ROADS & BRIDGES
The viability of EGGSHELL POWDER as a substitute in concrete
T
he viability of this exercise is justified by the presence of calcium carbonate (calcite) in eggshell powder, which is also found to be a fundamental chemical constituent in both cement and course aggregate. Data from the International Egg Commission reveals that when the total world output of eggs in 2018 is divided by the total world populace of 7.6 billion people, the average intake is 161 eggs per person per year. This translates into 76.7 million tonnes’ worth of eggshells being discarded to landfill. Given ongoing population growth, this figure continues to climb. From a construction perspective, calcite (CaCO3) not only serves the purpose of filling voids between cement granules; it also fasttracks the hydration process and affects the mechanical properties, workability and durability through nucleation, dilution and even chemical effects. In excess of 4% of the earth’s crust is composed of CaCO3. The most popular natural forms are marble, limestone and chalk. These are produced by the sedimentation of the shells of small fossilised shellfish, snails and coral over millions of years.
Chemical breakdown analysis During our research, X-ray powder diffraction (XRD) scans were performed on cement, sand and stone samples (the key ingredients in the concrete mix), as well as on eggshell powder in order to detemine the relative percentage of CaCO3 within each material. The XRD results that were obtained show that the chemical constituents of stone are quartz, albite, microcline,
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IMIESA November/December 2021
calcite, muscovite and kaolinite. The most dominant chemical constituent is quartz, at a percentage of 80.2%. For river sand, the chemical constituents are quartz, albite, microcline and muscovite. Again, the most dominant chemical constituent is quartz, at a percentage of 97.7%. In turn, the XRD results for eggshell powder confirm that the chemical composition is 100% calcite, while the composition breakdown for cement is alite, belite, ferrite, cubic, ortho, free lime, portlandite, periclase, arcanite, aphthitalite, calciolangbeinite, gypsum, bassanite, anhydrite, calcite, quartz and mullite. The most dominant chemical constituent is alite, at a percentage of 47.3%. These XRD scans also show that calcite exists within cement at a percentage of 3.6% and in stone (coarse aggregate) at a percentage of 1.3%.
Cement additives like fly ash help to promote a circular economy by recycling waste streams – but there are a host of other material options. A recent study by the authors presents the argument for the use of eggshell powder as an option for greener concrete production. By Bonke Mncwango* and Professor Dhiren Allopi** The chemical composition of eggshell powder is 100% calcite
Conclusions From these XRD findings, we would argue that there is a strong business case for further experimental investigation given the abundance of eggshells available. This experimental investigation should consider the inclusion of eggshells in a powdered form. Further evaluation will confirm whether its introduction can be used as a viable substitute in place of the 3.6% calcite in cement and 1.3% in stone in terms of final concrete strength and performance. *Research Student, Durban University of Technology **Durban University of Technology, Department of Civil Engineering and Sur veying
This is an edited version of the original paper. For the full paper, as well as references, please contact the authors at bmncwango@gmail.com.
