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Article

Enhancing the Performance of Ultra-High Performance Concrete Using Expansive Agent and Pre-Wetted Biochar to Produce a Synergistic Effect

by
Congbin Huang
1,
Zijian Zong
2 and
Peiyuan Chen
2,*
1
Anhui & Huaihe River Institute of Hydraulic Research, Hefei 230088, China
2
School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan 232001, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(8), 1348; https://doi.org/10.3390/buildings15081348
Submission received: 21 March 2025 / Revised: 14 April 2025 / Accepted: 16 April 2025 / Published: 17 April 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

This study explores the impact of an expansive agent (EA) on the performance of internally cured ultra-high-performance concrete (UHPC) using pre-wetted biochar to achieve a synergistic effect in mitigating autogenous shrinkage. Here, the biochar provides internal curing water to mitigate self-desiccation, while the EA generates expansive products to compensate shrinkage, collectively improving hydration. The results revealed that a portion of the internal curing water was consumed by the EA to form expansive products, thereby shortening the prolonged setting time typically observed in UHPC with pre-wetted biochar. The incorporation of pre-wetted biochar markedly increased the internal relative humidity of UHPC, reducing its 7 d autogenous shrinkage by 35.7%. Furthermore, the addition of 1–4% EA further decreased the autogenous shrinkage by 6.8–30.3% compared to the internally cured baseline. Notably, the inclusion of pre-wetted biochar slightly enhanced the 7 d and 28 d compressive strengths of UHPC, with further improvements achieved by adding EA (up to 2%). This demonstrates the effectiveness of this internal curing approach, which maintained or improved the compressive strength of UHPC.

1. Introduction

In recent years, ultra-high performance concrete (UHPC) has garnered considerable attention as an advanced engineering material with a high strength and excellent durability, which makes it a suitable material for complex projects, i.e., long-span bridge, high-rise building, and underwater structure [1,2,3]. Nevertheless, UHPC face a challenging problem of excessive autogenous shrinkage associated with its critical low w/b and refined pore structure. It has been reported that the UHPC’s autogenous shrinkage can reach 600–900 με depending on mixing proportions, posing a great risk of cracking [4,5].
Currently, there are two major solutions to the excessive autogenous shrinkage of UHPC through using expansive agents and internal curing, respectively. The former represents a traditional approach to mitigate the shrinkage in UHPC by introducing additional expansive products through the hydration of expansive agents. While the later one is the method providing additional water internally to compensate the water loss of UHPC owing to cement hydration, directly mitigating the self-desiccation and autogenous shrinkage. For instance, Liu et al. [6] studied the impact of a calcium oxide- and magnesium oxide-based expansive agent on the shrinkage and compressive strength in UHPC. An optimized amount of such expansive agent was recommended as 6% with a reduction in autogenous shrinkage by 35.74% while maintaining compressive strength. Liu et al. [7] investigated MgO-based expansive agent (MEA) with different reactivity on the performance of UHPC. The autogenous shrinkage of UHPC can be greatly reduced by 79.4% by adding 6% rapid MEA. Li et al. [8] claimed that MEA addition can be an effective way to mitigate the autogenous shrinkage in UHPC. The presence of 6% MEA reduced 59.5% autogenous shrinkage of UHPC.
Nevertheless, the hydration of expansive agents, normally taking place quicker than cement to produce expansive products, consumes mixing water [9,10]. Consequently, UHPC with a critical low w/b would face an aggravating shortage of water, resulting in both poor fluidity and impeded cement hydration. Liu et al. [7] observed that the fluidity of UHPC was decreased by 11.4–20.2% when adding 6–9% MEA due to competitive hydration of expansive agent and cement, whereas the 28 d compressive strength was reduced. The higher the reactivity of the MEA as well as its dosage, the poorer the fresh performance of the prepared UHPC [8]. Chen et al. [11] confirmed that an excessive dosage of MEA can caused obvious reductions in compressive strength of UHPC, where an optimized dosage of MEA was 6%. Zhang et al. [12] proposed using nano-materials to avoid a reduction in the compressive strength of UHPC with MEA.
On the other hand, internal curing for UHPC is still challenging, as its performance, i.e., compressive strength, is commonly compromised when adding improper water reservoirs [4,13]. It has been claimed that lightweight aggregates (LWA) and SAP that are commonly used in internally cured HPC may fail to be incorporated in UHPC, due to a reduction in compressive strength. For instance, Liu et al. [14] found that the size and molecular structure of SAP were the two key factors affecting the compressive strength of UHPC, where non-ionic type and smaller SAP caused less reductions in compressive strength of the prepared UHPC, owing to the smaller voids was created in the microstructure. Liu and Long et al. [5] reported that the presence of 30% pre-wetted fine LWAs resulted in a 35% reduction in the 56 d compressive strength in UHPC. The major reasons are that, (1) for LWAs, their sizes may be too large to be incorporated into UHPC composited by fine particles, in addition to their weak mechanical properties; and (2) the release of water from SAP would result in large voids in the microstructure of UHPC, weaking the mechanical strength of UHPC.
Biochar, a low-carbon, porous and fine material, is available as a potential internal curing agent. Zhang et al. [15] claimed that a proper amount (1 wt.%) of a biochar addition achieved a dual impact on the mitigation of drying shrinkage, enhancement in compressive strength, and preservation of natural sand. Dixit et al. [16] confirmed the feasibly of biochar as an internal curing agent, based on the results of improved hydration degree and enhanced compressive strength of UHPC. Du et al. [17] found that by adding 1% pre-wetted biochar, the autogenous shrinkage of UHPC can be reduced by 15% owing to internal curing effect, while enhanced the compressive strength of UHPC by 25%. Nevertheless, the dosage of biochar should be controlled in a very low content to avoid obvious reductions in the prepared UHPC’s compressive strength. Beyond experimental studies, numerical modeling [18,19] has provided complementary insights into UHPC’s early-age cracking mechanisms, particularly in quantifying shrinkage-induced stresses and fracture behavior [20,21,22,23].
In consideration of the above analysis, this paper proposes using expansive agent and pre-wetted biochar to produce a syntactic effect, not only to provide additional water for the quick hydration of expansive agent, but also give a high efficiency of autogenous shrinkage mitigation for UHPC. To this end, a comprehensive study was carried out to investigate the influence of combined use of expansive agent and pre-wetted biochar on the setting time, fluidity, internal relative humidity, autogenous shrinkage, hydration products, pore structure, compressive strength, microstructure, and micro-mechanical properties of the prepared UHPC. The syntactic effect and related mechanism were discussed and revealed [24].

