My primary reason for trekking across the Pacific wasn’t to see koalas in their natural habitat (even though that would be good reason), but actually to conduct research on fly ash geopolymers. Specifically, I am developing beneficial pathways for utilization for co-fired fly ash. Co-fired fly ash is a by-product of co-combustion of coal with biomass—a renewable source of energy. One potential reuse is the formation of geopolymers. These binders are formed by alkali-activating solid fly ash aluminosilicates. They can be used in the construction field as a less carbon intensive alternative to ordinary portland cement (OPC) concrete. In fact, in Australia they are already being commerically sold (see http://www.zeobond.com/). This research not only aimed to develop these co-fired fly ash geopolymers, but also to understand the science behind their formation.
First, we created many different mix designs utilizing three sources of ash (one coal fly ash and two co-fired ashes). Various alkali-activating solutions were formulated and added to these ashes to see if geopolymerization would occur. A quick test to see if you have actually created a geopolymer is to place the recently cured samples into water. If the matrix remains intact after a few days you have most likely created a geopolymer. Yet, if the matrix completely dissolves it’s back to the drawing board. Luckily we were able to create viable mix designs for each type of the ash.
I performed three analytical tests on the raw fly ash and the geopolymers (which I will explain over the next few posts). First, as I mentioned in the previous post, I utilized x-ray diffraction (XRD) to characterize the crystalline phases present in each sample. Samples were tested before and after geopolymerization to identify changes in the microstructure. All of the samples were pulverized to a powder before being placed in the holder for analysis. The crystalline structures in the samples are assumed to be randomly oriented when in this powder form and thus the diffraction pattern will reveal concentric rings of scattering peaks with the spacing between these rings dependent on the crystal lattice parameters (see Bragg’s Law). The final result is a plot of scattering intensity vs. the scattering angle, 2θ. The intensities and positions of the peaks are used to identify the crystalline components. In geopolymers the identification of zeolites can be an important step in understanding its unique structure. Furthermore, the detection of a hump in the intensity plot indicates there is an amorphous component to the structure. XRD is a powerful tool that when used properly can reveal detailed information on the chemical and crystallographic framework of a material.