10.1016/j.conbuildmat.2014.05.031
Characterization of mechanical and microstructural properties of palm oil fuel ash geopolymer cement paste
The palm oil fuel ash (POFA), obtained from burning of palm oil shells, husk and fibers, was collected from a mill at Johor State, south of Malaysia. The raw palm oil fuel ash was oven dried at 110 +- 5 degC for 24 h , sieved with a 300 μm sieve to remove large unwanted particles and incompletely combusted materials [49] and [50], and then it was ground by a modified Los Angeles machine [42]. The specific surface area after grinding was 0.915 m2/g. As can be seen, major components are SiO2 and CaO with concentrations of 47.37% and 11.83%, respectively with a low amount of Al2O3 (3.53%). The relatively high amount of CaO available in the POFA is most likely from lime and fertilizer [41]. It is worth to mention that POFA is a not toxic waste material in terms of heavy metals leachability [51]. 
Sodium hydroxide and sodium silicate were chosen as alkali activators in this investigation. The sodium hydroxide was in industrial-grade with minimum 99% purity. Industrial grade water glass (Na2SiO3) solution was chosen with a chemical composition of 15.33% Na2O, 31.28% SiO2, and 53% H2O. The alkaline activator selection was based on the recommendations in [34], [53], [54] and [55]. The alkali activation solution was prepared by mixing Na2SiO3 with NaOH within ratios ranged between 0.5 and 3.0 [34]. 
Preliminary experiments were conducted to study the alkali activation of POFA [56]. It revealed that raw POFA cannot be used without sieving and grinding because of the low compressive strength results which is related to the porous structure and high demands for the alkaline activator. Experimental program was designed in order to investigate the ability of POFA to be incorporated in geopolymer technology. NaOH solution was mixed with Na2SiO3 to produce six ratios (0.5, 1.0, 1.5, 2.0, 2.5, and 3) to prepare alkaline activator solution 24 h prior to use. Ground POFA and the alkaline activator were then mixed with two solid-to-liquid ratios (1.0, 1.32) as in [34], [53], [57] and [58]. Mix one with sodium silicate to sodium hydroxide ratio (0.5) was unable to be used; the geopolymer paste had such low workability that could not be cast in molds. 
The palm oil fuel ash was mixed directly with the alkaline activator. First the ash was discharged to the mixer pan, and then the alkaline was added and mixed for 1.0 min at a normal speed rate (gear one). Then, the mixer stopped for 10-20 s in order to scrap the un-mixed ash on the sides of the paddle and the pan, then the mixing was continued with a medium speed (gear two) for another 1.0 min in accordance to ASTM C305 [59]. All the geopolymer paste mixes were blended and produced with a small blender (HOBART Mixer). The fresh geopolymer paste was then cast into 50 x 50 x 50 mm iron molds in two layers immediately after mixing. To compact the specimen, each layer was vibrated by a vibrating table for 25-30 s. This procedure was used to remove the bubbles from the paste. After casting and vibrating, oven curing was applied directly at 60 degC for 2 h. The molds were then kept in the laboratory to reach ambient temperature. The specimens were kept in a plastic bags after de-molding to prevent any moisture loss and were stored to the date of testing [55].
sieve
300 μm
μm
raw palm oil fuel ash
dried
110 +- 5 degC
degC
raw palm oil fuel ash
dried
24 h
h
specific surface area
0.915 m2/g
m2/g
SiO2
concentrations
47.37%
%
CaO
concentrations
11.83%
%
Al2O3
concentrations
3.53%
%
sodium hydroxide
purity
99%
%
Na2O
chemical composition
15.33%
%
SiO2
chemical composition
31.28%
%
H2O
chemical composition
53%
%
Na2SiO3 with NaOH
ratios
ranged between 0.5 and 3.0
NaOH solution was mixed with Na2SiO3
ratios
0.5
NaOH solution was mixed with Na2SiO3
ratios
1.0
NaOH solution was mixed with Na2SiO3
ratios
1.5
NaOH solution was mixed with Na2SiO3
ratios
2.0
NaOH solution was mixed with Na2SiO3
ratios
2.5
NaOH solution was mixed with Na2SiO3
ratios
3
prepare alkaline activator solution
prior
24 h
h
Ground POFA and the alkaline activator
solid-to-liquid ratios
two solid-to-liquid ratios (1.0, 1.32
Mix one
sodium silicate to sodium hydroxide ratio
0.5
alkaline
mixed
1.0 min
min
mixer
stopped
10-20 s
s
mixing
continued
1.0 min
min
iron molds
50 x 50 x 50 mm
mm
layer
vibrated
25-30 s
s
oven curing
applied directly
60 degC
degC
oven curing
applied directly
2 h
h