Turning Waste Into Value-added Cement-Based Products - National Research Council Canada

A group of NRC scientists has been working to investigate how to turn "fly ash" from incinerated municipal solid waste into value-added cement-based products.

The research supports the Agency for Science, Technology and Research (Singapore) A*Star collaboration with the NRC Institute for Chemical Process and Environmental Technology (NRC-ICPET) and the NRC Institute for Research in Construction (NRC-IRC).

Dr. Mercier's (NRC-ICPET) work has led to the publication of an award winning paper published in Acta Crystallographica B – Structural Science and another article currently under review by the Journal of Applied Crystallography.

Fly ash is a by-product of solid waste incineration, a common disposal method used by many municipalities. Typically, limestone dust is used in the air pollution control system of these incinerators as a scrubbing agent for gases produced by combustion. The dust combines with toxic heavy metals present in the gas before the gas is discharged into the atmosphere. What remains is fly ash – chemical mixtures containing soluble heavy metals mixed with several mineral crystalline and amorphous (non-crystalline) phases.

Under the A*Star project led by NRC-ICPET's Dr. Pamela Whitfield and NRC-IRC's Dr. Lyndon Mitchell, the objective of Dr. Mercier's research is to investigate the potential for "apatites" to immobilize heavy metals, thereby stabilizing the chemical composition of the fly ash so that any toxic elements contained in the ash will not leach out of the material. The material can then safely be added to cement-based products - creating a value-added product from what was previously a toxic by-product of waste incineration. Apatite occurs in virtually all types of rocks and is also the mineral component of bones and teeth.

Dr. Mercier is an NSERC fellow working in NRC-ICPET's Energy Materials group. With colleagues Drs. Yvon Le Page, Pamela Whitfield, Lyndon Mitchell, Isobel Davidson and Tim White (Singapore), he authored the paper " Geometrical Parameterization of the Crystal Chemistry of P6₃/m Apatites: Comparison with Experimental Data and Ab Initio Results", published in Acta Crystallographica B – Structural Science. The journal's Editor-in-Chief named the group's paper as one of the four best papers published by this periodical in 2005. Dr. Mercier has a Bachelor of Engineering from the Royal Military College (Kingston, ON) and a M.Sc and Ph.D in Physics from the University of Ottawa.

Key to Mercier's research is the use of crystallographic diffraction techniques, such as X-ray powder diffraction, to analyze and define the crystal-chemical parameters of apatite containing heavy metals. These heavy, toxic metals could be lead, arsenic, cadmium or chromium. Along with Singaporean colleagues, a thorough and systematic approach to investigating the apatite crystal chemistry was developed. A crystal-chemical model of apatite was created by Mercier, linking the mineral's crystal structure to its chemical composition. The model was then used to compare experimental results of apatite materials studied by single crystal and Rietveld analysis. Mercier comments that a practical application of the crystal-chemical approach to studying apatite-type structures was clearly demonstrated by this research and is described in the paper submitted for publication to the Journal of Applied Crystallography.

"We show that least squares crystal-chemical parameters of P6₃/m apatite extracted directly by Rietveld refinement are nearly one order of magnitude more precise that those obtained through analysis of standard refinement results and more consistent with single-crystal results", he says.

With regard to the A*Star project work, it is apparent that an apatite mineral structure can produce stable chemical bonds with some toxic metals, thereby "locking in" or incorporating the toxic elements into the apatite crystal structure. The next step is to now prove that toxic metals do not leach out during subsequent processing of the fly ash.

The crystal-chemical parameterization of other structure types, in a similar way as applied to apatite, may indeed be suitable to the development of inorganic synthetic materials with "desired" physico-chemical properties. In particular, the crystal-chemical approach can be combined with ab initio quantum materials modeling methods in software packages developed in collaboration with Toth Information Systems Inc. These models can be used to predict the structures of hypothetical toxic metal-bearing compounds that have not yet been studied.

Finally, by developing and cataloguing the crystallographic features and chemical compositions of a range of apatite, using methods applied and refined by Dr. Mercier, apatite-type materials can be "engineered" to tailor structural and crystal-chemical properties that are specific to a range of applications. Potential applications include soil remediation, developing oxygen-ion conductors for the electrolyte in solid oxide fuel cells (SOFCs), human bone replacement and even dentistry.