This research work studies the compressive strength development and the relationship between compressive strength and thermal conductivity of an eco-friendly cementless mortar. The samples were prepared from a mixture of ground granulated blast furnace slag (GGBFS) and rice husk ash (RHA) with the addition of circulating fluidized bed combustion (CFBC) fly ash as an activator. The effect of RHA content on the compressive strength development and thermal conductivity of the mortars was investigated. Test results show that all of the mortar samples exhibited the lower strength than the OPC. However, the strength development rate was significant at the later ages of the cementless mortars. The compressive strength values of the mortar samples ranged from 29.6 to 34.5 MPa. The optimal mixture was found at 30% RHA content. The results of this study demonstrate a great potential for applying the cementless binder as a substitution of OPC for sustainable development.