Q. Why do gravel roads made of limestone get so much harder than roads made of other stones?
A. Limestone is abundant in Wisconsin, and it's the material of choice for the surface of gravel roads, and the base for roads paved with asphalt or concrete.
Limestone contains calcium carbonate, often with a mixture of magnesium carbonate.
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When chunks of limestone abrade against each other, small particles called "fines" are created, said Craig Benson, chair of the departments of civil and environmental engineering and of geological engineering at UW-Madison.
Fines partly dissolve when wet, Benson said, and, "The dissolved carbonate forms a glue between the particles when the water dries, which binds the particles together and creates a smooth and stiff roadbed."
Other stones lack this critical advantage, Benson said.
Even when dry, limestone is much weaker than concrete, he added, which hardens by a different mechanism. Limestone roads and roadbeds soften when water dissolves the carbonates.
Fortunately for road builders, limestone regains its hardness when it dries.