Comparison of embodied energies of Ordinary Portland Cement with Bayer-derived geopolymer products

Abstract

It has been shown that silicate-derived geopolymers can be manufactured with lower greenhouse gas emissions than can Ordinary Portland Cement (OPC). This is assuming that transport methods and distances are equal. In this paper, we have used published and newly-determined data, following closely with accepted life cycle assessment procedures, to evaluate the embodied energy of a new class of geopolymers, namely Bayer-derived geopolymers. These geopolymers utilise concentrated sodium aluminate solutions (Bayer liquor) with fly ash and other aluminosilicates to form geopolymers. Significantly, utilising this combination of industrial by-products can dramatically lower the embodied energy of Bayer-derived geopolymer. Under current industrial operations, Bayer liquor is recycled into the process after intensive treatment, in order to retain alumina and caustic soda however, under alternative configurations this impurity-laden stream could be removed, potentially enhancing environmental performance of both geopolymer and alumina production. In a reconfigured-system, with no allocation of embodied energy to the Bayer waste stream, this could reduce the embodied energy of the derived geopolymer concrete to as little as 0.33 GJ/t. Embodied energies of between 6 and 33% of those associated with OPC have been achieved depending on the process configuration and allocation assumptions used. Most importantly, such assessment indicates that for the first time, Bayer-derived geopolymer binders could be produced with embodied energy intensity at levels comparable to manufactured or recycled sand, gravel and stone.