Prediction of the Compressive Strength from Resonant Frequency for Low-Calcium Fly Ash–Based Geopolymer Concrete

This study focuses on using fly ash to produce a cement-free geopolymer concrete. The combination of potassium hydroxide (KOH) and potassium silicate solution (${\mathrm{K}}_2{\mathrm{SiO}}_3$) was used as the alkaline activator to initiate the geopolymerization reaction. The water immersion curing method was used. The concrete specimens were first cured at 60°C for 24 h, followed by ambient temperature curing until testing. The effect of concentration of KOH and the ratio of ${\mathrm{K}}_2{\mathrm{SiO}}_3$ to KOH (PS/PH ratio) on the compressive strength was investigated. The experimental results revealed that the geopolymer concrete activated with 14M KOH solution with the PS/PH of 2.0 exhibited the highest strength. This study also proposes a nondestructive method for the prediction of compressive strength. By using multiple regression technique, an accurate predictive relationship was established between the compressive strength and the longitudinal resonant frequency. The effectiveness and the accuracy of the proposed equation were proven by applying it to a previous study by another researcher.