2. Materials and Methods

2.1. Materials

Ordinary Portland cement (P·O 52.5, Huaxin Cement Co., Ltd., Huangshi, China), Class F fly ash (<75 μm) and Elkem 740 silica fume (0.1–0.3 μm) were used as the binders. The superplasticizer was a 300p polycarboxylate superplasticizer with a water-reducing rate of 25–30%. Copper-coated straight steel fibers (0.2 mm in diameter, 13 mm in length) were utilized as reinforcement. Biochar (<75 μm) with a specific surface area of 1042 m2/g was provided by Pingdingshan Green Source Activated Carbon Co., Ltd., sourced from Pingdingshan City, China. The expansive agent was a commercial CaO-type expansive agent with a specific surface area of 350 m2/g and a particle size D50 of 15.2 μm, which had a 3 d restrained expansion rate of 0.12% in water and a 7 d restrained expansion rate of 0.14%. Tap water and quartz sand (0.425–0.850 mm) were used. Table 1 gives the chemical compositions of the used cement, silica fume, fly ash and EA.

2.2. Mixing Proportion and Sample Preparation

The mixing proportions of UHPC are shown in Table 2, where the water/binder ratio (w/b) for all mixtures was 0.13. The w/b ratio was based on free mixing water only, and did not include the absorbed water in the surface saturated dry (SSD) biochar, which was pre-conditioned to a surface saturated dry state prior to mixing. The steel fiber content was 0.15% of the total mortar volume. Six mixtures were formulated to investigate the effect of EA contents synergized with biochar of 0.04 internal curing grade on the performance of UHPC. These mortars were, respectively, named U0, B2, B2/E1, B2/E2, B2/E3 and B2/E4, where the number following the E represented the content of expansive agent, with mass fractions ranging from 1 to 4%, respectively. The EA was incorporated as an additional component without replacing any existing binder materials, leading to a proportional increase in total binder content with higher EA dosage.
During mortar preparation, the biochar was pre-soaked in water for 24 h. Then, cement, silica fume, EA and fly ash were combined in specified proportions and stirred for 2 min using a Harbor mixer. Subsequently, the mixture of superplasticizer and water and SSD biochar was incorporated and stirred for 5 min to ensure the full effect of the superplasticizer. Finally, quartz sand and steel fibers were introduced and blended again for 3 min. The paste preparation process is the same as mortar but without the addition of quartz sand and steel fibers.

