A method for preparing a sulfate attack resistant concrete

Abstract

This invention provides a method for preparing sulfate-resistant concrete. The method includes: adding 8% to 12% PBL powder to freshly mixed ordinary Portland cement concrete; wherein the PBL powder is prepared by grinding phosphogypsum, magnesite, and limestone in a dry basis mass ratio of 4:2:1. By adding 8% to 12% PBL powder to ≥C30 ordinary Portland cement concrete, even after the corrosive medium sulfate has penetrated into the concrete, a sulfate attack reaction will not occur. Therefore, even in highly corrosive environments, it is not necessary to adopt engineering measures such as waterproofing layers, reducing the corrosivity of environmental water, drainage, soil replacement, lowering the groundwater level, and setting up protective layers as specified in DL / T5801—2019 "Technical Specification for Application of Sulfate-Resistant Concrete".

Description

Technical Field

[0001] The embodiments of the present invention relate to the field of concrete preparation technology, and more specifically, to a method suitable for preparing sulfate-resistant concrete. Background Technology

[0002] Concrete structures such as hydroelectric power stations, water conservancy projects, bridges, tunnels, coastlines, ports, and docks in most countries around the world suffer from sulfate attack, causing enormous economic losses of hundreds of billions of dollars. Therefore, sulfate attack has received high attention from governments and scholars worldwide. For over 100 years since Mihalys discovered "cement bacillus," scholars both domestically and internationally have conducted extensive and in-depth research on the mechanism of sulfate attack. They have discovered that three basic conditions must be met simultaneously for concrete to be susceptible to sulfate attack: the cement paste must contain a certain amount of calcium hydroxide and hydrated calcium aluminate; the microstructure of the cement paste must contain "channels" through which the corrosive medium penetrates the concrete; and the corrosive medium must contain a certain amount of sulfate. The mechanism of sulfate attack is as follows: when sulfate-containing corrosive media seeps into concrete through defects in cement stone, the corrosive media first reacts with calcium hydroxide in the hydration products to form gypsum (CaSO4•2H2O) or magnesium hydroxide (brucite). Then, the gypsum continues to react with calcium aluminate hydrate in the hydration products to form ettringite (3CaO•Al2O3•3CaSO4•32H2O). All three corrosion products have expansion characteristics, with ettringite being the dominant one. As the amount of ettringite or gypsum and brucite produced increases, their expansion effect will inevitably lead to cracking and spalling on the concrete surface. At this time, a large amount of sulfate-containing corrosive media seeps into the concrete, further aggravating the destructive degree of the sulfate attack reaction. This cycle repeats itself, eventually leading to the destruction of the concrete structure.

[0003] Based on the mechanism of sulfate attack and the engineering experience of preventing sulfate attack on concrete at home and abroad, my country's hydropower industry has revised and formulated DL / T5801-2019 "Technical Specification for Application of Sulfate-Resistant Concrete". This specification represents the advanced technical level at home and abroad in preventing sulfate attack on concrete. Specifically, Article 3.0.2 stipulates that the degree of sulfate corrosion on concrete in environmental water is classified into four levels: no corrosion, weak corrosion, moderate corrosion, and strong corrosion. Article 3.0.4 stipulates that for strongly corrosive environments, special tests and demonstrations should be conducted, and engineering measures such as waterproofing layers, reducing the corrosivity of environmental water, drainage, soil replacement, lowering the groundwater level, and setting up protective layers should be adopted according to specific circumstances. For weak and moderately corrosive environments, Article 4.1.1 stipulates that sulfate-resistant concrete should preferably use sulfate-resistant cement, and silicate cement, ordinary silicate cement, and slag silicate cement can also be used when admixtures are added. Article 4.1.2 stipulates that when silicate cement, ordinary silicate cement, and slag silicate cement are used, the tricalcium aluminate content in the cement should be less than 8%.

[0004] As can be seen from the above regulations, Articles 4.1.1 and 4.1.2 are fundamental. In weakly or moderately corrosive environments, sulfate-resistant concrete should preferably be prepared using sulfate-resistant cement, or using silicate cement, ordinary silicate cement, and slag silicate cement with admixtures, or using silicate cement, ordinary silicate cement, and slag silicate cement with a tricalcium aluminate content of less than 8%. In strongly corrosive environments, in addition to preparing sulfate-resistant concrete according to Articles 4.1.1 and 4.1.2, auxiliary engineering measures as stipulated in Article 3.0.4 are also required. However, the problems with these auxiliary engineering measures are: ① The cement paste of the concrete still contains a considerable amount of calcium hydroxide and hydrated calcium aluminate. In strongly corrosive environments, if there are quality problems with the auxiliary engineering measures, the concrete is still at risk of sulfate corrosion damage; ② The auxiliary engineering measures stipulated in Article 3.0.4 have three prominent problems: increased construction technology, extended construction period, and increased project cost.

