Construction and Building Materials 314 (2022) 125625
Available online 17 November 2021
0950-0618/© 2021 Elsevier Ltd. All rights reserved.
Hydration behavior of circulating fluidized bed fly ash (CFBFA) as a cementitious binder
Chun-Ran Wu
a, Bao-Jian Zhan
a, Zhi-Qiang Hong
a, Shi-Cai Cui
b, Peng Cui
a, Shi-Cong Kou
a,*aGuangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen Durability Centre for Civil Engineering, College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, Guangdong, PR China
bSchool of Architecture and Civil Engineering, Liaocheng University, Liaocheng 252000, Shandong, PR China
A R T I C L E I N F O Keywords:
Circulating fluidized bed fly ash (CFBFA) CFBFA-lime system
Compressive strength Hydration products
A B S T R A C T
In this study, the hydration behavior of circulating fluidized bed fly ash (CFBFA) as a cementitious binder was investigated in a CFBFA-lime system. Binary mixtures of CFBFA and lime were prepared with different weight ratios, and a series of experiments were performed to evaluate the hydration behavior of the mixtures. When the mixing weight ratio of CFBFA and lime was 5:1, the prepared mixtures could reach 17 MPa at 28 days. Hydration heat curves revealed that the release heat peak related to the hydration of CFBFA appeared earlier as the content of lime increased. This likely stemmed from the release of heat by the hydration of lime. Analysis of Ca(OH)2 consumption and chemically bound water revealed that the hydration reaction of mixtures can continue for a long time after Ca(OH)2 was depleted. In addition, the main hydration products of the CFBFA-lime system were C-S-H, C-A-S-H, AFm and AFt. The micro-morphology of the hydration products changed from needles to mesh- like and then to foil-like as the lime content in mixtures increased.
1. Introduction
Circulating fluidized bed coal-fired technology is a new effective coal-fired technology for power generation with many advantages, such as high efficiency, low pollution and high economic utility [1–3]. In recent years, circulating fluidized bed coal-fired technology has become increasingly mainstream in coal-fired power plants. By 2018, approxi- mately 3,000 circulating fluidized bed boilers had been built in China, accounting for 60% of the total worldwide, and the number of circu- lating fluidized bed boilers continues to increase. The applications of circulating fluidized bed boilers are wide, and the production of CFBFA (the by-product produced in the process of circulating fluidized bed coal combustion) is increasing [1–3]. By 2018, the annual emissions of CFBFA exceeded 100 million tons in China [4]. The released CFBFA not only enters farmland but also causes severe environmental pollution (e.
g., CFBFA exudates pollute water sources and soils, and its fine particles cause air pollution). However, CFBFA has completely different physi- cochemical properties compared with ordinary fly ash because of its different formation temperature: ordinary fly ash is formed at 1200–1400 ◦C, and CFBFA is formed at 850–900 ◦C [5–7]. Therefore, much of the insights that have been gained from studies on ordinary fly ash for decades do not apply to CFBFA, and the disposal and reuse of
CFBFA have become major challenges that need to be addressed.
Using CFBFA as a supplementary cementitious material (SCM) might be the most suitable way for the disposal of large quantities of CFBFA [8–10]. First, its pozzolanic activity has been confirmed by several re- searchers. Glinicki et al. showed that CFBFA can react with Ca(OH)2
produced by cement hydration and produce C-S-H and ettringite in a cement blend system [11]. Li et al. found that the pozzolanic reactivity index of CFBFA can reach 85.4% at 28 days of curing, which is higher than the minimum limited value of 70% for use as an SCM per the Chinese standard GB/T 1596–2005 [12]. Some studies have shown that the addition of CFBFA can improve the durability of concrete. Zahedi et al. found that using CFBFA as an SCM can enhance the resistance of concrete to ion penetration and alkali-silica reaction [10]. Maochieh et al. showed that CFBFA has a positive effect on the sulfate attack resistance of concrete [8]. Nevertheless, CFBFA is not considered a high- grade SCM because of its negative effects on concrete performance [5,12–13]. Li et al. reported that the addition of CFBFA has a negative effect on the rheology of concrete [5]. Chen et al. showed that the addition of CFBFA with high SO3 results in expansive products (AFt) formed in early ages as well as the expansion of cement pastes [12].
