Marine low-calcium ultra-high performance geopolymer concrete, preparation method and application thereof

Abstract

The application discloses a kind of marine low-calcium ultra-high performance geopolymer concrete and its preparation method and application.The concrete includes low-calcium active precursor powder, composite alkali activator, aggregate, steel fiber, silica fume and mixing water;The precursor powder is composed of F-grade low-calcium fly ash and metakaolin activated by mechanical grinding, and the total CaO mass fraction of the precursor powder is not higher than 5%.During preparation, the precursor powder and the aggregate are first dry mixed, and then the second dry mixing is carried out by adding steel fiber;After adding the aged sodium hydroxide-sodium silicate composite alkali activator and mixing water, wet mixing is carried out by physically adding silica fume or replacing part of the silica component in sodium silicate with silica fume;After pouring, it is sealed with a film and cured at room temperature.The method is beneficial to the dispersion of steel fiber and the maintenance of the humid environment required for alkali activation reaction, and is suitable for marine engineering structures, repair and reinforcement.

Description

Technical Field

[0001] This invention relates to the fields of alkali-activated cementitious materials, industrial solid waste resource utilization, and marine engineering construction materials, specifically to a marine low-calcium ultra-high performance geopolymer concrete, its preparation method, and its application. Background Technology

[0002] Foundations, slope protection structures, repair structures, and underwater structures in marine engineering are typically located in environments containing chloride, sulfate, and magnesium salts. When seawater enters the pores of cementitious materials, it can alter the internal composition and interfacial state of the material, thereby affecting the long-term service life of the structures.

[0003] Alkali-activated geopolymers, using fly ash, metakaolin, and other siliceous and aluminous materials as raw materials, can form an inorganic cementitious system. In existing technologies, when fly ash activity is insufficient, reaction conditions are often improved by refining or compounding it; steel fibers can be used to inhibit crack propagation; and silica fume can participate in the alkali-activated reaction as an ultrafine filler or a source of soluble silicon. However, in low-calcium systems, even with separate settings for raw material activation, fiber dispersion, silica fume introduction, and room-temperature curing, problems such as fiber agglomeration, uneven localized reactions, or excessively rapid water loss may still occur.

[0004] Therefore, there is a need to provide a low-calcium geopolymer concrete suitable for normal-temperature construction at marine engineering sites and its preparation method, so that the low-calcium precursor, silica fume, steel fiber and curing process can form a mutually compatible material system. Summary of the Invention

[0005] The purpose of this invention is to provide a low-calcium, ultra-high-performance geopolymer concrete for marine engineering, its preparation method, and its application. By controlling the composition of low-calcium active precursor powder, and using a graded dry mixing method with steel fiber added later, two methods of introducing silica fume, and a membrane sealing and room temperature curing process, a geopolymer concrete suitable for marine engineering is formed.

[0006] To achieve the above objectives, this invention provides a marine low-calcium ultra-high performance geopolymer concrete, comprising low-calcium active precursor powder, a composite alkali activator, aggregate, steel fiber, silica fume, and mixing water; the low-calcium active precursor powder comprises mechanically ground and activated Grade F low-calcium fly ash and metakaolin, and the total CaO mass fraction of the low-calcium active precursor powder is not higher than 5%; the composite alkali activator comprises sodium hydroxide and water glass; the silica fume is added by physical admixture or used to replace part of the silica component in the water glass.

[0007] Preferably, by weight, the F-grade low-calcium fly ash is 50-90 parts, the metakaolin is 10-40 parts, the aggregate is 100-300 parts, the silica fume is 5-50 parts, and the mixing water is 15-40 parts.

[0008] Preferably, the F-grade low-calcium fly ash has a median particle size of 1–30 μm, an amorphous material content of 60%–90%, and a CaO mass fraction of 1%–5%.

[0009] Preferably, the aggregate is low-activity quartz sand with a particle size of 0.20–2.50 mm or medium-activity granite with a particle size of 0.15–4.00 mm.

