UHPC (Ultra High Performance Concrete) doped with nano calcium carbonate and preparation method of UHPC

Abstract

The invention discloses UHPC (Ultra High Performance Concrete) doped with nano calcium carbonate and a preparation method thereof, and relates to the field of building materials. The UHPC is prepared from the following raw materials in parts by weight: 800 to 900 parts of cementing material, 450 to 550 parts of cement, 220 to 300 parts of local mineral admixture, 130 to 180 parts of silica fume and 3 to 8 parts of nano calcium carbonate, 600 to 700 parts of fine aggregate, 30 to 80 parts of optional coarse aggregate, 150 to 200 parts of steel fiber, an efficient water reducing agent (2.0 to 3.5 percent of the total amount of the cementing material) and 140 to 180 parts of water. The preparation method comprises the following steps: pre-stirring the aggregate, nano-mixing the rubber material, adding liquid step by step, dispersing the fiber, mixing and totally stirring. By utilizing the synergistic effect of the local material and the nano calcium carbonate, the self-shrinkage is obviously reduced, the high strength is maintained, the cost is reduced, the process controllability is good, and the method is suitable for high-performance structural engineering.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of building materials, and particularly relates to a UHPC concrete mixed with nano calcium carbonate and a preparation method thereof. BACKGROUND

[0002] Ultra-high performance concrete (UHPC) has shown great potential in important infrastructures such as bridges, high-rise buildings, offshore platforms and repair and reinforcement engineering due to its excellent mechanical properties (compressive strength > 120 MPa, flexural strength > 20 MPa), excellent durability and good compactness. The performance advantages mainly come from low water-binder ratio (usually ≤ 0.22), the addition of high-activity micro powder (such as silica fume), the selection of graded aggregate and the enhancement of steel fiber. However, the wide application of UHPC still faces two major challenges: one is the high production cost, mainly due to the use of high-quality cement, silica fume and other expensive cementitious materials and high-performance admixtures; the other is the significant early and later autogenous shrinkage, which is caused by the sharp decrease of internal humidity due to the ultra-low water-binder ratio and high cementitious material content, which easily leads to cracking risk and affects the structural durability and integrity.

[0003] In the prior art, the research on optimizing the mix proportion of UHPC to reduce the cost and shrinkage mainly focuses on the fine grading of aggregate, the large-scale addition of active mineral admixtures (such as fly ash, mineral powder, metakaolin) to replace cement, the optimization of fiber form, etc. Although these methods have achieved certain results, there are limitations: first, the use of a large amount of fly ash or mineral powder can reduce the cost and improve the later shrinkage, but may lead to slow early strength development; second, when using local aggregate, its physical properties (such as particle shape, grading) and chemical composition fluctuate, which greatly affects the workability and stability of mechanical properties, and it is difficult to meet the strict grading requirements of UHPC; third, the particle size of traditional mineral admixtures is relatively large (micron level), and the optimization ability of UHPC internal hydration product microstructure and pores is limited, which is difficult to simultaneously consider low shrinkage, high strength and excellent durability.

[0004] In order to overcome the above-mentioned defects, it is urgent to develop a new type of UHPC and its preparation method. The method should make full use of locally available raw materials (especially local mineral admixtures and conventional aggregate), and combine with new modified materials (such as nanomaterials), through scientific material component design and reasonable production process control, to significantly reduce the autogenous shrinkage (target value < 100 microstrain) and effectively improve the economy of UHPC on the premise of ensuring the high mechanical properties (compressive strength ≥ 150 MPa) and durability of UHPC. The present application is an innovative solution based on this demand. SUMMARY

[0005] The technical problems to be solved by the present application are to overcome the above-mentioned defects existing in the prior art, and a new UHPC and a preparation method thereof.

[0006] The technical scheme adopted by the present application to solve the technical problems is as follows: A UHPC concrete mixed with nano calcium carbonate is composed of the following raw materials in parts by weight, 800-900 parts of cementitious material, wherein the cementitious material comprises 450-550 parts of ordinary Portland cement, 220-300 parts of local mineral admixture, 130-180 parts of silica fume, and 3-8 parts of nano calcium carbonate; the local mineral admixture is slag powder of S95 grade or higher grade or Class I fly ash; An aggregate component, wherein the aggregate component comprises 600-700 parts of fine aggregate with a particle size of 0.15 mm-0.60 mm and 30-80 parts of coarse aggregate with a particle size of 2.0 mm-4.0 mm; 150-200 parts of steel fiber; 2.0%-3.5% of high-efficiency water reducing agent based on the total weight of the cementitious material on a dry basis; 140-180 parts of water.

