1 Introduction

The Natural Stone Council (NSC) is a collaboration of businesses and trade associations that have come together to promote the use of Genuine Stone in commercial and residential applications. By pooling resources, their goal is to increase the understanding of, preference for, and consumption of these natural products. Trade associations affiliated with the NSC include Allied Stone Industries, Elberton Granite Association, Indiana Limestone Institute, Natural Stone Institute, National Building Granite Quarries Association, and the National Slate Association.

Recognizing that green building was becoming a permanent element of the marketplace, the NSC established a Sustainability Committee made up of key industry members to elevate the issue of sustainability within the industry and provide a body responsible for planning and implementing relevant initiatives. In 2007, the NSC Sustainability Committee engaged in a partnership with the Center for Clean Products (CCP) at the University of Tennessee to assess current industry operations relating to dimensional stone production. Prior to this evaluation, the environmental implications of stone extraction and fabrication processes had received little attention compared to other industries. In particular, life-cycle inventory (LCI) data on natural stone products was limited, not well documented, and out-of-date. This information gap was partially due to the size and varying scale of industry members, the vast diversity of products and materials produced, and the global distribution of stone quarrying activities. As such, this work presents the most comprehensive survey to-date of the natural stone industry’s practices.

Provided in the following text are the results of the first phase of a three-year project launched by the NSC to benchmark and improve the environmental profile of the natural stone industry. Specifically, the information that follows is an initial LCI characterizing sandstone extraction and production operations in North America. These data will serve as a baseline from which industry best practices can be identified, comparisons to competing products can be made with regard to environmental considerations, and future research can be prioritized.

2 Sandstone Quarrying and Processing Operations

2.1 Sandstone

Sandstone is a sedimentary rock comprised of lithified sands. Most is primarily quartz sand or a mix of quartz and feldspar sands in conjunction with interstitial cementing materials including calcite, clay, iron oxides, and silica. The lithification process results in a hard, dense material that takes on the color of its components, most commonly tan to yellowish or tinted pink to dark red due to varying levels of iron oxide.

The commercial sandstone category encompasses many variations of texture and color. Common forms of sandstone include arkose which has a high feldspar content, graywacke which contains angular rock fragments, and conglomerate which contains rounded rock fragments. Other common stones included in this category are bluestone, a hard, dense feldspathic sandstone, brownstone, a reddish-brown stone taking its color from its high iron content, and flagstone, a sandstone or sandy slate that is easily split into large, thin slabs. Sandstone accounts for 15% of the stone produced in the United States, putting it in third place behind limestone and granite. It is quarried in 16 states with Arizona, New York, Ohio, Colorado, and Arkansas leading the industry. Sandstone’s primary uses include dressed stone for flagging, ashlars, and partially squared pieces as well as rough blocks for building and construction (Dolley 2007). Two general phases of sandstone production exist: quarrying and processing. Each of these phases is described below.

2.2 Sandstone Quarrying Operations

Extraction (more commonly referred to as quarrying) consists of removing layers or large pieces of stone from an identified and unearthed geologic deposit. Differences in the particular quarrying techniques used often stems from variations in the physical properties of the deposit itself—such as density, fracturing/bedding planes, and depth—financial considerations, and the site owner’s preference. Nevertheless, the process is relatively simple: locate or create (minimal) breaks in the stone, remove the stone using heavy machinery, secure the stone on a vehicle for transport, and move the material to storage. A flow diagram of typical quarrying operations is shown in Figure 1.

As shown in Figure 1, the first step in quarrying is to gain access to the sandstone deposit. This is achieved by removing the layer of earth, vegetation, and rock unsuitable for product—collectively referred to as overburden—with heavy equipment and transferring to onsite storage for potential use in later reclamation of the site. Additionally, a “plug” of poor-quality stone may sit atop the material that has commercial value; this plug must also be removed with the overburden. After the face of the deposit is exposed, the stone is removed from the quarry in layers or pieces. If bedding planes are visible, forklifts and/or steel wedges are driven between the strata to pry up the layers. Alternatively, loose pieces are scooped up with front-end loaders, dump trucks, or other equipment. Once the layers or pieces are secured on the heavy machinery, they are transferred to an inspection area for grading, temporary storage, and eventual shipment from the site. Sandstone of insufficient quality or size for current demand is stored on-site for future use, such as for site reclamation activities, or sent to a crushing facility to be used in other applications.

2.3 Sandstone Processing Operations

Processing operations include much more variation than extraction. Nevertheless, the general procedures begin with initial cutting, followed by application of a finish, and conclude with a second cutting or shaping step. Due to the array of stone products, the second and/or third steps may be eliminated, specifically when the product will have a “natural” appearance. Figure 2 depicts the fabrication process.