[0010] Preferably, the steel fiber is a short-cut copper-plated high-strength steel fiber with a diameter of 0.10 to 0.30 mm, a length of 6 to 20 mm, and a volumetric admixture of 1% to 3% of the total concrete volume.

[0011] The present invention also provides a method for preparing the aforementioned low-calcium ultra-high performance geopolymer concrete for marine engineering, the method comprising the following steps:

[0012] S1. Mechanically grind and activate F-grade low-calcium fly ash, and mix it with metakaolin to obtain low-calcium active precursor powder with a total CaO mass fraction of no more than 5%.

[0013] S2. Sodium hydroxide and water glass are mixed to obtain a composite alkali activator, and then sealed and aged.

[0014] S3. The low-calcium active precursor powder is first dry-mixed with the aggregate, and then steel fibers are added for a second dry-mixing.

[0015] S4. Add the composite alkali activator and mixing water to the material after the second dry mixing and perform wet mixing, and add silica fume by physical admixture, or replace part of the silica component in water glass with silica fume in S2.

[0016] S5. Pour the wet-mixed material into a mold, vibrate it, cover and seal it with a film, and cure it under normal temperature conditions.

[0017] Preferably, in step S1, the F-grade low-calcium fly ash sieved through a 45 μm sieve is ground to a median particle size of 5 μm, and then mixed with metakaolin at a mass ratio of 85:15.

[0018] Preferably, in step S2, the Si / Al molar ratio of the composite alkali activator is 1.0–4.0, the Na / Si molar ratio is 1.0–3.0, and the aging time is 24 h.

[0019] Preferably, in step S3, the first dry mixing speed is 300 r / min and the time is 3 min, and the second dry mixing speed is 300 r / min and the time is 2 min; in step S4, the wet mixing speed is 250 r / min and the time is 3 min, and when using physical external mixing, the mixing speed is 200 r / min for 3 min.

[0020] The present invention also provides the application of the marine low-calcium ultra-high performance geopolymer concrete in marine soil and rock foundation reinforcement, coastal protection, underwater leak sealing, coastal structure repair or offshore wind power foundation.

[0021] Compared with the prior art, the present invention has at least the following beneficial effects:

[0022] Activated low-calcium fly ash is blended with metakaolin to provide the precursor with a source of silica and aluminum suitable for alkali-activated reactions, while controlling the total CaO content to reduce the proportion of high-calcium components in the system. Dry mixing the precursor powder with the aggregate first allows for a more uniform coating of the powder on the aggregate surface; the addition of steel fibers in the subsequent dry mixing stage helps reduce fiber agglomeration through stirring and shearing. Silica fume, when added physically, can participate in particle size distribution adjustment; when introduced as a substitute for some of the silica in water glass, it can serve as a silicon source in the activator system. Sealed curing at room temperature reduces mixing water loss and maintains the humid environment required for alkali-activated reactions. These combined characteristics make the material system suitable for room-temperature preparation and use in marine engineering sites. Attached Figure Description

[0023] Figure 1 The X-ray diffraction pattern of the marine low-calcium ultra-high performance geopolymer concrete of the present invention is shown.

[0024] Figure 2 This is a macroscopic morphology image of the marine low-calcium ultra-high performance geopolymer concrete specimen of the present invention.

[0025] Figure 3 This is a scanning electron microscope image of the interface between steel fiber and geopolymer matrix in the marine low-calcium ultra-high performance geopolymer concrete of the present invention.

[0026] Figure 4 This is a spatial distribution diagram of steel fibers in the marine low-calcium ultra-high performance geopolymer concrete of the present invention. Detailed Implementation

[0027] The present invention will be further described below with reference to embodiments. Those skilled in the art should understand that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0028] In this invention, the low-calcium active precursor powder refers to a silica-alumina powder composed of mechanically ground and activated F-grade low-calcium fly ash and metakaolin. The total CaO mass fraction refers to the mass fraction of CaO introduced by each raw material in the precursor powder, weighted by mass. The composite alkali activator refers to an alkaline activation system formed by the compounding of sodium hydroxide and water glass. Physical admixture refers to adding silica fume to the base slurry after its formation; chemical substitution refers to replacing part of the silica component in the water glass with silica fume, and mixing them together during the preparation stage of the composite alkali activator.