Zahedi et al. found that CFBFA with high CaO content results in blended cement mortar with poor resistance to external sulfates [10]. There are
* Corresponding author.
E-mail address: [email protected] (S.-C. Kou).
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
https://doi.org/10.1016/j.conbuildmat.2021.125625
Received 7 February 2021; Received in revised form 27 September 2021; Accepted 8 November 2021
also likely several other factors that might affect the use of CFBFA as an SCM that have not yet been considered because of the complexity of industrial by-products. There is thus a need to study the hydration behavior of CFBFA as a cementitious binder to develop effective solu- tions for its subsequent applications.
Pozzolanic materials show pozzolanic activity by reacting with lime in the presence of water at ordinary temperatures to form cementitious compounds [14]. In this study, the hydration behavior of CFBFA was explored in a CFBFA-lime system. The advantages of using unhydrated lime are that its low-cost, and fewer CO2 emissions are produced during the manufacturing process; furthermore, the prepared CFBFA-lime binder can capture more atmospheric CO2 than portland cement dur- ing the natural carbonation process for the production of controlled low- strength materials, such as those used as fill materials for site formation [15].
The aim of this paper was to investigate the hydration behavior of CFBFA-lime binder. CFBFA and lime were mixed at different weight ratios to establish the CFBFA-lime system. The hydration heat, compressive strength, pore structure, consumption of Ca(OH)2, and chemically bound water content of CFBFA-lime binder were studied, and the hydration products were identified and quantified.
2. Materials and methods 2.1. Materials
The raw material of CFBFA was obtained from Jilin power plant, China. Table 1 shows the composites (chemical and mineral) of CFBFA, as determined by an X-ray fluorescence spectrometer (XRF; S4 Explorer, Bruker, Germany) and X-ray diffractometer (XRD; Bruker D8, Bruker, Germany) with Rietveld refinement. Fig. 1 shows the particle size dis- tribution (PSD) and morphologies of CFBFA, as measured by a laser particle analysis instrument (S3500, Microtrac Inc, USA) and scanning electron microscope (SEM; Quanta TM250 FEG, FEI Company, USA),
respectively. The PSD results show that the D (50) and D (90) values of CFBFA are 13.86 µm and 49.25 µm, respectively. In addition, the mor- phologies of CFBFA indicated that the CFBFA particle exhibits a rough surface and has varied shape characteristics.
AR grade lime (CaO content >98.00%, Macklin Company, China) was used as the binder. Corundum powder (α-Al2O3 content >99.99%, Macklin Company, China) was used as an internal standard in the quantitative X-ray diffraction (QXRD) analysis to quantify the mineral- ogical composition of samples [16].
2.2. Methods 2.2.1. Hydration heat
The hydration heat evolution of the paste mixtures was measured using an 8-channel isothermal calorimeter (TAM Air, TA Instruments, USA). To prepare test mixtures, the CFBFA and lime were first mixed evenly in a plastic container at different proportions (CFBFA : lime was 10:0, 10:1, 10:2, 10:3 and 10:4). The weighted distilled water (water-to- binder (binder = CFBFA +lime) ratio was 1.0) was added into the plastic container (which cannot react with alkali) and was mixed for 3 min by hand. Finally, approximately 8.5 g of fresh paste for each mea- surement was cast into ampoules for testing. The calorimetric data were recorded continuously at 20 ◦C for 72 h.
2.2.2. Specimen preparation
The mix proportions of specimens are shown in Table 2. The addition method (rather than the replacement method) with different lime dos- ages was used to explore the amount of lime required to react with a certain amount of CFBFA [17]. The CFBFA, lime, and distilled water were mixed evenly and cast into a steel mold (30 mm ×30 mm ×30 mm). After 24 h, the specimens were demolded and wrapped with plastic film to avoid water loss and carbonation as much as possible for curing. For curing ages of 7, 28, 90 and 180 days, the specimens were taken out for compressive strength tests. After compressive strength tests, the fractured specimens were further crushed into particles with sizes of<3.0 mm and then were stored in a plastic container with anhydrous ethanol for approximately 48 h. Finally, the particles were dried in a vacuum oven at 60 ◦C for approximately 72 h and then used in subsequent analyses.
