Preparation method and application of fly ash foam concrete integrating frost resistance and load bearing

Abstract

The invention discloses a preparation method and application of fly ash foam concrete with integrated frost resistance and load bearing properties. The preparation method of the fly ash foam concrete comprises: mixing cement, fly ash, quartz powder, quartz sand, polypropylene fiber, a foam stabilizer and manganese dioxide, stirring until uniform to obtain dry material, mixing the dry material, a water reducer and water uniformly to obtain slurry, adding a foaming agent to the slurry, stirring until uniform, standing for foaming, and curing to obtain fly ash foam concrete. The fly ash foam concrete solves the problems of low strength and poor frost resistance of ordinary foam concrete. Since the changes in pore structure parameters of fly ash foam concrete and ordinary concrete under freeze-thaw cycles are different, the compressive strength of the fly ash foam concrete can be accurately evaluated by using the method of the invention for evaluating the compressive strength of fly ash foam concrete based on the fly ash content.

Description

[0001] (This application is a divisional application of application number 202410220464.7, "Method for Estimating the Compressive Strength of Fly Ash Foam Concrete Based on Density," filed on February 28, 2024.) Technical Field

[0002] The invention belongs to the technical field of concrete, and in particular relates to a load-bearing and frost-resistant fly ash foam concrete and a preparation method and application thereof. Background Art

[0003] With the development of economy, energy consumption has become a major problem facing the world. The construction industry is characterized by high energy consumption and high carbon dioxide emissions. [1] , accounting for a large proportion of global energy consumption [2] Driven by modernization and economic growth, China has become a major energy consumer. According to the "2021 China Building Energy Consumption and Carbon Emissions Research Report," in 2019, China's total construction energy consumption reached 2.233 billion tons of standard coal, and carbon emissions reached 4.997 billion tons of carbon dioxide, accounting for 50.6% of national carbon emissions. Therefore, energy conservation and emission reduction are an inevitable trend in the development of the construction industry.

[0004] Green buildings are an important way to save energy in buildings. To promote their high-quality development, the state and local governments have successively promulgated a number of relevant policies, which clearly point out that it is necessary to improve the energy-saving level of new buildings, strengthen the promotion of green building materials and prefabricated buildings, promote green city construction, and form a green, low-carbon and sustainable construction and development mode. Under the guidance and support of the country, new wall materials have been gradually promoted and applied. At present, the types of new wall materials are increasing, and various performances are gradually optimized, but the ideal wall material with integrated thermal insulation structure is still under exploration and research and development. Since foam concrete is a lightweight and porous material with certain strength and excellent thermal insulation properties, it is an ideal innovation to apply it to the research of wall materials with integrated thermal insulation structure. However, a large amount of cement is used in the process of preparing foam concrete, which is not in line with the sustainable development concept of low-carbon energy conservation. [3-4] .

[0005] Fly ash is one of the major solid wastes in my country. It is a solid particle discharged after burning coal in thermal power plants. According to statistics, about 250kg to 300kg of fly ash is produced for every ton of coal burned. [7] As an industrial solid waste, fly ash can be used in foam concrete to replace part of the cement, which not only reduces the porosity of the slurry and improves the durability of the foam concrete, but also reduces resource consumption and environmental burden, and promotes the resource utilization of solid waste.

[0006] The application of foam concrete is becoming more and more widespread, but due to the presence of a large number of pores inside it, some foam concrete has problems such as low strength and poor frost resistance under the action of frost heave force. [5-6] As a result, the application of foam concrete in the north and south differs significantly. In the south, where the climate is warmer, the requirements for frost resistance are less stringent, and foam concrete meets performance requirements when used as a thermal insulation material for walls and roofs. However, in the colder north, foam concrete is required to possess not only good thermal insulation properties but also a certain degree of frost resistance. Therefore, effectively improving the frost resistance of foam concrete is a key issue in its promotion and application in cold regions. Summary of the Invention

[0007] In view of the deficiencies in the prior art, the present invention aims to provide a method for preparing load-bearing and frost-resistant fly ash foam concrete.

[0008] Another object of the present invention is to provide load-bearing and frost-resistant fly ash foam concrete obtained by the above preparation method.

[0009] Another object of the present invention is to provide a method for evaluating the compressive strength of fly ash foam concrete based on the fly ash content.

[0010] The purpose of the present invention is achieved through the following technical solutions.

[0011] A method for preparing load-bearing and frost-resistant fly ash foam concrete, comprising:

[0012] Step 1: Mix cement, fly ash, quartz powder, quartz sand, polypropylene fiber, foam stabilizer and manganese dioxide, and stir until uniform to obtain a dry material, wherein the ratio of the cement, fly ash, quartz powder, quartz sand, polypropylene fiber, foam stabilizer and manganese dioxide is (400-960): (240-600): (374-672): (160-288): (0.67-1.25): (0.4-1.3): (0.5-1.6) by mass;

[0013] In the step 1, the stirring is performed until uniform by stirring for 90 to 120 seconds to obtain a dry material.

[0014] In the step 1, the foam stabilizer is hydroxypropyl methylcellulose (HPMC).

[0015] Step 2: uniformly mixing the dry material, the water reducing agent and water to obtain a slurry, wherein the ratio of cement, the water reducing agent and water in the dry material is (400-960): (6-12): (334-600) by weight;

[0016] In step 2, the water reducing agent is polycarboxylic acid.

