Preparation Technology and Experimental Study of Fast-Hardening UHPC Repair Material

Abstract

The performance of concrete materials deteriorates under load fluctuation and long-term erosion from the external environment, leading to damage. In order to solve the problems of concrete materials needing repair, poor working performance and the low mechanical strength of ordinary concrete repair materials, this paper explores a series of experimental studies on the preparation of ultra-high performance concrete repair materials and solves the two technical problems of the low early strength and poor fluidity of ultra-high-performance concrete. The effects of different steel fibers and dosages of early strength agents on the working properties, mechanical properties and microstructure of fast-hardening UHPC are discussed. Finally, the mix ratio of fast-hardening UHPC with a 2 h compressive strength of up to 47 MPa and fluidity of up to 300 mm is obtained through the test, which is in line with practical applications in engineering and provides a reference for the design and analysis of fast-hardening UHPC repair materials.

1. Introduction

UHPC is a new type of ultra-high-performance cement-based composite material using a low water–binder ratio and a mix designed according to the principle of the closest packing to reduce the internal defects of the material, resulting in ultra-high strength, ultra-high toughness and ultra-long durability, while combining excellent impact resistance, wear resistance and adhesion [1]. Due to its dense matrix and excellent durability, UHPC has been widely used in the repair of bridges and other infrastructure in high-abrasion and high-corrosion environments in recent years, which significantly extends the service life of concrete structures under harsh corrosion environments [2].

With the erosion and cracking of concrete in major projects becoming more and more prominent, the rapid repair of concrete materials and structures has attracted much attention. The repair of cement concrete construction has become a serious problem and challenge for modern industrial buildings, especially for facilities that need to be put into use as early as possible during the repair process. Therefore, for facilities that need to be put into use quickly during the repair process, the repair material needs to have a high early strength. At the same time, after experiencing multiple load impacts, the repair material also needs to meet other mechanical properties to ensure that the repaired structure has long-term durability and stability.

However, UHPC as a quick repair reinforcement material still needs to solve two major technical problems [3]:

(1) Ultra-early strength performance improvement. Although the early strength of UHPC itself is high, and the compressive strength can reach more than 40 MPa in 1 day, UHPC does not have mechanical strength within 2 to 3 h, and the existing repair materials can reach 30 MPa in 3 h [2]. Therefore, it is necessary to optimize UHPC from the angles of fast solidification and early strength.

(2) Improved flow performance. UHPC is a cement-based material prepared under a very low water–binder ratio, and it is difficult to achieve high flow performance, especially when the setting time of concrete needs to be extended at the same time.

Therefore, based on the existing general mix ratio of UHPC, the effects of different early strength agents and steel fiber contents on the early performance of fast-hardening UHPC were studied in this paper.

2. Experiment

2.1. Experimental Materials

The cement studied was P·O52.5 cement produced by Anhui Tongling Conch Cement Co., Ltd. (No.132, Suburb of Tongling City, Tongling, China), with a density of 3.12 g/cm³. The main technical performance is shown in

Table 1. Main performance indices of the P·O52.5 Tongling Conch cement.

Specific Surface Area (m2/kg)	Water Consumption (%)	Setting Time (min)		Flexural Strength (MPa)		Compressive Strength (MPa)
		Initial Set	Final Set	3 Days	28 Days	3 Days	28 Days
405	26.7	130	172	5.5	8.7	31.2	66.5

. S95 mineral powder was made by Nanjing New Material Technology Co., Ltd. (No.688-2, Zhujiang Road, Xuanwu District, Nanjing, China), and its main technical performance is shown in

Table 2. Technical performance of the S95 mineral powder.

Technical Index	Firing Loss (%)	Vitreous Content (%)	Specific Surface Area (m2/kg)	Density (g/cm3)
Content (%)	0.8	99	440	2.8

. The fly ash was produced by a company in Taicang, Jiangsu, and it is Class FII with a fineness (45 μm sieve residue) of 22.5%, a density of 2.73 g/m³, a water requirement ratio of 98% and a 28-day activity index of 77%. The silica fume used in the experiment was produced by Elkem GMBH (No. 3966, Jindu Road, Xinzhuang Industrial Zone, Minhang District, Shanghai, China) in Norway. The model is Elkem Microsilica 940, with a density of 2.78 g/cm³, a specific surface area of 22 m²/kg, a SiO₂ content of 95% and a 7-day activity index of 115%. The main technical properties are shown in

Table 3. Main technical properties of the silica fume.