2.2.3. Pore structure of specimens
The pore structures of CF10, CF20, CF30 and CF40 at 90 curing days were determined using a mercury intrusion porosimeter (MIP; Autopore 9510, Micromeritics, USA). Stored specimens around 3 mm in size were used for the measurements. In these tests, the density, contact angle, and Table 1
Chemical and mineral compositions of CFBFA (wt%).
Chemical composites
SiO2 Al2O3 Fe2O3 CaO SO3 MgO K2O NaO2 TiO2 Other LOI
56.12 23.91 8.40 2.94 2.58 1.83 1.63 1.38 0.69 0.52 4.4
Mineral composites
Quartz Hematite Calcite Anhydrite Mullite Amorphous/unidentified
18.2 2.1 0.3 2.6 2.0 74.9
a Loss on ignition (LOI) at 1000 ◦C.
Fig. 1. Particle size distribution and micro-morphologies of CFBFA.
Table 2
Mix proportions of paste specimens
Specimens CFBFA (kg) Lime (kg) Water (kg) Water/binder ratio
CF10 1 0.1 0.77 0.7
CF20 1 0.2 0.84 0.7
CF30 1 0.3 0.91 0.7
CF40 1 0.4 0.98 0.7
surface tension of mercury were assumed to be 13.5335 g/mL, 130◦and 485 dynes/cm, respectively.
2.2.4. Quantitative X-ray diffraction analysis (QXRD)
The mineralogical composition of both CFBFA and hydrated pastes was determined using XRD with Cu Kα radiation (λ =1.5408 Å). The dried pastes were taken out and ground into power (<75 µm) by hand.
The obtained powder was used for TG and XRD tests. In these tests, the voltage and current were set to 40 kV and 40 mA, respectively. The scanning range, step size and scan speed were 5◦to 70◦(2θ), 0.01◦and 0.02◦/s, respectively. Corundum and the obtained powder were mixed in an agate mortar, ground by hand for approximately 15 min and then evenly mixed; the mixture was considered evenly mixed when the white powder could no longer be observed in the mixture. The mass ratio of corundum to obtained powder was 1:4. The diffraction data were analyzed using Xpert HighScore Plus 3.0e from PANalytical following a previously described method [18]. When the value of the R profile and weighted R profile was between 5 and 15 in the QXRD analysis, the goodness of fit was around 1. This is usually considered sufficiently accurate for QXRD analysis.
2.2.5. Thermogravimetric analysis
Thermogravimetric analysis (TGA) was performed using a thermal gravimetric analyzer (METTLER TGA2, Mettler Toledo, China). First, the paste particles were dried at 40 ◦C for 3 days, and then the dried samples were finely ground in an agate mortar. Finally, approximately 30 mg of ground samples were heated from 30 ◦C to 1000 ◦C under an N2 envi- ronment (50 mL/min), and the heating rate was 10 ◦C/min.
Taking into account the temperature ranges obtained in the TG analysis, the Ca(OH)2 consumption and chemically bound water content can be estimated in the mixtures. Based on the TG data, the consumption of Ca(OH)2 normalized to the CFBFA content was calculated and rescaled according to the following equation (1):
MCa(oH)2,consumption= MCa(oH)2,Initial− MCa(oH)2,Residue
MCFBFA,Initial× (1− LOICFBFA)
= MCaO,Initial×7456− WLCa(OH)2×7418 W1000◦∁× MCFBFA,Initial
MCFBFA,Initial+MCaO,Initial(1− LOICFBFA) (1)
where MCaO,Initial and MCFBFA,Initial are the initial content of CaO and CFBFA, respectively, which can be calculated based on Table 2, and WLCa(OH)2 is the weight loss stemming from the decomposition of Por- tlandite (between 380 and 460 ◦C) obtained using the tangent method [19]. In addition, the Ca(OH)2 consumption was determined based on the QXRD analysis following the approaches of previous studies [19–20].
