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Article

Investigation of Low-Calcium Circulating Fluidized Bed Fly Ash on the Mechanical Strength and Microstructure of Cement-Based Material

by
Wenyan Zhang
1,
Shuai Wang
1,
Liya Zhao
1,
Junsheng Ran
2,
Wenjing Kang
1,
Chunhua Feng
1 and
Jianping Zhu
1,*
1
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China
2
Architectural Science Research Institute Ltd., Zhengzhou 450053, China
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(3), 400; https://doi.org/10.3390/cryst12030400
Submission received: 23 February 2022 / Revised: 5 March 2022 / Accepted: 11 March 2022 / Published: 16 March 2022
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Abstract

:
This present study mainly focuses on the influence of low-calcium circulating fluidized bed fly ash (LCFA) on the mechanical property and microstructure of cement-based materials under different curing conditions. The mechanical properties test was conducted by changing variable parameters, such as LCFA content, the internal mixing ratio of LCFA and fly ash (FA), and dry and water curing conditions. Further, the hydration products and pore structure were analyzed using XRD, FT-IR, TG-DTG, NI, SEM, and BET micro-testing technology. The strength development law of LCFA on cement-based materials is discussed. The research results show that LCFA has a certain degree of self-hardening and can be used as a cementitious material in cement-based materials. Still, the loose and porous microstructure of LCFA leads to higher water requirements, which reduces the fluidity of cement-based pastes. Water curing is favorable for promoting the development of LCFA on the long-term compressive strength of cement-based materials. When the LCFA was added to the cement, the optimal substitution ratio was 20%, and the compressive strength at 91 days reached 101 MPa. In the case of compounding LCFA and FA, when the internal mixing ratio of LCFA/FA was 3 and the total content was 20%, the mechanical properties were the highest, and the compressive strength at 91 days was 92 MPa. The microscopic analysis result shows that the cumulative hydration heat of the samples decreased significantly with the increase of dosage of LCFA. The main hydration products of cement-based materials mixed with LCFA were AFt, C-S-H gel, and Ca(OH)2. AFt and C-S-H gels are critical to the strength development of OPC-LCFA samples. The active Al2O3 and active SiO2 in LCFA were involved in hydration reactions to promote the formation of C-A-H and C-S-H gel and effectively promote the development of the mechanical properties. Overdosages of LCFA would reduce the ettringite formation rate. FA is not conducive to AFt formation in the hydration process of OPC-FA samples.

1. Introduction

Cement has always been the primary raw material in the construction industry [1,2]. According to the statistics of consumables in China in March 2021, cement consumption was 19.7 million tons, up 33.1% from the previous year, thereby suggesting the huge social demand. At the same time, cement prices are gradually increasing. Secondly, cement production emits pollutants, such as dust, sulfur dioxide, and nitrogen oxides. From the point of view of economic savings and environmental protection, it has become a hot research topic to replace cement with industrial solid waste and study its influence on cement-based materials [3]. Presently, the application of pulverized coal combustion fly ash (FA) has been relatively mature and has been applied in practical engineering. However, many kinds of industrial solid wastes are still to be studied. The circulating fluidized bed fly ash is an industrial solid waste produced by a circulating fluidized bed combustion boiler. After gas-solid separation combustion, it is in urgent need of resource utilization. Recently, circulating fluidized bed combustion boilers have been recognized and favored by many power plants for their wide fuel adaptability, high combustion efficiency, and low combustion temperature. Circulating fluidized bed combustion boilers and pulverized coal boilers burn a ton of different amounts of coal-producing solid waste, the former about 30~40% more [4,5], resulting in circulating fluidized bed fly ash reserves, which increases year by year. Thus, the interest of several researchers is directed towards circulating fluidized bed fly ash resource utilization. The combustion temperature of the circulating fluidized bed combustion boiler is between 850 °C and 900 °C, which is lower than the pulverized coal boiler. As a result, the composition and properties of circulating fluidized bed fly ash are different from FA [6], indicating that circulating fluidized bed fly ash cannot be reused like FA. Although the circulating fluidized bed combustion boiler combustion desulfurization method dramatically reduces the furnace combustion caused by environmental pollution, the demand for fuel types is not high [7,8], leading to the discharge of industrial solid waste recycling. However, few power plants using the circulating fluidized bed combustion boiler desulfurization process operate differently. These conditions increase the difficulty of adequate and reasonable utilization of circulating fluidized bed fly ash. In different places, the performance of different circulating fluidized bed coal fly ash can produce more considerable differences [9], suggesting that the research findings of circulating fluidized bed fly ash cannot be directly applied to engineering. Several studies show the different characteristics between circulating fluidized bed fly ash and FA: the water requirement of circulating fluidized bed fly ash (about 50%) is much higher than FA [9,10,11]. Circulating fluidized bed fly ash has loose and porous microscopic morphology, and its shape is changeable, which is entirely different from FA [6,10,12,13,14]. The pozzolanic activity of circulating fluidized bed fly ash is different compared to FA, which is also prominent in the early stage of circulating fluidized bed fly ash [15,16,17].
The circulating fluidized bed fly ash has a more useable value than FA based on the above research. Thus, some scholars have studied circulating fluidized bed fly ash modification. The results show that mechanical grinding can change the particle size, pH value, and water demand of circulating fluidized bed fly ash and further improve the dispersion and uniformity of particles [6,14]. LiOH and Ca(OH)2 can also be used as alkali activators for circulating fluidized bed fly ash [18,19]. Presently, the mechanical properties of cementing materials with high-calcium circulating fluidized bed fly ash (CaO content higher than 10 wt%) have been studied [9,20,21,22,23]. However, few studies have reported the long-term mechanical properties of circulating fluidized bed fly ash that replaces ordinary Portland cement. Consequently, the study of low-calcium circulating fluidized bed fly ash (LCFA) is significant. It was thought worthwhile to further study the long-term properties of cement-based materials with LCFA.
Therefore, the present study focuses on exploring the influence of LCFA on the mechanical strength and microstructure of cement-based material. The effect of the dosage of LCFA and curing conditions on the long-term mechanical strength of cement-based material for 91 days is discussed. In addition, the mechanical properties of cement-based materials with a different mixing ratio of LCFA and FA are also studied. To further examine the development law of LCFA affecting the cementitious materials, the hydration products and hydration process of cement-based materials were analyzed. A scanning electron microscope technique, nanoindentation, and nitrogen isotherm adsorption analysis were used for the microstructure analysis tests.