[0029] like Figure 1 As shown, the X-ray diffraction pattern of the geopolymer concrete characterizes the mineral phase information in the molded body; the characteristic locations of silica, dolomite, NASH gel, and zeolite phases are marked in the figure. Figure 2 As shown, the molded specimen exhibits the macroscopic morphology of a fiber-reinforced geopolymer matrix. For example... Figure 3 As shown, the steel fibers and the geopolymer matrix form a contact fit at the interface region. Figure 4 As shown, the steel fibers are spatially distributed along the interior of the molded body.

[0030] An embodiment of the present invention provides a method for preparing marine low-calcium ultra-high performance geopolymer concrete, the method comprising the following steps.

[0031] Grade F low-calcium fly ash and metakaolin were selected as precursor raw materials. First, the Grade F low-calcium fly ash, sieved through a 45 μm sieve, was ground in a ball mill to refine its particle size to the required range. Then, the ground fly ash was mixed with metakaolin. The mass fractions of fly ash and metakaolin were 50–90 parts and 10–40 parts, respectively, and the total CaO mass fraction of the resulting precursor powder was controlled to be no higher than 5%. After grinding, the dense layer on the surface of the fly ash particles was broken, increasing the contact opportunity between the silica-alumina active phase and the alkali activator. Metakaolin supplemented the active silica-alumina components in the precursor, and the two together formed a low-calcium active precursor powder.

[0032] The composite alkali activator is prepared from sodium hydroxide and water glass. Sodium hydroxide and water glass are weighed according to the target Si / Al molar ratio and Na / Si molar ratio, and then stirred in an alkali-resistant container until the solid sodium hydroxide dissolves. The container is then sealed and aged. The aging time can be 24 hours. Aging gradually reduces the localized temperature difference caused by the exothermic reaction during dissolution, which helps maintain a uniform activator composition when dry materials are added subsequently.

[0033] Low-calcium reactive precursor powder and aggregate are first added to a forced mixer for initial dry mixing. The aggregate can be low-reactivity quartz sand or medium-reactivity granite. After the initial dry mixing, steel fibers are added for a second dry mixing. The steel fibers are short-cut copper-plated high-strength steel fibers with a diameter of 0.10–0.30 mm, a length of 6–20 mm, and a volumetric admixture of 1%–3% of the total concrete volume. Adding the steel fibers after the precursor powder and aggregate have completed the initial mixing allows for mechanical dispersion of the fibers after a stable dry-mix system has formed, thus reducing the possibility of fiber clumps forming when the fibers are added directly with a large amount of powder.

[0034] After the second dry mixing, aged composite alkali activator and mixing water are added to the mixer for wet mixing to obtain the base slurry. Silica fume can be introduced in two ways. In the physical admixture method, silica fume is added to the base slurry and then stirred at a lower speed to disperse the ultrafine particles in the slurry. In the chemical substitution method, silica fume replaces part of the silica component in water glass during the preparation of the composite alkali activator. The silica fume is mixed and aged together with the activator, and then added to the dry-mixed materials following the same wet mixing process. Both methods maintain the core process relationships of low-calcium precursor, staged dry mixing, and coating curing.

[0035] Example 1

[0036] Take 85 parts by weight of Grade F low-calcium fly ash, sieve it through a 45 μm sieve, and ball mill it to a median particle size of 5 μm. Mix it with 15 parts by weight of metakaolin for 5 min to obtain low-calcium active precursor powder. Mix sodium hydroxide and water glass to obtain a composite alkali activator, and seal and age for 24 h. Take 100 parts by weight of the aforementioned low-calcium active precursor powder and 100 parts by weight of low-activity quartz sand with a particle size not greater than 2 mm, and dry mix at 300 r / min for 3 min; add steel fiber with a volume ratio of 2%, and continue to dry mix at 300 r / min for 2 min. After adding the composite alkali activator and mixing water, stir at 250 r / min for 3 min; then add silica fume equivalent to 5% of the precursor powder mass, and stir at 200 r / min for 3 min. The resulting slurry is poured into a test mold, vibrated, covered and sealed, and cured at room temperature.

