UHPC recycled fine aggregate mortar containing embedded fibers and method of making

Abstract

The application provides a UHPC recycled fine aggregate mortar containing embedded fibers and a preparation method thereof, and aims at the problems of single resource utilization path of waste ultra-high performance concrete (UHPC), insufficient performance of recycled aggregate and poor dispersibility of externally added fibers. The waste UHPC containing fibers is mechanically crushed and sieved, and the undersize material with a particle size of 0.15-4.75 mm is taken to obtain UHPC recycled fine aggregate containing embedded fibers. The fibers are embedded in the interior of the aggregate particles or penetrate the surface of the aggregate particles after crushing. The recycled fine aggregate is used to replace part or all of the machine-made sand by using the equal-volume replacement method, and the high-performance mortar is obtained by mixing and stirring the recycled fine aggregate, cementing materials, water and water reducing agent. The method is simple in process, realizes high-value utilization of the waste UHPC, and the prepared mortar is excellent in working performance and mechanical performance.

Description

Technical Field

[0001] This application relates to the field of building materials technology, and more specifically, to a UHPC recycled fine aggregate mortar containing embedded fibers and its preparation method. Background Technology

[0002] Ultra-high performance concrete (UHPC) possesses high strength, high durability, and high impact resistance, and is widely used in projects such as long-span bridges, high-rise building joints, seismic reinforcement, and high-durability precast components. With the increasing use of UHPC, the amount of waste UHPC generated during its production, construction, and service is also increasing. The recycling of waste UHPC is of great significance for reducing its life-cycle cost and environmental impact.

[0003] In the field of construction solid waste recycling, preparing recycled aggregate from waste concrete through mechanical crushing and screening is a conventional technical approach. Ordinary recycled concrete aggregate suffers from defects such as high porosity, high water absorption, and poor crushing performance due to the presence of old mortar on its surface and the development of internal micro-cracks. To improve the performance of recycled aggregate mortar, existing technologies typically employ external fiber reinforcement. However, externally added fibers tend to disperse unevenly and accumulate locally during mixing, failing to provide toughening and crack resistance, and instead introducing weak interfacial zones, thus deteriorating the workability and mechanical properties of the mortar.

[0004] Waste UHPC can also be mechanically crushed and screened to obtain recycled fine aggregate. Due to the high density of the UHPC matrix, the resulting recycled fine aggregate has low porosity and high strength, exhibiting superior physical properties compared to ordinary recycled aggregate. Unlike ordinary recycled aggregate, after mechanical crushing, the steel fibers originally incorporated into waste UHPC are not entirely dispersed in a free state; some remain embedded within the aggregate particles or penetrate the aggregate surface, forming a fiber-embedded recycled aggregate structure. When these fibers are introduced into the mortar along with the aggregate, they can be evenly dispersed with the aggregate particles, thus avoiding the problem of uneven dispersion of externally incorporated fibers. Simultaneously, the fibers embedded in the aggregate can act as a bridge between the aggregate and the mortar interface, which is beneficial for improving the toughness of the mortar.

[0005] In addition, organic fibers such as polyoxymethylene (POM) fibers and TMA-modified polypropylene (PP) fibers have also been used in UHPC. After mechanical crushing, waste UHPC containing organic fibers also exhibits the characteristic of the organic fibers being partially embedded within the aggregate or penetrating the aggregate surface. However, research on the impact of UHPC recycled fine aggregate containing these embedded organic fibers on mortar properties is currently lacking.

[0006] Therefore, how to prepare high-performance mortar using UHPC recycled fine aggregate containing embedded fibers (including steel fibers and organic fibers), and how to clarify the influence of different fiber types on mortar performance, are technical problems that urgently need to be solved in this field. Summary of the Invention

[0007] To address the existing technical problems of limited UHPC resource utilization pathways, insufficient performance of recycled aggregates, and poor dispersion of externally incorporated fibers, this invention provides a UHPC recycled fine aggregate mortar containing embedded fibers and its preparation method. This method involves mechanically crushing waste UHPC containing fibers (steel fibers, polyoxymethylene fibers, or TMA-modified polypropylene fibers) and sieving it to obtain recycled fine aggregate with a particle size of 0.15~4.75 mm. The fibers in this fine aggregate are embedded within the aggregate particles or penetrate their surface after crushing, forming an embedded fiber structure. Subsequently, using manufactured sand as the reference aggregate, a volumetric substitution method is employed to partially or completely replace the manufactured sand, and the mixture is then combined with cementitious materials, water, and a water-reducing agent to obtain a high-performance mortar.

[0008] This invention utilizes the structural feature of pre-anchored fibers within recycled UHPC fine aggregates, allowing the fibers to be integrally introduced into the mortar along with the aggregate particles. This fundamentally avoids the problems of uneven fiber dispersion and localized enrichment inherent in traditional external fiber admixtures. Simultaneously, the embedded fibers act as a bridge at the aggregate-mortar interface, improving the density of the transition zone and the mechanical properties of the mortar. This preparation method is simple, requires no acid washing or chemical strengthening treatment of the recycled aggregates, and achieves high-value utilization of waste UHPC. The prepared mortar exhibits good workability and excellent mechanical properties.

