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. 2023 Aug 18;26(9):107651. doi: 10.1016/j.isci.2023.107651

Test study on mechanical properties of compound municipal solid waste incinerator bottom ash premixed fluidized solidified soil

Yawen Zhao 1, Na Zhang 1,3,∗, Xuemei Chen 2
PMCID: PMC10484961  PMID: 37694135

Summary

In this paper, the municipal solid waste incineration bottom ash (MSW-IBA) is used as the fine aggregate and firming agent component of the premixed fluid stabilized soil (PFSS). Through the mechanical strength test, and the hydration products and microstructure characterization, the effects of the NaOH content for MSW-IBA pre-aging treatment and activator content on the mechanical properties of the PFSS are studied. The results show that the mechanical strength of the prepared PFSS is closely related to the amount of NaOH and the activator. The increase in the amount of NaOH and activator can affect the formation of early hydration product Aft, and reduce the early strength. However, it can promote the depolymerization of glass phase in MSW-IBA and slag to form C-(A)-S-H and Mg-(A)-S-H gel, increase the microstructure density, and thus improve the late strength. The application of MSW-IBA in PFSS can achieve the recycling of solid waste.

Subject areas: Environmental chemical engineering, Soil

Graphical abstract

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Highlights

  • •

    The MSW-IBA is used instead of slag as a firming agent component

  • •

    The screened MSW-IBA particles are used as fine aggregates

  • •

    The application of MSW-IBA in PFSS can achieve the recycling of solid waste

Environmental chemical engineering; Soil

Introduction

In recent years, with the rapid development of urbanization construction and the rapid growth of urban population, the output of municipal solid waste (MSW) has increased year by year. According to the statistics of the China Environmental Statistics Yearbook 2019,1 the amount of MSW in China reached 228 million tons in 2018, which has seriously hindered the sustainable development of China’s economy and society. At present, the treatment of MSW is still based on landfill, composting, recycling, and incineration.2 However, incineration, as a mature, advanced, large treatment capacity and recyclable useful resources treatment technology, has been widely used in most cities of China. But at the same time, it also faces many new problems. The incineration of MSW can produce 20%–30% bottom ash (BA) of the mass of original MSW.3 Taking 2018 as an example, 117.06 million tons of MSW were actually incinerated nationwide, and the amount of bottom ash was about 23.41–35.11 million tons.1 Such a large number of BA disposal problems have not been well solved. In order to save the increasingly tense landfill site, reduce the disposal cost of BA, and the risk of secondary pollution, the recycling of BA is a feasible method to promote the sustainable development of society, and has become the focus of current research.

The municipal solid waste incineration bottom ash (MSW-IBA) is a heterogeneous mixture composed of glass, ceramics, molten-slag, unburned materials, etc.3 It is a large particle aggregate formed by irregular small particles, and the particle sizes are mainly concentrated in the range of 2–5 mm.4 The main components of MSW-IBA include about SiO2: 35%–40%, Al2O3: 10%–20%, Fe2O3: 5%–10%, CaO: 10%–20%, and a small amount of Na2O, K2O, MgO, and TiO2.5,6 The leaching concentrations of As, Cd, Cr, Cu, Pb, Ni, Zn, etc. in the incineration bottom ash are lower than the standard limit value of Identification standards for hazardous wastes-Identification for extraction toxicity (GB 5085.3-2007), and belong to general solid waste.7,8,9 At present, a great deal of research has been carried out at home and abroad on the resource utilization of MSW-IBA, but it is still in the development stage. Its utilization approaches are mainly concentrated in the sewage treatment system,9 landfill cover,10 unburned environmental protection bricks,11 as substitute aggregate of concrete12,13 and asphalt,14 and subgrade filling materials.15 Gao Z.X.16 studied the key technology of MSW-IBA used for subgrade filling, and the results showed that the construction technology of MSW-IBA subgrade pavement filling is basically the same as that of gravel subgrade and conventional pavement base, without special equipment and construction technology. Sheng P.L. et al.17 evaluated the feasibility of using MSW-IBA as fine aggregate in ultra-high-performance concrete (UHPC), and the results showed that due to the internal curing effect, adding appropriate amount of MSW-IBA into UHPC can improve the compressive strength of UHPC, while its mechanical strength decreases significantly when the amount is too high. The microscopic analysis shows that the addition of MSW-IBA promotes the hydration degree of UHPC, increases the compactness of matrix, and prevents the occurrence and expansion of cracks. Therefore, it is feasible to use MSW-IBA in UHPC. In addition, Li X.G. et al.18 and Zhang S.P. et al.19 showed that the MSW-IBA can be used as quartz sand and auxiliary cementitious material in autoclaved aerated concrete and dry hard concrete, with good application effect. Caprai V. et al.20 studied the replacement of sand in mortar by sodium silicate impregnated MSW-IBA. The results showed that the pore volume of MSW-IBA treated with sodium silicate impregnation decreased by 2.5 times, making the sand replacement rate reach 100%, and the rheological property of mortar increased to 38% compared with untreated control sample. Alderete N.M. et al.21 studied the application of MSW-IBA as a substitute for Portland cement in concrete, with the replacement amount reaching 20% of the binder. Maldonado Alameda A. et al.22 studied the potential use of MSW-IBA as a precursor for the synthesis of new alkali-activated cement. The results showed that MSW-IBA as a precursor for the new alkali-activated cement mainly depends on the particle size distribution and the ratio of Si to Al, which can be used as the only precursor or mixed with other precursors.