[0007] Preferably, the average particle size of the nano calcium carbonate is 20-80 nm, and the specific surface area is ≥25 m² / g.

[0008] Preferably, the fine aggregate is machine-made quartz sand or locally-made sand with equivalent performance, and the grading modulus is 2.2-2.7.

[0009] Preferably, the coarse aggregate is locally-made granite or quartz stone.

[0010] Preferably, the steel fiber is copper-plated straight steel fiber with a length of 6-12 mm and a diameter of 0.18-0.23 mm.

[0011] Preferably, the high-efficiency water reducing agent is a polycarboxylate-based high-efficiency water reducing agent.

[0012] A preparation method of a UHPC concrete mixed with nano calcium carbonate, comprising the following steps, (S1) fine aggregate and coarse aggregate are weighed according to the proportion and added into a mixer, and dry mixing is performed for 60 seconds; (S2) ordinary Portland cement, local mineral admixture, silica fume, and nano calcium carbonate are added, dry mixing is performed for 90-120 seconds, and mixing is uniform; (S3) a high-efficiency water reducing agent is dissolved in 75%-90% of the total mixing water to form a water reducing agent solution; (S4) most of the water reducing agent solution in (S3) is added into the mixer, and stirring is performed at a medium speed for 120-150 seconds to form a mortar matrix; (S5) reduce the stirring speed, evenly scattered into steel fiber; (S6) add the remaining water-reducing agent solution and the remaining mixing water, increase the stirring speed to high speed, stir for 180-240 seconds until a uniform mixture with an expansion degree ≥ 500 mm is obtained; (S7) after pouring into the mold and compacting, cover and maintain until initial setting, demold after 24 hours for subsequent curing.

[0013] Preferably, the local mineral admixture in (S2) is S95 grade slag powder.

[0014] Preferably, the high-speed stirring speed in (S6) is 200-250 revolutions per minute.

[0015] Preferably, the subsequent curing in (S7) is standard curing at 20±2°C and relative humidity ≥ 95%, or accelerated curing at 50-70°C for 3-7 days, or steam curing at 85-95°C for 18-24 hours.

[0016] The beneficial effects of the present application are as follows: (1) The present application significantly reduces autogenous shrinkage, and the addition of nano calcium carbonate is the key factor. The autogenous shrinkage (28 days) of Examples 1-3 is only 72-88 micro-strain, which is much lower than the 198 micro-strain of the comparative example. Example 3 shows that the late shrinkage is also very low; (2) The present application has excellent mechanical properties and economy. By using local admixtures (mineral powder, fly ash) and local aggregates, the cement dosage is reduced (Example 1 by 20%, Example 3 by 33%) and / or the aggregate gradation is adjusted, and the strength of the traditional high-cement UHPC (Example 1 / 2 at room temperature ≥ 155 MPa, Example 2 / 3 at high temperature ≥ 178 / 185 MPa) is still achieved or even exceeded, and the cost is reduced; (3) The process of the present application has strong applicability. The formula and process are suitable for room temperature curing to obtain high performance (Example 1 / 2 at room temperature), and are also compatible with steam curing to accelerate development (Example 2 / 3). Local resources can be fully utilized to prepare high-performance, low-shrinkage UHPC; (4) The present application has good workability and fiber dispersion. The proposed preparation process (aggregate pre-mixing, nanomaterial mixing of binder, stepwise liquid addition and fiber mixing) ensures an expansion degree ≥ 500 mm and uniform dispersion of fibers. DETAILED DESCRIPTION

[0017] The present application will be further described below with reference to examples.

[0018] The raw materials or chemical reagents used in the examples and comparative examples of the present application, unless otherwise specified, are obtained through conventional commercial channels; the experimental equipment used in the examples and comparative examples of the present application is a 60L planetary forced stirrer, a standard curing box (20±2°C, RH≥95%), a steam curing box (90±5°C), a pressure testing machine, and a flexural testing machine.

[0019] A UHPC concrete raw material with nano calcium carbonate added The UHPC concrete raw material with nano calcium carbonate added according to the example 1 comprises 480 parts of cement, 250 parts of S95 mineral powder, 160 parts of silica fume, 5 parts of nano calcium carbonate, 650 parts of machine-made quartz sand (0.15-0.6mm), 50 parts of machine-made granite gravel (2-4mm), 170 parts of copper-plated steel fiber (8mm, 0.20mm), 23.3 parts of polycarboxylic acid superplasticizer, and 140 parts of water.

