CN117809768A - Method for evaluating compressive strength of fly ash foam concrete based on density - Google Patents

Method for evaluating compressive strength of fly ash foam concrete based on density Download PDF

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CN117809768A
CN117809768A CN202410220464.7A CN202410220464A CN117809768A CN 117809768 A CN117809768 A CN 117809768A CN 202410220464 A CN202410220464 A CN 202410220464A CN 117809768 A CN117809768 A CN 117809768A
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fly ash
foam concrete
freeze
density
ash foam
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CN117809768B (en
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郝贠洪
孙浩
秦立达
王洪泽
侯永利
何晓雁
史勇
时金娜
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Inner Mongolia University of Technology
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Inner Mongolia University of Technology
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    • Y02W30/91—Use of waste materials as fillers for mortars or concrete

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Abstract

The invention discloses a method for evaluating the compressive strength of fly ash foam concrete based on density, wherein the preparation method of the fly ash foam concrete comprises the following steps: mixing cement, fly ash, quartz powder, quartz sand, polypropylene fiber, foam stabilizer and manganese dioxide, stirring to uniformity to obtain a dry material, mixing the dry material, a water reducing agent and water uniformly to obtain a slurry, adding a foaming agent into the slurry, stirring to uniformity, standing for foaming, and curing to obtain the fly ash foam concrete.

Description

Method for evaluating compressive strength of fly ash foam concrete based on density
Technical Field
The invention belongs to the technical field of concrete, and particularly relates to a method for evaluating compression strength of fly ash foam concrete based on density.
Background
With the development of economy, energy consumption has become a significant problem facing the world. The building industry has the characteristics of high energy consumption and high carbon dioxide emission, and occupies a large proportion in the global energy consumption. The implementation of energy conservation and emission reduction is a necessary trend of development in the field of buildings.
The novel wall material is gradually popularized and applied. At present, the types of novel wall materials are increased gradually, and all performances are optimized gradually, but ideal wall materials integrated with heat insulation structures are still under research and development. Because the foam concrete is a lightweight porous material, has certain strength and excellent heat preservation and insulation performance, and is an ideal innovation when being applied to researching a wall material integrating a heat preservation structure. However, a large amount of cement is used in the process of preparing the foam concrete, which does not conform to the sustainable development concept of low carbon and energy conservation.
According to statistics, about 250 kg-300 kg of fly ash is produced when 1t of coal is burned, the fly ash is used as industrial solid waste, and the fly ash is applied to foam concrete to replace part of cement, so that the porosity of slurry is reduced, the durability of the foam concrete is improved, the resource consumption and the environmental burden are reduced, and the recycling of the solid waste can be promoted.
The foam concrete is widely applied, but due to the fact that a large number of air holes exist in the foam concrete, the foam concrete has the problems of low strength, poor freezing resistance and the like under the action of frost heaving force. Therefore, the foam concrete has great application difference in the north and south. The weather in the south area is warmer, the requirements on the frost resistance of the material are not high, and the performance requirements are met when the foam concrete is used as a heat insulation material for walls and roofs. However, in the northern colder areas, the foam concrete is required to have good heat insulation performance and certain freezing resistance. Therefore, how to effectively improve the frost resistance of the foam concrete is a key problem for popularization and application in cold areas.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a preparation method of the load-bearing and frost-resistant fly ash foam concrete.
The invention also aims to provide the load-bearing and freezing-resistant fly ash foam concrete obtained by the preparation method.
The invention further aims to provide a method for evaluating the compressive strength of the fly ash foam concrete based on the fly ash mixing amount.
Another object of the invention is to provide a method for evaluating the compressive strength of fly ash foam concrete based on density.
The aim of the invention is achieved by the following technical scheme.
A preparation method of a load-bearing and frost-resistant fly ash foam concrete comprises the following steps:
step 1, mixing cement, fly ash, quartz powder, quartz sand, polypropylene fiber, a foam stabilizer and manganese dioxide, and stirring to be uniform to obtain a dry material, wherein the ratio of the cement to the fly ash to the quartz powder to the quartz sand to the polypropylene fiber to the foam stabilizer to the manganese dioxide is (400-960): (240-600): (374-672): (160-288): (0.67 to 1.25): (0.4 to 1.3): (0.5 to 1.6);
in the step 1, stirring is carried out for 90-120 s until the stirring is uniform, and then a dry material is obtained.
In the step 1, the foam stabilizer is hydroxypropyl methylcellulose (HPMC).
Step 2, uniformly mixing the dry material, the water reducer and water to obtain slurry, wherein the ratio of cement to the water reducer to the water in the dry material is (400-960): (6-12): (334-600);
in the step 2, the water reducing agent is a polycarboxylic acid.
In the step 2, the uniform mixing is realized by stirring for 120-240 s.
And step 3, adding a foaming agent into the slurry, stirring the mixture to be uniform, standing the mixture for foaming, and curing the mixture to obtain the fly ash foam concrete, wherein the ratio of cement to the foaming agent in the slurry is (400-960) in parts by weight: (14-20).
In the step 3, the foaming agent is hydrogen peroxide.
In the step 3, stirring is carried out for 60-90 s after the foaming agent is added, so that stirring is uniform, and the stirring rotating speed is 120-150 r/min.
In the step 3, standing and foaming are carried out for 24-30 hours.
In the step 3, the curing time is 28-35 days.
In the step 3, the temperature of the environment where the maintenance is located is 18-22 ℃, and the relative humidity of the environment where the maintenance is located is more than 95%.
A method for evaluating the compressive strength of fly ash foam concrete based on the mixing amount of fly ash comprises the following steps:
s1, preparing N fly ash foam concrete with the same density as a sample, wherein the fly ash mixing amounts of the N fly ash foam concrete in the sample are different, and the fitting formula of each fly ash foam concrete is obtained according to the following method: carrying out an anti-freezing test on the fly ash foam concrete, testing the porosity and compressive strength of the fly ash foam concrete after each freeze thawing in the anti-freezing test, and fitting the porosity and freeze thawing cycle times of the fly ash foam concrete to obtain a fitting formula:
wherein: />Is the porosity of the fly ash foam concrete, +.>The coefficient related to the mixing amount of the fly ash is t is the number of freeze thawing cycles;
In S1, N is 3 or more.