The premixed fluidized solidified soil (PFSS) is to mix the original soil source (waste soil) with special cementitious materials (firming agent), water, and additives evenly according to the needs of the project and the soil characteristics to form a mixture convenient for construction. The key technology for the preparation of PFSS is to optimize the composition of firming agent, and prepare PFSS with good fluidity, small dry shrinkage deformation, high compressive strength, good permeability, and durability. It can significantly save labor costs, and can coordinate the disposal of large solid wastes such as construction waste including engineering mud, low-quality industrial solid waste, etc. It can be used for the filling of municipal projects such as pipe gallery, temporary ground hardening, road engineering, and foundation engineering.23,24,25,26,27 As a new type of engineering material with excellent performance, green and environmental protection, the PFSS has broad application prospects because of its strong inclusiveness and wide adaptability, and will become a supporting solution for “waste-free city”. However, at present, the resource utilization of the MSW-IBA in China is still in the initial stage. There is no relevant technical research on the application of MSW-IBA as fine aggregate and firming agent component in PFSS. According to the characteristics of soft soil in Ningbo area, the MSW-IBA and its ground ash are used as fine aggregate and firming agent components. Through the regulation of its content and activator content, the mechanical strength of PFSS is optimized to meet the requirements for workability and mechanical strength of PFSS in various foundation pit and foundation cushion laying and backfilling construction projects, thereby achieving the recycling and utilization of solid waste and promoting the sustainable development of the construction industry.

Results and discussion

Effect of NaOH content on aging effect of MSW-IBA and mechanical strength of PFSS

According to previous experimental exploration and literature research, it is found that the unburned organic substances in MSW-IBA ground ash can react with alkali to release a large amount of gas under the action of activator, which can seriously affect the compressive strength of PFSS. In order to eliminate the influence of unburned organics in MSW-IBA on the performance of its products, the researchers tried various technical measures. Compared with the re-high temperature sintering, the pre-aging treatment of NaOH solution has technical, economic, and environmental benefits.29 According to the mix ratio in Table 1, where the content of MSW-IBA ground ash is 30% of the slag mass, the effects of NaOH concentration (4.0, 4.8, and 5.6 mol/L) and the content on the aging effect of MSW-IBA ground ash and the compressive strength of PFSS were studied. As shown in Figures 1A–1C, a large number of bubbles are generated in the aged MSW-IBA ground ash. When the NaOH concentration is 4.8 and 5.6 mol/L, the MSW-IBA ground ash has the larger gas emission and the better aging effect. With the increase of NaOH content, the time required for MSW-IBA ground ash aging treatment decreases.

Table 1.

Mix proportion of PFSS with MSW-IBA ground ash aged by different concentrations of NaOH

NaOH
(mol/L)		 Samples Soil
(g)		 Firming agent (%) Firming agent composition (%)
 Water (g)
Key component (%)
 Activator
 Water reducer
Slag MSW-IBA ground ash Cement Gypsum Sodium silicate
4.0 1 1000 30 45.5 19.5 25 10 18 1.5 580.2
2 1000 30 45.5 19.5 25 10 18 1.5 574.0
3 1000 30 45.5 19.5 25 10 18 1.5 567.8
4.8 4 1000 30 45.5 19.5 25 10 18 1.5 579.7
5 1000 30 45.5 19.5 25 10 18 1.5 573.3
6 1000 30 45.5 19.5 25 10 18 1.5 566.9
5.6 7 1000 30 45.5 19.5 25 10 18 1.5 579.2
8 1000 30 45.5 19.5 25 10 18 1.5 572.6
9 1000 30 45.5 19.5 25 10 18 1.5 566.0
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Figure 1.

Figure 1

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Effect of concentration and content of NaOH on compressive strength of PFSS

(A) 4.0 mol/L.

(B) 4.8 mol/L.

(C) 5.6 mol/L.

Figure 2 shows the compressive strength of PFSS with MSW-IBA ground ash treated with different concentrations and amounts of NaOH. It can be seen that the compressive strength of the samples increases with the increase of curing age. The PFSS prepared with MSW-IBA ground ash aged with 4.8 mol/L NaOH has high early compressive strength. When its content is 0.55 times the mass of MSW-IBA ground ash, the compressive strength of PFSS at 1, 7, and 28 days is 1.8, 2.7, and 3.5 MPa. When the NaOH concentration is 5.6 mol/L and the content is 0.6 times the mass of MSW-IBA ground ash, the compressive strength of PFSS at 1, 7, and 28 days is 0.9, 2.7, and 4.4 MPa, respectively. The results show that the compressive strength of PFSS is closely related to the concentration and amount of NaOH used for aging treatment. The PFSS with MSW-IBA ground ash treated with 4.8 mol/L NaOH has high early strength. When its concentration and content increase, it can reduce its early compressive strength and contribute to its later compressive strength. In comprehensive consideration of the contribution of NaOH concentration to the compressive strength of PFSS with MSW-IBA ground ash, 4.8 mol/L NaOH was selected for the subsequent experiment to pre age MSW-IBA ground ash.

Figure 2.

Figure 2

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Physical property of PFSS with MSW-IBA ground ash

(A) 4.0 mol/L.

(B) 4.8 mol/L.

(C) 5.6 mol/LF.

Effect of MSW-IBA ground ash content on workability and mechanical strength of PFSS

In addition to mechanical strength, fluidity is another important indicator of PFSS, which affects its construction operability. Table 2 shows the mix ratio of PFSS with different amounts of MSW-IBA ground ash. The MSW-IBA ground ash is pre aged with 4.8 mol/L NaOH. The ratio of its amount to the mass of MSW-IBA ground ash is 0.55, and the amount of activator is 18% of the total mass of slag and MSW-IBA ground ash. As shown in Figure 3A, the fluidity of PFSS decreases with the increase of the content of MSW-IBA ground ash. When the content is 10%, 30%, and 50% of the slag mass, the corresponding fluidity values are 183, 150, and 127 mm, respectively. It can be seen that the water demand for MSW-IBA ground ash is large.

Table 2.

Mix ratio of PFSS with different content of MSW-IBA ground ash

Samples Soil
(g)		 Content of curing agent (%) Curing agent composition (%)
 Water (g)
Key component
 Activator
 Water reducer
Slag MSW-IBA ground ash Cement Gypsum Sodium silicate
S-0 1000 30 65 0 25 10 18 1.5 600
S-10 1000 30 58.5 6.5(10) 25 10 18 1.5 600
S-30 1000 30 45.5 19.5(30) 25 10 18 1.5 600
S-50 1000 30 32.5 32.5(50) 25 10 18 1.5 600
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Figure 3.

Figure 3

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Physical property of PFSS with MSW-IBA ground ash

(A) Fluidity.

(B) Compressive strength.

Figure 3B shows the variation of compressive strength of PFSS with content of MSW-IBA ground ash. It can be seen that the compressive strength of the samples increases with the increase of curing ages. When the content of MSW-IBA ground ash is 30%, its compressive strengths at 1, 7, and 28 days are 1.4, 2.4, and 3.6 MPa, respectively; with the increase of the content of MSW-IBA ground ash, the compressive strength of PFSS at 1, 7, and 28 days showed a decreasing trend. When the content of MSW-IBA ground ash is 0%, 10%, 30%, and 50%, the corresponding 28-day compressive strength is 4.2, 4.0, 3.6, and 2.5 MPa, respectively. The results show that when the content of MSW-IBA ground ash is 30%, the PFSS prepared by MSW-IBA can basically meet the requirements for workability and mechanical strength of various foundation pit fills.30 However, due to the increase in the amount of MSW-IBA ground ash, the compressive strength of PFSS may decrease due to the insufficient amount of activator to fully stimulate the potential activity of MSW-IBA ground ash.

