Mechanistic Investigation for Solidification of Pb in Fly Ash by Alkali Mineral Slag—Calcium Chloroaluminate as an Example

Abstract

With the increase in municipal solid waste incineration, fly ash, its heavy metal content, and its disposal methods have attracted wide attention. This work investigates if the alkali-activated mineral slag gel solidification of heavy metals in fly ash has positive significance in promoting the harmless treatment of fly ash. This study obtained the optimal solidification conditions of fly ash from a grate incinerator, which are mineral slag content of 40%, activator content of 4%, and water content of 27.5%. Furthermore, the stability of synthesized calcium chloroaluminate is systematically investigated. The solidification effect of calcium chloroaluminate on Pb at pH = 10–13 was conducted at ambient temperatures from 15 °C to 35 °C to simulate the solidification environment of fly ash. The results show that the adsorption capacity of calcium chloroaluminate to Pb in a strongly alkaline environment is 0.1–3.5 mg/g. Pb is mainly solidified as lead-acid calcium chloroaluminate. This work provides a novel treatment strategy for fly ash.

1. Introduction

The rapid development of China’s economy and the continuous promotion of the construction of ecological civilization and new urbanization has caused the amount of domestic waste to increase rapidly, with an annual growth rate of 8%–10% [1]. According to the statistics of the China Statistical Yearbook 2021, China’s urban waste has reached 235 million tons, with the harmless treatment rate reaching 99.7% in 2020 [1]. There are many strategies for waste disposal, including sanitary landfills, incineration, etc. In recent years, sanitary landfills have lost their advantage due to the shortage of land resources, while waste incineration strategies are used widely because of the high degree of harmlessness, reduction, and resource utilization. In 2020, incineration accounted for 62.29% of the total harmless treatment, which has become one of the mainstream waste disposal methods.

However, incineration produces secondary pollution—fly ash, enriched with toxic and harmful substances such as heavy metals and dioxins. The mainstream power generation from waste incineration in China is grate furnace and fluidized bed technology, which produce 2%–5% and 8%–12% of the waste quantity, respectively [2]. The grate furnace method benefits from low and stable operation costs, and its proportion of use is much higher than that of the fluidized bed method worldwide.

Fly ash is usually composed of gray-white or dark-gray fine particles with different sizes, irregular shapes, and low moisture content. To control the acid gas in the flue gas during incineration, a large amount of absorbent, such as calcium oxide, is injected, so the fly ash is generally strongly alkaline [3]. The main components of fly ash are various oxides and chlorides. The characteristics of high calcium and high chlorine content have become a complex problem in the disposal of fly ash. The fly ash is enriched with heavy metals, soluble salts, and trace dioxins in the flue gas. Thus, fly ash is listed in the national hazardous waste inventory of China [4] and must be treated strictly. In addition, unclassified domestic waste is mixed with heavy metal residues. Moreover, due to the different volatility of heavy metals during incineration, they will migrate to various substances. Among them, moderately volatile heavy metals, including Cd, Pb, Zn, As, etc., are enriched into the fine particles of fly ash [5,6] Thus, if not appropriately handled before arriving at landfills, the heavy metals infiltrate rain in nature and create acid rain, which causes significant harm to the environment and the human body. With the increasing number of waste incineration plants in China, the fly ash produced by incineration is also increasing sharply. In 2014, the standard for pollution control in the municipal solid waste incineration of China started to focus on treating fly ash, with increasing disposal requirements. Therefore, how to reasonably and effectively carry out the harmless treatment and disposal of fly ash and actively open up a resource path are urgent problems to be solved.

The treatment of fly ash is generally solidification and stabilization. Its primary purpose is to prevent the dissolution of harmful substances, such as heavy metals, and make the toxic substances in fly ash chemically inert or stabilized. Among solidification and stabilization technologies, cement solidification became the best technology for treating toxic and hazardous wastes because of its high product strength, mature process, low price, and simple operation [7]. However, fly ash contains many soluble salts, which cause the fracture of the solidified body produced by cement solidification, reducing the solidification strength and increasing the permeability of heavy metals.

Alkali-activated cementitious material refers to a kind of cementitious material with particular force and durability prepared from materials with pozzolanic activity or potential hydraulic hardness at ambient temperature under the action of an alkali activator, which is called alkali cementitious material [8]. Compared with traditional Portland cement, alkali cementitious material has many advantages, including fast setting and hardening, high early strength of solidified body, the steady growth of later power, excellent frost resistance, corrosion resistance, impermeability, high temperature resistance, and durability. Thus, it has attracted much attention in recent years.