Specific Surface Area (m2/g)	SiO2 Content (%)	Water Demand Ratio (%)	7 Days Activity Index (%)
22	95	122	115

. The steel fibers were produced by Shandong Ruixing Building Materials Co., Ltd. (No.66, Huanghe Road, Tianqiao District, Jinan, China), with a diameter of 0.12 mm and a length of 15 mm. The quartz sands used have a particle size of 30–140 mesh for sand 1 and a particle size of 20–40 mesh for sand 2. The early strength agent, PRIORITY, used in the experiment is a cement-based fast-hardening mixed material, with a density of 2.86 g/cm³, designed and developed by the ettringite expansion technology accumulated by our research group and Shanghai Siqi Building Materials Co., Ltd. (No.66, Huanghe Road, Tianqiao District, Jinan, China). Its chemical composition is shown in

Table 4. Chemical composition of the early strength agent, PRIORITY.

Chemical Composition	CaO	SO3	Al2O3
Content (%)	40.9	28.1	20.5

. The product model of the retarder is HN-01, and it was analytically pure. The product model of the defoamer is SX-01. The product model of the water reducer is 540 P, and its appearance was a white powder. The mixing water was tap water.

2.2. Mix Ratio Design

The base mix of UHPC comprises a water–binder ratio of 0.21 and a sand–binder ratio of 1.12, with the raw materials (in mass ratio) consisting of 33.6% cement, 9% S95 mineral powder, 2% fly ash, 3% silica fume, 20% 20–40 mesh quartz sand, 32% 30–140 mesh quartz sand, 0.25% water-reducing agent and 0.05% defoaming agent. The precise quantity of each raw material is determined using the volumetric method. To investigate the influence of the early strength agent PRIORITY on the setting time and compressive strength of fast-hardening UHPC, the effects were examined by substituting 6%, 8% and 10% P·O52.5 cement with an equivalent mass of PRIORITY based on the fundamental mix proportions. Furthermore, to assess the impact of steel fiber content on the mechanical properties of fast-hardening ultra-high-performance concrete, varying amounts of steel fibers at 2%, 4%, and 6% by volume were incorporated, as outlined in

Table 5. Mix ratios of UHPC in experiment (kg/m³).

Mix	P·O52.5	S95MP	PFA	SF	Quartz Sand 1	Quartz Sand 2	PRIORITY	540P	SX-02	HN-01	Fiber
FC	940	238	53	78	590	885	0	8.8	1.2	0	0
FCS1	940	238	53	78	590	885	0	8.8	1.2	0	56
FCS2	940	238	53	78	590	885	0	8.8	1.2	0	112
FCS3	940	238	53	78	590	885	0	8.8	1.2	0	168
FCE1	772.8	238	53	78	590	885	168	8.8	1.2	2.4	0
FCE2	716.8	238	53	78	590	885	224	8.8	1.2	2.4	0
FCE3	660.8	238	53	78	590	885	280	8.8	1.2	2.4	0
FCS2E1	772.8	238	53	78	590	885	168	8.8	1.2	2.4	112
FCS2E2	716.8	238	53	78	590	885	224	8.8	1.2	2.4	112
FCS2E3	660.8	238	53	78	590	885	280	8.8	1.2	2.4	112
FCS1E2	716.8	238	53	78	590	885	224	8.8	1.2	2.4	56

Note: “FC” indicates the base group; the base group does not include the early strength agent or steel fiber in the mix, and the maintenance mode is natural maintenance; other groups are numbered on the basis of the base group. The letter “S” represents steel fiber, and the subscript numbers 1, 2 and 3 represent the steel fiber contents of 2%, 4% and 6%, respectively. The letter “E” represents the early strength agent, and the subscript numbers 1, 2 and 3 represent the early strength agent percentage contents of 6%, 8% and 10%, respectively.

.

2.3. Experimental Method

2.3.1. Setting Time Test

The setting time test of ultra-high-performance concrete was performed with reference to JGJ70-1990 “Test Method for Basic Properties of Building mortar” [4]. When the evenly mixed ultra-high-performance concrete was loaded into the test container, we paid attention to gently tapping the wall of the container, sealed the surface after smoothing it, and stored it at 20 °C. For the ratio with a faster setting time, the test frequency before the initial setting state was 3 min/time, and the test frequency between the initial setting state and the final setting state was adjusted to 1 min/time. The average value of the two test results was selected as the mortar setting time.