2.3. Test Methods

2.3.1. Setting Time

The Vicat needle apparatus was utilized to test the setting time of fresh paste in accordance with ASTM C191 [25].

2.3.2. Fluidity of Mortars

The freshly mixed mortar was poured into a mold positioned on the surface of the STNLD-3 fluidity tester following ASTM C1437 [26]. Then, the mold was lifted vertically and the device was simultaneously started, vibrating it 25 times. The diameter of the mortar was measured in orthogonal directions and the average value was calculated to determine its fluidity.

2.3.3. Compressive Strength

Specimens were cubic specimens of 50 mm side length. According to ASTM C192 [27], fresh mortar was poured into pre-prepared molds and then placed in a standard curing room. After the mortar had fully hardened, the molds were removed and the specimens were placed in water for 3 d, 7 d, and 28 d for compressive strength tests. The compressive strengths at each curing age were determined by analyzing triplicate specimens, with the corresponding

2.3.4. Hydration Products Analysis

The hydration products of 28 d paste were analyzed using a TGA Q5000 V3.17 Build 265 high-temperature thermogravimetric analyzer from TA Instruments (New Castle, DE, USA), heating from room temperature to 800 °C at 20 °C/min with nitrogen purging at 25 mL/min. Prior to the measurements, the 28 d-paste fragments were dried in vacuum conditions for 24 h and then pulverized into powder under 75 μm.

2.3.5. Internal Relative Humidity (IRH)

Pour the fresh mixture into pre-prepared cube molds with sides of 100 mm, with a pipe with screwable ends buried in the mortar in advance. Then, seal the mold and place it in a standard curing room. After the final setting of the mortar, unscrew the tube and insert a HMM100 humidity sensor from Vaisala HUMICAP® Technology (Louisville, CO, USA) into the tube. Recorded the internal humidity values every 12 h from the final setting of the mortar until 7 d.

2.3.6. Autogenous Shrinkage

The autogenous shrinkage test of mortar was performed in accordance with ASTM C1698 [28]. Fresh mortar was poured into corrugated tubes (420 mm in length) and then vibrated to remove excess air. The filled corrugated tubes are then placed on a wave plate and cured in standard curing room. The shrinkage from final setting to 7 d was monitored using a device equipped with a displacement transducer. Thermal effects were mitigated through environmental control and calibration, ensuring measured strains reflected purely autogenous shrinkage. Three specimens per group underwent triplicate testing, and the mean values and standard errors were determined.

2.3.7. Mercury Intrusion Porosimetry (MIP)

Using MicroActive AutoPore V 9600 MIP device from Micromeritics Instruments (Norcross, GA, USA) to measure the pore structure of 28 d paste. Prior to the measurement, the samples of 28 d paste were dried under vacuum conditions for 1 d to ensure complete dryness.

2.3.8. Microstructure Analysis

A Flex1000 SEM from JEOL (Tokyo, Japan) was employed to measure the microstructure of 28 d paste. The experimental samples were the same as those used for MIP. To meet the electrical conductivity requirements, the surface of the samples was pre-sputtered with gold particles for approximately 120 s prior to testing.

2.3.9. Nanoindentation Test

Nanoindentation testing was conducted to evaluate the micro-mechanical properties of 28 d pastes. Prior to testing, the samples were thoroughly dried under vacuum and then placed in 30 mm molds containing epoxy resin for solidification. The encapsulated samples were polished using sandpapers and diamond suspension on cloths to achieve a finer surface finish [29,30].
Following preparation, the polished samples were analyzed using a KLA-I micro-nanoindenter from KLA (Milpitas, CA, USA), where 231 indentations were systematically arranged in an 11 × 21 grid with a 10 μm spacing. Each indentation underwent 10 loading-unloading cycles, with the load linearly increasing to 2 mN, held at peak load for 2 s, followed by a gradual reduction to zero over a 5 s unloading phase [31]. The Oliver and Pharr method was utilized to calculate the elastic modulus for each indentation [32].