Mineral slag is the by-product of the blast furnace iron-making process and contains silicates and aluminosilicates. Research shows that when the mineral slag encounters a strongly alkaline aqueous solution (pH > 12), the mineral slag begins to depolymerize and undergo a hydration reaction [9], showing potential cementitious properties of the mineral slag. Therefore, alkaline substances are often selected as activators for chemical excitation. The main hydration products of alkali mineral slag solidified fly ash are hydrated calcium silicoaluminate (C-A-S-H), calcium silicate hydrate (C-S-H), ettringite, and AFm group compounds [10]. AFm group compounds belong to the layered double hydroxide system of boehmite, which has a similar structure to hydrotalcite. The structural unit comprises a positively charged main layer and a negatively charged intermediate layer (x = Cl⁻, OH⁻, SO₄²⁻, CO₃²⁻, and other anion groups) [11]. The AFm family mainly includes hydrated calcium aluminate (CaO·Al₂O₃·CaSO₄·10H₂O), hydrated calcium chloroaluminate (3CaO·Al₂O₃·CaCO₃·10H₂O, Friede l’s salt phase), and hydrated calcium carboaluminate phase (3CaO·Al₂O₃·CaCl₂·10H₂O) [11]. Similarly, the solidification of heavy metals by the AFm group is mainly realized through the ion exchange of Ca²⁺, Al³⁺, and other anion positions, and thus, stabilizes the heavy metals. In an environment containing chlorine, the product of fly ash solidified by alkali mineral slag tends to produce more calcium chloroaluminate.

Calcium chloroaluminate (3CaO·Al₂O₃·CaCl₂·10H₂O, Friede l’s salt phase) is a compound of the layered double hydroxide (LDH) system of boehmite. LDHs are layered bimetallic hydroxides composed of a positively charged metal hydroxide layer and a negatively charged interlayer anion or anion complex. Layered calcium aluminate hydrate is often called AFm phase. The general chemical formula of LDHs is [M²⁺_1−xM³⁺_x(OH)₂⁻]^x+[Aⁿ⁻_x/n]·mH₂O, which is anionic clay. Usually, the number of metal cations and hydroxyl ions in the main layer is fixed, while the anions in the inter-layer space are relatively easy to replace without damaging the layered structure [12]. The layered structure of the positive charge of LDHs possesses a large specific surface area, exchangeability of interlayer anions, collocation of lamellar cations, and tunable denaturation of pore size, which make LDHs widely used in catalysis, adsorption, ion exchange, and other fields as well as in daily chemicals, functional polymer materials, superconductors, oil fields, medicine, coatings, and environmental remediation, etc. [13,14].

Fly ashes contain heavy metals, including Cd, Pb, Zn, As, etc. The composition of incineration waste and the incineration environment will affect the quantity and form of the final migration and conversion of heavy metals. Pb, as a medium volatile heavy metal in fly ash, is very prone to exceed the standard, which is harmful to the nervous system, digestive system, and hemopoietic system of humans [15]. It is also [16] found that Cl can significantly enhance the volatility and transfer process of Pb to fly ash, with an amazing conversion rate of more than 90%. The sulfur in the incineration component causes Pb to mainly exist as PbSO₄. Thus, Pb is easily enriched in small fly ash particles, and most of them are in an unstable state. It is found that the proportion of carbonate-bound Pb increases with the decrease in fly ash particle size [8]. Thus, how to stabilize heavy metals such as Pb is very important.

Mineral slag has potential hydraulic properties. It can form hydraulic cementitious materials by adding an alkali activator [8]. It has certain solidification advantages for solid waste containing heavy metal ions and is a green environmental protection material with low energy consumption and low cost. The composition of fly ash is similar to these auxiliary cementitious materials. Therefore, the study of heavy metals in alkali-activated mineral slag cementitious solidified fly ash is of positive significance in promoting the harmless treatment of fly ash. Calcium chloroaluminate, as a product of alkali mineral slag solidification, shows an excellent binding effect on heavy metals and combines with the chlorine element in fly ash. Thus, this work investigates the solidification mechanism of calcium chloroaluminate on Pb and lays a theoretical foundation for recycling fly ash.