[0005] In recent years, to address the durability issue of concrete against sulfate attack in highly corrosive environments, researchers have begun to explore adding corrosion inhibitors or using patented products to concrete. However, no precedent has yet been found for improving concrete's sulfate resistance by pre-eliminating calcium hydroxide and hydrated calcium aluminate, which are easily corroded by sulfates. Furthermore, using corrosion inhibitors or patented technologies to formulate sulfate-resistant concrete presents a significant material cost problem, substantially increasing the overall cost of concrete projects. Summary of the Invention

[0006] In order to solve the technical problems existing in the background art, the present invention provides a method for preparing sulfate-resistant concrete, comprising: adding 8% to 12% PBL powder to freshly mixed ordinary silicate cement concrete;

[0007] The PBL powder is prepared by grinding phosphogypsum, magnesite and limestone in a dry basis mass ratio of 4:2:1.

[0008] As a further explanation of the present invention, the main mineral components of the PBL powder are gypsum dihydrate, magnesium carbonate and calcium carbonate.

[0009] As a further explanation of the present invention, the preparation process of the PBL powder includes:

[0010] Open-air stockpiled phosphogypsum should be air-dried or oven-dried at a temperature not exceeding 650°C.

[0011] Magnesite and limestone were used as grinding aids for phosphogypsum, and the mixture was ground at a dry basis mass ratio of 4:2:1. The grinding was carried out until the specific surface area was ≥400 m². 2 PBL powder can be prepared by increasing the yield to / kg.

[0012] As a further explanation of the present invention, when designing the concrete mix proportion, the PBL powder accounts for 8% to 12% of the total amount of cementitious materials. The phosphogypsum powder is added internally, replacing part of the mass of cementitious materials by equal mass. Magnesite powder and limestone powder are added externally, replacing part of the mass of fine aggregate by equal mass, in order to adjust and optimize the gradation of fine aggregate and participate in the secondary hydration reaction.

[0013] Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0014] This invention employs a grinding process to prepare a PBL powder using industrial solid waste from phosphogypsum, magnesite, and limestone. This powder is primarily used in the formulation of sulfate-resistant concrete. Compared to existing technologies, the invention achieves significant technical, economic, and social benefits, mainly reflected in the following three aspects.

[0015] 1. Technical Effects

[0016] (1) Regardless of the degree of sulfate erosion in the environment, as long as 8%~12% of PBL powder is added to ≥C30 ordinary silicate cement concrete, even if the eroding medium sulfate penetrates into the service concrete, sulfate erosion reaction will not occur.

[0017] (2) According to the test method specified in GB / T50082-2019 "Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete", the sulfate attack resistance of ordinary Portland cement concrete specimens mixed with 8%~12% PBL powder was tested. When the number of wet and dry cycles reached 150, the corrosion resistance coefficient of the concrete compressive strength was 98.5%.

[0018] (3) As long as 6%~8% of PBL powder is added to ≥C30 ordinary silicate cement concrete, even in a highly corrosive environment, it is not necessary to adopt engineering measures such as waterproofing, reducing environmental water corrosivity, drainage, soil replacement, lowering the groundwater level and setting up protective layers as required by DL / T5801—2019 "Technical Specification for Application of Sulfate-Resistant Concrete".

[0019] 2. Economic benefits

[0020] (1) Current technical specifications stipulate that sulfate-resistant cement should be used to prepare sulfate-resistant concrete, and its market price is about 600 yuan / t. However, the sulfate-resistant concrete prepared in this invention uses ordinary silicate cement, and its market price is about 400 yuan / t. The price difference between the two is about 200 yuan per ton.

[0021] (2) Using PBL powder to prepare sulfate-resistant concrete, assuming the concrete cementitious material dosage is 400 kg / m³, the technical admixture dosage is 10%, and the estimated market price is 1000 yuan / t, while the technical admixture dosage of the corrosion inhibitor is 8%, and the estimated market price is 2500 yuan / t, after comparative analysis, using PBL powder to prepare sulfate-resistant concrete can save 40 yuan / m³. 3 .

[0022] (3) In a highly corrosive environment, in addition to using high sulfate-resistant cement or corrosion inhibitors to prepare concrete, the current technology also requires engineering measures such as waterproofing, reducing the corrosivity of environmental water, drainage, soil replacement, lowering the groundwater level and setting up protective layers, in accordance with the technical specifications. However, using PBL powder to prepare sulfate-resistant concrete does not require any auxiliary engineering measures, which can significantly reduce the overall cost of the foundation engineering.

[0023] 3. Social benefits

[0024] Due to the low level of technology for the resource utilization and large-scale application of industrial waste such as phosphogypsum, magnesite, and limestone, the waste slag occupies a large amount of land resources and pollutes the ecological environment, with phosphogypsum being particularly problematic. Meanwhile, in my country's vast coastal areas, salt lakes, and saline soil regions, infrastructure and basic construction projects require large-scale sulfate-resistant concrete. The technology of this invention can consume large amounts of industrial waste, alleviate the environmental pressure on relevant enterprises, save significant land resources, reduce the pollution of the ecological environment by industrial waste, improve the technical level of sulfate-resistant concrete, extend the service life of sulfate-resistant concrete, and reduce the cost of sulfate-resistant concrete projects. Furthermore, compared with related corrosion inhibitors, the production of PBL powder can achieve zero pollution and zero CO2 emissions, demonstrating significant social benefits.

