Preparation method for improving stability of ultra-high performance concrete based on solid waste aggregate

Abstract

The invention discloses a preparation method for improving stability of ultra-high performance concrete based on solid waste aggregate, the method comprises the following raw materials in parts by mass: 70-80 parts of cement, 20-30 parts of silica fume, 100-145 parts of composite aggregate, 0.8-1.6 parts of polycarboxylate superplasticizer and 3-5 parts of steel fiber, the water-cement ratio is controlled to be 0.16-0.20, the composite aggregate is composed of type I and type II aggregate with specific particle size, and the composite aggregate is composed of type II aggregate with specific particle size. The UHPC is prepared through the steps of premixing treatment, staged stirring, compound doping, forming maintenance and the like. When the substitution rate of the solid waste-based spherical slag reaches 100%, the mixing amount of the polycarboxylate superplasticizer is decreased progressively according to the gradient, and the maximum decreasing proportion is 40%. According to the preparation method for improving the stability of the ultra-high performance concrete based on the solid waste aggregate, while the workability of UHPC is guaranteed, the use efficiency of the polycarboxylate superplasticizer is improved, the use amount is reduced, the side effect of the polycarboxylate superplasticizer is reduced, the compactness and the shrinkage resistance of the concrete are improved, solid waste resource utilization is achieved, and the cost is reduced. And a cooperative solution of performance improvement, cost control and environmental protection and efficiency improvement is formed.

Description

Technical Field

[0001] The present invention relates to the technical field of solid waste resource recovery and high-value utilization, specifically a preparation method for improving the stability of ultra-high performance concrete based on solid waste aggregates, and more specifically to a spherical slag screened from metallurgical by-product materials. The invention studies a method that can save admixture costs and inhibit the long-term autogenous shrinkage and drying shrinkage of the concrete matrix. Background Art

[0002] Ultra-high-performance concrete (UHPC) has become the material of choice for high-end projects such as bridges and building curtain walls, thanks to its exceptionally high compressive strength (120-180 MPa), exceptional durability, and low permeability. Its core performance stems from its dense matrix formed by an extremely low water-to-binder ratio (0.14-0.27), a characteristic that relies heavily on the dispersing effect of polycarboxylate superplasticizers (PCSs). PCSs ...

[0003] 1. Inefficient water reducers due to adsorption selectivity: Polycarboxylic acid-based water reducers preferentially adsorb on clay impurities and silica fume surfaces in sand and gravel aggregates, with clay adsorption reaching hundreds of times that of cement. To maintain construction performance, the amount of polycarboxylic acid-based water reducers used is forced to increase, directly driving up material costs.

[0004] 2. Negative effects of excessive polycarboxylate superplasticizers: High doses of polycarboxylate superplasticizers cause delayed hydration reaction, increased matrix porosity and capillary pores, exacerbated drying shrinkage (shrinkage rate increases by 30%-50%), and seriously affect the long-term volume stability and crack resistance of UHPC;

[0005] 3. Conflict between environmental protection and cost: Traditional UHPC relies on high-grade aggregates to reduce clay content, but high-quality aggregate resources are scarce and expensive, which is contrary to the concept of green building.

[0006] At the same time, aggregate properties are crucial to regulating UHPC performance. Studies have shown that spherical aggregates can significantly improve the fluidity of the mixture due to their smooth surface and low friction resistance, and theoretically reduce dependence on polycarboxylic acid-based water reducers.

[0007] Natural spherical aggregates are scarce, and the accumulation of industrial solid waste (such as metallurgical slag and fly ash) not only occupies land, but its disorderly discharge also brings environmental risks. Summary of the Invention

[0008] The present invention aims to provide a method for improving the stability of ultra-high performance concrete (UHPC) using solid waste aggregates. This method utilizes solid waste-based spherical slag to improve the efficiency and reduce the amount of polycarboxylate superplasticizer used, without compromising workability. This reduces the side effects of polycarboxylate superplasticizers and increases the compactness and shrinkage resistance of UHPC. This method is easy to operate, low-cost, and suitable for large-scale application. Using solid waste-based spherical slag to improve the volume stability of cement-based materials such as UHPC is highly feasible.

[0009] To achieve the above object, the present invention is implemented through the following technical solutions:

[0010] The material for preparing ultra-high performance concrete (UHPC) is prepared from the following components in parts by mass: 75 parts of cement, 25 parts of silica fume, 100-142 parts of composite aggregate, 0.9-1.5 parts of polycarboxylic acid-based water reducer, and 4 parts of steel fiber.

[0011] Furthermore, the cement is ordinary Portland cement.

[0012] Furthermore, the aggregate is river sand or spherical slag, and the fineness is 0-1.18mm (Type I) and 0-0.63mm (Type II).

[0013] Furthermore, the length of the steel fiber is about 12 mm.

