Development of Low-Viscosity UHPC Using Fly Ash Microbeads and Modified Polycarboxylic Acid Superplasticizer

Abstract

Rheological properties are essential to ultra-high performance concrete (UHPC), and it is necessary to guarantee a relatively lower viscosity to avoid fiber segregation and mechanical degradation. In this study, an innovative physical-chemical integrated approach, namely the simultaneous use of fly ash microbeads and a modified low-viscosity polycarboxylic acid superplasticizer (JN-PCE), was proposed to regulate the rheological performance of UHPC containing industrial by-products. The effect of varying microbead dosage, different superplasticizers, and their combined influence on the rheological parameters, mechanical characteristics, and microstructure evolution were systematically explored in this study. The results demonstrated that the addition of 1.5% JN-PCE led to significant improvements in the UHPC properties including a flow expansion of 775 mm, a static yield stress of 376.9 Pa, a dynamic yield stress of 188.01 Pa, a plastic viscosity of 160.87 Pa·s, and a 28-day compressive strength of 136.6 MPa. Moreover, when a combination of 10% microbeads and 1.5% JN-PCE was used, the UHPC exhibited a flow expansion of 730 mm, a static yield stress of 693.5 Pa, a dynamic yield stress of 542.90 Pa, a plastic viscosity of 202.40 Pa·s, and a 28-day compressive strength of 142.1 MPa. This study thus offers valuable insights into optimizing low-viscosity UHPC formulations using eco-friendly additives for construction applications.

1. Introduction

Ultra-high performance concrete (UHPC) is an innovative cement-based composite material, recognized for its remarkable strength, toughness, and durability. Its formulation involves optimizing the particle packing density and maintaining a low water-to-binder ratio (w/b < 0.25) [1], resulting in a dense microstructure that enhances its performance. UHPC has found widespread application in bridge strengthening, road repair, and the production of precast components [1,2,3]. However, the high cost and viscosity of UHPC paste have limited its large-scale use [4,5,6]. The rheological characteristics of UHPC directly affect the performance of UHPC such as the fiber distribution uniformity, porosity, and mechanical strength. Consequently, studying the rheological properties of UHPC paste and regulating key parameters, such as yield stress and viscosity, are essential to improve its flowability for pumping, pouring, and various construction applications [7,8,9].

Several factors influence the rheological behavior of UHPC including the particle packing density, specific surface area, chemical admixtures, moisture content change during hydration, and mixing temperature [10,11]. Currently, there are two effective methods for enhancing the rheological properties of UHPC: (1) adjusting the material composition and ratios, and (2) incorporating chemical admixtures [11]. Both approaches aim to increase the available free water, thereby reducing viscosity. In numerous studies focusing on the rheological behavior of UHPC paste, fly ash microbeads have been frequently employed to substitute some of the cementitious materials to enhance its rheological properties by adjusting its composition. Fly ash microbeads exhibit a notable “ball-bearing” effect [12,13], acting as inorganic agents that improve the workability of UHPC paste [10,14]. Additionally, the unique morphology of fly ash microbeads mitigates friction between cement particles, minimizing aggregation and facilitating the release of free water from flocculated particles [15]. Research shows that fly ash microbeads can increase the water film thickness at low water–binder ratios and reduce it at high water–binder ratios. This suggests that by appropriately increasing the amount of fly ash microbeads, the yield stress, and plastic viscosity of the cement-based paste can be reduced [16,17].

In terms of chemical admixtures, the commonly used polycarboxylic acid superplasticizer (PCE) is typically manufactured using methyl methacrylate (MPEG) and allyl ether (APEG). This choice is driven by the high specific surface area and agglomeration effects of silica fume, a key factor influencing the rheological behavior of UHPC paste. Consequently, introducing MPEG-PCE or APEG-PCE can effectively disperse silica particles, thereby reducing the yield stress and plastic viscosity of UHPC paste. Simultaneously, by considering the bonding interactions between particles from different cementitious material systems, it is possible to increase the amount of polycarboxylic acid (PCA) to enhance the flowability of UHPC paste without significantly affecting the particle packing density [11,18].

