Geopolymer, coal ash, alkaline activator, compression strength

Conversion of fly ash of coal into geopolymer concrete

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

              CHAPTER – 4


4.1 Physical Properties of Coal Ash and OPC
The coal ash was obtained by combustion of Lakhra coal, which is further used for
preparation of geopolymer. The OPC, used was Kohat Cement, obtained from the local
market. The physical properties of the coal ash and Kohat cement are displayed in Table
Table 4.1 Physical properties of Coal ash and OPC.
Serial No. Parameter Coal Ash Kohat cement
1 Specific gravity 2.1 3.15
2 Fineness 0.4 0.21
3 Size µm <63 <63
4 Colour Reddish brown Dark grey
4.2 Characterization of Lakhra Coal Ash
The Lakra coal ash was characterized by XRF, EDX and FTIR spectroscopy, in order to
investigate its chemical composition.
4.2.1 XRF Analysis
The chemical composition of the coal ash was determined by the XRF method. Results
are shown in table 4.2, it is evident from the Results that coal ash is classified as class F
because sum of SiO2 + Al2O3 + Fe2O3 more than 50%, CaO less than 18 % of, and carbon
less than 6%. According to ASTMC618-19.Bottom coal was characterized by the highest
content of silicone and Aluminum oxides which are necessary oxides for the
geopolymerization [52].
Table 4.2 Elemental composition of Coal Ash determined by XRF.
Oxides % age Oxides % age
SiO2 40.406 SrO 0.106
Al203 28.274 Cr2O3 0.082
Fe2O3 20.754 ZnO 0.039
TiO2 4.602 CuO 0.032
SO3 2.567 MnO 0.029
CaO 1.946 Y2O3 0.013
K2O 0.943 NiO 0.012
ZrO2 0.175 Ga2O3 0.008
4.2.2. EDX Analysis
The Elemental composition of coal ash was determined by Energy Dispersive X-ray,
results are shown in table 4.3. It can be observed from the results that major elements
present in Lakhra coal ash include Al, Si, K, Na, Fe, C, Mn, Ti and Mg. According to
EDX analysis Result shows that coal ash mostly composed of silica and alumina
components which are mostly required during synthesis of geopolymeric materials [53].
Element %
Al 22.262
Si 32.534
K 2.062
Ti 5.472
Mn 0.052
S 2.062
Figure 1.2 Elemental composition of Lakhra coal by EDX analysis.
4.2.3. FT-IR Analysis
FT-IR spectroscopy was done to characterize the functional group for the fly ash
as shown in the figure 1.3. The peak at 600 cm−1
corresponds to Si-O-Al stretching
vibration. Whereas peaks at 1067 cm−1 may be attributed to Si-O-Si asymmetric
stretching vibration. The peak located at 1608 cm−1
implies to H-O-H vibration [54]. The
broad component at 1076 cm−1
is due to the Si-O-Si and Al-O-Si asymmetric stretching
vibration [55].
500 1000 1500 2000 2500 3000 3500 4000
120 intensity (a.u)
wave number (cm-1)
Figure 4.1. FTIR spectra of Lakhra coal ash.
The results of compression strength of the prepared coal ash based geopolymer are
summarized in table 1.4 the results shows that the 7 days compression strength of KOH
and calcium hydroxide activated coal ash based geopolymer was 39 psi and 21 psi which
is high compared to NaOH and calcium hydroxide. Which can be further increased by
increasing KOH and calcium hydroxide concentration. Shamsad Ahmad et al studied the
effect of alkali hydroxide on compression strength of geopolymer concrete the results
shows that KOH-based mixtures exhibited higher strength compared to the respective
NaOH-based mixtures [56]. Duangrudee Chaysuwan et al also used KOH based
geopolymer concrete the results shows that highest compression of 34 MPa with 10 M
KOH, cured at 40 °C, 24 h and heat treated at 550 °C [57].
Table 4.3 Strength of geopolymer produced from Lakhra coal ash (8 M Na