Mechanical Strength, Permeability, and Micromechanics of Municipal Sludge Modified with Calcium-Containing Industrial Solid Waste and Powdered Construction Waste

Abstract

Each year, China produces a substantial amount of municipal sludge, industrial waste (slag, fly ash, and desulfurized gypsum), and construction waste, while its recycling rate is low. If not disposed in a properly and timely manner, this inequity can have serious environmental impacts. This study aimed to prepare a new type of modified sludge material with high strength, low shrinkage, and low permeability by curing municipal sludge with industrial waste (slag, desulfurized gypsum, and fly ash) and powdered construction waste. At specific maintenance ages, the modified sludge material was examined for shrinkage deformation, water content, compressive strength, and hydraulic conductivity. The modified sludge material was also tested by scanning electron microscopy (SEM + EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) tests. The hydration products, micromorphology, and elemental composition of modified sludge were also analyzed at specific maintenance ages. These analyses revealed the mechanism of solidification of municipal sludge by industrial waste and powdered construction waste and the changes in the microstructure of the sludge. The results showed that the compressive strength of the modified sludge ranged from 3.83 to 8.63 MPa, volumetric shrinkage ranged from 2.12 to 12.68%, and hydraulic conductivity ranged from 1.65 × 10⁻⁸ to 2.21 × 10⁻⁷ cm/s after 28 d of maintenance. The active substances, such as SiO₂, Al₂O₃, and CaO, in the industrial waste, powdered construction waste, and municipal sludge were subjected to a hydration reaction in an alkaline environment to produce dense blocks, agglomerates of C-S-H, ettringite, gismondine, and other hydration products. The compressive strength of the modified sludge increased, and its internal structure was dense.

1. Introduction

Municipal sludge has a high water content, poor mechanical properties, and contains abundant organic matter, heavy metals, pathogens, and other toxic and harmful substances. Municipal sludge can cause secondary pollution if improperly treated [1,2,3]. As the urbanization process continues to accelerate in China, the production of construction waste is increasing annually. Construction waste production accounts for 30–40% of total urban waste [4]. The main disposal methods for construction wastes in China continue to be landfilling and storage piles. These two treatment methods occupy considerable amounts of land and waste resources and contribute to safety hazards and environmental pollution. According to the Annual Report on Environmental Prevention and Control of Solid Waste Pollution in Large and Medium-sized Cities in China in 2020, the production of bulk industrial solid waste (tailings, fly ash, coal gangue, smelting slag, furnace slag, and desulfurization gypsum) was approximately 2.9 billion tons as of 2019 [5]. Although the bulk of industrial solid waste does not pose the dangers of hazardous waste, the generation of large quantities of wastes in storage piles occupies a considerable amount of land. Moreover, these circumstances have negative impacts on the goals of promoting solid waste reduction, resource conservation, and environmental protection (the concept of “reduction, resource, and harmless”) in the construction of an ecological civilization [6]. Therefore, there is an urgent need to find a low-carbon, economical method for the sound treatment and resource utilization of sludge, construction waste, and industrial solid waste.

