Cement Solidification/Stabilization: A Cost-Effective Soil-Remediation Strategy
Cement solidification/stabilization (S/S) of contaminated soil is a well-established and highly effective site-remediation strategy that offers significant sustainable-development and economic-performance advantages over most other environmental remediation methods.
Approved and used by the US EPA at numerous locations for more than 35 years, cement S/S effectively controls the leachability of soil contaminants and allows property owners and developers to beneficially reuse the treated soil. Since soil is not transported off site, truck traffic and emissions are significantly diminished, and material management expenses associated with excavation and disposal of contaminated soil, as well as replenishment with clean backfill material, are dramatically reduced.
By way of example, Holcim conducted a study on the impact of truck traffic on a project at the Toronto waterfront. The Ontario Ministry of the Environment initially considered removing the soil, disposing the contaminated material in a landfill, and replacing the excavated soil with clean backfill. The number of truck trips required for this remediation approach would be nearly 21 times the number of trucks (6,000 vs. 290) needed for a cement S/S treatment approach.
Cement S/S is very effective for remediating both metals and organic contaminants. The high pH of the cement converts characteristically hazardous heavy metals to nonhazardous insoluble metal hydroxides, thus preventing leaching into the groundwater or into surface water by precipitation. Based on testing for contaminant mobility, the reduced concentration of metals between untreated and cement S/S-treated soil is often four to five orders of magnitude, and leachable metals are not typically detectable. Two different cement S/S methods—in-situ and ex-situ—have been devised for remediating contaminated sites.
With both methods, the cement binding agent solidifies the contaminated soil, thereby locking up the contaminants and permanently isolating them from the environment. Both in-situ and ex-situ cement S/S technologies have proven to be very effective in the remediation of contaminated soils. According to the EPA, more than 22 percent of 126 source control treatment projects conducted between 2002 to 2005 used either in-situ or ex-situ cement S/S treatments.
In-situ cement S/S treatment
With the in-situ cement S/S method, contaminated soil is treated directly in place. The equipment used—an auger and excavator, or a modified excavator with a rotating drum—homogenizes the soil with dry cement or a slurry of cementitious product, injected directly into the mixing zone. Numerous mechanical devices have been used successfully in this manner. Some high-tech systems precisely meter the cement and water into the mixing zone, with help from an onboard computer and global positioning system (GPS) controls. Other systems are no more precise than a back hoe mixing cement into the soil until the operator “feels” mixing is complete.
One of the most favorable advantages to in-situ S/S treatment is the cost, which is typically half the expense of excavating and disposing contaminated soil in a landfill. From a sustainability perspective, the in-situ S/S method constitutes 100 percent resource recovery as the cement-treated contaminated soil can be recycled into a foundation material for future structures built on the site.
Ex-situ cement S/S treatment
With the ex-situ cement S/S method, contaminated soil is excavated and thoroughly blended with cement in a mechanical mixing device, such as a pug mill. The treated soil is then either placed back into the ground from where it was removed, used as a fill material at another site or disposed in a landfill, but at a much lower cost than untreated soil.
The ex-situ method is more expensive than in-situ treatment due to the ancillary equipment needed to process the soil. It also requires a larger footprint for the equipment. However, an important advantage of the ex-situ method is that the contaminated soil and cement can be metered together very accurately, which optimizes the amount of cement needed for effective remediation. In addition, the ex-situ S/S method usually can process a larger amount of soil than in-situ methods, depending on the size of the pug mill or other mixing device.
The ex-situ S/S method is often used when the contamination at a site is deeper than 25 feet. As a result, the soil is treated in lifts. The top 20 feet of soil is removed, then treated in a pug mill and stockpiled. The deeper material is then excavated and treated ex-situ. The treated soil is then placed back in the ground, reused in some other fashion or disposed off-site.
Materials do matter—the cement S/S options
Contamination issues and chemical conditions can vary dramatically from site to site, and a cement S/S treatment used at another location may not be the optimal solution for your site-remediation project. To develop a cost-efficient and reliable treatment strategy, contact our technical experts who have an in-depth knowledge of all the cement product options and who understand the base science of how the native soil chemistry and cement work together.
The key to success is to engage these qualified professionals very early in the planning process to discuss your project goals, customize a value-driven cement S/S solution, and facilitate material quality-control testing to ensure a high level of remediation performance.
CASE STUDY: Recycling Facility Site Remediation
A long-abandoned property in Lansing, Michigan, was the former site of an industrial barrel recycling facility. In the 1970s and 1980s, few, if any, quality-control measures were taken to determine what type of barrels could be processed safely at the facility. The recycling process consisted of power-washing the contents out of the barrels and allowing the rinsate to flow into the ground.
The site had numerous soil contaminants, including polyaromatic hydrocarbons, polychlorinated biphenyl and volatile organic compounds. The primary hazardous contaminant was lead, with soil concentrations ranging from 1,800 to 8,000 mg/kg total lead and 10 to 150 mg/L leachable lead. The original remediation plan called for excavation and disposal of the soil in a hazardous waste landfill, which was a very expensive option. The remediation team decided that treating the soil with portland cement (PC) in-situ would be a more cost-effective solution. The high pH of PC was expected to stabilize leachable lead to less than 5 mg/L and render the soil nonhazardous. After reviewing the modified remediation plan, Holcim recommended the use of cement kiln dust (CKD) instead of PC. CKD would provide the same reduction in TCLP-leachable lead as PC, but at a much lower cost.
The CKD in-situ treatment reduced leachable lead below hazardous waste levels (0.0035-3.8 mg/L), and the remediation was about half the cost of the original proposal. Furthermore, the original remediation plan would not have reduced the lead leachability but only contain it within the landfill.