2. Materials and Methods

2.1. Materials

The grate furnace fly ash (MSWI) used in this work was taken from the Gaoantun Waste Incineration Plant, Capital Steel Group, Beijing. In this plant, the waste furnace system adopted the SN grate incineration technology of TAKUMA, Japan. The generated flue gas was treated by the SNCR process and collected using a bag filter. The particle size distribution of fly ash is shown in Figure 1. The particle size of fly ash shows an approximately normal distribution from 0.1 μm to 100 μm. The XRD pattern of fly ash is shown in Figure 2. The main mineral components of fly ash include anhydrite (CaSO₄), calcite (CaCO₃), quartz (SiO₂), chloride (NaCl, KCl, CaClOH), etc., with low Si content and relatively high Cl content.

Table 1. The chemical composition of fly ash.

Sample	Grate Furnace Incineration Fly Ash (%)
CaO	30.80
Cl	14.72
Na2O	13.09
SO3	6.64
SiO2	5.32
K2O	4.92
MgO	3.44
Al2O3	2.07
Other	4.45
Loss on ignition	14.55

lists the chemical composition of the fly ash. It is observed that the fly ash contains 30.8% of CaO, 14.72% of Cl, 13.09% of Na₂O, and 6.64% of SO₃ with some heavy metal compounds.

Table 2. The physical properties of fly ash.

Physical Property	Instruments or Methods	Experimental Data
Water content	ASTM C311	0.73%
Specific surface area	TriStar 3020 physical breathing apparatus	4.4320 m2/g
pH	GB/T15555.12-1995	12.33

exhibits the typical physical properties of fly ash, which is a water content of 0.73%, and a specific surface area of 4.4320 m²/g with a pH of 12.33. Furthermore, the total amount and leaching concentration of heavy metals in fly ash are given in

Table 3. Total amount and leaching concentration of heavy metals in fly ash.

Heavy Metal	Fly Ash Leaching (mg/L)	Total Amount (mg/kg)	Leaching Rate (%)	GB 16889-2008
Ba	0.250	26.0	19.0	25
Cd	2.372	576	8.2	0.15
Cr	0.692	546.3	2.5	4.5
Cu	0.903	881.6	2.0	40
Ni	0.023	4.4	10.4	0.5
Pb	1.694	1689.6	2.0	0.25
Zn	2.295	2475.2	1.9	100

. As shown in Table 3, Pb exhibits a total amount of 1689.6 mg/kg with a leaching concentration of 1.694 mg/L, suggesting high content of Pb in fly ash.

The mineral slag used in this work was purchased from a company in Jiangsu. It is gray-white, with a specific surface area of 426 cm²/g and a density of 2.93 g/cm³.

Table 4. The chemical composition of mineral slag.

Component	CaO	SiO2	Al2O3	MgO	SO3	K2O	Other	Loss on Ignition
Mineral Slag (%)	38.47	30.99	16.74	9.87	2.62	0.39	0.26	0.66

lists the composition of mineral slag. It is observed that mineral slags contained 38.47% of CaO, 30.99% of SiO₂, 16.74% of 1Al₂O₃, and 9.87% of MgO.

2.2. Sample Preparation and Experimental Method

2.2.1. Preparation of Alkali Mineral Slag Cementitious Solidified Body

In the experiment, sodium hydroxide was selected as the activator. The fly ash, mineral slag, and sodium hydroxide were mixed evenly and stirred dry with a mixer for 1 min. Then, distilled water was added, and the mixture was stirred quickly for 2 min to create a slurry and transferred to a test block mold with a dimension of 2 cm × 2 cm × 2 cm. It was vibrated to enhance the compactness of the sample. To prevent the loss of water, a layer of fresh-keeping film was placed on the surface of the mold. The mold was removed after solidification for 24 h at ambient temperature, and the sample was cured for 5 days in distilled water at ambient temperature.

2.2.2. Preparation of Calcium Chloroaluminate

In this experiment, pure calcium chloroaluminate was synthesized by anhydrous aluminum chloride and calcium hydroxide in aqueous solution at ambient temperature. Firstly, AlCl₃ and Ca(OH)₂ were weighed with an equivalence ratio of 1:3. Then, Ca(OH)₂ and a large amount of distilled water were placed into the beaker with intense stirring to obtain a turbid state. Then, AlCl₃ was added with stirring. After reacting for a while, the mixture was stilled to obtain a white precipitate, which was washed with distilled water 3 times, and dried at 80 °C for 10 h to obtain a white powdery material. The synthetic equation of calcium chloroaluminate is as follows:

2AlCl₃ + 6Ca(OH)₂ + 4H₂O = 3CaO·Al₂O₃·CaCl₂·10 H₂O + 2CaCl₂

2.2.3. Leaching Toxicity Analysis of Heavy Metals

In this experiment, acetic acid buffer solution was used as the extracting agent to buffer the influence of the strong alkalinity of fly ash on the leaching environment, to simulate the landfill leachate erosion of landfill fly ash solidified body, and the leaching environment of harmful components. The leaching toxicity of heavy metals in fly ash and the solidified body was characterized according to HJ/T 300-2007 Solid waste-Extraction procedure for leaching toxicity- Acetic acid buffer solution method. The sample was crushed into small grains of less than 9.5 mm, and the corresponding amount of extracting agent was added to the fly ash with a solid–liquid ratio of 20:1 (L/kg). The mixture was covered and fixed in the oscillator and continuously shaken for 16–20 h at room temperature. After centrifugal filtration, the metal elements in the leaching solution were analyzed by atomic absorption spectrophotometer.

2.3. Characterizations and Analytical Methods

In this work, flame atomic absorption spectrometry (TAS-990, Beijing Puxie General Instrument Co., Ltd., Beijing, China) was applied to detect the adsorption of calcium chloroaluminate on Pb²⁺ in the solution. An X-ray diffractometer (XTRA, Syme Fisher Technologies, Waltham, MA, USA) was used to analyze the crystal substances in the samples. A scanning electron microscope (Nova Nano 450, FEI, Hillsboro, OR, USA) was used to characterize the micromorphology of the samples, and the elements in the samples were determined with the help of a large-area silicon drift detector spectrometer (X-MaxN, oxford-instruments, Oxford, UK). Fourier infrared spectroscopy (IRAffinity-1, Shimazu, Japan) was conducted to analyze the chemical bonds of the samples to infer the molecular structure of their substances. Then, X-ray photoelectron spectroscopy (PHI 5000, UIVAC-PHI, Japan) was carried out to analyze the samples before and after calcium-aluminate-solidified heavy metal. The chloride ion concentration was measured using an ion chromatograph (ICS2000, Syme Fisher Technologies, Waltham, MA, USA). A compressive strength test was conducted according to GB/T 50081-2019 Standard for test methods of concrete physical and mechanical properties.

3. Results and Discussion

3.1. Gelatinization of Fly Ash by Alkaline Mineral Slag in Grate Furnace

3.1.1. Leaching of Heavy Metals from Solidified Bodies under Different Mineral Slag Dosages

The loading of the activator NaOH was 5%, the amount of water was 40%, and the addition of mineral slag was controlled to be 0%, 10%, 20%, 30%, 40%, and 50% of the total weight of fly ash and minerals slag. The samples were cured at ambient temperature for five days. The leaching results of Pb are shown in

Table 5. Leaching of Pb from solidified bodies under different additions.

Different Mineral Slag Dosages	Leaching Concentration of Pb (mg/L)	Different Dosages of Activator	Leaching Concentration of Pb (mg/L)	Different Water Addition	Leaching Concentration of Pb (mg/L)
0%	1.629	0%	1.577	27.5%	0.106
10%	0.817	1%	0.978	30%	0.104
20%	0.716	2%	0.762	32.5%	0.099
30%	0.661	3%	0.325	35%	0.101
40%	0.071	4%	0.103	\	\
50%	0.009	5%	0.077	\	\
Landfill standard	0.25	\	0.25	\	0.25

.

As shown in Table 5, with increased mineral slag addition, the leaching value of Pb shows a downward trend. When the mineral slag addition is small, the leaching values of Pb are higher than the standard values of domestic waste landfills. With the increase in mineral slag loading, the leaching concentration of Pb decreases. This may be because the increase in mineral slag leads to more formation of hydrated calcium silicate (C-S-H), which could enhance the adsorption and wrapping ability of samples to heavy metals [17,18].

However, due to the low content of Si and Al in the fly ash of grate furnace incineration, it is necessary to add more Si-rich and Al-rich mineral slag to form a stable silicate aluminate hydration product. It was found that when the mineral slag content reached 40%, the leaching value of heavy metals was lower than the limit value of the landfill, and the formed solidified body had a relatively large capacity increase, which had no advantage for the final treatment in the landfill. Therefore, the mineral slag addition in this research was kept at 40%.