Considering the current research status, both fly ash microbeads and PCA are regarded as crucial for enhancing the rheological behavior of UHPC paste. However, relying solely on fly ash microbeads or PCA to regulate the rheological behavior of UHPC paste has its limitations [11,19]. To more effectively control the rheological behavior of UHPC paste and tailor it to diverse engineering applications, this study introduced modified a polycarboxylic acid superplasticizer (JN-PCE) in combination with fly ash microbeads and explored their synergistic effect on the fluidity and rheological parameters of UHPC paste. Building upon prior research that determined the optimal proportion of waste-based ultra-fine mineral admixtures in UHPC, this study investigated the rheological parameters of waste-based UHPC paste by adjusting its composition and incorporating chemical admixtures. The modified JN-PCE and fly ash microbeads were simultaneously utilized to enhance the flowability of waste-based UHPC paste. The ultimate objective was to meet the flowability standards required for self-compacting concrete. Additionally, this paper provides an in-depth analysis of the mechanisms behind the fluidity improvement in waste-based UHPC paste achieved through the synergistic effect of the modified JN-PCE and fly ash microbeads.

2. Materials and Methods

2.1. Raw Materials

The basic raw materials for the preparation of UHPC paste include Portland cement, ultrafine mineral admixture (UMA), and silica fume (SF). The cement, with the strength class of 42.5, was purchased from Xing’an, Guangxi. The UMA was composed of hydrated manganese slag from Longda, Quanzhou, Guangxi, and fly ash from Guigang, Guangxi. These two industrial by-products were first homogeneously mixed with a mass ratio of 1:1 and then mechanically ground to obtain the UMA. SF was purchased from Southeast New Sichuan. Fly ash microbeads were obtained from Haoquan New Materials in Beijing. The detailed oxide composition of the powders was analyzed by X-ray fluorescence (XRF), as shown in

Table 1. Main chemical composition of the cement, mineral admixture, and silica fume (%).

Oxides (wt%)	Cement	UMA	SF	MS
Na2O	0.56	0.61	0.07	1.26
MgO	2.15	2.77	1.05	0.97
Al2O3	5.14	26.31	0.36	20.65
SiO2	21.53	42.38	96.92	59.13
SO3	3.34	1.77	0.69	0.13
K2O	1.21	1.63	0.32	2.34
CaO	59.99	8.86	0.3	7.23
MnO	0.07	6.93	0.01	-
Fe2O3	4.57	5.95	0.08	5.22
Loss (950 °C)	4.63	1.65	2.80	0.86
Specific surface area (m2/g)	0.358	0.714	-	3.152
Density (kg/m3)	3.05	2.75	-	2.41

. The particle size distribution of these powders was determined by a particle size analyzer, as displayed in Figure 1.

Liquid dewatering agent A is a high-performance polycarboxylic acid (PCE) with a solid content of 40% and a dewatering rate of 30–40%. Liquid dewatering agent B is a modified superplasticizer (JN-PCE) with a solid content of 40% and a dewatering rate of 30–40%. This modified superplasticizer is synthesized by compounding PCE through a water solution polymerization method, utilizing allyl polyethylene glycol ether (APEG-2400) as the macromonomer and introducing functional monomers such as acrylic acid, acrylamide, and methacrylic acid.

The selected aggregate consisted of quartz sand with particle sizes of 20, 40, and 80 meshes in a ratio of 3:5:2. Copper-plated steel fibers with a diameter of 0.18–0.22 mm and a tensile strength of ≥2850 MPa were also included.

2.2. Mix Design

Based on experimental studies of the optimal UHPC mix design in our previous work, we conducted experiments to address the issue of high viscosity in solid waste-based UHPC by introducing fly ash microbeads and PCE. The specific mix ratios are detailed in

Table 2. Mix ratio design of the UHPC investigated in this study.

