1. Introduction
Self-leveling cement-based (SLC) pastes are highly adaptable construction materials composed of Portland cement, supplementary cementitious materials (SCMs), and chemical additives [1]. These pastes are distinguished by their high fluidity, excellent pumpability, and self-leveling capability, enabling them to form a smooth, uniform surface upon application without causing bleeding [2]. The fresh properties of SLC paste are essential for ensuring their optimal performance. Portland cement, the primary binder in SLC paste, is associated with significant carbon emissions [1,3]. Studies demonstrated that producing 1 kg of cement generates approximately 800 g of CO2, primarily due to the calcination of limestone and the combustion of fossil fuels [4]. With an annual global cement production of 4 billion tons, the cement industry is responsible for approximately 8% of total global CO2 emissions [5,6]. To address this environmental challenge, the most impactful approach involves substituting a portion of Portland cement with SCMs in cement-based products, thereby reducing overall cement production and associated carbon emissions [5].
A range of SCMs, such as ordinary fly ash, ceramic waste powder, waste marble powder, and blast furnace slag (BFS), are commonly used in engineering applications. The addition of SCMs reduces the amount of cement clinker required in concrete production while effectively preserving its mechanical performance and durability [7,8,9,10,11]. The influence of SCMs on concrete properties is primarily attributed to their pozzolanic reaction and filler effect [12]. Targan et al. [13] studied concrete containing natural pozzolan and fly ash as SCMs, observing that although early strength may decrease (e.g., a 15% reduction in 2-day compressive strength when 5 wt.% of cement was replaced with natural pozzolan), the long-term strength improved considerably (e.g., a 10% increase in 90-day compressive strength under the same replacement condition). Similarly, Elahi et al. [14] studied concrete’s resistance to chloride-ion penetration when incorporating fly ash, silica fume, and ground granulated BFS, demonstrating that SCMs substantially enhanced the resistance to chloride-ion penetration.
Over 100 million tons of CFB fly ash, a primary by-product of coal combustion, is generated annually in China [15,16]. However, a sustainable and safe approach for its effective application has yet to be established. Recently, there has been increasing interest in CFB fly ash as an SCM because of its significant pozzolanic activity. For example, Li et al. [16] showed that the pozzolanic reactivity index of CFB fly ash can exceed 80% after 28 days of curing, surpassing the 70% minimum threshold for SCMs established by the GB/T 1596-2005 standard [17]. Their research also showed that CFB fly ash interacts with Ca(OH)2, leading to the formation of cementitious compounds, including C–S–H, C–(A)–S–H, alkali ferrite monophosphate, and alkali ferrite triphosphate, playing a key role in enhancing the strength of the CFB fly ash–CaO system [15]. Currently, the majority of CFB fly ash is disposed of in landfills, resulting in severe environmental problems. Leachates from these landfills pose a risk of contaminating water sources and soil, while the fine particles contribute to air pollution [18,19]. The use of CFB fly ash as an SCM not only helps to significantly alleviate the aforementioned environmental issues but also reduces carbon emissions from cement-based products by decreasing the amount of clinker used. The carbon emissions associated with CFB fly ash are nearly zero, whereas cement clinker produces approximately 830 kg of CO2 per ton [5]. Guo et al. [20] demonstrated that incorporating 30 wt.% CFB fly ash can reduce the carbon emissions of the resulting cement-based materials by 18.8%. Therefore, the use of CFB fly ash as an SCM is emerging as a promising and viable strategy for large-scale disposal, with ongoing research exploring its broader applications [20,21,22,23].
