By Joshua Bowditch
Supply of concrete-suitable limestone is of economic interest and environmental concern for Lee County and all of southwest Florida. The basic components of building materials are geological resources which must be processed to some extent. Concrete in its various forms is integral to building construction in southwest Florida, because of its local availability, thermal mass, and hurricane resistance. The environmental concern with limestone mining includes detrimental impacts to natural habitats and agricultural establishments.
Quality of Material
As primary constituent of concrete, limestone greatly influences the characteristics of concrete1. The limestone material must be of suitable hardness and firmness for use in concrete1. In Florida the two general types of limestone are hard limestone that meets standards for high-value products such as aggregate for concrete, and relatively soft limestone which is limited to lower-value products such as road base and fill dirt2. This is an important factor that is often undifferentiated2. Limestone in the southwest Florida region is generally soft throughout its range, but unusually hard rock does occur, though it is very geographic-specific to an area of southeast Lee County3. It is important to note that limestone from this specific area supplies a multi-county region along the Interstate 75 corridor3.
Correlation with Concrete
One benefit of higher content of limestone in concrete is lower necessary content of cement. Crushed limestone, having a rough surface texture and being a calcareous material, results in good quasi-chemical bond, with the implication of allowing for less cement in the concrete1. While limestone is a raw material processed simply by crushing, cement is limestone that has undergone extensive pyroprocessing and grinding to exceeding fineness. Cement production is very energy-intensive and a major source of industrial emissions4. These emissions result from thermal decomposition of raw materials and combustion of fuels4. The importance of reducing the usage of cement is often overlooked.
Benefits of Concrete
In southwest Florida, concrete in its various forms is used profusely because of local availability of respective constituents, thermal mass, and hurricane resistance. The high thermal mass of concrete can be leveraged as a passive cooling strategy. Thermal mass is the ability of materials to retain a certain temperature, effectively reducing cooling loads5. Much of the heat absorbed by exterior concrete walls eventually dissipates to the outside. Inasmuch, heavy insulation at the building perimeter is not necessary for occupant comfort specific to climatic conditions of lower Florida. Thermal mass in combination with judicious insulation levels provides synergistic energy-savings benefits6.
It is acknowledged that mine lakes can adversely affect surrounding environments7. The specific area where high quality limestone occurs is surrounded by environmentally sensitive natural habitats and agricultural establishments. The edaphoclimatic conditions are characterized by seasonal fluctuation of the water table, with a pronounced wet season and severe drought conditions during the dry season8. The problem is that mine lakes will lower the water table of surrounding areas that are up-gradient of the mine lake waterline7. Therefore, in the dry season, drought conditions could be prolonged and intensified to the extreme of diminishing the vitality of plants and trees, from both an agricultural and conservation standpoint.
The conclusion to be made is that building materials, especially concrete products, should not be used flippantly. From an architectural standpoint, sustainability regarding the composition of building materials is indeed an important consideration. Offsetting the use of concrete is possible with expansive glass systems, and precast concrete as an alternative to conventional types of concrete.
1McCaulley, D. B., M. Mittelacher, J. L. Mross, J. P. Roebuck, and R. Winemberg. 1990. Florida Aggregates in Construction, Their Characteristics and Performance, Technical Report 90-01. Florida Concrete and Products Association, and Concrete Materials Engineering Council.
22009. Final Supplemental Environmental Impact Statement on Rock Mining in the Lake Belt Region of Miami-Dade County, Florida. U.S. Army Corps of Engineers (USACE). 1-1.
3Lampl-Herbert Consultants. 2007. Strategic Aggregates Study: Sources, Constraints, and Economic Value of Limestone and Sand in Florida. Florida Department of Transportation.
4van Oss, H. G. 2015. Cement. Page 16.1-16.32. In: 2013 Minerals Yearbook. United States Geological Survey (USGS).
5R-Values and U-Factors of Single Wythe Concrete Masonry Walls, TEK 6-2C. National Concrete Masonry Association, 2013.
6Sustainability. 2007. Designer’s Notebook, DN-16. Precast/Prestressed Concrete Institute.
7Maliva, R. G., K. Coulibaly, W. Guo, and T. M. Missimer. Simulations of Impacts of Sand and Rock Mining on Florida Coastal Plain Water Resources. Mine Water and the Environment 29:294-300
8Beever, J. and D. Thomas. 2006. Immokalee Rise/Pine Flatwoods Conceptual Ecological Model. Florida Fish and Wildlife Commission, and U. S. Fish and Wildlife Service.