fibers undergo thermal decomposition at 300-350°C, converting into gaseous escaping, exacerbating the mass loss.

[0090] 3.3 Residual compressive strength test

[0091] Refer to ASTM C109 / C109M standard, use 50mm cube specimen (see Figure 2 The residual compressive strength test was performed in part (b) of Figure 1. A closed-loop electro-hydraulic servo pressure test system was used to apply a compressive load at a constant rate of 1.8 kN / s in force control mode.

[0092] 3.3.1 Residual compressive strength

[0093] Figure 8 The residual compressive strength changes of SCECC with different CSP replacement rates after exposure to different temperatures, where (a) is the compressive strength and (b) is the compressive strength loss rate. Figure 8As can be seen in the figure, the residual compressive strength of SCECC decreases gradually as the temperature increases from room temperature (20°C) to 800°C. At 200°C, the strength decreases for R-0, R-25, R-50, and R-100 by 1.5%, 1.2%, 4.2%, and 5.8%, respectively. This decrease is only slight. This is due to the continuous hydration of unhydrated cement and fly ash in the high-temperature steam environment, which mitigates the strength degradation caused by free water evaporation and thermal deformation mismatch between particles. As the temperature rises to 400°C, the strength of SCECC gradually deteriorates. For example, the strength of the R-0 group drops to 51.8 MPa (a decrease of 24.7%) at 400°C. This is due to the dehydration of the CSH gel and the thermal decomposition of the PE fibers, which degrade the internal pore structure of the material. When the temperature rises to 600°C, the strength of the R-0 group drops to 33.6 MPa (a decrease of 51.1%). This is mainly due to the initial decomposition of the CSH gel. Notably, the strength of the R-100 group dropped to 23.9 MPa (a 62.7% decrease), an 11.6% decrease compared to the R-0 group. This is because the CaCO3 in the CSP decomposes at high temperatures to produce CaO and CO2. This process not only causes the material to shrink in volume but also forms a connected pore structure, significantly reducing the matrix's load transfer capacity. When the temperature reaches 800°C, the residual strength of the R-0 group is only 14.3 MPa (a 79.2% decrease), while the strength of the R-100 group drops to 11.5 MPa (a 82.0% decrease). At this point, the CSH gel has almost completely decomposed, and the decomposition of the calcium carbonate in the CSP has been essentially completed. The remaining brittle CaO phase and the high porosity together lead to a near-complete loss of strength.

[0094] In addition, regardless of the temperature, as the CSP replacement rate increases, the compressive strength of SCECC shows a trend of first increasing and then decreasing. Figure 8 As can be seen in part (a), the residual compressive strength of R-25 always performs best, thanks to the early pore structure optimization brought about by the micro-activity of CSP. However, when the CSP replacement rate reaches 100%, the high crushing value and low strength of CSP make its compressive strength value the lowest at each temperature. It is worth noting that the strength of the R-25 group after exposure to 400℃ for 2h is 60.6MPa, which only decreases by 15.3%, still within the range of high-strength concrete. When the temperature rises to 600℃ and 800℃, the porosity of the group with high CSP replacement rate increases sharply due to the decomposition of CaCO3, and the strength drops sharply.

[0095] Table 4 Changes in residual compressive strength of SCECC with different CSP replacement rates after exposure at different temperatures

[0096]

[0097] Note: The numbers in brackets in the table represent standard deviations.

[0098] 3.4 Residual axial tensile properties test

[0099] Dumbbell-shaped specimens (see Figure 2 The residual axial tensile properties test was carried out in part (c) of the test. The test system consists of a computer-integrated servo-controlled universal testing machine with a maximum load capacity of 100kN. The loading process adopts displacement control mode, and the axial load is applied at a constant rate of 0.5mm / min. In order to accurately measure the deformation characteristics of the specimen, two contact displacement meters with a range of 10mm are symmetrically arranged in the middle section with a gauge length of 80mm, and the deformation data are continuously collected at a sampling frequency of 1Hz. The load signal is obtained by the high-precision force sensor built into the testing machine, and the synchronous deformation data is recorded by an independent high-performance static data acquisition system. In addition, in order to observe the development of cracks in real time during loading, a thin layer of white paint is applied to the surface of the specimen, and the crack evolution is monitored in real time at a frequency of 0.2Hz during loading.

[0100] 3.4.1 Failure mode

[0101] Figure 9 The axial tensile failure modes of SCECC with different CSP replacement rates are shown in Figure 2. Figure 9 It can be seen that all groups exhibited significant multi-crack cracking, with no clear brittle failure. At both the ambient temperature of 20°C and the elevated temperature of 200°C, R-25 and R-50 exhibited more cracks than R-0 and R-100. For SCECCs with the same CSP replacement ratio, the number of cracks at 200°C was lower than at the ambient temperature of 20°C.