As discussed earlier, SLC paste demands superior fresh properties to achieve optimal performance. However, the inclusion of CFB fly ash as an SCM has been shown to compromise these fresh properties in cement-based pastes [19,24,25]. Zheng et al. [24] observed that increasing the CFB fly ash content significantly reduced the fluidity of blended cement paste, with a 30 wt.% dosage leading to a 20.3% reduction in fluidity. Li et al. [19] explained that the reduction in performance is due to the porous nature and rough surface texture of CFB fly ash, which leads to substantial water uptake. This excessive water uptake adversely affects the paste’s rheological characteristics, leading to higher yield stress and increased plastic viscosity. While the incorporation of CFB fly ash as an SCM negatively impacts the fresh properties of cement-based pastes, the existing research on this topic is still limited and lacks comprehensive detail, highlighting a significant gap in understanding the underlying mechanisms. Furthermore, the ability of water-reducing agents to enhance the fluidity of CFB fly ash-containing cement pastes, along with the compatibility between CFB fly ash and polycarboxylate superplasticizer (PCE), remains underexplored and insufficiently studied.
To address these gaps, two series of CFB fly ash-blended cement pastes were prepared. Series I pastes were prepared without the addition of PCE, maintaining a fixed water-to-binder ratio. Series II pastes, on the other hand, were modified with PCE to achieve a consistent fluidity of 240 ± 5 mm, while still preserving the same water-to-binder ratio. The research examined the fresh properties of these pastes, with a focus on fluidity, rheological behavior, bleeding rate, and the compatibility between CFB fly ash and PCE. The results provided valuable theoretical insights that can inform the effective use of CFB fly ash in SLC paste formulations.
2. Materials and Methods
2.1. Materials
The P I 52.5 Portland cement used in this study complied with the standards adjusted by GB 175-2007 [26]. CFB fly ash, sourced from the Jilin Power Plant in Jilin, China, was prepared by drying at 105 °C for 24 h, followed by cooling to room temperature. The prepared fly ash was subsequently used for chemical composition analysis, N2 adsorption–desorption isotherms, particle size distribution, micro-morphological analysis, and the preparation of CFB fly ash-blended cement paste. The chemical composition of samples was analyzed using an X-ray fluorescence spectrometer (S4 Explorer, Bruker, Berlin, Germany), with the results presented in Table 1. The analysis demonstrated that SiO2 and Al2O3 were the primary oxides in CFB fly ash, comprising over 80 wt.% of the total oxide content. Moreover, CFB fly ash demonstrated a significantly higher loss-on-ignition value compared to cement, primarily due to the presence of unburned carbon [27].
The N2 adsorption–desorption isotherms for both cement and CFB fly ash were obtained using a surface area analyzer (ASAP 2020, Micromeritics, Gwinnett, GA, USA), with the results presented in Figure 1. The isotherm curves, analyzed using the BET method, revealed that the specific surface areas of cement and CFB fly ash were determined to be 1.65 m2/g and 21.05 m2/g, respectively. The significantly higher values of CFB fly ash compared to cement can be attributed to its high porosity and the presence of unburned carbon.
The particle-size distribution of cement and CFB fly ash was analyzed using a laser particle size analyzer (S3500, Microtrac Inc., York, PA, USA). Figure 2 shows that CFB fly ash particles were smaller than cement particles. The D50 and D90 indices for CFB fly ash were measured at 13.9 µm and 49.9 µm, respectively, whereas for cement, these values were determined to be 21.5 µm and 53.2 µm.
The micro-morphologies of samples were examined using an SEM (Quanta TM250 FEG, FEI Company, Hillsboro, OR, USA). Figure 3 illustrates that the fly ash particles displayed irregular shapes with rough, porous surfaces. These morphological characteristics correlate with the results from the N2 adsorption–desorption and particle-size distribution analysis, offering a clear explanation for the elevated water demand associated with fly ash.
A commercially available methyl ether-based PCE (HPEG, Mingxuan Building Materials Co., Shenzhen, China), with a solid content of 50 wt.%, was employed as the water-reducing agent in this study.