In S1, the mixing amount of the fly ash in the fly ash foam concrete is 20-50%, and the mixing amount of the fly ash=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).
In S1, an antifreeze test is carried out according to industry standards.
In S1, the density is 500-1500 kg/m 3 。
S2, taking the fly ash mixing amount as an independent variable according to a fitting formula of N fly ash foam concreteFitting as dependent variable to obtain +.>Respectively mixing the fly ash with the relation formula of the mixing amount of the fly ash;
s3, willRespectively substituting the relation between the fly ash and the mixing amount of the fly ash into the fitting formula in S1Obtaining a porosity change formula taking the mixing amount of the fly ash and the freeze thawing cycle number as variables;
s4, fitting according to the porosity and compressive strength of the N fly ash foam concrete in the S1 after each freeze thawing to obtain a relation formula of the porosity and the compressive strength;
s5, substituting the porosity change formula taking the fly ash mixing amount and the freeze thawing cycle number as variables in the S3 into the relation formula of the porosity and the compressive strength in the S4 to obtain the quantitative relation of the fly ash mixing amount, the freeze thawing cycle number and the compressive strength of the fly ash foam concrete under the density.
A method for evaluating the compressive strength of fly ash foam concrete based on density, comprising the following steps:
s1, preparing N fly ash foam concrete with the same fly ash mixing amount as a sample, wherein the densities of the N fly ash foam concrete in the sample are different, and obtaining a fitting formula of each fly ash foam concrete according to the following method: carrying out an anti-freezing test on the fly ash foam concrete, testing the inter-particle pore volume and the compressive strength of the fly ash foam concrete after each freeze thawing in the anti-freezing test, and fitting the inter-particle pore volume and the freeze thawing cycle times of the fly ash foam concrete to obtain a fitting formula:wherein: />Is the inter-particle pore volume of the fly ash foam concrete,the coefficient related to the density of the fly ash foam concrete is t is the number of freeze thawing cycles;
in S1, N is 3 or more.
In S1, the mixing amount of the fly ash in the fly ash foam concrete is 20-50%, and the mixing amount of the fly ash=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).
In S1, the density is 500-1500 kg/m 3 。
In S1, an antifreeze test is carried out according to industry standards.
S2, taking the density of the fly ash foam concrete as an independent variable according to a fitting formula of N fly ash foam concrete, and taking the density of the fly ash foam concrete as an independent variableFitting as dependent variable to obtain +.>Respectively and density;
s3, willThe relation between the density and the formula of the fitting in S1 is substituted into +.>Obtaining a particle-to-particle pore volume change formula taking density and freeze thawing cycle times as variables;
s4, fitting according to the inter-particle pore volume and the compressive strength of the N fly ash foam concretes obtained in the S1 after each freeze thawing, and obtaining a relationship formula of the inter-particle pore volume and the compressive strength;
s5, substituting the inter-particle pore volume change formula taking the density and the freeze thawing cycle times as variables in the S3 into the inter-particle pore volume and compressive strength relation formula in the S4 to obtain the quantitative relation of the density, the freeze thawing cycle times and the compressive strength of the fly ash foam concrete under the fly ash mixing amount.
Compared with the prior art, the invention has the following beneficial effects:
1. the preparation method of the invention utilizes the industrial solid waste fly ash to develop, and the fly ash foam concrete solves the problems of low strength and poor freezing resistance of the common foam concrete: the freezing resistance grade is improved to be more than D50, so that engineering application requirements in severe cold areas are met;
2. Because the fly ash foam concrete contains a large number of bubbles, the pore structure of the fly ash foam concrete has obvious difference from that of common concrete, and under the action of freeze thawing circulation, the gel pores of the fly ash foam concrete with different fly ash mixing amounts are sequentially deteriorated to the inter-particle pores and the inter-particle Kong Xianghong pores, and the porosity is gradually increased. Under the action of freeze thawing circulation, the porosity of the fly ash foam concrete with different density grades is gradually increased, and the higher the density grade is, the stronger the freezing resistance is. 1200 kg/m 3 Above mentionedThe high-density fly ash foam concrete pores of the (2) are divided into gel pores, inter-particle pores and macro pores, wherein the inter-particle Kong Zhanbi is the largest, and the gel pores sequentially evolve to the inter-particle pores and the inter-particle Kong Xianghong pores under the action of freeze thawing circulation; 800 kg/m 3 The pores of the low-density fly ash foam concrete below are Kong Hehong pores among particles, do not contain gel pores, and are gradually deteriorated to macro pores under the influence of freeze thawing circulation.
3. Because the fly ash foam concrete is a porous and multiphase composite material, the pore structure parameters directly influence the mechanical property and the freezing resistance of the fly ash foam concrete. Meanwhile, the change of pore structure parameters of the fly ash foam concrete is different from that of the common concrete under the action of freeze thawing cycle, and the compressive strength of the fly ash foam concrete can be accurately estimated by means of the method for estimating the compressive strength of the fly ash foam concrete based on the fly ash mixing amount or the method for estimating the compressive strength of the fly ash foam concrete based on the density.