[0025] The above description is merely an overview of the technical solutions of the embodiments of this application. In order to better understand the technical means of the embodiments of this application and to implement them in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the embodiments of this application more obvious and understandable, specific implementation methods of this application are described below. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below. It should be understood that the drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention, wherein:

[0027] Figure 1 The images show the SEM scanning results of the blank group and the test group slurry specimens in Example 1 of this invention, where a is the blank group (× 500) and b is the test group (× 500). Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are also within the scope of protection of the present invention.

[0029] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of the phrase "embodiment" in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0030] Currently, in marine, salt lake, and saline soil areas, the following technical or engineering measures are commonly adopted to prevent concrete from being corroded by sulfates. A common drawback of these measures is that calcium hydroxide and hydrated calcium aluminate, which are easily corroded by sulfates, still exist in the cement paste. In highly corrosive environments, if the engineering measures have quality problems, the concrete still faces the risk of being damaged by sulfates. The different drawbacks of these measures are as follows:

[0031] (1) Reduce the water-cement ratio

[0032] When cement paste fills the voids in the aggregate and meets the excess coefficient requirement, reducing the water-cement ratio essentially means increasing the amount of cementitious material. Increasing the amount of cementitious material can improve the density and impermeability of concrete, thereby preventing the corrosive medium sulfate from penetrating into the concrete and causing sulfate attack. However, increasing the amount of cementitious material will increase the cost of concrete projects.

[0033] (2) Adjust the amount of cement used

[0034] Adjusting the cement dosage involves replacing part of the cement with mineral admixtures (fly ash, slag powder, or silica fume) to reduce the content of tricalcium silicate and tricalcium aluminate in the cementitious materials. The goal is to reduce the content of calcium hydroxide and hydrated calcium aluminate in the hydration products, thereby delaying or reducing the degree of sulfate erosion damage to concrete. However, in highly corrosive environments, additional auxiliary engineering measures are required according to the technical specifications, which will significantly increase the cost of the foundation engineering.

[0035] (3) Use sulfate-resistant cement

[0036] Sulfate-resistant cement is a special type of cement produced by adjusting or reducing the content of C3S and C3A in silicate cement clinker. It is only used to prepare sulfate-resistant concrete. Due to the limited number of domestic manufacturers, this cement is expensive and has high transportation costs. In addition, in highly corrosive environments, additional auxiliary engineering measures are required according to technical specifications, which will significantly increase the cost of foundation engineering.

[0037] (4) Add preservatives

[0038] Currently, there are many types of concrete sulfate-resistant corrosion inhibitors on the market, and their mechanisms of action are not entirely the same, but they are mainly reflected in two aspects: ① By adding corrosion inhibitors, the density of concrete is improved, the pore size of capillaries is refined, the concentration of sulfate penetrating into the concrete is reduced, and the rate at which the concrete is damaged by sulfate erosion is slowed down, thereby improving the concrete's resistance to sulfate erosion; ② By adding corrosion inhibitors, a targeted reaction is carried out with the sulfate penetrating into the concrete to generate water-insoluble barium sulfate, which blocks the channels for the corrosive medium sulfate to penetrate into the concrete, thereby preventing the corrosive medium sulfate from penetrating into the concrete and reacting with calcium hydroxide and hydrated calcium aluminate to undergo sulfate erosion, thereby improving the concrete's resistance to sulfate erosion. However, in highly corrosive environments, additional auxiliary engineering measures are required according to the technical specifications. At the same time, the market price of corrosion inhibitors is about 2,000 yuan / ton, which presents a problem of high material costs, which will significantly increase the cost of foundation engineering.

[0039] (5) CO2 curing

[0040] If CO2 curing is used for concrete, CO2 reacts with the hydration product calcium hydroxide to produce calcium carbonate, which can block the capillary pores that connect the inside and outside of the concrete, thereby improving the surface strength and density of the concrete, and thus improving the impermeability of the concrete to prevent the corrosive medium sulfate from penetrating into the concrete and causing sulfate attack. However, using CO2 curing will significantly increase the cost of concrete projects.

[0041] (6) Additional auxiliary engineering measures such as adding waterproof layers

[0042] In highly corrosive environments, even when using high sulfate-resistant cement to prepare concrete, auxiliary engineering measures must be taken in accordance with DL / T5801-2019 "Technical Specification for Application of Sulfate-Resistant Concrete". These measures have three prominent problems: complex construction process, long construction period and high project cost. However, this is a stopgap measure with no better solution.