2.2. Methods
2.2.1. Preparation of Cement-Based Paste
Two series of cement pastes, series I and II, were prepared to thoroughly assess the influence of fly ash on the fresh characteristics of cement paste [28]. The component proportions of the CFB fly ash-blended paste are listed in Table 2. In Series I, the cement content was partially replaced with 0–30 wt.% CFB fly ash, while the water-to-binder ratio remained at 0.5. This series was designed to examine the properties of cement paste without the addition of PCE. In Series II, the same water-to-binder ratio and CFB fly ash replacement levels as Series I were applied, but PCE was added in varying amounts depending on the fly ash content. Furthermore, the pastes in Series II were further adjusted to achieve consistent fluidity values of 240 ± 5 mm, allowing for the exploration of SLC paste properties with high and uniform fluidity facilitated by the inclusion of PCE.
2.2.2. Fluidity
The fluidity of the samples was measured using the spread diameter method following the standard GB/T 8077-2012 [29]. To determine fluidity, the prepared fresh paste (as shown in Table 2) was quickly poured into a cone mold with dimensions of 60 mm for height, 60 mm for base diameter, and 36 mm for top diameter. The mold was placed on a flat, smooth surface, and the paste was allowed to spread freely once the mold was removed. The flow diameter was then measured with a ruler, and the maximum observed flow diameter on the plate was recorded as the fluidity. The test was repeated three times, and the average fluidity values were calculated.
2.2.3. Rheological Properties
The rheological properties of the cement-based paste were determined using a rheometer (Figure 4a, Lamy RM100, Lamy Rheology Instruments, Champagne, France), with the experimental procedure illustrated in Figure 4b. For optimal accuracy, the test involved a 20 s pre-shear phase, followed by a 10 s rest period, with data collected during the deceleration stage [30,31]. The resulting data, including shear rate (γ) and shear stress (τ), were analyzed and fitted using the modified Bingham model (Equation (1)) [28,32]:
(1)
In the modified Bingham model, the parameters are defined as follows: τ0 represents the yield stress, which indicates the critical shear stress at the point of transition from elastic to plastic deformation; μ0 refers to the plastic viscosity, reflecting the material’s resistance to continuous flow; and φ0 represents a second-order term that describes shear thickening behavior. The ratio of φ0/μ0 can be used to characterize the type of rheological behavior: shear thickening (φ0 > 0), shear-thinning (φ0 < 0), or the standard Bingham model (φ0 = 0) [33].
2.2.4. Bleeding Rate
Considering the high fluidity (240 ± 5 mm) of the Series II pastes, bleeding was expected. To assess this, the bleeding rate was evaluated according to the T0520-2020 standard [34]. For the bleeding rate test, the fresh paste from Series II, with an initial volume denoted as V0, was placed in a pressure bleeding device (Figure 5). After a 10 min resting period, air was introduced into the device, and the pressure was increased to 0.8 MPa, where it was maintained for 5 min. The volume of water that bled out of the paste was measured as V1. The bleeding rate was calculated using the following equation:
(2)
where M represents the bleeding rate, V1 represents the volume of water that bled out, and V0 denotes the initial volume of the cement-based paste.2.2.5. Compatibility Between the CFB Fly Ash and PCE
The effectiveness of PCE in CFB fly ash-blended cement pastes was evaluated according to the standard GB/T 8077-2012 [29]. The component proportions of the pastes used in the PCE efficiency test are presented in Table 3. The control group comprised CFB fly ash-blended cement pastes without the addition of PCE, labeled as C-0, C-5, C-10, C-20, and C-30. The experimental group included pastes prepared with PCE, denoted as E-0, E-5, E-10, E-20, and E-30. For both groups, the PCE-to-binder ratio was consistently maintained at 0.2 wt.%. The fluidity of all pastes was adjusted to 180 ± 5 mm by regulating the water content. The PCE efficiency for the various CFB fly ash-blended cement pastes was determined using Equation (3):
(3)
where Es represents the efficiency of PCE, Wc is the water dosage used in the control group, and We represent the water dosage used in the experimental group.The PCE adsorption capacities of both cement and CFB fly ash were evaluated using the total organic carbon (TOC) method. The evaluation process consisted of three main steps, as outlined below:
(1) Preparation of the PCE diluent from deionized water
The PCE diluent was prepared by mixing deionized (DI) water with PCE in varying mass ratios. The diluents were labeled according to the dilution ratio of PCE, with designations such as SP-20, SP-50, SP-80, SP-120, SP-160, and SP-200. For example, SP-20 represents a dilution of PCE by a factor of 20 with DI water. The prepared PCE diluents were then used to investigate the adsorption behavior of PCE on cement particles, enabling an assessment of the PCE adsorption capacities.
