This paper presents the results of a study on the effects of the solids-to-liquid (S/L) ratio and of curing temperature on strength development, phase composition, and microstructure in geopolymers that prepared using a mixture of class-F fly ash (FA) and residual rice husk ash (RHA) and a solution of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The samples were prepared at various S/L ratios (2.4–2.8) and then cured at various curing temperatures (room temperature (RT)–90^°C) in order to investigate these effects. A compressive strength test was conducted on the samples after the curing process in order to determine the development of strength over time. Moreover, 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) was used to characterize the surface morphologies of the samples. The present study found that the S/L ratio and the curing temperature had significant effects on the properties of the geopolymer samples. The compressive strength of all of the geopolymer samples increased with curing age. The highest strength obtained from the samples that were prepared at an S/L ratio of 2.6 and a curing temperature of 60^°C. XRD and FTIR analyses showed that the major crystalline phases presented in the resultant samples were quartz, mullite, and cristobalite with the additional presence of minor zeolite phases. The results of the present study support the use of FA and RHA as solid waste materials in the preparation of geopolymers.