[0009] In a first aspect, the present invention provides a method for preparing UHPC recycled fine aggregate mortar containing embedded fibers, comprising the following steps: S1: Mechanically crush waste UHPC containing fibers, screen out the undersize material with a particle size of 0.15~4.75mm, pre-wet treat and then dry to obtain UHPC recycled fine aggregate containing embedded fibers; the fibers are embedded in the interior of the recycled fine aggregate particles or penetrate through their surface after crushing. S2: Using manufactured sand as the reference aggregate, the UHPC recycled fine aggregate containing embedded fibers is replaced with the manufactured sand using the equal volume replacement method; S3: Mix the UHPC recycled fine aggregate containing embedded fibers determined in S2 with cementitious materials, water, and water-reducing agent to obtain the mortar.

[0010] In existing technologies, the resource utilization of waste UHPC mainly focuses on its use as an auxiliary cementitious material after grinding, while research on its preparation as recycled aggregate is relatively limited. Furthermore, recycled aggregate from construction waste, due to its high porosity and low strength, typically requires complex strengthening treatments such as acid washing and polymer impregnation before it can be used, and its substitution rate is limited.

[0011] This invention utilizes the inherent density and high strength of waste UHPC matrix to directly crush it into recycled fine aggregate. This yields fine aggregate products with superior performance compared to ordinary recycled aggregates without any strengthening treatment, greatly simplifying the process. After mechanical crushing and screening, the original fiber portion of the waste UHPC remains embedded within or penetrates the surface of the aggregate particles, forming a fiber-embedded structure with the aggregate particles as the carrier. This structure couples the fiber distribution with the aggregate particle distribution; the fibers are introduced into the mortar mixing system as a whole along with the aggregate particles, and the uniformity of fiber dispersion is determined by the uniform distribution of the aggregate particles. Unlike free externally added fibers, which require mechanical stirring to disperse in the slurry and are prone to agglomeration, the dispersion of embedded fibers does not depend on the fiber's own dispersibility or the agglomeration effect caused by increased fiber content. This fundamentally avoids the technical problems of uneven fiber dispersion and localized enrichment associated with externally added fibers. Meanwhile, the fibers embedded in the aggregate play a bridging role in the aggregate-slurry interface transition zone, which can effectively inhibit the initiation and propagation of microcracks, improve the interfacial bonding state, and enhance the toughness and mechanical properties of the mortar.

[0012] Preferably, the fiber is one or more of steel fiber, polyoxymethylene fiber, or TMA-modified polypropylene fiber, mixed in any proportion.

[0013] Preferably, the mechanical crushing in S1 includes primary crushing and secondary crushing performed sequentially. After crushing and screening, the material is first pre-wetted and then dried in an oven at 60~80℃ for 12~48 h.

[0014] After mechanical crushing and screening, waste UHPC still contains unhydrated cementitious material particles in the surface and internal microcracks of the aggregate. Pre-wetting treatment brings the aggregate to a saturated surface-dry state, allowing moisture to penetrate into the aggregate through microcracks and pores on the surface, contacting the unhydrated cementitious material. During the subsequent drying process at 60-80℃, the increased temperature accelerates the hydration reaction rate of the unhydrated cementitious material, further hydrating the unhydrated cement particles and active mineral admixtures within the aggregate, generating hydration products such as CSH gel. These hydration products fill the microcracks and pores on the aggregate surface, blocking the moisture penetration channels and making the aggregate surface denser. The drying temperature is controlled at 60-80℃; too low a temperature results in insufficient hydration reaction, while too high a temperature may damage the internal fiber structure of the aggregate or cause microcracks to expand. A drying time of 12-48 hours ensures that the unhydrated cementitious material is fully hydrated in the pre-wetting-drying cycle, effectively strengthening the density of the aggregate surface. After the above treatment, the water absorption rate of recycled aggregate is significantly reduced, and it is less likely to adsorb free water in mortar when mixed with cementitious materials, thereby improving the workability of mortar and the compactness of the interface transition zone.

[0015] Preferably, the fineness modulus of the UHPC containing embedded fibers is 3.0 to 3.4.

[0016] Preferably, the bulk density of the UHPC containing embedded fibers is 1200~1350 kg / m³. 3 .

[0017] Preferably, the amount of UHPC recycled fine aggregate containing embedded fibers in S2 is determined by the following formula:

[0018] in The dosage of recycled fine aggregate in UHPC containing embedded fibers, The amount of manufactured sand that was replaced. The bulk density of manufactured sand, The bulk density is that of UHPC recycled fine aggregate containing embedded fibers.

[0019] Preferably, the cementitious material in S3 is cement or a mixture of cement and mineral admixtures; the mineral admixtures are one or more of fly ash, silica fume, and slag powder; the mineral admixtures account for 10% to 30% of the total mass of the cementitious material.

[0020] Preferably, the mortar mix proportion in S3 is as follows by mass: 400-500 parts cementitious material, 200-250 parts water, 0.3-1.0 parts water-reducing agent, and 200-1200 parts UHPC recycled fine aggregate containing embedded fibers.