[0102] 3.4.2 Crack width and crack density

[0103] In order to quantitatively analyze the effects of CSP replacement rate and temperature on the tensile failure mode of SCECC, Figure 9 Binarization was performed, and a reference line parallel to the axial tensile direction was established. The number of intersections between the cracks and the reference line and the pixel length were counted and measured to obtain the crack width and crack density. Figure 10 The average crack width and crack density of SCECC at different temperatures were statistically analyzed.

[0104] Figure 10 The average crack width (a) and crack density (b) of SCECC with different CSP replacement rates at different temperatures. Figure 10As can be seen in part (a) of the figure, the crack width shows a significant increasing trend with increasing temperature. At 20°C, the crack width of the R-0 group is 152μm, and at 200°C it increases to 160μm, an increase of 5.3%. However, the addition of CSP effectively slows down the increase in crack width. In the R-25 group, the crack width increases from 162μm to 164μm, an increase of only 1.2%, which indicates that an appropriate amount of CSP substitution can effectively control the increase in crack width, thereby maintaining good crack control ability. Figure 10 As can be seen in part (b), the crack density initially increases with the increase in CSP replacement rate within the range of 0-100%, and gradually decreases after reaching a peak. For example, at room temperature, the crack density of R-25 and R-50 increased by 8.1% and 16.2% respectively compared with the R-0 group. The addition of CSP reduces the cracking stress of SCECC, making cracks more likely to occur, thereby increasing the crack density. However, the crack density of the R-100 group is only 0.3mm -1 , which is significantly lower than that of other groups, indicating that the high CSP replacement rate reduces the crack control of SCECC.

[0105] Table 5 Average crack width and crack density of SCECC at different temperatures with different CSP replacement rates

[0106]

[0107]

[0108] 3.4.3 Axial tensile stress-strain curve

[0109] Figure 11 Figure 2 is the tensile stress-strain curve of SCECC axial with different CSP replacement rates at 20℃ and 200℃. Figure 11 As can be seen from the figure, all SCECC specimens exhibit typical strain hardening behavior under axial tensile loads at both 20°C and 200°C. This behavior can be divided into three stages: first, the elastic stage, in which the material matrix dominates load transfer, and the bridging effect of the PE fibers is not yet activated. The stress-strain relationship increases linearly until the initial crack occurs. Next, the pseudo-strain hardening stage begins. The bridging effect of the PE fibers effectively suppresses local crack propagation, and stress is redistributed through mechanical interlocking at the fiber-matrix interface. The fluctuations in the curves are due to the energy released when the fibers slip or break. The peaks in this stage represent the ultimate tensile strength and ultimate strain of the material. Finally, when the fiber bridging capacity reaches a critical threshold, the stress transfer mechanism fails, and the curve enters a monotonic decay region, manifesting as a gradual decrease in bearing capacity.

[0110] 3.4.4 Tensile properties

[0111] Figure 12 The initial cracking strength, initial strain, ultimate tensile strength, and ultimate tensile strain of SCECC with different CSP replacement ratios at different temperatures are shown in Table 6. The specimens were fractured due to high-temperature exposure at 400°C and 800°C, so no relevant data are available.

[0112] Table 6 Initial cracking strength, initial strain, ultimate tensile strength and ultimate tensile strain of SCECC with different CSP replacement ratios at different temperatures

[0113]

[0114]

[0115] Note: The numbers in brackets in the table represent standard deviations.

[0116] Figure 12 The ultimate tensile strength (a) and ultimate strain (b) of SCECC with different CSP replacement rates at different temperatures. Figure 12 It can be seen that at room temperature, as the CSP replacement rate increases, the ultimate tensile strength and ultimate tensile strain of SCECC first increase and then decrease. The ultimate tensile strength and ultimate tensile strain of R-25 are 7.03MPa and 5.63%, while those of R-50 increase to 7.63MPa and 5.78%, which are significantly improved compared to R-0. This shows that an appropriate amount of CSP replacement rate has a positive effect on the tensile properties and deformation capacity of the material. However, the ultimate tensile strength (5.94MPa) and strain (3.01%) of the R-100 group decrease. This indicates that a higher CSP replacement rate may lead to a decrease in the bridging ability between the material matrix and the fiber, thereby reducing the tensile properties.