[0020] A preparation method of a UHPC concrete with nano calcium carbonate added According to the components and weight parts of the UHPC concrete raw material with nano calcium carbonate added according to the example 1, first, 650 parts of machine-made quartz sand and 50 parts of machine-made granite gravel (particle size 2-4mm) are accurately weighed and put into a planetary forced stirrer, and a dry mixing program is started to continue for 60 seconds to ensure that the aggregates are initially mixed uniformly. Then, 480 parts of cement, 250 parts of S95 mineral powder, 160 parts of silica fume, and 5 parts of nano calcium carbonate are added in sequence according to the ratio, the stirring speed of the stirrer is adjusted to medium, and dry mixing is continued for 100 seconds to make all the powders and aggregates fully dispersed and mixed. Subsequently, 23.3 parts of polycarboxylic acid superplasticizer is dissolved in 140 parts of water to prepare a superplasticizer solution, about 85% (about 132 parts of the water solution) of which is added uniformly to the stirrer at medium speed (100-150r / min) for 135 seconds to form a uniform slurry. Finally, the stirring speed is lowered, 170 parts of copper-plated steel fiber is evenly scattered, the remaining superplasticizer solution (containing 8 parts of water) and the reserved 8 parts of water are added, and the stirring is continued at high speed (200-250r / min) for 210 seconds until the fiber is fully dispersed (the actual extension is 580mm); immediately, the mixture is poured and vibrated to compact for 15 seconds, covered with a film to keep moist, and demolded after 24 hours and moved into a 20±2°C, RH≥95% standard curing box.

[0021] In order to evaluate the effects of the preparation method of the UHPC concrete with nano calcium carbonate added according to the example 1 of the present application in terms of mechanical properties, shrinkage control, and workability, the following standard test methods are used for comprehensive evaluation: Workability test: According to GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures", the slump flow method is used for determination. Freshly mixed concrete is filled into the slump cone and then lifted vertically, and the average value of the expansion diameter in two vertical directions is measured (target ≥500mm).

[0022] Compressive strength test: 100mm cube specimens were prepared according to GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". The specimens were cured under standard curing conditions (20±2°C, RH≥95%) for 3d, 7d, and 28d, respectively, and tested using a 2000kN pressure testing machine at a loading rate of 1.0MPa / s.

[0023] Flexural strength test: According to the same standard GB / T 50081-2019, a 100mm×100mm×400mm prism specimen was used, with a three-point loading span of 300mm, and a loading of 0.08MPa / s until failure.

[0024] Self-shrinkage test: Following the RILEM TC-232 TCD recommended method, the axial deformation of a 100mm×100mm×400mm specimen was monitored from 24h to 28d after demolding using an embedded strain gauge or a non-contact laser displacement sensor (accuracy ±1μm). The micro-strain value (με) was calculated in an unconstrained environment with a constant temperature of 20±1°C and a constant humidity of ≥98%RH.

[0025] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate (CCO3) according to Example 1 of this invention, the 28-day autogenous shrinkage value significantly decreased to 72 microstrain (με). This is attributed to the effective incorporation of nano-calcium carbonate. The nanoparticles, acting as efficient nucleation sites, promoted more uniform and dense precipitation and growth of hydration products (CSH gel), optimizing the microstructure. This refined structure effectively alleviated the capillary negative pressure caused by hydration consumption and self-drying effects within the ultra-low water-cement ratio system, thereby significantly reducing macroscopic shrinkage strain. Simultaneously, the rigid filling effect of nano-CaCO3 itself also physically constrained shrinkage deformation.

[0026] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate (CPC), as described in Example 1 of this invention, the compressive strength reached 160 MPa after 28 days under standard curing conditions. This high strength was achieved through optimized formulation and the effect of nano-CaCO3. On one hand, S95-grade mineral powder (250 parts) and silica fume (160 parts) replaced a large amount of cement (480 parts, lower than conventional formulations), increasing the total amount of hydration products and improving the interfacial transition zone through the pozzolanic effect of the active mineral admixtures. On the other hand, nano-CaCO3 not only acts as crystal nuclei to promote earlier and more complete hydration, but its particles also act as ultrafine fillers, significantly reducing the porosity of the matrix and increasing its density. The stepwise addition of liquid and thorough mixing process ensured uniform mixing of the cementitious materials, nanoparticles, and aggregates.