[0021] Secondly, the present invention provides a UHPC recycled fine aggregate mortar containing embedded fibers.

[0022] Thirdly, the present invention provides an application of UHPC recycled fine aggregate mortar containing embedded fibers in building materials.

[0023] In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention provides a method for preparing UHPC recycled fine aggregate mortar containing embedded fibers. The recycled fine aggregate can be obtained simply by mechanically crushing and screening waste UHPC. No pretreatment such as acid washing, polymer impregnation, or carbonization modification of the aggregate is required, and no additional fiber is needed. The process is greatly simplified, significantly reducing the preparation cost of recycled aggregate mortar and facilitating its industrial application.

[0024] 2. This invention utilizes the embedded fiber structure of waste UHPC fibers, which are embedded in the aggregate particles or penetrate their surface after crushing. This allows the fibers to be introduced into the mortar mixing system as a whole along with the aggregate particles. The uniformity of fiber dispersion is determined by the uniform distribution of the aggregate particles, fundamentally avoiding the technical problems of uneven dispersion and local enrichment of traditional externally added fibers during the mixing process.

[0025] 3. The UHPC recycled fine aggregate mortar containing embedded fibers prepared by the method of the present invention can achieve a significant improvement in mechanical properties under the condition of 100% replacement of manufactured sand. The UHPC recycled fine aggregate mortar containing embedded steel fibers can achieve a 28-day compressive strength of 65.9 MPa and a flexural strength of 10.86 MPa, which are 31.75% and 20.80% higher than the benchmark mortar without recycled aggregate, respectively, breaking through the technical prejudice of mechanical property deterioration under high replacement rate of recycled aggregate. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This paper illustrates schematic diagrams of different fiber UHPC breakage processes provided in embodiments of this application. Figure 2 The microstructure images of different fiber-regenerated UHPC fine aggregates provided in the embodiments of this application are shown; wherein Figure 2 a is a microscopic morphology diagram of RFA-N; Figure 2 b is the RFA-P microstructure diagram; Figure 2 c is the RFA-T microstructure diagram; Figure 2 d is the RFA-S microstructure diagram; Figure 3 A flowchart illustrating the preparation process of recycled fine aggregate mortar according to an embodiment of this application is shown; Figure 4 The diagram shows the flowability variation of different fiber-recycled fine aggregate mortars provided in the embodiments of this application; Figure 5 The diagram shows the variation in dry density of different fiber-recycled fine aggregate mortars provided in the embodiments of this application; Figure 6 The diagram shows the variation in water absorption rate of different fiber-recycled fine aggregate mortars provided in the embodiments of this application; wherein... Figure 6 a is a graph showing the change in water absorption rate of RFA-N; Figure 6 b is a graph showing the change in water absorption rate of RFA-P; Figure 6 c is a graph showing the change in water absorption rate of RFA-T; Figure 6 d is a graph showing the change in water absorption rate of RFA-S; Figure 7 The diagram shows the variation of compressive strength of different fiber-recycled fine aggregate mortars provided in the embodiments of this application; wherein Figure 7 a is a graph showing the change in compressive strength of RFA-N; Figure 7 b is a graph showing the change in compressive strength of RFA-P; Figure 7 c is the graph showing the change in compressive strength of RFA-T; Figure 7 d is the graph showing the change in compressive strength of RFA-S; Figure 8 The diagram shows the variation of flexural strength of different fiber-recycled fine aggregate mortars provided in the embodiments of this application; wherein Figure 8 a is a graph showing the variation in flexural strength of RFA-N; Figure 8 b is a graph showing the change in flexural strength of RFA-P; Figure 8 c is a graph showing the change in flexural strength of RFA-T; Figure 8 d is the graph showing the change in flexural strength of RFA-S; Figure 9 The microstructure images of different fiber-recycled fine aggregate mortars at 100% replacement rate provided in the embodiments of this application are shown; wherein Figure 9 a represents the baseline group (DB). Figure 9 a1 is Figure 9 Enlarged view of a; Figure 9 b is the RFA-N group. Figure 9 b1 is Figure 9 Enlarged view of b; Figure 9 c represents the RFA-P group. Figure 9 c1 is Figure 9 Enlarged view of c; Figure 9 d represents the RFA-T group. Figure 9 d1 is Figure 9 Enlarged view of d; Figure 9 e represents the RFA-S group. Figure 9 e1 is Figure 9 Enlarged view of e; Figure 10 This paper presents EDS (Energy Dispersive Spectroscopy) analysis diagrams of the aggregate-slurry interface region of mortar with different fiber recycled fine aggregates at a 100% replacement rate, as provided in the embodiments of this application. The yellow / red boxes indicate the EDS data acquisition points. Figure 10 a is the SEM topography image of the baseline group (DB). Figure 10 a1 and 10a2 are EDS energy spectra of two characteristic regions in 10a; Figure 10 b is the SEM morphology image of the RFA-N group. Figure 10 b1 and 10b2 are EDS energy spectrum diagrams of two characteristic regions in 10b; Figure 10c is the SEM topography image of the RFA-P group. Figure 10 c1 and 10c2 are EDS energy spectra of two characteristic regions in 10c; Figure 10 d is the SEM morphology image of the RFA-T group. Figure 10 d1 and 10d2 are EDS energy spectra of two characteristic regions in 10d; Figure 10 e is the SEM topography image of the RFA-S group. Figure 10 e1 and 10e2 are EDS energy spectra of two characteristic regions in 10e; Figure 11 The figure shows the test results of porosity of different fiber-recycled fine aggregates provided in the embodiments of this application at 100% admixture; wherein Figure 11 Figure a shows the results of the DB pore content test. Figure 11 b is a graph showing the results of RFA-N pore content testing; Figure 11 c is the result of RFA-P pore content test; Figure 11 d is the result of RFA-T pore content test; Figure 11 e is a graph showing the results of RFA-S pore content testing; Figure 11 f is a graph showing the air content test results of four types of recycled UHPC fine aggregates at 100% admixture. Detailed Implementation