Drawings
FIG. 1 shows the mass loss rate of the fly ash foam concrete prepared in examples 1-4 after being subjected to 0, 15, 25, 35, 50, 65 freeze-thawing cycles;
FIG. 2 shows the compression strength loss rate of the fly ash foam concrete prepared in examples 1-4 after being subjected to 0, 15, 25, 35, 50 and 65 freeze thawing cycles;
FIG. 3 shows the mass loss rate of the fly ash foam concrete prepared in examples 3 and 5-7 after being subjected to 0, 15, 25, 35, 50, 65 freeze-thawing cycles;
FIG. 4 shows the compression strength loss rate of the fly ash foam concrete prepared in examples 3 and 5-7 after being subjected to 0, 15, 25, 35, 50 and 65 freeze thawing cycles;
FIG. 5 is a photograph of the fly ash foam concrete and the magnesium-based salt foam concrete before and after the freeze-thawing cycle, wherein (a) is the form of the fly ash foam concrete prepared in example 7 before the freeze-thawing cycle, (b) is the form of the fly ash foam concrete prepared in example 7 after the freeze-thawing cycle for 25 times, (c) is the form of the magnesium-based salt foam concrete before the freeze-thawing cycle, and (d) is the form of the magnesium-based salt foam concrete after the freeze-thawing cycle for 10 times;
FIG. 6 is a graph showing the relationship between compressive strength and freeze-thawing cycle times of the fly ash foam concrete prepared in examples 1 to 4;
FIG. 7 shows the fly ash loadingAs an independent variable, a 1 Fitting results of dependent variables;
FIG. 8 shows the fly ash loadingAs an independent variable, b 1 Fitting results of dependent variables;
FIG. 9 is a graph of compressive strength versus freeze-thaw cycle times for fly ash foam concrete of different density grades;
FIG. 10 is a densityAs an independent variable, a 2 Fitting results of dependent variables;
FIG. 11 is a densityAs an independent variable, b 2 Fitting results of dependent variables;
FIG. 12 is a graph showing the relationship between the porosity and the number of freeze thawing cycles of the fly ash foam concrete prepared in examples 1 to 4;
FIG. 13 shows the fly ash loadingIs an independent variable,l 1 Fitting results of dependent variables;
FIG. 14 shows the fly ash loadingIs an independent variable, m 1 Fitting results of dependent variables;
FIG. 15 shows the fly ash loadingIs an independent variable, n 1 Fitting to dependent variablesResults;
FIG. 16 is a mathematical model of freeze-thaw damage of fly ash foam concrete with different fly ash loadings;
FIG. 17 is a graph showing the relationship between the pore volume and the number of freeze thawing cycles between fly ash foam concrete particles prepared in examples 3 and 5 to 7;
FIG. 18 is a density ofIs an independent variable,l 2 Fitting results of dependent variables;
FIG. 19 is a densityIs an independent variable, m 2 Fitting results of dependent variables;
FIG. 20 is a densityIs an independent variable, n 2 Fitting results of dependent variables;
FIG. 21 is a mathematical model of freeze-thaw damage of fly ash foam concrete of different densities;
FIG. 22 is a graph showing the variation of pore volume ratio of different sizes of fly ash foam concrete and magnesium-based salt foam concrete with the number of freeze-thaw cycles.
In the figures, the "fly ash mixing amount 20%" or "20%" is example 1, the "fly ash mixing amount 30%" or "30%" is example 2, the "fly ash mixing amount 40%" or "40%" is example 3, and the "fly ash mixing amount 50%" or "50%" is example 4; "500kg/m 3 Or density 500kg/m 3 "is example 7, 800kg/m 3 Or "density 800kg/m 3 "is example 6, 1200, kg/m 3 Or "density 1200kg/m 3 "is example 5, 1500kg/m 3 Or "density 1500kg/m 3 "is example 3.
Detailed Description
The fly ash foam concrete of the present invention, and the preparation method and application thereof are described in detail below with reference to the accompanying drawings and examples.
The cement in the following examples is Jidong P.O42.5 grade Portland cement, the main chemical composition of the cement is as follows:
the physical performance indexes of the cement are as follows:
the fly ash in the following examples is grade II fly ash produced by Donghua power plant, and the chemical composition and physical properties of the fly ash are as follows:
The fineness of the quartz sand in the following examples is 40-70 meshes, and the particle size range is 0.25-0.45 mm; the fineness of the quartz powder is 200-400 meshes, and the particle size range is 0.037-0.075 mm.
The polypropylene fibers in the following examples were purchased from the gallery bissen building materials, inc., and the performance indexes are as follows:
the foaming agent hydrogen peroxide is hydrogen peroxide (H) with concentration of 30wt% produced by Tianjin metallocene chemical reagent factory 2 O 2 ）。
Polycarboxylic acid: the water reducing rate is 37 percent.
Hydroxypropyl methylcellulose: the viscosity is 20 ten thousand mpa.s, and Shanghai ministerial chemical industry and technology limited company.
The catalyst in the following examples was manganese dioxide (MnO) 2 ) Analytically pure, produced by the field of metallocene chemical reagent plant.
The water in the examples described below is tap water.
The density in the following examples is dry density, and the dry density test is carried out according to the Chinese industry standard JGT266-2011 foam concrete.
Compressive strength: according to the test of Chinese industry standard JGT266-2011 foam concrete.
Fly ash foam concrete antifreeze test: according to the technical procedure of foam concrete application of the industry standard JGJ/T341-2014, the main flow of the freeze thawing cycle test is as follows: taking out the test piece (100 mm ×100 mm ×100 mm cube) cured for 28d, placing into an electrothermal blowing drying oven, preserving heat at 60+ -5deg.C for 24h, and oven drying at 80deg.C to constant quality; after the test piece is cooled, the mass of the test piece is weighed as Placing the test piece into a water tank with the temperature of (20+/-5) ℃ and keeping for 48 hours; and finally, taking out the test piece, wiping off surface moisture, putting the test piece into a freezing chamber which is used for simulating a durability damage experiment system of engineering materials in natural environment and is cooled to below minus 15 ℃ in advance, and carrying out repeated freeze-thawing cycle experiments, and stopping the freeze-thawing cycle experiments when the test piece is damaged, the mass loss rate of the test piece exceeds 5% or the compressive strength loss rate exceeds 25%, wherein each freeze-thawing cycle experiment comprises the following steps: freezing in air at minus temperature (-20+ -2deg.C) for 6h, and thawing in water tank at plus temperature (20+ -5deg.C) for 5h. Drying the test piece at 80 ℃ after each freeze thawing cycle experiment, and weighing the mass +.>Compressive strength tests were performed.
Wherein, the mass loss rate:，/>: mass loss rate of test piece after n times of freeze thawing cycles, unit (%); />: the mass of the test piece before freeze thawing cycle; />: the mass of the test piece after n times of freeze thawing cycles.