3.1.2. Leaching of Heavy Metals from Solidified Bodies under Different Dosages of Activator

The mineral slag addition was kept at 40% of the total mass of fly ash and mineral slag, the water loading at 30%, the solidification time for five days at ambient temperature, and activator NaOH loading as 0%, 1%, 2%, 3%, 4%, and 5%, respectively, to investigate the activator dosage on leaching behaviors of heavy metals from solidified bodies. The leaching concentration of Pb is listed in Table 5.

As observed from Table 5, the leaching concentration of Pb in cement is lower than the landfill limit when the addition of alkali activator NaOH reaches 4% of the total mass of fly ash and mineral slag. In the solidification process of fly ash, 40% mineral slag is added, so more alkali activator NaOH is needed to destroy the silicon aluminum vitreous structure in mineral slag. The effect of activator content on the solidification of heavy metals is obvious. The hydration products formed after the dissolution of Si and Al elements in mineral slag can effectively stabilize the heavy metals in fly ash. Si and Al in hydration products are mainly provided by mineral slag, while Ca and Si in mineral slag are higher, and fly ash from the grate furnace also provides a large amount of Ca, which may promote the formation of hydration products C-S-A-H and C-S-H. Moreover, fly ash from the grate furnace also provides a large number of Cl and S elements, promoting the formation of calcium chloroaluminate and ettringite.

3.1.3. Leaching of Heavy Metals from Solidified Bodies under Different Water Addition

To investigate the effect of water addition on the leaching behavior of heavy metal from solidified bodies, the mineral slag loading is 40% of the total amount of fly ash and mineral slag, the activator NaOH addition is 4% with a curing time of 5 days at ambient temperature, and distilled water loading is 27.5%, 30%, 32.5%, and 35%, respectively. The leaching concentration of Pb is shown in Table 5.

As shown in Table 5, the amount of water added exhibits little effect on the solidification of fly ash, and the corresponding moisture contents of the solidified bodies are 15.81%, 15.97%, 16.32%, and 16.75%, respectively. Therefore, all the solidified bodies corresponding to the four amounts of water added can enter the landfill. Among them, the sample with water loading of 27.5% possessed the lowest moisture content, which is beneficial to heavy metal solidification.

Therefore, the optimal formulation of fly ash solidified body from an alkali mineral slag solidification grate furnace is as follows: 40% mineral slag addition, 4% activator addition, and 27.5% water addition. The solidified body was cured in water at room temperature for 5 days for the heavy metal leaching experiment. The leaching concentration of Pb was 0.106 mg/L with a moisture content of 15.81% and a compressive strength of 16.28 MPa.

3.1.4. Solidification Effect Analysis

The SEM-EDS spectra of fly ash cement and mineral slag are shown in Figure 3. Figure 3a gives the micromorphology of the mineral slag particles, and uneven lumps and dense structures are observed. The EDX mapping suggests that the main components of mineral slag are Ca, Al, and Si. Therefore, certain excitation methods are needed to destroy its structure to show the potential hydraulic property of the mineral slag. The fly ash was prepared according to the optimal proportion and cured for 5 d. The hydration reaction was terminated by immersing the sample in anhydrous ethanol. After crushing, the small bricks in the interior of the solidified body were selected and dried for scanning electron microscopy analysis. The microstructure of the solidified body is shown in Figure 3b. Compared with the loose and dispersed particles of the original fly ash, a large number of hydration colloids appear in the solidified body. This structure is compact and dense, which could significantly inhibit the leaching behavior of heavy metals into the environment.

Figure 4a gives the XRD pattern of mineral slag, which exhibits a large number of disorderly peaks, indicating the amorphous structure of mineral slag with potential activity. The XRD patterns of fly ash before and after solidification are shown in Figure 4b. It is observed that the peak intensities in Figure 4b are greatly weakened. The gel-solidified bodies are mainly stable products, such as calcium chloroaluminate, C-S-H, and a large number of amorphous silicate structures. It can be speculated that chlorine salt in fly ash is hydrated to form calcium chloroaluminate, and alkali mineral slag solidified fly ash has a very significant inhibitory effect on chlorine salt in fly ash.

The Fourier transform infrared spectra of fly ash and gel-solidified bodies are shown in Figure 4c. The absorption peaks around 792 cm⁻¹ and 530 cm⁻¹ correspond to the symmetric stretching vibration and antisymmetric stretching vibration of Al-OH in the octahedral structure of Al(OH)₆ in calcium chloroaluminate, respectively, which confirms the formation of calcium chloroaluminate in the solidification process [19].

