From low-grade clay to promising SCM: The potential of calcined marl for low-carbon cement

This study investigates the potential of calcined marl as a single-source supplementary cementitious material (SCM) for low-carbon cement, serving as an alternative to limestone calcined clay cement (LC 3 ). Results revealed a critical thermal activation window between 700 and 800 °C, with the optimal condition identified at 790 °C for 2 h. Within this range, the overlapping decomposition of calcite and illite allows nascent CaO to act as a network modifier, facilitating the formation of a highly reactive amorphous phase. Conversely, calcination above 900 °C drastically reduced reactivity due to gehlenite recrystallization, surface enrichment of calcium, and particle agglomeration. At a 30% cement replacement level, hydration studies demonstrated that despite exhibiting higher CH retention and lower C-S-H Si/Ca and Al/Ca ratios than LC 3 , calcined marl cement achieved comparable mechanical performance (over 90% of the reference strength at 28 days). This strength development was driven by a physical-chemical synergy, where the intermixing of carboaluminate phases and C-S-H gel refined the pore structure, leading to a 6.8% increase in the mesopore fraction compared to the reference cement at 28 days. Furthermore, correlation analysis results indicate that the standard R³ test should be applied with caution when predicting the reactivity of marl. These findings position optimally calcined marl as a natural, highly efficient analog to LC 3 for sustainable binders.