In the plain Portland cement hydration system, chemically bound water represents the amount of water needed for cement to react, and it is usually used to indicate the reaction degree of plain Portland cement [19]. Similarly, the chemically bound water represents the amount of water needed for CFBFA to react and indicates the reaction degree of CFBFA. Therefore, the chemically bound water was calculated based on the TG-DTG analysis using the following equation (2):
BWCFBFA=BWTotal− BWCa(OH)2 (2)
where BWCFBFA is the chemically bound water content in the hy- dration products of CFBFA, BWTotal is the total chemically bound water content in the hydration products of lime and CFBFA, which is related to the weight loss (40–500 ◦C) [19], and BWCa(OH)2 is the chemically bound water content in the unreacted Ca(OH)2. Furthermore, the chemically bound water content was normalized by the CFBFA content with the following equation
BWCFBFA,normalization= BWCFBFA
W1000◦∁× MCFBFA,Initial
MCFBFA,Initial+MCaO,Initial(1− LOICFBFA) (3)
where BWCFBFA, normalization is the chemically bound water content that was normalized to per g CFBFA; W1000◦
C is the weight of the sample at 1000 ◦C; MCFBFA,Initial and MCaO,Initital are the mass of CFBFA and CaO consumed in the preparation of specimens, respectively; and LOICFBFA is the loss on ignition of CFBFA at 1000 ◦C.
2.2.6. Fourier transform infrared spectroscopy (FTIR)
FTIR tests were performed on the powder samples after they were passed through a 75-µm sieve using a Fourier transform infrared spec- trometer (Agilent Cary 660, Agilent Technologies Inc, USA) with an ATR diamond crystal. The scanning range, scanning resolution and scanning rate were set to 4000 to 400 cm−1, 2 cm−1 and 5 kHz scans, respectively.
The final FTIR spectrum was obtained with 32 scans.
2.2.7. Nuclear magnetic resonance (NMR)
27Al MAS NMR can be used to analyze the coordination state of Al in the crystalline and poorly crystalline phase. In this study, the coordi- nation state of the Al phase of CFBFA is affected by the occurrence of the hydration reaction. Therefore, 27Al MAS NMR was carried out to determine the Al phase-based hydration products. 27Al MAS NMR spectra for the original material of CFBFA and hydrated specimens of CF10, CF20, CF30 and CF40 at 90 curing days were collected at a fre- quency of 156.3 MHz using an NMR spectrometer (JNM-ECZ600R, JEOL Ltd., Japan). The spectrometer was equipped with a 3.2-mm probe head.
The spinning frequency, pulse length and relaxation delay were set to 12.0 KHz, 0.1 µs and 3 s, respectively. The NMR experimental spectra were deconvoluted using Peak Fit v4.12®.
3. Results and discussion
3.1. Heat of hydration
The hydration heat release of samples during the first 72 h is shown in Fig. 2. The first exothermic peaks were detected within the first 12 h, which was mainly related to the lime exothermic reaction (Fig. 2 (b)).
The second exothermic peak arose between 12 and 35 h and likely stemmed from the pozzolanic reaction of CFBFA with Ca(OH)2 produced from lime hydration (Fig. 2 (c)) [4,17]. The lime exothermic peaks appeared to be slightly delayed as the CFBFA content increased because the lime and water could be absorbed by the CFBFA and then delay the lime exothermic reaction [17]. The second peak appeared earlier as the lime content increased, which likely stemmed from the fact that the hydration of lime releases heat; thus, a higher relative content of lime in
Fig. 2.Heat evolution curves of pastes.
the mixture releases more heat, which leads to the earlier activation of CFBFA activity. The exothermic rate stemming from the pozzolanic re- action of CFBFA was low, and the exothermic time of CFPFA was long.
This is consistent with the results of previous studies showing that the silicon and aluminum in CFBFA react slowly with calcium hydroxide [21].
3.2. Compressive strength
Fig. 3 shows the compressive strength development of CF10, CF20, CF30 and CF40 as a function of curing age to 180 days. During the entire curing period, the compressive strength of CF10 was lower than that of CF20, CF30 and CF40. This might stem from the fact that the relative content of lime in CF10 is small, and sufficient Ca(OH)2 to react with CFBFA is lacking. Compared with CF10, the increase in the compressive strength of CF20 might stem from the fact that more lime can provide more Ca(OH)2 to react with CFBFA and produce more hydration prod- ucts. The highest compressive strength was observed for CF20, not CF30 and CF40. This indicates that the compressive strength of specimens did not increase linearly with the increase in lime content. This may be explained by the pore structure and hydration products, the details of which are discussed in the following sections. In addition, at the age of 28 days, the compressive strengths of CF10, CF20, CF30 and CF40 were
12.6, 17.0, 14.9 and 15.7 MPa, respectively. Although the compressive strength values of these mixtures were lower than those of ordinary cement pastes, they can be used for controlled low-strength materials, such as fill materials for site formation (1.0 MPa) [15].
