Development of cementless binder for low thermal conductivity materials: Reactive ultra-fine fly ash mixed with co-fired fly ash

Low thermal conductivity materials are now an essential direction for construction material applications as the public has become more aware of global warming over the past few years. The development of cementless binders is also helping to improve the problem of global warming. This study uses reactive ultra-fine fly ash (RUFA) and co-fired fly ash (CFA) to develop an innovative cementless binder. The RUFA-CFA mixture undergoes hardening without alkali activators at a temperature of 50 °C before it is de-molded for air curing. The resulting lightweight cementless specimens present excellent thermal insulation, mainly due to the formation of interstitial pores between RUFA particles. The porosity of RUFA-CFA cementless specimens ranges from 41% to 50% with minimum thermal conductivity of 0.10 W/m·K. Our analysis reveals that the ultra-fine RUFA particles adhere to the CFA surface formed C-S-H colloids and ettringite via continuous hydrated reactions maintained by the CFA. The results obtained in this study provide a valuable reference for developing cementless composites based on industrial waste materials. •We developed a novel cementless composite (CC) without alkali activators.•The CC binders are ultra-fine fly ash and co-fired fly ash.•The thermal conductivity of the CC is ultra-low (< 0.12 W/m·K).•The porosity of the CC is between 41% and 50%.