Construction aggregates

Production of rock and crushed stone is an “invisible” industry, one that exists almost everywhere but goes largely unnoticed. Only when the products of this industry are needed or when producers are in conflict with environmental or regulatory agencies is their existence given much attention. Stone and rock are available and used worldwide, primarily in the construction industry.

Background

The crushed stone and rock industry has been in existence since time immemorial. Ancient roads throughout the world were paved with stone that was either found in the desired size or crushed by animal or human power and sized with crude sieves. As the construction industry became more sophisticated and exacting, so did requirements for engineered building products. Today the engineered aspects of manufactured stone products extend not only to physical dimensions but also to the chemical quality of the products.

The term “aggregate” represents all types of crushed stone and rock, from sand and gravel to coarse crushed material. The aggregates industry is huge. In 2023, in the United States alone, this industry produced 2.8 billion tons of product valued at roughly $36 billion. According to Concrete Financial Insights in 2024, the industry is still recovering from the economic crisis of 2008. Consumption of crushed stone and sand and gravel grew at average growth rates of 2.5 percent and 1 percent from 2013 to 2023.

Two forces continually drive aggregate producers: low operating cost and low transportation cost. Crushed stone has a product value of approximately eight dollars per metric ton; therefore, the expense of extraction, sizing, and inventory must always be controlled. The expense of bulk transportation for relatively low-cost stone and rock products forces producers to locate near end users. Also, the drawbacks of end-user on-site storage of aggregates cause such storage to be maintained at the site of the producer, with delivery on a just-in-time basis. A common remark concerning aggregates is that they are “worn out” after a transportation distance exceeding 80 kilometers from their origin. This means that the expense of transportation overtakes the value of the product after that distance, so that a producer must find a new production site near the customer or lose market share to a competitor who will be willing to relocate near the customer.

Uses of Aggregates

Typical aggregates used as industrial products include sand and gravel as well as crushed sandstone, limestone, dolomite, granite, and marble. Chert, an agglomeration of minerals, is also frequently excavated and used as a “fill material.” For sandstone and limestone, there is a certain “pecking order,” with high-silica sandstone and high-calcium-content commanding higher prices. For example, chemical-grade limestone is used in chemical reaction technology as well as in pharmaceutical manufacturing.

The bulk of aggregate production, however, goes to a “sized product” that will meet the specifications of the end user. For example, building and highway construction projects demand a certain size aggregate to meet a particular need. The mixing of concrete demands a fine-sized rock product for increasing the strength of the mixture. Gravels are also used in concrete and can be seen in the concrete matrix as small marble-shaped material. “Riprap,” a name given to relatively large, football-sized rock products, is used to control in areas with damaging flows or to reinforce slump-prone areas such as highway embankments.

Dimension stone, a name frequently given to the largest stone products, is used for massive construction and ornamental purposes and is not considered an aggregate. Sources for dimension stone are scarce, requiring sites with very little or no disturbances in the stone deposit through faults, mud slips, cracks, or other geological irregularities. Dimension stones may include limestone and sandstone, marble and granite, and other rocks and minerals found in an undisturbed state. The Egyptian pyramids and older US and state government buildings are examples of construction using dimension stone. Marble and granite are frequently used for ornamental stone because of their durability and ability to be polished to an attractive gloss finish. Examples include large building ornaments, countertops, and headstones for graves.

Sand and gravel are found in areas with past or existing streamflow. In many cases, the segregating aspects of streamflow have sized the sand and gravel so that further sizing is minimized. Crushed stone, however, is produced in mining operations, and the operator has control over the size range of the finished product. For the most part, these operations are at the surface and are known as quarries. Some stone is produced in underground mining operations, with the high quality of the stone justifying the additional expense of this type of operation. Many quarries operating in a particularly desirable deposit of stone continue to pursue it by going underground.

Preparations for Mining

Before any significant expense is committed to locating and developing an aggregate production site, a market study is performed to determine the amount and quality of materials needed within a certain radius of transportation. Experienced aggregate producers usually have a “sixth sense” about the need for and location of their products. After the marketing study has been completed, a site is located by geological exploration techniques such as identification of surface outcrops of material or inference, through regional studies of rock type. Even surface vegetation can indicate the type of material beneath the surface. Limestone terrains, for example, do not support acidic-soil vegetation because of their high pH values.