[0027] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate (Example 1 of this invention), the mixture exhibited a spread of 580 mm and good cohesion with no segregation. The excellent workability primarily stemmed from the optimized aggregate gradation (650 parts fine sand + 50 parts coarse aggregate) and the effective dispersion of the high-efficiency water-reducing agent (2.6% dosage). Although nano-CaCO3 increased the specific surface area of ​​the powder, its extremely small particle size and step-by-step addition process (dry mixing with the binder first) ensured its thorough dispersion under the action of the water-reducing agent, without excessively increasing the slurry viscosity or water demand. Pre-mixing the aggregates ensured uniform dry materials, while step-by-step liquid addition controlled the rheological state of the slurry, avoiding the risk of uneven aggregate coating or segregation caused by a large amount of water added at once. The post-addition of steel fibers during thorough wet mixing also prevented fiber clumping from affecting flow.

[0028] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate (CAPC), as described in Example 1 of this invention, the flexural strength reached 26 MPa after 28 days. This high toughness primarily stems from two aspects: firstly, the synergistic effect of nano-CaCO3 and mineral admixtures optimizes the density and strength of the cement matrix, thereby improving its toughness; secondly, the nano-CaCO3 particles refine and strengthen the matrix-fiber interface, improving the bond performance between the fiber and the matrix, allowing medium-sized steel fibers (8 mm long) to more effectively perform their bridging and crack-resistant functions. The uniformly dispersed fibers also ensured the maximization of their crack-resistant effect.

[0029] Example 2 of a UHPC concrete raw material with added nano-calcium carbonate Example 2 of the UHPC concrete raw material with added nano-calcium carbonate includes: 500 parts cement, 230 parts grade I fly ash, 150 parts silica fume, 7 parts nano-CaCO3, 620 parts machine-made quartz sand (0.15-0.6mm), 80 parts machine-made granite crushed stone (2-4mm), 180 parts copper-plated steel fiber (8mm, 0.20mm), 26.6 parts high-efficiency water-reducing agent, and 150 parts water.

[0030] Example 2: A method for preparing UHPC concrete with added nano-calcium carbonate According to the components and weight parts of Example 2 of a UHPC concrete raw material with added nano-calcium carbonate, the mixer was started, and 620 parts of machine-made quartz sand and 80 parts of machine-made granite crushed stone (particle size 2-4mm) were added. The mixture was dry-mixed for 60 seconds to complete the aggregate premixing. Then, 500 parts of cement, 230 parts of Grade I fly ash, 150 parts of silica fume, and 7 parts of nano-calcium carbonate were added, and the mixture was dry-mixed at medium speed for 95 seconds to ensure that the cementitious materials and nanoparticles fully coated the aggregate. A mixed solution of 26.6 parts of high-efficiency water-reducing agent and 125 parts of water was prepared. 85% of the total solution (approximately 106 parts of aqueous solution) was slowly injected into the mixer, and the mixture was stirred at medium speed for 130 seconds to form a fluid mortar matrix. Next, 180 parts of copper-plated steel fiber (8mm×0.20mm) are sprinkled in at a reduced speed. Then, the remaining water-reducing agent solution (containing 19 parts of water) and the reserved 19 parts of water are added. The mixture is stirred at high speed for 220 seconds until it is uniform (expansion of 520mm). The mixture is poured into the mold and vibrated to compact it. After curing with a film for 24 hours, the specimens are demolded and divided into two groups: the first group is placed in a standard curing room at 20±2°C, and the second group is heated to 90±5°C at a rate of 15°C / h and steamed for 24 hours, and then naturally cooled to room temperature.

[0031] To evaluate the effects of Example 2 of the preparation method of UHPC concrete with added nano-calcium carbonate of the present invention on high-temperature curing adaptability and mechanical development, the following standard test methods were used for comprehensive evaluation: Steam curing strength test: In addition to standard curing, the steam curing procedure of GB / T 50082-2009 "Test Methods for Long-term Performance and Durability of Ordinary Concrete" is added: After demolding, the temperature is raised to 90±5°C at a rate of 15°C / h, kept at a constant temperature for 24 hours, and then naturally cooled. The immediate compressive / flexural strength after steam curing is then tested.

[0032] Simultaneous monitoring of shrinkage: For specimens prepared in the same batch and cured at room temperature, the 28-day auto-shrinkage value was measured using the same non-contact displacement method (EN 12617-4 standard) as in Example 1 to eliminate interference from the curing regime.