(2) Preparation of the PCE diluent from the synthetic cement pore solution
A synthetic solution was prepared to examine the adsorption behavior of PCE on CFB fly ash within a cement pore solution, following the procedure described by Plank et al. [35]. In step 1, the PCE diluent was prepared by mixing the synthetic cement pore solution with PCE at varying mass ratios. These diluents were labeled C-SP-20, C-SP-50, C-SP-80, C-SP-120, C-SP-160, and C-SP-200, with the numbers indicating the respective dilution ratios of PCE in the synthetic cement pore solution.
(3) Preparation of samples for TOC analysis
The samples for the TOC tests were prepared according to the proportions presented in Table 4. Briefly, 6 g of cement or CFB fly ash were combined with 30 mL of the respective PCE diluent and mixed for 5 min. After mixing, approximately 10 mL of the mixture were centrifuged at 5000 rpm, and the resulting mixture was then collected and diluted with a 1 mol/L HCl solution to prepare the samples for TOC analysis.
3. Results and Discussion
3.1. Fresh Properties (Series I)
Figure 6 (series I) illustrates the effect of CFB fly ash dosage on the fluidity of the fresh paste. A nearly linear decrease in the spread diameter was observed as the CFB fly ash content increased, with a correlation coefficient of 0.99971. The observed finding suggests a strong inverse relationship between the amount of fly ash and the paste’s fluidity. At a 20 wt.% ash dosage the fluidity of the paste dropped by 51.4%. When the fly ash content exceeded 20 wt.%, the fresh paste completely lost its fluidity, indicating that higher fly ash proportions enhance the cohesion between paste particles, thereby limiting their ability to flow and spread. The observed effect can be attributed to the higher specific surface area and smaller particle size of CFB fly ash (see Figure 1 and Figure 2) compared to cement. Combined with our previous article [36], it was observed that for cement pastes with fluidity values of 162, 136, 113, and 90 mm, the corresponding water-to-cement ratios were approximately 0.456, 0.413, 0.375, and 0.337, respectively. At the same fluidity levels, the water demand of CFB fly ash-blended cement pastes increased as the CFB fly ash content increased. Specifically, for pastes containing 5 wt.%, 10 wt.%, 15 wt.%, and 20 wt.%, the water demand increased by approximately 9.6%, 21.0%, 33.3%, and 48.3%, respectively. This indicates that the water demand for CFB fly ash was significantly higher than that of cement. Therefore, the higher water absorption of CFB fly ash during mixing decreased the available water content between particles, which increased resistance to particle movement and consequently lowered the fluidity of the cement-based paste [19].
Figure 7 presents the rheological behavior of fresh CFB fly ash-blended paste (series I). Table 5 compiles the rheological parameters derived from Equation (1), including R2, plastic viscosity, and yield stress. The R2 values for all the rheological data exceeded 0.99, indicating a strong correlation with the modified Bingham model. Furthermore, as shown in Figure 7, an increase in ash content led to a proportional rise in shear stress at a constant shear rate, which can be attributed to the enhanced cohesion between the particles. This trend corresponded with the observed decrease in fluidity, providing further evidence of the effect of CFB fly ash content on the rheological behavior of the paste.