2.3.2. Fluidity Test

The determination of fluidity for this paper was carried out according to the experimental steps in GB/T2419-2005 “Method for Determination of Fluidity of cement mortar” [5]. After the mortar materials of different cementing systems were mixed into pulp according to their respective mixing processes, the surface of the slurry was scraped into the test mold, and the mold was quickly lifted to observe the flow of the slurry on the fluidity plate. After the slurry was stabilized, the two values with the most significant expansion degree were measured, and the average value was taken as the final result of the fluidity.

2.3.3. Flexural and Compressive Strength Tests

As the object of the experiment is a rapid repair material with a fast setting rate, the mold was cured for about 1 h for de-molding treatment when forming in the mold, and the relevant samples were placed in the natural environment (temperature is 25 °C) for curing. This experiment’s flexural and compressive strength tests were based on the standard of GB/T17671-2021 “Test Method for the Strength of cement mortar” [6]. The dimensions of 40 mm × 40 mm × 160 mm were used for flexural strength test, and the dimensions of 100 mm × 100 mm × 100 mm were used for compressive strength test. The test equipment was the YAW-300 microcomputer (China Zhejiang Xuantian Technology Co., Ltd., Hangzhou, China) controlled electro-hydraulic servo flexural and compression integrated machine. The bending strength loading rate was 50 N/s, and the compressive strength loading rate was 1.6 MPa/s.

2.3.4. Microstructure Analysis

After the specimen was cured to age, a small piece inside the sample was soaked in anhydrous ethanol to terminate hydration, and a relatively small hardened paste was selected for SEM testing [7]. In order to enhance the conductivity of the test block, the surface of the sample was gilded. The interfacial transition zone of hydration products was observed and analyzed.

3. Experimental Results and Analysis

3.1. Influence of Mechanical Properties of Fast-Hardening UHPC

Table 6. Effect of steel fibers on performance of fast-hardening UHPC.

Group Number	Setting Time (min)	Fluidity (mm)		1-Day Intensity (MPa)		7-Day Intensity (MPa)
		Initial Fluidity	30 min	Flexural Strength	Compressive Strength	Flexural Strength	Compressive Strength
FC	>60 min	320/330	310/320	3.6	31	11	60
FCS1	>60 min	300/310	290/300	4.1	35	12	68
FCS2	>60 min	290/300	280/290	4.4	41	13	78
FCS3	>60 min	280/290	260/270	5.3	45	15	85

shows the effect of steel fibers on the performances of fast-hardening UHPC.

As seen from Table 6, since no early strength agent was added to the four groups of samples, the solid setting times were all greater than 60 min. With the increase in steel fiber content, the fluidity of UHPC decreases obviously, and the 1-day compressive strength increases. It can be seen from Figure 1 and Figure 2 that when the steel fiber content is 2%, the expansion of the UHPC mixture after standing for 30 min is 290 mm, which is maneuverable, and the 1-day compressive strength of UHPC is 35 MPa. When the steel fiber content is 4%, the 1-day compressive strength of UHPC dramatically increases, but the expansibility decreases to 280 mm after 30 min, and the operability is poor. When the steel fiber content increases from 2% to 6%, the compressive strength increases by 22.2% in one day, but there is no fluidity after 30 min. Steel fibers can significantly improve the compressive strength and flexural strength of the UHPC matrix, especially the flexural strength [8]. Overall, the steel fiber content of fast-hardening UHPC should be 4%. However, due to the large amount of mixing materials in the actual construction and the on-site wind and sun, the solidification and hardening speeds of the material are significantly accelerated. The expansion degree of 280 mm after 30 min in the laboratory cannot meet the construction requirements, and the construction performance needs to be further improved while maintaining the early strength.