[0017] In the step 2, the uniform mixing is achieved by stirring for 120 to 240 seconds.

[0018] Step 3: Add a foaming agent to the slurry, stir until uniform, let it stand for foaming, and cure to obtain fly ash foam concrete. The ratio of cement to foaming agent in the slurry is (400-960): (14-20) by mass.

[0019] In step 3, the foaming agent is hydrogen peroxide.

[0020] In the step 3, after adding the foaming agent, stirring is performed for 60 to 90 seconds to achieve uniform stirring, and the stirring speed is 120 to 150 r / min.

[0021] In step 3, the foaming time is 24 to 30 hours.

[0022] In step 3, the curing time is 28 to 35 days.

[0023] In step 3, the temperature of the curing environment is 18-22° C., and the relative humidity of the curing environment is >95%.

[0024] A method for evaluating the compressive strength of fly ash foam concrete based on fly ash content comprises the following steps:

[0025] S1. Prepare N fly ash foam concretes with the same density as samples. The fly ash content of the N fly ash foam concretes in the samples is different. Obtain a fitting formula for each fly ash foam concrete according to the following method: subject the fly ash foam concrete to a frost resistance test. After each freeze-thaw cycle in the frost resistance test, test the porosity and compressive strength of the fly ash foam concrete. Fit the porosity of the fly ash foam concrete and the number of freeze-thaw cycles to obtain the fitting formula: P1 = l1t 2 +m1t+n1, where: P1 is the porosity of fly ash foam concrete, l1, m1 and n1 are coefficients related to the fly ash content, and t is the number of freeze-thaw cycles;

[0026] In S1, N is greater than or equal to 3.

[0027] In S1, the fly ash content in the fly ash foam concrete is 20-50%, and the fly ash content = the mass of the fly ash in the fly ash foam concrete / (the mass of the cement in the fly ash foam concrete + the mass of the fly ash in the fly ash foam concrete).

[0028] In S1, antifreeze test is carried out according to industry standards.

[0029] In S1, the density is 500~1500kg / m 3 .

[0030] S2, according to the fitting formula of N fly ash foam concrete, the fly ash content is taken as the independent variable, l1, m1 and n1 are taken as the dependent variables for fitting, and the relationship between l1, m1 and n1 and the fly ash content is obtained;

[0031] S3, substitute the relationship between l1, m1 and n1 and fly ash content into l1, m1 and n1 of the fitting formula in S1, and obtain the porosity change formula with fly ash content and freeze-thaw cycle number as variables;

[0032] S4, according to the porosity and compressive strength of N fly ash foam concretes after each freeze-thaw in S1, the relationship formula between porosity and compressive strength is obtained by fitting;

[0033] S5. Substitute the porosity change formula in S3 with fly ash content and freeze-thaw cycles as variables into the porosity and compressive strength relationship formula in S4 to obtain the quantitative relationship between fly ash content, freeze-thaw cycles and compressive strength of fly ash foam concrete at this density.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. The preparation method of the present invention utilizes fly ash, an industrial solid waste, to develop a foamed concrete. The fly ash foamed concrete solves the problems of low strength and poor frost resistance of conventional foamed concrete: the frost resistance grade is increased to above D50, meeting the requirements for engineering applications in severely cold regions.

[0036] 2. Because fly ash foamed concrete is a porous, multiphase composite material, its pore structure parameters directly affect its mechanical properties and frost resistance. Furthermore, due to the large number of bubbles in fly ash foamed concrete, its pore structure differs significantly from that of ordinary concrete. Under freeze-thaw cycles, fly ash foamed concrete with varying fly ash content deteriorates sequentially from inter-gel pores to inter-particle pores, and then to macropores, with a gradual increase in porosity.

[0037] The changes in pore structure parameters of fly ash foamed concrete and ordinary concrete under freeze-thaw cycles are different. With the help of the method for evaluating the compressive strength of fly ash foamed concrete based on the fly ash content of the present invention, the compressive strength of the above-mentioned fly ash foamed concrete can be accurately evaluated. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The mass loss rate of the fly ash foam concrete prepared in Examples 1 to 4 after being subjected to 0, 15, 25, 35, 50, and 65 freeze-thaw cycles;

[0039] Figure 2The compressive strength loss rate of the fly ash foam concrete prepared in Examples 1 to 4 after being subjected to 0, 15, 25, 35, 50, and 65 freeze-thaw cycles;

[0040] Figure 3 The mass loss rate of the fly ash foam concrete prepared in Example 3 and Examples 5 to 7 after being subjected to 0, 15, 25, 35, 50, and 65 freeze-thaw cycles;

[0041] Figure 4 The compressive strength loss rate of the fly ash foam concrete prepared in Example 3 and Examples 5 to 7 after being subjected to 0, 15, 25, 35, 50, and 65 freeze-thaw cycles;

[0042] Figure 5 The figures are photos of fly ash foam concrete and magnesium-based salt foam concrete before and after freeze-thaw cycles, wherein (a) shows the morphology of the fly ash foam concrete prepared in Example 7 before freeze-thaw cycles, (b) shows the morphology of the fly ash foam concrete prepared in Example 7 after 25 freeze-thaw cycles, (c) shows the morphology of the magnesium-based salt foam concrete before freeze-thaw cycles, and (d) shows the morphology of the magnesium-based salt foam concrete after 10 freeze-thaw cycles.

