Abstract
In this article, an investigation is reported on development of strength in South African fly ash
(FA) – based
geopolymer mixtures. Locally available Class F, FA from one of the coal power stations was used in
the investigation. The alkali-activator used consisted of sodium silicate (SS) and sodium hydroxide
(SH) mixed in varied ratios of 1.0, 1.5, 2.0, 2.5 and 3.0 SS to SH. The SS of silicate modulus =
2.5 was used but the SH concentration in the activator was varied to 10, 12, 14M NaOH. Mortars of
2.25 aggregate/binder ratio were used to prepare 50 mm cubes. In preparing mortar mixtures, the
liquid to solids (L/S) ratios were varied to L/S
= 0.3, 0.4, 0.5 and 0.6. Mortar cubes were cast and cured at 80oC for 7 days then tested for
compressive strength. It was found that all three parameters consisting of SS/SH ratio of the
activator, concentration of NaOH
used in the activator and the L/S ratio, showed significant influence upon compressive strength
development.
The optimum strength of the geopolymer mortar mixtures was obtained at SS/SH = 2.0,
12M NaOH
concentration ad L/S = 0.5.