Conversion of fly ash of coal into Geopolymer concrete

Conversion of fly ash of coal into Geopolymer concrete

ABSTRACT Traditional cementing materials (OPC) produced a lot of greenhouse gases such
CO2, NOx, SO3 which cause various environmental problem, It has been estimated that
for the production of one ton of cement, about two tons of raw materials (i.e. limestone
and shale) is consumed, and approximately one ton of carbon dioxide (CO2) and nitrogen
oxide (NO) gasses emitted (i.e. 0.87 ton of CO2 and 3 kg of NO). In contrast geopolymers
is ecofriendly cement material that exhibits good mechanical properties, thermal
resistance, highly durability and greenness. Geopolymer is commonly known as
inorganic aluminohydroxide polymer which is synthesized predominantly from silicon
and aluminum rich sources mainly coal ash and GGBS with alkaline liquid. In this work,
geopolymers were synthesize from coal ash derived from Larkra (ASTM class f). The
geopolymers were cured at three different curing conditions (Room Temperature, 60 ℃
and 80 oC) for 7 days. XRF, EDX FT-IR were used to characterize Coal ash,
geopolymers paste and their compressive strength were found out. The results reveled
that compressive strength of geopolymers depends on kOH concentration, curing
conditions and also coal ash to potassium hydroxide and Ca (OH2) mass ratio. Significant
Compressive strength of 0.226 Ksi=1.55 Mpa was obtained with 12M KOH ,coal ash to
potassium hydroxide ratio of 3:1 , 10 % Ca(OH2) at 6OOC

Keywords: Geopolymer, coal ash, alkaline activator, compression strength

The conversion of coal ash into geopolymer has been extensively studied, a brief review
of literature over the past few years is given below.
Alexandre Silva de Vargas et al. synthesized geopolymer alkali-activated fly ash
produced with combined NaOH and Ca(OH)2 activators. The compressive strength test
results shows that the M15 and M25 mixes produced using combined NaOH and
Ca(OH)2 activators achieved 18 and 28 MPa after 1 and 7 days respectively[48].
Adriano Michael Bernardin et al. synthesized geopolymer from bottom coal ash and
calcined paper alkali activator like of different concentration 5,10,15 M, sodium silicate
were used. Significant Compressive strength were obtained with alkali concentration of
15 M NaOH, bottom ash and calcined paper ratio of 2:1 [49].
N A Lioyd and B V Rangan, et al. studied that geopolymer concrete may be formed
from the reaction of a source material having sample amount of silica and alumina with
alkaline liquids. Who worked with fly ash –based geopolymer concrete presented. They
used to find out the effects of major factors which the chemistry of geopolymer concrete
.He also suggested a simple method for the design of geopolymer concrete mixture [44].
Rashidah Mohamed Hamidi et al. reported that for the preparation of geopolymer it
involves the uses of aluminosilicates materials along with alkali activators as a major
starting materials anyhow former will be fly ash, slag or kaolin and the most popular used
as sodium hydroxide (NaOH) and sodium silicates (Na2SiO3) respectively .Geopolymer
samples prepared under the fixed curing temperatures (60 ℃) and curing time was
reported (1day).This paper also discuss furthermore the effects of NaOH concentration
towards the properties of fly ash based geopolymer product. Based on the given results
obtained, the optimum 12 M NaOH concentration at which geopolymer performs the best
mechanical was achieved. They used Fourier transform infrared spectroscopy (FTIR) for
structural determination, scanning electron microscopy (SEM), to know about the
morphology the mechanical properties (Flexural strength ) was done by Universal testing
machine (UTM) [50].
Nakshatra B Singh et al. reported that large amount of waste entering into environment
from different industries which have has great challenging in the disposal and effects on
the environment, especially fly ash which is added into environment from various sources
one of them is thermal power plants, which is composed of alumino silicates materials
and generating a big environmental problem. In last decade it has been found that
geopolymer may be used for the solution of waste problems and environmental issues.
Geopolymer is an inorganic polymer first of all introduced by Davdovits Polymer
concrete can be supposed as an innovative and alternative material to the ordinary
Portland cement concrete .Uses of fly ash as a raw material reduces the waste production
of thermal power plants and protects the environment. Geopolymer concrete have very
high initial strength and resistance to an aggressive atmosphere .the properties of
geopolymer cement/mortar/concrete have been under various conditions is also variable.
Fire resistance properties and 3D printing technology have also been studied [44].
M Shahib Al Bari et al. studied that geopolymer is a kind of concrete technologies to
use the fly ash from burning of coal residue. They also studied the effects of calcination
at 1000 ℃ on five samples of fly ash. All fly ash was activated with alkali activators
having a mass proportion of NaSiO3 to NaOH was kept constant at pH 12. Compression
test was performed at 3, 7, 14, 21, 28, and 56 days to obtain the influenced of calcination
to the compressive strength. They also found that amorphous phase of fly ash dropped
after calcination as all compression test result was less than 1 Mpa. The calcination also
effects the geopolymerization all of the setting time exceeded up to seven days causes no
compressive strength at early hours. According to them the compressive strength of
geopolymer pastes using fly ash were less than 1 Mpa at 56 days .Clearly, the reduction
of CaO content reduced the compressive strength .the highest strength Was 32.96 Mpa
gained by j fly ash on the Si/Al ratio of 2.23. The lowest strength was 3.14 Mpa obtained
by L fly ash on the Si/Al ratio of 2.17. Recalcination at 1000 ℃ on fly ash greatly
decreased compressive strength of geopolymer paste into less than 1Mpa at 56 days on
geopolymer paste using fly ash [51].