Curing technology is used as a safe and efficient disposal method. Municipal sludge is converted into a solidified mass with an organized structure by adding a curing agent, allowing the municipal sludge to be recycled [7]. At present, researchers worldwide have completed much research on curing municipal sludge. Yang et al. [8,9] tested the unconfined compressive strength, shrinkage characteristics, and leachate toxicity of modified sludge. Their results showed that the unconfined compressive strength was 365.3 KPa, the linear shrinkage was 4.4%, and the heavy metal leachate met the requirements of national specifications after 28 d of curing. He et al. [10] selected lime, cement, sludge, and fly ash as materials to modify sludge. The resulting compressive strength and permeability coefficient were 114.1 KPa and 1.2 × 10⁻⁷ cm/s, respectively, at 7 d under natural indoor maintenance conditions. The modified sludge materials also met basic landfilling requirements. Li et al. [11] used composite conditioning agents such as quicklime and fly ash to dewater and cure sludge and found that the modified sludge material maintained an unconfined compressive strength of 400 KPa and a straight shear strength of 130 KPa after 1 a of maintenance. Liu et al. [12] used slag, desulfurization gypsum, and fly ash to modify municipal sludge. They performed tests to obtain the shear strength parameters, hydraulic conductivity during wet and dry cycles, and service performance of the modified sludge in resisting breakdown due to rainfall. Chen et al. [13] selected cement and fly ash as curing agents and cured contaminated sludge by sodium hydroxide alkali excitation. Studies have shown that fly ash can improve the unconfined compressive strength of modified sludge, and the best fixation effect on organic matter is produced when it is mixed with 0.2% sodium hydroxide. By conducting indoor and outdoor mechanical tests, Lin et al. [14] found that a modified sludge had a low bulk density, low soil particle density, high water content, high porosity ratio, and high liquid-plastic limit. The cured material also contained flaky mineral particles, hydrated calcium silicate, needle-like calcium alumina, and organic flocs with a loose structure and many internal inter-particle pores. Li et al. [15,16] used cement as a curing agent to explore the mechanism governing the change in compressibility of modified sludge from the perspective of water morphology and transformation. Qian et al. [17] used cement as the main agent and fly ash as an auxiliary agent to cure industrial wastewater sludge. The results of their study showed that the strength of the sludge was significantly increased after curing. Taki et al. [18] used lime to solidify and stabilize sewage sludge. The geotechnical engineering properties of the modified sludge, such as unconfined compressive strength, permeability coefficient, expansion, and contraction, were investigated by experiments. Changjutturas et al. [19] used fly ash ground polymer curing to stabilize metal plating sludge. The microscopic pore structure and the basic mechanical characteristics of the modified sludge were investigated. Kaling Sanghwa et al. [20] used cement, lime, ladle slag, or hydroxyapatite for the curing and stabilization of dye sludge. They investigated the suitability of modified sludge as landfill cover material and obtained the unconfined compressive strength and permeability coefficient of the modified sludge. In summary, this study found that current research has focused on using cement, fly ash, and lime as curing agents to solidify and stabilize sludge. These curing materials are high-energy materials that do not meet the ecological concepts of low carbon and environmental protection. Therefore, there is an urgent need to develop a new type of low-carbon, environmentally friendly sludge curing agent.

The objective of this study was to prepare a new type of modified sludge material with a high strength, low shrinkage, and low permeability by curing municipal sludge with industrial waste (slag, desulfurized gypsum, and fly ash) and powdered construction waste. This research used industrial waste (slag, fly ash, and desulfurization gypsum) and powdered construction waste as the main materials to develop a new sludge curing material. Concurrently, the recycling and safe disposal of multi-source solid waste (industrial slag, construction waste, and municipal sludge) were achieved. In this study, the mechanical strength and water infiltration of modified sludge were investigated by unconfined compressive tests, volumetric shrinkage tests, and infiltration tests. Additionally, the surface micromorphology, chemical characteristics, and mineralogical composition of the modified sludge were examined by a combination of swept surface electron microscopy (SEM + EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanism governing the solidification of municipal sludge using industrial and construction waste was revealed.

2. Materials and Test Methods

2.1. Materials

Municipal sludge was collected from a wastewater treatment plant in Wuhan; it comprised 80–90% water, was highly viscous, and emitted a strong smell. The construction waste aggregate was provided by a construction waste treatment plant in Wuhan. The components of construction waste mainly include waste concrete, bricks and tiles, etc. The construction waste was milled and sieved to particle sizes less than or equal to 1 mm. The slag powder, fly ash, and desulfurization gypsum were purchased from SinoCen Smartec Co., Ltd., Wuhan. The slag powder was gray-white, V500-I-grade, with a specific surface area of 428 m²/kg and a particle size of 1–38 µm. The fly ash was a grey-black powder, V1500-I grade. The gypsum used for desulfurization was a pale-yellow powder. The main contents and chemical compositions of municipal sludge, construction waste powder, slag powder, fly ash, and desulfurization gypsum are shown in

Table 1. Chemical composition and content of test materials.