3. Results and Discussions

3.1. Setting Time

Figure 1 shows the setting time of UHPC. It is evident that incorporating pre-wetted biochar prolonged the setting time of UHPC as the initial and final setting times of B2 were, respectively, increased by 55.3% and 35.0%. This could be likely due to the fact that some of the water in pre-wetted biochar was released into the fresh UHPC during the mixing process. When EA was added, the prolong in setting time of B2 was gradually overcome, as the UHPC with EA has a shorter setting time than B2. Specially, by adding 1% to 4% EA, the initial and final setting times were, respectively, reduced by 27.1–52.5% and 13.2–38.4%, compared with those of B2. This is because EA hydrates fast to produce expansive agents, thus accelerating the setting process of the prepared UHPC. This suggests that the setting process of UHPC can be controlled by adjusting the dosage of EA, thereby providing greater flexibility in optimizing the construction performance of UHPC [33].

3.2. Fluidity

Figure 2 shows the fluidity of UHPC. The presence of pre-wetted biochar can improve the fluidity of UHPC, with the initial fluidity of B2 increasing by 10.0%, compared to U0. This improvement can be attributed to the internal curing water released by the biochar, which may enhance the effectiveness of the superplasticizer, thereby improving the initial fluidity of UHPC [5,34]. In contrast, the addition of EA reduces the fluidity of UHPC with pre-wetted biochar, which is closely related to the dosage of EA. For instance, as the dosage of EA increases from 1% to 4%, the fluidity of UHPC decreases by 2.4–13.8% compared to U0. In addition, the flowability of B2/E4 was only 11.6 cm, indicating that a high dosage of EA can significantly affect the workability of UHPC with pre-wetted biochar, due to its quick hydration process.

3.3. IRH

Figure 3 presents the IRH of UHPC. It can be found that the IRH of U0 decreased rapidly from 95% to 65%, indicating that the occurrence of a prominent self-desiccation resulting from the drying capillary pores of UHPC. In contrast, the addition of pre-wetted biochar can significantly increase IRH as the IRH of B2 was 42% higher than U0. This suggests that pre-wetted biochar effectively mitigates early self-desiccation by releasing internal curing water and replenishing moisture loss in UHPC [35,36]. When additional EA was added to the UHPC with pre-wetted biochar, the IRH of these UHPC decreased compared to B2, though the IRH of the UHPCs were all still above 80% at 7 d. Notably, the IRH of B2/E4 at 7 d remained at 84.3%, which was 32.8% higher than that of U0 (63.5%). This suggests that pre-wetted biochar can effectively mitigate the negative effect of EA on the IRH of UHPC, maintaining higher internal humidity, which is essential for inhibiting autogenous shrinkage of UHPC.

3.4. Autogenous Shrinkage

Figure 4 shows the autogenous shrinkage of UHPC. The autogenous shrinkage of U0 increased rapidly within the first 2 d, reaching 689.9με, which accounted for 75.6% of the total autogenous shrinkage during the test period. This significant early-age shrinkage indicates an elevated risk of cracking, primarily due to the low mechanical strength and elastic modulus of UHPC at this stage. Following this, the rate of autogenous shrinkage slowed, reaching 913.1με at 7 d. The addition of pre-wetted biochar effectively mitigated the autogenous shrinkage of UHPC as the autogenous shrinkage of B2 was 35.7% lower than that of U0. This reduction is attributed to the internal curing water released by the biochar, which mitigates self-desiccation in capillary pores, thereby inhibiting the development of autogenous shrinkage in UHPC.
When the combined addition of EA and pre-wetted biochar was used, the autogenous shrinkage of UHPC was further reduced. Compared to B2, the addition of 1–4% EA mitigated the autogenous shrinkage of UHPC by 6.8–30.3%, with higher EA dosages resulting in greater reductions. This synergistic effect arises from the mutual reinforcement between EA and biochar, where the internal curing water released from biochar accelerated the hydration of EA and promoted the formation of expansive products, while the water consumption of EA improved the internal curing efficiency of biochar. This interaction refines the pore structure more effectively, thereby further reducing the autogenous shrinkage of UHPC.

