Former Sydney Steel Plant - Sydney, Nova Scotia

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Cement-Based Stabilization of Soil Contaminated with Fuel Oil and Coal Tar

The Challenge

Approximately 100,000 tonnes of industrial fill are contaminated with fuel oil and coal tar as a result of decades of storage and handling of these substances in the near-shore area adjacent to the mill’s former shipping piers. The variety of the fill material represents a major technical challenge – slag, ash and reworked silty till have very different grain-size distributions and chemical attributes. Another challenge is the requirement for ongoing excavation de-watering in the high-permeability shoreline fill materials. The project is part of the larger reclamation and redevelopment of the 170-hectare plant site.

The Solution

NS Lands, the provincial Crown Corporation responsible for the reclamation of the steel plant site, retained SEACOR Environmental to help select a remedial approach. A performance- based competition resulted in Hazco Environmental Services being contracted to excavate and perform cementbased solidification/stabilization (S/S) on the bulk of the material, with a smaller portion bioremediated at an on-site facility. This is the first time that cement-based S/S technology has been employed on a large scale in Atlantic Canada.

Results

The solidified/stabilized material is meeting all of the project criteria including strength requirements of 0.34 MPa and maximum permeability of 1 x 10-6 cm/s. When the remediation stage is complete, a monitoring program will be implemented to confirm the performance of the cement-based S/S material and gauge the effects on groundwater contamination and flow. 

What is Cement-Based S/S?

Introduction

Solidification/stabilization (S/S) is a widely used treatment for the management and disposal of a broad range of contaminated materials and wastes – particularly those contaminated with substances classified as hazardous in the United States. The treatment involves mixing a binding reagent into the contaminated substance. This process protects human health and the environment by immobilizing contaminants within the treated material, preventing them from migrating to plants, animals and humans.

S/S treatment has been used to treat radioactive wastes since the 1950s and hazardous wastes since the 1970s. S/S continues as a cornerstone treatment technology for the management of radioactive and hazardous wastes, as well as site remediation and Brownfield redevelopment. The U.S. Environmental Protection Agency (EPA) considers S/S an established treatment technology. EPA has selected S/S treatment for 24% of its Superfund source-control remedial actions.

How S/S Works

S/S treatment involves mixing a binding reagent into the contaminated substance. Although the terms solidification and stabilization sound similar, they describe different effects that the binding reagents create to immobilize hazardous constituents. Solidification describes changes in the physical properties of a contaminated substance. The desired changes usually include an increase in compressive strength, a decrease in permeability, and encapsulation of hazardous constituents. Stabilization refers to chemical changes of the hazardous constituents in the treated substance. The desired changes include converting the constituents into a form that is less soluble, mobile, or toxic.

Effects of Binding Reagents on Waste – Inorganic & Organic

Commonly used binding reagents include Portland cement, cement kiln dust (CKD), and a number of proprietary reagents. Portland cement is a generic material principally used in concrete for construction. This material is also a versatile S/S binding reagent with the ability to both solidify and stabilize a wide variety of wastes. Portland cement-based mix designs have been the most popular S/S treatments and have been applied to a greater variety of wastes than any other S/S binding reagent.

Cement is frequently selected for the reagent’s ability to:

  • chemically bind free liquids
  • reduce the permeability of the waste form
  • encapsulate waste particles, surrounding them with an impermeable coating
  • chemically fix hazardous constituents by reducing their solubility
  • help reduce the toxicity of some contaminants

This is accomplished by bringing about physical changes to the waste form and, often, chemical changes to the hazardous constituents themselves. Cement-based S/S has been used to treat wastes that have either or both inorganic and organic hazardous constituents.

Due to the great variation of waste constituents and media, a mix of reagents should be designed specifically for each waste that is to be treated. Mix designs often include by-products or additives in addition to Portland cement. Fly ash is often used to capitalize on the pozzolanic effect of this material when mixed with hydrating Portland cement. CKD and slag have minor cementitious properties and are sometimes used for economy. Lime can be used to adjust pH or to drive off water utilizing the high heat of hydration produced by these S/S binders.

Project Team
Owner Nova Scotia Lands Inc.
Engineer SEACOR Environmental Inc.
Contractor Hazco Environmental Services Inc.

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