Effect of Mix Parameters on Strength of Geopolymer Mortars - Experimental Study

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 and L/S = 0.5