Calcium-free geopolymer construction blocks from spent fluid catalytic cracking catalyst: Formulation and optimisation for curing at ambient temperature

The development of more sustainable masonry construction materials requires alternatives to conventional clay and concrete units, whose production entails high energy consumption and substantial CO₂ emissions. This study aims to upscale the synthesis of ambient-cured geopolymer masonry units derived from residues of spent Fluid Catalytic Cracking (FCC) catalyst by developing, first, a binder and then a mortar (binder + sand) with adequate structural stability and durability for industrial production. The optimised formulation, comprising 15 M NaOH, sodium silicate (Ms = 0.85), an activator-to-FCC ratio of 1.1, and 65 wt% sand, yielded a dense, crack-free mortar. The curing process was optimised to enable early demoulding after 24 h of pre-curing at ambient temperature without moisture protection, followed by six days of sealed curing. This regime promoted stable volumetric behaviour and efficient strength development without thermal input. The optimised mortar achieved 28-day compressive and tensile strengths of 28.8 MPa and 3.9 MPa, respectively, thereby validating the efficacy of the methodology employed. When scaled up to full-sized construction blocks, one solid and two hollow geometries, the FCC-based blocks exhibited apparent densities above 2000 kg/m³ , open porosity between 27% and 30%, and compressive strengths ranging from 13 to 24 MPa, exceeding standard requirements for load-bearing applications.