[0037] The geopolymer paste prepared using the method in this embodiment has a 28-day compressive strength of 106 MPa, a geopolymer concrete has a 28-day compressive strength of 150 MPa, a standard fluidity of 182 mm, a chloride ion binding rate of 82%, and the specimen strength increases by 5-15% after continuous immersion in simulated seawater at a concentration of 35 ppt for 56 days.

[0038] Example 2

[0039] The steps for preparing the precursor powder, the composite alkali activator, the graded dry mixing, wet mixing, and the film curing are the same as in Example 1. The aggregate is replaced with medium-reactivity granite with a particle size of 0.15–4.00 mm. The mineral composition on the granite surface can form an interface with the geopolymer matrix, making it suitable for coastal structures with structural load-bearing requirements.

[0040] The geopolymer concrete prepared using the method in this embodiment has a 28-day compressive strength of 152 MPa, a chloride ion binding rate of 79%, and a strength increase of 5-10% after immersion in simulated seawater (35 ppt) for 56 days.

[0041] Example 3

[0042] Similar to Example 1, but with the introduction of silica fume through chemical substitution. Silica fume was added during the preparation of the composite alkali activator, replacing 10% of the silica component in the water glass, and aged together with it. Subsequently, it was prepared according to the aforementioned first dry mixing, second dry mixing, wet mixing, casting, and film sealing curing steps. In this embodiment, silica fume participates in subsequent reaction processes as a partial source of silicon in the activator system.

[0043] The geopolymer concrete prepared using the method described in this embodiment has a 28-day compressive strength of 155 MPa, a slurry fluidity of 175 mm, and a chloride ion binding rate of 80%. The test results show that the mechanical strength and durability indicators fluctuate very little under both silica fume introduction methods, indicating that this invention has good compatibility with both process routes.

[0044] Comparative Example 1

[0045] Based on Example 1, a high-calcium system was used as a comparative process. 30 parts by weight of the F-grade low-calcium fly ash in the precursor powder of Example 1 were replaced with S95 slag powder, increasing the CaO mass fraction of the precursor powder to over 15%; the remaining steps—meta-kaolin, composite alkali activator, aggregate, silica fume, steel fiber, graded dry mixing, and room-temperature film curing—remained consistent. This comparative process was used to investigate the impact of changes in the calcium composition of the precursor powder on the material system.

[0046] The concrete prepared using this comparative method had a 28-day compressive strength of 65 MPa and a chloride ion binding rate of 51%. However, after 56 days of immersion in 35 ppt seawater, the strength retention rate was only 68%. The calcium phase in slag forms C-(A)-SH gel under alkaline activation conditions. This type of gel develops strength rapidly in the early stages; therefore, while the 28-day compressive strength of 65 MPa is lower than that of the embodiment in this invention, it is still at a certain level. However, during long-term immersion in seawater, free calcium ions react with SO42- in the seawater... 2- Mg 2+The gypsum-type or magnesium-type expansion corrosion reaction occurs, causing the gel structure to gradually disintegrate, with a strength retention rate of only 68% after 56 days. This contrasts sharply with the 5-15% improvement in mechanical properties in Example 1, demonstrating that the high-calcium system has an inherent defect in long-term service performance in marine environments. The low-calcium design of this invention, which controls CaO to within 5%, is the core prerequisite for ensuring marine engineering durability.

[0047] Comparative Example 2

[0048] Based on Example 1, a comparative process without adding silica fume was used. The precursor powder, composite alkali activator, aggregate, steel fiber, and mixing water were all treated according to the physical addition of silica fume method, but without adding silica fume. This comparative process was used to investigate the role of silica fume in the aforementioned low-calcium geopolymer system.

[0049] The geopolymer paste prepared using this comparative method has a 28-day compressive strength of 56 MPa, a 28-day compressive strength of 102 MPa, a paste fluidity of 110 mm, and a chloride ion binding rate of 60%.

