Providing sustainable alternatives to conventional concrete is becoming a global imperative, considering its significant footprints and versatile applications. The work by Washington State University (WSU)’s Laboratory for Advanced & Sustainable Cementitious Materials (ASCM) highlights a bolder approach to “greening” the cement and concrete industries.
According to the U.S. Environmental Protection Agency (EPA), the United Nations Environment Programme (UNEP), “Humans are consuming resources and producing waste at a greater scale than ever before and per capita consumption levels are projected to increase with continued development.” In the global context, the total volume of material resources extracted or harvested worldwide reached nearly 60 billion metric tons per year in 2007, with nonrenewable resource extraction accounting for 60% of global extraction. According to the World Resources Institute, “1/2 to 3/4 of annual resource inputs to industrial economies is returned to the environment as wastes within just one year.” The United States consumed 46% more materials on a per capita basis in the year 2000 than in 1975.
In 2013, 530 million tons of construction and demolition (C&D) debris were generated. The pie chart shows the 2013 generation composition for C&D. Portland cement concrete is the largest portion (67%), followed by asphalt (18%). Each year, every person on earth consumes about 1 ton of concrete. For a small town with 53,000 people, this translates to an energy consumption of 660 average U.S. households and 3,710 tons of annual CO2 emissions.
A cementitious binder made from coal fly ash presents a great business opportunity and contributes to environment and resource conservation, by diverting such industrial byproduct from waste stream into value-added applications. The WSU group recently patented an application in which they created a durable concrete with coal fly ash—a byproduct of coal combustion primarily from coal-fired power plants captured at the top of boilers—as the only binder and no need for cement or any heat treatment. The key innovation is the use of graphene oxide (GO) at 0.02% by weight of Class C fly ash to catalyze its hydration and form calcium-aluminate-silicate-hydrate (C-A-S-H) gel at ambient temperature. This has presented a great contrast to many advances made in the use of alternative cementitious binders to partially replace cement, thus highlighting a bolder approach to “greening” the cement and concrete industries.
In the context of diverting fly ash from the hazardous waste stream and reducing CO2 emissions, the alkali-activated fly ash geopolymer is being considered as one of the main treatments to solidify fly ash and as a sustainable construction material. U.S. fly ash production increased from 40.4 million tons in 1974 to 53.4 million tons in 2013. 23.3 million tons (43%) of fly ash were used beneficially in 2013. By 2033, it is forecasted that U.S. fly ash production will be 54.6 million tons and 35.7 million tons for utilization.
In order to improve the solidification of fly ash as construction material, graphene oxide (GO, a graphite-sourced nano-material) was employed in this work. Recent years have seen a growth in innovative research on the application of GO in engineering materials.
- GO shows great potential as a concrete admixture because it is a two-dimensional carbon sheet with an aspect ratio up to 30,000 or higher
- It features a Young’s modulus of ~0.3 TPa
- An intrinsic strength of ~112 GPa
- Is highly hydrophilic
Different from graphene, GO is hydrophilic and more compatible with water, facilitating its dispersion in fresh concrete.
In the work by WSU’s ASCM, the experimental results suggested that GO regulated the different elemental mole ratios to facilitate the formation of fly ash hydrates with improved mechanical strength, as GO showed the ability to selectively affect the distribution of different hydration precursors.
This work is significant as it shows that recent development of nano-size materials has given us the powerful tool to understand the world of molecules and atoms in the fly ash or cement hydration, which presents an opportunity to engineer concrete at molecular-level.
Civil engineers have been mainly focusing on manipulating materials at meso- and macro-levels. For the sustainable advancement of the construction industry, research need to enable across all scales (nano, micro, meso, and macro) and utilize this “bottom-up” capability of nanomaterial.
These findings are described by Gang Xu and Xianming Shi-(WSU) and Jing Zhong-(Harbin Institute of Technology, China), in the article titled “Influence of graphene oxide in a chemically activated fly ash”, recently published in the journal Fuel: https://www.sciencedirect.com/science/article/pii/S0016236118306562.
The technology reported in this work has been protected by a U.S. patent application and the fly ash pervious concrete slabs have been demonstrated on the WSU Pullman campus.
To read the details of the findings and the full article titled “Graphene Oxide Enables The Clinker-Free Concrete Based On Coal Fly Ash”, please go to: https://sciencetrends.com/graphene-oxide-enables-the-clinker-free-concrete-based-on-coal-fly-ash/
For the PDF download of the EPA titled “Advancing Sustainable Materials Management: 2013 Fact Sheet Assessing Trends in Material Generation, Recycling and Disposal in the United States”, please go to: https://www.epa.gov/sites/production/files/2015-09/documents/2013_advncng_smm_fs.pdf