What is slurry? The dictionary defines slurry as “a watery mixture of insoluble matter.”
The word describes a physical state and not a mixture’s components; countless kinds of slurry exist across many industries. Therefore, the environmental effects of a given slurry can only be determined by understanding its individual constituents.
In the road construction industry, when diamond grinding concrete highways, water used to cool cutting blades combines with hardened cement paste and aggregate particulates. This byproduct, concrete grinding residue (CGR), is commonly called slurry. How CGR is handled varies greatly across the country. In some regions, it is spread along adjacent slopes as the saw cutting or grinding operation moves down the roadway. Other times, and in all areas with closed drainage systems, CGR is collected and hauled to off-site locations for processing and disposal.
Engineers, state environmental departments, and others have shown some trepidation regarding the effects of slurry on roadside vegetation, soil, and drainage. Their concerns have led to limits being placed on how much slurry can be discharged along the roadside during the grinding process. However, because off-site slurry disposal is costly for state DOTs—and for taxpayers—researchers have been working to identify CGR’s precise ecological effects, as well as how CGR disposal can be optimized.
To determine the effect of discharging slurry, the question becomes: What is slurry made of? The contents of slurry reflect the contents of the concrete itself—mostly mineral—and possible compounds present in the cooling water. Multiple laboratory tests have been done to break down the components of slurry, finding that, as per criteria for identifying hazardous waste under U.S. Code of Federal Regulations, Title 40, Part 261, the elements and compounds present in tested slurry are non-ignitable, non-corrosive, and nontoxic; therefore, it can be considered a nonhazardous waste. The California DOT (Caltrans) tested slurry samples and compared results to California’s Title 22 hazardous waste standards, finding that concrete slurry’s organic and inorganic constituents displayed no hazardous characteristics. Ninety-six-hour acute toxicity testing showed no toxicity characteristics and concluded that the slurry represented no toxic threat to public health or to the environment. Elevated soil pH has been the sole concern raised by slurry testing; however, in the Caltrans tests the pH characteristics of disposed slurry did not exceed Title 22 standards.
A recent study conducted on behalf of the Minnesota DOT (MnDOT) found that fresh CGR materials collected for research purposes were comprised predominantly of silica (SiO2) at 53.12% and lime (CaO) at 16.82%. These compounds are also the major ones found in concrete materials. (It should be noted that while respirable silica on the jobsite can pose a health hazard to workers, silica in CGR is mixed with a substantial amount of water and does not become free or airborne after deposition on the site.)
The lime (along with some trace minerals) found in CGR can be beneficial to plant life. Many DOTs regulate the deposition of slurry in terms of its lime equivalency, often called calcium carbonate equivalency (CCE). Lime equivalency, expressed as a percentage, is the acid-neutralizing capacity of a carbonate rock relative to that of pure calcium carbonate (e.g., calcite). For pure calcite the value is 100%; for pure dolomite the value is 108.5%. Actual lime equivalency of most limestone will vary from these percentages due to impurities in the rock, as well as the fact that most commercially available limestones have a mixture of calcite and dolomite rather than either in its pure form.
Four studies in recent years give evidence of the often-beneficial effect of depositing CGR slurry along the roadside. All four studies showed that soil quality is not harmed by slurry deposition, and at least some vegetation responds favorably to it.
Minnesota DOT study
Most recently, principal investigator Halil Ceylan of the Institute for Transportation, Iowa State University, conducted studies on behalf of MnDOT evaluating slurry deposit impact on vegetation and soils. In January 2019, findings were published in a MnDOT report titled, “Concrete Grinding Residue: Its Effect on Roadside Vegetation and Soil Properties.”
Tests included depositing slurry that had been collected from a slurry tank at a Minnesota construction site onto a controlled field site in Iowa. CGR application rates were 0, 10, 20, and 40 dry tons per acre. Properties of soils and plants were assessed before the application in October 2016 and at points in time one month, six months, and one year after the CGR application from November 2016 to October 2017. According to the report, vegetation at the research site included common roadway plants such as “cool-seasoned grasses (e.g., Canada wild rye, Virginia wild rye), warm-seasoned grasses (e.g., Indian grass, switchgrass), forbs (e.g., showy golden rod, heath aster, common milkweed, wild bergamot, smooth blue aster, Queen Anne’s lace, cup plant, Maximilian sunflower), and legumes (e.g., wild white indigo, crown vetch).” Two roadsides along I-90 in Minnesota, where CGR had previously been applied, were also tested in 2016-17, with a total of 18 soil sampling points on the roadside experiments.
The MnDOT study demonstrated that in some locations, the increase in soil pH enhanced plant growth, and some of the minerals present in CGR, such as calcium and magnesium, were determined likely to contribute nutritive benefits. As stated in the report, testing performed at the control site “indicated that the application of CGR up to 40 dry tons/acre did not significantly affect soil physical properties and plant biomass; however, the plant coverage percentage of warm-seasoned grasses and the legumes were promoted, and the chemical properties of soil were significantly influenced. The changed chemical properties of soil after addition of CGR were due to the rich content of metallic compounds (CaO, MgO, etc.) in CGR, but changes such as elevated pH, alkalinity, contents of nutrients, and CEC are beneficial for vegetation from the perspective of improvements on plant growth and soil quality, especially for acidic soils.” Researchers also found that initial higher pH levels did not persist after one year.