Preparing two identical test pieces, wherein one test piece is subjected to an anti-freezing test, the other test piece is used as a control test piece, the control test piece is placed in a room under the room temperature condition, and after each freeze-thawing cycle experiment is completed, the test piece which completes the freeze-thawing cycle experiment and the control test piece are taken to test the compressive strength at the same time, so that the compressive strength loss rate of the test piece of the freeze-thawing cycle experiment is calculated.
Compression strength loss rate:
wherein:: the compressive strength loss rate of the test piece after n times of freeze thawing cycles is in units of (%); />: controlling the compressive strength of the test piece in units of MPa; />: compressive strength of test piece after n times of freeze thawing cycles, and unit (MPa).
Hole structure test: the test piece was cut into cubes of 40mm×40mm using a cutter, placed in a vacuum water-retention machine, and vacuum-retained for 24 hours, and tested using a Nuclear Magnetic Resonance (NMR) to obtain porosity, pore volume. The nuclear magnetic resonance instrument is kept at a constant temperature of 32 ℃ and the scanning range of the magnet is 0-60 mm.
Examples 1 to 7
A preparation method of a load-bearing and frost-resistant fly ash foam concrete comprises the following steps:
step 1, mixing cement, fly ash, quartz powder, quartz sand, polypropylene fiber, a foam stabilizer and manganese dioxide in a stirring barrel, and stirring for 120s by using a manual stirrer to obtain dry materials, wherein the ratio of the cement, the fly ash, the quartz powder, the quartz sand, the polypropylene fiber, the foam stabilizer and the manganese dioxide is X, and the foam stabilizer is hydroxypropyl methyl cellulose (HPMC);
step 2, mixing and stirring the dry material, the water reducer and water for 120s until the mixture is uniform to obtain slurry in a uniform flow state, wherein the ratio of the cement to the water reducer to the water in the dry material is Y, and the water reducer is polycarboxylic acid according to parts by weight;
And 3, adding a foaming agent into the slurry, stirring at a rotating speed of 120r/min for 60s to be uniform, pouring into a 100mm multiplied by 100mm mold, standing and foaming for 24h, and curing for 28 days (relative humidity is more than 95%) in a standard curing chamber at 18-22 ℃ to obtain the fly ash foam concrete, wherein the ratio of cement to the foaming agent in the slurry is Z, and the foaming agent is hydrogen peroxide.
The X value, Y value, Z value, fly ash blending amount and density are shown in Table 1.
TABLE 1
Fly ash loading = mass of fly ash/(mass of cement + mass of fly ash) in table 1.
Comparative example 1
The density of the prepared load-bearing and heat-insulating integrated foam concrete with reference to publication No. CN111410506A is 500kg/m 3 Magnesium-based salt foam concrete of (b). The preparation method of the magnesium-based salt foam concrete specifically comprises the following steps: (1) Mixing 259 parts by weight of water-chlorostone and 224 parts by weight of water, stirring until white crystals in the solution disappear, and adding 5.8 parts by weight of JSM-1 type polycarboxylate superplasticizer to obtain a mixed solution A; (2) Mixing 435 parts by weight of light burned magnesium oxide, 185 parts by weight of fly ash, 40 parts by weight of silica fume and 1.6 parts by weight of polypropylene fiber to obtain a mixture B; (3) Stirring the mixture B at a speed of 200r/min for 10min to uniformly disperse the polypropylene fibers in the mixture B; (4) Adding the mixed solution A into the mixture B, and uniformly stirring at the speed of 200 r/min; (5) Sequentially adding 5.0 parts by mass of hydrogen peroxide and 0.8 part by mass of hydroxypropyl methylcellulose, and stirring at a speed of 550r/min for 100s to obtain a mixture C; (6) Pouring the mixture C for molding, standing for 24 hours, demolding, and naturally curing for 28 days.
And (3) performing a freeze-thawing cycle test on the fly ash foam concrete of the examples 1-4, and researching the change rule of the mass loss rate and the compressive strength loss rate of the fly ash foam concrete with different fly ash mixing amounts along with the freeze-thawing cycle times.
The mass loss rates of fly ash foam concrete with different fly ash contents after being subjected to 0, 15, 25, 35, 50 and 65 freeze thawing cycles are shown in table 2 and fig. 1.
Table 2 mass loss (%) of fly ash foam concrete after different freeze-thaw cycles.
Note that: in the table "-" indicates that the freeze-thaw cycle experiment stop condition has been reached, and the freeze-thaw cycle experiment is stopped.
As can be seen from Table 2 and FIG. 1, the mass loss rate of the fly ash foam concrete with different fly ash contents is increased with the increase of the freeze-thawing cycle times. The fly ash foam concrete is a porous material, saturated water in the pores is frozen under the action of freeze thawing circulation, and the expansion pressure generated by the freezing expansion on the pore walls of the pores is gradually increased, so that the pore walls of the fly ash foam concrete form cracks and are gradually expanded, and the pores are mutually communicated. Therefore, as the freeze-thawing cycle proceeds, the surface layer and edges of the fly ash foam concrete gradually peel off. When the number of freeze thawing cycles is fixed, the quality loss rate of the fly ash foam concrete is firstly reduced and then increased along with the increase of the mixing amount of the fly ash. Analyzing the reason: the fly ash has the function of filling micro aggregates, and extremely fine micro beads and fragments are uniformly distributed in the cement paste, so that pores and capillary holes are filled; meanwhile, the silicate glass body of the fly ash can promote the secondary hydration of cement to generate hydrated calcium silicate gel, thereby playing a role in reinforcing slurry. When the mixing amount of the fly ash is increased, the redundant fly ash causes part of bubbles to be broken to form deformed holes and communicating holes, the porosity is increased, and the mass loss rate is increased.
The compressive strength results of the fly ash foam concrete with different fly ash contents after being subjected to 0, 15, 25, 35, 50 and 65 freeze thawing cycles are shown in table 3, and the compressive strength loss rate is shown in fig. 2.