3.2. Stability Analysis of Calcium Chloroaluminate

3.2.1. SEM Analysis

Calcium chloroaluminate synthesized in this experiment is a white powder solid with uniform particles. Figure 5a gives SEM microstructures of calcium chloroaluminate particles. It is observed that calcium chloroaluminate exhibits a flat layered hexagonal structure with a dimension of 1–5 μm, with a certain degree of aggregation.

3.2.2. Stability Analysis

Furthermore, the release concentration of chloride ions of calcium chloroaluminate under different pH is investigated to judge the existing state of calcium chloroaluminate at different pHs. It could provide a scientific basis for selecting a calcium chloroaluminate application environment.

Figure 5b shows the chloride ion release concentration of calcium chloroaluminate under different pHs. It can be found that when pH = 1, the chloride ion release concentration is very high, reaching 4963 mg/L. The release concentration of chloride ions is below 200 mg/L within a pH between 2 and 12. The release concentration of chloride ions rapidly increases to 2976 mg/L at a pH of 14. Thus, calcium chloroaluminate has a wide range of adaptability to pH value, which could be more stable in the range of pH = 2–13.

Figure 5c gives XRD spectra of calcium chloroaluminate under different pH conditions. Calcium chloroaluminate itself has a certain alkalinity, which is neutralized in an acidic environment. As observed from the XRD pattern, all the samples absorbed carbon dioxide from the air to produce calcium carbonate. When pH = 1, the solid phase changed completely, and characteristic peaks of CaCO₃ and Al(OH)₃ were observed, indicating that calcium chloroaluminate could not exist stably in an environment with pH below 1. When pH = 14, Ca₃Al₂(OH)₁₂ and calcium carbonate were observed in the sample, indicating the reaction of calcium chloroaluminate and high concentration of OH⁻, decreasing the stability of calcium chloroaluminate. Figure 5d shows FT-IR spectra of calcium chloroaluminate at different pH values. At a pH of 1, there is no octahedral structure of Al(OH)₆, and at pH = 14, there is no vibration peak of H-O-H in the intermediate water of the two-layer structure. Thus, FT-IR can also confirm the decomposition of calcium chloroaluminate and the formation of corresponding products.

3.3. Analysis of Pb Curing by Hydration Products—Calcium Chloroaluminate

3.3.1. Adsorption Experiments of Pb by Calcium Chloroaluminate at Different Temperatures and pH

Figure 6 exhibits the effect of pH and temperature on the adsorption of dissolved lead by calcium chloroaluminate. As observed from Figure 6a, the adsorption capacity of calcium chloroaluminate decreased with an increase in temperature, and the adsorption capacity obviously decreased at 35 °C. This is because the increase in temperature increases the solubility of calcium chloroaluminate, resulting in a decrease in the mass of solid calcium chloroaluminate, so the adsorption amount of Pb decreases significantly. Therefore, high temperature is not beneficial to the adsorption of Pb by calcium aluminate chloride.

Figure 6b shows that calcium chloroaluminate has a certain adsorption capacity for dissolved lead under strong alkali conditions. Calcium chloroaluminate exhibits an adsorption capacity of 3.5 mg/g at a pH of 11. With the increase in pH value, the adsorption capacity of lead is gradually decreased. When the pH value is 13, the adsorption capacity is around 2.8 mg/g. This may be because calcium chloroaluminate gradually decomposes at a high pH value, thus inhibiting the adsorption of Pb.

3.3.2. XRD Analysis of Cured Products at Different Temperatures and pH

Figure 7 is the XRD spectrum of the solidified products of calcium chloroaluminate to Pb at different temperatures and pH conditions. According to the analysis of search match software, lead-acid calcium chloroaluminate and calcium carbonate are produced after solidification. It is also found that the peak intensity of the main product is enhanced and with a shifted position after solidification. The shift is produced by ion substitution. Calcium carbonate is produced by the reaction between an alkali solution and CO₂ in the air. The solidified products at 15 °C and 25 °C are observed, which show little difference. Furthermore, the intensity of the characteristic peak for calcium carbonate at 35 °C is enhanced, indicating a high-temperature benefit for the formation of calcium carbonate in combination with CO₂ in the air. Therefore, there is not enough Cl⁻ position to replace Pb(OH)₃⁻, resulting in a small output of lead-acid calcium chloroaluminate (3CaO·Al₂O₃·Ca (Pb(OH)₃) 2nH₂O), and the solidification effect of Pb is also weakened. It is observed that the peak strength of lead-acid calcium chloroaluminate is decreased with the increase in pH at the same temperature, indicating that the enhancement of alkalinity promotes the dissolution of calcium chloroaluminate.

