Chemical dust suppression

Dust suppression c1Dust on construction sites is a common cause of nuisance to neighbours. To reduce this, many sites damp down dusty surfaces during dry weather using large quantities of (normally) tapwater, which rapidly evaporates in warm, dry weather and needs to be replaced to be effective – often at a time when there may be increased stress on mains water supplies.

However, by adding small quantities of relatively harmless chemicals to the mix. the need to respray can be dramatically reduced. These chemicals fall into two main groups – hydroscopic minerals that both hold and reabsorb moisture, and chemical “glues” that bind particles together into larger clumps, making them less likely to become airborne.

Hydroscopic minerals such as calcium chloride are mixed with water during spraying, and if applied to previously damped surfaces, or after rain, rapidly penetrate 7-10cm into the ground. By absorbing moisture in the surface layers, they maintain moisture content, increasing the shear strength of the surface reducing dust and surface wear. In dry weather, the mineral will dry out to some extent during the day, but will reabsorb moisture at night. In wet weather, heavy rain drives the mineral deeper into the soil, but as the surface dries, capillary action will return the mineral to the surface, maintaining it’s effectiveness in variable weather. The product has been used by the quarry industry for a number of years, and has shown to be effective on heavily used haul roads for in excess of four weeks. In addition to it’s moisture retaining properties, calcium chloride is also a mild weed suppressant, but unless used at excessive concentrations, has little impact on local wildlife and ecology.

Chemical “glues” such as calcium magnesium acetate (CMA) are normally applied “waterless” by spreading with a modified grit spreader onto a clean surface. Rather than binding the soil surface to prevent dusting, they instead bind individual fine dust particles (such as PM10, and to a certain extent PM2.5) into larger clumps that are more resistant to airborne suspension, remaining on the road surface for dispersal on vehicle wheels or removal by road sweepers. Clearly, the effectiveness of such chemicals will reduce with time (depending upon the level of dusting present and the rate of dispersal on vehicle wheels) requiring regular reapplication to control dust in the longer term. CMAs have been trialled extensively by Transport for London since 2011 in attempts to reduce PM10 pollution in the capital, and with regard to environmental impacts in their report they state: “The TfL report stresses that CMA is non-toxic, presents no significant risk of corrosion and is harmless to plants and water supplies. The calcium and magnesium components, the report says, can ‘benefit’ soil, just as liming a garden improves permeability, while the acetate portion biodegrades naturally.

Clearly the two systems act in different ways, and understanding their effect would determine exactly which might be most useful on construction sites. However, as a rule of thumb, hydroscopic minerals such as calcium chloride appear to work well on absorbent surfaces such as haul roads by maintaining a damp surface and minimising surface breakdown, whilst chemical glues are effective on hard, relatively clean, infrequently swept surfaces subject to light residual dusting from vehicle movement such as tarmac access roads.

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