Effect of alkali-activator and rice husk ash content on strength development of fly ash and residual rice husk ash-based geopolymers

• A mixture of fly ash and residual rice husk ash is introduced as new binder material.
• The alkali-activator and rice husk ash affects the strength development significantly.
• The strength peaked at an optimum activator concentration and rice husk ash content.
• The compressive strength of all geopolymers increased with ageing time.
• The geopolymer system included an amorphous gel phase and a crystalline zeolite phase.

This study combines various proportions of class-F fly ash (FA) and residual rice husk ash (RHA) with an alkaline solution to produce geopolymers. All of the geopolymer samples were cured at 35 °C and at 50% relative humidity until the required testing ages. The effects of the RHA content (0–50%) and of the concentration of the sodium hydroxide (NaOH) solution (8–14 M) on the compressive strength development of the samples were then investigated. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to examine the microstructural properties of the samples. Further, scanning electron microscopy (SEM) coupled with energy dispersive spectrometer (EDS) was used to characterize sample surface morphologies and compositions. Results found that the samples prepared with a NaOH concentration of 10 M and a RHA content of 35% exhibited the highest compressive strength and that increasing the NaOH concentration and RHA content beyond these values exhibited decreasing compressive strength. Chemical analysis showed that the major crystalline phases presented in the resultant geopolymer were quartz, mullite, and cristobalite. Furthermore, minor zeolite phases were detected in all of the geopolymer samples. The results of the present study support FA and RHA as promising solid waste materials for use in the production of geopolymers.