The influence of the NaOH solution on the properties of the fly ash-based geopolymer mortar cured at different temperatures

Abstract

In this study, geopolymer mortar was produced using Class F fly ash from the thermal power plant in Kütahya Seyitömer (Turkey). The changes caused by the geopolymerization on the properties of the final product were investigated by applying curing on geopolymer mortars in different NaOH concentrations at different temperatures and for different curing times. The purpose of this process was to determine the relationship between alkali solution concentration, curing temperature and curing time. In order to determine the effect of NaOH concentration on geopolymer mortars, three different molarities of NaOH concentrations (3M, 6M and 9M) were used together with sodium silicate (water glass) solution. The samples were cured at two different temperatures (65 and 85°C). Physical properties such as porosity, bulk density, apparent density and water absorption, and mechanical properties such as flexural strength and compressive strength were determined from the 7-day geopolymer mortar samples after the curing process. As a result, this study determined that curing temperature and curing time had an effect on the physical properties of the geopolymer mortars. It was observed that NaOH concentration had a clear effect on the properties of the mortar cured at 85°C. Compressive strength values of 21.3MPa and 22MPa were obtained from the mortar of 6M concentration cured at 65°C for 24h and from a sample of the same mortar cured at 85°C, respectively. Compressive strength values of the geopolymer mortars cured at 85°C increased depending on the curing time and the increase in NaOH concentration. Given the strength values obtained, the optimal thermal curing temperature and the optimal NaOH concentration were 85°C and 6M, respectively.