Effect of activator content on mechanical strength of PFSS

The development of mechanical strength of PFSS mainly comes from the hydration of firming agents.31 However, in addition to Portland cement, active mineral admixtures such as fly ash and slag are also important components of the firming agent, which can hydrate to form ettringite crystals, C-S-H and C-A-S-H gel, and other hydration products to cement soil particles.32,33 Therefore, it can greatly reduce the consumption of Portland cement, save costs, and increase its green environmental characteristics. Table 3 shows the mix ratio of PFSS with different amounts of activator. The content of activator is 16%, 18%, 20%, 22%, and 24% by mass of slag and MSW-IBA ground ash, respectively. It can be seen from Figure 4A that the PFSS prepared from MSW-IBA ground ash treated with 125 mL of NaOH has the highest compressive strength when the amount of activator is 18%, with compressive strengths of 1.8, 2.6, and 3.9 MPa at 1, 7, and 28 days, respectively. In Figure 4B, the PFSS prepared from MSW-IBA ground ash treated with 155 mL of NaOH has the highest compressive strength when the amount of activator is 22%, with compressive strengths of 0.9, 2.9, and 6.3 MPa at 1, 7, and 28 days, respectively. It can be seen that when the amount of pre-aging NaOH is low, the early compressive strength of the sample is higher, but the later compressive strength is lower. When the amount of pre-aging NaOH is high, the early compressive strength of the sample is lower, but the later compressive strength is higher. When the alkali concentration is high, it is not conducive to the formation of ettringite to contribute to the early strength, but it can effectively stimulate the activity of slag and MSW-IBA to form C-(A)-S-H gel, which is conducive to the later strength growth. When the alkali concentration is relatively low, it is beneficial for the formation of needle-like ettringite to contribute to its early strength.34

Table 3.

Mix ratio of PFSS with different activator contents

Samples Soil (g) Firming agent (%) Curing agent composition (%)
 Water (g)
Key component
 Activator
 Water reducer
Slag MSW-IBA ground ash Cement Gypsum Sodium silicate
125–16 1000 30 45.5 19.5 25 10 16 1.5 573.3
125–18 1000 30 45.5 19.5 25 10 18 1.5 573.3
125–20 1000 30 45.5 19.5 25 10 20 1.5 573.3
125–22 1000 30 45.5 19.5 25 10 22 1.5 573.3
125–24 1000 30 45.5 19.5 25 10 24 1.5 573.3
155–16 1000 30 45.5 19.5 25 10 16 1.5 566.9
155–18 1000 30 45.5 19.5 25 10 18 1.5 566.9
155–20 1000 30 45.5 19.5 25 10 20 1.5 566.9
155–22 1000 30 45.5 19.5 25 10 22 1.5 566.9
155–24 1000 30 45.5 19.5 25 10 24 1.5 566.9
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Figure 4.

Figure 4

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Effect of activator content on compressive strength of PFSS

(A) 125 mL (amount of NaOH for aging treatment).

(B) 155 mL (amount of NaOH for aging treatment).

Compressive strength of PFSS with MSW-IBA

Table 4 shows the mix ratio of PFSS with MSW-IBA, with the MSW-IBA content as a percentage of the mass of soil and solidified agent. The effect of MSW-IBA content on the workability and mechanical strength of PFSS was studied under the condition that the amount of activator was 18% of the slag mass. Under the same fluidity conditions, the water consumption increases with the increase of MSW-IBA content, due to the high porosity of MSW-IBA and large water demand. Figure 5 shows the variation of compressive strength of PFSS with different amounts of MSW-IBA with curing ages. From Figure 5, the early strength (1 day) of PFSS significantly decreases with the increase of MSW-IBA content. However, when the content is 30%, the compressive strength of A-30 sample increases significantly with the increase of curing age, and its 28-day compressive strength can reach 5.2 MPa, which is equivalent to the compressive strength of the S-0 sample without MSW-IBA in Table 3. This result indicates that MSW-IBA can be used as fine aggregate to improve the compressive strength of PFSS, thus achieving multiple economic, environmental, and technical benefits. MSW-IBA particles mainly play a skeleton support role in PFSS, which can reduce the volume changes caused by the formation of early hydration products and later shrinkage during the hardening process of PFSS. At the same time, due to the porous nature of MSW-IBA particles, it can provide growth space for the hydration products, and increase their bonding ability with soil particles and the hydration products of the firming agent, which is conducive to the later strength development of PFSS.

Table 4.

Mix ratio of PFSS with MSW-IBA

Samples Soil (g) Content of curing agent (%) Curing agent composition (%)
 Fine aggregate (%)
 Water (g)
Key component
 Activator
 Water reducer
Slag Cement Gypsum Sodium silicate MSW-IBA
A-10 1000 30 65 25 10 18 1.5 10 600
A-20 1000 30 65 25 10 18 1.5 30 650
A-30 1000 30 65 25 10 18 1.5 50 700
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Figure 5.