[0050] Compared to Example 1, without the addition of silica fume, the compressive strength of the concrete in this comparative example decreased from 150 MPa to 102 MPa, a reduction of 32%; the flowability decreased from 182 mm to 110 mm, no longer possessing good workability; and the chloride ion binding rate decreased from 82% to 60%. The simultaneous and significant decrease in these three core indicators demonstrates the important role of silica fume in this system. Physical filling optimizes particle size distribution, thereby improving flowability at low water-cement ratios; physical refining of pores enhances the physical retention of chloride ions; and chemical participation in the polymerization reaction improves the intrinsic strength of the matrix.

[0051] Comparative Example 3

[0052] Based on Example 1, a comparative process of heat curing was adopted. The steps of precursor powder, composite alkali activator, aggregate, steel fiber, silica fume, and graded dry mixing were kept consistent with the implementation method of physical external silica fume addition; after demolding, the molded body was placed under sealed conditions at 60°C for 28 days. This comparative process was used to examine the process differences between film-sealed room temperature curing and heat curing.

[0053] The concrete prepared using this comparative method showed a slight increase in compressive strength, but the entire process requires a constant-temperature curing chamber. This approach sacrifices ease of construction and significantly increases energy costs in exchange for limited strength gains, making it impractical for large-volume, long-distance projects such as coastal protection and underwater sealing.

[0054] Comparative Example 4

[0055] Based on Example 1, a comparative process using uncrushed fly ash was employed. Grade F low-calcium fly ash (screened at 45 μm) was directly mixed with metakaolin without ball milling activation. The remaining components and preparation steps were consistent with the method using physically added silica fume. This comparative process was used to investigate the effect of mechanical grinding and activation of fly ash on the preparation of precursor powders.

[0056] The geopolymer paste prepared using this comparative method had a 28-day compressive strength of 54 MPa, while the concrete had a 28-day compressive strength of only 110 MPa. This comparative example verifies the crucial role of mechanical grinding activation. The dense glassy structure on the surface of the undisturbed fly ash particles severely restricts the contact between the alkali-activated solution and the internal amorphous silica-alumina phase. After grinding, D... 50 Refining the particle size from its original state (typically about 20-30 μm) to 5 μm increases the specific surface area several times over, and the large amount of active internal phase is exposed after the glassy outer shell breaks, significantly enhancing the degree of polymerization reaction under room temperature conditions. Without activation, a large amount of raw material does not actually participate in the reaction and exists only as inert filler, therefore its strength is far from meeting ultra-high performance standards. This proves that grinding and activation is not a simple physical refinement, but an indispensable pre-process for achieving ultra-high performance at room temperature in this invention.

[0057] During casting, mechanical vibration can be used depending on the size of the molded body, with a vibration time of 1–3 minutes. After molding, cover the mold surface with plastic film and allow the material to stand at room temperature without special heating equipment. After demolding, continue to cover and seal the molded body for curing. Covering helps maintain the moisture content within the system within the range required for the reaction, while reducing the chance of carbon dioxide in the air coming into contact with the activator.

[0058] The materials of this invention can be used in marine engineering applications such as marine soil and rock foundation reinforcement, coastal protection dikes, underwater leak sealing and repair, coastal wharves or bridge piers, and offshore wind power foundations. For repair and leak sealing applications, the aggregate particle size can be controlled within a range suitable for the construction thickness; for structure forming applications, quartz sand or granite aggregate can be selected according to the structural dimensions.

[0059] This invention establishes a continuous process relationship through low-calcium precursor control, fly ash activation, graded dry mixing with added steel fibers, silica fume introduction, and room-temperature curing with a film-sealed coating. Precursor composition control provides a source of silicon and aluminum for the alkali-activated reaction; graded dry mixing improves the fiber distribution in the dry-mix system; different silica fume introduction methods adapt to different process settings; and film sealing provides a humid environment for the room-temperature reaction process. Therefore, the prepared geopolymer concrete is suitable for the preparation and application of marine engineering materials.

