3.1 single-factor experiment
With reference to Huang's research(Huang et al. 2015), the liquid-solid ratio of alkali activator (water glass) content and the modulus of alkali activator have a great influence on the preparation reaction. Set up single-factor experiments based on several types of indicators. The solidify temperature is 25°C. The compressive strength was measured after 7 days of solidify. The single-factor experiment ratio is shown in Table 5.
The experimental results of the influence of the liquid-solid ratio in the single-factor experiment are shown in Fig. 5. It can be obtained when the content of the alkali activator is 10% and the modulus of the alkali activator is 1.5. The compressive strength of the gel reaches the maximum when the liquid-to-solid ratio is about 0.24. The liquid-to-solid ratio affects the fluidity, concentration and stirring(Yahya et al. 2015) of the reactants, and should not be too small. But it can't be lower, otherwise the cavity and the dilution effect(Barbosa et al. 2000) will affect the intensity.
The results of the single factor experiment on the influence of the dosage of the alkaline activator are shown in Fig. 5. It can be obtained when the liquid-solid ratio is 0.27 and the modulus of the alkali activator is 1.5. The compressive strength of the gel reaches the maximum when the content of the alkali activator is about 13%. In addition to the stimulating effect, the alkaline activator also provides the reactant. However, too much stimulant will destroy the proper silicon-to-aluminum ratio, and will cover the surface of the reactant to prevent the reaction from proceeding(Alonso &Palomo 2001, Somna et al. 2011).
Table 5
Single factor experiment ratio
Num
|
Liquid-solid ratio
|
Alkaline activator dosage(%)
|
Alkaline activator modulus
|
1
|
0.21
|
10
|
1.5
|
2
|
0.24
|
10
|
1.5
|
3
|
0.27
|
10
|
1.5
|
4
|
0.30
|
10
|
1.5
|
5
|
0.27
|
7
|
1.5
|
6
|
0.27
|
10
|
1.5
|
7
|
0.27
|
13
|
1.5
|
8
|
0.27
|
16
|
1.5
|
9
|
0.27
|
10
|
0.9
|
10
|
0.27
|
10
|
1.2
|
11
|
0.27
|
10
|
1.8
|
3.2 Orthogonal experiment
Set appropriate parameters according to the results of single factor experiment, and carry out orthogonal experiment. In this section of the experiment, "liquid-solid ratio", "alkali activator dosage", and "alkali activator modulus" are orthogonal factors, and each factor is set to 3 levels. Design three-factor and three-level orthogonal experiments to analyze its relationship with the properties of geopolymer materials and determine the best combination parameters of each raw material. The factor level table is shown in Table 6. According to the principle of orthogonal experiment, the calculation method of design orthogonal experiment table range analysis and results is shown in Table 7.
Table 6
Orthogonal three-factor three-level table
Level/factor
|
Alkaline activator dosage(%)
|
Alkaline activator modulus
|
Liquid-solid ratio
|
1
|
11
|
1.3
|
0.22
|
2
|
13
|
1.5
|
0.24
|
3
|
15
|
1.7
|
0.26
|
Table 7
Experiment number
|
Alkaline activator dosage
|
Alkaline activator modulus
|
Liquid-solid ratio
|
Yi(Compressive strength)
|
1
|
1
|
1
|
1
|
73.28
|
2
|
1
|
2
|
2
|
80.79
|
3
|
1
|
3
|
3
|
91.56
|
4
|
2
|
1
|
2
|
82.72
|
5
|
2
|
2
|
3
|
89.53
|
6
|
2
|
3
|
1
|
95.47
|
7
|
3
|
1
|
3
|
78.51
|
8
|
3
|
2
|
1
|
79.51
|
9
|
3
|
3
|
2
|
77.93
|
Kj1
|
y1 + y2 + y3
|
y1 + y4 + y7
|
y1 + y6 + y8
|
|
Kj2
|
y4 + y5 + y6
|
y2 + y5 + y8
|
y2 + y4 + y9
|
|
Kj3
|
y7 + y8 + y9
|
y3 + y6 + y9
|
y3 + y5 + y7
|
|
kj1
|
K11/m = 81.88
|
K21/m = 78.17
|
K31/m = 82.75
|
|
kj2
|
K12/m = 89.24
|
K22/m = 83.28
|
K32/m = 80.48
|
|
kj3
|
K13/m = 78.65
|
K23/m = 88.32
|
K33/m = 86.53
|
|
R
|
max{k1i}-min{k1i} =10.59
|
max{k2i}-min{k2i} =10.15
|
max{k3i}-min{k3i} =18.16
|
|
Optimal
|
A2
|
B3
|
C3
|
|
According to the results of the orthogonal experiment, the best experiment ratio is 13% of the alkali activator, the modulus of the alkali activator is 1.7, and the liquid-solid ratio is 0.26. It can be seen from the extreme contrast that the relative importance of the three factors (alkali activator content, alkali activator modulus, liquid-solid ratio) that affect the compressive strength of the gel is different. Among them, the liquid-solid ratio has the greatest influence, and the influence of the dosage of the alkali activator and the modulus of the alkali activator is relatively small.