Figure 5

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Compressive strength of PFSS with different content MSW-IBA

According to the mix ratio design of PFSS in Table 5, the effects of pre-aging NaOH content and activator content on the compressive strength of PFSS were studied. The content of MSW-IBA is 30% of the total mass of soil and firming agent, and the content of its ground ash is 30% of the mass of slag. As shown in Figure 6, the mechanical strength of PFSS is closely related to the amount of NaOH solution used for aging treatment and the amount of activator added. From Figure 6A, when the amount of NaOH is 125 mL and the amount of activator added is 18%, the compressive strength of 125-18 sample at 28 days is higher, up to 3.9 MPa, but 1-day compressive strength is relatively lower, only 0.6 MPa; from Figure 6B, when the content of NaOH is 155 mL and the activator content is 24%, the compressive strength of 155-24 sample is relatively high, with 1.0, 2.3, and 3.3 MPa for 1, 7, and 28 days, respectively; from Figure 6C, when the content of NaOH is 185 mL and the activator content is 22%, the compressive strength of 185-22 sample at 1, 7, and 28 days is 1.1, 3.1, and 4.6 MPa, respectively. The results show that increasing the amount of activator is beneficial to the development of early compressive strength of the PFSS, and with the increase of the amount of NaOH solution used for aging treatment, the mechanical strength of the PFSS with MSW-IBA significantly increases. This is closely related to the composition of hydration products.

Table 5.

Mix ratio design of PFSS with MSW-IBA and its ground ash

Samples Soil
(g)		 Content of firming agent(%) Firming agent composition (%)
 Fine aggregate (%)
 Water (g)
Key component
 Activator
 Water reducer
Slag MSW-IBA ground ash (30%) Cement Gypsum Sodium silicate MSW-IBA
125–18 1000 30 45.5 19.5 25 10 18 1.5 30 623.3
125–22 1000 30 45.5 19.5 25 10 22 1.5 30 623.3
125–24 1000 30 45.5 19.5 25 10 24 1.5 30 623.3
155–18 1000 30 45.5 19.5 25 10 18 1.5 30 616.9
155–22 1000 30 45.5 19.5 25 10 22 1.5 30 616.9
155–24 1000 30 45.5 19.5 25 10 24 1.5 30 616.9
185–18 1000 30 45.5 19.5 25 10 18 1.5 30 610.5
185–22 1000 30 45.5 19.5 25 10 22 1.5 30 610.5
185–24 1000 30 45.5 19.5 25 10 24 1.5 30 610.5
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Figure 6.

Figure 6

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Mechanical strength of PFSS with MSW-IBA and its ground ash

(A) 125 mL.

(B) 155 mL.

(C) 185 mL.

Hydration products

MSW-IBA ground ash is mainly composed of active oxides such as SiO2, Al2O3, and MgO, with potential hydration activity. The early hydration products of PFSS under the action of activator are mainly unreacted Ca-Mg-Al-Si-O glass phase, as well as SiO2, ettringite (Aft), calcium silicate hydrate (C-(A)-S-H), and magnesium silicate hydrate (M-(A)-S-H). As shown in Figures 7A and 7B, when the amount of activator is 16%, there is still a glass phase bulging peak in the hydration product, and no obvious Aft peak is observed. However, with the increase of activator content, Ca-Mg-Al-Si-O glass phase depolymerized and polymerized with the active silicon provided by sodium silicate to generate C-(A)-S-H and Mg-Al-S-H gel. In the later stage of hydration, the main hydration products are SiO2 and C-(A)-S-H, which contribute to its mechanical strength. Figures 7C and 7D show the variation of hydration product composition with curing age for 125-18 and 155-22 samples, respectively. As shown in Figure 7D, when the amount of NaOH and activator increases, it can effectively promote the active reaction of MSW-IBA to form C-S-H to contribute to the mechanical strength of PFSS, so the 155-22 sample has excellent mechanical strength. The results show that when the alkali concentration is high, it is not conducive to the formation of ettringite to contribute to the early strength, but it can effectively stimulate the activity of slag and MSW-IBA to form C-(A)-S-H gel, which is conducive to the later strength growth.35 When the alkali concentration is relatively low, it is beneficial for the formation of needle-like ettringite to contribute to its early strength, which is consistent with the compressive strength results.