Raw Materials	Main Chemical Composition/%
	CaO	SiO2	Al2O3	Fe2O3	MgO	P2O5	K2O	SO3	TiO	Others
Municipal sludge	6.62	37.19	17.19	10.87	3.25	14.65	3.00	4.09	1.11	2.03
Construction waste powder	35.07	39.87	8.93	6.03	2.01	0.17	2.31	3.09	0.76	1.76
Slag	36.82	26.75	19.66	0.32	11.10	0.36	0.29	2.65	0.94	1.11
Fly ash	3.70	47.70	37.53	4.55	0.94	0.36	1.62	1.04	1.40	1.16
Desulfurized gypsum	45.36	2.38	0.90	0.48	0.62	0.02	0.18	49.16	0.05	0.85

.

2.2. Test Methods

The materials were proportioned using an orthogonal design test method. Varying amounts of municipal sludge, lime, NaOH, industrial waste (slag, desulfurized gypsum, and fly ash), and powdered construction waste were combined and mixed well. The mixture was layered and pressed into a cylindrical test mold 50 mm in diameter and 100 mm in height. We obtained modified sludge with specific ingredient ratios as samples MS1–MS9. The mixture included 45% municipal sludge, 24.75–41.25% powdered construction waste, approximately 3.67–16.18% powdered slag, 3.24–11.00% fly ash, 3.24–11.00% desulfurization gypsum, 1% quicklime, and 1% NaOH. The material formulations of samples MS1–MS9 are shown in

Table 2. Material proportioning table for MS1–MS9 samples.

Sample Number	Construction Waste Powder: Slag: Fly Ash: Desulfurized Gypsum	Construction Waste Powder (%)	Slag (%)	Fly Ash (%)	Desulfurized Gypsum (%)
MS1	9:5:3:3	24.75	13.75	8.25	8.25
MS2	9:3:2:2	30.94	10.31	6.88	6.88
MS3	9:1:1:1	41.25	4.58	4.58	4.58
MS4	9:5:2:1	29.12	16.18	6.47	3.24
MS5	9:3:1:3	30.94	10.31	3.44	10.31
MS6	9:1:3:2	33.00	3.67	11.00	7.33
MS7	9:5:1:2	29.12	16.18	3.24	6.47
MS8	9:3:3:1	30.94	10.31	10.31	3.44
MS9	9:1:2:3	33.00	3.67	7.33	11.00

. The samples were subjected to mechanical, micromechanical, and hydraulic conductivity testing.

2.2.1. Mechanical Properties

The compressive strength of the sludge samples MS1–MS9 was evaluated using a microcomputer-controlled electronic pressure tester (YAW–2000, Jinan Zhongluchang Testing Machine Manufacturing Co., Ltd., Jinan, China) with an axial displacement of 0.01 mm/s after they had been naturally cured for 3, 7, 14, 21, and 28 d. At these maintenance ages, changes in the sample volume and water content of the modified sludge samples were also assessed. The water content of the modified sludge samples was determined by the drying method. Two samples were selected from each group, and the average value was considered to be the final water content value.

2.2.2. Hydraulic Conductivity

The samples of modified sludge were submerged in pure water for vacuum saturation after 28 d of maintenance. To analyze the samples’ hydraulic conductivity, an environmental geotechnical flexible wall permeameter (PN3230M, GEOEQUIP, America) was employed. The test was conducted with upper and lower counter pressures of 30 and 60 KPa, respectively, and an enclosing pressure of 300 KPa. Each sample was a cylinder with 50 mm diameter and 100 mm height.

2.2.3. Micromechanics

MS7 sample aliquots with maintenance ages of 3, 7, and 28 d were tested. SEM + EDS (Gemini SEM 300, Zeiss, Oberkochen, Germany), XRD (SmartLab SE, Rigaku, Japan), and FTIR (Nicolet 6700, Thermo Scientific, America) were used to analyze the hydration products, micromorphology, and elemental composition of sample MS7.