[0043] Based on this, the present invention provides a method for preparing sulfate-resistant concrete. The method involves adding 8% to 12% PBL powder to freshly mixed ordinary silicate cement concrete. The main mineral components of PBL powder activate the activity of cement hydration products, promoting the secondary hydration reaction of calcium hydroxide and calcium aluminate hydrate in the hydration products. This pre-emptively eliminates calcium hydroxide and calcium aluminate hydrate in the cement stone that are easily eroded by sulfates, and transforms them into ettringite, magnesia, and calcium aluminate hydrate, which have lower solubility and more stable physicochemical properties. Even if the corrosive medium sulfate penetrates into the concrete in service, sulfate erosion reaction will not occur, thereby completely solving the durability problem of concrete damaged by sulfate erosion. The sulfate-resistant concrete is made with ordinary silicate cement, not sulfate-resistant or high-sulfate-resistant cement. Even in highly corrosive environments, it does not require engineering measures such as waterproofing, reducing environmental water corrosivity, drainage, soil replacement, lowering the groundwater level, and setting up protective layers as specified in DL / T5801—2019 "Technical Specification for Application of Sulfate-Resistant Concrete". The PBL powder is prepared by a simple grinding process using phosphogypsum, magnesite, and limestone industrial solid waste in a dry basis mass ratio of 4:2:1. Its main mineral components are dihydrate gypsum, magnesium carbonate, and calcium carbonate. The secondary hydration reaction is the process by which the main mineral components in the PBL powder react with calcium hydroxide and hydrated calcium aluminate in the cement hydration products to generate ettringite, magnesia, and hydrated calcium aluminate.

[0044] Under normal circumstances, during the setting and hardening process of concrete, due to the influence of factors such as the degree of cement hydration reaction, water evaporation, and changes in ambient temperature and humidity, the microstructure of cement stone exhibits three defects: insufficient gel density, capillary pores connected to the outside environment, and microcracks. These are the fundamental causes of the long-term performance and durability deterioration of concrete. Furthermore, calcium hydroxide and calcium aluminate hydrate, the hydration products, exist as final products in the cement stone. When service concrete is exposed to corrosive environments such as coastal areas, salt lakes, or saline soils, the corrosive medium, sulfate (Na+),... + Mg 2+ PBL powder can easily penetrate into concrete through microstructural defects in cement paste, and will react with calcium hydroxide and calcium aluminate hydrate in the hydration products to cause sulfate corrosion, leading to cracking and spalling of the concrete surface, and in severe cases, even destruction of the concrete structure. This invention incorporates 8%~12% PBL powder into freshly mixed concrete to solve the following technical problem:

[0045] 1. Solve the durability problem caused by insufficient density of the gel in cement stone.

[0046] Normally, calcium hydroxide and calcium aluminate hydrate in cement hydration products exist stably in cement stone as final hydration products, coating the surface of cement particles. This mode of existence inhibits the hydration rate and degree of C3S and C3A in cement clinker, resulting in insufficient gel polymerization and thus insufficient density of the cement stone, which adversely affects the durability of concrete. Adding 8%–12% PBL powder to fresh concrete utilizes its main mineral components to activate the hydration products, promoting secondary hydration reactions of calcium hydroxide and calcium aluminate hydrate to generate ettringite, magnesia, and calcium aluminate hydrate, which have lower solubility and more stable physicochemical properties. This increases the degree of cement hydration and gel polymerization, thereby improving the density and impermeability of the cement stone, making it difficult for the corrosive medium sulfate to penetrate into the concrete through the cement stone, thus solving the durability problem caused by insufficient gel density in the cement stone.

[0047] 2. Solve the durability problem of capillary pores in cement stone being connected to the outside environment.

[0048] Normally, only about one-third of the water used in concrete mixing participates in the cement hydration reaction, while the remaining two-thirds occupy part of the concrete's volume. After the water evaporates, it leaves many capillary pores in the cement paste that are connected to the outside environment, which will have a serious adverse effect on the durability of concrete. If 8% to 12% of PBL powder is added to fresh concrete, the early (3-day) formation and expansion effect of ettringite, a product of the secondary hydration reaction, can refine the capillary pores and increase their osmotic pressure. Furthermore, the formation, deposition, and filling of brucite, another product of the secondary hydration reaction, in the capillary pores, along with its "expansion" effect, can block the capillary pores, isolating them from the outside environment. This improves the concrete's impermeability and makes it difficult for the corrosive medium sulfate to penetrate the concrete, thus solving the durability problem caused by the interconnectedness of capillary pores in the cement paste with the outside environment.

[0049] 3. Solve the durability problem of numerous micro-cracks in cement stone.

[0050] Normally, concrete undergoes volume shrinkage during the setting and hardening process. When this volume shrinkage is constrained, numerous microcracks will form within the concrete, severely impacting its durability. Adding 8%–12% PBL powder to fresh concrete utilizes the synergistic expansion effect of the secondary hydration products, ettringite and magnesia, to compensate for the concrete's volume shrinkage throughout the process. This prevents the formation of microcracks within the cement paste, thereby improving the concrete's impermeability and making it difficult for the corrosive medium sulfate to penetrate. This addresses the durability issue caused by microcracks in the cement paste.

[0051] In summary, as described in points 1, 2, and 3, adding 8% to 12% PBL powder to freshly mixed concrete can comprehensively improve the microstructure morphology of cement stone and completely solve various existing defects in cement stone. This invention improves the impermeability of concrete from the perspective of the density of the microstructure of cement stone, thereby enhancing the concrete's resistance to sulfate attack. This is only an auxiliary technical effect produced by this invention.

[0052] 4. Solve the durability problem of hydration products being easily corroded by sulfates.