Figure 7.

Figure 7

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The composition of hydration products and

(A) 1 day (varies with the dosage of activator).

(B) 28 days (varies with the dosage of activator).

(C) 125-18 sample (changes with curing age).

(D) 155-22 sample (changes with curing age).

Fourier transform infrared spectroscopy

Figure 8 shows the infrared spectrum of PFSS samples with different amounts of activator after 28 days hydration. As shown in Figure 8, the absorption peaks around 3440 and 1644 cm−1 are caused by OH− bending vibration in water, which is the bound water in the hydration product C-(A)-S-H gel. The absorption peak near 466 cm−1 is caused by the bending vibration of O-Si-O,36 the absorption peak near 782 cm−1 is caused by the symmetric stretching vibration of Al-O-Si bonds or Si-O-Si bonds 1,17,18 the absorption peak near 1030 cm−1 is caused by the asymmetric stretching vibration of Si-O in the SiO4 tetrahedron in C-(A)-S-H, and the absorption peak near 3628 cm–1 is caused by the stretching vibration of Ca(OH)2 hydroxyl groups. The results show that activators Na2SiO3 and NaOH can effectively promote the hydration of MSW-IBA ground ash and slag to form C-(A)-S-H gel and improve the mechanical strength of PFSS with MSW-IBA.

Figure 8.

Figure 8

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The infrared spectrum of PFSS samples

Mercury intrusion porosimetry

The mechanical properties of PFSS are largely affected by its microstructure. Figure 9 shows the porosity and pore size distribution of samples 125-18 and 155-22 after hydration for 1 and 28 days, and the pore structure characteristics are shown in Table 6. From Figure 9, the porosity of 125-18 and 155-22 samples was 24.9% and 37.2%, respectively after curing for 1 day. As the hydration reaction progressed, the microstructure of the sample gradually became dense, and the porosity decreased to 22.9% and 19.0% after 28 days, respectively. The corresponding most probable pore size was also reduced from 75.9 and 88.7 nm to 51.0 and 55.1 nm, respectively. Therefore, the compressive strength of the sample increases significantly with the increase of curing age. However, it is worth noting that with the increase in the amount of NaOH and activator, the porosity of the sample is higher in the stage of early, and the most probable pore size is also larger. However, with the curing age increasing to 28 days, the porosity and the most probable pore size decrease significantly of samples, indicating that the increase of the amount of NaOH and activator is conducive to the activation and depolymerization of active admixtures such as slag and MSW-IBA ground ash, and a large number of gel-like hydration products such as C-(A)-S-H and M-A-S-H are formed in the later stage to contribute to the mechanical strength, which is consistent with the compressive strength results shown in Figure 4.

Table 7.

Water reducing agent used

Name Type Water-reducing rate (%) Appearance
Polycarboxylate-based superplasticizer PC-1021 ≥25 white to pale pink
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Figure 9.

Figure 9

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Pore structure characteristics of samples

(A) Porosity.

(B) Pore distribution curves.

Table 6.

Pore structure characteristics of samples

Samples Porosity (%) Most can be several aperture (nm) <10 nm (%) 10–100 nm (%) >100 nm (%)
125-18-1d 24.9 75.9 0.0 55.5 44.5
125-18-28d 22.9 51.0 0.3 83.1 16.6
155-22-1d 37.2 88.7 0.4 60.8 38.8
155-22-28d 19.0 55.1 0.8 75.3 23.9
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SEM

Figure 10 show the microstructure of hydration products of PFSS samples with curing age. As shown in Figures 10B and 10E, a large amount of needle-like Aft can be seen in the matrix of 155-22 and 125-18 samples after hydration for 7 days, contributing to the early mechanical strength of the PFSS. Compared to 155-22 sample, the 125-18 sample havs a relatively high Aft content and thick grain, resulting in higher early strength. At the same time, a large number of square crystals can also be seen in the matrix, which are embedded on the surface of C-S-H gel. However, compared to 125-18 sample, the 155-22 sample has a higher content of square crystals. Relevant literature shows that square crystal is an ordered crystal product in alkali-activated materials. The square crystal phase is closely combined with gel, which can improve the volume shrinkage of alkali slag reaction to a certain extent. However, due to the high amount of NaOH used in the aging treatment of 185-22 sample, there is no obvious Aft in the sample hydrated for 7 days. After 28 days of hydration, the component of the firming agent is fully hydrated, and a large amount of C-S-H or C-A-S-H gel with reticular structure can be seen in the sample matrix, which can be cemented with other hydration products to form a dense spatial network structure to wrap the soil particles to form a consolidation body with certain mechanical strength. Therefore, the compressive strength of the sample increases significantly after 28 days of standard curing. According to the results of compressive strength, hydration products, and microstructure of PFSS, the increase of alkali concentration is not conducive to the formation of hydration products Aft to contribute to its early strength, but will promote the formation of gel-like hydration products such as C-S-H, C-A-S-H, and M-A-S-H to contribute to its late strength.