[0014] Furthermore, the mineral composition of the silica fume and cement is as follows:

[0015]

[0016] Furthermore, the composite aggregate components are added to ultra-high performance concrete (UHPC) in four combinations of type I and type II according to 3:7, 3:9.9, 4.3:7 or 4.3:9.9.

[0017] Furthermore, when the solid waste-based spherical slag replacement rate in the composite aggregate reaches 100%, the dosage of the polycarboxylic acid-based water-reducing agent is adjusted in a decreasing gradient of 0.2 parts per time.

[0018] Furthermore, the temperature rise curves of the mixtures with different proportions were recorded by the real-time hydration heat monitoring system, and a synergistic optimization model of the water reducer dosage and aggregate grading was established in combination with the mercury intrusion porosity test results of the 28-day-old specimens.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The excellent flow-promoting properties of spherical slag significantly improve the fluidity of the mixture, theoretically reducing reliance on polycarboxylate superplasticizers. This allows UHPC to maintain workability while reducing the amount of polycarboxylate superplasticizer by 30-40%. The hard texture of spherical slag reduces UHPC shrinkage. This is due to the fact that its hard texture effectively suppresses matrix shrinkage. The coordinated use of spherical slag and polycarboxylate superplasticizers reduces UHPC shrinkage by over 300 με in its natural state.

[0021] 2. The method of the present invention is easy to operate, low-cost, and can be put into practical application. Utilizing industrial metallurgical waste can improve the shrinkage resistance and volume stability of UHPC, and can significantly reduce the cost of natural sand and admixtures. It can not only reduce the demand for polycarboxylate-based water-reducing agents in UHPC, but also realize the resource utilization of solid waste, forming a synergistic solution of "performance improvement, cost control, and environmental protection efficiency improvement." BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a spherical slag sample;

[0023] Figure 2(a) shows the hydration heat release increment curve of each UHPC mixture within 3 days;

[0024] Figure 2(b) shows the cumulative change curve of hydration heat release of each UHPC mixture within 3 days;

[0025] Figure 3(a) shows the pore distribution curves of each UHPC specimen after 28 days of curing;

[0026] Figure 3(b) shows the porosity change curves of each UHPC specimen after 28 days of curing;

[0027] Figure 4 is the test result of the fluidity of the mixture;

[0028] Figure 5(a) Long-term shrinkage curve of the specimen under closed state;

[0029] Figure 5(b) shows the long-term shrinkage curve of the specimen in the unsealed state. DETAILED DESCRIPTION

[0030] In the description of the present invention, unless otherwise specified, "plurality" means two or more; terms such as "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," and "tail" indicate positions or relationships based on those shown in the accompanying drawings. These terms are intended solely to facilitate the description of the present invention and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present invention. Furthermore, terms such as "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0031] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "connected" and "connection" should be understood in a broad sense. For example, they can refer to fixed connection, detachable connection, or integral connection; mechanical connection, electrical connection; direct connection, or indirect connection through an intermediary. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0032] First embodiment:

[0033] Weigh 300g of Type I river sand, 700g of Type II river sand, 750g of 42.5# cement, 250g of silica fume, 15g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0034] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0035] Second embodiment:

[0036] Weigh 430g of type I spherical slag, 990g of type II spherical slag, 750g of 42.5# cement, 250g of silica fume, 15g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0037] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0038] Third embodiment:

[0039] Weigh 300g of Type I river sand, 990g of Type II spherical slag, 750g of 42.5# cement, 250g of silica fume, 15g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0040] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0041] Fourth embodiment:

[0042] Weigh 430g of Type I spherical slag, 700g of Type II river sand, 750g of 42.5# cement, 250g of silica fume, 15g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0043] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0044] Fifth embodiment:

[0045] Weigh 430g of type I spherical slag, 990g of type II spherical slag, 750g of 42.5# cement, 250g of silica fume, 13g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0046] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0047] Sixth embodiment:

[0048] Weigh 430g of type I spherical slag, 990g of type II spherical slag, 750g of 42.5# cement, 250g of silica fume, 11g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0049] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0050] Seventh embodiment:

[0051] Weigh 430g of type I spherical slag, 990g of type II spherical slag, 750g of 42.5# cement, 250g of silica fume, 9g of polycarboxylic acid water reducer, and 40g of steel fiber according to the mass ratio;

[0052] First, dry mix silica fume, cement, and polycarboxylic acid-based water reducer for 3 minutes. Use the friction of the aggregate to thoroughly mix the steel fiber and aggregate for later use. At room temperature, weigh tap water with a water-cement ratio of 0.18. While the mixer is rotating slowly, gradually add water to the dry mixture, and it will become fluid in about 3 minutes. Once the mixture begins to show sufficient fluidity, manually add the mixture of steel fiber and aggregate to the mixer and stir for 1 minute. Then switch to high-speed stirring mode and continue for 3 minutes. The mixture can be used to measure fluidity. It is formed using a 25×25×180mm mold and demolded after curing in a curing box for 48 hours. Three specimens are grouped together. Each formula has a plastic film-sealed specimen for testing in a sealed state and a specimen for testing in an unsealed state. The shrinkage value is tested with a micrometer.

