John Hema and Dr. Fernando Fonseca, Civl and Environmental Engineering
Fossil fuels are used in modern power plants throughout the world to produce electrical energy. This process increases air pollution and generates millions of tons of a by-product known as fly ash. Fly ash is the inorganic residue that remains after pulverized coal is burned. This residue rapidly accumulates and is often disposed of in landfills, filling them to capacity. One solution to reducing the amount of waste has been to use fly ash in concrete.
Fly ash is commonly added to cement or used as a partial replacement for cement in a concrete mixture. The use of these by-products offers environmental advantages by diverting the material from landfills, reducing the energy investment in processing virgin materials, conserving virgin materials, and decreasing pollution. Usually about twenty to thirty-five percent of Portland cement is substituted with fly ash, consequently reducing the amount of cement needed to make concrete. Since fly ash is less than half the price of Portland cement, consumers save money when building roads, bridges, buildings and other community projects with concrete that contains fly ash.
Not only does fly ash offers economical advantages, it also improves the performance and quality of concrete. Fly ash affects the plastic properties of concrete by improving workability, reducing water demand, reducing segregation and bleeding, and lowering heat of hydration. It also increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction. Although concrete made with fly ash gains strength more slowly than concrete made with only Portland cement, its ultimate strength is equal to or exceeds the strength of the latter.
With the current major worldwide agenda to reduce greenhouse gas emissions, the pressure is on conventional coal-fired utilities to burn renewable fuels such as waste product or biomass fuels as a “lowest cost” option for reducing emissions. The incineration of waste products or biomass fuels will aid in preserving precious fossil fuels and in reducing air pollution. However, this new material will produce a new type of fly ash with chemical properties entirely different from that of coal fly ash. If utility companies were to start generating electrical energy through biomass incineration, there would have to be a market for biomass fly ash, which can only be created by proving that biomass will have no detrimental effects to concrete.
To evaluate the effects of biomass fly ash on concrete, two control groups of concrete specimens were made in order to observe the changes in compressive strength, flexural strength and durability of the specimens as biomass fly ash was added into the following mixes. The control groups consisted of pure Portland cement concrete and concrete with Class C fly ash. Currently, design regulations allow for Class C coal fly ash to be used commercially. The test groups consisted of concrete specimens made of (1) pure wood fly ash, (2) 10% herbaceous co-fired fly ash, and (3) 20% herbaceous co-fired fly ash.
Initially, it was hypothesized that the biomass fly ash would not have enough detrimental effects on concrete as to prevent its use in construction. The test results for compressive strength show that for the first 28 days pure Portland cement concrete and concrete with Class C fly ash had higher compressive strengths than did the test groups. However, after 28 days, the compressive strength of the test group started to exceed that of the control group. Although more time is needed to conduct 91 day and one-year tests, the results show a trend of biomass concretes developing compressive strength greater than pure and coal fly ash concretes.
Flexural strength tests for the concrete specimens were conducted after allowing the concrete to cure for 56 days. Each one of the specimen groups exhibited comparable strengths with pure Class C fly ash being the strongest. It can be concluded that the addition of biomass into concrete does not significantly reduce the flexural strength of concrete.
Durability tests are on-going since the amount of time required to test one specimen group is approximately five months. Thus far, no significant differences have been observed between the control and test groups.
Presently, the results are encouraging and have already provoked additional research to be conducted on this subject. The biggest obstacle that I faced was the amount of time I invested into this research. The curing times as well as the amount of specimens needed for testing contributed to long hours in the lab. Also, ASTM required that certifications be obtained in order to perform the various tests and operate the testing apparatuses. This added to my frustrations since nobody at BYU knew how to work some of the equipment. Several times I had to turn to professors at other universities to teach me how to use the equipment and other times I just had to learn it myself.
On the whole, I have gained invaluable experience in material science and a better understanding of the properties of concrete. I have had two internships over the past couple of summers and both have remarked on the experience I had in doing research. It has helped me greatly to organize and to solve unique problems. Hopefully, the research I have conducted will continue to progress so that the use of biomass will one day be allowed in commercial construction.