Multiscale investigation of sustainable spalling-resistant ultra-high performance concrete containing sludge-derived slag derivatives and recycled polypropylene fibre

This study investigates the fire performance and microstructural evolution of ultra-high performance concrete (UHPC) incorporating ground sludge-derived slag (GSDS) as a partial cement replacement, sludge-derived slag (SDS) as a fine aggregate replacement (less than 4 mm) and recycled polypropylene (PP) fibres for spalling prevention. A multiscale approach was adopted to investigate the effect of GSDS addition on phase assemblages and degradation mechanisms of ordinary Portland cement (OPC) and OPC/GSDS systems at elevated temperatures, correlating microscopic observations with macroscopic spalling behaviour. The findings revealed that the accelerated hydration in OPC/GSDS samples promoted incorporation of aluminates (Al) into C-S-H and increased its degree of polymerisation, thereby enhancing resistance to high temperature-induced dehydration and improving strength retention up to 600 °C. Mercury intrusion porosimetry (MIP) further confirmed that GSDS contributed to a denser microstructure at elevated temperatures. Spalling tests showed that larger SDS aggregates induced interfacial microcracks, while PP fibres formed interconnected vapour-release pathways through tangential microcracks around the aggregates and empty fibre channels, collectively increasing permeability. With 12 kg/m³ of recycled PP fibre, explosive spalling was completely prevented, reducing mass loss to 12.2 % compared to 100 % in the control UHPC. These results highlight the effectiveness of combining GSDS, SDS and PP fibres to develop a sustainable, spalling-resistant UHPC. This study provides valuable insights into the use of waste-derived supplementary cementitious materials (SCMs), alternative aggregates and fibres in enhancing the fire performance of UHPC, contributing to sustainable development in the built environment.