G V P, Bhagath Singh and K V L, Subramaniam
(2019)
Studies on Aluminosilicate Binder using
Alkali activated Low-calcium Fly ash.
PhD thesis, Indian institute of technology Hyderabad.
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Abstract
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.
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