In summary, the best ratio of pure slag preparation gelled body obtained by orthogonal experiment is: the content of alkali activator is 13%, the modulus of alkali activator is 1.7, and the liquid-solid ratio is 0.26. It was verified by experiments that the compressive strength of the gel under this condition (solidify time : 7 days) reached 96.07 MPa.
3.3 The influence of attapulgite content
According to the experimental results (Fig. 6), when the addition amount of attapulgite is small, the reason for the increase in strength should be that the particle size of attapulgite and slag are different, and mutual filling makes the overall structure more compact and stable. When the content of attapulgite gradually increased more than 6.5%, the strength began to decrease significantly. This may be because an excessive amount of attapulgite reduces the intensity of the hydration reaction. According to experiments, the content of attapulgite is determined to be 4%. Although the strength is not improved compared with the gel without attapulgite, it still maintains a high level.
3.4 Compressive strength of solidified soil
It can be seen from the results (Fig. 7) that with the incorporation of chromium-containing sludge, the strength of the gel decreases significantly. From 10–80%, the strength gradually decreases. Through analysis, the active components in chromium-containing sludge, such as silicon-aluminum salt and active binding components, are far lower than the related components contained in the slag, which also reduces the strength of the hydration reaction. In addition, the sulfur and chlorine in chromium-containing sludge and other materials may affect the stable structure of the gel; the heavy metal components in the sludge also affect the progress of the hydration reaction, which may be due to the low solubility of some heavy metal compounds. And its participation in the reaction is relatively small, making the hydration reaction not as smooth as before. In addition, some researchers(Chi 2004) believe that the water absorption rate may affect the strength of the solidified body. The water absorption rate of chromium-containing sludge is significantly lower than that of slag, which reduces the cementitious components produced in the mixture and ultimately reduces the compressive strength. According to the "Concrete Strength Inspection and Evaluation Standard" (GB/T 50107 − 2010), the minimum strength standard C7.5 has a strength of 10.8MPa. When the sludge blending amount exceeds 30%, its strength is already lower than 10MPa, and the strength of 20% blending amount is still close to the strength requirement of C7.5 in the "Concrete Strength Inspection and Evaluation Standard". According to the US EPA standard (landfill strength requirement ≥ 0.35MPa), the compressive strength of the solidified body is required for landfill. The compressive strength test was carried out after solidify for 7 days, and the results are shown in Figure. According to the "Concrete Strength Inspection and Evaluation Standard" (GB/T 50107 − 2010), the minimum strength standard C7.5 has a strength of 10.8 MPa. According to the experimental results, regardless of the addition of attapulgite, the amount of chromium-containing sludge in the solidified body should not exceed 20%, but the addition of attapulgite can greatly improve its mechanical properties. This is because the raw material is mainly blast furnace slag. The added attapulgite can not only serve as fine aggregates to fill the internal defects of the solidified body, but also because the attapulgite adsorbs some heavy metals, the content of heavy metals that hinder the hydration reaction can be reduced. Attapulgite promotes the hydration reaction due to the stabilization of heavy metal ions(Wang et al. 2021b), which enhances the compressive strength of the solidified body.