3.1 Chemicals and Reagents
All the chemicals used were analytical grade. Coal ash obtained from lakara
Sindh Pakistan. Calcium hydroxide, potassium hydroxide (KOH) and sodium hydroxide
(NaOH) were purchased from Merck.
3.2 Coal ash
In this study low calcium ash were used for the synthesis of geopolymer. Coal ash was
reddish brown in color and on the basis of chemical composition silicon and aluminum
oxide contents are classified as class F according to ASTMC618.
3.3 preparation of alkaline activator solution
Alkaline solution of various concentration 8, 10 and 12 M of NaOH, KOH and 10 %
solution of calcium hydroxide were prepared to chemically activate the coal ash
conversion to geopolymer.
3.4 Sand and aggregate
Locally available sand and aggregate are used in this study
3.5 Geopolymer synthesis
About 75 g of Coal ash was taken and mixed with alkali activator solutions i.e. NaOH
and KOH of different concentration in a beaker. The solid to liquid ratio between the coal
ash and activator solution was 3:1. (About 75% of coal ash and 25 % alkali activator)
The mixture was stirred on a magnetic stirrer to obtain a homogeneous mixture. Extra
amount of water were also added to paste to obtained paste consistency according to
ASTM C187. Then known amount of calcium hydroxide was added to the paste and
stirred for 20 min and then poured in to wooden Molds. The geopolymer paste was
further used for testing the mechanical strength.
3.6 Characterization
The geopolymer prepared in the laboratory was characterized by various
instrumental techniques, which are discussed as under.
3.6.1 FT-IR Spectroscopy
FTIR analysis of samples was carried out through FT-IR spectrophotometer
(Perkin Elmer, Spectrum II), integrated with ATR sample base diamond plate. The
spectra of the samples were recorded at resolution of 4 cm-1
and in scan range of 4000 to
450 cm-1
by NTOS2 software.
3.6.2 X-Rays Diffraction (XRD) Analysis
The crystalline structural phases of the geopolymer were determined by X-ray
diffractometer (Xpert Philip). The wavelength used was 1.54 A⁰
and the source of
radiation was CuKα. The diffract grams were recorded at wavelength range of 10 to 70
3.6.3 SEM and EDX Analyses
Scanning electron Microscope (Model JEOL-Jsm-5910; Japan) was used to
analyses the surface morphology of adsorbent material. The elemental determination was
done through Energy Dispersive X-ray technique, using X-ray detector coupled with
3.7 Compressive strength
The geopolymer paste was mixed with sand and aggregate in the ratio of 2:1 and then
casted into molds to make slabs of the dimension 150 x 150×150 cm cube. The
geopolymer slabs were placed in water tank and cured for 7 days and then subjected for
test of compression strength.
The compressive strength of the synthesized geopolymer was mortar cubes was measured
in the universal testing machine. The cubes were placed in the machine opposite to their
casting phase.It is describe in figure below 1.1.
Fig 1.1 Geopolymer concerte Testin