2.2.2. Concrete
Concrete mixtures (Table 3) were designed for all the mortar
mixtures of Table 2 containing 40% alkaline liquid. An alkaline
liquid content of 40% was selected based on the previous works
on fly ash based geopolymer concrete [32]. Mixture 2 was designed
with 6% OPC instead of 5% OPC in the corresponding mortar mixture.
Considering average unit weight of fly ash based geopolymer
concretes reported in previous works and trial mixtures tested for
this study, the final unit weight of the concrete was taken as
2420 kg/m3. The total binder content was constant as 400 kg/m3.
The other mixture variables such as percentage of OPC content
and total alkaline solution and ratio of alkaline solutions remained
same as those in the corresponding mortar mixtures.
2.2.3. Paste
Setting time was tested on geopolymer paste mixes having the
same proportions of binder and alkaline activator of the corresponding
mortar mixes with the fine aggregate excluded.
To investigate the microstructure of OPC blended fly ash based
geopolymers, two paste mixes were prepared containing OPC as
10% and 50% of the total binder (OPC and fly ash). The results were
compared with those of a control fly ash geopolymer paste sample.
Alkaline solution with the ratio of sodium silicate to sodium
hydroxide solution of 2.5 was used as 40% of the total binder. Samples
were cured in room temperature (20–23 C) and investigated
for general microstructural features after 28 days of age.
2.2.4. Designation of mixtures
Mixtures have been designated in terms of the variable components
of the mixture; namely percentage of OPC in the total binder
(P5, P6, P8, P10 and P12), percentage of alkaline activator to the
total binder (A35, A40 and A45) and the ratio of sodium silicate
to sodium hydroxide solution (SS/SH as R1.5, R2.0 and R2.5).
2.3. Casting and curing of test specimens
The mixing of geopolymer mixtures involves two general steps:
preparation of the alkaline activator solution and final mixing of all
ingredients. The alkaline activator solution was prepared about
20 min before final mixing with the other ingredients. The sodium
hydroxide solution and sodium silicate solution of desired quantity
were mixed together in the laboratory and left in a water bath at
room temperature to cool down. The exothermic reaction caused
by the mixture of solutions produces heat; hence it requires cooling
before adding to other ingredients. The geopolymer mortars
were mixed manually in a laboratory pan to obtain a uniform mixture.
Saturated surface dry (SSD) sand and the binders (fly ash and
OPC) were mixed thoroughly before adding the activator solution.
Premixed alkaline activator solution was then added gradually and
mixing was continued for another 4–6 min until a consistent mixture
was obtained. The similar procedure of mortar mixing was followed
for the mixing of concrete. Coarse and fine aggregates,
which were prepared in saturated surface dry condition, and the
binders (fly ash and OPC) were dry-mixed thoroughly in the mixing
pan for two minutes before adding the activator solution. Fresh
mortar mixture was cast in cube moulds (50 50 50 mm3) and
concrete mixture was cast in cylindrical concrete moulds of
100 mm in diameter and 200 mm in height. The moulds were filled
in two layers and each layer was compacted on a vibrating table.
They were then stored in a controlled temperature of 20–23 C
and relative humidity 65 ± 10%. Samples were removed from the
mould after 24 h of casting and left in air to cure at 20–23 C and
relative humidity 65 ± 10% until tested to ensure a consistent environment
for all samples rather than a variable ambient condition.
The control geopolymer mixture (Mix 1) was de-moulded three
days after casting, as it was too soft to remove from the mould after
24 h due to its slow setting at this temperature.