[0060] The order in which precursor powder, aggregate, and steel fiber are added should not be reversed. When steel fiber and precursor powder are added simultaneously, a large amount of ultrafine powder easily adheres to the fiber surface, and the fiber contact points formed in the initial mixing stage are not easily separated by subsequent wet mixing. When the precursor powder and aggregate are mixed first, followed by the addition of steel fiber, the movement and shearing zones formed by the aggregate particles during mixing help to carry the fiber into the homogenized dry mix. Because the steel fiber is acted upon by both the aggregate and powder in the second dry mixing stage, the fiber can remain relatively dispersed before the liquid phase is added, thus providing conditions for the formation of a homogeneous matrix after wet mixing.

[0061] There is a synergistic relationship between low-calcium control and the introduction of silica fume. The precursors primarily consist of fly ash and metakaolin, with the total CaO mass fraction controlled to be no higher than 5%, avoiding the use of high-calcium slag as a necessary precursor. When silica fume is physically added, it enters the already formed base slurry as ultrafine particles; when chemically substituted, it acts as a substitute for some of the silica components in water glass, entering the activator. In both methods, silica fume does not alter the compositional boundaries of the low-calcium precursor. Because the particle size of silica fume is smaller than that of commonly used aggregates, it can be distributed within the space formed by the precursor powder and aggregate particles, thus helping to adjust the particle arrangement within the slurry.

[0062] Film sealing and curing should be maintained throughout the early stages after casting and the continued curing stage after demolding. After casting, the areas of the material surface in contact with the outside air are the first to experience moisture loss. Film covering prevents rapid surface moisture loss and reduces the chance of carbon dioxide entering the material surface. Because the curing environment maintains relatively stable humidity conditions, the alkali-activated system can continue to react at room temperature, thus avoiding reliance solely on external heating equipment to maintain reaction conditions. For large-volume components, the film can be used in an overlapping manner, with the overlapping areas kept sealed to ensure continuous coverage of the entire molded surface.

[0063] When using quartz sand aggregate, select particles with similar particle sizes and dry-mix them together with the precursor powder. When using granite aggregate, remove surface dust and free impurities before adding it to the mixer. When the aggregate dosage changes within the range of 100–300 parts by weight, it is advisable to simultaneously check whether the activator and mixing water can form a continuous slurry from the dry-mixed material. The addition amounts of sodium hydroxide and water glass in the composite alkali activator should be adjusted according to the target molar ratio. The portion of water glass that provides silica can be replaced by silica fume. By maintaining the correspondence between these components, different aggregate selections and different silica fume introduction methods can all adhere to the same material system.

[0064] Therefore, this invention does not simply list low-calcium precursors, steel fibers, silica fume, and room-temperature curing in parallel. Instead, it employs a continuous technical approach through low-calcium precursor control, fly ash activation, two-stage dry mixing with added steel fibers, two methods of introducing silica fume, and film-sealed curing. These features work together to seamlessly integrate the reaction conditions of the low-calcium precursor, the fiber distribution, and the post-forming curing conditions, thus forming a geopolymer concrete process suitable for marine engineering preparation and application. In summary, the aforementioned technical means work together in the raw material preparation, mixing and molding, and curing stages. Because the processes of low-calcium precursors, staged dry mixing, and film-sealing are continuously arranged, the material can be prepared according to predetermined steps under room-temperature conditions, thus meeting the construction needs of marine engineering components and repair materials.

[0065] The above-mentioned technical features work synergistically: a low-calcium precursor determines the raw materials for the gelation reaction; staged dry mixing establishes the fiber dispersion conditions; silica fume is introduced to adjust the slurry composition; and a film-sealed environment maintains the curing process. Each step is seamlessly connected, and the continuous preparation process is not replaced by changing any single material component.

