Low-modulus lightweight high-strength concrete and method for preparing same

Abstract

The application discloses low-elastic-modulus light-weight high-strength concrete and relates to the technical field of building materials. The concrete comprises the following raw materials in a mass ratio: cement: 480-600 kg / m 3 ; fly ash: 240-300 kg / m 3 ; silica ash: 80-100 kg / m 3 ; hollow glass microbeads: 64-80 kg / m 3 ; ceramic sand: 150-220 kg / m 3 ; modified rubber particles: 120-180 kg / m 3 ; ceramsite: 225-330 kg / m 3 ; water reducing agent: 2.4-3.0 kg / m 3 ; defoaming agent: 1.6-2.0 kg / m 3 The application further provides a preparation method of the concrete. The method that rubber particles are wrapped with fast-hardening sulphoaluminate cement solves the problem of weak adhesion of an interface transition zone formed by rubber and cement-based materials, solves the problem of easy peeling of rubber under load, and guarantees the mechanical properties of rubber light-weight aggregate concrete.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a low-modulus lightweight high-strength concrete and its preparation method. Background Technology

[0002] With urbanization, buildings are increasingly characterized by high-rise structures, large spans, and diverse structures, while environmental conditions are becoming more complex and variable. To meet structural requirements, most buildings currently use high-strength concrete, known for its high strength and durability. However, its significant weight is amplified with increasing building height and bridge spans, creating a vast opportunity for the development of high-strength lightweight aggregate concrete. Besides its low weight, high-strength lightweight aggregate concrete exhibits significantly superior thermal insulation, heat insulation, moisture retention, fire resistance, and seismic resistance compared to ordinary concrete. Therefore, using lightweight aggregate concrete not only saves on cement and steel reinforcement, resulting in direct economic benefits, but also reduces foundation treatment costs, increases usable floor space, and extends the service life of projects, yielding significant indirect economic benefits. Furthermore, artificial lightweight aggregates can be produced from industrial waste such as fly ash and coal gangue, silt, and calcined ash from municipal solid waste. This type of aggregate is a green aggregate, which can further reduce the mining and utilization of natural aggregates. However, research has found that under high loads, high-strength lightweight aggregate concrete exhibits explosive brittle failure, which poses certain safety hazards in its engineering applications.

[0003] With the booming development of the automotive industry, the number of waste car tires has increased dramatically, making the rational recycling of waste tires an urgent matter. In recent years, crushing waste rubber tires and using them as coarse and fine aggregates in concrete has gradually become an important way to recycle waste tires. This study analyzed the effects of rubber powder content and type on the compressive and flexural strength of cement concrete by replacing a portion of sand with rubber powder of equal volume. The test results showed that the content and type of rubber powder had a significant impact on the strength of the concrete, with the overall strength decreasing as the rubber powder content increased. Chinese patent CN112341104A, "High-Performance Road Rubber Concrete and Its Preparation Method," describes the use of rubber powder obtained from crushed tires, after modification, in high-performance concrete, giving it characteristics of high compressive strength, high toughness, high deformation resistance, and high durability, for use in highway pavement engineering under heavy traffic. Chinese patent CN117361977A, entitled "A High-Performance Rubber Concrete and Its Preparation Method," describes the use of sodium hydroxide to treat rubber powder, resulting in high-performance rubber concrete with characteristics such as high workability, high compressive strength, and high durability. This solves the problems of low workability and severe deterioration of compressive strength in existing rubber concrete. While the above patents all utilize rubber particles obtained from crushed waste tires in ordinary concrete to obtain high-performance rubber concrete with excellent properties, they do not conduct related research on the application of rubber particles in lightweight aggregate concrete. Furthermore, existing rubber concrete technologies still suffer from defects such as the rubber's tendency to float during molding and its tendency to peel off under load. Summary of the Invention

[0004] To mitigate the brittleness of lightweight aggregate concrete and broaden the application scenarios of rubber particles, this invention proposes a low-modulus lightweight high-strength concrete and its preparation method. Furthermore, by coating the rubber particles with rapid-hardening sulfoaluminate cement, the weak bond in the interfacial transition zone between rubber and cementitious materials is addressed, resolving the issue of rubber easily peeling off under load and thus ensuring the mechanical properties of the rubber-modified lightweight aggregate concrete to a certain extent.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A low-modulus, lightweight, high-strength concrete comprises the following raw materials in the indicated mass ratios:

[0007] Cement: 480~600kg / m³ 3 ;

[0008] Fly ash: 240~300kg / m³ 3 ;

[0009] Silica fume: 80~100kg / m³ 3 ;

[0010] Hollow glass microspheres: 64~80kg / m 3 ;

[0011] Ceramic sand: 150~220kg / m 3 ;

[0012] Modified rubber granules: 120~180kg / m 3 ;

[0013] Expanded clay aggregate: 225~330kg / m³ 3 ;

[0014] Water-reducing agent: 2.4~3.0kg / m³ 3 ;

[0015] Defoamer: 1.6~2.0kg / m³ 3 ;

[0016] The water-cement ratio is 0.22 to 0.24, and the cement, fly ash, and silica fume are cementing materials. The water-cement ratio is the mass ratio of water to cementing materials.

[0017] Furthermore, the cement is low-heat silicate cement, wherein the mineral content is: C2S: 42-46 wt%, C3S: 30-40 wt%, C3A: 1-3 wt%, C4AF: 15-19 wt%.

[0018] Furthermore, the fly ash is Class I fly ash with a specific surface area of ​​350 m². 2 / kg, median diameter 16.4um.

[0019] Furthermore, the silica fume is ultrafine silica fume with a specific surface area of ​​400 m². 2 / kg, water requirement ratio <125%, and 7-day activity index of silica fume ≥105%.

[0020] Furthermore, the hollow glass microspheres are white powder with a particle size range of 70–80 μm, a water content of ≤0.25%, and a flocculation rate of ≥96%.

[0021] Furthermore, the ceramsite is a self-made high-strength ceramsite with a particle size range of 4.75–10 mm.

[0022] Furthermore, the ceramic sand is obtained by crushing self-made ceramic granules, wherein the cumulative sieve residue percentage of sand particles with a diameter > 4.75 mm is 0%, the cumulative sieve residue percentage of sand particles with a diameter > 2.36 mm is 36%, the cumulative sieve residue percentage of sand particles with a diameter > 1.18 mm is 61%, the cumulative sieve residue percentage of sand particles with a diameter > 0.60 mm is 78%, the cumulative sieve residue percentage of sand particles with a diameter > 0.30 mm is 91%, and the cumulative sieve residue percentage of sand particles with a diameter > 0.15 mm is 100%, and the fineness modulus is 3.34.

[0023] Furthermore, the self-made high-strength ceramsite is made by firing high-titanium slag, ferrochrome slag, and microsilica powder as the main raw materials, and industrial magnesium oxide and industrial alumina as auxiliary raw materials through high-temperature solid-state firing. Specifically, it includes: 25wt% high-titanium slag, 45wt% ferrochrome slag, 10wt% microsilica powder, 12wt% industrial magnesium oxide, and 8.0wt% industrial alumina.

[0024] The apparent density of the self-made high-strength ceramsite is 1450 kg / m³. 3 Water absorption rate ≤5.0wt% in 24h, compressive strength ≥10MPa.

[0025] Among them, the utilization rate of heavy metal industrial solid waste such as high-titanium slag and ferrochrome slag can reach up to 80 wt%.

[0026] The mechanism of the high-temperature solid-state firing method is as follows: based on the MgO-Al2O3-SiO2-CaO system, a quaternary system phase diagram is drawn using FactSage 6.0 thermodynamic simulation software. With the help of the phase diagram, the heavy metals of high-titanium slag and chromium-iron slag are further dissolved into the mineral phase lattice of the ceramic particles; Cr 2+ and Cr 3+ The ionic radii are 0.073 nm and 0.062 nm, respectively, while Mg 2+ And Al 3+ The ionic radii are 0.072 nm and 0.054 nm, respectively. Since the ionic radii of chromium and magnesium are very close, chromium can easily replace magnesium or aluminum and dissolve into the crystal lattice of cordierite and spinel. Testing showed that the heavy metal leaching amount of the self-made high-strength ceramsite was 0.237 mg / L, far below the national standard.