Sample	CT (%)	SF/(%)	UMA/(%)	MS (%)	B/S	W/B	PCE/(%)	JN-PCE/(%)	Steel Fiber/ (Vol.%)
UHPC-(MS0-JN0)	80	10	10	0	1.1	0.17	1.5	/	2
UHPC-(MS10-JN0)	70			10				/
UHPC-(MS20-JN0)	60			20				/
UHPC-(MS0-JN1.5)	80			0			/	1.5
UHPC-(MS10-JN1.5)	70			10			/
UHPC-(MS20-JN1.5)	60			20			/

. “UHPC-(MS××-JN××)” was used to represent different mixtures and their corresponding hardened specimens, in which “MS××” means the amount of fly ash microbeads, and “JN××” means the amount of JN-PCE in the mixture. In this table, “UHPC-(MS0-JN0)” denotes the control group without MS and JN-PCE, while “UHPC-(MS10-JN0)”, “UHPC-(MS20-JN0)”, “UHPC-(MS0-JN1.5)”, “UHPC-(MS10-JN1.5)”, and “UHPC-(MS20-JN1.5)” represent various combinations of MS dosage at 10% and 20%, along with the JN-PCE dosage at 1.5% with the MS dosage at 10% and 20%, or JN-PCE dosage at 1.5%. In this way, the effect of JN-PCE and MS can be well-revealed through a comparison of different mixtures.

2.3. Test Method

2.3.1. Mechanical Properties of UHPC

The mechanical properties of UHPC were assessed in accordance with GB/T 17671-2021, titled “Test Method of Cement Mortar Strength (ISO Method)”. The specimen size used as 40 × 40 × 160 mm, and testing was conducted after curing for 3 d, 7 d, and 28 d.

2.3.2. Rheological Properties of UHPC Paste

The flowability and slump ability of the UHPC paste were evaluated based on GB/T 50080-2016, “Standard for Test Method of Performance on Ordinary Fresh Concrete”. The yield stress and viscosity of UHPC paste ere determined using a concrete rheometer (Shanghai Tongrui, Shanghai, China) and tested following the guidelines outlined in ASTM C1874-19, “Standard Test Method for Measuring the Rheological Properties of Cementitious Materials Using a Coaxial Rotational Rheometer”. The testing procedure for assessing the rheological performance of the UHPC paste is illustrated in Figure 2, with the testing temperature maintained at 28 ± 2 °C.

Considering the time-dependent rheological characteristics of cementitious materials, the measurement of rheological parameters for the UHPC paste was conducted within 30 ± 5 min after the cementitious materials were mixed with water. The experimental setup for testing the rheological parameters of the UHPC paste is shown in Figure 3. To better understand the shear behavior of the UHPC paste, a graphical simulation of its shear state is presented in Figure 4. In Figure 4, R₁ represents the rotor diameter, R₂ denotes the cylinder diameter, R_c indicates the plug radius (the boundary between the sheared and non-sheared regions), and ‘r’ refers to the actual shear radius [20].

Moreover, due to the inherent limitations of the instrument itself, the shear rate utilized in this experiment approximated the shear conditions (shear rate ≤ 10 s⁻¹) encountered in engineering scenarios such as mixing, stirring, transportation, pumping, and casting. UHPC paste is considered as a complex non-Newtonian fluid. For pastes that exhibit a linear relationship between shear stress and shear rate, the Bingham model is typically applied for dynamic rheological parameter analysis, as per Formula (1). In cases where pastes show a nonlinear relationship between shear stress and shear rate, the Herschel–Bulkley (H–B) model is commonly used for dynamic rheological parameter analysis, as per Formula (2). However, for the shear stress and shear rate profiles observed in this experiment, which demonstrated nonlinearity and shear thickening behavior, a modified Bingham model was employed for dynamic rheological parameter fitting analysis [10,21,22], as defined in Formula (3).

Bingham model:

$$\mathrm{\tau }={\tau }_0+\mathrm{\eta }\dot{\gamma }$$

(1)

H–B model:

$$\mathrm{\tau }={\mathrm{\tau }}_0+\mathrm{K}{\dot{\mathrm{\gamma }}}^{\mathrm{n}}$$

(2)

Modified Bingham model:

$$\mathrm{\tau }={\mathrm{\tau }}_0+\mathrm{\eta }\dot{\mathrm{\gamma }}+\mathrm{c}{\dot{\mathrm{\gamma }}}^2$$

(3)

where τ is the shear stress, Pa; ${\tau }_0$ is the yield stress, Pa; η is the plastic viscosity, Pa·s; γ is the shear rate, s⁻¹; K is the consistency coefficient, and n is the consistency index.