[0053] Normally, cement paste contains calcium hydroxide and hydrated calcium aluminate, which are easily corroded by sulfates. When the corrosive medium, sulfate, penetrates into the concrete in service, a sulfate attack reaction will occur. However, if 8%–12% PBL powder is added to fresh concrete, its main mineral components simultaneously activate the cement hydration products, promoting a secondary hydration reaction of calcium hydroxide and hydrated calcium aluminate, converting them into sulfates such as ettringite, magnesia, and hydrated calcium aluminate. Under these conditions, even if the corrosive medium, sulfate, penetrates into the concrete in service, a sulfate attack reaction will not occur. This is the core or key technology of this invention. This invention, by preemptively eliminating the easily corroded calcium hydroxide and hydrated calcium aluminate, completely solves the durability problem of concrete subjected to sulfate attack.

[0054] 5. Addressing the need for auxiliary engineering measures in highly corrosive environments.

[0055] In highly corrosive environments, besides using sulfate-resistant cement or high-sulfate-resistant cement to prepare sulfate-resistant concrete, it is also necessary to implement engineering measures such as waterproofing layers, reducing the corrosivity of ambient water, drainage, soil replacement, lowering the groundwater level, and setting up protective layers, as stipulated in DL / T5801—2019 "Technical Specification for Application of Sulfate-Resistant Concrete". However, if 8%–12% PBL powder is added to fresh concrete to pre-eliminate calcium hydroxide and hydrated calcium aluminate in the cement paste, which are easily corroded by sulfates, a sulfate attack reaction will not occur even if the corrosive medium sulfate penetrates into the concrete. Therefore, in highly corrosive environments, concrete containing PBL powder no longer requires sulfate-resistant cement or high-sulfate-resistant cement, nor does it require auxiliary engineering measures, thus simplifying the construction process, shortening the construction period, and reducing project costs.

[0056] 6. Solve the technical challenges of large-scale utilization of industrial waste residue.

[0057] Industrial wastes such as phosphogypsum, magnesite, and limestone are typically used to produce low-value-added building materials or products. However, due to geographical or transportation cost limitations, the resource utilization of industrial waste is usually localized, preventing the achievement of large-scale utilization. This results in large amounts of industrial waste being dumped in the open, occupying land resources and polluting the ecological environment. If industrial wastes such as phosphogypsum, magnesite, and limestone are used as functional materials for concrete, and high-value-added PBL powder is prepared, it can be used as a functional admixture or corrosion inhibitor for sulfate-resistant concrete. In mildly corrosive environments, sulfate-resistant or moderately sulfate-resistant cement is no longer needed to prepare sulfate-resistant concrete; in strongly corrosive environments, high-sulfate-resistant cement is no longer needed, and no auxiliary engineering measures are required. Regardless of the corrosive environment, simply adding 6% to 8% PBL powder to ordinary silicate cement concrete can fundamentally and completely solve the problem of sulfate-resistant concrete durability. In the vast coastal, salt lake, and saline soil regions, the scale of concrete required for infrastructure and basic construction projects is enormous. Using PBL powder as a carrier can consume large amounts of industrial waste such as phosphogypsum, magnesite, and limestone, enabling the resource utilization and large-scale utilization of industrial waste. This solves the technical challenges of large-scale utilization of industrial waste and has significant practical implications.

[0058] The main objectives of this invention include three aspects, as detailed below:

[0059] (1) Provide the best technical solution for concrete anti-sulfate erosion engineering measures.

[0060] The main mineral components of PBL powder are gypsum dihydrate, magnesium carbonate, and calcium carbonate. Under normal temperature conditions, gypsum dihydrate can undergo a secondary hydration reaction with calcium aluminate hydrate in cement hydration products to generate ettringite with expansion properties; magnesium carbonate can undergo a secondary hydration reaction with calcium hydroxide in cement hydration products, also known as the alkali-carbonate reaction, to generate brucite with expansion properties; calcium carbonate can undergo a secondary hydration reaction with calcium aluminate hydrate in cement hydration products to generate calcium carboaluminate hydrate. The above mechanism explains that the main mineral components in PBL powder can simultaneously activate the activity of cement hydration products. That is, the secondary hydration reaction can improve the degree of cement hydration reaction and the degree of gel polymerization, thereby improving the density of the cement stone itself. The expansion characteristics of the secondary hydration reaction process and reaction products and their coupling effect can compensate for the volume shrinkage of concrete throughout the process, prevent shrinkage cracks in concrete, and improve the impermeability of concrete. The secondary hydration reaction can pre-eliminate calcium hydroxide and calcium aluminate hydrate, which are easily corroded by sulfates, and convert them into ettringite, magnesia and calcium aluminate hydrate, which do not react with sulfates. Therefore, the purpose of this invention is to incorporate 8% to 12% PBL powder into concrete, thereby simultaneously improving the density and impermeability of the concrete. This process eliminates calcium hydroxide and hydrated calcium aluminate in the cement paste, which are easily corroded by sulfates, and transforms them into ettringite, magnesia, and hydrated calcium aluminate that do not react with sulfates. Even if the corrosive medium sulfate penetrates into the concrete in service, no sulfate corrosion reaction will occur. This fundamentally and thoroughly solves the durability problem of concrete subjected to sulfate corrosion. Even in highly corrosive environments, it is no longer necessary to use high sulfate-resistant cement to prepare concrete according to technical specifications, nor is it necessary to take any auxiliary engineering measures.