3. Results and Discussions

3.1. Compressive Strength, Volumetric Shrinkage, and Water Content

Figure 1 shows the compressive strength of the modified sludge samples MS1 to MS9 at specific curing ages. The compressive strength of all modified sludge samples increased when increasing the curing age. The compressive strength of the modified sludge samples increased rapidly over the initial 14 d of maintenance. After 14 d of maintenance, the rate of compressive strength increase slowed substantially until reaching a steady state. During days 3–14 of maintenance, the compressive strength of samples MS1–MS9 increased from 0.34–1.22 MPa to 1.77–7.44 MPa. When cured for 28 d, the compressive strength reached 3.83–8.63 MPa. Among the samples, sample MS7 had the greatest compressive strength and sample MS6 had the smallest. The above data show that in the pre-conservation period (0–14 d), a violent hydration reaction occurred between industrial waste, powdered construction waste, and municipal sludge substances under alkaline conditions to produce ettringite, calcite, and hydration gel products. The samples had a certain strength and internal density. In addition, as the maintenance age increased, the water content of the modified sludge decreased, and the hydration reaction proceeded more slowly.

Figure 2 shows the volumetric shrinkage of modified sludge samples MS1 to MS9 at specific curing ages. The rate of volumetric shrinkage of the modified sludge samples showed a trend of first increasing and then stabilizing with an increasing maintenance age. The volumetric shrinkage of samples MS3, MS6, and MS9 varied greatly, increasing from an initial cured shrinkage of 1.63–2.96% to 9.02–12.68%. The volumetric shrinkage of samples MS3, MS6, and MS9 was greater than 4% at 28 d of maintenance, and cracks appeared on the sample surface at 3–7 d of maintenance. However, the volumetric shrinkage of samples MS1, MS2, MS4, MS5, MS7, and MS8 increased slowly from 0.17–1.49% to 2.12–3.47%. The volumetric shrinkage rate stabilized after 14 d of maintenance to a final volumetric shrinkage rate of less than 4%.

Figure 3 shows the water content of the modified sludge samples MS1 to MS9 at specific maintenance ages. As this figure shows, the water content of the modified sludge samples decreased rapidly at first, then more slowly with an increasing maintenance age. The active substances inside the modified sludge reacted chemically with free water substances in the pre-conservation period (0–14 d) to produce calcium alumina and hydrated gelling substances. However, during the conservation process, the modified sludge samples were subjected to internal water evaporation by the elevated temperatures, and the rate of the water content decrease began to slow. The pattern of compressive strength variation also conformed to these two stages. With an increasing curing age, the hydrated cementitious material generated inside the samples increased along with the compressive strength, while the water content decreased.

3.2. Hydraulic Conductivity

Figure 4 shows the hydraulic conductivity curves of modified sludge samples MS1, MS2, MS4, MS5, MS7, and MS8. The hydraulic conductivity demonstrated a pattern of rapid decrease followed by gradual stabilization. The hydraulic conductivity coefficients of samples MS1, MS2, MS4, MS5, MS7, and MS8 were 4.51 × 10⁻⁸, 6.53 × 10⁻⁸, 4.40 × 10⁻⁸, 2.55 × 10⁻⁸, 1.65 × 10⁻⁸ cm/s, and 2.21 × 10⁻⁷ cm/s, respectively. The hydraulic conductivity values of MS1, MS2, MS4, MS5, and MS7 were less than 1.0 × 10⁻⁷ cm/s, demonstrating a good impermeability. This low permeability is a result of the industrial waste, powdered construction waste, and municipal sludge being subjected to a hydration reaction under alkaline conditions, generating minerals such as C-S-H, calcite, ettringite, and gismondine. These substances filled the pores inside the modified sludge, blocking the connectivity of the pore channels and increasing its density.

3.3. Micromechanics

Figure 5 shows the X-ray diffraction patterns of sample MS7 at specific conservation ages. As shown in Figure 5, the characteristic diffraction peaks of C-S-H, calcite, ettringite, natrolite, nekoite, gismondine, and gypsum appear in the modified sludge. After 3 d of maintenance, the diffraction peaks of NaOH were found from the X-ray diffraction pattern of the modified sludge. This indicated that the alkali excitation reaction was incomplete. With an increasing conservation age, the diffraction peaks of NaOH disappeared, the diffraction peaks of SiO₂ decreased, and those of C-S-H, ettringite, calcite, and gypsum increased. In addition, after 28 d of maintenance, new gismondine diffraction peaks appeared. This phenomenon indicates that slag–fly-ash–desulfurization gypsum contains a considerable amount of active substances such as SiO₂, CaO, and Al₂O₃, which undergo hydration reactions under alkali excitation and SO₄²⁻ conditions to produce a substantial amount of minerals, such as C-S-H, ettringite, tamarugite, and gismondine [21]. The generated minerals fill the pores between the particles of the material and enhance the cementation between them, resulting in a denser internal structure and increased strength in the modified sludge. Meanwhile, the formation of ettringite consumes much free water, which can fix a substantial amount of free water in the modified sludge [22].