3.5 The leaching concentration of the solidified body
Table 8
List of leaching concentrations
CCS content
|
Cr (mg/L)
|
Cu (mg/L)
|
Pb (mg/L)
|
Add attapulgite or not
|
×
|
√
|
×
|
√
|
×
|
√
|
10%
|
102.60
|
52.10
|
0
|
20.16
|
146.50
|
14.40
|
20%
|
120.40
|
92.73
|
18.60
|
67.77
|
59.40
|
1.00
|
30%
|
130.00
|
100.20
|
54.00
|
112.86
|
6.50
|
0
|
40%
|
140.60
|
130.50
|
123.20
|
181.26
|
4.70
|
0
|
50%
|
163.50
|
161.46
|
211.40
|
235.71
|
2.10
|
0
|
60%
|
207.20
|
211.14
|
297.40
|
333.36
|
0
|
0
|
70%
|
254.20
|
223.38
|
351.30
|
423.81
|
0
|
0
|
80%
|
346.70
|
267.84
|
422.90
|
538.02
|
0
|
0
|
Table 9
The solidify efficiency of heavy metals in different content after adding attapulgite
Dosage/ solidify efficiency (%)
|
10%
|
20%
|
30%
|
40%
|
50%
|
60%
|
70%
|
80%
|
Cr
|
90.64
|
91.67
|
94.00
|
94.14
|
94.20
|
93.68
|
94.27
|
93.98
|
Cu
|
96.20
|
93.61
|
92.91
|
91.45
|
91.11
|
89.52
|
88.58
|
87.32
|
Pb
|
98.57
|
99.95
|
100
|
100
|
100
|
100
|
100
|
100
|
According to HJ/T 300–2007 "Solid Waste-Leaching Toxicity Leaching Method-Acetic Acid Buffer Solution Method", the solidified body after 28 days of solidify time is subjected to leaching test, and then the solidification effect of heavy metals is comprehensively investigated. The leaching of Cr, Cu, Pb is shown in Table 8 and Fig. 8.
The leaching level of Cr can be seen from the Fig. 8 above. After the 28-day solidify time is over, the chromium leaching level increases as the sludge content increases. After 28 days of solidify, the internal reaction of the solidified soil proceeded sufficiently to form a stable polymer structure(Barbosa et al. 2000). This structure strengthens the restraint on heavy metals. The leaching concentration of the composite solidified body with attapulgite added is generally lower than that of the pure slag solidified body without adding attapulgite. In addition, when the content of chromium-containing sludge increases, the compressive strength of the solidified body decreases. The compressive strength shows the compactness and stability of the solidified body itself. The higher the compressive strength, the better the ability to solidify heavy metals. When the content of chromium-containing sludge reaches 60%, the leaching level of chromium after adding attapulgite is relatively close to the leaching level without adding attapulgite. It may be because at this dosage, the amount of slag in the solidified body is not large and cannot maintain a certain strength, and the dosage of attapulgite is also small and cannot absorb more chromium. However, as the content of chromium-containing sludge continues to increase, the strength decreases, and the chromium in the solidified body is more likely to enter the leachate, and then be absorbed by the more effective attapulgite in the liquid(Ma et al. 2021). It can be seen that by adding attapulgite, the compressive strength of the solidified body can be increased, and the leaching concentration can be reduced accordingly.