[0033] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate in Example 2 of this invention, the 28-day autogenous shrinkage value was controlled at 88 με, lower than that of conventional UHPC. Although slightly higher than Example 1 (partly attributed to the slight influence of higher fly ash content and slightly higher coarse aggregate ratio on early hydration dynamics), its shrinkage value of 88 με was still far lower than the comparative example. This is also due to the key role of nano-CaCO3: its nucleation and filling effects counteracted the tendency for early microstructure weakening that might result from the slightly lower early activity of local Class I fly ash compared to mineral powder (responding more slowly with the cement hydration product Ca(OH)2), effectively improving the density of the matrix and reducing the shrinkage driving force.

[0034] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate in Example 2 of this invention, and followed by steam curing at 90°C for 24 hours, the compressive strength significantly increased to 185 MPa. High-temperature steam curing significantly accelerated the pozzolanic reaction process of the cementitious materials (especially fly ash and silica fume). The activity of fly ash was rapidly activated under high temperature and humidity, reacting with Ca(OH)₂ to generate a large amount of additional CSH gel. Silica fume also reacted more rapidly at high temperatures. Nano-CaCO₃, in this process, not only acted as an inert filler but also served as a nucleation center for accelerated hydration at high temperatures, promoting the formation of a larger quantity and denser hydration products. Although the coarse aggregate had a slight dilution effect on the ultra-high performance, its good gradation with the fine aggregate and the ultra-high strength of the matrix ensured the interfacial strength, ultimately leading to a breakthrough in overall strength.

[0035] Testing revealed that, in Application Example 2 of this invention, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate, further cost reduction was achieved by utilizing lower-priced Grade I fly ash and slightly increasing coarse aggregate. The optimized formula reduced the use of expensive cement (500 parts vs. 480 parts in Example 1, still lower than conventional methods) and silica fume (150 parts vs. 160 parts in Example 1), while introducing lower-priced industrial byproduct fly ash. Appropriately increasing the proportion of locally produced coarse aggregate (machine-made granite crushed stone) (80 parts) to replace some fine aggregate slightly reduced workability (spreadability 520 mm), but this was compensated for by the excellent matrix fluidity controlled by a high-efficiency water-reducing agent (3.0%) and nano-CaCO3, still meeting construction requirements. Comprehensive material substitution achieved cost optimization.

[0036] Testing revealed that, according to Example 2 of this invention, the flexural strength of steam-cured specimens prepared using a method for preparing UHPC concrete with added nano-calcium carbonate reached 32 MPa. High-temperature curing not only improved the matrix strength but also accelerated the "healing" and strengthening of the interface between the matrix and steel fibers, resulting in superior interfacial bonding performance in a short period. The strengthening effect of nano-CaCO3 on the interface was even more pronounced under high-temperature curing. The combination of these two factors allows the fibers to more effectively transfer stress and delay debonding under bending loads, thereby achieving higher flexural strength and toughness. The excellent compatibility between the formulation and the high-temperature process is evident here.

[0037] Example 3 of a UHPC concrete raw material with added nano-calcium carbonate Example 3 of the UHPC concrete raw material with added nano-calcium carbonate includes: 400 parts cement, 300 parts S95 mineral powder, 170 parts silica fume, 6 parts nano-CaCO3, 670 parts machine-made quartz sand (0.15-0.6mm), 40 parts machine-made granite crushed stone (2-4mm), 160 parts copper-plated steel fiber (12mm, 0.18mm), 24.6 parts high-efficiency water-reducing agent, and 160 parts water.

[0038] Example 3: A method for preparing UHPC concrete with added nano-calcium carbonate According to the components and weight parts of Example 3 of a UHPC concrete raw material with added nano-calcium carbonate, 670 parts of machine-made quartz sand and 40 parts of machine-made granite crushed stone were added to a mixer and dry-mixed for 60 seconds. Then, 400 parts of cement, 300 parts of S95 slag powder, 170 parts of silica fume, and 6 parts of nano-calcium carbonate were added, and the mixture was dry-mixed at medium speed for 105 seconds to complete the solid-phase mixing. A solution of 24.6 parts of high-efficiency water-reducing agent and 140 parts of water was prepared. 85% of the total solution (approximately 119 parts of aqueous solution) was added to the mixer and mixed at medium speed for 125 seconds to form a dense paste. After reducing the speed, 160 parts of copper-plated steel fiber (12mm × 0.18mm) were evenly sprinkled in. Then, the remaining solution (containing 21 parts of water) and the reserved 20 parts of water were injected, and the mixture was high-speed mixed for 240 seconds until the fluidity met the standard (spread 540mm). After the concrete is poured and vibrated to compact it, it is covered with a film and left to stand for 24 hours. After demolding, all specimens are moved into a steam curing chamber and heated to 90±5°C at a rate of 15°C / h. After maintaining the temperature for 24 hours, the specimens are allowed to cool naturally to room temperature to complete the curing process.