2.3.3. Microstructure of UHPC

Samples of UHPC cubes aged for 7 d or 28 d were subjected to drying in an oven at 60 °C for 6 h. Subsequently, these samples were observed using a field emission scanning electron microscope (GeminiSEM 300, Oxford, UK) to examine the microstructure of the UHPC specimens.

3. Results and Discussion

3.1. The Flowability of UHPC Paste

The flowability of the UHPC paste was controlled through the incorporation of fly ash microbeads and JN-PCE. According to the standards used for classifying the filling ability of self-compacting concrete in

Table 3. Filling grade classification of self-compacting concrete.

Performance Indices	Performance Level	Technical Requirements
Slump expansion (mm)	SF1	550–655
	SF2	660–755
	SF3	760–850
Spreading time T500 (s)	VS1	≥2
	VS2	<2

, the pouring expansion and spreading time T500 for UHPC with different mix proportions were determined and are presented in

Table 4. Influence of physicochemical action on flow parameters of UHPC paste.

Test No.	Slump Expansion (mm)	Spreading Time (s)	Slump Depth (mm)	Free Fall Time (s)	Filling Property
					Slump Expansion Grading	Spreading Time Grading
UHPC-(MS0-JN0)	650	18	270	23	SF1	VS1
UHPC-(MS10-JN0)	560	24	280	29	SF1	VS1
UHPC-(MS20-JN0)	490	40	260	35	-	VS1
UHPC-(MS0-JN1.5)	775	10	290	13	SF2	VS1
UHPC-(MS10-JN1.5)	730	12	290	17	SF2	VS1
UHPC-(MS20-JN1.5)	520	36	250	22	-	VS1

. It revealed that as the microbead content increased, the pouring expansion and slump degree of UHPC paste gradually decreased while the spreading time and free fall time gradually increased. The slump expansion for the reference UHPC-(MS0-JN0) group was measured as 650 mm. In contrast, UHPC-(MS10-JN0) and UHPC-(MS20-JN0) exhibited a reduction of 13.8% and 24.6%, respectively. When JN-PCE was added, the expansion flowability of UHPC-(MS0-JN1.5) was significantly improved, reaching 775 mm. This was 19.2% higher than that of UHPC-(MS0-JN0). However, when considering the combined impact of fly ash microbeads and JN-PCE, the expansion flowability of UHPC-(MS0-JN1.5) decreased to 730 mm, which was 12.3% lower than that of UHPC-(MS0-JN0). As for UHPC-(MS20-JN1.5), its flowability was only 520 mm. This indicates that the incorporation of more fly ash microbeads tends to inhibit the filling property of UHPC paste. This phenomenon might be attributed to the presence of a large amount of ultrafine particles under a low water–cement ratio. The thickness of the water film between particles is therefore reduced, consequently diminishing the expansion flowability of the UHPC paste [23]. Based on the findings presented in Table 4, it can be concluded that when the microbead content is set at 10% and the JN-PCE dosage as 1.5%, the prepared UHPC paste still possesses satisfactory flowability and can meet the requirements for the pouring expansion of UHPC.