TABLE 3 compressive Strength (MPa) of fly ash foam concrete (test piece) after freeze-thaw cycle
Note that: in the table "-" indicates that the freeze-thaw cycle experiment stop condition has been reached, and the freeze-thaw cycle experiment is stopped.
As can be seen from Table 3, the compressive strength of the fly ash foam concrete before freezing and thawing increases and decreases with the increasing amount of the fly ash. The fly ash exerts the micro-aggregate effect in the fly ash foam concrete, and a proper amount of fly ash particles are dispersed in the cement slurry to fill the pores, so that the strength of the fly ash foam concrete is improved. However, as the amount of fly ash added increases, the proportion of cement replaced by the fly ash increases, resulting in a decrease in cement hydration products, and thus a decrease in compressive strength. Along with the increase of the freeze thawing cycle times, the compressive strength of the fly ash foam concrete with different fly ash mixing amounts is continuously reduced.
As can be seen from fig. 2, under the same freeze-thawing cycle times, the loss rate of the compressive strength of the fly ash foam concrete is firstly reduced and then increased along with the increase of the mixing amount of the fly ash. Analyzing the reason: when the mixing amount of the fly ash is smaller, hydrated calcium silicate generated in the process of promoting cement hydration of the fly ash can be used as a filler to fill pores among cements, so that the pore structure is improved, and the freeze-thawing cycle resistance of the fly ash foam concrete is improved; when the mixing amount of the fly ash is large, the redundant fly ash causes partial bubbles to be broken and combined, and meanwhile, the water absorption rate of the test block is increased, so that the freezing resistance is reduced, and the strength loss rate is increased.
According to the industrial standard JGJ/T341-2014 foam concrete application technical specification, the freezing resistance of the test piece is evaluated according to the average mass loss rate of not more than 5% and the compressive strength loss rate of not more than 25%. Therefore, the fly ash foam concrete with the fly ash mixing amount of 20%, 30%, 40% and 50% has the best freezing resistance with the fly ash mixing amount of 30% and has the freezing resistance grades of D35, D65, D50 and D25 respectively.
Fly ash foam concrete of example 3 and examples 5 to 7 (density 1500kg/m 3 、1200 kg/m 3 、800 kg/m 3 、500 kg/m 3 ) And (3) performing a freeze-thawing cycle test, and comparing the change conditions of the frost resistance of the fly ash foam concrete with different density grades after the fly ash foam concrete undergoes the freeze-thawing cycle action by taking the mass loss rate and the compressive strength loss rate as indexes.
The mass loss rates 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 thawing cycles are shown in table 4 and fig. 3.
TABLE 4 Mass loss Rate of test pieces after different freeze-thaw cycles (%)
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped.
As can be seen from table 4 and fig. 3, the mass loss rate of the fly ash foam concrete with different densities gradually increases during the freeze-thawing cycle, but the mass loss rate increases relatively less in the early stage of the freeze-thawing cycle, and the mass loss rate of the fly ash foam concrete increases significantly after the number of freeze-thawing cycles reaches a certain critical point. The fly ash foam concrete with different densities has critical points affecting the freezing resistance, the internal structure of the fly ash foam concrete is changed after the critical points are reached, the freezing-thawing damage degree is obviously increased, and the freezing resistance is gradually reduced.
The mass loss rate of the fly ash foam concrete with different densities shows a trend of gradually increasing along with the increase of the freeze thawing cycle times, but the earlier-stage mass loss of the test piece with lower density is more obvious, and the later-stage high-loss state of the test piece with higher density only occurs. This is because the fly ash foam concrete is soaked for 48 hours before the freeze-thawing cycle (only once, not before each freeze-thawing cycle), the low density test piece has a large porosity, and will reach a saturated state during the soaking process, while the high density test piece is not fully saturated. Therefore, the low-density fly ash foam concrete has large water freezing capacity in the early stage of freeze thawing cycle, the pore wall of the low-density fly ash foam concrete has poor capability of resisting expansion stress, and the pore wall is gradually broken under the action of freeze thawing cycle. The high-density fly ash foam concrete has compact internal structure, lower water saturation and lower mass loss in the early stage of freeze thawing cycle; and after reaching the freezing-thawing cycle critical point, the pore structure of the fly ash foam concrete is frozen by the internal pore water to generate a frost heaving effect, the pore walls are broken, and pores are mutually communicated.
The compressive strengths of the fly ash foam concretes with different densities after being subjected to 0, 15, 25, 35, 50 and 65 freeze thawing cycles are shown in table 5, and the loss rates of the compressive strengths are shown in fig. 4.
TABLE 5 compressive Strength (MPa) of fly ash foam concrete (test piece) after freeze-thaw cycle
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped.
As can be seen from Table 5, the compressive strength of the fly ash foam concrete of different densities was continuously decreased as the number of freeze-thawing cycles was increased. Because fly ash foam concrete with different densities has different freeze-thawing cycle resistance, the test piece has different freeze-thawing cycle times when reaching the stop condition of the freeze-thawing cycle test, and the higher the density level is, the better the freezing resistance is.
As can be seen from fig. 4, the compression strength loss rate of the fly ash foam concrete with different densities increases with the increase of the freeze-thawing cycle times, the compression strength loss rate at the early stage slowly increases, and the compression strength loss rate at the later stage increases faster. This indicates that there is a critical freeze-thaw cycle time, the compression strength of the fly ash foam concrete drops slowly before the critical point, the loss rate of compression strength is less, and after exceeding the critical freeze-thaw cycle time, the loss rate of compression strength increases rapidly. Meanwhile, the lower the density is, the lower the critical freeze-thawing cycle times of the fly ash foam concrete are, and the higher the critical freeze-thawing cycle times of the reverse density is. The main reason is that the fly ash foam concrete with smaller density has large porosity and thinner pore wall, has weaker capacity of resisting freeze thawing damage, can generate certain damage in the early stage of freeze thawing cycle, and gradually loses compressive strength. The fly ash foam concrete with higher density does not reach a saturated state in the early stage of freeze thawing cycle, and has certain capacity of resisting freeze thawing cycle damage; as the number of freeze thawing cycles increases, the water absorption saturation of the fly ash foam concrete test piece is continuously increased, the damage range of the inner air hole wall of the fly ash foam concrete test piece under the action of frost heaving force is gradually increased, and the compression strength loss rate is increased.