[0027] The calculations using the FactSage 6.0 thermodynamic simulation software are based on the main mineral phases of the ceramsite: magnesium aluminum spinel (MgAl2O4) and cordierite (Mg2Al4Si5O4). 18 Using MgO as a baseline, the range of Al2O3, SiO2, and CaO contents was calculated by fixing the MgO content. The calculated ranges for each oxide content are: MgO (7.96–19.32 wt%), Al2O3 (20.08–43.50 wt%), SiO2 (44.54–71.96 wt%), and CaO (0–7.5 wt%).

[0028] The proportions of each main raw material and auxiliary raw material are determined based on the range of oxide content in the MgO-Al2O3-SiO2-CaO system calculated by the thermodynamic software, including: 25wt% high-titanium slag, 45wt% chromium ferrochrome slag, 10wt% silica fume, 12wt% industrial magnesium oxide, and 8.0wt% industrial alumina.

[0029] Furthermore, the high-titanium slag is a water-quenched slag, wherein the content of magnesium, aluminum and silicon oxides is more than 40 wt%, the content of calcium and titanium oxides is 50 wt%, and the content of Fe2O3 is 6 wt%; preferably, the high-titanium slag is the water-quenched slag of Sichuan Weigang Steel Group.

[0030] Furthermore, the ferrochrome slag contains more than 85 wt% magnesium aluminum silicon oxide, 6 wt% Fe2O3, and 8 wt% Cr2O3.

[0031] Furthermore, the silicon micro powder contains ≥95wt% silicon oxide, the industrial alumina contains ≥98wt% Al2O3, and the industrial magnesium oxide contains ≥99wt% MgO.

[0032] Furthermore, the modified rubber particles have a particle size of 2.36–4.76 mm and an apparent density of 1316 kg / m³. 3 The bulk density is 636 kg / m³. 3 .

[0033] Further, the modification method of the modified rubber particles is as follows: SA, sodium hypochlorite, concentrated hydrochloric acid and deionized water are mixed in a volume ratio of V(NaClO):V(HCl):V(H2O)=4:1:100 to prepare a mixed solution, then rubber particles are added to the mixed solution, the volume ratio of the mixed solution to the rubber particles is 2:1, and then placed in a constant temperature water bath at 25℃ for 10 minutes, then filtered, washed repeatedly with deionized water until the pH of the washing water is neutral, filtered again, and dried in an oven at 80℃ for 12 hours to complete the surface oxidation of the rubber particles; SB, the surface-oxidized rubber particles are added to water glass and mixed evenly, then sulfoaluminate cement is added and stirred until each rubber particle is coated with a layer of sulfoaluminate cement, and finally dried in an oven at 40℃ to obtain the modified rubber particles.

[0034] The purpose of SA is to oxidize the surface of rubber particles, thereby increasing the number of surface polar functional groups, increasing roughness, and improving wettability.

[0035] Furthermore, the mass ratio of the surface-oxidized rubber particles to water glass and sulfoaluminate cement is 1:0.1:0.2.

[0036] Furthermore, the water glass is an aqueous solution of Na2SiO3 with a modulus of 3.1.

[0037] Since the main component of water glass is sodium silicate, its functions are twofold: firstly, water glass acts as a binder, effectively bonding rubber particles to sulfoaluminate cement, preventing the rubber from detaching from the cement; secondly, sodium silicate is a soluble inorganic silicate that reacts with calcium hydroxide produced during cement hydration to form calcium silicate gel. This gel fills the capillary pores between the rubber particles and the cement matrix, increasing the interfacial bond strength and reducing the impact of the rubber particles on the concrete strength. Furthermore, sodium silicate also improves the impermeability and corrosion resistance of concrete, enhancing its durability.

[0038] Furthermore, the water-reducing agent is polycarboxylate superplasticizer 540P; preferably, the water-reducing agent is Sika 540P polycarboxylate high-performance water-reducing agent, a white powder with high water reduction rate and high plasticity retention.

[0039] Furthermore, the defoamer is a polyether powder defoamer, and preferably, the defoamer is a Degussa polyether powder defoamer.