3.2. The Rheological Behavior of UHPC Paste

3.2.1. Effect of Fly Ash Microbeads on the Static Yield Stress

The static yield stress of UHPC is the ultimate shear stress at which the UHPC paste initiates its flow. In our experiment, the concrete rheometer maintained a constant shear rate of 0.03 s⁻¹ for 25 s. As illustrated in Figure 5, as the microbead content increased, the static yield stress of UHPC paste displayed a pattern of initially decreasing and then increasing. The static yield stress measured for UHPC-(MS10-JN0) was 417.2 Pa, whereas for UHPC-(MS20-JN0) and UHPC-(MS0-JN0), the tested static yield stresses were 1789.5 Pa and 1796.7 Pa, respectively. In comparison to UHPC-(MS0-JN0), UHPC-(MS10-JN0) showed a substantial decrease of 76.8% in static yield stress, while UHPC-(MS20-JN0) displayed a minimal reduction of 0.04%. This suggests that a 10% microbead dosage significantly reduces the static yield stress of UHPC paste, while a 20% microbead dosage has little effect on diminishing the static yield stress of UHPC paste. The primary reason behind this observation may be that under low-dosage conditions (10%), the fly ash microbeads likely act as lubricants, facilitating particle movement within the UHPC paste and reducing the static yield stress. Conversely, under high-dosage conditions (20%), due to the low water–cement ratio, fly ash microbeads may not fully disperse and tend to agglomerate, which limits their ability to reduce the static yield stress [18]. Consequently, even with increased dosage, there was minimal change in the yield stress when fly ash microbeads reached 20% in the UHPC matrix.

3.2.2. Effect of Fly Ash Microbeads on the Dynamic Yield Stress and Viscosity

The effect of fly ash microbeads on the dynamic yield stress and viscosity of UHPC was also explored. The dynamic yield stress of UHPC represents the minimum stress required to sustain the flowing state of UHPC paste. As for the viscosity of the UHPC, it can be interpreted as the internal friction resistance within a fluid that hinders its flow. Therefore, it reflects the rate of paste deformation dictated by the inherent properties of materials [18]. As depicted in Figure 6, a notable linear relationship between the shear rate and shear stress was observed for UHPC-(MS0-JN0), UHPC-(MS10-JN0), and UHPC-(MS20-JN0). The Bingham model was employed for dynamic rheological parameter analysis. For every mixture, a high correlation coefficient of 0.998 was obtained for the linear fitting, indicating the good applicability of the Bingham model. According to the fitting results, it was observed that the dynamic yield stress (τ₀) of UHPC first declined from 606.76 Pa to 434.12 Pa, and then increased to 932.82 Pa with an increase in fly ash microbead dosage. This trend aligns with the earlier observations of the static yield stress of UHPC under the impact of fly ash microbeads. Moreover, with the increase in the amount of fly ash microbeads, the plastic viscosity (η) of UHPC paste showed a decrease from 296.97 Pa·s to 244.57 Pa·s and then further to 149.46 Pa·s. Compared with UHPC-(MS0-JN0), the plastic viscosity of UHPC-(MS10-JN0) and UHPC-(MS20-JN0) was reduced by 17.6% and 49.7%, respectively. These results emphasize that microbeads exerted a weakening effect on both the dynamic yield stress (τ₀) and plastic viscosity (η) of the UHPC, providing compelling support for the use of microbeads as a physical factor to regulate the rheological parameters of UHPC paste.

3.2.3. Effect of JN-PCE on the Static Yield Stress

As illustrated in Figure 7, when JN-PCE was added, the static yield stress of the UHPC-(MS0-JN1.5) paste was tested as 376.9 Pa. This value represents a substantial 79.0% reduction compared with the static yield stress of the UHPC-(MS0-JN0) paste presented in Figure 5. It is evident that the effect of JN-PCE was slightly more pronounced on the reduction in static yield stress of UHPC than that observed with microbeads alone (76.8%). The static yield stress of UHPC paste under the joint influence of microbeads and JN-PCE was examined. The outcomes revealed that the static yield stress of UHPC-(MS10-JN1.5) and UHPC-(MS20-JN1.5) paste were measured as 693.5 Pa and 2678.7 Pa, respectively, significantly lower than that of UHPC-(MS0-JN0) paste. In the comparison of UHPC-(MS10-JN1.5) and UHPC-(MS10-JN0), it was observed that the static yield stress increased with the addition of JN-PCE. A similar phenomenon was also observed for UHPC-(MS20-JN1.5) and UHPC-(MS20-JN0). This might be attributed to the adsorption of JN-PCE onto the surface of fly ash microbeads, impairing the effect of JN-PCE on the static yield stress. When two different water-reducing agents were used individually, JN-PCE was found to be more significant in reducing the static yield stress of UHPC paste. This can be attributed to the modification process, which introduces carboxyl and other hydrophilic groups to the surface of cement particles, thus increasing the thickness of the hydration film and enhancing the dispersion capacity of cement particles. Additionally, amides and methyl groups, functioning as hydrophobic groups, can adjust the hydrophilic-hydrophobic properties of JN-PCE, reduce its surface tension, and release more free water. The incorporation of acrylate polyether macromolecules increases the side chains of the composite-modified polycarboxylic acid macromolecules. Simultaneously, it improves the spatial blocking effect of JN-PCE. This, in turn, weakens the interaction forces between cement particles and allows for more spherical particles to be introduced, serving a lubricating function [11,24,25,26]. Therefore, when the microbead dosage was set at 10%, it effectively reduced the static yield stress of UHPC.

