Utilization of oil shale residue from Alberta Taciuk process in phosphogypsum slag cement: compressive strength, hydration behavior, and microstructural characteristics

This study investigates the feasibility of using oil shale residue (OSR), a byproduct of the Alberta Taciuk Process, as a supplementary cementitious material (SCM) in phosphogypsum-slag cement (PSC). OSR replaced slag at 20 %, 40 %, and 60 % by mass. The effects on compressive strength, hydration behavior, and microstructure were systematically evaluated. Results showed that 20 % and 40 % OSR replacement improved the 28-day compressive strength by 30.5 % and 17.4 %, respectively. This enhancement was attributed to increased formation and polymerization of calcium–aluminosilicate–hydrate (C-(A)-S-H) gels at later hydration stages. Analysis of hydration products confirmed reduced ettringite (AFt) content and increased chemically bound water, indicating a shift in hydration mechanisms. Fourier transform infrared spectroscopy (FTIR) and energy dispersive spectroscopy (EDS) revealed the formation of highly polymerized, Al-rich C-(A)-S-H gels. However, early-age strength declined due to OSR's limited reactivity. Although hydration heat analysis indicated accelerated initial dissolution and shortened induction periods, the effect was not sustained beyond 24 h. Microstructural observations showed that OSR particles enhanced matrix densification only when sufficient C-(A)-S-H was present. At 60 % replacement, gel deficiency and poor particle bonding reduced overall strength. These findings highlight OSR's dual role as both filler and reactive component. Its performance depends on balancing dosage and hydration product availability. This study provides insight into utilizing OSR for low-carbon PSC development and supports broader sustainable cement strategies.