Aggregates
Aggregate
is the formal name for crushed rock, for rock broken up before
use. Limestone or dolomite are the most common kinds of rock crushed
for aggregate. One very visible use of aggregate is for "gravel"
roads, roads where layers of crushed rock provide a surface superior
to that provided by soil or earth.
Each piece of aggregate comes from the rock crusher as an angular
fragment. These rock fragments never quite fit together again,
leaving many small gaps, or pores, between solid bits of rock.
Water can drain easily through these pores, but they remain open,
even when compressed by a heavy load, because of contacts between
strong, difficult-to-compress, pieces of aggregate. Limestone
and dolomite make the best aggregate because they are relatively
soft. Sharp edges break off, leaving rounded edges in contact
with your 80,000 mile tires. Soft rocks are also easier on rock
crushers than hard rocks would be.
Good think limestone is appropriate for aggregate. Crushed quartzite
is used for road metal. Quartzite is harder than steel, but this
quartzite is brittle and it shatters into splinters in the crusher.
Roads surfaced in quartzite aggregate are long lasting but hard
on tires. Edges remain sharp for years and a fragment can penetrate
a tire if wedged into the tread.
Russia also has few good sources of limestone or dolomite for
road metal. For two centuries, this proved an advantage. Armies
of Napoleon and Hitler got bogged down in muddy Russian roads.
Supply lines were unreliable, cavalry and tanks immobilized, artillery
left deployed in a most inefficient manner. While the U.S.A. built
a network of strategic defense highways (the Interstates) and
a farm-to-market system of paved roads, Russia viewed highways
as potential invasion routes and allowed its surface transportation
system to remain dominated by canals, rivers and railroads. Today,
this lack of surface transportation infrastructure poses a serious
challenge to agricultural efficiency in the former Soviet Union.
Subsistence farmers might survive without good roads, but unreliable
or costly transportation raise to cost and threaten the quality
of food supplies.
Aggregate is even more important for paved highways than it is
for gravel roads. Water is a highway's enemy. The first attempts
to construct a log road through the Great Black Swamp of northwest
Ohio resulted in a turnpike that continuously sank into the mud.
Water-saturated soil (mud) flows under pressure. It moves to the
side, not simply downward. In some places along Ohio's log road,
construction crews lost count of how many logs had sunk out of
sight into this apparently bottomless swamp. Freezing water is
also destructive. Water expands as it freezes, making small holes
larger and breaking apart the pavement.
A well-engineered highway includes ditches and a bed of aggregate
to drain away the water. Pavement is supported by a thick bed
of aggregate, compacted by heavy rollers so that it will not deform
further by traffic, but retaining many pores through which water
can escape into drainage ditches. Aggregate is also used to isolate
foundations from damaging effects of expansive soils.
The main factor that determines the price of aggregate is the
cost of transportation from quarry to customer. A quarry 25 miles
from a job might ship 8 loads per truck per day to that job, while
a quarry 50 miles away is limited to 4 loads per truck per day.
Most aggregate is used within 50 miles of the quarry from which
it is extracted. Loading and unloading railroad cars or barges
with aggregate raise costs.
Limestone quarries impact the environment in a variety of ways.
Truck traffic (noise, exhaust, dust, traffic accidents, roads
damaged by heavy loads) is the most common complaint. Quarry operators
usually purchase buffer strips that keep dust and noise from the
quarry contained, but rock is frequently loosened by blasting.
Quarry blasts, even those too light to damage nearby structures,
disturb the neighbors. It is not uncommon for a quarry operator
to install a temporary vibration monitor to prove that ground
motions from blasts fall within permit limitations. Shots while
the monitor is running tend to be only fraction the size of normal
shots, but lawyers for the quarry use this technical information
to silence complaints. Once the vibration monitoring contractor
leaves, blasts return to their normal levels. This is difficult
to prove unless a permanent seismograph station is in operation
within 10 or 20 miles of the quarry.
Some limestone quarries extend below the water table. When this
occurs, pumps are needed to keep equipment dry. In some cases,
the limestone is low permeability and water wells are not seriously
drawn down. However, some quarries have drained the water from
aquifers a mile or more from the quarry. Where laws regarding
groundwater ownership and theft are vague in this matter, property
owners seeking restoration of their water supply face an uphill
fight.
The fact that many quarries fill with water after they close shows
that they are connected to groundwater aquifers (most geologists
already know this, but it is frequently useful to point to evidence
more obvious to the average citizen).
Limestone forms on the floor of warm tropical seas. Unlike many
chemicals, calcium carbonate is less soluble in warm water than
in cold water. Many mollusks and coral colonies grow shells of
calcium carbonate in a crystalline form called aragonite. After
these animals die, seawater dissolves some of this chemical. When
CaCO3-saturated water moves from cold depths into warm shallow
waters, it precipitates out of solution but in the more difficult
to dissolve crystalline structure of calcite. Limestone we mine
today represents deposition on the floors of prehistoric oceans.
Today, thick beds of limestone and dolomite (MgCO3) are accumulating
in The Bahamas and in shallow seas of the western Pacific.
