Class is obtained from burning of harder, older

Class C fly ash
produced from the burning of younger lignite or sub-bituminous coal, in addition
to having pozzolanic properties. Class C fly ash also has some self-cementing
properties. In the presence of H2O, it hardens and gets stronger
over time. Class C fly ash generally encompasses more than 20 % of lime (CaO).
Disparate Class F, self-cementing Class C fly ash does not require an
activator. Alkali and sulfate (SO4) contents are normally higher in
Class C fly ashes 21.

Class F fly ash
is obtained from burning of harder, older anthracite, and bituminous coal.
Class F fly ash is pozzolanic in nature and contains less than 7 % of lime
(CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class
F fly ash require a cementing agent, such as Portland cement, hydrated lime or
quicklime, which is react with water and form cementitious compounds.
Otherwise, adding a chemical activator like sodium silicate (water glass) to a
Class F ash can form a geopolymer 21.

Class F fly ash
based geopolymers have been mainly proposed as new insulating materials which
possess several properties such as low cost and simple processing conditions,
high maximum application temperature, non-flammability, etc. than other widely
used traditional insulating materials like polystyrene, polyurethane, melamine
and glass wool, glass foams, and perlite. Thermal insulating materials which
have low thermal conductivity, as well as acceptable mechanical strength, are
important properties of energy efficient building blocks 1,22.

Geopolymer
material has an amorphous microstructure and resembles like high-performance
zeolite, such as very good acid and fire resistance and also produces high
compressive strength. The earlier literature showed that adding some minerals
to the geopolymer can increase the performance. The first geopolymer
applications were building product developed in 1973-1976 23.

Mostly,
silicon 24 and aluminum 25,26 power and hydrogen peroxide solution
19,27–29 is utilized as a blowing agent to generate porous components from
geopolymeric slurries, however the redox reactions of metallic silicon (Si) and
aluminum (Al) are very strong, due to utilization of high alkaline system
10,19. In the preparation of fly ash-based geopolymer, the feedstock fly ash
is usually mixed with an alkali activator, such as sodium hydroxide (NaOH) or
potassium hydroxide (KOH) solution for the geopolymerization process. Alkali
solution used to activate the precursor by dissolving them into Si(OH)4
and Al(OH)4 monomers. The most common used alkaline activators in
geopolymerization are a combination of sodium silicate and sodium hydroxide.
Sodium hydroxide is used to react with the silicate and aluminum by adding Na+
ion.  Further Na+ ion is only
used during the polymerization 30.

The
exact mechanism through geopolymer hardening and setting occurs is not yet understood.
However, the utmost proposed mechanisms for the geo-polymerisation includes the
following stages that proceed in parallel and thus, it is difficult to be
distinguished: (i) dissolution of Si and Al from the solid alumino-silicate
materials in the strong alkaline aqueous solution, (ii) formation of
geopolymers precursors (oligomers species) consisting of polymeric bonds of
Si-O-Al and/or Si-O-Si type, (iii) poly-condensation of the oligomers to form a
three-dimensional alumino-silicate framework and (iv) bonding of the unreacted
solid particles and filler materials into the Geopolymeric structure and
hardening of the entire system into a final solid polymeric structure. Fly ash, which is rich in silica and alumina, has full
potential to use as one of the source material for Geopolymer binder. It
is the main solid waste generated from the coal combustion in the power
stations. Since the Worldwide electric power industry depends on heavily use of
coal as a primary energy source; enormous quantities of fly ash are generated
every year. According to 2000 estimation, the annual worldwide fly ash
production was more than 600 million tons. In India, the annual production of
fly ash is nearly 110 million ton and its generation is likely to reach 170
million tons. Presently, as per the Indian Ministry of Environment and Forest
figures, only 20 % to 30 % of fly ash is utilized in manufacturing of cement,
concrete, construction, block and tiles and some disposed off in landfills and
embankments, but a huge amount is unutilized which causes several environmental
issues related to air, soils and surface and ground-water pollution. The use of
fly ash in the preparation of geopolymeric materials for construction purposes
has been and continues to be subject of many research studies 31.

In
recent years, class F fly ash based geopolymer foam has attracted attention
from the scientific community due to its high potential as a green alternative
to conventional foams based Portland cement, and as a precursor for the production
of foams by appropriate pore forming agents 32,33. In previous studies, the
commonly used pore-forming agents include aluminum powder which liberates
hydrogen gas 34-37, metallic silica which produces silica fumes 38,39,
hydrogen peroxide which liberates oxygen gas 1,40,. Recent literature also
reported few other pore forming agents such as sodium hypochlorite 41,
perborates 42, and limestone 43. All the reported pore forming agents have
drawback either of metallic toxicity, harsh reaction condition or use of the
hazardous reagent.

In this paper, we
prepared class F fly ash geopolymer at temperature 100 °C using sodium
carbonate as mild alkali activator as well as a pore-forming agent. This
geopolymer is a Portland cement-free, non-autoclaved, foamed, composed of
alkali-activated class F fly ash, wherein, the CO2 liberation during
the activation process leads to make a pore network.

Author: