Studies on Aluminosilicate Binder using Alkali activated Low-calcium Fly ash

The broad objective of the work presented in this thesis is to consistently produce an aluminosilicate binder using the typical low calcium fly ash available in India with an efficient use of alkaline activators. Fly ash is used as the source m aterial considering its ready availability in large quantities. The main challenges associated with the use of fly ash as a source material include: gauging its reactive potential, determining the influence of process variables on the dissolution of fly ash and identifying the parameters which influence the strength achieved by alkali activation of fly ash. Producing an aluminosilicate binder using fly ash often involves the excessive use of activators and very high concentrated alkaline solutions. The work outlined in this thesis addresses the key issue of quantifying the contribution of fly ash in an activated system. A fundamental understanding of fly ash dissolution in an alkaline environment and the influence of activated system parameters on the aluminosilicate production is developed. The outcome of the work is a procedure for determining the most effective composition of the alkali activator required to achieve the maximum ultimate compressive strength for a given fly ash. This thesis is focused on developing a fundamental understanding of the activation process of low calcium fly ash using a combination of sodium hydroxide and sodium silicate. Fly ash characterization is performed using multiple techniques to establish the typical range of parameters in multiple fly ash samples collected from different sources. The reactive components in the glassy phase of low calcium fly ash are identified and quantified. The dissolution of the glassy phase in low calcium fly ash in an alkaline environment at different curing temperatures is investigated. The experimental challenge in quantifying the progress of reaction in an alkali activated fly ash system is overcome and a new XRD-based direct decomposition technique is established and calibrated. The extent of dissolution of the fly ash glassy phase and the reaction product contents are directly determined by applying the direct decomposition technique to alkali activated fly ash. The product developed in the alkali activated fly ash system is identified with a sodium aluminosilicate hydrate. An investigation of the activating solution composition on the sodium aluminosilicate produced and the strength development in the alkali activated fly ash systems is presented. The role of the different reactive oxide ratios and total reactive contents on the sodium aluminosiliactes product formed and the influence of the product content on the compressive strength achieved from the alkali-activated fly ash are evaluated. The requirement of initial alkalinity achieved with the sodium hydroxide in the activated mix is established for the reactive glassy content available in the fly ash. The initial solution molarity of sodium hydroxide and temperature enhance the rate of glassy dissolution resulting in a faster gain in strength. The reactive alumina content in fly ash is shown to determine the maximum ultimate compressive strength achieved by alkali activation. The composition of the activated system considering the relative proportions of reactive silica, reactive alumina and sodium are shown to significantly influence the sodium aluminosilicates produced. The ratios of the reactive oxides in the activated fly ash system required to achieve the maximum compressive strength are established. Finally, a procedure for determining the composition of the most efficient activator, with the required sodium hydroxide and sodium silicate, for achieving the maximum ultimate compressive strength for a given fly ash is developed. The work presented in this thesis forms the basis for developing an optimized activator for a low calcium fly ash to produce an aluminosilicates binder meeting specific performance requirement in structural applications