[0061] (2) Provide new technical approaches for the resource utilization and large-scale utilization of industrial solid waste.

[0062] During cement production, 3%–5% gypsum dihydrate is already added to the cement clinker. However, this amount is only to address the issue of excessively rapid cement setting time and cannot completely convert the hydrated calcium aluminate in the hydration products into ettringite. But if phosphogypsum powder is further added to concrete, under normal temperature conditions and in the early stages of cement hydration, its main mineral component, gypsum dihydrate, can completely convert hydrated calcium aluminate into ettringite, which does not react with sulfates. This not only improves the density and impermeability of concrete but also enhances its resistance to sulfate attack. This objective law provides a new technical approach for the resource utilization and large-scale application of phosphogypsum. Simultaneously, if an appropriate amount of magnesite powder is added to concrete, under normal temperature conditions, its main mineral component, magnesium carbonate, undergoes the formation of calcium hydroxide, converting all of the calcium hydroxide into brucite, which does not react with sulfates. This not only improves the density and impermeability of concrete but also enhances its resistance to sulfate attack. This objective law provides a new technical approach for the resource utilization and large-scale application of magnesite tailings. In addition, an appropriate amount of limestone powder needs to be added to the concrete to convert the remaining small amount of hydrated calcium aluminate into hydrated calcium carboaluminate that does not react with sulfates, and to provide a guarantee for the formation and stability of ettringite, that is, to prevent the ettringite crystals from changing from the "trisulfite type" to the "monosulfite type".

[0063] Based on the above principles, a PBL powder is prepared using phosphogypsum, magnesite, and limestone. This powder can simultaneously activate the activity of cement hydration products, promoting the secondary hydration reaction of calcium hydroxide and calcium aluminate hydrate in the cement hydration products, and converting them into ettringite, magnesite, and calcium aluminate hydrate, which do not react with sulfates. The effects of PBL powder on the microstructure and properties of concrete are mainly reflected in the following four aspects: ① The secondary hydration reaction almost completely consumes calcium hydroxide and calcium aluminate hydrate, which are easily corroded by sulfates, and the final products, ettringite, magnesite, and calcium aluminate hydrate, do not react with sulfates; ② The secondary hydration reaction can increase the degree of cement hydration reaction and the degree of gel polymerization, thereby improving the density of the cement stone itself; ③ The alkali-carbonate reaction takes place in the pore solution of concrete, and the reaction products fill, deposit, or block the capillaries, which can improve the density of the cement stone microstructure; ④ The synergistic expansion effect of the secondary hydration reaction products, ettringite and magnesite, can compensate for the volume shrinkage of concrete throughout the process, preventing shrinkage cracks in concrete.

[0064] In summary, the mechanism of the secondary hydration reaction between PBL powder and calcium hydroxide and calcium aluminate hydrate in the hydration products, as well as the influence of the secondary hydration reaction process, product characteristics, and coupling effects on the microstructure and performance of concrete, provide theoretical support for the application of industrial solid wastes such as phosphogypsum, magnesite, and limestone in sulfate-resistant concrete. Therefore, the purpose of this invention is to use PBL powder as a carrier to formulate ultra-high performance sulfate-resistant concrete, thereby improving the resource utilization and large-scale utilization of industrial solid wastes such as phosphogypsum, magnesite, and limestone.

[0065] (3) Significantly reduce the cost of basic engineering projects

[0066] Adding 8% to 12% PBL powder to ordinary silicate cement concrete can completely convert the calcium hydroxide and hydrated calcium aluminate, which are easily corroded by sulfates, into ettringite, brucite, and hydrated calcium aluminate, which do not react with sulfates. Even if the corrosive medium sulfate penetrates into the concrete, sulfate corrosion will not occur. Therefore, the purpose of this invention is to eliminate the need for sulfate-resistant cement to be used in concrete preparation according to technical specifications, and to eliminate the need for any auxiliary engineering measures, thereby significantly simplifying the construction process, shortening the construction period, and reducing the cost of foundation engineering.

[0067] Specifically, the present invention provides a method for preparing sulfate-resistant concrete, comprising: adding 8% to 12% PBL powder to freshly mixed ordinary silicate cement concrete; wherein the PBL powder is prepared by grinding phosphogypsum, magnesite and limestone in a dry basis mass ratio of 4:2:1.

[0068] The main mineral components of this PBL powder are gypsum dihydrate (CaSO4•2H2O), magnesium carbonate (MgCO3), and calcium carbonate (CaCO3).

[0069] By utilizing the main mineral components of industrial waste such as phosphogypsum, magnesite, and limestone, a secondary hydration reaction is initiated with calcium hydroxide and calcium aluminate hydrate in cement hydration products. This pre-emptively eliminates calcium hydroxide and calcium aluminate hydrate, which are easily corroded by sulfates, and converts them entirely into ettringite, brucite, and calcium aluminate hydrate, which do not react with sulfates. Even if the corrosive medium sulfate penetrates into the concrete, sulfate corrosion will not occur, fundamentally and completely solving the problem of concrete's durability against sulfate corrosion. The effects of preparing PBL powder using phosphogypsum, magnesite, and limestone on the microstructure and properties of concrete are mainly reflected in the following three aspects:

[0070] (1) It can comprehensively improve various defects in the microstructure.