3.3.3. FT-IR Analysis of Cured Products at Different Temperatures and pH

Figure 8a gives the FT-IR spectra of calcium chloroaluminate solidified Pb at 25 °C under different pH values. The characteristic absorption peaks of the solidified product were highly consistent with those of calcium chloroaluminate. The strong and wide peaks at 3646 cm⁻¹ and 3480 cm⁻¹ correspond to the vibration of the hydroxyl group (O-H root) of structural water, and the peak at around 1620 cm⁻¹ is ascribed to the bending vibration peak of H-O-H of interlayer water in calcium chloroaluminate. These peaks gradually weaken with the decrease in pH and slightly shift, which is due to the change of ion radius resulting from the replacement of Cl⁻ by Pb(OH)₃⁻ in calcium chloroaluminate lattice [20,21]. At 791 cm⁻¹ and 530 cm⁻¹, the absorption peak intensity of Al-OH changed without a shift, indicating that Pb did not change the chemical bond of Al-OH [19]. Moreover, the absorption peak of CO₃²⁻ at 1420 cm⁻¹ increases with the increase in alkalinity, indicating that CO₂ in the air is combined in the dissolution of calcium chloroaluminate [22]. It is proved that the stronger the alkalinity is, the more enhanced the solubility of calcium chloroaluminate, so the solidification effect of lead is weakened.

The FT-IR spectra of the solidified products of calcium chloroaluminate to Pb at different temperatures with a pH of 11 are shown in Figure 8b. With the decrease in temperature, the absorption peak at 1620 cm⁻¹ which corresponds to interlayer water, changes significantly. Moreover, the changes in absorption peaks at 3646 cm⁻¹ and 3480 cm⁻¹ suggest the occurrence of ion substitution [18]. The higher the temperature, the sharper the peak corresponding to carbonate at 1420 cm⁻¹, corresponding to the generated calcium carbonate [22]. At low temperatures, a more significant change is observed, indicating more ions are replaced, which results in greater changes in the crystal structure. Therefore, the weakening effect of the increase in temperature on the adsorption of Pb by calcium chloroaluminate is consistent with previous studies [23].

3.3.4. SEM Analysis of Cured Products

The solidified products at 15 °C and pH = 11 were further investigated by scanning electron microscope (SEM). The SEM pattern is shown in Figure 9. It is observed that calcium chloroaluminate has lost its regular hexagonal layered structure with its original flake structure of about 5 μm, which became a rectangular block prism structure with a dimension of 20 μm. The EDS test results of the surface scan of the regular part and the amorphous residue with a flat periphery are shown in

Table 6. EDS test results of calcium chloroaluminate solidified Pb.

Rectangular Block Prism Structure			Amorphous Residue
Element	Mass Analysis	Atomic Fraction	Element	Mass Analysis	Atomic Fraction
C K	15.98	29.89	O K	48.79	71.95
O K	34.52	48.48	Na K	4.52	4.64
Al K	1.17	0.97	Al K	4.41	3.86
Cl K	1.41	0.90	Cl K	8.88	5.91
Ca K	32.27	18.09	Ca K	20.57	12.11
Au M	11.92	1.36	Au M	11.16	1.34
Pb M	2.72	0.30	Pb M	1.67	0.19
Total amount	100.00		Total amount	100.00

.

The reason why the Au element is detected is that a gold plating operation was conducted before detection. Because a small amount of residue adhered to the surface of the prism during surface scanning, some elements with a relatively small proportion also appear. Therefore, the main elements of the sample are judged as Ca and O, and the mass ratio of Ca and O is close to 1:3. Combined with the XRD and FT-IR results, it is confirmed that the sample is calcium carbonate with adsorption of a certain amount of Pb. It is speculated that some calcium chloroaluminate is dissolved and recrystallized during solidification:

$$3{\mathrm{CaO}\mathrm{Al}}_2{\mathrm{O}}_3{\mathrm{CaCl}}_{2}10{\mathrm{H}}_2\mathrm{O}+3{\mathrm{CO}}_3^{2-}\to 3{\mathrm{CaCO}}_3\downarrow +2\mathrm{Al}{\left(\mathrm{OH}\right)}_3\left(\mathrm{am}\right)\downarrow +{\mathrm{Ca}}^{2+}+2{\mathrm{Cl}}^{-}$$

$$3{\mathrm{CaO}\mathrm{Al}}_2{\mathrm{O}}_3 {\mathrm{CaCl}}_{2}10{\mathrm{H}}_2\mathrm{O}+{\mathrm{CO}}_3^{2-}\to 3{\mathrm{CaO}\mathrm{Al}}_2{\mathrm{O}}_3 {\mathrm{CaCO}}_{3}10{\mathrm{H}}_2\mathrm{O}+2{\mathrm{Cl}}^{-}$$