According to the industrial standard JGJ/T341-2014 foam concrete application technical specification, the freezing resistance of the test piece is evaluated according to the average mass loss rate of not more than 5% and the compressive strength loss rate of not more than 25%. The fly ash foam concrete of examples 3 and 5-7 had freeze resistance ratings of D50, D35, D25 and D15, respectively.
The fly ash foam concrete prepared in example 7 and the magnesium-based salt foam concrete prepared in comparative example 1 all belong to foam concrete. 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 after freeze thawing cycles of 0, 15 and 25 are shown in Table 6.
TABLE 6 Mass loss Rate of test pieces after different freeze-thaw cycles (%)
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped.
TABLE 7 compression strength loss Rate (%)
TABLE 7
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped.
The mass loss rate of the fly ash foam concrete prepared in the example 7 and the magnesium-based salt foam concrete prepared in the comparative example 1 is less than 5% when the freeze-thawing cycle experiment is stopped, and the requirements of the stop experiment are not met, but the compressive strength loss rate of the fly ash foam concrete under the corresponding freeze-thawing cycle times is more than 25%, so that the freeze-thawing cycle experiment is stopped.
The frost resistance grade of the magnesium-based salt foam concrete prepared in comparative example 1 is D5, and the fly ash foam concrete prepared in example 7 has better frost resistance.
The number of freeze-thawing cycles that the fly ash foam concrete can withstand is different from that of the magnesium-based salt foam concrete, and fig. 5 (a) shows the form before the fly ash foam concrete has freeze-thawing cycles, fig. 5 (b) shows the form after the fly ash foam concrete has freeze-thawing cycles for 25 times, fig. 5 (c) shows the form before the magnesium-based salt foam concrete has freeze-thawing cycles, and fig. 5 (d) shows the form after the magnesium-based salt foam concrete has freeze-thawing cycles for 10 times. As can be seen from FIGS. 5 (a) - (d), the two foam concrete surfaces were smoother before the freeze-thawing cycle, macroscopic pores were distributed, and the pore size distribution of the magnesium-based salt foam concrete surface was more uniform than that of the fly ash foam concrete before the freeze-thawing cycle. With increasing freeze-thaw cycles, both foam concretes showed different degrees of damage. When the fly ash foam concrete reaches freeze thawing damage (freeze thawing cycle is 25 times), the surface of a test piece becomes rough, pore walls are broken, pore diameters of the pores become large and are communicated, and phenomena of small cracks, corner peeling and fiber exposure appear. The magnesium-based salt foam concrete has serious freeze thawing damage, not only the pore wall fracture pore diameter becomes large, but also the phenomena of large cracks and large corner drops which extend transversely and longitudinally and are penetrated are generated on the surface of a test piece. This is mainly because magnesium-based salt foam concrete has poor water resistance, is easily hydrolyzed under the action of water during a freeze-thawing cycle, and gradually cracks under the action of freezing expansion force. This also explains why magnesium-based salt foam concrete has a greater loss of quality and strength than fly ash foam concrete.
Comparative example 2
The relationship between the compressive strength and the number of freeze-thawing cycles of the fly ash foam concrete of each example was fitted according to the compressive strength and the number of freeze-thawing cycles in table 3, respectively, as shown in fig. 6. As can be seen from fig. 6, as the number of freeze-thawing cycles increases, the compressive strength of the fly ash foam concrete with different fly ash contents gradually decreases and linearly decreases. The compressive strength was fitted as follows:
wherein:for compression strength->Is the coefficient related to the mixing amount of the fly ash, +.>Is the number of freeze thawing cycles. Mixing the fly ash with->As an independent variable ++>Fitting again for dependent variables:
，
，/>the mixing amount of the fly ash is as follows. The fitting results are shown in fig. 7 and 8. Therefore, the compressive strength change formula taking the mixing amount of the fly ash and the freeze thawing cycle number as variables is as follows:
wherein: />Is resistant toCompressive strength (I)>The mixing amount of the fly ash is; />Is the number of freeze thawing cycles.
Comparative example 3
The relationship between the compressive strength and the number of freeze-thawing cycles of the fly ash foam concrete of each example was fitted based on the compressive strength and the number of freeze-thawing cycles obtained in table 5, as shown in fig. 9. As can be seen from fig. 9, as the number of freeze-thawing cycles increases, the compressive strength of the fly ash foam concrete of different density grades gradually decreases and linearly decreases. The compressive strength was fitted as follows:
Wherein:for compression strength->Is a coefficient related to density, and t is the number of freeze-thawing cycles. Density->As an independent variable ++>Fitting again for dependent variables:
，/>，/>the fitting results are shown in fig. 10 and 11 for the density of the fly ash foam concrete.
Therefore, the compressive strength change formula taking the number of freeze thawing cycles and the density as variables is as follows:
wherein: />For compression strength->For freeze thawing cycle number, +.>Is the density of the fly ash foam concrete.
The porosity of the fly ash foam concrete prepared in examples 1 to 4 is shown in Table 8 as a function of the number of freeze thawing cycles.
TABLE 8 porosity of test pieces after freeze-thaw cycle (%)
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped.
The relationship between the porosity of the fly ash foam concrete with different densities and the number of freeze thawing cycles is shown in Table 9.
TABLE 9 porosity of test pieces after freeze-thaw cycle (%)
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped. The "×" indicates that the number of freeze-thaw cycles has too little effect on the experimental results, and is negligible.
The porosities after freeze thawing cycles of the fly ash foam concrete prepared in example 7 and the magnesium-based salt foam concrete prepared in comparative example 1 are shown in table 10.