3.2. Effect of Steel Fibers on Microstructure of Fast-Hardening UHPC

In order to further understand the microstructure of the fast-hardening UHPC steel fibers, the samples that have been hydrated for 2 h and 7 days were tested by SEM. In the SEM photo, a well-stacked crystal can be observed in the microstructure within 2 h. Through spectral analysis, it is concluded that this crystal is a struvite crystal, and excessive magnesium oxide is distributed around it, forming an alkaline skeleton. As seen in Figure 3, due to the short reaction time of the 2 h sample, the hydration reaction is still in progress. It can be seen that there are still many pores between the stacked crystals. The joints between the steel fibers and the matrix are not filled with hydration products, which also limits the development of its mechanical properties to a certain extent. With the continuous increase in hydration time, the crystal nuclei of hydration products continue to grow, making the gel more closely connected, and the gaps between crystals are also filled by newly generated hydration products, resulting in further density improvement. With the further development of strength, a micro-system with unhydrated MgO particles as the skeleton and hydrated crystal products as the binder is formed in the slurry in Figure 4, which is transformed into a hardened body with high mechanical properties. The addition of steel fibers effectively reduces the microscopic cracks in the UHPC matrix, and the steel fibers are closely cemented with the hydration products to avoid stress concentration in the matrix, which is the main reason for improving the mechanical properties [8]. Compared with the two age samples, after 7 days of structural hydration, the sample has higher density and higher compressive strength in macroscopic properties.

3.3. Effect of Early Strength Agent on Performance of Fast-Hardening UHPC

The performance and dosage of early strength agents play critical roles in setting time for UHPC. Selecting the appropriate dosage of early strength agent is an important measure for improving the construction operability and early mechanical properties of a fast-hardening UHPC mix. Based on the benchmark mix ratio of UHPC, the influence of the dosage of the early strength agent on the performance of UHPC is shown in

Table 7. Effect of early strength agent on performance of fast-hardening UHPC.

Group Number	Setting Time (min)	Fluidity (mm)		Intensity (MPa)		Intensity (MPa)
		Initial Fluidity	30 min	Flexural Strength	Compressive Strength	Flexural Strength	Compressive Strength
FC	>60 min	320/330	310/320	3.6 (1 day)	31 (1 day)	11 (7 days)	60 (7 days)
FCE1	48 min	310/280	300/290	2.6 (2 h)	27 (2 h)	7 (1 day)	54 (1 day)
FCE2	39 min	300/280	280/270	4.2 (2 h)	35 (2 h)	8 (1 day)	63 (1 day)
FCE3	26 min	280/260	0	4.5 (2 h)	38 (2 h)	10 (1 day)	68 (1 day)

.

It can be seen from Table 7 that the effects of different dosages of early strength agents in the fast-hardening UHPC system are significantly different. From the fluidity and setting time perspective, the fast-hardening UHPC with an early strength agent content of 6% has an excellent initial flow of 310 mm, a 30-minute flow of 290 mm, and a long setting time with sufficient construction time. The fast-hardening UHPC with an early strength agent content of 8% has excellent fluidity, the initial expansion degree is more significant than 300 mm, and the setting time of the UHPC mixture is moderate, which meets the construction requirement that the construction time is greater than 30 min. For the fast-hardening UHPC with an early strength agent content of 10%, the setting time is less than 30 min, which does not meet the requirements, the initial flow is low, and the clumping loses the flow after 30 min, which does not meet the construction requirements.

As shown in Figure 5 and Figure 6, the early flexural and compressive strengths of UHPC prepared with a 6% early strength agent content are lower, and the bending strength and compressive strength of UHPC decrease by 27.8% and 13% compared with the one-day baseline group. The bending strength and compressive strength of UHPC prepared with 8% and 10% early strength agents are indifferent, and the 2 h bending strengths reach 4.2 MPa and 4.5 MPa, respectively. The compressive strengths reach 35 MPa and 38 MPa, respectively. The bending strength and compressive strength of the 6% early strength agent increased by 14.3% and 11.5% in 2 h compared with the benchmark group in 1 day. The bending strength and compressive strength of the 6% early strength agent increased by 62.3% and 50.6% in 1 day compared with the benchmark group.

Considering the significant influences of fluidity and setting time on the working performance of UHPC, the setting time and initial flow of the FCE3 group do not meet the construction requirements, and the 2 h bending and compressive strengths of the FCE1 group are low, which does not meet the 2 h fast-hardening standard. Overall, the content of early strength agents in fast-hardening UHPC should be 8%.