[0043] Figure 6 The relationship between the compressive strength of the fly ash foam concrete prepared in Examples 1 to 4 and the number of freeze-thaw cycles;

[0044] Figure 7 Fly ash content The fitting results of as the independent variable and a1 as the dependent variable;

[0045] Figure 8 Fly ash content The fitting results of as the independent variable and b1 as the dependent variable;

[0046] Figure 9 The relationship between the porosity and the number of freeze-thaw cycles of the fly ash foam concrete prepared in Examples 1 to 4;

[0047] Figure 10 Fly ash content The fitting result is the independent variable and l1 is the dependent variable;

[0048] Figure 11 Fly ash content The fitting result is the independent variable and m1 is the dependent variable;

[0049] Figure 12 Fly ash content is the fitting result of the independent variable and n1 is the dependent variable;

[0050] Figure 13The mathematical model of freeze-thaw damage of fly ash foam concrete with different fly ash content is presented;

[0051] Figure 14 The volume ratio of pores of different sizes in fly ash foam concrete and magnesium-based salt foam concrete changes with the number of freeze-thaw cycles.

[0052] In the above figure, "fly ash content 20%" or "20%" is Example 1, "fly ash content 30%" or "30%" is Example 2, "fly ash content 40%" or "40%" is Example 3, "fly ash content 50%" or "50%" is Example 4; "500kg / m 3 " or "density 500kg / m 3 "For Example 7, 800kg / m 3 Or "density 800kg / m 3 "For Example 6, 1200kg / m 3 Or "density 1200kg / m 3 "For Example 5, 1500kg / m 3 Or "density 1500kg / m 3 " is Example 3. DETAILED DESCRIPTION

[0053] The fly ash foam concrete of the present invention and its preparation method and application are described in detail below with reference to the accompanying drawings and examples.

[0054] The cement in the following examples is Jidong P.O42.5 grade ordinary Portland cement, and the main chemical composition of the cement is as follows:

[0055]

[0056] The physical properties of cement are as follows:

[0057] Initial setting time / min Final setting time / min Compressive strength / MPa Flexural strength / MPa 161 280 56.3 7.6

[0058] The fly ash in the following examples is Grade II fly ash produced by Donghua Power Plant. The chemical composition and physical properties of the fly ash are as follows:

[0059]

[0060] In the following examples, the fineness of the quartz sand is 40-70 mesh, and the particle size range is 0.25-0.45 mm;

[0061] The fineness of quartz powder is 200-400 mesh, and the particle size range is 0.037-0.075mm.

[0062] The polypropylene fibers in the following examples were purchased from Langfang Shuangsen Building Materials Co., Ltd., and their performance indicators are as follows:

[0063]

[0064]

[0065] The foaming agent hydrogen peroxide was hydrogen peroxide (H2O2) with a concentration of 30 wt% produced by Tianjin Damao Chemical Reagent Factory.

[0066] Polycarboxylic acid: water reduction rate 37%.

[0067] Hydroxypropyl methylcellulose: viscosity is 200,000 mPa.s, produced by Shanghai Chenqi Chemical Technology Co., Ltd.

[0068] The catalyst in the following examples is manganese dioxide (MnO2), analytically pure, produced by Tianjin Damao Chemical Reagent Factory.

[0069] In the following examples, water is tap water.

[0070] The density in the following embodiments is dry density, and the dry density test is conducted in accordance with the Chinese industry standard JGT266-2011 "Foamed Concrete".

[0071] Compressive strength: tested according to Chinese industry standard JGT266-2011 "Foamed concrete".

[0072] Fly ash foam concrete frost resistance test: According to the industry standard JGJ / T341-2014 "Technical Specifications for the Application of Foam Concrete", the main process of the freeze-thaw cycle test is as follows: take out the specimen (100mm×100mm×100mm cube) that has been cured for 28 days, put it into an electric blast drying oven and keep it at (60±5)℃ for 24 hours, and then dry it at 80℃ until it is constant; after the specimen is cooled, weigh its mass as M0, and put the specimen into a (20±5)℃ water tank and keep it at 48h; finally, take out the specimen, wipe off the surface moisture, and place it in a freezer that has been pre-cooled to below -15℃ in a simulation of the natural environment of the engineering material durability damage test system. Perform multiple freeze-thaw cycle tests. Stop the freeze-thaw cycle test when the specimen is damaged, the mass loss rate of the specimen exceeds 5%, or the compressive strength loss rate exceeds 25%. Each freeze-thaw cycle test includes: freezing in air at a negative temperature of (-20±2)℃ for 6h and thawing in a water tank at a positive temperature of (20±5)℃ for 5h. After each freeze-thaw cycle test, dry the specimen at 80℃ and weigh the mass M after cooling. n , carry out compressive strength test.

[0073] Among them, the mass loss rate is: ΔM n : mass loss rate of the specimen after n freeze-thaw cycles, unit (%); M0: mass of the specimen before freeze-thaw cycles; M n : The mass of the specimen after n freeze-thaw cycles.