2. Materials and Methods

The low-calcium circulating fluidized bed fly ash (LCFA) used in this experiment was obtained from the Ruiping Power Plant in Henan Province. The median particle size was 20.4 µm, and the specific surface area was 1248 m2/kg. The cement used in this research was P.O42.5 ordinary Portland cement (OPC) of Qianye Cement Co. in Henan Province. The pulverized coal combustion fly ash (FA) was sourced from the Yangcheng Power Plant in Shanxi Province. The median particle size was 20.7 µm, and the specific surface area was 1039 m2/kg. The chemical compositions of LCFA, OPC, and FA were tested using X-ray fluorescence spectroscopy (XRF). The main chemical compositions are shown in Table 1. The probability distribution and cumulative particle size distribution of LCFA and FA are shown in Figure 1. The polycarboxylate superplasticizer (J) was provided by Sanrui Polymer Materials Co. in Shanghai Province. The water-reducing effect of the polycarboxylate superplasticizer was diluted to about 20% by adding water.

2.1. Experimental Proportion and Sample Preparation

The experimental program was designed in the following three series. In series 1, the LCFA effect as binder material on the mechanical properties of cement-based materials was investigated by changing the dosage of LCFA. In series 2, the effect of FA as binder material on the mechanical properties of cement-based materials was investigated by adjusting the amount of FA, which was compared with LCFA. In series 3, when the total content of LCFA and FA was 20% and 40%, the mechanical properties of cement-based materials with different internal content (LCFA:FA = 3:1, 1:1, and 1:3) were studied. The fluidity of the experimental samples was controlled between 180 ± 5 mm (except for O-L-10). The practical mix proportion and the fluidity results are shown in Table 2. The curing condition was set to water curing and dry curing with 60% R.H. at 20 °C.

2.2. Experimental Method

The samples were prepared using square steel molds of 40 mm × 40 mm × 40 mm, which were mainly used to test the compressive strength at different ages. This was according to GB/T 1346–2011 “Test methods for water requirement of normal consistency, setting time, and soundness of the Portland cement” [24]. The slurry was stirred, injected into the prepared mold, and then vibrated for 60 s. After scraping, the slurry was kept in the standard curing room for 24 h. Next, after demudding for 1 day, the samples were placed at a constant temperature in the curing (dry) room at 20 ± 2 °C, with humidity of 60 ± 5% and the water curing (water) box at 20 ± 2 °C for 3 days, 7 days, 28 days, and 56 days, respectively. The water curing samples were selected for the relevant tests. Some randomly chosen samples were dried in a vacuum furnace at 45 °C for 24 h and ground into powder using a three-head grinder.