Table 5 demonstrates that both the plastic viscosity and the yield stress of the blended cement paste (series I) increased with the incorporation of CFB fly ash. The paste without CFB fly ash demonstrated a yield stress of only 3.3 Pa. However, at a 30 wt.% ash dosage, the yield stress experienced a substantial increase to 503.9 Pa, representing more than a 150-fold increase. Similarly, the plastic viscosity increased from 1.0 to 5.1 Pa·s at a 30 wt.% CFB fly ash dosage, reflecting a four-fold increase.
The examination of fluidity and rheological characteristics of the series I pastes demonstrated that the incorporation of CFB fly ash had an obvious adverse effect on the fresh properties, especially in terms of fluidity. The observed effect was primarily due to the high water demand of CFB fly ash, which adsorbs a significant amount of water within the paste. Thus, the reduced availability of free water limits particle lubrication, leading to decreased fluidity. This reduction increases resistance to particle movement, resulting in lower fluidity as well as higher viscosity and yield stress [19]. Further, the rough and irregular particle morphology of CFB fly ash inevitably exerts a negative influence on the fresh properties of cement-based paste, though it is not the dominant factor [37]. This adverse effect limits the potential use of ash as an SCM. Therefore, enhancing the fresh properties of ash-blended cement paste while maintaining its hardening characteristics necessitates the incorporation of a water-reducing agent along with CFB fly ash.
3.2. Fresh Properties (Series II)
Figure 8 presents the rheological characteristics of the fresh series II pastes, demonstrating a pronounced increase in shear stress with rising shear rate. Furthermore, the data demonstrate a strong correlation with the modified Bingham model (R2 ≥ 0.99). The pastes labeled as FCF-0, FCF-5, FCF-10, FCF-15, FCF-20, FCF-25, and FCF-30 displayed distinct thickening behavior, with φ0 values of 0.0007, 0.0008, 0.0010, 0.0012, 0.0013, 0.0025, and 0.0035, respectively. The observed thickening effect was characteristic of cement-based mixtures with high fluidity and may be attributed to the formation of hydroclusters. Shear forces acting on the material cause hydrodynamic forces to overcome interparticle repulsion, leading to the formation of clusters, as explained by cluster theory [38,39,40,41].
Further analysis was performed using Equation (1) to examine the relationship between plastic viscosity, yield stress, and the CFB fly ash dosage (Figure 9a,b). The results revealed that an increase in ash dosage led to a marked rise in yield stress, highlighting that higher fly ash content contributed to greater resistance to flow in the paste. Despite this, the fluidity of all CFB fly ash-blended pastes remained consistent, adjusted by the addition of PCE. However, the correlation between the ash dosage and yield stress was weak (R2 = 0.9224). Similarly, with an increase in CFB fly ash dosage, a slight rise in the plastic viscosity of the paste was observed. However, the correlation remained weak (R2 = 0.72674), as shown in Figure 9b. The weak correlations between ash dosage and both plastic viscosity and yield stress indicated that the fresh properties of high-fluidity CFB fly ash-blended cement pastes had not been sufficiently optimized. This issue may be associated with the compatibility between the ash and the PCE [42].
Figure 10a,b illustrate the trends in the bleeding rate of blended pastes (series II) as a function of ash and PCE dosages, respectively. Figure 10a revealed that the bleeding rate of the fresh paste increased as the ash dosage rose, with a weak linear correlation observed between the CFB fly ash dosage and the bleeding rate (linear equation: y = −1.104 + 0.47x; R2 = 0.864). Figure 10b reveals a pronounced linear increase in the bleeding rate of the CFB fly ash-blended paste with increasing PCE dosage (linear equation: y = −0.49 + 6.82x; R2 = 0.986). These findings suggest that the higher bleeding rate in CFB fly ash-blended cement pastes may be influenced by the compatibility between CFB fly ash and PCE [42,43].
