What lignin sulfonate is, and how it holds a road together

Where it comes from

Lignin sulfonate comes from wood. The gravel roads manual published by the Federal Highway Administration describes it as a by-product of the pulp milling industry, and says it's "sometimes called 'tree sap' in the field."¹ That's the stuff in our tank.

If you want the chemistry, the federal feed rules give it in one line. They describe lignin sulfonates as the ammonium, calcium, magnesium or sodium salts of the extract of spent sulfite liquor, made from wood (or from two plant fibers, abaca and sisal).⁴

Road engineers file it with the organic non-petroleum dust treatments. The FHWA's 2013 handbook puts lignosulfonates in that group, along with tree resins, tree oils, vegetable oils and molasses-based products.²

How it holds a road together

Road dust is the fine material in a gravel surface. The gravel roads manual says it about as plainly as I would. Every gravel road gives off some dust under traffic.¹ A binder's whole job is keeping those fines in the road where they belong.

"These treatments act as 'glue' that agglomerates the fines and coarser particles together."

The gravel roads manual adds that these products work best mixed into the surface gravel, where they give the soil particles the cohesion to stick together.¹

The lignin we apply is also described as changing a little in the sun. Over time some of it becomes insoluble from solar heating, and that forms a water- and dust-resistant layer on top.

Where it works best

A binder needs something to bind. The product we apply is described as especially good on roads with more dirt and sand in them, and the gravel roads manual says these products work best mixed into the surface gravel.¹ So when I look at a road, the first thing I want to know is what the surface is made of.

Climate counts too. The FHWA's additive selection chart rates lignin good in damp-to-dry and dry climates and poor in wet ones, with a note that it may leach out in heavy rain.² Our summers here in the Rogue Valley are dry. NOAA's 1991–2020 normals for Medford give just 1.25 inches of rain across June, July and August combined.⁵ The timing guide covers what that means for when to book.

Its limits, said plainly

• It's water soluble. The FHWA handbook says most products in this group dissolve in water and may leach out in heavy rain.² The real risk is rain on a fresh treatment. As the Forest Service guide puts it, "if applied just before a rain, the material may wash away."³
• It controls dust. It won't rebuild a road. The same handbook says these products hold dust well but don't add enough strength to count as soil stabilizers.² Potholes and washboard need road work, not a binder, and I'd rather tell you that before we spray than after.
• Poor gravel shortens it. The Forest Service guide calls for higher rates or more frequent treatment when a road surface is low in fines (under 10 percent passing a No. 200 sieve) or poorly bladed.³ It recommends repairing the surface first, and I'd say the same.
• It's a yearly job. The gravel roads manual notes that virtually every method of dust control needs treating again each year.¹ Ours is built to last the season, so plan on doing it again the next summer.

If you grade your own road, you'll like this part. The FHWA says roads treated with this group can be kept up with the usual gravel road methods, such as grader blading after light rain.²

Other uses: an animal feed additive

Lignin sulfonates turn up outside road work too. Federal regulation 21 CFR 573.600 permits them in animal feed, within set limits, as a pelleting aid, as a binding aid when flaking feed grains, as a surfactant in molasses used in feeds, or as a source of energy.⁴

That's a feed rule, not a road-safety finding, and I won't stretch it into one. What I can tell you is that the product we apply is described as safe for animals, plants and people, non-corrosive and non-toxic.

Streams, ponds and wells nearby

The USDA Forest Service's dust palliative guide covers every product category, and two of its cautions apply to lignin. First, it notes that the organic non-petroleum products are the most likely to show high BOD results (biochemical oxygen demand, a water-quality measure).³ Second, it gives this advice.

"Determine prior to application if significant migration (water drainage) might occur from the treated area into local streams, ponds, and lakes."

So before we spray, tell me where your road drains. A creek at the bottom of the lane, a pond, a well head close to the road, a garden or a pasture are all things I want to hear about, and we'll talk them through with you first.

How we apply it

Lignin sulfonate is the one product we apply, and I've been putting it down on Southern Oregon roads for more than 20 years. The job itself is simple. We set an appointment, I pre-wet the road with water, and then the truck sprays the lignin. You can drive on it right away. On a hot summer day it's fully dry in about two hours, and it's built to last the season.

We treat everything from a short gravel driveway to miles of private road and multi-acre lots. The details of the process are on how it works. Weighing lignin against chlorides, oils or polymers? See dust control products compared. And if you're still not sure it suits your road, give me a call.