2.3. Methods of Analysis

(1)
Workability property
The fluidity of the slurry was tested using a truncated cone circular mold. After measuring the diameters perpendicular to each other, the average value of the two diameters was taken as the fluidity of the sample. The water requirement of normal consistency was tested as per the GB/T 1346–2011 “Test methods for water requirement of normal consistency, setting time, and soundness of the Portland cement” [24].
(2)
Compressive strength
The compressive strength at different ages was tested following the GB/T 17671–1999 “Method of testing cements-Determination of strength (ISO)” [25].
(3)
Micro measurement and analysis methods.
In this experiment, the TAM air thermal activity tracker was used to measure the heat flow and cumulative heat during slurry hydration. The hydrated samples (3 days, 7 days, 28 days, and 91 days) were ground and heated to a constant mass in a drying oven at a constant temperature of 105 °C. An analytical balance with accuracy of 0.0001 g was used to accurately weigh 1 g of powder sample. The sample was burned in a furnace at 1000 °C for 3 h and the quality difference of the sample before and after burning was the content of non-evaporative water. The X-ray diffraction (XRD) instrument was based on the SmartLab equipment developed by RIKEN, Japan. The diffractograms of the powder samples were tested from 5° to 75° at the rate of 10°/min. Fourier transform infrared (FT-IR) spectra were tested using a Bucks HP9 2FX Fourier transform infrared spectrometer. The Thermogravimetric (TG)-Differential Scanning Calorimeter was based on the STA8000 model developed by PerkinElmer, USA. The temperature was increased from 50 °C to 1000 °C in nitrogen at the rate of 10 °C/min. Nanoindentation (NI) is a method to characterize the micromechanical properties of samples. In this experiment, the micromechanical properties of the samples were tested by the Hystron TI-Premier type nanoindentation technique. The microscopic results were obtained using scanning electron microscopy (SEM) of the Merlin Compact type developed by Carl Zeiss NTS GmbH, Germany. The pore structure of the powder samples was obtained using adsorption of nitrogen (BET). Pore diameter distribution and cumulative pores of the samples were tested using a Tristar 3020 physisorption instrument.

3. Results and Discussion

3.1. Basic Characteristics

As seen from Table 1, there was an insignificant difference between the chemical composition of LCFA and FA. The CaO and SO3 content in LCFA was lower than the circulating fluidized bed fly ash used in other studies [15,23,26]. The loss on ignition of LCFA was a lot higher than OPC and FA. The primary reason is the low-temperature combustion of the circulating fluidized bed combustion boiler that fails to thoroughly burn the carbon in the fuel. Figure 2 shows the mineral composition of LCFA and FA. As seen from Figure 2, quartz, hematite, anhydrite, and calcite were the main mineral components of LCFA, and the main mineral composition of FA was mullite, quartz, and hematite. Figure 3 shows the microscopic morphology of LCFA and FA, respectively. From Figure 3, it can be seen that the surface of LCFA was rough and uneven, with loose and porous microstructure, similar to CFB fly ash used in other studies [6]. Figure 3 shows that the surface of FA was smooth and uniformly spherical with dense microstructures. This indicates that LCFA and FA differed in mineral composition and micromorphology. From Table 2, it can be seen that to adjust the fluidity to reach the target value of 180 ± 5 mm, the amount of the polycarboxylate superplasticizer increases with the increase of LCFA. To achieve fluidity, a cement-based slurry (O-L-4) containing 40% LCFA requires more polycarboxylate superplasticizers (1.43 wt%). However, the amount of polycarboxylate superplasticizer did not change considerably with the increase of FA. The water requirement of normal consistency is a method to characterize the water requirement of cement-based materials. Figure 4 presents the water requirement of normal consistency of different admixtures of LCFA. It shows that the water requirement of normal consistency increased with the increase of the LCFA admixture. Figure 4 further indicated that the water requirement of normal consistency presents a proportional relationship with the mixing amount of LCFA. When the LCFA admixture was 40%, the water requirement of normal consistency was 38.6%, which was 10.8% higher than the sample without LCFA. The results indicate that the water requirement for LCFA was significant, which was consistent with the results of other studies [9,10,11]. Figure 5 shows the self-hardening compressive strength of LCFA. The results show that LCFA does not have compressive strength at 3 days of hydration. The compressive strength increases with curing age, but the growth rate was low, indicating that LCFA had self-hardening properties.