[0074] Two identical specimens were prepared, one for frost resistance test and the other as control specimen. The control specimen was placed in a room at room temperature. After each freeze-thaw cycle experiment was completed, the compressive strength of the specimen that had completed the freeze-thaw cycle experiment and the control specimen were tested at the same time, and then the compressive strength loss rate of the specimen in the freeze-thaw cycle experiment was calculated.

[0075] Compressive strength loss rate:

[0076] Where: Δf n : Loss rate of compressive strength of specimen after n freeze-thaw cycles, unit (%); f n : compressive strength of control specimen, unit (MPa); f cn : Compressive strength of specimen after n freeze-thaw cycles, unit (MPa).

[0077] Pore ​​structure testing: The specimens were cut into 40 mm × 40 mm × 40 mm cubes using a cutting machine and placed in a vacuum water-retaining chamber for 24 hours. Porosity and pore volume were determined using a nuclear magnetic resonance (NMR) spectrometer. The NMR spectrometer was maintained at a constant temperature of 32°C, with a magnetic scanning range of 0 to 60 mm.

[0078] Examples 1 to 7

[0079] A method for preparing load-bearing and frost-resistant fly ash foam concrete, comprising:

[0080] Step 1: Cement, fly ash, quartz powder, quartz sand, polypropylene fiber, foam stabilizer and manganese dioxide are mixed in a mixing bucket and stirred with a manual mixer for 120 seconds until uniform to obtain a dry material, wherein the ratio of cement, fly ash, quartz powder, quartz sand, polypropylene fiber, foam stabilizer and manganese dioxide is X by mass, and the foam stabilizer is hydroxypropyl methylcellulose (HPMC);

[0081] Step 2: Mix the dry material, water reducer and water for 120 seconds until uniform, to obtain a uniform fluid slurry, wherein the ratio of cement, water reducer and water in the dry material is Y by weight, and the water reducer is polycarboxylic acid;

[0082] Step 3: Add a foaming agent to the slurry, stir at a speed of 120 r / min for 60 seconds until uniform, pour into a 100 mm × 100 mm × 100 mm mold, let it stand for foaming for 24 hours, and cure in a standard curing room at 18-22° C. for 28 days (relative humidity>95%) to obtain fly ash foam concrete, wherein the ratio of cement to foaming agent in the slurry is Z, and the foaming agent is hydrogen peroxide.

[0083] The X value, Y value, Z value, fly ash content and density are shown in Table 1.

[0084] Table 1

[0085]

[0086] In Table 1, the fly ash content = mass of fly ash / (mass of cement + mass of fly ash).

[0087] Comparative Example 1

[0088] The density of the load-bearing and heat-insulating integrated foam concrete prepared with reference to the publication number CN111410506A is 500kg / m 3 Magnesium-based salt foam concrete. A preparation method of magnesium-based salt foam concrete specifically comprises the following steps: (1) mixing 259 parts by mass of hydrochlorite with 224 parts by mass of water, stirring until white crystals disappear in the solution, and then adding 5.8 parts by mass of JSM-1 type polycarboxylate water reducer to obtain a mixed solution A; (2) mixing 435 parts by mass of light-burned magnesium oxide, 185 parts by mass of fly ash, 40 parts by mass of silica fume and 1.6 parts by mass of polypropylene fiber to obtain a mixture B; (3) stirring the mixture B at a rate of 200 r / min for 10 minutes to uniformly disperse the polypropylene fiber in the mixture B; (4) adding the mixed solution A to the mixture B and stirring at a rate of 200 r / min; (5) adding 5.0 parts by mass of hydrogen peroxide and 0.8 parts by mass of hydroxypropyl methylcellulose in sequence, stirring at a rate of 550 r / min for 100 seconds to obtain a mixture C; (6) casting the mixture C into a mold, standing for 24 hours, demoulding, and naturally curing for 28 days.

[0089] Freeze-thaw cycle tests were conducted on the fly ash foamed concrete of Examples 1 to 4 to study the variation of the mass loss rate and compressive strength loss rate of the fly ash foamed concrete with different fly ash content as the number of freeze-thaw cycles increased.

[0090] The mass loss rates of fly ash foam concrete with different fly ash contents after 0, 15, 25, 35, 50, and 65 freeze-thaw cycles are shown in Table 2 and Figure 1 shown.

[0091] Table 2 Mass loss rate of fly ash foam concrete after different freeze-thaw cycles (%)

[0092] Example Fly ash / % 0 times 15 times 25 times 35 times 50 times 65 times Example 1 20 0 0.6 1.0 1.3 1.9 — Example 2 30 0 0.3 0.5 0.7 1.0 1.2 Example 3 40 0 0.4 0.6 0.9 1.2 1.6 Example 4 50 0 0.8 1.3 1.9 — —

[0093] Note: “—” in the table means that the freeze-thaw cycle experiment has been stopped due to the freeze-thaw cycle experiment stopping condition.