[0039] To evaluate the volume stability, steam curing efficiency, and interfacial strengthening effects of Example 3 of the preparation method of UHPC concrete with added nano-calcium carbonate under a high-volume mineral powder system, the following standard test methods were used for comprehensive evaluation: Post-drying shrinkage test: According to ASTM C157 / C157M-17 "Standard Test Method for Length Change of Hardened Concrete", the specimens cured at 90°C for 24 hours were transferred to a constant temperature and humidity chamber (20±2°C, RH50±3%), and the length change increment (με) from day 1 to day 14 was measured to evaluate the long-term volume stability.

[0040] Fiber-matrix interface analysis: SEM-EDS scanning electron microscopy (JSM-7800F, accelerating voltage 15kV) was used to characterize the morphology and elemental distribution of the interface region of the fractured specimen after steam curing, and to verify the strengthening effect of nano-CaCO3 on the interfacial transition zone (ITZ).

[0041] Workability verification: Test the slump flow time of self-compacting concrete according to EN 12350-5:2019, and record the T value. 500 (Time required to reach 500mm spread) Verify fiber dispersion uniformity.

[0042] Testing revealed that, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate (Example 3 of this invention), and with a significantly increased local mineral powder content of 300 parts (compared to only 400 parts cement) and steam curing at 90°C for 24 hours, the compressive strength still reached 178 MPa. The high content of S95 mineral powder (300 parts) significantly replaced cement (400 parts), heavily relying on its activity under high-temperature activation. Nano-CaCO3 (6 parts) played a crucial role as both an "activator" and a "structural stabilizer": its nucleation effect greatly promoted the pozzolanic reaction of the mineral powder, enabling it to react rapidly in the alkaline environment provided by cement hydration products; its filling effect effectively compensated for structural porosity defects that might have resulted from reduced cement content in the early stages. Despite a significant reduction in cement usage, the system still achieved excellent final strength thanks to the synergy of nanomaterials and high-temperature acceleration.

[0043] Testing revealed that, in Example 3 of this invention, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate, and followed by steam curing and 14 days in a 50% RH environment, the increase in later-stage shrinkage was only approximately 42 με, demonstrating the excellent volume stability of the UHPC produced by this patent. Although most hydration is complete after high-temperature curing, some degree of drying shrinkage still occurs under low humidity conditions. The optimized ultra-dense microstructure and low-permeability matrix of nano-CaCO3 significantly hinder the migration and loss of internal moisture, effectively resisting shrinkage stress caused by changes in environmental humidity. Even in high-volume mineral powder systems, the low-porosity matrix ensures excellent later-stage volume stability.

[0044] Testing revealed that, in Application Example 3 of this invention, after treatment with a method for preparing UHPC concrete incorporating nano-calcium carbonate, the use of longer and finer steel fibers (12 mm long, 0.18 mm diameter) in conjunction with an optimized matrix still maintained good workability (spread 540 mm) and a flexural strength of 28 MPa. To compensate for the potential adverse effects of a significant reduction in cement content on matrix toughness and interfacial strength, steel fibers with a larger aspect ratio were used. Thanks to the overall reinforcement and interfacial strengthening effect of nano-CaCO3 on the matrix (even with low cement content), the matrix strength is sufficiently high to form a strong bond with the long fibers. The stepwise mixing process (especially the final high-speed mixing for 240 seconds) ensured the uniform dispersion of these more easily agglomerated long fibers. The optimized rheological properties of the matrix (good plastic viscosity and yield stress) also ensured good fiber encapsulation and distribution, meeting workability requirements and ultimately guaranteeing sufficient flexural strength and fracture toughness.

[0045] Comparative Example 1: A conventional UHPC concrete raw material without nano-calcium carbonate The conventional UHPC concrete raw material comparison example 1 without nano-calcium carbonate includes: 600 parts cement, 200 parts silica fume, 700 parts machine-made quartz sand (0.15-0.6mm) (excluding coarse aggregate), 180 parts copper-plated steel fiber, 25.6 parts high-efficiency water-reducing agent, and 150 parts water (water-binder ratio of approximately 0.22). (Note: no mineral powder / fly ash and nano-CaCO3 were added).

