Mechanical Characteristics and Micro-Mechanism of Modified Dredged Sludge Based on Calcium-Containing Solid Waste Used as Landfill Cover Materials

Kejian Shang

doi:10.3390/pr10030451

Abstract

In order to prepare a new type of landfill covering material for closure, we used industrial calcium-containing waste (construction rubbish, slag, desulfurized gypsum and fly ash) to modify the dredged urban sludge. Shrink, unconfined compression, shear and infiltration tests were performed to obtain the volume shrinkage, compressive strength, shear strength and permeability coefficient of the modified sludge, as well as the permeability coefficient under the action of wet and dry cycles. Comprehensive characterization of the modified sludge using X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy with energy dispersive spectroscopy detection methods, resulted in the hydration products, molecular groups and microstructure characteristics of the modified sludge and revealed the modification mechanism of calcium-containing waste to sludge. After natural curing for 28 d, the volume shrinkage rate of the modified sludge sample was 2.6~8.3%, the unconfined compressive strength was 7.9~14.5 MPa, the cohesion force c was 179~329 kPa, and the internal friction angle φ was 42.59~53.60°. After six wet and dry cycles, there were no cracks in the modified sludge; the permeability coefficient of the modified sludge reached stability at 0.84–11.1 × 10−7 cm/s; and the permeability coefficient of MS7 sample was less than 1 × 10−7 cm/s, which met the engineering anti-seepage requirements of the landfill closure cover. The industrial calcium-containing waste by alkali formed C–S–H and C–A–S–H gelled geopolymer, which filled the gaps between soil particles to form a strong soil cement skeleton. Therefore, the mix ratio of sludge:construction waste:slag:fly ash:desulfurized gypsum was 50:22:15:8:5. Calcium-containing solid waste modified sludge can be used as a cover material for landfill closure.

Highlights

Deposited sludge in dynamic and static water environments is extensively distributed in rivers and lakes around the world
The volume shrinkage of the modified dredged sludge samples with calcium-containing waste is shown in Figure 1, from which it can be seen that the volume shrinkage of the modified sludge samples under natural curing conditions increased with the prolongation of time, and it mainly occurred in the first 7 days
The unconfined compressive strength of the modified dredged sludge samples with calcium-containing waste is shown in Figure 2, from which it can be seen that the unconfined compressive strength of the modified sludge samples increased rapidly from

Summary

Introduction

Deposited sludge in dynamic and static water environments is extensively distributed in rivers and lakes around the world. In China, the volume of dredged sludge generated annually is quite large. Statistics indicate that more than 20 billion m3 of dredged sludge is currently deposited in rivers and lakes in China and is increasing at a rate of about million m3 per year [1]. As a fine-grained soil rich in water content and clay content, has some organic matter and a small amount of organic or inorganic pollutants. With particles mainly arranged in honeycomb and flocculent fabrics, dredged sludge is characterized by high porosity, high compressibility, and low bearing capacity [2].

Methods

Results

Conclusion