Figure 6 shows the surface micromorphology of sample MS7 at specific maintenance ages. The images show that the surface of the sample appears sticky, spherical, and lumpy, with a minor amount of agglomerated material after 3 d of maintenance. The mineral particles inside the modified sludge were loosely packed, and substances such as rod-shaped gypsum particles and spherical fly ash particles were unreactive. By 28 d of curing, the spherical fly ash particles on the surface of sample MS7 had disappeared, and a considerable amount of needle-like, lumpy, and agglomerated colloidal material had appeared. The substances were stacked vertically, and the material structures were relatively near each other. Combined with the results of the XRD pattern analysis, the needle-like, lumpy, and agglomerated gelling substances may be hydration products, such as C-S-H, calcite, and ettringite. The results of an energy dispersive spectroscopy (EDS) showed that the modified sludge primarily contained O, Ca, Si, C, and Al after 28 d. The elemental contents were 43.07, 18.49, 15.78, 8.40, and 5.86% for O, Ca, C, Si, and Al, respectively. This also accounts for the generation of gels in the modified sludge samples [23,24].

Figure 7 shows the FTIR spectral curves of sample MS7 at specific conservation ages. From the FTIR map of the modified sludge sample at 3 d, it is evident that the strong wave peak at 471.80 cm⁻¹ is due to the symmetric stretching vibration of Si-O-Si in silica-oxygen tetrahedra, the strong peak at 1031.51 cm⁻¹ is due to the asymmetric stretching vibration of Al-O/Si-O, the weak peaks at 875.27 cm⁻¹ and 1422.26 cm⁻¹ are typical C-O bond asymmetric stretching vibration peaks in carbonates, and the peak at 1114.30 cm⁻¹ is an S-O bond asymmetric stretching vibration peak. Combined with the XRD analysis, the main material generation may be C-S-H, ettringite, calcite, nekoite, and gismondine [25]. The peak bands at 471.80 cm⁻¹ and 1031.51 cm⁻¹ widened with an increasing maintenance age. The reason may be that under the action of alkali-excited NaOH, the silicate polymerization reaction gradually increased in strength, and eventually C-S-H, ettringite, and other minerals were produced [26].

4. Conclusions

The aim of this study was to prepare a new type of modified sludge material with a high strength, low shrinkage, and low permeability using industrial waste (slag, desulfurized gypsum, and fly ash) and powdered construction waste, in order to cure municipal dewatered sludge in an alkaline environment. We examined the patterns of change in the volumetric shrinkage deformation, compressive strength, hydraulic conductivity, and microstructure of modified sludge materials. We also systematically evaluated the macroscopic mechanical properties and microscopic mechanisms acting on the modified sludge materials at specific maintenance ages. After 28 d of maintenance, the water content of the modified sludge material ranged from 6.09 to 15.53%, the volumetric shrinkage from 2.12 to 12.68%, the compressive strength from 2.89 to 8.63 MPa, and the permeability coefficient from 1.65 × 10⁻⁸ to 2.21 × 10⁻⁷ cm/s. The shrinkage rate of samples MS3, MS6, and MS9 was greater than 4%, while that of samples MS1, MS2, MS4, MS5, MS7, and MS8 was less than 4%. Sample MS7 demonstrated the maximum compressive strength at 8.63 MPa, and the hydraulic conductivities of samples MS1, MS2, MS4, MS5, and MS7 were all less than 1.00 × 10⁻⁷ cm/s. The active substances in industrial waste, powdered construction waste, and municipal sludge, such as SiO₂, Al₂O₃, and CaO, generate dense blocks, agglomerates, C-S-H, ettringite, and other hydration products under the action of alkali excitation. This resulted in an increased strength and internal density of the modified sludge.