The potential source of aggregate is identified and outlined more precisely through the digging of test pits for samples of the materials or by drilling test holes and analyzing the samples obtained. Again, the inherent size of the naturally occurring materials and the estimated expense of preparing them to market specifications are critical points that help determine whether a site is to be developed.

If an aggregate site is to be produced, mineral leases are obtained from the mineral owners, who usually own the aggregate as well as deeper minerals. Sometimes the aggregate producer purchases the property outright if the location is environmentally sensitive or if a particularly long-lived aggregate source is identified. If the property is leased, the lease is usually for a fixed term, perhaps ten years, with options to continue the lease for additional terms. A royalty is paid to the owner of the minerals according to the market value of the products. The royalty usually varies from 2 percent to 10 percent of the aggregate’s selling price.

After the quarry dimensions have been verified, applications to develop the site are made to the local, state, and federal agencies that will regulate all aspects of site operation. The effects of the mining operation on air, and surface water, wildlife, archaeological sites, and various natural processes must be determined. This process can be extremely expensive and consume several years. An environmental impact study and report may be necessary in certain areas. This a very detailed and comprehensive report that incorporates all the environmental factors to be considered in locating the pit or quarry.

Aggregate Mining Operations

When all applicable permits and releases have been obtained, mining of the aggregate begins. For sand and gravel, the products can be recovered using simple excavation equipment such as front-end loaders, pan scrapers, or dredges. The stone in quarries, however, has inherent strength such that it is usually necessary to use explosives to fragment the material and make it suitable for bulk transport from the quarry. Blasting is used to dislodge stone from the “face,” or quarry wall. Almost any commercial explosive can be used for this purpose, but a mixture of ammonium nitrate and fuel oil has emerged over the past several decades as the most cost-efficient explosive. Dynamite is still used but in smaller quantities than before.

Several rows of blast holes are drilled at predetermined distances from the free quarry face. These distances are engineered to ensure efficient rock breakage without unnecessary “fly” rock, or rock that is propelled outside the desired blasting area. Explosive cord is also extensively used in blasting operations. This product, with an almost instantaneous ignition rate, is especially useful when “delay” blast rounds are used. In delay blasting, millisecond delay sequences cause the first row of blast holes to be initiated, then subsequent rows rearward from the free face to be ignited after predetermined delays. This technique, now standard in nearly all quarry blasting procedures, results in much greater blasting efficiency.

Dimension stone may also be removed by explosives, but the drill holes are very closely spaced, and the explosive is limited to the amount necessary to cut the block of dimension stone to the desired size and not fragment it further. High-pressure water jets or rock-cutting saws are also used in removing dimension stone from the quarry. Since the value of the dimension stone depends on the largest physical units minable, the stone must be gently dislodged so that it remains intact.

Blasting, while indispensable to the crushed stone industry, is also one of the predominant problems associated with it. The noise, vibration, and dust resulting from blasting operations, combined with the fact that quarries are usually located near populated areas, frequently bring ongoing conflict between producers and nearby residents. As a result, blasting is done carefully, and its side effects are measured to ensure that it is done within the guidelines set and enforced by regulatory agencies.

The blasted material, after it is removed from the quarry face, is loaded by front-end loaders or shovels into large trucks for transportation to the surface preparation plant. The timing of the quarry floor loading operation is a continually challenging problem involving machinery size and speed, operator efficiency, and positioning. Occasionally it is necessary to reduce the sizes of blasted fragments that are too large to be loaded and transported easily with existing equipment, and small amounts of explosives may again be necessary on the quarry floor. Transportation from the quarry floor is usually by truck, belt conveyor, or bucket conveyor, depending on the operating system in effect at the quarry. Removal of dimension stone poses a special problem because the large sizes demand gentle handling and require vehicles capable of transporting the large, heavy units of stone. Large track-mounted crawlers, rubber-tired vehicles, or cranes are frequently used for this task.

Sizing the Material

Blasted materials are transported from the quarry to be reduced in size (“comminuted”) using adjustable crushers, sorted into various sizes to suit the customer’s specifications, washed to remove unwanted extraneous materials such as clays, and stockpiled awaiting shipment. Typically, crushers operate by chewing on the material (“jaw crushers”) or passing the material through rollers (“roll crushers”). When crushed, the material assumes sizes varying from dust-sized particles to pieces equivalent to the crusher opening. Most of these crushed products, even the fine sizes, have some value, so the sizing operation is indispensable to an efficient quarry serving multiple customers.