[0053] After experimental testing and analysis:

[0054] Regarding the contents described in the above 1-7 embodiments and drawings;

[0055] 1. Analysis of hydration thermal behavior:

[0056] As shown in Figures 2(a) and (b), the incremental and cumulative curves of the heat release of the mixture with hydration time show that, as shown in Figure 2(a), when the spherical slag completely replaces the natural sand, the hydration heat release rate of U2 reaches its peak about 24 hours later than that of U1. As shown in Figure 2(b), the total hydration heat release of U2 is significantly lower than that of U1.

[0057] The comb-like polymers of PCE are adsorbed on the surface of cement particles through electrostatic repulsion and steric hindrance, delaying the cement hydration process;

[0058] With the decrease of PCE content, the peak of hydration heat release rate of mixture U5-7 shifts to the left relative to U2, and the heat release gradually increases but is still less than that of U1;

[0059] The porous layered structure of clay and the high specific surface area of ​​silica fume intensify the adsorption of PCE. The clay component in natural sand has a strong adsorption capacity for PCE.

[0060] 2. Correlation between porosity and permeability:

[0061] From the pore size distribution image in Figure 3(a), the capillary pores of U1-4 are much larger than those of U5-7. The proportion of capillary pores >10 nm in the PCE reduction group (U5-7) is significantly lower than that in U1-4, which verifies that reducing PCE can improve the impermeability of UHPC.

[0062] As shown in Figure 3(b), U3 and U4 may have worse pore distribution and higher total porosity due to aggregate gradation factors;

[0063] U1 and U2 have similar pore distribution and porosity, which means that the amount of PCE used determines the porosity and has nothing to do with the adsorption object.

[0064] 3. Liquidity impact mechanism:

[0065] like Figure 4 From the trend of the experimental results, it can be seen that the content of PCE and river sand has a significant impact on the fluidity of UHPC;

[0066] Completely replacing river sand with spherical slag can increase flow by 17.60%. This result supports the use of less PCE in UHPC. When the PCE dosage is reduced by 40%, U7 still has a flow gain of 6.16% compared to the control group.

[0067] The spherical ball effect of spherical slag helps the mixture to greatly reduce internal friction and promote the fluidity of fresh concrete;

[0068] 4. Adsorption performance analysis:

[0069] UHPC contains a large amount of silica fume and clay. The porous layered structure of the clay and silica fume adsorbs water reducers more effectively than cement. Therefore, clay and silica fume absorb more water reducer molecules. UHPC usually contains many times more water reducer than ordinary concrete.

[0070] After the adsorption of more water-reducing agent molecules, the concentration of polycarboxylate water-reducing agent on the surface of cement particles will be very low, which will seriously affect the dispersion of PCE. Not only will the fluidity of the mixture be very poor, but the flow loss will be rapid. Excessive PCE will limit the upper limit of concrete performance.

[0071] However, using pre-sand washing to remove most of the soil components will increase economic costs;

[0072] 5. Contraction and expansion mechanism in UHPC:

[0073] First of all, the shrinkage of UHPC is mainly divided into two parts: autogenous shrinkage and drying shrinkage:

[0074] Autogenous shrinkage is mainly caused by the volume reduction caused by the hydration reaction of the cementitious material, including chemical shrinkage caused by the hydration reaction of the cement and autogenous shrinkage caused by the reduction of free water content in the UHPC after the hydration reaction.

[0075] Drying shrinkage is mainly caused by the loss of moisture on the UHPC surface. When the external relative humidity is lower than the internal relative humidity of the concrete, the free water inside the UHPC migrates to the outside, generating capillary negative pressure and causing the volume to decrease.

[0076] The shrinkage evolution of the UHPC specimens in the sealed and unsealed states over 360 days in Figures 5(a) and (b) is analyzed. The sealed state mainly reflects the chemical shrinkage and self-drying shrinkage of the specimens, while the unsealed state reflects the drying shrinkage caused by the exchange of moisture between the specimens and the outside world.