3.5. TG-DTG Analysis

Figure 5 shows the 28 d TG-DTG curves of UHPC pastes. Three decomposition peaks of DTG curves represent the decompositions of AFt/C-S-H (80–105 °C), Ca(OH)2 (400–500 °C), and CaCO3 (600–800 °C), respectively [37,38]. The addition of pre-wetted biochar effectively promoted the hydration of UHPC, as evidenced by a 4% higher total mass loss in B2 compared to U0. This increase can be attributed to the internal curing effect of biochar. When EA was added into UHPC with biochar. The total mass loss of the UHPC was further increased compared to B2, as indicated by the significantly higher of endothermic peaks in DTG curves. Specifically, by adding 1–4% EA, the total mass loss of UHPC was increased by 4.6–13.8%. This suggests that the addition of EA in UHPC with biochar enhances the overall amount of hydration products. Consequently, it implies that part of the internal curing water may contribute to the hydration of EA in addition to cement hydration.

3.6. Compressive Strength

Figure 6 shows the compressive strength of UHPC. As shown in the figure, the addition of pre-wetted biochar slightly reduces the compressive strength at 3 d compared to U0. Nevertheless, as the curing time progresses, the compressive strength significantly improves. For example, the compressive strength of B2 is comparable to that of U0 at 7 d and increases by 12.0% at 28 d. This suggests that pre-wetted biochar could slightly delay the early cement hydration, resulting in inhibited development of the 3 d compressive strength of B2. On the contrary, it does not negatively affect the 28 d compressive strength. This is attributed to the internal curing water stored within the biochar, which is gradually released at later stages, promoting the cement hydration and generating additional hydration products, thereby significantly improving the compressive strength at 28 d.
When the combined addition of EA and pre-wetted biochar is used, the compressive strength of UHPC is closely related to the dosage of EA. Specifically, when the dosage of EA is 2%, the compressive strength reaches its highest at all curing ages, with a 5% increase compared to U0. This change indicates that the internal curing water carried by the biochar promotes the hydration of EA. The hydration of EA not only can mitigate autogenous shrinkage of UHPC but also fills the pores in the UHPC, improving its density and thus enhancing compressive strength. However, when the dosage of the expanding agent is excessively high, although the compressive strength at 28 d is close to that of the control group, the compressive strength decreases compared to the B2 group with 2% dosage. This could be attributed to the fact that excessive EA content leads to moisture competition and microstructural inhomogeneity, which reduces the strength.

3.7. Pore Structure

Figure 7 shows the pore size distribution of UHPC. It is evident that the pores in each group of UHPC were mainly concentrated in the 10–100 nm range, classified as capillary pores [39]. When pre-wetted biochar was added, the volume of capillary pores was increased compared to U0. This increase is likely attributed to the porous structure of biochar, which creates additional pores in the UHPC, leading to an increase in the volume of capillary pores. Nevertheless, the combined use of pre-wetted biochar and EA can reduce the volume of capillary pores in UHPC, indicating a synergistic effect in refining the pore structure. Upon a quantitative pore structure analysis, it was shown that the pores (50–100 nm) of B2/E2 were reduced by 23% compared to B2, and this pore refinement corresponded to a 5% increase in its compressive strength. This reduction is due to the additional water supplied by biochar, which promotes the hydration of EA, generating expansion products (e.g., AFt, C-S-H) that fill and densify the capillary pores.
In addition, the peak capillary pores of B2/E4 were slightly higher than those of B2/E2, and an increase in pore volume greater of 1000 nm, suggesting that the expansive product of 4% EA was less effective in pore filling due to water competition between the EA and cement hydration. This is consistent with the TG-DTG data, where B2/E4 shows a smaller increase in hydration products of AFt and C-S-H relative to B2/E2.

3.8. Microstructure Analysis

Figure 8 shows the SEM images of UHPC at 28 d. As can be seen from Figure 8a, the microstructure of UHPC is highly dense, with no visible pores. When pre-wetted biochar was added, the significant pore structures in UHPC were observed as shown in Figure 8b. This phenomenon is attributed to the intrinsically porous nature of biochar during its production process. These pores are a key factor enabling biochar to function as an internal curing agent. Moreover, the interface between the biochar and the matrix exhibits strong bonding without visible cracks, indicating that the addition of biochar does not negatively affect the overall density of the UHPC, thereby contributing to an improvement in its compressive strength.
Furthermore, it can be observed that with the incorporation of EA, expansion products such as C-S-H are dispersed in the matrix, which promotes the reduction in self-shrinkage. However, it is difficult to find needle-like AFt, which may be wrapped the dense microstructure. B2/E3 and B2/E4 exhibited minor micro-cracks near EA hydration sites, likely due to over-expansion, which corroborates its lower compressive strength relative to B2/E2. It is worth noting that there are noticeable cracks in the SEM image (Figure 8d) of the UHPC, and the sporadic distribution of micro-cracks and the correlation with EA dosage suggest a material-level origin, which is a probable result of sampling artifacts and drying [40].