[0028] 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 embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0029] The cementing material used in this invention is Lafarge P·O 42.5R ordinary Portland cement. The main components and physical properties of the cement are shown in Tables 1 and 2. The water-reducing agent is a powder water-reducing agent provided by Shanxi Feike New Material Technology Co., Ltd., and the mixing water is municipal tap water from Chengdu.

[0030] Table 1 Main components of ordinary Portland cement

[0031] Table 2 Physical properties of ordinary Portland cement

[0032] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0033] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0034] In the description of this invention, it should be understood that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0035] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] To enable those skilled in the art to better understand this application, the following embodiments are provided to illustrate in detail a UHPC recycled fine aggregate mortar containing embedded fibers and its preparation method.

[0037] Example Example 1: Preparation of Recycled Aggregate According to the mixing ratios in Table 3, the raw materials were weighed, mixed, molded, and cured under standard conditions (temperature 20±2℃, humidity ≥95%) for 28 days to obtain four types of waste UHPC matrices: those containing steel fibers, those containing TMA-modified polypropylene fibers, those containing polyoxymethylene fibers, and those without fibers. Figure 1 As shown in the schematic diagram of UHPC crushing with different fibers, each waste UHPC matrix was subjected to primary crushing by a jaw crusher and secondary crushing by an impact crusher. The crushed material was then passed through a 4.75 mm square hole sieve, and the sieve-passing material was pre-wetted and then dried in an 80℃ oven for 24 h to obtain four types of UHPC recycled fine aggregates with particle sizes ranging from 0.15 to 4.75 mm. According to the fiber type, they were labeled as UHPC recycled fine aggregates with embedded steel fibers (RFA-S), UHPC recycled fine aggregates with embedded TMA modified polypropylene fibers (RFA-T), UHPC recycled fine aggregates with embedded polyoxymethylene fibers (RFA-P), and UHPC recycled fine aggregates without fibers (RFA-N).

[0038] Table 3 Mix proportions and mechanical properties of UHPC

[0039] After crushing, the microstructures of the four aggregates are as follows: Figure 2 As shown, in RFA-S, the steel fibers are partially embedded inside the aggregate particles or penetrate their surface, exhibiting a fiber-aggregate intercalation; in RFA-P, the polyoxymethylene fibers are partially embedded inside the aggregate particles or penetrate their surface; in RFA-T, the TMA-modified polypropylene fibers are partially embedded inside the aggregate particles or penetrate their surface; RFA-N contains no fibers, and the aggregate surface exhibits a rough morphology. The physical property parameters of the four types of UHPC recycled fine aggregates are shown in Table 4.

[0040] Table 4 Physical properties of four types of UHPC recycled fine aggregates

[0041] Example 2 Mortar Preparation The cementitious material used in this embodiment is Lafarge P·O 42.5R ordinary Portland cement, the water-reducing agent is a powder water-reducing agent, and the mixing water is tap water. The aggregates are manufactured sand and the four types of UHPC recycled fine aggregates (RFA-N, RFA-P, RFA-T, RFA-S) prepared in Example 1. The particle size range of the manufactured sand is 0~4.75 mm, and the natural bulk density is 1608.3 kg / m³. 3 It has a fineness modulus of 3.18, which classifies it as coarse sand.

[0042] As shown in Table 4, the bulk density of the four types of UHPC recycled fine aggregates is lower than that of manufactured sand (1608.3 kg / m³). 3 If the traditional equal-volume substitution method is used, the absolute volume of recycled aggregate is greater than that of manufactured sand for the same mass, which will damage the bulk density of the mortar. Therefore, this embodiment adopts the equal-volume substitution method for mix design, that is, keeping the absolute volume of the replaced manufactured sand unchanged, and calculating its actual usage based on the bulk density of various recycled aggregates.

[0043] Equal volume substitution mix design: Using manufactured sand as the reference aggregate, the equal volume replacement method was adopted, with replacement rates set at 0%, 25%, 50%, 75%, and 100%, respectively. The actual amount of the four types of UHPC recycled fine aggregates at each replacement rate was determined by converting their bulk density to the bulk density of manufactured sand. The specific mix proportions are shown in Table 5.

