Keywords

1 Introduction

Incineration is one of the most widely used methods to treat the MSW. Even though incineration is efficient for treating MSW, one of the shortcomings of this technology is the significant production of solid residues, e.g., fly ash [1]. MSWI fly ash is regarded as hazardous material mainly due to its high content of heavy metals. Therefore, MSWI fly ash needs solidification/stabilization treatment before landfill disposal [2]. A deep understanding of the pozzolanic properties of MSWI fly ash has important theoretical significance in the solidification/stabilization treatment of MSWI fly ash.

At present, many studies have been conducted to investigate the pozzolanic properties of coal fly ash, mineral powder and other materials. Zhang et al. [3] studied the pozzolanic properties of mineral slag, steel slag and coal fly ash under the action of NaOH. The results showed that the mineral slag obtained the best effect, followed by steel slag and coal fly ash. Shi et al. [4] found that 4% CaCl2.2H2O can significantly improve the later strength of lime-pozzolan cement. Sivapullaiah et al. [5] found that the adding of CaSO4 accelerated the increase of UCS value of coal fly ash. Sun et al. [6] studied the effect of different alkali activator (NaOH, Na2CO3, Na2SiO3) on slag and coal fly ash, and the alkali activator constituted of NaOH and Na2SiO3 performed the best. It is seen from the above studies that the effects of activators adding on the pozzolanic behaviors of coal fly ash, mineral slag and steel slag have been well studied. However, few researches have been carried out to focus on the pozzolanic properties of MSWI fly ash.

The aim of this study is to investigate the pozzolanic properties of MSWI fly ash in the solidification process. Four groups of solidification tests were conducted on the MSWI fly ash samples, which were solidified by deionized water, Na2CO3, CaSO4 and Na2SiO3 solutions, respectively. After that, the UCS and water immersion tests were carried out on the solidified samples to investigate the influence of the kind and proportion of activators on the pozzolanic properties.

2 Materials and Methods

2.1 Materials

MSWI fly ash.

The MSWI fly ash used in this study was collected from a MSWI plant with the burning environment of circulating fluidized bed in Hangzhou, China. The MSWI fly ash is dark yellow in colour, as shown in Fig. 1. The chemical composition was determined by an energy dispersive X-ray spectrometer. The test result was shown in Table 1. It was observed that the MSWI fly ash contained a certain amount of CaO, SiO2, Al2O3 and Fe2O3, and the content of CaO was as high as 44.1%.

Fig. 1.
figure 1

MSWI fly ash

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Table 1. Chemical compositions of MSWI fly ash (%)
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Activator.

Three types of activators were used in this study, i.e., Na2CO3, CaSO4 and Na2SiO3. Na2CO3 was selected to take into account the high CaO content in fly ash. Na2CO3 not only plays a role in activating the pozzolanic reactivity [5], but also reacts with CaO to generate the precipitate of CaCO3 to produce a function of auxiliary cementation. CaSO4 is a widely used activator [6], which may generate hydration reaction with pozzolanic materials. Na2SiO3 is an activator which is commonly used to promote the solidification process of Portland cement [7].

2.2 Experimental Process

Four groups of solidification tests were conducted on the MSWI fly ash samples, as shown in Table 2. B0 was solidified by water with a water-cement ratio of 0.35, and cured for 7 and 28 days, which served as the reference group. B1-1 and B1-2 was solidified under the similar condition to B0, except that 3% and 6% Na2CO3 was added to the fly ash, respectively. 3 and 6% CaSO4 were added in B2-1 and B2-2, respectively, and 3 and 6% Na2SiO3 were used in B3-1 and B3-2, respectively. The groups B1, B2 and B3 were performed to investigate the effects of Na2CO3, CaSO4 and Na2SiO3 adding and their proportions on the solidification capacity of MSWI fly ash.

Table 2. Program of solidification/stabilization treatments
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Taking B2-1 for an example, the experimental processes were presented as follows:

  1. (a)

    The MSWI fly ash samples were dried at the temperature of 105 ± 5 °C, and mixed with 3% Na2CO3, then stirred at a water-cement ratio of 0.35. After that, the mixture was filled into a self-designed moulding cylinder in three lifts. As shown in Fig. 2, the moulding cylinder consists of a split PVC tube with the inner diameter of 36 mm and the height of 80 mm, a layer of geotextile lined between the sample and the PVC tube, and two porous stones placed on the bottom and top of the PVC tube, respectively. After filling and compacting of the mixture, the moulding cylinder was placed in an incubator with a temperature of 20 ± 2 °C and a humidity of ≥95% for curing. After curing for 24 h, the moulding cylinder was removed, and the sample was continued to be maintained for 7 and 28 days.

    Fig. 2.
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    Cylindrical mold for solidifying fly ash

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  2. (b)

    Subsequently, the UCS test was conducted on the solidified sample by using a servo mechanical press (CMT4000, China). The servo mechanical press can supply a maximum force of 30 kN with an accuracy of 1 N and a loading rate of 2 mm/min. The UCS value was determined by using the formula: P = F/A, where P is the compressive strength (MPa), F is the total maximum load recorded at the point of fracture (N), and A is the area of loaded surface (mm2).