[0046] Comparative Example 1: A method for preparing conventional UHPC concrete without nano-calcium carbonate According to the components and weight proportions of a conventional UHPC concrete raw material without nano-calcium carbonate (Example 1), 700 parts of machine-made quartz sand (excluding coarse aggregate) were weighed and added to a mixer for dry mixing for 30 seconds. Then, 600 parts of cement and 200 parts of silica fume were added, and the mixture was dry mixed at medium speed for 90 seconds. Separately, 25.6 parts of high-efficiency water-reducing agent were dissolved in 150 parts of water, and 85% of the total solution (approximately 128 parts of aqueous solution) was added to the mixer and mixed at medium speed for 120 seconds to form a high-viscosity mortar. 180 parts of copper-plated steel fibers (12mm × 0.18mm) were added evenly with reduced speed, followed by the remaining solution (containing 22 parts of water) and 25 parts of reserved water. The mixture was then mixed at high speed for 200 seconds until the fibers were dispersed (expansion 510mm). The final mixture was poured, vibrated to compact, covered with a film for moisture curing, and demolded after 24 hours. All specimens were placed in a standard curing room at 20±2°C and RH≥95% for performance comparison.

[0047] To evaluate the effectiveness of the conventional UHPC concrete preparation method without nano-calcium carbonate of the present invention in Comparative Example 1 in terms of benchmark performance and defect characterization, the following standard test methods were used for comprehensive evaluation: Parallel control tests: The workability (spreadability), compressive / flexural strength, and self-shrinkage tests all adopted the same national standards (GB / T series) and equipment parameters as in Example 1 to ensure data comparability.

[0048] Microscopic porosity analysis: The cumulative pore volume and most probable pore size distribution of the 28-day specimens were determined by mercury indentation method (Micromeritics AutoPore V, according to GB / T21650.1-2008) to explain the microscopic mechanism of high shrinkage.

[0049] Testing revealed that, after treatment with a conventional UHPC concrete preparation method without nano-calcium carbonate (CPC), the 28-day autogenous shrinkage value of Comparative Example 1 reached as high as 198 με. This high shrinkage was primarily determined by the ultra-low water-cement ratio (0.22) and the high content of cement (600 parts) and silica fume (200 parts). The lack of nucleation and filling effects from nano-CaCO3 resulted in relatively coarse structures of hydration products, with internal pore structures less refined than those of systems incorporating nanomaterials. The severe self-drying effect and high chemical shrinkage, in the absence of nanoparticle microstructure regulation, triggered significant capillary negative pressure, leading to a macroscopic shrinkage strain far exceeding that of the embodiments of this invention. The high total amount of binder material was also a major contributing factor to the high shrinkage.

[0050] Testing showed that, after treatment with a conventional UHPC concrete preparation method without nano-calcium carbonate (NCC) according to Comparative Example 1, the 28-day compressive strength of this invention was 168 MPa (standard curing). This strength mainly relied on a higher amount of cement (600 parts) and silica fume (200 parts), achieving high density through the total amount of hydration products and silica fume micro-filling. Although the strength met the target (close to Example 1), the cost was high material costs (cement usage was 20-50% higher than in the example of this invention) and significant shrinkage problems. Without the synergistic effect of nano-CaCO3, relying solely on highly reactive materials resulted in higher packing efficiency and cost.

[0051] Testing revealed that, after treatment with a conventional UHPC concrete preparation method without nano-calcium carbonate, the flexural strength of Comparative Example 1 was 24 MPa (28 days). This strength indicates that the fibers played a role; however, due to the lack of nanomaterial optimization, the matrix's inherent toughness and interfacial bonding with the fibers were relatively weak (in the nanomaterial-doped system). The stress transfer efficiency was low, and the fiber's crack-resistant performance was not maximized, resulting in a flexural strength lower than that of the embodiments of this invention incorporating nano-CaCO3 (26-32 MPa).

[0052] Testing revealed that the cementitious material cost of Comparative Example 1, after treatment with a conventional UHPC concrete without nano-calcium carbonate and its preparation method, was significantly higher than that of the embodiments of this invention. This was mainly due to the high cement content (600 parts) and the relatively high silica fume content (200 parts), both of which have unit prices far exceeding those of local mineral admixtures (mineral powder / fly ash). The Comparative Example did not utilize any inexpensive local mineral admixtures and lacked the technological support to reduce the use of expensive materials like nano-CaCO3, resulting in a high overall cost. Its economic efficiency was far inferior to the various optimized embodiments of this invention.

