Supersulfated cement based on mechanically activated fired brick powder: Fresh properties, mechanical strength and microstructural characteristics

This study explores the use of mechanically activated waste fired brick powder (FBP) as a substitute for Ground Granulated Blastfurnace Slag (GGBS) in Supersulfated Cement (SSC) formulations. Mortars and pastes were manufactured with GGBS replacement levels ranging from 0 wt% (FBP0) to 100 wt% (FBP100). The FBP10 combination achieved the lowest fresh consistency, increasing by 17 % compared to the reference. Higher FBP contents increased consistency due to greater water demand. All combinations exhibited a pH peak within the first 24 h after mixing, followed by a gradual decrease and stabilization by 48 h. The pH level was governed by the GGBS replacement rate, with lower values observed at higher FBP contents. The onset and progression of setting were closely linked to pH changes. Setting times decreased with increasing FBP content, due to reduced ettringite formation, higher water affinity, and enhanced nucleation from FBP’s finer particles. Mortars cured in water showed slight expansion, while air-cured samples exhibited shrinkage up to 0.138 % (FBP5), attributed to limited water availability for ettringite and the formation of Calcium Silicate Hydrate (C-S-H) gels. At 90 days, FBP10 improved compressive strength by 16.8 % over the reference; substitutions up to 50 % maintained or enhanced the mechanical performance of FBP0. Microstructural analyses (Thermogravimetry, X-Ray Diffraction and Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy) confirmed ettringite and C-S-H gels as the only hydration products. These results demonstrate the potential of FBP as a sustainable and technically viable precursor for SSC, reducing reliance on industrial by-products while maintaining or improving cement performance. • Fired Brick Powder (FBP) influenced the workability and setting time of SuperSulfated Cement (SSC). • FBP reduced the pH values of SSC compared to the reference SSC. • FBP reduced the free shrinkage of SSC compared to the reference SSC. • FBP improved the mechanical properties of the reference SSC. • Ettringite and C-S-H gels were the only hydration products in FBP-SSC.