[0117] Table 6 shows that the initial cracking strength and initial strain of SCECC decrease with increasing CSP substitution rate. This is related to the weakening of the matrix by the low strength of CSP. When the temperature rises to 200°C, the tensile properties of all SCECCs decrease. This is due to the evaporation of internal pore water, which increases the porosity and weakens the bridging effect between the matrix and the fiber. PE fibers enhance tensile strength by bridging cracks, so fiber bridging strength largely determines the tensile properties of the material. However, SCECCs with lower substitution rates (such as R-25) still maintain good tensile properties. The R-25 group achieved an ultimate tensile strength of 6.74 MPa and a strain of 4.53%, which are slightly higher than those at room temperature. This demonstrates that an appropriate amount of CSP can still improve the tensile properties of SCECC at 200°C.

[0118] 4. Microstructure

[0119] SCECC samples were microscopically analyzed using a Phenom scanning electron microscope (SEM). After cutting and drying, the samples were affixed to a sample stage using double-sided conductive adhesive and then vacuum-sprayed with gold to enhance conductivity. The micromorphology was observed using the SEM.

[0120] Figure 13 The EDS images of the ITZ between the aggregate and the matrix in SCECC with different CSP replacement rates at room temperature are shown under a scanning electron microscope. The ITZ (cross-sectional transition zone) is obtained by scanning through the aggregate line (yellow dotted circle) along the direction of the black arrow in the SEM image. Figure 13 As can be seen in the figure, the ITZ around the QP is wider than that around the CSP, indicating that the bond between the CSP and the cement matrix is stronger at similar particle sizes. Generally, the bond strength between aggregate and matrix is influenced by sliding friction (which is related to aggregate surface roughness). Therefore, the rough surface properties of the CSP lead to a higher interlocking force with the cement matrix. Consequently, the bond strength at the CSP-matrix interface is higher.

[0121] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for improving the high temperature resistance of engineering cement-based composite materials, characterized in that: Coral sand powder with a median particle size of 150-250 μm is used to replace quartz powder with an average particle size of 100-150 μm in the engineering cement-based composite material at a volume replacement rate of 0-50%, wherein the particle size ratio of the coral sand powder to the quartz powder is (1.2-2.0):1, and the fineness modulus of the coral sand powder is 1.9-2.1, and the value of the volume replacement rate is not 0%.

2. The method according to claim 1, characterized in that The specific surface area of the coral sand powder is 450-550m 2 / kg, the moisture content of the dried coral sand powder is less than 0.5%.

3. The method according to claim 1, characterized in that The particles with a particle size of 100-150 μm account for more than 90% of the quartz powder; and the silicon dioxide content of the quartz powder is ≥98.5%.

4. The method according to claim 1, wherein The volume replacement rate is negatively correlated with the median particle size of the coral sand powder, specifically: When the volume replacement rate is 15-50%, coral sand powder with a median particle size of 150-200 μm is used to replace quartz powder; When the volume replacement rate is 0-15%, coral sand powder with a median particle size of 200-250 μm is used to replace quartz powder, and the value of the volume replacement rate is not 0.

5. A high temperature resistant engineering cement-based composite material based on coral sand powder, characterized in that: Measured by unit volume mass, it is composed of the following raw materials: cement 600kg / m 3 、Fly ash 600kg / m 3 , quartz powder 216-432kg / m 3 , coral sand powder 0-226kg / m 3 , polyethylene fiber 19.4kg / m 3 , water reducing agent 3.6kg / m 3 , defoaming agent 1.8kg / m 3 and seawater 348kg / m 3 ; and the amount of the coral sand powder is not 0.

6. The high temperature resistant engineering cement-based composite material based on coral sand powder according to claim 5, characterized in that: The seawater is artificial seawater; the components of the artificial seawater are: NaCl 24.53g / L, MgCl2·6H2O 5.2g / L, Na2SO4 4.09g / L, CaCl2 1.16g / L, KC1 0.695g / L, NaHCO3 0.201g / L, KBr 0.101g / L and H3BO3 0.027g / L.

7. The high temperature resistant engineering cement-based composite material based on coral sand powder according to claim 5, characterized in that: The polyethylene fiber has a length of 12 mm, a diameter of 24 μm, and an elastic modulus greater than 116 GPa.

8. The high temperature resistant engineering cement-based composite material based on coral sand powder according to claim 5, characterized in that: The cement is P.II 52.5R Portland cement; and / or the water reducer is a polycarboxylic acid-based high-efficiency water reducer.

9. The method for preparing a high-temperature resistant engineering cement-based composite material based on coral sand powder according to any one of claims 5 to 8, wherein: The following steps are involved: Cement, fly ash, quartz powder, coral sand powder and a defoaming agent are stirred and mixed to obtain a dry material mixture; a mixture of seawater and a water reducer is added to the dry material mixture and stirred to obtain a slurry; polyethylene fiber is added to the slurry to obtain the high-temperature resistant engineering cement-based composite material based on coral sand powder.

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