EFFECTS OF FLY ASH AVERAGE PARTICLE SIZE, CAO, AND SI/AL ON COMPRESSIVE STRENGTH OF FLY ASH-BASED GEOPOLYMER AND ITS STRENGTH MODEL

As rule of thumb, compressive strength of geopolymer made of fly ash and water glass cured after 1 day at 80 degrees C is close to that after 28 days at room temperature, and can be used to shorten testing time. The strength growth profiles at room temperature follow the first order kinetic, different from the flat profiles at room temperature after 1 day at 80 degrees C. Three key material parameters affecting compressive strengths of fly ash-based geopolymers are water glass, fly ash, and the (water glass) liquid to (fly ash) solid ratio. The effects of water glass with moduli (of 1.5 and 1.8) and solid contents (from 11.5 to 46 wt.%) and liquid to solid (L/S) ratio (from 0.45 to 0.75) were studied using a given fly ash. The results show that water glass with higher solid content has higher strength. Water glass with higher modulus at a higher L/S ratio or lower modulus at a lower L/S ratio can have higher strength. Compressive strength can be as high as 57 MPa from this study, exceeding the 52.5 strength rating and close to the 62.5 rating. Eleven fly ash samples with various SiO2/Al2O3 (0.68 to 2.38), CaO (1.90 to 13.66%), and D-50 (4.5 to 122.7 mu m) were used to study their effects on compressive strength and develop their strength model using water glass modulus of 2.4 and solid content of 46 wt% at the water glass to fly ash ratio of 0.65. Higher CaO or lower D-50 shows higher compressive strength. An empirical strength model as functions of SiO2/Al2O3 (Si/Al), CaO, and D-50 were developed to study the effect of Si/Al. Higher Si/Al does not necessarily have higher strength, depending on the Si/Al, CaO, and D-50 values. Developing the strength growth model for water glass, fly ash, and their ratio is recommended.