In modern application, Portland cement is the most common binding agent. It was developed in the mid-18th century in England is used in place of pozzolana179 and is the result of heating limestone and clays to high temperatures, grinding the result (clinker) and adding other constituents like gypsum to the mix.180 This is then added to sand, gravel and water to form concrete. There are other binding agents which are used on their own or as part of a mixture with Portland cement including fly ash or organic plant materials like hemp.
Fly ash is the powdery residue leftover from coal fired kilns and power plants. The ash which is now captured magnetically, in the flues or on the bottom of the ovens is often a discarded by product of energy production. Per the EPA, of the 110 million tons of coal combustion residuals (CCRs) including fly ash, bottom ash or boiler slag generated in 2012 about 40% was recycled and 60% discarded into landfills.181 Due to its chemical makeup, fly ash can be used at about 60% of the cement mixture of concrete with the remaining portion coming from Portland cement.182 This project will not incorporate fly ash concrete due to the environmental impact, the fact that it is not produced in the state and because it must be used in conjunction with Portland cement anyway.
There are a few reasons why concrete in its most typical form is often said to be the most environmentally impactful building material. The cement industry is said to produce about 5% of the
176 P.K. Mehta, "Natural pozzolans: Supplementary cementing materials in concrete," CANMET Special Publication, SP86-8E, (1987).
177 Ibid.
178 F. Massazza, "Concrete resistance to seawater and marine environment," il Cemento 1 (1985): 3-25.
179 Robert G. Blezard, "The history of calcareous cements," Lea's chemistry of cement and concrete, 4 (1998): 1-23.
180 “User Guidelines for Waste and Byproduct Materials in Pavement Construction,” Report FHWA-RD-97-148 U.S. Deparment of Transportation, Federal Highway Administration accessed November 15, 2016 http://www.fhwa.dot.gov/publications/research/infrastructure/pavements/97148/072.cfm.
181“Frequent Questions the Coal Ash Disposal Rule,” Environmental Protection Agency, accessed November 12, 2016, https://www.epa.gov/coalash/frequent-questions-about-coal-ash-disposal-rule#1.
182 Michael Thomas, “Optimizing the use of Fly ash in concrete,” Portalnd Cement Association, (2007).
global CO2; for an indispensable building material, whose industry is growing 2.5% annually183 with the production of cement reportedly expected to rise from 2.55 billion tons as of 2006 to around 4 billion tons by 2050, this has significant environmental repercussions.184 The reason cement causes so much CO2 is due to the incredibly high energy output needed to heat limestone and clay to produce Portland cement products. Per the United Nations Environment Programme to produce 1 ton of cement you need 4.7 million BTUs or about 400 pounds of coal, generating almost a ton of CO2.185 Portland cement is usually heated in kilns powered by fossil fuels and the heating of limestone also releases CO2, so the entire process is a high source of greenhouse gases, one of the highest, per the EPA.186
There has been research and application of other alternatives to Portland cement and fly ash with varying degrees of success. Perhaps the most well-known is called Hempcrete. This type of concrete uses part of the hemp plant mixed with lime and water to form a lower density, lower strength bio-composite material. There are some benefits of hempcrete over traditional concrete like its neutral or even negative carbon footprint, its lighter weight, breathable, has good insulating properties and thermal mass. However, the lower strength means it must be used on conjunction with other supporting materials and requires the necessary hemp production to supply the high silica shivs.187 Use of hempcrete could be implemented in some types of residential architecture if the necessary hemp production could begin in the state for the future, but for this project it will not be used.
Another extremely interesting and intriguing supplementary replacement for Portland cement was developed by a company called Calera. The process includes capturing CO2 from flue gas and directing it through a source of alkalinity and calcium to form calcium carbonate. The source of alkalinity can be from industrial waste products like calcium hydroxide or from other separate sources like caustic soda and calcium chloride, both by products of other chemical processes. Most interesting is the ability to direct the CO2 emissions through salt water which contains sufficient alkalinity and calcium naturally, the result is the same calcium carbonate which can be used as a replacement for Portland cement.188 Calera’s system has undergone some different processes and use of saltwater may require more energy
183 “Cement Sustainability Initiative,” World Business Council for Sustainable Development, accessed November 17, 2016.
184 “IPCC Fourth Assessment Report: Climate Change 2007,” 7.4.5.1 Cement. Intergovernmental Panel on Climate Change, accessed November 14, 2016, http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch7s7-4-5.html
185 “”Greening Cement Production has a Big Role tro Play in Reducing Greenhouse Gas Emissions,” Environmental Science Alert, United Nations Environment Programme, accessed November 29, 2016, http://na.unep.net/geas/science/alert_2010_10.php.
186 Lisa J. Hanle, “CO2 Emissions Profile of the U.S. Cement Industry,” Environmental Protection Agency, (2004), accessed November 27, 2016, https://www3.epa.gov/ttn/chief/conference/ei13/ghg/hanle.pdf
187 Steve Allin, Building with Hemp, ( Seed Press, 2005), 146
188 Calera (website), accessed November, 28, 2016. http://www.calera.com/.
than other methods. Regardless, the ability to capture up to 90% of emissions from power plants to make a carbon neutral building material is interesting and further research should be done in the future when more information is available on the process and resulting calcium carbonate cement product.
If we as architects and builders are to use concrete in Hawai‘i a few things should be addressed. We should attempt to use as many local products as possible to expand the local economy and to reduce the environmental impact of imported building products. This can be done by using local aggregates to create a more regional feel and culturally connected material. We should try to supplement or replace Portland cement whenever possible due to the absorbent amount of CO2
produced in the process. Finally, we should try to use pre-cast or pre-stressed concrete to speed up the building process, reduce costs and attempt to develop an architectural language of simplicity.
Out of these goals, supplementing and replacing Portland cement is the most difficult. Local aggregates are in abundance and have been proven to create high quality concrete. Grace Pacific Rocky Mountain Prestress (GPRM Prestress, LLC) provides a wide assortment of Prestress / Precast Concrete Institute (PCI) certified structural and architectural products. Completely replacing Portland cement would be difficult with the current technology however there is some evidence to suggest that a creation of a local Pozzolana and production of lime from local limestone could be one solution.
Figure 8.5: Calera CO2 Capturing, The Process
Source: http://www.calera.com/beneficial-reuse-of-co2/process.html accessed November 3, 2016