Manufacturing Impacts: Natural Stone vs. Precast Concrete
Precast concrete is a popular building material that is cast in a mold or form. This process is done off-site from a project to control the product quality and reduce waste. The material is used in both indoor and outdoor projects for wall panels, flooring, and even kitchen countertops. Gray is often the color choice for concrete products, but there are several color options available that are achieved through the use of dyes, pigments, and acid stains.
Some of the ingredients in precast concrete are natural materials, such as coarse and fine aggregates, cement, water, and admixtures. The concrete industry has made efforts to reduce their significant environmental impact by substituting some of these ingredients with fly ash, a byproduct of coal production. Because they are achieving a reduction in the embodied carbon with this strategy, they often market their products as “green.” There are a few sustainable characteristics to precast concrete including its thermal mass. The factory environment uses less material than concrete created on site and often the materials come from local sources, which can reduce transportation costs and emissions. However, as you can see from the manufacturing process outlined below, the multi-step and energy intensive processes involved in manufacturing precast concrete create a much larger environmental footprint than natural stone.
A precast concrete wall panel with a repeating pattern. Photo Credit: Wikimedia Commons/ Acabashi
Manufacturing Precast Concrete vs. Natural Stone
To manufacture precast concrete, raw materials are required to first make cement. Minerals are mined out of the ground and then transported to a cement manufacturer or plant. The minerals are heated at extremely high temperatures to create the cement which requires the use of a lot of energy and produces a significant amount of harmful emissions including sulfur dioxide, nitrogen oxide, and carbon monoxide. A mold and steel reinforcement are also needed for the precast concrete. The mold must be created and the steel must be acquired and transported to a steel manufacturer, where it is melted and cast into reinforcing rods. Aggregates are also required, which must be mined, crushed, and sorted. There are other additives and products required depending on the desired finish. The materials are mixed with water, put into molds, and cured into the final product. Polishing, sealing, and/or waxing may also be necessary, depending on the end use of the precast concrete.
In contrast, natural stone requires only quarrying, fabricating, finishing, and transporting. No additional materials or chemicals are required to create natural stone. The Earth naturally forms the material over time. This means natural stone really is a natural material and has many other attributes, including its durability, recyclability, and wide range of aesthetics. Natural stone can be used in many different indoor and outdoor applications as well and contains no Volatile Organic Compounds (VOCs), meaning it does not emit any harmful gases, making it a healthy material.
Sustainability Concerns
From an overall sustainability perspective, natural stone has a lower environmental footprint than precast concrete. This is due to the minimal resources used to quarry, fabricate, finish, and transport natural stone. As the graph below demonstrates, precast concrete has a higher global warming potential (GWP) than natural stone. Every step in the precast concrete manufacturing process requires the use of a lot of energy, which contributes to their total GWP. The larger the GWP, the more that a given gas warms the Earth compared to carbon dioxide (CO2) over the same timeframe. The natural stone industry is often criticized for utilizing raw materials that are not rapidly renewable. However, natural stone is abundant within the Earth. Other industries also utilize the supply of natural stone as it is a significant ingredient in concrete, porcelain, and other manmade products.
The impacts of these processes have been documented and systematically compared against other materials using the same environmental criteria. This information is a valuable resource when selecting a sustainable material for a project.
The embodied carbon quantities displayed were estimated based on the following:
Unite of Measure: Global Warming Potential (kg CO2 eq)
Functional Unit: 1m2
Scope: Raw Material Extraction, Transportation, Manufacturing (A1-A3)
Natural Stone: Industry-Wide EPDs for Cladding, Flooring & Countertops
Pre-Cast Concrete Cladding: Industry-Wide EPD for Architectural Precast Panels, 150 lbs per ft3, 4” thick
Engineered Quartz: Average of three individual manufacturer EPDs
Terrazzo: Average of three individual manufacturer EPDs
These characteristics and attributes also make natural stone a great choice when seeking a green building rating certification within the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) or the International Living Future Institute’s Living Building Challenge (LBC). The different manufacturing processes and impacts of building materials are being documented in product labels such as Environmental Product Declarations (EPDs) and Health Product Declarations (HPDs). These labels make it even easier to qualify for points and credits within LEED and other green building rating systems using natural stone. The labels document and quantify environmental information on the life cycle of a product and allow you to make comparisons and informed decisions on materials that fulfill the same function. The product labels are also intended to demonstrate that the health and environmental claims are transparent, accurate, and meet defined standards. Industry-wide EPDs are available for natural stone cladding, flooring/paving, and countertops. HPDs are available for common natural stone types used in the dimension stone industry including granite, limestone, marble, quartzite, onyx, sandstone, slate, travertine, and more. (See also: Environmental and Health Product Labels for Natural Stone.)
To further advance these issues, the natural stone industry has been working diligently over the years to make continuous improvements in each area and step of the process. This includes reducing water use, energy use, improving the efficiency of the transportation of natural stone, properly managing and adaptively reusing quarry sites, and many others.
Technology advancements in the natural stone industry have led companies to create zero waste strategies that use every part of a stone block. Photo Credit: Coldspring
Stone wall designs can be quickly and easily fabricated using computer numerically controlled machinery, with any waste being used in stone byproducts. Photo Credit: Stephanie Vierra
The natural stone industry has created a certification system that determines if a quarry or fabricator meets defined sustainability standards in key areas of importance. This makes it easier to find and use natural stones that meet the standard, simplifying the process of choosing the right material that not only looks and performs well, but also has the least amount of impact on the environment. The standard is also accepted within the LBC, further ensuring its use in sustainable building projects. The metrics of the Natural Stone Sustainability Standard can be used to vet stone suppliers even if they have not yet achieved certification. More information is available via the Natural Stone Institute or NSF International.
