Fire-damaged recycled brick and concrete aggregates: enhancing early strength and reducing costs

Fire-damaged construction debris poses both a challenge and an opportunity for sustainable material recovery. This study investigates the use of 14-year-old, fire-damaged recycled brick aggregates (RBA) and recycled concrete aggregates (RCA) as partial replacements for natural coarse aggregates (NCA) in concrete up to 70%, utilizing a two-step mixing (TSM) method. Results indicate that Concrete mixes incorporating 20% and 40% fire-damaged RCA and RBA demonstrated notable improvements in mechanical performance. Tensile strength increased by approximately 20%, while compressive strength improved slightly by 2–3% compared to conventional NCA concrete. Early strength development was also enhanced, with 7-day compressive strengths reaching over 73% of the 28-day values, around 10% higher than the control mix. Microstructural analysis confirmed the development of a denser ITZ and reduced porosity at lower replacement levels, which helped restrain crack propagation. However, at higher replacements (55% and 70%), a notable increase in pore size (from 1–5 to 8–15 µm) and ITZ width (~ 5–7 to ~ 12–18 µm) indicated a decline in matrix compactness and structural integrity. Furthermore, the aggregate cost analysis indicated that 20% and 40% replacements could reduce overall material costs by 15% and 28%, respectively, while maintaining structural integrity. These results highlight the feasibility of incorporating fire-damaged recycled aggregates for sustainable, cost-effective concrete production without compromising performance.