Choosing the Right Dust Suppressant

Suppressants used to control dust on unpaved roads can range from water to palliative materials including brine solutions based on sodium chloride (NaCl), calcium chloride (CaCl₂), and magnesium chloride (MgCl₂). While lignin and asphalt emulsions, as well as natural clays and plant oils may also be used, chloride solutions are the predominant dust control option in most regions. Water is most commonly used as a temporary dust suppressant on construction sites and in gravel pits and quarries. Although water moisture helps bind fines to gravel to prevent dust, it must be applied frequently to maintain adequate moisture. This means high labor and equipment costs, particularly under arid conditions where water evaporates quickly.

The two most frequently used dust suppressant options are calcium chloride and magnesium chloride, which are hygroscopic (moisture attracting) materials that draw moisture from the air to provide extended dust suppression.

Calcium Chloride

Calcium chloride (CaCl₂) is a hygroscopic salt that draws moisture from the air to form a solution in road gravel that keeps road surfaces constantly damp, even in hot, dry conditions. The moisture helps to bind particles together to create a hard and compact road surface. Because it penetrates several inches into the road base, calcium chloride also contributes to overall road surface stability. It depresses the freezing point of the moisture in the road surface, to help minimize frost heave damage in the winter.

Either liquid or solid forms of calcium chloride can be used to control dust. OxyChem, the leading supplier of calcium chloride, supplies LIQUIDOW™ Technical Grade Calcium Chloride Solution, which is available in 28% to 42% concentrations of calcium chloride. DOWFLAKE™ Xtra, also produced by OxyChem, is 83-87% pure calcium chloride in a solid flake form (this makes DOWFLAKE™ Xtra a more potent dust control agent, pound-for-pound, than other flaked calcium chloride formulations, which are typically 77% calcium chloride). Also available is Anhydrous Calcium Chloride 94-97% Mini Pellets, a highly concentrated solid form product useful both in direct applications and in preparing liquid dust control solutions in the field.

To learn more about the properties of calcium chloride, see Calcium Chloride: A Guide to Physical Properties.

Magnesium Chloride

Magnesium chloride (MgCl₂) is also a hygroscopic salt. It is most commonly supplied in liquid form at a concentration of about 30%. Liquid magnesium chloride and liquid calcium chloride exhibit similar water attraction properties. However, under hot, dry conditions, CaCl₂ remains liquid, while MgCl₂ does not, resulting in lower dust suppression capabilities under these conditions. Application rates and environmental profiles of the two materials are very similar.

Comparing the Effectiveness of CaCl₂ and MgCl₂

Calcium chloride and magnesium chloride are often the two dust suppressant materials considered by road authorities. While, calcium chloride and magnesium chloride are similar materials, differences in the concentration of active ingredients in commercially available products can make a significant difference in how efficiently and economically they can be used to control dust.

Similar solution forming characteristics – The dust control performance of both CaCl₂ and MgCl₂ is based on their ability to keep road surfaces damp. When applied to a gravel road, calcium chloride and magnesium chloride suppress dust by attracting moisture from the air, creating a solution that dampens the road surface. Once CaCl₂ and MgCl₂ are applied to a gravel road surface, the solution volume in the road surface changes as it seeks equilibrium with the moisture content of the air. If relative humidity is high, the solution volume grows. If conditions are dry, the opposite occurs. Calcium chloride remains liquid in hot, dry weather, extending dust control under conditions when magnesium chloride solidifies and provides little if any dust protection.

Higher Concentration of CaCl₂ Products is a Critical Advantage – The chemical action of CaCl₂ and MgCl₂ may be similar, but the actual performance of commercially available products can be very different. Commercial calcium chloride products are available in higher concentrations than magnesium chloride dust suppressants, which means a crew using the same amount of each material can treat more road using CaCl₂ than with MgCl₂. Or, less calcium chloride than magnesium chloride will be required to control dust on the same length of roadway. The tables below illustrate calcium chloride’s concentration advantage in comparisons of liquid and of dry products.

When comparing the performance of liquid products, it is important to note that LIQUIDOW™ Technical Grade Calcium Chloride Solution is available in concentrations of calcium chloride ranging from 28% to 42% and that the concentrations used for dust control are typically 35% to 38%. In contrast, commercial liquid magnesium chloride products are typically supplied at a concentration of 30%. This is a significant distinction as shown in the table below:

Coverage Comparison of Commercial Liquid Dust Control Products

* Based on U.S. Forest Service typical application rates² LIQUIDOW 38% is OxyChem's recommended concentration for logging and mine haul roads.

The next table shows that CaCl₂’s concentration advantage is even more pronounced when dry flake products are compared. While DOWFLAKE™ Xtra 83-87% Calcium Chloride Flakes contain 83% to 87% purified calcium chloride, typical magnesium chloride flake products are sold at a concentration of just 47%. The data below shows 45% less DOWFLAKE™ Xtra would be required to treat one lane mile than a typical magnesium chloride commercial flake product.

Dust Control Coverage Comparison of Commercial Dry Flake Products

*84% used for comparison calculation
** Application rates for solid products are calculated to deliver the identical active ingredient to the road as U.S. Forest Service typical liquid application rates.²

It is important to note that liquid and solid OxyChem calcium chloride products also enjoy a concentration advantage over other commercial calcium chloride products. While LIQUIDOW™ is available in concentrations up to 42%, other liquid calcium chloride products contain no more than 35% to 38% calcium chloride. The 83-87% calcium chloride concentration of DOWFLAKE™ Xtra far exceeds the typical 77% concentration of other flake calcium chloride products.