[0077] Data analysis results:

[0078] 1. The shrinkage of the specimens in the sealed state tends to be stable after 14 days, while the shrinkage of the specimens in the unsealed state is basically stable after 28 days, and the duration of drying shrinkage is longer;

[0079] Autogenous shrinkage occurs primarily in the early stages of concrete, while drying shrinkage continues throughout the concrete's service life;

[0080] 2. Shrinkage behavior in a closed state:

[0081] (1) Early expansion phenomenon:

[0082] Figure 5(a) shows that U2 has an expansion phenomenon before reaching the age of 3 days. The reason is that the heat released by cement hydration increases the temperature inside the concrete, causing expansion;

[0083] (2) Contraction inhibition law:

[0084] After 14 days, the shrinkage of the sealed specimens entered the plateau region. The shrinkage trends of specimens U1-4 show that the maximum shrinkage gradually decreases with increasing spherical slag replacement. The shrinkage trends of specimens U5-7 indicate that the reduction in PCE reduces porosity, which helps suppress matrix shrinkage. During this period, specimens U3 and U4, while having greater porosity than U1, exhibit smaller shrinkage values.

[0085] Influence of spherical slag replacement rate: In U1-U4, as the spherical slag replacement rate increases, the maximum shrinkage value gradually decreases (U7 decreases by 197.66με compared with U1);

[0086] Porosity-shrinkage contradiction: U3 / U4 has higher porosity than U1, but due to the rigid support of spherical slag, the shrinkage value is still lower than U1. Therefore, it is concluded that the hardness of the aggregate (spherical slag > river sand) can offset the porosity disadvantage and inhibit the shrinkage deformation of the matrix.

[0087] 3. Shrinkage evolution in non-sealed state:

[0088] As shown in Figure 5(b), the shrinkage evolution trend of the specimen in the non-sealed state changed from rising to gentle after 28 days, but the shrinkage amplitude increased; this reflects the superposition of autogenous shrinkage and drying shrinkage.

[0089] After 28 days of shrinkage, the shrinkage inhibition effect of the spherical slag in U2-U6 was average, and the shrinkage difference between them was not obvious. Among them, the long-term drying shrinkage of U1 was more obvious (the 28-day shrinkage difference: U7 was 312.89με lower than U1), possibly due to the water absorption of river sand.

[0090] It can be concluded that the shrinkage inhibition effect of U2-U6 is general and the difference is not significant. U7 has a significant shrinkage inhibition effect through the synergistic effect of spherical slag + PCE reduction, achieving the optimal anti-shrinkage performance.

[0091] It will be understood that the above embodiments are merely exemplary embodiments for illustrating the principles of the present invention, and the present invention is not limited thereto. Those skilled in the art will appreciate that various modifications and improvements can be made without departing from the spirit and substance of the present invention, and such modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A method for improving the stability of ultra-high performance concrete based on solid waste aggregates, characterized in that: The following steps are involved: (1) Raw material ratio: by mass, including 70-80 parts of cement, 20-30 parts of silica fume, 100-145 parts of composite aggregate, 0.8-1.6 parts of polycarboxylic acid water reducer, and 3-5 parts of steel fiber. The water-cement ratio is controlled at 0.16-0.20; (2) Premixing: dry-mix cement, silica fume and polycarboxylic acid water reducer, and pre-disperse steel fiber and composite aggregate; (3) Stirring in stages: First, stir the dry material at a low speed, then add water in stages and stir at a medium speed to form a matrix slurry; (4) Composite incorporation: The pre-dispersed steel fiber-aggregate composite system is incorporated into the matrix slurry in batches and stirred at high speed to form a uniform mixture; (5) Molding and curing: The mixture is injected into the mold and vibrated to form the mixture. After 48±2 hours of standard curing, the mixture is demoulded and the volume stability is monitored under constant temperature and humidity conditions.

2. The method according to claim 1, wherein: The composite aggregate is composed of type I aggregate with a particle size of 0.1-1.18 mm and type II aggregate with a particle size of 0.1-0.63 mm, with a mass ratio of (3-4.3):(7-9.9), wherein the type I aggregate is river sand or solid waste-based spherical slag, and the type II aggregate is solid waste-based spherical slag.

3. The method according to claim 2, wherein: When the solid waste-based spherical slag replacement rate in the composite aggregate reaches 100%, the dosage of the polycarboxylic acid-based water reducer is adjusted in a gradually decreasing manner of 0.15-0.25 parts per time.

4. The method according to claim 1, wherein: In the step (3), the low-speed stirring speed is 100±20 rpm, and the duration is 2-4 min; the medium-speed stirring speed is 250±30 rpm, and the duration is 2-4 min; and the high-speed stirring speed is 450±50 rpm, and the duration is 2.5-3.5 min.

5. The method according to claim 1, wherein: The steel fiber is a straight copper-plated steel fiber with a length of 12±1 mm, an aspect ratio of 60-80, and a tensile strength of ≥2000 MPa.

6. The method according to claim 1, wherein: The method also includes a volume stability optimization step, in which the temperature rise curves of mixtures with different proportions are recorded by a real-time hydration heat monitoring system, and a synergistic optimization model of water reducer dosage and aggregate grading is established in combination with the mercury intrusion porosity test results of 28-day-old specimens.

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