The first step in sandstone processing is a primary cutting or shaping of the material. This is often accomplished for sandstone using a circular blade saw, but a splitter or hand tools, such as axes and mauls, can also be implemented. When operating a circular saw, a continuous stream of water over the saw is required in order to dissipate heat generated by the process; sufficiently elevated temperature can cause major machine and material damage. Natural faced products, such as veneer or flooring, may be completed with this step, while other products require a finishing application, secondary cutting, or both.

An array of finishing applications exists, and each uses specific types of equipment to accomplish the resulting appearance. Polished or honed finishing as well as a thermal treatment are frequently given to sandstone products, but others are also possible. The former is applied manually and/or mechanically through the use of polishing pads, while thermal finishes are applied with a flame or blow torch apparatus.

A secondary shaping step may be necessary if the product includes any features or custom size or shape. For this step, a circular saw is again commonly implemented for sandstone. Cooling water is again necessary to maintain an appropriate temperature at the stone-blade interface. Once a product is completed, it is packaged and stored for shipment or direct sale. Sandstone of insufficient quality or size for current demand is stocked on-site for future use, crushed for use in paving and construction applications, or stored for site reclamation activities.

3 LCI Methodology

3.1 LCI Data Collection

Information for this study was acquired through the distribution of a technical data collection tool.

This survey was developed by the Center for Clean Products after touring approximately 15 stone quarries and processing facilities located throughout the United States, and through extensive consultation with industry experts and quarry operators. Choosing a diverse array of facilities was key to this process as a broad understanding of stone industry operations was needed to fashion questions that apply to all members. As such, facilities of diverse magnitudes, locations, and products were toured during the beginning half of 2007.

The survey was distributed to sandstone quarries and processing facilities throughout North America in January of 2008. Responses were received, follow-up conducted, and the resulting data aggregated and analyzed in the period from March to July 2008.

3.2 Quality of LCI Data Set

The dataset presented in this report represents over 62,000 tons of quarried sandstone and nearly 32,000 tons of sandstone products generated in North America; this includes both masonry and landscape products as well as dimensional products. Data also reflects a diversity of operations with respect to size and location. Respondents indicated net annual quarry production ranging from approximately 2,000 tons to 35,000 tons, while processors reported a span of 500-29,000 net tons/year. Quarry data was submitted from companies located in 25% of the 16 states where U.S. sandstone quarries were active in 2006 (Dolley 2007). Reporting processing facilities are located in 4 states.

Due to the limited amount of data able to be supplied by reporting sandstone processors, fabrication data is withheld in order to protect proprietary information.

3.3 LCI Boundaries

3.3.1 Sandstone Quarry Operations

The LCI for quarry operations includes the inputs and outputs for each of the processes depicted in Figure 1. Specifically, processes and operations represented in the inventory presented in this report include:

• Removal of overburden using heavy equipment
• Transfer of overburden to on-site storage
• Quarry operations required to remove stone from deposit including drilling, prying, and use
  of slight explosive charges.
• On-site transport of stone using heavy equipment.
• Transport of scrap stone to on-site storage
• Onsite generation of energy and compressed air
• Capture and treatment of wastewater
• Upstream production of energy and fuels

Equipment and ancillary materials (e.g. drill bits, maintenance items) are listed in Tables 5 and 6 but have not been included in this inventory.

3.3.2 Sandstone Processing Operations

The LCI for sandstone processing operations includes the inputs and outputs for each of the processes depicted in Figure 2. Specifically, processes and operations represented in this portion of the inventory include:

• Primary shaping of stone into less-refined pieces, such as flagstone or veneer
• Application of a surface finish or texture
• Secondary shaping of stone into specific products
• Packaging of finished sandstone products for shipment
• On-site transport of stone using heavy equipment, such as forklifts
• Transport of scrap stone to on-site storage or reclamation
• Onsite generation of energy and compressed air
• Capture and treatment of wastewater and other waste materials such as dust
• Upstream production of energy and fuels

Equipment and ancillary materials (e.g. drill bits, maintenance items) are listed in Tables 5 and 6 but have not been included in this inventory.

Since a fabrication facility often processes more than one stone type, each facility was categorized as a “sandstone” facility if the majority of their production was indicated to be sandstone. Under this condition, all of respondents who are labeled “sandstone” processors indicate that at least 70% of their production is sandstone.

 

References

Dolley, T.P. 2007. Stone, Dimension. USGS 2006 Minerals Yearbook. Vol. 1, Metals & Minerals.
http://minerals.usgs.gov/minerals/pubs/myb.html