[0040] This invention also provides a method for preparing low-modulus lightweight high-strength concrete, comprising the following steps:

[0041] S1. Weigh the raw materials of each mass, and place cement, fly ash, silica fume, hollow glass microspheres and ceramsite in a mixing machine for 60 seconds, preferably in a forced concrete mixer. Then add water, water-reducing agent and defoamer in sequence and continue mixing for 2 minutes. Finally, add modified rubber particles and ceramsite, continue mixing, and then shape and cure to obtain the low elastic modulus lightweight high strength concrete.

[0042] Furthermore, the low-modulus lightweight high-strength concrete has the following properties: concrete density ≤ 1750 kg / m³ 3 The slump is ≥200mm, the spread is ≥380mm, the 28d compressive strength is ≥45MPa, and the elastic modulus is ≤25GPa. The elastic modulus value is taken as the secant modulus value under 30% of the axial compressive strength stress.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] (1) In this invention, rubber particles have the characteristics of low density and low elastic modulus. When applied to lightweight high-strength concrete, they will not increase the density of concrete. At the same time, because rubber has a good energy absorption effect, it can reduce the stress concentration phenomenon of concrete under load, improve the impact resistance of lightweight high-strength concrete, and reduce the brittleness of concrete.

[0045] (2) This invention applies the rubber particles after the waste tires are crushed to lightweight high-strength concrete, providing a new idea for the resource recycling of waste tires. In addition, the rubber particles in the concrete are regarded as "elastic bodies" due to their good elasticity, which can absorb strain energy, limit the occurrence and expansion of cracks, and thus improve the frost resistance of lightweight high-strength concrete.

[0046] (3) In this invention, sodium hypochlorite is used to oxidize the surface of rubber particles to increase the hydrophilicity and surface roughness of the rubber particles. Furthermore, water glass is used as a binder to coat the rubber particles with a layer of sulfoaluminate cement that can be rapidly hydrated. The continuous hydration of sulfoaluminate cement further strengthens the interfacial bonding strength between the rubber particles and the cement matrix, improves the interfacial performance of the two, and to a certain extent alleviates the possibility of damage caused by inconsistent deformation between the rubber particles and the cement matrix under load, thereby improving the load-bearing capacity of concrete.

[0047] (4) In this invention, industrial solid waste containing heavy metals, such as high-titanium slag and chromium slag, is used to replace natural silicon-alumina raw materials such as clay to make high-strength ceramsite and ceramsite sand. This not only reduces the pollution of the natural ecological environment by heavy metal industrial solid waste, but also alleviates the excessive consumption of natural silicon-alumina raw materials such as clay, which is in line with the current low-carbon and environmentally friendly production concept. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0049] Example 1

[0050] As a preferred embodiment of the present invention, the composition of a low-modulus lightweight high-strength concrete is shown in Table 1.

[0051] Table 1

[0052] raw material <![CDATA[Mass / kg / m 3 > Low-heat silicate cement 480 fly ash 240 silica ash 80 Hollow glass microspheres 64 Pottery Sand 220 Modified rubber granules 120 Expanded clay 330 Water reducing agent 2.4 Defoamer 1.6

[0053] In this embodiment, the water-to-binder ratio is 0.22.

[0054] Example 2

[0055] As a preferred embodiment of the present invention, the composition of a low-modulus lightweight high-strength concrete is shown in Table 2.

[0056] Table 2

[0057] raw material <![CDATA[Mass / kg / m 3 > Low-heat silicate cement 540 fly ash 270 silica ash 90 Hollow glass microspheres 72 Pottery Sand 185 Modified rubber granules 150 Expanded clay 277.5 Water reducing agent 2.7 Defoamer 1.8

[0058] In this embodiment, the water-to-binder ratio is 0.23.

[0059] Example 3

[0060] As a preferred embodiment of the present invention, the composition of a low-modulus lightweight high-strength concrete is shown in Table 3.