[0044] Table 5. Experimental mix proportions

[0045] Weigh out the cementitious materials, water, water-reducing agent, and corresponding aggregates according to the mix proportions in Table 5. First, place the cement and water-reducing agent in a mortar mixing pot and mix at low speed for 1 minute until homogeneous. Add water and mix at low speed for 30 seconds to form a neat paste. Add the corresponding aggregates, mix at low speed for 30 seconds, and then mix at high speed for 90 seconds to obtain the mortar. No fiber agglomeration or sedimentation was observed during the mixing process of each fiber group.

[0046] After the obtained mortar was tested for fluidity according to GB / T 2419-2005, it was poured into a 40 mm × 40 mm × 160 mm triple mold for molding. After natural curing for 24 hours, the mold was removed, the mortar was numbered, and it was placed in a constant temperature and humidity curing chamber at 20℃ and 98% humidity for 28 days. The specific preparation process is as follows: Figure 3 As shown.

[0047] Example 3: The fluidity of the mortar was tested. This embodiment tests the flowability of mortars prepared with the four types of UHPC recycled fine aggregates obtained in Example 1 at different replacement rates. Referring to the Chinese standard "Test Method for Flowability of Cement Mortar" (GB / T 2419-2005), the flowability of mortars with different mix proportions in Example 2 is tested. Using the reference mortar (DB group, flowability 205 mm) without recycled aggregate as a control, the flowability of mortars with four types of recycled aggregates (RFA-N, RFA-P, RFA-T, and RFA-S) at replacement rates of 25%, 50%, 75%, and 100% is tested.

[0048] The flowability test results of four types of UHPC recycled fine aggregate mortars at different replacement rates are as follows: Figure 4 As shown, as the RFA-N content increased from 0 to 100%, the mortar fluidity remained at 205 mm, the same as the baseline group. This is because RFA-N is obtained by crushing UHPC with a dense matrix, has a dense structure, and has a weak adsorption capacity for free water. In addition, both RFA-N and manufactured sand are coarse sand with similar particle size distribution, so they do not significantly change the flow resistance of the mortar.

[0049] As the replacement rate increased from 0 to 100%, RFA-P reduced the fluidity from 205 mm to 180 mm, a decrease of 12.2%, the largest decrease. This is because after the polyoxymethylene fibers were broken, some were exposed on the surface of the aggregate. During the mixing process, the exposed fibers and aggregates intertwined to form a mesh structure, which increased the internal frictional resistance of the mortar. At the same time, the water absorption of the exposed fibers further reduced the free water content of the mortar. RFA-T also showed a downward trend in fluidity, but the decrease was relatively small at 9.76%. In contrast, increasing the dosage of RFA-S increased the fluidity from 205 mm to 230 mm, an increase of 12.2%.

[0050] The above results indicate that within the replacement rate range of 25% to 100%, the 28-day compressive strength of the four types of UHPC recycled fine aggregate mortars is higher than that of the benchmark group. This demonstrates that the UHPC recycled fine aggregate provided by this invention can effectively improve the mechanical properties of mortar at different replacement rates, with 100% full replacement showing the best effect. Ordinary recycled concrete fine aggregates, due to the low strength and high porosity of the parent concrete, have a large amount of loose, old mortar adhering to the surface of the aggregate particles after crushing, and internal micro-cracks developing, resulting in a water absorption rate and surface friction coefficient much higher than that of natural sand. During mortar mixing, this type of aggregate absorbs a large amount of free water on the one hand, and the rough surface increases the frictional resistance between particles on the other. The combined effect of these two factors causes the mortar fluidity to decrease sharply with increasing replacement rate. Existing studies have shown that when ordinary recycled fine aggregates replace 100% of natural sand, the mortar fluidity can decrease by 50% to 80%.

[0051] In contrast, the UHPC recycled fine aggregate in this invention has a dense matrix with extremely low porosity, and the aggregate particles themselves have high strength and few surface attachments, resulting in a much weaker adsorption of free water compared to ordinary recycled aggregates. Even though the exposed surfaces of POM fibers and TMA fibers negatively impact flowability, the flowability at 100% replacement still remains above 180 mm, meeting the flowability requirements for mortar construction; and RFA-S achieves a significant improvement in flowability at 100% replacement. These results demonstrate that the UHPC recycled fine aggregate provided by this invention significantly outperforms traditional recycled aggregates in terms of workability.

[0052] Example 4: Dry Density Test This embodiment tests the dry density of four types of UHPC recycled fine aggregate mortars with different substitution rates prepared in Example 2. Referring to the Chinese standard "Test Method for Performance of Autoclaved Aerated Concrete" (GB / T 11969-2008), the samples from each group cured to 28 days in Example 2 were weighed under natural conditions and under oven drying to constant weight, respectively. The moisture content, natural density, and dry density of the samples with different recycled fine aggregate content were calculated.