3.3. Compatibility Between the CFB Fly Ash and PCE
The CFB fly ash-blended cement paste of series II demonstrates that the addition of PCE effectively improved the fluidity and rheological properties. This improvement was attributed to the electrostatic repulsive forces of PCE, which dispersed the cement and CFB fly ash particles, reducing their water demand and lowering resistance to particle movement. Therefore, the resistance to particle movement decreased, leading to a significant improvement in the fluidity of the CFB fly ash-blended cement paste [44].
To evaluate the compatibility between ash and PCE, the effectiveness of PCE in enhancing the properties of cement paste blended with varying dosages of CFB fly ash was assessed (Figure 11), showing that the PCE efficiency in the control paste without CFB fly ash was 52.2%. However, as the ash dosage increased to 5, 10, 20, and 30 wt.%, the PCE efficiency decreased progressively by 13.2%, 26.4%, 40.0%, and 48.1%, respectively. The observed finding suggests that the presence of CFB fly ash significantly reduced the PCE’s effectiveness. A linear analysis further revealed a strong negative correlation between the CFB fly ash dosage and PCE efficiency (linear equation: y = 49.5 − 0.82x; R2 = 0.9863). These results confirm that increasing CFB fly ash dosages significantly decreased PCE efficiency, indicating poor compatibility between CFB fly ash and PCE.
To further assess the compatibility between ash and PCE, the TOC method was used to measure the adsorption capacities of PCE on both ash and cement particles. Figure 12 demonstrates that as the PCE concentration increased from 5 to 50 g/L, the adsorption capacities of PCE on both cement and CFB fly ash particles increased linearly. In particular, the PCE adsorption capacity per unit mass of ash was significantly higher than that of cement. At PCE concentrations of 5, 6.3, 8.3, 12.5, 20, and 50 g/L, the PCE adsorption capacities per unit mass of CFB fly ash were 1.6, 1.8, 1.7, 1.8, 1.4, and 1.6 times greater than that of cement, respectively. The observed finding highlights that CFB fly ash had a far superior ability to adsorb PCE compared to cement particles in the blended paste.
However, since PCE functions as a dispersant upon adsorption onto the particle surface, its dispersion performance is somewhat proportional to its adsorption density [44]. Therefore, when evaluating the compatibility between CFB fly ash/cement and PCE, it is crucial to consider the specific surface area of the solid phase. The ash exhibited a significantly larger BET-specific surface area, approximately 12.6 times that of cement (Figure 1). Considering the differences in the adsorption capacities of PCE on CFB fly ash and cement, the adsorption density of PCE on ash particles is shown in Figure 13. It is evident that the adsorption density of PCE on cement particles was significantly higher than on ash particles. This observation confirms that the adsorption capacity of PCE on ash is much lower than on cement, further supporting the conclusion of poor compatibility between CFB fly ash and PCE.
Based on the results presented, the poor compatibility between ash and PCE can be attributed to the surface adsorption of PCE on ash particles. This behavior may be influenced by factors such as ion concentrations (e.g., S2− and Ca2+), the pH of the pore solutions, the surface zeta potential of ash particles, and the presence of unburned carbon in the CFB fly ash [45,46,47,48,49]. Other studies indicate that unburned carbon, due to its porous structure and large specific surface area, adsorbs more PCE than cement and SCM particles, thereby reducing the water-reducing efficiency of PCE [49]. The incorporation of CFB fly ash as an SCM reduced the Ca2+ concentration and pH of the paste while increasing the S2− ion concentration. The lower pH weakened the dispersibility of PCE, thereby diminishing its water-reducing efficiency [46]. Simultaneously, Ca2+ facilitated PCE adsorption by forming calcium “bridges” on mineral phases with weak affinity, as it neutralized negatively charged surfaces and provided additional adsorption sites for the anionic PCE molecules. However, S2− ions competitively adsorbed on positively charged surfaces, further inhibiting PCE adsorption [46,47]. Theoretically, CFB fly ash altered the ion concentration (e.g., S2− and Ca2+) and pH in the paste, which synergistically modulated PCE adsorption behavior and water-reducing efficiency. This, in turn, compromised the fresh properties of the cement-based paste. These factors will be further investigated in future research. Further, the strong positive linear correlation observed between the bleeding rate of the blended cement paste and ash dosage can be attributed to the adsorption kinetics, which will also be explored in subsequent studies [48].