[0061] Table 3

[0062] raw material <![CDATA[Mass / kg / m 3 > Low-heat silicate cement 600 fly ash 300 silica ash 100 Hollow glass microspheres 80 Pottery Sand 150 Modified rubber granules 180 Expanded clay 225 Water reducing agent 3.0 Defoamer 2.0

[0063] In this embodiment, the water-to-binder ratio is 0.24.

[0064] Comparative Example 1

[0065] This comparative example is a lightweight high-strength concrete, which is the same as that in Example 1 except that no modified rubber particles are added.

[0066] Comparative Example 2

[0067] This comparative example is a lightweight high-strength concrete, which is the same as Example 1 except that the modified rubber particles are replaced with ordinary rubber particles.

[0068] The concrete samples from each embodiment and comparative example were tested, and the test results are shown in Table 4.

[0069] Table 4

[0070]

[0071] As shown in Table 1, compared with Examples 1 to 3, as the amount of rubber particles in the composition increases, the elastic modulus of concrete gradually decreases and the toughness increases. However, the compressive strength of the concrete also tends to decrease, but the overall strength performance and workability of the concrete are good. Compared with Example 1 and Comparative Example 1, rubber particles can significantly reduce the elastic modulus of concrete and improve its toughness. Compared with concrete without rubber particles, the elastic modulus of low elastic modulus lightweight high-strength concrete decreases by about 29.6%. Compared with Example 1, Comparative Example 1, and Comparative Example 2, after the rubber particles are oxidized with sodium hypochlorite, and then coated with sulfoaluminate cement using water glass as a binder, the disadvantage of the decrease in concrete strength caused by rubber particles can be alleviated. Moreover, the rubber particle concrete coated with sulfoaluminate cement has higher strength, lower elastic modulus, and better workability. Therefore, the concrete prepared by this invention has the characteristics of low elastic modulus, lightweight, and high strength.

[0072] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention used to illustrate the technical solutions of the present invention, and are not intended to limit the invention, nor are they intended to limit the patent scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. That is to say, any changes or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but whose technical problems are still consistent with the present invention, should be included within the protection scope of the present invention. In addition, the direct or indirect application of the technical solutions of the present invention to other related technical fields are similarly included within the patent protection scope of the present invention.

Claims

1. A low-modulus, lightweight, high-strength concrete, characterized in that, Raw materials including the following mass ratio: Cement: 480~600 kg / m³ 3 ; Fly ash: 240~300 kg / m³ 3 ; Silica fume: 80~100kg / m³ 3 ; Hollow glass microspheres: 64~80kg / m 3 ; Ceramic sand: 150~220kg / m 3 ; Modified rubber granules: 120~180kg / m 3 ; Expanded clay aggregate: 225~330kg / m³ 3 ; Water-reducing agent: 2.4~3.0 kg / m³ 3 ; Defoamer: 1.6~2.0 kg / m³ 3 ; The water-cement ratio is 0.22~0.24, and the cement, fly ash, and silica fume are cementing materials. The water-cement ratio is the mass ratio of water to cementing materials. The modification method of the modified rubber particles is as follows: SA, sodium hypochlorite, concentrated hydrochloric acid and deionized water are mixed in a volume ratio of V(NaClO):V(HCl):V(H2O) = 4:1:100 to prepare a mixed solution. Rubber particles are then added to the mixed solution, with a volume ratio of mixed solution to rubber particles of 2:

1. The solution is then placed in a constant temperature water bath at 25℃ for 10 minutes, filtered, and washed repeatedly with deionized water until the pH of the washing water is neutral. After filtration, the solution is dried in an oven at 80℃ for 12 hours to complete the surface oxidation of the rubber particles. SB, the surface-oxidized rubber particles are mixed with water glass. After uniform mixing, sulfoaluminate cement is added and stirred until each rubber particle is coated with a layer of sulfoaluminate cement. Finally, the mixture is dried in an oven at 40℃ to obtain the modified rubber particles.

2. The low-modulus lightweight high-strength concrete according to claim 1, characterized in that, The cement is a low-heat silicate cement, wherein the mineral content is: C2S: 42~46wt%, C3S: 30~40wt%, C3A: 1~3wt%, C4AF: 15~19wt%.