3.2.4. Effect of JN-PCE on the Dynamic Yield Stress and Viscosity

As depicted in Figure 8, when subjected to the influence of JN-PCE alone, a nonlinear relationship existed between shear rate and shear stress for UHPC-(MS0-JN1.5). Consequently, the modified Bingham model (Quadratic version) was employed for dynamic rheological parameter analysis, yielding a strong correlation coefficient of 0.996. Similar to the linear model, the quadratic Bingham model also utilizes η and c to describe the nonlinear characteristics between the shear rate and shear stress. As shown in Figure 8, the dynamic yield stress (τ₀) of UHPC-(MS0-JN1.5) was determined to be 188.01 Pa, and the plastic viscosity (η) was 160.87 Pa·s. Compared with UHPC-(MS0-JN0), UHPC-(MS0-JN1.5) showed a significant reduction of 69.0% in dynamic yield stress (τ₀) and a notable decrease of 45.8% in plastic viscosity (η). Note that the direct comparison of the results from the linear and quadratic Bingham model might be inappropriate. Indirect analysis as also conducted here. When the shear rate varied from 1 to 4 s⁻¹, the obtained results exhibited a linear trending. Therefore, the linear Bingham model can be applied at this specified stage, and the corresponding results can be compared. It was observed that for UHPC-(MS0-JN0), the change rate of shear stress was much steeper than that of UHPC-(MS0-JN1.5). This indicates that when the linear Bingham model was applied, the derived dynamic yield stress from UHPC-(MS0-JN1.5) was still larger than that of UHPC-(MS0-JN1.5). This also suggests that the JN-PCE can effectively mitigate the yield stress of the UHPC binder, which is consistent with the static yield stress results.

In cases where the JN-PCE and microbeads were introduced together, a clear linear relationship between the shear rate and shear stress was observed for UHPC-(MS10-JN1.5) and UHPC-(MS20-JN1.5). Hence, the linear Bingham model was utilized for dynamic rheological parameter analysis on these two mixtures. In these scenarios, the correlation coefficients of the linear fitting for UHPC-(MS10-JN1.5) and UHPC-(MS20-JN1.5) attained 0.996 and 0.986, respectively. The dynamic yield stress (τ₀) of UHPC was found to initially decrease from 606.76 Pa to 542.90 Pa and then rise to 1276.10 Pa. Correspondingly, the plastic viscosity (η) of UHPC underwent a reduction from 296.97 Pa·s to 202.40 Pa·s initially, and then further to 132.70 Pa·s. This reduction in plastic viscosity aligned with the impact of microbeads, which also exhibited a gradual decrease with the increase in microbead content. On the other hand, the combination of JN-PCE and fly ash microbeads showed a synergistic effect on viscosity. In the comparison of UHPC-(MS10-JN1.5), UHPC-(MS20-JN1.5), and UHPC-(MS0-JN0), it was observed that the viscosity of UHPC containing JN-PCE and fly ash microbeads was lower than that of the reference sample. The addition of JN-PCE accentuated the reduction in UHPC plastic viscosity. This phenomenon can be attributed to the inclusion of APEG class polyether macromolecules in JN-PCE, which are readily adsorbed onto silicates and mitigate their agglomeration. This, in turn, enhances the flow state of silicates, becoming a pivotal factor in improving the high fluidity of UHPC and reducing its plastic viscosity [11].