Based on the above analysis, it is evident that further exploration is necessary to effectively use SLC paste in practical engineering applications, such as self-leveling ground and foundation pit backfilling. Moreover, beyond PCE, the compatibility of other types of water-reducing agents with CFB fly ash also requires further investigation. Meanwhile, future studies should focus on the mechanical properties, durability, and wear resistance of self-leveling cement-based materials.
4. Conclusions
This study innovatively utilized CFB fly ash as an SCM in the preparation of SLC paste. This approach not only helps to consume stockpiled CFB fly ash, reducing the environmental pollution caused by its accumulation, but also decreases the amount of cement required for producing SLC paste, providing significant environmental benefits. The research addresses the gaps in understanding the effects of CFB fly ash on the fresh properties of SLC paste and its compatibility with PCE. The findings provide valuable theoretical insights to support the application of CFB fly ash in SLC paste formulations. The main findings are summarized as follows:
(1) Increasing the CFB fly ash dosage significantly decreased fluidity while increasing the yield stress and plastic viscosity of the cement-based paste.
(2) The incorporation of PCE effectively mitigated the negative effects of ash on the fluidity of the blended cement paste. However, as the CFB fly ash dosage increased, higher PCE dosages were required to maintain similar fluidity levels, which consequently led to an increase in the paste’s bleeding rate.
(3) The adsorption capacity of PCE on ash was lower than that on cement particles. Furthermore, an increase in the ash dosage led to a decrease in PCE efficiency.
C.-R.W.: writing—original draft, data curation. W.T.: writing—review and editing. Y.-L.H.: writing—review and editing, B.-J.Z.: writing—review and editing, S.-C.K.: writing—review and editing, supervision, project administration. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.
The authors report that there are no competing interests to declare.
Footnotes
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Enlarge this image.Figure 3. Micro-morphologies of the CFB fly ash at 400× (a) and 6000× (b) magnifications.
Enlarge this image.Figure 5. (a) Configuration of the pressure bleeding apparatus, (b) working schematic.
Enlarge this image.Figure 8. Rheological properties of fresh CFB fly ash-blended pastes (series II).
Enlarge this image.Figure 9. Yield stress (a) and plastic viscosity (b) of the fresh CFB fly ash-blended cement pastes (series II).
Enlarge this image.Figure 10. Bleeding of the fresh CFB fly ash-blended cement paste (series II) with increasing CFB fly ash (a) and PCE dosages (b).
Enlarge this image.Figure 11. Efficiency of PCE in the blended paste with different CFB fly ash dosages.
Enlarge this image.Figure 12. Adsorption capacity of PCE on a unit mass of the CFB fly ash and cement.
Enlarge this image.Figure 13. Adsorption density of PCE on a surface area unit of the CFB fly ash and cement.
Composition of cement and CFB fly ash (wt.%).
| SiO2 | Al2O3 | CaO | MgO | Fe2O3 | SO3 | Na2O | K2O | Others | LOI | |
|---|---|---|---|---|---|---|---|---|---|---|
| Cement | 19.52 | 4.78 | 63.40 | 2.70 | 3.64 | 1.87 | 0.32 | 0.92 | 1.24 | 1.62 |
| CFB fly ash | 56.12 | 23.91 | 2.94 | 1.83 | 8.40 | 2.58 | 1.38 | 1.63 | 1.21 | 4.40 |
Proportions of the CFB fly ash-blended cement paste.