Figure 8 shows the comparison between the leaching of copper after adding attapulgite and the case without adding attapulgite. It can be seen from the figure that the solidified body after adding attapulgite does not help the absorption of Cu2+. The reason may be that the addition of attapulgite is not conducive to the development of strength compared with pure slag. When the mixing amount of chromium-containing sludge gradually increases and exceeds the adsorption/solidification limit of the composite solidified body, compared with the solidified body prepared from pure slag, the solidified body with attapulgite is not as stable as pure slag because of the mineral structure. Therefore, the effect of solidify of the silicon-aluminum polymer structure is relatively less obvious. In addition, the selective adsorption characteristics and adsorption balance of attapulgite clay minerals have led to different adsorption/solidification effects on different heavy metals(He et al. 1999), and the adsorption capacity of unmodified attapulgite for Cu should not be as good as that of the others. So there is a reason why the leaching concentration of copper is increased when the amount of doping increases in the later period. In an environment where both Pb2+ and Cu2+ exist, the Cu2+ absorption sites of attapulgite will be preempted by Pb(Xu et al. 2021). A further explanation comes from the study of Du et al.(Du et al. 2016). They believe that Pb2+ is preferentially adsorbed on attapulgite over Cu2 + and occupies the adsorption sites on the amino group, which inhibits the adsorption of Cu2+.
From Fig. 8, it can be seen that the leaching concentration of Pb has a different trend from that of Cr and Cu. As the content of chromium-containing sludge increases, the leaching concentration of Pb gradually decreases, until the content reaches 50% and there is almost no leaching. It can be seen that the leaching effect of Pb in an acidic environment is different than other heavy metal ions. Studies(Muhammad et al. 2018) pointed out that the leaching level of Pb2+ in an aggressive environment (such as acid) will drop sharply because it is in an amorphous environment, and the chemical reaction relationship at this time hinders the performance of leaching. Luo Zejiao(Luo et al. 2014) also found through experiments that the leaching of Pb is very insignificant in a near neutral environment, and after adding an alkaline leaching agent represented by NaOH, the leaching of Pb begins to increase significantly. The research of Chen Haojing(Chen 2008) supports this view. He conducted fly ash leaching experiments and found that the leaching ratio of Pb in a pH greater than 3 to neutral environment is basically 0. It can be explained that in this experiment, the solidified target chromium-containing sludge is acidic, and with the addition of alkaline slag (the amount of chromium-containing sludge is reduced), (gelled body/solidified body) acidic corrosive medium environment began to change, and the leaching of Pb also manifests itself. In addition, it can be seen that before and after adding attapulgite, the leaching concentration of lead during the leaching process of the solidified body decreases as the dose increases. This also verifies that the appearance of alkali slag breaks the original acidic medium leaching environment and makes the leaching effect more obvious. Although the trend is the same, the composite solidified body added with attapulgite shows a better solidification effect. Compared with before adding attapulgite, the leaching concentration of Pb after adding attapulgite is greatly reduced, the largest decrease is more than 90%, and the undetected content appears earlier(It reaching 30%). The leaching concentration of lead in the solidified body prepared from pure slag does not reach zero until the content reaches 60%. It shows that after adding attapulgite, the solidify effect of the solidified body on Pb is obviously improved.
Through comparison, it can be found that attapulgite can reduce the leaching concentration of heavy metals such as Cr and Pb within a certain range. Attapulgite has significant adsorption/solidification effects on several types of heavy metals. From Table 9, it can be seen that the solidification efficiency of the solidified body after adding attapulgite is maintained at a relatively high level. The adsorption effect of attapulgite on Cu is not ideal. This can be used to modify the attapulgite in subsequent experiments to improve its adsorption capacity, so that it can effectively adsorb Cu2+ in the presence of Pb2+.