  3. (c)

    The water immersion test was also conducted on the solidified sample after curing for 7 days. The sample bulk with an area of about 2 cm2 was immersed by deionized water in a beaker. Then, the sample bulk was kept immersion and observed the shape change carefully. The time durations for recording the observations were 10, 30 min, 1, 2, 4, 12 h, 1, 3, 7, 10, and 14 days.

3 Results and Analysis

3.1 Effect of Activator Proportion on the UCS Value

Figure 3 shows the UCS value of the solidified MSWI fly ash samples. The UCS value of the reference group (0% activator added) was obtained as 0.28 and 0.50 MPa after curing for 7 and 28 days, respectively. It was observed that the UCS value of the solidified samples under the action of the activators significantly improved when compared with the reference group. This indicated that all the three activators were likely to stimulate the pozzolanic capacity of the MSWI fly ash. As seen from Fig. 3(a), the UCS value of the solidified samples after curing for 7 days tended to increase with an increase in activator proportion. Among the three activators, CaSO4 performed best. When the adding proportions of CaSO4 were 3 and 6%, the UCS values of the solidified samples reached 1.18 and 1.96 MPa, respectively, which were 318.02 and 592.58% higher than that of the reference group. As shown in Fig. 3(b), the UCS values of the solidified samples cured for 28 days also remarkably increased with the activator proportions for the cases of adding Na2CO3 and Na2SiO3. However, the UCS value of the solidified sample achieved the peak value of 2.90 MPa for the case of adding 3% CaSO4, and decreased to 2.23 MPa when the CaSO4 proportion was raised up to 6%. This might be associated with the formation of ettringite (AFt) in the fly ash solidification process under the action of CaSO4. The AFt has the characteristic of swell effect, which tended to cause the solidified samples to slightly break [8]. This phenomenon was also found in the solidification process of sludge by the authors and was verified by scanning electron microscopy (SEM) [9].

Fig. 3.
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UCS values of the solidified samples with different activator proportions

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3.2 Effect of Curing Age on the UCS Value

The effects of curing age on the UCS value of the solidified MSWI fly ash samples are given in Fig. 4. As the curing age increased, the UCS values of solidified samples increased, which implied that the solidification processes were likely to progress with time. It was observed in Fig. 4(a) that CaSO4 performed the best on stimulating the pozzolanic activity among all the three activators when the adding proportion was 3%. When activated by CaSO4, the UCS values of the solidified samples after curing for 7 and 28 days reached 1.18 and 2.90 MPa, respectively, which were 318.02 and 478.24% higher than that of the reference group. The performance of Na2CO3 took the second place, and followed by Na2SiO3. As shown in Fig. 4(b), when the adding proportion of activators was 6%, the case of adding CaSO4 again performed the best after curing for 7 days, however, the UCS value of this case increased slightly after curing for a longer time of 28 days. For the curing time increased from 7 days to 28 days, the UCS values of the solidified samples significantly increased from 1.37 to 2.92 MPa for the case activated by Na2CO3, and from 0.99 to 2.84 MPa for the case activated by Na2SiO3.

Fig. 4.
figure 4

UCS values of the solidified samples under different curing ages

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3.3 Water Immersion Behavior

The water immersion behaviors of the solidified samples are showed in Table 3. The sample of the reference group gradually collapsed after immersing in the water for 7 days. Similar results were also observed in the cases of adding 3 and 6% CaSO4, and the samples were damaged in 7 and 10 days, respectively. This may be associated with the structural weakness induced by water. However, no visible damage was observed in the samples solidified by adding Na2CO3 and Na2SiO3 after immersing in the water for 14 days.

Table 3. Water immersion test results
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4 Summary and Conclusion

The findings from this study are summarized as follows

  1. (1)

    The reference group, with no activator added, had UCS values of 0.28 and 0.50 MPa after curing for 7 and 28 days, respectively. The solidified sample cured for 7 days collapsed after immersing in the water for 7 days.

  2. (2)

    When the sample was solidified with the action of the activator Na2CO3, its UCS value increased with increasing Na2CO3 mixing proportion and curing time. The 28-day UCS values reached 2.92 MPa for the samples with Na2CO3 mixing proportion of 6%, which was 483.43% higher than the reference group. No collapse was observed in the 7-day solidified sample even if it was immersed in the water for 14 days. Similar behavior was observed in the solidified sample added with Na2SiO3, however, its UCS value was lower than that added with Na2SiO3 under the given mixing proportion and curing time.

  3. (3)

    When the sample was solidified with the action of CaSO4, its UCS value achieved the peak value of 2.90 MPa for the case of adding 3% CaSO4, and decreased to 2.23 MPa when the CaSO4 proportion was raised up to 6%. The solidified samples cured for 7 days collapsed after immersing in the water for 7–10 days.

From the perspectives of compressive strength and water tolerance of the solidified MSWI fly ash, the optimal activator among the three ones studied in this work was Na2CO3, followed by Na2SiO3, and the last was CaSO4.