The change trend in the influence of microbeads and JN-PCE on dynamic yield stress (τ₀) and plastic viscosity (η) is depicted in Figure 9. Notably, the results revealed that the dynamic yield stress (τ₀) of UHPC-(MS10-JN1.5) was lower than that of UHPC-(MS20-JN1.5), while the plastic viscosity (η) of UHPC-(MS10-JN1.5) was higher than that of UHPC-(MS20-JN1.5). In comparison to the control group, both the dynamic yield stress (τ₀) and plastic viscosity (η) of UHPC experienced varying degrees of reduction under the combined influence of microbeads and JN-PCE. Simultaneously, the corresponding collapse expansion degree provided in Table 4 indicates that the co-action of microbeads with a 10% dosage and JN-PCE with a 1.5% dosage proved the most effective in adjusting the rheological parameters of the UHPC paste. This finding strongly supports the use of microbeads and JN-PCE as physical and chemical factors to regulate the rheological parameters of UHPC paste. Moreover, the collapse expansion degree of UHPC paste can meet the level requirements for self-compacting concrete collapse expansion degree SF2.

3.3. Mechanical Properties of UHPC

The impact of microbeads and water reducers on the mechanical properties of UHPC is detailed in

Table 5. Influence of physicochemical action on the mechanical properties of UHPC.

Test Number	Flexural Strength (MPa)						Compressive Strength (MPa)
	3d		7d		28d		3d		7d		28
	μ	σ	μ	σ	μ	σ	μ	σ	μ	σ	μ	σ
UHPC-(MS0-JN0)	28.4	0.3	31.8	1.0	32.8	2.4	86.9	1.2	98.8	2.4	126.8	1.8
UHPC-(MS10-JN0)	31.7	1.5	32.2	1.3	35.2	2.1	91.9	0.5	101.6	1.7	138.2	2.4
UHPC-(MS20-JN0)	27.7	1.7	28.7	1.1	29.3	1.7	79.4	1.4	106.5	3.6	116.9	2.2
UHPC-(MS0-JN1.5)	28.2	0.6	29.6	0.7	30.8	1.9	96.0	1.9	111.3	0.8	136.6	1.5
UHPC-(MS10-JN1.5)	23.6	2.3	26.1	1.6	31.4	2.0	98.9	1.6	108.3	1.3	142.1	2.7
UHPC-(MS20-JN1.5)	22.7	2.6	23.2	1.7	27.5	1.2	82.3	2.8	104.8	1.5	131.6	1.6

Note: μ is the average strength (unit: MPa) and σ is the standard deviation (unit; MPa).

. As illustrated in Figure 10, with an increase in microbeads under the influence of PCE or JN-PCE, the compressive strength of UHPC at 3 days and 28 days initially increased and subsequently decreased. Specifically, when the microbead content was set at 10%, the compressive strength of UHPC with PCE at 3 days experienced a 5.8% increase. From Figure 10, it can be deduced that the particle size of the microbeads fell between that of silica fume and ultrafine mineral admixture. This implies that an appropriate quantity of microbeads can enhance the packing density of the prepared UHPC, leading to improved particle filling. Furthermore, the pozzolanic reaction of the fly ash microbeads can contribute to the compressive strength of the UHPC samples. Consequently, the compressive strength at both early and later ages for UHPC containing a small amount of microbeads is promoted. However, when the fly ash microbead content reached 20%, the compressive strength of UHPC was decreased compared with the reference sample. This strength reduction can be attributed to the dilution effect of fly ash microbeads. An excessive replacement of Portland cement with fly ash microbeads causes the dilution effect, and the positive contribution of fly ash microbeads cannot totally compensate for the dilution effect. Therefore, the compressive strength of UHPC-(MS20-JN0) was lower than UHPC-(MS0-JN0).