3.2. Hydration Heat

The effects of LCFA dosage on hydration heat flow and the cumulative hydration heat of the OPC-LCFA sample after 70 h hydration were studied. The results are shown in Figure 6. Yan Peiyu et al. [27] divided the hydration process of cement-based materials into five stages: pre-induction (I), induction (II), reaction acceleration (III), reaction deceleration (IV), and stabilization (V). The amount of LCFA does not affect the number of exothermic peaks of cement-based materials. The exothermic peaks of the first stage occur rapidly within a few minutes. It was also found that increasing the dosage of LCFA progressively delayed the induction period (stage II). According to the Krstulovic–Dabic model, it is known that the induction period was a process in which active ions such as [SiO4]4−, [AlO4]5−, Ca2+, and Mg2+ in the pore solution of the sample gradually accumulated and reached the critical concentration. This indicated that the length of the induction period was related to the pH value of the pore solution. Therefore, the addition of LCFA would reduce the pH value of pore solution in cement-based materials, mainly due to the total amount of OPC, which decreased with the increase of LCFA. The acceleration and deceleration of hydration formed the second exothermic peak of hydration. The results show that the time and intensity of the second hydration exothermic peak extended and decreased with the increase of LCFA. When the dosage of LCFA was 20% and 40%, the second exothermic peak of hydration was prolonged by 8.57 h and 15.74 h, respectively, compared to the control group without LCFA. Further, compared with the control group without LCFA, the intensity of the second exothermic peak decreased by 0.59 mW/g and 1.13 mW/g, respectively. The key reason for the decrease of the second exothermic peak with the increase of LCFA content was the rise of LCFA, which reduced the pH value in the pore solution of the sample, resulting in the decrease of the dissolution rate of OPC and the formation of hydration products.
Figure 6 shows the cumulative heat of hydration of LCFA with different dosages. The results showed that the cumulative hydration heat of the samples decreased significantly with the increase of LCFA dosages. The cumulative hydration heat of the control sample was 75.08 J/g at 70 h hydration which was 22.5% and 68.1% higher than the sample mixed with 20% and 40% LCFA, respectively. This indicated the necessity to maintain a degree of alkaline pH in pore solutions.

3.3. Compressive Strength

3.3.1. Binary Compressive Strength

Figure 7 shows the influence of curing conditions and LCFA content on the compressive strength of OPC-LCFA samples. In general, the compressive strength of the samples increased with increasing cured age. Figure 7a,b, respectively, show the effects of dry and water curing on the compressive strength of OPC-LCFA samples. Figure 7a shows that the compressive strength of the samples decreased with the increase of LCFA admixture. From Figure 7b, it can be seen that water curing was more favorable to the strength development of the samples. The strength of the samples showed a trend of increasing and then decreasing with the increase of LCFA admixture. After 91 days of water curing, the 20% LCFA admixture sample obtained the highest compressive strength of 101 MPa. The compressive strength of the sample containing LCFA (O-L-3) was only 3.6 MPa lower than the control sample. Therefore, the optimal dosage of LCFA was 20%.
Figure 8 shows the influence of curing conditions and FA content on the compressive strength of OPC-FA samples. Figure 8a,b show the effects of dry and water curing on the compressive strength of OPC-FA samples, respectively. Overall, the compressive strength of OPC-LCFA samples was superior to that of OPC-FA samples. After 91 days of curing in water, the sample mixed with LCFA (O-L-1) was 1 MPa lower than the corresponding OPC-FA sample, and the sample mixed with LCFA (O-L-2) was 15 MPa higher than the corresponding OPC-FA sample. The results show that LCFA was more favorable than FA to replace OPC. It increased the dosage and evidently improved the strength of the sample.

3.3.2. Compressive Strength

Figure 9 shows the influence of curing conditions and content on the compressive strength of OPC-LCFA-FA (20% LCFA + FA) samples. In general, the compressive strength of samples decreased first and then increased with the increase of LCFA content. Figure 9a,b show the effects of dry and water curing on the compressive strength of OPC-LCFA-FA samples, respectively. Further, water curing promotes the strength development of OPC-LCFA-FA samples. As can be seen from Figure 9, after 91 days of water curing, the strength of the sample (O8-LF-31) increased by 9.6 MPa compared with the sample (O8-LF-13), and the compressive strength of the sample (O8-LF-31) at 91 days was 92 MPa. Figure 10 shows the effect of water curing conditions and content on the compressive strength of OPC-LCFA-FA (40% LCFA + FA) samples. Figure 10a,b show the effects of dry and water curing on the compressive strength of OPC-LCFA-FA samples, respectively. The variation trend of sample strength with LCFA content was the same as that of the OPC-LCFA-FA (20% LCFA + FA) sample. The strength of the OPC-LCFA sample was higher than the corresponding OPC-LCFA-FA sample. Table 2 shows the total amount of polycarboxylate superplasticizer used in OPC-LCFA-FA samples, which decreased with the increase of FA/LCFA content. The results show that FA can improve the fluidity of OPC-LCFA but reduce the compressive strength of cement-based materials. Optimal program for OPC-LCFA-FA: 20% compounding at LCFA: FA = 3.
To sum up, the strength of the OPC-LCFA sample was higher than OPC-FA. The large specific surface area of LCFA allowed the absorption of water to form a water film. The loose and porous structure provides space for free water that makes further hydration of OPC conducive. Therefore, the results suggest that LCFA may have a water-carrying effect.