3.3. Pore structure
The intrusion and extrusion curves for MIP tests are shown in SF 4.
There were obvious ink-bottle pores in the four samples. This affects the accuracy of the pore size distribution. Further analysis revealed that the ink-bottle effect caused 0.193, 0.198, 0.200 and 0.204 mL of mercury to be detained in ink-bottle pores per g of CF10, CF20, CF30 and CF40, respectively. This indicates that the ink-bottle effect was similar among the four samples and suggests that the pore size distribution of the samples was accurate. The pore size distributions of specimens at 90 days are shown in Fig. 4a. The porosity of CF10, CF20, CF30 and CF40 was 54.0%, 45.6%, 45.2% and 46.7%, respectively. CF10 had higher porosity than other samples, indicating that fewer hydration products formed. This result is consistent with the compressive strength results shown in section 3.2. The pores in cementitious composites are typically divided into harmless pores (<20 nm), less harmful pores (between 20 nm and 100 nm) and harmful pores (>100 nm) according to the pore diameter [22]. The pore size distribution was determined based on this classification of pores (Fig. 4b). The volumes of less harmful pores were similar to those in CF20, CF30 and CF40 and greater than those in CF10.
In addition, the harmful pore volume in CF10 was greater than that in CF20, CF30 and CF40. The low content of Ca(OH)2 means that there was not sufficient Ca(OH)2 to react with CFBFA and thus produce enough hydration products to fill the internal pores. This finding, coupled with the compressive strength results of CF20, CF30 and CF40, indicates that the pore structure analysis alone cannot reveal changes in the strength of these specimens.
3.4. Consumption of Ca(OH)2
The consumption of Ca(OH)2 can be calculated based on the TG and QXRD data, which are provided in SF 1 and SF 2. Although anti- carbonation was considered in this study, some samples were still carbonated (see blue areas in SF 1). In addition, the change in the Ca (OH)2 content can be observed from the green areas in SF 1. Ca(OH)2
was nearly depleted at 7, 28 and 90 days for CF10, CF20 and CF30, respectively. There were still a few residual Ca(OH)2 in CF40 even after 180 curing days. Similar results were obtained from the QXRD analysis (SF 2).
Fig. 3. Compressive strength development of specimens.
Fig. 4.Pore size distributions of hydrated specimens at the curing age of 90 days.
The consumption content of Ca(OH)2 based on TG-DTG and QXRD and calculated with equation (1) is shown in Fig. 5a and b. The con- sumption of Ca(OH)2 based on TG-DTG was similar to that obtained based on the QXRD analysis, which verifies the reliability of the experimental results. The consumption content of Ca(OH)2 in CF10, CF20 and CF30 peaked at 28, 28 and 90 days, respectively. The con- sumption content of Ca(OH)2 in CF40 increased throughout the whole curing period. This finding indicates that Ca(OH)2 in CF10, CF20 and CF30 was depleted, which limits the reaction degree of CFBFA, and different types of hydration products with different Ca/(Si +Al) ratios were formed in different specimens. The maximum consumption of Ca (OH)2 was approximately 0.09, 0.20 and 0.31 g per g of CFBFA in CF10, CF20 and CF30, respectively. The maximum consumption of Ca(OH)2
reached approximately 0.38 g per g of CFBFA in CF40 at 180 days.
During the early curing period (7 and 28 days), the Ca(OH)2
consumption of CF40 (approximately 0.12 and 0.23 g per g of CFBFA, respectively) was lower than that of CF30 (approximately 0.15 and 0.27 g per g of CFBFA, respectively). This indicates that the high Ca(OH)2
content could reduce the reaction rate of CFBFA during the early curing period. According to previous studies, this can be explained by the fact that the high Ca(OH)2 content leads to the precipitation of the hydration products on the surface of CFBFA, slowing the dissolution of alumina and silicate phases from the CFBFA particles [14].