Al(OH)₃ colloid is in an amorphous state, which is not shown in the XRD pattern. At the same time, Al(OH)₃ colloid will also adsorb some Pb. The amorphous residue has irregular morphology with a small size, which is stacked in flakes and layers, including Al, Cl, Ca, O, and other elements. At the same time, it also has a binding effect on Pb. It is speculated that the amorphous Al(OH)₃ and lead-acid calcium chloroaluminate are intertwined. It is also possible that the substitution of Pb(OH)₃⁻ destroys the previous regular structure and creates lead-acid calcium chloroaluminate irregular flocs.

3.3.5. XPS Analysis

The XPS spectra of Pb before and after calcium chloroaluminate solidification are shown in Figure 10a, and the changes in element binding energy before and after solidification are shown in

Table 7. XPS binding energy of main elements before and after calcium chloroaluminate curing.

Spectrum Lines	Peak Position before Curing/ev	Peak Position after Curing/eV
O1s	531	532
C1s	284.4	284.45
Ca2p	346.6	347.25
Cl2p	198.05	198.35
Pb4f		138.85

. Two Pb spectral peaks were observed at the solidified product, suggesting the successful solidification of Pb. Further, XPS analysis software was used to separate the peaks of Pb. The local amplification of Pb binding energy is shown in Figure 10b. The peak at around 138.85 eV and 143.4 eV corresponded to Pb4f_7/2 and Pb4f_5/2 of Pb. The peak intensities of Ca and Cl have a downward trend while O content is increased. At the same time, O, C, Ca, and Cl are observed. O1s, C1s, and Ca2p exhibit a chemical shift of 1 eV, 0.05 eV, and 0.65 eV. Moreover, the chemical shift of Cl2p is 0.3 eV, indicating that the chemical bond interaction between Pb and calcium aluminate chloride surface occurs. Therefore, it can be concluded that Pb may enter the structure of calcium chloroaluminate in the form of Pb(OH)₃₋ instead of Cl⁻ after binding with O to form a new complex. It is speculated that the reduction of Ca element is released during the dissolution–recrystallization process of calcium chloroaluminate.

4. Conclusions

Fly ash contains heavy metals, which are harmful to the environment and humans. How to dispose of fly ash and solidify the heavy metals has attracted much attention. In this work, alkali mineral slag is applied to solidify heavy metals in fly ash, which could form a solidification body in the existence of an activator. It is found that mineral slag and activator exhibit great influence on the solidification ability of heavy metals, while water addition has little influence on the leaching concentration of heavy metals. Furthermore, the optimal formulation for the solidification of fly ash from a grate incinerator is obtained: mineral slag of 40%, an activator of 4%, and water loading of 27.5%. After curing in water for 5 days at room temperature, the obtained gelled solid meets the requirements of GB 16889-2008 Standard for pollution control on the landfill site of municipal solid waste, possesses a certain compressive strength, and can be entered into the landfill.

A large amount of chlorine salt in the fly ash is stabilized in calcium chloroaluminate, which solves the solidification problem of heavy metals and the negative impact of chlorine salt on the hydration process and compressive strength. Herein, pure calcium chloroaluminate is synthesized. Furthermore, its solidifying behavior to Pb in different conditions is systematically investigated. It is found that calcium chloroaluminate exhibits a layered hexagon structure with stability in the range of pH 2–13. The adsorption capacity of Pb by calcium chloroaluminate in a strongly alkaline environment is 0.1–3.5 mg/g. With the increase in pH and temperature, the solubility of calcium chloroaluminate increases gradually, and the solidification effect of calcium chloroaluminate on Pb becomes weaker and weaker. Pb mainly replaces the position of Cl⁻ in calcium chloroaluminate with Pb(OH)₃⁻ to form lead-acid calcium chloroaluminate. This work provides a novel strategy for the disposal of fly ash with heavy metals.