3.4. Effect of Early Strength Agent on Microstructure of Fast-Hardening UHPC

Before the hydration reaction of cement begins, the amorphous calcium aluminate is hydrated to produce calcium ettringite, which fills the void to obtain the early strength, as shown in Figure 7. The chemical reaction mechanism is shown as follows:

12CaO∙7Al₂O₃ + 21CaSO₄ + 9Ca(OH)₂ + 215H₂O → 7(3CaO∙Al₂O₃∙3CaSO₄∙32H₂O)

(1)

Figure 8, Figure 9 and Figure 10 show the microscopic morphologies of the same sample in the fast-hardening UHPC FCE group after curing for 2 h, 7 days and 28 days. The 28-day microstructure photo shows that the hydrated products of aluminate cement ettringite bonded with each other to form the basic skeleton, and the hydrated AH₃ gel filled the skeleton to form a dense microstructure. The sample cured for 2 h already had strength due to the action of the early strength agent, but there were still more unhydrated particles due to the short hydration time. It can be seen from the microstructure graph of the 7-day aluminate cement that some gel is formed within a few hours of hydration, and only a tiny amount of ettringite crystals can be seen in the microstructure. The gel gradually turns into aluminum hydroxide crystals with the increase in hydration time. In addition, a large number of AFt columns can be seen in the structure, and the longer the curing time, the higher the density of AFt, which is consistent with the fast-hardening UHPC microscopic test in Zhou’s study [3]. The curing of the aluminate gelling system is similar to OPC to a certain extent. The lamellar or acicular structures generated by hydration exist in interlacing, forming a stable skeleton structure, densely filled with alumina hydroxide gel. The microstructure of CAC has low porosity and improved compactness, showing mechanical solid properties [9].

3.5. Fast-Hardening UHPC Performance Verification Results

Analysis of Working and Mechanical Properties

Based on the base mix ratio of the FC group, steel fibers and the early strength agent PRIORITY were added to study the effects of UHPC on the working time and mechanical properties under the coupling effect of the two materials, as shown in

Table 8. Effects of coupling of early strength agent and steel fibers on the properties of fast-hardening UHPC.

Group Number	Setting Time (min)	Fluidity (mm)		2 h Intensity (MPa)		1-Day Intensity (MPa)
		Initial Fluidity	30 min	Flexural Strength	Compressive Strength	Flexural Strength	Compressive Strength
FCS2E1	42 min	300/310	310/320	8	40	10.7	74
FCS2E2	35 min	290/300	280/290	12	47	14.8	82
FCS1E2	37 min	300/310	280/290	9	42	11	71
FCS3E2	30 min	280/290	260/270	10.6	49	12.7	84

.

It can be seen from Figure 11 and Figure 12 that the 2 h strength of the FCS2E2 group can reach 47 MPa, which is 2% higher than that of the FCS2E1 early strength agent, the 2 h bending strength increases by 33% and the 2 h compressive strength increases by 15%. Compared with FCS1E2, steel fiber content increased by 2%, the 2 h flexural strength increased by 25% and the 2 h compressive strength increased by 10.6%. After the steel fiber content is increased, flexural strength and compressive strength are improved to various degrees. Adding 2% steel fibers has a higher efficiency for flexural strength than for compressive strength. Compared with FCS3E2, steel fiber content decreased by 2%, the 2 h flexural strength increased by 11.2% and the 2 h compressive strength decreased by 4%. As the fiber content continues to increase, the fibers will agglomerate, and too many fibers will increase weak interfaces. When the weak interface is superior to the reinforcement of fibers, the strength of concrete will be reduced [10].

According to the EDS spectrum analysis of FCS2E2 at the 2 h and 7-day ages in Figure 13 and Figure 14, it can be seen that under the combined action of mineral powder and PRIORITY, the hydration products of the optimal group FCS2E2 at the ages of 2 h and 7 days are the same, mainly in the forms of 3CaO·Al₂O₃, 3CaSO₄·32H₂O and A1(OH)₃. From the 2 h and 7-day images, it can be seen that the peak value of hydration products increases with age, which is also related to the high-strength development of FCS2E2. In this system, no unstable substance C₂AH₈ is found, and adding mineral powder and the early strength agent PRIORITY transforms the unstable phase into a relatively stable substance. In ordinary Portland cement, the addition of the early strength agent PRIORITY is usually considered as an inert material filling, while in FCS2E2, the addition of the early strength agent PRIORITY can react not only with the main mineral CA of FCS2E2 but also with the hydration product of FCS2E2. The reaction equation is shown as follows:

3CA + CaCO₃ + 14H₂O → C₃A·CaCO₃·11H₂O + Al₂O₃·3H₂O

(2)

3C₂AH8 + 2CaCO₃ + H₂O → 2C₃A·CaCO₃·11H₂O + Al₂O₃·3H₂O

(3)