[0053] The dry density test results of four types of UHPC recycled fine aggregate mortars at different replacement rates are as follows: Figure 5 As shown, the dry density of the RFA-N, RFA-P, and RFA-T mortar groups all showed a trend of first increasing and then decreasing with the increase of the replacement rate, reaching the maximum value at a replacement rate of 25% (RFA-N, RFA-P) or 50% (RFA-T), respectively, which was 2.78%~3.71% higher than the baseline group; it dropped to the lowest value at 100% replacement, which was 0.15%~1.66% lower than the baseline group. The dry density of the RFA-S group fluctuated, and at 100% replacement, it was actually 1.22% higher than the baseline group.

[0054] The above results show that, at a low substitution rate of 25% to 50%, the rough surface of UHPC recycled fine aggregate can provide nucleation sites for hydration products and physical attachment points for residual fibers, which together improve the compactness of the interfacial transition zone and increase the dry density. At a high substitution rate of 75% to 100%, although the dry density decreases due to the lower bulk density of the aggregate compared to manufactured sand, the decrease is less than 2%, which is significantly better than that of ordinary recycled fine aggregate.

[0055] Example 5: Water Absorption Rate Test This embodiment tests the water absorption rate of four types of UHPC recycled fine aggregate mortars prepared in Example 2 under different substitution rates. The samples from Example 4, dried to constant weight, were completely immersed in water for 24 hours. The mass of the samples was measured at different immersion times (0 h, 0.5 h, 1 h, 2 h, 4 h, 6 h, 8 h, 12 h, 24 h), and the water absorption rate of each sample at different times was calculated. The variation of the water absorption rate of the four types of UHPC recycled fine aggregate mortars with immersion time is shown in [the table below]. Figure 6 .

[0056] The water absorption rates of all four UHPC recycled fine aggregate mortars under 24-hour soaking conditions were lower than those of the baseline group, indicating that the incorporation of UHPC recycled fine aggregate can reduce the water absorption rate of the mortar. Looking at different fiber types, the water absorption rates of the RFA-N, RFA-P, and RFA-T mortar groups showed a pattern of first decreasing and then increasing with increasing replacement rate. The lowest values ​​appeared at replacement rates of 25% or 50%, decreasing by 9.77% to 14.33% compared to the baseline group; while the highest values ​​appeared at 100% replacement, decreasing by 2.28% to 9.45% compared to the baseline group, still lower than the baseline group. The RFA-T group exhibited the lowest water absorption rate at all replacement rates, which corresponds to its higher dry density at the corresponding replacement rates. The water absorption rate of the RFA-S group fluctuated with the increase of the replacement rate, reaching a minimum of 2.74% at 100% replacement, which was 10.75% lower than the baseline group; and a maximum of 2.97% at 50% replacement, which was 3.26% lower than the baseline group.

[0057] There is a good negative correlation between the water absorption rate and dry density of the four types of UHPC recycled fine aggregate mortars: when the replacement rate is 25%~50%, the dry density of the mortar increases and the structure becomes denser, while the water absorption rate decreases accordingly; when the replacement rate is 100%, the dry density drops slightly and the water absorption rate rises, but none of them exceed the benchmark group.

[0058] Ordinary recycled concrete fine aggregates, due to their high porosity and the presence of a large amount of old mortar adhering to their surface, typically have a water absorption rate 3 to 5 times that of natural sand. Mortar prepared with ordinary recycled fine aggregates shows a significant increase in water absorption rate with increasing replacement rate; at 100% replacement, the water absorption rate can increase by more than 50% compared to the baseline group. In contrast, the water absorption rate of mortar prepared with 100% replacement of UHPC recycled fine aggregates in this invention is reduced by 2.28% to 10.75% compared to the baseline group, and the absolute water absorption rate is consistently below 3.0%. The results indicate that the UHPC recycled fine aggregates provided by this invention not only do not degrade the impermeability of mortar like ordinary recycled aggregates, but also significantly reduce the water absorption rate and improve its durability.

[0059] Example 6 Compressive Strength Test This embodiment tests the compressive strength of four types of UHPC recycled fine aggregate mortars prepared in Example 2 under different replacement rates. Referring to the Chinese standard "Test Method for Strength of Cement Mortar" (GB / T 17671-2021), the compressive strength was tested using an electronic compression-flexure integrated testing machine with a loading rate of 2400±200 N / s. The compressive strength test results of the four types of UHPC recycled fine aggregate mortars at 7 days and 28 days are shown below. Figure 7 .

[0060] At 7 days, the compressive strength of all mortars incorporating recycled UHPC fine aggregate was no lower than that of the benchmark group. RFA-N reached 38.07 MPa at 25% replacement, an increase of 3.68% compared to the benchmark group; RFA-P, RFA-T, and RFA-S reached 42.64 MPa, 45.79 MPa, and 50.78 MPa respectively at 100% replacement, representing increases of 16.1%, 24.7%, and 38.3% compared to the benchmark group. The reinforcing effect of the fiber-reinforced group was significantly better than that of the fiber-free group RFA-N, and both achieved their maximum values ​​at 100% replacement.

[0061] At 28 days of age, all four types of UHPC recycled fine aggregates achieved their maximum compressive strength at a 100% replacement rate. The RFA-N group achieved 53.63 MPa, a 7.2% increase compared to the baseline group; the RFA-P group achieved 57.26 MPa, a 14.5% increase; the RFA-T group achieved 54.71 MPa, a 9.4% increase; and the RFA-S group achieved 65.90 MPa, a 31.75% increase. The reinforcing effect of the four aggregates was ranked as RFA-S > RFA-P > RFA-T > RFA-N, consistent with the pattern observed at 7 days of age.