[0053] In summary, although Comparative Example 1, after being treated with a conventional UHPC concrete preparation method without nano-calcium carbonate, achieved a 28-day compressive strength comparable to Example 1 (168 MPa vs 160 MPa) under standard curing, its cementitious material composition (600 parts cement + 200 parts silica fume) resulted in significantly higher autogenous shrinkage (198 με vs 72 με, a difference of 126 με). Furthermore, due to the lack of use of local mineral admixtures and reliance on high-cost silica fume, the cost of the main materials was approximately 30% higher than that of Example 1. This fully highlights the breakthrough advantages of Example 1 of this invention in achieving ultra-low shrinkage and high economy through the scientific incorporation of nano-calcium carbonate (5 parts) and S95 mineral powder (250 parts).

Claims

1. A UHPC concrete with added nano-calcium carbonate, characterized in that: It is composed of the following raw materials in parts by weight. The cementitious material consists of 800-900 parts, including 450-550 parts of ordinary Portland cement, 220-300 parts of local mineral admixtures, 130-180 parts of silica fume, and 3-8 parts of nano-calcium carbonate. The local mineral admixture is S95 grade or higher slag powder or Class I fly ash; The aggregate component includes 600-700 parts of fine aggregate with a particle size of 0.15mm-0.60mm and 30-80 parts of coarse aggregate with a particle size of 2.0mm-4.0mm. 150-200 parts steel fiber; A high-efficiency water-reducing agent, wherein the water-reducing agent accounts for 2.0%-3.5% of the total dry weight of the cementitious materials; 140-180 parts water.

2. The UHPC concrete with added nano-calcium carbonate according to claim 1, characterized in that: The average particle size of the nano-calcium carbonate is 20-80 nm, and the specific surface area is ≥25 m² / g.

3. The UHPC concrete with added nano-calcium carbonate according to claim 1, characterized in that: The fine aggregate is manufactured quartz sand or locally manufactured sand with equivalent properties, and the gradation modulus is 2.2-2.

7.

4. The UHPC concrete with added nano-calcium carbonate according to claim 1, characterized in that: The coarse aggregate is locally sourced machine-made granite crushed stone or quartz crushed stone.

5. The UHPC concrete with added nano-calcium carbonate according to claim 1, characterized in that: The steel fiber is a copper-plated straight steel fiber with a length of 6-12mm and a diameter of 0.18-0.23mm.

6. The UHPC concrete with added nano-calcium carbonate according to claim 1, characterized in that: The high-efficiency water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent.

7. A method for preparing UHPC concrete with added nano-calcium carbonate, characterized in that: Includes the following steps, (S1) Weigh the fine and coarse aggregates according to the proportion and add them to the mixer, then dry mix for 60 seconds; (S2) Add ordinary Portland cement, local mineral admixtures, silica fume and nano calcium carbonate, dry mix for 90-120 seconds, and mix evenly; (S3) Dissolve the high-efficiency water-reducing agent in 75%-90% of the total mixing water to form a water-reducing agent solution; (S4) Add most of the water-reducing agent solution from (S3) to the mixer and stir at a medium speed for 120-150 seconds to form a mortar matrix; (S5) Reduce the stirring speed and evenly sprinkle in the steel fibers; (S6) Add the remaining water-reducing agent solution and the remaining mixing water, increase the stirring speed to high speed, and stir for 180-240 seconds until a uniform mixture with an expansion of ≥500mm is obtained. (S7) After pouring into the mold and compacting, cover and cure until initial setting. Remove from the mold 24 hours later for subsequent curing.

8. The method for preparing UHPC concrete with added nano-calcium carbonate according to claim 7, characterized in that: The local mineral admixture in (S2) is S95 grade slag powder.

9. The method for preparing UHPC concrete with added nano-calcium carbonate according to claim 7, characterized in that: The high-speed stirring speed in (S6) is 200-250 rpm.

10. The method for preparing UHPC concrete with added nano-calcium carbonate according to claim 7, characterized in that: The subsequent maintenance described in (S7) is standard maintenance at 20±2°C and relative humidity ≥95%, or accelerated maintenance for 3-7 days at 50-70°C, or steam maintenance for 18-24 hours at 85-95°C.