Sizing may be based on a number of physical principles, such as the ability to pass through or be rejected by sized openings (sieving), the tendency to drop out of a fluid stream (liquid classification), or the ability to be propelled through the air to a certain distance (air classification). If the materials have particular physical or chemical characteristics (magnetism, for example), other sizing operations can be designed to take advantage of them. Sieves (screens) frequently have a vibration device attached to them to prevent clogging and expedite the sorting process. Screening can be fully wet or dry, but merely moist materials cannot be separated in a screening operation.

The end result of sizing is having several sized products in which the size range within a sized product is quite narrow. If necessary, sized materials can be further classified by washing techniques designed to remove the fine sizes inherent in the crushing process. Practical sizes of quarry products range from dust sizes of a few microns in diameter to cubic-shaped dimension stone having dimensions exceeding 6 or 9 meters. For most applications, however, practical sizes include sands from 1.65 millimeters through gravels around 0.6 centimeter to 1.2 centimeters in diameter, to stone sizes 2.5 centimeters to 7.6 centimeters in diameter, to fist-sized material used for foundation bases in construction, to football-sized riprap. The ultimate goal of a stone and rock producer is to market 100 percent of the products generated in its operation. While this is a practical impossibility, the most efficient producers have been able to come remarkably close to this goal.

Environmental and Citizenship Concerns

Virtually all the activities of quarry operation have some undesirable side effects. Siltation problems are inherent in stream-based operations. Blasting generates noise, dust, and vibrations. Trucking activities do the same, and they present a hazard to mixed automobile traffic if public roads are used for transportation. Surface facilities are prone to emitting dust, and fluid classification equipment produces sludge that must be handled. While not toxic in themselves, these by-products from aggregate production are objectionable and must be dealt with according to stringent regulations from municipal, state, and federal agencies. The closer an aggregate producer is to populated areas, the greater the inherent problems are (and the harder it is for the producer to be considered a good community citizen), but moving farther from these areas brings increased transportation costs and may make the producer uncompetitive. Moreover, most quarries are below the water table, which means that a constant inflow of groundwater must be pumped to a nearby creek or river. Although this water is frequently used in the crushing and sizing processes, it must still be pumped, routed, stored, and eventually disposed. Desludging equipment may also be necessary, further increasing the complexity of the operation.

Most successful aggregate producers have recognized and come to terms with the environmental consequences of their businesses. Blasting procedures are carefully engineered to minimize unwanted side effects and are scheduled at times that are least objectionable to those living or working nearby. A bag-house (a type of giant vacuum-cleaner bag) traps dusts produced by crushing operations and, in turn, frequently creates its own marketable product. Electrostatic precipitators are also used to remove and collect dust-sized material. Fines (powder or very small particles) from classification facilities are collected in sediment ponds that can be periodically “mined” to yield a marketable product. In addition to being a good environmental citizen, an aggregate producer should have a visually pleasing facility if it is located in or near populated areas. Well-kept grounds, clean equipment, and storage areas free of debris are marks of the conscientious operator.

Operational Lifespan

Although a large sand and gravel pit or stone quarry may remain open for decades, there is an ultimate lifespan to each location. Besides the restrictions of mining to the edges of the deposit or to the legal property limits of the miners’ leases, there is a practical depth beyond which a quarry must either go underground or face significantly increased costs of lifting the material to the surface. If the decision is made to abandon the mine site, a large excavation remains that may encompass many hectares and be as much as 60 to 90 meters in depth. With the closing of the pit or quarry, the excavation quickly fills with water, and an impoundment, sometimes having very steep sides, becomes part of the landscape. initially removed to expose the minable aggregate must be hauled away or stored nearby. that was removed and discarded during the quarrying operation may now be visually objectionable and must be remedied.

If all these remedial activities are included as a part of the overall life cycle of the pit or quarry, then a desirable residential location may be created. It is common to see upscale housing developments built around old pits and quarries, with the impoundments becoming favorite targets of sport fishers. They are also frequented by migratory birds and other wildlife favoring large bodies of freshwater.

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