3.4. Non-Evaporated Water Content

The non-evaporated water contents of OPC-LCFA and OPC-FA samples are shown in Figure 11. The non-evaporative water content of the hardened slurry measured the hydration degree of the sample [28]. Figure 11 shows the growth rate of the non-evaporative water content of cement-based materials, which gradually slowed down with the increase of cured age. This decreasing trend is due to the hydration of cement, which occurs mainly in the early stages. For the OPC-FA sample, the non-evaporative water content decreased with the increase of FA content, which was significantly lower than the control sample. For the OPC-LCFA sample, when the dosage was 20%, the difference between the curve of non-evaporative water content and the curve of the control sample was very small and even 2.8% higher than the control sample for 28 days hydration. When the LCFA content was 40%, the non-evaporative water content curve was significantly different from the control sample; however, it was still higher than the OPC-FA sample. The results of non-evaporative water content are consistent with the compressive strength of samples. The results showed that the hydration product formation rate of the OPC-LCFA sample was higher than the OPC-FA sample.

3.5. Microscopic Analysis Results

3.5.1. XRD Analysis

Figure 12 shows the XRD pattern of the OPC-LCFA sample at 28 days. As can be seen from Figure 12, the main hydration products of the OPC-LCFA sample were Ca(OH)2, CaSO4·2H2O, ettringite (AFt), unhydrated C3S, C2S, and quartz. The main hydration products of OPC-LCFA were almost similar to the control sample, indicating that the addition of LCFA did not change the mineral phase of cement-based materials. The diffraction peak intensity of quartz increases with the increase of LCFA, which is due to the increase of LCFA content. The diffraction peak intensity of AFt and CaSO4·2H2O decreases with the increase of LCFA content.

3.5.2. FT-IR Analysis

The study [29] showed that the FT-IR characteristic spectra of ettringite consisted of OH− vibrational band (3635 cm−1), crystalline water vibrational band (3420 cm−1, 1640 cm−1, 1625 cm−1), sulfate vibrational band (1120 cm−1, 620 cm−1, 420 cm−1), and Al-O vibrational band (870 cm−1, 550 cm−1). The FT-IR curves of OPC-LCFA samples and OPC-FA samples at 28 days hydration are shown in Figure 13. The FT-IR curves of the control samples clearly show the OH− vibrational band (3644 cm−1), the crystalline water vibrational band (3380 cm−1, 1648 cm−1), the sulfate vibrational band (1099 cm−1, 420 cm−1), and the Al-O vibrational band (874 cm−1). The sulfate vibration band (1099 cm−1) does not appear at ① in Figure 13, indicating that the sulfate vibration band (1099 cm−1) gradually disappeared with the increase of LCFA admixture. Figure 13 shows that the addition of FA causes the disappearance of the crystalline water vibrational band (3380 cm−1, 1648 cm−1) and the sulfate vibrational band (1099 cm−1) of the cement-based material. The formation process of ettringite was related to the variation of water molecule vibration band at 3420 cm−1 and SO42− vibration band at 1120 cm−1, where SO42− essentially determines the stability of ettringite [30]. The results corresponded with the results of compressive strength of the samples, indicating that ettringite formation rate determines the compressive strength development of OPC-LCFA samples. At the same time, overdosages of LCFA would reduce ettringite formation rate. FA is not conducive to AFt formation in the hydration process of OPC-FA samples.

3.5.3. TG-DTG Analysis

Figure 14 shows the TG and DTG test results of the OPC-LCFA and OPC-FA sample at 28 days of hydration. Amorphous phases play an important role in cement-based materials, especially C-S-H gels [18]. Therefore, a thermal gravimetric technique analysis is used to access the C-S-H gel fraction. Previous studies have shown that the DTG curves have peaks at around 150 °C for the dehydration decomposition of AFt and C-S-H [18], another peak at about 450 °C for the dehydration decomposition of Ca(OH)2 [18], and a peak ranging from 600 to 800 °C was observed for the decarbonation of CaCO3 [31]. As seen from the first decomposition peak in Figure 14, the decomposition peak of the sample (O-L-2) was similar to that of the control sample (O-L-0). C-S-H crystallinity was poor, so its diffraction peak was not detected in Figure 12 [32]. The increase of LCFA decreased the Ca(OH)2 content, indicating that the addition of LCFA would reduce the mineral phase of Ca(OH)2 in the sample. The decomposition peak of CaCO3 may be due to the carbonization of the sample. Compared with the control sample, the AFt and C-S-H gel decomposition peaks of OPC-FA samples decreased significantly. Figure 7b and Figure 8b show that the compressive strength of OPC-LCFA samples was higher than that of OPC-FA samples. The results showed that more AFt and C-S-H gels were formed in the hardened OPC-LCFA slurry. Thus, AFt and C-S-H gels determined the development of the compressive strength of cement-based materials. This suggests that the synergistic effect of OPC and LCFA was superior to OPC and FA.

