Ross P. Williams

Influence of calcium compounds on the mechanical properties of fly ash geopolymer pastes

The influence of calcium compounds (CaO and Ca(OH)(2)) on the mechanical properties of fly ash based geopolymers has been studied. Calcium compounds were substituted in fly ash at 1, 2 and 3 wt%, respectively. Curing of the geopolymers was performed at ambient temperature (20 degrees C) and 70 degrees C. Addition of calcium compounds as a fly ash substitute improved mechanical properties for the ambient temperature cured samples while decreasing properties for the 70 degrees C cured samples. Seven days compressive strength of the ambient temperature cured samples increased from 11.8 (2.9) to 22.8 (3.8)MPa and 29.2 (1.1)MPa for 3% CaO and 3% Ca(OH)(2) additions, respectively.

The First 20 Hours of Geopolymerization: An in Situ WAXS Study of Flyash-Based Geopolymers

This study followed the first 20 h of flyash geopolymerization at 70 °C using time resolved Wide Angle X-ray Scattering (WAXS). The extent of dissolution of the amorphous phase of the flyash was determined to range from 29% to 54% for the different formulations trialed. The dissolution rate of the flyash significantly reduced after the first 5 h for all samples. During the formation stage of the geopolymer there were significant temporal variations in the chemistry of the dissolved solution due to the rate of flyash dissolution, with a relative standard deviation of 20%, 57% and 24% for the Si/Al, Na/Al and H/Si ratios, respectively. Utilizing the Power Law, scattering in the low angle region of the WAXS pattern combined with the geopolymer peak area yielded a measure which correlated with the compressive strength—providing a new method to measure the flyash dissolution and geopolymer formation processes independently. The evolution of several zeolite-like phases was followed, noting there are different formation mechanisms involved even within the same sample. Four samples were examined with compressive strengths ranging from 14(2)–50(9) MPa, each was synthesized with flyash from Collie Power Station (Western Australia) activated with sodium silicate solution of varying concentrations.

Methods for geopolymer formulation development and microstructural analysis

Alkali-activated materials (AAMs) and geopolymers have been extensively studied, although widespread commercialisation has been hampered, in part, by the use of precursors that are rarely homogeneous and are generally poorly characterised. Even when precursors are well characterised, their extent of reaction during geopolymer synthesis is not well known, leading to a disparity between targeted and actual compositional ratios. Small variations in compositional ratios, particularly Si:Al, can lead to dramatic changes in physical properties. A process for characterising precursors, focussing on their reactive component, will be described here, followed by methods that can be used to determine the extent of reaction in the final product. Characterising the final product is important, but it does not reveal what processes occur between mixing the precursors and setting of the solid geopolymer. We will also describe a method that can be used to track dissolution of precursors and subsequent evolution of the alkali-activated product, thus providing a more comprehensive picture of geopolymerisation. This paper demonstrates a link between precursor characterisation and the extent of reaction in order to provide those working with alkali-activated materials with additional knowledge enabling them to manufacture reproducible, high-quality products.