| Name | Cement (wt.%) | CFB Fly Ash (wt.%) | Water-to-Binder Ratio | PCE to the Binder (wt.%) | |
|---|---|---|---|---|---|
| Series I | CF-0 | 100 | 0 | 0.5 | - |
| CF-5 | 95 | 5 | |||
| CF-10 | 90 | 10 | |||
| CF-15 | 85 | 15 | |||
| CF-20 | 80 | 20 | |||
| CF-25 | 75 | 25 | |||
| CF-30 | 70 | 30 | |||
| Series II | FCF-0 | 100 | 0 | 0.5 | 0.15 |
| FCF-5 | 95 | 5 | 0.23 | ||
| FCF-10 | 90 | 10 | 0.44 | ||
| FCF-15 | 85 | 15 | 0.77 | ||
| FCF-20 | 80 | 20 | 1.21 | ||
| FCF-25 | 75 | 250 | 1.63 | ||
| FCF-30 | 70 | 30 | 2.19 |
Component of cement pastes prepared for the PCE efficiency tests.
| Name | Cement (wt.%) | CFB Fly Ash (wt.%) | Water-to-Binder Ratio | PCE-to-the-Binder (wt.%) | Fluidity (mm) |
|---|---|---|---|---|---|
| C-0 | 100 | 0 | 0.49 | 0 | 180 |
| E-0 | 0.23 | 0.2 | 176 | ||
| C-5 | 95 | 5 | 0.53 | 0 | 178 |
| E-5 | 0.29 | 0.2 | 181 | ||
| C-10 | 90 | 10 | 0.58 | 0 | 181 |
| E-10 | 0.36 | 0.2 | 178 | ||
| C-20 | 80 | 20 | 0.67 | 0 | 183 |
| E-20 | 0.46 | 0.2 | 179 | ||
| C-30 | 70 | 30 | 0.76 | 0 | 180 |
| E-30 | 0.55 | 0.2 | 182 |
Proportions of samples for the TOC tests.
| Number | Cement | CFB Fly Ash | PCE Diluent | PCE Diluent Type | Notes |
|---|---|---|---|---|---|
| 1 | 6 g | 0 | 30 mL | SP-20 | Experimental group for evaluating the adsorption capacity of PCE on cement particles |
| 2 | SP-50 | ||||
| 3 | SP-80 | ||||
| 4 | SP-120 | ||||
| 5 | SP-160 | ||||
| 6 | SP-200 | ||||
| 7 | 0 | 6 g | 30 mL | C-SP-20 | Experimental group for assessing the adsorption capacity of PCE on CFB fly ash particles |
| 8 | C-SP-50 | ||||
| 9 | C-SP-80 | ||||
| 10 | C-SP-120 | ||||
| 11 | C-SP-160 | ||||
| 12 | C-SP-200 | ||||
| C-1 | 6 g | 0 | 30 mL | Deionized water | Control group for evaluating the organic carbon content in the cement |
| C-2 | 0 | 6 g | Synthetic cement pore solution | Control group for evaluating the organic carbon content in the CFB fly ash | |
| C-3 | 0 | 0 | SP-50 | Control group for analyzing the organic carbon content in PCE | |
| C-4 | 0 | 0 | SP-160 |
Rheological parameters (series I) obtained using the modified Bingham equation.
| Parameters | CF-0 | CF-5 | CF-10 | CF-15 | CF-20 | CF-25 | CF-30 |
|---|---|---|---|---|---|---|---|
| Correlation coefficient (R2) | 0.99735 | 0.99622 | 0.99698 | 0.99524 | 0.99627 | 0.99627 | 0.99456 |
| Yield stress (Pa) | 3.3 | 26.2 | 75.2 | 154.8 | 243.5 | 356.5 | 503.9 |
| Plastic viscosity (Pa·s) | 1.0 | 1.3 | 1.9 | 2.7 | 3.3 | 4.4 | 5.1 |
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Details
; Tang, Wei 2 ; Yan-Lin, Huo 3 ; Bao-Jian Zhan 2 ; Shi-Cong Kou 1
1 School of Civil Engineering and Transportation, Guangzhou University, Guangzhou 510006, China;
2 Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China;
3 School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China;