[0066] In the preparation of precursor powders, the grinding degree of fly ash can be selected based on the median particle size and amorphous matter content of the original fly ash. When the initial particle size of the fly ash is large, a 45 μm sieve can be used first to remove coarse particles that are not conducive to uniform grinding; then ball milling can be performed. The ball-milled fly ash should be cooled to room temperature before being mixed with metakaolin to avoid the temperature change of the powder affecting subsequent weighing. Low-speed stirring can be used during mixing to ensure uniform contact between the two precursor raw materials. If the CaO content introduced into the precursor powder exceeds 5%, the total CaO mass fraction should be brought back to the limit range by adjusting the batch and ratio of fly ash or metakaolin raw materials.

[0067] When mixing sodium hydroxide and water glass, it is advisable to first add the predetermined amount of water glass to the alkali-resistant container, and then slowly add the sodium hydroxide. The container should have a sealable lid to reduce moisture loss during aging. The heat generated during the dissolution of sodium hydroxide will change the local temperature of the activator; therefore, it should be sealed and allowed to stand after the solid has completely dissolved. After aging, the compound alkali activator should be added to the mixer while maintaining uniformity. If local sedimentation occurs in the aging solution, it should be stirred until homogeneous before use without adding any additional components.

[0068] The forced mixer used for the first dry mixing stage can be a vertical shaft mixer or a planetary mixer. After adding the precursor powder and aggregate, the mixing paddle continuously shears and tumbles the material, causing the precursor powder to distribute between and on the surface of the aggregate particles. Steel fibers are added after the first dry mixing stage. The steel fibers are added dispersedly along the material flow area within the mixer to avoid fiber entanglement caused by a single concentrated addition. The second dry mixing stage ends when there are no obvious bundles of steel fibers in the dry mixture. This two-stage dry mixing process places the fiber dispersion process after the aggregate and precursor powder have formed a preliminary homogeneous system, which is beneficial for maintaining a relatively uniform spatial distribution of fibers during subsequent wet mixing.

[0069] In the embodiment of physical admixture of silica fume, the silica fume can be added after the basic slurry has been formed. Due to the fine particle size of silica fume, it can be added in stages with a reduced stirring speed. After dispersion, the silica fume occupies the gaps between smaller particles and forms a continuous slurry with the precursor powder, activator, and water. In the embodiment of chemical substitution, silica fume participates in the preparation of the composite alkali activator, replacing the portion of water glass that provides silica. When the substitution ratio changes, the target molar ratio of the activator should be kept within the disclosed range. The two introduction methods are not required steps in the same batch of materials; one method can be selected during construction based on the material preparation conditions.

[0070] The mixing water can be municipal tap water that meets the requirements for construction mixing water. After adding the activator, the mixing water is wet-mixed with the dry materials, allowing the activator to penetrate between the precursor powder and aggregate. During wet mixing, the uniformity of the slurry should be observed; if lumps of dry powder are found, the wet mixing time can be extended until the materials are uniform, but the order of adding steel fibers during dry mixing should not be changed. Pouring should be carried out as soon as possible after wet mixing to minimize the impact of moisture evaporation from the alkali activator in an open environment on the mixing state.

[0071] The molding mold can be a steel mold or an alkali-resistant mold that matches the dimensions of the intended product. During pouring, the slurry is added in layers and vibrated, with the vibration intensity limited to expelling entrained air bubbles without causing aggregate segregation. When covering with a film, the film should cover the mold opening and the exposed material surface; after demolding, the film continues to cover the molded body. For underwater leak sealing or on-site repairs, molding tools can be prepared according to the dimensions of the construction area, and then mixing and filling can be completed according to the preparation sequence described above. For coastal protection or foundation reinforcement scenarios, a template adapted to the structural dimensions can be set up in the component pouring area before pouring.

[0072] Quartz sand and granite aggregates are alternative options. When using quartz sand, its low activity and stable particle size can be utilized to form a relatively stable aggregate support system; when using granite, its surface mineral components can be used to form an interface with the slurry. Regardless of the aggregate chosen, the main process should remain the same: low-calcium precursor powder, composite alkali activator, steel fiber added after grading and dry mixing, silica fume introduction, and sealed film curing at room temperature.

