Experimental study of the clay activation methods to develop geopolymer binder

In this research, the potential of low-grade clays to produce cement-like material was investigated. In a process called alkali activation or geopolymerisation, the initial structure of clay, which is one of the aluminosilicate resources that can be used for geopolymer production, is broken down into individual aluminate and silicate tetrahedrons using a highly alkaline solution. These released individual tetrahedrons are then rearranged into a new structural configuration under specific temperature and pressure in order to form a three-dimensional network structure. This new product exhibits excellent strength, similar to that of Ordinary Portland cement (OPC). However, geopolymer is an environmentally-friendly substitute for OPC, with studies showing that it releases less CO2 into the atmosphere compared to Portland cement production. In fact, the cementitious property of aluminosilicates mixed with alkalis was observed many decades ago, in 1930, German scientist Hans Kuhl observed the cementitious properties of slag activated by potassium hydroxide. Many studies have since been conducted on this type of material, mainly concentrating on the use of fly ash, blast furnace slag, or Metakaolin, for the synthesis of geopolymers. However, the limited availability of these source materials in some geographic areas necessitates investigating the potential of other aluminosilicate resources for this purpose. Clay, which is cheap and abundantly available, has a good potential to produce geopolymer.  The utilization of low-grade clay for geopolymer synthesis is not well-established, with limited studies published in the literature. Widespread availability of low-grade clay makes it a potentially highly suitable alternative for large scale production of geopolymer. However, the internal structure of low-grade clays, makes it less reactive towards geopolymerisation compared to other resources. Therefore, the pre-treatment techniques are required to alter the structure of the clay and increase its reactivity towards geopolymerisation. In the present study, four different types of low-grade clay were supplied from construction contractors in Melbourne to be assessed as raw material for geopolymer synthesis. No information was provided regarding the specific location of the construction sites. In the initial stage of this research, all clays were characterised using advanced techniques such as X-ray Fluorescence, X-ray diffraction, Scanning Electron Microscopy, Laser Granulometry, and Thermogravimetry to identify the most suitable candidate. Also, due to the poor reactivity of low-grade clays, they can be treated to change their structure and increase their reactivity. Calcination was selected as the first treatment technique, and different calcination temperatures were applied to the clay to obtain a higher specific surface area and a finer particle size distribution, which could subsequently lead to better reactivity. The second treatment technique adopted in this study was mechanical activation. Mechanical energy in the form of shear and impact is imparted to the clay particles to change their structure and increase their reactivity. Optimal parameters for mechanical activation were also determined based on the largest specific surface area and finest particle size distribution achieved. In the next phase, geopolymer mortars were cast and their mix design was optimised for the geopolymer made from the untreated clay. Parameters such as alkaline modulus (the ratio of Na2O to SiO2 in the alkaline solution), dosage (mass of Na2O to mass of clay), and water to solids ratio were adjusted to optimise compressive strength. The curing temperature employed was 80°C, 100°C, and 120°C.  In the next phase, the strength optimisation process was repeated for geopolymer mortars made from calcined and mechanically activated clays. Microstructural studies were conducted on all the synthesis routes to elucidate the underlying micro-level parameters which determined the strength development. Advanced techniques such as Nuclear Magnetic Resonance spectroscopy, Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, Computerised Tomography, Atomic Absorption Spectroscopy were implemented to correlate the micro and macro-level properties. Based on the results of the previous phases, calcination was deemed a more efficient treatment technique for increasing the reactivity of the clay. Therefore, for the geopolymer concrete studies, calcined clay was used. The short and long-term properties of the geopolymer concrete were also studied to investigate the potential of clay-based geopolymer for concrete manufacture.