Third-party Application Recommendations Confirm CaCl₂’s Potency Advantage – Application rate recommendations of government authorities in the U.S. and Canada reflect the greater concentration and higher resulting dust suppression potency of calcium chloride solutions compared to commercial solutions of magnesium chloride. Environment Canada has published best practices that recommend an application rate of 1.4 to 2.3 l/m² of magnesium chloride to achieve the same dust suppression capability as calcium chloride applied at a rate of 0.9 to 1.6 l/m².¹ The USDA Forest Service found that a magnesium chloride application rate of 0.30 to 0.50 gal/yd² was necessary to achieve the same dust suppression performance as 0.20 to 0.35 gal/yd² of calcium chloride.²

CaCl₂ Provides Additional Concentration-Related Savings – Lower application rates for calcium chloride can also translate into transportation, man hour and equipment cost-savings, as well as reduced impact on the environment.

Environmental, Health & Safety Profiles are Similar – Independent research into the environmental, health and safety impact of calcium chloride and magnesium chloride in dust control applications has found that concerns related to the use of these materials are similar and can be effectively managed through proper handling and application procedures and by adhering to recommended application rates.

The U.S. Department of Agriculture Forest Service reported that soil samples taken after multiple dust control test projects using calcium chloride and magnesium chloride showed similar effects for both chemicals: increases in chloride levels in soils were below thresholds for concern. Data from tree samples taken as part of the same studies showed increased chloride levels but chloride levels were not deemed a significant long-term threat to vegetation survival. One of the tests, involving application of calcium chloride on roadway adjacent to the Tucannon River in Southeast Washington, resulted in “no significant change in calcium- or chloride-ion levels in the water”.³

Environment Canada application recommendations for liquid calcium chloride and magnesium chloride are the same for both materials, supporting similar impact and management requirements.¹ The U.S. Department of Agriculture Forest Service also provides identical management recommendations for magnesium chloride and calcium chloride.²

Product Availability – Product availability may also be a factor in choosing between CaCl₂ and MgCl₂. OxyChem supplies solid as well as liquid CaCl₂ products across North America.

Learn more about calcium chloride versus magnesium chloride.

¹Best Practices for the Use and Storage of Chloride-based Dust Suppressants, Environment Canada, February 2007.
²Dust Palliative Selection and Application Guide, USDA Forest Service, P. Bolander and A. Yamada, November 1999.
³Surface-Aggregate Stabilization with Chloride Materials, U.S. Department of Agriculture Forest Service, December 2006.

Well Head Brines

Well head brine solutions – also referred to as mineral well or oil well brines – are sometimes applied to unpaved roads under the mistaken belief that they offer the same dust suppressant advantages as commercially available calcium chloride and magnesium chloride, yet at lower cost. While the hygroscopic performance of defined concentrations of pure calcium chloride and magnesium chloride are well known, the moisture attraction and resulting dust control effectiveness of other brine solutions is less certain. The lower concentrations of hygroscopic material typically found in well head brines means far more material can be required to achieve dust control, increasing transportation and application expense.

Inadequate dust suppression – The two factors that determine dust suppressant capabilities are the composition and the concentration of brine material. Composition is important because some salts are highly hygroscopic (calcium chloride and magnesium chloride) and others, such as sodium chloride, are not. Well head brines tend to contain higher levels of sodium chloride and low levels of CaCl₂ and MgCl₂. Sodium chloride is a solid under most summer climate conditions and offers no dust suppression capability. In one study on a ¾ mile section of gravel road, LIQUIDOW™ 38% calcium chloride from OxyChem was compared to an oil field brine containing 15% sodium chloride and only 12% calcium chloride and magnesium chloride combined. Controlled sampling over seven weeks found that LIQUIDOW™ – with more than three times as much hygroscopic salt – controlled dust three times better than the brine.

Environmental Issues – By using more brine to achieve dust control, the amount of dust suppressant material introduced into the environment is increased, potentially increasing impact. The varying content of brines can also pose a concern. In some jurisdictions, regulations may require permits for the use of well head brine to treat roadways.¹What’s more, the relatively high concentrations of sodium chloride in well head brines can be detrimental to soil structure and soil permeability, posing additional environmental risk.

Higher overall treatment costs – Well head brine composition can vary widely depending on the local source and is often significantly lower in hygroscopic salt content compared to LIQUIDOW™ calcium chloride or other commercial CaCl₂ or MgCl₂ solutions. The concentration of hygroscopic salts in the brine is important because the lower the concentration, the more material required to effectively suppress dust and the higher the transportation costs and the application cost per mile of treated road. In one comparison, 0.46 gal/yd² of well head brine was required to achieve the same hygroscopic capacity as 0.28 gal/yd² of LIQUIDOW™ 38% Calcium Chloride. A 3,700 gallon truck making three trips per day could treat 3.4 miles per day with LIQUIDOW™ 38% Calcium Chloride and only about 2.1 miles per day with the well head brine. These costs can quickly offset any per-gallon cost savings on the material itself.

Learn more about the comparative performance of calcium chloride and well head brines.

¹Guidelines for Selecting Dust Suppressants to Control Dust and Prevent Soil Erosion, Michigan Department of Natural Resources and Environment, Office of Pollution Prevention and Compliance Assistance, 2010.