Microstructural characterisation of Biocrete™ Organo-mortar

Abstract

An innovative composite material, Biocrete, has been extensively characterised to determine its microstructure and specifically whether any geopolymer has been formed. The aim of the Biocrete project, as developed by Flexitech Pty Ltd, is to produce an acid-resistant mortar suitable for application within sewer tunnels and other aggressive environments. To this end a composite consisting of organic and inorganic polymers was desired. This thesis focused on discovering whether geopolymers had been produced within Biocrete and if not, what conditions are necessary to initiate their creation. As a four-phase composite comprising, theoretically, geopolymer, organic polymer, plaster and sand, the interplay between the various chemical reactions was determined by stripping Biocrete down into its constituent parts and using analytical techniques. Hence, the individual reactions and microstructural features were identified. It was found that the alumino-silicate starting materials, fly ash and metakaolin, could react with the alkali activator, a mix of potassium hydroxide and potassium silicate. This reaction could produce a fused, hard material, identified as geopolymer, but only when the activator was used at high concentrations. For a successful reaction the required H2O:K2O ratio of the solution was found to be below twenty. This level of alkalinity is not available within the current Biocrete formulation. When the remaining portions of Biocrete were introduced to solutions of this concentration, undesired reactions took place. Plaster, which works quite well in stabilising and strengthening Biocrete, sets extremely quickly to alkali solutions and does not bond to itself or other components. The acrylic and epoxy additions form a film throughout the microstructure of Biocrete, plating out in voids and air pockets. The addition of potassium hydroxide decreased the effectiveness of thesefilm by decreasing the water available and causing the polymers to solidify earlier and with less cohesion. The epoxy system, added in two parts, does not fulfil its purpose due to its inability to set. The two parts simply do not react together under the prevailing conditions during Biocrete cure. It was concluded that Biocrete in its current formulation does not form a geopolymer, and is rather an acrylic-modified plaster composite with fillers and aggregates. Future work on Biocrete should focus on removing the need for plaster to regulate setting time and finding an organic polymer that is tolerant of very alkaline conditions. Simplifying the mixture is also a worthwhile goal to reduce inconsistency and unidentified reactions. A major sideline throughout this project was gauging the usefulness of different characterisation techniques. Scanning Electron Microscopy, with associated Energy-dispersive Spectroscopy, proved the most useful in viewing the microstructures and identifying individual features. Both secondary electron and backscattered electron images were used in this regard. The contrast in backscattered images between aluminium/silicon and calcium regions was particularly valuable. Nuclear magnetic resonance using aluminium was chosen as the definitive guide as to how far the geopolymer reaction had progressed by comparing 4-fold and 6-fold coordination peaks. Computer microtomography was successful in obtaining a three dimensional image of the bulk microstructure, revealing that the organic polymer phase is continuous and coats the air voids within the structure.