3.5. Chemically bound water
Fig. 6 shows the chemically bound water content of CF10, CF20, CF30 and CF40, which was determined using equation (3). The chemi- cally bound water content of CF10, CF20 and CF30 stabilized (approx- imately 0.12, 0.19 and 0.24 g per g of CFBFA, respectively) after 28, 90 and 90 days, respectively. In addition, the chemically bound water of CF40 progressively increased and finally reached 0.25 g per g CFBFA at 180 days. This indicates that the reaction degree increases with the lime content in the mixtures, and at 180 days, the CFBFA in CF30 and CF40 has a similar reaction degree. Analysis of the consumption of Ca(OH)2
indicated that the time for the chemically bound water to reach a con- stant state lagged behind that of the depletion of Ca(OH)2. This sug- gested that the reaction of CFBFA can continue for a long time after Ca (OH)2 was depleted [23–24]. Before 90 days, the chemically bound water of CF40 was lower than that of CF30, which confirms that high amounts of Ca(OH)2 would inhibit the reaction of CFBFA, and this is consistent with the results obtained in the last section.
3.6. FTIR analysis
The FTIR spectra of CF10, CF20, CF30 and CF40 for the entire curing period along with the original CFBFA are shown in Fig. 7. In the CFBFA spectra, the main broad bands near 1019, 778 and 464 cm−1 correspond to Si-O-T (probably Si or Al) vibrations, and the latter two bands (778 and 464 cm−1) indicate the presence of quartz, which was not involved Fig. 5.Consumption of Ca(OH)2 based on (a) TG-DTG and (b) Q-XRD.
Fig. 6. Chemically bound water content of specimens.
in the hydration of CFBFA [14,25–26]. As the curing time and Ca(OH)2
content increased, the main broad band near 1019 cm−1 in the original CFBFA shifted to lower frequencies around 965 cm−1. This indicated that Si-based hydration products formed as the hydration reaction pro- gressed, which is most likely the C-S-H gel [14,27]. Although the vi- bration bands of Al-O–H were located between 900 and 1200 cm−1, Al-O was difficult to detect in the spectra between 900 and 1200 cm−1 because of overlap with the vibration bands of Si-O [25].
3.7. 27Al NMR analysis
The coordination state of Al in the crystalline and poorly crystalline phase can be accurately identified by 27Al MAS NMR. The 27Al MAS NMR spectra of CF10, CF20, CF30 and CF40 at 90 curing days, as well as the spectra of CFBFA, are shown in Fig. 8. In the CFBFA spectra, the centerband resonances near 61.0, 57.0 and 50.5 ppm are tentatively assigned to Al [4] (around 40–80 ppm), 39.0 ppm in the area of Al [5]
(around 15–40 ppm), and 3.0 and − 1.0 ppm in the area of Al [6] (around
− 20–15 ppm). Comparison of CFBFA and hydrated specimens of CF10, CF20, CF30 and CF40 at 90 curing days revealed that the spectra peaks of hydrated specimens are mainly distributed in the area of Al [4] and Al [6]; several new peaks appeared and were probably associated with the hydration products. In the area of Al [4], the spectra peak around 69.0 ppm was related to the Al for Si substitution in C-S-H [28]. In the area of Al [6], the peaks around 13.5 ppm originated from the ettringite (AFt), those around 10.0 ppm were attributed to monosulfate (AFm) or a hydrate-AFm phase and those around 5.0 ppm were attributed to the third aluminate hydrate phase, which most likely form on the surface of the C-S-H phase [28–31]. In addition, a characteristic peak associated with the steady Al [4] from alumina phases present in CFBFA can be observed near 61.5 ppm in the spectra from both the original material and the hydrated specimens [14,28,31–33].
To obtain quantitative information on the fractions of Al species with different tetrahedral coordination structures, the data shown in Fig. 8 were quantitatively analyzed by deconvolution using PeakFit® soft- ware, and the results are summarized in Table 3. The relative amounts of Al species associated with unreacted CFBFA in CF10, CF20, CF30 and CF40 were 78.3%, 44.8%, 21.6% and 22.9%, respectively. The amounts of hydration products of CF10, CF20, CF30 and CF40 were 18.5%, 54.1%, 78.4% and 77.0%, respectively. CF30 and CF40 have similar fractions of Al species present in the unreacted CFBFA and hydrated products, which indicated that there was sufficient calcium for the hy- dration reaction of Al phases in CF30. Approximately 22.9% of the Al phase exists in CFBFA in a stable state and thus cannot be hydrated.
3.8. SEM analysis
The micro-morphology of CF10, CF20, CF30 and CF40 at 90 days is shown in Fig. 9. The micro-morphology of the four specimens differed.