[0062] The above results demonstrate that UHPC recycled fine aggregate can significantly improve the compressive strength of mortar when it replaces 100% manufactured sand, overcoming the technical bias that mechanical properties deteriorate under high replacement rates of recycled aggregate. Among them, UHPC recycled fine aggregate with embedded steel fibers (RFA-S) shows the most outstanding reinforcing effect, which is attributed to the mechanical interlocking and anchoring effect between the hook / groove structure formed by the crushed steel fibers and the cement matrix.

[0063] Example 7 Flexural Strength Test This embodiment tests the flexural strength of four types of UHPC recycled fine aggregate mortars prepared in Example 2 under different replacement rates. Referring to the Chinese standard "Test Method for Strength of Cement Mortar" (GB / T 17671-2021), the flexural strength was tested using an electronic compression-flexure integrated testing machine with a loading rate of 50±5 N / s. The flexural strength test results of the four types of UHPC recycled fine aggregate mortars at 7 days and 28 days are as follows: Figure 8 As shown.

[0064] At 7 days, the flexural strengths of RFA-N, RFA-P, RFA-T, and RFA-S under 100% replacement were 8.18 MPa, 8.87 MPa, 8.35 MPa, and 9.33 MPa, respectively, representing increases of 15.0%, 24.8%, 17.4%, and 31.2% compared to the baseline group. The 7-day flexural strengths of the fiber groups were all superior to those of the non-fiber group RFA-N, but RFA-T was slightly lower than that of RFA-P. This may be related to the residual morphology of the two fibers after breakage and the interfacial bonding state with the matrix.

[0065] At 28 days of age, the flexural strengths of the four aggregates with 100% replacement reached 10.17 MPa, 10.52 MPa, 10.49 MPa, and 10.86 MPa, respectively, representing increases of 13.1%, 17.0%, 16.7%, and 20.8% compared to the baseline group. The flexural strength improvement of all fiber groups exceeded 16%, and the order of improvement was RFA-S > RFA-P > RFA-T > RFA-N, consistent with the compressive strength trend.

[0066] The above results demonstrate that UHPC recycled fine aggregate has a sustained enhancing effect on the flexural strength of mortar. Unhydrated cementitious particles in UHPC recycled fine aggregate can undergo secondary hydration in the mortar, generating hydration products such as hydrated calcium silicate gel and ettringite crystals, which fill the microcracks and pores between the aggregate and the mortar, improving the compactness of the interface transition zone. The fibers remaining in the aggregate can bridge cracks during mortar stress, absorbing fracture energy through fiber pull-out and deformation, effectively inhibiting the propagation of microcracks and the formation of macrocracks. The synergistic effect of both factors enables UHPC recycled fine aggregate mortar to achieve a qualitative improvement in flexural strength compared to ordinary recycled aggregate mortar under 100% full replacement conditions, providing performance assurance for the application of high-replacement-rate recycled mortar in engineering scenarios with high requirements for crack resistance and flexural strength.

[0067] Example 8: Microstructure and Energy Dispersive Spectroscopy Analysis This embodiment observes and analyzes the microstructure of four types of UHPC recycled fine aggregate mortars with a 100% replacement rate in Example 2. Each group of mortars with a 100% replacement rate in Example 2 (RFA-N, RFA-P, RFA-T, RFA-S) and the benchmark group of mortar (DB) without recycled aggregate were selected. The micromorphology of each group of mortars was observed using scanning electron microscopy (SEM), and the elemental composition of the interface region was characterized by energy dispersive spectroscopy (EDS).

[0068] Depend on Figure 9 It can be seen that the benchmark group mortar ( Figure 9 a) The mortar contains numerous pores and has a loose structure. After incorporating four types of UHPC recycled fine aggregates, the pores inside the mortar were significantly reduced, and the density of the structure was significantly improved, indicating that the incorporation of UHPC recycled fine aggregates can optimize the microstructure of the mortar.

[0069] RFA-N group ( Figure 9 b) Numerous needle-like ettringite crystals and flocculent hydrated calcium silicate gels are interwoven and distributed around the recycled fine aggregates, forming a dense hydration product aggregation zone near the aggregates. No obvious cracks or pores are observed in the interface transition zone; RFA-P group ( Figure 9 c) and RFA-T group ( Figure 9 d): Polyoxymethylene fibers and TMA-modified polypropylene fibers are distributed within the matrix, with flocculent hydrated calcium silicate gel adhering to the fiber surface. The fiber-slurry interface is well bonded, with no obvious debonding or interface cracks observed; RFA-S group ( Figure 9 e): The steel fibers are interspersed in the cement paste, and the fibers are tightly wrapped by flocculent hydrated calcium silicate gel, with a tight bond and no obvious debonding or cracks.