Figure 11.

Figure 11

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Chemical composition of MWS-IBA ground fine ash and soil

(A) XRD.

(B) SEM.

Figure 12.

Figure 12

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Compressive strength test

(A) Testing process.

(B) Apparent morphology of the sample after testing.

Figure 10.

Figure 10

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Changes of micro-morphology of PFSS with curing ages

(A) 1 day (125-18 Sample).

(B) 7 days (125-18 Sample).

(C) 28 days (125-18 Sample).

(D) 1 day (155-22 Sample).

(E) 7 days (155-22 Sample).

(F) 28 days (155-22 Sample).

(G) 1 day (185-22 Sample).

(H) 7 days (185-22 Sample).

(I) 28 days (185-22 Sample).

Conclusion

In this paper, the effects of MSW-IBA and its ground ash as fine aggregates and firming agent components on the mechanical strength of PFSS are studied, and the conclusions are as follows.

  • 1.

    The mechanical strength of PFSS is closely related to the concentration and amount of NaOH used for MSW-IBA pre-aging treatment, as well as the amount of activator. When the alkali concentration of the slurry is relatively low, the sample can form a large number of needle-like Aft to contribute to its early strength; when the alkali concentration of the slurry is relatively high, the amount of Aft in the sample is relatively small, and the early strength is low, but the amount of C-S-H gel like hydration products is relatively high, and the microstructure is compact, so it has a high late strength.

  • 2.

    The main hydration products that contribute to the early strength of PFSS with MSW-IBA are hydration product Aft, while the main hydration products that contribute to the development of later strength are C-(A)-S-H and Mg-(A)-S-H.

  • 3.

    The PFSS prepared with 30% MSW-IBA by mass of soil as fine aggregate and 30% MSW-IBA ground ash by mass of slag as firming agent component has excellent mechanical strength and microstructure characteristics when the NaOH content to MSW-IBA mass ratio is 0.7, and the activator content is 22%. It can provide a PFSS with adjustable strength and excellent performance for various foundation pit filling construction.

Limitations of the study

In this paper, the study of MSW-IBA as fine aggregate is not deep enough, in order to consider the influence of activator on fine aggregate. In actual industrial application, the temperature cannot guarantee under the standard curing temperature; the mechanical strength of the PFSS will change. Therefore, it is necessary to systematically study the effect of temperature on the mechanical properties of PFSS.

STAR★Methods

Key resources table

REAGENT or RESOURCE SOURCE IDENTIFIER
Deposited data
The amount of MSW in China China Environmental Statistics Yearbook 2019 https://www.mee.gov.cn/hjzl/sthjzk/sthjtjnb/202108/W020210827611248993188.pdf
The leaching concentrations of harmful metal elements Identification standards for hazardous wastes-Identification for extraction toxicity https://ebook.chinabuilding.com.cn/zbooklib/bookpdf/probation?SiteID=1&bookID=97075
Mechanical strength range of PFSS Technical standard for filling engineering of premixed fluidized solidified soil https://www.doc88.com/p-73247000271483.html
[Database]: [Dataset for ‘‘Test study on mechanical properties of compound municipal solid waste incinerator bottom ash premixed fluidized solidified soil’’] Mendeley Data https://doi.org/10.17632/GKC753R7XM.1 https://data.mendeley.com/preview/gkc753r7xm
Software and algorithms
Microsoft Excel Microsoft Excel v.16.0
OriginPro OriginLab Origin64
Open in a new tab

Resource availability

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Na Zhang (zhangna@nbu.edu.cn).

Materials availability

This study did not generate new unique reagents.

