Waterborne polyurethane modified CFBFA cementitious materials properties and mechanism

This study addresses the challenge of directly and efficiently utilising circulating fluidised bed fly ash (CFBFA), which is difficult to employ due to its high calcium, high sulfur, and low reactivity, by introducing environmentally friendly waterborne polyurethane (WPU) as a modifier. A systematic investigation was conducted to explore the effects of WPU on the mechanical properties and hydration mechanism of CFBFA-based cementitious materials. The results indicate that with a WPU dosage of 1%, the material exhibits optimal compressive and flexural strengths at all ages. Its 28-day compressive strength increases from 15.97 to 21.87 MPa, an improvement of 36.94%; and its flexural strength increases from 3.83 to 4.03 MPa, an improvement of 5.22%. An appropriate WPU amount, via polar groups like carboxyl and amino, coordinates with inorganic ions to form an organic–inorganic interpenetrating network. This promotes early hydration, increases gel products, inhibits carbonation, and refines pore structure (more harmless/less harmful pores), improving compactness and interfacial bonding. In summary, unlike previous WPU-modified cement-based materials or alkali-activated geopolymer systems, this study achieves alkali-free activation of high-calcium CFBFA solely through WPU, revealing a novel ‘promotion and encapsulation’ mechanism and providing a new paradigm for the high-value utilisation of low-reactivity solid waste.