[0070] Depend on Figure 10The EDS analysis results showed that the aggregate-slurry interface region of the RFA-N group was rich in elements such as Ca, Si, O, Al, and S. Similarly, the fiber surfaces of the FA-P and RFA-T groups were enriched with Ca, Si, and O elements, confirming that hydrated calcium silicate gel was indeed attached to the fiber surface. In the EDS spectrum of the matrix surrounding the steel fibers in the RFA-S group, the Fe element signal was weak, while the steel fibers themselves did not contain Ca or Si. However, the relative intensities of Ca, Si, and O elements were relatively high, further confirming that the flocculent material encapsulating the fibers was hydrated calcium silicate gel.

[0071] Further, the mortar groups (RFA-N, RFA-P, RFA-T, RFA-S) with a replacement rate of 100% from Example 2, and the benchmark mortar group (DB) without recycled aggregate were selected. The porosity of each mortar group was tested using a porosity analyzer. The results are shown in [Figure 1]. Figure 11 .

[0072] DB has a high air content, with pores mainly consisting of large pores ranging from 100 to 5000 μm. After incorporating four types of UHPC recycled fine aggregates, the porosity of the mortar in the 10–100 μm pore size range increased significantly compared to the baseline group, while the number of large pores in the 100–5000 μm range decreased significantly. This indicates that the incorporation of UHPC recycled fine aggregates can effectively refine the pore size distribution of the mortar. The total air content of the four types of UHPC recycled fine aggregates at a 100% replacement rate was lower than that of the baseline group.

[0073] The microstructure and properties of four types of UHPC recycled fine aggregate mortars at 100% replacement rate were systematically verified by combining SEM-EDS, pore structure analysis, and macroscopic mechanical property testing. SEM-EDS observations showed that after the incorporation of the four recycled fine aggregates, a dense hydration product enrichment layer was formed in the aggregate-slurry interface transition zone. The interfacial bonding between the embedded fibers (POM, TMA, and steel fibers) and the matrix was good, and the rough surface of the fiber-free group (RFA-N) also promoted the nucleation and growth of hydration products. Pore structure analysis showed that the content of 10–100 μm micropores increased, the content of 100–5000 μm macropores decreased, and the total air content decreased in the four mortars, resulting in a significantly refined pore size distribution.

[0074] The above results corroborate each other, indicating that UHPC recycled fine aggregate can achieve structural densification through multiple mechanisms such as interface transition zone strengthening, fiber bridging anchoring, and pore size distribution refinement. Ultimately, it endows the mortar with excellent mechanical properties under 100% full replacement conditions, proving the universality and reliability of the technical solution of this application.

[0075] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0076] Although preferred embodiments of the present application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present application.

[0077] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes said element.

[0078] The above provides a detailed description of the UHPC recycled fine aggregate mortar containing embedded fibers and its preparation method. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for preparing UHPC recycled fine aggregate mortar containing embedded fibers, characterized in that, Includes the following steps: S1: Mechanically crush waste UHPC containing fibers, screen out the undersize material with a particle size of 0.15~4.75mm, pre-wet and then dry to obtain UHPC recycled fine aggregate containing embedded fibers; S2: Using manufactured sand as the reference aggregate, the UHPC recycled fine aggregate containing embedded fibers is replaced with the manufactured sand using the equal volume replacement method; S3: Mix the UHPC recycled fine aggregate containing embedded fibers determined in S2 with cementitious materials, water and water-reducing agent to obtain the mortar.

2. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The fiber is one or more of steel fiber, polyoxymethylene fiber, or TMA-modified polypropylene fiber, mixed in any proportion.

3. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The mechanical crushing in S1 includes primary crushing and secondary crushing performed sequentially. After crushing and screening, the material is first pre-wetted and then dried in an oven at 60~80℃ for 12~48 h.

4. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The fineness modulus of the UHPC recycled aggregate containing embedded fibers is 3.0~3.

4.

5. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The bulk density of the UHPC recycled aggregate containing embedded fibers is 1200~1350 kg / m³. 3 .

6. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The amount of UHPC recycled fine aggregate containing embedded fibers in S2 is determined by the following formula: in The dosage of recycled fine aggregate in UHPC containing embedded fibers, The amount of manufactured sand that was replaced. The bulk density of manufactured sand, The bulk density is that of UHPC recycled fine aggregate containing embedded fibers.

7. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The cementitious material in S3 is cement or a mixture of cement and mineral admixtures; the mineral admixtures are one or more of fly ash, silica fume, and slag powder; the mineral admixtures account for 10% to 30% of the total mass of the cementitious material.

8. The method for preparing UHPC recycled fine aggregate mortar containing embedded fibers according to claim 1, characterized in that, The mortar mix proportion in S3, by mass parts, is: 400-500 parts cementitious material, 200-250 parts water, 0.3-1.0 parts water-reducing agent, and 200-1200 parts UHPC recycled fine aggregate containing embedded fibers.

9. A UHPC recycled fine aggregate mortar containing embedded fibers prepared by the method for preparing UHPC recycled fine aggregate mortar containing embedded fibers as described in any one of claims 1 to 8.

10. The application of the UHPC recycled fine aggregate mortar containing embedded fibers as described in claim 9 in building materials.

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