3. The low-modulus lightweight high-strength concrete according to claim 1, characterized in that, The fly ash is Class I fly ash with a specific surface area of ​​350 m². 2 / kg, median diameter 16.4µm; the silica fume is ultrafine silica fume with a specific surface area of ​​400m². 2 / kg, water requirement ratio <125%, 7d activity index of silica fume ≥105%; the hollow glass microspheres are white powder with a particle size range of 70~80μm, water content ≤0.25%, and floating rate ≥96%.

4. The low-modulus lightweight high-strength concrete according to claim 1, characterized in that, The ceramsite is a self-made high-strength ceramsite with a particle size range of 4.75~10mm; the ceramsite sand is obtained by crushing self-made ceramsite, wherein the cumulative sieve residue percentage of sand particles with a diameter >4.75mm is 0%, the cumulative sieve residue percentage of sand particles with a diameter >2.36mm is 36%, the cumulative sieve residue percentage of sand particles with a diameter >1.18mm is 61%, the cumulative sieve residue percentage of sand particles with a diameter >0.60mm is 78%, the cumulative sieve residue percentage of sand particles with a diameter >0.30mm is 91%, the cumulative sieve residue percentage of sand particles with a diameter >0.15mm is 100%, and the fineness modulus is 3.

34.

5. A low-modulus, lightweight, high-strength concrete according to claim 4, characterized in that, The self-made high-strength ceramsite is made from high-titanium slag, ferrochrome slag, and microsilica powder as the main raw materials, and industrial magnesium oxide and industrial alumina as auxiliary raw materials, through high-temperature solid-state firing. Specifically, it comprises: 25 wt% high-titanium slag, 45 wt% ferrochrome slag, 10 wt% microsilica powder, 12 wt% industrial magnesium oxide, and 8.0 wt% industrial alumina. The apparent density of the self-made high-strength ceramsite is 1450 kg / m³. 3 24h water absorption rate ≤5.0wt%, cylinder compressive strength ≥10MPa.

6. A low-modulus, lightweight, high-strength concrete according to claim 5, characterized in that, The high-titanium slag is a water-quenched slag, wherein the content of magnesium, aluminum, and silicon oxides is above 40 wt%, the content of calcium and titanium oxides is 50 wt%, and the content of Fe2O3 is 6 wt%; the content of magnesium, aluminum, and silicon oxides in the chromium slag is above 85 wt%, the content of Fe2O3 is 6 wt%, and the content of Cr2O3 is 8 wt%; the content of silicon oxide in the silicon micropowder is ≥95 wt%, the content of Al2O3 in the industrial alumina is ≥98 wt%, and the content of MgO in the industrial magnesium oxide is ≥99 wt%.

7. The low-modulus lightweight high-strength concrete according to claim 1, characterized in that, The modified rubber particles have a particle size of 2.36~4.76mm and an apparent density of 1316kg / m³. 3 The bulk density is 636 kg / m³. 3 .

8. The low-modulus lightweight high-strength concrete according to claim 1, characterized in that, The mass ratio of the surface-oxidized rubber particles to water glass and sulfoaluminate cement is 1:0.1:0.2; the water glass is an aqueous solution of Na2SiO3 with a modulus of 3.

1.

9. The low-modulus lightweight high-strength concrete according to claim 1, characterized in that, The water-reducing agent is polycarboxylate water-reducing agent 540P; the defoamer is a polyether powder defoamer.

10. A method for preparing low-modulus lightweight high-strength concrete according to any one of claims 1 to 9, characterized in that, Includes the following steps: S1. Weigh the raw materials of each mass, mix cement, fly ash, silica fume, hollow glass microspheres and ceramsite for 60 seconds, then add water, water-reducing agent and defoamer in sequence and continue mixing for 2 minutes, finally add modified rubber particles and ceramsite, continue mixing, and then mold and cure to obtain the low elastic modulus lightweight high strength concrete. The low-elasticity, lightweight, high-strength concrete has the following properties: concrete density ≤ 1750 kg / m³. 3 The slump is ≥200mm, the spread is ≥380mm, the 28d compressive strength is ≥45MPa, and the elastic modulus is ≤25GPa. The elastic modulus value is taken as the secant modulus value under 30% of the axial compressive strength stress.

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