Florida DOT study
In August 2016, the University of Florida submitted to the Florida DOT (FDOT) a report titled Concrete Debris Assessment for Road Construction Activities. Included in the study was an evaluation of the potential environmental impacts associated with CGR. Because FDOT testing has shown that CGR is not a hazardous waste, tests conducted for the study focused on measuring the pH of CGR samples over a range of liquid-to-solid ratios. At different times during sampling and analysis, pH was found to range from 11.0 to 12.4, depending on the amount of liquid present. Researchers concluded that roadside deposition of CGR should not be a concern, but that appropriate CGR best management practices should be observed.
Nebraska Department of Roads (NDOR) study
Investigators from the Department of Agronomy and Horticulture at the University of Nebraska-Lincoln, published a 2015 report titled Evaluation of Concrete Grinding Residue (CGR) Slurry Application on Vegetation and Soil Responses along Nebraska State Hwy 31 for NDOR. The two-year study evaluated the effect of CGR application on soil chemical properties, existing vegetation, and rainfall runoff.
Researchers conducted tests along two state highway sections, one consisting of loam and one consisting of silt loam soils. Vegetation for all locations was predominantly cool season grasses. The lime equivalency ranged from 13% to 28%. In July 2013, controlled slurry treatments were applied at locations along Nebraska State Highway mile marker 36. The application rates of dry slurry (0% moisture) were 0, 4.1, 8.2, 16.4, and 32.9 tons per acre for each treatment. Multiplying by an average lime equivalency of 13%, slurry rates applied were converted to lime equivalent rates. These rates were 0, 0.5, 1.1, 2.1, and 4.3 tons lime equivalent per acre, respectively. In June 2014, slurry treatments were applied along Nebraska State Highway mile marker 34. Application rates of dry slurry were 0, 5.5, 10.9, 21.8, and 43.7 tons per acre for each treatment. With an average lime equivalency of 28%, the lime equivalent rates were 0, 1.5, 3.1, 6.2, and 12.3 tons lime equivalent per acre, respectively.
For both the 2013 and 2014 one-time CGR slurry applications, no change was observed in runoff volume, runoff chemistry, ground cover, or species composition. The highest CGR application (40 dry tons per acre) increased soil sodium and pH in the short term (one month) but did not persist after one year following the CGR application. The study authors concluded that CGR discharge of up to 40 dry tons per acre can safely be applied in a uniform layer one time to roadsides with medium textured soils.
“It should be noted that deposition of CGR at a rate of 40 dry tons per acre is significantly in excess of that seen in a typical diamond grinding operation,” said John Roberts, executive director of the International Grooving & Grinding Association (IGGA).
North Dakota State University study
NDSU conducted research in conjunction with the IGGA beginning in 2009. The team studied five CGR samples from various regions (California, Michigan, Nebraska, Washington, and Minnesota) and determined their effect on smooth brome (Bromus inermis Leyss) and soil chemical response.
Testing contained three phases: (1) determination of chemical composition and characteristics of CGR; (2) determination of what effect CGR has on the mechanical properties of the soil; and (3) determination of what effect CGR has on plant growth.
Chemical composition of the samples was found to have a high pH, but otherwise was within toxicity limits outlined by the EPA—a finding consistent with other CGR testing. Most hazardous compounds tested for were entirely absent. It should also be noted that even with no history of CGR deposition, surface soils frequently contain many of the hazardous compounds tested for.
NDSU research results further indicated that CGR applied at 40 tons per acre or less was not harmful to the mechanical properties of the soil. CGR had the beneficial effect of increasing the shoot biomass of smooth brome and had a negligible effect on trace metals in both the soil and smooth brome. As in other tests of CGR’s effect on vegetation, the smooth brome took up calcium, an essential nutrient, from the CGR.
A win-win situation
The IGGA has developed a Best Management Practice guide on the proper handling and disposal of CGR. Most important is to monitor and control the pH of the slurry, which is easily done by controlling the flow and acidity of the cooling water used in the process.
A consistent finding from all research studies conducted is that slurry has no negative effect on roadside vegetation or soil; in fact, it bears agricultural liming potential. If CGR slurry deposited along roadsides can be utilized as a liming agent, it can promote plant health while minimizing the costs associated with hauling slurry off-site for disposal—a win-win situation for all involved.
“The collection, processing, and placement of CGR in limited landfill space costs taxpayers money, expends additional natural resources, and wastes an opportunity to benefit the environment,” said Roberts. “In a world where sustainability, resource conservation, and environmental stewardship are becoming increasingly necessary, recycling CGR as a soil enhancement makes more sense than ever.”