(This article is reproduced with permission from the February 1995 issue of "Technology News," a publication of the Iowa Transportation Center at Iowa State University.)

Study results: One application of bentonite, a natural clay, controls dust on unpaved limestone roads for two or three years.

Springlike days in January reminded us that warmer weather is just around the corner, soon to be followed by the hot dog days of summer. Road maintenance crews fighting clouds of snow today will, in just a few short months, be battling clouds of dust on Iowa's unpaved secondary roads.

Now's the time to plan your summer dust control program for unpaved roads.
An Iowa State University study says bentonite clay is one effective treatment.

Dust control is the number-one summer maintenance problem on unpaved roads, according to a survey of county and highway engineers published in Better Roads, April 1992. To address this problem, researchers and highway maintenance practitioners are working to find innovative and affordable ways to control dust.

One promising product being tested is bentonite, a naturally occurring sodium montmorillonite clay. Ken Bergeson, associate professor of civil engineering at Iowa State University, recently completed a two-year study, Bentonite Treatment for Economical Dust Reduction on Limestone Surfaced Secondary Roads, HR-351.

Test results indicate that for long-term treatment (two to three years), bentonite is an effective and less expensive alternative to chemical treatments on limestone roads.

In cooperation with Tama, Appanoose, and Hancock counties and the Iowa Department of Transportation secondary road staff, Bergeson selected one-mile sections of limestone-surfaced road for testing. Test sections were prepared by blading and windrowing loose surface material to one side. Bentonite was then spread along the wind-row and bladed several times to blend it into the surface material. At the same time, a 0.4 percent soda ash solution was sprayed on the surface as a dispersing agent, and the surface was wet-mixed by motor graders to a consistency of two- to three-inch slump concrete. Two graders worked in tandem to provide rapid mixing, preventing the agglomerations that form when bentonite is mixed with water.

Each test section was divided into five subsections. The first section was a control section, and the other four sections were treated with bentonite in amounts ranging from three to 12 percent (by weight of aggregate).

All four treated sections showed a significant reduction in dust. Final results show that from a cost/benefit stand-point, an optimum level of treatment is about eight percent (by weight of aggregate).

Bergeson says the results of the study indicate that bentonite has several benefits over the most commonly used dust control products, calcium chloride and lignin sulfonate.

One drawback with both calcium chloride and lignin sulfonate is that they are effective only while they remain directly on the road surface. When potholes and/or washboarding problems develop after either of these products is applied, blading the surface to correct these problems minimizes the effectiveness of the dust control materials.

Unlike calcium chloride and lignin sulfonate, bentonite is blended with road surface material when it is applied and adheres to pieces of surface material like an "electrochemical glue." Bentonite's effectiveness is not reduced by grading or other maintenance activities, such as applying another layer of limestone, says Bergeson.

The top micrograph shows a sample obtained from an untreated control section of road. Very few small particulates are adhering to the larger particle surfaces, and few agglomerates of smaller particles are present. The bottom micrograph shows a sample from the seven percent benonite treated section. Notice the abundance of small particles attached to larger particle surfaces, and agglomerates of smaller particulates. Both samples were obtained from a test road in Tama County in August 1992.

A second benefit of bentonite is its environmental friendliness, Bergeson says. Iowans are increasingly concerned about possible environmental damage associated with chemical dust control materials such as calcium chloride. Bergeson says that bentonite is a naturally occurring mineral containing no salt and poses no danger to the environment. And it does not contribute to vehicle rusting, as calcium chloride does.

In addition, bentonite is more cost effective than other dust control materials. Researchers estimate the cost of bentonite treatment at half the cost of calcium chloride treatment. Not only are initial application costs lower, but bentonite has a much longer effectiveness period, reducing costs even more.

Because bentonite's bonding properties survive alternating wet and dry periods and even Iowa's long winter freeze, its term of effectiveness is not months but years. One application, according to Bergeson, reduces dust by 60 to 70 percent the first year, 50 to 60 percent the second year, and 30 to 40 percent the third year. These results compare very favorably to the normal three-month total period of effectiveness for calcium chloride.

Bergeson says that, depending on the desired amount of dust control, maintenance supervisors may want to apply bentonite to limestone roads every two to three years.

Bergeson's research also addresses possible problems associated with using bentonite as a dust control measure. Researchers had been concerned that moisture might make bentonite treated roads slippery, but braking distances and braking handling on bentonite treated sections of road were comparable to braking results on dry control sections.

One disadvantage of bentonite is its inability to bind with the gravel used on some secondary roads. Made of igneous rock, gravel (like bentonite) has a negative electric surface charge, so bentonite won't adhere to it.

Limestone, however, has a positive charge, which allows bentonite to form a bond and adhere to it. For this reason, bentonite works best on crushed limestone. Bergeson estimates that limestone comprises the majority of Iowa's unpaved road surfaces.

Although Bergeson believes bentonite is an excellent dust control material, he says maintenance supervisors will still prefer other materials for gravel roads and for short-term treatments where maximum dust control is desired. For example, calcium chloride may still be the preferred spot treatment in front of houses.

"Bentonite is not a cure-all. There are still some applications for calcium chloride."