Table 10 porosity of test pieces after freeze-thaw cycle (%)
Note that: in the table "-" indicates that the test piece has reached the freeze-thawing cycle test stop condition, and the freeze-thawing cycle test is stopped.
Example 8
Because the fly ash foam concrete contains a large number of bubbles, the pore structure of the fly ash foam concrete is obviously different from that of common concrete. To investigate 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 classes, reference (TADA S, NAKANO S 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 foam concrete performance and control technique [ J ]. Material guide, 2012, 26 (07): 141-144+153.): gel pores (pore diameter <50 nm), particle pores (pore diameter 50 nm-50 μm), macropores (pore diameter >50 μm). The variation of pore volume ratios of different sizes of fly ash foam concrete and magnesium-based salt foam concrete with the number of freeze-thawing cycles is shown in fig. 22. Table 11 shows the porosity, gel pores (< 50 nm), particle pores (50 nm to 50 μm), macro pores (> 50 μm) and compressive strength of the fly ash foam concrete prepared in examples 1 to 4 according to the number of freeze thawing cycles.
TABLE 11
The pore structure of the fly ash foam concrete with different fly ash doping amounts is gradually deteriorated under the action of freeze thawing cycle, and as can be seen from the table 11, the gray entropy correlation degree between the pore structure parameters and the compressive strength is calculated by taking the pore volume value corresponding to the porosity, the inter-gel pores (< 50 nm), the pore volume value corresponding to the inter-particle pores (50 nm-50 μm) and the pore volume value corresponding to the macro pores (> 50 μm) as comparison sequences according to gray entropy correlation analysis methods (Liu Xin, eastern, xue Huijun, etc.. Gray entropy correlation analysis of the cement solidification arsenic sandstone strength and the pore structure evolution [ J ]. Agricultural engineering report, 2020,36 (24): 125-133 ]), as shown in the table 12.
Table 12
By comparing the gray entropy correlation of the pore structure parameters, the gray entropy correlation of the compressive strength of the fly ash foam concrete with different fly ash contents after the freeze thawing cycle and the pore structure parameters is sequentially that the porosity is equal to the inter-particle pore volume, the gel is equal to the macro pore volume. Thus, the porosity is a major factor causing a decrease in compressive strength.
The porosity and freeze-thaw cycle times of table 11 were fitted to obtain the relationship between the porosity and freeze-thaw cycle times of fly ash foam concrete with different fly ash contents as shown in fig. 12. As can be seen from fig. 12, as the number of freeze thawing cycles increases, the porosity of the fly ash foam concrete with different fly ash contents gradually increases, and the porosity increases nonlinearly, and the nonlinear correlation coefficient after fitting each scatter plot is greater than 0.90, and the correlation of the two is remarkable. The porosity fitting formulas are as follows:
Wherein: />For porosity->And t is the number of freeze thawing cycles, which is a coefficient related to the mixing amount of the fly ash. Mixing the fly ash with->As an independent variable, will->As a function of the variables,fitting again:
；
；
the fitting result is shown in fig. 13-15.
Therefore, the porosity change formula taking the mixing amount of the fly ash and the freeze thawing cycle number as variables is as follows:
formula (1)
The scatter diagram of the compressive strength and the porosity of the fly ash foam concrete with different fly ash contents under the action of freeze thawing cycle and the freeze thawing damage model established after regression analysis are shown in fig. 16, and the mathematical model of the freeze thawing damage is shown in the following formula:
formula (2)
The determination coefficient of the regression analysis of the data is about 0.985, and the fitting degree is higher, so that the quantitative relation between the compressive strength and the porosity after the freeze thawing cycle of the fly ash foam concrete with different fly ash contents can be predicted by the model, and the frost resistance of the fly ash foam concrete can be evaluated.
Substituting the formula (1) into the formula (2) to obtain:
formula (3)
Example 9
Table 13 shows the porosity, gel pores, particle pores, macro pores and compressive strength of the fly ash foam concrete prepared in example 3 and examples 5 to 7 according to the number of freeze thawing cycles.
TABLE 13
According to Table 13, ash entropy correlation analysis was performed on the pore structure parameters of the fly ash foam concrete at different density levels. Taking compressive strength as a reference sequence, taking porosity, pore volume values corresponding to pores among gel, pore volume values corresponding to pores among particles and pore volume values corresponding to macro pores as comparison sequences, and calculating gray entropy correlation degree between pore structure parameters and compressive strength according to a gray entropy correlation analysis method, wherein the gray entropy correlation degree is shown in table 14.
TABLE 14
As can be seen from Table 14, the main factor affecting the reduction in compressive strength of fly ash foam concrete of different density grades under the effect of freeze-thaw cycles is the inter-gel pore volume. Because the low-density fly ash foam concrete almost has no gel pores, the inter-particle pore volume with larger compressive strength degree is selected as a research object. The relationship between the inter-particle pore volume and the number of freeze-thawing cycles of the fly ash foam concrete with different density grades is obtained by fitting the inter-particle pore volume and the number of freeze-thawing cycles in table 13 as shown in fig. 17.
As can be seen from FIG. 17, the inter-particle pore volume of the fly ash foam concrete with different densities has a tendency to gradually increase with the number of freeze thawing cycles and increases nonlinearly, except for the density of 500kg/m 3 The nonlinear correlation coefficient of the test piece is larger than 0.7, the fitting of each other scatter diagram is larger than 0.90, and the correlation of the two is obvious. The fit formula for each inter-particle pore volume is as follows:
wherein: />Is inter-particle pore volume>And t is the number of freeze-thawing cycles, which is a coefficient related to density. Density of independent variable->Dependent variable->Fitting again:
；
；
，
is a density-related coefficient, +.>The fitting results are shown in fig. 18-20 for density.
Therefore, the inter-particle pore volume change formula taking the density and the freeze-thaw cycle number as variables is as follows:
formula (4)
Wherein:is inter-particle pore volume>Is the density; t is the number of freeze-thawing cycles.