[0094] From Table 2 and Figure 1As can be seen, the mass loss rate of fly ash foam concrete increases with increasing freeze-thaw cycles, regardless of fly ash content. This is because fly ash foam concrete is a porous material. During freeze-thaw cycles, saturated water in the pores freezes, and the resulting expansion exerts a gradually increasing pressure on the pore walls. This causes cracks to form and expand within the pore walls, interconnecting the pores. Consequently, as freeze-thaw cycles progress, the surface and edges of the fly ash foam concrete gradually flake off. For a given number of freeze-thaw cycles, the mass loss rate of fly ash foam concrete decreases first and then increases with increasing fly ash content. Analysis suggests that fly ash acts as a micro-aggregate filler, with its extremely fine microbeads and debris evenly distributed within the cement paste, filling pores and capillaries. Furthermore, the silicate glass in fly ash promotes secondary hydration of cement, forming a hydrated calcium silicate gel that strengthens the paste. When the fly ash content is further increased, the excess fly ash causes some bubbles to rupture and form deformed pores and connected pores, which increases the porosity and mass loss rate.

[0095] The compressive strength results of fly ash foam concrete with different fly ash content after 0, 15, 25, 35, 50, and 65 freeze-thaw cycles are shown in Table 3. The compressive strength loss rate is shown in Table 3. Figure 2 shown.

[0096] Table 3 Compressive strength of fly ash foam concrete (specimens) after freeze-thaw cycles (MPa)

[0097] Number of freeze-thaw cycles Example 1 Example 2 Example 3 Example 4 0 times 21.8 23.0 22.1 17.4 15 times 20.6 22.1 21.2 15.9 25 times 19.1 21.6 20.0 14.3 35 times 17.4 20.4 18.9 12.4 50 times 15.30 19.6 17.6 — 65 times — 18.0 16.0 —

[0098] Note: “—” in the table means that the freeze-thaw cycle experiment has been stopped due to the freeze-thaw cycle experiment stopping condition.

[0099] Table 3 shows that the compressive strength of fly ash foam concrete before freeze-thaw cycles first increases and then decreases with increasing fly ash content. Fly ash exerts its micro-aggregate effect in fly ash foam concrete, with appropriate fly ash particles dispersed in the cement paste to fill pores, thereby increasing the strength of the concrete. However, as the fly ash content increases, the proportion of cement replaced by fly ash increases, resulting in a decrease in cement hydration products and a decrease in compressive strength. With increasing freeze-thaw cycles, the compressive strength of fly ash foam concrete decreases at all levels of fly ash content.

[0100] Depend on Figure 2It can be seen that under the same number of freeze-thaw cycles, the compressive strength loss rate of fly ash foam concrete first decreases and then increases with increasing fly ash content. Analysis of the reasons: When the fly ash content is low, the hydrated calcium silicate produced by fly ash during cement hydration can act as a filler to fill the pores between the cement and improve the pore structure, thereby enhancing the fly ash foam concrete's ability to resist freeze-thaw cycles. When the fly ash content is high, the excess fly ash causes some bubbles to rupture and merge, while also increasing the water absorption rate of the test block, resulting in a decrease in frost resistance and an increase in strength loss.

[0101] According to the industry standard JGJ / T341-2014, "Technical Specifications for the Application of Foamed Concrete," the frost resistance of test specimens is assessed based on an average mass loss rate of no more than 5% and a compressive strength loss rate of no more than 25%. Therefore, the frost resistance grades of fly ash foamed concrete with fly ash content of 20%, 30%, 40%, and 50% are D35, D65, D50, and D25, respectively. The frost resistance of fly ash foamed concrete with a fly ash content of 30% is the best.

[0102] The fly ash foam concrete (density 1500kg / m 3 , 1200kg / m 3 , 800kg / m 3 , 500kg / m 3 ) conducted freeze-thaw cycle tests, and used mass loss rate and compressive strength loss rate as indicators to compare the changes in the anti-freeze performance of fly ash foam concrete with different density grades after freeze-thaw cycles.

[0103] The mass loss rates of the fly ash foam concrete prepared in Examples 3 and 5 to 7 after being subjected to 0, 15, 25, 35, 50, and 65 freeze-thaw cycles are shown in Tables 4 and Figure 3 shown.

[0104] Table 4 Mass loss rate of specimens after different freeze-thaw cycles (%)

[0105]

[0106] Note: “—” in the table means that the specimen has reached the freeze-thaw cycle test stop condition and the freeze-thaw cycle test is stopped.

[0107] From Table 4 and Figure 3The results show that the mass loss rate of fly ash foam concrete of different densities gradually increases during freeze-thaw cycles, but the growth rate is relatively small in the early stages of the freeze-thaw cycle. Once the number of freeze-thaw cycles reaches a certain critical point, the mass loss rate of fly ash foam concrete increases significantly. Fly ash foam concrete of different densities has a critical point that affects its frost resistance. After reaching this critical point, its internal structure changes, the degree of freeze-thaw damage increases significantly, and its frost resistance gradually decreases.

[0108] The mass loss rate of fly ash foam concrete of varying densities shows a gradual increase with the number of freeze-thaw cycles. However, specimens with lower density experience more pronounced mass loss early in the freeze-thaw cycle, while specimens with higher density experience high mass loss later in the freeze-thaw cycle. This is because the fly ash foam concrete undergoes a 48-hour soaking period before freeze-thaw cycles (only once, not before each freeze-thaw cycle). Low-density specimens have a high porosity and reach saturation during the soaking process, while high-density specimens are not fully saturated. Consequently, low-density fly ash foam concrete can freeze a large amount of water in the early stages of the freeze-thaw cycle, resulting in a poor ability of its pore walls to resist expansion stress and gradual rupture of the pore walls under the effects of freeze-thaw cycles. High-density fly ash foam concrete has a compact internal structure, low water saturation in the early stages of the freeze-thaw cycle, and minimal mass loss. However, after reaching the critical point of the freeze-thaw cycle, the pore structure of the fly ash foam concrete freezes, causing a frost heave effect, rupturing the pore walls and interconnecting the pores.

