Upcycling of Waste Materials: Green Binder Prepared with Pure Coal Fly Ash

Abstract

Extensive studies have focused on the beneficial use of coal fly ashes (CFAs) as a partial replacement for cement in concretes or mortars, in the effort of reducing the environmental footprint and embodied energy of concrete materials while removing CFAs from the waste stream. In this study, a pure fly ash paste (PFAP) was developed in place of ordinary portland cement paste (OPCP). This PFAP was prepared at room temperature and without direct alkali activation, unlike geopolymer or other fly-ash-based cementitious materials that need to be cured at a relatively high temperature or activated by alkalis. The samples were prepared using only the as-received class C coal fly ash, water, and a very small amount of borax (Na2B4O7). On average, the PFAP featured 28-day compressive strength of about 36 MPa, and micro-nano hardness and elastic modulus 29% and 5% higher than the OPCP, respectively. The PFAP also featured 15% lower gas permeability, 27% higher Cl− diffusivity, 20% lower dry density, and considerably higher surface resistivity and bulk resistivity. These mechanical, durability, and other properties of the PFAP make it a viable environmentally friendly (green) construction binder suitable for a host of structural and nonstructural applications. Advanced characterization of the raw material and PFAP pastes was employed to elucidate the hydration mechanisms of this green binder. While this work only showcases the properties of one specific CFA, the obtained knowledge sheds light on the role of class C CFAs in the hydration process and may benefit the expanded use of various CFAs in cementitious materials.