According to table 13, a scatter diagram of compressive strength and inter-particle pore volume of fly ash foam concrete with different densities under the action of freeze thawing cycle and a freeze thawing damage model established after regression analysis are shown in fig. 21, and a mathematical model of freeze thawing damage is shown in formula (5). The determination coefficient of the regression analysis of the data is about 0.989, and the fitting degree is higher, so that the quantitative relation between the compressive strength and the inter-particle pore volume of the fly ash foam concrete with different densities after the freeze thawing cycle can be predicted by the model, and the frost resistance of the fly ash foam concrete can be evaluated.
Formula (5)
Wherein:is inter-particle pore volume>Is compressive strength. P of formula (4) 2 Substituting formula (5) to obtain: />
Equation (6).
Under the action of freeze thawing circulation, the gel pores of the novel fly ash foam concrete with different fly ash mixing amounts are sequentially deteriorated to the inter-particle pores and the inter-particle Kong Xianghong pores, and the pore size distribution curve area and the porosity are gradually increased. Under the action of freeze thawing circulation, the novel fly ash foam concrete with different density grades gradually increases in porosity, and the higher the density grade is, the stronger the freezing resistance is. The novel high-density fly ash foam concrete air holes are divided into gel inter-particle holes, inter-particle holes and macro-holes, wherein the inter-particle Kong Zhanbi is largest, and the gel inter-particle holes sequentially evolve to the inter-particle holes and the inter-particle Kong Xianghong holes under the action of freeze thawing circulation; the novel low-density fly ash foam concrete pores are inter-particle pores and macro-pores, and are affected by freeze thawing circulation, and gel inter-particle pores are gradually deteriorated to macro-pores.
Example 10
The fly ash foam concrete prepared in the example 4 is subjected to 35 freeze-thawing cycle experiments according to the fly ash foam concrete freezing resistance test, and the actual compressive strength is tested to be 18.9 MPa.
The fly ash mixing amount and the freeze thawing cycle number (35 times) of the fly ash foam concrete prepared in example 4 are substituted into the formula (3), and the compressive strength is calculated to be 18.97 MPa, and the error is 0.40%.
The fly ash mixing amount and the freeze-thawing cycle number (35 times) of the fly ash foam concrete prepared in example 4 were substituted into comparative example 2, and the compressive strength was calculated to be 16.75 MPa with an error of 12.83%.
The density and the freeze-thawing cycle number (35 times) of the fly ash foam concrete prepared in example 4 were substituted into comparative example 3, and the compressive strength was calculated to be 19.80 MPa with an error of 4.76%.
The density and the freeze-thawing cycle number (35 times) of the fly ash foam concrete prepared in example 4 are substituted into the formula (6), and the compressive strength is calculated to be 19.6MPa, and the error is 0.039%.
The foregoing has described exemplary embodiments of the invention, it being understood that any simple variations, modifications, or other equivalent arrangements which would not unduly obscure the invention may be made by those skilled in the art without departing from the spirit of the invention.

Claims (5)

1. A method for evaluating the compressive strength of fly ash foam concrete based on density, which is characterized by comprising the following steps:
s1, preparing N fly ash foam concrete with the same fly ash mixing amount as a sample, wherein the densities of the N fly ash foam concrete in the sample are different, and obtaining a fitting formula of each fly ash foam concrete according to the following method: carrying out an anti-freezing test on the fly ash foam concrete, testing the inter-particle pore volume and the compressive strength of the fly ash foam concrete after each freeze thawing in the anti-freezing test, and fitting the inter-particle pore volume and the freeze thawing cycle times of the fly ash foam concrete to obtain a fitting formula ：Wherein: />Is the inter-particle pore volume of the fly ash foam concrete, < > and the like>The coefficient related to the density of the fly ash foam concrete is t is the number of freeze thawing cycles;
s2, taking the density of the fly ash foam concrete as an independent variable according to a fitting formula of N fly ash foam concrete, and taking the density of the fly ash foam concrete as an independent variableFitting as dependent variable to obtain +.>Respectively and density;
s3, willThe relation between the density and the formula of the fitting in S1 is substituted into +.>Obtaining a particle-to-particle pore volume change formula taking density and freeze thawing cycle times as variables;
s4, fitting according to the inter-particle pore volume and the compressive strength of the N fly ash foam concretes obtained in the S1 after each freeze thawing, and obtaining a relationship formula of the inter-particle pore volume and the compressive strength;
s5, substituting the inter-particle pore volume change formula taking the density and the freeze thawing cycle times as variables in the S3 into the inter-particle pore volume and compressive strength relation formula in the S4 to obtain the quantitative relation of the density, the freeze thawing cycle times and the compressive strength of the fly ash foam concrete under the fly ash doping amount;
the preparation method of the load-bearing and frost-resistant fly ash foam concrete comprises the following steps:
step 1, mixing cement, fly ash, quartz powder, quartz sand, polypropylene fiber, a foam stabilizer and manganese dioxide, and stirring to be uniform to obtain a dry material, wherein the ratio of the cement to the fly ash to the quartz powder to the quartz sand to the polypropylene fiber to the foam stabilizer to the manganese dioxide is (400-960): (240-600): (374-672): (160-288): (0.67 to 1.25): (0.4 to 1.3): (0.5-1.6), wherein the foam stabilizer is hydroxypropyl methyl cellulose;
Step 2, uniformly mixing the dry material, the water reducer and water to obtain slurry, wherein the ratio of cement to the water reducer to the water in the dry material is (400-960): (6-12): (334-600), wherein the water reducer is polycarboxylic acid;
and step 3, adding a foaming agent into the slurry, stirring the mixture to be uniform, standing the mixture for foaming, and curing the mixture to obtain the fly ash foam concrete, wherein the ratio of cement to the foaming agent in the slurry is (400-960) in parts by weight: (14-20).
2. The method of claim 1, wherein in S1, N is 3 or more.
3. The method according to claim 1, wherein in S1, the fly ash content in the fly ash foam concrete is 20 to 50%, and the fly ash content = mass of fly ash in the fly ash foam concrete/(mass of cement in the fly ash foam concrete + mass of fly ash in the fly ash foam concrete).
4. The method according to claim 1, wherein 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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