[0109] The compressive strength of fly ash foam concrete with different densities after 0, 15, 25, 35, 50, and 65 freeze-thaw cycles is shown in Table 5. The compressive strength loss rate is shown in Table 5. Figure 4 shown.

[0110] Table 5 Compressive strength of fly ash foam concrete (specimens) after freeze-thaw cycles (MPa)

[0111]

[0112]

[0113] Note: “—” in the table means that the specimen has reached the freeze-thaw cycle test stop condition and the freeze-thaw cycle test is stopped.

[0114] Table 5 shows that the compressive strength of fly ash foam concrete of varying densities decreases with increasing freeze-thaw cycles. Because fly ash foam concrete of varying densities has varying abilities to resist freeze-thaw cycles, the number of freeze-thaw cycles required for the specimens to reach the freeze-thaw cycle test termination condition also varies. Higher density grades are associated with better frost resistance.

[0115] Depend on Figure 4It can be seen that the compressive strength loss rate of fly ash foam concrete of different densities increases with the number of freeze-thaw cycles, with the compressive strength loss rate rising slowly in the early stages and increasing rapidly in the later stages. This suggests the existence of a critical number of freeze-thaw cycles. Before this critical point, the compressive strength of fly ash foam concrete decreases slowly, resulting in a low compressive strength loss rate. After exceeding this critical number of freeze-thaw cycles, the compressive strength loss rate increases rapidly. Furthermore, the lower the density of fly ash foam concrete, the lower the critical number of freeze-thaw cycles, while the higher the density, the higher the critical number of freeze-thaw cycles. This is primarily because fly ash foam concrete with lower density has higher porosity and thinner pore walls, making it less resistant to freeze-thaw damage. It suffers some damage in the early stages of freeze-thaw cycles, leading to a gradual loss of compressive strength. The fly ash foam concrete with higher density did not reach the saturated state in the early stage of freeze-thaw cycle, and had a certain ability to resist freeze-thaw cycle damage; with the increase of freeze-thaw cycles, the water absorption saturation of the fly ash foam concrete specimens continued to increase, the scope of damage to the internal pore wall under the action of frost heave force gradually increased, and the compressive strength loss rate increased.

[0116] According to the industry standard JGJ / T341-2014, "Technical Specifications for the Application of Foam Concrete," the frost resistance of test specimens is assessed based on an average mass loss rate of no more than 5% and a compressive strength loss rate of no more than 25%. Therefore, the frost resistance grades of the fly ash foam concrete in Examples 3 and 5-7 were D50, D35, D25, and D15, respectively.

[0117] The fly ash foamed concrete prepared in Example 7 and the magnesium-based salt foamed concrete prepared in Comparative Example 1 are both foamed concrete. The mass loss rates of the fly ash foamed concrete prepared in Example 7 and the magnesium-based salt foamed concrete prepared in Comparative Example 1 after 0, 15, and 25 freeze-thaw cycles are shown in Table 6.

[0118] Table 6 Mass loss rate of specimens after different freeze-thaw cycles (%)

[0119]

[0120] Note: “—” in the table means that the specimen has reached the freeze-thaw cycle test stop condition and the freeze-thaw cycle test is stopped.

[0121] Table 7 Compressive strength loss rate (%) of the fly ash foam concrete prepared in Example 7 and the magnesium-based salt foam concrete prepared in Comparative Example 1 after different freeze-thaw cycles

[0122] Table 7

[0123]

[0124] Note: “—” in the table means that the specimen has reached the freeze-thaw cycle test stop condition and the freeze-thaw cycle test is stopped.

[0125] The mass loss rates of the fly ash foam concrete prepared in Example 7 and the magnesium-based salt foam concrete prepared in Comparative Example 1 when the freeze-thaw cycle experiment was stopped were both less than 5%, which did not meet the requirements for stopping the test. However, the compressive strength loss rates under the corresponding freeze-thaw cycle numbers exceeded 25%, so the freeze-thaw cycle experiment was stopped.

[0126] The frost resistance grade of the magnesium-based salt foamed concrete prepared in Comparative Example 1 is D5, and the frost resistance of the fly ash foamed concrete prepared in Example 7 is better.