3.5.4. Nanoindentation Analysis

Figure 15 shows the results of measuring the microscopic elastic modulus of the sample using the nanoindentation technique. The elastic modulus of different mineral phases was different. The elastic moduli of the main mineral phases of cement hydration were determined using Constantinides [33]. Values were as follows: capillary pores (0~13 GPa), low-density (LD) C-S-H (13~22 GPa), high-density (HD) C-S-H (22~33 GPa), Ca(OH)2 (33~50 GPa), and the unhydrated fraction (>50 GPa). Figure 15a,b show the elastic modulus distribution for the control sample and OPC-LCFA (O-L-2) sample, respectively. It was observed that the figure contained the above four major stages, indicating that LCFA did not affect the type of hydration products of cement-based materials. The sample frequency distribution and Gaussian fitting curves are shown in Figure 16a,b. For the fitting results in Figure 16, the most probable elastic modulus of the samples was determined. The most probable elastic modulus of the samples is shown in Figure 17. In water curing conditions, the most probable elastic modulus of the OPC-LCFA sample (O-L-2) was lower, and the most probable elastic modulus of LD C-S-H and HD C-S-H was higher than the control sample. The results indicated that appropriate LCFA was beneficial for improving the pore structure and compressive strength of cement-based materials by water curing.

3.5.5. SEM Analysis

Figure 18 shows the SEM results of the hardened slurry of OPC-LCFA at 28 days of hydration. The three SEM images in Figure 18 are taken from the same sample. The microscopic morphology of straight AFt, fibrous C-S-H gel, and hexagonal sheet Ca(OH)2 was observed. To further determine the effect of LCFA and FA on C-S-H gel formation in cement-based materials, the energy-dispersive spectrum (EDS) of the OPC-LCFA (O-L-2) sample and OPC-FA (O-F-2) sample was tested, and the results are shown in Figure 19. Figure 19a,b show EDS results of O-L-2 and O-F-2 samples, respectively. Figure 19c shows the contents of Ca, Si and Al in the EDS results of O-L-2 and O-F-2 samples. The figure shows that the Ca/Si and Ca/Al ratios on the hydrated surface of LCFA were 2.057 and 4.192, respectively. However, the Ca/Si and Ca/Al ratios on the hydrated surface of FA were 1.038 and 1.464, respectively. Therefore, C-S-H gel and C-A-H gel produced by OPC-LCFA samples were higher than those produced by OPC-FA samples. The results showed that the pozzolanic activity of LCFA was superior to that of FA.

3.5.6. BET Analysis

The pore diameter distribution affects the mechanical properties of the cement-based sample [34]. The pore diameter distribution of OPC-LCFA samples at 28 days of hydration is shown in Figure 20. As seen from Figure 20, with the increase of LCFA content, the pores within 10 nm in the sample gradually decreased (except O-L-4), indicating that an appropriate amount of LCFA can effectively reduce gel pores and tiny capillary pores in hardened slurry. This may be attributed to the formation of hydrogen in the following hydration reactions:
CaO + H2O => Ca(OH)2, and
2Al + 3Ca(OH)2+ 6H2O => 3CaO·Al2O3·6H2O + 3H2
The total amount of OPC decreased with the increase of LCFA, which led to a gradual decrease in the occurrence of reaction (1), thereby affecting the formation of hydrogen in reaction (2). The study shows that the pores of cement composite can be divided into harmless pores (<50 nm) and harmful pores (>50 nm) [35]. When the pore diameter was between 50 nm and 200 nm, the increase of LCFA first decreased the porosity and then increased it, and the porosity of the OPC-LCFA sample (O-L-2) with 20% LCFA was the lowest. Therefore, under the condition of water curing, the addition of LCFA improves the porosity of cement-based materials. Thus, the optimal LCFA content was 20%.

