Concrete with sustainable fillers at elevated temperatures: A review

Fillers such as fly ash, slag, and biochar offer potential solutions for addressing carbon emissions from cement manufacturing and improving waste management. However, concrete with fillers experiences severe thermal damage at elevated temperatures due to issues like thermal incompatibility, pore pressure build-up, thermal stress, and phase transformation. This paper offers a comprehensive review of how fly ash, slag, and biochar impact concrete when subjected to high temperatures. It reviews phase stability, alterations in microstructure, thermal damage, and mechanical behaviour, as well as approaches to improve concrete's fire resistance. Fly ash and slag reduce microcracks in concrete during heat exposures by consuming free portlandite (Ca (OH)₂) during cement hydration, while biochar mitigates pore pressure in the matrix. However, fillers lower concrete's thermal conductivity, increasing temperature gradients and reducing fire resistance. A mix of steel and polypropylene fibers enhances fire resistance more effectively than using a single fiber type.