[0127] The number of freeze-thaw cycles that fly ash foam concrete and magnesium-based salt foam concrete can withstand is different. Figure 5 (a) is the state of fly ash foam concrete before freeze-thaw cycle. Figure 5 (b) is the morphology of fly ash foam concrete after 25 freeze-thaw cycles. Figure 5 (c) is the morphology of magnesium-based salt foam concrete before freeze-thaw cycle. Figure 5 (d) is the morphology of magnesium-based salt foam concrete after 10 freeze-thaw cycles. Figure 5 As shown in (a) to (d), before the freeze-thaw cycle, the surfaces of the two foam concretes were relatively smooth and distributed with visible pores. The pore size distribution of the magnesium-based salt foam concrete was more uniform than that of the fly ash foam concrete before the freeze-thaw cycle. With the increase in the number of freeze-thaw cycles, both foam concretes suffered varying degrees of damage. When the fly ash foam concrete reached freeze-thaw damage (25 freeze-thaw cycles), the surface of the specimen became rough, the pore walls ruptured, the pore diameter became larger and connected, and small cracks, edge peeling, and fiber exposure appeared. The freeze-thaw damage of the magnesium-based salt foam concrete was more serious. Not only did the pore walls rupture and the pore diameter increase, but also large cracks extending horizontally and vertically appeared on the surface of the specimen, and large pieces of edges and corners fell off. This is mainly because the water resistance of the magnesium-based salt foam concrete is poor. It is easily hydrolyzed under the action of water during the freeze-thaw cycle and gradually cracks under the action of freezing expansion force. This also explains why the mass loss and strength loss of the magnesium-based salt foam concrete are greater than those of the fly ash foam concrete.

[0128] Comparative Example 2

[0129] According to the compressive strength and freeze-thaw cycle number in Table 3, the relationship between the compressive strength and freeze-thaw cycle number of the fly ash foam concrete of each embodiment is fitted, as shown in FIG. Figure 6 As shown. Figure 6 It can be seen that with the increase of freeze-thaw cycles, the compressive strength of fly ash foam concrete with different fly ash content shows a gradual decrease and decreases linearly. The compressive strength fitting formulas are as follows:

[0130] f h =a1+b1t

[0131] Where: f h is the compressive strength, a1 and b1 are coefficients related to the fly ash content, and t is the number of freeze-thaw cycles. As the independent variable, a1 and b1 are fitted again as the dependent variables:

[0132]

[0133] is the fly ash content. The fitting results are as follows Figure 7 and Figure 8 Therefore, the formula for compressive strength change with fly ash content and freeze-thaw cycles as variables is as follows:

[0134]

[0135] Where: f h is the compressive strength, is the fly ash content; t is the number of freeze-thaw cycles.

[0136] Example 8

[0137] Since fly ash foam concrete contains a large number of bubbles, its pore structure is significantly different from that of ordinary concrete. In order to further study the effect of different pore sizes on the frost resistance of fly ash foam concrete, the pores of fly ash foam concrete are divided into three categories: gel pores (pore size <50nm), particle pores (pore size 50nm~50μm), and macropores (pore size >50μm) with reference to (TADA S, NAKANOS. Microstructural approach to properties of moist cellular concrete [C] Proceedings Autoclaved Aerated Concrete, Moisture and Properties. Amsterdam: Elsevier, 1983: 71-89. and Li Xiangguo, Liu Min, Ma Baoguo, et al. Influence of pore structure on the performance of foam concrete and control technology [J]. Materials Guide, 2012, 26 (07): 141-144+153.). Figure 14 Table 8 shows the porosity, inter-gel pores (<50 nm), inter-particle pores (50 nm to 50 μm), macropores (>50 μm) and compressive strength of the fly ash foam concrete prepared in Examples 1 to 4 as the number of freeze-thaw cycles changes.

[0138] Table 8

[0139]

[0140]

[0141] The pore structure of fly ash foam concrete with different fly ash contents gradually deteriorates under the action of freeze-thaw cycles. As shown in Table 8, the porosity, pore volume corresponding to gel pores (<50nm), pore volume corresponding to interparticle pores (50nm~50μm), and pore volume corresponding to macropores (>50μm) are used as comparison sequences. According to the gray entropy correlation analysis method (Liu Xin, Shen Xiangdong, Xue Huijun, et al. Gray entropy correlation analysis of strength and pore structure evolution of cement-solidified arsenic sandstone [J]. Transactions of the Chinese Society of Agricultural Engineering, 2020, 36(24): 125-133.), the gray entropy correlation between pore structure parameters and compressive strength is calculated, as shown in Table 9.

[0142] Table 9

[0143] Pore ​​structure parameters Gray entropy correlation Porosity 0.9913 Intergel pore volume 0.9903 Interparticle pore volume 0.9912 Macropore volume 0.9854

[0144] By comparing the ash entropy correlations between pore structure parameters, we found that the ash entropy correlations between the compressive strength of fly ash foam concrete after freeze-thaw cycles and pore structure parameters with different fly ash content were porosity > interparticle pore volume > intergel pore volume > macropore volume. Therefore, porosity is the primary factor contributing to the reduction in compressive strength.

[0145] The porosity and freeze-thaw cycles in Table 8 were fitted to obtain the relationship between the porosity and freeze-thaw cycles of fly ash foam concrete with different fly ash content. Figure 9 As shown. Figure 9 It can be seen that with the increase of the number of freeze-thaw cycles, the porosity of fly ash foam concrete with different fly ash content shows a trend of gradual increase, and the growth is nonlinear. The nonlinear correlation coefficients after fitting the scatter plots are all greater than 0.90, and the correlation between the two is significant. The porosity fitting formulas are as follows:

[0146] P1=l1t 2 +m1t+n1, where P1 is porosity, l1, m1, n 1 is the coefficient related to the fly ash content, and t is the number of freeze-thaw cycles. As independent variables, l1, m1, n1 are used as dependent variables and fitted again:

[0147]

[0148]

[0149] The fitting results are as follows Figures 10-12 shown.