[0073] In the above embodiments, each mass fraction is converted based on the mass of the low-calcium active precursor powder. For different construction scales, the dosage of each component can be proportionally increased or decreased; when increasing or decreasing, the total CaO mass fraction of the precursor powder, the molar ratio of the activator, the volumetric content of steel fiber, and the method of introducing silica fume should be kept within the disclosed limits. When the specifications of the mixing equipment are changed, the material can be made uniform by controlling the actual duration of dry mixing and wet mixing.

Claims

1. A marine low-calcium ultra-high performance geopolymer concrete, characterized in that: The mixture includes low-calcium active precursor powder, composite alkali activator, aggregate, steel fiber, silica fume, and mixing water; the low-calcium active precursor powder includes mechanically ground and activated F-grade low-calcium fly ash and metakaolin, and the total CaO mass fraction of the low-calcium active precursor powder is not higher than 5%; the composite alkali activator includes sodium hydroxide and water glass; the silica fume is added by physical admixture or used to replace part of the silica component in the water glass.

2. The marine low-calcium ultra-high performance geopolymer concrete according to claim 1, characterized in that: By weight, the F-grade low-calcium fly ash is 50-90 parts, the metakaolin is 10-40 parts, the aggregate is 100-300 parts, the silica fume is 5-50 parts, and the mixing water is 15-40 parts.

3. The marine low-calcium ultra-high performance geopolymer concrete according to claim 1, characterized in that: The F-grade low-calcium fly ash has a median particle size of 1–30 μm, an amorphous material content of 60%–90%, and a CaO mass fraction of 1%–5%.

4. The marine low-calcium ultra-high performance geopolymer concrete according to claim 1, characterized in that: The aggregate is low-activity quartz sand with a particle size of 0.20–2.50 mm or medium-activity granite with a particle size of 0.15–4.00 mm.

5. The marine low-calcium ultra-high performance geopolymer concrete according to claim 1, characterized in that: The steel fiber is a short-cut copper-plated high-strength steel fiber with a diameter of 0.10 to 0.30 mm and a length of 6 to 20 mm. The volumetric dosage is 1% to 3% of the total volume of concrete.

6. A method for preparing marine low-calcium ultra-high performance geopolymer concrete according to any one of claims 1 to 5, characterized in that: The method includes the following steps: S1. Mechanically grind and activate F-grade low-calcium fly ash, and mix it with metakaolin to obtain low-calcium active precursor powder with a total CaO mass fraction of no more than 5%. S2. Sodium hydroxide and water glass are mixed to obtain a composite alkali activator, and then sealed and aged. S3. The low-calcium active precursor powder is first dry-mixed with the aggregate, and then steel fibers are added for a second dry-mixing. S4. Add the composite alkali activator and mixing water to the material after the second dry mixing and perform wet mixing, and add silica fume by physical admixture, or replace part of the silica component in water glass with silica fume in S2. S5. Pour the wet-mixed material into a mold, vibrate it, cover and seal it with a film, and cure it under normal temperature conditions.

7. The method for preparing marine low-calcium ultra-high performance geopolymer concrete according to claim 6, characterized in that: In step S1, the F-grade low-calcium fly ash sieved through a 45 μm sieve is ground to a median particle size of 5 μm, and then mixed with metakaolin at a mass ratio of 85:

15.

8. The method for preparing marine low-calcium ultra-high performance geopolymer concrete according to claim 6, characterized in that: In step S2, the Si / Al molar ratio of the composite alkali activator is 1.0 to 4.0, the Na / Si molar ratio is 1.0 to 3.0, and the aging time is 24 h.

9. The method for preparing marine low-calcium ultra-high performance geopolymer concrete according to claim 6, characterized in that: In step S3, the first dry mixing speed is 300 r / min and the time is 3 min, and the second dry mixing speed is 300 r / min and the time is 2 min; in step S4, the wet mixing speed is 250 r / min and the time is 3 min, and when the physical external mixing method is adopted, the mixing speed is 200 r / min for 3 min.

10. The application of marine low-calcium ultra-high performance geopolymer concrete according to any one of claims 1 to 5 in marine soil and rock foundation reinforcement, coastal protection, underwater sealing, coastal structure repair or offshore wind power foundation.

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