4. Conclusions

In this paper, the influence of low-calcium circulating fluidized bed fly ash (LCFA) on mechanical properties of cement-based materials under different curing conditions was studied, and the influence of LCFA and FA on cement-based materials was further compared. The effects of LCFA on the microstructure of cement-based materials were investigated by XRD, FT-IR, TG-DTG, SEM, and BET. The results are summarized below.
  • The main mineral components of LCFA were quartz, hematite, anhydrite, and calcite. LCFA had certain self-hardening properties, but the high water requirement of LCFA was not favorable for developing the fluidity of cement-based materials. Therefore, attention is needed for selecting LCFA content in cement engineering applications. The addition of LCFA decreased the cumulative hydration heat of cement-based materials.
  • The compressive strength of OPC-LCFA samples was better than OPC-FA samples, and the water curing was more favorable to the strength development of OPC-LCFA samples. The optimal dosage of LCFA as a binder to replace OPC was 20%, and the compressive strength of the sample reached 101 MPa after 91 days of water curing. The optimal mixing ratio of LCFA-FA was LCFA: FA = 3:1, and the total amount of LCFA and FA was 20%.
  • The microscopic test results show that the main hydration products of OPC-LCFA samples were AFt, C-S-H gel, and Ca(OH)2. Further, AFt and C-S-H gels determined the strength development of OPC-LCFA samples. The formation rate of AFt and C-S-H gel mineral phases in the OPC-LCFA sample was higher than in the OPC-FA sample. The pozzolanic activity of LCFA was better than FA, which is essential for the resource utilization of LCFA. The appropriate amount of LCFA is beneficial to reduce the porosity and improve the pore structure of cement-based materials.
  • Compared with the OPC-FA sample, the OPC-LCFA sample had a faster hydration product formation rate, more compact pore structure, and higher compressive strength.
  • LCFA has potential application value as a binder, and its mechanical properties can meet engineering needs. To promote the resource utilization of LCFA, it is critical to further investigate the durability of OPC-LCFA.

Author Contributions

Data curation, W.Z., S.W. and J.Z.; investigation, L.Z. and S.W.; supervision, J.Z.; writing—original draft, W.Z., S.W., J.R. and C.F.; writing—review and editing, W.K. and S.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (51908198), Key Public Welfare Special Project of Henan Province (201300311000), China Postdoctoral Science Foundation (2018M632774), Henan Province Housing Urban and Rural Construction Plan (HNJS-2020-K26), Postdoctoral Program of Henan Province (001802025), and Henan Outstanding Foreign Scientists’ Workroom (GZS2021003).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (51908198), Key Public Welfare Special Project of Henan Province (201300311000), China Postdoctoral Science Foundation (2018M632774), Henan Province Housing Urban and Rural Construction Plan (HNJS-2020-K26), and Henan Outstanding Foreign Scientists’ Workroom (GZS2021003).