[0150] Therefore, the porosity change formula with fly ash content and freeze-thaw cycles as variables is as follows:

[0151]

[0152] The freeze-thaw damage model established after the scatter plot of compressive strength and porosity of fly ash foam concrete with different fly ash content under freeze-thaw cycles and regression analysis is as follows: Figure 13 The mathematical model of freeze-thaw damage is shown as follows:

[0153] f=33.707-0.60181P1 Formula (2)

[0154] f is the compressive strength of fly ash foam concrete. The determination coefficient of the regression analysis of this data is about 0.985, and the fitting degree is high, indicating that this model can be used to predict the quantitative relationship between the compressive strength and porosity of fly ash foam concrete with different fly ash contents after freeze-thaw cycles, so as to evaluate the frost resistance of fly ash foam concrete.

[0155] Substituting P1 in formula (1) into formula (2), we get:

[0156]

[0157] Under the action of freeze-thaw cycles, the gel interpores of the new fly ash foam concrete with different fly ash content deteriorate to interparticle pores and then to macropores, and the pore size distribution curve area and porosity gradually increase.

[0158] Example 9

[0159] The fly ash foam concrete prepared in Example 4 was subjected to 35 freeze-thaw cycle tests according to the fly ash foam concrete frost resistance test, and its actual compressive strength was tested to be 18.9 MPa.

[0160] Substituting the fly ash content and the number of freeze-thaw cycles (35) of the fly ash foamed concrete prepared in Example 4 into formula (3), the compressive strength was calculated to be 18.97 MPa with an error of 0.40%.

[0161] The fly ash content and the number of freeze-thaw cycles (35) of the fly ash foamed concrete prepared in Example 4 were substituted into Comparative Example 2, and the calculated compressive strength was 16.75 MPa, with an error of 12.83%.

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[0169] The above is an exemplary description of the present invention. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by other skilled in the art without expending creative labor falls within the scope of protection of the present invention.

Claims

1. A method for evaluating the compressive strength of fly ash foam concrete based on fly ash content, and a method for preparing load-bearing and frost-resistant fly ash foam concrete comprising: Step 1: Mix cement, fly ash, quartz powder, quartz sand, polypropylene fiber, a foam stabilizer and manganese dioxide, and stir until uniform to obtain a dry material, wherein the ratio of the cement, fly ash, quartz powder, quartz sand, polypropylene fiber, foam stabilizer and manganese dioxide is (400-960): (240-600): (374-672): (160-288): (0.67-1.25): (0.4-1.3): (0.5-1.6) by mass, and the foam stabilizer is hydroxypropyl methylcellulose; Step 2, uniformly mixing the dry material, the water reducer and water to obtain a slurry, wherein the ratio of cement, the water reducer and water in the dry material is (400-960): (6-12): (334-600) by weight, and the water reducer is polycarboxylic acid; Step 3, adding a foaming agent to the slurry, stirring until uniform, standing for foaming, and curing to obtain fly ash foamed concrete, wherein the ratio of cement to foaming agent in the slurry is (400-960): (14-20) by weight; The method for evaluating the compressive strength of fly ash foam concrete based on fly ash content comprises the following steps: S1. Prepare N fly ash foam concrete samples with the same density. The fly ash content of the N fly ash foam concrete samples is different. Obtain a fitting formula for each fly ash foam concrete according to the following method: subject the fly ash foam concrete to a frost resistance test. After each freeze-thaw cycle in the frost resistance test, test the porosity and compressive strength of the fly ash foam concrete. Fit the porosity of the fly ash foam concrete to the number of freeze-thaw cycles to obtain a fitting formula: , where: is the porosity of fly ash foam concrete, is the coefficient related to the fly ash content, t is the number of freeze-thaw cycles; S2, according to the fitting formula of N fly ash foam concrete, the fly ash content is taken as the independent variable, Fitting as the dependent variable yields The relationship between them and fly ash content respectively; S3, will Substitute the relationship between fly ash content and the fitting formula in S1 respectively , the porosity change formula with fly ash content and freeze-thaw cycles as variables was obtained; S4, according to the porosity and compressive strength of N fly ash foam concretes after each freeze-thaw in S1, the relationship formula between porosity and compressive strength is obtained by fitting; S5. Substitute the porosity change formula in S3 with fly ash content and freeze-thaw cycles as variables into the porosity and compressive strength relationship formula in S4 to obtain the quantitative relationship between fly ash content, freeze-thaw cycles and compressive strength of fly ash foam concrete at this density.

2. The method according to claim 1, characterized in that In S1, N is greater than or equal to 3.

3. The method according to claim 1 or 2, characterized in that In S1, the fly ash content in fly ash foam concrete is 20~50%, and the fly ash content = the mass of fly ash in fly ash foam concrete / (the mass of cement in fly ash foam concrete + the mass of fly ash in fly ash foam concrete).

4. The method according to claim 3, characterized in that In S1, the density is 500~1500 kg / m 3 .

5. The method according to claim 1, wherein In step 3, the foaming agent is hydrogen peroxide.

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