[0071] Under normal circumstances, during the setting and hardening process of concrete, due to the influence of factors such as the degree of cement hydration reaction, water evaporation and changes in ambient temperature and humidity, the microstructure of cement stone has various defects such as insufficient gel density, capillary pores connected to the outside world and microcracks caused by volume shrinkage. These defects are the fundamental reason for the significant reduction in the impermeability of concrete and are also the channels through which corrosive media penetrate into the concrete in service. Adding 8%–12% PBL powder to fresh concrete can produce the following technical effects: ① The main mineral components of PBL powder simultaneously activate the hydration products, promoting the secondary hydration reaction of calcium hydroxide and calcium aluminate in the hydration products, and transforming them into more physicochemically stable ettringite, magnesia, and calcium aluminate hydrate, thereby increasing the degree of cement hydration reaction and gel polymerization, and thus improving the density of the cement paste itself; ② The alkali-carbonate reaction takes place in the pore solution of the concrete, and its reaction products are calcite and magnesia, which are deposited or filled in the capillaries, blocking the capillaries that connect the cement paste to the outside, thereby improving the density of the cement paste microstructure; ③ Utilizing the synergistic expansion effect of the secondary hydration reaction products ettringite and magnesia, the volume shrinkage of concrete can be compensated throughout the process, avoiding shrinkage cracks in the cement paste. Therefore, adding 8%–12% PBL powder to fresh concrete can comprehensively improve various defects already existing in the microstructure of concrete, thereby comprehensively improving the density and impermeability of concrete.

[0072] (2) It can eliminate hydration products that are easily corroded by sulfates in advance.

[0073] Normally, calcium hydroxide and calcium aluminate hydrate, the hydration products of cement, exist stably in the concrete as final products. However, in corrosive environments, calcium hydroxide and calcium aluminate hydrate are unstable and can undergo sulfate attack reactions with corrosive media. The amount of ettringite (or accompanying gypsum) produced by the reaction and its expansion effect are the root causes of cracking, spalling, and damage to concrete structures. If 8% to 12% of PBL powder is added to fresh concrete, its main mineral components will undergo a secondary hydration reaction with the cement hydration products, thus eliminating the calcium hydroxide and calcium aluminate hydrate that are easily attacked by sulfates in advance and converting them all into ettringite, brucite, and calcium aluminate hydrate that do not react with sulfates. Even if the corrosive medium sulfates penetrate into the concrete, sulfate attack reactions will not occur, fundamentally and completely solving the durability problem of concrete subjected to sulfate attack.

[0074] (3) It can improve the formation and stability conditions of ettringite.

[0075] If all the calcium aluminate hydrate in the hydration products were converted into ettringite, the concrete might experience excessive expansion due to the presence of only dihydrate gypsum, which is undesirable. However, if the amount of dihydrate gypsum is reduced, the initially formed trisulfide ettringite (AFt) can continue to react with the calcium aluminate hydrate to form monosulfide ettringite (AFm). This significant crystal transformation of ettringite can lead to a "strength reduction" phenomenon in the concrete. Therefore, this invention utilizes limestone powder to replace a portion of the dihydrate gypsum, allowing it to continue reacting with the small amount of calcium aluminate hydrate that did not participate in the secondary hydration reaction to form calcium aluminocarbonate hydrate, which has no expansion properties. This keeps the concrete expansion within a suitable range and also prevents the transformation of the ettringite crystal form from AFt to AFm, thereby improving the formation and stability conditions of ettringite and preventing the "strength reduction" phenomenon in the concrete.

[0076] Furthermore, the method for preparing the above-mentioned PBL powder includes:

[0077] Open-air stockpiled phosphogypsum should be air-dried or oven-dried at a temperature not exceeding 650°C.

[0078] Magnesite and limestone were used as grinding aids for phosphogypsum, and the mixture was ground at a dry basis mass ratio of 4:2:1. The grinding was carried out until the specific surface area was ≥400 m². 2 PBL powder can be prepared by increasing the yield to / kg.

[0079] Furthermore, when designing the concrete mix proportion, the PBL powder accounts for 8% to 12% of the total cementitious material. Among them, the phosphogypsum powder is added internally, replacing part of the cementitious material by equal mass; the magnesite powder and limestone powder are added externally, replacing part of the fine aggregate by equal mass, in order to adjust and optimize the gradation of the fine aggregate and participate in the secondary hydration reaction.

[0080] To preemptively eliminate calcium hydroxide and calcium aluminate hydrate, which are easily corroded by sulfates, from the hydration products, phosphogypsum powder (of equal mass) from PBL powder is used to replace cementitious materials. This allows most of the calcium aluminate hydrate to be converted into ettringite, and limestone powder is used to replace fine aggregates to adjust and optimize the gradation of the fine aggregates. A small portion of the calcium aluminate hydrate is also converted into calcium aluminocarbonate hydrate, which provides a technical guarantee for the formation and stabilization of ettringite. Simultaneously, magnesite tailings powder is used to replace fine aggregates to adjust and optimize the gradation of the fine aggregates and to completely convert calcium hydroxide into brucite. Under these conditions, even if the corrosive medium sulfate penetrates into the service concrete, sulfate corrosion will not occur. To achieve this goal, the present invention employs an orthogonal design method to optimize the technical formulation, optimal dosage, and concrete performance of PBL powder. Based on the simultaneous improvement and enhancement of concrete mixture performance, compressive strength at different ages, and resistance to sulfate attack, the dry basis mass ratio of phosphogypsum, magnesite, and limestone is determined to be 4:2:1, and the optimal dosage of PBL powder is 8%~12%.