3.6 XRD analysis
It can be seen from the XRD pattern (Fig. 9) that the structure of the gelatinous mineral prepared by the alkali-excited slag is mainly calcium aluminum yellow feldspar, CSH gel (free calcium oxide and calcium sulfate in the blast furnace slag and the silicon-aluminum component reacted under the action of the alkaline activator to form hydrated calcium silicate and aluminum), the whole is an amorphous product. According to research(Puertas et al. 2005, Liu et al. 2017), when the ratio of SiO2/Al2O3 is lower than 4.5 (this experiment uses the main raw material slag, the ratio is lower than 4.4, and the content of attapulgite is extremely small) hydration product is CSH Gel Mainly. After mixing the sludge, the peak intensity of the yellow feldspar decreases, indicating that the amorphous heavy metal components should be embedded in the gel solidification body. After the addition of attapulgite, more diffraction peaks of heavy metal salts appeared, indicating that heavy metals were bound by the C-S-H gel and transformed into a class of amorphous substances. The above action causes the heavy metal components to be solidified. After adding attapulgite, the XRD pattern did not change too much, which may be due to the low content of attapulgite (4%). In addition, no more obvious characteristic peaks appeared after adding attapulgite, indicating that the addition of attapulgite did not change the product of the cementing material, and it was only used as an adsorbent for heavy metal ions and as an aggregate in the slurry to improve the performance of the cementing material.
3.7 FTIR analysis
Perform FTIR test on pure gel and solidified body, and analyze the results.
According to the analysis of FTIR results (Fig. 10), whether it is made of pure gel or solidified body mixed with chromium-containing sludge, several samples have relatively wide vibration bands near the wave numbers of 3350cm− 1 and 1640cm− 1. These two places correspond to tensile vibration and bending vibration of H-O-H respectively, indicating that bound water is formed after the completion of the hydration reaction(Bernal et al. 2011). There are many weak absorption peaks near 3500cm− 1-4000cm− 1 in the wavenumber, which should be the stretching vibration of the hydroxyl group in the hydrated aluminum compound(Navarro et al. 2010). The peak near the 1412cm− 1 wavenumber is the bending vibration of O-C-O, which has a certain relationship with the reaction of alkali metal oxides with CO2 (Andini et al. 2008, Yousuf et al. 1993). In the FTIR results of the pure slag gelatinous body, an absorption peak appeared at around 935cm− 1 wavenumber and was significantly weakened in the solidified body, and an absorption peak appeared at 861cm− 1 wavenumber (Si-O bending vibration) in the solidified body, which may be due to The Si-OT (T: Si or Al) stretching vibration produced by the tetrahedron of CSH, it indicates that the hydration reaction produces CSH gel(Kovtun et al. 2015). The wave number range of about 421 cm− 1 is caused by the bending vibration of the Si-O-Si and O-Si-O bonds. The right shift or weakening of absorption peaks such as Si-O-T stretching vibration, Si-O-Si and Al-O-Si symmetric stretching vibration should be attributed to the substitution of heavy metals for Si in the CSH structure or the influence of heavy metals on the degree of polymerization of CSH, it Changed the binding environment of the gel(Bakharev et al. 2003). After adding attapulgite soil, H-O-H tensile vibration appeared near the wave number of 3350 cm− 1, and bending vibration of H-O-H appeared near the wave number of 1640 cm− 1. The wave number and peak value did not change significantly, indicating that the generation of bound water was not affected. At the same time, it can be observed that after the addition of attapulgite, the wave number of the main peaks before and after the solidification of heavy metals decreases and the band shift phenomenon becomes smaller. This may be due to the ionization of the attapulgite with heavy metal ions after the addition of attapulgite. The exchange effect produces a change in vibration energy(Mao et al. 2019). The change of the peak value of each wave is also affected by the guarantee of the hydration degree of the cementitious material after the excellent adsorption performance of attapulgite for heavy metals. The silicon-aluminum-oxygen in the reaction product is single negatively charged, which plays a role in balancing the charge and stably bonding the heavy metal ions in the gel. In addition, compared with the case where attapulgite is not added, the FTIR results of the solidified body of attapulgite show that there are more faint and disordered peaks from 420cm− 1 to 680cm− 1. This shows the vibration of the free silica of Si-O-Si(Zhang et al. 2021), and its band shifts to the left, showing that the more active free silica gradually decomposes and participates in the solidify process.