In summary, although the 10% early strength agent content can significantly improve the early strength of fast-hardening UHPC, the initial flow is poor; there is no flow in 30 min, and the short setting time cannot meet the site construction conditions. The setting time and fluidity of the fast-hardening UHPC with an 8% early strength agent are better, and the 2 h flexural strength and compressive strength are significantly improved compared to the 6% early strength agent. The 6% steel fiber content significantly affects fluidity, and the bending strength decreases compared with 4% steel fiber content over 2 h. Compared with 4% steel fibers, the flexural strength and compressive strength of 2% steel fiber samples decreased by 25.7% and 13.4%, respectively. Therefore, FCS2E2 is selected as the optimal mix ratio.

The UHPC with PRIORITY added belongs to the class of repair materials with fast setting, fast hardening and rapid development of early strength. This experiment was carried out at −10~−10~5 °C in December, which reduces the overall material strength by about 15% [11]. The results of FCS2E2 of the optimal group in Table 7 are verified through tests. Figure 15 and Figure 16 show the flexural strength and compressive strength values of FCS2E2 of the optimal group and FC of the reference group at different ages. The results show that the compressive strength of FCS2E2 in the optimal 2 h age group is 47.0 MPa, which increases by 34% compared with that of FC1 at the age of one day. The strength develops rapidly at the age of one day, reaching 82.0 MPa. After one day, the strength develops slowly, but the strength still increases in general, reaching 96.7 MPa at the age of seven days. It reaches 108 MPa in 28 days.

The compressive strength of the benchmark group increases by 59.5% during the FC ages of 1–7 days and 26.8% during the ages of 7–28 days. It can be seen that the improvement of the compressive strength performance of FC is mainly concentrated in the early stage, and the strength improvement is little in the later stage. The compressive strength of FCS2E2 in the experimental group increases by 42.7% at 2 h to 1 day of age, 15.2% at 1–7 days and 10.5% at 7–28 days. It can be seen that the improvement of the compressive strength of FCS2E2 mainly occurred at the ages of 2 h to 1 day. The reason is that incorporating an 8% early strength agent promotes the 2 h compressive strength of fast-hardening UHPC to increase to 47.0 MPa significantly, and the setting time is more significant than 30 min. The construction performance and early strength both meet the design requirements. The experimental results show that the early strength of UHPC is promoted by adding PRIORITY in the preparation of fast-hardening UHPC. The compressive strength of FCS2E2 at 28 days is 24% higher than that of FC at 28 days. In the later stage of the hydration reaction, the strength of the matrix material reaches a higher degree, and the mechanical properties of the steel fibers are further strengthened on this basis.

4. Conclusions

Based on the comparison of experimental data, this study has optimized the materials of different gelling systems, obtained the base ratio with fast setting speed, high early strength and excellent performance, and analyzed its microscopic hydration reaction. On this basis, ultra-high-performance concrete is prepared by combining the closest packing theory. The properties of ultra-high-performance concrete with fast setting and fast hardening are evaluated in terms of fluidity, setting time, compressive strength, flexural strength and microstructure. The main conclusions are drawn as follows:

(1) The early strength agent PRIORITY can improve the early compressive strength of UHPC, but it can reduce the fluidity of UHPC and shorten its setting time. The optimal content of PRIORITY in fast-hardening UHPC should be 8%.

(2) The effects of different contents of steel fibers in the fast-hardening UHPC system are significantly different. The optimum content of steel fibers prepared with fast-hardening UHPC is 4%. The reason for the high strength improvement rate of steel fibers for fast-hardening UHPC in the early stage is that although the matrix material has been coagulated and solidified in the early stage of hydration, due to the short hydration time, the strength has not yet developed, and it can easily be destroyed under the action of external forces. Adding steel fibers overlapping in the matrix material forms a random fiber network skeleton, which sustains part of the external force. The occurrence and expansion of microcracks and macroscopic cracks are limited.

(3) The 28-day compressive strength of the primary group and the optimal group of the prepared fast-hardening ultra-high-performance concrete reached 82 MPA and 108 MPa, respectively. The 7-day bending strength of the baseline group and the optimal group reached 11.3 MPa and 14.8 MPa. The optimal group’s 2 h flexural strength and compressive strength reached 12 MPa and 47 MPa, respectively, showing excellent mechanical properties.