[0081] Chemical properties of PBL powder:

[0082] (1) Since the main mineral components of PBL powder are gypsum dihydrate, magnesium carbonate and calcium carbonate, they are all slightly soluble in water and do not react chemically with water. Therefore, PBL powder will not deteriorate when it comes into contact with water and is not afraid of moisture, rain or soaking during storage and transportation.

[0083] (2) The main mineral components in PBL powder can simultaneously activate the activity of cement hydration products under normal temperature conditions, promote the secondary hydration reaction of calcium hydroxide and calcium aluminate hydrate, and generate ettringite, magnesia and calcium aluminate hydrate that do not react with sulfate.

[0084] (3) Adding 8% to 12% PBL powder to ordinary silicate cement concrete will prevent sulfate attack even if the corrosive medium sulfate penetrates into the concrete. Therefore, when the concrete is in a highly corrosive environment, it is no longer necessary to take auxiliary measures as required by the technical specifications.

[0085] The Influence of PBL Powder on Concrete Performance

[0086] (1) C30 concrete reference mix proportion used in the experiment

[0087]

[0088] (2) Instructions for use of PBL powder

[0089] The optimal dosage of PBL powder should be 8% to 12% of the total cementitious materials. Among them, phosphogypsum powder should be added internally, replacing part of the cementitious materials by equal mass, and its dosage should be controlled within the range of 4% to 6% of the total cementitious materials. Magnesite powder or limestone powder should be added externally, replacing part of the fine aggregate by equal mass, in order to adjust and optimize the gradation of fine aggregates. The dosage of magnesite powder should be controlled within the range of 4% to 6% of the cement dosage, and the dosage of limestone powder should be controlled within the range of 2% to 3% of the cement dosage.

[0090] (3) When 10% PBL powder was added to the C30 concrete standard mix proportion, a comparative experiment was conducted with concrete without PBL powder. The cohesiveness, water retention and fluidity of the mixture were significantly improved.

[0091] (4) To verify the effect of PBL powder on the microstructure density of concrete, 10% PBL powder was added to the concrete. Two groups of slurry specimens were prepared according to the mass ratio of cementitious materials, PBL powder, water, and pumping agent: one control group and one experimental group. After standard curing for 28 days, the specimens were analyzed by SEM scanning. The results showed that the microstructure density of cement paste was significantly improved, such as... Figure 1 As shown.

[0092] (5) In order to verify the effect of PBL powder on the compressive strength of concrete, 10% of PBL powder was added to the C30 concrete standard mix proportion, and 6 groups of concrete specimens were prepared, namely 3 groups of blank group and 3 groups of test group. After standard curing for 7d, 28d and 56d respectively, the compressive strength test was carried out on the specimens at different ages. The results showed that the compressive strength of the test group at different ages was better than that of the blank group, as shown in Table 1.

[0093]

[0094] (6) In order to verify the effect of PBL powder on the sulfate resistance of concrete, 10% of PBL powder was added to the C30 concrete standard mix proportion, and 5 groups of concrete blocks were prepared. After standard curing for 28 days, the sulfate resistance of the concrete blocks was tested, as shown in Table 2.

[0095]

[0096] In summary, the analysis demonstrates that the technical solution of using PBL powder prepared from industrial solid wastes such as phosphogypsum, magnesite, and limestone for sulfate resistance in concrete is feasible.

[0097] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing sulfate-resistant concrete, characterized in that, include: Add 8% to 12% of PBL powder, which accounts for 8% to 12% of the total cementitious material, to freshly mixed ordinary silicate cement concrete. The PBL powder is prepared by grinding phosphogypsum, magnesite, and limestone in a dry basis mass ratio of 4:2:1; the main mineral components of the PBL powder are gypsum dihydrate, magnesium carbonate, and calcium carbonate. When designing concrete mix proportions, the PBL powder content accounts for 8% to 12% of the total cementitious materials. Among them, phosphogypsum powder is added internally, replacing part of the cementitious materials by equal mass; magnesite powder and limestone powder are added externally, replacing part of the fine aggregate by equal mass, in order to adjust and optimize the gradation of fine aggregates and participate in secondary hydration reactions.

2. The method according to claim 1, characterized in that, The preparation process of the PBL powder includes: Open-air stockpiled phosphogypsum should be air-dried or oven-dried at a temperature not exceeding 650°C. Magnesite and limestone were used as grinding aids for phosphogypsum, and the mixture was ground at a dry basis mass ratio of 4:2:

1. The grinding was carried out until the specific surface area was ≥400 m². 2 PBL powder can be prepared by increasing the yield to / kg.

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