3.9. Micro-Mechanical Properties

Figure 9 illustrates the micro-mechanical properties of 28 d U0, B2, B2/E2, and B2/E4 samples, analyzed by nanoindentation. U0 exhibits a relatively uniform elastic modulus distribution, indicating a more consistent hydration process. In contrast, in B2, more localized areas of more than 50 GPa modulus of elasticity are shown, suggesting that the biochar enhances internal curing but introduces some hydration inhomogeneity. Generally, areas above 50 GPa represent unhydrated cement clinker, while areas within 50 GPa correspond to gel products formed during hydration [41,42]. With the incorporation of EA in B2/E2 and B2/E4, the region greater than 50 GPa decreases, whereas the modulus of elasticity distribution becomes more continuous and refined, suggesting an improvement in matrix densification due to enhanced hydration kinetics.
The elastic modulus results were further deconvoluted through the Gaussian method, as illustrated in Figure 10. The elastic modulus can be classified into four phases in decreasing order: unhydrated cement particles, high-density C-S-H, low-density C-S-H, and porous phase, according to previous studies [43,44], with π, μ, and σ, respectively, denoting the volume fraction, mean, and standard deviation of each phase. Comparing Figure 10a,c, the simultaneous addition of EA and biochar notably raised the volume fraction of low-density C-S-H from 0.21 in U0 to 0.5 in B2/E2, while the volume fraction of unhydrated cement particles decreased by 13%. This indicates that cement hydration was significantly promoted. Moreover, compared to U0, the elastic modulus of high-density C-S-H and low-density C-S-H increased by 4.59 GPa and 2.1 GPa, respectively, suggesting that incorporating EA effectively densified the microstructure of UHPC. With the addition of EA, as shown in Figure 10d, the gradual transition in modulus values observed in B2/E2 and B2/E4 suggests that EA not only accelerates hydration but also enhances structural continuity in the hardened matrix, thereby reducing localized stress concentrations.

4. Conclusions

This study examined the effects of an EA on the performance of internally cured UHPC incorporating pre-wetted biochar, with a focus on autogenous shrinkage and compressive strength. The key findings are summarized as follows:
  • The hydration of EA, which consumed water and generated expansive products rapidly, accelerated the setting process of internally cured UHPC. This mitigated the prolonged setting time caused by the leakage of internal curing water from pre-wetted biochar. For example, adding 1–4% EA reduced the initial and final setting times by 27.1–52.5% and 13.2–38.4%, respectively, compared to the reference sample (B2). However, the initial fluidity of internally cured UHPC decreased with the addition of EA.
  • Internal curing with pre-wetted biochar significantly increased the IRH of UHPC compared to the control sample (U0). Although the rapid hydration of EA slightly lowered the IRH, it remained above 70% in all EA-modified mixtures. As a result, the 7 d autogenous shrinkage of B2 was reduced by 35.7% compared to U0. Furthermore, incorporating 1–4% EA further decreased autogenous shrinkage by 6.8–30.3% relative to B2.
  • TG-DTG analysis revealed that adding EA to UHPC with biochar increased the total quantity of hydration products. This suggested that a portion of the internal curing water contributed to EA hydration in addition to cement hydration. This dual hydration process created a synergistic effect, refining the pore structure through EA-induced expansion and enhanced cement hydration facilitated by the internal curing water.
  • The inclusion of pre-wetted biochar slightly improved the 7 d and 28 d compressive strengths of UHPC, highlighting the effectiveness of this internal curing approach. The addition of EA initially enhanced these strengths, peaking at a 2% EA dosage, before declining with higher EA content. Additionally, EA incorporation led to a more uniform and refined elastic modulus distribution, indicating improved matrix densification due to accelerated hydration kinetics.

Author Contributions

C.H.: investigation, methodology, writing—original draft preparation, funding acquisition, resources. Z.Z.: investigation. P.C.: conceptualization, writing—reviewing and editing, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the Scientific Research Project of Anhui Province (Huaihe River Water Resources Commission of the Ministry of Water Resources) Water Resources Research Institute (JCZKY202404).