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 00400 g001 550
Figure 1. Probability distribution and cumulative particle size distribution of LCFA and FA.
Figure 1. Probability distribution and cumulative particle size distribution of LCFA and FA.
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Crystals 12 00400 g002 550
Figure 2. Mineral composition of LCFA and FA.
Figure 2. Mineral composition of LCFA and FA.
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Crystals 12 00400 g003 550
Figure 3. Microstructure of LCFA and FA.
Figure 3. Microstructure of LCFA and FA.
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Crystals 12 00400 g004 550
Figure 4. Water requirement of normal consistency of different admixtures of LCFA.
Figure 4. Water requirement of normal consistency of different admixtures of LCFA.
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Crystals 12 00400 g005 550
Figure 5. Self-hardening compressive strength of LCFA.
Figure 5. Self-hardening compressive strength of LCFA.
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Crystals 12 00400 g006 550
Figure 6. Hydration heat flow and cumulative hydration heat.
Figure 6. Hydration heat flow and cumulative hydration heat.
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Crystals 12 00400 g007 550
Figure 7. Compressive strength of OPC-LCFA samples. (a) Dry. (b) Water.
Figure 7. Compressive strength of OPC-LCFA samples. (a) Dry. (b) Water.
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Crystals 12 00400 g008 550
Figure 8. Compressive strength of OPC-FA samples. (a) Dry. (b) Water.
Figure 8. Compressive strength of OPC-FA samples. (a) Dry. (b) Water.
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Crystals 12 00400 g009 550
Figure 9. Compressive strength of OPC-LCFA-FA (20% LCFA + FA) samples. (a) Dry. (b) Water.
Figure 9. Compressive strength of OPC-LCFA-FA (20% LCFA + FA) samples. (a) Dry. (b) Water.
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Crystals 12 00400 g010 550
Figure 10. Compressive strength of OPC-LCFA-FA (40% LCFA + FA) samples. (a) Dry. (b) Water.
Figure 10. Compressive strength of OPC-LCFA-FA (40% LCFA + FA) samples. (a) Dry. (b) Water.
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Crystals 12 00400 g011 550
Figure 11. Non-evaporated water content of OPC-LCFA and OPC-FA samples.
Figure 11. Non-evaporated water content of OPC-LCFA and OPC-FA samples.
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Figure 12. XRD patterns of OPC-LCFA sample at 28 days.
Figure 12. XRD patterns of OPC-LCFA sample at 28 days.
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Figure 13. FT-IR curves of OPC−LCFA and OPC−FA samples at 28 days.
Figure 13. FT-IR curves of OPC−LCFA and OPC−FA samples at 28 days.
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Figure 14. TG and DTG analysis of OPC-LCFA and OPC-FA sample at 28 days.
Figure 14. TG and DTG analysis of OPC-LCFA and OPC-FA sample at 28 days.
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Figure 15. Nanoindentation results of samples at 28 days. (a) Control. (b) O-L-2.
Figure 15. Nanoindentation results of samples at 28 days. (a) Control. (b) O-L-2.
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Figure 16. Frequency distribution and Gaussian peak fitting results of samples at 28 days. (a) Control. (b) O-L-2.
Figure 16. Frequency distribution and Gaussian peak fitting results of samples at 28 days. (a) Control. (b) O-L-2.
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Figure 17. The most probable elastic modulus variation of samples at 28 days.
Figure 17. The most probable elastic modulus variation of samples at 28 days.
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Figure 18. SEM images of OPC-LCFA sample at 28 days.
Figure 18. SEM images of OPC-LCFA sample at 28 days.
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Figure 19. EDS results of OPC-LCFA (a) samples and OPC-FA (b) samples at 28 days, (c) contents of Ca, Si and Al in the EDS results of O-L-2 and O-F-2 samples.
Figure 19. EDS results of OPC-LCFA (a) samples and OPC-FA (b) samples at 28 days, (c) contents of Ca, Si and Al in the EDS results of O-L-2 and O-F-2 samples.
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Figure 20. Pore diameter distribution of OPC-LCFA samples at 28 days.
Figure 20. Pore diameter distribution of OPC-LCFA samples at 28 days.
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Table
Table 1. Main chemical composition of raw materials wt%.
Table 1. Main chemical composition of raw materials wt%.
SiO2Al2O3Fe2O3CaOMgONa2OK2OSO3TiO2LOI
LCFA51.7929.464.546.290.890.321.882.771.401.12
OPC18.464.984.1862.813.820.440.782.970.380.69
FA51.1027.908.805.140.680.621.921.661.420.71
Table
Table 2. Experimental mix proportion and the fluidity results.
Table 2. Experimental mix proportion and the fluidity results.
SeriesCodeLCFA
/wt%		FA
/wt%		OPC
/wt%		W/B
/wt%		J/B
/wt%		Fluidity/mmCuring Condition
1O-L-1010000652.22—Dry curing
O-L-000100350.30184Water
curing		Dry curing
O-L-1100900.50180
O-L-2200800.80180
O-L-3300701.10182
O-L-4400601.43180
2O-F-1010900.30180
O-F-2020800.33176
O-F-3030700.33182
O-F-4040600.27177
3O8-LF-31155800.67175
O8-LF-111010800.48181
O8-LF-13515800.47182
O6-LF-313010600.85183
O6-LF-112020600.60178
O6-LF-131030600.37185
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Zhang, W.; Wang, S.; Zhao, L.; Ran, J.; Kang, W.; Feng, C.; Zhu, J. Investigation of Low-Calcium Circulating Fluidized Bed Fly Ash on the Mechanical Strength and Microstructure of Cement-Based Material. Crystals 2022, 12, 400. https://doi.org/10.3390/cryst12030400

AMA Style

Zhang W, Wang S, Zhao L, Ran J, Kang W, Feng C, Zhu J. Investigation of Low-Calcium Circulating Fluidized Bed Fly Ash on the Mechanical Strength and Microstructure of Cement-Based Material. Crystals. 2022; 12(3):400. https://doi.org/10.3390/cryst12030400

Chicago/Turabian Style

Zhang, Wenyan, Shuai Wang, Liya Zhao, Junsheng Ran, Wenjing Kang, Chunhua Feng, and Jianping Zhu. 2022. "Investigation of Low-Calcium Circulating Fluidized Bed Fly Ash on the Mechanical Strength and Microstructure of Cement-Based Material" Crystals 12, no. 3: 400. https://doi.org/10.3390/cryst12030400

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