In the case of JN-PCE, the increase was more pronounced, with a 13.8% rise at 3 days and a 12.1% increase at 28 days. This indicates that the enhancing effect of microbeads on the compressive strength of UHPC during the entire hydration process surpassed that of PCE. This might be attributed to the strong reducing effect and its promotion on the early-age cement hydration of JN-PCE. By harnessing JN-PCE’s exceptional water-reducing properties, the effective particle-filling capabilities of microbeads within the cementitious system, and the abundant active components found in microbeads, a synergistic effect is achieved, further enhancing the mechanical properties of UHPC.

As for the flexural strength, when 10% of fly ash microbeads were added into UHPC, the flexural strength slightly improved due to better particle packing. When the fly ash microbead content reached 20%, the flexural strength of UHPC was decreased due to its dilution effect. On the other hand, JN-PCE appeared to impair the flexural strength when used synergistically with fly ash microbeads. JN-PCE might cause the fly ash microbeads to more easily float and aggregate in the slurry due to density difference effects, forming a weak interfacial layer. Therefore, the flexural strength of UHPC containing both JN-PCE and fly ash microbeads was slightly lower than that containing only fly ash microbeads.

3.4. Microstructure Characterization of UHPC

A secondary electron (SE) detector was utilized in the SEM observations of the UHPC matrix. The SEM images in Figure 11 provide valuable insights into the microstructure evolution of hardened UHPC. After curing for 3 d, the image of the blank group (a) revealed a substantial presence of fibrous and gel-like C-S-H on the surface of the UHPC, along with some flake-like hydrated minerals. The overall structure displayed larger pores and a relatively loose composition. Comparatively, the image of UHPC subjected to the combined influence of microbeads and JN-PCE (Figure 11c) at 3 d portrayed a denser overall structure. Most of the hydrated products appeared gel-like and were distributed more uniformly. Notably, numerous spherical depressions or facets were encased by gel-like C-S-H, suggesting that the addition of microbeads effectively enhances particle filling, resulting in fewer pores in the early-stage test blocks and a more compact structure. On the other hand, it was observed that the surface of the fly ash microbeads in Figure 11b–d was relatively smooth and did not seem to be etched, indicating that the reactivity of the microbeads was low and that it played a major role in particle packing in the body of UHPC.

As the hydration process advanced from 3 days to 28 days, an increasing quantity of hydration products, including calcium hydroxide, were generated within the cementitious system. During this period, the SiO₂ and Al₂O₃ active components within the ultrafine mineral admixtures present in the UHPC reacted with the generated calcium hydroxide, initiating secondary hydration reactions. This led to a significant improvement in the compactness of UHPC, as evident in Figure 11b at 28 d. Fly ash microbeads could also be detected in the corresponding SEM images. Similar to those presented at 3 d, these fly ash microbeads persistently exhibited a smooth surface, implying that even after 28 days of curing, the degree of reaction for fly ash microbeads is limited. Additionally, it was observed that these fly ash microbeads were densely encapsulated by the hydration products. The interfacial transition zone between the microbeads and surrounding mortar was exceptionally narrow, further evidence of the high-density microstructure of the UHPC prepared in this study.

4. Conclusions

This study investigated the combined effect of JN-PCE and fly ash microbeads on the performance of UHPC. The following conclusions were drawn:

• Either the sole use of JN-PCE or its combined application with fly ash microbeads can effectively guarantee the excellent flowability in UHPC.
• A low content of fly ash microbeads effectively reduces the static/dynamic yield stress of UHPC. However, a high content contributes less to regulating it.
• The modified JN-PCE exhibited superior water-reducing and viscosity-reducing performance and helped improve the mechanical properties of UHPC.
• The combined use of fly ash microbeads and JN-PCE can reduce the viscosity of UHPC and improve its compressive strength.
• Under the combined use of microbeads and JN-PCE, UHPC exhibited high hydration levels, minimal pores, and exceptional compactness. The microbeads and JN-PCE effectively regulated the fluidity of waste-based UHPC. This research on their dual effects on the rheological parameters of UHPC holds significant promise for the widespread promotion and application of UHPC on a larger scale.

This study primarily focused on the macro mechanical properties and rheological properties. Further microstructure characterization should be conducted, and the synergistic effect of fly ash microbeads and the viscosity-reducing admixture should also be explored in more depth.