Method details

Raw materials

The waste soil was taken from the Yucai Wantang Power Engineering Project, Ningbo City, Zhejiang Province. The soil depth was 1.5∼2.5 m, and it belongs to clay. The original soil obtained is dried to constant weight at a constant temperature of 110°C, impurities such as rocks are removed, crushed by a crusher, and passed through 1.0 mm square mesh sieve. The MSW-IBA is from Ningbo Bohan Environmental Protection Technology Co., Ltd. As a fine aggregate, the original MSW-IBA passes through 0.3 mm square mesh sieve to remove the powder for standby. The MSW-IBA ground fine ash, which is a component of the firming agent, is ground by a ball mill (30 min) to approximately 400 mesh (0.038mm). The Portland cement (P. O 42.5) is from Ningbo Conch Cement Co., Ltd.; slag (S95) is from Ninghai Hongji New Material Co., Ltd.; and water reducing agent (see table below) is from Suzhou Xingbang Additives Co., Ltd., as shown in below table.

The modulus of sodium silicate n=3.3, wherein the content of Na2O is 8.3% and the content of SiO2 is 26.5%; the dihydrate gypsum (98%, commercial purity) and sodium hydroxide (analytical purity, 96.0%) are both from Shanghai Aladdin Reagent Co., Ltd.

The mineral composition and micromorphology of MSW-IBA ground ash and soil were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM), as shown in below figure.

The main mineral composition of both is quartz, and MSW-IBA ground ash also contains a certain amount of calcium magnesium aluminosilicate glass phase, gypsum, and aluminides; In addition to quartz, the soil also contains minerals such as magnesium silicate and illite. The ground MSW-IBA exhibits various micro morphologies due to its diverse composition. The prismatic polyhedron may be quartz minerals, while the spherical polyhedron may be magnesium silicate minerals.

Specimen preparation

The PFSS is composed of construction waste soil and firming agent, with the content of firming agent accounting for 30% of the soil mass; The firming agent consists of Portland cement, slag, gypsum, activator (sodium silicate, n=1.2), and water reducing agent. The amount of slag, cement and gypsum added is 65%, 25% and 10% by mass of the firming agent, respectively. The amount of water reducing agent added is 1.5% by mass of the firming agent, and the amount of activator added is a percentage of the total mass of slag and MSW-IBA ground ash. Firstly, the MSW-IBA ground ash is pre aged, mixed with a certain concentration of NaOH, and allowed to stand for several hours until the gas in the MSW-IBA is completely released. It is mixed with soil, firming agent, and fine aggregate to prepare PFSS slurry. The slurry was molded into a plexiglass mold with a diameter of 40 mm and a height of 80 mm, and cured to the required age under standard curing conditions of 20 ± 2°C and 95 ± 3% relative humidity.

Test methods

The fluidity of slurry is tested according to Methods for testing uniformity of concrete admixture (GB/T8077-2012).28 The compressive strength is conducted using an electro-hydraulic servo universal testing machine (WDW-100) with a loading rate of 1 mm/min. The testing process is shown in below figure.

The samples used for microscopic testing was soaked in isopropanol to its hydration reaction. Before samples preparation, vacuum drying is required for at least 72 hours. The samples for XRD analysis are ground into a powder of 70 μm, and the XRD data were collected at a scanning speed of 8 °/min in the 2θ angle range of 5° to 80°. The samples used for micro pore structure test were split into 3-5 mm fragments and tested by the mercury intrusion porosimetry (MIP). The samples used for micro morphology test were split into thin slices, and the fracture surface was viewed by scanning electron microscope (SEM). Fourier transformed infrared spectroscopy (FTIR) was performed using a Thermo Scientific Nicolet IS10 spectrometer. The spectral analysis was carried out in the range of 4000-400 cm-1.

Acknowledgments

The authors appreciate the Open Fund Project of Sichuan Urban Solid Waste Energy and Building Materials Conversion and Utilization Technology Engineering Research Center (GF2022YB006) and the K.C. Wong Magna Fund in Ningbo.

Author contributions

Y.W.Z.: Investigation, Formal analysis, Conceptualization, Methodology, Software: Data curation, Writing – Original draft preparation, Writing – Reviewing, and Editing; N.Z.: Project administration, Supervision, Visualization, Writing – Reviewing, and Editing; X.M.C.: Resources, Formal analysis, Methodology, Reviewing, and Editing.

Declaration of interests

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.

Inclusion and diversity

We support inclusive, diverse, and equitable conduct of research.

Published: August 18, 2023

Data and code availability

  • •

    All original data has been deposited at Mendeley Data and is publicly available as of the date of publication. DOI is listed in the key resources table.

  • •

    All data were analyzed with standard programs and packages, as detailed in the key resources table.

  • •

    Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

  • •

    All original data has been deposited at Mendeley Data and is publicly available as of the date of publication. DOI is listed in the key resources table.

  • •

    All data were analyzed with standard programs and packages, as detailed in the key resources table.

  • •

    Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

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