In CF10, a large number of unhydrated CFBFA particles can be observed, and the hydration product of C-S-H resembled diverging needles, which Fig. 7.FTIR spectra of CFBFA and hydrated specimens.
Fig. 8. 27Al NMR spectra of CFBFA and hydrated specimens after curing for 90 days.
grow outwards from the CFBFA particles. In CF20, the unhydrated CFBFA particles were almost completely covered by hydration products.
Meanwhile, the needle-like C-S-H was not apparent, and more C-S-H with aciculate and mesh was observed. In CF30 and CF40, more foil-like C-S-H and a small amount of Ca(OH)2 sandwiched in C-S-H can be observed. The results of the micro-morphology analysis coupled with the results of section 3.3 reveal that there are more unhydrated particles in CF10, and the fewer hydration products are not enough to fill its harmful pores (the harmful pore volume of CF20, CF30 and CF40 is 20.53%, 10.80% and 16.31% lower than that of CF10, respectively) between unhydrated particles, which results in its high porosity. Although the micro-morphology of CF20, CF30 and CF40 was obviously different, they all show similar porosity (45.6%, 45.2% and 46.7% for CF20, CF30 and CF40, respectively). This may indicate that the porosity of speci- mens is more closely related to the reaction degree of CFBFA than to the type of hydration products. Based on the aforementioned SEM obser- vations, differences in the content of Ca(OH)2 led to clear differences in the micro-morphology of C-S-H formed in the hydrated specimens. The transition of the micro-morphology from needles to the foil-like C-S-H appeared to be related to the concentration of Ca(OH)2 in the mixtures.
This might mainly stem from the difference in the Ca/Si ratio of the Si phase hydration products, and the correlation between the morphology of C-S-H with the Ca/Si ratio discussed in previous studies [34].
4. Conclusion
The hydration behavior of CFBFA as a cementitious binder in a CFBFA-lime system was examined in this study. The main conclusions are detailed below.
(1) The hydration products of CFBFA can show certain strength performance. At 28 days, CF20 showed the highest compressive strength (ca. 17 MPa) compared with CF10, CF30 and CF40.
These binders can be used to control low-strength materials, such as fill materials for site formation (which exceed the 1.0 MPa strength requirements at 28 d).
(2) The hydration heat curves show that the release heat peak related to the hydration of CFBFA appeared earlier as the content of lime increased.
(3) The reaction degree of CFBFA is limited in CF10 and CF20 because of the depletion of Ca(OH)2. CFBFA has a similar reaction degree in CF30 and CF40 at 180 days. In addition, the hydration reaction continued for a long time after Ca(OH)2 was depleted.
(4) In the CFBFA-lime system, the main hydration product of the Si phase was C-S-H, and the Al phases in CFBFA were transformed into AFm, AFt and C-A-S-H during hydration.
(5) The micro-morphology of the hydration products changed from needles to mesh-like and then to foil-like as the lime content in CFBFA-lime binder increased, which might stem from the different Ca/Si ratios of the hydration products.
Table 3
Deconvolution results of 27Al NMR spectra of specimens at 90 days of curing
Specimens Al [4] Al [6]
CFBFA Peak position 69.0 61.1 57.1 50.6 13.5 10.0 5.0 3.0
Integral (%) – 44.1 12.2 16.4 – – – 14.4
CF10 Peak position 68.6 61.3 57.0 51.1 – 10.1 5.0 3.0
Integral (%) 7.5 56.9 10.0 6.9 – 5.1 5.9 4.5
CF20 Peak position 68.7 61.5 56.5 – 13.1 10.1 5.0 2.8
Integral (%) 11.3 37.5 4.2 – 2.2 37.4 3.2 3.1
CF30 Peak position 68.8 61.8 – – 13.0 10.1 4.7 –
Integral (%) 12.8 21.6 – – 13.8 44.5 7.3 –
CF40 Peak position 69.1 61.5 – – 13.2 10.0 4.7 –
Integral (%) 7.9 22.9 – – 2.7 47.8 18.6 –
Fig. 9.Micro-morphology of hydrated specimens at 90 days of curing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was funded by the Key-Area Research and Development Program of Guangdong Province (No. 2019B111107003).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.conbuildmat.2021.125625.
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