Data Availability Statement

Data are available on request.

Acknowledgments

The Huaihe River Water Resources Commission of the Ministry of Water Resources is appreciated for the financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Buildings 15 01348 g001
Figure 1. Setting time of UHPC.
Figure 1. Setting time of UHPC.
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Figure 2. Fluidity of fresh UHPC.
Figure 2. Fluidity of fresh UHPC.
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Figure 3. IRH of UHPC over 7 d.
Figure 3. IRH of UHPC over 7 d.
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Figure 4. Autogenous shrinkage of UHPC over 7 d.
Figure 4. Autogenous shrinkage of UHPC over 7 d.
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Figure 5. TG-DTG thermograms of 28 d UHPC pastes.
Figure 5. TG-DTG thermograms of 28 d UHPC pastes.
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Figure 6. Compressive strength of UHPC.
Figure 6. Compressive strength of UHPC.
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Figure 7. Pore size distribution of 28 d UHPCs.
Figure 7. Pore size distribution of 28 d UHPCs.
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Figure 8. SEM images of UHPC cured at 28 days: (a) U0; (b) B2; (c) B2/E1; (d) B2/E2; (e) B2/E3; (f) B2/E4.
Figure 8. SEM images of UHPC cured at 28 days: (a) U0; (b) B2; (c) B2/E1; (d) B2/E2; (e) B2/E3; (f) B2/E4.
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Figure 9. Contour mappings of elastic modulus of UHPCs: (a) U0; (b) B200-4; (c) B2-4/E2; (d) B2-4/E4.
Figure 9. Contour mappings of elastic modulus of UHPCs: (a) U0; (b) B200-4; (c) B2-4/E2; (d) B2-4/E4.
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Figure 10. Deconvolution results of elastic modulus for U0 (a), B2 (b), B2/E2 (c) and B2/E4 (d).
Figure 10. Deconvolution results of elastic modulus for U0 (a), B2 (b), B2/E2 (c) and B2/E4 (d).
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Table 1. Chemical compositions of the used cement, fly ash, and silica fume (%).
Table 1. Chemical compositions of the used cement, fly ash, and silica fume (%).
SiO2CaOFe2O3Al2O3MgOK2OOthers
Cement25.3253.114.015.633.810.837.29
Silica fume97.10.430.610.930.350.190.39
Fly ash59.33.383.8325.191.11.16.1
Expansive agent4.2873.810.895.551.320.2313.92
Table 2. Mix proportion (kg/m3).
Table 2. Mix proportion (kg/m3).
U0B2B2/E1B2/E2B2/E3B2/E4
Cement971971971971971971
Silica fume155155155155155155
Fly ash169148148148148148
Quartz sand997997997997997997
Water171171171171171171
Expansive agent009.7119.4229.1338.84
Biochar02121212121
Steel fiber187187187187187187
Superplasticizer252525252525
Total binder *129512951304.711314.421324.131333.84
* Total Binder = cement + silica fume + fly ash/biochar + EA.
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MDPI and ACS Style

Huang, C.; Zong, Z.; Chen, P. Enhancing the Performance of Ultra-High Performance Concrete Using Expansive Agent and Pre-Wetted Biochar to Produce a Synergistic Effect. Buildings 2025, 15, 1348. https://doi.org/10.3390/buildings15081348

AMA Style

Huang C, Zong Z, Chen P. Enhancing the Performance of Ultra-High Performance Concrete Using Expansive Agent and Pre-Wetted Biochar to Produce a Synergistic Effect. Buildings. 2025; 15(8):1348. https://doi.org/10.3390/buildings15081348

Chicago/Turabian Style

Huang, Congbin, Zijian Zong, and Peiyuan Chen. 2025. "Enhancing the Performance of Ultra-High Performance Concrete Using Expansive Agent and Pre-Wetted Biochar to Produce a Synergistic Effect" Buildings 15, no. 8: 1348. https://doi.org/10.3390/buildings15081348

APA Style

Huang, C., Zong, Z., & Chen, P. (2025). Enhancing the Performance of Ultra-High Performance Concrete Using Expansive Agent and Pre-Wetted Biochar to Produce a Synergistic Effect. Buildings, 15(8), 1348. https://doi.org/10.3390/buildings15081348

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